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Proceedings ArticleDOI

A CMOS 128-APS linear array integrated with a LVOF for highsensitivity and high-resolution micro-spectrophotometry

30 Apr 2010-Proceedings of SPIE (International Society for Optics and Photonics)-Vol. 7726, pp 772616

AbstractA 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.

Topics: CMOS sensor (56%), CMOS (51%), Optical filter (51%), Time delay and integration (50%)

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].

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 [2], 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 [2].
  • 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 [7], 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 [5] 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 [1].
  • Figure 15 shows the die photo of this microspectrometer.
  • Therefore by multiplexing the APS in this linear array, the interested spectrum can be scanned.

5. CONCLUSION

  • 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 integrated with a LVOF for high-
sensitivity and high-resolution micro-spectrophotometry
Chi Liu*, Arvin Emadi, Huaiwen Wu, Ger De Graaf, Reinoud F. Wolfenbuttel
Faculty of EEMCS, Department for ME/EI, Delft University of Technology,
Mekelweg 4, 2628 CD Delft, The Netherlands
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 high-
resolution 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 (7.2μ300μm). 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.
Keywords: Capacitive Transimpedance Amplifier, Correlated Double Sampling, Active Pixel Sensor, IC-compatible
microspectrometers, LVOF, Variable Integration Time
1. INTRODUCTION
1.1 LVOF Microspectrometers
Microspectrometers have found application in many fields due to their small size and their low requirement on the
sample volume. Figure 1 shows a general diagram of a dispersive microspectrometer. 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. A LVOF is actually a resonator cavity with
a wedge shape. It is based on the theory of Fabry-Perot interference and the transmitted wavelength of the LVOF varies
linearly with the cavity thickness. Therefore, a wide spectrum can be covered by the LVOF and no focusing element is
needed. The optical resolution is mainly determined by the reflectance of the highly reflective coatings on the cavity
walls. 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].
Figure 1 A general diagram for a spectrometer
*c.liu-2@student.tudelft.nl; phone +31 (0)15 2785745; fax +31 (0)15 2785755; ei.ewi.tudelft.nl
Optical Sensing and Detection, edited by Francis Berghmans, Anna Grazia Mignani, Chris A. van Hoof,
Proc. of SPIE Vol. 7726, 772616 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.854584
Proc. of SPIE Vol. 7726 772616-1
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Transmittance
1
1
140
1
n
n
nm
a
x
xmm
λ
λ
==
Figure 2 Principle of the Linear Variable Optical Filter (LVOF) [2]
1.2 Photodetection in LVOF Microspectrometers
A high-quality microspectrometer requires a custom-designed imaging system covered by the LVOF. The combined
limitations of the LVOF design and the imaging system determine the performance of the microspectrometer in terms of
spectral resolving power and light detection limit due to optical throughput (etendue). 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. These parameters determine
the quality of this microspectrometer. Unfortunately, reduced spectrometer dimensions are difficult to combine with high
resolving power and etendue. A reduced optical throughput can be compensated for in a special mode of operation of the
photodetector. Increasing the integration time of the detector is a solution. The spectral resolution should not be
compromised by the photodetector design. The photodetector array specification in terms of element dimensions (pitch)
and number of elements should be sufficient to cover the resolution by the LVOF. The taper angle of the LVOF, as
published in [2], is sufficient for a spectral resolution of 2nm on the wavelength range between 540nm and 720nm over
an LVOF length extending over 1mm. This LVOF will generates a dispersed pattern with a spatial bandwidth of 1/14.4
μm
-1
. According to the Nyquist Sampling Theorem, a pixel pitch is required as small as 7.2μm.
There are commercialized image sensors and linear detector arrays, but their key performances are not optimized for on-
chip photodetection in microspectrometers. There are two problems in the photodetection after the LVOF filtering,
making the commercial image sensors less suitable:
(1) Low luminosity [3]. The function of the filter is to remove most of the input optical power by destructive
interference. Therefore, only a small amount of light reaches the detector array. Thus the imaging system should be
capable of low illumination detection.
(2) CMOS-compatibility. The smallest overall system dimensions are obtained when fabricating a LVOF right on top of
the imaging system with the MOMEMS technology. This is an essential feature in applications such as lab-on-a-chip [4].
