Download Self-amplified CMOS image sensor using a - Everest

Self-Amplified CMOS Image Sensor using a Current-Mode
Readout Circuit
Patrick M. Santosa,b,c , Davies W. de Lima Monteirob and Patrick Pittetc
a Graduate
Program in Electrical Engineering - Federal University of Minas Gerais - Av.
Antônio Carlos 6627, 31270-901, Belo Horizonte, MG, Brazil;
b Department of Electrical Engineering, DEE, Federal Center for Technological Education of
Minas Gerais, MG, Brazil;
c Institut des Nanotechnologies de Lyon, INL, CNRS UMR5270;
Université de Lyon, Lyon, F-69003, France;
Université Lyon 1, Villeurbanne, F-69622, France - INSA de Lyon, Villeurbanne, F-69621;
ABSTRACT
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.
Keywords: Image sensors, CMOS technology, Current-mode.
1. INTRODUCTION
If one looks several years ago one can see that the first approaches to readout the MOS/CMOS pixel signal were
through its output voltage.1, 2, 3 The output signal swing was about 1 V ,3 although the supply voltage could
reach 12 V and the common used supply voltage was 5 V . This supply voltage level was enough to produce a
very high signal at the output of the external amplifier placed between the column reader and the ADC. Putting
this together with a suited sensor design one could reach dynamic ranges and signal-to-noise ratios (SNR) as
high as 61 dB 2 and 51 dB,4 respectively.
In recent years the feature size of CMOS processes became smaller, which permitted high resolution into
image sensors, with more integrated functionality, but also with reduced sensitivity.5, 6 At the same time, several
improvements were made in order to achieve better results in several figures of merit, such as: SNR, dynamic
range(output signal swing), fill factor, bandwidth, power consumption and number of integrated functionalities.4, 6, 7, 8, 9, 10, 11, 12, 13 Including the studies regarding what is called mixed-mode (voltage and current modes
Further author information: (Send correspondence to P.M.S.)
P.M.S.: E-mail: [email protected], Telephone: +55 31 3319 6838
D.W.L.M.: E-mail: [email protected], Telephone: +55 31 3409 3416
P.P.: E-mail: [email protected]
Optical Sensing and Detection III, edited by Francis Berghmans, Anna G. Mignani,
Piet De Moor, Proc. of SPIE Vol. 9141, 914128 · © 2014 SPIE
CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2052589
Proc. of SPIE Vol. 9141 914128-1
together) and purely current-mode, today there can be found a wide variety of sensors and readout circuits with
better achievements in one or more of these figures of merit.14, 15, 16, 17, 18, 19, 20
The sensor presented in this paper is a mixture of sensor modifications approach together with a new concept
of readout. A matrix of CMOS pixels was designed to perform output signal amplification inside itself as if it
was a very large MOS transistor. The output current is modulated by the light intensity at which the matrix
is exposed to. The goal is to build a sensor with amplification function integrated into it and that is able to
achieve high signal swing (intended to increase the dynamic range). In the next sections the operation, control
and design are presented as well as the simulations results supporting their understanding.
2. SELF-AMPLIFICATION
The concept was presented in a previous paper21 and will be shortly described here. Each pixel of a CMOS
image sensor matrix will have a current source controlled by the output voltage of the control pixel. The control
pixel is selected by the electronic rolling shutter that is commonly used for signal readout. If the pixel is just
producing the output current and it’s not the control one for that readout clock cycle it will be called a scene
pixel. For the sake of concept demonstration the output voltage of the basic 3T pixel with integrated photodiode
was used to modulate the current produced in each pixel. This voltage will be called the matrix gate voltage
VGM io (see Figure 2). A transconductance amplifier (source follower) was put inside the pixel to produce the
output current inside the pixel.
