- Email: [email protected]
A CVD-diamond based beam profile monitor for undulator radiation
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
A CVD-diamond based beam profile monitor for undulator radiation a . C. Schulze-Briesea,*, B. Ketterera, C. Pradervanda, Ch. Bronnimann , a a b b C. David , R. Horisberger , A. Puig-Molina , H. Graafsma a
Paul Scherrer Institute, Swiss Light Source, WSLA 217, CH-5232 Villigen PSI, Switzerland b ESRF, B.P. 220, F-38043 Grenoble Cedex, France
Abstract First results obtained with prototypes of a new type of beam profile monitor based on semi-transparent CVDdiamond are presented. The devices consists of a few micrometer thin membrane on a Si-substrate. Ti/Al-pixel are generated lithographically on the membrane while the signal wires are deposited on a dielectric in order to minimise signal pickup. Synchrotron radiation tests at 8 keV showed good signal homogeneity, spatial resolution and linearity for low resistivity diamond membranes. Considerable difficulties were encountered in the test of a detector grade diamond membrane. They are most likely related to enhanced surface conductivity and capacitive effects. # 2001 Published by Elsevier Science B.V. PACS: 07.85; 41.85 Keywords: Synchrotron radiation; Beam profile monitor; CVD-diamond; Automatic beamline alignment
1. Introduction Precise measurement of the position of undulator radiation is hampered by the superposition of radiation emitted by the upstream and downstream bending magnets. Since traditional 4-blade monitors measure the beam intensity at the fringes of the undulator central cone, their sensitivity to the wider bending magnet background is enhanced. We propose to overcome this difficulty with a new type of undulator beam position and *Corresponding author. Tel.: +41-56-310-4533; fax: +4156-310-3151. E-mail address: [email protected] (C. Schulze-Briese).
profile monitor based on a thin CVD-diamond membrane, which is sensitive in the region of the central cone only. Because of the much higher flux of the undulator in the region of the central cone, the positional information suffers minimal bending magnet radiation contamination. Several groups have reported on the design and test of quadrant X-ray Beam Position Monitors both for white beam and monochromatic synchrotron radiation [1–3]. The reasons for the choice of the material are manifold. CVD-diamond has outstanding mechanical, thermal and electronic properties and can be produced in large areas. It is UHV-compatible and extremely radiation hard when produced appropriately. Moreover, its sur-
0168-9002/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 8 1 - 9
C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
faces can be polished to few nanometer roughness but most importantly, thin diamond membranes are semi-transparent to X-rays with energies greater than 3 keV. Therefore, a CVD-diamond based XBPM can stay permanently in the beam and maybe used as a sensor in a feedback system to stabilise the beam position. With the advances in the focussing of undulator radiation from low emittance synchrotron sources an accurate control of the beam position has become more and more important. In addition to the control of the beam position, it is very desirable to monitor the beam profile at several positions along the beam path in particular when the energy is changed frequently like in MAD experiments. The profile information not only allows to detect misalignment of the optical components but also to derive the focus position e.g. to focus on the detector. Finally, the information can be used for automatic alignment of the beamline. We report here on the design and test of a prototype of a profile, position and intensity monitor based on a thin CVD-diamond membrane. The profile information is obtained by depositing an array of metal contacts on the diamond surface and routing the measured photocurrent of each pixel to a charge sensitive amplifier chip. The measured beam profile and position is calibration independent.
