Patent Grant
7479694
1. Field of the Invention
This invention relates to methods for fabricating integrated circuits on and in flexible membranes, and to structures fabricated using such methods.
2. Description of Related Art
Mechanically and thermally durable free standing dielectric and semiconductor membranes have been disclosed with thicknesses of less than 2 μm. (See commonly invented U.S. Pat. No. 4,924,589, and U.S. patent application Ser. No. 07/482,135, filed Feb. 16, 1990, now U.S. Pat. No. 5,103,557, both incorporated herein by reference). This disclosure combines the novel use of these technologies and other integrated circuit (IC) processing techniques to form ICs as membranes typically less than 8 μm thick. This approach to IC fabrication falls under the generic industry-established title known as Dielectric Isolation (DI), and is inclusive of subject areas such as Silicon-on-Insulator (SOI) and Silicon-on-Sapphire (SOS). ICs formed from dielectric and semiconductor membranes can reduce significantly the number and complexity of processing steps presently used to provide complete IC device isolation; dielectric isolation techniques that provide dielectric isolation on all surfaces of the individual circuit devices comprising the complete IC are not as yet widely used in volume IC fabrication. Integrated Circuits are defined as commonly understood today when referring to SSI, MSI, LSI, VLSI, ULSI, etc. levels of circuit complexity.
This invention is directed to a general method for the fabrication of integrated circuits and interconnect metallization structures from membranes of dielectric and semiconductor materials. The fabrication technology in accordance with this invention is referred to herein as Membrane Dielectric Isolation (MDI), and the circuits made from it as circuit membranes. The novel use of materials. and processing techniques provides for the fabrication of high temperature, mechanically durable, large area free standing membranes (greater than 1 square cm in area) from low stress dielectric and/or semiconductor films. These membranes permit the application (continued use) of most of the established integrated processing methods for the fabrication of circuit devices and interconnect metallization.
In accordance with the invention, an integrated circuit is formed on a tensile low stress dielectric membrane comprised of one layer or a partial layer of semiconductor material in which are formed circuit devices and several layers of dielectric and interconnect metallization. Also, a structure in accordance with the invention is a tensile membrane of semiconductor material in which are formed circuit devices with multiple layers of tensile low stress dielectric and metallization interconnect on either side of the semiconductor membrane.
The membrane structure is a processing or manufacturing structure for enabling the manufacture of novel and more cost effective integrated circuits. This is in addition to an objective to manufacture an integrated circuit, or portion thereof, in a membrane or thin film form.
The general categories of circuit membranes that can be made by this invention are:
1. Large scale dielectric isolated integrated circuits formed on or from semiconductor or non-semiconductor substrates.
2. Multi-layer interconnect metallization circuits formed on or from semiconductor or non-semiconductor substrates.
The primary objectives of the MDI fabrication technology disclosed herein are the cost effective manufacture of high performance, high density integrated circuits and integrated circuit interconnect with the elimination or reduction of detrimental electrical effects on the operation of individual circuit devices (e.g. diodes, transistors, etc.) by completely isolating with a dielectric material each such circuit device from the common substrate upon which they are initially fabricated, and therefore, from each other, and to provide a more versatile and efficient physical form factor for the application of integrate circuits. Some of the benefits of the MDI IC fabrication process are the elimination or reduction of substrate current leakage, capacitive coupling and parasitic transistor effects between adjoining circuit devices. The MDI IC fabrication process benefits extend to several other categories of IC fabrication such as lower IC processing costs due to fewer IC isolation processing steps, greater IC transistor densities through the capability to use established IC processing techniques to fabricate interconnect metallization on both sides of a MDI IC circuit membrane, and greater IC performance through novel transistor structures.
The strength of the MDI processes is primarily drawn from two areas:
(1) The ability to make a large area flexible thin film free standing dielectric membrane, typically framed or suspended or constrained at its edges by a substrate frame or ring, or bonded frame or ring. This membrane is able to withstand a wide range of IC processing techniques and processing temperatures (of at least 400° C.) without noticeable deficiency in performance. The present dielectric materials that meet these requirements are silicon dioxide and silicon nitride films when prepared with specific low stress film deposition recipes for instance on equipment supplied by Novellus Systems, Inc. Dielectric free standing films created by CVD process methods such as silicon carbide, boron nitride, boron carbon nitride aluminum oxide, aluminum nitride, tantalum pentoxide, germanium nitride, calcium fluoride, and diamond have been produced, and can potentially be used as one of the dielectric materials in a MDI circuit membrane when deposited at an appropriate level of surface stress. Advances in the technology for making low stress dielectric films will likely produce additional free standing films that can be used as described herein.
(2) The ability to form a uniform thin film single crystal semiconductor substrate either as the primary substrate of semiconductor devices or as a carrier substrate upon which semiconductor devices could be grown epitaxially. Several methods toward this end are disclosed herein, and other techniques which are modifications thereof exist. Further, in certain applications polycrystalline semiconductor membranes such. as polysilicon can be used in substitution for monocrystalline material.
It is the combination of the use of low stress free standing dielectric films with the-appropriate processing qualities and membrane or thin film single crystalline (monocrystalline), polycrystalline or amorphous semiconductor substrate formation that provides much of the advantage of the MDI IC fabrication process. The following methods are encompassed within the present disclosure:
The MDI circuit fabrication process in one embodiment starts with a semiconductor wafer substrate, and results in an IC in the form of a circuit membrane where each transistor or semiconductor device (SD) in the IC has complete dielectric isolation from every other such semiconductor device in the IC. Only interconnect at the specific electrode contact sites of the semiconductor devices provides electrical continuity between the semiconductor devices. The primary feature of the MDI process is complete electrical isolation of all semiconductor devices of an IC from all of the intervening semiconductor substrate on which or in which they were initially formed and to do so at lower cost and process complexity than existing bulk IC processing methods. Other features of the MDI process are vertical electrode contact (backside interconnect metallization), confined lateral selective epitaxial growth, non-symmetric dopant profiles, and the use of a MDI circuit membrane to serve as a conformal or projection mask for lithography processing. Even if the initial substrate with which MDI processing begins with is the most commonly used semiconductor silicon, the resulting IC need not be composed of silicon-based devices, but could be of any semiconductor device material such as GaAs, InP, HgCdTe, InSb or a combination of technologies such as silicon and GaAs grown on a silicon substrate through epitaxial means. Silicon is an inexpensive and well understood semiconductor substrate material with superior mechanical handling properties relative to most other presently established semiconductor materials. The MDI process is not limited to starting with a silicon substrate and the process definition of MDI is not dependent on use of silicon; however, there are presently clear advantages to using silicon as a starting semiconductor substrate, and the chief embodiment disclosed herein of MDI uses a starting semiconductor substrate material of silicon.
The benefits to fabricating an IC with the MDI process are significant over prior art methods, some of these benefits being:
In some semiconductor technologies it is not necessary to have complete isolation between each transistor or semiconductor device, such as certain applications using polycrystalline or amorphous TFTs (thin film transistors). This is not a limitation on the MDI process, because semiconductor device side wall isolation is an option in the MDI process. What is novel is that the MDI process provides general methods by which thin films or membranes of dielectric and semiconductor materials can be formed into a free standing IC or circuit membrane.
a to 1j show a dielectric and semiconductor membrane substrate in cross-section.
a, 3b show dielectric membranes with semiconductor devices.
a to 6i show a circuit membrane Air Tunnel structure.
a to 9j show fabrication of a MOSFET in a membrane.
a to 10d show fabrication of a transistor by lateral epitaxial growth on a membrane.
a to 11f show vertical MOSFET and bipolar transistors formed on a membrane.
a to 12g show transistor fabrication on a membrane using confined laterally doped epitaxy.
h to 12j show cross-sections of selective epitaxial growth on a membrane.
a to 13d show cross-sections of multi-chip modules.
a, 16b show multi-chip nodules in packages.
a to 17c show soldering of bond pads of a circuit membrane to a die.
a, 19b show bonding and de-bonding of a die to a circuit membrane.
a to 22c show formation of a metal trace in a circuit membrane by a lift-off process.
a, 23b show use of a buried etch stop layer to form a circuit membrane having a thinner inner portion.
a to 28b show a coil for the device of
a to 29k show portions of a source-external radiation valve for direct write lithography device.
l to 29n and 29p show use of fixed freestanding membrane lithography masks.
a to 31c show cross-sections of a display formed on a membrane.
a, 32b show bonding of two circuit membranes.
c and 32d show die cut from a circuit membrane and bonded onto a rigid substrate.
The MDI process is the formation of an IC or interconnect metallization circuit as a free standing dielectric and/or semiconductor circuit membrane. Each semiconductor device comprising an IC circuit membrane is a semiconductor device optionally isolated from adjoining semiconductor devices, and where each semiconductor device is formed on or in a membrane of semiconductor material typically less than 8 μm in thickness. The overall thickness of a circuit membrane is typically less than 50 μm and preferably less than 8 μm. The dielectric membrane is compatible with most higher temperature IC processing techniques.
MDI Fabrication Process
Several process variations can be used to form the thin film or membrane of semiconductor material for use in the MDI process. Additional related methods for forming semiconductor membranes may exist or come into existence and are included in the MDI technology.
Examples of some of the methods that can be used for forming silicon single crystal thin films are:
1. Heavily boron doped (typically greater than 1018 atoms/cm2) etch stop layer (formed by diffusion, implant or epitaxy) with optional epitaxial SiGe (less than 20% Ge) anti-autodoping overlayer layer and optional epitaxial layer.
