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% IEEE 2002 paper for Sensor Journal
%
% Contact: Hubert Kroha
%          Max-Planck-Institut fuer Physik
%          Foehringer Ring 6
%          D-80805 Muenchen
%          Tel.: +49 (89) 32354-435
%          Fax:  +49 (89) 3226704
%          Email: kroha@mppmu.mpg.de
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\begin{document}

\title{Studies of Semitransparent Optoelectronic Position Sensors}

\author{F.~Bauer, V.~Danielyan, S.~Horvat, and H.~Kroha$^{\ast}$
\thanks{Manuscript received April 30, 2002.}
\thanks{F.~Bauer, V.~Danielyan, S.~Horvat, and H.~Kroha are with the
Max-Planck-Institut f\"ur Physik, F\"ohringer Ring 6, D-80805 Munich, Germany,
(tel.: +49-89-32354-0, fax: +49-89-32354-305).}
\thanks{Permanent address of F.~Bauer: CEA Saclay, DSM, DAPNIA, F-91191 Gif-sur-Yvette Cedex,
France (e-mail: florian@hep.saclay.cea.fr).}
\thanks{Permanent address of V.~Danielyan: Yerevan Physics Institute, 375036 Yerevan,
Armenia (e-mail: danielya@mppmu.mpg.de).}
\thanks{S.~Horvat is on leave of absence from Ruder Bo\v{s}kovi\'{c} Institute, 
10001 Zagreb, Croatia (e-mail: sandra@mppmu.mpg.de).}
\thanks{Corresponding author: H.~Kroha, Max-Planck-Institut f\"ur Physik,
F\"ohringer Ring 6, D-80805 Munich, Germany (tel.: +49-89-32354-435,
fax: +49-89-32354-305, e-mail: kroha@mppmu.mpg.de).}
}
\maketitle

\begin{abstract}
Novel semitransparent optoelectronic position sensors
(`ALMY sensors')
have been developed for high-precision multi-point position and angle
measurements of collimated laser beams over a large measurement range.
The sensors consist of a thin film of amorphous silicon deposited
on a glass substrate between two transparent layers of crossed strip electrodes. 
They provide a position resolution of on the order of a micrometer
over sensitive areas of several square centimeters.
A transmittance of $80-90\%$ has been achieved for 780~nm laser light
produced by diode lasers. We report about recent
optimizations of the sensor performance and tests of the long-term
stability under laser illumination and of the radiation tolerance
at high neutron doses.
As expected, the radiation hardness of the amorphous silicon sensors exceeds
the one of crystalline silicon devices.
The custom designed readout electronics allows for operation at sufficiently low
laser intensities in order to prevent significant degradation of the performance 
of the amorphous silicon sensors under illumination with laser light.
\end{abstract}

\begin{keywords}
Silicon strip sensors, amorphous silicon, laser beams, laser diodes, alignment monitoring
\end{keywords}
%
\begin{figure}[h]

\begin{center}                            %Bild zentrieren
\begin{minipage}[t]{145mm}                %Minipage der Breite 155 mm
\begin{picture}(170,89)                   %Bild ist 155 mm breit, 81 mm hoch
\put(0,81){\special{em:graph asi_sd3}}    %Bild 0 mm nach rechts,
\end{picture}                             %81 mm nach oben verschieben
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\end{center}
\vspace{8mm}

\caption{Structure of the ALMY sensors.\label{sensor}}
\end{figure}
\vspace{-1mm}
%
%
\begin{figure}[ht]
\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/mdt_electronics_new_bw.eps,width=1.00\linewidth}}

\caption{ALMY sensor module with readout electronics. The transparent sensor is glued
across a cutout in the 60~mmm~$\times$~60~mm size readout electronics boards. The bond pads of
the strip electrodes outside the sensitive and transparent area are visible.
\label{module_new}}
\vspace{-3mm}
\end{figure}
%
\vspace{-6mm}

