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Single-Photon Resolution CMOS Integrating Image Sensors
Procedia Chemistry Procedia Chemistry 1 (2009) 1355–1358 www.elsevier.com/locate/procedia
Proceedings of the Eurosensors XXIII Conference
Single-Photon Resolution CMOS Integrating Image Sensors T. Baechler*, S. Neukom, C. Lotto, N. Blanc CSEM, Photonics Division, Technopark, Zürich, Switzerland
Abstract A CMOS image sensor – realized in a commercially available 0.18µm CIS technology – for near “single-photon detection” applications is presented. The pixel array features 1.2e- readout noise and 2.5e- overall noise comprising dark current shot noise effects at 60 frames per second (16µs row-time). The 11µm x 11µm pixel (fill factor of 63%) provides a pixel conversion gain of 50µV/e-. High-dynamic range of over 150dB is achieved. Further research on noise in charge detector circuits has resulted in the invention of a novel ultra-low-noise pixel featuring in-pixel amplification enabling single-electron resolution imaging. A typical sense node referred readout noise of 0.9e- and an overall sense node referred noise floor of 1.5e- are measured at a pixel conversion gain of 300µV/e-. Keywords: Single-Photon Detection; CMOS Image Sensor; In-Pixel Amplification; Ultra-Low Noise; Ultra-Low Light; High-Dynamic Range.
1. Introduction Many customer-specific image sensing applications require single-photon resolution detection capabilities. Scientific, space, medical, industrial inspection and automation, safety and security, biometry, and automotive products are asking for most advanced low-light, low-noise, and high-dynamic range technologies. For a very long time, probably since Albert Einstein described the law of the photoelectrical effect caused by absorption of quanta of light (photons) in 1905 (and made him win the Nobel Prize in Physics in 1921), it became a very ambitious research goal to capture single photons and if possible even on a per pixel basis. The benefits of such research activities are manifold – for example resulting in considerable reduction of X-ray doses in medical consultations, in improved diagnostic procedures using microscopy, spectroscopy, and fluorescence imaging methods, in discovering new further away galaxies and planets, or in better surveillance and safety cameras for automotive and traffic applications. With the discovery of the point-contact transistor in 1947 at Bell Labs and later with the fast-paced developments in (C)MOS technologies, image sensing solutions have been developed, first based on chargedcoupled devices (CCD), and have finally led to CMOS image sensing (CIS) technologies. CCD was certainly the preferred technology platform for low-light applications – due to very low dark current levels below 1pA/cm2 – but CIS technologies are catching up and profit from the availability of on-imager electronics at low additional cost.
* Corresponding author: Fax: +41 44 497 14 00 E-mail address: [email protected]
1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.338
1356
T. Baechler et al. / Procedia Chemistry 1 (2009) 1355–1358
With the availability of buried photodiodes (BPDs) in dedicated CIS technologies, CMOS sensors are becoming the preferred technology for many high performance low-light imaging applications. Currently available BPDs are showing dark current level in the pA/cm2 range, therefore, getting very close to CCD performance levels. CSEM has a long tradition in image sensor design for applications requiring specialized high-performance imagers not available as standard products on the market. The presented devices combine ultra-low-light pixels with a very highdynamic range and dedicated low-noise readout circuitry.
2. Technical features of the low-light high-dynamic image sensor Recently, a 256x256 pixel image sensor (fully scalable to a mega-pixel imager) was designed, integrated and characterized [1]. The sensor was implemented in UMC’s 0.18μm CIS technology featuring buried photodiodes, double poly and an optimized metal backend (thinner inter-metal oxide layers). A pixel schematic that combines low-noise and very high-dynamic range is shown in figure 1b. Compared to a conventional CMOS 4T-pixel (4 transistor-pixel) as represented in figure 1a, a diode-connected logarithmic-compression transistor Tlog is added. For many imaging applications low-noise and high-dynamic range at the same time is a key point. log
reset reset
Tlog
transfer transfer
sense node sense node
select
column line
select
Bias
Bias
(a)
(b)
Fig. 1. (a) Conventional CMOS 4T-pixel; (b) High-dynamic 5T-pixel.
