The CMOS buried multi-junction (BMJ) detector with multiple outputs has distinct spectral responses that may be exploited for applications such as bio-chemical analysis. We tackle here dark current issue by identifying different components inside the detector structure. The identification methods are based on the observation of bias and temperature dependence, as well as measurements of test detector chip integrating different design variations. Surface thermal generation may become predominant when the detector size shrinks, thus causing dark current degradation. To prevent this effect, we propose a low-sized detector structure with passivation of all its surrounding Si/SiO2 interface areas.
Also for the detector readout, we present a multi-channel charge-amplifier architecture with noise analysis. Effects of noise coming from amplifiers and related to the coupled detector’s dynamic conductances are illuminated. To pick up weak signals, synchronous detection can be implemented. A BDJ (Buried Double Junction) detector chip designed with a switched-phase architectural approach gives a minimum detectable signal of 15μlx@555nm or 1μlx@555nm at 27°C or – 10°C, for an integration time of 3s or 45s respectively.
The feature size of the CMOS processes decreased during the past few years and problems such as reduced dynamic range have become more significant in voltage-mode pixels, even though the integration of more functionality inside the pixel has become easier. This work makes a contribution on both sides: the possibility of a high signal excursion range using current-mode circuits together with functionality addition by making signal amplification inside the pixel. The classic 3T pixel architecture was rebuild with small modifications to integrate a transconductance amplifier providing a current as an output. The matrix with these new pixels will operate as a whole large transistor outsourcing an amplified current that will be used for signal processing. This current is controlled by the intensity of the light received by the matrix, modulated pixel by pixel. The output current can be controlled by the biasing circuits to achieve a very large range of output signal levels. It can also be controlled with the matrix size and this permits a very high degree of freedom on the signal level, observing the current densities inside the integrated circuit. In addition, the matrix can operate at very small integration times. Its applications would be those in which fast imaging processing, high signal amplification are required and low resolution is not a major problem, such as UV image sensors. Simulation results will be presented to support: operation, control, design, signal excursion levels and linearity for a matrix of pixels that was conceived using this new concept of sensor.
For biomedical microanalysis systems requiring implementation of optical signal generation and detection, we propose a package of VHDL-AMS functions to allow co-simulations of optical path, opto-electronic elements and associated electronics. This package contains a set of functions, which may be used for functional description of parts of microanalysis systems. An overview of simulation techniques shows that VHDL-AMS allows continuous-time simulation of polychromatic optical signals needed by the wavelength shifting nature of fluorescence. Indeed, directivity of optic path is well managed by VHDL-AMS using directional ports. By design, optical signals are easily simulated together with associated command and processing electronic circuits. Inspired by RF simulation techniques, the proposed description of polychromatic optical signals lies on a discretization of spectra. This format allows each optic band to be processed independently by models. The array data structure available in VHDL-AMS provides a compact form to device descriptions and to optical signal connexions. Fluorescence is modelled with absorbance and emission spectra, and optical couplings are described using results of geometric-optic analysis. A “spectral plug-in” has been developed, to be connected to output-power models of LASER-LED reported in the literature. Furthermore, a physical model of the CMOS Buried Double Junction (BDJ) detector has been described. Models of optic and electronic parts include a modulated LASER source, fibre optic, fluorochrom, BDJ detector and Constant Voltage Threshold (CVT) analogue-to-digital signal conversion. The system-level simulations, with Variable-Time Synchronous Detection (VTSD) are performed using the “Advanced-MS” environment. The validity domain of this approach as well as limitations of the available VHDL-AMS simulators (especially in terms of convergence and simulation time) are discussed.
For sensitive fluorescence detection requiring weak signal recovery, we propose a novel digital synchronous detection method. It is based on a voltage/time duality concept which, compared to a conventional approach, consists of transformation of constant sampling rate with voltage measurement into variable-time sampling with constant threshold voltage. This Variable-Time Synchronous Detection (VTSD) method ensures a constant SNR over a large dynamic range, with optimised measuring rate. It can be implemented without any precise analogue-to-digital converter. A CMOS photodetection system with implementation of this VTSD method together with charge amplification is designed and tested. The results confirm its ability to recover photocurrent signals at femto-Ampers levels.
We present a fluorescence detection system for capillary analysis. It is designed using a CMOS BDJ (Buried Double p-n Junction) detector which can be operated either as a photodiode or as a wavelength-sensitive device. Noise-reduction techniques such as signal pre-amplification and synchronous detection are implemented to boost the sensitivity of measurements. The system indicates fluorescence intensity for concentration determination, and average wavelength of fluorescence spectrum for molecular discrimination. The system has been tested by measuring two widely used fluorophores (FITC and Rhodamine B) in different concentrations. A 407-nm blue laser diode and a 532-nm green YAG compact laser have been respectively employed for their excitation. The illuminated volume inside the capillary is about 5 nl. The best results have been obtained with FITC, enabling as low as 10-10 M to be detectable.
We have designed a cost-effective, fiber optic bundle-based detection system for microarray fluorescence measurements. A bundle, fabricated with thin-cladding fibers of 50-μm in core diameter, is used for spot excitation and collection. The collected optical signal is detected by a CMOS BDJ (Buried Double p-n Junction) detector, which can be operated either as a photodiode or as a wavelength-sensitive device. For improving measuring rate of a microarray, we have proposed a direct spot scanning technique, which is based on a prior knowledge about the predefined microarray's mask pattern, and operates to bring the bundle successively over each spot for single-point measurement. It is implemented with a microarray registration procedure to determine the spots' positions. The detection system with implemented scanning technique has been tested using microarray samples. Its scanning operation has been verified by comparing the determined spots' coordinates to the microarray image.
CMOS photodiodes have increasingly been used for biomedical purposes. They offer many technical and economic advantages for their on-chip integration and system miniaturization. With the aim of developing portable instruments for micro-analysis, we have investigated the CMOS BDJ detector for fluorescence detection. Like conventional photodidoes, the CMOS BDJ detector can be used as a photodetector. In addition to that, it can also be employed as a wavelength-sensitive device.