Browsing by Author "Golovleva, M."

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    AC-coupled n-in-p pixel detectors on MCz silicon with atomic layer deposition (ALD) grown thin film
    (Elsevier, 2021-01-11) Gädda, A.; Ott, Jennifer; Bharthuar, S.; Brücken, Erik; Kalliokoski, Matti; Karadzhinova-Ferrer, A.; Bezak, M.; Kirschenmann, S.; Litichevsky, V.; Golovleva, M.; Martikainen, Laura; Winkler, Alexander; Chmill, V.; Tuovinen, E.; Luukka, P.; Härkönen, Jaakko; Department of Electronics and Nanoengineering; Hele Savin Group; University of Helsinki; Ruder Boskovic Institute; Advacam Oy
    We report initial characterization of our novel sensor process solutions with AC-coupled n+/p−/p+ pixel detectors made on 150 mm diameter p-type Magnetic Czochralski silicon (MCz-Si) wafers. The pixels were segmented in a 52 × 80 dual column array and designed to be AC capacitive coupled. The resistive coupling between pixels, allowing quality assurance probing prior the flip chip bonding, was realized with thin film metal-nitride resistors fabricated by sputtering deposition. This approach allows us to omit punch-through resistor structures, which reduces the overall process complexity. Moreover, our previous studies have emphasized that applying ALD Aluminum Oxide (Al2O3) field insulator and passivation layer results in negative net oxide charge and thus additional p-spray or p-stop surface current termination structures are not necessary. Our focused application is a radiation-hard ALD AC-coupled pixel detector to be used in future particle physics experiments, such as the High-Luminosity Large Hadron Collider (HL-LHC), as well as photon counting applications. The pixel detectors were tested at Helsinki Institute of Physics (HIP) Detector laboratory and Ruđer Bošković Institute (RBI). We show measurement data of pixel detectors and other test structures. For the TiN resistors surrounding pixels, the resistance values were measured to be about 15kΩ. Data of electrical properties, full depletion voltage and leakage current are shown as well. Our Transient Current Technique (TCT) measurements indicated clear pixel segmentation with excellent homogeneity. For further study, AC-coupled sensors were hybridized to PSI46dig read out chips (ROC) by flip-chip interconnection technique and tested with a radioactive source.
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    Characterization of Heavily Irradiated Dielectrics for Pixel Sensors Coupling Insulator Applications
    (FRONTIERS MEDIA SA, 2022-01-19) Bharthuar, S.; Golovleva, M.; Bezak, M.; Brücken, E.; Gädda, A.; Härkönen, J.; Karadzhinova-Ferrer, A.; Kramarenko, N.; Kirschenmann, S.; Koponen, P.; Luukka, P.; Mizohata, K.; Ott, J.; Tuominen, E.; Helsinki Institute of Physics; University of Helsinki; Hele Savin Group; Department of Electronics and Nanoengineering
    An increase in the radiation levels during the high-luminosity operation of the Large Hadron Collider calls for the development of silicon-based pixel detectors that are used for particle tracking and vertex reconstruction. Unlike the conventionally used conductively coupled (DC-coupled) detectors that are prone to an increment in leakage currents due to radiation, capacitively coupled (AC-coupled) detectors are anticipated to be in operation in future collider experiments suitable for tracking purposes. The implementation of AC-coupling to micro-scale pixel sensor areas enables one to provide an enhanced isolation of radiation-induced leakage currents. The motivation of this study is the development of new generation capacitively coupled (AC-coupled) pixel sensors with coupling insulators having good dielectric strength and radiation hardness simultaneously. The AC-coupling insulator thin films were aluminum oxide (Al2O3) and hafnium oxide (HfO2) grown by the atomic layer deposition (ALD) method. A comparison study was performed based on the dielectric material used in MOS, MOSFET, and AC-coupled pixel prototypes processed on high resistivity p-type Magnetic Czochralski silicon (MCz-Si) substrates. Post-irradiation studies with 10 MeV protons up to a fluence of 1015 protons/cm2 suggest HfO2 to be a better candidate as it provides higher sensitivity with negative charge accumulation on irradiation. Furthermore, even though the nature of the dielectric does not affect the electric field within the AC-coupled pixel sensor, samples with HfO2 are comparatively less susceptible to undergo an early breakdown due to irradiation. Edge-transient current technique (e-TCT) measurements show a prominent double-junction effect as expected in heavily irradiated p-type detectors, in accordance with the simulation studies.
