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

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 H2O-P2O5 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 Å/min. Under the same conditions deposited silicon dioxide had an etch rate of 0-25 Å/min depending on the method of preparation, and elemental silicon 3 Å/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 (94.5% H3PO4) 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 SiO2 as a junction seal. A good deal of attention has recently been focused on silicon nitride (Si3N4) as a semiconductor junction seal, partly or fully replacing silicon dioxide. The reason for this is that Si3N4 is a better mask than SiO2 against diffusants of all kinds, including water, oxygen and sodium. The better masking properties of Si3N4 go hand-in-hand with lower etch rates in HF or buffered HF. Silicon nitride films prepared by various deposition processes (SiCl4 + NH3, SiH4 + NH3 and d-c plasma) were reported to exhibit “very slight” diffusion of sodium (1). These films all had low etch rates in buffered HF (~10Å/min). This meant that new ways of etching contact holes had to be developed because photoresist material, now used for etching SiO2, does not mask sufficiently against buffered HF at the excessively long etching times necessary (2 hours for a film 1200Å thick). Silicon nitride films have a reasonably fast etch rate (200-300 Å/min) in concentrated HF (48%), but it was found that the ordinary photoresist materials, as used for SiO2, do not mask sufficiently against the concentrated HF. Even if a good etching mask for concentrated HF were available, the etching of a contact hole in Si3N4 which is deposited on an SiO2 film will present the problem of undercutting because the underlying SiO2 is etched much faster than Si3N4 in concentrated HF. 1 Composition: 40g NH4F + 80 ml H2O +15 ml 48% HF IMTEC Imtec acculine, inc. Pub: 15-A143-0298 Page 2 This paper reports etching experiments with Si3N4, SiO2, and Si in phosphoric acid. It was found that Si3N4 has a high etch rate in refluxed boiling phosphoric acid, compared to SiO2 and elemental Si. Accordingly, phosphoric acid can be applied as an etchant for Si3N4 using deposited SiO2 as a mask.