Patterning thick diffused junctions on CdTe

Dividing the detector crystal into discrete pixels enables making an imaging detector, in which the charge collected by each pixel can be read separately. Even if the detector is not meant for imaging, patterns on the crystal surface may be used as guard structures that control and limit the flow of charges in the crystal. This has been exceedingly hard for the detector crystals having thick diffused layers. The paper reports a patterning method of the thick diffused junctions on CdTe. The patterning method of In-diffused pn-junction on CdTe chip is demonstrated by using a diamond blade. The patterning is done by removing material from the pn-junction side of the chip, so that the trenches penetrate the diffused layer. As the trenches extend deeper into the bulk than the junction, the regions separated by the trench are electrically isolated. Electrical characterization results are reported for the strips separated by trenches with various depths. The strip isolation is clearly seen in both measured leakage currents and inter-strip resistances. The paper also represents the first images of the pixelated CdTe chips bump bonded to the Medipix2 readout chip. The images were taken with the Sr90 source. The chips had 110 µm x 110 µm and 55 µm x 55 µm grid spacing. The images demonstrate the imaging capability of the pixelated chips and give an idea on the uniformity and quality of the pixelization method.