Vesper Mic Drops Under Water
Piezoelectric MEMS Are Immune to Dust and Liquids
By Mike Demler
Vesper Technologies, a 17-person Boston-based startup, is the first to bring piezoelectric techniques to the MEMS-microphone market. It’s offering an alternative to capacitive MEMS microphones, which are currently the norm in smartphones, tablets, and other consumer electronics. To waterproof capacitive MEMS, however, manufacturers must incur the further expense of adding a separate protective layer. But this layer muffles the incoming sound, so designers must compensate by increasing the gain (and power) in the signal-processing chain.
Piezoelectric MEMS are natively immune to impairment from dust and liquids, and they can also function as hydrophones. Vesper guarantees its devices will continue to work after up to seven days immersed in tap water, or even salty or soapy water. The company has its roots at the University of Michigan, where Robert Littrell (now Vesper’s CTO) developed the unique piezoelectric acoustic-sensor technology as part of his doctoral thesis. He worked with Professor Karl Grosh on this effort. CEO Matt Crowley previously founded and handled business development for Sand 9, another piezoelectric startup, after several years on staff at Boston University.
Initial funding for Littrell’s research came from NASA, which was looking for a waterproof microphone. The piezoelectric technology attracted investments from microphone manufacturers AAC Technologies and XinGang Electronics, which are also commercializing the devices. Together with angel and VC funding, Vesper has garnered a total of $10 million in early-stage investments. To expand its market beyond mobile devices, it plans to establish a U.S. distributor network.
The company’s business model is to sell its products in wafer form to microphone manufacturers, which assemble the components into packages for delivery to customers. Knowles is currently the largest MEMS-microphone supplier, followed by AAC Technologies, Goertek, Infineon, STMicroelectronics, and several Chinese and Korean companies (see MCR 12/21/15, “Smartphones Gain Microphone IQ”).
According to market-research firm IHS, approximately 3.6 billion MEMS microphones shipped last year, and the market is on track to hit 6 billion in 2019. Smartphones constitute the largest application segment, but the microphones appear in other consumer electronics such as noise-canceling headphones as well as voice-enabled IoT and consumer devices. Since beginning production in June, Vesper has already shipped more than one million units of its VM1000 product, shown in Figure 1. GlobalFoundries builds the piezoelectric MEMS at its Singapore fabs.
Figure 1. The Vesper VM1000, a piezoelectric MEMS microphone. The tiny device matches the package size and pinout of current capacitive MEMS microphones, so it can serve as a drop-in replacement. (Chip photo: Vesper)
A Slice of Piezo
As Figure 2 shows, a MEMS capacitor comprises two parallel metalized plates separated by an air gap. The backplate electrodes have fixed positions, but a hinge enables the diaphragm to move in response to sound-induced air pressure. Perforations in the backplate allow air to flow freely through the cavity. As the diaphragm moves, the distance between it and the fixed electrodes varies, changing the coupling capacitance. To better measure this capacitance, the microphone control ASIC uses a charge pump to precharge the diaphragm. The change in capacitance creates a change in this voltage that varies with the sound’s amplitude and frequency. These voltages changes are small, so the ASIC contains an amplifier that boosts the signal.
Figure 2. Cross-section of a capacitive MEMS microphone. The diaphragm moves in response to acoustic-pressure changes, varying the backplate coupling capacitance. By precharging the diaphragm, the microphone converts the changing capacitance into a voltage signal.
One problem with MEMS capacitor microphones is that the air gap and ventilated backplate render them inherently susceptible to the effects of dust and moisture. Dust accumulation inside the capacitor impairs its sensitivity, and liquids can cause complete failure. The backplate air gaps can also introduce noise. In addition, these devices only function when a voltage is applied to the diaphragm, resulting in a constant power drain in “always listening” applications.
By contrast, Vesper employs an alternate MEMS structure that’s immune to these issues. Manufacturing the piezoelectric sensors requires fewer mask layers than MEMS capacitor microphones. As Figure 3 shows, the piezoelectric elements comprise cantilevered aluminum-nitride plates, which generate a variable electric charge when sound waves cause them to flex. The thin-film sputtering process is similar to that for manufacturing piezoelectric RF filters (see MCR 8/22/16, “Akoustis Challenges Broadcom FBAR”).
Figure 3. Cross-section of Vesper’s piezoelectric MEMS microphone. The cantilevered structure moves in response to sound energy, and the piezoelectric material generates a current directly without the need for a charge pump.
Since the piezoelectric effect doesn’t require an air gap, dust and moisture don’t affect the electrical characteristics of such devices. In addition, because piezoelectric materials produce their own current, the microphone can omit precharge circuits. This capability allows Vesper to implement a wake-on-sound feature that keeps the ASIC in a low-power sleep mode until sound is detected. As a result, power consumption in listening mode is much lower than for capacitive microphones.
