- Email: [email protected]
Charge collection mapping of a novel ultra-thin silicon strip detector for hadrontherapy beam monitoring
Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Charge collection mapping of a novel ultra-thin silicon strip detector for hadrontherapy beam monitoring Mohamed Bouterfa a,n, Geoffrey Alexandre b, Eduardo Cortina Gil b, Denis Flandre a a b
ICTEAM Institute, Universite catholique de Louvain, Louvain-la-Neuve, Belgium IRPM Institute, Universite catholique de Louvain, Louvain-la-Neuve, Belgium
art ic l e i nf o
a b s t r a c t
Available online 30 May 2013
In precise hadrontherapy treatments, the particle beam must be monitored in real time without being degraded. Silicon strip detectors have been fabricated over an area as large as 4.5 4.5 cm2 with ultra low thickness of 20 μm. These offer the following considerable advantages: significantly reduced beam scattering, higher radiation hardness which leads to improved detector lifetime, and much better collection efficiency. In a previous work, the novel sensor has been described and a global macroscopic dosimetry characterization has been proposed. This provides practical information for the detector daily use but not about the local microscopic knowledge of the sensor. This work therefore presents a micrometric-accuracy charge-collection characterization of this new generation of ultra-thin silicon strip detectors. This goal is reached thanks to a 1060 nm-wavelength micrometric-sized laser that can be positioned relatively to the sensor with a submicron precision for the three different axes. This study gives a much better knowledge of the inefficient areas of the sensor and allows therefore optimization for future designs. & 2013 Elsevier B.V. All rights reserved.
Keywords: Silicon strip detector Radiation therapy Dosimetry Low material budget
1. Introduction Radiation therapy contributes nowadays to the cure of approximately 23% of all cancer patients according to Ref. [14], mostly based on photons or electrons which represents one of the most important and widespread techniques for cancer treatment over the world. Hadrontherapy, which uses massive particles like protons and carbon ions, is gaining increasing interest thanks to its better ballistic precision by exploiting the Bragg Peak effect. After Ref. [1], the progression of the number of patients being treated with protons looks exponential since 1993. For quality assurance (QA) purposes, the beam characterization is made before the patient irradiation. Then the detector is removed to prevent it from degrading the beam through scattering and energy straggling. For optimal treatment, it is necessary to monitor the beam in real time in order to know the actual dose deposited into the patient body. If material budget is not considered during its design, the detector may generate Multiple Coulomb Scattering (MCS) as well as energy straggling and therefore, the lateral penumbra and distal falloff expand which represents a risk for surrounding healthy tissues. Various detectors have been built and
n
Corresponding author. Tel.: +32 486468868. E-mail address: [email protected] (M. Bouterfa).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.05.139
characterized in Refs. [2,3] but they still show relatively high degradation of the beam [4]. To reduce the detector impact on the treatment beam, we designed and fabricated innovating silicon strip detectors over an area as large as 4.5 4.5 cm2 with an ultra low thickness of 20 μm, which represents a material budget as low as 0.02% of radiation length. In addition to a reduced beam scattering, ultra-thin detectors offer the following considerable advantages: a higher radiation hardness [5] which leads to an improved detector lifetime and a much better collection efficiency [6]. In a previous work [15], the novel sensor, irradiated under a proton beam, has been briefly described and a global macroscopic dosimetry characterization has been proposed. The latter characterization provides practical information for the detector daily use, however it does not help to improve the local microscopic knowledge of the sensor. To complete the novel sensor study, we characterized it by performing a micrometric-accuracy mapping of the charge collection using a 1060 nm-wavelength laser. The aim of this paper is to present the measurement results in order to validate the novel structure and to clarify which are the most efficient areas of the sensor in terms of charge collection. Accordingly, the second section gives a description of the sensor and presents the readout system used in the experimental setup. Eventually, it shortly describes the laser irradiation system and its characteristics.
M. Bouterfa et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
The results and discussion section finally reports the collection mapping measurements and discusses it.