2. O2 (oxide) and N2 (Nitride) implant etch stop barrier layer. Implant concentrations are typically between 10 to 100 times less for formation of an etch stop barrier layer than that required to form a buried oxide or nitride dielectric isolation layer as presently done with a standard thickness silicon substrate.
3. Buried oxide etch stop barrier layer formed from a porous silicon layer.
4. High precision double sided polished substrate and masked timed chemical etch back of back-side.
5. Electro-chemical etch stop.
6. Buried etch stop layer formation through anodic or thermal wafer bonding in combination with precision substrate polishing and chemical etching.
There are many established methods for forming thin semiconductor substrates or membranes. The MDI process requires that the semiconductor membrane forming process (thinning process) produce a highly uniform membrane typically less than 2 μm thick and that the surface tension of the semiconductor membrane be in low tensile stress. If the membrane is not in tensile stress, but in compressive stress, surface flatness and membrane structural integrity will in many cases be inadequate for subsequent device fabrication steps or the ability to form a sufficiently durable free standing membrane.
The use of highly doped layers on the surface or near the surface of the substrate formed by diffusion, implant or epitaxial means is an established method for forming a barrier etch stop layer. A heavily doped boron layer will etch 10 to 100 times slower than the rest of the substrate. However, if it is to form an effective uniform membrane surface, autodoping to the lower substrate and to the upper device layer must be prevented or minimized. This is accomplished in one method by epitaxially growing a SiGe layer of less than 4,000 Å (1 Å=10−10m) and less than 25% Ge on either side of the barrier etch stop. The SiGe layers and the barrier etch stop layers are subsequently removed after formation of the membrane in order to complete device dielectric isolation.
The MDI process for forming a dielectric membrane requires that the dielectric material be deposited in net surface tensile stress and that the tensile surface stress level be 2 to 100 times less than the fracture strength of the dielectric. Consideration is also given to matching the coefficient of thermal expansion of the semiconductor material and the various dielectric materials being used in order to minimize the extrinsic net surface stress of the membrane. Thermally formed silicon dioxide forms as a strongly compressive film and most deposited dielectrics currently in use form typically with compressive surface stress. High temperature silicon dioxide and silicon nitride dielectric deposited films with tensile surface stress levels 100 times less than their fracture strength have been demonstrated as large area free standing membranes consistent with the requirements of the MDI process.
It is not a requirement of the MDI process that if a semiconductor material is used in the fabrication of a circuit membrane, that the semiconductor material be capable of forming a free standing membrane. The dielectric materials that are optionally used to isolate any semiconductor devices in the circuit membrane can provide the primary structural means of the resulting circuit membrane as implied by the general fabrication methods presented below.
The ability to form large durable temperature tolerant low tensile stress films of both semiconductor and dielectric materials as components or layers of a substrate for the fabrication of integrated circuits and interconnect structures is unique to the MDI process. The large free standing semiconductor and dielectric membrane substrates of the MDI process provide unique structural advantages to lower the cost and complexity of circuit fabrication and enhance the performance of circuit operation.
The MDI process can be broadly described as two methods, depending on which method of forming the semiconductor substrate thin film is selected. The sequence of steps of the two process methods presented below may be utilized in a different order depending on processing efficiencies. Formation of polysilicon or a-Si (amorphous silicon) circuit devices on a dielectric membrane without the starting use of a semiconductor substrate is disclosed below, but is not categorized as a method.
Method #1
Method #1 fabricates the desired semiconductor devices of an IC on a standard thickness semiconductor substrate 10 prior to the deposition of the low stress dielectric membrane and release of membrane structure 14 by etching the back side of the substrate. (
The semiconductor devices will typically be fabricated from vapor phase epitaxial depositions. Well known epitaxial fabrication methods such as SEG (Selective Epitaxial Growth), ELO (Epitaxial Lateral Overgrowth), MOCVD (Metal-Organic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy) may be employed.
In the embodiment of
The original substrate upon which the MDI circuit membrane is formed serves as retaining frame (or ring) 18 for the MDI circuit membrane after the backside of the substrate is etched. The backside etch of the substrate leaves the frame 18 of the substrate to hold the resulting circuit membrane. The width of frame 18 is sufficient to prevent the surface forces of the circuit membrane from causing the frame to crack. A width typically of less than 400 mils (1 cm) is sufficient. This is done by growing thermal oxide and or depositing a film typically of silicon nitride (about 5,000 Å to 7,500 Å thick) on the backside 14 of the substrate and then etching a window or opening in dielectric layer the size and shape of the desired circuit membrane. The substrate is then etched through dielectric window or dielectric mask. The shape of the backside window opening (or dielectric mask) is typically rectangular, although it can vary in shape.
The barrier etch stop layer 12 is then selectively etched to leave only the original surface 10 layer of the silicon substrate. A barrier etch stop layer could also be formed on the surface of the silicon substrate if the layer preserves sufficient crystalline structure required for subsequent epitaxial processing, surface stress (typically less than 108 dynes/cm2 tensile) and there is a selective etch procedure for the silicon substrate versus the barrier etch stop layer, and if need be, the applied barrier etch stop layer can subsequently be removed. The backside 14 is etched away by TMAH (tetra-methyl ammonia hydroxide) or other appropriate selective silicon etchant (see below).
An alternative technique to trench isolation as suggested by either steps 3 or 6 of Method #1 in order to laterally isolate transistor devices of the circuit membrane is to use the well known LOCOS (LOCal Oxidation of Silicon) isolation method. LOCOS could be applied as part of the processing of step 6 (process steps 4 or 5 in Method 12 below) as shown in
The thermal oxide isolation created by the LOCOS method may change the net tensile surface stress of the semiconductor (substrate) membrane layer. The deposition of low stress dielectric films on either side of the semiconductor layer prior to LOCOS processing will offset most compressive effects of the oxide formation. Device isolation by the LOCOS process when applied to a MDI circuit membrane is more effective than current bulk processing because of the shallow depth of the semiconductor substrate layer. This also allows near optimum density of the circuit devices with respect to minimum device isolation separation. The easy incorporation of LOCOS into the MDI processing methods also is an indication of the general compatibility of the MDI process with existing IC fabrication techniques.
The remaining substrate 10 (see
An alternative to forming a barrier etch stop layer is to polish the silicon wafer 10 on both sides to a thickness uniformity specification such that when the substrate is selectively time etched from the back side, the etch can be stopped prior to reaching the dielectric membrane 20 and will leave an acceptably uniform thickness for the active device substrate. This method is mechanically difficult in that it requires the wafer to be exactingly polished on both sides to achieve the uniform thickness.
The dielectric membrane 20, 36 is formed as part of the interconnect metallization dielectric and as a layer over the two-sided interconnect metallization 35 as shown in
Two sided interconnect metallization 35, with low stress dielectric material 36 (see
The alignment of lithography tools on the back side of the substrate is performed through the transparent dielectric material as shown in
Method #2
IC processing steps used on the top side and backside of the semiconductor membrane substrate are well known and not unique in application to the semiconductor substrate membrane; nearly any semiconductor process technique can be applied.
Method #2 fabricates a thin low stress semiconductor substrate membrane 20, such as the one shown in
The back side etch of the semiconductor substrate 14 (see
The uniformity of the semiconductor substrate thickness is important to the uniformity of the operating characteristics of the semiconductor devices of an IC, and to lithography processing steps. The above-described embodiments use etch stop techniques when selectively etching the semiconductor substrate material to form the desired substrate membrane thickness.
The interconnect circuit membrane can also be formed on a glass or quartz substrate 23 (typically less than 50 mils thick). In this case a tensile film 25 of polysilicon (as taught by L. E. Trimble and G. E. Celler in “Evaluation of polycrystalline silicon membranes on fused silica” J. Vac. Sci. Technology B7(6), November/December 1989) is deposited on both sides of the substrate 23, as shown in
Low Stress Silicon Dioxide and Silicon Nitride Deposition Recipes
Low stress dielectric membranes have been manufactured consistent with MDI requirements for low stress high temperature dielectric films. These membranes were produced on Novellus Systems, Inc. (San Jose, Calif.) Concept One dielectric deposition equipment, but are not limited to such equipment. Low stress is defined relative to the silicon dioxide and silicon nitride deposition made with the Novellus equipment as being less than 8×108 dynes/cm2 (preferably 1×107 dynes/cm2) in tension. Acceptable surface stress levels of different dielectrics made on various equipment may vary widely.
The following are two typical recipes used to produce the dielectric membranes of silicon dioxide or silicon nitride required by the MDI process. Variations of these recipes can be used to emphasize specific characteristics of the deposited dielectric membrane; these recipes are not the only recipes to achieve the requirements of the MDI process on the Novellus equipment and are not limitations on the MDI process. Small variations in the parameters of the recipes can produce changes in the material structure, etch rate, refractive index, surface stress, or other characteristics of the deposited dielectric material.
