Test structures for ISFET chemical sensors

Simple test structures and measurements were studied for determining their usefulness for the online monitoring of the aging effects on pH ion sensitive field effect transistor (ISFET) chemical sensors used in automatic systems. Devices consisted of long drain and field transistors and metallic meanders. Current and capacitance measurements were carried out with standard instruments. Results showed that not all the test structures and their related parameters were equally sensitive, and in some cases tests at a high temperature were necessary to accelerate the desired effects. The metallic meander structure proved to be quite sensitive to epoxy-coating degradation. For dielectric passivating layers, the parasitic transistors also showed sensitivity, especially when electrolyte was heated