增加深次微米接觸窗微影製程聚焦深度的解析度增強技術

Abstract

本論文敘述減光式相位移光罩之工作原理及其於實際製程運用上所碰到的問題並針對變形照明濾光片之設計提出一種名為「照明圖法」的新穎作圖法以便讓此二種解析度增強技術能有效整合於深次微米接觸窗層曝光製程上。文章內容主要分成三大部分。 第一部份(第二到第四章)是探討邊緣型減光式相位移光罩其增進聚焦深度之機制及在實際運用上所面臨的問題。經由將光軸方向上的成像光強分布以一連串的餘弦函數來近似藉以了解每一對繞涉光之干涉情況後，我們可以清楚的了解到具有對稱相位移圖案之光罩設計所以能增進聚焦深度的原因是由於其提供了破壞性干涉來平衡由於建設性干涉所造成的離焦光強減弱的現象。藉此我們導出一個簡單的方程式來表達邊緣型減光式相位移光罩其增進聚焦深度的能力。此方程式顯示聚焦深度之增長只與曝光波長(l)和投影透鏡之數值孔徑(NA)有關。由於增加照明角度會增長破壞性干涉在光軸方向之週期進而減弱其補償離焦光強的能力，因此增加光源之空間非同調度會減少聚焦深度改善。另一方面，在使用邊緣型減光式相位移光罩對密集接觸窗圖案進行曝光時，由於當缺少建設性干涉的緣故會使得側葉光強大於中央光強造成圖形轉移失敗，此推論意味著此種相位移方法只能運用於圖形週期大於 l/NA 之接觸窗圖案。在研究中發現極小的球面相差會對影像光強在最佳成像位置附近造成非常顯著的不對稱離焦行為，這是由於透鏡上面的相差會造成建設性干涉與破壞性干涉向不同離焦方向位移所致。上述分析經由0.17微米接觸窗曝光實驗得到證實。在實驗中，我們特別留意觀察光罩圖案週期對聚焦深度之影響進而闡明密集接觸窗與疏離接觸窗所需改善離焦容忍度機制之不同。 第二部份(第五章)介紹一個名為「照明圖法」的作圖方法來輔助分析光曈面上繞射光場分布與照明角度間的關係。一個照明圖是由許多圓圈組成﹔每個圓圈可視為光瞳投影在照明面上之影像而其圓心離原點的距離等於其所對應之繞設光繞射向量在光瞳面上投影的大小。藉由此作圖法，任意照明角度所造成的光瞳光場分布可以經由計算在照明圖上重疊的圓圈數目得到。照明圖上任兩相交圓之共弦表示可造成兩繞射光對光軸有相同繞設角度的光源位置，這是定義最佳偏軸照明位置的必要條件之一。照明圖法對有效光源位置的選擇極有助益。 最後，也是此論文最重要的部份(第六章)，我們提出一個結合變形照明技術與邊緣型減光式相位移光罩的整合型解析度增強技術來輔助深次微米接觸窗層曝光製程。照明濾光片的設計目的在綜合所有光罩圖形之有效照明位置以補救減光式相位移光罩無法輔助密集接觸窗成像之缺點。為了在單一曝光中同時保有偏軸照射與同軸照射的優點，我們利用照明圖法得到使用能消去0階繞射光的光罩設計來曝出密集接觸窗的結論。另一方面，疏離接觸窗之曝光則需要具有相位移180度的0階繞射光的光罩設計使得其能於較高空間非同調度照明下仍能維持足夠的聚焦深度。我們將此整合型解析度增強技術運用在使用248奈米波長與0.55數值孔徑機台條件的0.17微米接觸窗曝光製程中。實驗結果不但證明此解析度增強技術可以同時改善所有圖形週期之聚焦深度，並且也顯示我們設計的變形照明結構可以降低由球面相差引起的最佳聚焦位置漂移問題。實驗結果顯示最小可解析之圖形週期可推進至0.39微米，同時可以得到一個離焦容忍度為0.63微米而能量寬容度為6%的矩形共同製程窗。

This thesis describes the working principles and implementation issues of the attenuated phase-shifting mask (PSM) and proposes an instructive graphic method named the illumination chart for customized illumination aperture filter (CIF) design such that these two resolution enhancement techniques (RETs) can be combined synergistically for deep sub-micron contact level printing. It is divided into three parts. The first part of this text (chapter 2-4) discusses the enhancement mechanism and implementation issues of a rim-type attenuated PSM. The mechanism of focus latitude enhancement for contact/via hole printing is explained by approximating the axis intensity distribution of the image as a series of cosine functions to characterize the interference between each pair of diffraction beams. It is found a phase-shifting mask with symmetrical assisting features improves the depth of focus (DOF) by introducing the destructive interference to counterbalance the intensity attenuation from the constructive interference as defocus. A simple formula was derived to represent the capability of focus latitude enlargement. It shows the extent of enhancement depends on the exposure wavelength and the numerical aperture (NA) of the projection lens only. Increasing the degree of partial coherence degrades the focal range enlargement because a larger illumination angle elongates the destructive interference pattern in the optical-axis direction to weak its ability for intensity compensation. On the other hand, the lack of the constructive interference in dense hole imaging fails the mask pattern transfer, which limit the application of the phase-shifting method to pattern pitches greater than l/NA. A tiny amount of spherical aberration results in prominent asymmetrical defocus behavior because the wave deformation in the projection lens shifts the distribution of constructive and destructive interference patterns to opposite defocus directions. The printing characteristics of 0.17 mm contact hole using an 18%-transmission rim-type attenuated PSM are investigated to corroborate the above analysis. The dependence of DOF on the pattern duty is stressed to elucidate the difference in mechanisms of focus latitude improvement for sparse holes and periodic dense holes. The second part of this work (chapter 5) introduces a novel graphic method named the illumination chart to give an aid for analyzing the effect of the illumination angle on the diffraction order distribution in the pupil. An illumination chart is composed of several disks; each of which can be regarded as the pupil image projected in the illumination plane with its center offset equal to the vector of a diffraction order. By this graphic method, the interference condition from a specific source position can be determined by counting the number of overlapping disks. The common chart of two overlapping disks represents the source positions to diffract the corresponding diffraction orders with equal propagation angles to the optical axis, which is the necessary condition for optimum off-axis illumination position determination. The illumination chart method is useful in effective source selections. Finally, also the most important part of this work (chapter 6) presents a practical resolution enhancement technique that integrates the modified illumination technique and the rim-type attenuated PSM for deep sub-micron contact hole printing. A customized illumination aperture filter is synthesized by collecting effective source elements for every pattern pitch to remedy the inability of the attenuated PSM for dense patterns. To preserve the merits of off-axis illumination (OAI) to dense patterns and on-axis illumination to sparse patterns in a single exposure, the illumination chart suggests a zeroth-order-eliminated mask design for dense patterns. On the other hand, a zeroth-order-phase-shifted mask design is required to sustain enough DOF for sparse hole patterning under higher illumination incoherence. We applied this integrated RET to 0.17 mm contact hole printing using a 248 nm, 0.55 NA exposure system. The experimental results show our CIF illumination not only balance the DOF enhancement throughout the pattern pitches but also suppress the best focus shift from spherical aberration. The minimum pitch is pushed to 0.39 mm, and a common process window of 0.63 mm DOF at 6% exposure latitude is achieved.