|Publication number||US20050005421 A1|
|Application number||US 10/916,196|
|Publication date||13 Jan 2005|
|Filing date||11 Aug 2004|
|Priority date||13 Sep 2002|
|Also published as||US6667189, US7132307, US20040179705|
|Publication number||10916196, 916196, US 2005/0005421 A1, US 2005/005421 A1, US 20050005421 A1, US 20050005421A1, US 2005005421 A1, US 2005005421A1, US-A1-20050005421, US-A1-2005005421, US2005/0005421A1, US2005/005421A1, US20050005421 A1, US20050005421A1, US2005005421 A1, US2005005421A1|
|Inventors||Zhe Wang, Qingxin Zhang|
|Original Assignee||Knowles Electronics, Llc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (3), Referenced by (27), Classifications (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The invention relates to a method of manufacturing a silicon condenser microphone, and more particularly, to a method of manufacturing a high performance silicon condenser microphone using a silicon micro-machining process.
Silicon condenser microphones have long been an attractive research and development subject. Various microphone designs have been invented and conceptualized by using silicon micro-machining technology. Despite various structural configurations and materials, the silicon condenser microphone consists of four basic elements: a movable compliant diaphragm, a rigid and fixed backplate (which together form a variable air gap capacitor), a voltage bias source, and a pre-amplifier. These four elements fundamentally determine the performance of the condenser microphone. In pursuit of high performance; i.e., high sensitivity, low bias, low noise, and wide frequency range, the key design considerations are to have a large size of diaphragm and a large air gap. The former will help increase sensitivity as well as lower electrical noise, and the later will help reduce acoustic noise of the microphone. However, the large diaphragm requires a large span of anchored supports and correspondingly a large backplate. Also, a large air gap requires a thick sacrificial layer. These present major difficulties in silicon micro-machining processes. Due to constraints of material choices and intrinsic stress issues in silicon micro-machining, the silicon microphones reported so far have not achieved sensitivity of more than 20 mV/Pa.
Miniaturized silicon microphones have been extensively developed for over sixteen years, since the first silicon piezoelectric microphone reported by Royer in 1983. In 1984, Hohm reported the first silicon electret-type microphone, made with a metallized polymer diaphragm and silicon backplate. And two years later, he reported the first silicon condenser microphone made entirely by silicon micro-machining technology. Since then a number of researchers have developed and published reports on miniaturized silicon condenser microphones of various structures and performance.
Some of these reports include the following:
U.S. Pat. No. 5,870,482 to Loeppert et al reveals a silicon microphone. U.S. Pat. No. 5,490,220 to Loeppert shows a condenser and microphone device. U.S. patent application Publication 2002/0067663 to Loeppert et al shows a miniature acoustic transducer. U.S. Pat. No. 6,088,463 to Rombach et al teaches a silicon condenser microphone process. U.S. Pat. No. 5,677,965 to Moret et al shows a capacitive transducer. U.S. Pat. Nos. 5,146,435 and 5,452,268 to Bernstein disclose acoustic transducers. U.S. Pat. No. 4,993,072 to Murphy reveals a shielded electret transducer.
However, none of the silicon condenser microphones mentioned above has been reported to achieve sensitivity above 20 mV/Pa. In terms of conventional condenser microphones (i.e. non-silicon), very few products can have sensitivity as high as 100 mV/Pa. For example, Bruel & Kjoer, Denmark (B&K) has only one microphone available with this high sensitivity (B&K 4179, 1-inch diameter). Its dynamic range is about 140 dB (200 Pa) and frequency range is 5-7 kHz. However, this microphone must be fit onto a bulky pre-amplifier and requires a polarization voltage of 200V
A principal object of the present invention is to provide an effective and very manufacturable method of fabricating a silicon condenser microphone having high sensitivity and low noise.
Another object of the invention is to provide a silicon condenser microphone design having high sensitivity and low noise.
A further object of the invention is to provide a method for fabricating a silicon condenser microphone using via contact processes for a planar process.
Yet another object of the invention is to provide a method for fabricating a silicon condenser microphone using buckling of a composite diaphragm to prevent stiction in a wet release process.
