Production of smooth, defect-free silicon surfaces is essential for the fabrication of precise three-dimensional devices. Micromachining usually involves anisot ropic etching in alkaline media such as potassium hydroxide-isopropanol mixtures (KOHIIPA). 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 KOHIIPA 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 sudace 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.

slow also indicates that urea-water hydrogen bonding is only short-lived (cf. a rate of ca. 103 sec-1 for waterwater proton exchange in pure water). This agrees with observations that in water the presence of structure-breaking ions slows down the exchange rate.41 ConclusionsWe can draw the following conclusions about the structures and interactions in a concentrated urea solution. (i) The long-range order characteristic of pure water is destroyed and is not replaced by extended structures involving urea molecules, (ii) There are no urea dimers or higher polymers, (iii) There are extensive short-range, short-lived interactions between water and urea, involving hydrogen bonds to the urea NH2's and possibly to the urea C=0.These conclusions fit the model13 which proposes that urea displaces the water equilibrium from a bulky species involving long-range order to a dense species involving only short-range structure, by resembling water in its capability to form hydrogen bonds but by having the wrong geometry to take part in extended water structures. The resemblance to water explains the high solubility of urea.

Publication costs assisted by the Petroleum Research FundThe reversibly adsorbed hydrogen and deuterium on zinc oxide at -195 and -183°( type III) has been examined by adsorption and ir techniques. The isotherms show a significant isotope effect and yield isosteric heats at low coverage of 2.2-2.5 kcal. Infrared studies and the temperature and pressure dependence show this hydrogen is present in molecular form and is limited to 5 to 10% of the surface. Poisoning studies with water suggest it occurs on the same sites as the dissociative type I hydrogen. Interpretation of the spectral results and the isotope effect suggest that there is a considerable barrier to rotation on the surface. The possible role of such a species in the ortho-para hydrogen conversion is discussed.

No abstract

No abstract