If the pixel is selected that means it is the control pixel and it will be its output voltage that will appear to all
of the other pixels through the VGM io connection. If it is not selected it will act as a scene pixel, just producing
current. The transistor Mf csel controls when the current will be outsourced, setting the pixel in the integration
mode (off state) or in the readout mode (on state).
pixel
VDD
Rst
Integration Mode
Mrst
Msn
VGM
Row
iDS
io
Mrow
VGM io
Rvgm
VG
Ld
Mcol
MLd
1
M
Mf c
Rst
0
1
SDio
M ode
Col
VG
LdM
Mf c
Readout Mode
τint
Mode
0
VGLdpx
0
VGLdM
0
VGpx
sel
MSD
Ld
Figure 1. Simplified schematics of self-amplified matrix.
Shutter Clock
VGM
1
0
Figure 2. Signals used to control the matrix operation.
All pixel currents will be drawn into the same active charge transistor (MSDLd ) forming the output current
of the whole matrix: iDS M . The matrix, during the readout will behave as large MOS transistor producing a
variable current controlled in the rolling shutter readout frequency.
Under the same biasing conditions the larger the matrix the higher the value of iDS M . Since the cathode
voltage of the photodiode will be degraded by the photocurrent, in the dark the matrix will produce its maximum
current. The saturation process will occur if the integration time is long enough to produce a very small current
(since a very low VGM io will be available), regarded the amplifiers biasing condition. To limit power consumption
and respect heat dissipation rules inside the integrated circuits, only few pixels may have the transconductance
amplifier to produce current.
3. DESIGN, OPERATION AND CONTROL
The self-amplified matrix operation process is very close to the one with the 3T pixel approach during the
integration. The signal diagram of the Figure 2 shows the main signals. The Rst(reset) signal sets the reference
Proc. of SPIE Vol. 9141 914128-2
voltage at the photodiode cathode. Signal Mode is used to block iDS M during the integration mode (t ≤ τint )
and to permit its flow during the readout mode.
Since the pixel has two amplifiers, that one that receives the photodiode signal will be called signal amplifier
and the one used to produce iDSM will be called modulation amplifier . Both amplifiers must be biased and
this is done by VGLdpx and VGLdM . The value VGpx , for VGLdpx , must be chosen according to the application or
scene. Scenes with high levels of illuminance should lead to high levels of VGLdpx , since it will reduce the gain of
the source follower.
The choice of VGM will be made depending the desired transfer characteristic that is desired from the matrix
transistor (see Figures 3 and 4). While VGLdpx controls the VGM io swing, VGLdM controls how much current
is produced in the matrix. The values presented are for an arbitrary illustrative matrix with 16 pixels (4×4),
whose design will be detailed next. An example of simulated control signals can be seen in the Figure 6. In this
simulation the readout is made after an integration time (τint ) of 500 µs at 10 MHz. A delay of 100 ns was
arbitrated for the reset pulse. For the example, a 1 kΩ resistor was put in series between MSDLd and the ground,
simulating a post-readout circuit. The smaller this impedance the greater the value of iDS M .
Transfer Characteristics: i
DS
−3
1
vs V
GM
M
x 10
VG
= 1.00 V
VG
= 1.50 V
VG
= 2.00 V
VG
= 3.30 V
io
− V
G
=0.65 V
M
V
G
8
V
G
− VG
=1.00 V
Ldpx
= 1.00 V
= 1.50 V
LdM
LdM
LdM
0.8
io
LdM
LdM
0.9
Transfer Characteristics: iDS vs VGM
−4
x 10
Ldpx
VG
= 2.00 V
V
= 3.30 V
LdM
7
G
LdM
LdM
0.7
6
0.6
5
(A)
(A)
M
DS
M
i
i
DS
0.5
4
0.4
3
0.3
2
0.2
1
0.1
0
0
0.2
0.4
0.6
0.8
1
1.2
V
GM
1.4
1.6
1.8
2
(V)
0
0
0.2
0.4
0.6
0.8
1
1.2
V
GM
io
1.4
1.6
1.8
2
(V)
io
Figure 3. Transfer characteristic for VGLdpx = 0.65 V
Figure 4. Transfer characteristic for VGLdpx = 1.0 V
The only readout circuit designed∗ was the modulation amplifier, since the main goal of this project was to
assess the self-amplifying concept. The connection of the MSDLd to the ground actually does not exists and
it is used only for simulation purposes. In the real circuit that transistor terminal is open in order that an
ammeter or any other post-readout circuit (filter or ADC) can be connected and perform iDS M measurements.
Two situations for this external circuit input impedance were taken into account during the simulations: 1 Ω
(Figures 3, 4, 9 and 10) and 1 kΩ (Figures 6, 7 and 8). The pixel layout can be seen at the Figure 5.
The photodiode area was arbitrarily made bigger (≈ 220 µm2 ) than usual pixels in order to facilitate the
assessment of the proposed self-amplifying scheme and current-mode readout process with a large phodiode. The
choice of a PMOS as a reset transistor also makes the pixel bigger than it needs, but allows maximum voltage
reference value at the cathode. The pixel measures are 20 µm×23 µm (L×H), providing a fill factor of 45,8 %.
As can be seen in the Figures 3 and 4, the value of VGM is used to adjust the matrix transistor transconductance (derivative of iDSM with respect to VGM io ). The value of VGpx will limit the maximum value of iDS M
available at the output, since VGM io will be smaller (by a source follower smaller gain). This was achieved with
16 pixels with the previous layout and could be made different if a matrix transconductance is desired to be
smaller or higher, according to a specific application.
∗
In a 0.35 µm process with maximum supply voltage of 3.3 V .
Proc. of SPIE Vol. 9141 914128-3
Control signals for the 4×4 matrix
−4
x 10
3
GM
2
iDS
1
V
i
DS
M
io
(A)
2
(V)
4
M
V
GM
io
0
5
5.002
5.004
5.006
5.008
5.01
5.012
5.014
0
5.016
Time (s)
−4
x 10
Voltage Signals (V)
3.5
3
2.5
Mode
V
2
G
M
1.5
V
G
px
1
Shutter clk
0.5
0
5
5.002
5.004
5.006
5.008
5.01
5.012
5.014
5.016
Time (s)
t
Figure 5. Layout of the proposed pixel in 0.35 µm process.
Figure 6. Control signals:
3.3 V ,τint = 500 µs
−4
x 10
VGpx
=
= 0.65 V ,VGM
The transient behavior of iDSM is presented in the Figures 7 and 8. The voltage limitation imposed by
a higher value of VGpx is shown in the graphics as the maximum value of iDSM is different in the two shown
situations. In the graphics one can notice three time instants chosen to take iDS M measurements or, in other
words, stop the integration mode and start the readout mode. They correspond to different points in the transfer
characteristic and as one approaches matrix gate voltages (VGM io ) bellow 0.85 V † , the non-linearity of iDS M
becomes more pronounced. So, correct values of τint should be set by the mean scene illuminance.
Transient of i
DS
−4
5
= 0.65 V
for different control pixel illumiances − V
G
px
M
x 10
−4
5
4.5
x 10
Transient of iDS for different control pixel illumiances − VG = 1.00 V
M
px
0 lux
1000 lux
10000 lux
4.5
4
← t =50 µs
4
1
3.5
3.5
3
iDS (A)
2
M
0 lux
1000 lux
10000 lux
← t =150 µs
i
DS
M
(A)
3
2.5
2.5
2
2
1.5
1.5
← t =50 µs
1
1
1
← t =150 µs
0
2
← t =300 µs
0.5
3
3
0
0
0.5
1
1.5
2
2.5
3
3.5
t =300 µs
0.5
4
4.5
Time (s)
0
5
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Time (s)
−4
5
−4
x 10
x 10
Figure 7. Transient of iDS M , VGLdpx = 0.65 V
Figure 8. Transient of iDS M , VGLdpx = 1.0 V
The integration time can be very small compared to the ordinary 3T voltage-mode pixel approach. Associating
this fact with a proper arrange of the post-readout circuitry‡ it is possible to operate this sensor at very high
speeds. As the designed pixel is large, the overall resolution of the designed matrix (14 lines×7 columns) is low.
Sensing ultraviolet irradiation present at the occurrence of corona effect around transmission lines or sensing gas
leakage using infrared detectors could be possible applications of a sensor with such characteristics.
†
‡
This value obviously will vary with the technology and the overall gain of the amplifiers chain.
This is still under investigation.
