Dry micromachining for high aspect ratio microsensors and optical switches in silicon.

Abstract

Precise dry micromachining technologies have been developed in an electron cyclotron resonance (ECR) plasma source for the fabrication of high aspect ratio microstructures in polyimide, Ni, ceramic, and single crystalline Si with fast etch rate, high selectivity, vertical sidewall profile, and smooth surface morphology. These high aspect ratio elements are crucial for high performance micro-electromechanical systems. Various micromachined transducers were fabricated by etching with the ECR source. High aspect ratio polyimide was etched in an O$\sb2$ plasma for fabricating Ni microresonators and ceramic micromolds. Polyimide microstructures that were 32 $\mu$m thick and 1 $\mu$m wide were obtained. Interdigitated Ni comb fingers with aspect ratio $>$10:1 were also demonstrated by electroplating through the polyimide molds, whereas piezoelectric ceramic microstructures that were 9 $\mu$m tall and 4 $\mu$m wide were formed by micromolding. In addition, Si microstructures with $>$30:1 aspect ratio were etched using a Cl$\sb2$ plasma generated by the ECR source. A novel deep etch-shallow diffusion process was developed. It involved a first deep, high aspect ratio dry etching step, followed by a short B diffusion to fully convert the microstructures into p$\sp{++}$ Si, and then a second dry etching step to remove the heavily B doped Si layer at the bottom to separate adjacent elements. This process has been applied to fabricate Si microresonators with larger thicknesses ($>$50 $\mu$m) compared to those fabricated by the conventional deep B diffusion process. Optical switches consisting of arrays of vertical Si micromirrors were designed and fabricated by the deep etch-shallow diffusion process. These 50-$\mu$m-tall, released micromirrors were supported by 800 $\mu$m long, 3 $\mu$m wide folded beams and electrostatically actuated by 150 interdigitated comb fingers. Electrical measurements showed that a mirror displacement of 34 $\mu$m can be obtained by applying 30 V dc bias. At atmospheric pressure, the Si micromirrors showed a resonant frequency of 987 Hz. Therefore, the micromirror switches can be operated up to the resonant frequency and achieve large mirror displacement. At a light wavelength of 1.55 $\mu$m, the reflectivity of Au-coated vertical Si mirrors was 85%. Dynamic response of the Si micromirror switches showed that the switching function was successfully demonstrated at frequencies up to 1.2 kHz.