Study of the annealing effect of low-temperature oxide on the etch rate in TMAH solutions for micro-heater applications

Suspended structures are frequently used in some micro-electro-mechanical system (MEMS) devices. They are usually protected and supported by silicon oxide. Because the suspended structures are released from the silicon substrate in various etchants, the etch resistivity of the oxide layer is very important for suspended structure fabrication. In this paper, we improve the etch resistivity of low temperature chemical vapor deposition (CVD) silicon oxide layers by using annealing process to substitute thermal oxide grown in a high-temperature and high-cleanness furnace. We prepared thermal oxide and CVD oxide samples and then measured the etch rates after tetramethyl ammonium hydroxide (TMAH) anisotropic etching process and dry etch process. According to the experiment data, we found that the etch rates of CVD oxide were slightly decreased in dry etch and obviously decreased in TMAH solution. The etch rates of 600°C annealed TEOS based oxide and 600 °C annealed SiH4 based oxide in 25 wt.% TMAH at 80°C are 1.1 and 0.7 Å/min respectively. The etch resistivity of CVD oxide can compete the etch resistivity of thermal oxide when annealing temperature is up to 600°C. The stress characteristics of annealed oxide layer were also measured in this paper. The stress has a minimum value at 600°C annealing temperature. In our work, a single crystalline silicon heater was successfully completed by using annealed CVD oxide as passivation layer. The heater with a resistance of 200 ohms glowed as bias current is up to 38 mA. We also simulated and characterized the temperature distribution of the microheater. The ANSYS simulation shows the central temperature of the microheater is 1135°C as the bias power is 0.59 W.