High-temperature piezoresistive pressure sensor based on implantation of oxygen into silicon wafer

Abstract

Silicon on insulator (SOI) substrates can be prepared using ion implantation of oxygen. For piezoresistive detection, the top layer (0.2 μm thickness) of silicon is used as the active material due to its excellent monocrystalline properties. The piezoresistive effect of the top silicon layer of the SOI wafer is analyzed using a cantilever structure. Results show that under certain doping concentration conditions, the longitudinal piezoresistive coefficients of 〈1 1 0〉 crystal direction silicon decrease with temperature, while transverse piezoresistive coefficients are less affected by temperature. At 300 °C, Si 〈1 1 0〉 crystal direction has larger longitudinal and transverse piezoresistive coefficients, which make it suitable for high temperature piezoresistive pressure sensor production. The pressure sensor chip structure is simulated and analyzed using the finite element method. The pressure gauge chips are manufactured using MEMS techniques. The manufactured sensors are measured with an applied pressure from 0 to 6.0 MPa at 300 °C. The test results show that the sensitivity is approximately 30 mV/(mA MPa), the non-linearity is less than 1.5‰FS, and the repeatability is less than 0.3‰FS. This research shows that the SOI piezoresistive pressure sensor could reliably work at high temperatures up to 300 °C.

Introduction

For many years, piezoresistive silicon strain gauges have been widely used as the detection elements in pressure sensors. Their main characteristics include a high sensitivity, high mechanical stability due to their monocrystalline structure, small size, and low cost. Piezoresistive silicon strain gauges are integrated within the silicon diaphragm structure, but strain gauges are electrically isolated from the silicon diaphragm as shown schematically in Fig. 1. The reverse junction ensures electrical insulations between the strain gauges themselves and from the substrate. This insulation method has a limited operating temperature range of below 125 °C due to the higher leakage current of the PN junction [1]. The need for a semiconductor pressure sensor which can operate at temperatures above 125 °C is constantly on the rise across many industries. An ideal piezoresistive pressure sensor should combine single crystal silicon resistors with oxide isolation [2], [3], [4], [5]. The SOI piezoresistive pressure sensor cross-sections are shown schematically in Fig. 1. Due to its perfect insulation, the SOI solution is not affected by the traditional problem of rising leakage current at high temperatures, and allows piezoresistive pressure sensors with an operating temperature range up to 300 °C.

The fabrication of SOI wafers has proceeded along two main routes [6]. The first has been generically described as the bonded wafer approach. As the name implies, two wafers are bonded together via the thermally grown silicon oxide (SiO₂). Many fabrication and separation methodologies have been developed for the bonded wafer approach including macroscopic grinding or etching the back of the silicon wafer until the desired film thickness is achieved. The second route is the implantation of oxygen into the polished silicon wafer, explicitly denoted as the separation by implantation of oxygen into silicon (SIMOX) as shown schematically in Fig. 2. SIMOX techniques have been developed over 20 years for use in the production of microelectronic devices. Two main steps are needed to manufacture a SIMOX structure: a deep ion implantation of oxygen followed by a high temperature annealing. The dose used is 1.8 × 10¹⁸ O⁺ cm⁻², far higher than is conventionally used in ion implantation. Its implanted profile has a Gaussian distribution-like shape with a peak in the middle (at a certain distance from the surface depending on the implant energy) and tails on both sides. The implantation must be done at a temperature higher than 500 °C in order to avoid the amorphization of the substrate, which could lead to the formation of a polysilicon top layer after annealing. The other parameters, such as the total implanted dose, and the direction and energy of implantation are also important and require good control to obtain high quality SIMOX substrates. During high temperature annealing (>1300 °C), the oxygen originally located further away from the peak gradually gathers around the peak and reacts with the silicon to form oxide as well. As a result, a layer of silicon dioxide can be formed underneath a thin film silicon wafer with fewer defects. Thicknesses of 2350 Å for the silicon top layer and 3000 Å for the buried oxide can be achieved. In this paper, SOI wafers are fabricated using the SIMOX technique and now commercially available, are used to manufacture the novel high temperature pressure sensors.