Building a Product Line
A complete MEMS microphone is a two-chip system comprising the transducer and a signal-processing ASIC. Regardless of the transducer type, the chips require manufacturing in different processes, but vendors can then mix and match the sensor and ASICs to offer a variety of products. Digital microphones also include an analog-to-digital converter to output the acoustic signal in a form that processors can handle directly.
Vesper’s first product is the VM1000. It ships in a tiny 3.76mm x 2.95mm package that’s just 1.1mm thick. The six-pin layout on the bottom of the package matches industry standards, enabling the device to serve as a drop-in replacement for capacitive microphones.
The VM1000 uses a simple analog chip that integrates an operational amplifier and a low-dropout (LDO) voltage regulator. The LDO handles external VDD supplies ranging from 1.6V to 3.6V. At the maximum supply level, the amplifier draws 145 microamps, generating output signals in the tens of millivolts. Vesper manufactures the analog ASIC in a 180-micron CMOS process.
The industry-standard test for measuring microphone sensitivity uses a 1kHz tone to generate a 94dB sound-pressure level (SPL), which is equivalent to one pascal (Pa). This SPL represents a very loud sound, akin to a jackhammer 50 feet away. The sensitivity of the VM1000 is typically –38dBV, equivalent to roughly a 13mV output, and is on a par with that of Knowles capacitor microphones.
Hey Siri, Wake Up!
Vesper has begun sampling two additional designs: the VM2000 and VM1010. The VM2000 is essentially the same as the VM1000, but the analog ASIC includes a differential output amplifier to increase the device’s noise immunity and dynamic range. The power-supply rejection ratio (PSRR) improves from –55dB to –70dB, and the acoustic-overload point for 10% total harmonic distortion increases from 125dBSPL to 135dBSPL. The tradeoff is a 50% greater active supply current—220 microamps. The VM2000 is currently sampling, and the company expects it to be generally available in 4Q16.
Next up is the VM1010. As Figure 4 shows, this design adds a latching comparator to the analog ASIC. By connecting one comparator input to the piezoelectric transducer and the other to a threshold reference voltage, customers can implement an always-on device that wakes up on sound. When the piezoelectric transducer senses sound pressure exceeding the threshold, the comparator output switches to generate a wake-up signal for the speech processor. The threshold is adjustable, so users can set it accordingly for noisy or quiet environments. In the always-on sleep mode, the VM1010 draws just 3 microamps; in active mode, it draws essentially the same current as the VM1000. The company expects it to enter volume production in 1Q17.
Figure 4. Schematic for VM1010 piezoelectric MEMS microphone. The two-chip package combines the piezoelectric transducer with a small analog ASIC, which provides the signal buffer and amplification. The VM1000 omits the comparator, mode input, and differential output.
The VM1010’s wake-on-sound function creates additional opportunities. In a burglar alarm, for example, it can detect breaking glass before thieves can even enter a building. In high-crime areas, it can immediately alert police to dangerous sounds such as gunshots, screams, or crashing cars.
Developing a Sound Strategy
The popularity of voice-command user interfaces such as Amazon’s Alexa, Apple’s Siri, OK Google, and Microsoft’s Cortana is building demand for MEMS microphones. For example, the iPhone 6 contains three microphones, and the supersized iPhone 6s has four. The extra mics improve sound quality by enabling noise cancellation and by increasing accuracy for hands-free speakerphone operation. To enable far-field voice-command detection, digital assistants use beamforming with an array of microphones. The Amazon Echo employs seven microphones, but other designs have even more.
To serve these new multisensor applications, capacitor-microphone manufacturers have integrated multiple diaphragms on the same die. That solution is problematic, however, since a failure of one element makes the others unusable as well. Vesper says its cantilevered piezoelectric structure avoids these types of failures.
The startup sees further growth opportunities beyond voice microphones. Its transducer can respond to ultrasonic waves, potentially offering a low-power alternative to the infrared-based proximity detectors in all smartphones. By coupling the sensor with the appropriate ASIC, the device can detect changes in atmospheric pressure, serving as an altimeter or barometer.
In a rapidly growing market such as MEMS microphones, which some forecast to exceed $1 billion in annual revenue, a new disruptive technology such as Vesper’s piezoelectric transducer is sure to draw imitators. To protect itself, the company has licensed related patents from the University of Michigan, and it has filed for or already received several others from the U.S. Patent Office.
Employing the reusable MEMS component, Vesper can quickly develop new products by spinning low-complexity ASICs, and it has on its 2017 roadmap a digital microphone as well as an improved SNR model. Its future will depend on investors’ patience as it ramps to volume production. The current staffing is too small to pursue all the available opportunities, so we expect Vesper will need additional funding to build out its product line. Having a proven product and customers, it should easily raise the necessary capital. The company could also be an attractive acquisition target for AAC or another large microphone vendor. In the meantime, designers seeking a waterproof or always-on microphone should talk to Vesper.
Price and Availability
Vesper withheld pricing for the VM1000, but we expect it carries a small premium over capacitor microphones owing to its waterproof properties. The MEMS device and supporting ASIC are available now. For more information, access www.vespermems.com/products/vm1000.