Browsing by Subject "heterogeneous integration"

Data acquisition and diagnostics for chemical and biological analytes are critical to medicine, security, and the environment. Miniaturized and portable sensing systems are especially important for medical and environmental diagnostics and monitoring applications. Chip scale integrated planar photonic sensing systems that can combine optical, electrical and fluidic functions are especially attractive to address sensing applications, because of their high sensitivity, compactness, high surface specificity after surface customization, and easy patterning for reagents. The purpose of this dissertation research is to make progress toward a chip scale integrated sensing system that realizes a high functionality optical system integration with a digital microfluidics platform for medical diagnostics and environmental monitoring.

This thesis describes the details of the design, fabrication, experimental measurement, and theoretical modeling of chip scale optical sensing systems integrated with electrowetting-on-dielectric digital microfluidic systems. Heterogeneous integration, a technology that integrates multiple optical thin film semiconductor devices onto arbitrary host substrates, has been utilized for this thesis. Three different integrated sensing systems were explored and realized. First, an integrated optical sensor based upon the heterogeneous integration of an InGaAs thin film photodetector with a digital microfluidic system was demonstrated. This integrated sensing system detected the chemiluminescent signals generated by a pyrogallol droplet solution mixed with H2O2 delivered by the digital microfluidic system.

Second, polymer microresonator sensors were explored. Polymer microresonators are useful components for chip scale integrated sensing because they can be integrated in a planar format using standard semiconductor manufacturing technologies. Therefore, as a second step, chip scale optical microdisk/ring sensors integrated with digital microfluidic systems were fabricated and measured. . The response of the microdisk and microring sensing systems to the change index of refraction, due to the glucose solutions in different concentrations presented by the digital microfluidic to the resonator surface, were measured to be 95 nm/RIU and 87nm/RIU, respectively. This is a first step toward chip-scale, low power, fully portable integrated sensing systems.

Third, a chip scale sensing system, which is composed of a planar integrated optical microdisk resonator and a thin film InGaAs photodetector, integrated with a digital microfluidic system, was fabricated and experimentally characterized. The measured sensitivity of this sensing system was 69 nm/RIU. Estimates of the resonant spectrum for the fabricated systems show good agreement with the theoretical calculations. These three systems yielded results that have led to a better understanding of the design and operation of chip scale optical sensing systems integrated with microfluidics.

Patient outcomes for colon and esophageal cancers depend on early detection and surveillance of the progression of dysplasia (abnormal cellular growth indicative of pre-cancer) and malignancy (cancer) in the epithelium of these organs. The present standard of care for detection and surveillance of these diseases is to inspect the surface of the colon and esophagus searching for high-risk tissue using white light endoscopy (WLE), which provides imaging of the organ surface. While WLE is effective for the detection of some high-risk tissues such as colon polyps, other types of high-risk tissue, such as flat dysplasia, may not be visually observable to the physician by WLE. The limited efficacy of WLE results in reduced adenoma detection rates, unnecessary resection of healthy tissues, incomplete resection of high-risk tissues, and necessitates random sampling of esophageal or colon regions known to be at elevated risk for flat dysplasia, such as in cases of Barrett’s esophagus or inflammatory bowel disease. Patient outcomes for colon and esophageal cancer can be improved if the capabilities of WLE can be augmented to readily detect the microstructural and biochemical characteristics of these diseases in tissues during endoscopy.

Diffuse reflectance spectroscopy (DRS) is a label free optical technique that can be used to sense the morphological and biochemical changes that occur when normal colon or esophageal tissue becomes pre-cancerous or cancerous, and DRS has the potential to be used to augment the performance of traditional WLE. However, the reliance of conventional DRS systems on optical fibers for reflectance collection limits system performance, limits scalability, and constrains the design options.

Custom thin film silicon (Si) photodetectors (PDs) offer a high performance, scalable, and highly customizable alternative to optical fibers for reflectance photon collection in tissue DRS applications. Si PDs improve performance over optical fibers since they have superior numerical apertures (NAs) and collect light from a larger fraction of the probe surface; they are highly scalable due to Si processing technology; they can be geometrically optimized for specific applications; and they can be implemented on a variety of substrates, including flexible or conformal, and can be implemented as an attachment to an endoscope. And arrays of Si PDs can be used perform spatially resolved DRS (SRDRS), which adds depth sensitivity to DRS by simultaneously measuring the tissue reflectance at multiple illumination-collection distances.

This dissertation describes the design, fabrication, optoelectronic characterization, phantom studies with reduced scattering coefficient (μ_s^' (λ)) and absorption coefficient (μ_a (λ)) extraction, and preliminary clinical testing on ex-vivo human colon tissue of the first Si thin film multi-pixel SRDRS array optimized for a specific tissue measurement application. The illumination-collection geometry of the Si PD based SRDRS sensor array described herein was designed using Monte Carlo simulation to maximize performance when characterizing human colon tissue. The design was then implemented using 10 μm thick crystalline Si PD arrays heterogeneously integrated onto a transparent substrate and packaged for phantom and clinical ex-vivo human tissue testing. The performance of the fabricated sensors was initially evaluated on synthetic liquid tissue phantoms, with comparisons of the experimentally measured diffuse reflectance from these phantoms compared to Monte Carlo simulated diffuse reflectance, yielding a mean discrepancy of 8.4% over the 450 to 650 nm wavelength range.