As illustrated in figure 1b, each pixel contains, besides a buried photodiode (BPD) and a transfer gate, a reset switch, a source follower and a select switch. During integration the transfer gate is kept on a low potential above 0V allowing charges to spill-over into the sense node when the full-well capacity of the BPD is reached. The voltage on the sense node is logarithmically proportional to the (high) illumination level. Under low-light conditions the fullwell level is not reached and the BPD is read out by pulling the transfer gate to a high potential. The obtained voltage depends linearly on the (low) illumination level. The analog readout chain from pixel to output amplifier was carefully optimized for lowest possible electronic noise: • The bandwidth of the pixel source follower including column bus was made as small as possible to avoid excess noise on the column lines. • The column amplifier implements CDS (Correlated Double Sampling) in case of low-light, i.e. when the pixel operates in the linear mode. DDS (Double Data Sampling) is used in the logarithmic mode. • The column amplifier output is sampled twice to be able to compensate the kTC-noise of its switched-cap operation by subtracting both samples externally.
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T. Baechler et al. / Procedia Chemistry 1 (2009) 1355–1358
3. Measurement results of the low-light image sensor Two captured images under extreme illumination levels are shown. Even with low-light conditions at the mlux level, all image features (see figure 2a below) can be observed at 60fps (at 16µs row-time). By switching to the highdynamic mode, which decreases the frame rate to 30fps, extreme light intensities are imaged without saturation and this for over 7 decades of illumination levels in the scene (figure 2b).
(a)
(b)
Fig. 2. (a) Low-light scene (at 12mlux); (b) High-dynamic scene with measured 150dB dynamic range.
4. Technical features of the amplifying ultra-low-noise 4T-pixel To achieve even better low-noise performance, an amplifying ultra-low-noise 4T-pixel (figure 3a below), optimized for the same frame rate, has been developed and successfully measured [2, 3]. Such in-pixel amplifier provides, on the one hand, high pixel conversion factor, which effectively reduces downstream readout circuit noise, and on the other hand, optimized column bandwidth for minimum noise [4]. This pixel is controlled using standard CDS operation sequence.
transfer sense node
select_n
reset_n column line Rload
(a) Fig. 3. (a) Amplifying ultra-low-noise 4T-pixel; (b) Readout noise distribution for 140 pixels.
(b)
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T. Baechler et al. / Procedia Chemistry 1 (2009) 1355–1358
The select transistor is activated during all phases of the pixel readout sequence. During the reset phase, the reset transistor establishes a short-circuit feedback path between the input and the output of the amplifier device, i.e. between the sense node and the column line. During this phase, the sense node voltage is set to a value holding the common-source transistor in saturation. At the end of the reset phase the reset transistor is turned off and the common-source amplifier is operated in an open-loop configuration for reset level readout. The signal level readout phase is started by transferring photo-electrical charge from the BPD onto the sense node. As the amplifier device is still operated in an open-loop configuration, the decrease of the sense node voltage leads to an amplified increase of the column voltage. Due to amplification of about a factor 10, the impact of downstream readout circuits is efficiently reduced. The remaining readout noise is dominated by the in-pixel amplifier’s performance.
5. Comparative results and further work Comparative results of both pixels are given in table 1 (please take note of the different frame rates and dynamic range depending on the capturing mode of the 5T-pixel). Compared to SPADs (single-photon avalanche diodes), which need internal counters to integrate incoming photons and therefore have poor fill factor (e.g. 1.1% [5]), these novel integrating 4T-imaging devices are featuring a fill factor of 48%. These pixel structures are the most sensitive CMOS-pixels available today and are fully scalable to any array size without degradation in performance. Table 1. Summary of measurement results of the presented low-light pixels.
