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High-performance ultra-small single crystalline silicon microphone of an integrated structure
Microelectronic Engineering 67–68 (2003) 508–519 www.elsevier.com / locate / mee
High-performance ultra-small single crystalline silicon microphone of an integrated structure T. Tajima a , *, T. Nishiguchi a , S. Chiba a , A. Morita a , M. Abe a , K. Tanioka a , N. Saito a , M. Esashi b a
b
NHK Science and Technical Research Laboratories, 1 -10 -11 Kinuta, Setagaya, Tokyo 157 -8510, Japan Tohoku University New Industry Creation Hatchery Center, 01 Aza-Aoba, Aramaki, Aoba, Sendai 980 -8579, Japan
Abstract We have succeeded in fabricating an ultra-small condenser microphone that has excellent acoustic characteristics, and excellent reliability and mass-producibility, with an integrated structure made from single-crystalline silicon, a material that has high tensile strength. This is owing to the use of a bonded wafer, which is prepared using powder silicon oxide as a glue (SODIC method), and precise control of the thickness of the diaphragm, a thin film that vibrates under acoustic pressure. The microphone’s acoustic characteristics are: wide dynamic range with excellent linearity up to 10 Pa, wide frequency range of 75 Hz–24 kHz, and high sensitivity of 2 47 dB (0 dB 5 1 V/ Pa). Since it is made of single-crystalline silicon, it is robust and thermally resistant. Moreover, it has suitability to mass production, because it is fabricated with a semiconductor process. 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon microphone; Silicon MEMS; Bonded wafer; Etch-stop
1. Introduction The microphones that are currently used in broadcasting have superior acoustic characteristics, such as wide bandwidth and high sensitivity. However, the manufacture of these microphones requires processes such as the assembly and precise adjustment of many components, so the making of ultra-small, inexpensive microphones has been a difficult problem. For that reason, there is a strong demand for the development of the next generation of microphones, which will have excellent suitability for mass production as well as being ultra-small and having high performance. A condenser microphone that is constructed mainly of silicon has been attracting attention as a possible way of achieving those qualities. This microphone, which is called a * Corresponding author. Tel.: 181-3-5494-3268; fax: 181-3-5494-3278. E-mail address: [email protected] (T. Tajima). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00108-4
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silicon microphone, is fabricated by making use of silicon MEMS technology. Research has been actively conducted mainly in Europe, and high-performance microphones were reported [1–4]. However, since it has been difficult to form a diaphragm (a thin film that vibrates under acoustic pressure) that has good characteristics on a silicon substrate, further improvement on the performance of the microphone is not so easy. We have been investigating problems concerning silicon microphone structure and fabrication technology and have been grappling with research aimed at achieving breakthroughs in those areas. We have introduced the new approach of fabricating the diaphragm and the other parts that form the heart of the microphone (i.e. the microphone capsule) as an integrated structure made of singlecrystalline silicon, a material that has an extremely high tensile strength [5]. We also developed a proprietary micromachining technology for implementing that fabrication. The result is the world’s first successful prototype condenser silicon microphone that is ultra-small and also has excellent acoustic characteristics, reliability, and mass-producibility [6,7]. In this paper, we describe the structure, process technology and main characteristics of this silicon microphone, which is gathering interest as the next-generation microphone for uses ranging from broadcasting to consumer uses.
2. Basic structure and operating principle of the condenser microphone The basic structure of the condenser microphone is illustrated in Fig. 1. The capsule part of this microphone has a simple structure in which two plates, called the diaphragm and the back plate, are positioned close together facing each other. The two plates form a condenser (capacitor). A bias voltage is applied between those two plates across a high resistance of at least 100 MV, as shown in the figure, to charge the capacitor formed by the two plates. When acoustic pressure made the diaphragm vibrate, the capacitance changes, which changes the voltage between the two plates. The change in voltage becomes the audio output signal. For a condenser microphone, it is necessary to prevent excessive damping of the diaphragm vibration. For that purpose, holes, called acoustic holes, are made in the back plate to allow air to flow in and out of the cavity between the two plates. The condenser microphone offers the advantages of a simple structure and the ability to obtain a flat
Fig. 1. Schematic structure of a conventional condenser microphone.
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frequency characteristic and wide dynamic range over a broad frequency band by optimizing the material and shape of the diaphragm and the structure of the cavity.
3. Design concept of a silicon microphone Because of the special features of the condenser microphone such as those described in the previous section, much is being expected of silicon condenser microphones in terms of ultra-small size, mass-producibility, etc., considering that they are made mainly of silicon and fabricated with MEMS processes. The development of these devices has been proceeding mainly in Europe [1–4]. In most cases, the diaphragm has been formed by building up polycrystalline silicon or other materials on a single-crystalline silicon substrate (silicon wafer) by chemical vapor deposition (CVD), and highperformance microphones were reported. However, diaphragms made of these materials have limits in bandwidth, dynamic range, and reliability, because they have insufficient mechanical strength and buckling (bending of the layer by compressive forces that arise during fabrication) occurs. Since we consider the development of an ultra-small, high-performance silicon microphone by the conventional methods described above to be difficult, we have investigated new silicon microphone structures and fabrication techniques. As a result, we decided on the design policy of implementing the microphone capsule structure that is illustrated in Fig. 2 with single-crystalline silicon. This is an attempt to respond to expectations for a high performance silicon microphone that can be mass produced by fabricating the diaphragm and other parts that constitute the heart of the microphone as a single unit made of single-crystalline silicon, a material which has a tensile strength 15 times that of steel. In the rest of this paper, we describe a new silicon microphone design method and a newly developed single-crystalline silicon bonded wafer and fabrication technology for implementing that method, as well as the fabrication accuracy of this process.
3.1. Design requirements In this section we describe the design of the diaphragm and the length of the cavity (i.e. the thickness of the cavity layer) [8–12], which are important elements of the microphone capsule. The
Fig. 2. Structure of a silicon microphone capsule. (a) Top, (b) cross sectional, and (c) bottom view.
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bias voltage is strongly related to the design. We selected a bias voltage of 48 V, which is the value currently used for the condenser microphones employed in broadcasting.
3.1.1. Diaphragm Thinner diaphragms have better sensitivity and frequency characteristics. However, the practical processing accuracy for diaphragm thickness during manufacture (61 mm) must be considered, so the design thickness for the diaphragm was selected as 5 mm. The sensitivity of the microphone is proportional to the surface area of the diaphragm. Accordingly, it is better to design the diaphragm surface area to be large from the viewpoint of sensitivity. For silicon microphones fabricated by MEMS technology, however, ultra-small size and mass-producibility (low cost) are important expectations, so there are obviously certain constraints on the size of the diaphragm surface area. The surface area is also related to the design of the microphone capsule air gap, as described below. Consideration on these requirements led us to a design surface area of 2 3 2 mm 2 . 3.1.2. Air gap The air gap of the microphone capsule is strongly related to microphone sensitivity and operating stability. Those relationships are shown in Fig. 3 [8–12]. The stability shown in Fig. 3a is derived from the condition that the diaphragm bent by the electrostatic force recovers spontaneously to the original position after the force is removed. The stability m is defined as [12] m 5 d 3b Sm /(´a Sb E b2 ), where Sm is stiffness given as Sm 5 4.2Et m3 S / h(1 2 n )(a / 2)4 j, and d b is the air gap, ´a is the dielectric constant of the air, Sb is the area of the back plate, Eb is bias
Fig. 3. Relationship of air gap to (a) stability and (b) sensitivity. The value 7 in (a) indicates the recommended one as the stable operation of the condenser microphone. When the air gap takes the target value of 15 mm, stability and sensitivity will be 24 and 7, respectively.
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voltage, E is Young9s modulus, t m and S are thickness and area of the diaphragm, n is Poisson9s ratio, and a is the length of the back plate. Because the stability is proportional to the third power of the air gap, the longer the air gap, the more stably the microphone operates. Since stable operation of the microphone requires a stability value of 7 or higher [10], the air gap must be at least 10 mm. Sensitivity, on the other hand, is inversely proportional to the air gap, as shown in Fig. 3b. Therefore, the smaller the air gap, the greater the sensitivity. In the present work, the emphasis is on stability of the microphone capsule, targeting sensitivity that is equivalent to that of conventional small broadcast microphones, 7 mV/ Pa. We therefore set the air gap to 15 mm, for which case the stability value is 24 as shown in Fig. 3a. This provides a sufficient margin of stability.
3.2. Fabrication technology As can be understood from the design considerations described above, the important issue in making a prototype of this microphone is how to form a strong, thin, buckle-free diaphragm. To achieve that, we used a bonded single-crystalline wafer as the substrate for the microphone capsule. We also developed process technology for realizing the integrated-structure microphone capsule by etching to form the thin diaphragm and bonding layer from that substrate. These fabrication processes are illustrated in Fig. 4a–f.
3.2.1. Making the bonded wafer As shown in Fig. 4a and b, the bonded wafer that serves as the substrate for the microphone capsule is made by putting two single-crystalline silicon wafers (referred to as simply wafers in the following) together by a powder silicon oxide bonding method (SODIC) [13]. Before the wafers are put together, an etch-stop [14] layer (described below) is formed on the part of the wafer that will become the diaphragm. The SODIC method used to put the wafers together is a bonding technique in which silicon oxide
Fig. 4. Fabrication process of the silicon microphone capsule.
