Flip-chip Packaging Structure of HEMTs Devices for Millimeter-wave Applications
Chang, Edward Yi
In recent years, the rapid development for wireless communication and imaging system has pushed the operational frequency for wireless communication to millimeter-wave or sub millimeter-wave bands. These frequency bands have several advantages, including wide transmission bandwidth, high transmission speed, and high signal resolution. In order to realize the targeted applications, the packaging technology plays an important role to provide the transmission path from chip to substrate, the heat dissipation, environment protection. In the chip-level packaging, the flip-chip is the more promising approach for millimeter-wave applications in comparison with conventional wire bonding. The advantages of the flip-chip interconnection are short path to reduce the parasitic effect and compact product size. This dissertation presents the study on the flip-chip packaging structure of HEMT devices for millimeter-wave applications. The flip-chip packaged In0.7Ga0.3As MHEMT device was firstly demonstrated on Al2O3 substrate. By adopting the optimized design for the flip-chip transition, the packaged device shows almost similar RF performance as the bare die up to 60 GHz. In addition, a two-stage gain block at 60 GHz was designed and fabricated using the microwave integrated circuit (MIC) approach to demonstrate the applicability for V-band applications. The MHEMT device was flip-chip packaged on Al2O3 substrate with the matching circuit. The gain block exhibited a small signal gain of 9 dB at 60 GHz, indicating the feasibility of MIC approach for millimeter-wave applications. The thermal stress induced by the coefficient of thermal expansion (CTE) mismatch between the chip and the substrate can distort the flip-chip structure. BCB material was used as the underfill to improve the reliability and mechanical property of the flip-chip structure. The good dielectric property of BCB exhibited better RF characteristics up to 100 GHz as compared to the conventional epoxy-based underfill. From the results of thermal cycling test and shear force test, BCB underfill can effectively improve the reliability of the flip-chip structure. The flip-chip on board (FCOB) technology bypassed the chip-level package to achieve cost-effective millimeter-wave package. The RO 3210 polymer substrate was the promising substrate because its dielectric property is similar to Al2O3 substrate. The packaged device with the optimal flip-chip structure exhibited good RF results up to W-band. An exopy-based underfill was applied to improve the reliability without the characteristic degradation of the device. Analytical results revealed that the proposed packaging structure maintained a low minimum noise figure of 3 dB with 6 dB of associated gain at 62 GHz. In addition, the impact of bonding temperature on the device performance was also investigated. The degradation in RF performance was observed at higher bonding temperature. The reason of the degradation was mainly due to the mismatch in the CTE between the GaAs chip and the polymer substrate. From the equivalent circuit extraction from S-parameter measurements, the higher parasitic values occurred during the higher bonding temperature. The applicable bonding condition could reduce the thermal stress without degrading the RF performance.
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