A CMOS 128-APS linear array integrated with a LVOF for highsensitivity and high-resolution micro-spectrophotometry
Abstract: A linear array of 128 Active Pixel Sensors has been developed in standard CMOS technology and a Linear Variable Optical Filter (LVOF) is added using CMOS-compatible post-process, resulting in a single chip highly-integrated highresolution microspectrometer. The optical requirements imposed by the LVOF result in photodetectors with small pitch and large length in the direction normal to the dispersed spectrum (7.2μ;m×300μm). The specific characteristics of the readout are the small pitch, low optical signals (typically a photocurrent of 100fA~1pA) and a much longer integration time as compared to regular video (typically 100μs~63s). These characteristics enable a very different trade-off between SNR and integration time and IC-compatibility. The system discussed in this paper operates in the visible part of the spectrum. The prototype is fabricated in the AMIS 0.35μm A/D CMOS technology.
Summary (3 min read)
1.1 LVOF Microspectrometers
- Microspectrometers have found application in many fields due to their small size and their low requirement on the sample volume.
- A Linear variable Optical Filter (LVOF) combined with a detector array is a suitable principle for the realization of a high-resolution microspectrometer, where the LVOF replaces the traditional grating as a dispersion component.
- It is based on the theory of Fabry-Perot interference and the transmitted wavelength of the LVOF varies linearly with the cavity thickness.
- Complete LVOF fabrication involves CMOS-compatible deposition of the top and the bottom dielectric mirrors and a tapered layer in between, as shown in Figure 2 .
1.2 Photodetection in LVOF Microspectrometers
- A high-quality microspectrometer requires a custom-designed imaging system covered by the LVOF.
- The spectral resolution of the microspectrometer is primarily determined by the LVOF design, whereas etendue is limited by the optical design and imposes the required detection limit of the detector in terms of minimum optical intensity.
- The photodetector array specification in terms of element dimensions and number of elements should be sufficient to cover the resolution by the LVOF.
- The taper angle of the LVOF, as published in , is sufficient for a spectral resolution of 2nm on the wavelength range between 540nm and 720nm over an LVOF length extending over 1mm.
- Thus the imaging system should be capable of low illumination detection.
2. SMART CTIA-APS
- A CTIA-APS linear array with 128 elements has been designed and fabricated within this framework, where CTIA is short for Capacitive Transimpedance Amplifier and APS is short for Active Pixel Sensor.
- This detector array is designed based the LVOF developed in .
- The idea is to take advantage of the special optical pattern generated by the LVOF and to implement an IC-compatible photodetection module suitable for operation at low illumination intensities.
2.1 Detector Array
- As discussed, the detector should be qualified in three aspects: (1) Small pitches along the filter length for high spatial sampling frequency; (2) Large light-sensitive area, for ensuring maximum sensitivity and SNR; (3) IC compatibility.
- The first two problems can be solved by applying a linear array of strip pixels.
- The extension of pixel length should compensate for the narrow pixel width.
- The nwell-psubstrate junction is selected for photodetection for an optimized responsivity in the visible light range.
2.2 Active Pixel Sensor with CTIA
- Image sensors usually apply a junction capacitor as the charge-to-voltage convertor, which is not good for linearity; the popular 3T-APS has its light sensitive areas in proportional with its junction capacitance, which resulted in a limited sensitivity.
- The sensitivity can be improved by enlarging the pixel length and by decreasing the integration capacitor; (2) The link between the accumulated charge and the integration capacitor is avoided.
- The linearity can be improved by implementing the integration capacitor as a poly-to-poly structure; (3) The amplifier enables the implementation of T-type switches with large off-resistance , and thus long integration time.
- A two-stage circuit is chosen for large output swing.
2.3 In-pixel CDS
- Correlated Double Sampling as the traditional technique for reducing low-frequency noise is also applied here.
- The schematic  and the timing chart are shown in Figure 5 .
2.4 Variable Integration Time
- Two different controls are applied for the photo detection, fixed integration time control and fixed voltage difference control.
- Therefore besides the fixed integration time control, the fixed voltage difference control is also introduced to boost both the dynamic range and the signal-to-noise ratio.
