Walter Scott, Jr. College of Engineering

Graduate Exam Abstract

Guangwei Yuan
Ph.D. Final
May 07, 2008, 9.00am-11.00am
Wagar 107B
Characterization of integrated optical waveguides
Abstract: <br>
The need for biosensors has dramatically increased over the last decade. A widely used medical biosensor is the blood glucose biosensor that helps to save thousands of lives each year. Integrated optical biosensors are attractive among all available biosensors for their low-cost, compactness, and portability, especially in general medical use that would save millions of dollars each year in pre-therapy examination and clinical test cost. Besides public health use, these advanced biosensors would be used in the war on terrorism by alerting troops to biological attacks. Although advantages of using integrated optical waveguide deices are obvious, the investigations are far from complete. There are many problems that need to be addressed in new device development. At the Optoelectronics Research Lab in ECE at CSU, we explore the issues of design, modeling and measurement of integrated optical waveguide devices of interest, such as optical waveguide biosensors and on-chip optical interconnects. The measurement of this sensor requires the aid of either a commercial near-field scanning optical microscope (NSOM) or new proposed buried detector arrays. This novel lab-on-chip optical waveguide biosensor meets the needs for low-trace biological detection without florescent chemical agent aids. The fabrication of the complete sensor with buried detectors was done using the 0.35 micrometer CMOS process at Avago Technologies, Fort Collins, Colorado. The detailed fabrication processes were described. Experimental results were demonstrated for using NSOM and buried detectors. The ultimate sensor can be made so compact as to enable portable applications. By implementing the sensor array on a silicon chip, multiple analytes, such as viruses, DNA and chemical agents, can be sensed simultaneously. The local evanescent-field array coupled (LEAC) sensor was first used to detect pseudo-adlayers on the waveguide top surface. These adlayers include SiNx, crystal bond, and photoresist. The field modulation that was obtained based on NSOM measurement was approximately 80% for a 17 nm SiNx adlayer that was patterned on the waveguide using plasma reactive ion etching. Later, single and multiple regions of immunoassay complex adlayers were analyzed using NSOM. The most recent results demonstrated the capability of using this sensor to differentiate immunoassay complex region with different surface coverage ratio. For full and zero coverage CRP immunoassay adlayer, the difference in optical intensity modulation is more than 30%. The study on buried detectors revealed a higher sensitivity of the sensor to a thin organic film on the waveguide. By detecting the optical intensity decay rate, the sensor was able to detect a few nanometer film with 1.7 dB/mm/nm sensitivity. In bulk material analysis, this sensor demonstrated more than 15 dB/mm absorption coefficient difference between organic oil and air upper claddings. In further analysis, the optical interference phenomena was observed after the adlayer region that indicated the first order leaky waveguide mode in addition to the fundamental mode was excited by the existence of the adlayer. These experimental results matched well with numerical simulations using beam propagation method (BPM). In conjunction with the biosensor study, fiber-to-waveguide coupling, waveguide polishing and photolithography techniques were also investigated. Two generations of on-chip optical interconnect chips were designed, modeled and measured. The first generation chip consists of optical waveguide test structures and leaky-mode waveguide coupled photodetectors. A 16-node H-tree waveguide was used to deliver light into photodetectors and characterized. Photodetectors at each end node of the H-tree were measured using near-field scanning microscopy. The 0.5 micrometer wide photodetector demonstrated up to 80% absorption rate over just a 10 micrometer length. This absorption efficiency is the highest among reported leaky-mode waveguide coupled photodetectors. The responsivity and quantum efficiency of this photodetector are 0.35 A/W and 65%, respectively. The second generation chip incorporated modifications based on the first generation design. The CMOS circuitry was for the first time added into the chip to recover the optical clock. The second generation chip exploits a thicker upper cladding that obeys the design rules and accommodates a second metal layer. Analysis revealed the effect of upper cladding index on attenuation coefficients. For example, some chips pulled out before deposition of upper cladding layer were experimentally studied. Due to low refractive index contrast, the absorption coefficient drops about 70%. In contrast, the waveguide loss in the H-tree splitter region drops dramatically. Based on the observation, a new fabrication process is proposed. To better understand the performance of leaky-mode waveguide coupled photodetectors, a one-dimensional finite difference model is used to calculate leaky mode profiles, propagation constants and attenuation coefficients. The numerical calculation demonstrates a great match with the near field scanning optical microscopy results.
Adviser: Dr. Kevin L. Lear
Co-Adviser: NA
Non-ECE Member: Dr.David S. Dandy (Chemical & Biological Engineering)
Member 3: Dr.Tom W. Chen
Addional Members: NA
Program of Study: