含矽嵌段共聚物自組裝之行為研究與應用: 從溶液態到塊材及薄膜

Self-assembly and Applications of Silicon-Containing Block Copolymers: from Solution to Bulk and Thin Film

羅廷亞(Lo, Ting Ya)

指導教授 ： 何榮銘

國立清華大學/工學院/化學工程學系/博士(2016年)

摘要

一直以來，微電子科技產業總是不斷追求更快的微型處理器以及更高儲存密度的硬碟設備。隨著時代的演進，如同著名的”摩爾定律”所指出，每單個電腦晶片上的場效電晶體數目約每18個月將會增漲一倍。傳統之光微影蝕刻術(photolithography)，對於新世代的高密度元件製備之成本將會過於昂貴。為了延續一貫的產品規格之演進，製備更小、更快及更便宜的裝置，新的微影蝕刻之圖案化技術乃是大勢所趨。嵌段共聚物可自組裝形成多種不同的圖案，且其尺度可達十奈米；依此嵌段共聚物自組裝為薄膜將可作為蝕刻的遮罩，再轉移至基材上，此種新型之圖案化技術，即稱作『嵌段共聚物蝕刻術(block copolymer lithography) 。
利用嵌段共聚物自組裝獲得不同的奈米結構，需仰賴一系列不同體積組成之嵌段共聚物的合成。本研究提出一個簡易方式，不須合成不同之共聚物，僅使用單一組成之含矽雙嵌段共聚物：聚苯乙烯-聚二甲基矽氧烷 (polystyrene-block-polydimethylsiloxane (PS-PDMS))，即可達到不同之奈米結構的製備。由於聚苯乙烯-聚二甲基矽氧烷本身具極強之相分離強度，於溶劑揮發時將經歷明確的相轉換過程(clear-cut phase transition)，因此可經由選擇不同的選擇性溶劑，當聚苯乙烯嵌段於高濃度玻璃化後，即可製備不同的奈米微結構，以達到自組裝形態的多樣性。由於玻璃化形成之奈米微結構為一介穩態(metastable phase)，於實際應用上熱處理為元件製程過程中不可避免的步驟，可能導致相轉換的發生，本研究乃針對形成之柱狀(cylinder)及雙螺旋二十四面體(gyroid)介穩態結構，使用時間解析小角度X光散射(time-resolved small angle X-ray)實驗以及三維穿透式電子顯微鏡的影像重建技術探討其可能之相轉換行為，了解介穩態結構如何經過有序-有序相轉換(order-order transition)過程而回到穩定態結構。
應用上，達到大規模定向且有序的薄膜型態(特別是垂直定向的柱狀或板狀結構)為嵌段共聚物蝕刻術的首務。對於聚苯乙烯-聚二甲基矽氧烷，由於其聚二甲基矽氧烷本身的自由能過低，將導致自由表面的聚集，形成平行排列的奈米微結構。本研究我們提出一個嶄新的概念，利用『熵-驅使 定向方式(entropic-driven orientation)，使用具有星狀結構(star architectures)的嵌段共聚物系統，平衡其界面的作用力，獲得垂直定向的奈米結構薄膜。由實驗和理論模擬結果顯示，隨著分支數目(arm number)的增加，將可有效的抑制其介面的作用力，獲得垂直定向的柱狀及層板狀結構之奈米薄膜。
嵌段共聚物蝕刻術為二維圖案化之製程。如何製備更為複雜的三維裝置為技術研發的新趨勢。本研究使用溶劑退火(solvent annealing)的方式，可精確地得到多種不同的型態的奈米薄膜，結合含矽嵌段的高抗蝕刻特性，經過反應性離子蝕刻處理，可製備多種不同的矽氧化物高低差圖紋(topographic SiOx pattern)。由此方式所製備而得的矽氧化物高低差圖紋，將可作為具有高低差的基材，進一步利用逐層堆疊(Layer-by-layer sequential process)的方式來進行引導式自組裝(directed self-assembly)，製備多層的奈米圖案薄膜；結合由上而下(top down)的蝕刻技術(lithography)以及由下而上(bottom up)的自組裝(self-assembly)材料，提供具有高度可行性的方法製備三維奈米微結構。

