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  • 1.
    Badel, Xavier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Electrochemically etched pore arrays in silicon for X-ray imaging detectors2005Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    Digital devices have now been introduced in many X-ray imaging applications, replacing slowly traditional photographic films. These devices are preferred as they offer real time imaging, easy handling and fast treatment of the images. However, the performance of the detectors still have to be improved in order to increase the image quality, and possibly reduce the X-ray dose, a vital parameter for medical use. In this thesis, three different new detector concepts have been investigated. All designs use pore arrays, which are ideal starting structures to form pixellated detectors.

    Electrochemical etching of n-type silicon in aqueous hydrofluoric acid solution (HF) has been studied to form these pore arrays. A broad range of pores have been fabricated with diameters varying from 200 nm to 40 µm and with depths reaching almost the wafer thickness, thus leading to very high aspect ratios. The technique was also found to be suitable for the formation of other types of structures such as pillars and tubes on the sub micrometer scale. The etching is based on the dissolution of silicon in HF under anodic bias and a supply of positive electrical carriers (holes). As holes are the minority carriers in n-type silicon, they are usually photo-generated. In this work an alternative technique, based on hole injection from a forward-biased pn junction, has been successfully pioneered.

    The first X-ray imaging detector concept presented in the thesis consists of a silicon charge coupled device (CCD) in proximity with a scintillating screen. The screen is made from a pore array having reflective pore walls and filled with CsI(Tl), emitting photons at a wavelength of 550 nm under X-ray exposure. The secondary emitted photons are light-guided by the pore walls and then detected by the CCD pixels. Detectors were fully fabricated and characterized. This concept provides good spatial resolution with negligible cross talk between adjacent pixels. The dependences of the detector efficiency on pore depth and on the coating of the pore walls are presented. Although most of the produced detectors had a detective quantum efficiency of about 25%, some detectors indicate that efficient scintillating screens can be achieved approaching the theoretical limit as set by poissonian statistics of the X-ray photons.

    The two other detector designs require the formation of vertical pn junctions, i.e. in the pore walls. In one concept the secondary emitted photons are detected by photodiodes located in the pore walls. This would lead to high charge collection efficiency as the photons do not have to be guided to one end of the pore. However, high noise due to the direct detection of X-rays in the diodes is expected. The other concept is based on generation of electron-hole pairs in a semiconductor and the ‘3D’ detector, where an array of vertical electrodes is used to separate the charges via an electric field. To uniformly dope the inside of deep pores, both boron diffusion and low-pressure chemical vapor diffusion of boron-doped poly-silicon were shown to be successful techniques. This was confirmed by SIMS profiles taken through the pore wall thickness. Finally, the possibility to form individual junction in each pore was shown. The diodes were electrically characterized, demonstrating good rectifying behavior and sensitivity to light.

  • 2.
    Badel, Xavier
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Domeij, Martin
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Doping of electrochemically etched pore arrays in n-type silicon: processing and electrical characterization2005Ingår i: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 152, nr 4, s. G252-G258Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Silicon p-n diodes formed in the walls of deep pores have been electrically characterized. The pores were electrochemically etched in low-doped n-type silicon substrates, and the entire pore array was doped p(+) by boron diffusion at 1050 degrees C. Two different process flows were investigated to disconnect the p(+) layers from one pore to another. The first consists of removing a few micrometers of silicon at the top of the sample using reactive ion etching after diffusion while the second enables the prevention of doping at the top of the pore walls with an oxide, acting as a barrier during diffusion. Current-voltage and capacitance-voltage characteristics of p-n junctions are presented and related parameters, such as the serial resistance and the ideality factor are discussed. The results show good rectifying behavior of the diodes with a reverse current about four to five decades smaller than the forward current. Measurements with two pores connected in a transistor-like configuration (p(+)/n(-)/p(+)), were also performed. Device simulations were used to examine the device behavior. Finally, our results demonstrate that pores could work as individual detector pixels for moderate reverse voltages, suitable for radiation imaging applications.

