Eschke, J.

Ghosh, P.

Kolb, B.

Malygina, H.

Niebur, W.

Zumbruch, P.

GSI Repository

Calibration of a laser scanning system for quality assurance of CBM prototypesilicon microstrip sensors ∗P. Ghosh1,2, J. Eschke2,3, W. Niebur2,3, H. Malygina1,2, P. Zumbruch2, and B. Kolb21Goethe Universita¨t, Frankfurt am Main, Germany; 2GSI, Darmstadt, Germany; 3FAIR, Darmstadt, GermanyFor the characterization and Quality Assurance (QA) ofprototype sensors produced for the Silicon Tracking Sys-tem (STS) at the Compressed Baryonic Matter Experiment(CBM), a infrared pulsed Laser Testing System (LTS) hasbeen developed. The main aim for the LTS is to scan andcharacterize the prototype sensors. These QA scans areintended to understand charge sharing in the interstrip re-gion and investigate uniformity of sensor performance inthe active area of prototype sensors. The prototype sensorCBM02 which has 256 strips with a pitch of 50 μm on eachside has been investigated in the LTS [1]. The strips on thesensors are wire bonded to connectors on a board and read-out via self-triggering n-XYTER prototype electronics.The goal for the LTS is to have automatized quality as-surance tests in a controlled manner at several thousandspositions across the sensor with focused infra-red laserlight (σspotsize ≈ 15 μm). The duration (∼10 ns) andpower (few mW) of the laser pulses are selected such thatthe absorption of the laser light in the 300 μm thick sili-con sensors produces about 24 000 electrons,which is sim-ilar to the charge created by minimum ionizing particles(MIP) there.The wavelength of the laser was chosen to be1060 nm because the absorption depth of infra-red lightwith this wavelength is of the order of the thickness of thesilicon sensors [2].Figure 1: Schematic representation of the laser setup.A schematic view of the measurement setup is shownin Figure 1. The laser light is transmitted through a 6 μmthick optical fiber to a two-lens focusing system, which fo-cuses the light to a spot of about 15 μm diameter and theworking distance is about 10 mm. Figure 2 shows a depen-dence of the distanceto the sensor surface as a function ofthe number of strips fired with a signal just above thresh-old. With this measurement the proper focus distance hasbeen achieved. Figure 3 shows the charge sharing func-tion between neighboring strips represented in the form offraction of amplitudes collected by the individual strip.∗Work supported by HIC4FAIR, HGS-HIRe and H-QMFigure 2: Dependence of distance to focuser from the sen-sor surface as a function of strips fired.Figure 3: Charge division (fraction of amplitude) in theinterstrip region as a function of position of laser spot.The LTS has recently been upgraded with motor controlsusing EPICS [3] and programs have been written to stepover the active area make several measurements automati-cally. Figure 4 shows an operator interface of the runningsystem using EPICS tools. The next step is to integrate thedata acquisition software DABC [4] plugin for the EPICSposition information for data taking, logging and furtheranalysis using the Go4 analysis software.Figure 4: Run time operator interface for the LTS.References[1] J.M. Heuser et al., CBM Progress Report 2011[2] P.O’Connor et al., Proc. of SPIE Vol. 6276 62761W-1, p.2[3] EPICS: http://www.aps.anl.gov/epics[4] DABC: http://dabc.gsi.deGSI SCIENTIFIC REPORT 2013 NQM-CBM-14doi:10.15120/GR-2014-1-NQM-CBM-14 29

Calibration of a laser scanning system for quality assurance of CBM prototype silicon microstrip sensors

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Calibration of a laser scanning system for quality assurance of CBM prototype silicon microstrip sensors

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