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Feature Article Think Small! issue 1, Volume 1:

MEMS microphones break design mould

Silicon micromachined microphones have recently begun to emerge as a competitor technology to the electret condenser microphone (ECM), a device first developed by Bell Labs way back in the 1960s. Today, MEMS microphones amount to just tens of millions of units, but by 2009 will have carved out an estimated $500 million piece of the $2 billion total market for microphones — on average every third microphone will be based on MEMS.
While companies like Panasonic dominate the manufacture of electret microphones, a handful of companies such as market leader Knowles Acoustics and recent entrants, Sonion and Akustica, are supplying this hot market with analog and digital MEMS microphones for mobile phones, notebooks, and consumer products with audio input such as video recorders, digital cameras, dictation devices, and PDAs, in fact, any device requiring an audio input.
So far, the development of the MEMS microphone market has been technology-push oriented. The ECM is entrenched, inexpensive ($0.50 or less) and in terms of traditional metrics such as frequency response and sensitivity it captures sound relatively faithfully. Micromachined microphone chips can match and extend the performance of existing devices, for instance using sensor arrays. Silicon microphones also offer advantages to the OEM in the form of improved manufacturing methods (reliability, yield, assembly cost) combined with robustness. They also offer additional functionality, such as the ability to incorporate multiple microphones into portable electronic devices for noise suppression and beam forming.
The potential for smaller footprint components and resistance to electromagnetic interference also supports new cell phone designs, for example. Moreover, MEMS microphones meet price points set by electret microphones by leveraging established high-volume silicon manufacturing processes. This combination of size, performance and functionality and low cost are highly desirable for OEMs and consumers alike.


Advantage MEMS
MEMS microphones offer the following advantages over ECMS:

 

  • Surface mount designs, PCB mounting and reductions in footprint and height
  • Improved vibration / temperature performance and functionality
  • Integration with CMOS processes and on chip electronics, e.g. with amplifiers, A/D converters, and also with chip-scale packaging methods
  • Directionality, noise cancellation using microphone arrays
  • Immunity to RF and EMI for digital microphones

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    A key driver for the early adoption of MEMS microphones in mobile phone applications is the ability to use reflow soldering processes. During manufacturing, high costs are generally incurred when manual processes are required. MEMS microphone integration is a combination of surface mount approach and an ability to with withstand reflow temperatures up to 260°C, plus auto pick-and-place tools to install the microphone without impacting performance. This is not possible with ECMS, and being able to treat a micro-phone like any other semiconductor component increases yield and translates into lower costs for manufacturers.
    MEMS devices can also be made smaller. A MEMS condenser microphone typically consists of a flexible diaphragm and electrically charged back-plate with damping holes. The diaphragm and back-plate form a capacitor, and the impinging sound pressure waves act to change the capacitance. Surface and bulk micromachining processes are both employed. A key advantage is that the back-plate and diaphragm are up to ten times smaller than the most compact ECM, allowing for further savings in footprint and reduced design restrictions. Another advantage is the lower susceptibility to vibration as a result of the lower diaphragm mass.
    High humidity and temperatures can also affect microphone performance. In warm climates, or when the phone is left in a car in the sun, temperatures can reach as much as 85°C inside the cell phone chamber, exceeding the functional capacity of the ECM but not that of the MEMS equivalent. In a MEMS microphone, a constant charge can be maintained on the diaphragm, resulting in better isolation from power supply noise.

    Designer’s dream
    The small footprint affords mobile system designers greater freedom. Thin MEMS sensors of around 1 mm in height lend themselves to mobile phones with “clamshell” designs, where each half contributes to the thickness of the phone, and the component height is critical. ECMS have reached dimensions of 4 mm × 1.5 mm without the acoustic boot and are unlikely to shrink much further without compromising performance.
    Integrating MEMS microphones chips with other CMOS processes can also lead to additional functional benefits. Preamplifiers and analog to digital converters can be placed on the same chip with the sensor, for example.
    Digital microphones also feature robust signal output that is immune to the EMI or RF interference that can compromise the optimum acoustic during signal routing, e.g. in a laptop computer. This position is ideally in the bezel at the top of the display, facing the talker, to allow voice tracking and ambient noise suppression. The cables do not need to be shielded, which is also an advantage in small cell phone designs.
    Other forms of signal disturbance can include parasitic capacitance. Here, a densely integrated microphone and ASIC in a chip scale package, such as supplied by Sonion, can dramatically reduce parasitic electrical elements, for example.

    MEMS microphone suppliers
    Currently, the leading supplier of MEMS microphones to the open market is us company Knowles Acoustics, that has well over 80% of the market. Knowles produces both conventional microphones and MEMS microphones, and has agreements with Sony Semiconductor, MEMSCAP, Kyushu Corporation and Austria Microsystems (that makes the ASIC). Foundries with an interest in microphones include MEMSCAP (France), Microfab Bremen (Germany), in addition to Silex Microsystems (Sweden), Micralyne (Canada) and APM (Taiwan). Other manufacturers include Sonion (Denmark), Akustika (US) and Memstech (Singapore)..
    Sonion introduced a digital MEMS microphone in late 2005. It outsources the sensor and carrier substrate wafer production to MEMS foundries such as MicroFab Bremen and its asic wafers to CMOS foundries. Chip scale packaging and assembly is performed at its back-end facility in Roskilde, Denmark. Akustica, which entered the market with a digital microphone in February, is a fully fabless company. It outsources sensor manufacturing to a CMOS fab and subsequently to MEMS foundries to perform the final etching steps of the MEMS structure.

    Voice over IP
    The number of laptops has grown quickly in recent years, and the Gartner Group predicts 60 million units will be sold in 2006. Multiple microphones (two or three) are now starting to be employed in high-end notebooks for VoIP applications. The microphones are used with beam-forming software for noise cancellation and echo removal, which allows the user to speak to the device from any position. Knowles Acoustics has developed an algorithm that targets this application, and Akustica is developing microphone arrays.
    Companies like Microsoft and Intel have for some time advocated directional audio sources for speech recognition, although high speed Internet and VoIP has really driven interest in this segment. Small size and immunity to interference make digital microphones well suited to this challenging acoustic application, and will help to propel this to a considerable market opportunity in the next five years.

    Markets and applications
    One of the near term challenges has been the higher component manufacturing cost as a result of the lower volumes compared to ECMS. The mobile phone is an opportunity for MEMS technology due to the huge market size (estimated 900–1,000 million units by the end of the decade). MEMS microphone manufacturers are without exception targeting this opportunity, which will drive the early adoption of such sensors.
    In 2005, we estimate that some 80 million MEMS microphones were sold for a total of $56 million (see Table), primarily to the cell phone market (see table). This amounts to just 5–10% of the total microphone market, which itself amounted to 1.3 billion ECM units and was worth around $600 million. The selling price is set by ECMS and is around $0.50 or less for analog microphones, while a premium of 20–30% is added for digital devices, in high volume. The differential is larger at lower volumes. By 2010, the overall market will be worth $680 million, predominantly for mobile phones, notebooks, digital cameras, camcorders, PDAs and other appliances such as dictation recorders. There will be modest penetration of automotive markets, leveraging arrays to allow noise suppression in hands-free speakerphones, for example. Hearing aids are challenging to produce, e.g. have a very low noise floor, and will remain a small market.