12th International "Hiroshima" Symposium on the Development and Application of Semiconductor Tracking Detectors at Hiroshima, Japan

Study of time resolution of low-gain avalanche detectors
University Of Tsukuba K. Onaru, K. Hara, D. Harada, S. Wada, KEK K. Nakamura, Y. Unno
Sample
Name

50A
50B
50C
50D
80A
80B
80C
80D

Low Gain Avalanche Detector

60

ActiveThickness [µm]5040302050

80Time resolution(ps)150

Ø Fast signal＋High S/N ratio @gain~10= high time resolution
ü Time resolution ~30ps @FNAL Test beam

10N++
P+P bulkP++

Figure 1: Photos of pad (a) and strip (b) detectors

PhysicalThickness [µm]Table 1: Sample list of HPK LGAD samples.

p+ Dose
Low to HighABCDABCDn N+ in P type silicon detector• P+ layer under N+ readout implant-> High Electric Field at P+ - N++ junction-> Avalanche

LGAD structure

Al

Strip-type(KEK/HPK)Pad-type(ATLAS/HPK)

00.2

0.4

0.6

0.8

n Timing Detector for future colliders• Additional time information tohits->reduce wrong track combinations
tracks-> Identify the vertex position(30ps time resolution = 1cm)
Ø 4D detector+ Time information
500keV γ-ray
TOF-PETn TOF-PET• Cancer diagnostic equipment
cancer• Measure annihilation position from differenceof flight times of annihilation gamma rays
• Time resolution = position resolution ofannihilation point
500keV γ-ray

Vthreshold/Vpeak

0

Applications of LGAD

1Time resolution measurement using β-rays

Measurement system

Amp + pad sensor

n We measured time resolution in FNAL beam test ~ 30ps
• Is it possible to measure time resolution more easily using a beta source?
• A measurement system was developed using FlashADC CAEN DT5742 CAEN DT5742 FlashADC
5GHz 10bit time sampling(200ns FS)
0.25mV 12bit ADC(1Vpp)
n Time resolution measurement• Stack 2 pad-type sensors (lower->Trigger&signal, upper->signal)①Noise cut (Max ADC value > 450ADC ~ 100mV , time difference of Max ADCs < 3ns)
②Find time exceeding Vthreshold = factor × Vpeak③Evaluate the time difference of two overlapping sensors④Evaluate the time resolution of each sensor
lb2` JMmH lJ9kdy.h8d9k%%
𝜎 𝑇# − 𝑇% = 𝜎 𝑇# + 𝜎 𝑇% ,𝜎 𝑇# ≈ 𝜎 𝑇%ReYR *?MM2H Rk#Bi 8 :afb arBi+?2/ *T+BiQ` .B;BiBx2`③time difference distribution
ü Time resolution using beta-rays ~ 36ps②Signal & fit_2pX 3 @ CMm`v jy - kyRd

PCsensorSensor in The temperature chamber

42①Max ADC distribution

41

ADC

Upper sensor ADC④Time resolution

Time resolution(ps)thTrigger threshold

Flash ADC

-20degC

40
39

Vthreshold=factor ×Vpeak

38

𝜎 𝑇# − 𝑇%37
36Signal

35Vpeak0NoiseAC‒LGAD TCAD Simulationn Concerns• Enough gain?
• Signal separation?
n Search for parameters to satisfy both
DC Al
N++
P+

AC Al

P bulkP++

Al
AC-LGAD structure

N++
P+

Al00
2e-8Time(ns)1e-8Time(ns)2e-8Collected charge(e)n MIP signal simulation
80000• MIP is incident on the center electrode 70000
60000
and measured charge collection
50000++
n N concentration40000
ü N++ high(low resistance)
30000-> Charge flows to the next electrodes 20000
10000
ü N++ low(high resistance)
0-> No signal on the next electrode
0.00E+00
but charge collection is slow
20
18
n Gain16
ü Enough gain
14
Gain ~10 @N++:1e18, P+:5e1612n Gain uniformity
ü Uniform gain is obtainable in entire region

P bulk
P++-HV(V)

Next to next
01e-8Time(ns)2e-8

Low N++ concentration

GainN++ concentration

5.00E+17
1.00E+18
5.00E+18

Gain ~10

1.00E+19

Gain =1
2.00E+16

Gain

No gain300

Next90000

Gain decreases

250

6e-7

High N++ concentration

IR-laser measurement for strip LGAD

200

Center

1e-8n AC-LGAD is considered as a structure to solve these problems• Dope N++ in entire sensitive area• P+ is doped similarly -> uniform amplification• Read signals from AC coupled Al electrodesn designed an AC-LGAD using
TCAD simulation

6e-7

6e-7

n Evaluated gain uniformity for strip type LGAD
• Focused IR-laser was incident, and the position dependence of gain wasevaluated from the amount of charge collected at each position
ü The gain decreases toward the edge of the P+ amplification layer
ü No gain at the position where there is no P+ amplification layer

150

Current(A)Current(A)Current(A)100Time difference (ns)Time(ns)

Lower sensor ADC

50

4.00E+166.00E+16P+ concentration

Center + next
NextCenter108642Center electrode0020

Gain uniformity

Next electrode406080100

MIP Incident position(µm)

Summaryn We are developing LGAD for 4D detectorn Developed measurement system using Flash ADC and beta source. Can evaluate the time resolution to the same level as in the beam testn TCAD Simulated AC-LGAD structure. AC-LGAD with enough and uniform gain can be designed
This research was supported in part by KAKENHI 19H94393 and US-Japan Science Cooperation Program (2018)