- Email: [email protected]
First results on NUV-SiPMs at FBK
Nuclear Instruments and Methods in Physics Research A 718 (2013) 371–372
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First results on NUV-SiPMs at FBK A. Ferri n, A. Gola, C. Piemonte, T. Pro, N. Serra, A. Tarolli, N. Zorzi Fondazione Bruno Kessler (FBK-IRST), Centro per i Materiali e i Microsistemi, Via Sommarive, 18, I-38100 Povo di Trento (TN), Italy
a r t i c l e i n f o
abstract
Available online 12 October 2012
In this paper we show selected results on the first release of Near Ultra Violet SiPM technology (NUVSiPM) produced at FBK. In particular, we focus our attention on the photo-detection efficiency (PDE) performance. The PDE in the near-UV part of the light spectrum is mainly limited by the quantum efficiency term since the photo-generation takes place in a very shallow region of the silicon. Thus, besides using a p þ -on-n junction configuration to have an avalanche triggered by the electrons, we need to implement a very shallow p þ layer. In this context, we will show that our NUV-SiPM technology features a quantum efficiency higher than 80% in the measured range between 360 and 420 nm. This allows to reach a PDE of about 30% at 9 V over-voltage on a SPAD featuring 50 50 mm2 cell size and 45% fill factor. We will also show other important features of the device such as noise, breakdown voltage temperature dependence and single-cell response uniformity to prove its functionality. & 2012 Elsevier B.V. All rights reserved.
Keywords: SiPM NUV PDE
1. Introduction In many SiPM applications, the detector is coupled to a scintillator which has a peak emission wavelength close to the blue limit of visible light. This is the case of LYSO (420 nm) or LaBr (380 nm): two crystals used in many PET research activities and products. Standard FBK n/p technology features good results with LaBr [1] despite being optimized for green light. To further improve the performances we are exploring different technology solutions such as a new p/n technology with a PDE optimized for the Near UltraViolet (NUV) wavelengths. In this contribution we will show some results of the first NUV SiPM batch produced at FBK: this includes preliminary functional measurements and PDE measurements, along with a comparison with FBK n/p technology.
2. Preliminary functional measurements A preliminary set of functional measurements is performed to characterize the new technology. The first measurement is a reverse current–voltage characteristics of a 1 1 mm2 SiPM with a 50 mm cell to estimate the breakdown voltage [2]. The measurement is repeated at different temperatures (from 20 1C to þ20 1C with a step of 5 1C). The breakdown voltage ranges between 14.7 V and 15.8 V, with a calculated temperature dependence of 27 mV=1C.
A second characterization is performed to evaluate the dark count rate as a function of the over-voltage and temperature. The SiPM is placed in a thermostatic chamber along with a preamplifier and the output signal is sent to a 1 GHz oscilloscope. The digitized data are transferred to a PC where a LabVIEW program performs the online analysis. Long frame waveforms (time length Z 1 ms) are acquired with a random trigger and a pulse identification is performed. Knowing the occurrence times of the events, we can plot an histogram of the time distance between consecutive pulses. We expect an exponential distribution of the pulses with an excess of events occurring at short time distance due to after-pulsing. This is very clear in a log–log plot with logarithmic time binning where the expanded time scale allows to better distinguish poissonian events from afterpulse events. The pulse rate is finally extracted from an exponential fit on the poissonian part of the histogram. The results for three different temperature (20 1C, 0 1C and þ20 1C) are shown in Fig. 1. The dark count rate is acceptable, still the technology has to be improved from this point of view. The single cell response uniformity is then evaluated with a pulsed LED. The signal of the SiPM is integrated in a window of 20 ns synchronous with LED pulse. The charge histogram relative to 2.5 V over-voltage is shown in Fig. 2. Up to six peaks are clearly distinguishable, showing a good single cell response uniformity. We estimated that the main limitation of the single peak resolution is due to electronic noise. 2.1. PDE measurements
n
Corresponding author. E-mail address: [email protected] (A. Ferri).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.10.021
To perform PDE measurements, an halogen lamp is used as continuous white light source. The wavelength selection is done
372
A. Ferri et al. / Nuclear Instruments and Methods in Physics Research A 718 (2013) 371–372
Fig. 1. Dark count rate as a function of the over-voltage for different temperatures: þ 20 1C (top line), 0 1C (middle line) and 20 1C (bottom line).
Fig. 3. PDE results for a single 50 mm SPAD as a function of wavelength and for different over-voltages (from bottom to top: 2,4,6 and 9 V). The dashed line correspond to the estimated maximum PDE (the product of the measured QE and the fill factor).
Fig. 2. Typical SiPM charge spectrum in response to a pulsed LED source. The bias voltage is set to 2.5 V over-voltage. Up to six peaks are visible which reflects a good single cell response uniformity.