  • 3.
    Badel, Xavier
    et al.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Galeckas, Augustinas
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Linnros, Jan
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Kleimann, P.
    LENAC, Université Claude Bernard Lyon-I, Villeurbanne.
    Fröjdh, C.
    Mitthögskolan, Department of Electronics.
    Petersson, C. Sture
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Improvement of an X-ray imaging detector based on a scintillating guides screen2002Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 487, nr 1-2, s. 129-135Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An X-ray imaging detector has been developed for dental applications. The principle of this detector is based on application of a silicon charge coupled device covered by a scintillating wave-guide screen. Previous studies of such a detector showed promising results concerning the spatial resolution but low performance in terms of signal to noise ratio (SNR) and sensitivity. Recent results confirm the wave-guiding properties of the matrix and show improvement of the detector in terms of response uniformity, sensitivity and SNR. The present study is focussed on the fabrication of the scintillating screen where the principal idea is to fill a matrix of Si pores with a CsI scintillator. The photoluminescence technique was used to prove the wave-guiding property of the matrix and to inspect the filling uniformity of the pores. The final detector was characterized by X-ray evaluation in terms of spatial resolution, light output and SNR. A sensor with a spatial resolution of 9 LP/mm and a SNR over 50 has been achieved using a standard dental X-ray source and doses in the order of those used at the moment by dentists (around 25 mR).

  • 4.
    Badel, Xavier
    et al.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Kumar, R. T. R.
    Atomic Physics Division, Stockholm University.
    Kleinmann, P.
    LENAC, Univ. Claude Bernard Lyon-I, Villeurbanne.
    Linnros, Jan
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Formation of ordered pore arrays at the nanoscale by electrochemical etching of n-type silicon2004Ingår i: Superlattices and Microstructures, ISSN 0749-6036, E-ISSN 1096-3677, Vol. 36, nr 1/3, s. 245-254Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electrochemical etching has been studied to structure n-type silicon substrates at the nanoscale. In this work, well-ordered pore arrays with diameters in the range of 150-500 nm and depths up to 50 mum have been fabricated. The pores were successfully formed by anodic etching in (100)oriented n-type silicon wafers of low-resistivity, typically 1 Omegacm, using aqueous hydrofluoric acid solutions. The lithographic step was performed in a thermally grown oxide using a stepper and dry oxide etching technique. Two types of oxide openings and pitch sizes were tested. The smallest oxide opening realised at this stage was 0.5 mum for a pitch of 1 mum. Stable pore formation was obtained and the smallest pore size obtained was about 200 nm with an aspect ratio close to 100.

  • 5.
    Badel, Xavier
    et al.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Linnros, Jan
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Electrochemical etching of n-type silicon based on carrier injection from a back side p-n junction2003Ingår i: Electrochemical and solid-state letters, ISSN 1099-0062, E-ISSN 1944-8775, Vol. 6, nr 6, s. C79-C81Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A technique for electrochemical etching of n-type silicon in aqueous hydrofluoric acid is presented. This technique differs from photoelectrochemical etching because the holes (positive carriers) needed for the dissolution reaction to occur, are not photogenerated. The principle developed here is to inject these positive carriers using a p-n junction under forward bias formed at the back side of the sample. Drift-diffusion of holes through the wafer thickness allows a chemical dissolution reaction at the interface with the electrolyte. To enable holes diffusing through the wafer the minority carrier lifetime must be sufficiently high making the technique well adapted for high resistivity silicon. However, extension to low resistivity wafers has been achieved. Results show the possibility of forming pore arrays and diverse 3D structures.

  • 6.
    Badel, Xavier
    et al.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Linnros, Jan
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Janson, M. S.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Österman, J.
    KTH, Tidigare Institutioner                               , Mikroelektronik och informationsteknik, IMIT.
    Formation of pn junctions in deep silicon pores for X-ray imaging detector applications2003Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 509, nr 1-3, s. 96-101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The formation of pn junctions in deep silicon pores has been studied for a new concept of X-ray imaging detectors. The sensitive part of the device is an array of CsI(Tl) columns formed by filling a silicon matrix of pores having pn junctions in their walls. Under X-ray illumination, the CsI(TI) scintillator emits photons that are collected by the pn junctions. Relatively high signal collection efficiency is expected. However, the formation of pn junctions inside pore walls represents a challenging step in the detector fabrication. In this work pore matrices were fabricated in n-type silicon by deep reactive ion etching and by photo-electrochemical etching. The pn junctions were formed either by boron diffusion or deposition of boron doped poly-silicon. Various techniques were used to analyze the transverse depth profiles of boron atoms at different pore depths. The study shows successful results for pn-junctions formed both by diffusion and by poly-silicon deposition.

