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Ion micro-beam diagnostics with photodetectors
Nuclear Instruments and Methods in Physics Research B 209 (2003) 340–344 www.elsevier.com/locate/nimb
Ion micro-beam diagnostics with photodetectors L. Cosentino a, P. Finocchiaro a,*, A. Pappalardo a, A. Hermanne b, H. Thienpont b, M. Vervaeke b, B. Volckaerts b, P. Vynck b a
INFN Laboratori Nazionali del Sud, via S.Sofia 44, Catania, I-95125, Italy b Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium
Abstract We have developed two techniques for microscopic ion beam imaging and profiling, both based on scintillators, particularly suitable for applications in deep lithography with protons (DLP) or with heavier ions. The first one employs a scintillating fiberoptic plate and a CCD camera with suitable lenses, the second makes use of a small scintillator optically coupled to a compact photomultiplier. We have proved the possibility of spanning from single beam particles counting up to several nA currents. Both the devices are successfully exploited for on-line control of proton beams, down to a beam size of less than 50 lm, in the framework of DLP application. Ó 2002 Elsevier B.V. All rights reserved. PACS: 42.82.Cr; 41.85.Qg; 41.85.Ew; 07.77.Ka; 29.27.Eg Keywords: Beam diagnostics; Micro-beam; Photodetectors; Lithography; Micro-optics
1. Introduction One of the interesting goals of the optoelectonics in the field of data communication, is to increase the transmission bandwidth by making use of fibre bundles coupled to arrays of integrated lasers (like e.g. VCSEL) [1]. As a possible solution for the optical coupling between a VCSEL and the related fibre bundle, a lens-based approach has recently been proposed. This raises the problem of producing arrays of spherical micro-lenses, along with the related mechanical supports and other micro-components like mirrors and prisms. One of
*
Corresponding author. Tel.: +39-095-542-284; fax: +39095-714-1815. E-mail address: fi[email protected] (P. Finocchiaro).
the envisaged solutions is to make use of the deep lithography with protons (DLP), which allows to produce several microscopic optical and mechanical structures [2]. The main purpose of DLP is to use a microscopic proton beam to produce a controlled damage in PMMA samples, which are later run through a selective chemical treatment (etching, swelling) in order to realize the needed microstructures. It is rather evident that controlling size and shape of the proton beam becomes of utmost importance, as the ‘‘a posteriori’’ check of the developed samples shows. This is why we decided to build two different devices, described in the following, improving previous schemes which proved to be useful in an environment of low-intensity radioactive beam diagnostics [3–5].
0168-583X/$ - see front matter Ó 2002 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(02)02021-9
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
2. The l-SFOP beam imaging sensor This device is mainly meant to get live images of the beam intensity distribution on the transverse plane. To this purpose we employed a scintillating fiber optic plate (SFOP), made from a bundle of terbium-glass scintillating fibres and a compact CCD camera watching it (Fig. 1). Each fibre in the bundle is 10 lm in diameter, while the overall plate size is 25 25 1:6 mm3 . From a practical point of view, we decided to make these first experiments in air. Hereby we separate the accelerator beam pipe, which is under vacuum, from our setup by means of a 75 lm thin aluminum window. A first collimation stage, made from an aluminum block 12 cm long, has a 1 mm square aperture. This insures an output beam size of the order of 1 mm2 , as compared to the input one typically of several square millimeters; moreover, such a long collimator provides an output beam with a high degree of parallelism (low divergence). The second, and final, collimation stage is a lithographic nickel mask featuring an array of high precision round holes of decreasing diameter from 1 mm to 20 lm. The thickness of this specific mask is 300 lm, which guarantees a full stop of the 11 MeV proton beam.
Fig. 1. Scheme of the l-SFOP beam sensor.
