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Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors
KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.
KTH, Superseded Departments, Microelectronics and Information Technology, IMIT.ORCID iD: 0000-0002-5260-5322
LENAC, Univ.́ Claude Bernard Lyon-I, Villeurbanne.
Department of Information Technology, Mid-Sweden University, Sundsvall.
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2004 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, no 3, 1001-1005 p.Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
2004. Vol. 51, no 3, 1001-1005 p.
Keyword [en]
CsI(TI), Pixellated detectors, Scintillating wave-guides, X-ray imaging, Cesium compounds, Imaging techniques, Metallizing, Optical resolving power, Optical waveguides, Phosphors, Scanning electron microscopy, Signal to noise ratio, Silicon, X rays, CsI(Tl), Pixellated detectors, Scintillating waveguides, X-ray imaging, Charge coupled devices
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:kth:diva-4959DOI: 10.1109/TNS.2004.829579ISI: 000222644400035ScopusID: 2-s2.0-3342928886OAI: diva2:7216
QC 20100831. Konferens: Nuclear Science Symposium/Medical Imaging Conference/13th International Workshop on Room-Temperature Semiconductor X-and Gamma-Ray Dectectors/Symposium on Nuclear Power Systems, Portland, OR, OCT 19-25, 2003.Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2011-10-26Bibliographically approved
In thesis
1. Electrochemically etched pore arrays in silicon for X-ray imaging detectors
Open this publication in new window or tab >>Electrochemically etched pore arrays in silicon for X-ray imaging detectors
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
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.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. viii, 69 p.
Trita-FTE, ISSN 0284-0545 ; 2005:1
Electrophysics, Elektrofysik
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
urn:nbn:se:kth:diva-137 (URN)
Public defence
2005-03-07, Sal C1, KTH-Electrum, Isafjordsgatan 22, Kista, 10:15
QC 20100831Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2010-08-31Bibliographically approved

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