1. Ultra-fast Silicon Detectors Hartmut F.-W. Sadrozinski
SCIPP – UC Santa Cruz
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Low-Gain Avalanche Detectors (LGAD)
Timing Resolution
Radiation Effects
Development of 4-D Detectors

2. Effect of Timing at the HL-LHC
UFSD introduces a 4th dimension in the partcicle coordinates allowing the disentangling of complicated events. to
to+Dt x y
Timing z
Longitudinal view z y
Transverse view x
Timing layer
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Hartmut F.-W. Sadrozinski, Abraham Seiden, Nicolò Cartiglia,
“4-Dimensional Tracking with Ultra-Fast Silicon Detectors“, arxiv

3. Low-Gain Avalanche Detectors (LGAD)
Principle:
Add to n-on-p Silicon sensor an extra thin p-layer below th junction which increases the E-field so that charge multiplication with moderate gain of occurs without breakdown.
High Doping Concentration: High Field
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Manufacturers of LGAD (30 µm – 300 µm):
CNM Barcelona (RD50, ATLAS-HGTD)
HPK Hamamatsu
FBK Trento (INFN)
very similar behavior with exception of breakdown voltage and special design features .
50µm
50µm
35µm

4. LGAD Thickness Effects
UFSD Simulation WF2
Nicolo Cartiglia developed a full sensor simulation to optimize the sensor design
F. Cenna et al, “Weightfield2: a Fast Simulator for Silicon …..”, NIMA796 (2015)
Available at
LGAD Pulses
It includes many “bells & whistles” required for the detailed description of the signals, i.e. charge generation, drift and collection, electronics shaping
LGAD Thickness Effects
Rise time
dV/dt
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
50μm
G=10
Landau fluctuations
Radiation Effects
Pulse Distortions

5. Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep 13 2017
Timing with Silicon
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
𝜎 𝑡 2 = 𝜎 𝑇𝑖𝑚𝑒𝑊𝑎𝑙𝑘 2 + 𝜎 𝐿𝑎𝑛𝑑𝑎𝑢𝑁𝑜𝑖𝑠𝑒 2 + 𝜎 𝐷𝑖𝑠𝑡𝑜𝑟𝑡𝑖𝑜𝑛 2 + 𝜎 𝐽𝑖𝑡𝑡𝑒𝑟 2 + 𝜎 𝑇𝐷𝐶 2
𝜎 𝑇𝑖𝑚𝑒𝑊𝑎𝑙𝑘 = [ 𝑉 𝑡ℎ 𝑆/ 𝑡 𝑟𝑖𝑠𝑒 ] 𝑅𝑀𝑆 ∝ 𝑁 𝑑𝑉 𝑑𝑡 𝑅𝑀𝑆 , 𝜎 𝐽𝑖𝑡𝑡𝑒𝑟 = 𝑁 𝑑𝑉/𝑑𝑡 ≈ 𝑡 𝑟𝑖𝑠𝑒 𝑆/𝑁
Maximize slope dV/dt (i.e. large and fast signals)
Signal ~ gain, expect jitter ~ 1/G
Minimize noise N
Time walk is corrected by using constant-fraction discriminator CFD

6. LGAD Capacitance & Current Amplifier (BBA)
Noise vs. C for different BW:
Data
WF2
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Keep area small!
LGAD area 1x1 mm2 -> 3x3 mm2
Capacitance (50 μm): 2 pF -> 20 pF

7. LGAD Test in Particle beams and 90Sr β-Telescope
To understand MIP signals from LGAD pre-rad and post-rad several low-noise single-channel, 2-channel and 4-channel broad band current amps (BBA) were developed by
UC Santa Cruz, Kansas Univ., CERN, FNAL.
(In addition commercial BBA amplifiers employing “bias-T” connection were used, but with somewhat reduced performance.)
They were used in beam tests at CERN and FNAL and in 90Sr β-telescope.
UC Santa Cruz boards:
(Ned Spencer, Max Wilder, Zach Galloway)
Input impedance: 23 , BW >1 GHz
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Now ASICs: expect improved noise performance
Roberta Arcidiacono et al, “TOFFEE”, 11 am
Nicolo Cartiglia, Angelo Rivetti et al (INFN Torino) , Tue Poster session
Christophe De LaTaille et al , ALTIROC (Omega), Tue am Session.
(being tested in HGTD beam test at CERN)

8. Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep 13 2017
Dynamic Testing
β-source : explore LGAD performance without position information in house and in time
process parameters, geometrical variations, operating bias & temperature, …..
Strong temperature dependence of gain
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Investigate resolution vs. temperature:
only gain matters!
Investigate resolution vs. thickness
……but only for the same thickness: observe Landau fluctuations at large gain.

