Patent Application
20060093170
The invention relates to a sensing element of a silicon condenser microphone and a method for making the same, and in particular, to a silicon microphone structure without a dedicated backplate that has perforated plates attached directly to a movable diaphragm.
The silicon based condenser microphone also known as an acoustic transducer has been in a research and development stage for more than 20 years. Because of its potential advantages in miniaturization, performance, reliability, environmental endurance, low cost, and mass production capability, the silicon microphone is widely recognized as the next generation product to replace the conventional electret condenser microphone (ECM) that has been widely used in communication, multimedia, consumer electronics, hearing aids, and so on. Of all the silicon based approaches, the capacitive condenser type of microphone has advanced the most significantly in recent years. The silicon condenser microphone is typically comprised of two basic elements which are a sensing element and a pre-amplifier IC device. The sensing element is basically a variable capacitor constructed with a movable compliant diaphragm, a rigid and fixed perforated backplate, and a dielectric spacer to form an air gap between the diaphragm and backplate. The preamplifier IC device is basically configured with a voltage bias source (including a bias resistor) and a source follower preamplifier. Although there have been numerous embodiments of the variable capacitor on silicon substrates, each prior art example includes a dedicated backplate in the construction of the microphone sensing element. Table 1 lists typical examples which employ various materials in the fabrication of a microphone sensing element.
The references in Table 1 are the following: (1) D. Hohm and G. Hess, “A Subminiature Condenser Microphone with Silicon Nitride Membrane and Silicon Backplate”, J. Acoust. Soc. Am., Vol. 85, pp. 476-480 (1989); (2) J. Bergqvist et al., “A New Condenser Microphone in Silicon”, Sensors and Actuators, A21-23 (1990), pp. 123-125; (3) W. Kuhnel et al., “A Silicon Condenser Microphone with Structured Backplate and Silicon Nitride Membrane”, Sensors and Actuators A, Vol. 30, pp. 251-258 (1991); (4) P. Scheeper et al., “Fabrication of Silicon Condenser Microphones Using Single Wafer Technology”, J. Microelectromech. Systems, Vol. 1, No. 3, pp. 147-154 (1992); (5) U.S. Pat. No. 5,146,435 and U.S. Pat. No. 5,452,268; (6) J. Bergqvist et al., “A Silicon Microphone Using Bond and Etch-back Technology”, Sensors and Actuators A, Vol. 45, pp. 115-124 (1994); (7) Zou, Quanbo, et al., “Theoretical and Experimental Studies of Single Chip Processed Miniature Silicon Condenser Microphone with Corrugated Diaphragm”, Sensors and Actuators A, Vol. 63, pp. 209-215 (1997); (8) U.S. Pat. No. 5,490,220 and U.S. Pat. No. 4,870,482; (9) M. Pedersen et al., A Silicon Microphone with Polyimide Diaphragm and Backplate”, Sensors and Actuators A, Vol. 63, pp. 97-104 (1997); (10) P. Rombach et al., “The First Low Voltage, Low Noise Differential Condenser Silicon Microphone”, Eurosensor XIV, The 14th European Conference on Solid State Transducers, Aug. 27-30, 2000, pp. 213-216; (11) M. Brauer et al., “Silicon Microphone Based on Surface and Bulk Micromachining”, J. Micromech. Microeng., Vol. 11, pp. 319-322 (2001); (12) PCT Patent Application No. WO 01/20948 A2.
The inclusion of a dedicated backplate in the microphone sensing element normally leads to manufacturing complications due to its special definitions in material and processing method. The required masking levels as well as the processing issues relating to overlay and spacing between the diaphragm and backplate normally result in a complex and high cost fabrication.
Therefore, an improved structure for a silicon microphone is needed that enables the fabrication process to be simplified at a reduced cost. In particular, a novel design for the variable capacitor component is desirable so that fewer masking levels are needed to produce a microphone sensing element with improved performance.
One objective of the present invention is to provide a microphone sensing element that does not include a dedicated backplate component.
A further objective of the present invention is to provide a simplified method for fabricating a microphone sensing element.