341
A remotely controlled device allows to perform the coarse and fine positioning of the mask in front of the first collimator, in order to select a particular aperture to be used for the irradiation. A second remotely controlled stage allows to translate the PMMA sample in the proton beam. Finally, the SFOP and its CCD camera are installed in front of the beam. In Fig. 2 we show a picture of the described setup. When the beam impinges on the SFOP it produces scintillation light, which is detected by the camera and displayed on a computer screen by means of a frame grabber device. In Fig. 3 we show a sample picture taken with a primary beam current of 30 pA and a mask hole of 150 lm (the current on the sensor was well below 1 pA). The corresponding measured X and Y diameters are 133 and 139 lm (FWHM), as can be seen in Figs. 4 and 5. We remark that the SFOP is also sensitive to photons, namely X and gamma rays. X-rays are ruled out because of the nickel mask thickness, as comes out from a simple evaluation of all the possible enegy levels available and the attenuation coefficient in nickel. Gamma rays are produced rather copiously by the proton beam, both inside the long aluminum collimator and on the nickel mask. This is clearly seen in the two plots, where the larger bell-shaped curve is produced by gamma rays and has an FWHM of about 1 mm, consistent with the 1 mm collimator size. The gamma background is quite useful for the precision alignment between the collimator and the mask: Figs. 4 and 5 show how the X alignment
Fig. 2. The l-SFOP beam sensor in the irradiation setup.
342
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
Fig. 3. Live picture of a 30 pA proton beam after the 1 mm collimator and the lithographic mask while selecting a 150 lm aperture.
Fig. 5. Y profile of the previous picture. The measured width is 139 lm for the beam spot and 1264 lm for the gamma background (FWHM), in good agreement with the collimator and mask apertures.
profiles. It basically consists of a 1 1 0:2 cm3 CsI(Tl) scintillator optically coupled to a compact photomultiplier (Hamamatsu 5774) by means of a prism-shaped lightguide. In front of the scintillator we should place a beam stopper with two 20 lm slits, perpendicular to each other. A suitable single 1D translation (at 45° with respect to both slits) would allow to reconstruct the X and Y beam intensity profiles. Unfortunately producing such a stopper is not straightforward, therefore for the time being we fixed an aluminum beam stopper
Fig. 4. X profile of the previous picture. The measured width is 133 lm for the beam spot and 1099 lm for the gamma background (FWHM), in good agreement with the collimator and mask apertures.
was pretty good, while there was a slight misalignment in the Y direction.
3. The l-SBBS beam sensor 3.1. The l-SBBS as a profiling sensor The l-SBBS (scintillator based beam sensor) device is meant to reconstruct the x and y beam
Fig. 6. Scheme of the l-SBBS beam sensor: aluminum stopper (a), CsI scintillator (b), lightguide (c), photomultiplier (d).
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
343
Fig. 8. Proton count rate on l-SBBS as a function of the square of the beam diameter.
Fig. 7. X and Y profiles as measured with the l-SBBS beam sensor. Upper part: the raw data; lower part: the derivative represents the X and Y profiles.
with two sharp edges perpendicular to each other, as shown in Fig. 6. The output signal from the photomultiplier is handled by an I–V converter, which converts the anodic current into a more friendly voltage one that is fed into an ADC; at the same time the I–V converter provides the unperturbed pulse output, useful in case of single particle counting. A 1D scan of the beam with this device, by means of a high precision translation stage, provides incremental information about the fraction of beam stopped by the aluminum. The derivative of the measured function, after suitably scaling the translation axis by square root of 2 (cos 45°), represents the intensity profile along the x and y directions in the transverse plane, as shown in Fig. 7. 3.2. The l-SBBS as a beam current sensor The l-SBBS has also been used for single beam particle counting, as each proton impinging on the scintillator produces a characteristic scintillation pulse. As long as the beam rate is low enough (below 105 particles per second) a discriminator
with the proper threshold setting is sensitive to the beam particles and blind to the background radiation pulses. With a fixed primary beam current we selected several different intensities by using different collimators, and then counted the number of detected protons in 20 s. The beam stopper in front of the scintillator was removed for these integral measurements and the sensor was positioned in front of the collimator hole. Fig. 8 shows the count rate as a function of the square of the collimator diameter, that is expected to be roughly proportional to the beam current emerging from the collimator itself and hitting the sensor. 4. Discussion The starting point of our development was the need for a better control of the DLP process, in order to have an on-line check of the beam quality and as a consequence to improve the overall quality of the final samples. Our results tell us that a beam of several tens fA intensity (105 particles per second) and a few tens lm width can be imaged on-line using the nearly off-the-shelf l-SFOP sensor. We have also proved that such a device is truly interactive, indeed during our tests its display screen was installed on the accelerator console in order to help the operators. While in most cases the standard accelerator equipment was unable to sense any beam, the 8-bit frame grabber reading out the l-SFOP was often close to saturation because of the large amount of scintillation light produced by the sensor. Moreover, the short decay
344
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
time of the terbium-glass light, about 3 ms, gives no appreciable afterglow: this means that even fast beam fluctuations, in terms of intensity, position, shape or size, can be observed on-line. To prove this we have also recorded digital movies while moving the mask up and down in front of the beam, showing the different holes sliding in and out on the display in real time. Concerning the l-SBBS we have to admit that even though it is very sensitive and powerful, it has been to some extent overruled by the l-SFOP with respect to beam profiling: the latter showed to be surprisingly more sensitive than expected. However, should a space resolution below 20 lm be needed, l-SBBS with a high precision beam stopper could be the optimum solution. On the other hand l-SBBS has shown to be a friendly and reliable device to measure the deposited dose: it can count the beam particles one by one at very low intensity and at the same time it provides a DC output signal which can be used for its absolute self-calibration versus the count rate. At higher beam intensity only the DC output is meaningful and its absolute calibration still holds.
