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.