Piezo Effect Boosts MEMS Microphones, Fingerprint Sensors
Mike Rosa, Ph.D.
The MEMS industry is getting a push from companies interested in taking advantage of the piezoelectric effect to build next-generation microphones and fingerprint sensors—two high-volume products that could have a major impact on the 200mm semiconductor landscape.
New materials and architectures for MEMS microphones and fingerprint sensors—both major revenue streams for the MEMS industry—use the piezoelectric effect: the ability of certain materials to generate an electric charge in response to applied mechanical stress (see sidebar, “The Piezoelectric Effect”).
Though challenges remain, technologists are working with Applied Materials to develop a new class of MEMS devices that leverage this important technology.
High SNR Microphones
MEMS-based microphones—one of the highest-volume MEMS devices produced today—are undergoing a seismic shift in capabilities (see figure 1). A move is underway from single parallel plate, capacitive-coupled microphones to piezo-based microphones, soon to be followed by electrostatic comb finger-based designs and then optical-based devices. While it is the piezo-based design that has captured attention recently, experts agree it will take time to develop the necessary materials and process technologies required to mass produce the devices with high reliability.
Figure 1. Microphone architectures that leverage new materials and new sound-wave detection schemes promise increased signal-to-noise performance. (Source: IHS)
A recent forecast by Yole Développement (Lyon, France) puts shipments of MEMS microphones in 2016 at over 4.4B units, exceeding 7B units by 2020, a nearly 14% compound annual growth rate (CAGR) over the forecast period.
Microphones used inside the cabins of cars and trucks are expected to be a key driver, provided that the signal-to-noise ratio (SNR) can be improved enough to filter out street noises. Most cars and trucks still use older electret-style devices. However, a change is underway as the SNR of MEMS-based microphones improves, the number of microphones per vehicle increases, and more emphasis is placed on triangular noise cancellation and directionality.
Improved SNR Opens Doors
Matt Crowley, CEO of Boston-based Vesper, a piezo-MEMS microphone startup, expects that the increased SNR capability (beyond 70dB) of next-generation microphones “will open up numerous additional applications in consumer and industrial markets,” ushering in an era of voice-activated, recognition-based technologies.
PVD-sputtered piezo materials, including scandium-doped aluminum nitride (ScAlN), are enabling much of this improved performance. Theoretical calculations demonstrate that the higher the scandium concentration, the more improved the SNR will be.
Crowley calculates that for every 5% of scandium (Sc) there is a corresponding increase of 1dB of SNR, which is significant from both application performance and manufacturing standpoints.
Today there are readily available single alloyed targets containing less than 10% Sc. However, for higher concentrations the target alternatives are not well developed. Applied Materials, in conjunction with target manufacturers, is tackling the development of higher-concentration Sc-doped single alloy targets, as well as the robust deposition processes needed to deliver them in a manufacturing environment. The goal is to enable deposition processes to support Sc concentrations up to 43%—the empirically proven limit before there is a roll off in the achieved piezoelectric coefficient.
Reliable Fingerprint Sensors
Capacitive fingerprint sensors are being used increasingly in smartphones and other mobile devices (see figure 2). However, from a manufacturability standpoint, their popularity is due simply to the CMOS compatibility of their device structure. Despite this, the capacitive-type fingerprint sensor is prone to reliability issues caused by moisture, dirt and other surface contamination on fingers, and therefore is ripe for reinvention. Enter the piezobased fingerprint sensor, where Sc also plays a role.
Figure 2. The conceptual capacitive sensor shown has an array of 57,000 pixels; each pixel is a capacitive device with CMOS control circuitry underneath the MEMS structure. While popular, this architecture is prone to security flaws and reliability issues caused by dirt and moisture. (Source: Solid State Fingerprint Scanners: A Survey of Technologies, Philip D. Wasserman, Guest Researcher, NIST,Gaithersburg, MD, December 26, 2005)
Unlike capacitive-based fingerprint sensors, piezo-based devices detect the electrical impedance of both ridges and gaps on the epidermal layer of skin. Furthermore, ultrasonic sense mechanisms provide the ability to acquire images beneath the skin’s surface for optional additional security (see figure 3, which shows a conceptual diagram of a piezo-based fingerprint sensor).
