Lateral etch study of thin films using hydrofluoric acid chemistries for MEMS devices

摘要

Microelectromechanical systems (MEMS) device processing requires an understanding of sacrificial layer release etch times. This is extremely important, especially when the sacrificial oxide film is etched under long narrow diaphragms and complex channel geometries. The characterization of the release time as a function of the etch rate and selectivities is necessary to optimize the device performance. In this study, three different oxide films (SiO_2, PSG, BPSG) were used to characterize the effects of lateral undercut etching under various diaphragm designs and channels. Etch length vs. time using 49% wt. HF concentration was obtained. Temperature dependence when etching the PSG film was also observed. Laser Scanning Microscopy (LSM) as a non-destructive optical measurement method was used to measure the lateral undercut distance in the different MEMS designs. Since sacrificial etching is based on chemical reactions and diffusion principles, a linear etch rate is not obtained. The relative doping concentrations and processing conditions of these thin films control the lateral etch characteristics. Optimized etch times for the selected thin films during sacrificial release of MEMS devices with different geometries can be calculated. Also, current and future theoretical modeling work on predicting release times can be optimized by using the experimental etch data in this report.