5. Conclusions The two sensors we developed for microscopic ion beam imaging and profiling, both based on scintillators, showed to be quite promising for applications in deep lithography with protons and with heavier ions. Their surprisingly good performance make them suitable candidates for any other application where ion micro-beams are involved.
References [1] H. Thienpont et al., Proc. IEEE 88 (6) (2000) 769. [2] B. Volckaerts et al., Asian J. Phys. 10 (2) (2001) 195; B. Volckaerts et al., Proceedings of the International Conference on Optical MEMS, 2000 IEEE/LEOS, Kauai, Hawaii, 2000, p. 103. [3] L. Cosentino, P. Finocchiaro, IEEE Trans. Nucl. Sci. 48 (4) (2001) 1132. [4] P. Finocchiaro et al., Nucl. Instr. and Meth. A 437 (1999) 552. [5] P. Finocchiaro, Proceedings of the 15th International Conference on the Application of Accelerators in Research and Industry, 4–7 November 1998, Denton, Texas, USA.
Ion micro-beam diagnostics with photodetectors L. Cosentino a, P. Finocchiaro a,*, A. Pappalardo a, A. Hermanne b, H. Thienpont b, M. Vervaeke b, B. Volckaerts b, P. Vynck b a
INFN Laboratori Nazionali del Sud, via S.Sofia 44, Catania, I-95125, Italy b Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussel, Belgium
Abstract We have developed two techniques for microscopic ion beam imaging and profiling, both based on scintillators, particularly suitable for applications in deep lithography with protons (DLP) or with heavier ions. The first one employs a scintillating fiberoptic plate and a CCD camera with suitable lenses, the second makes use of a small scintillator optically coupled to a compact photomultiplier. We have proved the possibility of spanning from single beam particles counting up to several nA currents. Both the devices are successfully exploited for on-line control of proton beams, down to a beam size of less than 50 lm, in the framework of DLP application. Ó 2002 Elsevier B.V. All rights reserved. PACS: 42.82.Cr; 41.85.Qg; 41.85.Ew; 07.77.Ka; 29.27.Eg Keywords: Beam diagnostics; Micro-beam; Photodetectors; Lithography; Micro-optics
1. Introduction One of the interesting goals of the optoelectonics in the field of data communication, is to increase the transmission bandwidth by making use of fibre bundles coupled to arrays of integrated lasers (like e.g. VCSEL) [1]. As a possible solution for the optical coupling between a VCSEL and the related fibre bundle, a lens-based approach has recently been proposed. This raises the problem of producing arrays of spherical micro-lenses, along with the related mechanical supports and other micro-components like mirrors and prisms. One of
*
Corresponding author. Tel.: +39-095-542-284; fax: +39095-714-1815. E-mail address: fi[email protected] (P. Finocchiaro).
the envisaged solutions is to make use of the deep lithography with protons (DLP), which allows to produce several microscopic optical and mechanical structures [2]. The main purpose of DLP is to use a microscopic proton beam to produce a controlled damage in PMMA samples, which are later run through a selective chemical treatment (etching, swelling) in order to realize the needed microstructures. It is rather evident that controlling size and shape of the proton beam becomes of utmost importance, as the ‘‘a posteriori’’ check of the developed samples shows. This is why we decided to build two different devices, described in the following, improving previous schemes which proved to be useful in an environment of low-intensity radioactive beam diagnostics [3–5].
