- Email: [email protected]
Proton beam writing using the high energy ion nanoprobe LIPSION
Nuclear Instruments and Methods in Physics Research B 231 (2005) 372–377 www.elsevier.com/locate/nimb
Proton beam writing using the high energy ion nanoprobe LIPSION F. Menzel a
a,*
, D. Spemann a, J. Lenzner b, J. Vogt a, T. Butz
a
Nuclear Solid State Physics, Faculty of Physics and Geosciences, University of Leipzig, Linne´str. 5, 04103 Leipzig, Germany b Semiconductor Physics, Faculty of Physics and Geosciences, University of Leipzig, Linne´str. 5, 04103 Leipzig, Germany Available online 17 March 2005
Abstract Proton beam writing (PBW) is a very unique technique capable of the direct creation of three dimensional structures with a very high aspect ratio. Since the high energy ion nanoprobe LIPSION has a very high spatial resolution and is therefore well suited for the creation of structures in the micrometre range or below, it is planned to establish the PBW technique at the University of Leipzig. The results of the first proton beam writing experiments at the LIPSION nanoprobe are presented in this article. Structures with high aspect ratio and smooth side walls with an edge definition of 0.2 lm were created in negative SU-8 photo resist using 2.25 MeV protons. Furthermore, investigations were carried out concerning the mechanical stability of single free standing walls in order to collect information for the targeted production of samples with smaller feature sizes in the submicrometre range. Up to now, wall widths down to 1.5 lm were achieved. However, smaller feature sizes could not be obtained due to beam spot fluctuations which enlarge the wall width by a factor of three. Self-supported structures were produced using 2.25 MeV protons and subsequently 1.5 MeV helium ions demonstrating the stability and accuracy of these real three dimensional structures. In addition, different methods for online dose normalization were tested showing that ionoluminescence is the most suitable method for this purpose. 2005 Elsevier B.V. All rights reserved. PACS: 7.78.+s; 81.05.Lg; 78.60.Hk Keywords: Proton beam writing; SU-8; Dose normalization; Ionoluminescence
1. Introduction *
Corresponding author. Tel.: +49 341 9732 707; fax: +49 341 9732 708. E-mail address: [email protected] (F. Menzel).
During the last years the development of methods for the production of structures in the micrometre range have become more important due to
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.01.085
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
the progressive development for smaller components in technical devices. Proton beam writing is a direct writing technique able to produce three dimensional structures in the micrometre and submicrometre range [1,2] with high aspect ratio. Buried [3,4] or self-supported structures [5] can be created by using positive or negative photo resists like PMMA and SU-8, respectively. These structures can be applied directly, e.g. in microoptics as optical wave guides [3,6] and microlenses [7] or used as templates for the production of metallic structures by electroplating [8], e.g. microstamps for imprinting procedures. Furthermore, the creation of structures made of silicon is a new innovative way of proton beam writing [9,10]. Since the high energy ion nanoprobe LIPSION has a very high spatial resolution of less than 0.5 lm [11], it is well suited for the micromachining of structures with feature sizes of some hundreds of nanometers. In July 2003, the University of Leipzig started the research unit ‘‘Architecture of nano- and microdimensional building blocks’’. Within this framework proton beam writing will be established in order to create microstructures, e.g. for the patterned growth of ZnO nanowires. We report on the first results of proton beam writing at the LIPSION laboratory.
