Mechanisms of chemical anisotropic etching of single crystals consistently applicable throughout Micro/Meso-scopic domains

该项目的目的是阐明单晶（如硅和石英）在从原子到微米范围内的各向异性蚀刻机制。阐明了两种主要硅蚀刻剂蚀刻特性的差异；即，KOH和四甲基氢氧化铵（TMAH）溶液，与这些蚀刻机制的关系。我们还实验研究了少量杂质对蚀刻速率、各向异性和蚀刻表面形貌的影响。我们利用蒙特卡罗模拟，通过计算晶体表面扭结和步骤处的原子去除速率，分析验证了蚀刻机理。蒙特卡罗模拟基于一个考虑氢氧基附着在暴露表面上导致原子键减弱的模型。硅（111）及其附近的蚀刻机理得到了很好的解释。宏观蚀刻行为显然是由存在于硅上的台阶的活跃性决定的（111）。该模型可以很好地解释各向异性根据蚀刻物质的不同而发生的逆转。我们最近发现，溶液浓度的变化也会导致各向异性的逆转。因此，各向异性的逆转不是蚀刻剂的单一问题，而是通过步骤和扭结的活跃性变化而更普遍接受的现象。我们正在进一步研究除（111）以外的其他取向的蚀刻模型。与硅平行，我们还使用与硅相同的方法表征了石英的各向异性蚀刻特性。我们评估了石英晶体的蚀刻速率作为取向的函数。这使我们能够通过模拟来预测三维蚀刻轮廓。该技术可有效地设计用于制造MEMS器件的石英刻蚀工艺。

The aim of the project is to clarify anisotropic etching mechanisms of single crystals such as silicon and quartz throughout all scales from atomistic to micrometer range. We clarified the difference in etching characteristics of two major etchants for silicon ; i.e., KOH and Tetra Methyl Ammonium Hydroxide(TMAH) solutions, in relation to those etching mechanisms. We also experimentally studied the effects of a small amount of impurities in the etchant that affect the etching rate, its anisotropy, and etched surface morphologies. We analytically verified the etching mechanism by calculating atomic removal rates at kinks and steps of a crystal surface using Monte-Carlo simulation that is based on a model considering weakening of atomic bonds caused by attachment of OH-base on the exposed surface. Etching mechanisms are well explained for silicon (111) and its vicinity. Macroscopic etching behavior is apparently dominated by the activeness of steps existing on silicon (111). This model can well explain the reverse in the anisotropy according to the difference in etching species. We recently found that the reverse in anisotropy also happens by a change in concentration of the solution. Thus the reverse in anisotropy proved not being a singular problem of the etchant, but more commonly acceptable phenomena by the change in activeness of the steps and kinks. We are further investigating the etching models for other orientations than (111).In parallel to the silicon we also characterized the anisotropic etching properties of quartz, using the same methodologies applied to silicon. We evaluated the etching rate of quartz crystal as a function of orientations. This allowed us a 3-D etching profile prediction by using a simulation. This technology is effective to design etching process of quartz for fabricating MEMS devices.