Fig. 4. Comparison of the PDE dependence on over-voltage between NUV and n/p technology for a fixed wavelength (360 nm). For short l the avalanche is mainly triggered by electrons in p/n and by holes in n/p. This leads to a steeper increase of the PDE for the NUV technology.
with a monochromator and the light is sent to a single 50 mm SPAD (with the same layout of a SiPM cell), to eliminate any crosstalk effect. Then the SPAD signal is amplified and digitized with a 1 GHz oscilloscope and transmitted to a computer where a LabVIEW program performs the acquisition and real time data analysis to extract the pulse count rate with the same procedure discussed for dark measurements. The measurement is repeated for every wavelength in the range between 350 nm and 450 nm (with a step of 10 nm) and for different over-voltages. For every over-voltage a dark measure is performed as well to estimate the number of fake events. The rate of incident photon at each wavelength is measured replacing the SPAD with a calibrated photodiode (UDT 221). The PDE is finally calculated subtracting the dark rate from the pulse rate in light and dividing by the incident photon rate. The results of measurements taken for different over-voltages (2, 4, 6 and 9 V) are shown in Fig. 3. It can be seen that a peak value of 30% PDE can be achieved with 9 V over-voltage at 360 nm. The dashed line corresponds to the estimated maximum PDE, i.e., the product of the measured QE (on a test photodiode) and the fill factor. Measured PDE of NUV SiPMs are then compared to PDE of standard n/p devices. Fig. 4 reports the comparison at 360 nm as a function of the overvoltage. The NUV technology has a steeper increase in the PDE because the avalanche is mainly triggered by electrons, while in n/p technology it is triggered mainly by holes. At 9 V over-voltage the ratio between the PDE of the two technologies reaches a factor of 2:5.
3. Conclusions We presented the first results of the Near Ultra Violet SiPM technology (NUV-SiPM) produced at FBK. The functional measurements on a 1 1 mm2 SiPM with 50 mm cell size show that the device is working properly. The optical characterization of a SPAD with the same cell structure of the SiPM evidences a PDE as high as 30% at 360 nm and 9 V over-voltage which is a rather good value considering a F.F. of 45%. For the same over-voltage and wavelength, PDE in NUV-SiPM is a factor 2.5 greater than in our standard technology. We are currently working to improve the noise and after-pulse performance of this new technology.
Acknowledgments This work is partially supported by the EU FP7 project SUBLIMA, Grant Agreement N 241711 and the MEMS2 agreement between FBK, PAT and INFN. References [1] R.I. Wiener, et al., Timing and energy characteristics of LaBr3[Ce] and CeBr3 scintillators read by FBK SiPMs, in: Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), IEEE, 2011. [2] C. Piemonte, et al., IEEE Transactions on Nuclear Science NS-54 (1) (2007) 236.
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First results on NUV-SiPMs at FBK A. Ferri n, A. Gola, C. Piemonte, T. Pro, N. Serra, A. Tarolli, N. Zorzi Fondazione Bruno Kessler (FBK-IRST), Centro per i Materiali e i Microsistemi, Via Sommarive, 18, I-38100 Povo di Trento (TN), Italy
a r t i c l e i n f o
abstract
Available online 12 October 2012
In this paper we show selected results on the first release of Near Ultra Violet SiPM technology (NUVSiPM) produced at FBK. In particular, we focus our attention on the photo-detection efficiency (PDE) performance. The PDE in the near-UV part of the light spectrum is mainly limited by the quantum efficiency term since the photo-generation takes place in a very shallow region of the silicon. Thus, besides using a p þ -on-n junction configuration to have an avalanche triggered by the electrons, we need to implement a very shallow p þ layer. In this context, we will show that our NUV-SiPM technology features a quantum efficiency higher than 80% in the measured range between 360 and 420 nm. This allows to reach a PDE of about 30% at 9 V over-voltage on a SPAD featuring 50 50 mm2 cell size and 45% fill factor. We will also show other important features of the device such as noise, breakdown voltage temperature dependence and single-cell response uniformity to prove its functionality. & 2012 Elsevier B.V. All rights reserved.
Keywords: SiPM NUV PDE
1. Introduction In many SiPM applications, the detector is coupled to a scintillator which has a peak emission wavelength close to the blue limit of visible light. This is the case of LYSO (420 nm) or LaBr (380 nm): two crystals used in many PET research activities and products. Standard FBK n/p technology features good results with LaBr [1] despite being optimized for green light. To further improve the performances we are exploring different technology solutions such as a new p/n technology with a PDE optimized for the Near UltraViolet (NUV) wavelengths. In this contribution we will show some results of the first NUV SiPM batch produced at FBK: this includes preliminary functional measurements and PDE measurements, along with a comparison with FBK n/p technology.
2. Preliminary functional measurements A preliminary set of functional measurements is performed to characterize the new technology. The first measurement is a reverse current–voltage characteristics of a 1 1 mm2 SiPM with a 50 mm cell to estimate the breakdown voltage [2]. The measurement is repeated at different temperatures (from 20 1C to þ20 1C with a step of 5 1C). The breakdown voltage ranges between 14.7 V and 15.8 V, with a calculated temperature dependence of 27 mV=1C.
