高分子材料在電子束照射與奈米轉印上的行為與應用

Abstract

非主流微影技術（alternative techniques）在近二十年蓬勃發展，目的在取代光學微影技術，高價格機台與光波長極限讓光學微影發展受限制。而能夠取代光學微影最具代表性的技術是電子束與奈米壓印微影技術，電子束微影最主要的特質如下所示： 1. 具備非常高解析度圖案製造能力，甚至可達原子等級。 2. 非常彈性的技術，適用於各種材料。 然而，電子束微影在產速上受到限制，造成電子束產品高單價，目前在微影主流競爭上，仍未完全取代光學微影，而奈米壓印微影是近年來新興的技術，具備高產速製造奈米元件的特性，其中熱壓印微影在圖案解析度上，沒有光學微影繞射或是電子束散射以及其他微影技術的問題，因此而受到矚目。 在本論文中，我們焦點將放在高分子材料於電子束與奈米壓印技術兩部分研究上：

（1）高分子在電子束照射下的行為與應用 線性壓克力高分子（PMMA）受到電子束照射下會產生結構上的轉變，而成為交聯壓克力高分子被定義為雙性高分子，並在敏感度曲線顯現出來。因此，這類高分子可在同一層薄膜上同時展現出正與負型光阻特性並產生環形圖案，此現象可歸類為電子束的散射與熱效應所造成，而使得環形圖案半徑與電子束劑量的關係圖顯示出兩個區域，交界劑量定義在600 □C/cm2，對此現象我們建立電子束的散射與熱效應的半實驗數學行為模式，結果顯示在低劑量時，電子束散射效應控制了解析度，而在高劑量時，熱效應控制了解析度。而化學結構的變化上，壓克力高分子受電子束低劑量照射時（< 30 □C/cm2），會產生雙鍵，而在電子束高劑量照射下（> 240 □C/cm2），則雙鍵會產生聚合反應，並產生交聯結構。如此藉由電子束來控制壓克力高分子線性與交聯結構，非常適合應用在半導體領域中的正與負型光阻，並能夠在同一圖層上製造出正與負型光阻，非常有利於半導體圖案設計。不論正、負型光阻，其對比度與臨界劑量皆隨著厚度增加，對比度仍然以正型光阻較為優良。 環氧樹脂材料（SU-8）也能夠利用電子束技術來製造斜面與曲面結構，我們先對SU-8, PMMA, 與 KrF 三種光阻研究其對比度、敏感度、蝕刻與熱性質，並找出適合製造三維結構的高分子。在這些光阻中，環氧高分子（SU-8）顯示出最低對比度與最佳熱穩定度，所以被認定為可製造立體圖案的最佳高分子。我們所製造三維結構的方法是將環氧樹脂薄膜分成五個區域曝光，分別施加不同劑量，以做成斜面結構。並討論氧電漿處理後表面結構、反應機構與疏水性質，結果顯示表面經過電漿處理會增加表面粗糙度，而減少表面釋氣現象增加環氧樹脂膜的穩定度。並評估電子束劑量與環氧樹脂斜面角度的關係，利用曝光區域的擴大來控制斜面角度，結果顯示當曝光單位距離為10 □m，可達到較平坦的表面結構。由同樣的方式可製造出凹面與凸面結構，並可製造出具實用價值的螺旋結構，此可應用在微流體晶片上，會有曲面結構的產生主要是因為電子束散射的原因，電子束散射模糊了階梯狀結構，讓曲面更加光滑。然而，隨著曝光單位距離加大時，階梯狀結構則會慢慢出現，而所有的曲面皆是藉由電子顯微鏡來分析曲面結構。同樣的利用單位曝光距離改變，可製造出不同曲率半徑的凹面與凸面結構，而這種結構可用於控制流體在晶片上流動的速度，以減少微流體晶片上微幫浦的使用個數。另外，螺旋結構可縮小設計為流體晶片的體積，我們討論了螺旋結構的製造方式，並製造出螺旋結構的原始模型。

