We report simulated and experimental image quality for contact imaging, a method for imaging objects close to the sensor surface without intervening optics. This technique preserves microscale resolution for applications that can not tolerate the size or weight of conventional optical elements. In order to assess image quality, we investigated the spatial resolution of contact imaging, which depends on the sensor size as well as the distance between objects and the sensor surface. We studied how this distance affects image quality using a commercial optical simulator. Simulation results show that the image quality degrades as objects move away from the sensor surface. To experimentally validate these results, an image sensor was designed and fabricated in a commercially available three metal, two poly, 0.5 m CMOS technology. Experiments with the contact imager corroborate the simulation results. Two specific applications of contact imaging are demonstrated.Index Terms-CMOS analog integrated circuits, contact imaging, geometrical optics, image resolution, image sensors, mixed signal analog-digital integrated circuits.
Abstract-This paper reports a novel integrated circuit for fluorescence sensing. The circuit implements a differential readout architecture in order to reduce the overall noise figure. The circuit has been fabricated in a commercially available 0.5 µm CMOS technology. Preliminary results show that the reset noise is reduced by a factor of 1.42 and the readout noise by a factor of 9.20 when the pixel is operated in differential mode versus singleended mode. Spectral responsivity characteristics show that the photodiodes are most sensitive at 480 nm. Using a commercially available emission filter, the sensor was able to reliably detect a concentration of Fura-2 as low as 39 nM. The sensor was used to perform ratiometric measurements and was able to reliably detect a free calcium concentration of 17 nM.
We report an improved design and successful demonstration of single photon avalanche diode (SPAD) detectors fabricated in a standard nwell 0.5 µm CMOS technology.The detectors are implemented as circular junctions between p+ and nwell regions. Two techniques are used to suppress perimeter breakdown: guard rings at the edges of the junctions, formed using lateral diffusion of adjacent nwell regions, and a poly-silicon control gate over the diffused guard rings and surrounding regions. The detectors exhibit a breakdown voltage of -16.85 V, ~4 V higher than simple diode structures in the same technology. The detector exhibits a thermal event rate of 16000 counts/s at room temperature at an excess bias voltage of 1.15 V.
Abstract-There is a growing interest in developing low cost, low power, highly integrated biosensor systems to characterize individual cells for applications such as cell analysis, drug development, environmental monitoring, and medicine. In such micro-systems, it's desirable to track individual cells in real time in order to steer cells using on-chip micro-actuators or monitor the movement of motile cells. To address this requirement, we are developing an embedded optical image sensor, called a contact imager, for imaging of a biological specimen directly coupled to the chip surface. The designed CMOS image sensor comprises an array of active pixel sensors (APS), logic and control signal generation, and readout circuits. The pixel layout has a pitch of 8.4 µm (24 λ). The design was fabricated in a commercially available 0.5 µm CMOS technology. The imager was first characterized on the bench as a normal CMOS image sensor, and then as a contact imager with microbeads (16 µm) placed directly on the chip surface. After further packaging with bio-compatible material, the chip was tested with cells cultured directly on the chip surface. Test results confirm successful detection of both beads and cells.
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