To enable extraction of tissue optical properties from measured tissue diffuse reflectance, two inverse DRS methods were developed which relied on look-up tables of Monte Carlo simulated diffuse reflectance. One method extracts spectrally constrained values of μ_s^' (λ) and μ_a (λ) from measurements of diffuse reflectance independently for each PD in the SRDRS sensor. A second model uses the radial dependence of the PDs in the SRDRS sensor to both spatially and spectrally constrain the extracted values of μ_s^' (λ) and μ_a (λ). The average percent error between expected (known) and extracted μ_s^' (λ) and μ_a (λ) values of eight liquid phantoms used to evaluate the inverse models was 13.25% and 31.42%, respectively, for the first inverse method, and 6.38% and 8.38%, respectively, for the second, for diffuse reflectance measurements spanning the 450 nm to 750 nm wavelength range. These results indicate that use of three PDs in the SRDRS sensor can reduce the error for tissue optical property extraction.

Further assessment of the Si PD array performance and tissue compatibility was conducted on ex-vivo porcine esophageal tissue, and an experimental set-up to enable clinical measurements of resected human colon tissue samples was developed in collaboration with physicians in the Duke University Medical Center: Dr. N. Lynn Ferguson, MD, Assistant Professor of Pathology in the Duke University Department of Pathology, Dr. Katherine Garman, MD, Associate Professor of Medicine in the Duke University Department of Medicine, and Dr. Deborah Fisher, MD, Associate Professor of Medicine in the Duke University Department of Medicine.

SRDRS measurements on excised normal and tumorous human colon tissue were performed with a prototype thin film Si PD based SRDRS probe, and the absorption and reduced scattering optical properties of the tissue were extracted. The extracted optical properties are comparable to previously reported values optical properties for similar tissue and indicate that the prototype system is able to extract similar tissue information as other more expensive and less scalable tissue spectroscopic systems.

The thin film Si PD based SRDRS probe and experimental work presented herein contributes to the field of Si PD based tissue DRS by being the first demonstration of a thin-film Si PD based DRS or SRDRS system in direct contact with human and animal tissues. The development, experimental validation, and demonstrated application to analysis of SRDRS measurements of human colon tissue, of a method for tissue optical property extraction described herein is the first inverse method to demonstrate an improvement of optical property extraction in a Si PD based system by utilizing the spatial dependence of reflectance, compared to a single PD based extraction method. The novel inverse model extraction method presented herein, which utilizes multiple Monte Carlo populated LUTs of diffuse reflectance to spatially and spectrally constrain extracted optical properties, is a contribution to the field of tissue optical property extraction from SRDRS measurements. Finally, the report of μ_s^' (λ) and μ_a (λ) properties for unfrozen ex-vivo human colon tissue adds to the sparsely reported data of this type in the literature, contributing to the general field of biophotonics.

The integration of planar on-chip light sources is a bottleneck in the implementation of portable planar chip-scale photonic integrated sensing systems, integrated optical interconnects, and optical signal processing systems on platforms such as Silicon (Si) and Si-CMOS integrated circuits. A III/V on-chip laser source integrated onto Si needs to use standard semiconductor fabrication techniques, operate at low power, and enable efficient coupling to other devices on the Si platform.

In this thesis, thin film strain compensated InGaAs/GaAs single quantum well (SQW) separate confinement heterostructure (SCH) edge emitting lasers (EELs) have been implemented with patterning on both sides of the thin film laser under either growth or host substrate support, with the devices metal/metal bonded to Si and SiO₂/Si substrates. Gain and index guided lasers in various configurations fabricated using standard semiconductor manufacturing processes were simulated, fabricated, and experimentally characterized. Low threshold current densities in the range of 250 A/cm2 were achieved. These are the lowest threshold current densities achieved for thin film single quantum well (SQW) lasers integrated on Si reported to date, and also the lowest reported, for thin film lasers operating in the 980 nm wavelength window.

These thin film EELs were also integrated with photolithographically patterned polymer (SU-8) waveguides on the same SiO₂/Si substrate. Coupling of the laser and waveguide was compared for the cases where an air gap existed between the thin film laser and the waveguide, and in which one facet of the thin film laser was embedded in the waveguide. The laser to waveguide coupling was improved by embedding the laser facet into the waveguide, and eliminating the air gap between the laser and the waveguide. Although the Fresnel reflectivity of the embedded facet was reduced by embedding the facet in the polymer waveguide, leading to a 27.2% increase in threshold current density for 800 &mum long lasers, the slope efficiency of the L-I curves was higher due to preferential power output from the front (now lower reflectivity) facet. In spite of this reduced mirror reflectivity, threshold current densities of 260 A/cm2 were achieved for 1000 &mum long lasers. This passively aligned structure eliminates the need for precise placement and tight tolerances typically found in end-fire coupling configurations on separate substrates.