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powder that is deposited by the soot deposition method 1 is softened by heat treatment to serve as glue. The thickness of the bonding material (bonding layer) can be controlled by the time of the silicon oxide powder deposition. The thickness of the bonding layer determines the air gap of the microphone capsule, as described below. Therefore, an arbitrary air gap can be obtained by setting the deposition time to the appropriate value. That gives this method the advantage of a high degree of freedom in microphone capsule design. The stop layer mentioned above is a boron-doped layer formed on the silicon wafer. The etching rate of the boron-doped silicon layer is smaller than that of the non-doped silicon. Making use of that property, the etch-stop layer makes it possible to form the diaphragm. To realize a thin and precise diaphragm, the etch-stop layer must be formed with a high degree of accuracy. For that reason, we studied optimization of the heat treatment during formation of the etch-stop layer and developed technology for highly accurate formation of that layer.
3.2.2. Formation of the diaphragm and back plate Prior to the etching process for fabricating the diaphragm, a silicon oxide layer is formed on the surface of the bonded wafer as shown in Fig. 4c. Then, an etching mask is formed by a photolithography technique. Next, the single-crystalline silicon in the area where the silicon oxide layer has been removed is etched away down to the etch-stop layer, as shown in Fig. 4d, to obtain a diaphragm of the specified thickness. Then, as shown in Fig. 4e, an etching mask is formed in the same way on the back plate side of the wafer, and the acoustic holes are formed in the back plate by etching. 3.2.3. Formation of the cavity The cavity is formed by removing the bonding layer between the diaphragm and the back plate by etching with a HF solution, using the back plate as an etching mask. The capsule was naturally dried after the HF etching, thus obtaining the integrated microphone capsule shown in Fig. 4f. No sticking happened, because the air gap is large enough for the diaphragm and the back plate to spontaneously separate. 3.3. Fabrication accuracy We evaluated the fabrication accuracy of the prototype microphone capsule. The thickness of the diaphragm was measured with a Fourier transform infrared spectrometer (FTIR), taking the refractive index of the etch-stop layer (the effect of the boron doping) into consideration [15]. The external appearance was observed with an optical microscope and the surface flatness was measured with a laser displacement gauge. The thickness of the back plate and the air gap are obtained from the steps formed at the end face of the bonded wafer. Figs. 5 and 6 are photographs showing the external appearance of the diaphragm and back plate of the microphone capsule. The cross sectional structure showing the measured dimensions is presented in Fig. 7. The diaphragm had a surface area of 2.132.1 mm 2 and a thickness of 4.2 mm (the target value is 5 mm). There was no bending of the diaphragm surface, and the surface flatness was within 0.5 mm. The 1
A method of depositing silicon oxide powder by CVD.
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Fig. 5. Optical micrograph of the diaphragm of the silicon microphone capsule.
Fig. 6. Optical micrograph of the back plate of the silicon microphone capsule.
Fig. 7. Measured dimensions of the silicon microphone capsule.
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air gap was 14.3 mm, whereas the target value was 15 mm. We thus confirmed that the diaphragm and air gap could be formed to the specified dimensions within a tolerance of 1 mm. This proves that the method described here can produce a thin diaphragm that has a flat surface with no buckling, and that it is possible to fabricate a fine-structured microphone capsule close to design values with good fabrication accuracy.
4. Measurement of characteristics Here we describe a basic evaluation of the mechanical characteristics of the test-fabricated diaphragm and the acoustic characteristics of the microphone.
4.1. Mechanical characteristics of the diaphragm The mechanical characteristics of the diaphragm were evaluated by measuring the relation between the applied pressure and electrostatic capacitance of the microphone capsule. The measurement system is illustrated in Fig. 8. The microphone capsule is attached to a pressure vessel and a static pressure is applied to the diaphragm by a pressurization cylinder. The changes in the capacitance between the diaphragm and back plate that arise are measured with a micro-capacitance meter. The results are shown in Fig. 9. The electrostatic capacitance varies linearly with the pressure, with no evident hysteresis. Furthermore, the diaphragm did not rupture or exhibit other such problems even at pressures up to 10 Pa, confirming durability. From these results, we conclude that the singlecrystalline silicon diaphragm exhibits ideal elastic deformation, and, because microphones are ordinarily subjected to acoustic pressures of about 1 Pa, this microphone capsule has a wide dynamic range with excellent linearity.
4.2. Basic evaluation of acoustic characteristics We measured the frequency characteristics and sensitivity of the prototype microphone and evaluated its basic characteristics. With a free-standing microphone capsule, sound waves can also reach the back surface of the diaphragm and cancel out the vibration of the diaphragm. To prevent that, the microphone capsule was installed in a microphone case to form a back cavity. The external
Fig. 8. Set-up for the measurement of the pressure–capacitance characteristics.
