- Email: [email protected]
Recent detector developments at SINTEF (industrial presentation)
Nuclear Instruments and Methods in Physics Research A 409 (1998) 147—151
Recent detector developments at SINTEF (industrial presentation) Berit Sundby Avset, Lars Evensen, Geir Uri Jensen, Sjur Mo, Kari Schj+lberg-Henriksen*, Trond Westgaard SINTEF Electronics and Cybernetics, Department of Microsystems, Forskningsveien 1, P.O. Box 124 Blindern, N-0314 Oslo, Norway
Abstract Results from SINTEF’s research on radiation hardness of silicon detectors, thin silicon detectors, silicon drift devices, reach-through avalanche photodiodes, and detectors with thin dead layers are presented. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction to radiation detectors at SINTEF
2. ATLAS Prototypes
Silicon radiation detectors are a main research area at the Department of Microsystems at SINTEF. In addition, there is substantial activity within the fields of Micro Electro-Mechanical Systems and Micro Opto-Mechanical Systems. Department of Microsystems belongs to SINTEF Electronics and Cybernetics, one of nine institutes of the SINTEF foundation. The SINTEF foundation carries out contract research and development projects for industry and the public sector on a non-profit basis. SINTEF has contributed to progress in detector processing in developing polysilicon resistor biasing, double-metal systems with polyimide dielectric, dedicated guard designs for high-voltage operation, and more. SINTEF has designed and manufactured most types of silicon radiation detectors. This article presents new results in current research areas.
An important task in recent years has been to improve the radiation hardness of silicon radiation detectors. The new high-energy particle physics experiments will have accumulated particle fluences which are orders of magnitude higher than in previous experiments. SINTEF has designed a singlesided n—in—n microstrip detector of dimensions 60]60 mm2 for analogue read-out [1]. The detector has 1025 strips of which 512 are read-out strips. The strips are biased with polysilicon resistors connected to the strips with metal “bridges”. Individual p-stops are boron implanted for electrical isolation between the n-strips. In order to increase the interstrip resistance the p-stops surround the n-type strip ends. The guard structure reduces the electric field in the detector edge region and stabilises the field distribution even when the bulk material properties change with radiation exposure [2]. In the initial operation phase, before type inversion has occurred, depletion starts from the p—n junction on the back side of the detector. In this phase, a multiguard structure with field plates on the p-side
* Corresponding author. E-mail: [email protected]
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 5 4 - 0
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enables high-voltage operation of the detector. After type inversion of the bulk material has occurred, the detector is depleted from the n—p junction at the n-type strips on the front side, and the backside multiguard is inactive. The ATLAS prototype detectors were tested electrically before irradiation. The depletion voltages were in the range 60—70 V, and the leakage current measurements showed the breakdown voltage to be above 300 V. At 100 V, the total leakage current was about 1.4 lA. A special process improvement reduced the average pin-hole count to below 2 pinholes per detector. The detector performance after proton irradiation was tested at CERN. The leakage current level increased from 0.1 to 10 lA/cm2 at 300 V bias [1]. Even these high leakage current levels meet the ATLAS performance requirements. The work on ATLAS prototype detectors continues at SINTEF, with the new series A¹¸AS97. For these detectors, the design is altered because all ATLAS prototype suppliers should use equivalent designs. The major changes are an increased strip pitch and a revised implementation of the p-stop structures.
3. Thin detectors The energy loss of an ion passing through a thin silicon detector is given by the atomic number Z, the mass number A, and the energy E of the ion. A *E—E telescope for isotope identification and energy determination of fragments from nuclear collisions can be made by combining a thin detector with a conventional silicon detector. 10 lm thin silicon detectors have been made for the *E—E telescopes in the CHICSi1 detector system [3]. The CHICSi detector telescope is shown in Fig. 1. The resolution of the telescope is given by the thin detector’s thickness and its thickness variation. The thin detectors are made as p—i—n diodes in thin etched membranes in 280 lm thick silicon wafers. The detectors have a 10.0]10.0 mm2 active area on a 10.2]10.2 mm2 membrane surrounded by a 1.1 mm wide supporting frame. Measurements with a Tencor Alpha-Step 500 profilometer indicate 1 Celsius Heavy Ion collaboration, Uppsala, Sweden.
