Page 11 - Spring 2009

Fig 12. Single chip silicon condenser microphone.
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Fig 13. Knowles SiSonic® MEMS microphone cross-section.
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measurements were made using a 28V DC bias and they reported open-circuit sensitivity measurements comparable to conventional microphones. In their conclusion, Hohm and Hess predicted that “silicon microphones will probably find applications in all fields where small dimensions rather than high signal-to-noise ratios are desired.” This was the first subminiature condenser microphone with a diaphragm
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on one single silicon chip. Bernstein and Borenstein described a MEMS microphone with a 1 mm diaphragm sup- ported by springs, a plated gold perforated backplate and an
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will not affect the CMOS devices.
Neumann and Gabriel 37 described an integrated CMOS
MEMS microphone in 2003 that is the basis for the Akustica MEMS microphone product launched in 2006. The diaphragm is formed from the top level metal and oxide lay- ers of the CMOS wafer, after circuit fabrication is complete. A serpentine metal and oxide mesh pattern repeats within the diaphragm area and underlying sacrificial polysilicon is
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described work at the Federal Institute of Technology in Lausanne on a silicon microphone that had a highly perforated backplate and a 5 volt bias. They described a wafer fabrication process they claimed was suit- able for high volume manufacturing. The diaphragm and backplate were fabricated on two separate wafers that were
subsequently joined together using wafer bonding.
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less than 1 mm x 1 mm.
In 1990 Bergqvist et al.
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presented a MEMS microphone fabri- cated on one silicon wafer in 1992. The diaphragm and high- ly perforated backplate was formed with vapor deposited sil- icon nitride and used an aluminum sacrificial layer to form the gap between the diaphragm and backplate (Fig. 12). A flat frequency response from 100 Hz to 14 kHz was reported from the structure. A detailed discussion of the early devel- opment of MEMS microphones in the 1980’s and early 1990’s
in 2003 when Knowles released the SiSonic® surface mount
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Today Knowles has shipped in excess of 300 million MEMS microphones into general consumer applications including cell phones, digital cameras
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microns by a series of support posts.
In 2005 Sonion, a Danish audio transducer manufactur-
er, launched the SiMic,® 32 consisting of a MEMS sensing ele- ment and a signal conditioning application-specific integrat- ed circuit (ASIC) both mounted directly to a silicon substrate carrier. The all-silicon microphone package measured 2.6
Scheeper et al.
can be found in Sessler (1996)
The first commercialization of MEMS microphones was
A conformal coated polymer is then deposited over the mesh
Knowles began development of MEMS microphones in the early 1990’s. At that time the target appli- cation was hearing aids but the focus changed to the con- sumer market where they were able to exploit opportunities
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The Knowles microphone incorpo- rates a MEMS element and a complimentary metal oxide microscope (CMOS) die combined in a surface mount acoustic package. The sensor die consists of a compliant diaphragm separated from a highly perforated, rigid back- plate (Fig. 13). The diaphragm is fabricated from 1 micron thick polysilicon and has a 0.5 mm effective diameter. The diaphragm separation from the backplate is maintained at 4
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MEMS microphone.
area to form an airtight seal over the cavity.
In 2006 Weigold et al.39 described a MEMS microphone
with a 0.5 mm diaphragm mounted on springs to maximize sensitivity using a low bias voltage. The backplate is formed from the device layer of a SOI (Silicon on Insulator) wafer and the 1 micron thick diaphragm is formed using polysili- con deposition. Figure 16 shows a scanning electron micro- scope (SEM) micrograph of the diaphragm top surface show- ing the spring support mechanism. This technology is the basis for the Analog Devices MEMS microphone.
We have traced the evolution of MEMS microphone tech- nology through the years from the early work by Sessler to the
in the mobile handset market.
and Bluetooth headsets.
24 28 and Scheeper et al. (1994).
The development of the Sonion MEMS microphone began in 1993 as collaboration with the Microelectronics Center at the Technical University of Denmark (DTU). Pulse is a division of Technitrol who pur- chased all of Sonion, including the Sonion MEMS micro- phone. More information on Pulse can be found on the web.34 Another approach to MEMS microphones is the integra- tion of the MEMS sensor element and the sensor electronics
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proposed an inte- grated microphone with a digital output. The microphone sensing element was a polyimide structure deposited on a standard CMOS circuit wafer (Fig. 14). The advantage of polyimide is that it can be deposited at low temperatures that
mm x 1.6 mm x 0.865 mm.
on-chip buffer amplifier. Pedersen et al.
etched away to form a suspended diaphragm (Fig. 15).
Fig 14. Polyimide capacitive microphone.
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10 Acoustics Today, April 2009