In accordance with the objects of this invention a silicon condenser microphone is achieved. The silicon condenser microphone of the present invention comprises a perforated backplate comprising a portion of a single crystal silicon substrate, a support structure formed on the single crystal silicon substrate, and a floating silicon diaphragm supported at its edge by the support structure and lying parallel to the perforated backplate and separated from the perforated backplate by an air gap.
Also in accordance with the objects of this invention a method of fabricating a silicon condenser microphone having high sensitivity and low noise is achieved. A single crystal silicon substrate (P−) is provided. First ions (P+) of a first conductivity type are implanted into the single crystal silicon substrate to form a pattern of acoustic holes in a central portion of the substrate. Second ions (N−) of a second conductivity type opposite the first conductivity type are implanted into the substrate and surrounding the pattern of acoustic holes to form a backplate region. Third ions (P+) of the first conductivity type are implanted overlying the pattern of acoustic holes. Fourth ions (N+) of the second conductivity type are implanted overlying a portion of the backplate region not surrounding the pattern of acoustic holes to form an ohmic contact region. A front side nitride layer is deposited overlying the backplate region. A back side nitride layer is deposited on an opposite surface of the substrate. A front side sacrificial oxide layer is deposited overlying the front side nitride layer. A back side sacrificial oxide layer is deposited overlying the back side nitride layer. First trenches are etched through the front side sacrificial oxide layer to the ohmic contacts, and to the substrate off the backplate region. The first trenches are filled with a first polysilicon layer which is patterned to form polysilicon caps overlying the first trenches and to form polysilicon endplates surrounding the pattern of acoustic holes. A first oxide layer is deposited overlying the patterned first polysilicon layer. The first oxide layer is etched to the polysilicon layer followed by a thin oxide deposition to form the tiny holes for first dimples overlying the endplates. A second polysilicon layer is deposited overlying the first oxide layer and filling the first dimple holes. The second polysilicon layer is etched to form a functional layer of a composite diaphragm and its lead-out to a bond pad. A second oxide layer is deposited overlying the first oxide layer and the functional diaphragm. A narrow and continuous opening on the second oxide layer is etched on an inner edge of the functional diaphragm. A third polysilicon layer is deposited overlying the second oxide layer and filling the openings whereby a portion of the second oxide layer is enclosed between the second and third polysilicon layers to form a compressive layer of the composite diaphragm. The third polysilicon layer is patterned to remain filling the narrow and continuous opening to form a protective layer over the compressive layer of the composite diaphragm. The first and second oxide layers are etched followed by a thin oxide deposition to form second dimple holes overlying the first dimples. A deep oxide trench etching is made through the endplates and the sacrificial oxide layer to the substrate to form the supporting struts. The first and second oxide layers are etched to make anchor openings to the polysilicon caps, endplates, and bond pads. A nitride layer is deposited overlying the second oxide layer and filling the second dimple holes, the oxide trenches and the anchor openings. The nitride layer is patterned to expose the bond pads and the composite diaphragm within the second dimples. Thereafter, the backside sacrificial oxide layer is removed and the backside nitride layer is patterned. From the backside, the silicon substrate is etched away to the backplate region. The pattern of acoustic holes is selectively etched away. The backside nitride layer and the frontside nitride layer exposed by the acoustic holes are etched away from the backside. The frontside sacrificial oxide layer is removed using a wet etching method wherein the compressive layer of the composite diaphragm causes the composite diaphragm to buckle in a direction away from the backplate region. After drying, the protective layer and the compressive layer of the composite diaphragm are removed wherein the functional diaphragm flattens to complete fabrication of a silicon condenser microphone.
