Helsingin yliopisto


Helsingin yliopiston verkkojulkaisut

University of Helsinki, Helsinki 2006

Designing measuring system for silicon diode IV-curve characterization in low temperatures

Summary of "Mittauslaitteiston suunnittelu piidiodien vuotovirtamittauksiin matalissa lämpötiloissa".

Petri Sane

Master's thesis, March 2006.
University of Helsinki, Faculty of Science, Department of Physical Sciences and Helsinki Institute of Physics, Accelerator Laboratory.

This study concentrated on radiation induced defects in silicon and on silicon detector radiation hardness in general. The aim was to design and test measurement system for IV-curve characterization in low temperatures, samples were planar 8x8 mm silicon diodes, processed with either Magnetic Czochralski-method (MCz) or Float Zone-method(FZ).

The setup being used in future proton irradiations, materials inside the irradiation chamber were chosen in order to minimize the activation levels during the irradiations.

System is composed of two major components, sample pad and plastic mask with contact probes.

Sample pad (Figure 15=Kuva 15 in the thesis) was designed to carry signal by bonding wires separately from diodes backside, guard ring and front side to the pad. The sample pad was epoxy substrate PCB-pad that had golden contact areas for bonds to attach. The backside of the diode was glued to the pad with conductive silver paste and carbon cement for grip in low temperatures.

Sample pad (Figure 16&17) was attached with screws to aluminum plane connected to cryochambers cold finger. For better thermal conductance, electrically non-conductive silicon paste was added between the cold finger and aluminum plane.

A plastic mask acting as support for the probe needles and electrical insulator was placed on top of the PCB-pad. The mask had a gap on its centre for beam to pass. On the mask, four holes were drilled on top of the pad's contact areas.

The probe needles that would be pushed trough the holes to the PCB, were made of gilded steel. Needles' contacts were secured with springs that would push the needles on the PCB. From the needles, the signal was transmitted via quick disconnect connector-junction and then to high voltage cables.

Signal-to-Noise-ratio in system was taken into account by gilding all the steel contacts and shielding cables sufficiently.

More detailed description of the technical setup can be found in the thesis in Finnish.

During the testing, four diodes were measured in low temperatures. Two of them were non-irradiated MCz-diodes with very low (1E-7A) leakage current in RT and another two were irradiated with 50 MeV protons with a fluence of 10E14/cm2 1MeV proton corresponding fluence. The aim for the measurements was to test temperature correlation of leakage current and to determine the current measurement limit of the setup. IV-characterizations were measured in the range of 70 K to RT. Depending on the leakage current in RT, leakage current corresponded to the temperature change as predicted. Measuring limit was detected in around 1E-10A region. For irradiated diodes, cooling down from 290 K to 190 K lowered the leakage current by 6 decades.

Title page of the publication

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© University of Helsinki 2006

Last updated 31.03.2006

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