Multilayer SiO2-B2O3-Na2O films on Si for optical applications

The fabrication of glass by the sol-gel technique has attracted considerabl e 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 porosit y may allow the insertion of a dopant species, such as nano-crystals for op tical non-linearity, into the host glass. Here we have investigated SiO2-B2 O3-Na2O compositions, to obtain low process temperatures, and to prepare Si O2 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 te chnique in a humidity-controlled chamber, and rapid-thermal-annealing. The results show good quality in terms of homogeneity, absence of cracks, and v ersatility in the studied compositions. However, films prepared using rapid thermal annealing show a high residual carbon concentration. This causes s trong optical absorption, and inhibits the phase separation and leaching as sociated with the Vycor technique. Reduction of carbon content has been inv estigated through adaptation of the heat treatment.