Silicon Dioxide and Oxynitrides

The only other plasma-enhanced CVD film that has seen wide use in integrated circuit manufacture is the plasma oxide film. We say so-called because it is not truly Si02, but rather SiOxNyHz. In fact, it is just this ability to modify the film stoichiometry that makes these films so valuable. Many of the film characteristics change depending on this stoichiometry, so it allows a freedom to alter film characteristics that is not possible with thermally-grown films. 

Plasma oxide can be grown from a number of oxidizers plus SiH4. Among these are N20, 02, C02 and even TEOS (tetraethoxysilane). Generally, 02 is not used as it too often leads to homogeneous nucleation. The preferred reactants have proven to be SiH4 and N20, so we will restrict our discussion to these. Films grown in both cold-wall and hot-wall reactors will be considered. 

Plasma oxide has found utility in high-frequency applications for dual-layer isolation,8 because of its low dielectric constant and high breakdown voltage. Also, it is in compression when deposited, so that it can be used as the dielectric when thick films (2 to 5 microns) are needed. Such thick films when deposited by thermal CVD (which is deposited in tension) tend to crack. One final advantage to the use of plasma oxide rather than plasma nitride is that 

If desired, plasma oxide films can be doped much as the plasma nitride film we discussed earlier. In fact, doping with boron and phosphorus has been carried out as an alternative to the standard atmospheric-pressure thermal CVD process for BPSG.11 12 The latter process has the drawbacks of high defect density and poor thickness uniformity, so it was hoped that plasma BPSG would be an improvement. However, there are differences in the films in terms of H2 and N2 content, and their effect on reflow temperature, intrinsic stress and passivation effectiveness had to be examined. 

Depositions were done in a hot-wall tubular plasma reactor at a frequency of 410 kHz. Deposition temperatures ranged from 300° to 400°C, and pressures from 830 mTorr to 1.5 Torr. Gases used were SiH4, N20, 02, PH3 and B2H6, with the latter two diluted by argon. The boron and phosphorus concentrations were adjusted by changing the reactant gas mixture. 

---