Patent Application
20080308524
This invention relates to the field of electrochemical deposition and more particularly to the field of electrochemical deposition using conformable contact masks that are formed separate from a substrate to control deposition, such as for example in Electrochemical Fabrication (e.g. EFAB™) where such masks are used to control the selective electrochemical deposition of one or more materials according to desired cross-sectional configurations so as to build up three-dimensional structures from a plurality of at least partially adhered layers of deposited material.
A technique for forming three-dimensional structures (e.g. parts, components, devices, and the like) from a plurality of adhered layers was invented by Adam L. Cohen and is known as Electrochemical Fabrication. It is being commercially pursued by Microfabrica Inc. (formerly MEMGen® Corporation) of Burbank, Calif. under the name EFAB™. This technique was described in U.S. Pat. No. 6,027,630, issued on Feb. 22, 2000. This electrochemical deposition technique allows the selective deposition of a material using a unique masking technique that involves the use of a mask that includes patterned conformable material on a support structure that is independent of the substrate onto which plating will occur. When desiring to perform an electrodeposition using the mask, the conformable portion of the mask is brought into contact with a substrate while in the presence of a plating solution such that the contact of the conformable portion of the mask to the substrate inhibits deposition at selected locations. For convenience, these masks might be generically called conformable contact masks; the masking technique may be generically called a conformable contact mask plating process. More specifically, in the terminology of Microfabrica Inc. such masks have come to be known as INSTANT MASKS™ and the process known as INSTANT MASKING™ or INSTANT MASK™ plating. Selective depositions using conformable contact mask plating may be used to form single layers of material or may be used to form multi-layer structures. The teachings of the '630 patent are hereby incorporated herein by reference as if set forth in full herein. Since the filing of the patent application that led to the above noted patent, various papers about conformable contact mask plating (i.e. INSTANT MASKING) and electrochemical fabrication have been published:
The disclosures of these nine publications are hereby incorporated herein by reference as if set forth in full herein.
The electrochemical deposition process may be carried out in a number of different ways as set forth in the above patent and publications. In one form, this process involves the execution of three separate operations during the formation of each layer of the structure that is to be formed:
After formation of the first layer, one or more additional layers may be formed adjacent to the immediately preceding layer and adhered to the smoothed surface of that preceding layer. These additional layers are formed by repeating the first through third operations one or more times wherein the formation of each subsequent layer treats the previously formed layers and the initial substrate as a new and thickening substrate.
Once the formation of all layers has been completed, at least a portion of at least one of the materials deposited is generally removed by an etching process to expose or release the three-dimensional structure that was intended to be formed.
The preferred method of performing the selective electrodeposition involved in the first operation is by conformable contact mask plating. In this type of plating, one or more conformable contact (CC) masks are first formed. The CC masks include a support structure onto which a patterned conformable dielectric material is adhered or formed. The conformable material for each mask is shaped in accordance with a particular cross-section of material to be plated. At least one CC mask is needed for each unique cross-sectional pattern that is to be plated.
The support for a CC mask is typically a plate-like structure formed of a metal that is to be selectively electroplated and from which material to be plated will be dissolved. In this typical approach, the support will act as an anode in an electroplating process. In an alternative approach, the support may instead be a porous or otherwise perforated material through which deposition material will pass during an electroplating operation on its way from a distal anode to a deposition surface. In either approach, it is possible for CC masks to share a common support, i.e. the patterns of conformable dielectric material for plating multiple layers of material may be located in different areas of a single support structure. When a single support structure contains multiple plating patterns, the entire structure is referred to as the CC mask while the individual plating masks may be referred to as “submasks”. In the present application such a distinction will be made only when relevant to a specific point being made.
In preparation for performing the selective deposition of the first operation, the conformable portion of the CC mask is placed in registration with and pressed against a selected portion of the substrate (or onto a previously formed layer or onto a previously deposited portion of a layer) on which deposition is to occur. The pressing together of the CC mask and substrate occur in such a way that all openings, in the conformable portions of the CC mask contain plating solution. The conformable material of the CC mask that contacts the substrate acts as a barrier to electrodeposition while the openings in the CC mask that are filled with electroplating solution act as pathways for transferring material from an anode (e.g. the CC mask support) to the non-contacted portions of the substrate (which act as a cathode during the plating operation) when an appropriate potential and/or current are supplied.
