An automatic beam focusing system for MeV protons

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1 Nuclear Instruments and Methods in Physics Research B 231 (2005) An automatic beam ocusing system or MeV protons C.N.B. Udalagama *, A.A. Bettiol, J.A. van Kan, E.J. Teo, M.B.H. Breese, T. Osipowicz, F. Watt Centre or Ion Beam Analysis (CIBA), Department o Physics, National University o Singapore, Blk S12, 2 Science Drive 3, Singapore , Singapore Available online 23 March 2005 Abstract An automatic ocusing system or MeV protons has been developed. The ocusing system utilises rapid real time proton induced secondary electron imaging o a calibration grid coupled with a modiied Gaussian it in order to take into account the enhanced secondary electron signal rom the calibration grid edge. The ocusing system has been successully applied to MeV protons ocused using a coupled triplet coniguration o magnetic quadrupole lenses (Oxord triplet). Automatic beam ocusing o a coarse beamspot o approximately (5 3.5) micrometres in the X and Y directions to a sub-micrometre beamspot o approximately ( ) micrometers was achieved at a beam current o about 50 pa. Ó 2005 Elsevier B.V. All rights reserved. PACS: Jx; Lc; Va Keywords: Proton beam writing; Automatic MeV proton beam ocusing; Secondary electron imaging 1. Introduction * Corresponding author. address: scip0216@nus.edu.sg (C.N.B. Udalagama). The Proton Beam Writing (PBW) acility at the Centre or Ion Beam Applications (CIBA) is used or multidisciplinary research in photonics, microluidics, bio-physics and semiconductor micromachining [1 4]. Unlike e-beam writing technology and electron microscopy, MeV proton ocusing technology is still in its development phase and is necessarily more complex because o the high momentum o the proton beam. Focusing is usually achieved using a combination o magnetic quadrupole lenses, and carried out by manually adjusting lens current power supplies whilst minimizing the dimensions o a proton induced luorescence image rom a suitable target. Alternatively, X/$ - see ront matter Ó 2005 Elsevier B.V. All rights reserved. doi: /j.nimb

2 390 C.N.B. Udalagama et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) or sub micron ocusing, the beam spot can be manually minimized by mapping proton induced signals (e.g. rom characteristic X-rays (PIXE), backscattered protons (RBS), ionoluminescence (IBIL), transmitted ions (STIM), proton induced secondary electrons etc.) whilst scanning the proton beam across a resolution standard. Since these manual modes o proton beam ocusing are inherently slow and dependent on user perception, training and experience, there is a need or an expeditious, rapid, reproducible and accurate way o beam ocusing and beamspot size determination. We have developed a system that maps secondary electron emission o a proton beam scanning across a calibration grid and produces X and Y line scans as the proton beam traverses horizontal and vertical edges. A modiied Gaussian unction is itted to the edge proiles and the FWHMs o the modiied Gaussian unctions in both the X and Y directions are minimised by computer control o the quadrupole lens currents. With this procedure, an MeV proton beam can be ocused to submicron spot sizes in 2 10 min, depending on the starting conditions. Due to the copious number o secondary electrons generated, the ocusing procedure can be carried out in real time and allows or automatic, rapid and accurate ocusing. 2. Description o hardware and sotware An automated beam ocusing system requires rapid monitoring o the variation o the FWHM o the beam spot in both X and Y directions, in response to the changes made to the magnetic quadrupole lens currents. This has been achieved in our system by using a combination o sotware developed in-house along with data acquisition computer cards rom National Instruments Ò or data acquisition and computer cards rom Oxord Microbeams Ò or the remote control o their OM52E quadrupole power supplies. The sotware written in the C++ programming language in the.net environment, utilizes the National Instruments Ò NIDAQ and IMAQ libraries or data acquisition, beam control and data presentation while using the libraries rom Numerical recipes in C++ [5] or some o the mathematical routines. Since our approach utilises rapid imaging, we have had to develop our own ast imaging system which uses proton induced secondary electrons. The process o the detection o the proton induced secondary electrons is via a scintillator which is connected to a voltage output based photomultiplier tube. The electron collection eiciency is bolstered by the application o a positive bias voltage (2.5 3 kv) around the scintillator. Further details o this system will be described in a uture publication, but essentially ollows the system described by Teo et al. [6]. At CIBA, three magnetic quadrupole lenses operating in the Oxord triplet coniguration are used or beam ocusing. Here the lens arrangement ollows a CDC (converge diverge converge) lens coniguration, where the irst 2 lenses are coupled, i.e. ed in series with the same current via one power supply. In this system, the demagniications in the X and Y plane (horizontal and vertical) are unbalanced and or the Singapore proton beam writing line are 228 and 60 respectively. However, this coniguration lends itsel to automatic ocusing because the ocus in the two orthogonal beam directions are decoupled to a irst approximation. In particular it is possible to optimize the beam spot in the vertical direction with minimal disruption to the horizontal ocus, although the decoupling is not so pronounced when the procedure is reversed. More details on the Oxord Triplet beam optics may be ound in [7], and, details o the Singapore proton beam line optics may be ound in [8,9]. Fitting unction: For ocusing MeV protons, the beam is scanned across a calibration grid which exhibits pronounced sharp edges, and line scans extracted in both the horizontal (X) and vertical (Y) directions. A mathematical it can then be used to extract the FWHM o the beamspot in both the X and Y directions. Previously, when using PIXE, STIM or RBS signals, the FWHM has been extracted using a complementary error unction [8,10]. However, or proton induced electron emission there is an enhanced electron emission at the edge, and thereore this unction needs to be mod-

