Module Design

In the module design the MPI engagement mainly concentrated on the developments of a cost effective design for radiation tolerant silicon strip sensors, and a design for the endcap modules concentrating on the mechanical and thermal performance.

The MPI evaluated the the p-on-n sensor concept using n-type bulk silicon sensors with implanted p-type strips. Supported by the University of Dortmund and by the CERN ATLAS group a program including simulation studies, sensor design, prototyping, irradiation and data analysis was conducted. In this design, a multi guard-ring structure reduces the electric fields in the edge regions, such that after type inversion these sensors can be operated up to 500V. The p-on-n sensor technology developed at the HLL needs only five photolithographic steps reducing the costs of such sensors. In order to simplify the sensor technology further, the MPI design replaced the commonly used polysilicon bias resistors by implanted resistors formed by a medium dose boron implantation.

Driven by the results and price inquiries from the semiconductor industry, the SCT collaboration decided to change the sensor baseline from n-on-n to p-on-n early 1998, which resulted in substantial cost savings for SCT.

Motivated by findings of the RD48 (ROSE) collaboration suggesting that an increased oxygen concentration reduces the radiation induced acceptor defects, thereby resulting in a significantly lower operation voltage after irradiation the MPI investigated the influence of the oxygen diffusion. Indeed a significantly reduced operation voltage compared to standard sensors was measured. Oxygenated sensors are therefore used for part of the inner endcap modules, which are exposed to the highest radiation dose.

All types of sensors were designed at the HLL and the designs were transferred to the CiS company in Erfurt, Germany.

Related documents:

L. Andricek et al., Radiation hard strip detectors for large-scale silicon trackers NIMA436 (1999) 262-271.
L. Andricek et al., Design and test of radiation hard p⁺ n silicon strip detectors for the ATLAS SCT NIMA439 (2000) 427-441.
L. Andricek et al., Radiation-hard strip detectors on oxygenated silicon, IEEE Trans. Nucl. Sci. 49 (2002) 1117-1121.
L. Andricek et al., Single sided p ⁺ n and double sided silicon strip detectors exposed to fluences up to 2 10¹⁴/cm² 24 GeV protons, NIMA409 (1998) 184-193.
P.P. Allport et al., Comparison of the performance of irradiated p-in-n and n-in-n silicon microstrip detectors read out with fast binary electronics, NIMA450 (2000) 297-306.
3^rd RD48 Status Report, CERN/LHCC 2000-009.

Starting in 1997 the MPI SCT group took a leading role in the development of the module design concentrating on mechanical and thermal issues. A common research project of the MPI, the Universität Freiburg, QMW London, IHEP Protvino, NIITAP (Zelenograd), and NIIGraphite (Moscow), supported by INTAS (Ref 99-249) was conducted.

As a result of this research, the original design of the mechanical support structure (spine) based on a BeO substrate proved to have insufficient thermal properties, and was replaced by thermal pyrolytic graphite (TPG) with a 5 times better thermal conductivity. Using finite element simulations the thermal behavior of the spine was optimized to ensure stable operation and low silicon temperature (-10 degrees Celsius). Also mechanical properties like the bowing of modules have been investigated.

The series production of all spines for SCT endcap modules has been carried out as an ISTC project by the Russian Institutes.