- Email: [email protected]
Magnetic Czochralski silicon strip detectors for Super-LHC experiments
Nuclear Instruments and Methods in Physics Research A 636 (2011) S79–S82
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Magnetic Czochralski silicon strip detectors for Super-LHC experiments a ¨ onen ¨ ¨ Esa Tuovinen a,, Jaakko Hark , Panja Luukka a, Teppo Maenp a¨ a¨ a, Henri Moilanen a, b a Ivan Kassamakov , Eija Tuominen a b
Helsinki Institute of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland Department of Micro and Nanosciences, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland
a r t i c l e in fo
abstract
Available online 1 May 2010
High resistivity and high oxygen concentration of silicon wafers can be beneficial for the radiation hardness of silicon detectors. Wafers of Magnetic Czochralski silicon (MCz-Si) can be grown with a resistivity of a few kO cm and with well-controlled, high oxygen concentration. According to the beam test results presented in this paper, n-type MCz-Si bulk, p-strip readout detectors with can be operated with acceptable signal-to-noise ratio up to the irradiation fluence of 1 1015 cm 2 1-MeV neutron equivalent. The improved radiation hardness compared to that of traditional p-in-n Float Zone silicon (p-in-n FZ-Si) detectors can be explained by better electric field distribution inside MCz-Si detectors. The difference between the distributions is clearly shown by Transient Current Technique (TCT) measurements, presented in this paper. Thus, strip detectors made on n-type MCz-Si are a feasible option for the outer tracker layers of the potential upgrade of the Large Hadron Collider (LHC), the Super-LHC. This corresponds approximately 95% of the total area of silicon detectors in the Super-LHC. & 2010 Elsevier B.V. All rights reserved.
Keywords: Silicon Particle detectors Radiation hardness Transient current technique Beam tests
1. Introduction In the potential luminosity upgrade of the CERN Large Hadron Collider (LHC), the Super-LHC, fluences of fast hadrons are expected to increase by an order of magnitude. In the tracking layers of the large LHC experiments, such as ATLAS and CMS, silicon strip detectors are expected to receive fluences up to 1 1015 1-MeV neq/cm2. In the innermost tracking layers made of silicon pixel detectors, the situation is even more challenging as the fluences are expected to exceed 1 1016 1-MeV neq/cm2. Such operating conditions require very strong research and development of silicon detectors. Approximately 95% of the silicon detectors needed for the LHC upgrade will be placed in the layers of strip detectors. Therefore, the chosen silicon strip detector technology must also be cost-effective. Oxygen improves the radiation hardness of silicon detectors after charged irradiations [1]. Oxygen can react with the radiation induced defects in silicon thus reducing the degradation of the electrical properties of silicon detectors. Magnetic Czochralski Silicon (MCz-Si) can be manufactured with high and well determined concentration of oxygen. Many previous studies have shown that for particle detection in high-radiation environment, MCz-Si may offer an advantage with respect to depletion voltage when compared to Float-Zone silicon (FZ-Si) or Diffusion Corresponding author. Tel.: + 358 9 470 22365; fax: + 358 9 470 26080.
E-mail addresses: esa.tuovinen@helsinki.fi, [email protected] (E. Tuovinen). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.089
Oxygenated FZ-Si (DOFZ) [2–4]. MCz-Si is also available in large quantities. Furthermore, most of the commercial manufacturers are familiar with the processing of MCz-Si. In this paper, we present the results of the characterization of silicon pad detectors with Transient Current Technique and Capacitance–Voltage measurements. Furthermore, we present the signal distributions acquired from full-size 768 channel silicon strip detectors measured by a reference telescope in a particle beam. In addition, we present simulation results of the charge collection of a p-in-n MCz-Si detector irradiated up to the fluence of 1 1015 1-MeV neq/cm2.