2. Experimental details For the experiments the negative photo resist SU-8 2010 from MicroChem Corp. was used. It was spin coated on silicon wafers of approximately 1.5 cm · 1.5 cm size at two different final rotational speeds of 1000 rpm and 3000 rpm in order to produce layer thicknesses of approximately 20 lm and 10 lm, respectively. After spin coating the samples were soft-baked in an convection oven at 90 C for 5, 10, and 15 min in order to study the influence of bake time on the mechanical stability of the structures. However, within this range no influence of the bake time on the stability of the structures was observed. Therefore, a bake time of 10 min was used for all subsequent experiments. The exposure experiments were carried out at the LIPSION laboratory with 2.25 MeV protons and additionally 1.5 MeV helium ions for the pro-
373
duction of self-supported structures. The proton beam was focused down to approximately 250 nm at a current of 1 pA, while for the helium ion beam of the same diameter, a current of 0.1 pA was used. Since a low beam current of the order of 1 pA is difficult to measure directly, it was determined by comparing its RBS yield from a scan of stainless steel with that obtained at high beam current. For the proton exposure of the resist layers a dose of 30 nC/mm2 was applied [1], whereas for the helium exposure 3 nC/mm2 was used, because the stopping power of the resist for 1.5 MeV helium ions is roughly 12 times higher than that for 2.25 MeV protons [12]. For this purpose, the beam was scanned at constant speed over the sample assuming a constant beam current. Up to now, due to the lack of a dedicated beam scanning software, all structures consist of lines and rectangles. After exposure, the samples were baked on a contact hot plate for 1 min at 65 C and 2 min at 95 C, then immersed in MicroChemÕs developer for at least 3 min in order to remove the unexposed resist, rinsed with isopropyl alcohol, and finally dried in an air flow. In recent experiments the post-exposure bake was omitted in order to prevent ‘‘T-topping’’, the formation of thin resist layers hanging over the top surface of the written structures [13]. The visualisation of the written structures was performed by scanning electron microscopy using 20 keV electrons. For this purpose, the structures were coated with gold.
3. Structures The first rectangular structures which were written at LIPSION have a very good edge definition of 0.2 lm up to 0.5 lm (Fig. 1(a)) and an aspect ratio better than 20. As can be seen in Fig. 1(a), the edge definition is limited by irregularities resulting from the scanning procedure. With increasing height of the structures, straggling effects become more prominent leading to a significant broadening of the structures higher than 50 lm (Fig. 1(b)). These effects are also visible in Fig. 1(c). This structure which was written in an
374
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
Fig. 1. SEM images of structures written in SU-8 using 2.25 MeV protons with (a) overall good edge definition, but edge irregularities due to the scanning procedure, (b) broadening of the structure base by straggling effects and (c) illustration of ion range in the resist.
area near the edge of the silicon substrate where the spin coated resist was much thicker reveals the ion range in the resist. In this case, the 2.25 MeV protons reached a depth of 83 lm in the resist while they penetrate thinner resist layers entirely and come to rest in the silicon substrate. During the development procedure the structure detached from the substrate. The ion range of 83 lm in SU-8 resist was compared with TRIM simulations of various different resist materials [12]. Epoxy (cast) with a density of 1.18 g/cm3 and a composition of C18H19O3 shows the best agreement with the experimental result. Besides rectangular structures, single free standing walls were produced by scanning single lines. The resulting wall width depends on the beam diameter and on the dose. Fig. 2(a) shows free standing walls with very smooth sidewalls, but large wall width due to a broad beam diameter because the sample was not placed exactly in the beam focus. The lines in Fig. 2(b) were created with three different doses of 70, 35, and 10 nC/mm2 proving that the wall width decreases with decreas-
ing ion dose. This is due to the Gaussian beam shape which results in larger effective beam spot sizes for higher exposure doses. The dependency of the wall stability on the wall width is also recognizable in this image. The waves in the two walls with the lower dose are clear indications of wall instabilities and are not caused by beam fluctuations during the writing procedure as can be seen from the straight edge at the wall bases. However, besides the waves, smaller ripples are also visible in this image. Some of these ripples which are visible at nearly all free standing walls and enlarge the wall width by a factor of three, have a regular structure while others are irregular. For the regular ones a frequency of 50 Hz was determined. In order to test some speculations about the origin of these ripples, e.g. mechanical vibrations or magnetic stray fields despite an active magnetic field compensation system [14], single lines were written under different experimental conditions. However, the origin is unknown up to now. Furthermore, self-supported structures were created (Fig. 3(a)). For this purpose, 9.6 lm broad
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
375
Fig. 2. SEM images of walls micromachined in SU-8. (a) Smooth and stable walls with a height of 27 lm and a comparably large width of 3 lm. (b) Walls with a height of 20 lm showing the decreasing wall stability with decreasing wall width.
