Implementation of MeV heavy-ion implantation into a microstructures with a defined pattern structure using a nuclear microprobe

Implementation of MeV heavy-ion implantation into a microstructures with a defined pattern structure using a nuclear microprobe Oleksandr Romanenko a , Petr Malinsky a,b,* , Vladimir Havranek a , Anna Mackova a,b a Nuclear Physics Institute of the Czech Academy of Sciences, Hlavní 130, ˇ Reˇ z 250 68, Czech Republic b Department of Physics, Faculty of Science, J. E. Purkinje University, Ceske Mladeze 8, Usti nad Labem 400 96, Czech Republic ARTICLE INFO Keywords: Ion beam writing Ion implantation Micropatterning Polymeric mask ABSTRACT Facilities equipped with electrostatic accelerators for ion beam analysis often perform high-energy ion beam implantation to dope target materials. A typical task of ion implantation is the uniform placement of ions over a large area using a wide-profile raster-scanned beam. Implantation of ions in a specific pattern (ion beam lithography) is also possible; however, this requires the use of masks, which are typically outsourced for production. Laboratories equipped with a nuclear microprobe can accomplish ion beam lithography without the need for masks, using the direct-write method, although the ion mass and energy may be limited by the magnetic rigidity of the microprobe focusing system. The aim of this work is to demonstrate the possibility of heavy ion lithography without the need for outsourced masks, using a nuclear microprobe as an auxiliary technique. The validity of the proposed method was established by implantation of 2.5 MeV gold ions into glass in a pattern with a minimum feature size of 3 μ m. The potential challenges of this method are described in detail. The most critical issue of this method is material incompatibility. 1. Introduction The ion microprobe has gained a good reputation in the field of ion beam analytical techniques as well as in proton beam lithography. By focusing an ion beam down to micrometer-scale dimensions and scanning a selected area of the sample, ion beam methods such as PIXE, RBS and ERDA have been elevated to a new level. As demonstrated by Amato et al. [1], Calligaro et al. [2], Torrisi et al. [3], Valter et al. [4] and Yamazaki et al. [5], it became possible not only to determine the elemental composition of the material, but also its spatial distribution. The high penetration depth of MeV protons has enabled proton beam lithography to create three-dimensional microstructures in photoresist with a high aspect ratio [6,7], which is unattainable using electron or laser beam lithography. Furthermore, the modification of materials that exhibit greater radiation resistance in comparison to photoresists necessitates the utilisation of ions heavier than protons and also typically possess higher energy level. The ion beam microprobe’s ability to use medium-heavy ions, such as nitrogen or oxygen allows the modification of the glass [8]. Another application of a ion beam microprobe is ion implantation, ranging from a single ion [9,10] to the desired fluence. However, this application is limited by the ion mass. This is due to the fact that the magnetic rigidity of the most quadrupole systems in conventional nuclear microprobe is insufficient to focus ions from the 5th or 6th periods of the periodic table at MeV energies. Consequently, the implantation of such heavy ions is conventionally undertaken through the utilization of masks and broad-beam scanning. It is evident that this requires fabrication of a multitude of masks, each corresponding to a distinct pattern. Despite the relatively low cost of producing a mask with a feature size ≥50 µm using laser cutting technology, this approach is not feasible when minimum detailing at the 1 µm level is required. In this work, a method for implanting high-energy heavy ions in a desired pattern is presented, which can be performed in-house in laboratories equipped with an electrostatic accelerator used for ion beam analysis methods, without the need for costly outsourcing of mask production. The method is based on ion beam microprobe as an auxiliary technique, with PMMA employed as a low-cost resist. 2. Experiment 2.1. Sample preparation Microscope Slides (25 ×75 ×1 mm) from Knittel Glass were used as * Corresponding author at: Nuclear Physics Institute of the Czech Academy of Sciences, Hlavní 130, ˇ Reˇ z 250 68, Czech Republic. E-mail address: [email protected] (P. Malinsky). Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, B journal homepage: www.elsevier.com/locate/nimb https://doi.org/10.1016/j.nimb.2025.165920 Received 13 August 2025; Received in revised form 3 October 2025; Accepted 27 October 2025 Nuclear Instruments and Methods in Physics Research B 569 (2025) 165920 Available online 31 October 2025 0168-583X/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ). a substrates to better visualize the processes occurring at each stage due to their transparency. Slides were cut into three parts to obtain square samples with a side length of 25 mm. Poly(methyl methacrylate) (PMMA) was used as the resist material. PMMA powder was dissolved in acetone at a weight ratio 1:8. It was then spin-coated onto the substrate at 2500 rpm for 120 s, resulting in a 10 μ m thick PMMA film on the glass. 