2. Detector processing and irradiations The strip detectors and pad detectors used in this study were processed at the cleanroom facilities of Micronova, the Research Centre for Micro- and Nanotechnology of the Helsinki University of Technology. Maximum six lithography mask levels were used in the process. The starting material was 4-in. n-type silicon wafers. MCz-Si wafers were 300 mm thick with the nominal resistivity of 900 Ocm. FZ-Si wafers were 285 mm thick with the nominal resistivity of 4 10 kO cm. The crystal orientation of both was /1 0 0S. For the strip devices the p-type implant is segmented and readout. Detailed process description is given in Ref. [5]. Each strip detector had 768 channels with the pitch of 50 mm, the strip width of 10 mm, and the strip length of 3.9 cm. The total area of each strip detector was 4.1 4.1 cm2. Pad
E. Tuovinen et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S79–S82
detectors with an active area of 25 mm2 were fabricated with similar process, but without the mask level patterning bias resistors. The strip detectors were irradiated up to the total fluence of 3 1015 1-MeV neq/cm2 at the University of Karlsruhe and at Universite Catholique de Louvain with 25 MeV protons and with 3–45 MeV neutrons, respectively. The devices were not annealed before characterization with the beam telescope or the TCT setup. Pad detectors were irradiated at the Department of Physics of the University of Helsinki or at CERN Proton Syncrotron (PS) with 9 MeV protons and with 24 GeV/c protons, respectively.
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Effective Doping Concentration (cm–3)
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Capacitance as a function of voltage was measured from irradiated pad detectors in order to determine the detector full depletion voltages after different irradiation fluences. Effective doping concentrations (Neff) of the irradiated detectors were deduced from the full depletion voltages. Fig. 1 presents the evolution of Neff as a function of the radiation fluence for both MCz-Si and Fz-Si detectors. A hardness factor of 5 was used for the scaling of the 9 MeV proton fluences to 1-MeV neutron equivalent fluences. As clearly seen in Fig. 1, as the fluence increases, the changes in Neff are much more severe in Fz-Si than in MCz-Si. It is worth remembering that the increase in the detector depletion voltage, i.e. in the biasing of the detector, has consequences for the power dissipation of the detector. Transient Current Technique (TCT) measurements were performed in order to get an insight of the electric field distribution inside the irradiated silicon pad detectors [7,8]. Measurements were performed using excitation by red laser (wavelength 678 nm) of the cryogenic TCT setup of the CERN RD39 Collaboration [9]. The TCT-signals obtained from MCz-Si and Fz-Si detectors irradiated with 24 GeV/c protons to the fluence of 8.1 1014 1-MeV neq/cm2 are presented in Fig. 2. The diode was illuminated with laser from the p + side. The TCT measurements were performed at 100 V above the detector full depletion or at 500 V, depending of which of the two options provided lower biasing voltage for the detector to be measured. The data presented is not trapping corrected. As seen in Fig. 2, the two detectors undergone the same irradiation and annealing history, show opposite trends in current transient responses. Even without trapping correction, it is apparent that at given fluence, 8.1 1014 1-MeV neq/cm2, the electric field distribution in MCz-Si detector is more profound on
7
0.5
4 3
1.5 x 10–8
Fig. 2. TCT pulse shapes measured from n-type pad detectors irradiated with 24 GeV/c protons to the fluence of 8.1 1014 1-MeV neq/cm2. Diodes were illuminated from the p + side with a red laser ðlÞ ¼ 678 nm. Bias voltages were 475 and 500 V for MCz-Si and Fz-Si detectors, respectively. Measurements were performed at 240 K. The curves are scaled in order to help the comparison between the shapes of the pulses.
the front side with respect to the FZ-Si device. This is an advantage of the MCz-Si material, because at very high fluences, the electric field and thus the charge collection is strongly enhanced by the weighting potential on the segmented side of the detector.
4. Full systems with MCz strip sensors In order to study the performance of Cz-Si strip detectors in full-chain system tests, the detectors were placed in 225 GeV muon beam at CERN H2 station. Silicon Beam Telescope (SiBT) was used as reference telescope. The SiBT consists of eight layers of Hamamatsu HPK silicon detectors with 60 mm readout strip pitch and intermediate strips [10]. The detectors are attached to APV25 readout [11] and CMS data acquisition system. The SiBT has an active area of 4 4 cm2 and is able to measure reference tracks with an accuracy of 4 mm. The telescope is described in detail in Ref. [12] and the SiBT data analysis is explained in detail in Ref. [13]. The temperature of the detectors was between 8 and 10 3 C during the tests. However, the detectors irradiated up to the fluence of 3 1015 1-MeV neq/cm2 were tested at the temperature of 30 3 C using an external cold finger. Fig. 3 presents the signal distributions measured from MCz-Si detectors that were irradiated with three different fluences, up to 3 1015 1-MeV neq/cm2. For reference, a signal measured from a non-irradiated Fz-Si detector is also shown. The signal counts have been scaled in order to help to compare the amount of collected charge. The detector irradiated up to the fluence of to 3 1015 1-MeV neq/cm2 is no longer operating properly. On the other hand, the detector irradiated to the fluence of 1 1015 1-MeV neq/cm2 is still collecting charge with the efficiency of about 50%. The average noise for the detectors is less than 2 ADC, as reported in Ref. [6]. Thus, the signal-to-noise ratio after the fluence of 1 1015 1-MeV neq/cm2 is still over 10.
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5. Modeling and simulation of the charge collection in irradiated detectors
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Fig. 1. Effective doping concentration Neff as a function of fluence for Fz-Si and MCz-Si pad detectors irradiated with 9 MeV protons.
In order to get information for the simulation of the charge collection in irradiated detectors, the MCz-Si detector irradiated with 9 MeV protons to the fluence of 1 1015 1-MeV neq/cm2 was measured using red laser by the CERN RD39 cryogenic-TCT setup.
E. Tuovinen et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S79–S82
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The sample was irradiated from the p + contact. Data presented in Fig. 4 is not trapping corrected. Since the TCT-signal, i(t), is proportional to the electric field, E(x), the red laser reveals the electric field inside the detector. The data shows clearly Double Junction behavior [14]. The second peak starts to take over when the voltage increases. This means that there is electric field on the segmented front side of the strip detector. This electric field (1st peak) is enforced by the Weighting Field-effect allowing higher Charge Collection Efficiency (CCE). The Double Junction behavior is not observed in Fz-Si detectors used in this study. During the SiBT beam tests it was observed that the charge collected by irradiated MCz-Si detectors depends on the detector bias voltage. To explain this result, charge collection was simulated using a Matlab program. The program takes into account the trapping by radiation induced defects, namely 7 and 2 ns for electrons and holes, respectively, and the effect of weighting field [15,16]. The shape of the electric field distribution was given as a program input parameter. Double peak electric field distribution was used. The electric field distributions used in the simulation are presented in Fig. 5. Zero electric field at detector bulk was chosen for simulation. Using zero field decreases collected charge at lower voltages, but has little effect to the shape of the curve. Overall using zero field leads to slightly higher trapping times than using non-zero electric field at the detector bulk. The simulation scales the magnitude of the electric field and therefore the magnitudes of the curves are given in arbitrary units. The experimental SiBT-data can be split into two parts according to the detector bias voltage. At the lower voltages ranging from 0 to 400 V, the data can be fitted and explained by assuming the electric field with double peak structure so that the electric field is split in the ratio of 35:65 for front and back junctions, respectively. When the detector bias voltage reaches 400 V, the model has to be modified in order to fit the increase in collected charge from 10 to 20 ADC channels. Adjusting trapping parameters did not work since the increase in trapping time required is very high. However, the experimental data can be fitted by the electric field gradually shifting towards the front contact. Above 500 V, the electric field is stronger in the front contact than in the back contact. The TCT results presented in Fig. 4 also indicate that at high operating voltages the electric field might be higher in the front contact. This shift from back contact to front contact coincides with the detector ‘‘depletion’’. The charge collection efficiency of a MCz-Si detector irradiated up to the fluence of 1 1015 1-MeV neq/cm2 was also deduced from a
8.5
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Fig. 4. TCT pulse shapes measured from n-type MCz-Si pad detectors irradiated with 9 MeV protons to the fluence of 1 1015 1-MeV neq/cm2. p + contact of the detector was illuminated with red laser ðlÞ ¼ 678 nm during the measurement.
