Development of a bump bonding interconnect technology for GaAs pixel detectors

Pixel detectors made of semi-insulating GaAs have already been successfully tested for their application as tracking detectors in high energy physics experiments. For the operation of large arrays of pixel sensors, a technology allowing a two dimensional interconnect with a high yield is necessary. For this purpose an indium based bump bonding process has been developed. Especially the generation of the indium bumps on the surface of the GaAs pixel detector was optimized. The development of these processing steps is described. In order to investigate the influence of these steps and of the bonding procedure on the detector performance, I–V-characteristics of bump bonded GaAs pixel detectors were analyzed.     

Radiation and cryogenic test results with a monolithic GaAs preamplifier in C-HFET technology

We report test results with a monolithic GaAs preamplifier fabricated in industrial C-HFET technology irradiated with a total dose of 10¹⁴ neutrons/cm² and 100 Mrad γ radiation and operated under cryogenic conditions. The measured gate current of the input transistor of a few nA increases by < 10% after irradiation. For a 330 μm input FET width, the equivalent noise charge (ENC) is typically 145 electrons per pF total input capacitance at a shaping time of 25 ns (bipolar) before irradiation and changes approximately by 10% after irradiation. Under cryogenic operation the corresponding input referred noise decreases by roughly a factor of two.     

Investigation of a bump bonding interconnect technology for GaAs pixel detectors

Pixel detectors made of semi-insulating GaAs have already been successfully tested for their application as tracking detectors in high energy physics experiments. For the operation of large arrays of pixel sensors, a technology allowing a two-dimensional interconnect with a high yield is necessary. For this purpose a bump bonding process was developed. In this paper the, yield and the resistance of the interconnections is analyzed. With more than 2700 interconnections on special test structures the resistance of a single bump-bond is consistent with zero Ohm and the yield was determined to be 92%.     

Measurements on a GaAs microstrip detector with 50 GeV/c electrons

A microstrip gallium arsenide detector (thickness 508 μm, strip width 100 μm, strip pitch and readout pitch 200 μm) has been tested in a 50 GeV/c electron beam at CERN. Using the low noise Viking preamplifier chip (shaping time 1.5 μs) signal to noise ratios up to 25 were measured depending on bias voltage and angle of incidence. Applying the so-called η-algorithm (using the impact position-sensitive charge sharing between adjacent strips) a resolution down to σ 20 μm could be obtained.