Parameter
High-dynamic 5T-pixel
Amplifying ultralow-noise 4T-pixel
Unit
Pixel size
11 x 11
11 x 11
µm2
Fill factor
63
48
%
60
Hz
Pixel conversion gain
50
300
µV/e-
Readout noise
1.2
0.9
e-
Overall noise
2.5
1.5
e-
87 (no log)
dB
Frame rate (at 16 µs row-time)
Dynamic range
60 / 30 (lin-log)
72 / >150 (lin-log)
Different versions – with resolutions up to 1.3Mpixel, on-imager 12-bit ADCs, and on-imager lin-log decision – of a CMOS low-light imager family based on the high-dynamic 5T-pixel have been designed. Regarding the amplifying ultra-low-noise 4T-pixel, a small test image sensor array is under construction. Further investigations about pixel array effects, such as photo response non-uniformity (PRNU) and fixed pattern noise (FPN), have to be made in order to fully characterize and extend the performance limits of this very promising, novel, and compact ultra-low-noise amplification pixel.
References 1. Neukom S, Baechler T. A Low-Noise CMOS Imager with 1.2e- Readout Noise, 2.5e- Overall Noise and over 140dB Dynamic Range at 60fps. In: Proceedings EOS Conference on Frontiers in Electronic Imaging. Munich, Germany, June 15-16, 2009. 2. Lotto C, Seitz P. A Low-Noise CMOS Imaging Pixel with In-Pixel Voltage Amplification. In: Proceedings EOS Conference on Frontiers in Electronic Imaging. Munich, Germany, June 15-16, 2009. 3. Lotto C, Seitz P. Synchronous and Asynchronous Detection of Ultra-Low Light Levels. In: Proceedings 2009 International Image Sensor Workshop. Bergen, Norway, June 25-28, 2009. 4. Lustenberger F, Seitz P. Highly Sensitive Solid-State Image Sensor. Patent EP1643754, 2006. 5. Niclass C et al. Arrays of Single Photon Avalanche Diodes in CMOS Technology: Picosecond Timing Resolution for Range Imaging. In: Proceedings 1st Range Imaging Research Day. Zurich, Switzerland, 2005.
Proceedings of the Eurosensors XXIII Conference
Single-Photon Resolution CMOS Integrating Image Sensors T. Baechler*, S. Neukom, C. Lotto, N. Blanc CSEM, Photonics Division, Technopark, Zürich, Switzerland
Abstract A CMOS image sensor – realized in a commercially available 0.18µm CIS technology – for near “single-photon detection” applications is presented. The pixel array features 1.2e- readout noise and 2.5e- overall noise comprising dark current shot noise effects at 60 frames per second (16µs row-time). The 11µm x 11µm pixel (fill factor of 63%) provides a pixel conversion gain of 50µV/e-. High-dynamic range of over 150dB is achieved. Further research on noise in charge detector circuits has resulted in the invention of a novel ultra-low-noise pixel featuring in-pixel amplification enabling single-electron resolution imaging. A typical sense node referred readout noise of 0.9e- and an overall sense node referred noise floor of 1.5e- are measured at a pixel conversion gain of 300µV/e-. Keywords: Single-Photon Detection; CMOS Image Sensor; In-Pixel Amplification; Ultra-Low Noise; Ultra-Low Light; High-Dynamic Range.
1. Introduction Many customer-specific image sensing applications require single-photon resolution detection capabilities. Scientific, space, medical, industrial inspection and automation, safety and security, biometry, and automotive products are asking for most advanced low-light, low-noise, and high-dynamic range technologies. For a very long time, probably since Albert Einstein described the law of the photoelectrical effect caused by absorption of quanta of light (photons) in 1905 (and made him win the Nobel Prize in Physics in 1921), it became a very ambitious research goal to capture single photons and if possible even on a per pixel basis. The benefits of such research activities are manifold – for example resulting in considerable reduction of X-ray doses in medical consultations, in improved diagnostic procedures using microscopy, spectroscopy, and fluorescence imaging methods, in discovering new further away galaxies and planets, or in better surveillance and safety cameras for automotive and traffic applications. With the discovery of the point-contact transistor in 1947 at Bell Labs and later with the fast-paced developments in (C)MOS technologies, image sensing solutions have been developed, first based on chargedcoupled devices (CCD), and have finally led to CMOS image sensing (CIS) technologies. CCD was certainly the preferred technology platform for low-light applications – due to very low dark current levels below 1pA/cm2 – but CIS technologies are catching up and profit from the availability of on-imager electronics at low additional cost.