Fig. 1. The CHICSi detector telescope. (Courtesy of the Celsius Heavy Ion Collaboration)
that the etch depth variation is as small as $0.1 lm on each wafer, and that the surface roughness due to etching is in the range 2—4 nm. The over all membrane thickness variation across a wafer is measured to be less than $0.25 lm, which implies that the uniformity of the starting material is the most important parameter with respect to high yield [3]. Evaluation experiments at CHIC have shown excellent isotope separation capabilities of these detectors. Fig. 2 shows a point plot of *E versus E for a tested telescope. The resolution is sufficient to separate 6Li from 7Li, and inspection of the measurement data reveals that 7Be and 9Be are resolved as well. This is not evident from Fig. 2 as the production of Be fragments was low for this particular reaction.
4. Drift devices Drift devices represent an alternative to conventional strip and pixel detectors. The drift detectors introduce low capacitive loads for the read-out amplifier. Good position resolution is obtained with a reduced number of read-out channels.
B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
Fig. 2. *E vs. E plot for intermediate mass fragments emitted at 140° from 150-500 MeV protons incident on a Kr gas target. Light gray indicates the higest density of fragments. The full scale readings are approx. 31Mev for E-axis and 23MeV for *Eaxis. (Courtesy of the Celsius Heavy Ion Collaboration.)
A silicon drift detector is the solid-state version of the gas drift chamber. P—n junction strips are made on both sides of the device and are biased to create an electric field directing electrons to the anode. The drift time of the electrons determines the position in one dimension. By dividing the anode into a multi-anode structure, two-dimensional resolution can be obtained. SINTEF has been working on drift devices since 1986. The most recent project is the processing of prototypes designed by the STAR Collaboration. These are advanced two-dimensional drift detectors to be used in the inner tracking detector in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
5. Avalanche photodiodes In collaboration with the Norwegian company ame,2 SINTEF has developed a process for manufacturing reach-through avalanche photodiodes [4]. These diodes are operated fully depleted.
2 ame as, Horten, Norway
149
Photons are absorbed in the long absorption zone, and the resulting electric signals are amplified in the short multiplication zone. The length of the absorption zone makes reach-through APDs well suited for infrared detection, and the diodes developed by ame and SINTEF have a detection range of 600—1100 nm. Fig. 3 shows the responsivity as a function of the operating voltage for a diode of diameter 800 lm. At the operating voltage (275 V), the diode has a gain of about 100, and the dark current is only 2.9 nA. This is remarkably low for reach-throughtype APDs. The diameter of the diodes manufactured at SINTEF varies from 800 to 2000 lm. Fig. 4 shows measurements of the gain uniformity on an 800 lm diode.
6. Thin dead layers In several applications, e.g. in medical imaging, there is a need for detection of radiation with short penetration depth in silicon. Examples are detection of low-energy electrons, alpha particles or soft X-rays. A typical detector for two-dimensional position detection and energy measurement in these applications is a p—i—n pixel detector. The radiation enters at the uniformly doped n-side of the detector, and the charge signal is collected by p-type doped pixels on the p-side of the detector. In order to avoid complete or partial loss of signal due to recombination of electrons and holes, the detector must have a thin dead layer on the n-side. Thin dead layers are achieved by reducing the implantation energy and dose of the n-type dopant. This gives less effective impurity gettering during processing, which results in higher bulk leakage current levels. The overvoltage capability of the detector can be reduced because charge injection can occur at the n-side contacts. In order to investigate these problems a test series was processed with phosphorus, arsenic, and antimony as n-type dopants. Process simulations, as that of Fig. 5, indicated that dead layers of 0.2 lm thickness could be achieved with phosphorus, while arsenic and antimony could give values around 0.1 lm.
