Development of economic MeV-ion microbeam technology at Chiang Mai University

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Introduction
The ion microbeam technology is an extension and also a promotion of general ion beam technology. An ion microbeam is an ion beam of micrometer or sub-micrometer in the beam diameter. Together with integrated imaging techniques, microbeams allow precisely defined locations on the sample to be irradiated. The main applications of microbeam are in two aspects: (1) processing materialshighly localized and highly controlled ion quantity irradiation of the sample, and (2) analyzing materialsimaging/mapping and probing of the sample. Microbeam at MeV energy levels is more advantageous than lower energy microbeam in applying ion beam analysis techniques such as particle-induced X-ray emission (PIXE) for mapping analysis of samples and MeV-ion irradiation of materials for more effective modification or fabrication.
Since the first microbeam facilities were developed in the mid-1990s [1], nowadays, there are about 50 microbeam facilities all over the world [2].
Beam spots with a diameter down to about two micrometers can be obtained by collimating the beam with pinhole apertures or with a drawn capillary. Sub-micrometer beam spot sizes can be achieved by focusing the beam using various combinations of electrostatic or magnetic lenses. Both methods are used at present. Certainly the method of using focusing devices is considerably expensive compared to the method of using beam confining devices. For example, the cost of a triplet quadrupole magnetic focusing lens system for MeV-ion beams, although highly variable, can be up to million US dollars, while that of a beam collimation and aperture system for the same energy ion beams may only be about a hundredth of the former.
Moreover, utilization of such a system requires high stability of a power supply system. Based on our limited resources and locally practical power supply quality, we have adopted the more economic ways to establish the MeV microbeam technology at Chiang Mai University (CMU) with some technical developments in adapting such microbeams to the micrometersubmicrometer levels for applications in MeV-ion-beam lithography (at present) as well as imaging/mapping (under development). Two techniques have been developed or adopted, namely programmable proximity aperture and tapered glass capillary. Utilization of the programmable proximity aperture for MeV ion beam lithography was initially developed by the groups in both University of Jyvaskyla and CMU [3]. Although the technique does not have a comparable resolution with that of using the focused proton beam lithography technique [4], it has the significant advantage of fast writing especially for large areas [5]. The use of tapered glass capillary for focusing MeV light ion beam has been developed in a number of studies (e.g. [6][7][8]). As a novel and new way to focus ion beams, some theories involved such as ion transmission and focusing mechanism and effect have been investigated. The technique has been demonstrated to be cost-effective in obtaining focused ion beams for various applications though the beam size currently remains yet at micrometer in minimum. A noticeable difference between the aperture and the capillary techniques is in the focusing effect which is described by the focusing factor (f) as f = (Outlet beam current density)/(Inlet beam current density).
For the aperture method, always f = 1, while for the capillary method, f > 1 [9].

Facilities
The CMU MeV microbeam facility is based on our 1.7-MV Tandetron tandem accelerator (High Voltage Engineering Europa, or HVEE) and its beam line. As shown in Fig Ionoluminescence (IL) analyses, respectively. For microbeam lithography, systems of a programmable L-shape micro-aperture [10] and a tapered glass micro-capillary [11] have been developed and installed in the chamber, respectively.
The micro-aperture system, as shown in Fig and on the exposed sample area, which should be in principle proportional to the ratio between the two areas, is predetermined by calibration. Then, in the experiment the charge input by the beam onto the aperture blades transformed to the charge on the sample [10].
The tapered glass microcapillary system, as shown in Fig. 3, consists of a 0.8-mm beam collimator to guide the beam into the capillary tube, the glass tapered capillary tube fixed in an Al frame, and a rotator which is connected by a carbon fiber rod with the Al frame to move the glass tube away or into the beam direction. The sample holder is X-Y moveable with a translation repeatability of 4 µm and an accuracy of 6 µm. The electronic and software drivers of the system are in-house developed including the slider backlash compensate. In the front of the holder a 120-µm pinhole slit is installed to limit the final beam size for routine irradiation experiments. A Faraday cup is placed beside the sample on the holder to monitor the beam current. Various sized glass capillary tubes (as shown in Fig. 4) can be in-house fabricated using a home-developed glass microcapillary puller based on the induction heating technique [13]. The glass capillary in different sizes is changeable to install in the capillary holder. After each time of the installation, beam alignment is firstly carried out with the help of using a laser beam. A laser source is installed under the beam line just in the front of the endstation and the laser beam is deflected by a rotatable mirror inside the beam line to direct to the capillary (the laser beam was originally well aligned with the ion beam). Simple manual adjustment of the capillary can finally achieve a satisfying alignment by looking at the best laser beam spot at the target position through the capillary (certainly this is a time-consuming process). The microbeam quality at the target as a function of the distance between the capillary exit and the target was studied. As seen in Fig. 5 for example, the quality of the beam spot is generally improved when the beam core homogeneity increases as increasing of the distance up to about 2 cm, while the halo region increases too, indicating the ion fluence decreasing in the core region and thus less PMMA modification. Based on the studying result, in irradiation experiments the distance between the capillary exit and the sample surface is normally set not greater than 20 mm depending on the capillary size.