2. Materials and methods 2.1. Sensor design and fabrication A cross-section SEM photograph of a 20 μm thick fabricated device is depicted in Fig. 1. In this subsection, we first describe the substrate, then the strips design, thirdly the interstrip insulation and eventually, the thinning step and the back metallization.
2.1.1. Substrate features Thick Silicon On Insulator (SOI) 3-in. wafers have been used as substrates for the device fabrication. Top silicon layer, buried oxide (BOX) and back substrate thicknesses are respectively 20 μm, 0:5 μm and 525 μm. Silicon is boron pre-doped with a resistivity of 10–30 Ω cm, which represents an equivalent impurity level ranging from 1.32 1015 cm−3 to 4.36 1014 cm−3. The p-type substrate has been demonstrated in Refs. [7–9] to have a better radiation hardness as well as a better charge collection efficiency than the n-type one. Furthermore, hadrons irradiation damage in silicon may result in a type inversion for n-type substrates, which is not the case for p-type substrates [10]. Effective impurity level increases with the accumulated ionizing dose [10]. As a consequence, the low impurity level of a high resistivity substrate will be degraded as the accumulated dose increases and therefore represents an unnecessary over cost.
2.1.2. Strips description Schematic cross-section of the SOI device is shown in Fig. 2. In the top silicon layer, 81 N+ arsenic-doped strips are processed with a pitch of 500 μm. Strips length is 4 cm. The implanted width of each strip is 20 μm while the aluminum width is 13 μm. A rectangular guard-ring of 4.2 4.6 cm2 surrounds the 81 strips. The guard ring implant width is 20 μm as well. The implantation was performed through a 45 nm-thick thermal oxide with a 65 keV energy and a 1016 cm−2 dose. Dopants were activated and diffused at 1050 1C for 1 h.
557
2.1.3. Interstrip insulation According to Ref. [11], oxide trapped charges increase with the total ionizing dose to a saturation level of about 1012 cm−2. Consequently, all p-type substrates need to prevent this shorting phenomenon from occurring by positively doping the silicon superficial layer either locally in between strips (p-stops) or over the whole surface (p-spray). Since the p-stop technique needs an additional lithographic step, the simpler p-spray was preferred. According to a simulation performed on ATLAS from the SILVACO suite, the level of acceptor dopants needed to cancel the electron channel is a bit less than 1017 cm−3 since the superficial electron concentration is as weak as about 5 105 cm−3 after implantation. To reach the needed impurity level, boron was implanted with an energy of 13 keV and a dose of 2.5 1012 cm−2 through 45 nm of thermal oxide. Boron and Arsenic dopants diffused during the same process step with the diffusion parameters mentioned above. 2.1.4. Thinning and back metalization A centered square of 4.5 4.5 cm2 of the back substrate and the BOX layer is wet-etched from the backside using TMAH in order to release the top silicon layer as a membrane after coating the strips with the PROTEK B3 protective resist. The purpose of leaving an unthinned framework is to have a device with sufficient robustness to be easily handled without breaking. Eventually, 150 nm of aluminum is then deposited on the overall backside. A fabricated 20 μm thick device seen from the backside is depicted in Fig. 3. 2.2. EmXX readout system The detector signals were read out thanks to the emXX interface (Fig. 4), which has been developed by Ion Beam Applications (IBA) to measure the dose and dose rate within p-type silicon detectors at null bias. The emXX consists of two TERA chips designed by TORINO INFN division and fabricated by WELLHOFERSCANDITRONIX. Each Tera chip reads out, through current-tofrequency converters, up to 64 analog channels simultaneously and converts the integrated charge to a number of counts with good noise performances, sensitivity and radiation hardness [12,13]. Actually, the emXX imposes a 3 mV reverse bias to the p-type detector to ensure the reverse mode biasing since the Tera chip is not functional with forward biasing. 2.3. Laser irradiation system
Fig. 1. Cross-section SEM photograph of a strip on ultra-thin substrate.
The experimental setup is shown in Fig. 5. The sensor (wafer) is glued on the interface PCB which lies on the sub-micron resolution moving platform along X, Y and Z axes. The laser diode is fixed and emits 1060 nm photons perpendicularly to the sensor surface. The equivalent energy of such photons is 1.17 eV, which is just above the silicon band gap. In that way, the excess energy, which will be transferred to phonons and consequently lost as thermal crystalline vibrations, is very low. The wavelength straggle is reported by the manufacturer to be less than 1 nm of full width at half
Fig. 2. Schematic cross-section of the ultra-thin device.