* Corresponding author: Fax: +41 44 497 14 00 E-mail address: [email protected]
1876-6196/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.proche.2009.07.338
1356
T. Baechler et al. / Procedia Chemistry 1 (2009) 1355–1358
With the availability of buried photodiodes (BPDs) in dedicated CIS technologies, CMOS sensors are becoming the preferred technology for many high performance low-light imaging applications. Currently available BPDs are showing dark current level in the pA/cm2 range, therefore, getting very close to CCD performance levels. CSEM has a long tradition in image sensor design for applications requiring specialized high-performance imagers not available as standard products on the market. The presented devices combine ultra-low-light pixels with a very highdynamic range and dedicated low-noise readout circuitry.
2. Technical features of the low-light high-dynamic image sensor Recently, a 256x256 pixel image sensor (fully scalable to a mega-pixel imager) was designed, integrated and characterized [1]. The sensor was implemented in UMC’s 0.18μm CIS technology featuring buried photodiodes, double poly and an optimized metal backend (thinner inter-metal oxide layers). A pixel schematic that combines low-noise and very high-dynamic range is shown in figure 1b. Compared to a conventional CMOS 4T-pixel (4 transistor-pixel) as represented in figure 1a, a diode-connected logarithmic-compression transistor Tlog is added. For many imaging applications low-noise and high-dynamic range at the same time is a key point. log
reset reset
Tlog
transfer transfer
sense node sense node
select
column line
select
Bias
Bias
(a)
(b)
Fig. 1. (a) Conventional CMOS 4T-pixel; (b) High-dynamic 5T-pixel.
As illustrated in figure 1b, each pixel contains, besides a buried photodiode (BPD) and a transfer gate, a reset switch, a source follower and a select switch. During integration the transfer gate is kept on a low potential above 0V allowing charges to spill-over into the sense node when the full-well capacity of the BPD is reached. The voltage on the sense node is logarithmically proportional to the (high) illumination level. Under low-light conditions the fullwell level is not reached and the BPD is read out by pulling the transfer gate to a high potential. The obtained voltage depends linearly on the (low) illumination level. The analog readout chain from pixel to output amplifier was carefully optimized for lowest possible electronic noise: • The bandwidth of the pixel source follower including column bus was made as small as possible to avoid excess noise on the column lines. • The column amplifier implements CDS (Correlated Double Sampling) in case of low-light, i.e. when the pixel operates in the linear mode. DDS (Double Data Sampling) is used in the logarithmic mode. • The column amplifier output is sampled twice to be able to compensate the kTC-noise of its switched-cap operation by subtracting both samples externally.
1357
T. Baechler et al. / Procedia Chemistry 1 (2009) 1355–1358
3. Measurement results of the low-light image sensor Two captured images under extreme illumination levels are shown. Even with low-light conditions at the mlux level, all image features (see figure 2a below) can be observed at 60fps (at 16µs row-time). By switching to the highdynamic mode, which decreases the frame rate to 30fps, extreme light intensities are imaged without saturation and this for over 7 decades of illumination levels in the scene (figure 2b).
(a)
(b)
Fig. 2. (a) Low-light scene (at 12mlux); (b) High-dynamic scene with measured 150dB dynamic range.