8
x 104 40V 390V 540V 590V
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Fig. 3. Signal distributions of irradiated MCz-Si detectors and FZ-Si reference detector. The data has been collected from different test beam runs and therefore the number of recorded events used in off-line data analysis is different. Thus, the y-axis has been scaled from 0 to 100. The operating voltages were 600 and 400 V for the irradiated detectors and the reference detector, respectively.
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pad detector by TCT setup and CCE method [17] using infrared laser (1060 nm) (IR laser diode MCz-Si data set). IR laser TCT data coincides well with the non-clustered SiBT data. Differences between traditional clustering and non-clustered data have been discussed in detail in Ref. [18]. The experimental data and the simulation results are shown in Fig. 6.
6. Conclusions In this study, we have characterized irradiated n-type Float Zone silicon (FZ-Si) and Magnetic Czochralski silicon (MCz-Si) detectors. We have shown that in hard radiation environment, Mcz-Si detectors are more resistive to the changes in the detector depletion voltage and in the effective doping concentration. Furthermore, unlike FZ-Si detectors, MCz-Si detectors do not experience space charge sign inversion even at high radiation fluences. We have measured signal distributions from full-size 768 channel silicon strip detectors by SiBT reference telescope at CERN H2 particle beam. We have demonstrated that p-in-n MCz-Si detector irradiated up to the fluence of 1 1015 1-MeV neq/cm2 is still operational for particle detection, with efficiency of about
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Strip detectors made on p-in-n MCz-Si detectors are a feasible option for the radiation environment of the outer tracker layers of the potential upgrade of the LHC. This corresponds approximately 95% of the total area of silicon detectors in the Super-LHC. In addition, n-type MCz-Si is a cost-effective solution.
25
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Acknowledgments 10
This study has been partially funded by the Academy of Finland. The research has been done in the framework of CERN RD39 and RD50 collaborations. Authors would like to thank all the people and institutes who have helped with detector irradiation, sample preparation and beam tests.
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Voltage [V] Fig. 6. Charge collection in irradiated MCz-Si detectors, experimental data and simulation results. The experimental data and simulations correspond when assuming double peak electric field distribution.
50% and with signal to noise ratio of more than 10 using APV25 readout ASIC. We have presented simulation results of the charge collection of a MCz-Si detector irradiated up to the fluence of 1 1015 1MeV neq/cm2. We have shown that our experimental results can be explained only by assuming double peak electric field distribution inside the detector. Having a strong electric field near collecting junction is beneficial for the detector charge collection. The double peak distribution has been observed only in MCz-Si detectors.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