Fig. 3. SEM images of self-supported structures created with 2.25 MeV protons and subsequently 1.5 MeV helium ions. (a) The smallest supported walls are bent due to a loss of mechanical stability caused by omitting the post-exposure bake. In (b) the helium ion range in the resist of 6 lm is visible. (c) Some of the slits are covered by SU-8 resist due to ‘‘T-topping’’ effects.
support walls with a distance of 8 lm, 19 lm, 28 lm, and 37 lm to each other were created using 2.25 MeV protons. The supported walls were written with 1.5 MeV helium ions which have a range of 6 lm in the SU-8 resist (Fig. 3(b)). This is in good agreement with TRIM simulations of Epoxy (cast). The supported walls are 1.7 lm, 2.6 lm, 6.3 lm, and 11.2 lm wide and have a separation distance of 10 lm to each other. This structure
was created two times in order to test the influence of the development procedure on the quality of the structure. One structure was developed including the post-exposure bake step (Fig. 3(c)). For this structure strong ‘‘T-topping’’ effects were observed. Some of the unexposed slits are completely covered by thin resist layers. Apart from this, the supported walls are very stable both between the supporting walls and at the protruding levitated endings. The
376
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
large roughness of the sidewalls is caused by the fact that the structures are produced by just a single scan, where beam spot fluctuations are not averaged out. Therefore, for the second structure (Fig. 3(a) and (b)) each wall was scanned two times and the post-exposure bake was omitted. As can be seen the slits of this structure are now uncovered, however, the smallest supported walls are bent indicating a lower mechanical stability.
4. Dose normalization Up to now, the proton beam writing experiments were carried out with a constant scan speed assuming a constant beam current. However, taking into account that there might be beam current instabilities, an on-line method for dose normalization on a pixel by pixel basis is needed to ensure an accurate exposure dose. For this purpose, four different methods were tested. From RBS measurements during the exposure 0.0012 counts/pixel and from PIXE 0.0063 counts/pixel were obtained
(pixel size: 100 nm · 100 nm). Collecting secondary electrons as suggested by Bettiol et al. [15] resulted in 0.0322 counts/pixel. Furthermore, in order to avoid upcharging of the resist during exposure which significantly influences the observed SE yield, the deposition of a thin metal layer on top of the resist is necessary. In conclusion, these three methods are not sensitive enough for dose normalization. Finally, a dose normalization method using ionoluminescence tested by Udalagama et al. [16] was carried out. For this purpose, single photons caused by ionoluminescence of the SU-8 resist were detected during the writing procedure using a photo channel multiplier tube C982P from PerkinElmer. During first tests using 2.25 MeV protons at a beam current of 0.54 pA on a 20 lm thick SU-8 resist layer a count rate of 22000 cps, i.e. 200 counts/pixel (pixel size: 400 nm · 400 nm) was observed. The single lines shown in Fig. 4(a) were written using ionoluminescence for dose normalization which proves that this method works well. In order to test its capability the beam energy stabilization of the accelerator
Fig. 4. SEM images of dose normalization tests using ionoluminescence with (a) normal experimental conditions, (b) with unstable beam current conditions and three different constant scan speeds, and (c) with unstable beam current conditions and ionoluminescence dose normalization.
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
was switched off and settings were used that resulted in a very unstable beam current. Under these conditions single lines were written using a constant scan speed without dose normalization (Fig. 4(b)) as well as written using the ionoluminescence dose normalization (Fig. 4(c)). In Fig. 4(b) massive effects due to under-exposure caused by beam current fluctuations are visible, while the even wall width of the lines shown in Fig. 4(c) proves that the dose normalization by ionoluminescence works well.