Structurally Enhanced Low Stress Dielectric Circuit Membranes
MDI Air Tunnel Interconnect Structure
The impedance of metal conductors (traces) that make up the interconnect metallization between semiconductor devices and passive circuit elements such as capacitors or resistors must be given careful design consideration at operational frequencies in excess of 100 MHz. The dielectric constant of a dielectric (insulating) material is a primary determining factor when consideration is given to the use of the material. Polyimide materials conventionally used in the construction of interconnect structures have dielectric constants typically ranging between 2 and 3.5. The dielectric constants of CVD silicon dioxide and silicon nitride typically range upward from 3.5. The ideal dielectric is vacuum, gaseous or air with a dielectric constant of approximately unity. The fabrication of an interconnect structure that isolates the majority of a conductor's surface area with vacuum or gaseous dielectric results in a near optimum condition for high speed operation of that structure. Trace or conductor structures called “Air Bridges” are fabricated on the surface of an IC contacting the surface of the IC only periodically. This periodic contact provides mechanical support and or electrical contact with a low net dielectric constant of isolation. Air Bridges are conventionally used in the fabrication of microwave circuits; such circuits have operational frequencies in the GHz range.
a, 6b and 6c show cross-sectional views of a conductor structure 50 internal to dielectric layer 52, 52a, but which has a gaseous primary surface dielectric contact with only periodic mechanical contact 50-a, 50-b, 50-c, like the conventional Air Bridge. The conductor 50 is held suspended in the gaseous dielectric without contacting the surrounding or enclosing solid material structures.
The Air Tunnel structure can be fabricated on any semiconductor substrate in addition to an MDI circuit membrane. The structural dielectric materials used are selected from the same group of low stress dielectrics used in the fabrication of the MDI circuit membranes discussed above. The fabrication method for the Air Tunnel provided here can also be extended to the gaseous dielectric isolation of semiconductor devices and passive circuit elements in a circuit membrane.
The Air Tunnel structure in one embodiment is fabricated with CVD processing techniques; alternatively, ECR (Electron-Cyclotron-Resonance) plasma CVD processing may soon provide an alternative deposition method. The gaseous dielectric separation of a conductor or a semiconductor device is accomplished by forming a sacrificial CVD film of a-Si, polysilicon or alternate material (typically dielectric material) that can be deposited by CVD means and selectively etched versus the other MDI circuit membrane material layers. Anticipating the use of Air Tunnel interconnect structures in the fabrication of a circuit membrane, semiconductor devices are isolated by trench isolation of each device, and depositing a thin layer of oxide or nitride (typically less than 2,000 Å thick) over exposed device surfaces, and then depositing a film of a-Si. The thickness of the a-Si film and the width of the isolation trench are chosen such that the trench is plugged or filled evenly leaving the surface over the plug relatively planar. This plug technique is facilitated by CVD process technology which deposits films conformally on all interacting surfaces. Subsequent Air Tunnel interconnect structures are completed and the a-Si layers are removed by a silicon selective etchant; the etchant accesses the aSi through the etch-vias as explained below.
The fabrication method of Air Tunnel interconnection is independent of the underlying substrate. The underlying substrate could b a MDI circuit membrane as shown in
The Air Tunnel fabrication process begins. (see
The resulting three films. 76, 78, 82 of a-Si are relatively planar with minor surface features along the edges where conductors 50 are directly beneath. These surface features can be reduced by shallow thermal oxidation of the a-Si surface and subsequent stripping of the oxide (not shown).
An alternative method that can be used to planarize the a-Si and metal films isto spin coat a thick polymer over substrate 75, low stress dielectric 77, first a-Si layer 79, and second a-Si layer 81 after the application of the second a-Si film 81. (This a-Si film 81 is deposited in thickness equal to the combination of the former second and third a-Si films.) The polymer 85 used is selected to have a RIE rate nearly identical to the that of the a-Si film. The polymer 85 is completely removed and the etch of the a-Si is continued as desired, see
The a-Si film stack is then patterned to form trenches that will become the supporting columns 50-a, 50-b, 50-c of an over layer of low stress dielectric. These support columns are typically placed in a non-continuous manner along conductors 50 (as shown in
The etch-vias 62 in the dielectric are placed near dielectric support columns as shown in
The etch-vias 62 serve as access ports for gaseous material (such as N2 or air) which allow the effective circuit dielectrics to enter the circuit membrane. The etch-vias 62 also serve to maintain equalized pressure between the internal interconnect structure and the external environment of the surface of the circuit.
The following sequence of processing steps for fabricating an Air Tunnel assumes that the substrate being processed has an existing layer of low stress dielectric film on which to start:
Steps 6 through 8 from above can be replaced with the following alternative steps for forming a planarized conductor layer:
6b. Deposit a-Si film with a thickness equal to the thickness of the conductor and the desired separation over the conductor.
7b. Deposit planarizing polymer with a etch rate that is very similar to that of the a-Si film, as shown by
8b. RIE until all polymer material has been removed, as shown by
The principle attributes of the Air Tunnel are:
A MOSFET device connected by Air Tunnels is shown in cross-section as an example in
MDI Circuit Membrane Advantaaes
The fabrication of circuit membranes provides the capability to fabricate and use integrated circuits in novel ways such as:
1. Back side interconnect metallization, backside SD electrode contact, and through-the-dielectric membrane interconnect metallization signal routing. (See
2. The use of optical communication (see
The optical receiver of the MDI circuit membrane is typically less than 5,000 Å in thickness. A receiver of this thickness will absorb only a portion of the optical signal fluence striking it, the remainder of the fluence will pass through and out the opposite side of the receiver. This permits a second or third receiver to be positioned in the path of the optical transmitter to receive the same signal. The limiting conditions that determine the number of receivers that can be associated with one transmitter are the output fluence of the optical transmitter and the thickness of the transceiver substrate. This ability to optically transmit to several receivers simultaneously has the benefits of reducing circuit complexity (versus a transceiver structure at every circuit membrane interface) and performance (no transceiver (repeater) propagation delay).
3. The vertical bonding of two or more circuit membranes to form a three dimensional circuit structure, as shown in
4. MDI Nanometer width gate MOSFET device fabrication methods. The ability to fabricate a MOSFET device with a gate region length of less than 0.5 μm (500 nm) presently requires lithograph tools with greater resolution capability than the current high volume optical stepper tools. Several methods are presented here for the fabrication of MOSFET with gate lengths of less than 500 nm and capable of gate lengths less than 25 nm. These methods take advantage of the MDI process to form transistor gate regions of less than 500 nm (0.5 μm) without lithographic means are described hereinafter.
5. Sensor diaphragms formed with integrated circuits fabricated on conventional rigid substrates.
6. Fabrication of electrically isolated semiconductor devices on both sides of the semiconductor layer of a circuit membrane.
V-Groove Transistor Region Gate Formation Method
a to 9e show a sequence of steps for forming a p-channel or n-channel (npn) transistor with an opposed gate electrode and a gate width of less than 25 nm (250 Å). The process steps (see
The sequence of process steps for one embodiment is:
It should be clear that the. opposed gate electrode which is much wider that the actual length of the gate width could have been fabricated on top of the transistor with the source and drain contacts, or after the gate region of the transistor was formed.
The reliable fabrication of the gate region 194 at a known length is dependent on etch rate control. Present etch rate techniques support the capability to form a gate region 194 length of less than 25 nm. A CVD dielectric could also be used, as the gate implant mask instead of a metal plug 90. This gate fabrication method can also be done in bulk silicon and is not limited to the MDI process. It is important to note that the sub-micron resolution of the gate region was achieved without lithographic means capable of sub-micron resolution.
f, g, h, i & j show the process sequence for substrate-imaged nonlithographic MOS transistor gate region or Bipolar emitter region formation. This process takes advantage of the novel ability of the MDI structure to accommodate process steps on either side of the membrane substrate. This process can form a minimum feature size of less than 100 nm and does so by the use of anisotropic etching epitaxial processes.
f shows a MDI substrate of semiconductor 189 and low stress dielectric 191 formed by one of the methods presented above. The dielectric layer 191 on one side has been patterned and the semiconductor layer 189 anisotropically etched through to the dielectric layer 193 on the opposite side. The thickness of the semiconductor layer 189 is selected to be less than 2 μm, and the dielectric layers 191, 193 are less than 1 μm thick each, though these dimensions can be scaled to achieve any transistor dimensions. The size of the patterned opening 195 in the dielectric is chosen such that the etched opening 197 onto the underlying surface of the dielectric is a desired width, less than 100 nm. Widths of any dimension can be created, however, widths of less than 100 nm are the primary objective of this process which cannot be achieved by optical means. The patterned dielectric 191 has a 1,000-2,000 Å layer 199 of low stress silicon nitride to provide high selectivity versus the opposed dielectric layer-during a subsequent RIE process step.
g shows an opening 197 formed through the dielectric layer 193 opposite the patterned layer 191. The anisotropically etched semiconductor layer 189 with its opening 197 onto the under side of the opposite sid dielectric was used as a substrate-imaging mask, and th dielectric layer 193 was etched by RIE (dry) processing. This substrate-imaged opening 197 can be less than 100 nm in diameter and will become the gate region of a MOS transistor or the emitter region of a bipolar transistor.
j shows the finished MOS transistor. The side opposite the gate electrode 205 is stripped of dielectric, planarized and etched to a thickness of less than 2,500 Å. It was then trenched 207 for source/drain isolation a layer of low stress dielectric 209 deposited and source/drain electrodes 211, 213 formed.
It should be clear to some skilled in the art that a bipolar transistor can be formed with somewhat similar fabrication steps.
MDI Lateral Epitaxial Grown Transistor Gate Region Fabrication
a, b, c, & d show a method for forming a MOSFET transistor with transistor gate region lengths of sub-micron dimensions, without the requirement for lithography tools with sub-micron CD resolution capability. This method is an extension of the above-described MDI process technology.