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B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
Fig. 3. Responsivity and dark current curves for an avalanche photodiode manufactured at SINTEF. (Courtesy of ame).
Fig. 4. Uniformity of responsitivity measured at ame for an avalanche photodiode of diameter 800 lm. (Courtesy of ame).
Electrical measurements indicate a leakage current level in the range 2—5 nA/cm2 for detectors of 280 lm thickness. This level is a factor 10 higher than the level achieved with SINTEF’s standard process. The results are still satisfactory for most applications. The most promising results for dead layers as thin as about 0.12 lm were obtained using arsenic as dopant.
7. Future prospects The main focus of SINTEF’s research on radiation detectors will continue to be on radiation hard detectors for high-energy physics and imaging detectors for medical applications and materials science. The imaging activities will be included in a Norwegian Programme for microtechnology, the
B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
151
Fig. 5. Net simulated doping concentration (in cm~3) of phosphorus as a function of depth in (lm) and the corresponding results from spreading resistance measurements (SRP).
k-tech programme, which is currently being established. As a part of this programme, a major upgrade of SINTEF’s laboratory facilities for 150 mm silicon wafer processing is planned.
[2] B.S. Avset, L. Evensen, Nucl. Instr. and Meth. A 377 (1996) 397. [3] L. Evensen et al., IEEE Trans. Nucl. Sci. NS-44(3) (1997) 629. [4] R.J. McIntyre, IEEE Trans. Electron Devices ED-13 (1966) 164.
References [1] L. Evensen, T. Westgaard, Nucl. Instr. and Meth. A 392 (1997) 206.
II. TRACKING (SOLID STATE)
Recent detector developments at SINTEF (industrial presentation) Berit Sundby Avset, Lars Evensen, Geir Uri Jensen, Sjur Mo, Kari Schj+lberg-Henriksen*, Trond Westgaard SINTEF Electronics and Cybernetics, Department of Microsystems, Forskningsveien 1, P.O. Box 124 Blindern, N-0314 Oslo, Norway
Abstract Results from SINTEF’s research on radiation hardness of silicon detectors, thin silicon detectors, silicon drift devices, reach-through avalanche photodiodes, and detectors with thin dead layers are presented. ( 1998 Elsevier Science B.V. All rights reserved.
1. Introduction to radiation detectors at SINTEF
2. ATLAS Prototypes
Silicon radiation detectors are a main research area at the Department of Microsystems at SINTEF. In addition, there is substantial activity within the fields of Micro Electro-Mechanical Systems and Micro Opto-Mechanical Systems. Department of Microsystems belongs to SINTEF Electronics and Cybernetics, one of nine institutes of the SINTEF foundation. The SINTEF foundation carries out contract research and development projects for industry and the public sector on a non-profit basis. SINTEF has contributed to progress in detector processing in developing polysilicon resistor biasing, double-metal systems with polyimide dielectric, dedicated guard designs for high-voltage operation, and more. SINTEF has designed and manufactured most types of silicon radiation detectors. This article presents new results in current research areas.
