Fabrication of catheter-tip and sidewall miniature pressure sensors

Silicon-diaphragm miniature pressure sensors, which use the piezoresistive effect, were developed for biomedical applications. We fabricated two types of sensors; that is, a catheter-tip (1.2-mm outside diameter, 0.17 mm thick) and a sidewall (1.4 × 3.45 × 0.22 mm) sensor, both having a thin circular diaphragm. Their diaphragms, 10 µm in thickness and 0.55 mm in diameter, were formed by an electrochemical etching method. Since the stability of pressure sensors is the most important requirement for precise pressure measurements, attractive approaches have been investigated to improve stability. The major instabilities of the present miniature pressure sensors are electrical drifts caused by leakage currents and thermal disturbances related to packaging stress. A shield and a guard plate can prevent the device from leakage currents. A thick supporting rim structure of the sensor and mounting on a stainless steel support with elastic material contribute to eliminate the packaging stress. For the purpose of easy lead attachment to the catheter-tip sensor, we use a unique structure having deep contact holes in deposited thick polysilicon layer (0.05 mm thick). Experimental results are as follows: Initial drift after power up was improved to about one tenth. Thermal disturbances, as temperature zero shift, thermal transient response, and temperature cycle hysteresis were greatly reduced. Low-temperature zero shift of 0.2 mmHg/°C was obtained using a simple temperature compensation method. Long-term drift was 0.6 mmHg/day. The catheter of 1.8-mm outer diameter having two sidewall sensors has been satisfactorily used for the study of urodynamics.