The Etching of Silicon Nitride in Phosphoric Acid with Silicon Dioxide as a Mask

Abstract

The water content of phosphoric acid in etching silicon nitride and silicon dioxide plays an important role. An increase in water content increases the etch rate of silicon nitride and decreases the etch rate of silicon dioxide. The highest possible temperature for a fixed water content at atmospheric pressure in the system is realized by boiling the liquid and refluxing the vapor phase. Refluxed boiling phosphoric acid at 180°C was found to be a useful etchant for silicon nitride films. The etch rate is 100 Aå/min. Under the same conditions deposited silicon dioxide had an etch rate of 0–25 Aå/min depending on the method of preparation, and elemental silicon 3 Aå/min. Etch rates of silicon nitride, silicon dioxide, and silicon in refluxed boiling phosphoric acid were measured as a function of temperature (and concentration) in the range of 140°–200°C. All etch rates increased with temperature. The “apparent” activation energies are 12.7, 27.6, and 26.4 kcal/mole, respectively. The etch rate of silicon nitride in phosphoric acid of constant concentration was measured as a function of temperature only. In this case the “real” activation energy was 22.8 kcal/mole. The difference in etch rate between silicon nitride, deposited silicon dioxide, and silicon offers a technique for etching contact holes in silicon nitride using deposited silicon dioxide as a mask. Such a technique was used successfully in making transistors with silicon nitride over as a junction seal.