Fig. 3. The 4.5 4.5 cm2 membrane is 20 μm thick.
558
M. Bouterfa et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
0.07
Beam Spot (mm)
0.06 0.05 0.04 0.03 0.02 0.01 21.5
22
22.5
23
23.5
24
24.5
Relative Position (mm)
Fig. 4. EmXX readout system (right) connected to the detector through the printed circuit board (left).
Fig. 6. Longitudinal profile of the laser spot size according to the Knife-Edge method.
Fig. 7. Collected signal in log-scale versus the x position of the beam spot relatively to the central strip with unfocused beam.
The first subsection presents the collection measurements along X-axis and Z-axis in order to correctly position the central strip along X-axis and to focus the beam on the wafer surface. Once the laser spot is localized and focused, the second subsection reports a more precise collection profiling along X-axis in order to extract the diffusion length of minority carriers. 3.1. Laser positioning and focusing measurements
Fig. 5. Experimental setup consisting of the sensor PCB and the laser diode.
maximum. Using the Knife-Edge method, we characterized the longitudinal profile of the spot size in order to find its minimum size. The spot being Gaussian-shaped, we measured its standard deviation for various Z. As can be seen in Fig. 6, the minimum size is 7:5 μm.
3. Results and discussions For clarity purposes, we only report the central strip measurements. According to Ref. [15], interstrip collection standard deviation is indeed as low as 0.68% for 62 MeV protons. We assume that it is still verified for 1060 nm photons. We positioned the sensor in a way that the Z-axis corresponds to the wafer surface axe. The Y- and X-axis respectively correspond to the longitudinal and orthogonal strip directions.
Fig. 7 depicts the collected current on the central strip versus the “x” position of the beam spot. If the beam is focused, its standard deviation size is about 7:5 μm. Because of the strip aluminum shading, which is as a reminder 13 μm wide, the signal should reach a minimum at the strip center. The curve in Fig. 7 does not behave in a such way because the beam is not focused. The collected signal as a function of the “z” position is shown in Fig. 8. We notice that there indeed exists a minimum which illustrates the aluminum shading effect. The best focusing “z” corresponds to the minimizing “z” in Fig. 8. We also observe that the well is as narrow as few hundreds of micrometers. For precise positioning and focusing purposes, x- and z-profiles centered on the minimum were performed iteratively by increasing the spatial resolution at every iteration step. 3.2. Diffusion length extraction Fig. 9 shows the measured collected current (log-scale in arbitrary units) as a function of x-position. Contrary to what can be expected, there are two diffusion lengths instead of one,
M. Bouterfa et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
559
the highly doped N+ region that significantly degrades the lifetime. As a consequence to the latter phenomenon, the collection efficiency decreases by one order of magnitude in about 25 μm only, as can be seen in Fig. 9. Consequently, the major part of the silicon between two adjacent strips is highly inefficient in terms of collection.
4. Conclusion
Fig. 8. Collected signal in log-scale versus the z-position of the wafer.
In this paper, we present a micrometric-accuracy mapping of the charge collection of a novel ultra-thin silicon detector for hadrontherapy application using a 1060 nm-wavelength laser. The measurement results validate the novel structure and clarify which are the most efficient areas of the sensor in terms of charge collection. In fact, collection efficiency decreases by one order of magnitude at only 25 μm far from the strip which means that the major part of the silicon between two adjacent strips is highly inefficient in terms of collection. References
Fig. 9. Collected current in log-scale in arbitrary units for a beam focused at various x-positions.
meaning that there are two effective minority carrier lifetimes (ELT). Close to the strip the diffusion length is about 8:5 μm, corresponding to an ELT of 0:02 μs. At about 50 μm distance and further, diffusion length and ELT increase to 105 μm and 3 μs, respectively. We first notice that the ELT is more than 150 times lower close to the implanted areas of the strips, which may be explained by
[1] J.-M. Lagniel, Status of the hadrontherapy projects in Europe, in: Proceedings of PAC07, Albuquerque, USA, 2007, p. 127. [2] A. La Rosa, et al., Nuclear Instruments and Methods in Physics Research Section A 565 (2006) 833. [3] N. Givehchi, et al., Nuclear Instruments and Methods in Physics Research Section A 572 (2007) 1094. [4] M. Bouterfa, et al., Towards a new generation of ultra-thin p-type silicon strip detectors for hadrontherapy beam monitoring, in: Proceedings of ANIMMA11, Ghent, Belgium, 2011. [5] D. Menichelli, et al., Nuclear Instruments and Methods in Physics Research Section A 583 (2007) 109. [6] M. Bouterfa, D. Flandre, Journal of Circuits, Systems and Computers, submitted for publication. [7] G. Casse, et al., Nuclear Instruments and Methods in Physics Research Section A 535 (2004) 362. [8] G. Casse, Nuclear Instruments and Methods in Physics Research Section A 487 (2002) 465. [9] G. Casse, IEEE Transactions on Nuclear Science NS-47 (2000) 527. [10] M. Moll, Radiation Tolerant Sensors for Solid State Tracking Detectors, 〈http:// www.cern.ch/rd50〉. [11] G. Pellegrini, et al., Nuclear Instruments and Methods in Physics Research Section A 566 (2006) 360. [12] G. Mazza, et al., IEEE Transactions on Nuclear Sciences NS-52 (2005) 847. [13] A. La, Rosa, et al., Nuclear Instruments and Methods in Physics Research Section A 586 (2008) 270. [14] K. Ando, et al., Relative Biological Effectiveness in Ion Beam Therapy, International Atomic Energy Agency, Technical Reports Series No. 461, Vienna, Austria, 2008, ISBN 978-92-0107807-0. [15] M. Bouterfa, et al., Characterization of ultra-thin silicon strip detectors for hadrontherapy beam monitoring, in: Proceedings of I2MTC2013, Minneapolis, USA, 2013, p. 1088.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Charge collection mapping of a novel ultra-thin silicon strip detector for hadrontherapy beam monitoring Mohamed Bouterfa a,n, Geoffrey Alexandre b, Eduardo Cortina Gil b, Denis Flandre a a b
ICTEAM Institute, Universite catholique de Louvain, Louvain-la-Neuve, Belgium IRPM Institute, Universite catholique de Louvain, Louvain-la-Neuve, Belgium
art ic l e i nf o
a b s t r a c t
Available online 30 May 2013
In precise hadrontherapy treatments, the particle beam must be monitored in real time without being degraded. Silicon strip detectors have been fabricated over an area as large as 4.5 4.5 cm2 with ultra low thickness of 20 μm. These offer the following considerable advantages: significantly reduced beam scattering, higher radiation hardness which leads to improved detector lifetime, and much better collection efficiency. In a previous work, the novel sensor has been described and a global macroscopic dosimetry characterization has been proposed. This provides practical information for the detector daily use but not about the local microscopic knowledge of the sensor. This work therefore presents a micrometric-accuracy charge-collection characterization of this new generation of ultra-thin silicon strip detectors. This goal is reached thanks to a 1060 nm-wavelength micrometric-sized laser that can be positioned relatively to the sensor with a submicron precision for the three different axes. This study gives a much better knowledge of the inefficient areas of the sensor and allows therefore optimization for future designs. & 2013 Elsevier B.V. All rights reserved.
Keywords: Silicon strip detector Radiation therapy Dosimetry Low material budget
1. Introduction Radiation therapy contributes nowadays to the cure of approximately 23% of all cancer patients according to Ref. [14], mostly based on photons or electrons which represents one of the most important and widespread techniques for cancer treatment over the world. Hadrontherapy, which uses massive particles like protons and carbon ions, is gaining increasing interest thanks to its better ballistic precision by exploiting the Bragg Peak effect. After Ref. [1], the progression of the number of patients being treated with protons looks exponential since 1993. For quality assurance (QA) purposes, the beam characterization is made before the patient irradiation. Then the detector is removed to prevent it from degrading the beam through scattering and energy straggling. For optimal treatment, it is necessary to monitor the beam in real time in order to know the actual dose deposited into the patient body. If material budget is not considered during its design, the detector may generate Multiple Coulomb Scattering (MCS) as well as energy straggling and therefore, the lateral penumbra and distal falloff expand which represents a risk for surrounding healthy tissues. Various detectors have been built and
n
Corresponding author. Tel.: +32 486468868. E-mail address: [email protected] (M. Bouterfa).
