Juha Kalliopuska

Planar edgeless silicon detectors for the TOTEM experiment

Silicon detectors for the Roman Pots of the the large hadron collider TOTEM experiment aim for full sensitivity at the edge where a terminating structure is required for electrical stability. This work provides an innovative approach reducing the conventional width of the terminating structure to less than 100 /spl mu/m, still using standard planar fabrication technology. The objective of this new development is to decouple the electric behavior of the surface from the sensitive volume within a few tens of micrometers. The explanation of the basic principle of this new approach together with the experimental confirmation via electric measurements and beam test are presented in this paper, demonstrating that silicon detectors with this new terminating structure are fully operational and efficient to under 60 /spl mu/m from the die cut.

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.

Test beam results of 3D silicon pixel sensors for the ATLAS upgrade

Results on beam tests of 3D silicon pixel sensors aimed at the ATLAS Insertable B-Layer and High Luminosity LHC (HL-LHC) upgrades are presented. Measurements include charge collection, tracking efficiency and charge sharing between pixel cells, as a function of track incident angle, and were performed with and without a 1.6 T magnetic field oriented as the ATLAS inner detector solenoid field. Sensors were bump-bonded to the front-end chip currently used in the ATLAS pixel detector. Full 3D sensors, with electrodes penetrating through the entire wafer thickness and active edge, and double-sided 3D sensors with partially overlapping bias and read-out electrodes were tested and showed comparable performance.

Edge pixel response studies of edgeless silicon sensor technology for pixellated imaging detectors

Silicon sensor technologies with reduced dead area at the sensors perimeter are under development at a number of institutes. Several fabrication methods for sensors which are sensitive close to the physical edge of the device are under investigation utilising techniques such as active-edges, passivated edges and current-terminating rings. Such technologies offer the goal of a seamlessly tiled detection surface with minimum dead space between the individual modules. In order to quantify the performance of different geometries and different bulk and implant types, characterisation of several sensors fabricated using active-edge technology were performed at the B16 beam line of the Diamond Light Source. The sensors were fabricated by VTT and bump-bonded to Timepix ROICs. They were 100 and 200 μ m thick sensors, with the last pixel-to-edge distance of either 50 or 100 μ m. The sensors were fabricated as either n-on-n or n-on-p type devices. Using 15 keV monochromatic X-rays with a beam spot of 2.5 μ m, the performance at the outer edge and corners pixels of the sensors was evaluated at three bias voltages. The results indicate a significant change in the charge collection properties between the edge and 5th (up to 275 μ m) from edge pixel for the 200 μ m thick n-on-n sensor. The edge pixel performance of the 100 μ m thick n-on-p sensors is affected only for the last two pixels (up to 110 μ m) subject to biasing conditions. Imaging characteristics of all sensor types investigated are stable over time and the non-uniformities can be minimised by flat-field corrections. The results from the synchrotron tests combined with lab measurements are presented along with an explanation of the observed effects.

Characterization of M-π-n CdTe pixel detectors coupled to HEXITEC readout chip

Segmentation of the anode-side of an M-π-n CdTe diode, where the pn-junction is diffused into the detector bulk, produces large improvements in the spatial and energy resolution of CdTe pixel detectors. It has been shown that this fabrication technique produces very high inter-pixel resistance and low leakage currents are obtained by physical isolation of the pixels of M-π-n CdTe detectors. In this paper the results from M-π-n CdTe detectors stud bonded to a spectroscopic readout ASIC are reported. The CdTe pixel detectors have 250 μm pitch and an area of 5 × 5 mm2 with thicknesses of 1 and 2 mm. The polarization and energy resolution dependence of the M-π-n CdTe detectors as a function of detector thickness are discussed.

Edgeless planar semiconductor sensors for a Medipix3-based radiography detector

We study the influence of active edges on the response of edge pixels by comparing simulations of the electrostatic-potential distribution to position-defined measurements on the energy deposition. A laser setup was used to measure the edge-pixel response function and shows the sensitive edge is only about 2 μm from the physical edge. 3D reconstruction of tracks from high-energy pions and muons, produced at the SPS H6 test beam facility at CERN, enabled to relate the energy deposition at edge pixels to the particles interaction depth. A clear correlation is observed between the simulated electric-field distortion and the reconstructed interaction-depth dependent effective size.

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.

Active-Edge planar silicon sensors for large-area pixel detectors

We study the influence of active edges on the response of edge pixels by comparing simulations of the electrostatic-potential distribution to position-defined measurements on the energy deposition. A laser setup was used to measure the edge-pixel response function and shows the sensitive edge is only about 2 µm from the physical edge. 3D reconstruction of tracks from high-energy pions and muons, produced at the SPS H6 test beam facility at CERN, enabled to relate the energy deposition at edge pixels to the particles interaction depth. A clear correlation is observed between the simulated electric-field distortion and the reconstructed interaction-depth dependent effective size.

Pixelized M-π-n CdTe detector coupled to Medipix2 readout chip

We have realized a simple method for patterning an M-π-n CdTe diode with a deeply diffused pn-junction, such as indium anode on CdTe. The method relies on removing the semiconductor material on the anode-side of the diode until the physical junction has been reached. The pixelization of the p-type CdTe diode with an indium anode has been demonstrated by patterning perpendicular trenches with a high precision diamond blade and pulsed laser. Pixelization or microstrip pattering can be done on both sides of the diode, also on the cathode-side to realize double sided detector configuration. The article compares the patterning quality of the diamond blade process, pulsed pico-second and femto-second lasers processes. Leakage currents and inter-strip resistance have been measured and are used as the basis of the comparison. Secondary ion mass spectrometry (SIMS) characterization has been done for a diode to define the pn-junction depth and to see the effect of the thermal loads of the flip-chip bonding process. The anode and cathode-sides of a 6 × 6 mm2 diodes were patterned with a diamond blade and flip-chip bonded to the Medipix2 readout chips. First imaging results with an X-ray source show reduced polarization effect and edgeless detector behavior for the anode-side patterned detector.