In
The epitaxial growth process in the gate region trench 208 is stopped when the remaining separation distance “d” between the approaching silicon sidewalls 220 is equal to the desired gate region length. The doping concentration is changed for forming the actual gate region 222, and the epitaxial growth is continued until the gate region trench is closed, as shown in
The width of the initial gate region trench 208 and the epitaxial growth rate can be controlled with an error of less than 5 nm. This allows the length d of the gate region to be determined by the precision with which the gate region trench 208 can be formed, the deposition rate control of the lateral epitaxial growth 220 and the capability to create the desired doping transitions. The result of this approach means that a gate region length d of 25 nm or less can be formed. This is far beyond the current optical lithographic methods which are presently expected to reach production capability of 120 nm by the end of the decade.
d shows the completion of the transistor with source and drain electrodes 226, 224, and an opposed gate electrode 230. The surface of the epitaxially grown gate region 222 is not smooth, but the backside of the gate region 222 is smooth, due to its formation against the underlying low stress dielectric membrane 204. The gate electrode 230 is more easily and reliably formed by patterning the underlying dielectric membrane 204 from its backside, thereby forming the opposed gate electrode 230. The backside of the gate region 222 is etched free of dielectric with an opening significantly larger than the length d of the epitaxially grown gate region 222; portions of the source 210 and drain 206 regions are also exposed with the gate region due to lithographic tool limitations. Gate oxide isolation 232 is thermally grown or deposited to the required thickness, and a metal or silicide electrode is formed 230.
The overlap of the gate electrode 230 over the source 210 and drain 206 regions of the transistor channel results undesirably in performance limiting capacitance. The reduction of the capacitance from the overlapping gate electrode structure can be achieved by either epitaxial graded doping of the source and drain regions at the interface to the gate region growth over the gate region, to form an extended gate region, or by offset of the gate electrode so that the source side edge of the electrode aligns approximately with the source/gate interface of the transistor channel. These methods are disclosed below. The above-described MOSFET transistor structure can be converted to a bipolar transistor structure as shown in
The MDI circuit membrane structure provides the capability to position electrode contacts of semiconductor devices at their backside (or opposed side). This provides significant savings in reduced processing steps to achieve device isolation and new transistor structures.
a shows a MDI circuit membrane (dielectric 240 and semiconductor 242 films) in cross-section on which additional semiconductor layers were formed epitaxially to achieve either an npn or pnp transistor structure 244. The epitaxial layer 244-a that corresponds to the gate region can be formed with a thickness of less than 25 nm.
c shows a bipolar transistor formed by a well known method but on a low stress dielectric membrane 254, and without the requirement to form a lateral conductive path (typically referred to as a buried layer). Formed in semiconductor. film 256 are emitter 258, base 260, and collector 262 regions. The collector contact 264 is instead formed directly under the emitter 258. It should also be apparent that there are non-silicon semiconductor transistor structures that can be restructured with a backside contact as depicted in
The extension of the gate electrode 250-a beyond the gate region in the fabrication of the MOSFET transistor structure of
The Epitaxially Extended Gate 244-b (see.
The extended gate region 244-b of
The Gate Electrode Offset (GEO) structure is shown in
The transistor structure of
MDI Transistor Fabrication Based on Confined Laterally Doped Epitaxy
Selective growth of high quality epitaxial films in a confined volume was reported in a paper (“Confined Lateral Selective Epitaxial Growth of Silicon for Device Fabrication,” Peter J. Schubert, Gerold W. Neudeck, IEEE Electron Device Letters, Vol. 11, No. 5, May 1990, page 181-183). This “CLSEG” (Confined Lateral Selective Epitaxial Growth) method was developed to achieve dielectric isolation of crystalline silicon.
a, b and c show the use of a MDI circuit membrane to produce, a confinement cavity for the epitaxial growth of crystalline silicon. The original objective of CLSEG, to fabricate a dielectric isolated semiconductor substrate, is not an objective here because the MDI process is itself a dielectric isolation process. What is taught here is the application of the capability for growing epitaxial films in a confinement cavity within the context of the MDI process, and a processing method for using confined lateral epitaxial growth to fabricate MOSFET transistors with arbitrarily narrow gate region lengths without the use of lithographic means.
MDI confined lateral doped epitaxial closely controls transistor geometry in all three dimensions: height (as deposited semiconductor film thickness), width (placement of isolation trenches) and source, gate, drain regions (epitaxial deposition thickness). This makes the device operational characteristic more predictable with lower manufacturing complexity.
The MDI Confined Lateral Epitaxy method begins with a membrane of two films, one film 280 of semiconductor (silicon) and one film 282 of low stress dielectric. This membrane 280, 282 is made by one of the methods disclosed above. Dielectric filled isolation trenches 284 (see
The exposed semiconductor film 280 is then anisotropically etched back in two directions under the dielectric over layer 290 and along the isolation trenches 284 as shown in
A smooth contamination free seed crystal wall 292-a, 292-b is required for the subsequent epitaxial growth. The crystal seed wall 292-a, 292-b will form at an angle dependent upon the crystallographic orientation of the semiconductor film. A <100> silicon film will form a seed wall angle of approximately 54.74° as measured from the horizonal, and a <110> silicon film will form a seed wall 292-a, 292-b with an angle 90° (as shown in
The selective epitaxial growth of crystalline semiconductor proceeds from the seed crystal wall 292-a, 292-b. The npn or pnp regions of silicon MOSFET transistors are epitaxially grown (see
d shows two pairs 294, 296 of CMOS transistors formed as mirror images of each other in two confinement cavities; this was a design choice because a single confinement cavity could have been formed by additional trench dielectric isolation of the initial confinement window opening. Depending on the method of epitaxial growth, gate region lengths “g” of less than 25 nm can be fabricated. In contrast, prior art MOSFET transistor gate regions are predominately formed by optical lithographic means and are presently limited to minimum gate region lengths of approximately 0.5 μm (500 nm).
e shows the CMOS transistor pairs 294, 296 with electrode contacts 294-a, 294-b, 294-c, 296-a, 296-b, 296-c. The isolated gate electrodes 300, 302, 304, 306 each span the narrower gate region length of the transistor. The gate electrode is formed by standard lithographic processes, and may be many times of greater length than the gate region length of the transistor. For example, a lithographically defined gate electrode of 0.5 μm and a transistor gate region length of 25 nm are possible. Gate electrode capacitance attributed to the gate electrode overlap of the source or drain regions can be reduced as described above with the GGD, EEG, and GEO methods.
The MDI Confined Lateral Epitaxy method is not limited to MOSFET transistors or silicon as a semiconductor material. The MDI Confined Lateral Epitaxy method can be used to fabricate silicon bipolar transistors, or applied to other semiconductors such GaAs or InP which can be epitaxially formed. An example of a bipolar transistor is shown in
The semiconductor layer of a MDI circuit membrane provides various methods to pack circuit devices densely, mix semiconductor types or simplify the fabrication steps.
i shows a MDI circuit membrane composed of a silicon layer 295 and a low stress dielectric layer 297. The dielectric layer 297 has been patterned to form open areas 299 into which semiconductor material was deposited by selective epitaxial growth. The selectively deposited semiconductor 299 can be an semiconductor other than silicon, or silicon. Semiconductor devices can be fabricated in the epitaxial islands 299 on the backside of the MDI semiconductor layer.
j shows a MDI circuit membrane composed of a silicon layer 295 and a low stress dielectric layer 297. The dielectric layer 297 has been patterned and silicon islands 299 formed through selective epitaxial growth in the same manner as described for
The structures
Dielectric Membranes as Sensor on Rigid Substrates
The mechanical (physical) and thermal properties of the nitride and oxide free standing membranes made with the Novellus equipment are similar to silicon membranes presently being used in the making of ICs with integrated sensors. Examples of such silicon membrane or diaphragm based sensors are those made for sensing acceleration, pressure or temperature, but not limited to these areas. The availability of such oxide and nitride dielectrics for general application is novel with this disclosure.
The use of such free standing dielectrics membranes significantly reduces the complexity of producing a membrane with non-conducting properties in a silicon or semiconductor fabrication environment. The low stress dielectric membranes have selective etchants that allow the dielectrics to be processed independently in the presents of silicon. The known passivating capability and inert chemical nature of silicon nitride, and the relatively similar coefficient of thermal expansion of silicon, silicon dioxide and silicon nitride increases the operating environmental range of sensors versus organic dielectrics.
Sensor diaphragms of low stress dielectric membranes produced on rigid substrates (typically semiconductor) can in most cases directly replace silicon membranes or be used in conjunction with a silicon membrane formed on a rigid (conventional) semiconductor substrate. No new fabrication methods are required to integrate the low stress MDI process dielectrics into such sensor ICs.
The following discloses additional applications of the MDI process, and of forms of MDI circuit membranes and extensions of the MDI IC process. For example, a MDI circuit membrane with few or without active circuit devices is called more specifically an interconnect circuit membrane. When the interconnect circuit membrane is used to interconnect conventional ICs in die form, the interconnect circuit membrane becomes a multiple chip module (MCM) interconnect circuit membrane as discussed immediately below. This does not limit the application of a MDI circuit membrane with active and passive circuit devices from being used in the fabrication of a MCM.
MDI Multi-Chip Module Interconnect Circuit Membrane
A MDI circuit membrane composed primarily of interconnect metallization of one or several internal interconnect levels can be fabricated on a substrate to provide electrical interconnections between the bond (signal) pads of various individual dice (ICs) as in the well established packaging technique called multiple chip module (MCM). A MDI circuit membrane made for this typ of application is called multi-chip module Interconnect Circuit Membrane (ICM).
The multi-chip module interconnect circuit membrane is formed using the MDI process and conventional semiconductor processing techniques. The interconnect circuit membrane may have several thousands of die metal bond pad contacts or bonding points. The position of these metal contact or bonding points on the surface of the interconnect circuit membrane can be arbitrarily placed.