5. Conclusion and outlook In the first proton beam writing experiments at the high energy nanoprobe LIPSION structures with feature sizes down to 1.5 lm with an edge definition of 0.2 lm up to 0.5 lm were created. Furthermore, self-supported structures were produced and a dose normalization method using ionoluminescence was tested successfully. However, due to fluctuations of the beam spot position the feature sizes are significantly larger than the beam diameter. Therefore, further investigations will concentrate on determining the origin of the disturbing fluctuations in order to overcome this limitation. For the creation of mechanically stable structures with smaller feature sizes thinner resist layers will be used in the future. The production of dedicated structures for the single ion bombardment of living cells at LIPSION is planned as well. Furthermore, a new scan program for writing arbitrarily shaped structures will be developed and implemented in co-operation with the University of Melbourne and experiments concerning the creation of structures in silicon and of buried structures in positive PMMA resist are planned in the future.
377
Acknowledgment The authors would like to thank Dr. H. Herrnberger for his support during the preparation of resist layers. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the research unit grant FOR 522.
References [1] J.A. van Kan, T.C. Sum, T. Osipowicz, F. Watt, Nucl. Instr. and Meth. B 161–163 (2000) 366. [2] 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. [3] M.L. Bibra, A. Roberts, S.D. Dods, Nucl. Instr. and Meth. B 168 (2000) 47. [4] Y. Gonin, F. Munnik, F. Benninger, S. Mikhaı¨lov, Appl. Surf. Sci. 217 (2003) 289. [5] J.A. van Kan, J.L. Sanchez, B. Xu, T. Osipowicz, F. Watt, Nucl. Instr. and Meth. B 158 (1999) 179. [6] T.C. Sum, A.A. Bettiol, H.L. Seng, I. Rajta, J.A. van Kan, F. Watt, Nucl. Instr. and Meth. B 210 (2003) 266. [7] A.A. Bettiol, T.C. Sum, J.A. van Kan, F. Watt, Nucl. Instr. and Meth. B 210 (2003) 250. [8] J.A. van Kan, A.A. Bettiol, F. Watt, Nucl. Instr. and Meth. B 181 (2001) 258. [9] P. Polesello, C. Manfredotti, F. Fizzotti, R. Lu, E. Vittone, G. Lerondel, A.M. Rossi, G. Amato, L. Boarino, S. Galassini, M. Jaksic, Z. Pastuovic, Nucl. Instr. and Meth. B 158 (1999) 173. [10] 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. [11] D. Spemann, T. Reinert, J. Vogt, D. Dobrev, T. Butz, Nucl. Instr. and Meth. B 190 (2002) 312. [12] J.F. Ziegler (Ed.), The Stopping and Range of Ions in Matter, Vols. 2–6, Pergamon Press, New York, 1977–1985. [13] J.A. van Kan, private communication, 2004. [14] D. Spemann, T. Reinert, J. Vogt, J. Wassermann, T. Butz, Nucl. Instr. and Meth. B 210 (2003) 79. [15] A.A. Bettiol, I. Rajta, E.J. Teo, J.A. van Kan, F. Watt, Nucl. Instr. and Meth. B 190 (2002) 154. [16] C.N.B. Udalagama, A.A. Bettiol, J.A. van Kan, Frank Watt, Nucl. Instr. and Meth. B 210 (2003) 256.
