W. Treberspurg

Measuring doping profiles of silicon detectors with a custom-designed probe station

Silicon detectors are often used in High Energy Physics (HEP) experiments as tracking and vertexing devices. Many scientific institutes are equipped with setups able to electrically characterize those detectors e.g. for quality assurance reasons. Such probe stations can be easily extended to measure resistivities and doping profiles in the bulk material and in doped regions by using the Spreading Resistance Probe (SRP) technique. After an introduction to the method, this paper describes how an existing probe station, that has been used for electrical measurements on strip detectors, has been modified to perform SRP measurements. The presented results prove that the method is reliable and capable of characterizing doping regions as thin as one micron. Beside profiling implants, SRP measurements have the potential to deliver the basis for investigations of bulk material defects in heavily irradiated samples.

Optimizing the quality of silicon strip sensors produced by Infineon Technologies Austria AG

The tracking systems of most modern particle physics experiments are realized by silicon based sensors. The size of such systems has continuously increased and nowadays a sensitive area of several 100 m2 has to be covered. This large amount of sensors might exceed the production capabilities of existing companies and institutes. Therefore the Institute of High Energy Physics of the Austrian Academy of Sciences (HEPHY) and the European semiconductor manufacturer Infineon Technologies Austria AG developed together a production process for p-on-n strip sensors. Although the first prototype run has shown a promising quality, it has been observed that weak strips exist, which are mainly located at distinctive areas on each wafer. At these areas the affected parameters are correlated to each other. A similar behaviour could be reproduced with a smaller second batch, whose sensors have been used for further analysis and advanced measurements. This paper sums up the characteristic behaviour of the specific effect and presents different possibilities how to cure the sensors. The systematic accumulation of weak strips can be traced back to a specific operation during the fabrication process. All data strongly indicate that the effect is caused by local charging effects on an isolating layer.

Backside doping profiles of irradiated silicon detectors

Silicon detectors are used in High Energy Physics (HEP) experiments as tracking and vertexing devices. The damage caused by radiation is of special interest for sensors to be used at the HL-LHC. The doping profiles of highly irradiated sensors can neither be measured with common capacitance voltage methods nor with methods of chemical analysis. Nevertheless, they need to be known for damage modelling or for simulations of the sensor performance. In this paper it is shown that highly neutron irradiated doping profiles can be measured by using a spreading resistance probe technique. It turned out that the implantation depth of the profiles of active dopants decreases with increasing fluences.

Comparison of n-side strip isolation methods for silicon sensors

Precision experiments at electron-positron-colliders and b-factories demand high position resolution and low material budget for precise particle tracking. These requirements are fulfilled by thin double-sided silicon detectors (DSSDs). However, due to the low signals of thin sensors a careful sensor design is required in order to achieve high charge collection efficiency. In this paper we investigate the p-stop and the p-spray blocking method for strip isolation on the n-side of DSSDs with n-type bulk. We compare three different p-stop patterns: the common p-stop, the atoll p-stop and a combined p-stop pattern, whereas for every pattern four different geometric layouts are considered. Test sensors featuring these p-stop patterns and the p-spray blocking method were tested in a 120 GeV/c hadron beam at the Super Proton Synchrotron (SPS) at CERN (Geneva, Switzerland), where one variant of the atoll p-stop pattern performed best. The results of these tests are used to design the DSSDs for the Belle II experiment at KEK (Tsukuba, Japan).

Results of an electron beam test with prototype silicon sensors manufactured by Infineon Technologies Austria AG

The demand on silicon based sensors continuously increased since they have been used the first time in particle physics for tracking purposes. In accordance with this development the Institute of High Energy Physics of the Austrian Academy of Sciences (HEPHY) and the European semiconductor manufacturer Infineon Technologies Austria AG engaged in a cooperation to develop prototype p-on-n silicon strip sensors. The sensors of two independent batches with slightly varying production processes are evaluated. To investigate their performance, modules have been assembled with an analogue readout chip (APV25) and operated in an electron beam test. An already well-studied problem of poorly isolated strips, restricted to a small region of the sensor could be further investigated at one sensor and has proven to be cured at the others. Therefore charge sharing effects and their dependency on the bias voltage have been investigated on different regions of the sensors. Furthermore the recorded data of the modules, including one gamma irradiated, document the functionality of the devices.

The spatial potential and electric field distribution of irradiated silicon diodes

The hadron radiation background of modern experiments in high energy physics causes crucial changes of the bulk material properties of p+- n - n+ silicon structures. In particular the n-type bulk material inverts to a p-type one and the effective doping concentration becomes non-uniformly distributed. This effect results in the build up of a double junction inside silicon sensors. Its impact on the signal generation was earlier analysed by several measurement methods. This paper presents the first results of a custom developed surface potential measurement method, which is applied on neutron irradiated samples. For this purpose the spatial distribution of the electric field inside the irradiated diodes is investigated for different bias voltages. The edges of depleted diodes are therefore contacted at different positions and the local potential is electrically measured. This requires a complex sample preparation and measurement procedure. The results confirm the generation of a double junction as a consequence of the type inversion process.

Comparing Spreading Resistance Profiling and C-V characterisation to identify defects in silicon sensors

The quality and functionality of a silicon sensor strongly depends on the effective doping concentration of the active silicon bulk. The creation of additional defects, by certain steps in the production or through irradiation in a particle beam, can heavily influence its performance. Several methods exist to characterise the bulk material for a silicon sensor. C-V characterisation is a widely implemented, non-destructive method to extract the depth profile of Neff. A technique which is rarely used at laboratories developing silicon sensors is Spreading Resistance Profiling (SRP) which directly measures the resistivity of the silicon. We will show, that a comparison of measurements from these two methods can yield important information on the defect concentration in the bulk of the silicon. To demonstrate the technique, we investigated a sensor material where the active region was reduced using a deep diffusion process which is assumed to create additional defects in the bulk.