An 11.6aF/kPa Mechanical Stress Sensor With 0.808% Temperature-Drift Oscillator for Flip-Chip Packaging

During the package processes, including chip attachment and encapsulation, the stress caused by the thermal and mechanical load is usually generated in integrated circuit chips. This brief developed a mechanical stress sensor in a standard 180nm CMOS process for runtime stress measurement. The contributions done in this brief are summarized as follows: 1) A temperature-compensation RC oscillator circuit converts the warpage of the die induced by the mechanical stress into the frequency output. It even can measure the thermally induced stresses with a wide temperature range. The oscillator has a frequency variation of -0.808% from temperature stability measurements from -40 degrees C to 120 degrees C. 2) Rather than the complicated calibrations and advanced measurement equipment, the frequency can be easily measured with the chip area less than 0.569 mm(2) and the power consumption less than 22.9 mu W. 3) As for the sensing array, the sensitivity and stability of the metal-insulator-metal (MIM) and metal-oxide-metal (MOM) capacitors are verified. The measured average sensitivity of the test chip using a MIM capacitor with 0.1736 I3z/kPa is more sensitive than that using a MOM capacitor with 0.102 I3z/kPa. The linear fit curves of the output frequency have the coefficient of determi-nation (R-2) of 0.9983 for MIM and 0.9959 for MOM, indicating an excellent linear response of the sensor circuit.

Automated summary

This study developed a mechanical stress sensor for runtime stress measurement during the package processes of integrated circuit chips. The sensor, fabricated in a standard 180nm CMOS process, utilizes a temperature-compensation RC oscillator circuit to convert the mechanical stress-induced die warpage into frequency output, enabling measurement of thermally induced stresses over a wide temperature range. Unlike complicated calibrations and advanced measurement equipment, the frequency can be easily measured with a small chip area (<0.569 mm(2)) and low power consumption (<22.9 mu W). The sensitivity and stability of metal-insulator-metal (MIM) and metal-oxide-metal (MOM) capacitors in the sensing array are verified, with MIM exhibiting higher sensitivity and excellent linear response of the sensor circuit.