Silicon nitride elaborated by low pressure chemical vapour deposition from Si 2H 6 and NH 3 at low temperature

Abstract

For the elaboration of silicon nitride at a temperature of up to 600°C, plasma-enhanced chemical vapour deposition is generally used and the resulting films present a large amount of electrical defects. In addition, this technique is expensive and is not very well suited for mass production. We present a study of the low pressure chemical vapour deposition of silicon nitride in a horizontal hot-wall furnace. Preliminary experiments at 600°C were carried out, in which we tested the SiH 2Cl 2-NH 3 system. We show that it leads to an unsatisfactory thickness homogeneity and to a poor chemical quality of the film. A second attempt was made using SiH 4 and NH 3. The kinetic results compared with those for Si deposition from SiH 4 showed that, similarly to the case of the addition of PH 3 to SiH 4, NH 3 addition causes an important reduction of the Si supply to the film with poor thickness homogeneity on the wafer. Consequently, we tested the Si 2H 6-NH 3 system at temperatures between 555 and 645°C, with the pressure lower than 200 mTorr. The resulting kinetic data were evaluated, taking into account their variation along the wafer and along the load. The material properties were systematically assessed by ellipsometry measurements, and some samples were characterized by Fourier transform IR spectroscopy. In addition, the passivation capability and the insulating properties were tested by buffer HF etching and I–V measurements respectively. The first general result is that the films are Si rich, with only the lowest Si 2H 6/NH 3 ratio giving stoichiometric Si 3N 4, and that the deposition rate can reach high values (20 Å min −1 or more) for high values of the ratio (according to what can be obtained in the case of Si deposition from Si 2H 6) but with a high Si content in the resulting film. The buffer HF etching rate is low, with a minimum when the Si content is at a maximum, despite the fact that the density of Si-bonds can be quite high (about 5%). The I–V characteristics of the different films show that the stoichiometric films present a breakdown field of about 7 MV cm −1 and that the Si-rich films have a semi-resistive behaviour.