A. Ruzin, IEEE Trans. Nucl. Sci. NS-46 (1999) 1310. D. Creanza, et al., Nucl. Instr. and Meth. A 579 (2007) 608. G. Pellegrini, et al., Nucl. Instr. and Meth. A 552 (2005) 27. ¨ onen, ¨ J. Hark et al., Nucl. Instr. and Meth. A 518 (2004) 346. ¨ onen, ¨ J. Hark et al., Nucl. Instr. and Meth. A 514 (2003) 173. P. Luukka, et al., Nucl. Instr. and Meth. A 604 (2009) 254. V. Eremin, Nucl. Instr. and Meth. A 372 (1996) 388. V. Eremin, et al., Nucl. Instr. and Meth. A 372 (1996) 188. ¨ onen, ¨ J. Hark et al., Nucl. Instr. and Meth. A 581 (2007) 347. M. Demarteau, et al., D0 note 4308 (2003). M. French, et al., Nucl. Instr. and Meth. A 466 (2001) 359. ¨ ¨ et al., Nucl. Instr. and Meth. A 593 (2008) 523. T. Maenp a¨ a, M.J. Kortelainen, et al., Nucl. Instr. and Meth. A 602 (2009) 600. E. Verbitskaya, et al., Nucl. Instr. and Meth. A 583 (2007) 77. T.J. Brodbeck, et al., Nucl. Instr. and Meth. A 455 (2000) 645. G. Kramberger, et al., Nucl. Instr. and Meth. A 476 (2002) 645. B. Dezillie, et al., Nucl. Instr. and Meth. A 452 (2000) 440. ¨ ¨ et al., 2009, in: IEEE Nuclear Science Symposium Conference T. Maenp a¨ a, Record 832.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Magnetic Czochralski silicon strip detectors for Super-LHC experiments a ¨ onen ¨ ¨ Esa Tuovinen a,, Jaakko Hark , Panja Luukka a, Teppo Maenp a¨ a¨ a, Henri Moilanen a, b a Ivan Kassamakov , Eija Tuominen a b
Helsinki Institute of Physics, P.O. Box 64, FI-00014 University of Helsinki, Finland Department of Micro and Nanosciences, Helsinki University of Technology, P.O. Box 3500, FI-02015 TKK, Finland
a r t i c l e in fo
abstract
Available online 1 May 2010
High resistivity and high oxygen concentration of silicon wafers can be beneficial for the radiation hardness of silicon detectors. Wafers of Magnetic Czochralski silicon (MCz-Si) can be grown with a resistivity of a few kO cm and with well-controlled, high oxygen concentration. According to the beam test results presented in this paper, n-type MCz-Si bulk, p-strip readout detectors with can be operated with acceptable signal-to-noise ratio up to the irradiation fluence of 1 1015 cm 2 1-MeV neutron equivalent. The improved radiation hardness compared to that of traditional p-in-n Float Zone silicon (p-in-n FZ-Si) detectors can be explained by better electric field distribution inside MCz-Si detectors. The difference between the distributions is clearly shown by Transient Current Technique (TCT) measurements, presented in this paper. Thus, strip detectors made on n-type MCz-Si are a feasible option for the outer tracker layers of the potential upgrade of the Large Hadron Collider (LHC), the Super-LHC. This corresponds approximately 95% of the total area of silicon detectors in the Super-LHC. & 2010 Elsevier B.V. All rights reserved.
Keywords: Silicon Particle detectors Radiation hardness Transient current technique Beam tests
1. Introduction In the potential luminosity upgrade of the CERN Large Hadron Collider (LHC), the Super-LHC, fluences of fast hadrons are expected to increase by an order of magnitude. In the tracking layers of the large LHC experiments, such as ATLAS and CMS, silicon strip detectors are expected to receive fluences up to 1 1015 1-MeV neq/cm2. In the innermost tracking layers made of silicon pixel detectors, the situation is even more challenging as the fluences are expected to exceed 1 1016 1-MeV neq/cm2. Such operating conditions require very strong research and development of silicon detectors. Approximately 95% of the silicon detectors needed for the LHC upgrade will be placed in the layers of strip detectors. Therefore, the chosen silicon strip detector technology must also be cost-effective. Oxygen improves the radiation hardness of silicon detectors after charged irradiations [1]. Oxygen can react with the radiation induced defects in silicon thus reducing the degradation of the electrical properties of silicon detectors. Magnetic Czochralski Silicon (MCz-Si) can be manufactured with high and well determined concentration of oxygen. Many previous studies have shown that for particle detection in high-radiation environment, MCz-Si may offer an advantage with respect to depletion voltage when compared to Float-Zone silicon (FZ-Si) or Diffusion Corresponding author. Tel.: + 358 9 470 22365; fax: + 358 9 470 26080.
