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The recent development and study of silicon photomultiplier
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532 www.elsevier.com/locate/nima
The recent development and study of silicon photomultiplier Valeri Saveliev Obninsk State University of Nuclear Engineering, Studgorodsok 1, Obninsk, Russia Available online 25 August 2004
Abstract Recent developments and results from the study of a Silicon Solid State Photomultiplier (Si-PM) are presented. The basis of this new type of photodetector is a fine structure of microcells operating in the Geiger mode with an internal gain greater than 106 : Common signal output allows for the detector to be operated in the proportional mode, and to reach a dynamic range of 1:5 103 : Such photodetectors have shown single photon response at room temperature with a fast timing of 100 ps. They are compact, robust and non-sensitive to magnetic fields. Results show the detection of low-intensity light in single photon mode and the detection of minimal ionizing particles using a scintillation tile for hadron calorimetry. The silicon photomultiplier is suitable for wide application in scintillation calorimetry, medical application, etc. r 2004 Published by Elsevier B.V.
1. Introduction Silicon avalanche structures with Geiger mode operation, i.e., at a bias voltage exceeding the breakdown voltage, can be used for single photon detection if noise conditions are optimized (see for example [1]). The intrinsic gain of the avalanche structure when operated in the Geiger mode is of the order of 105 2106 and higher. The noise conditions of the Si avalanche structures are defined by the dark count rate as a result of breakdown events with a signal amplitude Corresponding author. Tel.: +49-40-8998-3276; fax: +49-
40-8998-4306. E-mail address: [email protected] (V. Saveliev). 0168-9002/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.nima.2004.07.184
equivalent to the photoelectron signal (Geiger mode operation) from thermally generated carriers in the depletion region of the avalanche structure. This gives the main limitation of increasing the sensitive area of Si avalanche structures operated in single photon counting mode at room temperature. Two main methods can be used for the reduction of the dark counting rate in Si avalanche structures:
Operation at low temperature [2], minimization of the depletion volume of the structure at room temperature [1].
These methods give the possibility to reduce the dark count rate in Si avalanche structures below the kHz level.
ARTICLE IN PRESS V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532
The avalanche process in Geiger mode operation in Si is not a self-quenching process which means that it is necessary to introduce a quenching mechanism. Modern technology gives the possibility to produce the Si avalanche microcells with an integrated quenching mechanism based on a Metal–Resistor–Semiconductor structure, where the precise resistive elements are embedded for each individual microcell and provides effective feedback for stabilization and quenching of the avalanche process [3]. Finally, the existing technology allows for the production of combinations of large numbers of microcells in very fine structures on a common substrate with common electrodes. Such Si avalanche structures provide a proportional mode operation for detecting a low flux of light photons and is known as a silicon photomultiplier.
2. Structure of silicon photomultiplier and principle of operation The schematic cross-section of a silicon photomultiplier is shown in Fig. 1. The basis of the microcells is a reach-through avalanche structure nþ ppþ [4]. On top of the avalanche microstructure is placed a semitransparent electrode, which can have an antireflection coating. The depletion region is represented by a thin nþ area and p area, of total thickness
Fig. 1. Cross-section of a silicon photomultiplier structure.