In the accompanying drawings forming a material part of this description, there is shown:
The present invention discloses a novel design and process for making a silicon condenser microphone. Referring now more particularly to
Referring now to
The thermal oxide layer 12 is removed, for example, by wet etching. Now a second thermal oxide layer 20 is grown on the surface of the substrate to a thickness of between about 270 and 330 Angstroms, as illustrated in
Referring now to
Now, a tetraethoxysilane (TEOS) oxide layer is deposited over the composite oxide/nitride layer on both the front and back sides of the wafer by LPCVD to a thickness of between about 1800 and 2200 Angstroms. Finally, a second nitride layer is deposited over the TEOS layer only on the back side of the wafer by plasma enhanced chemical vapor deposition (PECVD). This will provide an excellent mask for silicon etching by KOH on the backside of the wafer. The composite layer of thermal oxide, nitride, and TEOS oxide on the top side of the wafer is represented by 30 in
Now, sacrificial oxide layers are deposited on the front and back sides of the wafer as shown in
Referring now to
Now, a polysilicon layer 46 is deposited over the top oxide layer and within the trenches. Simultaneously, polysilicon 48 is deposited on the bottom oxide layer 42. The polysilicon layer is patterned to leave a polysilicon cap of about 1.5 μm in thickness over the filled trenches and elsewhere as shown in
Now the diaphragm is to be formed. An oxide layer 50 is deposited over the patterned polysilicon layer, as shown in
Now a layer of polysilicon 58 is deposited over the oxide layer 50 and filling the dimple holes to form the dimples 53, as shown in
As illustrated in
Referring now to
As illustrated in
The nitride layer 70 is etched using, for example, a combination of dry and wet etching to form openings 75 to bonding pads 46 and 59 and to clear the nitride from the area of the diaphragm.
A contact 81 is opened by a dry and wet etching process to the substrate surface, as shown in
Referring now to
Now, a KOH etching is performed using the composite layer 32 as mask, to open the back side of the wafer as shown in
Cr/Au as the sputtered ECE metal layer is etched. 83 is plated by Au about 2 microns thick and so remains. A blanket nitride stripping from the back side of the wafer removes layer 32 completely and also strips nitride layer 30 where it is exposed by the acoustic holes, as illustrated in
The wafer is now cut by a high speed spinning diamond cutter, called dicing. Now, the wafer is subjected to a dip in a hydrofluoric acid solution, preferably about 49% HF, for about 3.5 minutes. This dip removes the sacrificial oxide layer 40 through the backside opening as well as the frontside gaps, as shown in
Now, the protective layer 62 and the compressive layer 60 of the composite diaphragm are removed. First the polysilicon layer 62 is removed by a dry etching. A second dry etching step removes the PSG oxide layer 60. No masking is required in these removal steps because either polysilicon etching or oxide etching does not attack the other exposed layers. The two dry etching process steps have high selectivity to each other.
The completed microphone is shown in
A number of design variations are proposed to cover the sensitivity from 25 mV/Pa to above 100 mV/Pa.
TABLE I Design Variations 1 2 3 4 5 Diaphragm 2000 1000 1000 2000 2000 size (μm) Diaphragm 3 2 2 2 2 thickness (μm) Air Gap 8 8 8 8 8 (μm) Backplate 10 10 10 10 10 thickness (μm) Acoustic 20 30 40 40 40 hole (μm) Acoustic 60 84 100 100 84 hole pitch (μm) # acous. 850 95 75 300 425 holes Acoustic 10.80% 10.90% 15.30% 15.20% 12.20% perforation TABLE II
Bias volt. (V)
dB ref 1 V/Pa
Table I illustrates design parameter variations that have been reduced to practice for 5 sample dies. Table II illustrates the simulation results for the 5 sample dies. Important results are the bias voltage (=2/3 of the collapse voltage) and Sensitivity in mV/Pa. Over pressure is shown where deflection is less than 2/3 of the gap height. The design parameters of design variations 1, 4, and 5 enable high sensitivities above 100 mV/Pa while those of design variations 2 and 3 give lower sensitivities (33 mV/Pa) but a wider frequency response.
The microphone design and fabrication process of the present invention produces a high performance microphone with the highest sensitivity and lowest noise achieved. The microphone of the present invention includes a stress-free polysilicon diaphragm. The composite diaphragm design includes compressive buckling for anti-stiction. After release and drying, the compressive layers on the diaphragm are removed. The fabrication process is a planar process despite thick sacrificial layers. Via contacts are formed by polysilicon filling and self-doping.
While the 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 the invention.
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|Cooperative Classification||H04R19/005, Y10T29/49005|