Avalanche PhotoDiodes APDs

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• Avalanche Photo-Diodes (APDs)
• for the CMS
• Electromagnetic Calorimeter (ECAL)
• K.Deiters, Q.Ingram, D.Renker, T.Sakhelashvili
• Paul Scherrer Institute, Villigen, Switzerland
• I.Kronqvist, R.Rusack, A.Singovski, P.Vikas
• University of Minnesota, Minneapolis, USA
• A.Kuznetsov, Y.Musienko, S.Reucroft, J.Swain
• Northeastern University, Boston, USA
• Z.Antunovic, N.Godinovic, I.Soric
• University of Split, Croatia
• HEP2001, Budapest. July 13th 2001.

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APDs Outline
Requirements Performance Radiation
Hardness Conclusion
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The Detector
PbWO4 crystal
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APD structure
Photo-conversion electrons from the thin p-layer
induce avalanche amplification at the p-n
junction Electrons from ionising
particles traversing the bulk are not amplified
2 APDs (each 5 x 5 mm) mounted in a capsule
ready for gluing to a crystal
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APD Requirements
Operate in 4 Tesla field Radiation hard
(2.1013 n/cm2 250 kRad) Fast (? 10
nsec) Compatible with ECAL energy resolution
requirement Insensitivity to particles
traversing the diode Amplification Cheap
(122400 pieces) gt 8 year RD effort by
Hamamatsu (and initially EGG) in close
collaboration with CMS ECAL
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APD Impact on Energy Resolution

• ECAL energy resolution
• CMS design goal a 3, b0.5,
  c200 MeV
• APD contributions to
• a photo statistics (area, QE) and avalanche
  fluctuations (excess noise factor)
• b stability (gain, sensitivity to voltage,
  temperature variation, aging
• and radiation damage)
• c noise (low capacitance, serial resistance and
  dark current)

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APD Properties

• Active area (2 APDs per crystal) 5 x 5 mm
  (each)
• Quantum efficiency 75 at 430 nm
• Light collection within 20 nsec 99 1
• Operating voltage 380 V
• Gain (M) 50 (Max gt1000)
• Capacitance 80 pF
• Serial resistance 3 O
• Dark Current lt 50 nA ( 10 nA
  typical)
• Voltage sensitivity (1/MdM/dV) 3.15 / V
• Temperature sensitivity (1/TdM/dT) - 2.2 / V
• Excess noise factor 2.1
• Thickness sensitive to ionising particles 5 µm
• After radiation and accelerated aging equivalent
  to 10 years of LHC, ONLY quantity to change is
  the dark current, which rises to 5 µA

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APD Gain, V and T sensitivity
Voltage sensitivity
Operating Gain 50 gt
Temperature sensitivity
Well behaved up to gain 2000
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APD Capacitance, Quantum efficiency
PbWO peak emission
Vr
Q.E. is 75 at peak emission but APD insensitive
to traversing ionising particles (5µm effective
thickness)
APD is fully depleted at operating voltage
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Breakdown - Operating Voltage 1999 - 2001
Vb - Vr found important indicator of radiation
hardness
Absolute value should be large
Spread in Vb - Vr is small
First production 2001
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Reliability

• Production in 2000
• Few died (breakdown voltage drops below
  operating voltage)
• 1) in accelerated aging testing (80-90 deg)
• 2) in radiation testing (protons)
• gt Production stopped
• 1) Origin soon traced by Hamamatsu. Problem
  was solved.
• 2) Proved much harder. Complex with number
  of different causes
• over 6 months intensive RD by Hamamatsu
• review of radiation testing procedures at
  PSI
• gt Production restarted (3/2001)

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Radiation Hardness Conclusion

• Basic APD structure is radiation hard
• APDs found sensitive to Co ?-irradiation, not
  sensitive to neutrons
• problem at surface, not inside the silicon
• Solution modify geometry to reduce lateral
  fields (rounder corners, change spacings
  between structures, field clamps, etc.)
• basic APD structure unchanged.
• Plus screening of all APDs with Co
  ?-irradiation (500 kRad)
• Reject on lowered Vb, anomalous dark current
• followed by 2 weeks annealing/aging
  testing at 90 deg
• sampling (5) testing with 2 x 1013
  neutrons/cm2

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Cobalt screening results
Induced dark current almost completely
anneals after 2 weeks at 90 deg
Change in Vb after Co Irradiation
---------Vr---------------------------------------
--------------------------
X 1 day after Co irradiation after 10 days at
90 deg
Reject 2 APDs with change in Vb
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Radiation Hardness Status

• Of first 2700 APDs delivered in 2001, 3
  failed Co irradiation
• Investigation 1) All APDs from a few wafer
  positions are bad
• (over half the failures)
• 2) APDs from random wafer positions
• Hamamatsu 1) Bad position on mask. Reject all
  APDS from here
• 2) Dust, etc Reject APDs
  based on screening
• Other indicators Vb-Vr is abnormally low
• Discharge-like noise
• .. (still investigating)
• Status In next two Lots tested (7/01) only
  0.5 failed Co irradiation
• Of ca 500 APDs neutron irradiated,
  NONE have failed

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Neutron irradiation

• Neutron irradiation of APDs with the Minnesota Cf
  source (4 days)

All 25 APDs behave similarly Spread in induced
dark current largely due to flux non-uniformity
Dark current (log) vs time
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APD Summary

• The Hamamatsu APD meets all the specifications
• Radiation hardness proved hardest to achieve
• Now satisfactory. Expect to achieve gtgt 99
• Mass production rising to full rate (gt1000/week)

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Photodetectors for the CMS ECAL

• Photodetectors developed for the barrel (APDs)
  and end-cap (VPTs) of the CMS ECAL
• Fast, work in 4 T field, radiation hard
• Both meet the specifications necessary for CMS
  to find the Higgs in the favoured mass range
  (and much
  more.)