Beam test measurements on GaAs pixel detectors at various angles of incidence

Abstract A GaAs pixel detector constructed in Aachen has been tested in a 4 GeV electron beam at DESY. The experimental setup allowed tilting the detector with respect to the beam line with angles of incidence from 0° to 45°. The sensor-array consisted of 8 × 16 pixels with a size of 125 × 125 μm 2 each. The detector was made of a 250μm thick Freiberger SI-GaAs wafer. An improved contact was formed on the backside, allowing safe operation of the detector in the soft breakdown regime. A double metal technique allowed bonding the single pixels linearly to the readout-chip. Using the the fast PreMux128 preamplifier multiplexer chip ( τ p = 40 ns ) a signal to noise ratio of 29 was obtained for a beam angle of incidence of 0° increasing up to 38 for 45°. The spatial resolution obtained with an angle of incidence of 45° was (9.0 ± 6.0) μm while the resolution of the untilted detector is equal to the digital one (36.1μm). For these testbeam-measurements the detector was connected to the electronics via wire-bonds. For future experiments bump-bonding connections are required. The results of a process for the formation of bump-bond connections on GaAs pixeldetectors are shown.