Biomedical pressure sensor using buried piezoresistors

The improvement of the base line stability of a silicon diaphragm piezoresistive pressure sensor is described. When a miniature pressure sensor mounted on a catheter tube is used for biomedical instrumentation, its surface is subject to ionic contamination, which causes electrical instabilities in the form of base line drift. The resistance modulation, which is attributed to the field effect of outer impurity ions, is proved experimentally to be the dominant mechanism of the drift. To eliminate the electrical drift, it is necessary to keep the resistance of piezoresistors constant, even if undesirable impurity ions exist. For this reason, a novel buried piezoresistor structure is proposed and studied experimentally. p⁺ piezoresistors were formed on a thin n^- diaphragm 10 μm thich and were then buried under an n⁺ diffusion layer. This buried piezoresistor sensor gives an excellent stability (1.5 mmHg/day) in every atmosphere, including wet air.