A monolithic pressure-pH sensor for esophageal studies

This paper describes a single-chip sensor which has been developed to allow the diagnosis of esophageal abnormalities in the ambulatory patient. The chip consists of a capacitive pressure transducer and an ion-sensitive field-effect transistor (ISFET). The pressure transducer uses a boron etch-stopped diaphragm and a batch fabricated lead transfer. The pressure sensitivity is about 1000 ppm/mmHg with a temperature sensitivity of +30 ppm/°C. The ISFET has a pH sensitivity of about 50 mV/pH with a temperature coefficient of about 0.015 pH/°C. These temperature sensitivities are low enough that individual temperature trims of the transducers are eliminated in this application, even though temperature excursions of 30°C can occur simultaneously with pressure and pH changes during swallowing. The chip is compatible with use in a 3 mm OD catheter.