The multi-chip module interconnect circuit membrane can be formed on any substrate material that can meet the requirements for flatness specification, withstand the processing temperatures of the applied low stress dielectric(s), and can be selectively etched with respect to the low stress dielectric(s) to form the circuit membrane.
The interconnect circuit membrane can also be formed (see
The interconnect metallization traces in the circuit membrane form electrical connections from one or more of the contact pads of an IC to one or more contact pads of the other ICs. The diameter of these pads can vary from less than 0.5 mil (0.001″ or 25 μm) to several mils. The limiting factors of the size of the IC contact pads is the lithography method used in fabrication and the assembly method; the cost of such methods increase as pad diameters less than 3 mils are employed.
A die can be bonded to a circuit membrane with any of several techniques. Techniques such as compressive metal to metal bonding with such metals as Indium or gold (or their respective alloys), infrared thermal bonding, laser bonding, and vertically conductive adhesive films such as ZAF from 3M Corporation are examples.
If the interconnect circuit membrane 360 (see
Solder weld bonds are formed by using a heat source (not shown) such as a high intensity lamp, laser or metal instrument that can heat the solder to its melting point. The heat source melts the solder 367 (see
The solder 367 (see
An advantage of the high temperature tolerance of the interconnect circuit membrane which allows die solder bonding to be done without the application of any significant contact pressure between the interconnect circuit membrane and die, is the ability (see
A hermetic seal bond pad 372 extends continuously along the perimeter of the surface of the die. A hermetic seal of the surface of the die with the surface of the circuit membrane can be accomplished by the formation of an enclosing solder bond (comprising a solder well as described above for forming pad bonds) along the edg of the die. The hermetic seal is typically also a ground contact for the die.
The infrared solder bonding of die to the multiple chip module interconnect circuit membrane is a simple process which also allows the dies to be easily removed and replaced without damage to the interconnect circuit membrane. As shown in
The die 372 can be removed (see
The multi-chip module interconnect circuit membrane can be enhanced to include passive devices such as resistors, capacitors, and polysilicon or a-Si (amorphous-Silicon) TFTs (Thin Film Transistors). Such circuit elements can be fabricated into or onto the membranes due to the ability of the dielectric membranes to tolerate processing temperatures in excess of 400° C. These circuit elements can be fabricated internally to the interconnect circuit membrane structure as part of various specific layers or collectively on either external side of the interconnect circuit membrane. After the interconnect circuit membrane is freed from the substrate on which it is initially formed, it is strong enough to withstand continued semiconductor processing steps.
The multi-chip module interconnect circuit membrane can further be enhanced to include any degree of active single crystal semiconductor devices. This is consistent with the earlier discussion of the capability of bonding two or more MDI membranes to form a three dimensional IC, except in this case conventional dice (ICs) are bonded at the bond pads of the circuitry internal to a MDI circuit membrane which is also serving the purpose of interconnect circuit membrane.
The multi-chip module interconnect circuit membrane provides novel circuit cooling advantages due to its low mass structure. The thermal energy generation of an IC is from the face side of the IC. ICs attached-face down against the circuit membrane allows the thermal energy of an IC to be radiated and conducted through the circuit membrane to a liquid or solid heat sink means on the opposite side of the circuit membrane. The circuit membrane provides a very short thermal path from the IC to the heat sinking means. The ability directly to cool ICs in this manner is a unique feature of the MDI interconnect circuit membrane structure. The cooling efficiency that can be achieved by an IC that is fabricated with the MDI process as a circuit membrane is significantly improved versus standard thickness substrates by the obvious reduction in the thermal mass resistance of the semiconductor substrate.
d shows in cross-section a portion of a MCM MDI interconnect circuit membrane 329 including low stress dielectric and interconnect layer 329a and semiconductor layer 329b where two die 331, 333 are bonded face down at their signal pads 335a, 335b, 335c, 335d. The MDI circuit membrane 329 incorporates the pad drivers 337a, 337b, 337c, 337d that normally would be incorporated onto the die (IC). The primary advantages of integrating the IC pad drivers into the MCM are circuit performance, simplified use of mixed device technologies (Bipolar & CMOS), cooling and die size (IC real estate).
IC pad drivers are typically designed to meet a broad range of circuit design conditions which often results in a trade-off of performance. The designer of the MCM pad driver has a better understanding of the operational requirements of the pad driver, and therefore, can typically optimize the pad driver design for higher performance. Pad drivers in the MCM can be formed from Bipolar transistors while the IC can be CMOS. This achieves many of the benefits of BiCMOS without the fabrication complexity. The pad drivers of an IC typically generate more than half of the thermal energy of the IC especially at high operating speeds. Placing the pad drivers in the MCM provides a means to bring the primary thermal components of an IC into direct contact with heat sinking means. IC pad drivers are typically the largest transistor structures making up the IC. Moving the pad drivers of the IC into the MCM can result in a potential 5-10% reduction in the size of the overall IC.
The incorporation of low complexity ICs into a MCM MDI circuit membrane can reduce overall MCM assembly and parts costs. Such often used circuits as bus drivers or combinatorial logic can be incorporated into a MCM reducing parts and assembly costs without a corresponding increase in the cost to fabricate the MCM. (Once the decision has been made to include such circuits in a MCM MDI circuit membrane, there is no significant cost difference between the inclusion of 1,000 transistors or 10,000.) Yield of circuits in.a MCM circuit membrane can be addressed through redundant fabrication of the desired circuit devices; the circuit devices of the MCM circuit membrane comprise only a small amount of the total surface area, typically less than 10%. The circuit devices of the MCM circuit membrane can be tested for defects. A MDI functional tester membrane disclosed herein, can be used to perform this testing and, in the same step if so desired, to blow anti-fuses to enable the defect free circuits or fuses to disable the defective circuits.
The multi-chip module interconnect circuit membrane, like prior art PCBs (printed circuit boards), can provide access to the contact pads of the IC from the opposite side on which the IC is attached. This access permits in-circuit testing of the bonded IC to be performed by a separate interconnect circuit membrane fabricated to do such a functional test, and also permits electrical continuity testing of the traces fabricate din the multi-chip module interconnect circuit membrane, again performed by an interconnect circuit membrane test surface fabricated for that purpose.
The multi-chip module interconnect circuit membrane method could also be used to construct a functional IC tester surface with a high number of probe points (in excess of several thousand) for testing ICs while in wafer or die form (i.e., wafer or die sort testing), and also to wafer sort subsections of ICs such as an ALU (Arithmetic Logic Unit), FPU (Floating Point Unit), cache segment, etc.
A functional circuit membrane tester surface made with the MDI process offers advantages not presently available from present on-wafer IC membranes testers constructed from polymers such as continuous high temperature operation (greater than 100° C.) without deleterious effects due excessive material expansion or at-speed IC testing without EM interconnect trace coupling.
High numbers (in excess of 1000) of pads or traces of an IC or IC subsection can be contacted. The contact sites of the IC (die) can be arbitrary in position and all sites can be contacted at one time. The contact sites (pads or traces) can be less than 50 μm (2 mils) or even less than 2 μm in diameter. The probe points of the functional tester surface can be less than 50 μm or even less than 1 μm in diameter, and can be placed on center to center distances of less than twice the diameter of the probe points. (Functional probe points can be formed to be 12 μm in diameter and spaced on center to center distances of less than 24 μm.) The ability to form and contact high numbers of pads or traces with dimensions smaller than presently available in the industry is due to the ability of the MDI IC process to form multiple layers of interconnect with established semiconductor fabrication means resulting in a flexible and elastic membrane structure. The multiple layers of interconnect provide for denser contact test sites, the thin membrane structure provides for contact conformation with little required force (typically less than 10 psi), and the compatibility of the dielectric material with standard IC fabrication techniques allows contact of contact site dimensions less than 2 μm.
A MDI circuit membrane 332 used as an IC functional tester surface extends under fluid pressure “P” to make contact with the surface of a wafer or single die 339 to be tested as shown in
In the case of sub-section testing of a die, the detection of a defective sub-section before packaging permits the option of pinning-out the IC as one of several alternative product configurations, and thereby, making useable an otherwise scrapped IC.
In
Alternately the native metal oxide (approximately 30 Å thick) on the pads of a die can be rubbed through by the probe points of the functional tester surface by repeatedly moving the wafer or die several microns in a lateral manner when the tester surface is first brought into contact with the IC to be tested. The mechanical control presently available with most wafer or die handling equipment is sufficient to effect this rubbing action.
The interconnect circuit membrane can range in thickness from less than 2 μm to over 25 μm. A typical thickness per layer of interconnect (dielectric and metal traces) is 1 to 4 μm. Dielectric layer of several microns thickness may be used as a planarizing technique to lessen metal trace step height differentials that can develop when more than two interconnect layers are used. Planarization of metal traces can also be achieved by depositing the dielectric to a thickness inclusive of the metal trace. A channel that is at least as deep as the desired thickness of the metal trace is patterned in the dielectric. Then established lift-off techniques are used to fill the channel with metal.
a to 22c show this process. In
Planarization of the interconnect structure can be achieved by applying a thick layer of a planarizing polymer over the MDI dielectric film. The polymer must have an etch rate similar to that of the MDI dielectric film. The polymer is then removed completely by dry etch means and in the same process removing any dielectric surface features that extended into the polymer layer. This planarization process is identical to the optional planarization process disclosed in the process steps for the Air Tunnel. The only differences being the material being planarized is a MDI dielectric and not a-Si.
