Applications of suspended active electrothermal circuits.

Abstract

In this work, we investigate applications of active electrothermal circuits thermally isolated by micromachining techniques. The electrothermal integrated circuits (ETC) devices are fabricated using a commercial CMOS process such as MOSIS 1.2 mum HP process and one more post CMOS etching step to form the thermally isolated n-well structure. The etch opening is patterned by superimposing active, contact, via and pad opening four layers down to the silicon surface during the CMOS process followed the silicon etch by the etching solution of TMAH with water. The TMAH solution contained enough dissolved silicon to prevent the attack of the exposed aluminum interconnection layers during the etch. By independently controlling the bias on the suspended n and p type substrate, TMAH undercuts the n-well from the front end exposed silicon area while preserving the n-well region from being attacked. Many processing details are included in this thesis work. Several ETC devices were fabricated using this method including a thermal infrared detector and a thermally stabilized oscillator. Since the devices are fabricated using a commercial CMOS process, the first research effort emphasizes the investigation of the thermal properties of passive and active devices fabricated using the MOSIS foundry process. We developed two different applications using suspended active electrothermal circuits. A new type of infrared detector based on the heat balancing of a PMOSFET between the incoming radiation and heat loss to the surrounding air was developed. The output from the sensing circuitry yields a responsivity of 10<super>6</super> V/W with a 0.8 muA operating current. The measured detectivity is D*=3x107cm HzW-1 at 30 Hz. The tested responsivity is one order of magnitude higher than the reported performance from other passive uncooled VO₂ microbolometers. However, the noise generated by the PMOSFET reduces the detectivity. It has also been demonstrated that a higher D* of 108cm HzW-1 can be achieved with this CMOS technology imager. Using this infrared detector, an 8 x 8 infrared imager was designed, fabricated and tested. The size of this 8 x 8 imager is 1.3mm x 1.5mm. (Abstract shortened by UMI.) The second application is a 60 MHz thermal stable ring oscillator. Due to the limitation of the technology to all the devices inside the n-well, this ring oscillator is built entirely with PMOSFETs. Without the temperature regulation, the frequency temperature coefficient is 2290 ppm/&deg;C. The frequency temperature coefficient was reduced to 591 ppm/&deg;C by the introduction of the heating balancing heater.