529
527 mm and is illuminated by photons from above. There are p type layers of different doping levels adjacent to the nþ layer to suitably modify the field distribution across the structure. The first is a thin p type layer, and the second is a heavily doped pþ layer. The absorption of photons, and hence photogeneration, takes place mainly in the p-layer. The nearly uniform field here separates the electron– hole pairs and drifts them at velocities near saturation towards the nþ and pþ sides, respectively. When the drifting electrons reach the p layer near nþ ; they experience even greater fields and are accelerated by the high fields to sufficiently large kinetic energies to further cause impact ionization and release more electron–hole pairs which leads to an avalanche of impact ionization processes and provides an internal gain of amplification. The resistive layer on the top of the nþ layer is an important feature of the avalanche microcell structure with Geiger mode operation and provides a negative feedback in the local area of multiplication (quenching mechanism). The avalanche process increases the current through a resistive layer and a charge distribution accumulation on the Si-resistive layer interface. The result is a redistribution of the potential in the structure and an increasing electric field of opposite direction, which screens the initial electric field. The negative feedback produced causes a deceleration of the avalanche process and its termination. The resistive layer negative feedback is of a local nature due to very low tangential conductivity of the resistive layer. Modern technology allows for the production of very fine avalanche microstructures on a common substrate. The microphotograph in Fig. 2 shows the topology of the microcells on the Si substrate. The total number of microcells on the tested photodetector with a sensitive area of 1 mm2 is 1440. This value defines the dynamic range of the photodetector. All microcells are identical, independent and operate in single photon detection mode; this will be illustrated below. The output signal is defined as the sum of the Geiger mode signals from microcells triggered by the initial flux of photons.
ARTICLE IN PRESS 530
V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532
requires an increase of the thickness in order to maximize the absorption, but it is necessary to minimize the thickness of the depletion area in order to reduce the dark count rate. The absorption coefficient of light in Si depends on its wavelength and for l ¼ 400 nm the absorption coefficient is 5:4 106 cm1 : Therefore the thickness required to absorb 99.9 percent of the light is small ð2:33 mmÞ: To optimize the sensitivity in the green region of light a depletion region of 5 mm was chosen which gives the possibility to use a low resistive Si and low bias voltage. The total efficiency including the geometrical efficiency of Si-PM for the green range of the visible spectrum is estimated as 24%. Fig. 2. A microphotograph of the topology of the microcells on the Si substrate.
4. Noise consideration (dark count rate of Si-PM) 3. Efficiency of silicon photomultiplier The efficiency of Si-PM is a product of several factors and depends on the geometrical efficiency, absorption efficiency and triggering (Geiger process) probability E SiPM ¼ GEOM ð1 RÞð1 eax ÞGM
(1)
where GEOM - geometry efficiency of the sensitive area, GM - efficiency of Geiger process triggering in Si, R - reflection coefficient, a - coefficient of absorption. The geometrical efficiency is defined by topology and technological processes, which is equal to 0.6 for the present photodetectors. The quantum efficiency of the sensitive area is defined by the intrinsic QE of Si, the thickness of layers on top of the structure and the thickness of the depletion area and can be optimized for specific applications. The fraction of light transmitted to the sensitive volume is defined by layers on the top and the resistive layer and has been optimized for light in the green range (left edge of sensitive spectra). To improve the sensitivity in the short light wave band it is necessary to optimize the top contact technology. The thickness of the sensitive volume is defined by two factors. Efficient absorption of photons
To limit the dark count rate the sensitive area of the Si-PM is chosen as 1 mm2 and the number of microcells is 1440 with a size of 30 20 mm: With an optimized thickness of the depleted region of the Si-PM the total dark count rate of the photodetector structure at the level of a single photoelectron is 400 kHz at room temperature.
5. Characteristics of silicon photomultiplier The main characteristics of the Si-PMs used for the evaluation of the performance are shown in Table 1.
6. Silicon photomultiplier performance evaluation The single photon detection performance was tested with pulsed LEDs emitting at 470 nm peak wavelength, triggered over a gate of 50 ns. The low photon flux spectra detected by the silicon photomultiplier are shown in Fig. 3 for one photoelectron (a) and 3 photoelectrons (b), respectively. The spectrum shows the pedestal of the photodetector and photoelectron peaks for 1; 2; 3; . . . photons. The spectrum clearly shows the excellent single photon resolution and can give an estimation of the uniformity of the microcells of the
ARTICLE IN PRESS V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532 Table 1 Parameter
Value
Sensitive area Number of microcells Amplification gain of microcells Efficiency (green range of light) Fast timing (rising time) Recovery time Bias voltage Magnetic field sensitivity Mechanical properties
1 mm2 1440 4 106 24% 100 ps 100 ns 51.5 V No Possible mechanical contact
531
gain of amplification at a level of less than 25% FWHM. In order to demonstrate the feasibility of developing a high-quality TESLA hadron calorimeter and the possibility of calibration by minimal ionizing particles (mip) with scintillation tiles equipped with Si-PM the test measurements were performed. The goal is to develop a high granularity calorimeter systems working in high magnetic fields up to 4–5 T [5]. The main requirements of the photodetectors for TESLA hadron calorimeter are the possibility to detect the low-intensity flux of photons for calibration purposes and a wide dynamic range of photon flux detection in hadron shower measurements [6]. The sensitive element of the calorimeter structure is a 5 5 cm and 5 mm thick scintillation tile (BicronBC-408) with WLS readout fiber (Kuraray Y11) of 1 mm diameter. The quarter loop of the WLS fiber was embedded in a plastic body and one end was connected to the Si-PM as shown in Fig. 4.
