Novel embedded capacitive sensors for monitoring the cure of polyimide in multichip modules

Abstract

Efficient and cost-effective manufacturing techniques can be aided by built-in microsensors. In order to monitor the curing of polyimide, an important component of multichip modules two types of embedded capacitive sensors have been developed. One is composed of parallel plates, the other is formed by interdigitated electrodes. The capacitors fabricated on multiple UltradelR 4212 polyimide layers probe the dielectric constant and thickness of the polyimide. Capacitance and loss factor of the sensors are measured after curing the polyinide over the temperature range 100–350 °C, as a function of frequency from 100 kHz to 1 MHz. The interdigitated capacitance decreases during the curing cycle to a stable value indicating that the cure has taken place. Curing time-temperature measurements indicate that a temperature is critical for complete curing. The dielectric constant is calculated from the measured interdigitated capacitance using Schwartz–Christoffel transformation. The film thickness is determined from the parallel plate capacitance. The interdigitated capacitance increases by 4.5% due to humidity, however, when baked at 350 °C the capacitance returns to its initial value. Interdigitated capacitance changes by 10.8% with extra polyimide coating layers.