Performance of Si PAD sensors for a RICH detector

In this article a development is described to build a Hybrid Photon Detector using multicell Si pad detectors [I] with readout electronics originally developed for the readout of Si strip detectors. The principle of the readout is to route each pad with a thin metal line on top of a 5 w thick oxide to the periphery of the device, where a bond pad layout identical to that of a strip detector is implemented. This type of sensor is a straightforward solution when pad sizes are required which are either bigger or smaller than naturally covered by pixel detectors, where the one to one correspondence of the size of one electronics channel and the pixel dimensions of the sensor put practical limits to the size of a single cell. It is in general an attractive alternative to pixel detectors in cases where the number of cells are not too big. The application of pad sensors is clearly limited by the maximal number of pixels which can be read out in this way. A practical limit when using 4" sensor technology is about 6000 pads-with an area of each pad less than 1 mm'. Advantages of pad sensors are that there is no constraint on the readout electronics in terms of layout space and that a simple and robust connection technology with wire bonding exists. Disadvantages are increased capacitance from routing lines leading to higher noise and the limitations in number of cells for single devices.