Inhibition of pyramid formation in the etching of Si p(100) in aqueous potassium hydroxide-isopropanol

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Abstract
Production of smooth, defect-free silicon surfaces is essential for the fabrication of precise three-dimensional devices. Micromachining usually involves anisotropic etching in alkaline media such as potassium hydroxide-isopropanol mixtures (KOH/IPA). The quality of the etched surfaces is highly dependent on the etching conditions and surface inhomogeneities such as micropyramids or pits can present major problems, In the present investigation, the purity of the reagents used in the etchant and the effects of dissolved gases, such as oxygen and nitrogen, on surface finish have been evaluated for KOH/IPA baths. The purity of both the water and KOH used was found to be extremely important; the best surfaces were obtained with baths prepared with the lowest impurity content. Dissolved gases had marked effects on surface finish. Pyramid-free surfaces were obtained in oxygen-saturated etchants; this is attributed to the rapid reaction of oxygen with hydrogen produced at the surface during etching, thereby decreasing hydrogen bubble formation. The opposite was seen in solutions saturated with nitrogen where pyramid formation increased. The presence of IPA in the nitrogen-saturated bath improved the surface finish but did not prevent pyramid formation whereas with oxygen and IPA, defects were completely eliminated and surfaces with a roughness of less than 5 nm were obtained.