Next Generation Pressure Sensors In Surface Micromachining Technology

Abstract

One of the first MEMS products -the pressure sensor -has still room for innovation. We report a completely new pressure sensor generation based on a novel surface micromachining technology. Using porous silicon the membrane fabrication can be monolithically integrated with high synergy in an analogidigital semiconductor process suited for high volume production in an IC-fah. Only two mask tayers and one electrochemical ctching step are inserted at the beginning of a standard IC-process to transform the epitaxial silicon layer from the electronic process into a monocrystalline membrane with a vacuum cavity under it. Fig. 2: Pressure sensor membmne with vucuum cavity under it and implantedpiezzo resistors bn top.FABRICATION Fig, 1: Mtnihmnefabricatton iechnolo,y: (a) conventional hulk niicromachinirg wifh KUH etching and glass bonding to fabricate a membrnne over a 1-efirence vacuum. (b) using porous silicon technology a monoc ysiallinr membrarre i s jix-ied bj? eppita.xiul layer growth. The porous silicon is converted in a vucuiim caviy by sintering in H2 ntmosphere.The epitaxial layer is dcposited on a porous silicon layer. The cavity is formed by subsequent thermal rearrangement of this porous silicon during the epitaxial growth process and thc following high temperature difision processes [I]. The enclosed hydrogen in the cavity during the epitaxial deposition diffuses out, but no other gases can diffuse in. This leads to a good reference vacuum in thc cavity. The square membrane is deflected by 0.7 pm / bar. It is mcchanically robust against overload because of the ''floor stopper" when it touches the bottom of the cavity. A 1 bar sensor is overload proof to more than 60 bar. The signal is sensed by a piezoresistive Wheatstone bridge, which is a measurement principle with better linearity compared to capacitive sensors. The piezoresistors are fabricated using the same diffusion layers as the electronic circuit to achieve good matching properties. The bridge signal of 55 mV / bar is ampIified by the surrounding electronics. The mixedsignal semiconductor process is suited for the automotive temperature range of -40 .. +I40 "C. The circuit is designed to be ESD and EMC proof. TRANSDUCERS'OSThe 13th lnremational ConCerencc on Solid-State Sensors, Actuators and Microsystems, Seoul, Korea, June 5-9,2005 0-7803-8952-2/0.5/$20.00 0 2 0 0 s IEEE. 35

Review: Semiconductor Piezoresistance for Microsystems

“…This sensor is used to measure atmospheric and manifold pressure in electronic engine control systems. Researchers at Bosch developed a new technique for these piezoresistive pressure sensors using porous silicon and epitaxy to form a single crystal silicon membrane and vacuum cavity without bonding [263], [264]. This approach saves wafer real estate and is CMOS compatible.…”

Advanced silicon microstructures, sensors, and systems

“…From the point of view of silicon micromachining, the portfolio of currently available techniques includes wet etching in alkaline solutions with and without electrochemical biasing [4], anisotropic and isotropic reactive ion etching (RIE) including deep RIE (DRIE) also called advanced silicon etching (ASE) [5], silicon porosification [6], and laser ablation. Combinations of these techniques produce structures that were out of reach only a few years ago.…”

More than Moore

“…To fulfill these requirements, the MEMS structure is typically sealed at the end of the MEMS process [91]. As an example of such pre-CMOS MEMS technology, Bosch has developed a technology called advanced porous silicon membrane (APSM) [123]. This technology makes use of porous silicon to form a cavity underneath the surface of the monocrystalline silicon, which helps to make the wafer manufacturing process fully CMOS-compatible and turns the traditional bulk-micromachining process into a surface technology.…”

“…25 Bosch's APMS process schematic (left) and the corresponding SEM cross section (right)[123]. Reproduced with permission of Robert Bosch GmbH 150 M. Lantz et al…”