Fig. 3. The low photon flux spectra detected by silicon photomultipliers. (a) Mean of one photoelectron (b) mean of three photoelectrons.
silicon photomultiplier. Consider the single photon peak of the spectra. The contribution to the statistics of this peak is the statistics from signals generated by randomly distributed photons over the sensitive area of the silicon photomultiplier, i.e., over 1440 microcells. The width of this peak gives the estimation of the variation of the
Fig. 4. The sensitive element of a hadron calorimeter structure with silicon photomultipliers.
ARTICLE IN PRESS 532
V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532
and more detailed studies will be presented in future.
7. Conclusion
(a)
The design and technology to produce this novel type of Silicon avalanche photodetectors with Geiger mode operation based on a Metal–Resistor–Semiconductor Structure (Si-PM) were developed and the latest test measurements have shown stable results. Si-PM characteristics are already suitable for the requirements of the operation in the TESLA hadron scintillation tile calorimeter system with a strong magnetic field and includes the possibility of calibration with mip signals. Novel types of avalanche photodetectors could be interesting for applications in other areas, e.g., for detecting low photon flux, with fast time resolution as in Positron Emission Tomography (PET), for scintillation detector system and others.
(b) Fig. 5. Test result spectra with cosmic muons (mip signal). (a) Without mirror at the end of the WLS fiber, (b) with mirror at the end of the WLS fiber.
Fig. 5 shows the test result of spectra with cosmic muons as an illustration of the possibility for precision calibration of individual tiles of the TESLA hadron calorimeter. Fig. 5a shows the spectrum of low flux photons, detected by Si-PM without mirrors at the other end of WLS. The spectrum shows the pedestal of the photodetector and clearly shows the single photon spectrum mode. Fig. 5b shows the spectrum of mip signals with mirrors at the other end of WLS fiber, which gives significant improvement in light yield of the scintillation tile. These results are shown as an illustration of a real application of silicon photomultipliers
References [1] S. Vasile, P. Gothoskar, D. Sdrulla, R. Farrel, Photon detection with high gain avalanche photodiode arrays, IEEE Trans. Nucl. Sci. NS-45 (1998). [2] A. Bross, E. Flattum, D. Lincoln, S. Grunendahl, J. Warchol, M. Wayne, P. Padley, Characterisation and performance of visible light photon counters (VLPC for the upgraded D) detector at the Fermilab Tevatron, Nucl. Instr. and Meth. A 477 (2002) 172. [3] V. Saveliev, V. Golovin, Silicon avalanche photodiodes on the base of metal–resistor–semiconductor (MRS) structures, Nucl. Instr. and Meth. A 442 (2000) 223. [4] V. Saveliev, V. Golovin, Novel type of avalanche photodetector with Geiger Mode operation, Nucl. Instr. and Meth. A 518 (2004) 560. [5] TESLA Technical Design Report, DESY Polymers, Vol. 1, Elsevier, Amsterdam, 1987 (Chapter 5). [6] V. Korbel, V. Morgunov, V. Saveliev, Upcoming R&D, design and construction studies for the HCAL tile calorimeter, LC-DET-2001-050, 2001.
Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532 www.elsevier.com/locate/nima
The recent development and study of silicon photomultiplier Valeri Saveliev Obninsk State University of Nuclear Engineering, Studgorodsok 1, Obninsk, Russia Available online 25 August 2004
Abstract Recent developments and results from the study of a Silicon Solid State Photomultiplier (Si-PM) are presented. The basis of this new type of photodetector is a fine structure of microcells operating in the Geiger mode with an internal gain greater than 106 : Common signal output allows for the detector to be operated in the proportional mode, and to reach a dynamic range of 1:5 103 : Such photodetectors have shown single photon response at room temperature with a fast timing of 100 ps. They are compact, robust and non-sensitive to magnetic fields. Results show the detection of low-intensity light in single photon mode and the detection of minimal ionizing particles using a scintillation tile for hadron calorimetry. The silicon photomultiplier is suitable for wide application in scintillation calorimetry, medical application, etc. r 2004 Published by Elsevier B.V.
1. Introduction Silicon avalanche structures with Geiger mode operation, i.e., at a bias voltage exceeding the breakdown voltage, can be used for single photon detection if noise conditions are optimized (see for example [1]). The intrinsic gain of the avalanche structure when operated in the Geiger mode is of the order of 105 2106 and higher. The noise conditions of the Si avalanche structures are defined by the dark count rate as a result of breakdown events with a signal amplitude Corresponding author. Tel.: +49-40-8998-3276; fax: +49-
40-8998-4306. E-mail address: [email protected] (V. Saveliev). 0168-9002/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.nima.2004.07.184
equivalent to the photoelectron signal (Geiger mode operation) from thermally generated carriers in the depletion region of the avalanche structure. This gives the main limitation of increasing the sensitive area of Si avalanche structures operated in single photon counting mode at room temperature. Two main methods can be used for the reduction of the dark counting rate in Si avalanche structures:
Operation at low temperature [2], minimization of the depletion volume of the structure at room temperature [1].
These methods give the possibility to reduce the dark count rate in Si avalanche structures below the kHz level.
ARTICLE IN PRESS V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532
The avalanche process in Geiger mode operation in Si is not a self-quenching process which means that it is necessary to introduce a quenching mechanism. Modern technology gives the possibility to produce the Si avalanche microcells with an integrated quenching mechanism based on a Metal–Resistor–Semiconductor structure, where the precise resistive elements are embedded for each individual microcell and provides effective feedback for stabilization and quenching of the avalanche process [3]. Finally, the existing technology allows for the production of combinations of large numbers of microcells in very fine structures on a common substrate with common electrodes. Such Si avalanche structures provide a proportional mode operation for detecting a low flux of light photons and is known as a silicon photomultiplier.
2. Structure of silicon photomultiplier and principle of operation The schematic cross-section of a silicon photomultiplier is shown in Fig. 1. The basis of the microcells is a reach-through avalanche structure nþ ppþ [4]. On top of the avalanche microstructure is placed a semitransparent electrode, which can have an antireflection coating. The depletion region is represented by a thin nþ area and p area, of total thickness
Fig. 1. Cross-section of a silicon photomultiplier structure.
529
527 mm and is illuminated by photons from above. There are p type layers of different doping levels adjacent to the nþ layer to suitably modify the field distribution across the structure. The first is a thin p type layer, and the second is a heavily doped pþ layer. The absorption of photons, and hence photogeneration, takes place mainly in the p-layer. The nearly uniform field here separates the electron– hole pairs and drifts them at velocities near saturation towards the nþ and pþ sides, respectively. When the drifting electrons reach the p layer near nþ ; they experience even greater fields and are accelerated by the high fields to sufficiently large kinetic energies to further cause impact ionization and release more electron–hole pairs which leads to an avalanche of impact ionization processes and provides an internal gain of amplification. The resistive layer on the top of the nþ layer is an important feature of the avalanche microcell structure with Geiger mode operation and provides a negative feedback in the local area of multiplication (quenching mechanism). The avalanche process increases the current through a resistive layer and a charge distribution accumulation on the Si-resistive layer interface. The result is a redistribution of the potential in the structure and an increasing electric field of opposite direction, which screens the initial electric field. The negative feedback produced causes a deceleration of the avalanche process and its termination. The resistive layer negative feedback is of a local nature due to very low tangential conductivity of the resistive layer. Modern technology allows for the production of very fine avalanche microstructures on a common substrate. The microphotograph in Fig. 2 shows the topology of the microcells on the Si substrate. The total number of microcells on the tested photodetector with a sensitive area of 1 mm2 is 1440. This value defines the dynamic range of the photodetector. All microcells are identical, independent and operate in single photon detection mode; this will be illustrated below. The output signal is defined as the sum of the Geiger mode signals from microcells triggered by the initial flux of photons.
