Furuichi, T., Nagao, T., Yokura, H., Abe, R. et al., "Thin-Film Air Flow Sensors for Automotive using the MEMS Technologies," SAE Int. J. Passeng. Cars – Electron. Electr. Syst. 8(2):314-319, 2015, doi:10.4271/2015-01-0233.
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.