2.2. Fabrication of PMMA mask Proton beam writing was performed using a microprobe chamber in our laboratory [11]. A 2 MeV proton beam, focused to a spot of 1.3 × 4.0 µm 2 at a beam current of 180 pA was used for direct ion beam lithography. Although the optimal dose for MeV proton beam writing in PMMA is considered to be in the range of 80–150 nC/mm 2 [12], in this work we used 160 nC/mm 2 (1 ×10 14 protons/cm 2 ). This value was determined to be optimal for our setup based on our prior internal research. The same research also showed that the best results were obtained by performing the irradiation not in a single scan but in 10 consecutive loops, which was chosen to reduce the radiation-induced heating effect and the influence of beam current fluctuation. After irradiation, the development of microstructures was carried out using conventional dip development in a 3:7 water/isopropanol (IPA) solution (volume ratio). 2.3. Heavy ion implantation Implantation of heavy ions was carried out in the implantation chamber in our laboratory. Gold ions with an energy of 2.5 MeV were used for better visualization of the implanted area. Irradiation was performed in raster scanning mode using an electrostatic scanning system. The beam current was monitored throughout the entire irradiation, the beam current density was 25nA/cm 2 and the accumulated dose was 1 ×10 15 at/cm 2 . 2.4. Mask removing The final step in the creating of the material with heavy ions implanted in the desired pattern is the removal of the resist mask. This is achieved by subjecting the sample to acetone until the mask is fully dissolved. The sample has thus been prepared for utilisation in the intended application. 3. Results and discussion 3.1. Procedure The general scheme of the proposed method is illustrated in Fig. 1, with the experimental details conducted to validate the method are indicated in square brackets. The substrate selected for this study was soda-lime glass (Knittel Microscope Slides) due to its smooth surface and transparency. The latter was necessary for the purpose of visual observation of changes in the material caused by implanted heavy ions. The substrates were characterised by dimensions of 25 ×75 ×1 mm 3 . The PMMA was utilised as a resist due to its low cost and accessibility. The PMMA coating was prepared by means of spin-coating at 2500 rpm for a period of 2 min. Prior to the commencement of the experiment, the samples were placed under vacuum overnight to allow any volatile products to degas, if any remained. The 2 MeV proton beam that we used for ion beam writing exhibits a substantial penetration depth (Fig. 2), thereby ensuring that the Bragg peak does not exert any influence on the regions of PMMA or glass that are of interest with regard to implanted gold ions. The fluence for proton beam writing was set at 1 ×10 14 protons/cm 2 (160 nC/mm 2 ). In order to reduce the radiation-induced heat effect and the associated carbonisation of the irradiated area of PMMA [13], the ion beam scanning of the pattern was performed in 10 consecutive loops. The passage of protons through PMMA has been Fig. 1. Scheme of the experiment. Fig. 2. Penetration depth of 2 MeV proton beam in our PMMA/glass samples calculated using SRIM software [17]. O. Romanenko et al. Nuclear Inst. and Methods in Physics Research, B 569 (2025) 165920 2 shown to result in a reduction in the material’s dimensions, thereby modifying its reflectivity. This, in turn, enables the observation of the irradiated area through the use of an optical microscope [14]. In this work, we employed conference-related text (“ANP2024//Thessaloniki, Greece//22–27 September 2024” – referring to the conference on Applied Nuclear Physics) and our institute’s logo (“UJF” – stands for Ústav Jadern´ e Fyziky) as templates. Fig. 3a shows optical microscopes images, in which the irradiated areas are visible. In the next step, the samples were immersed in a water/IPA solution (volume ratio 3:7) for a period of 2 min for development. Optical microscopy demonstrated that PMMA had been effectively removed from the irradiated area, and revealed the presence of long streaks emanating from the sharp-edged regions (Fig. 3b). These streaks are associated with an electrostatic discharge that occurred during the proton beam writing. Although we used an electron gun to compensate of the accumulation of positive charge on the sample caused by the proton beam, it seems that the electron current was insufficient. The heavy ion implantation was performed in an implantation chamber connected to the same accelerator that was used for proton beam writing. The selection of 2.5 MeV Au 2+ ions was based on the accelerator’s capabilities and their penetration depth in PMMA and glass, which is 1 µm and 0.6 