0168-583X/$ - see front matter Ó 2002 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(02)02021-9
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
2. The l-SFOP beam imaging sensor This device is mainly meant to get live images of the beam intensity distribution on the transverse plane. To this purpose we employed a scintillating fiber optic plate (SFOP), made from a bundle of terbium-glass scintillating fibres and a compact CCD camera watching it (Fig. 1). Each fibre in the bundle is 10 lm in diameter, while the overall plate size is 25 25 1:6 mm3 . From a practical point of view, we decided to make these first experiments in air. Hereby we separate the accelerator beam pipe, which is under vacuum, from our setup by means of a 75 lm thin aluminum window. A first collimation stage, made from an aluminum block 12 cm long, has a 1 mm square aperture. This insures an output beam size of the order of 1 mm2 , as compared to the input one typically of several square millimeters; moreover, such a long collimator provides an output beam with a high degree of parallelism (low divergence). The second, and final, collimation stage is a lithographic nickel mask featuring an array of high precision round holes of decreasing diameter from 1 mm to 20 lm. The thickness of this specific mask is 300 lm, which guarantees a full stop of the 11 MeV proton beam.
Fig. 1. Scheme of the l-SFOP beam sensor.
341
A remotely controlled device allows to perform the coarse and fine positioning of the mask in front of the first collimator, in order to select a particular aperture to be used for the irradiation. A second remotely controlled stage allows to translate the PMMA sample in the proton beam. Finally, the SFOP and its CCD camera are installed in front of the beam. In Fig. 2 we show a picture of the described setup. When the beam impinges on the SFOP it produces scintillation light, which is detected by the camera and displayed on a computer screen by means of a frame grabber device. In Fig. 3 we show a sample picture taken with a primary beam current of 30 pA and a mask hole of 150 lm (the current on the sensor was well below 1 pA). The corresponding measured X and Y diameters are 133 and 139 lm (FWHM), as can be seen in Figs. 4 and 5. We remark that the SFOP is also sensitive to photons, namely X and gamma rays. X-rays are ruled out because of the nickel mask thickness, as comes out from a simple evaluation of all the possible enegy levels available and the attenuation coefficient in nickel. Gamma rays are produced rather copiously by the proton beam, both inside the long aluminum collimator and on the nickel mask. This is clearly seen in the two plots, where the larger bell-shaped curve is produced by gamma rays and has an FWHM of about 1 mm, consistent with the 1 mm collimator size. The gamma background is quite useful for the precision alignment between the collimator and the mask: Figs. 4 and 5 show how the X alignment
Fig. 2. The l-SFOP beam sensor in the irradiation setup.
342
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
Fig. 3. Live picture of a 30 pA proton beam after the 1 mm collimator and the lithographic mask while selecting a 150 lm aperture.
Fig. 5. Y profile of the previous picture. The measured width is 139 lm for the beam spot and 1264 lm for the gamma background (FWHM), in good agreement with the collimator and mask apertures.
profiles. It basically consists of a 1 1 0:2 cm3 CsI(Tl) scintillator optically coupled to a compact photomultiplier (Hamamatsu 5774) by means of a prism-shaped lightguide. In front of the scintillator we should place a beam stopper with two 20 lm slits, perpendicular to each other. A suitable single 1D translation (at 45° with respect to both slits) would allow to reconstruct the X and Y beam intensity profiles. Unfortunately producing such a stopper is not straightforward, therefore for the time being we fixed an aluminum beam stopper
Fig. 4. X profile of the previous picture. The measured width is 133 lm for the beam spot and 1099 lm for the gamma background (FWHM), in good agreement with the collimator and mask apertures.
was pretty good, while there was a slight misalignment in the Y direction.
3. The l-SBBS beam sensor 3.1. The l-SBBS as a profiling sensor The l-SBBS (scintillator based beam sensor) device is meant to reconstruct the x and y beam
Fig. 6. Scheme of the l-SBBS beam sensor: aluminum stopper (a), CsI scintillator (b), lightguide (c), photomultiplier (d).
