Ophthalmic tonometer with silicon micromachined structure

An electromechanical transducer with silicon micromachined die is described. The transducer is designed according to the Mackay-Marg principle of intraocular pressure (IOP) measurement. The transducer contains a 1 mm diameteer SS 316 central movable plunger attached to the silicon die of a piezoresistive pressure sensor for the 100 kPa pressure region. The 2 mm x 2 mm x 0.38 mm die has a 1 mm x 1 mm x 0.015 mm membrane with four piezoresistors in a Wheatstone bridge configuration. The plunger and die are packaged to the SS 316 body by means of standard IC mounting techniques. The transducer shows good static parameters with an output signal typically up to 40 mV of FSO for 60 mmHg of IOP. A very small deflection area of the cornea is obtained. The temperature characteristics are equal to a temperature coefficient of sensitivity (TCS) of -0.21% degrees C-1 per FSO. The dynamic output signal of the transducer versus time has a typical shape, well known from the Mackay-Marg description. An electronic system of the tonometer analyses the output dynamic, identifies the correct measurements, counts the IOP value, mean value and standard deviation of the measurement series.