X-Ray Lithography for Nanofabrication: Is There a Future?

Abstract

X-ray lithography has been first proposed almost 50 years ago, and the related LIGA process around 25 years ago. It is therefore a good time to make an analysis of the technique, with its pros and cons. In this perspective article, we describe X-ray lithography’s latest advancements. First, we report the improvement in the fabrication of the high aspect ratio and high-resolution micro/nanostructures. Then, we present the radiation-assisted synthesis and processing of novel materials for the next generation of functional devices. We finally draw our conclusion on the future prospects of the technique.

Advances in lithographic techniques for precision nanostructure fabrication in biomedical applications

“…However, XRL may not be suitable for the mass production of biomedical systems. The high cost associated with investing in a synchrotron source and producing complex x-ray masks, coupled with the relatively low throughput, restricts its widespread use in the biomedical industry [ 41 , 45 ].…”

“…Wavelength reduced to 193 nm or 248 nm [ 32 ] 65–130 nm [ 32 ] Improved resolution in comparison to traditional UV lithography [ 32 ] Shorter wavelengths are more easily reflected [ 33 ] Interference effects [ 32 ] Maximum total thickness variation is 0.5 µm [ 34 ] Low depth of focus [ 34 ] Extreme UV lithography Use UV light and mask to pattern a photoresist. Wavelength reduced to 13.5 nm [ 35 ] < 10 nm [ 36 ] Improved resolution in comparison to traditional UV lithography [ 36 ] Shorter wavelengths are more easily reflected [ 33 ] Low photon transmission efficiency [ 37 ] Defects in the photomask warp pattern [ 38 ] Secondary electrons cause blur [ 39 ] Increased stochastic pattern variations [ 40 ] X-ray lithography Use x-rays and mask to pattern a photoresist [ 41 ] 15 nm [ 42 ] Improved resolution in comparison to traditional UV lithography [ 42 ] Large substrate-mask distances do not cause diffraction/proximity effects until the feature width approaches 100 nm [ 43 ] Shorter wavelengths are more easily reflected [ 33 ] Masks are thin, fragile, and expensive [ 44 ] Secondary electrons cause blur [ 45 ] Electron beam lithography Use electrons to pattern a resist [ 46 ] > 10 nm [ 47 ] Precise control [ 48 ] Can pattern complex geometries [ 48 ] Beam can damage the substrate [ …”

Refractive X-ray optics: towards wavelengths of 13.5 nm and beyond

“… photons). At the same time, for the tasks of X-ray nanolithography, a number of research groups propose concepts of optical elements that are alternative to multilayer Bragg mirrors in modern EUV scanners [17]: diffraction optics (Fresnel zone plates [18]), refractive optics (biconcave parabolic lenses [19]), as well as composite (refractive-diffraction) kinoform lenses [20]. The use of the above types of mirrors can help increase the spectral range to the level of ~10% dE E , which, together with the use of the effect of total external reflection instead of diffraction from periodical multilayers, will make it possible to collect the radiation dose required for exposure of X-ray resists several times faster.…”

Reflective x-ray masks for x-ray lithography

“…X-ray lithography (XRL) along with its branches (including LIGA) is a versatile technology for microfabrication purposes compared to other types of lithography: optical, e-beam and ion-beam ones [1,2]. It provides new possibilities for creating micro-devices (MEMS and MOEMS) with a high aspect ratio in thick (∼1 mm) resists with almost vertical sidewalls [3], offering various resist illumination techniques (dynamic, maskless, interference ones, etc). Many x-ray optical elements, such as x-ray refractive lenses [4], collimators [5], and diffraction gratings [6] are manufactured by this technique.…”