Proton beam writing using the high energy ion nanoprobe LIPSION F. Menzel a
a,*
, D. Spemann a, J. Lenzner b, J. Vogt a, T. Butz
a
Nuclear Solid State Physics, Faculty of Physics and Geosciences, University of Leipzig, Linne´str. 5, 04103 Leipzig, Germany b Semiconductor Physics, Faculty of Physics and Geosciences, University of Leipzig, Linne´str. 5, 04103 Leipzig, Germany Available online 17 March 2005
Abstract Proton beam writing (PBW) is a very unique technique capable of the direct creation of three dimensional structures with a very high aspect ratio. Since the high energy ion nanoprobe LIPSION has a very high spatial resolution and is therefore well suited for the creation of structures in the micrometre range or below, it is planned to establish the PBW technique at the University of Leipzig. The results of the first proton beam writing experiments at the LIPSION nanoprobe are presented in this article. Structures with high aspect ratio and smooth side walls with an edge definition of 0.2 lm were created in negative SU-8 photo resist using 2.25 MeV protons. Furthermore, investigations were carried out concerning the mechanical stability of single free standing walls in order to collect information for the targeted production of samples with smaller feature sizes in the submicrometre range. Up to now, wall widths down to 1.5 lm were achieved. However, smaller feature sizes could not be obtained due to beam spot fluctuations which enlarge the wall width by a factor of three. Self-supported structures were produced using 2.25 MeV protons and subsequently 1.5 MeV helium ions demonstrating the stability and accuracy of these real three dimensional structures. In addition, different methods for online dose normalization were tested showing that ionoluminescence is the most suitable method for this purpose. 2005 Elsevier B.V. All rights reserved. PACS: 7.78.+s; 81.05.Lg; 78.60.Hk Keywords: Proton beam writing; SU-8; Dose normalization; Ionoluminescence
1. Introduction *
Corresponding author. Tel.: +49 341 9732 707; fax: +49 341 9732 708. E-mail address: [email protected] (F. Menzel).
During the last years the development of methods for the production of structures in the micrometre range have become more important due to
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.01.085
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
the progressive development for smaller components in technical devices. Proton beam writing is a direct writing technique able to produce three dimensional structures in the micrometre and submicrometre range [1,2] with high aspect ratio. Buried [3,4] or self-supported structures [5] can be created by using positive or negative photo resists like PMMA and SU-8, respectively. These structures can be applied directly, e.g. in microoptics as optical wave guides [3,6] and microlenses [7] or used as templates for the production of metallic structures by electroplating [8], e.g. microstamps for imprinting procedures. Furthermore, the creation of structures made of silicon is a new innovative way of proton beam writing [9,10]. Since the high energy ion nanoprobe LIPSION has a very high spatial resolution of less than 0.5 lm [11], it is well suited for the micromachining of structures with feature sizes of some hundreds of nanometers. In July 2003, the University of Leipzig started the research unit ‘‘Architecture of nano- and microdimensional building blocks’’. Within this framework proton beam writing will be established in order to create microstructures, e.g. for the patterned growth of ZnO nanowires. We report on the first results of proton beam writing at the LIPSION laboratory.
2. Experimental details For the experiments the negative photo resist SU-8 2010 from MicroChem Corp. was used. It was spin coated on silicon wafers of approximately 1.5 cm · 1.5 cm size at two different final rotational speeds of 1000 rpm and 3000 rpm in order to produce layer thicknesses of approximately 20 lm and 10 lm, respectively. After spin coating the samples were soft-baked in an convection oven at 90 C for 5, 10, and 15 min in order to study the influence of bake time on the mechanical stability of the structures. However, within this range no influence of the bake time on the stability of the structures was observed. Therefore, a bake time of 10 min was used for all subsequent experiments. The exposure experiments were carried out at the LIPSION laboratory with 2.25 MeV protons and additionally 1.5 MeV helium ions for the pro-
373
duction of self-supported structures. The proton beam was focused down to approximately 250 nm at a current of 1 pA, while for the helium ion beam of the same diameter, a current of 0.1 pA was used. Since a low beam current of the order of 1 pA is difficult to measure directly, it was determined by comparing its RBS yield from a scan of stainless steel with that obtained at high beam current. For the proton exposure of the resist layers a dose of 30 nC/mm2 was applied [1], whereas for the helium exposure 3 nC/mm2 was used, because the stopping power of the resist for 1.5 MeV helium ions is roughly 12 times higher than that for 2.25 MeV protons [12]. For this purpose, the beam was scanned at constant speed over the sample assuming a constant beam current. Up to now, due to the lack of a dedicated beam scanning software, all structures consist of lines and rectangles. After exposure, the samples were baked on a contact hot plate for 1 min at 65 C and 2 min at 95 C, then immersed in MicroChemÕs developer for at least 3 min in order to remove the unexposed resist, rinsed with isopropyl alcohol, and finally dried in an air flow. In recent experiments the post-exposure bake was omitted in order to prevent ‘‘T-topping’’, the formation of thin resist layers hanging over the top surface of the written structures [13]. The visualisation of the written structures was performed by scanning electron microscopy using 20 keV electrons. For this purpose, the structures were coated with gold.
