CTE (coefficient of thermal expansion) mismatch between two wafers has potential for brittle failure when large areas are bonded on top of one another (wafer to wafer or wafer to die bonds). To address this type of failure, we proposed patterning a polymer around metallic interconnects. For this project, utilized benzo cyclobutene (BCB) to form the bond and accommodate stress. For the metal interconnects, we used indium. To determine the benefits of utilizing BCB, mechanical shear testing of die bonding with just BCB were compared to die bonded just with oxide. These tests demonstrated that BCB, when cured for only 30 minutes and bonded at 200°C, the BCB was able to withstand shear forces similar to oxide. Furthermore, when the BCB did fail, it experienced a more ductile failure, allowing the silicon to crack, rather than shatter. To demonstrate the feasibility of using BCB between indium interconnects, wafers were pattered with layers of BCB with vias for indium or ENEPIG (electroless nickel, electroless palladium, immersion gold). Subsequently, these wafers were pattered with a variety of indium or ENEPIG interconnect pitches, diameters, and heights. These dies were bonded under a variety of conditions, and those that held a bond, were cross-sectioned and imaged. Images revealed that certain bonding conditions allow for interconnects and BCB to achieve a void-less bond and thus demonstrate that utilizing polymers in place of oxide is a feasible way to reduce CTE stress.
The Defense Advanced Research Project Agency has identified a need for low-standby-power systems which react to physical environmental signals in the form of an electrical wakeup signal. To address this need, we design piezoelectric aluminum nitride based microelectromechanical resonant accelerometers that couple with a near-zero power, complementary metal-oxide-semiconductor application specific integrated circuit. The piezoelectric accelerometer operates near resonance to form a passive mechanical filter of the vibration spectrum that targets a specific frequency signature. Resonant vibration sensitivities as large as 490 V/g (in air) are obtained at frequencies as low as 43 Hz. The integrated circuit operates in the subthreshold regime employing current starvation to minimize power consumption. Two accelerometers are coupled with the circuit to form the wakeup system which requires only 5.25 nW before wakeup and 6.75 nW after wakeup. The system is shown to wake up to a generator signal and reject confusers in the form of other vehicles and background noise.
The defense community desires low-power sensors deployed around critical assets for intrusion detection. A piezoelectric microelectromechanical accelerometer is coupled with a complementary metal-oxide-semiconductor comparator to create a near-zero power wakeup system. The accelerometer is designed to operate at resonance and employs aluminum nitride for piezoelectric transduction. At a target frequency of 160 Hz, the accelerometer achieves sensitivities as large as 26 V/g. The system is shown to require only 5.4 nW of power before and after latching. The combined system is shown to wake up to a target frequency signature of a generator while rejecting background noise as well as non-target frequency signatures.