Researchers at Brigham Young University (BYU) have developed an optical biosensor that can quickly analyze a sample to detect any type of nucleic acid based organism, such as viruses or bacteria. The biosensor's reliability over time is compromised due to water absorbing into the SiO2 waveguides of the chip. It was hypothesized that keeping the thin-film stress of the waveguides close to zero would slow or stop water absorption from occurring. Completion of this thin-film study relied upon a new plasma enhanced chemical vapor deposition (PECVD) machine and a new 3-D optical profilometer, both of which were installed in the BYU cleanroom. The new PECVD machine was much more capable than previous machines at controlling deposition parameters and was a critical component in accurately controlling the intrinsic stress of deposited films. The 3-D optical profilometer provided us a way to accurately measure the intrinsic stress of the films. Rib waveguides made from different stressed SiO2 films were fabricated over anti-resonant reflecting optical waveguide (ARROW) layers. The wafers were baked out, cleaved, and their initial throughputs recorded. All waveguides were placed in a humid environment and were removed periodically to check their optical throughput. The resulting measurements were normalized to the highest measured throughput to determine how throughput was changing over time. Rib waveguides with the lowest stressed SiO2 had the slowest rate of throughput change, dropping to 50% of the original throughput after 40 days in the humid environment. The +50 MPa stressed waveguides performed next best, dropping to 20% of the original throughput after 40 days. The +100 MPA stressed waveguides dropped to 20% throughput after 16 days while the -50 MPa stressed wafers dropped to 20% optical throughput after 7 days. Keeping the stress of the film as low as possible helped reduce the rate of water absorption, but did not eliminate it completely. A method involving the use of high index difference buried channel waveguides is shown to be effective at stopping the effects of water absorption in our waveguides.



College and Department

Ira A. Fulton College of Engineering and Technology; Electrical and Computer Engineering



Date Submitted


Document Type





ARROW, waveguide, stress, PECVD, film, water absorption