2. Technical realisation 2.1. Diamond materials Two different diamond species were used for the tests. Sample CSEM 1–3 were grown by means of hot filament deposition on p-Si(1 0 0) wafers, coated with 790 nm of SiO2 and 150 nm of Si3N4. The growth rate was 0.15 mm/h up to a thickness of 1.3 mm. Surface roughness was measured to be ( (RMS). The resistivity was found to be 250–400 A 109 O cm. The relatively small value is most likely due to the boron contamination of the filament wire resulting in a boron content of the diamond of 30 ppm [4]. The detector grade diamond De Beers was grown by a proprietary process (De Beers Industrial Diamonds, Ascot, UK). In order
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to reach a high charge collection efficiency, a 100 mm layer was grown, polished on the growth side, removed from substrate and subsequently thinned to 20 mm. The grain size should be between 10 and 20 mm, i.e. the membrane is formed by single grains. The membrane was then glued to a 300 mm Si-substrate by means of EPO-TEK H77. The resistivity of this sample was measured to be 5 1011 O cm and the surface roughness was ( (RMS). approximately 50 A 2.2. The APC chip The photocurrent from each channel will be routed to the preamplifier of an APC128 chip. The APC128 was originally designed as a readout chip for silicon strip detectors for HEP-applications. The ASIC chip consists of 128 low noise, low power preamplifiers, each followed by a 32 cell switched capacitor analog event pipeline, i.e. 32 time slices can be stored at a maximum frequency of 20 MHz [5]. A continuous read-out rate of 2.5 MHz was reported. The chip has an equivalent noise charge of 675 e+28 e/pF, which is negligible with respect to the expected signal levels. Presently the APC128 is redesigned in radiation hard DMILL technology which will facilitate its use close to intense synchrotron beams. Several APC chips can be read-out in series, i.e. the beam profile monitor can have n 128 pixel. For the present test of the prototype sensors a commercial 4-channel pico-amperemeter (LCAD4, GMS Frank Optic Products GmbH, Germany) was used. 2.3. Lithography Two protocols for the structuring of the metal pixel were developed: (a) starting with a dielectric ( thickness and (b) starting with a layer of 1000 A ( The tested sensors were Ti/Al layer of 150/1000 A. fabricated according to the latter scheme. Prior to processing the CSEM diamonds were cleaned in H2SO4 and H2O2 (2 : 1). A 5 5 mm2 opening was etched into the Si-substrate. The diamond surface was cleaned by means of Ar sputtering (5 min at 450 V, PAr ¼ 0:01 mbar) before the Ti and Al were sputtered. After creation of the pixel structure a
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( was sputtered and then Si3N4 layer of 1000 A removed on the position of the pixel. The tracks and contacts to the pixel are formed by an additional layer of Ti/Al. Finally an Al/Ti layer ( was sputtered on the backside. of 150/1000 A Fig. 1 shows the pixel structure. The sensors were then mounted on a chip carrier and the ten small and ten of the large pixel were wire bonded to the tracks of the carrier.
3. Results 3.1. Experimental setup The tests were carried out on the optics beamline at ESRF (BM5). The energy was set by means of a Si(1 1 1) double crystal monochromator to 8 keV and the beamsize was controlled with two
Fig. 1. Microscopic image of the pixel structure on a 1.3 mm thick membrane. The small pixel are 110 290 mm2 and the large ones 400 400 mm2. The width of the tracks is 15 mm and their pitch is 30 mm.
sets of double slits. The flux density of the unfocussed beam incident on the samples was 2 108 phts/s/100 mA/mm2 and the synchrotron current was between 45 and 80 mA during the tests. The sensors were mounted to a XY-stage and a Si-PIN diode was positioned at 15 cm distance to monitor the primary beam intensity. 3.2. CSEM sensors The current was measured as a function of the bias voltage applied to the membrane. In general, one would expect a linear increase of the signal which saturates at higher field levels. However, we could only observe an increase of the current from 0.45 to 0.75 pA when the field was turned on for CSEM 1 while it stayed constant for CSEM 2 independently of the field. The charge collection efficiency for the photocurrent of 0.3 pA is approximately 8% and the total is 21%. The total charge collection efficiency of the large and small pixel of CSEM 1 were found to be 22% and 25%, respectively. The homogeneity of response of the large pixel was found to be 0.84 0.15 and 1.07 0.08 pA for sensor 2 and 1 respectively, and 0.2 0.02 pA for the small pixel of the latter one. High resolution CCD images (pixel size: 3.3 mm, FWHM: 10 mm) were taken of the primary beam and of the beam as transmitted by the monitors at 45 and 90 cm distance in order to verify if the structured diamond membrane would give rise to phase contrast. The subtraction images exhibited a remaining RMS-noise between 2% and 2.5% vs. 11% for the background image alone. They showed no indication of the pixel structure. Due to the very small signal it was not possible to measure the resolution of the device with a small beam or to a map the response within one pixel. Fig. 2 shows the response curves of two adjacent 0.4 mm pixel as measured with an incident beam of 0.1 0.5 mm2. The FWHM of the two peaks is 0.44 and 0.52 mm, respectively. The linearity of response was measured by varying the size of the beam incident on a large pixel. The response is linear until part of the incident beam falls off the pixel. Moreover, the measurement showed that the pixel picks up some signal as far as 0.5 mm from its centre, in
C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
Fig. 2. Response of two adjacent 0.4 mm pixel.
