Thin detectors for the CHICSi ΔE-E telescope

A pilot series of 10 μm to 15 μm thin silicon detectors has been made for the ΔE-E telescopes in the CHICSi detector system. This system will operate at the CELSIUS heavy ion storage ring in Uppsala, Sweden. ΔE-E telescopes provide isotope identification and energy determination of fragments from nuclear collisions. The thin detectors are made as p-i-n diodes in thin etched membranes in 280 μm thick silicon wafers. The membranes are made with anisotropic etching using 25 wt.% tetramethylammonium hydroxide (TMAH) solution. The etch speed of this solution is very uniform across a wafer. As a result detectors with uniform thickness can be produced. The etch depth varies with less than ±0.3 μm over a wafer and the surface microroughness is in the range from 2 to 4 nm. Each detector has a 10.0 mm×10.0 mm active area on a 10.2 mm×10.2 mm membrane surrounded by a 1.1 mm wide supporting frame. The detectors have leakage currents in the active area of approximately 0.5 nA at 20 V. The breakdown voltage of the detectors is above 100 V. Evaluation experiments with telescopes consisting of a thin detector in combination with a thick detector have shown excellent isotope separation capabilities. Mass separation of ⁶Li and ⁷Li is clearly observable