A second characterization is performed to evaluate the dark count rate as a function of the over-voltage and temperature. The SiPM is placed in a thermostatic chamber along with a preamplifier and the output signal is sent to a 1 GHz oscilloscope. The digitized data are transferred to a PC where a LabVIEW program performs the online analysis. Long frame waveforms (time length Z 1 ms) are acquired with a random trigger and a pulse identification is performed. Knowing the occurrence times of the events, we can plot an histogram of the time distance between consecutive pulses. We expect an exponential distribution of the pulses with an excess of events occurring at short time distance due to after-pulsing. This is very clear in a log–log plot with logarithmic time binning where the expanded time scale allows to better distinguish poissonian events from afterpulse events. The pulse rate is finally extracted from an exponential fit on the poissonian part of the histogram. The results for three different temperature (20 1C, 0 1C and þ20 1C) are shown in Fig. 1. The dark count rate is acceptable, still the technology has to be improved from this point of view. The single cell response uniformity is then evaluated with a pulsed LED. The signal of the SiPM is integrated in a window of 20 ns synchronous with LED pulse. The charge histogram relative to 2.5 V over-voltage is shown in Fig. 2. Up to six peaks are clearly distinguishable, showing a good single cell response uniformity. We estimated that the main limitation of the single peak resolution is due to electronic noise. 2.1. PDE measurements
n
Corresponding author. E-mail address: [email protected] (A. Ferri).
0168-9002/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nima.2012.10.021
To perform PDE measurements, an halogen lamp is used as continuous white light source. The wavelength selection is done
372
A. Ferri et al. / Nuclear Instruments and Methods in Physics Research A 718 (2013) 371–372
Fig. 1. Dark count rate as a function of the over-voltage for different temperatures: þ 20 1C (top line), 0 1C (middle line) and 20 1C (bottom line).
Fig. 3. PDE results for a single 50 mm SPAD as a function of wavelength and for different over-voltages (from bottom to top: 2,4,6 and 9 V). The dashed line correspond to the estimated maximum PDE (the product of the measured QE and the fill factor).
Fig. 2. Typical SiPM charge spectrum in response to a pulsed LED source. The bias voltage is set to 2.5 V over-voltage. Up to six peaks are visible which reflects a good single cell response uniformity.
Fig. 4. Comparison of the PDE dependence on over-voltage between NUV and n/p technology for a fixed wavelength (360 nm). For short l the avalanche is mainly triggered by electrons in p/n and by holes in n/p. This leads to a steeper increase of the PDE for the NUV technology.
with a monochromator and the light is sent to a single 50 mm SPAD (with the same layout of a SiPM cell), to eliminate any crosstalk effect. Then the SPAD signal is amplified and digitized with a 1 GHz oscilloscope and transmitted to a computer where a LabVIEW program performs the acquisition and real time data analysis to extract the pulse count rate with the same procedure discussed for dark measurements. The measurement is repeated for every wavelength in the range between 350 nm and 450 nm (with a step of 10 nm) and for different over-voltages. For every over-voltage a dark measure is performed as well to estimate the number of fake events. The rate of incident photon at each wavelength is measured replacing the SPAD with a calibrated photodiode (UDT 221). The PDE is finally calculated subtracting the dark rate from the pulse rate in light and dividing by the incident photon rate. The results of measurements taken for different over-voltages (2, 4, 6 and 9 V) are shown in Fig. 3. It can be seen that a peak value of 30% PDE can be achieved with 9 V over-voltage at 360 nm. The dashed line corresponds to the estimated maximum PDE, i.e., the product of the measured QE (on a test photodiode) and the fill factor. Measured PDE of NUV SiPMs are then compared to PDE of standard n/p devices. Fig. 4 reports the comparison at 360 nm as a function of the overvoltage. The NUV technology has a steeper increase in the PDE because the avalanche is mainly triggered by electrons, while in n/p technology it is triggered mainly by holes. At 9 V over-voltage the ratio between the PDE of the two technologies reaches a factor of 2:5.
3. Conclusions We presented the first results of the Near Ultra Violet SiPM technology (NUV-SiPM) produced at FBK. The functional measurements on a 1 1 mm2 SiPM with 50 mm cell size show that the device is working properly. The optical characterization of a SPAD with the same cell structure of the SiPM evidences a PDE as high as 30% at 360 nm and 9 V over-voltage which is a rather good value considering a F.F. of 45%. For the same over-voltage and wavelength, PDE in NUV-SiPM is a factor 2.5 greater than in our standard technology. We are currently working to improve the noise and after-pulse performance of this new technology.
Acknowledgments This work is partially supported by the EU FP7 project SUBLIMA, Grant Agreement N 241711 and the MEMS2 agreement between FBK, PAT and INFN. References [1] R.I. Wiener, et al., Timing and energy characteristics of LaBr3[Ce] and CeBr3 scintillators read by FBK SiPMs, in: Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), IEEE, 2011. [2] C. Piemonte, et al., IEEE Transactions on Nuclear Science NS-54 (1) (2007) 236.