（2）在二氧化矽表面合成自組裝單層膜的研究及其在奈米壓印微影上的應用 在奈米壓印（nanoimprinting）技術中，母模（mold）與高分子在高溫高壓之下接觸，高分子因為超過玻璃轉移溫度而流動，形成與母模上圖案互補的圖形，在此種情況下，高分子與母模之間的作用力成為一個重要的議題，而母模與高分子之間的作用力強度可藉由表面張力計算來分析。一般而言，母模表面張力愈小，黏著力愈小，母模與高分子愈容易分離。在本實驗中，3,3,3-三氟丙烷基三氯矽烷[trichloro(3,3,3-trifluoropropyl)silane, FPTS] 與1H, 1H, 2H, 2H-全氟辛烷基三氯矽烷[trichloro(perfluorooctyl)silane, FOTS] 被用作在二氧化矽表面形成自組裝膜（self-assembled layer），來當成防止黏著層，使母模脫離高分子時，降低界面的黏著力。自組裝薄層表面官能基可藉由紅外線光譜儀來進行分析。而自組裝單層膜的品質則取決於晶片表面二氧化矽與反應物的浸泡時間與加熱去水縮合反應溫度，這些特性可藉由橢圓測厚儀與溶劑接觸角的測量分析，並可計算自組裝單層膜的表面自由能。結果顯示在母模上生成FPTS或是FOTS的自組裝單層膜可減少在奈米壓印過程中所產生的缺陷，並在壓印過程中能夠降低高分子膜表面的粗糙度。在母模上生成FOTS的自組裝單層膜可將其表面自由能降低約至8 mJ/m2，在壓印過程中消除了所有在高分子表面的缺陷，讓表面更加光滑，甚至提升了圖案解析度，而使得圖案解析度到達60 nm。由此可證實表面能是影響奈米壓印圖案品質的重要因素。 母模一般是經由微影技術定圖案並通過漫長半導體製成而成，過程十分繁瑣，使得母模的價格居高不下，利用熱固型高分子作為圖案並當成是母模的一部份，在超過熱塑性高分子的玻璃轉移溫度時，仍可保持圖案形狀，這樣的方法使得母模的製造流程不需經過蝕刻與去光阻製程，而大幅降低母模製造成本。使用熱固性高分子作為圖案模型進行熱壓的技術主要基於兩種可能性：（1）在矽晶片上製造熱固性圖案在半導體製程上是非常純熟的技術；（2）熱固型高分子因為其網狀交聯的結構，所以沒有明顯的玻璃轉移溫度，且其軟化溫度比熱塑性高分子高出甚多。在本實驗中，環氧樹脂類高分子被用來當作熱固性圖案結構，並熱壓熱塑性高分子，利用電子束製造出來的山丘狀圖案結構增加了環氧樹脂與矽晶片之間的接觸面積，進而增加環氧樹脂與矽晶片之間的附著力。奈米壓印技術製造出來的圖案將由電子顯微鏡來分析。由於高分子與高分子接觸時會有較大的黏著力，在分離製程時，會驅動熱塑性高分子產生收縮的情況，會使得圖案縮小，我們定義此為收縮因子（shrinkage factor），發現收縮因子與圖案大小有直接關係，此因兩高分子分離時，表面吸附力驅動熱塑性高分子填補分離時的位置所造成。另外，可利用熱塑性高分子上面的微透鏡圖案來翻印聚雙甲基矽烷（polydimenthyl siloxane，PDMS）的微透鏡。

Alternative techniques to cost-intensive or limited-access fabrication methods with nanometre resolution have been under development for nearly two decades. Two clear examples are electron beam and nanoimprint lithography technologies. The main attributes of the electron beam lithography are as following: 1. It is capable of very high resolution, almost to atomic level. 2. It is a flexible technique that can work with a variety of materials and almost infinite number of patterns. Nevertheless, electron beam lithography is a technique with limited throughput, leading to high costs in device production. The contest of lithography techniques for reliable fabrication of future integrated nanometer-scaled devices is not yet settled. Nanoimprint is an emerging lithographic technology that promises high-throughput patterning of nanostructures. Based on the mechanical embossing principle, nanoimprint technique can achieve pattern resolutions beyond the limitations set by the light diffractions or beam scatterings in other conventional technique. In this thesis, we focus on the subjects which based on the materials for electron beam and nanoimprint lithography:

(1)The behaviors of polymer under electron beam irradiation and the applications Structural transformation of polymers from linear to crosslinked structure by using electron beam irradiation has been defined as the zwitter-polymers. The novel sensitivity curve has been determined for the zwitter-polymer. The irradiation dose in the center can create a ring pattern due to simultaneous exhibition of the positive tone and negative tone of zwitter-polymer. The natural logarithm dependence of ring width and electron beam dose is linear in two ranges, irrespective of the dot design radius. The heating effect is identified from 600 □C/cm2. Mathematical modeling for the prediction of ring width for zwitter-polymer is achieved by considering the electron scattering and heating effects. The results of experimental measurement and modeling on ring width show a very good correlation. Furthermore, the polymer may exhibit either linear and crosslinked behavior depending on dosage of the electron beam irradiation. The property change from the structural transformation is suitable for application of positive and negative tone resists in semiconductor field. The contrast ratio and threshold dose both increase with increasing resist thickness for both the positive and negative tones, however, the positive tone exhibits better contrast than the negative tone. The epoxy material, SU-8, has been successfully fabricated to oblique, concave, and convex structures by a new electron beam technology. We study the contrast, sensitivity, etching, and thermal properties of SU-8, PMMA, and KrF resists and evaluate their suitability for the fabrication of these structures. Among these resists, SU-8 reveals the lowest contrast ratio, highest throughout, and best thermal stability, and so it becomes the candidate material for patterning the oblique structures. The technique that we have developed involves five regional exposures of a thick SU-8 resist layer with gradual increase of electron beam dosages. Furthermore, we discuss the surface morphology, reaction mechanism, and hydrophobicity after subjecting the SU-8 resist to a series of plasma treatments. The formation of surface nano-nodules during oxygen plasma treatment explains the surface hydrophobicity. Furthermore, oxygen plasma treatment increases the surface roughness of SU-8 polymer, while minimizing the outgassing problem and stabilizing the SU-8 film. We have carefully evaluated the effects of the electron beam writing dose and the design of the exposure area with respect to the inclined angle of the fabricated structure. Convex, concave, and spiral structures are fabricated successfully by using a gradient dosing strategy. The interface between two shot sections is smoothed by the electron scattering effect. The curved profile is fabricated at various curvatures, and characterized using scanning electron microscopy and a profiler. In addition, a spiral structure is fabricated that possesses the advantageous feature of having a small chip area. We discuss a method of characterizing the spiral structure.

(2) The studies of self-assembled monolayer on the silicon oxide surface and fabrication of microlens As all imprint techniques rely on contact between resist and mold, the wetting and adhesion characteristics of the polymer materials to the substrate are critical issues. The strength of adhesion between mold surface and resist is characterized by the amount of energy required to separate the two materials. In this study, trichloro(3,3,3-trifluoropropyl)silane (FPTS) and trichloro(1H, 1H, 2H, 2H- perfluorooctyl)silane (FOTS) are used for self-assembled monolayers (SAM) on mold (SiO2/Si) as releasing and anti-sticking layers for nanoimprint. Chemical reaction between the head groups of different fluorinated trichlorosilanes and the surface hydroxyl groups is investigated by FTIR. The SAM quality depends on immersion time and silanization temperature investigated through measurement of the ellipsometer and calculation of the surface energy. It has been demonstrated that less defect and lower roughness of the resist surface can be achieved by mold with SAMs of FOTS and FPTS. The mold with FOTS layer processes lower surface energy (8 mJ/m2) and smoother of the resist after imprinting. The surface energy of the SAM on mold (SiO2/Si) dictates the results in quality of better resist surface and the pattern formation. Mold fabrication for imprinting can be significantly simplified by using specialized crosslinking polymers for pattern definition on silicon wafer. The thermosetting polymer pattern can be used on silicon molds for imprint technology because of two possibilities: (1) the silicon oxide molds with thermosetting polymer pattern can be obtained by any conventional semiconductor technology; (2) thermosetting polymers have no obvious Tg because of cross-linking structure, whereas decreases the hardness as the temperature increasing over their Tg. In this work, the SU-8 resist is used as the thermosetting polymer pattern on silicon wafer for molds. Thermal properties of the thermosetting and thermoplastic polymers are tested for imprinting pattern and imprinted resist. The hill-like structure fabricated by electron beam strategy for thick film is used to increase the adhesion between pattern and silicon wafer. The resolution of the thermoplastic polymer resist pattern imprinted by thermosetting polymer pattern is investigated by SEM. The shrink factor of the feature size after separation between thermosetting polymer pattern and thermoplastic polymer resist is utilized to define the feature size after imprinting. In addition, a microlens of polydimenthyl siloxane (PDMS) has been fabricated by replication using the thermoplastic polymer resist after imprinting by the mold with microlens structure of the thermosetting polymer (SU-8).