3 C.N.B. Udalagama et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) iied. For electron emission thereore we have added another Gaussian unction to represent this edge eect. Given below is the mathematical treatment o the ÔmodiiedÕ error unction used in our it. Adopting a Gaussian shape, the normalized beam unction that represents the particle distribution o a beam with a given FWHM, centred around x = X 0 is given by BeamðxÞ ¼ 2 riiiiiii exp p ln 16 2 ðx X 0 Þ 2 I this beam is scanned over a sharp edge the collected yield will be obtained by integrating the above unction over all x ater been multiplied by a step unction. The step unction represents the sample edge and causes the beam unction to return a yield only in the region where there are primary particle interactions with the resolution standard. Let the sample edge be at x = a so that the step unction may be expressed as: 1 x 2 ½ 1; aš STEPðxÞ ¼ 0 x 2ða; 1Š: The yield collected by the detector will be given by YieldðX 0 ; ; aþ ¼ ¼ Z x¼þ1 Z x¼a : STEPðxÞBeamðxÞdx BeamðxÞdx ¼ 1 p 2 Er 2 iiiiiii!# x¼a ðx X 0 Þ ¼ 1 p 2 1 þ Er 2 iiiiiii!# ða X 0 Þ : I we now incorporate the enhanced electron emission by the augmentation o a Gaussian unction, the complete unction that represents the linescan due to an electron signal is given by F ðx 0 ; ; aþ ¼ 1 p 2 1 þ Er 2 iiiiiii!# ða X 0 Þ þ BeamðaÞ: Mathematical itting: The mathematical itting o the above unction to the linescan data was by means o the non-linear Levenberg Marquradt method. More details o this may be obtained rom [5]. The actual ormula used in the it is given below. The ive itted parameters are a, Herr, Hgau, Hlow and. p 2 iiiiiii!# F ðx 0 ; ; aþ ¼Herr 1 þ Er ða X 0 Þ 3. Results ln 16 þ Hgau exp ða X 2 0 Þ 2 þ Hlow: The beam resolution standard used was a 10 micron thick mesh standard abricated using proton beam writing and nickel electroplating, which eatures 15 lm holes and 15 lm grid bars [10]. In our preliminary investigations, we have used the simplest beam spot minimisation algorithm possible, that o incrementally stepping through each quadrupole lens current until a minimum FWHM is ound in each direction. Due to the decoupled nature o the X and Y ocus in the triplet coniguration, only 2 successive iterations in the X and Y directions were needed, taking around 2 10 min, depending on the initial ocus conditions. In the case described here, we ocused a 1 MeV proton beam to around 5 lm spot size (at around 50 pa current) and scanned the beam across the grid. These start conditions were chosen to relect the typical beam spot dimensions that can be achieved by reproducing the quadrupole lens currents at the best ocus conditions, the reproducibility in the ocus being limited by hysteresis. The 2D secondary electron map o the grid, and the corresponding X and Y line scans are shown in Fig. 1(a), (b) and (e). Fig. 1(c), (d) and () shows the results o the automatic ocusing, and indicates a spot size o lm. It should be mentioned at this stage that RBS and PIXE scanning over a

