奈米週期性干涉條紋的接合技術

Abstract

本論文研發關於在四吋晶圓上製做週期性奈米結構的步進對位干涉微影系統(SAIL)。利用兩道擴束後的高斯光束在干涉時使用近接遮罩法產生方型的干涉條紋圖形，本步進對位干涉微影系統可以在很高的成功率下，將小區域的干涉條紋接合成大面積。並且也可以利用本系統製做二維大面積結構。 可以大致將步進對位干涉微影系統區分為兩大部分：光學與機械方面。前者致力於可用的干涉條紋影像在干涉平面上產生，而其由很多不同的研究主題組成。而機械工程方面包含了定位系統、晶圓固定系統及量測系統，此為本論文之主要議題。平台的定位精度與長行程的移動範圍由複合式雙致動器平台達成，而定位訊號則由高穩定度高解析度的雷射干涉儀系統反饋回控制平台。 在步進對位干涉微影系統中，將小面積曝光區域接合成大面積最基本的便是掌握週期整數倍的移動步距以步進接合干涉條紋，因此在本論文所開發的幾何週期量測法便是在晶圓尚未顯影前便可便利的計算條紋週期。利用此幾何週期量測法配合位移感測器在250奈米週期結構的量測下可以得到小於0.5奈米的量測誤差。由於氬離子雷射腔體中的循環冷卻水會對於主要曝光光學桌產生一定程度的震動影響，在此考量下將此雷射置於另一張光學桌。然而雷射光源對於主要光學桌上的光學元件產生不小的相對運動，造成干涉區域的週期與對比會有不穩定現象。因此我們使用光源穩定技術將氬離子入射光源穩定於最小且可接受的飄移量，光源位置與角度的最終穩定結果在三個標準差下分別為3微米及10微徑度。 為了將單一干涉小區域佈滿整片4吋晶圓，我們提出了使用雙致動器的複合式伺服控制系統的接合方法。配合高解析的干涉儀建構一個標準絕對座標，由雙致動器平台提供大行程移動及高精度定位能力。複合式控制架構在這幾種不同取樣頻率的溝通介面下進行整合控制，最終的三軸(X，Y及Z軸轉角)定位精度在一個標準差下分別為 11.34奈米、9.25奈米及240.6奈徑度。最終本控制系統可以將定位誤差控制在10奈米左右，此定位精度高於十分之一的線寬大小便可接合條紋。 最後我們使用掃描式電子顯微鏡觀看600奈米及250奈米整片晶圓接合的一維結構與250奈米小面積的二維結構，配合我們利用影像所計算的週期加以驗證。而在重疊區的干涉條紋連續性則由掃描式電子顯微鏡連續觀看下加以證實，利用本論文提出的步進對位干涉微影系統在重疊區的確可以成功接合干涉條紋。

A step-and-align interference lithography (SAIL) system for manufacturing continuous periodic nano-structures on 4-inches wafers is developed. The system establishes a large square interference image by interfering two expanded Gaussian laser beams onto a proximity approach mask. The servo system then moves the wafer around to stitch the images into a large grating. The resulted patterning shows high successful rate and potential to achieve two-dimensional structures. The key technologies in the SAIL are roughly classified as optical and mechanical. The optical technique focuses on yielding an available interfered image on the substrate and is the fusion of many different topics. The mechanical engineering subsystem includes positioning system, wafer holding system, and measurement system. The positioning accuracy and long-distance movement of the wafer chuck is archived by a dual-actuated stage, and positioning signals are feedback by high-stability and high resolution laser interferometers. To stitch the small exposure area onto the full wafer, it is essential that the stage steps at integer multiple of the fringe period. The period measurement system by geometric method is able to calculate nanometer structure period before the development of the wafer. The measurements resulted in 250 nm periodical structures with good error bound under 0.5 nm. Since the circulation of cooling water in the laser tube causes vibration on the main optical table, it is necessary to place the argon-ion laser on a separate optical table. This generates serious relative motion between the two tables and has to be eliminated by a beam stabilization/steering system. The experiments show that the variance of the position shift and angle change is under 3μm and 10 μrad, 3σ, respectively. For stitching the single small interfered area over the 4' wafer, we propose a stitching method using a complex servo system composed of dual actuator positioning stage with laser interferometer. The system operation is then based on the high resolution laser interferometer coordinate. The dual actuator system provided a wide travel range using the linear positioning stage and fed the positioning error back to the piezo-actuated stage to achieve nanometer accuracy. Hybrid control architecture was introduced due to the different communication interface between the two sets of positioning systems. The stability of positioning system in X-axis, Y-axis, and Theta_Z are 11.34 nm, 9.25 nm, and 240.6 nrad, 1σ, respectively. The positioning system can exactly lock on to the target vibration with about 10 nm error. This is less than one tenth of the fringe period and is sufficient to satisfy the accuracy requirement. The micrographs of SEM in 600 nm and 250 nm one-dimensional structures of full wafer and 250 nm two-dimensional structures of single exposure have been shown and calculated the period of nanometer structures by SEM images. The continuity of the interference fringes in the wafer has been examined using SEM. There were no discontinuous of the fringes over a long distance in the crossover region.