9. “Beam test results of a 16 ps UFSD timing system”
N. Cartiglia et al., “Beam test results of a 16 ps timing system based on ultra-fast silicon detectors”, NIM. A850, (2017), 83–88.
3 identical 45 μm thick 1.3x1.3 mm2 LGAD produced by CNM
To extract the time stamp of the LGAD employ constant-fraction discrimination (CFD) to correct for time walk (CFD ≈ 20%).
Important: this can be reliably implemented in an ASIC.
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Timing resolution vs. # of UFSD averaged
Good matching of three LGAD
Time resolution of single UFSD:
~ 25 ps (240V)
Time resolution of average of 3 UFSD:
20 ps (200V) & 16 ps (240V)
Timing resolution agrees
with expectation σ(N) = σ(1)/N0.5
240V

10. HGTD beam tests at CERN SPS
Results from testbeam in August 2016 at CERN SPS H6 beam line with 120 GeV pions
DUTs in the middle of a beam telescope with 3 μm position resolution
Simultaneous data taking of beam telescope and oscilloscope, combination done offline
Gain uniformity in arrays of CNM run 9088
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
W7 HG22
W8 HG11

11. UFSD Characterizations in Beam Tests
FNAL Beam Test, A. Apresyan et al.
Position scan of the time resolution
in a 2x2 array of 3x3 mm2 pads, showing the improvement with lowered temperature.
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Occupancy scan of two 2x2 array of 3x3 mm2 pads showing different width of the inter-pad gap:
CNM (red) : 70µm
HPK (blue) : 100 µm

12. Radiation Hardness of LGAD
ATLAS HGTD will require radiation tolerance of LGAD up tp 5e15 neq/cm2 (CMS 1e15?)
The small thickness of the LGAD will mitigate the usual radiation effects in Si
Increase in leakage current (+ cooling to -30 oC):
Increase of depletion voltage due to acceptor creation
Decrease of collected charge due to trapping
Modification of electric field due to trapped charge (“Double-junction”)
A LGAD specific effect is the gain decrease due to acceptor removal in the gain layer.
Technology Mitigation:
Replacing Boron with Gallium as acceptor,. use Carbon enriched Silicon wafers:
first encouraging results, new FBK structure under irradiation as we speak.
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Z. Galloway et al, arXiv:
Data from C-V scans
Simulation from WF2

13. Radiation Hardness of LGAD
In parallel to the technology improvements, explore radiation performance of existing technology ( i.e. Boron multiplication layer) through operational mitigation, i.e. raised bias
Small thickness permits high voltage biasing ->
part of gain in the bulk, smaller rise time.
Radiation campaigns with CNM, HPK and FBK LGAD by RD50, ATLAS and CMS
Here report on 1mm HPK 50D (i.e. 50µm), data taken at -20 oC, and -30 oC.
Neutron fluence steps: 0, 1e14, 3e14, 6e14, 1e15, 2e15, 3e15 (HGTD), 6e15 n/cm2
Decrease of gain but also Decrease of rise time
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Z. Galloway et al, arXiv:

14. Gain and Resolution after neutron irradiation
Radiation campaigns with CNM, HPK and FBK LGAD by RD50, ATLAS and CMS
Loss of gain due to acceptor removal can be recovered by increase in bias voltage.
This works up to a fluence of about 1e15 n/cm2
there it becomes limited by breakdown.
Note that for 6e15 operation at -30 oC allows to reach much higher gain than at -20 oC.
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Time resolution in a scan of the CFD threshold showing very different behavior beyond a fluence of about 1e15 n/cm2
Z. Galloway et al, arXiv:

15. Post-rad Rise time -20C, neutron Irradiation
High gain (low fluence):
drift time limits rise time at low gain pre-rad
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
High fluence, low gain:
fast rise time from bulk multiplication!
Rise time decreases at high fluences (small gain), and again at very high gain due to fast charge collection.
Reality check: for fluence 6e15 n/cm2:
𝝈 𝑱𝒊𝒕𝒕𝒆𝒓 ≈ 𝐭 𝐫𝐢𝐬𝐞 𝑺 𝑵 =270ps/5
Z. Galloway et al, arXiv:

16. Efficiency for HPK LGADs
6e15 n/cm2 , -30 oC
Data are from the beta source scans of
neutron irradiated HPK-I 50D LGAD.
Pmax = max pulse height in event
Tmax = time of Pmax
The increase in gain with increased bias is visible
700 V
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Max Pulse height: Pmax
750 V

17. Efficiency for HPK LGADs
6e15 n/cm2 , -30 oC
Bias = 750 V, Gain = 7
Bias = 700 V, Gain= 4
Raw Pmax spectrum + noise
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Noise subtracted Pmax spectrum:
Bias = 700 V and 750 above noise

18. Efficiency, Noise Occupancy, Resolution HPK 50D
6e15 n/cm-2, -30 oC
Noise sigma typically N = 1.6 mV , Noise occupancy = c* exp[-0.5*(Vth/ N)2], c≈1
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Need Gain > 7
for good resolution
Satisfied for all
lower fluences

19. UFSD Time resolution vs. Fluence
Operating voltage increases from
about 300 V pre-rad to
about 600 V V above 3e14 n/cm2
The bias allows for about 10% head room
where the performance does not change much.
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
The time resolution achieved:
20 ps (pre-rad),
35 ps (after 1e15 n/cm2),
50 ps (after 6e15 n/cm2).

20. Pulse shapes for irradiated LGAD: -20C, neutron Irradiation
WF2
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Data

21. UFSD Timing Resolution
Smaller C
Low noise
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Thin sensors
The time resolution achieved with 50 µm LGAD with 1x1 mm2 pads is
20 ps (pre-rad), 35 ps (after 1e15 n/cm2), 50 ps (after 6e15 n/cm2):
-> success!
Their position resolution is 300 µm:
-> needs work

22. Next Step: from Pads to Pixels
High bias voltage capability of the LGAD require design details which have no gain. They are being shrunk but can’t be eliminated and limit the fill factor of pixelated LGAD
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
-> For pixels need to avoid segmenting the gain layer
Separation of position (pixel) and time (pad) measurement:
Double-sided LGAD (“iLGAD”)
Un-segmented gain layer results in single-sided AC-LGAD

23. Separating the Collection and the Gain
LGAD
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
n-LGAD
iLGAD
After Irradiation ?

24. Inverted LGAD “iLGAD”: 2-sided Readout
p-on-p
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
M. Carulla et al.
Produced by CNM:
First try with laser on 300 μm iLGAD is “lessons learned”,
i.e. can be explained by WF2 - which tells us to go thin!

25. WF2: MIP Pulse in 300 & 50 µm iLGAD
p-on-p
300 µm
p-on-p
50 µm
n-on-p
50 µm
5 ns
500 ps
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Thin p-on-p and n-on-p LGAD seem both to be viable, the holes are a only bit more delayed than in n-on-p.
Thick p-type LGAD rely on late hole collection: p-on-p not viable.

26. Less complex: Single-sided AC LGAD
Tested:
Single-sided pad LGAD
Un-segmented LGAD
(AC-coupled,
resistive n++ implant sheet, pixelated metal contact)
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
First laser test on-going
Could change the silicon sensor paradigm.

27. Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep 13 2017
Conclusions
Ultra-fast Silicon detectors are being realized in form of thin Low-gain Avalanche Detectors.
The time resolution achieved with 50 µm LGAD pre-rad is
20 ps for 1x1 mm2 pads (3pF) , 35 ps for 3x3 mm2 pads (20pF).
The radiation hardness is being improved with process engineering.
The present design gives 35 ps for 1e15 n/cm2, 50 ps for 6e15 n/cm2.
2nd generation UFSD are
thin double-sided iLGAD,
thin single-sided AC-LGAD with much simplified layout.
Improvements in performance are expected from ASICs , both existing (ALTIROC0) and in the planning stage.
….and read Helmuth’s 1982 IEEE paper!
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep

28. Contributors
V. Fadeyev, P. Freeman, Z. Galloway, C. Gee, V. Gkougkousis, B. Gruey, H. Grabas, C. Labitan, Z. Liang, R. Losakul, Z. Luce, F. Martinez-Mckinney, H. F.-W. Sadrozinski, A. Seiden, E. Spencer, M. Wilder, N. Woods, A. Zatserklyaniy, Yuzhan Zhao SCIPP, Univ. of California Santa Cruz, CA 95064, USA
R. Arcidiacono, B. Baldassarri, N. Cartiglia, F. Cenna, M. Ferrero, A. Staiano, V. Sola
Univ. of Torino and INFN, Torino, Italy
G. Pellegrini, S. Hidalgo, M. Baselga, M. Carulla,
P. Fernandez-Martinez, D. Flores, A. Merlos, D. Quirion
Centro Nacional de Microelectrónica (CNM-CSIC), Barcelona, Spain
V. Cindro, G.Kramberger, I. Mandić, M. Mikuž, M. Zavrtanik
Jožef Stefan Inst. and Dept. of Physics, University of Ljubljana, Ljubljana, Slovenia
K. Yamamoto, S. Kamada, A. Ghassemi, K. Yamamura
Hamamatsu Photonics (HPK), Hamamatsu, Japan
beam test crews
HGTD (CERN): Lucia Masetti & US LGAD R&D (FNAL):: Artur Apresyan
Students in bold
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
This work was supported by the United States Department of Energy, grant DE-FG02-04ER41286.
Part of this work has been financed by the European Union’s Horizon 2020 Research and Innovation funding program, under Grant Agreement no (AIDA-2020) and Grant Agreement no (ERC UFSD669529), and by the Italian Ministero degli Affari Esteri and INFN Gruppo V.
This work was partially performed within the CERN RD50 collaboration.

29. Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep 13 2017
Back-up
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep

30. Anything New in “Fast Timing with Silicon Detectors”?
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep

31. Fast Timing with Silicon Detectors: ‘82 -> ‘16
Helmuth Spieler (1982): Heavy Ions
UFSD (2016): MIPS
Signal
Heavy Ion ( >>> 10 MeV)
MIP (13keV) (+ Gain)
Sensors
Thin sensors
50μm
Over-depletion of sensors
Large over-voltage: saturated drift velocity
Capacitance (C =100pF) degrades
slew-rate dV/dt
Small-area sensors C < 20pF
Cooling to reduce “Plasma effect”
Cooling reduces leakage current,
decreases jitter: increases gain, faster drift
Electronics
Charge Sensitive Amp
Current Amp
Constant Fraction Discriminator.
CFD=50% is best at largest dV/dt
CFD = 20-30% eliminates time walk,
reduces Landau jitter
Electronics degrades τRise
2GHz SiGe, ASIC preserves τRise
Large noise compromises jitter
Low-noise amplifiers, ASICs (130nm)
Minimize inductance
Minimize wire bond length
Analysis
“Residual jitter δt “
WF2: Landau Noise and field distortions: 𝜎 𝐿𝑎𝑛𝑑𝑎𝑢𝑁𝑜𝑖𝑠𝑒 2 + 𝜎 𝐷𝑖𝑠𝑡𝑜𝑟𝑡𝑖𝑜𝑛 2
Resolution
8ps
25 ps
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep
Facit: Gain, modern electronics and WF2 makes H. Spieler’s HI sensors work for MIP’s

32. What is missing?: Landau Fluctuations
Beam Tests show that we can correct for time walk.
So assuming we can beat the time jitter with lower noise and higher gain, Landau fluctuations become the time resolution floor which we can’t go below.
They depend on the sensors thickness, independent of the gain.
Both thin sensors, and low noise (for low threshold) are required for good timing resolution.
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep

33. Neutron irradiated LGAD: Doping Profile
Loss of gain due to acceptor removal can be recovered by increase in bias voltage.
This works up to a fluence of about 1e15 n/cm2 .
Hartmut F.-W. Sadrozinski, UFSD, TWEPP, Sep