by St.J. Dixon-Warren
Engineering and Process Analysis Manager, Chipworks
The extraordinary success of the iPhone 4 lies in the superb integration of multiple sensor technologies in conjunction with a very slick software interface. In a recent series of MEMS Investor Journal articles, we reviewed the 9DoF motion sensing technology used in the iPhone 4. Here we will discuss the MEMS microphones that have been incorporated into the iPhone 4, and we will then provide a review of some other MEMS microphones seen by Chipworks in recent years. MEMS microphone suppliers apparently saw a 50% increase in shipments in 2010, and they expect to see a four fold increase by 2014.
The iPhone 4 incorporates two microphones in the body of the device, and a third was found in the headset leads. Knowles Technology earned the design win for the two primary audio sensing microphones, one in the handset and one in the headset lead. An Infineon fabricated microphone, likely used for background pickup for noise cancellation, was found at the opposite end of the handset body.
An iPhone 4 teardown, showing photographs and x-rays of the microphone packages, and the MEMS and ASIC die, is available from Chipworks. In this article, we will focus on the MEMS microphone die. We will discuss the fabrication and operation of these clever little pieces of technology.
MEMS microphones are capacitive sensing devices. In essence, they operate like a high frequency pressure sensor. They feature a diaphragm which is comprised of two capacitor plates that, under the influence of the sound wave, vibrate with respect to each other. This results in variation of the capacitance, that is then amplified by an associated ASIC to produce either an analog or digital output signal. In the case of the Knowles microphones in the iPhone 4, Chipworks believes they are analog devices, although we have not yet done a circuit analysis to prove this. We have not been able to identify the specific Knowles part numbers used.
The two Knowles primary audio sensing microphones contain a 1.1 mm2 MEMS die, with S4.10 die markings, shown in Figure 1 below. Chipworks has seen the S4.10 die in many different downstream products. We believe that this MEMS microphone die is used in the majority of Knowles’ current MEMS microphone product line up, with the distinction between the various different products being determined by the amplifier ASIC and the packaging. The S4.10 die is very simple. It features a MEMS diaphragm with two bond pad connections, one for the top plate and the other for the bottom plate. The S4.10 likely uses the back side of the die to form a ground connection.
Figure 1: Knowles S4.10 MEMS microphone die from the iPhone 4.
The S4.10 die is actually quite similar to a Knowles microphone die, with S2.14 die markings, shown in Figure 2. It was extracted from the SP0103BE3 product by Chipworks in 2006. The major difference is the S2.14 is 1.6 mm2, corresponding to about twice the die area when compared to the S4.10. The 50% shrink in the S4.10 die area will, of course, approximately double the yield of devices per wafer, thus resulting in a dramatic reduction in cost. The MEMS microphone diaphragm has essentially the same ~0.5 mm diameter on both parts; however, the S2.14 has four bond pads, one for the bottom plate, two for the top plate, and a separate ground connection. Thus, not only was the S2.14 larger and, hence, more expensive to make, its packaging and integration costs would have been higher, since twice the wire bonding was required.
Figure 2: Knowles S2.14 MEMS microphone die.
A detailed view of the edge of the S2.14 microphone diaphragm is presented in Figure 3. The top plate is covered with an array of small holes, which are required to allow air to escape from the cavity between the two plates during operation. They were also needed in the release etch step in the fabrication process, discussed below.
Figure 3: Knowles S2.14 MEMS die detail.
Figure 4 shows a cross section through the S2.14 MEMS microphone die. The top and bottom capacitor plates are suspended above a sealed cavity, which is formed from the back side of the die using a wet etch which was selective for the {111} plane of the silicon. The position of the plates has been somewhat distorted by the epoxy resin used by Chipworks to stabilize the sample for cross-sectioning.
Figure 4: Knowles S2.14 MEMS die cross section.
A detailed view of the edge of the microphone diaphragm is shown in Figure 5. The bottom plate is formed using a single layer of polysilicon (poly 1), while the top flexible diaphragm plate is comprised of a bilayer of silicon nitride and polysilicon (poly 2), perforated with the holes.
According to iSuppli, the Knowles MEMS microphone die are fabricated by Sony Semiconductor Kyushu Corp. The fabrication of the two membranes, separated by an air gap, would have depended on the spacer layer, likely silicon dioxide, which would have been removed during the release step through the holes in the top plate, likely with anhydrous HF. It is most probable that the back side cavity etch was performed before this final step, probably by using a KOH wet etch.
