砷化銦鎵元件覆晶封裝於低成本有機基板(RO 3210)之W-band應用

Abstract

本研究探討了利用低成本RO 3210有機基板作為mHEMT高頻覆晶封裝之可能性。本論文利用Au-to-Au熱壓法將被動與主動元件 (150 nm 砷化銦鎵mHEMT) ，覆晶封裝於RO 3210有機基板。覆晶接合搭配最佳化的補償設計，封裝後被動元件在高達110 GHz的頻段下，展現出低於15 dB的反射損失(return loss)以及低於1 dB的介入損失(insertion loss)。此最佳化的補償設計其後應用在mHEMT元件上。覆晶封裝前後，mHEMT元件在DC特性呈現出非常小的差異。在射頻特性量測中，由於覆晶封裝結構對於元件所造成的熱應力，使其介入增益(insertion gain)降低大約4 dB。為了改善封裝後元件的特性，我們使用低溫接合封裝來降低其熱應力；經過低溫封裝後的元件，在最高截止頻率高達110 GHz，封裝前後元件的介入增益僅僅少1 dB的差異。除此之外，我們也填入了底膠來改善覆晶封裝接合的可靠度，由於底膠樹脂相較於空氣，具有較高的介電常數(dielectric constant)與介電損耗(loss tangent)，因此元件的高頻特性會有所衰減。為了改善底膠造成的缺點，我們利用電磁模擬(EM simulation)來調整CPW模組上G-S(ground-signal)的間距，將最佳化的設計模組應用在被動元件上，底膠封裝後的元件，在高達70 GHz頻段以上，呈現出低於20 dB的反射損失，使其在W-band應用中能有良好的特性。另一方面，我們也利用上述最佳化的設計套用在主動元件上，在經過底膠封填後，mHEMT在高達110 GHz下，其介入增益僅呈現低於1 dB的差距。為了比較有機基板與商用氧化鋁基板的差異，我們也將mHEMT覆晶封裝於氧化鋁基板，並且量測其特性表現，其結果呈現出相當小的差距。由此可知，覆晶封裝於RO 3210有機基板，在W-band應用中展現出良好的高頻特性，而其成本則遠低於氧化鋁基板。此研究顯示RO 3210是高至W-band應用中，低成本覆晶封裝基板的良好選擇。

This study evaluates the feasibility of assembly mHEMT device using flip-chip technology on low cost Rogers RO3210 organic substrate for high speed and wireless communication applications up to W-band. The passive and active (In0.6GaAs0.4 mHEMT with 150 nm gate length) devices were flip-chip assembled on RO3210 organic substrate by Au-to-Au thermal compression bonding method. The flip-chip interconnect with optimized compensation design showed excellent performances with the return loss below 15 dB and the insertion loss less than 1 dB up to 110 GHz. This optimized interconnect structure was then used for mHEMT device packaging. The DC characteristics of the packaged device showed slight degradation after flip-chip bonding. For the RF characteristics, the measured insertion gain decayed about 4 dB due to thermomechanical stress generated from the chip bending in the flip-chip structure. In order to improve the package performance, low temperature bonding was introduced to reduce the stress and the bending of chip. After low temperature flip-chip bonding, the measured insertion gain of the packaged device decayed less than 1 dB up to 110 GHz as compared with the bare die device performance. In addition, an epoxy underfill was applied to improve the interconnect reliability. Due to its higher dielectric constant and loss tangent as compared to the air, underfill usually degrade the interconnect performance. In order to minimize the degradation, the EM simulation was employed for pre-matching design by modifying the gap size (ground-to-signal) of the CPW chip. With the optimized matching design, the flip-chip package with underfill exhibited less than 20 dB return loss above 70 GHz, which is excellent for W-band applications. For the active (mHEMT) device demonstration, the underfill effect was also taken into consideration for the pre-matching design. With the improved design, after underfill application, the mHEMT device showed less than 1 dB degradation (insertion gain) up to 110 GHz. For comparison, the demonstration of the mHEMT device on commercial alumina substrate is also presented. As compared to the alumina substrate, RO 3210 showed fair performance with much lower cost, showing its potential for low cost packaging application up to W-band frequencies.