Deposition and characterization of thick graded index SixOyFz films with low stress

This paper develops deposition techniques that are suitable for realizing low stress thick and crack free fluorine doped graded index silica (SixOyFz) films by the hollow cathode plasma enhanced chemical vapour deposition (HC-PECVD) system from a mixture of SiH4/O2/CF4. Silica films are deposited by varying O2, SiH4, CF4 flow rates, and RF powers. The refractive index, stress, deposition rate, chemical bonding state and chemical composition of such films have been investigated. The results indicate that low stress thick graded index silica film can be deposited by either (i) employing RF power below a certain value, which we shall refer it as 'threshold RF power' or (ii) increasing O2/SiH4 flow rate ratios. The threshold RF power is found to be 200 W using 50 sccm of O2, and 20 sccm of SiH4. At RF powers below this threshold value, no oxygen deficiency is observed in the film even at a higher CF4 flow rate. On the other hand, stressed silicon rich film results at CF4 flow rates of 10 sccm when the RF power is increased to 300 W. By increasing O2 flow rate to 100 sccm and decreasing SiH4 flow rate to 15 sccm, the plasma at 300 W RF power remains not oxygen deficient for up to 50 sccm of CF4 flow rates. These deposition parameters are employed to deposit 28 μm thick 1-D GRIN lens with a net stress of only 30 MPa (compressive), and the graded index profile is optically tested. Moreover, the effect of moisture on such films has been investigated and a simple preventative measure is proposed.