Multilayer SiO2-B2O3-Na2 O Films on Si for Optical Applications

The fabrication of glass by the sol-gel technique has attracted considerable attention in microelectronics, optics and other fields. For applications in integrated optics on silicon substrates, sol-gel offers the possibility of a great variety of compositions and structures. For example, the porosity may allow the insertion of a dopant species, such as nano-crystals for optical non-linearity, into the host glass. Here we have investigated SiO2-B2O3-Na2O compositions, to obtain low process temperatures, and to prepare SiO2 guiding layers with a porosity control using the Vycor glass method.Films of several micron thickness were prepared in many compositions of the SiO2-B2O3-Na2O system. The process included a two-step method to introduce the water while inhibiting crystal formation, a repetitive spin-coating technique in a humidity-controlled chamber, and rapid-thermal-annealing. The results show good quality in terms of homogeneity, absence of cracks, and versatility in the studied compositions. However, films prepared using rapid thermal annealing show a high residual carbon concentration. This causes strong optical absorption, and inhibits the phase separation and leaching associated with the Vycor technique. Reduction of carbon content has been investigated through adaptation of the heat treatment.