The ability of the interconnect circuit membrane to achieve reliable contact on the bond pads of a die (see
In
MDI Source-Integrated Light Valve (SLV) Direct Write Lithography Tool
The MDI process technology can be used to fabricate a circuit membrane with an nxm cell array of active radiation sources of X-ray, DUV (Deep Ultra Violet) or E-Beam sources that can be used to form lithographic patterns in a film of appropriately sensitive resist. The preferred embodiment in this disclosure uses a X-ray source. Variations of the electrodes, gas contents and structure of the radiation source cell can be made within the overall structure of this source integrated light valve embodiment. The preference for the X-ray source embodiment is its capability to pattern a smaller feature size.
The radiation source cells in the membrane are under computer control, so that this type of MDI membrane combines the functions of reticle or mask and a light or radiation source for.fabricating ICs. The radiation source cells of this MDI circuit membrane are positioned (aligned) over an area of the substrate to be patterned and at a uniform distance from or in contact with the substrate. Each cell then illuminates (exposes) various portions of the substrate directly beneath it as required. The MDI circuit membrane functioning in this manner is called herein a Source-Integrated Light Valve (SLV) and can generally be categorized as a Direct Write (maskless) Lithography Tool.
The array of Radiation Source Cells (RSC) that make up the patterning structure of the SLV, emit a collimated source of radiation. The exposure feature size of this collimated radiation source is dimensionally much smaller than the area the RSC occupies on the surface of the SLV. The emitted collimated radiation source is referred to as the Radiation Exposure Aperture (REA). The SLV is moved in a scanning X-Y manner over the substrate to be patterned. The dimensions of the scanning motions cause each REA of an RSC to pass over a portion of the substrate that typically is equal in area to the size of the RSC. The RSCs are operated in parallel, and the patterned areas on the substrate beneath each RSC adjoin each other and collectively form a much larger overall pattern on the substrate. This pattern can be as large as the underlying substrate. The scanning motion of the RSCs is generated by well known computer controlled mechanical motor driven stages or piezoelectric driven stages. The SLV or the substrate can be attached to the motion stage. Due to the short X-Y motions in the use of the SLV, and the desire to scan the RSCs with REAs of dimensions much less than 1 μm and in some cases with dimensions of less than 50 nm, piezoelectric motion stage drive is the preferred embodiment. As the REAs of the SLV circuit membrane are scanned over the substrate, they are turned on or off by computer controlled logic associated with each RSC to effect the desired exposure pattern to be made on the surface of the substrate. An example would be a RSC with an area of 25 μm by 25 μm and a REA of 0.1 μm in diameter. In order for the REA to expose an area the size of the RSC, a scanning motion of 25 μm by 0.1 μm would be made 250 times in the X-axis direction while translating 0.1 μm in the Y-axis direction after each scan.
The textured cathode 450 will typically be in the shape of a square array of rows and columns of metal cubes, with each cube having dimensions much smaller than the overall dimensions of the square cathode layer. As an example, if the cathode 450 is 4 μm2 in area, the dimension of each cube would be 0.1 to 0.5 μm on a side. The textured cathode cubes are made by established lithography techniques (E-Beam or DUV optical lithography) and anisotropic RIE processing. This cathode 450 configuration is a novel part of the SLV because of high density of cold electron emitters, and therefore, high electron fluence onto the anode 458. The cathode 450 emits electrons which are accelerated by the electric field 456 created by the voltage potential of the target or anode 458. When electrons reach the target 458, X-rays are emitted from the target 458. X-ray absorber materials 466 such as tungsten or gold are deposited as layers as part of the fabricated structure of the SLV membrane to provide a means to form the REA 470 and in back scatter layer 461 to prevent back scattered X-rays from reaching control logic devices 472 with interconnect 473 to other RSCS in the SLV. This RSC is related to the well known X-ray vacuum tube design. The space 474 separating the cathode and target is in partial vacuum. This separation distance can vary from less than 1 μm to 40 μm.
Although laser diodes that emit radiation with a wavelength less than approximately 6,000 Å do not presently exist, development of shorter wavelength laser diodes is on-going and contemplated as a radiation generator for the SLV. There is significant demand for diodes with shorter wavelengths for use in consumer and computer peripheral products like compact discs. Such devices in the not too distant future may come into existence and such laser diodes could possibly be integrated into the SLV as a radiation source generator of a RSC. The use of laser diodes is therefore contemplated. An additional application of the SLV (with a RSC of appropriate radiation capability) is radiation-induced CVD (chemical vapor deposition). The use of an SLV in this manner eliminates the need for mask and etch processing steps. The material of interest to be deposited, be it metal or dielectric, is deposited in a patterned form. This provides for a maskless, resistless, etchless process and it has significant cost and particulate contamination advantages. The SLV operating in this manner supplies radiation at the surface of a substrate where a mixture of gaseous compounds are present. This process step is mechanically the same as exposing a resist layer on the substrate as discussed above. The radiation from a RSC causes the compounds to react at the surface of the substrate leaving a deposition of material selectively on the irradiated portions of the surface of the substrate. This method of radiation-induced selective CVD has been demonstrated with the use of excimer lasers. Laser diodes that can emit radiation in the excimer frequency band of 75 to 250 nm do not presently exist.
MDI Source-External Particle Valve (SPV) Direct Write Lithography Tool
The MDI process technology can be used to fabricate a circuit membrane with an n×m cell array of individually controlled electrostatic or electromagnetic valves or shutters. These valves can be used to control the passage of charged particles through the circuit membrane to create an exposure pattern in a film of appropriately sensitive resist. The array of valves on the MDI circuit membrane is for practical purposes the same structure as the SLV structure shown in
The array of valve cells of this MDI circuit membrane is placed between a collimated ion particle source and at a uniform distance from or in contact with a substrate coated with a thin film of resist. The array is positioned (aligned) over an area of the substrate to be patterned and the array of valves, under individual computer control, allow ion (charged) particles to expose or not expose an area of resist coated substrate directly under beneath it. The MDI circuit membrane functioning in this manner is called herein a Source-External Particle Valve (SPV).
The collimated particle beam passed by a particle valve has a fixed exposure feature size called the particle Exposure Aperture (PEA). The SPV is moved in a scanning X-Y manner over the substrate to be patterned. The dimensions of the scanning motions cause each PEA of a particle valve to pass over a portion of the substrate that typically is equal in area to the size of the valve. The valves are operated in parallel (simultaneously), the patterned areas on the substrate beneath each valve adjoin each other and collectively form a much larger overall pattern on the substrate. The scanning motion of the valves is generated by well known computer controlled mechanical motor driven stages or piezoelectric driven stages. The SPV or the substrate can be attached to the motion stage. Due to the short X-Y motions in the use of the SPV, and the desire to scan the valves with PEAs of dimensions much less than 1 mm in diameter and in some cases less than 50 nm, piezoelectric motion stage drive is the preferred embodiment. As the valves of the circuit membrane are scanned over the substrate, they are turned on or off by computer controlled logic associate with each valve to effect the desired exposure pattern to be mad on the surface of the substrate. An example would be a valve with an area of 25 μm by 25 μm and a PEA of 0.1 μm in diameter. In order for the valve to expose an area the size of the valve, a scanning motion of 25 μm by 0.1 μm would be made 250 times in the X-axis while translating 0.1 μm in the Y-axis after each scan.
a shows a cross-section of two valves 502, 504 in a SPV 506. A SPV in the preferred embodiment has rows and columns of valves 502, 504 where each row and each column can include more than 1,000 valves resulting in a total count of valves in a SPV in excess of several million.
The valves of the SPV prevent the passage of ions or charged particles by using an electric or magnetic field of the same polarity to deflect approaching particles from the PEA. The PEA 514a, 514b is typically a square opening in the SPV circuit membrane 508. The PPEA 514a, 514b is circumscribed by a metal film 520a, 520b which creates a local electrostatic field over the PEA by applying a voltage potential, or by a metal wire 540 (coil) (shown in
The SPV 506 is typically operated in an enclosure (not shown) under vacuum. The charged ions or particles can be of any material that can be appropriately accelerated or caused to uniformly flood the surface of the SPV. Some examples of charged particles that have been used as sources for patterning resist films are electrons (negative), protons (positive) and gallium+ (positive). The SPV can be in close proximity or contact with the substrate to be patterned, or it can be positioned before imaging lens elements of the lithography equipment that could be used to enhance the focus of the particles beams formed by passing through the SPV circuit membrane or demagnify (reduce) the image generated by the SPV.
c and 29d are cross-sections that show different methods for implementing an electrostatic shutter or valve of the SPV.
e and 29f show a method for implementing an electromagnetic shutter or valve of the SPV.
Control logic is associated with each valve of a SPV, supplying voltage or current as necessary to the metal structure circumscribing the PEAs. In
The design of electro-static or electromagnetic valves for achieving the desired functions of the SPV can vary widely. The examples presented here are not intended to limit the scope of the design structure of such valves for use in a SPV.
MDI Mechanical Light Valve (MLV) Direct Write Lithography Tool
g through 29k show portions of a direct write lithography tool which uses repeated micro-machined mechanical electro-static shutter cell 550-k for the patterning of photonic or particle exposure source. The overall structure and function of this lithography tool is similar to the above described SLV and SPV tools. It is composed of rows and columns of pattern forming elements called shutter cells 550-k that are each approximately 25 μm by 25 μm.