\section{Introduction}
%
Novel semitransparent optoelectronic position sensors, the `ALMY sensors,'
have been developed for high-precision multi-point position and angle
measurements of collimated laser beams over a 
large measurement range~\cite{ALMY1}-\cite{ALMY5}.
The ALMY sensors consist of a thin
layer of hydrogenated amorphous silicon (a-Si:H) of $0.3-1~\mu$m
thickness deposited on a 0.5--1~mm thick polished glass substrate between
two electrode layers of indium-tin oxide (ITO) 
of $50-100~\mu$m thickness which are transparent in the visible light spectrum
(see Fig.~\ref{sensor}). A sensor module with readout electronics is shown
in Fig.~\ref{module_new}.
%
\begin{figure}[t]
\vspace{-4mm}
\mbox{\hspace{-2mm}\epsfig{figure=/home/pcatlas17/kroha/almy/trans-before_new.eps,width=1.12\linewidth}}

\vspace{-3mm}
\caption{Transmittance $T$ of a typical ALMY sensor at wavelengths of (a and b) 780~nm
and (c and d) 690~nm. Shown is (a and c) the uniformity of the transmittance
as a function of the laser beam position $(x,y)$
on the sensor surface and (b and d) the distribution of the transmittance from a scan of the
sensor (see text).
\label{transmission_before_new}}
\vspace{-2mm}
\end{figure}
%
\begin{figure}[ht]
\vspace{-20mm}

%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/sensi-irr.eps,width=1.10\linewidth}}
\mbox{\hspace{-1mm}\epsfig{figure=/home/pcatlas17/kroha/almy/sensi-before_new.eps,width=1.11\linewidth}}

\vspace{-10mm}
\caption{Relative photo current response $R$ (average normalized to one) 
of the same ALMY sensor as in Fig.~3. Shown is
(a) the response as a function of the laser beam position $(x,y)$ on the sensor surface
and (b) the distribution of the response from a scan of the sensor (see text).
\label{sensitivity_before_new}}
\vspace{-2mm}
\end{figure}
%
%
\begin{figure}[ht]
\mbox{\hspace{-7mm}\epsfig{figure=/home/pcatlas17/kroha/almy/irrad-compare-bw.eps,width=1.20\linewidth}}

\vspace{-8mm}
\caption{Current-voltage characteristics of an amorphous silicon photo diode (a) without 
and (b) with light exposure (intensity not calibrated) before (full line) and after
(dashed line) irradiation with $10^{14}$ neutrons/cm$^2$. 
\label{diode_new}}
\vspace{-2mm}
\end{figure}
%
%
\begin{figure}[ht]
\mbox{\hspace{-2mm}\epsfig{figure=/home/pcatlas17/kroha/almy/irrad-compare-one-bw.eps,width=1.20\linewidth}}

\vspace{-6mm}
\caption{Dark current as a function of strip number for the same ALMY sensor as in Fig.~5
at the nominal bias voltage of 1~V before (full line) and after (dashed line) irradiation with
$10^{14}$ neutrons/cm$^2$.
\label{strips_new}}
\vspace{-2mm}
\end{figure}
%
%
\begin{figure}[p]
\vspace{-7mm}
%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/posres-before-irr.eps,width=1.10\linewidth}}
%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/posres-after-irr.eps,width=1.10\linewidth}}
\mbox{\hspace{-11mm}\epsfig{figure=/home/pcatlas17/kroha/almy/posres-before-after_new.eps,width=1.24\linewidth}}

\vspace{-6mm}
\caption{Position measurement accuracy of the same ALMY sensor as in Fig.~3 
in the two sensor coordinates $x$ and $y$ (a and b) before and (c and d) after irradiation
with $10^{13}$ neutrons/cm$^2$. Shown are the
bdistributions of the differences $\Delta x$ and $\Delta y$ between
the measured and the true laser beam coordinates (see text).
\label{resol_new}}
%\end{figure}
%
%
%\begin{figure}[ht]
%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/defl-newnew.eps,width=1.10\linewidth}}
\vspace{-1mm}
\mbox{\hspace{-11mm}\epsfig{figure=/home/pcatlas17/kroha/almy/defl-before-after_new.eps,width=1.24\linewidth}}

