Commentary: Nano- and micromachining using laser plasma soft X-rays

Abstract

Investigating interactions of intense soft X-rays with solid surfaces and resulting phenomena is still challenging. One could expect material removal from the surfaces (ablation) caused by soft X-ray irradiation at a precision as high as the diffraction limit of the soft X-rays. The X-ray ablation technique could enable us to fabricate nano- and micro-structures on sufaces of even transparent materials such as silica glass, polymethylmethacrylate (PMMA) and polydimethylsiloxane (PDMS), which are highly valued for their use in the fields of nanometric chemical analysis and chemical reactions in medicine and biotechnology. For machining at high resolution, light with a short wavelength is preferable because of the diffraction limit. In the X-ray region, however, most of the materials have less optical absorption at shorter wavelengths, resulting in no energy transfer of the light to the materials, that is, no ablation. Therefore, photons in the range of 100 eV–1 keV are suitable for micromachining. So far, synchrotrons have been utilized for extensive studies on interaction of soft X-rays with inorganic materals such as silica glass. However, silica surfaces are etched at a low rate and the exposed surfaces are modified to Si-rich ones. In contrast, photo-etching of polymers has been realized. PMMA surfaces can be removed (etched) by X-ray irradiation without further chemical treatment. PMMA can also be etched by chemicals after X-ray irradiation, which is applied to fabricate micro-molds of PMMA and metals in the Lithographie Galvanoformung und Abformung (LIGA) process. In addition to synchrotrons radiation, soft X-rays from laserproduced plasma have been applied to PMMA ablation [1,2]. Barkusky et al. irradiated PMMA with focused laser plasma soft X-rays (LPSXs), using a Schwaltzschild objective that reflects X-rays at 13.5±1.0. Although they achieved high energy as high as 1 J/cm 2 , X-rays out of the