E-mail addresses: esa.tuovinen@helsinki.fi, [email protected] (E. Tuovinen). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.04.089
Oxygenated FZ-Si (DOFZ) [2–4]. MCz-Si is also available in large quantities. Furthermore, most of the commercial manufacturers are familiar with the processing of MCz-Si. In this paper, we present the results of the characterization of silicon pad detectors with Transient Current Technique and Capacitance–Voltage measurements. Furthermore, we present the signal distributions acquired from full-size 768 channel silicon strip detectors measured by a reference telescope in a particle beam. In addition, we present simulation results of the charge collection of a p-in-n MCz-Si detector irradiated up to the fluence of 1 1015 1-MeV neq/cm2.
2. Detector processing and irradiations The strip detectors and pad detectors used in this study were processed at the cleanroom facilities of Micronova, the Research Centre for Micro- and Nanotechnology of the Helsinki University of Technology. Maximum six lithography mask levels were used in the process. The starting material was 4-in. n-type silicon wafers. MCz-Si wafers were 300 mm thick with the nominal resistivity of 900 Ocm. FZ-Si wafers were 285 mm thick with the nominal resistivity of 4 10 kO cm. The crystal orientation of both was /1 0 0S. For the strip devices the p-type implant is segmented and readout. Detailed process description is given in Ref. [5]. Each strip detector had 768 channels with the pitch of 50 mm, the strip width of 10 mm, and the strip length of 3.9 cm. The total area of each strip detector was 4.1 4.1 cm2. Pad
E. Tuovinen et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S79–S82
detectors with an active area of 25 mm2 were fabricated with similar process, but without the mask level patterning bias resistors. The strip detectors were irradiated up to the total fluence of 3 1015 1-MeV neq/cm2 at the University of Karlsruhe and at Universite Catholique de Louvain with 25 MeV protons and with 3–45 MeV neutrons, respectively. The devices were not annealed before characterization with the beam telescope or the TCT setup. Pad detectors were irradiated at the Department of Physics of the University of Helsinki or at CERN Proton Syncrotron (PS) with 9 MeV protons and with 24 GeV/c protons, respectively.
0.9
FZ n–type MCZ n–type
0.8 0.7 TCT Signal [arb]
S80
0.6 0.5 0.4 0.3 0.2 0.1 0 0
3. Characterization
Effective Doping Concentration (cm–3)
x 1012
6
FZ CZ fit CZ fit FZ
5
1 Time [s]
Capacitance as a function of voltage was measured from irradiated pad detectors in order to determine the detector full depletion voltages after different irradiation fluences. Effective doping concentrations (Neff) of the irradiated detectors were deduced from the full depletion voltages. Fig. 1 presents the evolution of Neff as a function of the radiation fluence for both MCz-Si and Fz-Si detectors. A hardness factor of 5 was used for the scaling of the 9 MeV proton fluences to 1-MeV neutron equivalent fluences. As clearly seen in Fig. 1, as the fluence increases, the changes in Neff are much more severe in Fz-Si than in MCz-Si. It is worth remembering that the increase in the detector depletion voltage, i.e. in the biasing of the detector, has consequences for the power dissipation of the detector. Transient Current Technique (TCT) measurements were performed in order to get an insight of the electric field distribution inside the irradiated silicon pad detectors [7,8]. Measurements were performed using excitation by red laser (wavelength 678 nm) of the cryogenic TCT setup of the CERN RD39 Collaboration [9]. The TCT-signals obtained from MCz-Si and Fz-Si detectors irradiated with 24 GeV/c protons to the fluence of 8.1 1014 1-MeV neq/cm2 are presented in Fig. 2. The diode was illuminated with laser from the p + side. The TCT measurements were performed at 100 V above the detector full depletion or at 500 V, depending of which of the two options provided lower biasing voltage for the detector to be measured. The data presented is not trapping corrected. As seen in Fig. 2, the two detectors undergone the same irradiation and annealing history, show opposite trends in current transient responses. Even without trapping correction, it is apparent that at given fluence, 8.1 1014 1-MeV neq/cm2, the electric field distribution in MCz-Si detector is more profound on
7
0.5
4 3
1.5 x 10–8
Fig. 2. TCT pulse shapes measured from n-type pad detectors irradiated with 24 GeV/c protons to the fluence of 8.1 1014 1-MeV neq/cm2. Diodes were illuminated from the p + side with a red laser ðlÞ ¼ 678 nm. Bias voltages were 475 and 500 V for MCz-Si and Fz-Si detectors, respectively. Measurements were performed at 240 K. The curves are scaled in order to help the comparison between the shapes of the pulses.
the front side with respect to the FZ-Si device. This is an advantage of the MCz-Si material, because at very high fluences, the electric field and thus the charge collection is strongly enhanced by the weighting potential on the segmented side of the detector.
