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Novel Semi-Transparent Optical Position Sensors for high-precision alignment monitoring applications: 

Novel Semi-Transparent Optical Position Sensors for high-precision alignment monitoring applications Sandra Horvat, F.Bauer, V.Danielyan, H.Kroha Max-Planck-Institute for Physics, Munich,Germany 13.06.2002. Orlando, Florida

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Introduction ATLAS detector @CERN End-Cap Octant Sandra Horvat 1/13 MPI for Physics,Munich ALMY System: high-precision alignment monitoring designed for high energy physics experiments laser beam along the alignment line ALMY sensors measure its position Requirements: high position resolution high transmittance insensitivity to high magnetic fields long term illumination radiation hardness 23 m

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ALMY Optical Monitoring System Sandra Horvat 2/13 MPI for Physics,Munich Multi-point measurement of the laser beam position : collimated laser beams (690 nm and 780 nm) single mode fibers (SMF), gaussian beam profile semi-transparent silicon sensors for 2D position measurement, from the induced photo current distribution

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ALMY Optical Position Sensors Transparent active surface: chemical vapour deposition of a-Si:H layer (0.3-1.0 mm) from the plasma phase 2 layers of ITO electrodes (50-100 nm) glass substrate (0.5-1.0 mm) anti-reflective coating Custom designed readout electronics: analog signal multiplexed, amplified (I-V converter), digitized, transmitted to PC for online analysis readout speed: 10 ms up to 30 sensors serially to PC port only commercial components laser controler 64x64 diodes, 312.5 mm pitch, 20x20 mm2 6 cm 6 cm Sandra Horvat 3/13 MPI for Physics,Munich

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Setup for the Sensor Tests Scan of the sensor surface with a laser beam, using stepping motors of 1 mm positioning accuracy position resolution photo current response laser beam deflection transmittance long term illumination ~200 sensors Sandra Horvat 4/13 MPI for Physics,Munich

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Transmittance transmittance @780 nm: 80-90% transmittance @690 nm: 70-75% Minimized reflectivity: optimized thicknesses of a-Si:H and ITO layers anti-reflective coating on the back side of the glass up to 10 sensors along the 780 nm beam Sandra Horvat 5/13 MPI for Physics,Munich

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Photo Current Response Finite wedge angle of the glass substrate can cause the interference patterns, due to the reflected light. Efficiently suppresed! anti-reflective coating laser diodes with short coherence length uniform distribution over the whole surface laser beam profile remains undistorted Sensitivity: 0.1 A/W @690 nm 0.01 A/W @780 nm Saturation (strip current 1mA): 1 mW/cm2 @690 nm 10 mW/cm2 @780 nm Sandra Horvat 6/13 MPI for Physics,Munich

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Position Resolution 5.012 mm 5.746 mm Typical production batch: S/N>1000 local resolution: 1 mm overall resolution: 5 mm uniform distribution Sandra Horvat 7/13 MPI for Physics,Munich

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Laser Beam Deflection specially polished glass wafers parallel surfaces remain undeformed during the antireflecive coating deflection <5mrad Glass quality is unchanged during sensor production for T of up to 2000. uniform distribution, deflection angle < 5mrad Glass wafers: Sensors: Sandra Horvat 8/13 MPI for Physics,Munich

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Long Term Illumination (Staebler-Wronski effect) Photo current response after 1500 h of illumination with 690 nm @ 1mW/cm2 photo current response in a-Si:H deteriorates under illumination degradation in position resolution local sensitivity minimum increases with time dependance on the wavelenghth and the beam intensity Sandra Horvat 9/13 MPI for Physics,Munich

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Long Term Illumination 690 nm @ 1 mW/cm2 780 nm @ 10 mW/cm2 780 nm @ 10 mW/cm2 equivalent effect 1% 1.5-2.5% 3.5% Laser Beam, below saturation Laser Beam & daylight degradation at a constant rate depends on the absorbed light, rather than illumination intensity 500 h correspond to a tolerable position measurement error of 5 mm non-additive effect under an additional daylight illumination Sandra Horvat 10/13 MPI for Physics,Munich

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Radiation Hardness: 1014 neutrons/cm2,1 MeV eq. bias voltage: 1V I-V characteristics dark current Naked silicon cells: before and after irradiation (no degradation) Sandra Horvat 11/13 MPI for Physics,Munich

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Radiation Hardness: 1013 neutrons/cm2 Change in the transmittance: <1% Sensor module with electronics: Change in the photo current: <2% No degradation! Additionally: No degradation after g-irradiation (10 MRad). Sandra Horvat 12/13 MPI for Physics,Munich

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Summary ALMY sensors: transparent a-Si:H position sensors application in optical alignment monitoring systems Sensor performance : dynamic range 20x20 mm2 position resolution 5 mm high quality parallel glass substrate transmittance 80-90% @780 nm uniformity over the whole active surface radiation hard (1013 n/cm2, 10 g MRad) insensitive to high magnetic fields (Hall mobility <10-2 cm/Vs) negligible degradation under illumination with laser beam intensities below saturation (10 mW/cm2 @780 nm) optimized and verified in extensive tests larger-scale production under preparation Sandra Horvat 13/13 MPI for Physics,Munich