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Inert thermal anneals were performed at various temperatures to determine annealing kinetics of dry thermally grown SiO2 films on Si. Two stages of relaxation are demonstrated. The film relaxes quickly to an intermediate level, and then progresses more slowly toward full relaxation. The relaxation times to attain the fully relaxed refractive index, 1.460, and full ≤3% swelling were found to fall below typical oxidation times at T≥1150 °C, in concurrence with the experimentally observed breakpoint in the refractive index versus growth temperature data. It is concluded that the linear viscoelastic model is sufficient to quantitatively explain the breakpoints in refractive index for both wet and dry thermally grown oxide.

This work contributes to the understanding of anisotropic etching of silicon, for microsystems technology, by studying Si/Al selectivity during anisotropic etching of silicon in tetra-methyl ammonium hydroxide (TMAH). By under-etch experiments using a wagon-wheel mask pattern, Si/Al selectivity is studied in relation to trends in etch anisotropy, etched surface morphology, and variations of under-etch behavior with mask-edge angle. TMAH at 5 wt % is used, with or without the additives: dissolved silicon and ammonium persulfate. High Si/Al selectivity is accompanied by obvious changes in the roughness, flatness, and etch rate of the {100} cavity bottoms, by large changes in anisotropy as seen in under-etch rate curves, by lower ratio of {101} to {100} etch rates, and by more regular 〈101〉-oriented steps on non-{111} cavity sidewalls. The conditions are consistent with a low rate of attack of 〈101〉-directed periodic bond chains in the Si lattice.

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

Oxide dilation in thin films is analyzed using a Voigt viscoelastic model. If stress-dependent viscosity is used to model the dilation, a logarithmic time evolution is predicted. The form of the solution is in agreement with the non-Maxwellian behavior seen in experimental data. The analysis provides an estimate of the critical stress and low-stress viscosity of dry SiO2 films.