Biaxial piezoelectric MEMS mirrors with low absorption coating for 1550 nm long-range LIDAR

This paper presents the fabrication and characterization of a biaxial MEMS (MicroElectroMechanical System) scanner based on PZT (Lead Zirconate Titanate) which incorporates a low-absorption dielectric multilayer coating, i.e., a Bragg reflector. These 2 mm square MEMS mirrors, developed on 8-inch silicon wafers using VLSI (Very Large Scale Integration) technology are intended for long-range (>100 m) LIDAR (LIght Detection And Ranging) applications using a 2 W (average power) pulsed laser at 1550 nm. For this laser power, the use of a standard metal reflector leads to damaging overheating. To solve this problem, we have developed and optimised a physical sputtering (PVD) Bragg reflector deposition process compatible with our sol-gel piezoelectric motor. Experimental absorption measurements, performed at 1550 nm and show up to 24 times lower incident power absorption than the best metallic reflective coating (Au). Furthermore, we validated that the characteristics of the PZT, as well as the performance of the Bragg mirrors in terms of optical scanning angles, were identical to those of the Au reflector. These results open up the possibility of increasing the laser power beyond 2W for LIDAR applications or other applications requiring high optical power. Finally, a packaged 2D scanner was integrated into a LIDAR system and three-dimensional point cloud images were obtained, demonstrating the scanning stability and operability of these 2D MEMS mirrors.This research was funded by ECSEL Joint Undertaking (JU) grant number No. 826600 (project VIZTA).Peer ReviewedPostprint (published version

New challenges for piezoelectric MEMS

International audienceLead zirconate titanate, Pb(Zr,Ti)O$_3$ (PZT), has been the most studied piezoelectric thin film for MEMS actuators the last two decades due to its high transverse piezoelectric coefficient d31. The PZT technology is now available in several MEMS foundries and PZT thin film actuators are already integrated in commercialized devices like inkjet printheads, micro-speakers and autofocus camera. Emerging applications such as micro-mirror for LIDAR, haptics for human-machine interface, micro-pumps, as well as PMUT for fingerprint, medical probe or gesture recognition are also based on thin film PZT actuators. The potential of use of PiezoMEMS is thus expanding, but it may require to functionalize non silicon substrate (Glass, Flex, Metal,…), large surface (smart windows), curved surface (haptics). In some cases also, transparency is requested.One of the constraints that limit the use of PZT in microsystems is its high crystallization temperature, usually from 500°C to 700°C depending on deposition method/process. It is thus very complicated to integrate PZT on Si for Above IC applications (Temp <450 °C), glass substrate (optical applications, haptics) or flexible (low cost applications, large format) as require for some applications. Low temperature deposition processes and film transfer techniques are the main ways to bypass the thermal budget issue. At Leti, we developed a technological process that allows transferring films stack from silicon substrate to any other substrate. It was used for transferring high quality sol-gel deposited PZT films from silicon growth substrate to either Si, glass or polymer. The transferred PZT film keeps its high (100) texturation obtained during deposition, which is required for optimal piezoelectric properties. PZT based devices subsequently fabricated as test vehicles show excellent ferroelectric and piezoelectric properties. In this presentation, we will show the potential of this process for realizing non standard piezoMEMS devices such as capacitors on glass or transparent PZT membranes

High performance piezoelectric MEMS loudspeaker based on an innovative wafer bonding process

International audienceDespite a significant number of new structures in the past few years, MEMS loudspeaker still are not competitive in terms of performance compared to non-MEMS loudspeakers for free field applications. For industrial perspectives, a high sound pressure level on a wide frequency band is required, as well as a low total harmonic distortion. To widen the frequency range of MEMS loudspeakers, we propose to separate the actuating element from the radiating one, in order to separate design constraints to reach an optimal structure. In this paper, the lumped element model of the loudspeaker in presented, as well as the innovative manufacturing process. Finally, the computed frequency response is compared to the measured one. At the resonance, pressures as high as 110 $dBSPL$ at 1 kHz and at 10 mm are reported for an active surface of 36 mm$^2$, which is above the known state of the art for a loudspeaker with similar dimensions. Also, the flatness of the radiated sound pressure in a wide frequency range is closer to the ideal frequency response of loudspeakers than other MEMS loudspeakers, due to the piston mode of the moving rigid plate of the loudspeaker. The total harmonic distortion, mainly due to the nonlinearity of the piezoelectric transduction, is below 5% for reasonable sound pressure levels in the usable frequency band. The use of digital signal processing and of a dedicated packaging will allow our loudspeaker to advantageously replace the main or secondary one in smartphones

