AN INTEGRATED SENSOR FOR INVASIVE BLOOD-VELOCITY MEASUREMENT

Invasive measurements of physical parameters in blood are very important for the intensive care. The integrated flow sensor presented here has been developed as a part of an 'intelligent catheter' for measuring certain blood parameters. The sensor is based on the principle of hot-film anemometry. The temperature difference on a chip is measured by two diodes. One diode is heated by means of a polysilicon resistor in close proximity. This diode is (for thermal insulation) placed on a 5 mum x 260 mum x 260 mum silicon membrane. For the membrane formation the implanted oxide layer of SIMOX substrates (Separation by Implanted Oxygen) was used as a stop for backside etching. This technique allows batch processing and is compatible with a standard CMOS process. In addition, electronic CMOS devices can be mounted directly into the membrane. Switched capacitor circuits are used for sensor read-out electronics. The differential voltage at the diodes is amplified by means of a two-stage operational amplifier with an overall amplification of 238. The total chip size amounts to 1 mm x 5 mm. A change of flow velocity from 0 to 80 cm/s results in a change of the pulse width of 20 mus for a maximal chip temperature of 13 K relative to the ambient.