3. Structures The first rectangular structures which were written at LIPSION have a very good edge definition of 0.2 lm up to 0.5 lm (Fig. 1(a)) and an aspect ratio better than 20. As can be seen in Fig. 1(a), the edge definition is limited by irregularities resulting from the scanning procedure. With increasing height of the structures, straggling effects become more prominent leading to a significant broadening of the structures higher than 50 lm (Fig. 1(b)). These effects are also visible in Fig. 1(c). This structure which was written in an
374
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
Fig. 1. SEM images of structures written in SU-8 using 2.25 MeV protons with (a) overall good edge definition, but edge irregularities due to the scanning procedure, (b) broadening of the structure base by straggling effects and (c) illustration of ion range in the resist.
area near the edge of the silicon substrate where the spin coated resist was much thicker reveals the ion range in the resist. In this case, the 2.25 MeV protons reached a depth of 83 lm in the resist while they penetrate thinner resist layers entirely and come to rest in the silicon substrate. During the development procedure the structure detached from the substrate. The ion range of 83 lm in SU-8 resist was compared with TRIM simulations of various different resist materials [12]. Epoxy (cast) with a density of 1.18 g/cm3 and a composition of C18H19O3 shows the best agreement with the experimental result. Besides rectangular structures, single free standing walls were produced by scanning single lines. The resulting wall width depends on the beam diameter and on the dose. Fig. 2(a) shows free standing walls with very smooth sidewalls, but large wall width due to a broad beam diameter because the sample was not placed exactly in the beam focus. The lines in Fig. 2(b) were created with three different doses of 70, 35, and 10 nC/mm2 proving that the wall width decreases with decreas-
ing ion dose. This is due to the Gaussian beam shape which results in larger effective beam spot sizes for higher exposure doses. The dependency of the wall stability on the wall width is also recognizable in this image. The waves in the two walls with the lower dose are clear indications of wall instabilities and are not caused by beam fluctuations during the writing procedure as can be seen from the straight edge at the wall bases. However, besides the waves, smaller ripples are also visible in this image. Some of these ripples which are visible at nearly all free standing walls and enlarge the wall width by a factor of three, have a regular structure while others are irregular. For the regular ones a frequency of 50 Hz was determined. In order to test some speculations about the origin of these ripples, e.g. mechanical vibrations or magnetic stray fields despite an active magnetic field compensation system [14], single lines were written under different experimental conditions. However, the origin is unknown up to now. Furthermore, self-supported structures were created (Fig. 3(a)). For this purpose, 9.6 lm broad
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
375
Fig. 2. SEM images of walls micromachined in SU-8. (a) Smooth and stable walls with a height of 27 lm and a comparably large width of 3 lm. (b) Walls with a height of 20 lm showing the decreasing wall stability with decreasing wall width.
Fig. 3. SEM images of self-supported structures created with 2.25 MeV protons and subsequently 1.5 MeV helium ions. (a) The smallest supported walls are bent due to a loss of mechanical stability caused by omitting the post-exposure bake. In (b) the helium ion range in the resist of 6 lm is visible. (c) Some of the slits are covered by SU-8 resist due to ‘‘T-topping’’ effects.