\vspace{-6mm}
\caption{Laser beam deflection angles $\Theta_x$ and $\Theta_y$ in the two 
coordinates transverse to the beam for the same ALMY sensor as in Fig.~3
(a and b) before and (c and d) after irradiation with $10^{13}$ neutrons/cm$^2$.
\label{deflection_new}}
\end{figure}
%
%
\begin{figure}[ht]
\vspace{-8mm}
%\mbox{\hspace{-1mm}\epsfig{figure=/home/pcatlas17/kroha/almy/dip_780_day_high.eps,width=1.12\linewidth}}
\mbox{\hspace{-1mm}\epsfig{figure=/home/pcatlas17/kroha/almy/dip_690.eps,width=1.12\linewidth}}

\vspace{-7mm}
\caption{Local degradation of the relative photo current response 
of an ALMY sensor measured with laser scans (see text) after 120, 500 1000 and 1500 hours
of continuous illumination with a 690~nm laser beam of 3~mm diameter ($3\sigma$) and
1~mW/cm$^2$ intensity under dark conditions. The local sensitivity loss
follows the gaussian profile of the laser beam.
\label{dip}}
\end{figure}
%
%
\begin{figure}[ph]
\vspace{-2mm}

%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/all_aged_in_one_2.eps,width=1.10\linewidth}}
\mbox{\epsfig{figure=/home/pcatlas17/kroha/alignment/all_aged_in_one_bw_c.eps,width=1.03\linewidth}}

\vspace{-2mm}
\caption{Relative photo current response of four ALMY sensors monitored as a function of time
at the peak position of the laser beam profile
during continuous illumination over 520 hours with a 780~nm laser beam 
of 3~mm diameter ($3\sigma$) and 10~mW/cm$^2$ intensity under dark conditions.
\label{age_780}}
%\end{figure}
%
%
%\begin{figure}[ht]
%\mbox{\hspace{-8mm}\epsfig{figure=/home/pcatlas17/kroha/almy/aging_690_2.eps,width=1.17\linewidth}}
\mbox{\hspace{-8mm}\epsfig{figure=/home/pcatlas17/kroha/almy/age_690_780.eps,width=1.18\linewidth}}

\vspace{-4mm}
\caption{Time dependence of the relative photo current response (as in Fig.~10) of an ALMY sensor
during continuous illumination (a) with a 690~nm laser beam of 3~mm diameter ($3\sigma$)
and 1~mW/cm$^2$ intensity under dark conditions and (b) with a 780~nm laser beam of 3~mm diameter
and 10~mW/cm$^2$ intensity in the presence of daylight.
\label{age_690_780}}
\end{figure}
%
%
%\begin{figure}[ht]
%%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/aging_690.eps,width=1.10\linewidth}}
%\mbox{\hspace{-8mm}\epsfig{figure=/home/pcatlas17/kroha/almy/aging_690_2.eps,width=1.17\linewidth}}
%
%\vspace{-10mm}
%\caption{Time dependence of the relative photo current response (as in Fig.~10) of an ALMY sensor
%during continuous illumination over 1500 hours with a 690~nm laser beam of 3~mm diameter ($3\sigma$)
%and 1~mW/cm$^2$ intensity under dark conditions.
%\label{age_690}}
%\end{figure}
%
%
%\begin{figure}[ht]
%%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/ageday_780nm.eps,width=1.10\linewidth}}
%\mbox{\hspace{-8mm}\epsfig{figure=/home/pcatlas17/kroha/almy/ageday_780nm_2.eps,width=1.18\linewidth}}
%
%\vspace{-3mm}
%\caption{Time dependence of the relative photo current response (as in Fig.~10) of two ALMY sensors
%during continuous illumination over 520 hours with a 780~nm laser beam of 3~mm diameter
%and (a) 10~mW/cm$^2$ or (b) 60~mW/cm$^2$ intensity, respectively, in the presence of daylight.
%\label{ageday_780}}
%\end{figure}
%
%
\begin{figure}[ht]
\vspace{-24mm}
%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/trans-irr14.eps,width=1.10\linewidth}}
%\mbox{\epsfig{figure=/home/pcatlas17/kroha/almy/trans-irr.eps,width=1.10\linewidth}}
\mbox{\hspace{-2mm}\epsfig{figure=/home/pcatlas17/kroha/almy/trans-after_new.eps,width=1.12\linewidth}}