An example of a CC mask and CC mask plating are shown in
Another example of a CC mask and CC mask plating is shown in
Unlike through-mask plating, CC mask plating allows CC masks to be formed completely separate from the fabrication of the substrate on which plating is to occur (e.g. separate from a three-dimensional (3D) structure that is being formed). CC masks may be formed in a variety of ways, for example, a photolithographic process may be used. All masks can be generated simultaneously prior to structure fabrication rather than during it. This separation makes possible a simple, low-cost, automated, self-contained, and internally-clean “desktop factory” that can be installed almost anywhere to fabricate 3D structures, leaving any required clean room processes, such as photolithography to be performed by service bureaus or the like.
An example of the electrochemical fabrication process discussed above is illustrated in
Various components of an exemplary manual electrochemical fabrication system 32 are shown in
The CC mask subsystem 36 shown in the lower portion of
The blanket deposition subsystem 38 is shown in the lower portion of
The planarization subsystem 40 is shown in the lower portion of
Another method for forming microstructures from electroplated metals (i.e. using electrochemical fabrication techniques) is taught in U.S. Pat. No. 5,190,637 to Henry Guckel, entitled “Formation of Microstructures by Multiple Level Deep X-ray Lithography with Sacrificial Metal layers. This patent teaches the formation of metal structures utilizing mask exposures. A first layer of a primary metal is electroplated onto an exposed plating base to fill a void in a photoresist, the photoresist is then removed and a secondary metal is electroplated over the first layer and over the plating base. The exposed surface of the secondary metal is then machined down to a height which exposes the first metal to produce a flat uniform surface extending across the both the primary and secondary metals. Formation of a second layer may then begin by applying a photoresist layer over the first layer and then repeating the process used to produce the first layer. The process is then repeated until the entire structure is formed and the secondary metal is removed by etching. The photoresist is formed over the plating base or previous layer by casting and the voids in the photoresist are formed by exposure of the photoresist through a patterned mask via X-rays or UV radiation.
The '630 patent as well as the other conformable contact mask plating (i.e. Instant Mask Plating) and electrochemical fabrication (i.e. EFAB) publications noted above describe copper as a sacrificial material and nickel as a structural material. The copper is the preferred material for selective deposition while the nickel is the preferred material for blanket deposition. In most applications after formation of the nickel structure it is desirable to reveal or release it by separating it from the copper sacrificial material. The '630 patent proposes that this removal be performed by an etching operation and that useful etching compositions for selectively stripping copper from nickel structures include (1) solutions of ammonium hydroxide and copper sulfate or (2) solutions of ammonium hydroxide and sodium chlorite. This prior art patent indicates that a preferred etchant is Enstrip C38 commercially available from Enthone OMI. The patent goes further and indicates that etching can also be performed in the presence of (1) vibrations, e.g., ultrasound applied to the etchant or the substrate that was plated, and (2) pressurized jets of etchant contacting the metal to be etched.
In September of 1998, Adam Cohen placed an enquiry onto the “mems-talk” mailing list at http://mail.mems-exchange.org. In this enquiry Mr. Cohen indicated that he was seeking suggestions concerning a Cu etchant that didn't cause pitting or other damage to Ni. He further indicated that Enthone's Enstrip C38 caused pitting at least sometimes. In October of 1998, Mr. Cohen received three responses to this enquiry: (1) recommendation to use a copper etching process that showed no pitting problems with nickel—the etchant was HNO3:H3PO4:CH3COOH at 0.5:50.0:49.5 (volume) and was used at room temperature; (2) recommendation to use a caustic etchant and in particular Cu(NH3)4++ mixed with ammonia; and (3) recommendation to try 50% NH4OH mixed with 50% H2O2 in a 1:1 ratio.
A need remains in the field of conformable contact mask plating and electrochemical fabrication for improved post deposition processing and in particular for processes that separate copper from nickel while minimizing the pitting of nickel, and more particularly to provide an improved process of separating copper from nickel when the nickel structure has a complex geometry with copper needing to be removed from small but extended or even intricate passages within the nickel structure.