These objectives are achieved with a microphone sensing element which in its most basic embodiment features a movable diaphragm that is supported at its edges or corners by mechanical springs that are anchored to a conductive substrate through rigid pads. Each pad is disposed on a dielectric layer which acts as a spacer to define an air gap between the diaphragm and substrate. Attached to the sides of the diaphragm are perforated plates made from the same material layer as the diaphragm, pads, and mechanical springs. One or more of the pads have an overlying first electrode which is an island of a conductive metal material that is connected by wiring to external circuitry. A second electrode of the same material composition is formed on the conductive substrate and is wired to complete a variable capacitor circuit. In one embodiment (SOI version), the diaphragm, perforated plates, pads, and mechanical springs are coplanar and are made from the same silicon layer and the dielectric spacer is an oxide layer. Both the diaphragm and perforated plates may be rectangular in shape. The perforated plates are positioned between adjacent mechanical springs. Perforations preferably comprise multiple rows and columns of holes. An air gap exists in the dielectric spacer layer between the substrate and the perforated plates and a back hole is formed in the substrate below the diaphragm so that a sound signal has a free path to the diaphragm and thereby induces vibrations in the diaphragm. The diaphragm, mechanical springs, and perforated plates move up and down (perpendicular to the substrate) in a concerted motion during a vibration. This movement results in a capacitance change between the first and second electrodes which can be converted into an output voltage.
In a second embodiment wherein a silicon oxide layer such as tetraethyl orthosilicate (TEOS) is used as a sacrificial layer, the diaphragm, mechanical springs, pads, and perforated plates are all made from a thin polysilicon (poly 2) layer. The diaphragm with attached perforated plates may have bottom reinforcements that project below the bottom surface of the diaphragm that is aligned over a back hole in the substrate. The diaphragm may be square with four corners and four sides and with a perforated plate affixed to each side. Each of the four mechanical springs is formed in a lengthwise direction along a plane that passes through the center and a corner of the diaphragm and has two ends wherein one end is attached to the diaphragm and the other end is connected to a poly 2 anchor pad. Optionally, the mechanical springs are attached to the sides of the diaphragm and the perforated plates are affixed to the corners and portions of the adjoining diaphragm sides. The anchor pad or pad also serves as an electrical connection point. To reduce parasitic capacitance between the poly 2 anchor pad and the conductive substrate, the poly 2 anchor pad may not be coplanar with the diaphragm and may be raised away from the substrate by adding one or more dielectric oxide layers between the substrate and anchor pad. Another polysilicon (poly 1) pad may be interposed between the poly 2 anchor pad and the substrate to serve as an etch stop layer for oxide trench etching. A poly 2 filled trench in the shape of a wall continuously surrounds the inner edges of the interposed poly 1 pad. Vertical sections of the poly 2 anchor pad form a continuous ring around the edge of the poly 1 anchor pad and thereby protect the oxide layer beneath the poly 1 anchor pad from being etched away in a release process. The oxide layer between the interposed poly 1 pad and substrate is protected with another dielectric layer made of silicon nitride or the like that can resist or delay the oxide release etching used to form the air gap. To further reduce parasitic capacitance, a plurality of mesh patterned deep trenches filled with oxide may be formed in the conductive silicon substrate wherever they are overlaid by the mechanical springs and their anchor pads.
In a third embodiment, the diaphragm has four attached perforated plates and four mechanical springs that connect the diaphragm at its corners to four pads (anchor pads) as in the second embodiment. However, the mechanical springs, pads, and diaphragm are coplanar and made from the same polysilicon layer which is a first distance from the substrate. The diaphragm may have bottom reinforcements as in the second embodiment. However, each mechanical spring is anchored to a horizontal section of a base element that is supported by a vertical section comprised of sidewalls that have a top, bottom, and width. The base element is preferably made of silicon rich silicon nitride (SRN) that fills four trenches to form four sidewalls arranged in a square or rectangular ring. The horizontal section of the SRN base is formed on a pad which in one embodiment is an extension of a mechanical spring. Thus, the diaphragm and its attached perforated plates are suspended over an air gap and a back hole in the substrate. A first electrode may be non-planar and formed on the top of a horizontal section and adjacent pad. A second electrode is formed on the substrate.
A fourth embodiment is shown that is a modification to the first embodiment in which a corner or edge support for the mechanical springs is replaced by a “center support” configuration. A dielectric spacer layer that functions as a center rigid anchor pad is formed on the substrate below the center of the diaphragm and supports four mechanical springs that overlap on one end below a first electrode. The other ends of the mechanical springs are connected to the edge of the diaphragm. Each mechanical spring may have a rectangular shape with a lengthwise direction along one of two perpendicular planes that intersect at the center of the diaphragm and are perpendicular to the substrate. Along the lengthwise direction on either side of the mechanical springs are slots that separate the mechanical spring from the diaphragm. The back hole has four sections wherein one section is formed below each diaphragm quadrant defined by the two intersecting planes. The thickness of the dielectric spacer layer defines the thickness of the air gap between the diaphragm and substrate.
