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

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