An important task in recent years has been to improve the radiation hardness of silicon radiation detectors. The new high-energy particle physics experiments will have accumulated particle fluences which are orders of magnitude higher than in previous experiments. SINTEF has designed a singlesided n—in—n microstrip detector of dimensions 60]60 mm2 for analogue read-out [1]. The detector has 1025 strips of which 512 are read-out strips. The strips are biased with polysilicon resistors connected to the strips with metal “bridges”. Individual p-stops are boron implanted for electrical isolation between the n-strips. In order to increase the interstrip resistance the p-stops surround the n-type strip ends. The guard structure reduces the electric field in the detector edge region and stabilises the field distribution even when the bulk material properties change with radiation exposure [2]. In the initial operation phase, before type inversion has occurred, depletion starts from the p—n junction on the back side of the detector. In this phase, a multiguard structure with field plates on the p-side
* Corresponding author. E-mail: [email protected]
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 1 6 8 - 9 0 0 2 ( 9 7 ) 0 1 2 5 4 - 0
II. TRACKING (SOLID STATE)
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B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
enables high-voltage operation of the detector. After type inversion of the bulk material has occurred, the detector is depleted from the n—p junction at the n-type strips on the front side, and the backside multiguard is inactive. The ATLAS prototype detectors were tested electrically before irradiation. The depletion voltages were in the range 60—70 V, and the leakage current measurements showed the breakdown voltage to be above 300 V. At 100 V, the total leakage current was about 1.4 lA. A special process improvement reduced the average pin-hole count to below 2 pinholes per detector. The detector performance after proton irradiation was tested at CERN. The leakage current level increased from 0.1 to 10 lA/cm2 at 300 V bias [1]. Even these high leakage current levels meet the ATLAS performance requirements. The work on ATLAS prototype detectors continues at SINTEF, with the new series A¹¸AS97. For these detectors, the design is altered because all ATLAS prototype suppliers should use equivalent designs. The major changes are an increased strip pitch and a revised implementation of the p-stop structures.
3. Thin detectors The energy loss of an ion passing through a thin silicon detector is given by the atomic number Z, the mass number A, and the energy E of the ion. A *E—E telescope for isotope identification and energy determination of fragments from nuclear collisions can be made by combining a thin detector with a conventional silicon detector. 10 lm thin silicon detectors have been made for the *E—E telescopes in the CHICSi1 detector system [3]. The CHICSi detector telescope is shown in Fig. 1. The resolution of the telescope is given by the thin detector’s thickness and its thickness variation. The thin detectors are made as p—i—n diodes in thin etched membranes in 280 lm thick silicon wafers. The detectors have a 10.0]10.0 mm2 active area on a 10.2]10.2 mm2 membrane surrounded by a 1.1 mm wide supporting frame. Measurements with a Tencor Alpha-Step 500 profilometer indicate 1 Celsius Heavy Ion collaboration, Uppsala, Sweden.
Fig. 1. The CHICSi detector telescope. (Courtesy of the Celsius Heavy Ion Collaboration)
that the etch depth variation is as small as $0.1 lm on each wafer, and that the surface roughness due to etching is in the range 2—4 nm. The over all membrane thickness variation across a wafer is measured to be less than $0.25 lm, which implies that the uniformity of the starting material is the most important parameter with respect to high yield [3]. Evaluation experiments at CHIC have shown excellent isotope separation capabilities of these detectors. Fig. 2 shows a point plot of *E versus E for a tested telescope. The resolution is sufficient to separate 6Li from 7Li, and inspection of the measurement data reveals that 7Be and 9Be are resolved as well. This is not evident from Fig. 2 as the production of Be fragments was low for this particular reaction.
4. Drift devices Drift devices represent an alternative to conventional strip and pixel detectors. The drift detectors introduce low capacitive loads for the read-out amplifier. Good position resolution is obtained with a reduced number of read-out channels.
B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
Fig. 2. *E vs. E plot for intermediate mass fragments emitted at 140° from 150-500 MeV protons incident on a Kr gas target. Light gray indicates the higest density of fragments. The full scale readings are approx. 31Mev for E-axis and 23MeV for *Eaxis. (Courtesy of the Celsius Heavy Ion Collaboration.)
