Analysis of highly doping with boron from spin-on diffusing source

A large number of MEMS devices require thin diaphragms or cantilevers fabricated using bulk or surface micromachining. This paper presents a characterization of heavily doped p++ layers (well known as efficient etch-stop layers for anisotropic etching of Si in alkaline solution) processed using spin-on diffusants (SOD) sources. The doping profiles were simulated using TSUPREM4 and measured using SIMS. The stress induced in such layers-as resulted from our process-was 25 …40 MPa (tensile). The surface roughness was also investigated, and the measured average value Ra=30 nm (achieved after a diffusion of 5 h at 1150 °C) can be associated with the effect of the oxidation that took place simultaneously with the diffusion process. The paper presents the advantages and disadvantages offered by spin-on doping sources for diffusing heavily doped p++ layers.