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exhibited some cracks after the membrane had been structured, but it was still possible to bond most of the pixel. When exposed to synchrotron radiation, all pixel showed a very broad response curve that did not provide useful geometrical information. Moreover, the measured photocurrent was far below the expected level. It is believed that this surprising behaviour has its origin in a capacitive surface effect. As laid out by Mayer et al. [6], a hydrogen terminated diamond surface with adsorbed water molecules leads to an electron transfer from the diamond to the water, driven by the difference of the chemical potential of water and diamond. The holes in the diamond form an accumulation layer compensating the remaining the anions ðHCO 3 Þ in the water phase. Fig. 3 shows the response of three pixel to a bias voltage of 15 V without being exposed to X-rays. The charging cannot be explained by the bulk capacitance of the diamond and the very slow decay, exhibiting a fast and a slow component, does not match a simple RC-element.
4. Conclusions
Fig. 3. Charging of pixel after applying a bias voltage of 15 V. The signal of the large pixel #3 is multiplied by 0.1. At T¼ 400 s the X-rays are turned on, then off and finally on again. The charge collection efficiency as derived from the jump of pixel #1 is 18.6%.
agreement with the results shown in Fig. 2. Finally, it was verified within the limit of the noise that the tracks do not pick up spurious signal.
It was demonstrated that a beam monitor based on a thin CVD diamond membrane can be built, which provides good spatial resolution, homogeneity of response and linearity so that it can be used as a profile, position and intensity monitor. In order to improve the signal-to-noise ratio microwave plasma enhanced CVD diamond material will be employed in the future. Since temperature in excess of 7008C dehydrogenate the surface fully [6], problems as encountered with the De Beers sensor can be avoided by coating the ( thermal Si3N4 layer cleaned surface with a 1000 A at 8508C. The full scale undulator beam profile monitor will have 16 16 pixel and a thickness between 5 and 20 mm. The pixel dimensions can be chosen according to the beam size.
3.3. De Beers sensor Acknowledgements Due to the temperatures of up to 2008C during processing and the large thermal expansion coefficient of the epoxy, the De Beers sensor
It is a pleasure to thank M. Horisberger for the sputtering of the sensors, R. Baldinger, S. Streuli
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C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
and E. Kirk (PSI) for bonding and A.K. Freund (ESRF) for providing the beam time.
References [1] H. Sakae, H. Aoyagi, M. Oura, T. Ohata, H. Shiwaku, S. Yamamoto, H. Suhiyama, K. Tanabe, K. Kobashi, H. Kitamura, J. Synchrotr. Radiat. 4 (1997) 204. [2] D. Shu, T.M. Kuzay, Y. Fang, J. Barraza, T. Cundiff, J. Synchrotr. Radiat. 5 (1998) 636.
[3] P. Bergonzo, A. Brambilla, D. Tromson, R.D. Marshall, C. Jany, F. Foulon, C. Gauthier, V.A. Sole, A. Rogalev, J. Goulon, J. Synchrotr. Radiat. 6 (1999) 1. [4] W. Haenni, Centre Suisse d’E´lectronique et de Microtechnique SA, private communication, 2000. [5] R. Horisberger, D. Pitzl, Nucl. Instr. and Meth. A 326 (1993) 92. [6] F. Mayer, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett., 85 (16) (2000) 3472.
