Orientation- and concentration-dependent surfactant adsorption on silicon in aqueous alkaline solutions: explaining the changes in the etch rate, roughness and undercutting for MEMS applications
Orientation- and concentration-dependent surfactant adsorption on silicon in aqueous alkaline solutions: explaining the changes in the etch rate, roughness and undercutting for MEMS applications
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DOI：
10.1088/0960-1317/19/12/125011
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发表时间：
2009-12-01
影响因子：
2.3
通讯作者：
Ishibashi, K.
中科院分区：
文献类型：
--
作者：
Gosalvez, M. A.;Tang, B.;Ishibashi, K.
我们结合椭圆偏振光谱(SE)、傅里叶变换红外光谱(FT-IR)、动力学蒙特卡罗模拟(KMC)和凸角切角分析来表征和解释在碱性水溶液中添加少量表面活性剂(如Triton X-100在四甲基氢氧化铵(TMAH)中的作用。我们认为，表面活性剂作为一层薄层吸附在硅-腐蚀剂界面上，作为过滤器，通过减少到达表面的反应物分子的数量来调节表面反应活性。根据SE和FT-IR的测量，吸附层的厚度是一个与取向和浓度有关的量，这主要是由于H端面密度的取向依赖性和潜在氧化和刻蚀反应的相对速率的浓度依赖性，这直接影响到OH端基的数量。对于表面的部分OH覆盖，OH基团的水化有效地充当了表面活性剂分子水化壳的锚定位置，从而能够形成水合桥，从而放大表面活性剂的吸附密度。在高浓度下，该模型解释了精确的和邻近的Si{110}表面的刻蚀速率显著降低，而精确的和邻近的Si{100}表面的刻蚀速率变化很小。在低浓度下，它解释了两个家族的蚀刻速率是如何显著降低的。表面活性剂吸附的取向和浓度依赖关系解释了微米级湿法刻蚀图形在使用TMAH和TMAH+Triton获得的微米级图案中的巨大差异。
We combine spectroscopic ellipsometry (SE), Fourier transform infrared spectroscopy (FT-IR), kinetic Monte Carlo simulations (KMC) and convex corner undercutting analysis in order to characterize and explain the effect of the addition of small amounts of surfactant in alkaline aqueous solutions, such as Triton X-100 in tetra methyl ammonium hydroxide (TMAH). We propose that the surfactant is adsorbed at the silicon-etchant interface as a thin layer, acting as a filter that moderates the surface reactivity by reducing the amount of reactant molecules that reach the surface. According to the SE and FT-IR measurements, the thickness of the adsorbed layer is an orientation-and concentration-dependent quantity, mostly due to the orientation dependence of the surface density of H-terminations and the concentration dependence of the relative rates of the underlying oxidation and etching reactions, which have a direct impact on the number of OH terminations. For partial OH coverage of the surface, the hydration of the OH group effectively acts as an anchoring location for the hydration shell of a surfactant molecule, thus enabling the formation of hydration bridges that amplify the adsorption density of the surfactant. At high concentration, the model explains the large reduction in the etch rate of the exact and vicinal Si{110} surfaces, and the small changes in the etch rates for the exact and vicinal Si{100} surfaces. At low concentration, it explains how the etch rate for both families is significantly reduced. The orientation and concentration dependence of the surfactant adsorption explains the dramatic differences in the micron-scale wet-etched patterns obtained using TMAH and TMAH+Triton for microelectromechanical systems applications.