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    Characterization of magnetic Czochralski silicon devices with aluminium oxide field insulator: Effect of oxygen precursor on electrical properties and radiation hardness
    (IOP Publishing Ltd., 2021-05) Ott, J.; Bharthuar, S.; Gädda, A.; Arsenovich, T.; Bezak, M.; Brücken, E.; Golovleva, M.; Härkönen, J.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Kirschenmann, S.; Litichevskyi, V.; Luukka, P.; Martikainen, L.; Naaranoja, T.; Hele Savin Group; University of Helsinki; Advacam Oy; Ruder Boskovic Institute; Department of Electronics and Nanoengineering
    Aluminium oxide (Al2O3) has been proposed as an alternative to thermal silicon dioxide (SiO2) as field insulator and surface passivation for silicon detectors, where it could substitute p-stop/p-spray insulation implants between pixels due to its negative oxide charge, and enable capacitive coupling of segments by means of its higher dielectric constant. Al2O3 is commonly grown by atomic layer deposition (ALD), which allows the deposition of thin layers with excellent precision. In this work, we report the electrical characterization of single pad detectors (diodes) and MOS capacitors fabricated on magnetic Czochralski silicon substrates and using Al2O3 as field insulator. Devices are studied by capacitance-voltage, current-voltage, and transient current technique measurements. We evaluate the influence of the oxygen precursors in the ALD process, as well as the effect of gamma irradiation, on the properties of these devices. We observe that leakage currents in diodes before the onset of breakdown are low forall studied ALD processes. Charge collection as measured by transient current technique (TCT) is also independent of the choice of oxygen precursor. The Al2O3 films deposited with O3 possess a higher negative oxide charge than films deposited by H2O, However, in diodes a higher oxide charge is linked to earlier breakdown, as has been predicted by simulation studies. A combination of H2and O3 precursors results in a good compromise between the beneficial properties provided by the respective individual precursors.
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    Modeling the impact of defects on the charge collection efficiency of a Cadmium Telluride detector
    (IOP Publishing Ltd., 2021-08) Golovleva, M.; Bezak, M.; Bharthuar, S.; Brücken, E.; Gadda, A.; Harkönen, J.; Kalliokoski, M.; Kirschenmann, S.; Luukka, P.; Ott, J.; Tuuva, T.; University of Helsinki; Ludong University; Ruder Boskovic Institute; Hele Savin Group; LUT University; Department of Electronics and Nanoengineering
    Cadmium telluride is a favorable material for X-ray detection as it has an outstanding characteristic for room temperature operation. It is a high-Z material with excellent photon radiation absorption properties. However, CdTe single crystals may include a large number of extended crystallographic defects, such as grain boundaries, twins, and tellurium (Te) inclusions, which can have an impact on detector performance. A Technology Computer Aided Design (TCAD) local defect model has been developed to investigate the effects of local defects on charge collection efficiency (CCE). We studied a 1 mm thick Schottky-type CdTe radiation detector with transient current technique by using a red laser at room temperature. By raster scanning the detector surface we were able to study signal shaping within the bulk, and to locate surface defects by observing their impact on the CCE. In this paper we present our TCAD model with localized defect, and compare the simulation results to TCT measurements. In the model an inclusion with a diameter of 10 mu m was assumed. The center of the defect was positioned at 6 mu m distance from the surface. We show that the defect has a notable effect on current transients, which in turn affect the CCE of the CdTe detector. The simulated charge collection at the position of the defect decreases by 80 % in comparison to the defect-free case. The simulations show that the defects give a characteristic shape to TCT signal. This can further be used to detect defects in CdTe detectors and to estimate the overall defect density in the material.
