熱致超解析效應在光碟母模製程上的應用

Abstract

在製作光碟母模所使用的雷射微影(laser lithography)製程中，由於雷射光是直接照射在光阻層上，因此光阻層上的聚焦光點大小或是最小曝光區域就必須受限於於光學的繞射極限：0.82×l/NA，其中 l 是光源在真空中的波長， NA(numerical aperture)是物鏡的數值孔徑。根據這個公式，縮小光點最直接的辦法就是使用波長更短的光源或是NA值更高的物鏡。然而，由於光阻本身是一種對波長相當敏感的物質，而且在空氣中物鏡的NA極限值為1，因此上述的這兩種方法在實際應用上都會受到相當程度上的限制。 在本論文研究中，我們根據熱致超解析效應(thermal-induced super resolution)的原理提出的一個新的雷射微影技術，並且証明這項技術的確能有效的縮小光阻層上聚焦光點，因此藉著這項新的技術，將可利用現有的曝光設備製作出更高記錄密度的光碟母模。這項技術最主要是在光阻層上加了一金屬薄膜，並利用一聚焦在此薄膜上的雷射光點，因光點中心高熱部分造成薄膜光學性質的非線性變化，開啟一小於繞射光點的孔徑，而形成一小於繞射極限的光點。由實驗及模擬的結果指出，利用此一新的技術在現有的光學系統架構下可以使得光阻層上所形成的線寬縮小約60﹪。

In the typical laser lithography process which is used to make optical master disks, the laser beam is directly illuminated on the surface of photoresist layer, so the resulted exposed area or spot size is limited by the optical diffraction limit, approximated to 0.82×l/NA, where l is the free space wavelength of light source and NA is the numerical aperture of the object lens. There are two main approaches to reduce the spot size: reducing l and increasing NA. However the sensitivity of photoresist material depends on the wavelength of light source, and the maximum value of numerical aperture is 1 in air. Thus, the two schemes in reducing focused spot size are either with some complication or limited performance. In this thesis, we propose a new lithography technique that using the effect of thermal-induced super resolution and demonstrate that the technique can effectively reduce the exposed spot size on the photoresist layer, thus allowing disk mastering toward higher density using exiting light source and optics. The technique makes use of a mask layer on the photoresist layer. The material of mask layer, which has nonlinear optical properties, opens an aperture in high temperature area near to the center of the laser spot, focusing on the mask layer. Usually, the aperture size is much smaller than the laser spot, thus achieving thermal-induced super resolution. The simulation and experimental results reveal that the line width on the photoresist layer could be shrunk by more than 60﹪by using the new technique, thus effective in reducing line width for laser disk mastering and other lithography.