Thus a photodetection system with standard CMOS process is preferred for its low cost and high compatibility.
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 [2]. 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.
Proc. of SPIE Vol. 7726 772616-2
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AUU
APJb
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. Considering all the specifications, the pixel size was selected at
7.2 300mmmm´ .
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. A capacitive transimpedance amplifier is included in each pixel to avoid this problem, as shown in Figure 3
[5] [6].
ΦΦ
_
Φ
T-type Reset Switch
Switch 1 Switch 2
Switch 3
Cint
Figure 3 Capacitive Transimpedance Amplifier (CTIA) with a T-type switch
The introduction of the CTIA brings several benefits:
(1) The link between the light sensitive area and the charge-to-voltage conversion is avoided. 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 [7], and thus long integration
time. The readout time can then be traded for higher signal-to-noise ratio;
(4) By means of the virtual ground, the photodiode can be biased with a highly fixed biasing voltage, which reduces the
charge modulation effect dramatically and leads to higher accuracy. The amplifier schematic is shown in Figure 4. A
two-stage circuit is chosen for large output swing. The T-type switch across the input and output of the amplifier results
in an off-resistance much larger than feasible with a single pass switch.
Figure 4 Schematic of the in-pixel amplifier
Proc. of SPIE Vol. 7726 772616-3
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2.3 In-pixel CDS
Correlated Double Sampling as the traditional technique for reducing low-frequency noise is also applied here. The
schematic [5] and the timing chart are shown in Figure 5 .
Figure 5 Schematic and timing chart for the buffered CDS
2.4 Variable Integration Time
Two different controls are applied for the photo detection, fixed integration time control and fixed voltage difference
control. Both control principles are tested. The SNR and the dynamic range for the fixed integration time are [8] [9]:
DR=
max int
10
2
int
20log
()
leakage
r photo leakage
qIt
qI I t
σ
++
(1)
SNR=
int
10
2
int
20log
()
photo
r photo leakage
I
t
qI I t
σ
++
(2)
These expressions demonstrate that the SNR increases with integration time while the dynamic range decreases.
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 In the fixed voltage difference control, the time consumed by the
CTIA-APS output to increase from V1 to V2 is recorded, with the assistance of a reference clock [10]. 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. Each
pixel can then work independently, needing no other control signals;
(3) The A/D conversion can be finished in the time recording by clock counters;
(4) The dynamic range can be improved by designing the clock and the counter appropriately, unlike in the fixed
integration time control where it is jeopardized by the increasing of integration time. The working principle is shown in
Figure 6.
Proc. of SPIE Vol. 7726 772616-4
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Figure 6 Working principle for fixed voltage difference control
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.
Figure 7 Complete system diagram
Figure 8 Standard timing chart for one pixel operation in one cycle
Proc. of SPIE Vol. 7726 772616-5
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Citations
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Journal ArticleDOI
Abstract: A CMOS compatible P+/Nwell/Psub double junction photodiode pixel was proposed, which can efficiently detect fluorescence from CsI(Tl) scintillation in an X-ray sensor. Photoelectric and spectral responses of P+/Nwell, Nwell/Psub and P+/Nwell/Psub photodiodes were analyzed and modeled. Simulation results show P+/Nwell/Psub photodiode has larger photocurrent than P+/Nwell photodiode and Nwell/Psub photodiode, and its spectral response is more in accordance with CsI(Tl) fluorescence spectrum. Improved P+/Nwell/Psub photodiode detecting CsI(Tl) fluorescence was designed in CSMC 0.5 μm CMOS process, CTIA (capacitive transimpedance amplifier) architecture was used to readout photocurrent signal. CMOS X-ray sensor IC prototype contains 8 × 8 pixel array and pixel pitch is 100 × 100 μm2. Testing results show the dark current of the improved P+/Nwell/Psub photodiode (6.5 pA) is less than that of P+/Nwell and P+/Nwell/Psub photodiodes (13 pA and 11 pA respectively). The sensitivity of P+/Nwell/Psub photodiode is about 20 pA/lux under white LED. The spectrum response of P+/Nwell/Psub photodiode ranges from 400 nm to 800 nm with a peak at 532 nm, which is in accordance with the fluorescence spectrum of CsI(Tl) in an indirect X-ray sensor. Preliminary testing results show the sensitivity of X-ray sensor IC under Cu target X-ray is about 0.21 Vm2/W or 5097e−/pixel @ 8.05 keV considering the pixel size, integration time and average energy of X-ray photons.