Figure 5: Knowles S2.14 MEMS die cross section detail.
The Infineon MEMS microphone die found in the iPhone 4 is similar in many respects to the Knowles device. Figure 6 shows a photograph of the 1.35 mm x 1.25 mm Infineon E2002 MEMS microphone die. The microphone diaphragm is ~1.0 mm in diameter. The die features three bond pads, one for the top polysilicon plate, one for the bottom polysilicon plate, and a third which connects to a polysilicon guard ring. The Infineon E2002 die found in the iPhone is identical to that seen by Chipworks during the analysis of the Infineon SMM310E6433XT integrated silicon microphone.
Figure 6: Infineon E2002 MEMS microphone die from the iPhone 4.
The SMM310E6433XT is no longer advertised on the Infineon web site; however, according to iSuppli, Infineon now supplies its MEMS die to three Asian microphone suppliers, AAC Acoustic Technologies Holdings Inc., BSE Co. Ltd., and Hosiden Corp. These three suppliers, along with Knowles and Analog Devices, constitute the top five suppliers in the MEMS microphone market, with Knowles apparently holding nearly 80% of the market.
As an aside, it is worth noting that Analog Devices won the design win for the microphone in the fifth generation Apple iPod Nano (the new sixth generation Nano does not contain a microphone). It is interesting that Analog was not able to win a socket in the iPhone 4. Knowles and Analog Devices have recently concluded patent litigation, with the judge ruling in Analog Devices’ favor.
Figure 7 presents a photograph of the 1.0 mm x 1.0 mm Analog Devices MEMS microphone die found in the ADMP403, extracted from the iPod Nano (fifth generation). Apparently, a major part of Analog’s strategy to avoid infringement on Knowles’ patents was to base the fabrication of this device on its well established iMEMS process. The iMEMS process has been in use for many years for the production of inertial sensor products.
Figure 7: Analog Devices 4.8H MEMS microphone die from the iPod Nano.
MEMS microphones have evolved into a mature commodity product. The market is highly competitive. There are a number of other manufacturers, such as Akustica and MEMSTech, who continue to produce MEMS microphone products. Akustica microphones are of particular interest to the MEMS community, since it was the first company to produce a CMOS-based MEMS device. The MEMS diaphragm is formed using etch and release of the CMOS metallization layers. Figure 8 shows a SEM micrograph of the Akustica AKU2000 microphone diaphragm, which is formed using a serpentine pattern in the CMOS “metal 1” layer. A benefit of this approach is the ability to easily integrate signal processing circuitry onto the same die, thus allowing for single chip solutions. Akustica was acquired by Robert Bosch GmbH in August 2009.
Figure 8: Akustica AKU2000 microphone diaphragm.
Chipworks expects to see continued innovation in the MEMS microphone market. The innovation is likely to occur in the signal processing ASIC and the packaging, rather than in the MEMS part of the product. After all, Infineon has done rather well selling its original MEMS die to multiple microphone suppliers. There may not be a “Moore’s Law” governing the size of MEMS microphone devices, since the physics of sound require that the microphone diaphragm be large enough to interact with pressure variation induced by the sound waves. At 10 kHz, the wavelength is 34 mm, which is already much larger than the typical 0.5 mm diameter being used in commercial MEMS microphones. The diaphragm needs to be large enough, for a given plate thickness, such that the pressure variations from the sound waves for a human voice induce sufficient displacement of the capacitor plates to give a suitable signal for the ASIC amplifier. We can expect circuit designers to continue to make innovations in the design of the ASIC, thus creating lots of new work for the reverse engineer. Designers will likely move towards the integration of an ADC and digital signal processing within the microphone amplifier ASIC.
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St. J. (Sinjin) Dixon-Warren manages the Process Analysis group in the Technical Intelligence business unit at Chipworks. His group provides technical competitive analysis services to the semiconductor industry, currently with a special focus on the analysis of MEMS, CMOS images sensor, advanced CMOS and advanced power devices. He is the Sector Analyst for MEMS analysis at Chipworks. Dr. Dixon-Warren holds a PhD in physical chemistry from the University of Toronto and a BSc in chemistry from Simon Fraser University. Dixon-Warren joined Chipworks, in 2004, as a member of the process analysis group. He is author of about 50 publications and of about 100 Chipworks reports. Dr. Dixon-Warren can be reached at sdixonwarren@chipworks.com.
Copyright 2011 MEMS Investor Journal, Inc.
March 08, 2011 at 05:26 PM | Permalink
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