4. Full systems with MCz strip sensors In order to study the performance of Cz-Si strip detectors in full-chain system tests, the detectors were placed in 225 GeV muon beam at CERN H2 station. Silicon Beam Telescope (SiBT) was used as reference telescope. The SiBT consists of eight layers of Hamamatsu HPK silicon detectors with 60 mm readout strip pitch and intermediate strips [10]. The detectors are attached to APV25 readout [11] and CMS data acquisition system. The SiBT has an active area of 4 4 cm2 and is able to measure reference tracks with an accuracy of 4 mm. The telescope is described in detail in Ref. [12] and the SiBT data analysis is explained in detail in Ref. [13]. The temperature of the detectors was between 8 and 10 3 C during the tests. However, the detectors irradiated up to the fluence of 3 1015 1-MeV neq/cm2 were tested at the temperature of 30 3 C using an external cold finger. Fig. 3 presents the signal distributions measured from MCz-Si detectors that were irradiated with three different fluences, up to 3 1015 1-MeV neq/cm2. For reference, a signal measured from a non-irradiated Fz-Si detector is also shown. The signal counts have been scaled in order to help to compare the amount of collected charge. The detector irradiated up to the fluence of to 3 1015 1-MeV neq/cm2 is no longer operating properly. On the other hand, the detector irradiated to the fluence of 1 1015 1-MeV neq/cm2 is still collecting charge with the efficiency of about 50%. The average noise for the detectors is less than 2 ADC, as reported in Ref. [6]. Thus, the signal-to-noise ratio after the fluence of 1 1015 1-MeV neq/cm2 is still over 10.
2 1
5. Modeling and simulation of the charge collection in irradiated detectors
0 0
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1
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2
2.5
1 MeV Neutron Equivalent Fluence (cm–2)
3 x 1014
Fig. 1. Effective doping concentration Neff as a function of fluence for Fz-Si and MCz-Si pad detectors irradiated with 9 MeV protons.
In order to get information for the simulation of the charge collection in irradiated detectors, the MCz-Si detector irradiated with 9 MeV protons to the fluence of 1 1015 1-MeV neq/cm2 was measured using red laser by the CERN RD39 cryogenic-TCT setup.
E. Tuovinen et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S79–S82
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The sample was irradiated from the p + contact. Data presented in Fig. 4 is not trapping corrected. Since the TCT-signal, i(t), is proportional to the electric field, E(x), the red laser reveals the electric field inside the detector. The data shows clearly Double Junction behavior [14]. The second peak starts to take over when the voltage increases. This means that there is electric field on the segmented front side of the strip detector. This electric field (1st peak) is enforced by the Weighting Field-effect allowing higher Charge Collection Efficiency (CCE). The Double Junction behavior is not observed in Fz-Si detectors used in this study. During the SiBT beam tests it was observed that the charge collected by irradiated MCz-Si detectors depends on the detector bias voltage. To explain this result, charge collection was simulated using a Matlab program. The program takes into account the trapping by radiation induced defects, namely 7 and 2 ns for electrons and holes, respectively, and the effect of weighting field [15,16]. The shape of the electric field distribution was given as a program input parameter. Double peak electric field distribution was used. The electric field distributions used in the simulation are presented in Fig. 5. Zero electric field at detector bulk was chosen for simulation. Using zero field decreases collected charge at lower voltages, but has little effect to the shape of the curve. Overall using zero field leads to slightly higher trapping times than using non-zero electric field at the detector bulk. The simulation scales the magnitude of the electric field and therefore the magnitudes of the curves are given in arbitrary units. The experimental SiBT-data can be split into two parts according to the detector bias voltage. At the lower voltages ranging from 0 to 400 V, the data can be fitted and explained by assuming the electric field with double peak structure so that the electric field is split in the ratio of 35:65 for front and back junctions, respectively. When the detector bias voltage reaches 400 V, the