Anisotropic Etching of Three-Dimensional Shapes in Silicon

Abstract

As a result of the exposure to solution of different crystallographic facets during anisotropic etching of three-dimensional structures in silicon, the open-circuit potential of the semiconductor can change markedly. Using a ͑100͒Si substrate, masked to reveal ͑111͒ facets, we show that such a shift in potential can alter the chemical etch rates of the individual facets. The extent of the changes depends both on the facets exposed and on their relative areas. Because the surface geometry, and with it the silicon potential, change continuously in time, chemical etching must adapt continuously to these changes. This is an interesting example of a self-regulating system with a complex feedback mechanism. The effects described in this work clearly influence anisotropic etching ratios and are therefore important for the fabrication of microelectromechanical systems. Anisotropic etching of silicon in alkaline solutions is an important step in the technology of microelectromechanical systems ͑MEMS͒.1 Etching is employed to make high-definition, threedimensional structures in silicon wafers. These are required for a range of applications including miniature sensors for the measurement of pressure, flow, angular rate, and velocity. Other microsystems, such as filters, resonators, valves, pumps, and optomechanical connectors, can also be fabricated. The form that is obtained depends on the surface orientation of the wafer, the mask geometry, and the relative dissolution rates of the various crystallographic faces exposed during etching. The low etch rate of the ͑111͒ face with respect to that of other faces is exploited to produce deep grooves with flat walls, inverted pyramids, V-shaped grooves, and various other forms.1-3 For ͑100͒-oriented wafers, widely used in MEMS technology, the ratio of the etch rates of ͑111͒ and ͑100͒ faces, referred to here as the anisotropic ratio, is of vital importance for making high-resolution micromechanical structures. Although in the most favorable case the anisotropic ratio can exceed 100, 4 control and reproducibility at this level are poor. The composition of the etchant and the etching conditions as well as the ͑mis͒orientation of both the wafer surface and the mask edge are clearly important. Other factors which may play a role are dislocations in the silicon, defects at the silicon-mask interface, and impurities ͑even at very low concentrations͒ in the etchant.

Influence of Applied Potentials on Anisotropic Etching of Silicon Described Using Kinematic Wave Etch Model

“…Results above −875 mV vs SCE show the highest decrease as electrochemical oxidation of Si to SiO 2 becomes predominant, resulting in a passivated Si surface. 28 The results are consistent with cyclic voltammogramic measurements performed on large ͑111͒ surfaces. 3 The onset of electrochemical oxidation of the bulk ͑111͒ surface, reflected in the increase in anodic current in a cyclic voltammogram, coincides with the decrease in v k111 .…”

Anisotropy in the Anodic Oxidation of Silicon in KOH Solution

“…In this way we could study the electrochemistry starting with the original (100) surface and follow the evolution of the V-shaped grooves defined by (111) facets, finally obtaining pure (111) surfaces. 2,8 This approach has the extra advantage of allowing a comparison of (100) and (111) orientations in the same wafer. Potentiodynamic and potential-step measurements were performed.…”

Anisotropy in the wet-etching of semiconductors

“…The open-circuit potential changes by as much as 200 mV. Such changes can alter the etch rates of the individual facets and thus the anisotropy of the system [23]. Raisch et al used an explanation based on galvanic interaction between (1 0 0) and (1 1 1) faces to explain the stability of micropyramids formed on (1 0 0) surfaces during anisotropic etching [24].…”