It is an object of certain aspects of the invention to provide improved post deposition processing for structures produced by conformable contact mask plating or electrochemical fabrication.
It is an object of certain aspects of the invention to provide an improved process for separating copper from nickel.
It is an object of certain aspects of the invention to provide a generalized copper removal process that can be used to remove copper from a complex nickel structure without damaging the nickel.
Other objects and advantages of various aspects of the invention will be apparent to those of skill in the art upon review of the teachings herein. The various aspects of the invention, set forth explicitly herein or otherwise ascertained from the teachings herein, may address any one of the above objects alone or in combination, or alternatively may not address any of the objects set forth above but instead address some other object ascertained from the teachings herein. It is not intended that all of these objects be addressed by any single aspect of the invention even though that may be the case with regard to some aspects.
A first aspect of the invention provides an electrochemical fabrication process for producing a three-dimensional structure from a plurality of adhered layers, the process that includes: (A) supplying a plurality of preformed masks, wherein each mask includes a patterned conformable dielectric material that includes at least one opening through which deposition can take place during the formation of at least a portion of a layer, and wherein each mask includes a support structure that supports the patterned conformable dielectric material; (B) selectively depositing a first material onto the substrate to form a portion of a layer and depositing at least a second material to form another portion of the layer, wherein the substrate may include previously deposited material, and wherein one of the first material or the second material is a structural material and the other is a sacrificial material; (C) forming a plurality of layers such that each successive layer is formed adjacent to and adhered to a previously deposited layer, wherein said forming includes repeating operation (B) a plurality times; and (D) after formation of a plurality of layers, separating at least a portion of the sacrificial material from the structural material using an etching solution that includes ammonium hydroxide, a chlorite salt, and a nitrate salt; and wherein at least a plurality of the selective depositing operations include (1) contacting the substrate and the conformable material of a selected preformed mask; (2) in presence of a plating solution, conducting an electric current through the at least one opening in the selected mask between an anode and the substrate, wherein the anode including a selected deposition material, and wherein the substrate functions as a cathode, such that the selected deposition material is deposited onto the substrate to form at least a portion of a layer; and (3) separating the selected preformed mask from the substrate.
A second aspect of the invention provides a process for producing a structure, wherein the process includes: (A) supplying at least one preformed mask that includes a patterned conformable dielectric material that includes at least one opening through which deposition can take place during the formation of at least a portion of a layer, and wherein at the least one mask includes a support structure that supports the patterned conformable dielectric material; and (B) selectively depositing at least a first material onto the substrate, the depositing including (1) contacting the substrate and the conformable material of the preformed mask; (2) in presence of a plating solution, conducting an electric current through the at least one opening in the selected mask between an anode and the substrate, wherein the anode including a selected deposition material, and wherein the substrate functions as a cathode, such that the selected deposition material is deposited onto the substrate to form at least a portion of a layer; and (3) separating the selected preformed mask from the substrate; (C) depositing at least a second material onto the substrate after depositing the at least first material; and (D) after formation of at least one layer, separating at least a portion of the sacrificial material from the structural material using an etching solution that comprises ammonium hydroxide, a chlorite salt, and a nitrate salt.
A third aspect of the invention provides a process of etching a first material from a structure, including: (A) supplying a structure including at least a first material and a second material; and (B) placing the structure in an etching solution that includes ammonium hydroxide, a chlorite salt, and a nitrate salt.
Other aspects of the invention will be understood by those of skill in the art upon reviewing the teachings herein. Other aspects of the invention may involve apparatus that can be used in implementing one or more of the above method aspects of the invention. These other aspects of the invention may involve various combinations of the aspects presented above, addition of various features of one or more embodiments, as well as other configurations, structures, functional relationships, and processes that have not been specifically set forth above.
In some preferred conformable contact mask plating and electrochemical fabrication embodiments, deposition and etching of a sacrificial material, such as copper, are essential steps. The sacrificial material serves as a mechanical support for the structural material during structure formation. Additionally, since the sacrificial material, like the structural material, is conductive, additional material can be deposited over the entire layer without constraint. Thus the use of a sacrificial material eliminates virtually all geometrical restrictions, allowing the structural material on a layer to overhang and even be disconnected from that on the previous layer. Furthermore, the use of a sacrificial material may allow a broader range of structural materials to be used in that the sacrificial material can be deposited in a selective process (e.g. by a conformable contact mask process) while the structural material may be deposited in some other manner (e.g. blanket deposition) where fewer deposition limitations may exist.
