Interface engineering during plasma-enhanced chemical vapor deposition of porous/dense SiN_1.3 optical multilayers

We investigate the growth of dense/porous SiN_1.3 multilayers by dual-mode radiofrequency/microwave (RF/MW) plasma-enhanced chemical vapor deposition (PECVD) using in situ real-time spectroscopic ellipsometry (RTSE) and post-deposition atomic force microscopy (AFM). Dense (d-) SiN_1.3 films grown under high-energy ion bombardment on flat c-Si (001) substrates are very smooth (root mean square (RMS) roughness R=0.5 nm) and exhibit bulk-like optical properties (n=l.9 at 550 nm). In contrast, porous (p-) SiN_1.3 films deposited at low ion energies are porous and columnar, with n∼1.6 and R∼2 nm. When SiN_1.3 layers are grown under high ion-energy bombardment conditions on porous films, RTSE analyses reveal a rapid filling of surface porosity, at a rate depending on the incident precursor flux, before the total film thickness starts to increase. Films prepared under these conditions exhibit a lower average n (∼1.8), an ascending index depth profile, and a higher roughness (R>3 nm) than the layers deposited under identical conditions on initially flat surfaces. We also demonstrate that the use of Ar ion bombardment, under RF plasma conditions, is an effective method for decreasing the roughness of porous films. SiN_1.3 layers deposited under RF plasma conditions on such surface engineered substrates are dense with high and uniform n values. Such an approach has been successfully used to grow dense/porous multilayer optical interference filters. It efficiently stops roughness propagation through the multilayer stack and allows growing filters with optical characteristics essentially identical to those predicted from the design.