4 392 C.N.B. Udalagama et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) Fig. 1. (a) 2D proton induced secondary electron scan across a calibration grid, showing a beam spot resolution o around lm, (b) corresponding X line scan + it beore ocusing and (c) corresponding X scan + it, ater automatic ocusing. (d) 2D proton induced secondary electron scan across the same calibration grid ollowing automatic ocusing, showing a beam spot resolution o around lm, (e) Y line scan + it beore ocusing and () corresponding Y scan + it ater auto ocusing. straight edge results in a more accurate determination o the spot size, but since these processes produce a much reduced signal compared with secondary electron emission, they cannot be ei-

5 C.N.B. Udalagama et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) ciently used in the automatic ocussing process to sub-micron spot sizes. 4. Conclusions We have successully demonstrated the possibility o automatic beam ocusing using the Oxord triplet. Starting with a 5 lm coarse ocus, consistent with using preset quadrupole lens current or best ocus conditions, the automatic ocusing system was able to arrive at a sub-micrometer beamspot size within minutes. The crucial aspects o this work are the use o real time secondary electron imaging to increase data rates thereby reducing ocusing time, coupled with a modiied Gaussian it in order to take into account the enhanced secondary electron signal rom the calibration grid edge. Further work will involve the use o more robust itting algorithms, intelligent iterative ocusing algorithms and specially constructed calibration standards exhibiting sharper edges by van Kan et al. [these proceedings]. Reerences [1] T.C. Sum, A.A. Bettiol, J.A. van Kani, F. Watt, E.Y.B. Pun, K.K. Tung, APL 83 (2003) [2] F. Sun, D. Casse, J.A. VanKan, R. Ge, F. Watt, Tiss. Eng. 10 (2004) 267. [3] K. Ansari, J.A. van Kan, A.A. Bettiol, F. Watt, APL 85 (2004) 476. [4] E.J. Teo, E.P. Tavernier, M.B.H. Breese, A.A. Bettiol, F. Watt, M.H. Liu, D.J. Blackwood, Nucl. Instr. and Meth. B 222 (2004) 513. [5] William H. Press, Saul A. Teukolsky, William T. Vetterling, Brian P. Flannery Numerical Recipes in C++, 2nd Edition, Cambridge University Press, [2003]. [6] E.J. Teo, M.B.H. Breese, A.A. Bettiol, F. Watt, J. Vac. Sci. Technol. B 22 (2) (2004). [7] G.W. Grime, F. Watt, Beam Optics o Quadrupole Probe- Forming Systems Adam Hilger Ltd., Techno House, Redcli Way, Bristol BS1 6NX [1984]. [8] F. Watt, J.A. Van Kan, I. Rajta, A.A. Bettiol, T.F. Choo, M.B.H. Breese, T. Osipowicz, Nucl. Instr. and Meth. B 210 (2003) 14. [9] J.A. vankan, A.A. Bettiol, F. Watt Mat. Res. Soc. Symp. Proc. Vol. 777 T [10] F. Watt, I. Rajta, J.A. van Kan, A.A. Bettiol, T. Osipowicz, Nucl. Instr. and Meth. B 190 (2002) 306.