0168-9002/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2013.05.139
characterized in Refs. [2,3] but they still show relatively high degradation of the beam [4]. To reduce the detector impact on the treatment beam, we designed and fabricated innovating silicon strip detectors over an area as large as 4.5 4.5 cm2 with an ultra low thickness of 20 μm, which represents a material budget as low as 0.02% of radiation length. In addition to a reduced beam scattering, ultra-thin detectors offer the following considerable advantages: a higher radiation hardness [5] which leads to an improved detector lifetime and a much better collection efficiency [6]. In a previous work [15], the novel sensor, irradiated under a proton beam, has been briefly described and a global macroscopic dosimetry characterization has been proposed. The latter characterization provides practical information for the detector daily use, however it does not help to improve the local microscopic knowledge of the sensor. To complete the novel sensor study, we characterized it by performing a micrometric-accuracy mapping of the charge collection using a 1060 nm-wavelength laser. The aim of this paper is to present the measurement results in order to validate the novel structure and to clarify which are the most efficient areas of the sensor in terms of charge collection. Accordingly, the second section gives a description of the sensor and presents the readout system used in the experimental setup. Eventually, it shortly describes the laser irradiation system and its characteristics.
M. Bouterfa et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
The results and discussion section finally reports the collection mapping measurements and discusses it.
2. Materials and methods 2.1. Sensor design and fabrication A cross-section SEM photograph of a 20 μm thick fabricated device is depicted in Fig. 1. In this subsection, we first describe the substrate, then the strips design, thirdly the interstrip insulation and eventually, the thinning step and the back metallization.
2.1.1. Substrate features Thick Silicon On Insulator (SOI) 3-in. wafers have been used as substrates for the device fabrication. Top silicon layer, buried oxide (BOX) and back substrate thicknesses are respectively 20 μm, 0:5 μm and 525 μm. Silicon is boron pre-doped with a resistivity of 10–30 Ω cm, which represents an equivalent impurity level ranging from 1.32 1015 cm−3 to 4.36 1014 cm−3. The p-type substrate has been demonstrated in Refs. [7–9] to have a better radiation hardness as well as a better charge collection efficiency than the n-type one. Furthermore, hadrons irradiation damage in silicon may result in a type inversion for n-type substrates, which is not the case for p-type substrates [10]. Effective impurity level increases with the accumulated ionizing dose [10]. As a consequence, the low impurity level of a high resistivity substrate will be degraded as the accumulated dose increases and therefore represents an unnecessary over cost.
2.1.2. Strips description Schematic cross-section of the SOI device is shown in Fig. 2. In the top silicon layer, 81 N+ arsenic-doped strips are processed with a pitch of 500 μm. Strips length is 4 cm. The implanted width of each strip is 20 μm while the aluminum width is 13 μm. A rectangular guard-ring of 4.2 4.6 cm2 surrounds the 81 strips. The guard ring implant width is 20 μm as well. The implantation was performed through a 45 nm-thick thermal oxide with a 65 keV energy and a 1016 cm−2 dose. Dopants were activated and diffused at 1050 1C for 1 h.
557
2.1.3. Interstrip insulation According to Ref. [11], oxide trapped charges increase with the total ionizing dose to a saturation level of about 1012 cm−2. Consequently, all p-type substrates need to prevent this shorting phenomenon from occurring by positively doping the silicon superficial layer either locally in between strips (p-stops) or over the whole surface (p-spray). Since the p-stop technique needs an additional lithographic step, the simpler p-spray was preferred. According to a simulation performed on ATLAS from the SILVACO suite, the level of acceptor dopants needed to cancel the electron channel is a bit less than 1017 cm−3 since the superficial electron concentration is as weak as about 5 105 cm−3 after implantation. To reach the needed impurity level, boron was implanted with an energy of 13 keV and a dose of 2.5 1012 cm−2 through 45 nm of thermal oxide. Boron and Arsenic dopants diffused during the same process step with the diffusion parameters mentioned above. 2.1.4. Thinning and back metalization A centered square of 4.5 4.5 cm2 of the back substrate and the BOX layer is wet-etched from the backside using TMAH in order to release the top silicon layer as a membrane after coating the strips with the PROTEK B3 protective resist. The purpose of leaving an unthinned framework is to have a device with sufficient robustness to be easily handled without breaking. Eventually, 150 nm of aluminum is then deposited on the overall backside. A fabricated 20 μm thick device seen from the backside is depicted in Fig. 3. 2.2. EmXX readout system The detector signals were read out thanks to the emXX interface (Fig. 4), which has been developed by Ion Beam Applications (IBA) to measure the dose and dose rate within p-type silicon detectors at null bias. The emXX consists of two TERA chips designed by TORINO INFN division and fabricated by WELLHOFERSCANDITRONIX. Each Tera chip reads out, through current-tofrequency converters, up to 64 analog channels simultaneously and converts the integrated charge to a number of counts with good noise performances, sensitivity and radiation hardness [12,13]. Actually, the emXX imposes a 3 mV reverse bias to the p-type detector to ensure the reverse mode biasing since the Tera chip is not functional with forward biasing. 2.3. Laser irradiation system
Fig. 1. Cross-section SEM photograph of a strip on ultra-thin substrate.
The experimental setup is shown in Fig. 5. The sensor (wafer) is glued on the interface PCB which lies on the sub-micron resolution moving platform along X, Y and Z axes. The laser diode is fixed and emits 1060 nm photons perpendicularly to the sensor surface. The equivalent energy of such photons is 1.17 eV, which is just above the silicon band gap. In that way, the excess energy, which will be transferred to phonons and consequently lost as thermal crystalline vibrations, is very low. The wavelength straggle is reported by the manufacturer to be less than 1 nm of full width at half
Fig. 2. Schematic cross-section of the ultra-thin device.
Fig. 3. The 4.5 4.5 cm2 membrane is 20 μm thick.
558
M. Bouterfa et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
0.07
Beam Spot (mm)
0.06 0.05 0.04 0.03 0.02 0.01 21.5
22
22.5
23
23.5
24
24.5
Relative Position (mm)
Fig. 4. EmXX readout system (right) connected to the detector through the printed circuit board (left).
Fig. 6. Longitudinal profile of the laser spot size according to the Knife-Edge method.
Fig. 7. Collected signal in log-scale versus the x position of the beam spot relatively to the central strip with unfocused beam.
The first subsection presents the collection measurements along X-axis and Z-axis in order to correctly position the central strip along X-axis and to focus the beam on the wafer surface. Once the laser spot is localized and focused, the second subsection reports a more precise collection profiling along X-axis in order to extract the diffusion length of minority carriers. 3.1. Laser positioning and focusing measurements
Fig. 5. Experimental setup consisting of the sensor PCB and the laser diode.
maximum. Using the Knife-Edge method, we characterized the longitudinal profile of the spot size in order to find its minimum size. The spot being Gaussian-shaped, we measured its standard deviation for various Z. As can be seen in Fig. 6, the minimum size is 7:5 μm.
3. Results and discussions For clarity purposes, we only report the central strip measurements. According to Ref. [15], interstrip collection standard deviation is indeed as low as 0.68% for 62 MeV protons. We assume that it is still verified for 1060 nm photons. We positioned the sensor in a way that the Z-axis corresponds to the wafer surface axe. The Y- and X-axis respectively correspond to the longitudinal and orthogonal strip directions.
Fig. 7 depicts the collected current on the central strip versus the “x” position of the beam spot. If the beam is focused, its standard deviation size is about 7:5 μm. Because of the strip aluminum shading, which is as a reminder 13 μm wide, the signal should reach a minimum at the strip center. The curve in Fig. 7 does not behave in a such way because the beam is not focused. The collected signal as a function of the “z” position is shown in Fig. 8. We notice that there indeed exists a minimum which illustrates the aluminum shading effect. The best focusing “z” corresponds to the minimizing “z” in Fig. 8. We also observe that the well is as narrow as few hundreds of micrometers. For precise positioning and focusing purposes, x- and z-profiles centered on the minimum were performed iteratively by increasing the spatial resolution at every iteration step. 3.2. Diffusion length extraction Fig. 9 shows the measured collected current (log-scale in arbitrary units) as a function of x-position. Contrary to what can be expected, there are two diffusion lengths instead of one,
M. Bouterfa et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 556–559
559
the highly doped N+ region that significantly degrades the lifetime. As a consequence to the latter phenomenon, the collection efficiency decreases by one order of magnitude in about 25 μm only, as can be seen in Fig. 9. Consequently, the major part of the silicon between two adjacent strips is highly inefficient in terms of collection.
4. Conclusion
Fig. 8. Collected signal in log-scale versus the z-position of the wafer.
In this paper, we present a micrometric-accuracy mapping of the charge collection of a novel ultra-thin silicon detector for hadrontherapy application using a 1060 nm-wavelength laser. The measurement results validate the novel structure and clarify which are the most efficient areas of the sensor in terms of charge collection. In fact, collection efficiency decreases by one order of magnitude at only 25 μm far from the strip which means that the major part of the silicon between two adjacent strips is highly inefficient in terms of collection. References
Fig. 9. Collected current in log-scale in arbitrary units for a beam focused at various x-positions.
meaning that there are two effective minority carrier lifetimes (ELT). Close to the strip the diffusion length is about 8:5 μm, corresponding to an ELT of 0:02 μs. At about 50 μm distance and further, diffusion length and ELT increase to 105 μm and 3 μs, respectively. We first notice that the ELT is more than 150 times lower close to the implanted areas of the strips, which may be explained by
[1] J.-M. Lagniel, Status of the hadrontherapy projects in Europe, in: Proceedings of PAC07, Albuquerque, USA, 2007, p. 127. [2] A. La Rosa, et al., Nuclear Instruments and Methods in Physics Research Section A 565 (2006) 833. [3] N. Givehchi, et al., Nuclear Instruments and Methods in Physics Research Section A 572 (2007) 1094. [4] M. Bouterfa, et al., Towards a new generation of ultra-thin p-type silicon strip detectors for hadrontherapy beam monitoring, in: Proceedings of ANIMMA11, Ghent, Belgium, 2011. [5] D. Menichelli, et al., Nuclear Instruments and Methods in Physics Research Section A 583 (2007) 109. [6] M. Bouterfa, D. Flandre, Journal of Circuits, Systems and Computers, submitted for publication. [7] G. Casse, et al., Nuclear Instruments and Methods in Physics Research Section A 535 (2004) 362. [8] G. Casse, Nuclear Instruments and Methods in Physics Research Section A 487 (2002) 465. [9] G. Casse, IEEE Transactions on Nuclear Science NS-47 (2000) 527. [10] M. Moll, Radiation Tolerant Sensors for Solid State Tracking Detectors, 〈http:// www.cern.ch/rd50〉. [11] G. Pellegrini, et al., Nuclear Instruments and Methods in Physics Research Section A 566 (2006) 360. [12] G. Mazza, et al., IEEE Transactions on Nuclear Sciences NS-52 (2005) 847. [13] A. La, Rosa, et al., Nuclear Instruments and Methods in Physics Research Section A 586 (2008) 270. [14] K. Ando, et al., Relative Biological Effectiveness in Ion Beam Therapy, International Atomic Energy Agency, Technical Reports Series No. 461, Vienna, Austria, 2008, ISBN 978-92-0107807-0. [15] M. Bouterfa, et al., Characterization of ultra-thin silicon strip detectors for hadrontherapy beam monitoring, in: Proceedings of I2MTC2013, Minneapolis, USA, 2013, p. 1088.