Qlog. Logarithmic CMOS Pixel with Single Electron Detection. Yang Ni. New Imaging Technologies SA

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1 Qlog Logarithmic CMOS Pixel with Single Electron Detection Yang i ew Imaging Technologies SA Impasse de la oisette, Verrières le Buisson, F OPTRO, 2-4 February 2016, Paris, France

2 Why we need logarithmic sensing? Very wide instantaneous dynamic range Ø Extra-wide DR directly in pixel logarithmic compression Ø Extra-wide DR without any breakpoints Contrast indexed imaging Ø Constant contrast sensitivity over the DR Ø Stable image with ambient light invariance to some degree Ø Color consistency for better color imaging Modular design Ø o exposure time control, o AGC needed and instant accomodation Ø Easy colour processing and no explicit white balance

3 IT s Solar cell Log Pixel Design Research at Institut ational des Télécommunications, Evry, France : Ø Y. i, F. Lavainne, F. Devos, "CMOS compatible photoreceptor for high-contrast car vision", Intelligent Vehicle Highway Systems, SPIE's International Symposium on Photonics for Industrial Applications, Oct.-ov. 1994, Boston, pp Ø Y. i, K. Matou, "A CMOS Log Image Sensor with on-chip FP Compensation", ESSCIRC'01, Sept Villach, Austria, pp Physically exact dark reference on-chip FP correction High sensitivity o image lag High Quality Logarithmic Image 3T equivalent Low Light Performance

4 Voltage-mode Implementation REST state High T Low T VAB VAB Dark VAB Dark VAB VAB Illuminated Illuminated Temperature independent log response! How to suppress KTC noise? KTC noise or Johnson noise is always present and affects low light performance

5 Charge domain logarithmic compression PD TX FD charge transfert mechanism with CDS will suppress KTC Logarithmic response possible with charge domain logarithmic compression Logarithmic charge compression can be done by using electron evaporation effect during charge collection. VB0 Gdt ηe VB qv B kt dt η : d = ( ηe V B = V B0 qv kt q C PD B + G) dt Structure dependant constant

6 umeric resolved response VB0 = 0.2:0.05 : 0.5 T EXP =10ms :10ms : 40ms Residual electrons (a.u.) Residual electrons (a.u.) Photo-generated electrons (a.u.) Photo-generated electrons (a.u.) d = ( ηe qv kt B + G) dt; V B = V B0 q C PD

7 oise model d = ( ηe qv kt B + G) dt; V B = V B0 q C PD Ø At low photon flux, photo electrons are accumulated in the potential well. RMS noise is shot noise : n e = Ø At high photon flux, is stablized at logarithmic response. RMS results from John noise on PPD capacitance : n e = KTC q PPD

8 Some conclusions Ø Logarithmic charge compression is possible by using electron evaproation phenomenum Ø Logarithmic response can be Lin-Log or pure Log depending on initial barrier height Ø Lin response is exposure time dependant Ø Log response is exposure independant è Exposure dependant global response Ø Response is temperature dependant è Compensation mechanism needed Ø Implementation of logarithmic charge compression is very process dependent! We have developped a special low mask count recipe in a std 0.18um CMOS process with good performance for this purpose.

9 IT Qlog test chip 320x um pixel size Rolling shutter Analog differential outputs fixed testbed for process validation & process tuning flexible testbed for design verification > 10 lots with a lot of process splits and conditions

10 IT Qlog test results 1 Mean Level Value vs Faceplate illumination power Log response Output amplifier limited Mean Level Value (lsb) Lin response FPS=25Hz FPS=50Hz FPS=75Hz FPS=100Hz LSB= = 122uV ,0E- 02 1,0E- 01 1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 Faceplate illumination power (lux)

11 IT Qlog test results 2 Temporal oise Value vs Faceplate illumination power Photon shot noise PPD Johnson noise Temporal oise Value (lsb) FPS=25Hz FPS=50Hz FPS=75Hz FPS=100Hz 5 1 LSB= = 122uV ,0E- 02 1,0E- 01 1,0E+00 1,0E+01 1,0E+02 1,0E+03 1,0E+04 1,0E+05 Faceplate illumination power (lux)

12 IT Qlog test results 3 Ø From low photon flux response : CG I C CG = 38.9uV FD / e = 4.11 ff = 0.22LSB / e EXT lin Log 8000e(200mV ) Good match! V PI = 180mV Ø From high photon flux Log slope : C PPD = 16.6 ff 0.43 ff / um 2 Good match! n e = KTC q 11.4LSB PPD = 52e ü Working silicon demonstrates the behaviors predicted by theoretical model ü Reliable process parameters can be extracted from PTC curves

13 Photon counting capable pixel High CG can be an efficient (simple?) solution for photon-counting capable pixel But how about DR?? n e MAX = V CG FDMAX vn = < 0.28 CG n e = vn CG DR = n MAX CG > 360 uv / e CFD < 0. 44fF e vn CG vn =100µV MAX = 2800e 67dB CharacterizaMon of Quanta Image Sensor Pump- Gate Jots With Deep Sub- Electron Read oise Jiaju Ma, Dakota Starkey, Arun Rao & Eric Fossum Thayer School of Engineering, Darmouth College, Hanover, H A CMOS Image Sensor with 240μV/e- Conversion Gain, 200ke- Full Well Capacity and nm Spectral Response Satoshi asuno, Shunichi Wakashima, Fumiaki Kusuhara, Rihito Kuroda, Shigetoshi Sugawa Graduate School of Engineering, Tohoku University, Japan CMOS image sensor reaching 0.34 e- RMS read noise by inversion- accumulamon cycling Qiang Yao, Bart Dierickx, Benoit Dupont Caeleste, Mechelen, Belgium.

14 Photon counting Log pixel 1 Generation TX V LL PPD C FD Evaporation Shot noise KTC noise Log (L) Lin Dec MAX DR = C PPD PI = DR = 120dB = = V T ln(10)* C q Lin * V q + 20Log Lin Log PPD + 20 Log Dec C PPD = 2 ff, V PI = 100mV Lin Dec MAX = 1250 = 750 DR = 120 Log = 2175 = = CG MAX = = 292uV / e 3425 Photon counting is possible with 120dB DR!

15 Photon counting Log pixel 2 C PPD = 2 ff, V DR = 120dB PI = 100mV S S Lin _ Max = 20 log Lin = 61dB Lin Dec MAX = 1250 = 750 DR = 120 Log = 2175 = = 3425 S Log100 = 20log n Dec e_log = 38.5dB Log (L) ü ü ü ü High Image Quality is possible Suitable for small pixel S/ disparity between Lin/Log can be balanced by lower Vpin Precise TCAD should be necessary ü Process uniformity is a challenge! Completely opposite direction to current small pixel design optimization!!

16 Conclusion In this talk, we present new pixel design concept Qlog with very preliminary results. This design concept permits: ü Photon-counting sensitivity ü High dynamic range with Lin-Log response This concept is an extension of IT s pioneer work on high quality logarithmic pixel design into charge domain: ü The theorectical model has been validated by a silicon test chip. ü This model can be fitted easily from PTC curve with good precision. ü Actual testchip suffers from Vpin uniformity which has been estimated to +-10%. Investigation is on the way. OPTRO, 2-4 February 2016, Paris, France