The present invention is also a simple method of fabricating a microphone sensing element that requires fewer masks than most of the conventional silicon condenser microphones having a dedicated backplate. An exemplary process sequence involves forming a dielectric spacer layer on a conductive substrate such as doped silicon. The dielectric spacer layer may be comprised of silicon oxide. A membrane film that may be doped silicon or polysilicon is then formed on the dielectric spacer layer. Next, a hardmask comprised of one or more layers that will subsequently be used for fabricating a back hole is formed on the back side of the substrate. A first photo mask is employed to generate one or more vias in the membrane film that extend through the dielectric spacer layer to contact the substrate. After a conductive layer which may be a composite of two or more metals is deposited on the front side, a second photo mask is used to remove the conductive layer except for one or more islands on the membrane layer that are first electrodes and an island in one or more vias on the substrate that are second electrodes. Another photo mask is then employed to etch holes in portions of the membrane layer to define the perforated plates and form openings that define the edges of the perforated plates, mechanical springs, and pads. A fourth photo mask is used to etch an opening in the hard mask on the backside that allows KOH etchant or a deep RIE etch in a subsequent step to form a back hole in the substrate below the diaphragm. Finally, an etchant during a timed release step removes a portion of the dielectric spacer layer between the diaphragm and back hole to create an air gap so that the diaphragm becomes suspended over the air gap and the underlying back hole.
The simplest fabrication method to form the basic silicon microphone structure involves silicon-on-insulator (SOI) wafers. Those skilled in the art will appreciate that other fabrication methods including wafer-to-wafer bonding methods and polysilicon surface micromachining can be used to form the other embodiments or embodiments similar to those described herein.
The present invention is a sensing element for a capacitive condenser type of microphone that can readily be made with existing semiconductor materials and silicon micromachining processes. The figures are not necessarily drawn to scale and the relative sizes of various elements in the structures may be different than in an actual device. The present invention is based on the discovery that a high performance microphone sensing element may be constructed without a dedicated backplate component. A microphone working capacitance is achieved with a conductive substrate having a back hole formed therein and with perforated plates affixed to a movable diaphragm above the substrate. The diaphragm may be connected to mechanical springs attached to rigid anchor pads on a dielectric spacer layer disposed on the substrate.
Referring to
The pads 13c are anchored to the substrate 11 through a dielectric layer 12 that serves as a spacer so that the diaphragm 13a and perforated plates 13d are suspended over an air gap and a back hole (not shown) through which a sound signal may pass to induce a vibration in the diaphragm. In one aspect, the dielectric layer 12 is comprised of silicon oxide. This embodiment encompasses an SOI approach wherein the membrane film is comprised of silicon and the dielectric layer 12 is silicon oxide. Optionally, the dielectric layer 12 may be made of other dielectric materials used in the art and may be a composite with a plurality of layers therein.
Another important feature of the present invention is that a perforated plate 13d which is rectangular in shape is adjoined to each side of the diaphragm 13a. The perforated plate 13d has a lengthwise dimension equal to or less than the length of the diaphragm side to which it is attached, a width that is less than its lengthwise dimension, and has the same composition and thickness as the diaphragm 13a. Perforations consist of holes 19 that may be arranged in multiple columns and rows. The holes are needed to allow air ventilation and thus reduce the air damping in the narrow air gap (not shown) during vibrations.
There is a contact or first electrode 18a comprised of metal layers like Cr/Au above each pad 13c that serves as a connecting point to external wiring. Additionally, there are one or more second electrodes 18b with the same composition as a first electrode located on the front side of the substrate 11. A first electrode and second electrode are connected by wiring (not shown) to form a variable capacitor circuit. Again, for an illustrative purpose, the first and second electrodes 18a, 18b are shown as square in shape although rounded corners or rectangular shapes may be adopted. A first electrode 18a is smaller in length and width than the width c of a pad 13c to allow for some overlay error in processing. Optionally, the first and second electrodes 18a, 18b may be a single or composite layer comprised of Al, Ti, Ta, Ni, Cu, or other metal materials.
The first embodiment is further illustrated in a cross-sectional view in
A second embodiment of a sensing element in a backplateless silicon microphone according to the present invention is shown in
Returning to
Vertical sections of a rigid semiconductor layer preferably made of polysilicon are formed in the dielectric spacer stack comprised of thermal oxide layer 35, silicon nitride layer 36, and oxide layer 37 and contact the substrate 31 or the poly 1 layer 34 in certain regions outside the periphery of the diaphragm 41b. In one embodiment, the vertical sections are polysilicon filled trenches 38a, 38b, 40.
