The proposed research and educational efforts focus on the development and application of a new class of image sensor architectures with transformative effects on biomedical optical diagnostics. The aim is to further advance the field of near-infrared spectroscopy by developing miniaturized, multi-spectral, imaging sensors with pixel-level RF detection and signal processing. The proposed work will improve image resolution, reduce acquisition time, and enable portability of frequency-domain diffuse optical tomography (FD-DOT) systems. A mixed-mode design methodology for high-sensitivity optical imaging sensors optimized for phase-sensitive detection of RF-modulated optical signals will be explored. The optical and electrical performance of photodetector structures in nanometer-scale VLSI technologies will be evaluated with an emphasis on the affect of new high-k dielectric materials and metal layer stacking on photoresponse.
The resulting research will have broader impacts in many areas, including medical optical diagnostics, high-performance image sensor technology, and the semiconductor industry workforce. A central focus of the proposed activities is to broaden opportunities to students from underrepresented groups and engage students at all levels in interdisciplinary research and integrated circuit design training. A summer enrichment program will be launched, incorporating IC CAD tool training, coursework, a team-based research project, and visits to labs/companies in the greater Boston area. There is a strong mentoring and training component for students at all levels.
The measurement of absorption and scattering of NIR light upon interaction with biological chromophores and structures is rapidly evolving as a promising non-invasive technique for fundamental studies in neurobiology and in analyzing the structural and physiological properties of biological media. Frequency-domain (FD-NIRS) techniques, using sinusoidally-modulated excitation light, allow explicit separation of tissue absorption and scattering coefficients. These wavelength-dependent optical parameters can provide information about the concentration of important biological chromophores, including hemoglobin, water, fat, NIR-absorbing drugs, and the composition, density, and organization of biological tissue structures (cells, organelles). The key merits of NIR optical imaging techniques lie in the ability to design completely non-invasive instrumentation that can be made portable, unobtrusive, low-cost, low-power, and robust to motion artifacts. There are growing efforts to miniaturize the FD-NIRS for portable, point-of-care diagnostic applications. Compared to benchtop designs, CMOS integrated NIR sensors offer several performance advantages by drastically reducing noise and enabling realization of dense arrays of sensors to increase spatial mapping and detection at multiple tissue sites. This research project has made several key contributions to the area of portable optical imaging tools for biomedical applications. We have developed novel optical sensing architectures that can provide a higher level of integration, sensitivity, resolution and overall performance superior to current commercial imaing sensors for time-resolved NIR imaging applications. Several CMOS compatible photodetectors were studied to find a photodetector with sufficient responsivity and bandwidth for FD-NIRS studies. Simple photodiodes have good bandwidth but suffer from lowest responsivity. Phototransistors have sufficient high responsivty due to the built-in bipolar transistor, however, the speed is inferior and is only suitable to low frequency application. Among all of them, CMOS APD offers very excellent responsivity with decent bandwidth. Two CMOS APDs are fabricated and characterized using in an 180nm CMOS technology. The input noise of conventional transimpedance amplifier is a critical parameter for this application and is analyzed in-depth to reveal the fundamental limitation. The front-end SNR is greatly limited by the TIA input current noise. Resistive feedback TIA structures are able to deliver high transimpedance gain with low noise compared to other TIA topologies. The noise of the resistive feedback TIA can be further reduced using multistage voltage amplifier. However, the noise is still not small enough to effectively boost up the signal from the photodiode. The most feasible way to reduce the TIA noise is to reduce the TIA bandwidth. A novel frequency-mixing TIA (FM-TIA) with narrowband response is implemented to significantly reduce the front-end noise. The novelty of FM-TIA lies on combining the frequency translation and transimpedance amplification into one, which not only reduces noise but also simplifies the front-end design. The proposed FM-TIA is fabricated in a 180nm CMOS process to demonstrate its functionality. The measurement results show that the FM-TIA drastically reduces the noise compared with convential front-end amplifier designs with good linearity. The research results from this project have inspired the design of disruptive integrated image sensor technologies for a host of non-invasive studies of biological structures. The development of arrayed sensors with integrated photonic devices operating in the NIR region and complex signal processing for data extraction will suport a wide community of biological researchers using noninvasive, NIR optical techniques to: a) Characterize in vivo macroscopic optical properties (absorption and scattering) of multiply scattering tissues; b) Understand the spatial and temporal correlations between fMRI BOLD (blood oxygen level dependent) signals and changes in oxy- and deoxyhemoglobin concentration during brain activation; and c) Develop theories relating to the biophysical mechanism of correlated coupling between evoked potential activation of nerves and changes in the hemodynamic delivery of glucose and oxygen to local neural tissues through blood vessels. Further research on these topics is currently being pursued through the following grant program awarded based on preliminary results from this BRIGE grant: CAREER: Wireless Optical Sensors for High Resoultion Imaging of Biological Structures. The educational outreach program launched through this BRIGE grant involved an intensive internship program designed to give students an opportunity to (1) explore electrical engineering sub-disciplines in VLSI design and microelectronic circuits, (2) gain insights into graduate research, and (3) engage in interdisciplinary team work that will be useful throughout their careers. Three students were selected to participate in the summer enrichment program. The goal of the summer internship project was to design a miniaturized wireless optical sensor for use in near-infrared functional brain studies. Each student was assigned a graduate student mentor to study the design of the sensor front-end electronics and understand the design trade-offs (power, gain, bandwidth, sensitivity) involved in developing the optical sensor according to application-specific design specifications. Interns received training on lab instruments, computer-aided design (CAD) software tools, printed circuit board design and fabrication, and project background material.