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Fig. 9. Pressure–capacitance characteristic of the silicon microphone capsule.
appearance of that assembly is shown in Fig. 10 and its cross-sectional structure is illustrated in Fig. 11. The four square parts in the upper half of the photograph are diaphragms, each of which forms an independent microphone capsule. One of those was connected to the preamplifier (gain51). The frequency-sensitivity characteristics of this microphone are presented in Fig. 12. The measurements were made with the diaphragm squarely facing the sound source (08), which is the usual case, and with the diaphragm at a right angle to the sound source (908). The frequency range was from 75 Hz to 24 kHz, and the sensitivity was 247 dB (0 dB51 V/ Pa). The decrease in sensitivity in the frequency region below 120 Hz in Fig. 12 is the result of leakage of acoustic pressure from gaps in the microphone case and from the microphone capsules, because the lower the frequency, the more the sound pressure leaks into the case and compensates the vibration of the
Fig. 10. Photograph of the silicon microphone. Although four diaphragms (microphone capsule) are fabricated on one wafer, only one of them was connected to the preamplifier (Fig. 11).
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Fig. 11. Schematic structure of the present silicon microphone.
diaphragm. The increase in sensitivity in the region above 1 kHz is considered to be the effect of diffraction and reflection of sound waves by the microphone case and the casing of the preamplifier. We can infer this from the fact that the rise of sensitivity decreased when the microphone was turned to an angle of 908 relative to the sound source. Our calculation with low stress in consideration shows that the resonance frequency ( f0 ) of our diaphragm has a value around 20 kHz [8], but it has not been confirmed. Stress in the diaphragm and f0 are to be evaluated. The results presented above confirm that a microphone made with this ultra-small microphone capsule has excellent frequency characteristics and sensitivity, which are difficult to achieve with conventional silicon microphones. Furthermore, we believe that this microphone can achieve the same good frequency characteristics and sensitivity as those of the small microphones that are currently in use by improving the microphone case, increasing the gain of the preamplifier, and making other such
Fig. 12. Frequency characteristics of the silicon microphone. Solid line shows the characteristics measured with the diaphragm squarely facing to the sound source (08). Broken line shows that measured with the diaphragm at a right angle to the sound source (908).
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improvements. We can thus conclude that the design and fabrication methods used for this microphone capsule are very effective for implementing an ultra-small, high-performance silicon microphone.
5. Conclusion We have implemented the first ultra-small microphone that not only has excellent acoustic characteristics, but also has excellent reliability and mass-producibility. That was achieved by the development of technology for fabricating the diaphragm and back plate that are the heart of the microphone, which is to say the microphone capsule, as an integrated unit made from singlecrystalline silicon, a material that has 15 times the tensile strength of steel. This fabrication method forms a microphone capsule with high accuracy by using etching technology on a newly developed substrate made by bonding two single-crystalline silicon wafers together. Because this method allows a high degree of freedom in capsule design, we believe that this microphone, which is primarily intended for broadcast use, can easily be adapted to various other applications, in which different bias voltage is used. Accordingly, we believe that the technology that we have developed will serve as a basis for further research of ultra-small, high-performance microphones having advanced functions (directionality control, etc.) at a very low cost for uses ranging from broadcasting to consumer use. This will be achieved by further improvement of process accuracy, the development of microphone capsule arrays, and integration with electronic circuits.
References [1] P. Rombach, M. Mullenborn, U. Klein, K. Rasmussen, The first low voltage, low noise differential condenser silicon microphone, in: Eurosensors XIV, 2000, pp. 213–216. [2] A. Torkkeli, O. Rusanen, J. Saarilahti, H. Seppa, H. Sipola, J. Hietanen, Capacitive microphone with low-stress polysilicon membrane and high-stress polysilicon back plate, Sensors Actuators A85 (2000) 116–123. [3] M. Pedersen, W. Olthuis, P. Bergveld, An integrated silicon capacitive microphone with frequency-modulated digital output, Sensors Actuators A69 (1998) 267–275. [4] J. Bergqvist, F. Rudlf, A silicon condenser microphone using bond and etch-back technology, Sensors Actuators A45 (1994) 115–124. [5] M. Esashi, H. Fujita, I. Igarashi, S. Sugiyama, Micromachining and Micromechatronics, Baifukan, 1992. [6] T. Tajima, T. Nishiguchi, S. Kondo, N. Saito, S. Chiba, A. Morita, M. Esashi, Mechanical characteristics of a condenser silicon microphone, in: Institute of Electrical Engineers of Japan, Sensors and Micromachines Society Subcategory Conference, MSS-01-31, 2001, pp. 95–98. [7] T. Nishiguchi, T. Tajima, S. Kondo, N. Saito, A. Morita, M. Esashi, Prototype silicon microphone and evaluation of its acoustic characteristics, in: Proceedings of the Acoustic Society of Japan, 2001, pp. 641–642. [8] S. Chiba, A. Morita, K. Shoda, T. Tajima, F. Ando, Construction of a microphone consisting of an electro-acoustic conversion element on a silicon chip, in: Proceedings of the Acoustic Society of Japan, 3-6-18, 1999, pp. 557–558. [9] S. Chiba, A. Morita, F. Ando, K. Shoda, T. Tajima:, Considerations on the sensitivity of silicon microphones, in: Proceedings of the Acoustic Society of Japan, 3-P-31, 1999, pp. 533–534. [10] A. Mizoguchi, Design of a small directional condenser microphone, J. Acoust. Soc. Japan 31 (10) (1975) 593–601. [11] J.H. Jerman, The fabrication and use of micro-machined corrugated silicon diaphragms, Sensors Actuators A21–A23 (1990) 988–992. [12] A. Mizoguchi:, Doctoral Thesis, Nagoya University, Aichi, Japan, 1977. [13] SODIC (Soot Deposited Integrated Circuit) Technical Sheet, Nippon Ceramic Co. Ltd., Hatto Research Center.
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[14] C. Cabuz, K. Fukatsu, T. Kurabayashi, K. Minami, M. Esashi, Microphysical investigations on mechanical structures realized in p1 silicon, IEEE J. Micro Elector Syst. 4 (3) (1995) 109–118. [15] T. Nishiguchi, T. Tajima, S. Kondo, S. Chiba, A. Morita, F. Ando, N. Saito, M. Esashi, Measurement of silicon microphone diaphragm thickness by FTIR using the measured refractive index of a boron-diffused etch-stop layer, in: Extended Abstracts of 18th IEEJ Symposium on Sensor Micromachines and Their Application System, 2001, p. 60.
High-performance ultra-small single crystalline silicon microphone of an integrated structure T. Tajima a , *, T. Nishiguchi a , S. Chiba a , A. Morita a , M. Abe a , K. Tanioka a , N. Saito a , M. Esashi b a
b
NHK Science and Technical Research Laboratories, 1 -10 -11 Kinuta, Setagaya, Tokyo 157 -8510, Japan Tohoku University New Industry Creation Hatchery Center, 01 Aza-Aoba, Aramaki, Aoba, Sendai 980 -8579, Japan
Abstract We have succeeded in fabricating an ultra-small condenser microphone that has excellent acoustic characteristics, and excellent reliability and mass-producibility, with an integrated structure made from single-crystalline silicon, a material that has high tensile strength. This is owing to the use of a bonded wafer, which is prepared using powder silicon oxide as a glue (SODIC method), and precise control of the thickness of the diaphragm, a thin film that vibrates under acoustic pressure. The microphone’s acoustic characteristics are: wide dynamic range with excellent linearity up to 10 Pa, wide frequency range of 75 Hz–24 kHz, and high sensitivity of 2 47 dB (0 dB 5 1 V/ Pa). Since it is made of single-crystalline silicon, it is robust and thermally resistant. Moreover, it has suitability to mass production, because it is fabricated with a semiconductor process. 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon microphone; Silicon MEMS; Bonded wafer; Etch-stop
1. Introduction The microphones that are currently used in broadcasting have superior acoustic characteristics, such as wide bandwidth and high sensitivity. However, the manufacture of these microphones requires processes such as the assembly and precise adjustment of many components, so the making of ultra-small, inexpensive microphones has been a difficult problem. For that reason, there is a strong demand for the development of the next generation of microphones, which will have excellent suitability for mass production as well as being ultra-small and having high performance. A condenser microphone that is constructed mainly of silicon has been attracting attention as a possible way of achieving those qualities. This microphone, which is called a * Corresponding author. Tel.: 181-3-5494-3268; fax: 181-3-5494-3278. E-mail address: [email protected] (T. Tajima). 0167-9317 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0167-9317(03)00108-4
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silicon microphone, is fabricated by making use of silicon MEMS technology. Research has been actively conducted mainly in Europe, and high-performance microphones were reported [1–4]. However, since it has been difficult to form a diaphragm (a thin film that vibrates under acoustic pressure) that has good characteristics on a silicon substrate, further improvement on the performance of the microphone is not so easy. We have been investigating problems concerning silicon microphone structure and fabrication technology and have been grappling with research aimed at achieving breakthroughs in those areas. We have introduced the new approach of fabricating the diaphragm and the other parts that form the heart of the microphone (i.e. the microphone capsule) as an integrated structure made of singlecrystalline silicon, a material that has an extremely high tensile strength [5]. We also developed a proprietary micromachining technology for implementing that fabrication. The result is the world’s first successful prototype condenser silicon microphone that is ultra-small and also has excellent acoustic characteristics, reliability, and mass-producibility [6,7]. In this paper, we describe the structure, process technology and main characteristics of this silicon microphone, which is gathering interest as the next-generation microphone for uses ranging from broadcasting to consumer uses.
