Three new innovative demonstrators — 15 µm wide SiGe micromirrors, grating light valves and SiGe accelerometers — have been built in imec’s SiGeMEMS technology platform. With this CMORE technology platform, imec offers a generic CMOS-compatible MEMS process for the monolithic integration of MEMS devices directly on top of CMOS metallization.
The demonstrators were built in the frame of a Flemish Strategic Basic Research (SBO) project called Gemini by imec and its project partners, Ghent University (UGent) and Katholieke Universiteit Leuven (K.U. Leuven). They illustrate the broad applicability of the technology platform.
Imec’s SiGeMEMS technology is based on a MEMS-last approach, where the MEMS are processed after and on top of the CMOS circuits (Fig. 1). It enables monolithic integration of CMOS and MEMS, integrating MEMS devices with the driving and readout electronics on the same die. This leads to a better performance compared with other integration schemes — with a better signal-to-noise ratio through a reduced interconnect parasitic resistance and capacitance, a smaller die size and package, and lower power consumption.
1. This graphic illustrates a schematic cross-section of a MEMS device created with imec’s baseline SiGeMEMS platform.The SiGeMEMS platform is versatile. It consists of standard and optional modules that can be processed at ~450ºC above standard CMOS, with many possibilities to tune and optimize the modules. The standard modules provide, for example, a CMOS protection layer, MEMS via and poly-SiGe electrode, an anchor and poly-SiGe structural layer, and thin-film poly-SiGe packaging. Optional modules, such as optical, piezoresistive or probes, can be added depending on the application.
The platform’s flexible and modular approach allows application-specific tuning and optimization. An example is the thickness of the MEMS structural layer, which can vary between 300 nm and 4 µm. A 300 nm thick layer allows the manufacture of optical MEMS, such as the micromirrors and gratings discussed here.
For such devices, the process is extended to add various coatings with specific reflective properties. A 4 µm structural layer is used, for example, to create inertial sensors or actuators such as the Gemini accelerometer (Fig. 2). Other possible applications of the technology are µmicrophones, µspeakers, µsensors, probe-based memories and micropower generation.
2. An in-plane (left) and out-of-plane (right) SiGe accelerometer.The Gemini mirror design uses an innovative actuation mechanism, relying on six electrodes (using two possible electrode thicknesses of the SiGeMEMS platform). Two of the six electrodes serve as landing electrodes. The other four attracting electrodes are driven by two anti-phase saw tooth signals and two fixed analog voltage signals. By applying this signal scheme, the duty cycle of the mirror is modulated in an analog way.
Laser Doppler vibrometer measurements have confirmed the feasibility of analog pulse width modulation (PWM) for 15 µm wide SiGe micromirrors, which have been designed for use in a display system. The novel actuation mechanism enables the display of a large range of grayscale values. The six-electrode design combines the advantages of analog driving (no contouring effects) with that of a full-swing mirror movement (simpler optical system and higher response speeds).
A grating light valve is a MEMS reflection grating that produces bright and dark pixels in a display system, by controlled diffraction of incident light due to electrostatic deflection of microbeams. In the Gemini grating light valves, the microbeams are clamped beams suspended over an electrode, which can modulate the intensity of the diffracted light when an actuation voltage is applied to half of the beams. Display systems consisting of such a technology provide a high contrast ratio, high resolution and high brightness. Both the mirrors and grating light valves are realized with a 300 nm thick SiGe structural layer.
For the Gemini accelerometer, both in-plane and out-of-plane low-g designs have been proposed. Measurements of a fabricated out-of-plane accelerometer show that this device can sense the gravitation projection to the main sensing axis with an average sensitivity of 0.5 mV/g. This sensitivity is comparable to the state-of-the-art, but above-CMOS integration will greatly improve the state-of-the-art noise performance of such accelerometers. The accelerometers have been built with a 4 µm thick SiGe structural layer to obtain an improved capacitive readout of the in-plane devices.
Imec CMORE SiGeMEMS services include feasibility studies, design and technology development, prototyping and low-volume manufacturing. In addition to these services, imec recently announced a new SiGeMEMS foundry service. This service is based on the SiGeMEMS platform, fixing the options in a baseline process with a 4 µm SiGe mechanical layer. It is supported by mature design kits for the most important commercial MEMS design tools, and allows interested parties to develop their own MEMS designs for rapid prototyping.
In addition, for universities and research centers, there is a multi-project wafer (MPW) service. By gathering the designs of multiple customers on the same mask set, MPWs enable the fabrication of test structures and prototypes of devices at low cost. A first MPW run, scheduled for the end of 2010, will be processed on a wafer with a single metal layer, and is meant for initial prototyping. A second run, with full capability and with the SiGeMEMS devices on top of TSMC 0.18 µm CMOS, is scheduled for mid-2011.
Imec’s SiGeMEMS services are part of its CMORE offering. With CMORE, imec offers companies all the services needed to convert their ideas into smart packaged microsystem products. The CMORE toolbox contains a wide variety of device technologies (including high-voltage technologies, CMOS imagers, photonics or MEMS) and packaging capabilities (such as through-silicon vias and MEMS capping), as well as design expertise and testing and reliability know-how. Through its partners, imec can also offer a path to transfer the technology to the foundry for volume production.
— Ann Witvrouw, Mieke Van Bavel, and Jan Provoost, imec