To reduce parasitic capacitance between the pad 41d and substrate 31, the pad 41d may not be coplanar with the diaphragm 41b and may be raised away from the substrate (compared with the diaphragm) by inserting a dielectric layer which in this case is oxide layer 33 on certain regions of the substrate 31. Furthermore, the poly 1 layer 34 is interposed between the oxide layer 33 and thermal oxide layer 35 to serve as an etch stop to protect the oxide layer 33 when etching the trench 38b through the thermal oxide layer 35 and oxide layer 37. As a result, the filled trench 38b continuously surrounds the edge of the poly 1 layer 34. Note that portions of the oxide layer 37, silicon nitride layer 36, and thermal oxide layer 35 below the pad 41d and horizontal section 41a are completely enclosed within the filled trench 38a and within filled trench 38b and thereby the enclosed oxide layers 35, 37 are protected from an etch that is applied to form the air gap 48 in a release step. Additionally, the oxide layer 33 below the poly 1 layer 34 is protected by the silicon nitride layer 36 that can resist or delay the oxide etching in the release step.
From a top perspective in
In an enlarged view of one pad area shown in
Returning to
The diaphragm 41b, perforated plates 41e, and mechanical springs 41c are suspended over an air gap 48. The air gap 48 is between the perforated plates 41e and silicon nitride layer 36. The diaphragm 41b, perforated plates 41e, and mechanical springs 41c may have reinforcements 39 along their bottom sides that project downward toward substrate 31. Reinforcements 39 are preferably employed when the diaphragm 41b is thin (about 1 micron thick) and are not necessary when the diaphragm thickness is greater than about 3 microns. Note that the openings 43 separate the horizontal sections 41f of the poly 2 layer from the perforated plates 41e and pads 41d. There is a trench 49 with a ring shape in the horizontal section 41f of the poly 2 layer that isolates the horizontal section 41a below the second electrode 45.
The perspective in
In one aspect, the diaphragm 41b has essentially a square shape. A perforated plate 41e is adjoined to each side of the diaphragm 41b and has a rectangular shape with a lengthwise dimension that is equal to or less than the length of a diaphragm side and a width that is less than its lengthwise dimension. Perforations (holes) 42 are preferably arranged in multiple rows and columns and may have a square, rectangular, or circular shape as mentioned in the first embodiment. Surrounding the three unattached sides of the perforated plates 41e and pads 41d are the openings 43 which expose the silicon nitride layer 36 above the substrate 31 and separate the perforated plates and pads from the horizontal sections 41f. Reinforcements 39 help to strengthen the diaphragm 41b and in one embodiment are arranged like spokes radiating from the center of the diaphragm. Although eight reinforcements are depicted, those skilled in the art will appreciate that other reinforcement designs involving various patterns are equally feasible.
The second embodiment has an advantage over the first embodiment in that the reinforcement ring 39 around the top opening of the back hole 46 prevents acoustic leakage through the air gap 48 (as shown in
A third embodiment of a microphone sensing element according to the present invention is shown in
The back side of substrate 51 has a stack of layers in which a thermal oxide layer 52b is disposed on the substrate and a silicon nitride layer 53b is formed on the thermal oxide layer. A back hole 68 is formed in the substrate 51 wherein the opening in the front side is smaller than the opening in the back side when the back hole is formed by KOH etching. Alternatively, the back hole 68 may have vertical sidewalls as explained previously in the second embodiment. The back hole 68 extends vertically (perpendicular to the substrate) through thermal oxide layer 52b and silicon nitride layer 53b on the back side and also extends essentially vertical from the front side of the substrate through the thermal oxide layer 52 and silicon nitride layer 53 to form an upper edge 69 that preferably has a square shape (not shown) when seen from a top view.
An important feature is that an SRN base having horizontal and vertical sections 61a, 61b, respectively, is formed on, within, and below each pad 58c. The horizontal section 61a serves as an electrical connection base while the vertical sections 61b provide a rigid support for the pad 58c. A horizontal section 61a is disposed on the pad 58c and preferably has a square shape which is centered above the vertical sections. Vertical sections 61b are comprised of a ring shaped trench 60 that has four walls and is filled with the SRN layer that encloses a dielectric spacer stack (not shown) comprised of a lower thermal oxide layer 52, a middle LPCVD silicon nitride layer 53, and an upper PSG layer 56. In a preferred embodiment, the trench 60 for each SRN base has four sections that intersect in a square shape although a rectangular or circular shape is also acceptable.