µm, respectively. Irradiation was conducted utilising a raster scanning method, encompassing an area of 5 ×5 cm 2 . The current density was 25nA/cm 2 and the fluence of 1 ×10 15 at/cm 2 was achieved within 3.5 h. Despite the unavailability of a method for measuring the temperature of the sample itself within the chamber during the irradiation process in the current experimental configuration, an estimation was derived through the utilisation of the temperature data provided by the sample holder, which is equipped with a thermocouple. The recorded temperature of the sample holder during the irradiation did not exceed 30 ◦C, while the ambient temperature was 22 ◦C. This suggests that the temperature of the sample as a whole was below the glass transition temperature of PMMA (approximately 100 ◦C). Following the irradiation process, the PMMA film exhibit a dark colouration (Fig. 3c), suggesting that PMMA underwent a carbonisation process in the region where gold ions were present (Fig. 4). In the final step of the process, the samples with the prepared microstructures were immersed in acetone in order to remove the PMMA resist film. As demonstrated in Figure, the successful implantation of gold ions into the samples was achieved in accordance with the desired pattern in the microstructure, which was achieved by implanting Au ions and subsequently forming hydrocarbons through the irradiation of glass. The shape of the region with implanted ions is in precise alignment with the pattern obtained by proton beam writing, suggesting that the implantation process itself does not introduce any distortion into the final shape. The nature of the dark lines emanating from the pattern and associated with the electrostatic discharge, as mentioned earlier, will be discussed in the next subchapter. 3.2. Method-related problems The primary issue encountered during the experimental phase was the inadequate adhesion of PMMA to the glass substrate. It was demonstrated by some samples that the spin-coated PMMA film could be easily peeled off from the substrate (Fig. 5). Sometimes this phenomenon also occurred during other phases of the method (Fig. 6). This material incompatibility must be taken into account when selecting a resist material for a sample to be implanted. The adhesion of PMMA to glass can be enhanced by the utilisation of an interlayer, such as the selfcuring polyester poly(2-hydroxypropyl maleate) as cited by [15]. The second issue pertains to the presence of residual PMMA after development. If the PMMA in the irradiated area is not completely removed during development, this will have a subsequent impact on the implantation process. This phenomenon may be attributable to two potential causes: i) incorrect fluence during proton beam writing (PBW) or ii) insufficient development time. In Fig. 7a it can be seen that after development, the PMMA was completely removed from the irradiated area only in the upper left corner, while in other places it remained in varying amounts. The irradiated areas (during PBW) with undeveloped PMMA exhibited a black colouration analogous to that observed in the non-irradiated areas after gold implantation (Fig. 7b). Gold implanted in areas with undeveloped PMMA is located at a shallower depth compared to areas with developed PMMA, because the incident ions lose their energy on the PMMA residues. The situation was further exacerbated by the inability to remove these areas in the final step along with the rest of the PMMA (Fig. 7c). This phenomenon can be explained as follows: the thickness of the remaining undeveloped PMMA is less than or comparable to the penetration depth of gold ions into it (Fig. 8). Thus, there is Fig. 3. a) Change in refractive index of irradiated region caused by shrinkage of pmma under proton irradiation. b) Image of developed pmma mask. c) Sample after gold irradiation. O. Romanenko et al. Nuclear Inst. and Methods in Physics Research, B 569 (2025) 165920 3 no buffer of pure PMMA between the irradiation-modified part of PMMA and the glass substrate. The presence of this buffer zone plays a crucial role in mask removal, since modified PMMA does not dissolve in acetone. A similar explanation can be given for the lines associated with electrostatic discharge, with the only difference being that the nature of the modification of PMMA is caused not by irradiation, but by discharge. The final limitation of the method that merits attention to is the maximum attainable fluence. During the experimental process pertaining to the fabrication of microcapacitors in polyimide films using 2.5 MeV Au 2+ ions at a fluence of 6 ×10 15 at/cm 2 , the final stage of PMMA mask removal proved unsuccessful (Fig. 9). This does not necessarily imply that the maximum fluence for the method is below 6 ×10 15 at/ cm 2 . In order to achieve this fluence, a current density of 88nA/cm 2 was employed, with implantation over 6 h. The sample holder temperature during implantation was 47 ◦C, and the ambient temperature was 20 ◦C. This represents a substantial increase in