Bipolar-compatible epitaxial poly for smart sensors: stress minimization and applications

This paper presents the optimization of the fabrication process for bipolar-compatible epipoly for micromachining applications. The use of an epitaxial reactor to grow polysilicon enables the growth of monocrystalline silicon (for bipolar electronics) and polysilicon on top of oxide (for MEMS) in a single deposition step. However, after bipolar processing the early structures showed compressive strain in the epipoly layer, which then required careful MEMS design. The cause of this compressive strain is shown to be the oxidation steps in the bipolar process, The occurrence of this strain can be explained by the presence of oxygen in the epipoly. An alternative processing technique, where the epipoly is doped using implantation and shielded from oxidation by a nitride layer during further bipolar processing, yields epipoly layers without compressive strain. The full thermal budget of the bipolar process is used to diffuse and activate the implanted epipoly dopant. Functional thermal and electrostatic sensor and actuator structures have been fabricated to demonstrate the feasibility of this process. (C) 1997 Elsevier Science S.A.