1. Silicon Photomultiplier (SiPM) Readout Application Specific Integrated Chip (ASIC) for Timing
Huangshan Chen

2. What is Timing?
Something happened. t t0
It happened at this time!

3. What is Timing? Something happened. t t0 It happened at this time!
Delay
Time jitter t t0
It happened at this time!

4. An example about timing
Step 0
Step 1
Step 2
Step 3

5. From timekeeper to timing in medical imaging & particle physics experiment
Scintillator
Photon detector (SiPM)
Readout front-end electronics
Time to Digital Converter (TDC)

6. Outline Timing in Mu3e and EndoTOFPET-US project
SiPM + Front end electronics
What is SiPM?
Why it is suitable for timing?
What is the downside from SiPM for timing?
What does front end electronics look like?
What’s the main concern on the Front end electronics for timing?
Time to Digital Converter (TDC)
What does TDC look like?
What’s the main concern on TDC?
STiC v3 and plan
Summary

7. Mu3e Project Search for µ+ e+e+e-
Forbidden in standard model (BR < 10-52)
Clear signal for new physics
Signal:
3 tracks (2 positive, 1 negative)
One vertex
Coincidence signal
Kinematic property:
𝑝 𝑡𝑜𝑡 = 𝑝 𝑖 =0
𝐸 𝑡𝑜𝑡 = 𝐸 𝑖 = 𝑚 𝜇
Goal:
Sensitivity of 1 in 1016

8. Detector design Fiber detector and SiPM
Scintillating tile detector + SiPM

9. Timing Pixel: O(50 ns) Scintillating fibers: O(1 ns)
Scintillating tiles: O(100 ps)

10. PET/TOF-PET principles
PET is non invasive, diagnostic imaging technique for measuring the metabolic activity of cells in the human body
Tracer Injection
β+ radio-labeled tracer is injected in the patient
The positron annihilates with e- from tissue, forming back-to-back 511 keV photon pair
511 keV photons detected in time coincidence
Image reconstruction
Time of Flight (TOF) PET uses TOF information to reduce background from neighboring organs
𝐿 𝐴 + 𝐿 𝐵 = 𝐿 𝐿 𝐴 − 𝐿 𝐵 =𝑐( 𝑡 𝐴 − 𝑡 𝐵 )
patient
Detector A tA LA
𝐿 𝐴 = 𝐿+𝑐( 𝑡 𝐴 − 𝑡 𝐵 ) 2 𝐿 𝐵 = 𝐿−𝑐( 𝑡 𝐴 − 𝑡 𝐵 ) 2 tB L
Detector B LB

11. EndoTOFPET-US project
Endoscopic Time-of-Flight PET & Ultrasound
Goal: to develop a new multimudal tool for the research of new biomarkers for pancreas and prostate tumors
The PET detector system:
PET detector head mounted on a endoscopic ultrasound probe
External PET detector
Challenges:
Fusion between US and PET images
Asymmetric design
Excellent timing resolution: ~ 200ps FWHM
Spatial resolution: ~ 1mm

12. EndoTOFPET-US external plate
256 modules
4x4 LYSO:Ce crystal
4x4 MPPC matrix (Hamamatsu)
Front-end board - A (FEB-A)
Front end electronics: STiCv3
4096 channels
FEB-D
FPGA

13. Silicon Photomultiplier
Rquenching
GM-APD
n pixels ……
Vbias
Signal i1 i2 i3 in
isignal=i1+i2+i3+…+in n+ P-
p+ subst.
Vbias
Signal
Multiplication Zone
Metal
Rquenching
SiO2 E
Geiger mode APD in parallel
Quenching resistor for each pixel
Photon counting for low light intensity
High gain & fast timing

14. The evolution of the signal
(Shockley–Ramo theorem)
Ith
exponential growth hv hv E n+
Diffusion-assisted Avalanche Spreading + Photo-assisted Avalanche Spreading x e-
Dead space, 40ps, avalanche self-sustaining, h+

15. SiPM timing readout ASIC – basic principle
Rquenching
GM-APD
n pixels ……
Vbias
Signal i1 i2 i3 in
isignal=i1+i2+i3+…+in
isignal
vin = isignal * Rin
vamp = vin * A
Vthr
vtrigger t I V
Vthr
Amplifier
Comparator + -
vtrigger
vin
vamp
Rin

16. Time jitter determined by σsys/K σsys (Signal fluctuation): K (Slope):
the fluctuation of the carrier creation
the avalanche buildup timing uncertainty
thermal noise at the quenching resistor
the noise of the readout electronics
K (Slope):
the detector parasitic
the bandwidth of readout electronics
Signal + noise
Threshold
σsys
σt=σsys/K
Trigger
Time jitter

17. Jitter contribution from SiPM
Minor Carrier diffusion
Avalanche build up statistics
Signal
Threshold
Trigger
Time jitter E n+ hv
hv’
Avalanche propagation progress
Pixel uniformity
Signal + noise
Threshold
Trigger
Time jitter E n+ e- h+
Diffusion-assisted Avalanche Spreading + Photo-assisted Avalanche Spreading