2. Basic structure and operating principle of the condenser microphone The basic structure of the condenser microphone is illustrated in Fig. 1. The capsule part of this microphone has a simple structure in which two plates, called the diaphragm and the back plate, are positioned close together facing each other. The two plates form a condenser (capacitor). A bias voltage is applied between those two plates across a high resistance of at least 100 MV, as shown in the figure, to charge the capacitor formed by the two plates. When acoustic pressure made the diaphragm vibrate, the capacitance changes, which changes the voltage between the two plates. The change in voltage becomes the audio output signal. For a condenser microphone, it is necessary to prevent excessive damping of the diaphragm vibration. For that purpose, holes, called acoustic holes, are made in the back plate to allow air to flow in and out of the cavity between the two plates. The condenser microphone offers the advantages of a simple structure and the ability to obtain a flat
Fig. 1. Schematic structure of a conventional condenser microphone.
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frequency characteristic and wide dynamic range over a broad frequency band by optimizing the material and shape of the diaphragm and the structure of the cavity.
3. Design concept of a silicon microphone Because of the special features of the condenser microphone such as those described in the previous section, much is being expected of silicon condenser microphones in terms of ultra-small size, mass-producibility, etc., considering that they are made mainly of silicon and fabricated with MEMS processes. The development of these devices has been proceeding mainly in Europe [1–4]. In most cases, the diaphragm has been formed by building up polycrystalline silicon or other materials on a single-crystalline silicon substrate (silicon wafer) by chemical vapor deposition (CVD), and highperformance microphones were reported. However, diaphragms made of these materials have limits in bandwidth, dynamic range, and reliability, because they have insufficient mechanical strength and buckling (bending of the layer by compressive forces that arise during fabrication) occurs. Since we consider the development of an ultra-small, high-performance silicon microphone by the conventional methods described above to be difficult, we have investigated new silicon microphone structures and fabrication techniques. As a result, we decided on the design policy of implementing the microphone capsule structure that is illustrated in Fig. 2 with single-crystalline silicon. This is an attempt to respond to expectations for a high performance silicon microphone that can be mass produced by fabricating the diaphragm and other parts that constitute the heart of the microphone as a single unit made of single-crystalline silicon, a material which has a tensile strength 15 times that of steel. In the rest of this paper, we describe a new silicon microphone design method and a newly developed single-crystalline silicon bonded wafer and fabrication technology for implementing that method, as well as the fabrication accuracy of this process.
3.1. Design requirements In this section we describe the design of the diaphragm and the length of the cavity (i.e. the thickness of the cavity layer) [8–12], which are important elements of the microphone capsule. The
Fig. 2. Structure of a silicon microphone capsule. (a) Top, (b) cross sectional, and (c) bottom view.
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bias voltage is strongly related to the design. We selected a bias voltage of 48 V, which is the value currently used for the condenser microphones employed in broadcasting.
3.1.1. Diaphragm Thinner diaphragms have better sensitivity and frequency characteristics. However, the practical processing accuracy for diaphragm thickness during manufacture (61 mm) must be considered, so the design thickness for the diaphragm was selected as 5 mm. The sensitivity of the microphone is proportional to the surface area of the diaphragm. Accordingly, it is better to design the diaphragm surface area to be large from the viewpoint of sensitivity. For silicon microphones fabricated by MEMS technology, however, ultra-small size and mass-producibility (low cost) are important expectations, so there are obviously certain constraints on the size of the diaphragm surface area. The surface area is also related to the design of the microphone capsule air gap, as described below. Consideration on these requirements led us to a design surface area of 2 3 2 mm 2 . 3.1.2. Air gap The air gap of the microphone capsule is strongly related to microphone sensitivity and operating stability. Those relationships are shown in Fig. 3 [8–12]. The stability shown in Fig. 3a is derived from the condition that the diaphragm bent by the electrostatic force recovers spontaneously to the original position after the force is removed. The stability m is defined as [12] m 5 d 3b Sm /(´a Sb E b2 ), where Sm is stiffness given as Sm 5 4.2Et m3 S / h(1 2 n )(a / 2)4 j, and d b is the air gap, ´a is the dielectric constant of the air, Sb is the area of the back plate, Eb is bias
Fig. 3. Relationship of air gap to (a) stability and (b) sensitivity. The value 7 in (a) indicates the recommended one as the stable operation of the condenser microphone. When the air gap takes the target value of 15 mm, stability and sensitivity will be 24 and 7, respectively.
