A novel X-ray detector diode, optimized for angular independent (isotropic) dose response, is presented. The diode is designed as a silicon cube with p–n junctions on all six sides, which creates a device that is close to being symmetrical in 3D. The cube edge is 300 μm or 410 μm. Its manufacturing process is based on micromachining, featuring deep reactive ion etching (DRIE) of silicon-on-insulator (SOI) substrates, doping of vertical walls from gas phase dopants and re-fill of etched trenches with polysilicon. The variation in detector response to 6 MV X-rays, in a ±30° beam angle range, was at best ±0.5% for a cubic diode compared to ±3.3% for conventional diodes, which indicates improvement by a factor 7.

A cubic isotropic X-ray dose detector diode fabricated by DRIE of SOI substrates

An insight is given into the state-of-the-art 3D processing on 6 in. (150 mm) high resistivity silicon-on-insulator (SOI) wafers. The edgeless detector design offers some attractive properties for high-energy physics experiments and medical imaging studies, such as seamless tileability of the detectors with an inactive region width of about 10 μm. These detectors can be made very thin and the active edge avoids inhomogeneous electric fields and surface leakage currents. The paper summarizes the fabrication of edgeless detectors and the issues faced in the 3D processing on 6 in. SOI wafers. The fabrication process employed highly doped polysilicon filling in order to implement the active edges of the detector. Several planarization and ICP-etching steps were required. The layout had microstrip detectors with a pitch of 50 μm and sizes of 5×5 cm 2 and 1×1 cm 2 , and Medipix2 compatible 1.4×1.4 cm 2 pixel detectors. Also several test structures were fabricated. Electrical characterization of a 150-μm-thick edgeless diode showed low leakage currents, below 1 nA/cm 2 at full depletion. The single-strip measurements showed leakage currents of about 10 pA/cm, regardless of the detector size. Low breakdown voltage of about 20 V was observed for several detectors. This might be caused by cracking of the detector edges following self-dicing. A simplified process flow for the fabrication of edgeless detectors is presented. The process is straightforward and fast because it excludes multiple ion coupled plasma (ICP)-etching steps, slow and wafer-damaging polysilicon filling and planarization steps. First images of the prototype are presented.

3D processing on 6 in. high resistive SOI wafers: Fabrication of edgeless strip and pixel detectors

VTT has two decades experience on the manufacturing and design of the silicon radiation detectors. The activities cover a range of industrial devices as well as the devices for the physics experiments. The paper gives an overview on the new solid state detector development activity at VTT that comprise advanced X-ray photo diodes, through wafer interconnections for the photo diode arrays, thin silicon strip detectors, active-edge strip and pixel detectors with different biasing schemes, and full 3D 1 mm thick active edge silicon detectors.

Silicon radiation detector development at VTT

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Silicon radiation detector development at VTT