\vspace{-10mm}
\caption{Change $\Delta T$ of the transmittance of the ALMY sensor in Fig.~3 
at a wavelength of 780~nm 
after irradiation with $10^{13}$ neutrons/cm$^2$ (denotations as in Fig.~3).
\label{transmission_after_new}}
\end{figure}
%
%
\begin{figure}[ht]
\vspace{-23mm}
\mbox{\hspace{-2mm}\epsfig{figure=/home/pcatlas17/kroha/almy/sensi-after_new.eps,width=1.12\linewidth}}

\vspace{-10mm}
\caption{Change $\Delta R$ of the relative photo current response of the ALMY sensor in Fig.~4
after irradiation with $10^{13}$ neutrons/cm$^2$ (denotations as in Fig.~4).
\label{sensitivity_after_new}}
\end{figure}
%
%
%\begin{figure}[ht]
%\mbox{\hspace{-2mm}\epsfig{figure=/home/pcatlas17/kroha/almy/trans-irr-naked.eps,width=1.12\linewidth}}
%
%\vspace{-10mm}
%\caption{Change $\Delta T$ of the transmittance of a ALMY sensor
%at a wavelength of 780~nm
%after irradiation with $10^{14}$ neutrons/cm$^2$ (denotations as in Fig.~3).
%\label{transmission_after_naked}}
%\end{figure}
%
%

The thin films are produced by chemical vapour deposition from the plasma phase.
In contrast to amorphous silicon solar cells which are made with similar
techniques,
the two electrode layers of the sensors are segmented into orthogonal arrays 
of 64 strips with $312~\mu$m pitch and a width of $300~\mu$m covering
the whole sensor area of $20\times 20$~mm$^2$. Larger and smaller
sensor sizes are possible. 

Laser diodes coupled to single-mode optical fibers with collimator optics
are used to produce collimated laser beams with gaussian profile at wavelengths
of 690~nm or 780~nm near the end of the absorption spectrum of a-Si:H.
The two-dimensional centroide of the gaussian laser
beam profile projected on the sensor surface can be determined from the
measured photo current distributions on the top and bottom strips with
an accuracy of on the order of $1~\mu$m over the whole measurement range~\cite{ALMY1}-\cite{ALMY5}.

At 690~nm and 780~nm laser wavelength, transmittances of the sensors of $70-75\%$
and of $80-90\%$, respectively, have been reached (see Fig.~\ref{transmission_before_new}).
The high transmittance of the sensors for visible light allows for multiple
measurements of the same laser beam by more than 10 sensors positioned along the beam.
Such systems consisting of diode lasers and ALMY sensors have a variety of applications 
in the alignment monitoring systems of high energy physics experiments~\cite{ATLAS1}-\cite{ZEUS}.
For applications in high energy physics it is of special interest that the 
position resolution of the amorphous silicon sensors does not deteriorate 
in high magnetic fields~\cite{ALMY1}-\cite{ALMY4}
and under high radiation doses~\cite{ALMY5} in contrast to many crystalline
silicon devices. The sensors have also been tested in industrial applications.

In this paper we give an update on the sensor development and performance
and report about studies of the long-term stability and radiation
hardness of the ALMY sensors.

