Unstable Etching Of Si(110)with Potassium Hydroxide
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Unstable Etching Of Si(110)With Potassium Hydroxide Z. Moktadir, K. Sato, T. Shimizu and M. Shikida Department of Microsystem Engineering, Nagoya University, Nagoya, Japan e-mail : [email protected] Tel: +81 52 789 5289, Fax : +81 52 789 5032
ABSTRACT We present the experimental data for the morphological evolution of Si(110) etched with Potassium Hydroxide. The observed results are interpreted using a continuum equation. The results reveal the presence of unstable etching, which leads to the formation of a columnar structure on the surface. The early stage of the formation of this columnar structure can be explained by a linear theory. This instability is caused by anisotropic surface tension. INTRODUCTION In recent years, wet chemical anisotropic etching of silicon has become a widespread technique allowing the fabrication of three-dimensional structures on the surface and in the bulk of silicon [1, 2]. The surface quality after etching is of prime importance, for example, the performance of fluidic microdevices is strongly dependent on the quality of etched surfaces. This has generated much experimental and theoretical interest in order to understand the physical chemistry of wet chemical anisotropic etching of silicon [3, 4, 5, 6, 7]. Many analytical results [8, 9, 10] have been reported and many studies have shown that the anisotropy of the surface tension is one of the causes of the formation of facets during crystal growth [11, 12, 13]. In recent studies, the instability of growing surfaces and interfaces is caused by the thermodynamic instability induced by the surface tension [14]. In epitaxial growth experiments, instability can lead to mound formation [15]. The formation of mounds can be explained by the presence of linear instability during the smoothing of initially rough surfaces [16]. This instability is a result of asymmetric barriers for surface diffusion across or near the steps [17, 18]. In ion sputtering experiments, ripples are formed on the surface due to an early stage linear instability [19]. Numerical simulation of dynamics of ripple formation in sputter erosion shows that after a characteristic time, the non-linearity determines the surface morphology by destroying the ripples or generating a new rotated ripple structure [20]. In this paper, we analyze the etched Si(110) with potassium hydroxide (KOH). We will give an interpretation of observed results using a continuum equation of motion derived form thermodynamics. We find that etching of Si(110) with KOH is linearly unstable. The long wavelength modes grow in time while the small wavelength modes are dumping. The etching instability leads to the formation of a columnar structure oriented in the < 110 > direction. We used KOH solution of 34 wt.%. The etching temperature was 80oC. Different samples were etched at different times and rinsed using DI water solution after etching. The temperature variation was small enough to obtain accurate results. The initial roughness of the samples was small (some nanometers), and thus wil
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