4. Technical features of the amplifying ultra-low-noise 4T-pixel To achieve even better low-noise performance, an amplifying ultra-low-noise 4T-pixel (figure 3a below), optimized for the same frame rate, has been developed and successfully measured [2, 3]. Such in-pixel amplifier provides, on the one hand, high pixel conversion factor, which effectively reduces downstream readout circuit noise, and on the other hand, optimized column bandwidth for minimum noise [4]. This pixel is controlled using standard CDS operation sequence.
transfer sense node
select_n
reset_n column line Rload
(a) Fig. 3. (a) Amplifying ultra-low-noise 4T-pixel; (b) Readout noise distribution for 140 pixels.
(b)
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T. Baechler et al. / Procedia Chemistry 1 (2009) 1355–1358
The select transistor is activated during all phases of the pixel readout sequence. During the reset phase, the reset transistor establishes a short-circuit feedback path between the input and the output of the amplifier device, i.e. between the sense node and the column line. During this phase, the sense node voltage is set to a value holding the common-source transistor in saturation. At the end of the reset phase the reset transistor is turned off and the common-source amplifier is operated in an open-loop configuration for reset level readout. The signal level readout phase is started by transferring photo-electrical charge from the BPD onto the sense node. As the amplifier device is still operated in an open-loop configuration, the decrease of the sense node voltage leads to an amplified increase of the column voltage. Due to amplification of about a factor 10, the impact of downstream readout circuits is efficiently reduced. The remaining readout noise is dominated by the in-pixel amplifier’s performance.
5. Comparative results and further work Comparative results of both pixels are given in table 1 (please take note of the different frame rates and dynamic range depending on the capturing mode of the 5T-pixel). Compared to SPADs (single-photon avalanche diodes), which need internal counters to integrate incoming photons and therefore have poor fill factor (e.g. 1.1% [5]), these novel integrating 4T-imaging devices are featuring a fill factor of 48%. These pixel structures are the most sensitive CMOS-pixels available today and are fully scalable to any array size without degradation in performance. Table 1. Summary of measurement results of the presented low-light pixels.
Parameter
High-dynamic 5T-pixel
Amplifying ultralow-noise 4T-pixel
Unit
Pixel size
11 x 11
11 x 11
µm2
Fill factor
63
48
%
60
Hz
Pixel conversion gain
50
300
µV/e-
Readout noise
1.2
0.9
e-
Overall noise
2.5
1.5
e-
87 (no log)
dB
Frame rate (at 16 µs row-time)
Dynamic range
60 / 30 (lin-log)
72 / >150 (lin-log)
Different versions – with resolutions up to 1.3Mpixel, on-imager 12-bit ADCs, and on-imager lin-log decision – of a CMOS low-light imager family based on the high-dynamic 5T-pixel have been designed. Regarding the amplifying ultra-low-noise 4T-pixel, a small test image sensor array is under construction. Further investigations about pixel array effects, such as photo response non-uniformity (PRNU) and fixed pattern noise (FPN), have to be made in order to fully characterize and extend the performance limits of this very promising, novel, and compact ultra-low-noise amplification pixel.
References 1. Neukom S, Baechler T. A Low-Noise CMOS Imager with 1.2e- Readout Noise, 2.5e- Overall Noise and over 140dB Dynamic Range at 60fps. In: Proceedings EOS Conference on Frontiers in Electronic Imaging. Munich, Germany, June 15-16, 2009. 2. Lotto C, Seitz P. A Low-Noise CMOS Imaging Pixel with In-Pixel Voltage Amplification. In: Proceedings EOS Conference on Frontiers in Electronic Imaging. Munich, Germany, June 15-16, 2009. 3. Lotto C, Seitz P. Synchronous and Asynchronous Detection of Ultra-Low Light Levels. In: Proceedings 2009 International Image Sensor Workshop. Bergen, Norway, June 25-28, 2009. 4. Lustenberger F, Seitz P. Highly Sensitive Solid-State Image Sensor. Patent EP1643754, 2006. 5. Niclass C et al. Arrays of Single Photon Avalanche Diodes in CMOS Technology: Picosecond Timing Resolution for Range Imaging. In: Proceedings 1st Range Imaging Research Day. Zurich, Switzerland, 2005.