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    Multispectral photon-counting for medical imaging and beam characterization — A project review
    (Elsevier, 2022-09-11) Kirschenmann, S.; Bezak, M.; Bharthuar, S.; Brücken, E.; Emzir, M.; Golovleva, M.; Gädda, A.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Karjalainen, A.; Koponen, P.; Kramarenko, N.; Luukka, P.; Ott, J.; Petrow, H.; Siiskonen, T.; Särkkä, S.; Tikkanen, J.; Turpeinen, R.; Winkler, A.; Department of Electrical Engineering and Automation; Sensor Informatics and Medical Technology; University of Helsinki; LUT University; Helsinki Institute of Physics; Radiation and Nuclear Safety Authority; Detection Technology Oyj; Okmetic Oy; University of California, Santa Cruz
    Central focus of the MPMIB project – funded via the Academy of Finland's RADDESS 2018–2021 programme – has been research towards a next-generation radiation detection system operating in a photon-counting (PC) multispectral mode: The extraction of energy spectrum per detector pixel data will lead to better efficacy in medical imaging with ionizing radiation. Therefore, it can be an important asset for diagnostic imaging and radiotherapy, enabling better diagnostic outcome with lower radiation dose as well as more versatile characterization of the radiation beam, leading for example to more accurate patient dosimetry. We present our approach of fabricating direct-conversion detectors based on cadmium telluride (CdTe) semiconductor material hybridized with PC mode capable application-specific integrated circuits (ASICs), and will give a review on our achievements, challenges and lessons learned. The CdTe crystals were processed at Micronova, Finland's national research infrastructure for micro- and nanotechnology, employing techniques such as surface passivation via atomic layer deposition, and flip chip bonding of processed sensors to ASIC. Although CdTe has excellent photon radiation absorption properties, it is a brittle material that can include large concentrations of defects. We will therefore also emphasize our quality assessment of CdTe crystals and processed detectors, and present experimental data obtained with prototype detectors in X-ray and Co-60 beams at a standards laboratory.
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    Processing of AC-coupled n-in-p pixel detectors on MCz silicon using atomic layer deposited aluminium oxide
    (Elsevier, 2020-04-01) Ott, Jennifer; Gädda, A.; Bharthuar, S.; Brücken, E.; Golovleva, M.; Härkönen, J.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Kirschenmann, S.; Litichevskyi, V.; Luukka, P.; Martikainen, L.; Naaranoja, T.; Hele Savin Group; Advacam Oy; University of Helsinki; Ruder Boskovic Institute; Department of Electronics and Nanoengineering
    We report on the fabrication of capacitively (AC) coupled n+-in-p pixel detectors on magnetic Czochralski silicon substrates. In our devices, we employ a layer of aluminium oxide (Al2O3) grown by atomic layer deposition (ALD) as dielectric and field insulator, instead of the commonly used silicon dioxide (SiO2). As shown in earlier research, Al2O3 thin films exhibit high negative oxide charge, and can thus serve as a substitute for p-stop/p-spray insulation implants between pixels. In addition, they provide far higher capacitance densities than SiO2 due to their high dielectric constant, permitting more efficient capacitive coupling of pixels. Furthermore, metallic titanium nitride (TiN) bias resistors are presented as an alternative to punch-through or poly-Si resistors. Devices obtained by the above mentioned process are characterized by capacitance–voltage and current–voltage measurements, and by 2 MeV proton microprobe. Results show the expected high negative charge of the Al2O3 dielectric, uniform chargecollection efficiency over large areas of pixels, and acceptable leakage current densities.