Engineering Morphology of Surfaces by Oblique Angle Etching
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Engineering Morphology of Surfaces by Oblique Angle Etching Mehmet F. Cansizoglu, and Tansel Karabacak Department of Applied Science, University of Arkansas at Little Rock, Little Rock, AR, 72204 ABSTRACT During a typical chemical etching process growth front morphology generally generates an isotropic rough surface. In this work, we show that it is possible to form a rippled surface morphology through a geometrical self-assembly process using a chemical oblique angle etching technique. We observe in our Monte Carlo simulations that obliquely incident reactive species preferentially etch the hills that are exposed to the beam direction due to the shadowing effect. In addition, species with non-unity sticking (etching) coefficients can be re-emitted from the side walls of the hills and etch the valleys, which at the end can lead to the formation of ripples along the direction of the beam. This mechanism is quite different than the previously reported ripple formation during ion-beam bombarded surfaces where the incident ions have much higher energies, and surface and subsurface bulk atoms are physically removed (sputtered) from the substrate. We investigate the ripple formation process in our simulated surfaces for a wide range of etching angle and sticking coefficient values. INTRODUCTION Although it is a key parameter in microfabrication and nanofabrication processes, morphological evolution during etching, where the material from the surface is chemically removed, has got less attention compared to that of thin film deposition [1]. Contrary to the common belief, it has been shown that [1] etched surfaces can remarkably get rougher due to the competition between “shadowing and re-emission effects”. In shadowing effect, obliquely incident etching molecules can preferentially remove the tops of “hills” due to their higher heights, while lower lying “valleys” get shadowed and stay intact. Therefore, shadowing has a smoothening effect during etching process. On the other hand, etching particles with non-unity sticking coefficient (s), which defines the probability of etching a surface atom (for example, s = 1 stands for 100% probability of etching the surface atom, and s = 0.1 for 10%), can bounce off from the first impact point, get re-emitted, and finally etch another surface point after such multiple re-emission processes. As illustrated in Fig.1, re-emission can lead to the removal of atoms at valleys, and therefore, it has a roughening effect as opposed to smoothening shadowing effect. Unlike well studied oblique ion beam sputtering/erosion/etching process [2-4], where the material from the surface is “physically” removed (sputtered), to the best of our knowledge, no work has been reported yet for the morphological evolution of surfaces under the presence of an obliquely incident “chemical” etching beam. Boyd et al. has used an oblique reactive ion etching (RIE) flux incident on a patterned substrate in order to investigate the angular dependence of etching rate during RIE [5]. In this work, using Mo
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