Development of a Low-Power 700°C Micro heater in Low Temperature Cofire Ceramics

摘要

Ceramic-based gas sensors, which typically operate in the range from 400°C - 700°C, are found in a wide range of applications. A low temperature cofire ceramic (LTCC) low power micro heater was developed which operates at 700°C to meet the heating requirements for these sensors. The thermal impedance performance (and therefore the power consumption) has been optimized to allow for high temperature heater operation with the lowest possible power consumption. To minimize power, the overall thermal impedance from the heater to ambient must be maximized. The conduction impedance can be maximized by suspending the heater on narrow arms, increasing the conduction thermal impedance to 2000°C/W. The loss for convection would be proportional to the size of the sensor, which is minimized while still maintaining sensitivity and manufacturability of the heater. In most microelectronics applications, radiation is ignored; however, it is the dominant factor for the micro heater. Radiation shielding, however, offered a unique solution to this problem. Large area thin cavities have been fabricated by the incorporation of a polymer sheet during lamination, which burns out during the firing cycle. Multiple level cavities, typically 75 microns in thickness, have been demonstrated. Since the LTCC is hermetic, this cavity is actually in the partial atmosphere, minimizing many convective couplings between the layers. A series of parallel thin plates, usually between 4 to5 plates, will allow 100% of the radiation to be back reflected so that the surface of the last shield is at room temperature, although the first shield is near the temperature of the heater. Thus, all the radiation loss is recaptured and if the thermal conduction of the shields is optimized, radiation losses will be minimized.