1 citations


References
More filters

Proceedings ArticleDOI
TL;DR: This paper describes a methodology, using a camera simulator and image quality metrics, for determining the optimal pixel size, and it is shown that the optimalpixel size scales with technology, btu at slower rate than the technology itself.
Abstract: Pixel design is a key part of image sensor design. After deciding on pixel architecture, a fundamental tradeoff is made to select pixel size. A small pixel size is desirable because it results in a smaller die size and/or higher spatial resolution; a large pixel size is desirable because it results in higher dynamic range and signal-to-noise ratio. Given these two ways to improve image quality and given a set of process and imaging constraints an optimal pixel size exists. It is difficult, however, to analytically determine the optimal pixel size, because the choice depends on many factors, including the sensor parameters, imaging optics and the human perception of image quality. This paper describes a methodology, using a camera simulator and image quality metrics, for determining the optimal pixel size. The methodology is demonstrated for APS implemented in CMOS processes down to 0.18 (mu) technology. For a typical 0.35 (mu) CMOS technology the optimal pixel size is found to be approximately 6.5 micrometers at fill factor of 30%. It is shown that the optimal pixel size scales with technology, btu at slower rate than the technology itself.

110 citations


"A CMOS 128-APS linear array integra..." refers background in this paper

  • ...The SNR and the dynamic range for the fixed integration time are [8] [9]: DR= max int 10 2 int 20log ( ) leakage...

    [...]


Journal ArticleDOI
Abstract: This paper reports on the IC-compatible fabrication of vertically tapered optical layers for use in linear variable optical filters (LVOF). The taper angle is fully defined by a mask design. Only one masked lithography step is required for defining strips in a photoresist with trenches etched therein of a density varying along the length of the strip. In a subsequent reflow, this patterned photoresist is planarized, resulting in a strip with a local thickness defined by the initial layer thickness and the trench density at that position before reflow. Hence a taper can be flexibly programmed by the mask design to be from 0.001o to 0.1o, which enables the simultaneous fabrication of tapered layers of different taper angles. The 3D pattern of resist structures is subsequently transferred into Si or SiO2 by appropriate etching. Complete LVOF fabrication involves CMOS-compatible deposition of a lower dielectric mirror using a stack of dielectrics on the wafer, tapered layer formation and deposition of the top dielectric mirror. Design principle, processing and simulation results plus experimental validation of the technique on the profile in the resist and after transfer of the taper into Si and SiO2 are presented.

54 citations


"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 [1]....

    [...]

  • ...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 [1]....

    [...]


Journal ArticleDOI
Abstract: We have developed a new capacitive transimpedance amplifier (CTIA) that can be operated at 2 K, and have good performance as readout circuits of astronomical far-infrared array detectors. The circuit design of the present CTIA consists of silicon p-MOSFETs and other passive elements. The process is a standard Bi-CMOS process with 0.5 /spl mu/m design rule. The open-loop gain of the CTIA is more than 300, resulting in good integration performance. The output voltage swing of the CTIA was 270 mV. The power consumption for each CTIA is less than 10 /spl mu/W. The noise at the output showed a 1/f noise spectrum of 4 /spl mu/V//spl radic/Hz at 1 Hz. The performance of this CTIA nearly fulfills the requirements for the far-infrared array detectors onboard ASTRO-F, Japanese infrared astronomical satellite to be launched in 2005.

52 citations


01 Dec 2008

8 citations


"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 [4]....

    [...]


Frequently Asked Questions (1)
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.