Thin-Film Air Flow Sensors for Automotive using the MEMS Technologies

This paper presents two newly developed technologies of optimizing impurity diffusion concentration for silicon semiconductor material and controlling internal stress of the top SiN (Silicon Nitride) layer on a membrane of a silicon substrate to apply them to the manufacturing process of MEMS (Micro Electro Mechanical Systems) type air-flow sensor chips. Until today, in MEMS-type airflow sensors, poly-crystalline silicon (poly-Si) and platinum were widely used as a resistor material of key functional elements on a membrane of air-flow-rate measurement portion. The functional resistors on the membrane are required to monitor high temperatures of about 300 °C and to perform the self-heating operations at that temperature range because of the suppression of contaminant deposition by means of evaporation or incineration. However, the use of those resistor materials at such high-temperatures is very difficult because high-temperature use causes the problems of the sensing error expansion, the resistor material layer delamination etc. Therefore, we have developed a new MEMS-type air-flow sensor with high-temperature usable resistors which are formed in a single-crystal-semiconductor silicon of SOI (Single-crystal Si on insulator) substrate by applying a technique to optimize impurity diffusion concentration. In addition to the high-temperature use problem above, MEMS-type air-flow sensors have another problem that the membrane is fragile towards internal and external stress due to its extremely thin thickness. To solve this problem, we have developed a new technique to control the internal stress of the SiN layer of the membrane top surface using the LP-CVD (Low Pressure Chemical Vapor Deposition) method. Consequently, we have realized a new MEMS-type air-flow sensor with structurally strong membrane by making the SiN layer thicker.