The basic rules governing etching are as follows:
1. Selectivity: Etchants should only remove sacrificial materials. No or little effect on main materials should occur.
2. Completion: A sacrificial material needs to be removed completely.
3. Speed: The shorter the etching time, the higher the throughput.
4. Integration: An etching process should not damage delicate structures.
Wet etching is a fast, cheap process and can also remove materials from blind geometries. Usually, to remove a metal, it must be of an oxidized form so as to transition from the metallic to an ionic state. Therefore, the active ingredient in a metal etchant needs to be an oxidizing agent. Alternatively, electrochemical anodic etching provides the required oxidizing action by passing a current of cations from a workpiece. An acid or alkaline complexing agent may be included to increase the etching rate. Other additives may also be included. Common oxidizing agents used for stripping copper include chlorite, ferric chloride, cupric chloride, persulfate, organic nitro compounds, and peroxide.
In electrochemical fabrication, a fast reliable copper etching process without negative effect on structural material (e.g. nickel) and associated structures is desirable to achieve the final structures (e.g. microstructures).
Some common copper etchants were evaluated for use in electrochemical fabrication and are listed in
Although these etchants were reported to be nickel compatible, etchants with slow etching rate and bubble formation during the etching process were not considered further. A slow etching rate means more process time while vigorous bubble formation could induce stress in free standing structures such as beams and cantilevers, could break delicate microstructures, or could inhibit etchant access into small passages. Though most of the etchants were successful in removing thin sacrificial copper films, their slow etching rates and/or bubble formation make them impractical for removing relatively large amounts of copper in electrochemical fabrication or similar cases. Of the etchants evaluated the ENSTRIP® C-38 stripper had an etching rate of 460 μm/hr and appeared to be the most promising.
ENSTRIP® C-38 stripper (Enthone-OMI Inc. of New Haven, Conn.) is a two-component, ammoniacal immersion stripper designed to quickly remove copper from steel and stainless steel substrates. The recommended C-38 stripper is formed from two primary components, Enstrip C-38A at 75% by volume and Enstrip C-38B at 25% by volume. It is recommended that the Enstrip C-38 solution should only be operated within the pH range of 9.3 to 10.5 and within a temperature range from room temperature to a maximum of 38° C. If the solution pH becomes too low, it is recommended that 27% ammonium hydroxide be added in small increments until the pH is brought into the right range. It is believed that the two main components of the C-38 solution are sodium chlorite, NaClO2, and ammonium hydroxide, NH4OH. The C-38 solution can dissolve up to 8 ounces of copper per gallon of solution. The C-38 basic reaction mechanism is believed to be:
On the etching surface:
ClO2−+H2O+2e−→ClO−+2OH−
2Cu−2e−→2Cu+
Cu++2NH3→[Cu(NH3)2]+
In the bulk solution:
ClO2−+H2O+2e−→ClO−+2OH−
2[Cu(NH3)2]++4NH3−2e−→2[Cu(NH3)4]2+
C-38 does not attack nickel significantly. Experiments showed that the nickel corrosion rate in C-38 is only about 72 μm/yr. For a short etching time, the actual amount of etched nickel is negligible. To extend the range of electrochemical fabrication structural materials beyond nickel, the etching rates of other metals and alloys were tested in C-38. Samples with a known area and weight were immersed into C-38 at room temperature for a known time. The etching rate was calculated from the corresponding weight loss. The test results are listed in the following table. Compatibility of Metals and Alloys in C-38
Zinc is not suitable for use as an unprotected structural material but may be useful as a sacrificial material since it is quickly dissolved in C-38. All other metals and alloys that were tested were determined to be useful as structural materials when C-38 is the etchant.
