Ultra-thin active pixel sensor for high precision particle detection
The MPG Semiconductor Laboratory recently finalized the very first DEPFET module for the Vertex Detector of the Belle II experiment at the Japanese accelerator center KEK. The Belle II detector is expected to start operation in 2017, recording collisions between electrons and their antiparticles, positrons.
Made from silicon a thousand times more pure than conventional transistors or memory chips, the module integrates 200,000 DEPFET pixel cells on a surface area of eight square centimeters. (DEPFET stands for “depleted p-channel field-effect transistor.”) It was invented at the MPG HLL and is fabricated here exclusively.
The DEPFET component enables the detection of photons – or, as in this case, of high energy particles – with the utmost efficiency and precision. The fundamental process is very similar to what goes on in a conventional photo or video camera. However, the primary signal from individual photons or particles is very much smaller.
This is where the major advantage of the DEPFET comes into play: The tiny primary signal is amplified within the sensor itself. Thus the DEPFET is the sensor material and the first stage of amplification rolled into one. Arranging many DEPFETs in a matrix forms an image sensor with which a particle’s point of origin can be precisely determined. In the present configuration for the Belle II experiment this can be done with an accuracy of around one-hundredth of a millimeter.
Control of the pixels in a matrix and ultra-fast processing of the DEPFET signals require additional electronics, which have been developed and produced in collaboration with German universities. These electronics, in the form of application-specific integrated circuits (ASICs), are placed directly on the sensor substrate. The ASICs allow digitization of signals from the pixel matrix, as well as lossless data compression, to transmit them off-module at the highest speed (50,000 frames per second).
With this, the DEPFET matrix becomes a very complex module with maximal integration density, which despite all its complexity is extremely thin and light, so that the measurement accuracy of particle tracks is not distorted by the sensor material itself.
The HLL has developed a unique technology for this purpose that makes it possible to fabricate extremely thin and highly integrated sensor modules. In the process, the sensitive part of the module, the DEPFET matrix, is thinned by a customized etching technique to 75 micrometers, roughly the thickness of a human hair.
These bendable silicon films are supported by a monolithically integrated silicon frame, on which the readout and control electronics are mounted. The power supply and data lines run through a flexible ribbon cable, which is attached to the end of the module.
The HLL technology makes it possible to arrange the thin DEPFET matrices in a cylindrical form, without any further support, around the interaction point of the experiment. With that, the highly precise measurement of particle tracks is becoming reality.