\section{Readout electronics}
%
For each measurement of the laser beam position, the $2\times 64$ strip
electrodes of an ALMY sensor are read out by custom designed readout electronics.
The sensors are glued and wire bonded to readout boards which have
cutouts for the active area of the sensors (see Fig.~\ref{module_new}).
On the boards, the analog signals of all strips are multiplexed, amplified by a common
current-voltage converter, digitized and transmitted to a PC for online analysis.

Previously published test results~\cite{ALMY5} have been obtained
using custom designed integrated readout electronics in CMOS technology
~\cite{elec1}, \cite{elec2} with interfaces to the VME bus or to the PC serial port.
Recently a new version of the readout electronics has been developed~\cite{MPInote}
using only commercial components on an analog and a digital board which both
have a size of 60~mm~$\times$~60~mm (see Fig.~\ref{module_new}).
The digital board contains a
laser controller which allows for turning on and off laser diodes in
synchronization with the sensor readout in order to minimize the sensor illumination
time (see below). The readout speed for the whole sensor is 10~ms as with the
intergrated electronics version. With a custom designed interface card,
up to 30 sensors can be serially connected to the PC serial port. For larger
applications, an alternative CANBus interface is under development.

Both electronics versions have been tested to be radiation hard without
changes in the performance up to fluences
of $10^{13}$~neutrons/cm$^2$ (1~MeV neutrons equivalent).
The laser diodes with optical fibers and collimator optics also survived
this neutron fluence.

\section{Sensor performance}
%
The reflectivity of the sensor surfaces in the wavelength range of 690--780~nm
has been minimized by optimizing
the a-Si:H and ITO layer thicknesses on the front surface and by using 
state-of-the-art antireflective coating on the back side of the glass substrate
in order to increase the average transmittance for the laser beam 
and to suppress interference patterns in the sensor response and in the
transmittance across the sensor surface which can be caused by finite wedge
angles of the glass substrate. This design leads to a sufficiently uniform
distribution of the transmittance (see Fig.~\ref{transmission_before_new}) and of the
light absorption or sensor response, respectively (see Fig.~\ref{sensitivity_before_new}), over
the sensor surface such that the traversing laser beam profile remains undistorted
and the linearity of the position measurement is not degraded.
Also, laser diodes with short longitudinal coherence length are used in order
to minimize interference effects in the glass substrate.

The uniformity of
the sensor parameters is tested by scanning the surface with a laser beam
of about 3~mm diameter ($3\sigma$)
using a two-dimensional translation stage with stepping motors
of $1~\mu$m absolute positioning accuracy on which the sensor is mounted.
The transmittance of the test sensor and the deflection of the traversing laser beam
are measured with two additional ALMY sensors at known distances
using the measurement without test sensor as reference. The deflected
beam position is measured at two distances behind the test sensor
in order to disentangle the angular beam deflection from the parallel
displacement of the beam in the case of non-perpendicular incidence~\cite{MPInote}.
Care is taken to
prevent deflections of the laser beam due to temperature gradients in the air.

The top electrodes of the sensors act as Schottky contacts. The photo diode characteristics
of a strip crossing on a typical sensor is shown in Fig.~\ref{diode_new}.
The sensors are operated at a bias voltage of 1~V.
In this voltage range, the dark currents are very small, only
$10^{-7}-10^{-8}$~A/cm$^2$ (see Figs.~\ref{diode_new} and \ref{strips_new}), 
while the sensitivity of the
sensors is approximately 0.1~A/W at 690~nm and 0.01~A/W at 780~nm.
The readout electronics saturates at laser intensities of about 1~mW/cm$^2$
at 690~nm and 10~mW/cm$^2$ at 780~nm corresponding to a
maximum strip current of $1~\mu$A. The strip currents have been limited 
to $1~\mu$A in order to
limit the voltage drop along the strips which have non-negligible resistance
and thus to prevent a significant sensitivity variation across the sensor surface.

