毫微米波段覆晶封裝技術之研究與應用

Abstract

隨著近年來無線通訊市場的蓬勃發展，通訊產品的需求量大增，無線通訊系統之操作頻率也慢慢地提升到毫微米波段（Millimeter-Ｗaves）。發展毫微米無線通訊系統除了需要適當的積體電路元件外，低成本與高性能的封裝技術也十分重要。為了達成商品化的目標，低功率消耗、低成本、小尺寸及輕量化的封裝技術在微波方面的應用慢慢地變成一個不可或缺的角色。然而，當元件操作在毫微米波段時，目前的封裝結構將會伴隨著許多寄生效應。因此為了維持良好的高頻電性，晶片與封裝基板之間的轉接必須謹慎地考量與設計。打線技術（Wire-Bonding）為傳統常用的封裝技術，但會伴隨著很強的寄生電感效應，此效應可能會嚴重衰減封裝後微波元件之特性。 覆晶封裝（Flip-Chip）憑藉著許多優異的特點，如：較短之轉接路徑、較小之封裝尺寸以及較高之生產效率，吸引了許多來自不論是學術界或者是工業界的注目。然而，由於覆晶技術有著極短的轉接路徑，使得晶片與封裝基板非常地接近，進而產生會使高頻電性衰退的接近效應（Proximity Effect or Detuning Effect）。文獻中已發表了運用增加凸塊（Bump）高度、減少金屬重疊面積以及利用補償電路（Compensation）等等之結構設計可有效地抑制此接近效應。此外，覆晶封裝的可靠度也是一個相當關鍵的課題。利用底膠填充（Underfill）的技術可有效地改善其可靠度，但卻會引起電性衰退。另一方面，生產成本也是封裝技術中相當重要的議題。傳統的封裝結構中，陶瓷基板提供穩定及良好的物性和化性。但一般來說，陶瓷基板的生產製作成本較高。因此，利用特性相近且較低價的高分子基板取代傳統陶瓷基板可達到降低成本同時又可以維持良好電性的目的。 本論文探討覆晶封裝於毫微米波段之研究與應用，主要可區分為兩部分。第一部分為單石微波積體電路（Monolithic Microwave Integrated Circuit）之覆晶封裝的驗證。本研究配合一高阻抗之補償設計，利用覆晶技術將一60 GHz一進二出（Single-Pole-Double-Throw）之開關元件封裝接合至氧化鋁（Al2O3）基板上。覆晶封裝後之開關元件表現出相當良好的高頻特性。另一方面，本研究也將覆晶封裝應用在一個包含低頻振盪器（Oscillator）以及倍增器（Multiplier）之多晶片模組（Multi-Chip Module）上。除了多晶片模組的驗證之外，此振盪器以及倍增器也被個別地覆晶接合至基板上以驗證覆晶封裝對單一晶片之影響。其量測結果顯示，覆晶封裝並沒有造成元件特性衰退。此外，多晶片模組也表現出良好的低像噪（Phase Noise）以及高輸出功率。 第二部分則是關於覆晶封裝中材料評估方面之研究。一般來說，覆晶封裝需要底膠填充來改善其機械可靠度。但填入底膠會對覆晶封裝之高頻特性產生負面的影響，如：傳輸線阻抗不匹配，以及高頻的介電損耗。為了解決這些問題，本研究將一環氧樹脂之底膠填入覆晶結構並且量測驗證至67 GHz。搭配傳輸線阻抗匹配設計與低高頻損耗的底膠材料，此覆晶結構表現出極佳的高頻特性。此外，我們也對此結構做一系列的可靠度測試。其測試結果表現出極佳的機械可靠度。此外，覆晶基板也是本研究另一個材料評估的重點。為了降低覆晶封裝的生產成本，本研究將被動傳輸線以及主動高電子遷移率電晶體（High Electron Mobility Transistor）用覆晶技術接合至一低成本Rogers RO3210TM高分子基板上來做驗證以及評估。其量測及可靠度測試結果顯示此低成本高分子基板可運用在50 GHz以上的商業應用。

In recent years, with the demands for wireless communication systems increases rapidly, the operating frequency for the portable wireless is moving toward millimeter-waves. Millimeter-wave wireless communication systems require not only suitable functional IC components but also competent package with low cost and good interconnect performance. To meet the demands for commercial applications, package with low power consumption, low cost, small size, and light weight becomes indispensable. However, unlike low frequency applications, millimeter-wave frequencies introduce significant parasitics and therefore the interconnect between IC chips and packaging carriers must be carefully managed in order to maintain good electrical performance. Conventional bond-wire induces significant parasitic inductance and thus results in unwanted effects, which could deviate the IC performance after assembly, especially at millimeter-wave frequencies. Flip-chip interconnect has drawn lots of attentions for chip-level packaging at millimeter-wave frequencies due to several advantages over bond-wire, e.g., shorter interconnect length, smaller package size and higher throughput. However, at MMW frequency range, the proximity effect, or detuning effect, is a crucial issue for flip-chip due to the proximity of chip to substrate. The proximity effect may cause the flipped-chips to deviate from its original performance. Approaches like increasing the bump height, reducing the metal overlap and employing compensation design at the transition region have been proposed to improve flip-chip performance. In addition, flip- chip reliability is very crucial for industrial applications since it relies only on several metallic connections. Using underfill as a buffer layer between chips and carriers can significantly improve flip-chip reliability, but unfortunately, the trade-off is the underfill induced performance decay and deviation. Furthermore, cost-down is also very important for commercialization. Conventional ceramic-based carrier offers excellent chemical and physical properties but with higher cost. Using low-cost organic board might be a good solution to get lower cost with fair performance. However, the investigation for flip-chip on organic board is generally insufficient. This dissertation covers an overall study for flip-chip interconnect to apply at millimeter-wave frequencies. It can be divided into two parts. The first part is about active device packaging. Single MMIC chips and mm-wave modules were flip-chip assembled for demonstration. A V-band SPDT switch for half-duplex RF front-end switching was flip-chip assembled and RF characterized to 67 GHz. By adopting hi-compensation design, the packaged switch showed excellent frequency response and very low additional loss. Moreover, a V-band frequency source with a 7 GHz oscillator and a x8 multiplier was flip-chip assembled onto a multi-chip carrier. For comparison, both the oscillator and x8 multiplier were also bonded as individual chip. From the measurement results, the flip-chip technique did not have any detrimental effect and the assembled module showed excellent phase noise of -112 dBc/Hz @ 1 MHz offset with high output power of 11 dBm, demonstrating outstanding performance for millimeter-wave frequency generation. The second part is about material investigation in a flip-chip system. Underfill is generally required for improving flip-chip reliability. However, underfill in a flip-chip interconnect might introduce negative effects i.e., chip impedance mismatch and dielectric loss at millimeter-wave frequencies. To investigate and solve this issue, an epoxy-based was applied to a flip-chip structure and measured up to 67 GHz. By using pre-matching design and low-loss underfill, the flip-chip assembly exhibited excellent performances with return loss below -20 dB and insertion loss less than 0.6 dB. In addition, the reliability test revealed that the flip-chip assembly also performed excellent reliability. The other material investigation is about flip-chip carrier material. Low-cost Rogers RO3210TM organic laminate was employed to replace ceramic-based carrier for cost reduction and performance improvement. Both passive transmission lines and active discrete mHEMTs were flip-chip bonded onto RO3210TM. The test results showed that RO3210TM is a promising packaging carrier for commercial applications up to 50 GHz.