Fabrication Of Fresnel Zone Plates Research Articles

Fresnel zone plate sculptured out of diamond by femtosecond laser for harsh environments.

With the rapid development of micro-optical applications, there is an increasing demand for micro-optical elements that can be made with minimal processing steps. Current research focuses on practical functionalities of optical performance, lightweight, miniaturization, and easy integration. As an important planar diffractive optical element, the Fresnel zone plate (FZP) provides a compact solution for focusing and imaging. However, the fabrication of FZPs with high quality out of hard and brittle materials remains challenging. Here, we report on the fabrication of diamond FZP by femtosecond laser direct writing. FZPs with the same outer diameter and different focal lengths of 250-1000 µm were made via ablation. The fabricated FZPs possess well-defined geometry and excellent focusing and imaging ability in the visible spectral range. Arrays of FZPs with different focal lengths were made for potential applications in imaging, sensing, and integrated optical systems.

High efficiency and scalable fabrication of fresnel zone plates using holographic femtosecond pulses.

To meet the growing demand for photonic integration and device miniaturization, planar diffractive Fresnel zone plates (FZPs) are widely applied in integrated optical systems. However, challenges remain in fabricating FZPs with high efficiency and satisfying the requirement for cross-scale fabrication. This paper details a high efficiency method for fabricating ultrathin FZPs of different scales on metal films by using holographic femtosecond lasers. The FZPs are split into a series of element patterns that are printed in order by using corresponding modulated femtosecond pulses. The fabricated FZPs are spliced by the printed element structures with no FZP size limitation in theory. FZPs with an area varying across three orders of magnitude are presented to demonstrate the capability of cross-scale fabrication. The fabricated FZPs possess an excellent broadband focusing and imaging ability in the visible spectrum. Furthermore, the fabrication of other functional ultrathin lenses, such as axial multifocal zone plates, petal-like zone plates, and FZP arrays, is described, revealing the wide potential for the flexible and scalable fabrication method in on-chip integrated optical systems.

Fabrication of Fresnel zone plate in chalcogenide glass and fiber end with femtosecond laser direct writing

Chalcogenide glass (ChG) has excellent optical properties in the mid-infrared range. The micro-optical fiber devices made of ChG play a strong part in various infrared sensing and detection fields. In this work, Fresnel zone plate (FZP) was fabricated on the Ge-As-S ChG surface and the end face of ChG fiber through femtosecond laser direct writing. FZP with high optical quality was obtained by optimizing the laser writing parameters. Clear focusing pattern can be observed in the visible and infrared bands with the prepared FZP. The full width at half maximum (FWHM) of laser beam with wavelength of 1550 nm is recorded to be 10.39 μm at the focus of FZP, and the numerical aperture (NA) is 0.122. With a well-designed FZP on the fiber end face, the ChG fiber can achieve a self-focusing effect without the collimating and focusing lens. The FWHM of the 1550 nm laser beam at the focus is measured to be 7.46 μm, and NA is 0.162. The combination of FZP and ChG fiber can make the structure of the optical fiber more compact and has potential application value in enhancing the interaction between light and material in the infrared band.

Fabricating high-quality Fresnel zone plates by efficiently eliminating redeposition based on focused ion beam

Fresnel zone plates (FZPs) are significant optical objects for extreme ultraviolet and x-ray microscopy, both of which are powerful tools due to their high spatial resolution and large penetration depth. Fabrication of FZPs is challenging as it requires high accuracy to achieve the narrow outmost zone and large aspect ratio. The focused ion beam (FIB) has been practically demonstrated as a potential alternative to electron beam lithography for its precision, rapidness, and mask-free essence. However, the common redeposition effect in the FIB process is a major obstacle to distort actual patterns from designed ones, especially for FZPs with outmost zone width of 100 nm and simultaneously aspect ratio beyond 2. We proposed an efficient method noted as single-pixel line-assisted sputtering (SLAS) to eliminate redeposition in situ during FZP fabrication. Through systematically optimizing parameters predominating ion dosages and, particularly, the positions of SLAS, we successfully obtained high-quality FZPs with outmost zone width down to 100 nm, perpendicular side walls, aspect ratio of up to 3 and diameter of 38 μm in 1.5 h. The application of SLAS also provides instructive guidance for other FIB processes to finely modulate the nanostructures.