- The This control principle brings three benefits: (1) There is always a large amount of photons captured, even for low illumination levels.
- The photon shot noise will be the dominant noise source, which means a high SNR detection; (2) The control principles can be implemented with digital logic circuits and integrated into each pixel simply.
2.5 Circuit Diagram
- The circuit diagram of the readout is shown in Figure 7 (for simplicity the readout of only four pixels is shown).
- The standard timing chart of the pixel operation over one cycle is also presented in Figure 8.
3. DEVICE PERFORMANCE
- The prototype of this CTIA-APS array is designed and fabricated in standard CMOS technology, AMIS A/D 0.35μm.
- The initial tests used a halogen light source.
- A DAQ board controlled by a Lab View program is used for data acquisition, signal processing and generating control signals.
- The experimental results are to be discussed so as to demonstrate the performance of the device, including photodetectors’ responsivity, leakage current, linearity, temporal noise and the APS operations under both control principles.
3.1 Photodetectors: Spectral Response and Leakage Current
- The spectral response of the nwell-psubstrate junction has been tested.
- A calibrated photodiode ORIEL 71638 has been used as the reference.
- The ripples in the spectral response curve are believed to originate from the SiN layer deposited on the wafer, which causes interference.
- The leakage current contributes to the offset and the shot noise during the detection.
- Its effect should be estimated in advance.
3.3 Temporal Noise
- The temporal noise determines the minimum detectable signal.
- The measurement results are listed and discussed below.
- Both the reset noise and the flicker noise contributed by the CTIA can be eliminated largely by the correlated double sampling, while the thermal component of the readout noise can be reduced by averaging the multiple readout results.
- The noise of around 220μVrms is observed.
- This noise is quantified at the output of the CDS circuit in the dark condition, while the reset switch is kept on and the two CDS switches operates according to the standard timing chart for several cycles.
3.4 CTIA-APS operation: Fixed Integration Time Control
- Figure 12 shows the basic operation of this 128 CTIA-APS linear array, with half of the pixels illuminated while the rest set in the relatively dark condition.
- Their APS outputs are read out one by one sequentially through the multiplexer.
- Figure 12 Operation of pixels under Fixed Integration Time Control.
3.5 CTIA-APS operation: Fixed Voltage Difference Control
- For the low illumination detection, a long integration time can be applied to ensure enough signal energy.
- Under the fixed voltage difference control, the pixel adapts its integration time according to the sensed illumination.
- The noise level is constant for all illumination levels, allowing a sensitive detection even for small optical power.
4. SYSTEM CONFIGURATION WITH LVOF
- The prototype of this linear CTIA-APS array is fabricated in the AMIS 0.35µ C035M-D/A process.
- To form a complete optical micro-system, a linear variable optical filter is fabricated right on top of the photodetection system by IC-compatible reflow .
- Figure 15 shows the die photo of this microspectrometer.
- Therefore by multiplexing the APS in this linear array, the interested spectrum can be scanned.
- In this paper a CMOS APS linear array has been designed specifically for application in an LVOF-based microspectrometer.
- A buffered CDS circuit and a complete Capacitive Transimpedance Amplifier are integrated at every pixel to increase the readout speed and to enable the testing using a Fixed Voltage Difference Control.
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"A CMOS 128-APS linear array integra..." refers background in this paper
...The SNR and the dynamic range for the fixed integration time are  : DR= max int 10 2 int 20log ( ) leakage...
"A CMOS 128-APS linear array integra..." refers background or methods in this paper
...Complete LVOF fabrication involves CMOS-compatible deposition of the top and the bottom dielectric mirrors and a tapered layer in between, as shown in Figure 2 ....
...To form a complete optical micro-system, a linear variable optical filter is fabricated right on top of the photodetection system by IC-compatible reflow ....
"A CMOS 128-APS linear array integra..." refers background in this paper
...This is an essential feature in applications such as lab-on-a-chip ....
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Q1. What contributions have the authors mentioned in the paper "A cmos 128-aps linear array integrated with a lvof for high- sensitivity and high-resolution micro-spectrophotometry" ?
The system discussed in this paper operates in the visible part of the spectrum.