model has to be modified in order to fit the increase in collected charge from 10 to 20 ADC channels. Adjusting trapping parameters did not work since the increase in trapping time required is very high. However, the experimental data can be fitted by the electric field gradually shifting towards the front contact. Above 500 V, the electric field is stronger in the front contact than in the back contact. The TCT results presented in Fig. 4 also indicate that at high operating voltages the electric field might be higher in the front contact. This shift from back contact to front contact coincides with the detector ‘‘depletion’’. The charge collection efficiency of a MCz-Si detector irradiated up to the fluence of 1 1015 1-MeV neq/cm2 was also deduced from a
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Fig. 4. TCT pulse shapes measured from n-type MCz-Si pad detectors irradiated with 9 MeV protons to the fluence of 1 1015 1-MeV neq/cm2. p + contact of the detector was illuminated with red laser ðlÞ ¼ 678 nm during the measurement.
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Fig. 3. Signal distributions of irradiated MCz-Si detectors and FZ-Si reference detector. The data has been collected from different test beam runs and therefore the number of recorded events used in off-line data analysis is different. Thus, the y-axis has been scaled from 0 to 100. The operating voltages were 600 and 400 V for the irradiated detectors and the reference detector, respectively.
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Depth (cm) Fig. 5. Shapes of the electric field distributions used in the simulation of the charge collection in irradiated detectors.
pad detector by TCT setup and CCE method [17] using infrared laser (1060 nm) (IR laser diode MCz-Si data set). IR laser TCT data coincides well with the non-clustered SiBT data. Differences between traditional clustering and non-clustered data have been discussed in detail in Ref. [18]. The experimental data and the simulation results are shown in Fig. 6.
6. Conclusions In this study, we have characterized irradiated n-type Float Zone silicon (FZ-Si) and Magnetic Czochralski silicon (MCz-Si) detectors. We have shown that in hard radiation environment, Mcz-Si detectors are more resistive to the changes in the detector depletion voltage and in the effective doping concentration. Furthermore, unlike FZ-Si detectors, MCz-Si detectors do not experience space charge sign inversion even at high radiation fluences. We have measured signal distributions from full-size 768 channel silicon strip detectors by SiBT reference telescope at CERN H2 particle beam. We have demonstrated that p-in-n MCz-Si detector irradiated up to the fluence of 1 1015 1-MeV neq/cm2 is still operational for particle detection, with efficiency of about
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E. Tuovinen et al. / Nuclear Instruments and Methods in Physics Research A 636 (2011) S79–S82
Strip detectors made on p-in-n MCz-Si detectors are a feasible option for the radiation environment of the outer tracker layers of the potential upgrade of the LHC. This corresponds approximately 95% of the total area of silicon detectors in the Super-LHC. In addition, n-type MCz-Si is a cost-effective solution.
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Acknowledgments 10
This study has been partially funded by the Academy of Finland. The research has been done in the framework of CERN RD39 and RD50 collaborations. Authors would like to thank all the people and institutes who have helped with detector irradiation, sample preparation and beam tests.
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References
Voltage [V] Fig. 6. Charge collection in irradiated MCz-Si detectors, experimental data and simulation results. The experimental data and simulations correspond when assuming double peak electric field distribution.
50% and with signal to noise ratio of more than 10 using APV25 readout ASIC. We have presented simulation results of the charge collection of a MCz-Si detector irradiated up to the fluence of 1 1015 1MeV neq/cm2. We have shown that our experimental results can be explained only by assuming double peak electric field distribution inside the detector. Having a strong electric field near collecting junction is beneficial for the detector charge collection. The double peak distribution has been observed only in MCz-Si detectors.
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