  • 7.
    Badel, Xavier
    et al.
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Linnros, Jan
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Kleimann, P
    Norlin, B
    Koskiahde, E
    Valpas, K
    Nenonen, S.
    Petersson, Sture
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Frojdh, C
    Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors2004Ingår i: 2003 IEEE NUCLEAR SCIENCE SYMPOSIUM, CONFERENCE RECORD, VOLS 1-5, 2004, s. 1006-1010Konferensbidrag (Refereegranskat)
    Abstract [en]

    Silicon Charge Coupled Devices (CCD) covered with a scintillating film are now available on the market for use in digital medical imaging. However, these devices could still be improved in terms of sensitivity and especially spatial resolution by coating the CCD with an array of scintillating waveguides. In this work-, we fabricated such waveguides by first etching pores in silicon, then performing metallization or oxidation of the pore walls and finally filling the pores with CsI(TI). The resulting structures were observed using SEM and tested under X-ray exposure. The detector performances were also compared with simulations, indicating that the optimal pore depth for metallized macropore arrays is about 80 mum while it is around 350 mum for oxidized ones. This result, together with the roughness of the metal coating, explains why lower performances were measured experimentally with the metallized macropores. Indeed, our macropore arrays had depths in the range of 210-390 mum, which is favorable to oxidized structures.

  • 8.
    Badel, Xavier
    et al.
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Linnros, Jan
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Kleimann, P.
    LENAC, Univ.́ Claude Bernard Lyon-I, Villeurbanne.
    Norlin, B.
    Department of Information Technology, Mid-Sweden University, Sundsvall.
    Koskiahde, E.
    Metorex International Oy, Espoo.
    Valpas, K.
    Metorex International Oy, Espoo.
    Nenonen, S.
    Metorex International Oy, Espoo.
    Petersson, Sture
    KTH, Tidigare Institutioner, Mikroelektronik och informationsteknik, IMIT.
    Fröjdh, C.
    Department of Information Technology, Mid-Sweden University, Sundsvall.
    Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors2004Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, nr 3, s. 1001-1005Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Silicon charge-coupled devices (CCDs) covered with a scintillating film are now available on the market for use in digital medical imaging. However, these devices could still be improved in terms of sensitivity and especially spatial resolution by coating the CCD with an array of scintillating waveguides. In this paper, such waveguides were fabricated by first etching pores in silicon, then performing metallization or oxidation of the pore walls and finally filling the pores with CsI(TI). The resulting structures were observed using scanning electron microscopy and tested under X-ray exposure. Theoretical efficiencies of macropore arrays filled with CsI(TI) were also calculated, indicating that the optimal pore depth for metallized macropore arrays is about 80 mum while it is around 350 mum for oxidized ones. This result, together with the roughness of the metal coating, explains why lower SNR values were measured with the metallized macropores. Indeed, the macropore arrays had depths in the range of 210-390 mum, which is favorable to oxidized structures.

  • 9.
    Badel, Xavier
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Norlin, B.
    Department of Information Technology and Media, Mid-Sweden University, Sundsvall.
    Kleimann, P.
    LENAC, Université Claude Bernard Lyon-I, Villeurbanne.
    Williams, L.
    Applied Scintillation Technology, Harlow.
    Moody, S. J.
    Applied Scintillation Technology, Harlow.
    Tyrell, G. C.
    Applied Scintillation Technology, Harlow.
    Fröjdh, C.
    Department of Information Technology and Media, Mid-Sweden University, Sundsvall.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Performance of scintillating waveguides for CCD-based X-ray detectors2006Ingår i: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 53, nr 1, s. 3-8Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Scintillating films are usually used to improve the sensitivity of CCD-based X-ray imaging detectors. For an optimal spatial resolution and detection efficiency, a tradeoff has to be made on the film thickness. However, these scintillating layers can also be structured to provide a pixellated screen. In this paper, the study of CsI(TI)-filled pore arrays is reported. The pores are first etched in silicon, then oxidized and finally filled with CsI(TI) to form scintillating waveguides. The dependence of the detector sensitivity on pore depth, varied from 40 to 400 mu m here, follows rather well theoretical predictions. Most of the detectors produced in this work have a detective quantum efficiency of the incoming X-ray photons of about 25%. However, one detector shows that higher efficiency can be achieved approaching almost the theoretical limit set by Poisson statistics of the incoming X-rays. Thus, we conclude that it is possible to fabricate scintillating waveguides with almost ideal performance. Imaging capabilities of the detectors are demonstrated.

  • 10.
    Kleinmann, P.
    et al.
    LENAC, Universit́ Claude Bernard Lyon-I, Villeurbanne.
    Badel, Xavier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Toward the formation of three-dimensional nanostructures by electrochemical etching of silicon2005Ingår i: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, nr 18, s. 183108-1-183108-13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report a simple technique to form various kinds of three-dimensional structures in silicon. The process flow is only composed of two steps: lithography and electrochemical etching ("LEE"). The LEE process is an easy and low-cost solution for the fabrication of high-aspect-ratio structures such as walls, tubes, and pillars. Here we demonstrate the possibility to apply the LEE process on the submicrometer scale, indicating that it is a promising tool for silicon nanomachining.