Figure 3. The piezo-based fingerprint sensor is positioned above CMOS circuitry. (Source: Applied Physics Letters 106, 2015)
The key to enabling this functionality lies in the choice of piezo materials, including aluminum nitride (AlN), scandium-doped aluminum nitride (ScAlN) and other members of a CMOS-compatible class of PVD-sputtered materials with the right electrical and mechanical characteristics.
Professor David Horsley of the University of California at Berkeley’s Sensor and Actuator Center (BSAC) points out that ultrasonic receiver sensitivity lies in the combination of high piezoelectric coefficients affecting the transmitter sensitivity and a low relative permittivity. “These, in addition to a higher electromechanical coupling coefficient for AlN films doped with Sc, have shown that increased percentages of Sc enable an overall higher performing CMOS-compatible class of piezoelectric materials,” he said.
Today, Sc concentrations of up to 20% in AlN films have been experimentally demonstrated to improve device performance, with an almost linear increase in performance for continued increases in Sc concentrations.
For equipment OEMs the challenge becomes how to deliver robust, reliable sputtering processes for these films. ScAlN films are currently available as single-alloyed targets with Sc concentrations of less than 10% for use in RF filter devices.
Applied Materials is working closely with materials innovation provider Materion Corporation (Cleveland, Ohio) to develop single-alloy targets of ScAl, with concentrations exceeding 20%. This development work will apply to multiple MEMS applications from RF filters to micro-machined ultrasonic transducers (pMUTs) to high-force piezoelectric actuators.
Once suitable target materials have been developed, process development begins. Hardware elements are refi ned in concert with process recipe conditions to achieve the deposition rates, nonuniformities and across-wafer stress levels required to meet the on-wafer film requirements.
MEMS Sensor Sniff Exhausts
There are other trends likely to have notable effects on the MEMS industry. The recent automotive emissions scandals have had a positive effect on the automotive MEMS market, prompting increased adoption of more advanced exhaust after-treatment systems, such as the selective catalytic reduction (SCR) system based on MEMS sensors. Industry analysts estimate that improved emissions control alone may account for an additional dozen sensors or more per vehicle.
Humidity sensors, which aid in controlling in-cabin HVAC, are featured in more than 20% of vehicles today, and that number is rising. Sensors also play a role in improving the safety and lifetime of the battery pack in electric vehicles.
Whether it’s expanded functionality in the latest smartphone, advanced driver awareness systems, or regulatory forces driving market trends in passenger safety and emissions control (as is the case in the automotive market), the next generation of MEMS-based device applications will rely heavily on new materials and processes. And all of these applications must be developed on robust, reliable and competitively priced platform solutions.
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 Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic d-block element, it has historically been sometimes classified as a rare earth element, together with yttrium and the anthanoids. (Source: Wikipedia)
The Piezoelectric Effect
The piezoelectric effect is the ability of certain materials to generate an electric charge in response to applied mechanical stress. Lead zirconate titanate (PZT)—the most common piezo material—generates measurable piezoelectricity under applied mechanical stress. Piezoelectric sensors use this property to measure changes in displacement or applied mechanical force.
One of the unique characteristics of the piezoelectric effect is that it is reversible. Materials exhibiting the direct piezoelectric effect also exhibit the converse piezoelectric effect: the generation of mechanical stress when an electric field is applied.
The converse piezoelectric effect can be used to generate a mechanical (or actuation) force. One example is the conversion of a pulsed electrical signal into high frequency vibrational waves such as those used in medical ultrasound scans. A piezoelectric transducer in the wand a technician passes over the body generates an ultrasonic signal through high-frequency mechanical vibrations. In this example, with the correct design, the same piezoelectric device can generate an ultrasonic signal and also “listen” for return signals received.
Piezoelectricity was discovered in 1880 by French physicists Jacques and Pierre Curie. The word piezoelectric is derived from the Greek piezein, which means to squeeze or press, and piezo, which is Greek for “push.”