Referring to
Referring to
It is understood that a total of four SRN bases with horizontal sections 61a and vertical sections 61b are formed a similar distance from the edge 69 on substrate 51 and support the four pads 58c (
Returning to
There may be reinforcements 67 on the bottom surface of the diaphragm 58a that project downward toward the back hole 66 and the substrate 51. Reinforcements may not be necessary in an embodiment wherein the diaphragm is comprised of a polysilicon layer having a thickness of about 3 microns or greater. Although three reinforcements are depicted, a plurality of reinforcements 67 may be employed in a variety of designs including a spoke like pattern with an outer ring as illustrated previously for reinforcements 39 in the second embodiment. The reinforcements 67 are an integral part of the diaphragm 58a and have the same composition as the diaphragm.
From a top view in
The advantage of the third embodiment is that the SRN base serves as an anchor for a pad and overlying first electrode and thereby eliminates the need for a poly 1/oxide stack adopted in the second embodiment. Furthermore, no filled trenches are required for reducing substrate parasitic capacitance. However, the drawback is that formation of the SRN base is achieved with additional material deposition and etch processes.
All three embodiments anticipate a configuration wherein mechanical springs are attached to the center of each side of the diaphragm and a perforated plate is attached to adjacent sides of a diaphragm around a corner. In the exemplary embodiment depicted in
A fourth embodiment of a microphone sensing element according to the present invention is depicted in
Referring to
The dielectric spacer layer 12 has a thickness t5 and may be a single or composite layer comprised of one or more oxide layers, silicon nitride layers, or other dielectric layers. Furthermore, the dielectric spacer layer 12 may have a circular or square shape and has a width w2.
Another important feature of the fourth embodiment is that the back hole 26 is comprised of four sections. There is one section of back hole formed in each quadrant of the substrate defined by the planes X-X′ and Y-Y′. From a top down view, one back hole section is below the lower right quadrant of the diaphragm 13a while the other three sections of back hole 26 are located below the upper right, upper left, and lower left quadrants of the diaphragm, respectively. A first electrode 18a is disposed on the overlap region of the four mechanical springs above the dielectric spacer layer 12 while a second electrode 18b is formed on the substrate 11 outside the periphery of the diaphragm 13a and perforated plates 13d.
Referring to
This embodiment has the advantages of the first embodiment but also provides additional advantages in that fewer pads are required and there is less parasitic capacitance. Furthermore, the center support allows symmetric relaxing of any intrinsic stress and the fabrication process employed for the second and third embodiments may be used as well for the fourth embodiment.
All four embodiments of the microphone sensing element have a similar advantage over prior art in that the resulting silicon microphone has no dedicated backplate and thus can be produced at a lower cost than heretofore achieved. Furthermore, a microphone sensing element according to the present invention can exhibit good performance that is similar to results obtained from prior art microphone sensing elements with a dedicated backplate feature.
The present invention is also a method of forming a previously described silicon microphone sensing element. In one process sequence illustrated in
Referring to
Next, a hardmask comprised of one or more layers that will subsequently be used for fabricating a back hole is formed on the back side of the substrate. In one embodiment, the back side hard mask is comprised of a thermal oxide layer 15 grown by a well known LPCVD method on the substrate 11 and a silicon nitride layer 16 deposited by an LPCVD method on the thermal oxide layer. Note that the thermal oxide/silicon nitride hard mask is simultaneously grown on the membrane film 13 but is subsequently removed by well known wet chemical or dry etching methods.
A first photo mask is employed to generate one or more vias 17 in the membrane film 13 that extend through the dielectric spacer layer 12 to contact the substrate. For example, in an SOI approach a reactive ion etch or plasma etch may be used to transfer the openings in a photoresist layer through a silicon membrane film 13 followed by a wet buffered oxide etch (BOE) to remove the exposed dielectric spacer layer (oxide) 12 and extend the vias 17 to the substrate.
Referring to
Referring to
Referring to
Referring to
Referring to
It is understood that in addition to the microphone sensing element 10, a silicon microphone is also comprised of a voltage bias source, a source follower preamplifier, and wiring to connect the first and second electrodes to complete a variable capacitor circuit. However, these features are not shown in order to simplify the drawings and direct attention to the key components of the present invention. The resulting silicon microphone has a simpler fabrication sequence than prior art methods which include a dedicated backplate construction. Furthermore, the method of the present invention is less expensive to practice in manufacturing since fewer photomasks are required.
While this invention has been particularly shown and described with reference to, the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.
