MicroElectroMechanical Systems (MEMS) is a rapidly developing technology for electronic devices where small sensors and actuators can be manufactured by conventional silicon microelectronics processing. Microdevices such as accelerometers, pressure sensors, optical sensors, and flow sensors are widely used in the areas of biomedical, automobile, and communication industries. The major advantages of using MEMS technology are small size, light weight, fast response, high precision, and low cost.

Manufacturing of MEMS involves micromachining of silicon substrates to form anisotropic trenches 10 μm to 600 μm deep into the wafer. The etching process for MEMS is significantly different from conventional semiconductor applications, as deep etching is required with high etching rate, high anisotropy, and high selectivity with respect to the mask. Because of these requirements, metallic masks that have high selectivity are predominantly employed for MEMS fabrication. However, metallic masks generate unwanted metallic etching products which impedes further etching, due to their nonvolatility.

We investigated the potential of silicon-based masks such as SiO₂ and Si₃N₄ that do not generate non-volatile products. The etch system used in this study was the Applied Materials P5000 Mark II Reactive Ion Etcher. All the etching processes were carried out at room temperature and at a frequency of 13.5MHz. The etching gas was a mixture of CHF₃/CF₄ / Ar for mask etching, and a mixture of HBr/SiF₄/NF₃/He-O₂ for silicon trench etching.

Figure 1 shows the trench profiles using 3 μm thick SiO₂ and Si₃N₄ masks. After etching a 40 μm deep trench, the anisotropy of the SiO₂ masked sample was found to be very high at above 0.96. On the other hand, tapered trench profiles were obtained for the Si₃N₄masked samples even with both a 3 μm thick mask, and a 1 mm thick mask, and shows that tapering is not due to inadequate thickness of mask material. The etching selectivity of silicon with respect to the SiO₂ mask was about 120. The high selectivity was achieved by addition of oxygen that helped to lower the etch rate of the SiO₂ mask. The selectivity of silicon to the Si₃N₄ mask was only 18. That is, Si₃N₄ was more chemically attacked by HBr/SiF₄/NF₃, than SiO₂. This is also indicated by the rounder shape of the Si₃N₄ mask as compared to that of the SiO₂ mask (on masked wafer). Consequently, the lower selectivity for the Si₃N₄ mask resulted in less anisotropic trench profiles than for the SiO₂ mask.

Figure 1: Scanning Electron Micrographs after 30 minutes of etching.

Figure 2 shows their corresponding variation of etch rates with time for the 3 μm thick masks. As the trenches become deeper, the differences in trench depths of the 2μm and 7μm wide trenches become more obvious due to Reactive Ion Etching (RIE) lag.

Figure 2: Etch rates using SiO2 and Si3N4 masks as a function of time.

Although oxygen, which is available either from the etching gas or mask material, can react with the unsaturated etching products and affect sidewall passivation layer buildup, it is found that a small amount of oxygen is required to ensure formation of anisotropic trenches. Without sufficient oxygen, non-volatile etching products accumulate on the silicon surface and lead to micrograss formation during etching. The oxygen present in the etching gases can also increase the etching selectivity of the substrate silicon with respect to the SiO₂ mask during the etching process, which also ensures anisotropic trenches.

This work was done with great help of undergraduate student Koh Say Yong and postgraduate student Kok Kitt Wai.