A CVD-diamond based beam profile monitor for undulator radiation a . C. Schulze-Briesea,*, B. Ketterera, C. Pradervanda, Ch. Bronnimann , a a b b C. David , R. Horisberger , A. Puig-Molina , H. Graafsma a
Paul Scherrer Institute, Swiss Light Source, WSLA 217, CH-5232 Villigen PSI, Switzerland b ESRF, B.P. 220, F-38043 Grenoble Cedex, France
Abstract First results obtained with prototypes of a new type of beam profile monitor based on semi-transparent CVDdiamond are presented. The devices consists of a few micrometer thin membrane on a Si-substrate. Ti/Al-pixel are generated lithographically on the membrane while the signal wires are deposited on a dielectric in order to minimise signal pickup. Synchrotron radiation tests at 8 keV showed good signal homogeneity, spatial resolution and linearity for low resistivity diamond membranes. Considerable difficulties were encountered in the test of a detector grade diamond membrane. They are most likely related to enhanced surface conductivity and capacitive effects. # 2001 Published by Elsevier Science B.V. PACS: 07.85; 41.85 Keywords: Synchrotron radiation; Beam profile monitor; CVD-diamond; Automatic beamline alignment
1. Introduction Precise measurement of the position of undulator radiation is hampered by the superposition of radiation emitted by the upstream and downstream bending magnets. Since traditional 4-blade monitors measure the beam intensity at the fringes of the undulator central cone, their sensitivity to the wider bending magnet background is enhanced. We propose to overcome this difficulty with a new type of undulator beam position and *Corresponding author. Tel.: +41-56-310-4533; fax: +4156-310-3151. E-mail address: [email protected] (C. Schulze-Briese).
profile monitor based on a thin CVD-diamond membrane, which is sensitive in the region of the central cone only. Because of the much higher flux of the undulator in the region of the central cone, the positional information suffers minimal bending magnet radiation contamination. Several groups have reported on the design and test of quadrant X-ray Beam Position Monitors both for white beam and monochromatic synchrotron radiation [1–3]. The reasons for the choice of the material are manifold. CVD-diamond has outstanding mechanical, thermal and electronic properties and can be produced in large areas. It is UHV-compatible and extremely radiation hard when produced appropriately. Moreover, its sur-
0168-9002/01/$ - see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 2 8 1 - 9
C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
faces can be polished to few nanometer roughness but most importantly, thin diamond membranes are semi-transparent to X-rays with energies greater than 3 keV. Therefore, a CVD-diamond based XBPM can stay permanently in the beam and maybe used as a sensor in a feedback system to stabilise the beam position. With the advances in the focussing of undulator radiation from low emittance synchrotron sources an accurate control of the beam position has become more and more important. In addition to the control of the beam position, it is very desirable to monitor the beam profile at several positions along the beam path in particular when the energy is changed frequently like in MAD experiments. The profile information not only allows to detect misalignment of the optical components but also to derive the focus position e.g. to focus on the detector. Finally, the information can be used for automatic alignment of the beamline. We report here on the design and test of a prototype of a profile, position and intensity monitor based on a thin CVD-diamond membrane. The profile information is obtained by depositing an array of metal contacts on the diamond surface and routing the measured photocurrent of each pixel to a charge sensitive amplifier chip. The measured beam profile and position is calibration independent.