Highly transparent PZT capacitors on glass obtained by layer transfer process

International audienceTransparent ITO/PZT/ITO capacitors were fabricated on 200 mm glass substrate. The PZT films of 1 µm and 2 µm thickness were first grown on platinized Si wafer by sol-gel method, and then transferred onto glass substrate together with ITO electrodes following an innovative process. The obtained PZT based stacks on glass show an average transmission of about 70 % in the visible range. PZT films keep their preferred (100) orientation after transfer process. The capacitors exhibit ferroelectric, dielectric and piezoelectric properties comparable to standard non-transparent PZT films with metal electrodes. Transverse piezoelectric coefficient e31,f as high as 16 C/m² was measured for both PZT film thicknesses. This proof of concept opens the way to the fabrication of transparent piezoelectric actuators on glass for high performances haptic devices, as well as for other emerging applications like self-cleaning or functionalization of smart windows

Highly transparent PZT capacitors on glass obtained by layer transfer process

International audienceTransparent ITO/PZT/ITO capacitors were fabricated on 200 mm glass substrate. The PZT films of 1 µm and 2 µm thickness were first grown on platinized Si wafer by sol-gel method, and then transferred onto glass substrate together with ITO electrodes following an innovative process. The obtained PZT based stacks on glass show an average transmission of about 70 % in the visible range. PZT films keep their preferred (100) orientation after transfer process. The capacitors exhibit ferroelectric, dielectric and piezoelectric properties comparable to standard non-transparent PZT films with metal electrodes. Transverse piezoelectric coefficient e31,f as high as 16 C/m² was measured for both PZT film thicknesses. This proof of concept opens the way to the fabrication of transparent piezoelectric actuators on glass for high performances haptic devices, as well as for other emerging applications like self-cleaning or functionalization of smart windows

Low absorption optical coating on biaxial piezoelectric MEMS mirrors for 1550 nm long-range LIDAR

International audienceThis paper reports the first biaxial PZT (Lead Zirconium Titanate) piezoelectric MEMS mirror, which incorporated low-absorption dielectric multilayers coating, i.e. Bragg reflector. These MEMS-mirrors, developed on 8-inch silicon wafers, are dedicated to long-range LIDAR (LIght Detection And Ranging) applications with a high power 1550 nm pulsed laser

VALIDATION OF BENDING TEST BY NANOINDENTATION FOR MICRO-CONTACT ANALYSIS OF RF-MEMS SWITCHES

International audienceThis paper demonstrates the validity of a new methodology using a commercial nanoindenter coupled with electrical measurement on test vehicles specially designed to investigate the micro-scale contact physics. Dedicated validation tests and modelling are performed to assess the introduced methodology by analysing the response of gold contact with 5 µm² square bumps under various levels of current flowing through contact asperities. Contact temperature rising is measured leading to shifts of the mechanical properties of contact material and modifications of the contact topology. In addition, the data provide a better understanding of micro-contact behaviour related to the impact of current at low- to medium-power levels

Transparent PZT piezoelectric membranes for MEMS applications

International audienceLead zirconate titanate (PZT) is the most powerful piezoelectric material, especially for micro-actuator applications. The PZT based MEMS technology is today well established and mastered in industry. Thin film piezoelectric stacks are deposited onto silicon substrate and implement non-transparent layers (electrodes, metallization). One current great challenge is to integrate PZT, or other piezoelectric materials like AlN, in transparent MEMS devices for various application domains that may require transparency like haptics, human-machine interface, acoustics, self-cleaning. However, the crystallization at around 650-700°C of the PZT film does not allow it to be deposited directly on an SiO2 membrane and / or transparent electrode, such as ITO. PZT films are indeed most often grown on platinized Si substrate. A solution to this problem is to transfer the PZT thin film, together with a transparent electrode deposited on top of it, from a growth Pt/Si substrate to a host substrate (transparent or not), following a process recently developed [1]. We report here for the first time the realization of fully transparent ITO/PZT/ITO parallel plate capacitor on SiO2 membrane, obtained by layer transfer process (Figure 1). The piezoelectric membranes obtained on 200 mm Si wafer are electrically functional and show average transparency of 75% in visible spectrum. The actuation of membranes was demonstrated and measured under applied voltage. This proof of concept opens the way to the fabrication of transparent Piezoelectric Micromachined Ultrasonic Transducers (PMUT) devices for fingerprint application or else

Methodology to Analyze Failure Mechanisms of Ohmic Contacts on MEMS Switches

International audienceThis paper demonstrates the efficiency of a new methodology using a commercial nanoindenter coupling with electrical measurement on test vehicles specially designed to investigate the micro contact reliability. This study examines the response of gold contacts with 5 μm² square bumps under various levels of current flowing through contact asperities. Contact temperature rising is observed leading to shifts of the mechanical properties of contact material, modifications of the contact topology and a diminution of the time dependence creep effect. The data provides a better understanding of micro-scale contact physics especially failure mechanisms due to the heating of the contact on MEMS switches