The etching rate of copper in C-38 can be adjusted downward by diluting the full strength C-38. A plot of etching rates versus C-38 concentration is shown in
The C-38 wet etching process is followed by a drying process to remove the liquid from the microstructure. Because of the surface tension of the rinse water, the released free-standing structures can tend to stick to the substrate. Once a structure is attached to the substrate by sticking, the mechanical force needed to dislodge it usually is large enough to damage the structure. In some MEMS processes, it has been proposed that this problem be overcome by use of freezing-sublimation or a CO2 supercritical drying process. However, these techniques can be process intensive, time consuming and often require sophisticated high-pressure apparatus. In electrochemical fabrication, a relatively simple method is preferred. After rinsing the part, it is immediately transferred into an alcohol solution where the alcohol is made to replaces the water from around the structure. The structure is then immediately transferred to an oven at ˜60° C. for 5-10 minutes to evaporate the alcohol and dry the structure.
The preferred procedure for releasing structures (i.e. copper from nickel structures) produced by electrochemical fabrication involves surrounding the combined copper/nickel structure with a diluted C-38 etchant without any stirring. The preferred dilution is about one part C-38 by volume to about four to five parts H2O. In some embodiments though, the level of dilution may range from as low as about one part C-38 to about ten parts water and as high as undiluted C-38. The etching endpoint is reached when a blue substance stops appearing from the structure and in particular from any cavities or ports within the structure. The structure is then dipped into a Di water tank and is slowly moved through the water so as to displace the etchant with the water. The structure is then transferred to an alcohol tank where the structure is slowly moved through the alcohol to displace the water with alcohol and it is thereafter removed from the tank and dried in an oven.
Nickel is considered to be a slightly noble metal. It resists corrosion in many environments due to its high passivation tendency. Usually there is a passive oxide or hydrated oxide film on the nickel surface which produces good corrosion resistance. In neutral and moderately alkaline solutions, a passive surface layer of Ni(OH)2 and perhaps NiO forms on the nickel surface, while the passive film is possibly NiOOH in strongly oxidizing neutral and alkaline conditions such as in a C-38 environment (i.e. in an alkaline oxidizing solution).
Passive films protecting metals and alloys break down locally in certain corrosion environments and pitting results. Local points undergo anodic dissolution to form pits on the surface, while the major part of the surface remains passive. Usually, the diameter of pits is in the range of tens of micrometers and the depth of pits is equal to, or more than, their diameter. Obviously, formation of pits on nickel is unacceptable to microstructures. C-38 works well in etching copper without attacking nickel. However, occasionally pits have been observed to form on the nickel substrate and nickel deposits.
Additional preferred electrochemical fabrication etching processes add a corrosion inhibitor to the C-38 to help prevent pitting. The use of a corrosion inhibitor in combination with the etchant may be done alone or in addition to the above noted handling and checking preferences. The preferred inhibitor for use in etching electrochemical fabrication structures with a Chlorite based etchant like C-38 is sodium nitrate, NaNO3.
Corrosion inhibitors are chemical compounds which, when added in small concentration to a corrosion environment, can greatly increase the corrosion resistance of an exposed metal. It is known that nitrate can be used as a pitting inhibitor for steels, stainless steels, aluminum and its alloys. For nickel it is believed that the anti-pitting mechanism of NaNO3 is due to the preferential adsorption of NO3— on the nickel surface. In this way, NO3− ions prevent aggressive ions like ClO− from adsorbing on the surface to cause pitting. The presence of the nitrate can shift a pitting potential (Epit) to a more noble value. Its efficiency can be evaluated by a pitting scan which is a potentiodynamic polarization curve measurement in which Epit is determined from the anodic polarization curve as the potential where the current density sharply increases due to breakdown of the passive film and formation of pits. Pits initiate and grow above Epit, but not below. The more positive the Epit, the better the efficiency of the inhibitor.
A test was performed to determine if the present of NaNO3 could raise the Epit value. The test was performed using polished nickel disks having diameters of 1.27 cm. Pitting scans were conducted in 0.5 N NaCl solution with and without NaNO3 (1 g/100 ml) using an EG&G 273A Potentiostat/Galvanostat in accordance with ASTM G5 and G61. The scan rate was 0.166 mV/s. Epit increased by about 90 mV in the presence of 1 g/100 mL of NaNO3. An additional test indicated that when only 0.1 g/100 ml NaNO3 was added, no shift of Epit occurred. It is believed that a concentration of NaNO3 sufficient to raise the Epit value by about 10 mV would yield some improvement in performance though having it be raised to about 30 mV or more preferably by about 50 mV would be better. In any event, an effective quantity of an antipitting agent may be empirically determined by those of skill in the art in view of the teachings herein such that pitting is eliminated or brought down to a tolerable level.