The large signal-to-noise ratio of $>1000$ allows for a local position resolution
of the sensors of better than $1~\mu$m (for a strip pitch of $312~\mu$m and
laser beam diameters of $>2$~mm) and a dynamic range of $>10$.
The linearity of the position measurement over the whole sensitive area
depends on the uniformity of the sensor response which can be achieved in the
fabrication process (see Fig.~\ref{sensitivity_before_new}).
About 200 sensors with different types of glass substrate and antireflective coating
have been produced and tested. With the antireflective coating of the glass substrate,
a position resolution over the whole sensor surface of $1~\mu$m
has been achieved~\cite{ALMY5}. 
For the average sensor production batch with antireflective coating,
a position resolution of $\le 5~\mu$m was obtained (for a typical example see
Fig.~\ref{resol_new}). The resolution in each of the two sensor coordinates
$x$ and $y$ is determined as the width of the distribution of the difference
between the measured positions and the true positions given
by the stepping motor

In order to minimize the deflection of the traversing laser beam 
and the variation of the deflection over the whole sensor surface,
specially polished glass wafers with flat parallel surfaces are used
which do not get deformed during the antireflective coating of the
wafers (see~\cite{ALMY5}) and during sensor production at temperatures
of up to $200^\circ$C. The average laser beam deflection and the
uncertainty in the deflection are below $5~\mu$rad for the complete sensors
(see Fig.~\ref{deflection_new}).

\section{Long-term stability}
%
From the study of amorphous silicon solar cells, it is well known 
that the photo current response of a-Si:H deteriorates under illumination~\cite{wronski}.
A local degradation of the sensitivity and thus of the position measurement accuracy
has been observed for the ALMY sensors when they are continuously
illuminated by a laser beam at constant position~\cite{ALMY5}.
The position sensitivity of the ALMY sensors allows for detailed studies of the
dependence of the degradation effect on the laser intensity and wavelength
by monitoring as a function of time with laser scans the local sensitivity 
minimum created at the peak of the
laser beam profile with respect to the rest of the sensor area (see Fig.~\ref{dip}).
With this method, one does not have to rely on a precise calibration of the
laser intensity and profile when probing sensitivity
variations over a long time period.

The readout electronics is designed such that the maximum signal height
below saturation is reached for a 3~mm diameter ($3\sigma$) gaussian laser beam
with 780~nm wavelength and 10~mW/cm$^2$ intensity or with 690~nm wavelength and 1~mW/cm$^2$
intensity, respectively. The sensors are operated with a shielding tube against daylight 
which otherwise would saturate them.
The local degradation of the sensor response as a function of
time is shown in Figs.~\ref{age_780} and \ref{age_690_780}a for the two operation modes.
The degradation rate is constant at the relatively low laser intensities below saturation.
After 500 hours of continuous illumination at the above operating conditions
of the sensors, the sensitivity loss at the peak is $1.5-2.5\%$ at 780~nm (depending on the sensor)
and $1\%$ at 690~nm. In the latter case, $2.5\%$ degradation is reached after 1500 hours
of illumination. The results at different wavelengths support the concept that
the degradation rate depends on the light absorption in the amorphous silicon,
which varies with the wavelength, rather than just on the illumination intensity.

A $2.5\%$ local sensitivity loss is of similar size as the observed sensitivity
variations on the sensors after fabrication (see Fig.~\ref{sensitivity_before_new}).
It corresponds to a local absolute position measurement error of $\pm 5~\mu$m~\cite{ALMY5}.
Therefore there is no significant deterioration of the sensor performance during at least
500 hours of continuous illumination in the same spot at the maximum signal height.
The sensors will only be illuminated by the laser beam while they are read out during
a measurement.

The illumination with a 780~nm laser beam
of 10~mW/cm$^2$ intensity has been repeated in the presence of uniform illumination 
with daylight which produces the maximum signal current close to saturation.
Fig.~\ref{age_690_780}b shows, as a function of time, the additional local
degradation of the sensitivity
at the laser beam position relative to the response of the rest of the sensor
which is illuminated with daylight only. After 500 hours,
the additional local degradation is $3.5\%$ exceeding the degradation observed with
laser illumination under dark conditions.
The dependence of the degradation rate on the light absorption appears to be non-additive.