  • 11. Li, F.
    et al.
    Badel, Xavier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Wiley, J. B.
    Colloidal crystal wires from directed assembly2005Ingår i: Micro- and Nanosystems-Materials and Devices, Materials Research Society, 2005, s. 477-482Konferensbidrag (Refereegranskat)
    Abstract [en]

    Colloidal crystal wires with tubular-like packings are prepared by the directed assembly of spheres into cylindrical one-dimensional channels. Silica spheres are infiltrated into porous silicon membranes, treated with silane, and annealed. Single annealing cycles are found to result in colloidal crystal wires with varied packing geometries, while repeated annealing produces a thin translucent silica sheath around the wires. Packing in the wires varies with the relative channel diameter of the silicon membrane where typical wires contain 4 to 7 helical strands. Both chiral and achiral packing geometries are observed. The fabrication of these wires is discussed and the relationship between channel size and packing structure detailed.

  • 12.
    Linnros, Jan
    et al.
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Badel, Xavier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Materialfysik, Halvledarmaterial, HMA.
    Kleimann, Pascal
    Macro pore and pillar array formation in silicon by electrochemical etching2006Ingår i: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T126, s. 72-76Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electrochemical etching may be used to form high aspect-ratio pores and pillars in silicon. Starting from lithographically patterned surfaces, regular arrays of macro pores or pillars can be fabricated. The pitch and pillar) pore size must then scale with the depletion width, in turn set by the material resistivity. We review various results where the achievable pore diameter ranges from 100 mu m for high resistivity material to the submicron range for highly doped wafers. At slightly higher current density and using different patterns, pillars or walls may be formed. The fabricated structures may be further processed and we demonstrate oxidation, uniform wall doping and finally, filling of the structures to result in functional materials. Applications include both optical, microelectronic, material and bio-applications.

  • 13. Olsen, U. L.
    et al.
    Badel, Xavier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och Informationsteknik, IMIT.
    Di Michiel, M.
    Martin, T.
    Schmidt, S.
    Poulsen, H. F.
    Development of a high-efficiency high-resolution imaging detector for 30-80 keV X-rays2007Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 576, nr 1, s. 52-55Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A newly developed fabrication method makes the formation of deep structured scintillator screens possible. We demonstrate that electrochemical etching in silicon can be used to produce regular arrays of 120 mu m deep pores with a 4 mu m pitch. A layer of SiO2 is grown on the pore walls and CsI:Tl is melted into the pores, resulting in a structure with a high refractive index core surrounded by a quartz cladding, providing efficient light guiding. The efficiency and radiation hardness of the scintillator is evaluated in realistic environment at beamline ID15 at the ESRF synchrotron. The efficiency is measured to be a factor two higher than a planar YAG:Cc scintillator of equal thickness, while radiation damage is found to be neglectable for doses up to at least 2 x 10(4) Gy.

  • 14. Simon, Matthias
    et al.
    Engel, Klaus-Juergen
    Menser, Bernd
    Badel, Xavier
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Linnros, Jan
    KTH, Skolan för informations- och kommunikationsteknik (ICT), Mikroelektronik och tillämpad fysik, MAP.
    Challenges of pixelated scintillators in medical X-ray imaging2008Ingår i: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 591, nr 1, s. 291-295Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In current X-ray detectors, spatial resolution is limited by optical photon diffusion in the scintillator layer. A method to prevent photons from reaching neighboring pixels is the embedding of opaque walls within the scintillator. The realization of a pixelated scintillator faces, several challenges to obtain a good imaging performance, especially a high detective quantum efficiency (DQE). To maintain a high X-ray absorption, a high volume fill-factor is required. Losses of secondary light quanta have to be kept to a minimum to maintain an acceptable gain. Moreover, the signal per primary X-ray quanta should have a low variation with the depth of interaction to avoid a high secondary quantum noise (Swank-noise). Light scatter inside the scintillator causes both enhanced light loss and Swank-noise. For this work, a pixelated scintillator has been built from electrochemically etched silicon pore arrays, which are filled with cesium iodide (CsI:Tl). With a pixel pitch of 50 mu m, wall thicknesses of 6.5 mu m and pore depths of nearly 400 mu m are achieved. The modulation transfer function is 40% at 4 lp/mm and 10-20% at 8 lp/mm. The ability of the pores to transport light quanta from their origin to the photodiode is expressed in a light guiding efficiency, which is determined as 6.5% in the better cases. The maximal DQE(0) is 0.28, while the X-ray absorption with the given thickness and fill-factor is 0.57. The difference is explained by high Swank-noise due to optical scatter inside the CsI-filled pores, in agreement to Monte-Carlo simulations of the photon transport inside the pore array structure.

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