Orlando, Florida
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.
Introduction
ATLAS detector @CERN
ALMY System:
23 m
• high-precision alignment monitoring
• designed for
high energy physics experiments
• laser beam along the alignment line
• ALMY sensors measure its position
End-Cap Octant
Requirements:
• high position resolution
• high transmittance
• insensitivity to high magnetic fields
• long term illumination
• radiation hardness
Sandra Horvat
1/13
MPI for Physics,Munich
ALMY Optical Monitoring System
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
Sandra Horvat
2/13
MPI for Physics,Munich
ALMY Optical Position Sensors
Transparent active surface:
64x64 diodes,
312.5 mm pitch,
20x20 mm2
• 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
6 cm
Custom designed readout electronics:
6 cm
Sandra Horvat
• 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
3/13
MPI for Physics,Munich
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
Sandra Horvat
4/13
~200 sensors
MPI for Physics,Munich
Transmittance
transmittance @780 nm: 80-90%
Minimized reflectivity:
• optimized thicknesses of
a-Si:H and ITO layers
• anti-reflective coating on
the back side of the glass
transmittance @690 nm: 70-75%
up to 10 sensors
along the 780 nm beam
Sandra Horvat
5/13
MPI for Physics,Munich
Photo Current Response
Finite wedge angle
of the glass substrate
can cause the
interference patterns,
due to the reflected light.
Sensitivity:
0.1 A/W @690 nm
0.01 A/W @780 nm
Efficiently suppresed!
• anti-reflective coating
• laser diodes with
short coherence length
Saturation
(strip current 1mA):
1 mW/cm2 @690 nm
10 mW/cm2 @780 nm
• uniform distribution
over the whole surface
• laser beam profile
remains undistorted
Sandra Horvat
6/13
MPI for Physics,Munich
Position Resolution
5.012 mm
Typical production batch:
•
•
•
•
S/N>1000
local resolution: 1 mm
overall resolution: 5 mm
uniform distribution
5.746 mm
Sandra Horvat
7/13
MPI for Physics,Munich
Laser Beam Deflection
Glass wafers:
• specially polished glass wafers
• parallel surfaces
• remain undeformed during
the antireflecive coating
deflection <5mrad
Sensors:
Glass quality is unchanged
during sensor production
for T of up to 2000.
uniform distribution,
deflection angle < 5mrad
Sandra Horvat
8/13
MPI for Physics,Munich
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
Long Term Illumination
Laser Beam, below saturation
1%
690 nm @ 1 mW/cm2
• 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
Laser Beam & daylight
equivalent effect
780 nm @ 10
3.5%
mW/cm2
1.5-2.5%
780 nm @ 10 mW/cm2
Sandra Horvat
10/13
MPI for Physics,Munich
Radiation Hardness: 1014 neutrons/cm2,1 MeV eq.
Naked silicon cells: before and after irradiation
I-V characteristics
(no degradation)
dark current
bias voltage: 1V
Sandra Horvat
11/13
MPI for Physics,Munich
Radiation Hardness: 1013 neutrons/cm2
Sensor module
with electronics:
Change in the transmittance:
<1%
Change in the photo current:
<2%
No degradation!
Additionally:
No degradation after
g-irradiation (10 MRad).
Sandra Horvat
12/13
MPI for Physics,Munich
Summary
ALMY sensors: transparent a-Si:H position sensors
application in optical alignment monitoring systems
Sensor performance :
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•
•
•
•
•
•
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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