A silicon drift detector is the solid-state version of the gas drift chamber. P—n junction strips are made on both sides of the device and are biased to create an electric field directing electrons to the anode. The drift time of the electrons determines the position in one dimension. By dividing the anode into a multi-anode structure, two-dimensional resolution can be obtained. SINTEF has been working on drift devices since 1986. The most recent project is the processing of prototypes designed by the STAR Collaboration. These are advanced two-dimensional drift detectors to be used in the inner tracking detector in the STAR experiment at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
5. Avalanche photodiodes In collaboration with the Norwegian company ame,2 SINTEF has developed a process for manufacturing reach-through avalanche photodiodes [4]. These diodes are operated fully depleted.
2 ame as, Horten, Norway
149
Photons are absorbed in the long absorption zone, and the resulting electric signals are amplified in the short multiplication zone. The length of the absorption zone makes reach-through APDs well suited for infrared detection, and the diodes developed by ame and SINTEF have a detection range of 600—1100 nm. Fig. 3 shows the responsivity as a function of the operating voltage for a diode of diameter 800 lm. At the operating voltage (275 V), the diode has a gain of about 100, and the dark current is only 2.9 nA. This is remarkably low for reach-throughtype APDs. The diameter of the diodes manufactured at SINTEF varies from 800 to 2000 lm. Fig. 4 shows measurements of the gain uniformity on an 800 lm diode.
6. Thin dead layers In several applications, e.g. in medical imaging, there is a need for detection of radiation with short penetration depth in silicon. Examples are detection of low-energy electrons, alpha particles or soft X-rays. A typical detector for two-dimensional position detection and energy measurement in these applications is a p—i—n pixel detector. The radiation enters at the uniformly doped n-side of the detector, and the charge signal is collected by p-type doped pixels on the p-side of the detector. In order to avoid complete or partial loss of signal due to recombination of electrons and holes, the detector must have a thin dead layer on the n-side. Thin dead layers are achieved by reducing the implantation energy and dose of the n-type dopant. This gives less effective impurity gettering during processing, which results in higher bulk leakage current levels. The overvoltage capability of the detector can be reduced because charge injection can occur at the n-side contacts. In order to investigate these problems a test series was processed with phosphorus, arsenic, and antimony as n-type dopants. Process simulations, as that of Fig. 5, indicated that dead layers of 0.2 lm thickness could be achieved with phosphorus, while arsenic and antimony could give values around 0.1 lm.
II. TRACKING (SOLID STATE)
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B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
Fig. 3. Responsivity and dark current curves for an avalanche photodiode manufactured at SINTEF. (Courtesy of ame).
Fig. 4. Uniformity of responsitivity measured at ame for an avalanche photodiode of diameter 800 lm. (Courtesy of ame).
Electrical measurements indicate a leakage current level in the range 2—5 nA/cm2 for detectors of 280 lm thickness. This level is a factor 10 higher than the level achieved with SINTEF’s standard process. The results are still satisfactory for most applications. The most promising results for dead layers as thin as about 0.12 lm were obtained using arsenic as dopant.
7. Future prospects The main focus of SINTEF’s research on radiation detectors will continue to be on radiation hard detectors for high-energy physics and imaging detectors for medical applications and materials science. The imaging activities will be included in a Norwegian Programme for microtechnology, the
B.S. Avset et al. /Nucl. Instr. and Meth. in Phys. Res. A 409 (1998) 147—151
151
Fig. 5. Net simulated doping concentration (in cm~3) of phosphorus as a function of depth in (lm) and the corresponding results from spreading resistance measurements (SRP).
k-tech programme, which is currently being established. As a part of this programme, a major upgrade of SINTEF’s laboratory facilities for 150 mm silicon wafer processing is planned.
[2] B.S. Avset, L. Evensen, Nucl. Instr. and Meth. A 377 (1996) 397. [3] L. Evensen et al., IEEE Trans. Nucl. Sci. NS-44(3) (1997) 629. [4] R.J. McIntyre, IEEE Trans. Electron Devices ED-13 (1966) 164.
References [1] L. Evensen, T. Westgaard, Nucl. Instr. and Meth. A 392 (1997) 206.
II. TRACKING (SOLID STATE)