Semiconductor micropattern pixel detectors: a review of the beginnings

The innovation in monolithic and hybrid semiconductor 'micropattern' or 'reactive' pixel detectors for tracking in particle physics was actually to fit logic and pulse processing electronics with µW power on a pixel area of less than 0.04 mm2, retaining the characteristics of a traditional nuclear amplifier chain. The ns timing precision in conjunction with local memory and logic operations allowed event selection at > 10 MHz rates with unambiguous track reconstruction even at particle multiplicities > 10 cm-2. The noise in a channel was ~100 e- r.m.s. and enabled binary operation with random noise 'hits' at a level < 10-8. Rectangular pixels from 75 µm ´ 500 µm down to 34 µm ´ 125 µm have been used by different teams. In binary mode a tracking precision from 6-14 µm was obtained, and using analog interpolation one came close to 1 µm. Earlier work, still based on charge integrating imaging circuits, provided a starting point. Two systems each with more than 1 million sensor + readout channnels have been built, for WA97-NA57 and for the DELPHI very forward tracker. The use of 0.5 µm and 0.25 µm CMOS and enclosed geometry for the transistors in the pixel readout chips resulted in a radiation hardness of ~ 2 Mrad and >30 Mrad, respectively.