Microstructure release and test techniques for high-temperature micro hotplate

Abstract

We report several practical issues in the fabrication and high-temperature operation of micro suspended heating structures compatible with standard CMOS technology. Suspended microstructures are fabricated in a standard CMOS process and are released by post-process silicon etching. TMAH at 25wt% with 15~01% of IPA is found to greatly increase yield by reducing mechanical disturbances during etching. Electro-thermal properties of the polysilicon are investigated during high-temperature operation. Significant thermally -induced negative drift in resistance at high temperatures is found, and the impact on temperature control is discussed. Thermal isolation is found to be about SOWmW, and reliable operation is observed near 1000°C. INTRODUCTIONThis work concerns the fabrication of suspended microstructures in standard CMOS technology and micromachined post-processing. The microstructures are designed for micro-hot-plate-(MHP)-based thermal sensor applications, with operating temperatures of 500°C and higher. Within the context of this endeavor, this paper reports on two specific technical aspects of releasing and testing the suspended microstructures. (a) The microstructures are composed of layers of Si02 and SiN, with embedded polysilicon thermoresistors. This type of structure has been studied previously by substantial numbers of researchers and laboratories, [ 1-61, The release of such structures by post-process etching of the silicon substrate has been the subject of several investigations [ 1,7,8], where the mechanical behaviour of the microstructure during release is analyzed and/or controlled in some way, to avoid catastrophic failure of the device by mechanisms such as cracking of the structure, either during or after the release etch. In a similar vein, in this paper, we report on adding IPA to the TMAH etchant solution, and its impact on the cracking behaviour of the microstructures. (b) The second aspect that we consider is that of the behaviour of polysilicon thermoresistors during hightemperature operation. The polysilicon resistivity is

Reliability of freestanding polysilicon microheaters to be used as igniters in solid propellant microthrusters

“…The decrease of the resistance could be caused by the recrystallisation of polysilicon at high temperatures, as it has been described in the literature [21,25]. A drift of the electrical power after a few cycles above 40 mW/beam was observed (Fig.…”

High-temperature gas sensor using perovskite thin films on a suspended microheater

“…Under these conditions of high-temperature operation, the polysilicon heater material is not stable enough 9 to be used simultaneously as both a heater and a temperature sensor. Accordingly, we used an additional temperature sensor, and also a polysilicon resistor, located on one of the supporting arms closer to the substrate at a lower temperature, which can be used for temperature control.…”

“…It was found that at temperatures below a certain threshold, estimated to be between 400 and 500°C, poly-1 was stable enough. 9 Since it is expected that the heater resistive load will vary during the operation, a power supply integrated circuit delivering constant power is used in order to keep the temperature of the microhotplate constant. The power supply, based on an analog multiplier with feedback stages, delivers constant power within Ϯ1% when the microheater resistance changes within Ϯ50%, up to 40 mW on heater loads in the range 1-10 k⍀.…”

“…8. Effective use of MHPs at these unusually high temperatures raises practical issues 9 especially since, in order to allow for integration of an array of devices with powering and control circuitry on a single chip, implementation in a host standard process is desirable. Among others, the design must avoid the use of Al on the suspended MHP membrane.…”

Microhotplates with TiN heaters

“…The most obvious material that was first used as heater was polysilicon (polySi) [1,15,16]. Poly-Si is useful up to 550 • C, above which its resistivity becomes unstable [17,18]. Lately, heaters based on monocrystalline silicon were developed for CMOS hotplates but these MOSFET type heaters still suffer of a lack of stability at high temperature [19,20].…”