comparison to the preceding experiment, described in the previous subchapter, in which the sample holder temperature increased by a mere 8 ◦C above ambient. Consequently, it is not possible to definitively conclude that the failure was caused solely by the fluence value, due to the uncertainty of whether the sample temperature remained below the glass transition point of PMMA. However, it is evident that the maximum achievable fluence is limited not only by the implantation time, but also by the beam current density to avoid heating-related complications. In the work of Banyasz et al. [16], the Microposit 1450 J photoresist was utilised as a mask for the implantation of 1.6 MeV nitrogen ions at a fluence of 4 ×10 16 at/cm 2 into glass to fabricate phase gratings. It is unfortunate that the experiment details, particularly the nitrogen beam current density and mask fabrication method, are not available. In addition, the fact that nitrogen is an order of magnitude lighter than gold, which was used in this work, does not allow us to conclude whether the Microposit 1450 J photoresist is more suitable for this task than PMMA. 4. Conclusions The present study focuses on the implementation of high-energy heavy ion implantation into a pattern that can be applied in laboratories equipped with electrostatic accelerator and ion beam microprobe, that are often used for the performance of ion beam analysis. It was demonstrated that the implantation of heavy ions, such as gold, into a pattern can be accomplished in-house, without the need for outsorcing mask fabrication. The utilisation of proton beam writing as an auxiliary method is imperative in order to achieve this objective. The primary function of the resist is to create the pattern on the substrate, which is subsequently used for heavy-ion implantation. The method can be summarised as follows: i) firstly, the resist must be prepared on the substrate by spin coating; ii) secondly, the desired pattern must be written in the resist by proton beam writing; iii) thirdly, the resist must be developed; iv) fourthly, heavy ions must be implanted by broad beam; v) and fifthly, the resist from the substrate must be removed. The validity of this approach was demonstrated through the successful implantation of 2.5 MeV gold ions into glass in the pattern associated with the Applied Nuclear Physics Conference and the institute’s logo. The result proved that the method is effective. The most critical issue among Fig. 4. Sample after PMMA was removed. Fig. 5. Peeled PMMA film from glass substrate. Fig. 6. A square (left bottom corner) that came off during the development process ruined the experiment. O. Romanenko et al. Nuclear Inst. and Methods in Physics Research, B 569 (2025) 165920 4 others associated with the method is the material incompatibility between the resist used and the substrate, as well as resist damage. CRediT authorship contribution statement Oleksandr Romanenko: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation, Conceptualization. Petr Malinsky: Writing – review & editing, Methodology, Funding acquisition. Vladimir Havranek: Methodology. Anna Mackova: Writing – review & editing. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Petr Malinsky reports financial support was provided by Czech Science Foundation and Ministry of education, yout and sports of CZ. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research has been carried out at the CANAM (Centre of Accelerators and Nuclear Analytical Methods) infrastructure LM 2015056. The scientific results were obtained with the support of the GACR Project No. 23-06702S. The authors acknowledge the assistance provided by the Advanced Multiscale Materials for Key Enabling Technologies project, supported by the Ministry of Education, Youth, and Sports of the Czech Republic. Project No. CZ.02.01.01/00/22_008/0004558, Co-funded by the European Union.” – AMULET project. Data availability Data are available on https://doi.org/10.5281/zenodo.17258279 Fig. 7. Influence of undeveloped PMMA on the final implantation result. a) The structure was developed in water/IPA mixture for 2 min after PBW (fluence 1 ×10 14 at/cm 2 ); b) After implantation of gold ions with an energy of 2.5 MeV at the fluence of 1 ×10 15 at/cm 2 (implantation time was 3.5 h at current density 25nA/cm 2 ); c) The final result after mask removal. Fig. 8. Diagram of undeveloped resist adhesion to the substrate during ion implantation. Fig. 9. Failure to remove the PMMA mask from Polyimide substrate at the final stage. O. Romanenko et al. Nuclear Inst. and Methods in Physics Research, B 569 (2025) 165920 5 References [1] G. Amato, A. Battiato, L. Croin, M. Jaksic, Z. Siketic, U. Vignolo, E. Vittone, MicroIBA analysis of Au/Si eutectic “crop-circles”, Nucl. Instrum. Methods Phys Res. Sect. B Beam Interact. Mater. At. 348 (2015) 183–186, https://doi.org/10.1016/j. nimb.2014.10.004. [2] T. Calligaro, A. Banas, K. Banas, I.B. Radovi´ c, M. Brajkovi´ c, M. Chiari, A.-M. 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