Applications
Both programmed L-shaped blade aperture and tapered glass capillary systems were applied to MeV proton microbeam lithography. For operation, installation of the two systems was interchangeable within a four-column frame inside the endstation chamber ( Fig. 2b and Fig.   6). Normally the capillary microbeam had a relatively higher beam current or intensity due to a focusing effect compared with the aperture microbeam (e.g. at the same beam size, such as 100 µm, the former had a current of several nA whereas the latter had about 1 nA for 2 MeV proton beam), while the latter had a more homogeneous distribution of the beam intensity in the beam spot than the former which had a halo effect. Therefore, for high-quality lithography the aperture microbeam was applied, while for fast but not very high quality lithography the capillary microbeam was applied. The materials irradiated included either positive tone or negative tone poly(methyl methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS) and amorphous silica (SiO 2 ). Exposure characteristics of the materials to the microbeam were studied for fluence conditions satisfying production of good quality microstructures. In order to produce microstructures in "positive" resist tone of PMMA, the ion fluence Φ should satisfy the where Φ 0 and Φ x0 are the cleaning and onset of cross-linking ion fluences, respectively, whereas for producing "negative" tone resist the fluence should be Φ x∞ ≤ Φ, where Φ x∞ is the full cross-linking ion fluence. Φ 0 , Φ x0 and Φ x∞ were measured for various ion beam conditions [14] so that an applied ion fluence for a certain material could be determined from the database. In using apertures for microbeams, concerns are normally on the beam scattering caused divergence behind the aperture. However, our experimental results showed that in the utilization of the L-shaped aperture for microbeam, the aperture edge scattering did not significantly affect the pattern edge sharpness [3,15] so that the pattern quality could be guaranteed. Some examples of applications in MeV microbeam lithography using the L-shaped micro-aperture have been reported [10]. In a recent investigation on blister formation on PMMA induced by the aperture microbeam, we determined the blister-free condition of 2- MeV proton beam which should be with a flux less than 4.7 × 10 11 ions/cm 2 s. In the tapered glass micro-capillary writing of micro-patterns, a pattern generation system consisting of two parts has been developed. The first part is a stand-alone pattern design program that operates similarly to a paint program. The drawing pad has a selectable scale of 1000×1000 or 10000×10000 µm 2 with a resolution of 1 µm. The drawing tools consist of a line tool, a freehand drawing tool and a point tool with a selectable point size which is the beam spot size. The second part is the pattern writer, which is integrated into the x/y target stage motor control software. An x/y data pair is read into the program from a file, the exposure time between drawing points is set, and the pattern is automatically written to the target. Fig. 7 shows an example of patterns written in such way and the sharpness of the written pattern edge. The focusing factor of the tapered glass capillary was measured to be f = 2.7, which was fairly high. This is an advantage of the tapered capillary for microbeam lithography over the aperture chopped microbeam due to the focusing effect and thus higher beam intensity to lead to shorter processing time. In a recent new development, the applications of the tapered glass microcapillary system are being extended to MeV microbeam PIXE mapping/imaging ( Fig. 8 shows an example). This extension is simple with placing the target sample on the sample stage and x-y translating the stage step by step for the capillary-focused microbeam to perform PIXE analysis point by point on the sample surface.

Conclusion
Economic systems of the programmable L-shape micro-aperture and the tapered glass