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Electrochemically etched pore arrays in silicon for X-ray imaging detectors
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
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. , p. viii, 69
Series
Trita-FTE, ISSN 0284-0545 ; 2005:1
Keywords [en]
Electrophysics
Keywords [sv]
Elektrofysik
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-137OAI: oai:DiVA.org:kth-137DiVA, id: diva2:7220
Public defence
2005-03-07, Sal C1, KTH-Electrum, Isafjordsgatan 22, Kista, 10:15
Opponent
Supervisors
Note
QC 20100831Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2010-08-31Bibliographically approved
List of papers
1. Electrochemical etching of n-type silicon based on carrier injection from a back side p-n junction
Open this publication in new window or tab >>Electrochemical etching of n-type silicon based on carrier injection from a back side p-n junction
2003 (English)In: Electrochemical and solid-state letters, ISSN 1099-0062, E-ISSN 1944-8775, Vol. 6, no 6, p. C79-C81Article in journal (Refereed) Published
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.

Keywords
Carrier concentration, Diffusion in solids, Dissolution, Electric conductivity of solids, Electrochemistry, Etching, Hydrofluoric acid, Interfaces (materials), Positive ions, Substrates, Aqueous hydrofluoric acid, Carrier injection, Electrochemical etching, Photoelectrochemical etching, Radiation imaging detector, Silicon pore arrays, Silicon wafers
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4955 (URN)10.1149/1.1566533 (DOI)000182269700008 ()
Note
QC 20100831Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
2. Formation of ordered pore arrays at the nanoscale by electrochemical etching of n-type silicon
Open this publication in new window or tab >>Formation of ordered pore arrays at the nanoscale by electrochemical etching of n-type silicon
2004 (English)In: Superlattices and Microstructures, ISSN 0749-6036, E-ISSN 1096-3677, Vol. 36, no 1/3, p. 245-254Article in journal (Refereed) Published
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.

Keywords
Electrolytic polishing, Etching, Hydrofluoric acid, Lithography, Pore size, Silicon wafers, Dry oxide etching, Electrochemical etching (ECE), Pore arrays, Space charge region (SCR), Silicon
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4956 (URN)10.1016/j.spmi.2004.08.037 (DOI)000225425100028 ()2-s2.0-9644259379 (Scopus ID)
Note
QC 20100831Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
3. Toward the formation of three-dimensional nanostructures by electrochemical etching of silicon
Open this publication in new window or tab >>Toward the formation of three-dimensional nanostructures by electrochemical etching of silicon
2005 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 86, no 18, p. 183108-1-183108-13Article in journal (Refereed) Published
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.

Keywords
Aspect ratio, Etching, Lithography, Low energy electron diffraction, Machining, Masks, Nanostructured materials, Nanotechnology, Oxides, Electrochemical etching, Isotropic etching, Pillars, Process flows, Silicon
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4957 (URN)10.1063/1.1924883 (DOI)000229288700049 ()2-s2.0-20944451255 (Scopus ID)
Note
QC 20100831. Tidigare titel: Towards The Formation of 3D Nano-Structures by Electrochemical Etching of Silicon. Titel ändrad samt uppdaterad från Submitted till Published 20100831.Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
4. Improvement of an X-ray imaging detector based on a scintillating guides screen
Open this publication in new window or tab >>Improvement of an X-ray imaging detector based on a scintillating guides screen
Show others...
2002 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 487, no 1-2, p. 129-135Article in journal (Refereed) Published
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).

Keywords
dental imaging, pixel detector, scintillating screen, CsI(Tl), X-rays, silicon, pores
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-21771 (URN)10.1016/S0168-9002(02)00956-7 (DOI)000177177500021 ()
Note
QC 20100825. Konferens: 3rd International Workshop on Radiation Imaging Detectors, OROSEI, ITALY, SEP 23-27, 2001.Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2017-12-12Bibliographically approved
5. Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors
Open this publication in new window or tab >>Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors
Show others...
2004 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 51, no 3, p. 1001-1005Article 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.

Keywords
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
Identifiers
urn:nbn:se:kth:diva-4959 (URN)10.1109/TNS.2004.829579 (DOI)000222644400035 ()2-s2.0-3342928886 (Scopus ID)
Note
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: 2017-12-05Bibliographically approved
6. Performance of scintillating waveguides for CCD-based X-ray detectors
Open this publication in new window or tab >>Performance of scintillating waveguides for CCD-based X-ray detectors
Show others...
2006 (English)In: IEEE Transactions on Nuclear Science, ISSN 0018-9499, E-ISSN 1558-1578, Vol. 53, no 1, p. 3-8Article in journal (Refereed) Published
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.

Keywords
Pixellated detectors, Scintillating waveguides, X-ray imaging, Charge coupled devices, Detectors, Photons, Scintillation, Statistics, X rays, Pixellated detectors, Scintillating waveguides, X-ray imaging, Optical waveguides
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4960 (URN)10.1109/TNS.2005.862981 (DOI)000236473800001 ()2-s2.0-33645696279 (Scopus ID)
Note
QC 20100831. Uppdaterad från Submitted till Published 20100831.Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
7. Formation of pn junctions in deep silicon pores for X-ray imaging detector applications
Open this publication in new window or tab >>Formation of pn junctions in deep silicon pores for X-ray imaging detector applications
2003 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 509, no 1-3, p. 96-101Article in journal (Refereed) Published
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.

Keywords
pn junction, SCM, SEM, Silicon macropores, SIMS, SSRM, Photons, Porosity, Semiconductor junctions, X rays, X-ray imaging detectors, Radiation detectors
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4961 (URN)10.1016/S0168-9002(03)01556-0 (DOI)000185047700017 ()
Note
QC 20100831Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved
8. Doping of electrochemically etched pore arrays in n-type silicon: processing and electrical characterization
Open this publication in new window or tab >>Doping of electrochemically etched pore arrays in n-type silicon: processing and electrical characterization
2005 (English)In: Journal of the Electrochemical Society, ISSN 0013-4651, E-ISSN 1945-7111, Vol. 152, no 4, p. G252-G258Article in journal (Refereed) Published
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.

Keywords
Arrays, Computer simulation, Doping (additives), Electric currents, Electric potential, Electric resistance, Electrochemistry, Etching, Micrometers, Positive ions, Semiconductor junctions, Thermal diffusion, Transistors, Anisotropic alkaline etching, Boron diffusion, Pore arrays, Silicon substrates, Silicon
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4962 (URN)10.1149/1.1862263 (DOI)000228521200056 ()2-s2.0-18344377120 (Scopus ID)
Note
QC 20100831. Uppdaterad från In press till Published 20100831.Available from: 2005-03-03 Created: 2005-03-03 Last updated: 2017-12-05Bibliographically approved

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  • apa
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