In this work, a novel gas sensing platform using the TSMC 0.35μm CMOS-MEMS process was developed. Three-dimensional interdigitated sensing electrodes (3D IDEs) with a polysilicon microheater and a polysilicon thermometer were integrated in this CMOS-based platform. Compared to conventional 2D IDEs, our 3D IDEs not only extended the sensing surface area to the vertical sidewalls but also decreased the gap (the inter metal dielectric layer, IMD, thickness) between electrodes. The micoheater of 2.6kΩ resistance and the thermometer of 2kΩ resistance were designed to provide on-chip heating, which could facilitate the deposition and/or activation of the sensing material. The sensing material, monolayer protected gold nanocluster (AuC8), was coated onto the electrodes through the air brush spraying. The sensor performance was demonstrated with three compounds (Octane, Butanol, and Toluene) of concentrations in the range of 2000ppm to 5000ppm and manifested the good linearity and sensitivity.

 

The backend of CMOS processes for interconnects were utilized to provide microstructures which offer many potential advantages for sensors including low power consumption, low fabrication cost, high sensitivity and reliability. In the CMOS etching process, the design rules of a released (RLS) mask limit the minimum gap between electrodes to 3μm. Hence, 3D IDEs were used to decrease the gap between electrodes to 1μm (the IMD thickness) so as to increase the sensitivity of the designed sensor as well as to lower the resistance of the deposited sensing material. In addition, the consumption of the sensing material was reduced significantly.

 

When exposing to Toluene at different concentrations, the transient responses of the sensor were changed accordingly. The exceptional linearity of the sensor responses on targeted compounds at high concentrations was demonstrated. The great sensitivities, defined as the ratio of the impedance before and after exposing to the target gas, of the three compounds were obtained (Toluene: 3.66E-5/ppm, Octane: 3.30E-5/ppm, and Butanol: 5.71E-5/ppm). The differences in sensitivities are largely affected by the target gas and its affinity to AuC8 surface. These variations in sensitivity for different compounds can enhance the specificity of our CMOS-based gas sensor platform.