An array of shutter cells 550-k made on a MDI circuit membrane is placed between a collimated photonic or ion particle source and a substrate to be patterned, and can be used in projection or contact lithography methods to pattern such a substrate coated with a thin film of resist. The array is positioned (aligned) over an area of the substrate to be patterned and the array of shutter cells, each under individual computer control regulating the shutter opening or REA (Radiation Exposure Aperture) through the circuit membrane, exposes an area of the substrat directly beneath or corresponding to it. The MDI circuit membrane functioning in this manner is called herein a Mechanical Light Valv (MLV).
The size of the REA collimated exposure image passed by a shutter cell 550-k is variable. Further, the REA does not need to be a physical opening through the MDI circuit membrane for photonic sources, but only transparent to the incident photonic radiation. The MLV is moved in a scanning X-Y manner (shown by X-Y axis 554) over the substrate to be patterned. The dimensions of the scanning motions cause each REA of a shutter cell to pass over a portion of the substrate that typically is equal in area to the size of the shutter cell 550-k. The shutter cells 550-k are operated in parallel (simultaneous) manner; the patterned areas on the substrate beneath each shutter cell adjoin each other and collectively form a much larger overall pattern on the substrate. The scanning motion of the shutter cell is generated by well known computer controlled mechanical motor driven stages or piezoelectric driven stages. The MLV or the substrate can be attached to the motion stage. Due to the short X-Y emotions in the use of the MLV, and the desire to scan the shutter cells with REAs of dimensions much less than 1 μm in diameter and in some cases less than 50 nm, piezoelectric motion stage drive is the preferred embodiment.
As the shutter cells of the circuit membrane are scanned over the substrate, they are opened or closed by computer controlled logic 556-k associated with each shutter cell 550-h to effect the desired exposure pattern to be made on the surface of the substrate. An example would be a shutter cell with an area of 25 μm by 25 μm and a REA setting of 0.1 μm in diameter. In order for the REA to expose an area the size of the shutter cell, a scanning motion of 25 μm by 0.1 μm would be made 250 times in the X-axis while translating 0.1 μm in the Y-axis after each scan. The size setting of the REA is typically fixed for a given scan of the substrate, and vector motion of the REA can be used to improve exposure performance when there are areas of the substrate common to all shutter cells that do not require exposure such as is the case with via pattern layers.
The operation of the shutter cells regulates the opened/closed status of an opening that allows a fluence of the exposure source (photons or particles) to pass, and in the same operation dynamically determines the exposure CD for the lithography tool. The ability to change the minimum exposure CD or REA of the MLV is unique to this tool, versus the SLV and the SPV which have fixed exposure apertures. The ability to dynamically change the size of the REA allows the pattern or geometry fracture database of the IC to be partitioned into CD specific geometry sets per pattern layer of an IC. This provides the capability to optimize the performance of the lithography tool by patterning those portions of an exposure image with the smallest CD (processing rate is due to the size of the REA, therefore, the smaller the REA the greater the number of REA exposures), separately from other portions of the exposure image composed of larger CDs.
h and 29i show plan views of two different shutter arm structures of a shutter cell 550-k.
The shutter arm 560, 566 is positioned or moved by electro-static potentials. The shutter arm 560, 566 is provided with a voltage potential. Electrodes 580a, 580b positioned at either end of the maximum travel displacement of the central area of the shutter arm 566 are selectively set at opposite potentials sufficient to attract or repel the shutter arm a desired displacement. The shutter arm is held in physical contact with the electrode film beneath it by applying an opposite potential to the lower substrate electrode (not shown).
j is a cross section of part of the shutter cell through line j-j of
As shown in
Fixed Free Standing Membrane Lithography Masks
The MDI process can be used to form free standing membranes used as lithographic masks with fixed patterns. The MDI process can be used to form optical, X-ray and stencil masks lion or charged particle masks). The mask substrate is made from a low stress dielectric and optionally semiconductor material. In the preferred embodiment masks are made from oxide and nitride low stress dielectric made on the Novellus equipment and consistent with the formulation presented above or as variations of such low stress membranes formed on other oxide and nitride fabrication equipment. The patterning material is a semiconductor such as silicon, another dielectric material or a metal which is non-transmissive to a photonic exposure source. If the exposure source is an ion or charged particle beam, then the patterning is accomplished with voids (openings through the membrane) or a lack of material, or stencil.
l and 29m show in cross-section the formation of a fixed photonic mask. A membrane substrat 620 initially formed of dielectric layers 622, 624 and silicon 626 (patterning material shown with pattern) is formed as described above. The thickness of the dielectric layers 622, 624 of the membrane is typically 2 to 3 μm thick, and the silicon 626 may vary in thickness depending upon the desired sidewall angle of the silicon or the CD aspect ratio (ratio of layer thickness to the CD opening of the pattern). The silicon layer 626 can be patterned directly with established resist spin-on films, pattern generation lithography tools such as E-beam and selective silicon etch processing.
l shows in cross-section a patterned silicon layer 626 formed by first patterning an overlay of metal 630 such as tungsten, and then selectively etching the silicon layer 626 done to the dielectric layer 622. A metal pattern could also be formed by removing the silicon layer altogether and depositing a metal film, then subsequently patterning it, or by selectively undercutting the patterned silicon layer of
m shows in cross-section the structure of
In the case of X-ray masks gold is often used as the patterning film because it is a good X-ray absorber. Gold and the noble metals in general can be patterned only with wet processing techniques and or in combination with electroplating techniques. The silicon pattern 626 of
n and 29p show a stencil mask formed by patterning the dielectric layer 640 of a MDI silicon 642 and dielectric 640 membrane with a RIE process. The silicon layer 640 is then selectively wet etch removed to leave a stencil pattern as shown by the openings in the dielectric membrane 644l, 644b, 644c in
Stepper Contact Printer (SCP)
Pattern generation tools made from MDI circuit membranes can be used to pattern an appropriately sensitive resist film deposited on a substrate. MDI pattern generation tools refer to MDI circuit membrane applications such as the SLV, SPV and MLV lithographic tools, and the more conventional fixed pattern masks made on a low stress dielectric membrane with appropriate optical transmission characteristics (see above). A MDI fixed pattern mask is a single layer metallization pattern formed on a MDI dielectric membrane with one of the MDI fabrication methods presented above. A method of alignment to an existing image on a substrate through electrical inductive coupling is presented below; it is also incorporated into the MDI circuit membrane fixed pattern generation tool.
Contact printing methods are inexpensive and can provide nearly unlimited lithographic feature size imaging capability. However, the use of thick mask plates for image transfer to the resist on the substrate must come in contact with the resist to achieve the best image results. This contact is brought about by the application of strong vacuum forces that bend the rigid mask plate into conformal contact with the substrate. The result of applying such strong vacuum forces often causes small particles of the resist to adhere to the rigid mask which in turn requires the mask to be cleaned or, if undetected, prevents correct image transfer by nonconformal contact to the substrate or an imaging defect during subsequent exposures. Proximity printing is similar to contact printing except the mask is not brought into contact but is held several micrometers over the substrate called the proximity gap. The effectiveness of proximity printing to produce small feature size images is directly proportional to the size of the proximity gap. Further, contact printing has been limited to printing a complete substrate in one exposure instead of smaller portions of the substrate as is the case with conventional stepper projection lithography equipment. The time to pull vacuum during the contact printing process is lengthy, and therefore, making several contact print exposures on a single substrate would be prohibitive in time without consideration to the compounded mask cleanliness challenge and the mechanical complexity of vacuum application to uneven or overlapping ridge mask and substrate plate during contact.
Electro-Magnetic Lithogrphic Alignment Method
The lithographic pattern generation tools disclosed herein as the SLV, SPV, KLV, and fixed masks are capable of pattern generation with Critical Dimensions (CDs) or minimum pattern feature sizes of less than 50 nm. A fixed pattern generation tool, commonly referred to as a mask, can be made with the MDI circuit membrane.process (as disclosed above) with the capability of pattern generation CDs directly proportional to the wavelength of the exposure source. Potential exposure sources are UV, DUW, E-Beam and particle beam. (Fixed masks made with the MDI process are circuit membranes of typically less than 4 μm thickness and with a single layer of patterned material to be used as in conventional lithographic masks. Alternately, the MDI circuit membrane can be a stencil pattern, where the pattern to be generated is represented in the circuit membrane as trenches that go completely through the membrane. Fixed pattern stencil membranes are required for E-Beam or particle beam lithographic process, where the exposure source must physically pass through the mask to be imaged (patterned). Prototype particle beam stencil masks have been developed by Ion Microfabrication Systems.)
The utility of the CD generation capability of a pattern generation tool for the manufacture of ICs, however, is dependent on the ability to align or register multiple overlaying patterns. Accepted pattern alignment tolerances used presently in the semiconductor industry are typically ±25% of the CD value for a set of overlaying patterns.
Alignment for pattern generation with CDs of approximately 1 μm or greater can be done with optical microscopes by overlaying or matching alignment patterns on the substrate with corresponding patterns on the surface of the pattern generation tool. The microscope can view through the pattern generation tool, since it is typically made from transparent dielectric materials.
The preferred embodiments of the SLV, SPV, and MLV are to provide the capability to generate patterns that have critical dimensions of less than 100 nm. Such patterns require alignment methods capable of achieving pattern registration of less than ±25 nm. Optical alignment methods presently available cannot achieve such critical dimensions. The MDI pattern generation tools achieve alignment for critical dimensions of less than 100 nm through the use of electromagnetic proximity sensing.
