Description
Equipment control, process development and materials characterization for LPCVD poly-SiGe for MEMS applications are investigated in this work. In order to develop a repeatable process in an academic laboratory, equipment monitoring methods are implemented and new process gases are explored. With the dopant gas BCl3, the design-of-experiments technique is used to study the dependencies of deposition rate, resistivity, average residual stress, strain gradient and wet etch rate in hydrogen-peroxide. Structural layer requirements for general MEMS applications are met within the process temperature constraint imposed by CMOS electronics. However, the strain gradient required for inertial sensor applications is difficult to achieve with as-deposited films.
Approaches to reduce the strain gradient of LPCVD poly-SiGe are investigated. Correlation between the strain gradient and film microstructure is found using stress-depth profiling and cross-sectional TEM analysis. The effects of film deposition conditions on film microstructure are also determined. Boron-doped poly-SiGe films generally have vertically oriented grains -- either conical or columnar in shape. Films with conical grain structure have large strain gradient due to highly compressive stress in the lower (initially deposited) region of the film. Films with small strain gradient usually have columnar grain structure with low defect density. It is also found that the uniformity of films deposited in a batch LPCVD reactor can be improved by increasing the deposited film thickness, using a proper seeding layer, and/or depositing the film in multiple layers. The best strain gradient achieved in our academic research laboratory is 1.1 x 10^-6 mu-m^-1 for a ~3.5 mu-m thick film deposited at 410 degrees C in 8 hours, with a worst-case variation across a 150 mm-diameter wafer of 1.6 x 10^-5 mu-m^-1 and a worse-case variation across a load of twenty-five wafers of 7 x 10^-5 mu-m^-1. The effects of post-deposition annealing and argon implantation on mechanical properties are also studied. While the as-deposited film can achieve the desired mechanical properties, post-deposition processing at elevated temperatures can degrade the strain gradient.