CBM Progress Report 2006 - GSI

Detector Developments CBM Progress Report 2006
Development of Microstrip Sensors for the CBM Silicon Tracking System
J. M. Heuser and Chr. J. Schmidt, GSI, Darmstadt, Germany
We have designed the first prototype of a silicon
micro-strip sensor that can serve as a building block of
detector modules for CBM’s Silicon Tracking System.
Tracking Stations
The tracking stations are planar arrangements of
micro-strip sensors, grouped into several modules of either
the same or different length, that cover at given positions
downstream of the target the fiducial area approximately
between 2.5 and 25 degrees polar angle. Figure 1 visualizes
this station concept with a vertical orientation of the
modules and the long direction of the strips perpendicular
to the bending plane of the magnet for best momentum
measurement. The first and smallest station may be
as close as 30 cm to the target. The last and largest station
will be in about one meter distance from the target.
One of the key requirements of the Silicon Tracking System
is a low-mass design to achieve momentum measurement
with about 1% resolution. Silicon micro-strip detectors
are compatible with a low-mass design as the sensors
themselves are thin. With an appropriate module structure,
active readout electronics with its cooling requirements
and material involved may be avoided in the aperture.
The strongly inhomogeneous track density profile
makes a sectorized module structure necessary. Close to
the beam line, a module must comprise sensors with short
strips. Longer strips, realized either on single wafers or
by chaining two or more sensors, can be employed in the
regions further outside. Those different “sectors” must be
read out individually. This may be realized by routing the
strips’ analog signals from every sector through thin flat
multi-line cables to front-end electronics at the periphery
of the stations.
Figure 1: Silicon micro-strip sensors arranged into modules
of different lengths building up a tracking station.
30
Sensor Design
We focused on a sensor compatible with this detector
module structure. The design, shown in Fig. 2, addresses in
particular connectivity issues. We intend to employ doublesided
sensors with one strip orientation along the module’s
long axis, and a “stereo” direction on the second side. We
strived to accomplish that both sensor sides can be contacted
both at the top and the bottom edge. On the stereo
side, this requires interconnections on a second metal layer
between the strips of the two corner regions. This layout
will enable us to chain several sensors, thus forming long
strip sectors for the outer regions of the tracking stations.
A staggered arrangement of the contact pads was applied
allowing a wire-bonding scheme that conserves the correlation
of neighbouring channels through the chain up to the
readout electronics. Small stereo angles are preferred as
unavoidable dead area on the sensor sides is kept small.
Figure 2: (Left) Schematics of the sensor’s stereo side.
(Right) Chained sensors and their interconnections.
Detector R&D
The first sensors, to be produced in 2007 [1], will have
a thickness of 200–300 µm, are double-sided with 2×1024
AC-coupled strips of 50 µm pitch and feature a 15 ◦ stereo
angle between front and back side. They will be utilized
for the construction of a prototype detector module: an
assembly of chained sensors, readout electronics [2] and
mechanical support. Forthcoming prototypes will address
radiation hardness, the minimization of inactive area near
the edges, and will touch e. g. the biasing technique and
the layout of the guard rings structure. The development
of readout cables, made from fine-pitch aluminum traces
on polyimide material for minimum material budget, is a
particular important task. The capacitance of sensor and
cable at the input of the front-end electronics must be small
enough to achieve a sufficiently large signal-to-noise ratio.
References
[1] CiS Institut für Mikrosensorik gGmbH, Erfurt, Germany
[2] Chr. J. Schmidt et al., this report