ARTICLE IN PRESS 530
V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532
requires an increase of the thickness in order to maximize the absorption, but it is necessary to minimize the thickness of the depletion area in order to reduce the dark count rate. The absorption coefficient of light in Si depends on its wavelength and for l ¼ 400 nm the absorption coefficient is 5:4 106 cm1 : Therefore the thickness required to absorb 99.9 percent of the light is small ð2:33 mmÞ: To optimize the sensitivity in the green region of light a depletion region of 5 mm was chosen which gives the possibility to use a low resistive Si and low bias voltage. The total efficiency including the geometrical efficiency of Si-PM for the green range of the visible spectrum is estimated as 24%. Fig. 2. A microphotograph of the topology of the microcells on the Si substrate.
4. Noise consideration (dark count rate of Si-PM) 3. Efficiency of silicon photomultiplier The efficiency of Si-PM is a product of several factors and depends on the geometrical efficiency, absorption efficiency and triggering (Geiger process) probability E SiPM ¼ GEOM ð1 RÞð1 eax ÞGM
(1)
where GEOM - geometry efficiency of the sensitive area, GM - efficiency of Geiger process triggering in Si, R - reflection coefficient, a - coefficient of absorption. The geometrical efficiency is defined by topology and technological processes, which is equal to 0.6 for the present photodetectors. The quantum efficiency of the sensitive area is defined by the intrinsic QE of Si, the thickness of layers on top of the structure and the thickness of the depletion area and can be optimized for specific applications. The fraction of light transmitted to the sensitive volume is defined by layers on the top and the resistive layer and has been optimized for light in the green range (left edge of sensitive spectra). To improve the sensitivity in the short light wave band it is necessary to optimize the top contact technology. The thickness of the sensitive volume is defined by two factors. Efficient absorption of photons
To limit the dark count rate the sensitive area of the Si-PM is chosen as 1 mm2 and the number of microcells is 1440 with a size of 30 20 mm: With an optimized thickness of the depleted region of the Si-PM the total dark count rate of the photodetector structure at the level of a single photoelectron is 400 kHz at room temperature.
5. Characteristics of silicon photomultiplier The main characteristics of the Si-PMs used for the evaluation of the performance are shown in Table 1.
6. Silicon photomultiplier performance evaluation The single photon detection performance was tested with pulsed LEDs emitting at 470 nm peak wavelength, triggered over a gate of 50 ns. The low photon flux spectra detected by the silicon photomultiplier are shown in Fig. 3 for one photoelectron (a) and 3 photoelectrons (b), respectively. The spectrum shows the pedestal of the photodetector and photoelectron peaks for 1; 2; 3; . . . photons. The spectrum clearly shows the excellent single photon resolution and can give an estimation of the uniformity of the microcells of the
ARTICLE IN PRESS V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532 Table 1 Parameter
Value
Sensitive area Number of microcells Amplification gain of microcells Efficiency (green range of light) Fast timing (rising time) Recovery time Bias voltage Magnetic field sensitivity Mechanical properties
1 mm2 1440 4 106 24% 100 ps 100 ns 51.5 V No Possible mechanical contact
531
gain of amplification at a level of less than 25% FWHM. In order to demonstrate the feasibility of developing a high-quality TESLA hadron calorimeter and the possibility of calibration by minimal ionizing particles (mip) with scintillation tiles equipped with Si-PM the test measurements were performed. The goal is to develop a high granularity calorimeter systems working in high magnetic fields up to 4–5 T [5]. The main requirements of the photodetectors for TESLA hadron calorimeter are the possibility to detect the low-intensity flux of photons for calibration purposes and a wide dynamic range of photon flux detection in hadron shower measurements [6]. The sensitive element of the calorimeter structure is a 5 5 cm and 5 mm thick scintillation tile (BicronBC-408) with WLS readout fiber (Kuraray Y11) of 1 mm diameter. The quarter loop of the WLS fiber was embedded in a plastic body and one end was connected to the Si-PM as shown in Fig. 4.
