<title>Diffusion-induced dislocations in highly boron-doped silicon layers used for bulk micromachining applications</title>

Abstract

Boron-doped silicon layers with sufficiently high doping levels become effective stop-layers during the chemical etching of silicon in alkaline type solutions (KOH, NaOH, LiOH) or in EDP (ethylene-diamine-pyrocatechol). An advantageous chemical solution consisting in tetramethyl ammonium hydroxide (TMAH) with isopropyl alcohol (IPA), showing similar etching properties was also proposed. The property as a stop layer of the boron-doped silicon is currently used as the most convenient etch-stop technique, because it is easy to define the thickness of the structure by the depth of boron diffusion in silicon. However, the boron diffusion profile in silicon is not a step-like distribution, but it presents a continuous decrease of the concentration from the silicon surface to the bulk, that depending on the diffusion conditions, i.e. diffusion time, temperature and doping technique. It is therefore expected that such a decrease will result to a continuous variation of the etching rate and a consequently variation of the etching time with the diffusion depth. In this paper we present firstly the doping properties of the silicon layers doped by the termo-chemical method using chemical sources. It is shown that the doping properties vary within the boron-doped layers. Boron diffusion profiles determined by SIMS method and electrical method are presented in order show the specific behavior of the concentration distribution in the silicon bulk. Misfit dislocations are induced by the boron diffusion in silicon at high concentrations. The conditions of the generation of the misfit dislocations in the boron-doped layers depends on the processing conditions, especially on the diffusion time and temperature. We show that the density distribution of the misfit dislocations in the silicon bulk is not uniform after the boron prediffusion and diffusion processes. From the point of view of the micromechanical applications, the inhomogeneity of the structural and doping properties of the silicon layer can influence the stress properties of such silicon-doped layers. Therefore, in order to reduce the stress gradient in the silicon membranes and micromechanical elements, it is necessary to obtain layers with uniform material properties. Both the doping and structural properties of the boron doped layers are to be therefore better knowledged and controlled. However, the doping properties obtained after the boron doping by termo-chemical method or by implantation doping technique cannot provide uniformly doped silicon layers. Therefore, a careful chemical etching during the self-limitation process of the boron-doped silicon layers offers such a possibility, as it will be presented in the paper. In order to eliminate from the silicon doped layers the regions were the properties of the silicon layers are not uniform, it is necessary to control the chemical etching process which is the next important step in the bulk micromachining technology useful to prepare the micromechanical elements. These key parameters of the chemical etching process are the chemical etching rate and the chemical etching time. It is shown that it is possible to calculate the chemical etching rate and the chemical etching time for some specified etching conditions. Such a possibility allows to control the thickness of the micromechanical elements and to eliminate the stress gradient induced by the non-uniform doping and by the misfit dislocations in the silicon micromechanical elements.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.