18. Jitter contribution from SiPM
Thermal noise at the quenching resistor
Rquenching
GM-APD
n pixels ……
Vbias
Signal i1 i2 i3 in
isignal=i1+i2+i3+…+in
Signal + noise
Threshold
Trigger
Time jitter
Detector parasitic capacitance
Signal + noise
Threshold
Trigger
Time jitter
Rquenching
GM-APD
n pixels ……
Vbias
Signal i1 i2 i3 in
isignal=i1+i2+i3+…+in

19. Jitter contribution from SiPM
Dark noise pile up Effect
Trigger
Time jitter
Signal, no dark noise
Signal, dark noise pipe up
Threshold t
Rquenching
GM-APD
n pixels ……
Vbias
Signal i1 i2 i3 in
isignal=i1+i2+i3+…+in

20. Main concerns on the front end electronics
Rquenching
GM-APD
n pixels ……
Vbias
Signal i1 i2 i3 in
isignal=i1+i2+i3+…+in
Time jitter is determined by σsys/K
keep σsys (fluctuation) as small as possible
Low noise circuit design
Differential structure to reject common mode noise
Clean power/ground supply
keep K (slope) as large as possible
Design high bandwidth amplifier
Control the parasitic capacitance along the signal path
vin
Rin
Amplifier
vamp
Vthr - +
Comparator
vtrigger

21. Time to Digital converter…… 1 2 3 4 5 6 7 n
Event triggered
ΔT = clock period t
Tevent – T0 = n * ΔT
Main concerns:
Stable clock period
external clock + PLL
Good time resolution
Good linearity
Simple counter TDC:
Resolution: reference clock period (ΔT)
Range: counter full range * ΔT
Quantization error: ΔT / √12
High resolution  small range
Resolution < 200 ps 
clock frequency > 5 GHz !

22. TDC – towards higher resolution…… 1 2 3 4 5 6 7 n
Event triggered
ΔT = clock period t
Event triggered t
Tfine
Digital time interpolation: Subdivided one coarse time bin into multiple fine time bins
Resolution: ΔT / N_fine_bins
Interval: coarse counter full range * ΔT
Quantization error: ΔT / N_fine_bins / √12
Tevent – T0 = n * ΔT - Tfine n

23. TDC Non-Linearity Random events t Calibration &Correction
Bin Number (Time)
counts
Calibration &Correction
Bin Number
counts
Random events t

24. Silicon photomultiplier Timing Chip (STiC)
STiC v3: 64-ch SiPM readout ASIC for Time-of- Flight applications
UMC 0.18um CMOS technology
Time + Energy information based on timing measurements
Analog frontend + TDC + Digital part
Single-ended / differential input
SiPM bias tuning
Input signal
E threshold
T threshold
E trigger
T trigger
edges processed by TDC
Analog part
TDC
Digital part
Fully differential
Time binning 50 ps

25. Coincidence timing resolution measurement
Measurements performed inside temperature oven stable at 18 ˚C
3.1x3.1x15 mm3 crystals
SiPM: one cell of the Hamamatsu S CN(X) MPPC matrix
Differential readout connection of SiPM
Good energy resolution
Coincidence timing resolution (CTR) : 213 ps
STiC v3
22Na
LYSO
MPPC
FPGA PC
Oven: 18 ˚ C
FWHM :
213 ps
511 keV
1.2 MeV

26. EndoTOFPET-US external plate commissioning
FEB-A
Laser
Moving stage
Main board
[FEB-A QA setup]
[ Front View of external plate (without cover)]
crystal
[ External plate cart]
External plate
Coincidence event
[Hit map of coincidence events]
External plate
FDG
Probe

27. New version of STiC for Mu3e experiment
Requirement:
Timing: 100 ps
Event rate : 50 KHz/ch
Requirement:
Timing: 250 ps
Event rate : 300 KHz/ch
or 1.3 MHz/ch
Event rate limit for STiC v3:
Analog part
TDC
Digital part
Dead time depends on the input signal
Almost no rate limit 
Dead time: ns
> 30 MHz/ch 
Serial link ,160 Mbps per chip
~40 KHz/ch 

28. Customized LVDS driver for serial data linkG
Termination at far end
Pre-driver
LVDS driver
Vref
Common mode feed back
Vin
FARADAY cell
customized
Much smaller size
Using only 1.8V transistors
Less power
Higher data rate, simulation: 1.5 Gbps (FARADAY cell: 630 Mbps)
Test chip layout

29. Summary EndoTOFPET-US (200 ps FWHM) & Mu3e (100 ps sigma)
Time jitter determined by σsys/K
SiPM:
GM-APD, quenching, fast timing
carrier creation fluctuation, avalanche fluctuation, parasitic capacitance, thermal noise, dark count pipe up
Analog front-end:
low noise, differential; less parasitic, high bandwidth
TDC:
Clock counter; digital interpolation; Non-linearity
STiC:
64ch; differential analog front end + TDC + digital part; 214 ps FWHM CTR

30. Backup slides