Simo Eränen

Recent advances in processing and characterization of edgeless detectors

During past five years VTT has actively developed edgeless detector fabrication process. The straightforward and high yield process relies on ion-implantation to activate the edges of the detector. A recent fabrication process was performed at VTT to provide p-on-n edgeless detectors. The layout contained DC- and AC-coupled strip detector and pixel detectors for Medipix/Timepix readouts. The fabricated detector thicknesses were 50, 100 and 150 μm. Electrical characterization was done for 5 × 5 mm2 edgeless diodes on wafer level. All measured electrical parameters showed a dramatic dependence on the diode thickness. Leakage current was measured below 10 nA/cm2 at full depletion. Calculation using a theoretical approximation indicates the diode surface generation current of less than 300 pA. The breakdown voltages were measured to be above 140 V and increased as a function of diode thickness. Reverse bias of 10 V is enough to fully deplete designed edgeless diodes. Leakage current dependence of temperature was investigated for both p-on-n and previous n-on-n edgeless detectors and results show that the leakage current doubles for every 8.5 degree Celsius rise in temperature. TCAD device simulations reveal that breakdown occurs at the lateral p-n junction where the electric field reaches its highest value. Thick edgeless diodes have wider bulk space that allows electric potential to drop and causes smaller curvature of the equipotential lines. This releases the accumulation of electric field at the corner of anode and increases the breakdown voltage. A good match of the simulated and the measured capacitance-voltage curves enables identification of proper parameters used in the simulation.

Advanced Photodiode Detector For Medical CT Imaging: Design and Performance

This paper presents an advanced photodiode detector design for medical imaging applications, especially computerized tomography. The detector is silicon based and integrates the through-wafer interconnection technology into conventional front illuminated photodiode. The number of photodiode elements in the detector can be extended in 2D without reducing the photodiode active area. The signal of each photodiode element can be directly read out from the backside of the detector. Moreover, detectors can be tiled together in 2D without any physical limitation. A test photodiode detector was designed and demonstrated with 3times3 photodiode matrix arrangement in this paper. Different parameters were measured and analyzed from the demonstrated chip and test structures. The results show that the detector inherits most of the performance advantages from the conventional front illuminated photodiode, and all the parameters can either meet or exceed the requirements of modern CT systems.

Study of edgeless radiation detector with 3D spatial mapping technique

Edgeless radiation detector has gained increased attention due to its superiority in the defect-free edge fabrication and the capability to minimize the insensitive area at the detector edge. The doped edge in the edgeless detector is at the same potential with the back plane and causes a local distortion of the electric field at the detector edge. The deformed electric field alters the charge collection of the edge pixel and leads to an inaccurate charge interpolation. To study the influence of active edges on the response of edge pixels, we used an advanced X-ray based 3D spatial mapping technique to visually show the charge collection volumes of pixels. Various edgeless detectors with diverse polarities, thicknesses and edge-to-pixel distances were investigated. For the n-on-p (n+/p−/p+) edgeless detector, the mapping shows that the p-spray isolation method has the advantage of achieving a greater sensitive edge region compared to the p-stop method. And the p-on-p (p+/p−/n+) edgeless detector, reported for the first time, functions for both spatial and energy signals. The n-type edgeless detectors were studied together with a standard Medipix detector with the guard ring design. The results show that the edgeless detector is capable of maximally utilizing the edge region of the detector as the charge sensitive volume, while the standard Medipix detector has still vast insensitive region at the edge. The X-ray spectroscopic measurements with 241Am and 55Fe sources performed on all detectors gives a similar conclusion and proves the 3D spatial mapping results.

Novel ion detector for fusion plasma diagnostics

A novel thin silicon detector for the neutral particle analyzers (NPA) of the joint European Torus (JET) is introduced for studying plasma characteristics during the fusion experiments. The new ion detector would replace the presently used very thin scintillator - photomultiplier tube combination. The proposed new NPA detector is based on direct conversion of charge in silicon and approximate matching of the detector thickness with the ranges of the observed ions. A thin silicon detector is only weakly sensitive to photon and neutron backgrounds but detects highly ionizing ions efficiently. Even high energy gammas deposit only little energy in the thin detector allowing effective background discrimination through pulse- height-analysis. Thin silicon strip detectors have been fabricated by using the silicon-on-insulator (SOI) technology. Fabricated detectors have 6 or 26 mum thick high resistive silicon bonded on a conductive silicon support. The thinner detectors are designed to be used for the low energy NPA and the thicker ones for the high energy NPA. The presentation comprises a short introduction to the fabricated detector structures, TCAD simulations and results of the electrical and spectroscopic characterizations.

Radiation detectors fabricated on high-purity GaAs epitaxial materials

Epitaxial GaAs material shows a great potential in X-ray spectroscopy and radiography applications due to its high absorption efficiency and low defect density. Fabrication of pixel radiation detectors from high-purtity epitaxial GaAs has been developed further. The process is based on mesa etching for pixellisation and sputtering for metallization. The leakage currents of processed pad detectors are below 10 nA/cm2 at a reverse bias of 100 V and decrease exponentially with the temperature. Measurement with transient current technique (TCT) shows that electrons have a trapping time of 8 ns. Good spectroscopic result were obtained from both a pad detector and a hybridized Medipix GaAs detector.

Silicon radiation detector development at VTT

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.

Via-in-Pixel Design of Truly 2D Extendable Photodiode Detector for Medical CT Imaging

This paper presents an advanced design of the photodiode detector for medical imaging, especially for the X-ray computed tomography applications. In this paper, through wafer interconnection technology with via-in-pixel design is utilized to achieve the truly 2D extendable feature of the photodiode detector. The active area of each photodiode element on the detector can achieve the maximum active area with the fixed pitch size. Comparing to the via-off-pixel design, different parameters of the photodiode with via-in-pixel design were measured and analyzed from the demonstrated samples. The results show that the performances of the samples either meet or exceed the requirements of the modern X-ray computed tomography applications.

Simulations of 3D silicon radiation detector structures in 2D and 3D

Three dimensional (3D) detector structures yield faster charge collection times compared to the ones realized by using the planar (2D) technology. The charge collection time is greatly dependent on the location of the incident ionising radiation. In this paper, the charge collection performance of a rectangular 3D detector structure is simulated by using the finite-element simulation software ISE-TCAD. The surface effects, surface charge and surface recombination, are shown to slow down the charge collection by 50% to 70%. The charge sharing between the neighboring pixel is studied by using 2D simulations at 20 V bias and the charge collected by the pixel of the MIP entrance is shown to be more than 95% in a major part of the pixel volume. In addition, it was shown that great care should exercised when analyzing 3D detector structures using 2D simulation tools. When the simulated structure is thinned down, the internal detector characteristics are altered and, since the RC-constant is proportional to square of the detector thickness, the charge collection time may decrease unexpectedly

Investigation of voltages and electric fields in silicon semi 3D radiation detectors using Silvaco/ATLAS simulation tool and a scanning electron microscope

The structure of silicon semi three-dimensional radiation detector is simulated on purpose to find out its electrical characteristics such as the depletion voltage and electric field. Two-dimensional simulation results are compared to voltage and electric field measurements done by a scanning electron microscope.