The signals from the substrate coil are sensed by the corresponding alignment coil in the pattern generation tool. The pattern generation tool or the substrate are then moved in an X-Y and angular manner until predefined electrical parameter settings are achieved via the signals sensed from the substrate coil. This is possible due to the close proximity of the substrate and the pattern generation tool, and the integrated control logic electronics of the pattern generation tool. Alignment sensing accuracies smaller than 10 nm can be achieved in this manner. The electrode pads of the substrate coil typically have dimensions of less than 2 mils on a side, and the probe point electrodes that extend from the pattern generation tool to make contact with the substrat coil pads are typically less than 12 μm in diameter and height. A fluid pressure applied to the back side of the pattern generation tool may be used to bring the surface of the pattern generation tool in near contact (proximity) or full contact with the surface of the substrate during the alignment process.
Alternately,
Inexpensive piezoelectric motion control devices are commercially available from Burleigh Instruments, Inc. (Fisher, N.Y.) that demonstrate motion control capability of less than one. Angstrom; such piezoelectric precision motion has been demonstrated in the AFM (Atomic Force Microscope) equipment where motion control of less than one Angstrom is required.
MDI Circuit Membrane Flat Panel Display
Much of the cost of manufacturing a flat panel display (without consideration for yield loss) is the cost of the substrate on which it is fabricated. The substrat must meet requirements of flatness imposed by lithography tools and high temperatures imposed by various fabrication. steps.
An MDI circuit membrane flat panel display is formed on a rigid optically flat reusable substrate such as fused silica or fused quartz. Other substrates can be used to meet the requirements imposed by the fabrication processing steps. A release agent such as KBr or KBO2 is deposited onto the substrate and then a dielectric layer or layers are deposited to seal the release agent deposited film (as described above). The release agent is required to have a working temperature higher than the highest temperature of the various fabrication process steps employed and be readily soluble in a solvent that is not detrimental to the finished display device. As an example, KBr has a melting point of 734° C. and is readily soluble in DI water.
The flat panel display is conventionally composed of rows and columns of pixel elements which generate the gray shades or the RGB (Red, Green, Blue) colors of the display. The operation of the pixels results in the generation of the image on the display. The pixels of the display can be made with conventional active matrix LCD technology. The preferred embodiment of the MDI circuit membrane display uses redundant circuit devices at each pixel and the above referenced fine grain testing technology to correct for circuit fabrication defects.
Secondly, the KDI circuit membrane display makes use of electro luminescent phosphors, similar to the phosphors used in conventional TV and industrial monitors. The phosphors are applied after the completion of the fabrication of the circuitry of the display. The display circuitry is then used in the final manufacturing process steps to cause the selective deposition of the RGB phosphors to their respective RGB electrodes.
Additionally, the multiple chip module interconnect circuit membrane technology disclosed above also is employed to bond ICs at the edges of the display panel and at any desired location on the back of the panel. The ability to attach ICs is of significant importance in lessening the complexity of the design and manufacturing of the display. ICs bonded to the back of the display do not interfere with or obscure the visual operation of the phosphor based display, because the display is not back lit as is the case with LCD displays, but generates its own luminescence.
a shows the cross-section of a MDI circuit membrane display 700 while still attached to a reusable quartz fabrication substrate 702 with intervening release agent 703. The control logic circuit 704-1, 704-2, 704-3 for each RGB pixel has been fabricated from a-Si or polysilicon thin film transistors (TFTs), and electro-phoretic plating electrodes 708-1, 708-2, 706-3 (pads) have been formed as a final metallization process step over each RGB pixel logic circuit 704. The plating electrodes 708-1, 708-2, 708-3, one for each RGB phosphor of a pixel, can be selectively addressed and a specific plating potential applied. At the end of each row and column of pixels are bonding pads 710 for ICs (die). The ICs provide control and memory logic for turning individual pixels on or off, therefore generating a display image. It should be noted that the control and/or memory logic 712, 713 (see
b shows in cross section the MDI circuit membrane display 700 attached to a support frame 718 and released from the reusable fabrication substrate. The support frame 718 is bonded to the MDI circuit membrane 700 by an anodic method as one technique, and then the MDI circuit membrane 700 is scribed along the side of the fabrication substrate 702. This exposes the release agent 703 of
c shows the display after electro-phoretic plating of the individual RGB phosphors 708-1, 708-2, 708-3. Each RGB phosphor is plated separately by immersion of the display in a plating solution of a specific phosphor, and then using the control and pixel logic 704-1, 704-2, 704-3 of the display to select appropriate pixel elements (either Red or Green or Blue) and apply the desired plating electrical potential.
The substrate used to fabricate the flat screen display could also be a MDI membrane with a semiconductor device layer. Using a semiconductor based MDI membrane as the substrate to fabricate the flat screen display will not require any substantial difference in processing steps, however, due to the limited size of available silicon wafers (presently 8 inches) and the circular shape, the use of a semiconductor wafer as the starting substrate greatly limits the maximum size of the display.
Three Dimensional IC Structures
Three dimensional (3D) IC structures can be formed from MDI circuit membranes. This is a novel capability of the MDI IC membrane. The low stress dielectrics of silicon dioxide or silicon nitride can be formed to withstand working temperatures in excess of 400° C. This high working temperature capability allows the use of anodic or thermal (quartz to quartz or silicon to quartz) bonding procedures that are typically high temperature processes. The dielectric circuit membranes are optically transparent and thin, allowing the circuit membranes to be aligned very accurately.prior to bonding. As shown in
MDI circuit membranes 732, 746 are aligned using established optical alignment mark techniques and anodically or thermally bonded 752 to each other. The circuit membranes 732, 746 that are anodically or thermally bonded are complete of all necessary semiconductor device processing steps, and only require the completion of vertical interconnect metallization 754-1, 754-2. The final interconnect metallization 754-1, 754-2 is completed by etching vias 756-1, 756-2 through the last circuit membrane 732 to be bonded to electrical contacts on the circuit membrane 746 onto which it was bonded. Well known metal deposition and patterning process steps are applied. If additional circuit membrane ICs are to be bonded to a 3D IC structure, a dielectric coating is applied to the 3D structure to isolate any metallization as a necessary requirement to a subsequent bonding step.
Anodic bonding 752 is a glass-to-glass process. The MDI circuit membranes 732, 746 are each provided with a 1 μm thick or greater final deposition of silicon dioxide to form the bond 752 prior to the anodic bonding process step. Anodic processing is well known and there are numerous such techniques used in conventional glass plate manufacturing.
Thermal (fusion) bonding of silicon to quartz or glass substrates is an established technique. It can be used instead of anodic bonding methods to bond MDI circuit membranes.
The ability to use optical data transmission as an interconnect means (instead of metallization interconnect) between MDI circuit membrane layers 732, 746 is a direct consequence of the method of fabricating the 3D IC structure. Wave guides (not shown) between optical transceiver components can be formed from vias by bonding vias (openings) corresponding to the various optical transceiver devices on circuit membranes. The normally high transmissive nature of the circuit membranes and the short layer-to-layer distances between the circuit membranes allow integrated optical semiconductor devices to provide the communication function through vias or directly through certain dielectric materials.
Integrated circuits formed on a MDI circuit membrane can be cut from the circuit membrane in much the same manner as ICs or die are presently cut from a rigid wafer substrate in order to be packaged. This is possible due to the net low stress of the circuit membrane.
Once the circuit membrane substrate or wafer is cut into dice in the manner which is present practice, a pick and place bonding tool can pick up the die with slight vacuum pressure, align it over the substrate onto which it is to be bonded and bond it to the substrate.
c shows a 3D circuit structure of membrane dice (ICs) 764-a, 764-b, 764-c bonded by compression technique to a rigid substrate 770. This is accomplished by aligning metal pads 772-1, 1, 772-2, . . . , 772-k, . . . , 772-x on the rigid substrate 770 and metal pads 776-1, 776-2, . . . , 776-w on the membrane die 764-a, . . . , 764-c and then applying temperature and pressure (as explained previously). Also shown are bonding-out pads 778-1, 778-2. The surfaces are bonded together by the metal pads which also serve as circuit interconnect and thermal relief paths.
d shows a 3D circuit structure of membrane dice 764-a, 764-b, 764-c bonded by anodic or fusion (thermal) techniques to a substrate 770. The handling of the individual die 764-a, 764-b, 764-c can be done in a manner similar to that described above, and the bonding and interconnect completion methods are also as described above for bonding the circuit membranes together.
The benefits of such die assembly practice are dramatically increased circuit device densities and low thermal mass for efficient cooling. The handling equipment for making such a circuit assembly is presently available.
This disclosure is illustrative and not limiting; further modifications will be apparent to one skilled in the art in light of this disclosure and the appended claims.
This application is a continuation of commonly assigned U.S. patent application Ser. No. 09/775,598, filed Feb. 5, 2001, now U.S. Pat. No. 6,765,279, which is a continuation of U.S. patent application Ser. No. 09/027,959, filed Feb. 23, 1998, now abandoned, which is a division of U.S. patent application Ser. No. 08/850,749, filed May 2, 1997, now U.S. Pat. No. 5,985,693, which is a continuation of U.S. patent application Ser. No. 08/315,905, filed Sep. 30, 1994, now U.S. Pat. No. 5,869,354, which is a division of U.S. patent application Ser. No. 07/865,412, filed Apr. 8, 1992, now U.S. Pat. No. 5,354,695, all of which are incorporated by reference herein in their entireties.
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| Number | Date | Country | |
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| Child | 09027959 | US | |
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| Number | Date | Country | |
|---|---|---|---|
| Parent | 09775598 | Feb 2001 | US |
| Child | 10742282 | US | |
| Parent | 09027959 | Feb 1998 | US |
| Child | 09775598 | US | |
| Parent | 08315905 | Sep 1994 | US |
| Child | 08850749 | US |
