1. Vertically-integrated CMOS Geiger-mode avalanche pixel sensors
L. Pancheri, P. Brogi, G. Collazuol, G.-F. Dalla Betta, A. Ficorella,
P.S. Marrocchesi, F. Morsani, L. Ratti, A. Savoy-Navarro
University of Trento & TIFPA
University of Siena & INFN Pisa
University of Padova & INFN Padova
University of Pavia & INFN Pavia
Laboratoire APC, University Paris-Diderot/CNRS Paris
14th Topical Seminar on Innovative Particle and Radiation Detectors (IPRD16)
Siena, Italy, October 3-6, 2016

2. APiX2 project
“Development of an Avalanche Pixel Sensor for tracking applications”
Funded by INFN – CSN5
Project coordinator: Pier Simone Marrocchesi, INFN Pisa and University of Siena
Partners:
TIFPA and University of Trento,
INFN Pavia and University Pavia,
INFN Padova and University of Padova,
Laboratoire APC, Université Paris-Diderot/CNRS

3. Outline Introduction Sensor architecture Experimental results
Summary and future perspectives

4. APiX particle detector concept
Quenching
Particle
detection
Discriminators
Coincidence detector
Dark counts
Two Geiger-mode avalanche detectors (SPADs) in coincidence:
DCR = DCR1 DCR2 2DT
In-pixel coincidence: not feasible in SiPM optimized process (Hamamatsu, FBK, SensL …)
Integrated electronics is needed: CMOS process
V. Saveliev, US Patent. 8,269,181, N. D’Ascenzo et al., JINST 2014

5. Avalanche detectors – cross section
Standard 150nm CMOS process – no modifications
Avalanche diodes in deep nwell: isolated from substrate
Type 1:
Shallow step junction
Active thickness ~ 1μm
Type2:
Deep graded junction
Active thickness ~ 1.5μm
L. Pancheri et al., J. Selected Topics Quantum Electron, 2014

6. Single-photon timing resolution
Measured on 10-μm devices, with blue laser (470nm), 70ps FWHM
Type 1: 60ps FWHM Type 2: 170ps FWHM

7. Proof-of-concept demonstrator
2-layer pixel cross section:
Electronic readout on both layers
Metal shielding from optical cross-talk
Vertical interconnection by bump bonding

8. Pixel architecture
Pixel enable/disable
Pulse shortening: reduces the rate of accidental coincidence
Programmable pulse width: 750ps, 1.5ns, 10ns

9. Pixel architecture: storage
In-pixel 1-bit memory
Output register for fast data output

10. Sensor floorplan Wire bonding pads on chip 2: pre-integration test.
Bottom chip
Top chip
Wire bonding pads on chip 2: pre-integration test.
Final assembly
Top chip
Bottom chip

11. Pixel array 16 x 48 pixel array Pixel size: 50μm x 75μm
Splittings in detector type and area
Bump bonding pad
30μm x 30μm
35μm x 35μm
40μm x 40μm
43μm x 45μm
Pixels with different detector area
(unshielded)
Pixels with shielded detectors

12. Sensor micrographs
Bottom chip
Top chip

13. Bottom chip - Micrographs
Shielded
Unshielded

14. Bottom chip - Micrographs

15. Breakdown voltage uniformity
Measurements on 5 sample chips x 2 types x 196 devices per chip
Very good uniformity on-chip (s < 20mV)
Large difference (1V) between different chips for type 1

16. Dark Count Rate Distribution
VEX = 3.3V
VEX = 3.3V
Cumulative distribution, combined measurements on 3 chips
600 devices for largest size, 72 for smaller ones
Median DCR = 2.2kHz for largest cell size of both types

17. Crosstalk characterization
Crosstalk coefficient
CRm = DCRe ∙ DCRd ∙ 2∆T + K ∙ ( DCRe + DCRd )
Emitter (fixed)
Detector (scan)
Crosstalk map – Type 1, 25µm thickness

18. Crosstalk vs substrate thickness

19. Vertically-integrated assembly
Dark Count Rate vs. coincidence time DT
DCRCOINC = DCR1 x DCR2 x 2DT
T = 20°C
DT = 1.5ns
DT = 10ns
DT = 0.75ns
DCRCOINC = 27 counts/s mm2

20. b-source measurements
90Sr β source – 37kBq at 2mm distance from sensor
VEX = 2V T = 5°C Dt = 0.75ns
~ 2x10-3 counts/pixel s
Count rate ~0.5 counts/s mm2

21. Summary
Strengths:
Can be thinned to a few microns: low material budget
Timing resolution
Low power consumption
Weaknesses:
Radiation tolerance
Geometric efficiency: surface device guard ring and electronics
Opportunities:
Progress in SPAD production and 3D integration technologies: many major foundries active
Threats:
- Cost and accessibility of 3D integration processes

22. Future work Current prototype: Design of new prototype:
Test beam at CERN just finished: analysis of test beam results on-going
Radiation hardness
Design of new prototype:
Larger array
Improved fill factor
Optimized timing
Optimized power consumption

23. Thank you

24. Additional slides

25. Geiger-mode avalanche detectors
a.k.a. Single-Photon Avalanche Diodes (SPADs),
Silicon Photomultipliers
1 primary generated electron-hole pair:
very large current pulse ~ electrons
Vout
VBIAS > VBD
SPAD
Vout RQ
time
events

26. Experimental results - summary
Characterization of single-layer sensors:
Core supply current (at 1.8V): 8mA
Breakdown voltage uniformity
Dark count rate distribution
Cross-talk
Timing resolution
Characterization of vertically integrated sensors
Coincidence Dark Count Rate
Measurements with 90Sr β source
Test beam with protons at CERN (data analysis in due course)

27. Coincidence detection
Count rate in coincidence between
two pixels in the same column
Normalized rate:
𝐶 𝑅 𝑀𝑒𝑎𝑠 2∙𝐶 𝑅 1 ∙𝐶 𝑅 2 ∙∆𝑇
Cross-talk
Cross-talk

28. Photo-Detection Efficiency
Type 1: p+/nwell
Type 2: pwell/niso
Shallower junction:
better NUV – Blue efficiency
Wider depletion region:
Better red-IR efficiency
L. Pancheri et al., J. Selected Topics in Quantum Electron, 2015

29. DCR temperature dependence
Devices with 43μm x 45μm active area, but different DCR
Measurements from -30°C to 50°C with 10°C steps
Overvoltage: VOV = 3.3V

30. Breakdown probability (Pb)
IR light: uniform generation, Pb measured for a single photoelectron
Particles generate N primary electrons: PbN = 1 – (1-Pb)N
Example: single electron  Pb = 30%
10 electrons  Pb10 = 97%
L. Pancheri et al., J. Selected Topics in Quantum Electron, 2015

31. VBD extraction method VREF VBD extraction from I-V curves:
not possible
VBD extracted from the dark count rate vs. voltage curve
L. Pancheri et al., Proc. IEEE ICMTS, 2014

32. Dark Count Rate vs. bias
Active area: 43μm x 45μm

33. Effect of crosstalk in DCR distribution
Thickness 280 µm
Thickness 50 µm
Median DCR increase of 70% for 280µm sample and 400% for 50µm sample

34. Timing resolution IR laser (780nm) 50ps FWHM Diffuser Sensor
VEX = 1V
208ps FWHM
Diffuser
Sensor
Coincidence
output
Timing histogram between laser trigger and sensor coincidence output
N.B. Design not optimized for timing
2 pixels enabled