Stability polysilicon layers

The main goal of another microhotplate design was the replacement of all CMOS-metal elements within the heated area by materials featuring a better temperature stability. This was accomplished by introducing a novel polysilicon heater layout and a Pt temperature sensor (Sect. 4.3). The Pt-elements had to be passivated for protection and electrical insulation, so that a local deposition of a silicon-nitride passivation through a mask was performed. This silicon-nitride layer also can be varied in its thickness and with regard to its stress characteristics (compressive or tensile). This hotplate allowed for reaching operation temperatures up to 500 °C and it showed a thermal resistance of 7.6 °C/mW. 

Microstructure stabilization. Before complete removal of the sacrificial oxide, small cavities are etched around and under the mechanical polysilicon and down to the ground-plane polysilicon below the sacrificial oxide layer. These cavities are then backfilled with photoresist, resulting in pillars that support the polysilicon. In a subsequent masking operation, strips of photoresist are placed across the micromachined elements. This results in a web of photoresist material that holds the polysilicon elements in place after complete removal of the sacrificial oxide layer. Subsequent etching of the sacrificial oxide with a buffered... 

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