| 1. |
- Badel, Xavier
(author)
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Development of macropore arrays in silicon and related technologies for X-ray imaging applications
- 2003
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Licentiate thesis (other academic/artistic)abstract
- Digital devices have started to replace photographic film inX-ray imaging applications. As compared to photographic films,these devices are more convenient to obtain images and tohandle, treat and store these images. The goal of the presentstudy is to develop macropore arrays and related silicontechnologies in order to fabricate X-ray imaging detectors formedical applications, and in particular for dentistry. Althougha few detectors are already available on the market, theirperformances, such as the X-ray sensitivity, can still beimproved. In addition, the image quality, defined by thespatial resolution and the signalto- noise ratio (SNR), shouldbe sufficiently high to enable diagnosis and, as regard to thepatient health, the X-ray dose should be reduced to aminimum.Three detector concepts were investigated. All of themrequire the formation of a macropore array in silicon as afirst step in the detector fabrication. Even though deepreactive ion etching was used to form these macropore arrays,silicon electrochemical etching in aqueous hydrofluoric acid(HF) solution has been more intensively studied. The porespacing was fixed to about 50 µm in order to achieve aspatial resolution of 10 lp/mm, as required in dentalapplication. Pore depths up to 420 µm with diameterranging from 10 to 40 µm, depending on the detectorconcept, have been achieved. Electrochemical etching of siliconis, indeed, a very promising technique to fabricate high aspectratio structures and damage-free macropore arrays. Thistechnique is based on a silicon dissolution reaction involvingthe species of the HF solution, silicon atoms and holes, thepositive charge carriers. As holes are the minority carriers inn-type silicon, they are usually photogenerated. However, wealso developed an alternative technique based on hole injectionfrom a forwardbiased p-n junction, and the possibility to formmacropore arrays and diverse threedimensional structures wasdemonstrated.The first detector concept investigated consists of asilicon charge-coupled device (CCD) in proximity with ascintillating guide screen. This screen is made of a siliconmacropore array filled with CsI(Tl), emitting photons at awavelength of 550 nm (green light) under X-ray exposure. Thevisible light is then reflected on the walls of the pores inorder to be detected by the CCD pixels. Both oxide and metalcan be used as a reflective layer. Such detectors were fullyfabricated and characterized, showing good spatial resolutionand comparable results with currently available detectorsconcerning the SNR and the X-ray dose. The second detectorstudied in this thesis uses photodiodes, instead of a CCD, inorder to detect the photons emitted from the scintillator. Thisconcept would lead to high charge collection efficiency sincethe diodes are formed in the silicon pore walls, making thedistance between the generation and detection points of thevisible photons short. However, this design implies two majordifficulties in the detector fabrication: formation of p-njunction in the pore walls and formation of contacts to thediodes. Thus, boron diffusion from a solid source andlow-pressure chemical vapor deposition of boron-dopedpoly-silicon were experimented. Both techniques were shown tobe successful. The last detector concept is based on thegeneration of electron/hole pairs in the semiconductor bulkunder X-ray exposure. The generated charges would then becollected by electrodes going through the bulk, requiringformation of deep and narrow pores. Siliconphoto-electrochemical etching was used and 425-µm deeppores with a diameter of 14 µm were formed, resulting inan aspect ratio of ~ 30 and an active area of 90 %.
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| 2. |
- Badel, Xavier, et al.
(author)
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Doping of electrochemically etched pore arrays in n-type silicon : processing and electrical characterization
- 2005
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In: Journal of the Electrochemical Society. - : The Electrochemical Society. - 0013-4651 .- 1945-7111. ; 152:4, s. G252-G258
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Badel, Xavier, et al.
(author)
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Electrochemical etching of n-type silicon based on carrier injection from a back side p-n junction
- 2003
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In: Electrochemical and solid-state letters. - : The Electrochemical Society. - 1099-0062 .- 1944-8775. ; 6:6, s. C79-C81
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Journal article (peer-reviewed)abstract
- 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.
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| 4. |
- Badel, Xavier, 1977-
(author)
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Electrochemically etched pore arrays in silicon for X-ray imaging detectors
- 2005
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 5. |
- Badel, Xavier, et al.
(author)
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Formation of ordered pore arrays at the nanoscale by electrochemical etching of n-type silicon
- 2004
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In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:1/3, s. 245-254
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Journal article (peer-reviewed)abstract
- 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.
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| 6. |
- Badel, Xavier, et al.
(author)
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Formation of pn junctions in deep silicon pores for X-ray imaging detector applications
- 2003
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In: Nuclear Instruments and Methods in Physics Research Section A. - 0168-9002 .- 1872-9576. ; 509:1-3, s. 96-101
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Journal article (peer-reviewed)abstract
- 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.
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| 7. |
- Badel, Xavier, et al.
(author)
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Improvement of an X-ray imaging detector based on a scintillating guides screen
- 2002
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In: Nuclear Instruments and Methods in Physics Research Section A. - 0168-9002 .- 1872-9576. ; 487:1-2, s. 129-135
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Journal article (peer-reviewed)abstract
- 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).
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| 8. |
- Badel, Xavier, et al.
(author)
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Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors
- 2004
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In: IEEE Transactions on Nuclear Science. - : IEEE. - 0018-9499 .- 1558-1578. ; 51:3, s. 1001-1005, s. 1006-1010
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Journal article (peer-reviewed)abstract
- 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.
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| 9. |
- Badel, Xavier, et al.
(author)
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Performance of scintillating waveguides for CCD-based X-ray detectors
- 2006
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In: IEEE Transactions on Nuclear Science. - 0018-9499 .- 1558-1578. ; 53:1, s. 3-8
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Journal article (peer-reviewed)abstract
- 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.
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| 10. |
- Kleinmann, P., et al.
(author)
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Toward the formation of three-dimensional nanostructures by electrochemical etching of silicon
- 2005
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 86:18, s. 183108-1-183108-13
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Journal article (peer-reviewed)abstract
- 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.
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