The IEEE MEMS conference was held in Vancouver during January 2020. We attended the conference to meet with our customers and to see what new developments are being made in the field of MEMS. One area of interest was the MEMS Industry Workshop, where a number of top MEMS companies as well as new startups had the opportunity to present their technologies.
Bosch presented their new EpyC (Epi-Poly-Cycle) Process, which enables MEMS devices to be manufactured with up to 5 stackable polysilicon layers, each extending to 40 mm in thickness. The process is being promoted as “3D printing for MEMS” and is potentially superior to the use of wafer bonded stacks. The technology can support high volume applications and could fuel a new generation of MEMS technology. Sensirion also discussed their CMOSens® Technology, which enables a sensor component to be combined with analog and digital signal processing circuitry on CMOS. ST Microelectronics demonstrated their progress in developing stable PZT and ALN films for actuators using PVD/Solgel deposition. ST’s processes could be used in inkjet printers, autofocus systems, audio speakers, MEMS micromirrors (used in automotive and projection systems), and PMUT applications (such as fingerprint sensors).
Presentations were also given on the evolution of new devices, such as eye-tracking micromirrors from Adhawk Microsystems. This device tracks the movement of the eye based on an infrared scan of the cornea’s position, using MEMS micromirrors. The Chirp Microsystems MEMS ultrasonic sensors (“sonar on a chip”) was discussed, with applications in AR/VR, smartphones, automobiles, industrial machinery, and other ICT applications. NXP and TDK Invensense representatives both discussed yield and robustness issues in the production of high-volume MEMS devices for applications such as the automotive market. Chip yield and robustness are extremely important in the automotive market, where there is a continual push to improve yield to the sub-ppm level. Issues such as immunity to vibration, transient dynamics, package and PCB stress, and design/manufacturing errors (residual stringers, parts not releasing, etc.) were discussed by the presenters.
An interesting failure case was recounted at the conference during a presentation in the commercial session. The speaker discussed the effect of creating numbered labels by “penciling them on” to a packaged MEMS device in their laboratory. The labels created additional parasitic capacitance within the MEMS device and skewed its output. This error could have theoretically been simulated, but who would have thought that marking a test chip with a graphite pencil would change its performance! No doubt there are many other unexpected and interesting failures like this that are never presented at conferences, due to their commercial sensitivity.
Simulation was mentioned as key to understanding the robustness of a device, and whether a device can cope with high overload shocks and vibrations, either during use or during assembly and test. Coincidentally, Coventor presented a poster at the conference that used simulation to describe the effects of package deformation on sensor (gyroscope) performance. A portion of the poster is shown in Figure 1. A hybrid-approach combining FEA (finite element analysis) with a multi-physics analysis was used to model sub-pF changes in the output sensing capacitance of a MEMS gyroscope due to package deformation. You can find additional information on our website about this particular example, or contact us for a demo of the hybrid MEMS gyroscope model.
|Co-simulation of the package and transducer at a temperature of -40°C, highlighting displacement of mechanical connectors, sensing capacitor and comb stators||Simulation results show a step change at 20oC, which corresponds to the zero-stress temperature|
Figure 1: CoventorMP® model of MEMS Gyroscope displaying performance changes as a result of package deformation
When designing a MEMS device, the sensor and associated control circuitry must not only adequately control static deformation (like that shown in our example), but also control mechanical shock and vibration loads during packaging and handling, along with other vibrations and shocks that can affect operational performance. We have developed solutions to allow the simulation of vibration and shock loads, including design issues such as the coupling between packaging and devices, contact forces on overload stoppers, and other static and transient effects. These simulation capabilities address many of the design robustness and reliability issues discussed during this year’s IEEE MEMS conference. We look forward to learning about the next generation of MEMS process and device technology at future IEEE MEMS conferences, and are grateful to be a part of these amazing developments.