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voltage, E is Young9s modulus, t m and S are thickness and area of the diaphragm, n is Poisson9s ratio, and a is the length of the back plate. Because the stability is proportional to the third power of the air gap, the longer the air gap, the more stably the microphone operates. Since stable operation of the microphone requires a stability value of 7 or higher [10], the air gap must be at least 10 mm. Sensitivity, on the other hand, is inversely proportional to the air gap, as shown in Fig. 3b. Therefore, the smaller the air gap, the greater the sensitivity. In the present work, the emphasis is on stability of the microphone capsule, targeting sensitivity that is equivalent to that of conventional small broadcast microphones, 7 mV/ Pa. We therefore set the air gap to 15 mm, for which case the stability value is 24 as shown in Fig. 3a. This provides a sufficient margin of stability.
3.2. Fabrication technology As can be understood from the design considerations described above, the important issue in making a prototype of this microphone is how to form a strong, thin, buckle-free diaphragm. To achieve that, we used a bonded single-crystalline wafer as the substrate for the microphone capsule. We also developed process technology for realizing the integrated-structure microphone capsule by etching to form the thin diaphragm and bonding layer from that substrate. These fabrication processes are illustrated in Fig. 4a–f.
3.2.1. Making the bonded wafer As shown in Fig. 4a and b, the bonded wafer that serves as the substrate for the microphone capsule is made by putting two single-crystalline silicon wafers (referred to as simply wafers in the following) together by a powder silicon oxide bonding method (SODIC) [13]. Before the wafers are put together, an etch-stop [14] layer (described below) is formed on the part of the wafer that will become the diaphragm. The SODIC method used to put the wafers together is a bonding technique in which silicon oxide
Fig. 4. Fabrication process of the silicon microphone capsule.
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powder that is deposited by the soot deposition method 1 is softened by heat treatment to serve as glue. The thickness of the bonding material (bonding layer) can be controlled by the time of the silicon oxide powder deposition. The thickness of the bonding layer determines the air gap of the microphone capsule, as described below. Therefore, an arbitrary air gap can be obtained by setting the deposition time to the appropriate value. That gives this method the advantage of a high degree of freedom in microphone capsule design. The stop layer mentioned above is a boron-doped layer formed on the silicon wafer. The etching rate of the boron-doped silicon layer is smaller than that of the non-doped silicon. Making use of that property, the etch-stop layer makes it possible to form the diaphragm. To realize a thin and precise diaphragm, the etch-stop layer must be formed with a high degree of accuracy. For that reason, we studied optimization of the heat treatment during formation of the etch-stop layer and developed technology for highly accurate formation of that layer.
3.2.2. Formation of the diaphragm and back plate Prior to the etching process for fabricating the diaphragm, a silicon oxide layer is formed on the surface of the bonded wafer as shown in Fig. 4c. Then, an etching mask is formed by a photolithography technique. Next, the single-crystalline silicon in the area where the silicon oxide layer has been removed is etched away down to the etch-stop layer, as shown in Fig. 4d, to obtain a diaphragm of the specified thickness. Then, as shown in Fig. 4e, an etching mask is formed in the same way on the back plate side of the wafer, and the acoustic holes are formed in the back plate by etching. 3.2.3. Formation of the cavity The cavity is formed by removing the bonding layer between the diaphragm and the back plate by etching with a HF solution, using the back plate as an etching mask. The capsule was naturally dried after the HF etching, thus obtaining the integrated microphone capsule shown in Fig. 4f. No sticking happened, because the air gap is large enough for the diaphragm and the back plate to spontaneously separate. 3.3. Fabrication accuracy We evaluated the fabrication accuracy of the prototype microphone capsule. The thickness of the diaphragm was measured with a Fourier transform infrared spectrometer (FTIR), taking the refractive index of the etch-stop layer (the effect of the boron doping) into consideration [15]. The external appearance was observed with an optical microscope and the surface flatness was measured with a laser displacement gauge. The thickness of the back plate and the air gap are obtained from the steps formed at the end face of the bonded wafer. Figs. 5 and 6 are photographs showing the external appearance of the diaphragm and back plate of the microphone capsule. The cross sectional structure showing the measured dimensions is presented in Fig. 7. The diaphragm had a surface area of 2.132.1 mm 2 and a thickness of 4.2 mm (the target value is 5 mm). There was no bending of the diaphragm surface, and the surface flatness was within 0.5 mm. The 1
A method of depositing silicon oxide powder by CVD.
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Fig. 5. Optical micrograph of the diaphragm of the silicon microphone capsule.
Fig. 6. Optical micrograph of the back plate of the silicon microphone capsule.
Fig. 7. Measured dimensions of the silicon microphone capsule.
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air gap was 14.3 mm, whereas the target value was 15 mm. We thus confirmed that the diaphragm and air gap could be formed to the specified dimensions within a tolerance of 1 mm. This proves that the method described here can produce a thin diaphragm that has a flat surface with no buckling, and that it is possible to fabricate a fine-structured microphone capsule close to design values with good fabrication accuracy.
