COMPATIBILITY BETWEEN BIPOLAR READOUT ELECTRONICS AND MICROSTRUCTURES IN SILICON

Bulk micromachining with an electrochemical etch-stop is a well-established method for realizing membranes, beams and cantilevers in silicon. The use of an epitaxial layer for defining the depth of the passivation junction, and thus of the thickness of the micromechanical structure. strongly favours the use of bipolar components for the readout circuitry in an integrated mechanical smart silicon sensor. The mechanical properties of a micromachined structure, such as the resonance frequency and the sensitivity in an accelerometer, strongly depend on the thickness of the epitaxial layer. The performance of the integrated bipolar transistors, however, is also strongly affected by the width and doping concentration of the epitaxial layer. Both the micromechanical device and the active bipolar components need to operate properly in order to obtain integrated smart micromechanical sensors in silicon. Moreover, the influence of post-processing high-temperature steps on the performance of integrated bipolar components needs to be examined. In particular. the frequently applied thermal annealing in surface-micromachined structures for residual stress removal is of importance, but its effect on the operation of the active components is usually not considered or is taken for granted. These compatibility problems are addressed in this research. The effect of epitaxial thickness between 4 and 10-mu-m and impurity concentration between 10(14) and 10(17) cm-1 on the performance of integrated bipolar components is investigated using process modelling packages.