1. PIN current degradation Versus 3 MeV proton fluence 3 MeV proton (a)(b) (c)(d) Study of radiation damage in VCSELs and PINs for the optical links of the ATLAS inner tracker Optical links are used by the ATLAS inner detector modules for receiving timing, trigger and control signals, and for data transmission at 40 Mbit/sec. Radiation damage is expected for the high flux of charged particles and low energy neutrons produced in proton-proton collisions at  s=14 TeV. The fluences over a ten-year operation are expected for 2×10 14 neutrons and 1.5×10 14 charged hadrons per cm 2 for the SemiConductor Tracker (SCT), and is five times higher for the Pixel detector. VCSELPIN The Vertical-Cavity Surface-Emitting Laser (VCSEL) is made from a GaAlAs multi- quantum well structures emitting light at about 850 nm. The radiation is expected to cause atomic displacement damage by nuclear interactions. The defects can act as non-radiative recombination centers, which decreases the minority carrier lifetime and results to increase in laser threshold current. However, most of this damage can be removed by injection annealing. The radiation tolerance of VCSELs and PIN photodiodes (manufactured by Truelight Co. Taiwan) were studied with 3 MeV protons at Academia Sinica, 200 MeV protons at Indiana University Cyclotron Facility, and with 20 MeV (on average) neutrons at Cyclotron Research Centre, Louvain-la-Neuve. Two types of VCSELs (proton- implant and oxide-confined) were tested. We report results mostly of the oxide- confined VCSEL that has higher and uniform light power characteristics. SCT barrels SCT endcaps Pixels Barrel detector module Endcap detector modules ATLAS inner detector Optical links Pelletron tandem accelerator for tests with 3 MeV protons VCSEL and PIN components were irradiated inside a vacuum chamber. Measurements were made offline. Laser light power was detected by a large area PIN diode. PIN responsivity was measured with calibrated Laser light sources. M.L. Chu, S. Hou*, S.C. Lee, D.S. Su, P.K. Teng Academia Sinica, Taipei, Taiwan IUCF proton and LCU neutron beam testsIUCF LCU Irradiation tests were conducted with VCSEL and PIN components connected to Radiation hard optical fibers for online readout. Tests were also made for unbiased samples which were studied for annealing characteristics. VCSEL and PIN of 8 or 12-channel arrays were assembled on PC boards suitable for mounting to the test beam and MT type fiber connection. The readout of ATLAS inner detector is conducted with optical links for a total of 6.2 M channels for the SCT, and 140 M channels for the Pixel detector. The VCSEL optical power is approximately a linear function to the current. In an irradiation conducted with 200 MeV protons, the L-I curves (figure on right) show a linear degradation with The annealing of VCSEL with current injection is a quick process. The changes in annealing at the nominal operation current of 10 mA are illustrated for (a) laser light power, (b) laser threshold current, and (c) slope of L-I curves. These VCSELs were irradiated unbiased with 20 MeV neutrons to three fluencies of up to 7.7×10 14 n/cm 2. The distributions were fit to a recovery function of f(t) = f  -b exp(-t/  ). The curves are the fits to the recovery function. The recovery time (  ) is about 6 hours for all three fluencies. The permanent damage is characterized by the f  of the fit after annealing. The fit values are The degradation of oxide-confined VCSELs irradiated with 200 MeV protons are shown in (a) for three fluences. The light power and threshold current measured before irradiation (“+” marks) demonstrate the uniform performance between channels. The annealing was conducted for 80 hours with half of the channels biased at 6 mA. These channels show slower recovery than those biased at 10 mA. As functions of proton fluence, the L-I parameters are shown in (b,c,d) for degradations of the two types of VCSELs before and after annealing. laser threshold currents shifting to higher values. Parameters of linear fits are shown for (a) L/L 0 at 10 mA, (b) threshold current, and (c) slope. The VCSEL circuit is fabricated in a depth of less than 20 μm in a GaAs wafer. The 3 MeV protons can not penetrate the GaAs wafer and build an insulation layer at around 55 μm, which kills the lasing cavity at a fluence of about 3x10 14 p/cm 2. The annealing of the L-I parameters are shown for an VCSEL array receiving 2x10 14 p/cm 2. Large deviation is observed with the light power reduced by about 10% and the threshold current increased by 1.3 mA. These values are about twice of the degradation caused by 200 MeV protons. In comparison, the degradation caused by 20 MeV neutrons (~7% on light power) is compatible to 200 MeV protons. The epitaxial PIN diode has a high frequency response that matches well with the 850 nm light of the VCSEL. The thin active layer provides more radiation tolerance than bulk silicon PINs. The irradiation with 3 MeV protons causes an implantation layer at around 90 μm in Si wafer, however, the change in PIN responsivity is smooth (figure on left) and is consistent between channels with a RMS of 2%. The responsivity drops quickly to 10% at 3x10 14 (3 MeV) p/cm 2. While those irradiation with 200 MeV protons (figure on right) show a loss of 25%. The test with 200 MeV proton has included PIN diodes of another manufacturer (Centronic), which shows faster degradation curve than those (Truelight) used for ATLAS. The PIN responsivities shown were measured with the PIN surfaces perpendicular to the proton beam. The tests were also made with the PIN surfaces aligned in parallel to the proton beam and caused longer proton traversing distance inside the PIN active volume, The degradation thus obtained is twice larger. In summary, the VCSEL threshold current increases by less than 1 mA with 2×10 14 (200 MeV) p/cm 2 or (20 MeV) n/cm 2, and the light degradation is less than 10%. The epitaxial PIN diodes show a fast degradation at the beginning fluences and the PIN responsivity degrades by 20% for 2×10 14 (200MeV) p/cm 2. Both components show excellent radiation tolerance for the requirement of ATLAS experiment. [1] ATLAS Inner Detector Technical Design Report, CERN/LHCC/97-16/17, 30 April 1997. [2] D.G. Charlton, et al., Nucl. Instr. and Meth. A 443 (2000) 430. [3] M.L. Chu, et al., Nucl. Instr. and Meth. A 530 (2004) 293. [4] P.K. Teng, et al., Nucl. Instr. and Meth. A 497 (2003) 294. [5] L.S. Hou, et al., Nucl. Instr. and Meth. A 539 (2005) 105.