S. Fanget, F. Casset, S. Nicolas, C. Dieppedale, M. Allain, B. Desloges, G. Le Rhun
CEA - LETI,
Keywords: MEMS actuators, piezoelectric thin film, PZT, AlN, varifocal lens, PMUT, haptics, micro-scanner
Summary:Over the past three decades, huge progresses have been made in MEMS technologies. Hundreds millions of MEMS, principally: accelerometers, gyroscopes and microphones, are commercialized yearly. This should continue to increase with upcoming new applications related to IOT or autonomous cars. New devices will be needed as for example: environment sensors, particles sensors, biometrical sensors, optical MEMS, micro-projection systems or new Human-Machine Interfaces (HMI). Up to now, most of MEMS were focused on sensing with very few commercialized MEMS dedicated to actuation. Inkjet printheads and Micro-mirrors (DMD from TI) based on MEMS actuators remains the two exceptions with a forecasted market for 2017 of about 900 and 1000 US$M respectively. New opportunities exposed above could be the trigger for a rapid growth of the MEMS actuators market. Next killer application will probably be autofocus inside modules cameras for smartphones. As an illustration, STMicroelectronics and PoLigth jointly announced in 2014 a thin-film piezoelectric technology for a MEMS varifocal lens dedicated to autofocus. Although micro-actuation can be achieved using electrostatic or electromagnetic transduction, piezoelectric actuation remains the most promising approach to generate high forces while mastering power consumption. Most of commercialized piezo-actuators are currently based on bulk piezoelectric ceramics which present several limitations: difficulties for integration with MEMS, thickness and power consumption. LETI has been developing for more than 25 years a large know-how on piezoelectric thin films: Lead Zirconate Titanate (PZT) and Aluminum Nitride (AlN) for MEMS actuators. Using PZT or AlN thin films deposited by sol-gel method and sputtering respectively on a released silicon membrane, a powerful technological platform of has been developed at LETI to fabricate MEMS actuators for a large number of applications. This know-how has partially been transferred to STMicroelectronics before 2014 during the collaborative ENIAC JU Lab4MEMS project. The first part of this talk will be focused on piezo materials developments and improvements. The second one will be a review of several devices fabricated using the LETI’s Piezo-MEMS platform. The first presented device, a varifocal lens jointly developed with the startup Wavelens, integrates a piezo-actuator to modify the curvature of deformable membrane that encapsulates a liquid. The actuators displace the liquid towards the center of the lens and thus changes the curvature of the membrane and therefore the focal length. The second optical device is a micro-scanner based on an innovative half-circular piezoelectric actuators associated to a circular mirror asymmetrically mounted on two lateral torsion hinges. By this way, a translation excitation of actuators is converted into a rotational oscillation of the mirror. We will then describe an haptic interface fabricated on glass and based on the squeeze-film effect. It consists in modulating the friction between the user’s finger and a plate resonator this provides high granularity level of haptic sensations. The last presented device will be a set of acoustic micro-transducers, arranged in a matrix that can emits ultrasonic signals. This generic acoustic device is able to address a lot of applications like ultrasonic imaging, medical treatments, fingerprint sensors or new HMI.