Fabrication of a single crystalline silicon capacitive lateral accelerometer using micromachining based on single step plasma etching

Abstract

Silicon capacitive accelerometers are superior to their piezoresistive counterparts due to their high sensitivity and low temperature coefficient, especially for downscaled microsensors [ 11. Several processes have been developed for the fabrication of such devices, such as surface micromachining [2]; bulk micromachining using wet etching [ 3 ] or using plasma etching (SCREAM) [4]. Surface micromachining suffers from limited structural film thickness (< 2 pm), which limits the mass and sidewall capacitance value and, therefore, the sensitivity of the device. Bulk micromachining using wet etching depends on crystal orientation and usually results in tapered sidewalls, thus limiting the minimum lateral dimensions. Therefore, it is desirable to micromachine bulk silicon with directional plasma etching so as to achieve superior mechanical properties, small lateral dimensions and large vertical dimensions. Although the SCREAM process does fulfil these requirements, it is a rather complicated process involving several film deposition and etching steps. Furthermore, the resulting surface non-planarity make it very difficult to integrate on-chip circuits. In this paper, the fabrication of a single-crystalline capacitive lateral accelerometer using the SIMPLE (SIlicon Micromachining by single step PLasma Etching) technique is described. This technique uses .a CI,-based plasma chemistry which etches por lightly n-doped silicon anisotropically but heavily n-doped silicon isotropically [5] . In such a way free-standing single crystalline silicon microstructures can be patterned and released from the substrate in a single step plasma etching. It will be demonstrated that the technique is compatible with the fabrication of on-chip electronic circuits.