\section{Radiation hardness}
%
Amorphous silicon sensors are expected to be more tolerant against particle
irradiation than crystalline silicon devices. This is of particular importance 
for applications in the optical alignment monitoring systems
of high energy physics experiments~\cite{ATLAS1}-\cite{ZEUS}.
Detailed studies have been performed of the performance of ALMY sensors
from different production batches (different a-Si:H thickness, different glass substrates)
after irradiation with neutrons.

In Figs.~\ref{diode_new} and \ref{strips_new},
the current-voltage characteristics and the dark currents at the operating bias voltage
of sensors from the current production after optimization of the a-Si:H thickness and
of the glass substrate are compared before and after irradiation with a neutron fluence of 
$10^{14}$~neutrons/cm$^2$ (1~MeV neutrons equivalent).
No degradation of the current-voltage characteristics was observed and, in particular,
there is no significant increase of the dark currents up to this high neutron fluence.

In Figs.~\ref{resol_new}, \ref{deflection_new}, \ref{transmission_after_new} and \ref{sensitivity_after_new},
the position resolution, laser beam deflection, transmittance and photo response of the sensors
of the current production equipped with the new readout electronics (see Fig.~\ref{module_new})
are compared before and after irradiation
with a neutron fluence of $10^{13}$~neutrons/cm$^2$ (1~MeV neutrons equivalent). There is no
significant degradation of the essential performance parameters of the sensors including the electronics
up to this neutron fluence. The same is true for sensors from previous productions with thicker
a-Si:H layers and different glass substrates equipped with the previous integrated electronics version.

Sensor modules with readout electronics have not yet been tested with neutron fluences
above $10^{13}$~neutrons because the electronics is not expected to survive.
The transmission and beam deflection properties of the sensors and of the glass substrates alone
have been tested without readout electronics
up to neutron fluences of $10^{14}$~neutrons/cm$^2$ (1~MeV neutrons equivalent).
No significant deterioration in the average parameters is seen for the new sensors
as well as for the ones from previous productions and for all types of glass substrates used.
The variation of the transmittance and of the angular deflection over the sensor surface becomes slightly
larger with increasing neutron fluence.

The radition hardness results are confirmed by irradition tests of ALMY sensors of previous
productions by other groups. The sensors (without electronics) were shown to survive
$^{60}$Co $\gamma$ irradiation of 10~MRad and a neutron fluence of $1.1\cdot 10^{15}$~neutrons/cm$^2$ without
significant changes in transmittance and deflection~\cite{CMSirrad}. A degradation of the sensor response 
of $10\%$, $30\%$ and $40\%$ was observed after the $\gamma$ irradiation and after additional neutron
irradiation of $10^{14}$ and $1.1\cdot 10^{15}$~neutrons/cm$^2$, respectively, which is not far outside the
sensor production tolerances and does not affect the functionality of the sensors.

\section{Conclusions}
%
Transparent amorphous silicon position sensors (`ALMY sensors') have a wide range
of applications in optical alignment monitoring systems. The sensors fulfill the
particularly high demands on the position resolution, measurement range and radiation hardness
for applications in high energy physics experiments.
The performance of the sensors has been further optimized and verified in extensive tests. 
A new larger-scale sensor production is under preparation. 
The effect of degradation of the sensitivity
of the amorphous silicon under illumination and the radiation hardness of the sensors
including readout electronics under neutron irradiation have been studied in detail.
Deteriorations of the sensor performance due to these effects were found to be negligible
for the foreseen applications.

\begin{thebibliography}{99}

\bibitem{ALMY1}
W.~Blum, H.~Kroha, and P.~Widmann,
``A novel laser alignment system for tracking detectors
using transparent silicon strip sensors,''
{\sl Nucl.~Instr.~Meth.}, vol.~367, pp.~A413-A417, 1995.

\bibitem{ALMY2}
------,
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\end{thebibliography}

\end{document}