2. Technical realisation 2.1. Diamond materials Two different diamond species were used for the tests. Sample CSEM 1–3 were grown by means of hot filament deposition on p-Si(1 0 0) wafers, coated with 790 nm of SiO2 and 150 nm of Si3N4. The growth rate was 0.15 mm/h up to a thickness of 1.3 mm. Surface roughness was measured to be ( (RMS). The resistivity was found to be 250–400 A 109 O cm. The relatively small value is most likely due to the boron contamination of the filament wire resulting in a boron content of the diamond of 30 ppm [4]. The detector grade diamond De Beers was grown by a proprietary process (De Beers Industrial Diamonds, Ascot, UK). In order
231
to reach a high charge collection efficiency, a 100 mm layer was grown, polished on the growth side, removed from substrate and subsequently thinned to 20 mm. The grain size should be between 10 and 20 mm, i.e. the membrane is formed by single grains. The membrane was then glued to a 300 mm Si-substrate by means of EPO-TEK H77. The resistivity of this sample was measured to be 5 1011 O cm and the surface roughness was ( (RMS). approximately 50 A 2.2. The APC chip The photocurrent from each channel will be routed to the preamplifier of an APC128 chip. The APC128 was originally designed as a readout chip for silicon strip detectors for HEP-applications. The ASIC chip consists of 128 low noise, low power preamplifiers, each followed by a 32 cell switched capacitor analog event pipeline, i.e. 32 time slices can be stored at a maximum frequency of 20 MHz [5]. A continuous read-out rate of 2.5 MHz was reported. The chip has an equivalent noise charge of 675 e+28 e/pF, which is negligible with respect to the expected signal levels. Presently the APC128 is redesigned in radiation hard DMILL technology which will facilitate its use close to intense synchrotron beams. Several APC chips can be read-out in series, i.e. the beam profile monitor can have n 128 pixel. For the present test of the prototype sensors a commercial 4-channel pico-amperemeter (LCAD4, GMS Frank Optic Products GmbH, Germany) was used. 2.3. Lithography Two protocols for the structuring of the metal pixel were developed: (a) starting with a dielectric ( thickness and (b) starting with a layer of 1000 A ( The tested sensors were Ti/Al layer of 150/1000 A. fabricated according to the latter scheme. Prior to processing the CSEM diamonds were cleaned in H2SO4 and H2O2 (2 : 1). A 5 5 mm2 opening was etched into the Si-substrate. The diamond surface was cleaned by means of Ar sputtering (5 min at 450 V, PAr ¼ 0:01 mbar) before the Ti and Al were sputtered. After creation of the pixel structure a
232
C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
( was sputtered and then Si3N4 layer of 1000 A removed on the position of the pixel. The tracks and contacts to the pixel are formed by an additional layer of Ti/Al. Finally an Al/Ti layer ( was sputtered on the backside. of 150/1000 A Fig. 1 shows the pixel structure. The sensors were then mounted on a chip carrier and the ten small and ten of the large pixel were wire bonded to the tracks of the carrier.
3. Results 3.1. Experimental setup The tests were carried out on the optics beamline at ESRF (BM5). The energy was set by means of a Si(1 1 1) double crystal monochromator to 8 keV and the beamsize was controlled with two
Fig. 1. Microscopic image of the pixel structure on a 1.3 mm thick membrane. The small pixel are 110 290 mm2 and the large ones 400 400 mm2. The width of the tracks is 15 mm and their pitch is 30 mm.
sets of double slits. The flux density of the unfocussed beam incident on the samples was 2 108 phts/s/100 mA/mm2 and the synchrotron current was between 45 and 80 mA during the tests. The sensors were mounted to a XY-stage and a Si-PIN diode was positioned at 15 cm distance to monitor the primary beam intensity. 3.2. CSEM sensors The current was measured as a function of the bias voltage applied to the membrane. In general, one would expect a linear increase of the signal which saturates at higher field levels. However, we could only observe an increase of the current from 0.45 to 0.75 pA when the field was turned on for CSEM 1 while it stayed constant for CSEM 2 independently of the field. The charge collection efficiency for the photocurrent of 0.3 pA is approximately 8% and the total is 21%. The total charge collection efficiency of the large and small pixel of CSEM 1 were found to be 22% and 25%, respectively. The homogeneity of response of the large pixel was found to be 0.84 0.15 and 1.07 0.08 pA for sensor 2 and 1 respectively, and 0.2 0.02 pA for the small pixel of the latter one. High resolution CCD images (pixel size: 3.3 mm, FWHM: 10 mm) were taken of the primary beam and of the beam as transmitted by the monitors at 45 and 90 cm distance in order to verify if the structured diamond membrane would give rise to phase contrast. The subtraction images exhibited a remaining RMS-noise between 2% and 2.5% vs. 11% for the background image alone. They showed no indication of the pixel structure. Due to the very small signal it was not possible to measure the resolution of the device with a small beam or to a map the response within one pixel. Fig. 2 shows the response curves of two adjacent 0.4 mm pixel as measured with an incident beam of 0.1 0.5 mm2. The FWHM of the two peaks is 0.44 and 0.52 mm, respectively. The linearity of response was measured by varying the size of the beam incident on a large pixel. The response is linear until part of the incident beam falls off the pixel. Moreover, the measurement showed that the pixel picks up some signal as far as 0.5 mm from its centre, in
C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
Fig. 2. Response of two adjacent 0.4 mm pixel.