4. Measurement of characteristics Here we describe a basic evaluation of the mechanical characteristics of the test-fabricated diaphragm and the acoustic characteristics of the microphone.
4.1. Mechanical characteristics of the diaphragm The mechanical characteristics of the diaphragm were evaluated by measuring the relation between the applied pressure and electrostatic capacitance of the microphone capsule. The measurement system is illustrated in Fig. 8. The microphone capsule is attached to a pressure vessel and a static pressure is applied to the diaphragm by a pressurization cylinder. The changes in the capacitance between the diaphragm and back plate that arise are measured with a micro-capacitance meter. The results are shown in Fig. 9. The electrostatic capacitance varies linearly with the pressure, with no evident hysteresis. Furthermore, the diaphragm did not rupture or exhibit other such problems even at pressures up to 10 Pa, confirming durability. From these results, we conclude that the singlecrystalline silicon diaphragm exhibits ideal elastic deformation, and, because microphones are ordinarily subjected to acoustic pressures of about 1 Pa, this microphone capsule has a wide dynamic range with excellent linearity.
4.2. Basic evaluation of acoustic characteristics We measured the frequency characteristics and sensitivity of the prototype microphone and evaluated its basic characteristics. With a free-standing microphone capsule, sound waves can also reach the back surface of the diaphragm and cancel out the vibration of the diaphragm. To prevent that, the microphone capsule was installed in a microphone case to form a back cavity. The external
Fig. 8. Set-up for the measurement of the pressure–capacitance characteristics.
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Fig. 9. Pressure–capacitance characteristic of the silicon microphone capsule.
appearance of that assembly is shown in Fig. 10 and its cross-sectional structure is illustrated in Fig. 11. The four square parts in the upper half of the photograph are diaphragms, each of which forms an independent microphone capsule. One of those was connected to the preamplifier (gain51). The frequency-sensitivity characteristics of this microphone are presented in Fig. 12. The measurements were made with the diaphragm squarely facing the sound source (08), which is the usual case, and with the diaphragm at a right angle to the sound source (908). The frequency range was from 75 Hz to 24 kHz, and the sensitivity was 247 dB (0 dB51 V/ Pa). The decrease in sensitivity in the frequency region below 120 Hz in Fig. 12 is the result of leakage of acoustic pressure from gaps in the microphone case and from the microphone capsules, because the lower the frequency, the more the sound pressure leaks into the case and compensates the vibration of the
Fig. 10. Photograph of the silicon microphone. Although four diaphragms (microphone capsule) are fabricated on one wafer, only one of them was connected to the preamplifier (Fig. 11).
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Fig. 11. Schematic structure of the present silicon microphone.
diaphragm. The increase in sensitivity in the region above 1 kHz is considered to be the effect of diffraction and reflection of sound waves by the microphone case and the casing of the preamplifier. We can infer this from the fact that the rise of sensitivity decreased when the microphone was turned to an angle of 908 relative to the sound source. Our calculation with low stress in consideration shows that the resonance frequency ( f0 ) of our diaphragm has a value around 20 kHz [8], but it has not been confirmed. Stress in the diaphragm and f0 are to be evaluated. The results presented above confirm that a microphone made with this ultra-small microphone capsule has excellent frequency characteristics and sensitivity, which are difficult to achieve with conventional silicon microphones. Furthermore, we believe that this microphone can achieve the same good frequency characteristics and sensitivity as those of the small microphones that are currently in use by improving the microphone case, increasing the gain of the preamplifier, and making other such
Fig. 12. Frequency characteristics of the silicon microphone. Solid line shows the characteristics measured with the diaphragm squarely facing to the sound source (08). Broken line shows that measured with the diaphragm at a right angle to the sound source (908).
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improvements. We can thus conclude that the design and fabrication methods used for this microphone capsule are very effective for implementing an ultra-small, high-performance silicon microphone.
5. Conclusion We have implemented the first ultra-small microphone that not only has excellent acoustic characteristics, but also has excellent reliability and mass-producibility. That was achieved by the development of technology for fabricating the diaphragm and back plate that are the heart of the microphone, which is to say the microphone capsule, as an integrated unit made from singlecrystalline silicon, a material that has 15 times the tensile strength of steel. This fabrication method forms a microphone capsule with high accuracy by using etching technology on a newly developed substrate made by bonding two single-crystalline silicon wafers together. Because this method allows a high degree of freedom in capsule design, we believe that this microphone, which is primarily intended for broadcast use, can easily be adapted to various other applications, in which different bias voltage is used. Accordingly, we believe that the technology that we have developed will serve as a basis for further research of ultra-small, high-performance microphones having advanced functions (directionality control, etc.) at a very low cost for uses ranging from broadcasting to consumer use. This will be achieved by further improvement of process accuracy, the development of microphone capsule arrays, and integration with electronic circuits.
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