INHIBITION OF PYRAMID FORMATION IN THE ETCHING OF SI P[100] IN AQUEOUS POTASSIUM HYDROXIDE-ISOPROPANOL

Production of smooth, defect-free silicon surfaces is essential for th e fabrication of precise three-dimensional devices. Micromachining usu ally involves anisot ropic etching in alkaline media such as potassium hydroxide-isopropanol mixtures (KOH/IPA). The quality of the etched s urfaces is highly dependent on the etching conditions and surface inho mogeneities such as micropyramids or pits can present major problems. In the present investigation, the purity of the reagents used in the e tchant 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; th e best surfaces were obtained with baths prepared with the lowest impu rity content. Dissolved gases had marked effects on surface finish. Py ramid-free surfaces were obtained in oxygen-saturated etchants; this i s attributed to the rapid reaction of oxygen with hydrogen produced at the surface during etching, thereby decreasing hydrogen bubble format ion. The opposite was seen in solutions saturated with nitrogen where pyramid formation increased. The presence of IPA in the nitrogen-satur ated bath improved the surface finish but did not prevent pyramid form ation whereas with oxygen and IPA, defects were completely eliminated and surfaces with a roughness of less than 5 nm were obtained.