Fig. 3. The low photon flux spectra detected by silicon photomultipliers. (a) Mean of one photoelectron (b) mean of three photoelectrons.
silicon photomultiplier. Consider the single photon peak of the spectra. The contribution to the statistics of this peak is the statistics from signals generated by randomly distributed photons over the sensitive area of the silicon photomultiplier, i.e., over 1440 microcells. The width of this peak gives the estimation of the variation of the
Fig. 4. The sensitive element of a hadron calorimeter structure with silicon photomultipliers.
ARTICLE IN PRESS 532
V. Saveliev / Nuclear Instruments and Methods in Physics Research A 535 (2004) 528–532
and more detailed studies will be presented in future.
7. Conclusion
(a)
The design and technology to produce this novel type of Silicon avalanche photodetectors with Geiger mode operation based on a Metal–Resistor–Semiconductor Structure (Si-PM) were developed and the latest test measurements have shown stable results. Si-PM characteristics are already suitable for the requirements of the operation in the TESLA hadron scintillation tile calorimeter system with a strong magnetic field and includes the possibility of calibration with mip signals. Novel types of avalanche photodetectors could be interesting for applications in other areas, e.g., for detecting low photon flux, with fast time resolution as in Positron Emission Tomography (PET), for scintillation detector system and others.
(b) Fig. 5. Test result spectra with cosmic muons (mip signal). (a) Without mirror at the end of the WLS fiber, (b) with mirror at the end of the WLS fiber.
Fig. 5 shows the test result of spectra with cosmic muons as an illustration of the possibility for precision calibration of individual tiles of the TESLA hadron calorimeter. Fig. 5a shows the spectrum of low flux photons, detected by Si-PM without mirrors at the other end of WLS. The spectrum shows the pedestal of the photodetector and clearly shows the single photon spectrum mode. Fig. 5b shows the spectrum of mip signals with mirrors at the other end of WLS fiber, which gives significant improvement in light yield of the scintillation tile. These results are shown as an illustration of a real application of silicon photomultipliers
References [1] S. Vasile, P. Gothoskar, D. Sdrulla, R. Farrel, Photon detection with high gain avalanche photodiode arrays, IEEE Trans. Nucl. Sci. NS-45 (1998). [2] A. Bross, E. Flattum, D. Lincoln, S. Grunendahl, J. Warchol, M. Wayne, P. Padley, Characterisation and performance of visible light photon counters (VLPC for the upgraded D) detector at the Fermilab Tevatron, Nucl. Instr. and Meth. A 477 (2002) 172. [3] V. Saveliev, V. Golovin, Silicon avalanche photodiodes on the base of metal–resistor–semiconductor (MRS) structures, Nucl. Instr. and Meth. A 442 (2000) 223. [4] V. Saveliev, V. Golovin, Novel type of avalanche photodetector with Geiger Mode operation, Nucl. Instr. and Meth. A 518 (2004) 560. [5] TESLA Technical Design Report, DESY Polymers, Vol. 1, Elsevier, Amsterdam, 1987 (Chapter 5). [6] V. Korbel, V. Morgunov, V. Saveliev, Upcoming R&D, design and construction studies for the HCAL tile calorimeter, LC-DET-2001-050, 2001.