關鍵字

嵌段共聚物 ； 自組裝 ； 聚苯乙烯-聚二甲基矽氧烷

並列摘要

The industries of microelectronics constantly strives to increase the speed of microprocessors and the storage density of hard disk drives. Historically, the number of transistors on a computer chip has approximately doubled every 18 months, a trend known as “Moore’s law”. Photolithography, the traditional patterning methodology used to fabricate these devices, has become prohibitively expensive. Alternative patterning technologies that enable high-resolution and high-throughput at lower cost must be developed for the semiconductor manufacturers are to continue their historical pace of “smaller, faster and cheaper devices”. Block copolymers (BCPs) offer an attractive alternative patterning technology since they can self-assemble on length scales from a few to hundreds of nanometers, and can self-assemble into various morphologies. With the use of BCP thin films as the etching mask, the self-assembled nanopatterns can be transferred into the underlying materials, which is referred as BCP lithography.
For BCP self-assembly, to acquire a variety of nanostructures would require to synthesize a series of BCPs with different volume fractions. In this study, a simple method to create a variety of nanostructures via the self-assembly of a single-composition silicon-containing block copolymer (BCP) is developed. Herein, we demonstrate that, by using selective solvents for the self-assembly of a silicon-containing block copolymer, polystyrene-block-polydimethyl -siloxane (PS-PDMS), the phase behavior of intrinsic BCP can be enriched due to the strong segregation of the PS-PDMS enabling the diversity of the phase behavior of PS-PDMS/solvent mixtures and the clear-cut phase transitions during solvent evaporation. By taking advantage of the clear-cut order-order phase transitions of the PS-PDMS in solution and the vitrification of the PS domains upon solvent evaporation, a variety of metastable phases can be acquired by simply tuning the selectivity of solvent for casting; most interestingly, the final morphologies from the casting are independent of the corresponding evaporation rate. For practical uses, the self-assembled BCP samples may encounter further thermal treatment during the manufacturing process; it may give rise to the phase transformation from the cast morphologies. To examine the stabilities of those phases from casting, the forming cylinder and gyroid phases are investigated by time-resolved SAXS and electron tomography experiments. Phase transformation occurs during thermal treatment, indicating that the cast morphologies are metastable phases at which the formation of intrinsic lamellar phase through order-order transition (OOT) can be clearly clarified in reciprocal space and real space. Those results offer new insights into the phase behaviors of the silicon-containing BCPs for practical applications, in particular for BCP lithography.
For the applications in lithography, nanostructured thin films with oriented periodic arrays over large areas are desirable. Most engineering applications demand thin films in which the orientation of the structures, such as lamellae or cylinders, is perpendicular to the substrate. However, for the PS-PDMS BCP systems, the low surface energy PDMS block will prefer to wet the air free surface (air/polymer interface) due to the favorable enthalpic interactions which will result in the parallel oriented nanostructures. Herein, we suggest a new concept to give entropic-driven orientation using BCPs with star architectures to balance the interfacial interactions for minimum Gibbs free energy state. Star-block copolymers of PS-PDMS are used as an exemplary case to demonstrate the effect of architecture on the controlled orientation. As demonstrated experimentally and theoretically, induced perpendicular orientation of BCP nanostructures for cylinder- and lamella-forming PS-PDMS star-block copolymers can be achieved by increasing the arm number of the star-block copolymer to suppress the effect of interfacial interactions. Those results offer new opportunities for the applications of BCPs in the thin-film state by exploiting complex block architecture.
Thin films of block copolymers are widely seen as enablers for nano-fabrication of planar devices (2D devices). However, the inherently three-dimensional structure of block copolymer microdomains could enable them to make 3D devices and complex patterns. On the basis of the systematic studies with respect to the self-assembly of the silicon-containing BCPs, we aim to demonstrate the appealing applications by exploiting the strongly segregated PS-PDMS for 3D nanopatterning. By taking advantage of solvent annealing techniques, PS-PDMS thin films with different morphologies, can be acquired from a single-composition sample. Consequently, by taking advantage of high etching resistance of the silicon-containing block, various topographic SiOx can be fabricated after reactive ion etching treatment. The forming topographic SiOx patterns can be further used as a topographical substrate to give multi-layer nanopatterned thin films using a layer-by-layer sequential process via directed self-assembly (DSA). By combining top-down lithography and bottom-up self-assembly, this approach suggests a feasibility to fabricate three-dimensional nanopatterning for various applications.

並列關鍵字

block copolymer ； self-assembly ； PS-PDMS

參考文獻

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