Convex corner undercutting of {100} silicon in anisotropic KOH etching: the new step-flow model of 3-D structuring and first simulation results

Abstract

In this paper, the mechanism of convex corner (CC) undercutting of Si 100 in pure aqueous KOH solutions is revisited by proposing the step-flow model of 3-D structuring as a proper description of the observed phenomena. The basic idea is to conceive the Si 100 anisotropic etching process, on the atomic scale, as a "peeling" process of terraced {111} planes at 110 oriented steps to understand also the arising shape in Si 100 etching. On the basis of our new model, we are able to predict the microscopic three-dimensional (3-D) structure of the characteristic CC undercutting without any compensation etchmask structures. Furthermore, the theoretical description has been implemented in a new 3-D simulation tool. Its ability to calculate the shape of simple beam structures of different orientation is experimentally shown.[521]

Pattern induced convex corner undercutting of oriented silicon microridges in potassium hydroxide

“…When exposed to a KOH solution, they undergo undesirable opportunistic etching in an unconventional nature. Unlike the more uniform <111> defined slope of the <110> adjacent micro ridge wall, etching these convex corners forms random surfaces that cause severe damage to these corners in extreme cases (Enoksson et al 1997;Mayer et al 1990;Offereins et al 1991;Puers and Sansen 1990;Schroder et al 2001;Schroder and Obermeier 2000;Zhang et al 1996). The study further elucidate that microridges oriented further away from the <110> plane experience exacerbated structural anhilation compared to those oriented along the latter plane.…”

Three-Dimensional Simulation of Orientation-Dependent Wet Chemical Etching

“…Its extension and its implementation in our simulation tool have been described in detail in [3,4]. Employing numerical simulation we could demonstrate the applicability of the step flow model to complex three-dimensional structures.…”

“…The processing of large and bulky movable elements, which is the typical application of bulk micromachining, requires long etching times, since the substrate has to be etched in its whole thickness. Here, properly designed corner compensation structures derived from realistic simulations (as shown in [3,4], for example) play an essential role to avoid waste of chip area. Additionally, an accurate predictive simulation of the geometric shape of the flexible beams provides a reliable basis to keep the physical properties of the sensor element (stiffness, resonance frequencies, etc.)…”

“…<112>-direction for steps at convex corners), and the step velocity v i (step etch rate) is determined from experimental data [1,3,4]. Additional equations are set up to determine the position of cells j that are exposed by the proceeding of the etch front:…”

“…During the etch process the steplines move in <112> direction. This mechanism may be conceived as peeling off stable {111} planes on the surface in lateral direction [1,4]. The mathematical method used in our simulation approach was originally developed for digital image processing and later adapted to the purpose of efficient topography simulation [2,5].…”