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
343
Fig. 8. Proton count rate on l-SBBS as a function of the square of the beam diameter.
Fig. 7. X and Y profiles as measured with the l-SBBS beam sensor. Upper part: the raw data; lower part: the derivative represents the X and Y profiles.
with two sharp edges perpendicular to each other, as shown in Fig. 6. The output signal from the photomultiplier is handled by an I–V converter, which converts the anodic current into a more friendly voltage one that is fed into an ADC; at the same time the I–V converter provides the unperturbed pulse output, useful in case of single particle counting. A 1D scan of the beam with this device, by means of a high precision translation stage, provides incremental information about the fraction of beam stopped by the aluminum. The derivative of the measured function, after suitably scaling the translation axis by square root of 2 (cos 45°), represents the intensity profile along the x and y directions in the transverse plane, as shown in Fig. 7. 3.2. The l-SBBS as a beam current sensor The l-SBBS has also been used for single beam particle counting, as each proton impinging on the scintillator produces a characteristic scintillation pulse. As long as the beam rate is low enough (below 105 particles per second) a discriminator
with the proper threshold setting is sensitive to the beam particles and blind to the background radiation pulses. With a fixed primary beam current we selected several different intensities by using different collimators, and then counted the number of detected protons in 20 s. The beam stopper in front of the scintillator was removed for these integral measurements and the sensor was positioned in front of the collimator hole. Fig. 8 shows the count rate as a function of the square of the collimator diameter, that is expected to be roughly proportional to the beam current emerging from the collimator itself and hitting the sensor. 4. Discussion The starting point of our development was the need for a better control of the DLP process, in order to have an on-line check of the beam quality and as a consequence to improve the overall quality of the final samples. Our results tell us that a beam of several tens fA intensity (105 particles per second) and a few tens lm width can be imaged on-line using the nearly off-the-shelf l-SFOP sensor. We have also proved that such a device is truly interactive, indeed during our tests its display screen was installed on the accelerator console in order to help the operators. While in most cases the standard accelerator equipment was unable to sense any beam, the 8-bit frame grabber reading out the l-SFOP was often close to saturation because of the large amount of scintillation light produced by the sensor. Moreover, the short decay
344
L. Cosentino et al. / Nucl. Instr. and Meth. in Phys. Res. B 209 (2003) 340–344
time of the terbium-glass light, about 3 ms, gives no appreciable afterglow: this means that even fast beam fluctuations, in terms of intensity, position, shape or size, can be observed on-line. To prove this we have also recorded digital movies while moving the mask up and down in front of the beam, showing the different holes sliding in and out on the display in real time. Concerning the l-SBBS we have to admit that even though it is very sensitive and powerful, it has been to some extent overruled by the l-SFOP with respect to beam profiling: the latter showed to be surprisingly more sensitive than expected. However, should a space resolution below 20 lm be needed, l-SBBS with a high precision beam stopper could be the optimum solution. On the other hand l-SBBS has shown to be a friendly and reliable device to measure the deposited dose: it can count the beam particles one by one at very low intensity and at the same time it provides a DC output signal which can be used for its absolute self-calibration versus the count rate. At higher beam intensity only the DC output is meaningful and its absolute calibration still holds.
5. Conclusions The two sensors we developed for microscopic ion beam imaging and profiling, both based on scintillators, showed to be quite promising for applications in deep lithography with protons and with heavier ions. Their surprisingly good performance make them suitable candidates for any other application where ion micro-beams are involved.
References [1] H. Thienpont et al., Proc. IEEE 88 (6) (2000) 769. [2] B. Volckaerts et al., Asian J. Phys. 10 (2) (2001) 195; B. Volckaerts et al., Proceedings of the International Conference on Optical MEMS, 2000 IEEE/LEOS, Kauai, Hawaii, 2000, p. 103. [3] L. Cosentino, P. Finocchiaro, IEEE Trans. Nucl. Sci. 48 (4) (2001) 1132. [4] P. Finocchiaro et al., Nucl. Instr. and Meth. A 437 (1999) 552. [5] P. Finocchiaro, Proceedings of the 15th International Conference on the Application of Accelerators in Research and Industry, 4–7 November 1998, Denton, Texas, USA.