Anisotropic Etching of Crystalline Silicon in Alkaline Solutions: I . Orientation Dependence and Behavior of Passivation Layers

Abstract

The anisotropic etching behavior of single-crystal silicon and the behavior of SiO2 and Si3N4 in an ethylenediaminebased solution as well as in aqueous KOH, NaOH, and LiOH were studied. The crystal planes bounding the etch front and their etch rates were determined as a function of temperature, crystal orientation, and etchant composition. A correlation was found between the etch rates and their activation energies, with slowly etching crystal surfaces exhibiting higher activation energies and vice versa. For highly concentrated KOH solutions, a decrease of the etch rate with the fourth power of the water concentration was observed. Based on these results, an electrochemical model is proposed, describing the anisotropic etching behavior of silicon in all alkaline solutions. In an oxidation step, four hydroxide ions react with one surface silicon atom, leading to the injection of four electrons into the conduction band. These electrons stay localized near the crystal surface due to the presence of a space charge layer. The reaction is accompanied by the breaking of the backbonds, which requires the thermal excitation of the respective surface state electrons into the conduction band. This step is considered to be rate limiting. In a reduction step, the injected electrons react with water molecules to form new hydroxide ions and hydrogen. It is assumed that these hydroxide ions generated at the silicon surface are consumed in the oxidation reaction rather than those from the bulk electrolyte, since the latter are kept away from the crystal by the repellent force of the negative surface charge. According to this model, monosilicic acid Si(OH)4 is formed as the primary dissolution product in all anisotropic silicon etchants. The anisotropic behavior is due to small differences of the energy levels of the backbond surface states as a function of the crystal orientation.

Anisotropy in the Anodic Oxidation of Silicon in KOH Solution

“…These factors can account for the much higher etch rate of Si(100) and the observation that terraces on (111) surfaces are "stable" and that etching of Si(111) follows a step-flow mechanism. 1,2,17,18 (2) Anodic Oxidation: n-Type Si. Anodic oxidation of an n-type semiconductor can only occur by injection of electrons from surface bonds into the conduction band.…”

“…For example, chemical etching of silicon in alkaline solution is strongly anisotropic; the etch rate depends on the crystal face exposed to the etchant. 1 This dependence allows a range of characteristic forms to be etched in single-crystal silicon, a process widely used in MEMS (microelectromechanical systems). 2,3 Anodic oxidation of silicon is also an important step in MEMS technology.…”

Properties of light-emitting porous silicon photoetched in aqueous HF∕FeCl3 solution

“…Then, the electron energy of the pure HF solution with respect to vacuum is Ϫ4.36 eV ͑ l ͒. 32 After immersion of the silicon substrate into the electrolyte in the dark, the Fermi levels E F and E F, redox on both sides of the n-Si/electrolyte interface are brought to be the same energy level by a transfer of electrons from the silicon substrate into the electrolyte ͓Fig. 7͑a͔͒.…”

Effect of Low Temperature on the Fabrication of Microring Resonator by Wet Etching

“…Typically used etchants for Si include Potassium Hydroxide (KOH), thylenediamine-pyrocatechol-water (EDP) and hydrazine-water (Seidel et al, 1990). Hydrazine water and EDP handling must be done with special precaution because of its toxicity and instability.…”