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exhibited some cracks after the membrane had been structured, but it was still possible to bond most of the pixel. When exposed to synchrotron radiation, all pixel showed a very broad response curve that did not provide useful geometrical information. Moreover, the measured photocurrent was far below the expected level. It is believed that this surprising behaviour has its origin in a capacitive surface effect. As laid out by Mayer et al. [6], a hydrogen terminated diamond surface with adsorbed water molecules leads to an electron transfer from the diamond to the water, driven by the difference of the chemical potential of water and diamond. The holes in the diamond form an accumulation layer compensating the remaining the anions ðHCO 3 Þ in the water phase. Fig. 3 shows the response of three pixel to a bias voltage of 15 V without being exposed to X-rays. The charging cannot be explained by the bulk capacitance of the diamond and the very slow decay, exhibiting a fast and a slow component, does not match a simple RC-element.
4. Conclusions
Fig. 3. Charging of pixel after applying a bias voltage of 15 V. The signal of the large pixel #3 is multiplied by 0.1. At T¼ 400 s the X-rays are turned on, then off and finally on again. The charge collection efficiency as derived from the jump of pixel #1 is 18.6%.
agreement with the results shown in Fig. 2. Finally, it was verified within the limit of the noise that the tracks do not pick up spurious signal.
It was demonstrated that a beam monitor based on a thin CVD diamond membrane can be built, which provides good spatial resolution, homogeneity of response and linearity so that it can be used as a profile, position and intensity monitor. In order to improve the signal-to-noise ratio microwave plasma enhanced CVD diamond material will be employed in the future. Since temperature in excess of 7008C dehydrogenate the surface fully [6], problems as encountered with the De Beers sensor can be avoided by coating the ( thermal Si3N4 layer cleaned surface with a 1000 A at 8508C. The full scale undulator beam profile monitor will have 16 16 pixel and a thickness between 5 and 20 mm. The pixel dimensions can be chosen according to the beam size.
3.3. De Beers sensor Acknowledgements Due to the temperatures of up to 2008C during processing and the large thermal expansion coefficient of the epoxy, the De Beers sensor
It is a pleasure to thank M. Horisberger for the sputtering of the sensors, R. Baldinger, S. Streuli
234
C. Schulze-Briese et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 230–234
and E. Kirk (PSI) for bonding and A.K. Freund (ESRF) for providing the beam time.
References [1] H. Sakae, H. Aoyagi, M. Oura, T. Ohata, H. Shiwaku, S. Yamamoto, H. Suhiyama, K. Tanabe, K. Kobashi, H. Kitamura, J. Synchrotr. Radiat. 4 (1997) 204. [2] D. Shu, T.M. Kuzay, Y. Fang, J. Barraza, T. Cundiff, J. Synchrotr. Radiat. 5 (1998) 636.
[3] P. Bergonzo, A. Brambilla, D. Tromson, R.D. Marshall, C. Jany, F. Foulon, C. Gauthier, V.A. Sole, A. Rogalev, J. Goulon, J. Synchrotr. Radiat. 6 (1999) 1. [4] W. Haenni, Centre Suisse d’E´lectronique et de Microtechnique SA, private communication, 2000. [5] R. Horisberger, D. Pitzl, Nucl. Instr. and Meth. A 326 (1993) 92. [6] F. Mayer, M. Riedel, B. Mantel, J. Ristein, L. Ley, Phys. Rev. Lett., 85 (16) (2000) 3472.