研究石墨烯複合材料與其可調變電性之封裝應用

Abstract

隨著製程技術的提升，積體電路在近年迅速微縮，目前元件尺寸可達到奈米尺度。根據摩爾定律（Moore’s Law），積體電路上電晶體數量，大約每隔18個月便會加倍[1]，為達成這個需求，積體電路的結構已從平面變成垂直方向的整合。隨著積體電路的元件數量提升，這也代表著電腦的工作效率越高。但越多的運算便會導致更多的熱能產生，且隨著晶片的微縮，晶片間的電磁干擾也越發嚴重。因此，研發一個可同時具有高散熱且具有電磁屏蔽效果之複合材料，並且同時可以進行量產應用於實際封裝是一件非常重要的事。高分子聚合物是目前常見應用於封裝的材料，其質量輕、價格便宜且方便加工，但高分子聚合物的散熱效果非常差，即便添加大量導熱填料，熱傳導也只能達到 ~3W/m·K[2]，無法有效解決電子三維結構的積體電路熱聚集的問題；而應用於電磁波屏蔽的封裝技術中，常將金屬粉末與高分子聚合物進行混和，但此類複合材料通常需要耗費較高的成本，且有漏電得疑慮。 在本篇論文中，我們成功的研究出一種同時具有高散熱及電磁屏蔽效果的複合材料，目標是將同時具有超高導熱及導電能力的二維材料石墨烯(Graphene)與奈米纖維進行混和後，均勻散佈於高分子聚合物中；但由於石墨烯惰性的邊緣結構，難以和奈米纖維直接鍵結，因此我們運用一種新的混和技術，『溶液替換法』，藉由將石墨烯轉移到適合的溶液裡面，在與奈米纖維進行混和，形成具有高導電效果的薄膜，再將其散佈於高分子聚合物中，形成連續的散熱網路，並藉由高分子聚合物內均勻分散的片狀導電材料，提供電磁波屏蔽的效果。我們所研發的複合材料將高分子聚合物的熱傳導從原本的0.2 W/m·K提高至54 W/m·K，除此之外，由於我們將導電填料均勻分散於聚合物中，使得我們的複合材料在垂直方向擁有可調變的閾值電壓(1.5V~5V)，因此當外界有突發的ESD事件時，我們的複合材料將開啟保護機制；我們已經實際將複合材料進行封裝，並證實其真的有防護ESD事件的效果。不僅如此，由於其內部導電結構分布均勻的特性，也為其提供良好的屏蔽效果，在X-Band的波段應用下，可以達到34dB，在Near field也可以提供40%以上的屏蔽效果，且生產過程是使用微型混鍊射出機，可以大量生產，適合用於商業化的封裝材料。最後，我們將複合材料實際封裝在裝有IC(M1511)的晶片載具中，在封裝後，我們可以用熱顯像儀(TVS-500EX)發現其元件所產生的熱能被有效的消散，且電特性也有提升的現象。

Scaling of integrated circuits (IC) scaling has become exceptionally challenging in recent years, especially reaching the level of nanometers, that can accommodate billions of transistors. To keep up with the Moore's law, where the density of transistors doubles every 18 months, the design of IC has headed toward three-dimensional integration. Nevertheless, thermal issues and the electromagnetic interference (EMI) between the devices would become more severe in 3DICs. Therefore, it is very important to develop a novel strategy to address such problems, where packaging material with high thermal conductivity and EMI shielding effects might be a great solution. Commercial packaging material, such as polypropylene and epoxy would be light, cheap, and easy to process, but these polymers have low thermal conductivity, and did not exhibit EMI shielding abilities. Adding metal powders in polymers is a common method, but it usually increases the cost while causes concerns about the leakage current. In this paper, we successfully developed novel composites that have high thermal conductivity and EMI shielding effect, where we adopted a thermally and electrically conductive filler. Inside the filler, graphene provided excellent thermal as well as electrical conductivity, where the structure and mechanical properties of the filler were maintained by cellulose nanofibers (CNF), due to its ultra-high aspect ratios. Considering the difficulty of directly bonding graphene to CNF, we invented a new mixing technique, "solution replacement", by transferring graphene in a suitable solution, and mixed with CNF to form a graphene/CNF film with high electrical and thermal conductivity. The graphene/CNF film, which provides EMI shielding effect was uniformly dispersed in the polymer to serve as sheet-like conductive filler that construct a continuous heat dissipation network. By introducing the graphene/CNF filler, the thermal conductivity of polypropylene was increased 250 times (from 0.2 W/m·K to 54W/m·K). Our composite could also achieve an EMI shielding effect up to 32 dB and a controllable threshold voltage (1.2V~3.1V) in the vertical direction thanks to the 3D conductive network of graphene/CNF filler. In addition, a commercial IC (M1511) was packaged with our composites, where we further confirmed outstanding thermal dissipation and EMI shielding effect of our composites.