support walls with a distance of 8 lm, 19 lm, 28 lm, and 37 lm to each other were created using 2.25 MeV protons. The supported walls were written with 1.5 MeV helium ions which have a range of 6 lm in the SU-8 resist (Fig. 3(b)). This is in good agreement with TRIM simulations of Epoxy (cast). The supported walls are 1.7 lm, 2.6 lm, 6.3 lm, and 11.2 lm wide and have a separation distance of 10 lm to each other. This structure
was created two times in order to test the influence of the development procedure on the quality of the structure. One structure was developed including the post-exposure bake step (Fig. 3(c)). For this structure strong ‘‘T-topping’’ effects were observed. Some of the unexposed slits are completely covered by thin resist layers. Apart from this, the supported walls are very stable both between the supporting walls and at the protruding levitated endings. The
376
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
large roughness of the sidewalls is caused by the fact that the structures are produced by just a single scan, where beam spot fluctuations are not averaged out. Therefore, for the second structure (Fig. 3(a) and (b)) each wall was scanned two times and the post-exposure bake was omitted. As can be seen the slits of this structure are now uncovered, however, the smallest supported walls are bent indicating a lower mechanical stability.
4. Dose normalization Up to now, the proton beam writing experiments were carried out with a constant scan speed assuming a constant beam current. However, taking into account that there might be beam current instabilities, an on-line method for dose normalization on a pixel by pixel basis is needed to ensure an accurate exposure dose. For this purpose, four different methods were tested. From RBS measurements during the exposure 0.0012 counts/pixel and from PIXE 0.0063 counts/pixel were obtained
(pixel size: 100 nm · 100 nm). Collecting secondary electrons as suggested by Bettiol et al. [15] resulted in 0.0322 counts/pixel. Furthermore, in order to avoid upcharging of the resist during exposure which significantly influences the observed SE yield, the deposition of a thin metal layer on top of the resist is necessary. In conclusion, these three methods are not sensitive enough for dose normalization. Finally, a dose normalization method using ionoluminescence tested by Udalagama et al. [16] was carried out. For this purpose, single photons caused by ionoluminescence of the SU-8 resist were detected during the writing procedure using a photo channel multiplier tube C982P from PerkinElmer. During first tests using 2.25 MeV protons at a beam current of 0.54 pA on a 20 lm thick SU-8 resist layer a count rate of 22000 cps, i.e. 200 counts/pixel (pixel size: 400 nm · 400 nm) was observed. The single lines shown in Fig. 4(a) were written using ionoluminescence for dose normalization which proves that this method works well. In order to test its capability the beam energy stabilization of the accelerator
Fig. 4. SEM images of dose normalization tests using ionoluminescence with (a) normal experimental conditions, (b) with unstable beam current conditions and three different constant scan speeds, and (c) with unstable beam current conditions and ionoluminescence dose normalization.
F. Menzel et al. / Nucl. Instr. and Meth. in Phys. Res. B 231 (2005) 372–377
was switched off and settings were used that resulted in a very unstable beam current. Under these conditions single lines were written using a constant scan speed without dose normalization (Fig. 4(b)) as well as written using the ionoluminescence dose normalization (Fig. 4(c)). In Fig. 4(b) massive effects due to under-exposure caused by beam current fluctuations are visible, while the even wall width of the lines shown in Fig. 4(c) proves that the dose normalization by ionoluminescence works well.
5. Conclusion and outlook In the first proton beam writing experiments at the high energy nanoprobe LIPSION structures with feature sizes down to 1.5 lm with an edge definition of 0.2 lm up to 0.5 lm were created. Furthermore, self-supported structures were produced and a dose normalization method using ionoluminescence was tested successfully. However, due to fluctuations of the beam spot position the feature sizes are significantly larger than the beam diameter. Therefore, further investigations will concentrate on determining the origin of the disturbing fluctuations in order to overcome this limitation. For the creation of mechanically stable structures with smaller feature sizes thinner resist layers will be used in the future. The production of dedicated structures for the single ion bombardment of living cells at LIPSION is planned as well. Furthermore, a new scan program for writing arbitrarily shaped structures will be developed and implemented in co-operation with the University of Melbourne and experiments concerning the creation of structures in silicon and of buried structures in positive PMMA resist are planned in the future.
377
Acknowledgment The authors would like to thank Dr. H. Herrnberger for his support during the preparation of resist layers. This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the research unit grant FOR 522.
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