Proc. of SPIE Vol. 9141 914128-4
4. LINEARITY
Based on the transient behavior of iDSM one can now present the real output expected from this sensor: the
effective current. The effective current, or ∆iDS M , is defined as being the difference between the iDS M
measured in the dark and the iDS M measured with the current control pixel. This current difference will be the
real output of the self-amplified matrix. If the measurements are made in the portion of the transfer characteristic
whose non-linearity is not so pronounced a linear output can be achieved. This must be done by setting the
proper integration time (τint ) in such a way that VGM io does not decrease below 0.85 V , as explained in the
previous sections. This will heavily depend on the biasing voltages levels and the illuminance at the control
pixel.
The different time instants selected for measurement (see Figures 7 and 8) provide different values of ∆iDS M
(see Figures 9 and 10)§ . The range of photocurrents in these simulations goes from dark, Iphd ≈ 3.5 pA, to
the maximum expected¶ for the designed photodiode: Iphmax ≈ 645 pA (for 10,000 lux), resulting in an input
dynamic range of 45 dB. Currents as high as 1.155 mA can be achieved (VGpx = 0.33 V and VGM = 3.3 V ).
Naturally this is an extreme situation and elevated currents like this could also lead to a high measurement errors
(even with accuracy levels in the order of 0,5 %).
The gain of the matrix amplifier will vary with: (i) its size; (ii) the aspect ratio of all transistors (changing
the overall gain of the source followers), specially with the aspect ratio of Mf c , since it impacts the aspect ratio
of the matrix transistor; (iii) The biasing voltages VGLdpx and VGLdM . This gain will affect directly the linearity
and linear range of operation of the sensor.
These parameters not only affect the gain, but also the bandwidthk . The matrix design was made to permit a
very short settling time at the output of the source followers, and consequently setting very fast current transients
at the output.
∆i
−3
1
int
τ
= 150µs − measured
τ
= 300µs − measured
τ
= 150µs − fit linear
τ
= 300µs − fit linear
int
int
0.8
vs Control Pixel Illuminance for Different τ
−V
G
∆i
= 0.65 V
int
DS
−4
px
4.5
int
0.9
DS
M
x 10
int
τ
= 150µs − measured
τ
= 300µs − measured
τ
= 150µs − fit linear
τ
= 300µs − fit linear
int
int
4
vs Control Pixel Illuminance for Different τ
M
x 10
int
int
3.5
−V
G
= 1.00 V
px
0.7
3
0.6
2
(A)
r2 = 0.99756 →
M
2.5
DS
∆i
∆i
DS
M
(A)
r = 0.99564 →
0.5
2
0.4
1.5
2
← r = 0.99972
0.3
1
0.2
0.5
0.1
0
2
← r = 0.99995
0
1
2
3
4
5
6
7
8
9
10
0
0
1
2
3
Figure 9. ∆iDS M
VGLdpx = 0.65 V
3
4
5
6
7
8
9
10
3
Control Pixel Illuminance (× 10 lux)
Control Pixel Illuminance (× 10 lux)
versus control pixel illuminance,
Figure 10. ∆iDS M
VGLdpx = 1.0 V
versus control pixel illuminance,
5. IMAGER CONCLUSIONS
It was shown that is possible to integrate the amplification function inside the sensor array. The use of currentmode operation for its readout can considerably improve the output signal excursions specially when associated
§
At these figures the scale of the vertical axis were made different purposely to show the details about linearity.
Considering the photodiode area and a quantum efficiency 0.2 A/W .
k
This study is not finished and is still under investigation.
¶
Proc. of SPIE Vol. 9141 914128-5
amplifier with a high and controllable gain. Although the designed pixel was made bigger than usual 3T ones,
the design can be changed to achieve particular conditions of operation or applications∗∗ .
6. ACKNOWLEDGMENTS
We thank Prof. Guo-Neng, Lu, of the Université Claude Bernard for his support and observations. We also
thank to the National Council for Scientific and Technological Development (CNPq), Brazil, the Coordination
for the Improvement of Higher Education Personnel (CAPES), Brazil, and the Minas Gerais State Research Aid
Fund (FAPEMIG) for the financial support.
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∗∗
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