High-aspect-ratio Si etching for microsensor fabrication

Abstract

Etching of high-aspect-ratio Si structures for microsensors was carried out in a Cl2 plasma generated by a multipolar electron cyclotron resonance (ECR) source. Si etch rate is found to increase with microwave and rf power, and it is the highest at 1 mTorr over the range of 0.5–50 mTorr. Optical emission spectroscopy (OES) and mass spectrometry (MS) were used to monitor the etch process to provide precise control for the sensor fabrication. The Si emission intensity at 288.1 nm and the mass spectrometric signal for 63SiCl+ follow the Si etch rate directly. The selectivity of etch masks decreases at higher rf power and Ni has higher selectivity than SiO2. Si etch rate is reduced for structures with smaller linewidth, but this microloading effect decreases at lower pressure. For the etch conditions used, the Si etch rate remains almost constant when flow rate is changed, but the optical emission and mass spectrometric signals of the etch products are found to decrease with increasing flow. Endpoint detection is applied to monitor etching of the resonators. Both OES and MS provide a clear indication when the polycrystalline Si is etched down to the oxide layer. With a Cl2 plasma at 3 mTorr, 100 W microwave power, 100 W rf power, and 8 cm below the ECR source, high-aspect-ratio microstructures in Si with vertical profile and smooth morphology were etched. These sensing elements are 28 μm deep with 1.5 μm gap, providing a high aspect ratio of 19 in Si. The width of the elements was reduced by oxidation followed by wet etching of the dry etched microstructures to provide better sensitivity for the microsensors.