The MPP’s primary objective while developing the modules of the silicon strip detector was to design low-cost, radiation-hard sensors. Its researchers also developed the mechanical design of the end-cap modules and optimized its mechanical and thermal properties.
The MPP pursued the p-in-n concept of using p-type implants in an n-type silicon substrate to develop the sensors. With the support of the University of Dortmund and the ATLAS group at CERN it carried out a development program comprising simulation studies, sensor design, prototyping, sensor irradiation and data analysis.
The program also included developing a multi-guard-ring structure for the sensors. This reduces the electric fields in the edge regions of the sensors, allowing them to be operated safely at depletion voltages of up to 500 volts even after type inversion.
The p-in-n sensor technology developed requires only five processing steps and thus reduces the costs of the sensors. In order to reduce the costs even further, the MPP replaced the commonly used and expensive polysilicon bias resistors by implanted resistors. These are simply implanted during the processing steps by means of a medium dose of boron.
Motivated by the research findings of the CERN RD48 (ROSE) collaboration, the MPP used a process whereby oxygen is diffused into some of the sensors as they are being manufactured in order to increase its concentration. This significantly reduces the radiation-induced silicon defects so that the sensors can still be operated at a lower operating voltage even after being subject to high levels of irradiation. The MPP tested and confirmed the concept on prototypes. These oxygenated sensors are used for the innermost end-cap modules.
All sensors were developed in the MPG semiconductor laboratory and manufactured at CiS, a semiconductor plant in Erfurt, Germany.
The MPP has coordinated the production of about 3,000 silicon sensors for the end-caps of the silicon strip detector including the pilot production. These sensors were delivered to the MPP, where they were tested before being handed over to the institutes involved for assembly.
One of the main tasks at the MPP was to assemble 424 modules for the inner rings of the end-caps. The picture on the right shows a wire bonding tool in action. Wire bonding is a method for interconnecting the sensors with each other and with the read-out electronics by means of thin, ultrasonically welded wires.
End-Cap Construction, Integration, and Tests
The MPP played its part in integrating the silicon strip detector into the ATLAS detector at CERN. Its primary concern was the thermal shielding of the detector. The silicon strip detector and the downstream transition radiation tracker each comprise three components: The cylindrical part around the interaction zone of the proton beams and the two end-caps. The three components of the silicon strip detector were introduced separately into the corresponding components of the transition radiation tracker. A functional test was then conducted for the whole system using particles of cosmic radiation. The MPP group was involved in setting up and integrating the trigger for the data acquisition.