CBM Progress Report 2006 - GSI

CBM Progress Report 2006 Detector Developments
Silicon Microstrip Sensor Prototypes for CBM
M. Merkin, D.Karmanov, N.Baranova
Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University
N.Egorov, S.Golubkov, A.Sidorov
Research Institute of Material Science and Technology, Zelenograd, Moscow
We have started to explore the design and the production
of double-sided microstrip sensors for CBM's Silicon
Tracking System, with focus on thin detectors and
radiation tolerant design features.
Double-sided strip sensors: Thin, radiation hard
Silicon sensors are essentially not commercial products.
Custom geometries and a variety of special requirements
have to be taken into account for a given application.
For the CBM experiment, the challenge is to establish
reliable technologies for the processing of both sides of
thin silicon wafers. The goal is to manufacture low-mass
double-sided sensors with long, fine-pitch charge
collecting strips, laid out in a radiation tolerant design.
There is very limited experience in the world on the
production of double-sided radiation tolerant sensors.
R&D steps
For this project, High Reflectivity Wafers (HiREF®)
from Wacker-Siltronic have been used. The planned R&D
steps are:
(1) Sensor design and mask production:
a. Single-sided test detectors with 50 µm strip pitch
and DC or AC readout.
b. Double-sided sensors with 50 µm strip pitch and
15º stereo angle between front-and back sides.
Strips poly-silicon biased, AC-coupled readout.
(2) Production of 250 µm sensors.
(3) Optimization of the sensor design.
(4) Technology optimization of the production process.
(5) Production of 200 µm sensors.
(6) Detailed tests of the produced sensors.
First results from single-sided sensors
Two batches of test wafers have been processed in steps
(1a) and (2) to qualify the 250 µm wafer material for the
forthcoming production: One batch of sensors with 2 cm
long single-sided DC strips of 50 µm pitch on 2 × 4 cm 2
area, and one batch with single-sided AC-coupled strips
of the same design. Measurements with the DC strip
Leakage current, nA
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Sensor #4041
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Bias voltage, V
Figure 1: Current-voltage characteristics of a single-sided
microstrip sensor on a HiREF® silicon wafer.
sensors are shown in Fig.1. The IV-characteristics
indicates good wafer material, with full depletion at around
100 V and high-voltage breakdown beyond 500 V. The
leakage current for the given resistivity of 3 kOhm and the
given thickness of 250 µm is reasonable: about 5 nA/strip.
We can therefore proceed with the R&D steps as planned.
Design and production of double-sided sensor
The study of double-sided strip sensors matching the CBM
specifications is in the center of our project. The design of
a prototype sensor was recently completed (step 1b).
Figures 2 and 3 show the corner regions of the N-side, and
P-side. The stereo angle of the P-strips with respect to the
N-strips is clearly seen, as well as the small DC probe pads
and the double-row of larger AC readout pads. The design
takes into account several features to enhance the radiation
tolerance of the detector: Poly-silicon bias resistors on both
the P and the N sides, P-stop implants between the strips
on the N-side, and a multi-guard ring structure. The
production masks have been fabricated. The complete set
consists of 17 masks: 7 for the P-side, 9 for the N-side, and
one additional mask for fiducial marks.
After full characterization of the test sensors, the
production of double-sided microstrip sensor prototypes
will be started with 250 µm wafers or thinner.
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Figure 2: N-side design.
Figure 3: P-side design.