An experiment was performed to determine the effect of the presence of NaNO3 on the copper etching rate. The determined etching rate of copper foil in C-38 containing 1 g/100 ml NaNO3 was 430 μm/hr compared to 460 μm/hr without NaNO3 suggesting that the presence of NaNO3 has only a small effect on copper etching and that the effectiveness of the etchant remains. Experiments have also shown that pitting is reduced when etching with C-38 in combination with a small amount of NaNO3 (sodium nitrate). It is believed that the concentration of C-38 may be lowered to about 0.5 g/100 ml and still have obtain a benefit from the process and raised well above the 1 g/100 ml concentration level without bringing harm to the etching process though a point may be reached where little additional benefit is added by the increased concentration.
Wet chemical sacrificial etching is dependent on the reacting species reaching the etching surface (e.g. by diffusion). If the etching area is relatively large and open to the etchant, and the etching length of the sacrificial layer is short (e.g. <100 μm), the etchant can always be sufficiently supplied at the etching front. This etching mode is called reaction-limited etching. However, if the etching length is very long compared to the channel width such as in narrow channel etching or where the etchant flow is severely restricted due to cavities or structures with irregularly shaped interfaces, the etchant may be depleted at the etching front. This is known as the diffusion-limited etching mode. In this mode, etching may become extremely slow or even stop.
To eliminate the limitation of diffusion of chemical species in wet chemical etching, it is believed that some form of electrochemical anodic etching may be used to assist in the removal of copper particularly from complex geometries such as narrow passages and blind cavities. Besides the chemical etching effect of an etchant itself on copper, electrochemical anodic etching provides also for anodic dissolution by passing current through the etchant to the surface to be etched. In addition, the applied electric field can drive copper ions through the etchant away from the structure being etched toward a cathode while simultaneously attracting anions to the surface of the structure, thus creating higher material transfer rate and helping to bring unreacted fluid closer to the copper front due to mass conservation effects.
Preliminary electrochemical anodic etching of both the DC and biased AC type were investigated for use with electrochemical fabrication produced structures. C-38 was used as the etchant. Based on these preliminary investigations, electrochemical etching seems to be a promising copper etching technique.
Various other embodiments of the present invention exist. Some of these embodiments may be based on a combination of the teachings herein with various teachings incorporated herein by reference. Some embodiments may not use any blanket deposition process and/or they may not use a planarization process. Some embodiments may involve the selective deposition of a plurality of different materials on a single layer or on different layers. Some embodiments may use blanket depositions processes that are not electrodeposition processes. Some embodiments may use selective deposition processes on some layers that are not Instant Mask processes and are not even electrodeposition processes. Some embodiments may use nickel as a structural material while other embodiments may use different materials such as gold, silver, or any other electrodepositable materials that can be separated from the copper and/or some other sacrificial material. In some embodiments, the depth of deposition will be enhanced by pulling a CC mask away from the substrate as deposition is occurring in a manner that allows the seal between the conformable portion of the CC mask and the substrate to shift from the face of the conformal material to the inside edges of the conformable material.
In view of the teachings herein, many further embodiments, alternatives in design and uses of the instant invention will be apparent to those of skill in the art. As such, it is not intended that the invention be limited to the particular illustrative embodiments, alternatives, and uses described above but instead that it be solely limited by the claims presented hereafter.
This application is a continuation of U.S. patent application Ser. No. 10/434,294, filed May 7, 2003, which in turn claims benefit of U.S. Provisional Patent Application No. 60/379,134 which was filed on May 7, 2002. Each of these applications is incorporated herein by reference as if set forth in full herein.
This invention was made with Government support under Grant Number DABT63-97-C-0051 awarded by DARPA. The Government has certain rights.
| Number | Date | Country | |
|---|---|---|---|
| 60379134 | May 2002 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10434294 | May 2003 | US |
| Child | 12046431 | US |