Atomic layer deposition frequency-multiplied Fresnel zone plates for hard x-rays focusing

The design and fabrication of Fresnel zone plates for hard x-ray focusing up to 25 keV photon energies with better than 50 nm imaging half-pitch resolution is reported as performed by forming an ultrananocrystalline diamond (UNCD) scaffold, subsequently coating it with atomic layer deposition (ALD) with an absorber/phase shifting material, followed by back side etching of Si to form a diamond membrane device. The scaffold is formed by chemical vapor-deposited UNCD, electron beam lithography, and deep-reactive ion etching of diamond to desired specifications. The benefits of using diamond are as follows: improved mechanical robustness to prevent collapse of high-aspect-ratio ring structures, a known high-aspect-ratio etch method, excellent radiation hardness, extremely low x-ray absorption, and significantly improved thermal/dimensional stability as compared to alternative materials. Central to the technology is the high-resolution patterning of diamond membranes at wafer scale, which was pushed to 60 nm lines and spaces etched 2.2-μm-deep, to an aspect ratio of 36:1. The absorber growth was achieved by ALD of Ir, Pt, or W, while wafer-level processing allowed to obtain up to 121 device chips per 4 in. wafer with yields better than 60%. X-ray tests with such zone plates allowed resolving 50 nm lines and spaces, at the limit of the available resolution test structures.

Fabrication of soft x-ray zone plates using carbon, titanium as sacrificial layer

In this paper, we report a novel process for the fabrication of Fresnel zone plate (FZP) in poly methyl methacrylate (PMMA) using ultra-thin titanium film on carbon as sacrificial layer by electron-beam lithography at 30 keV with proximity correction. The process involves fabrication of free standing structure with high mechanical stability. A study was undertaken to understand the interaction of an electron beam in the resist stack, leading to the optimization of the process for high aspect ratio structures in a single step of exposure and development. The negative tone of PMMA was also utilized to provide mechanical strength to the structure. FZPs of 800 μm diameter were designed and fabricated for a capillary discharge based argon x-ray laser operating at 46.9 nm, to have a focal length of 25 mm. Each FZP had 165 zones with an outer most zone width of 1.34 μm, and a numerical aperture of 17 mrad.

Rapid Prototyping of Fresnel Zone Plates via Direct Ga+ Ion Beam Lithography for High-Resolution X-ray Imaging

A significant challenge to the wide utilization of X-ray microscopy lies in the difficulty in fabricating adequate high-resolution optics. To date, electron beam lithography has been the dominant technique for the fabrication of diffractive focusing optics called Fresnel zone plates (FZP), even though this preparation method is usually very complicated and is composed of many fabrication steps. In this work, we demonstrate an alternative method that allows the direct, simple, and fast fabrication of FZPs using focused Ga(+) beam lithography practically, in a single step. This method enabled us to prepare a high-resolution FZP in less than 13 min. The performance of the FZP was evaluated in a scanning transmission soft X-ray microscope where nanostructures as small as sub-29 nm in width were clearly resolved, with an ultimate cutoff resolution of 24.25 nm, demonstrating the highest first-order resolution for any FZP fabricated by the ion beam lithography technique. This rapid and simple fabrication scheme illustrates the capabilities and the potential of direct ion beam lithography (IBL) and is expected to increase the accessibility of high-resolution optics to a wider community of researchers working on soft X-ray and extreme ultraviolet microscopy using synchrotron radiation and advanced laboratory sources.

Fabrication of Fresnel zone plates by ion-beam lithography and application as objective lenses in extreme ultraviolet microscopy at 13 nm wavelength

Fresnel zone plates are used for imaging at extreme ultraviolet and soft x-ray wavelengths. Fabricating these zone plates is challenging due to small structure sizes (<150 nm) and complex nanostructuring processes. Fabrication techniques such as electron-beam lithography followed by etching and electroplating processes have been developed over the years. We are reporting on the development of a technique incorporating focused gallium ion-beam lithography to fabricate Fresnel zone plates with 120nm outermost structure size in a process that combines pattern exposure and structure transfer in one single step. The fabricated zone plates were successfully applied in a microscopic setup at λ=13 nm wavelength.

Fabrication of Fresnel zone plates with 25 nm zone width using extreme ultraviolet holography

In this article we report the fabrication of Fresnel zone plates (FZP) by recording the interference pattern of converging and diverging spherical waves at extreme ultraviolet wavelength ( λ = 13.5 nm ). The spherical waves are generated by two concentric annular parent zone plates. The recorded pattern shows a factor of two reduction in the smallest feature size (outermost zone width) compared to its parent counterparts that generate the spherical waves. FZPs with a zone width down to 16 nm have been recorded with this technique. We also present scanning transmission X-ray microscopy (STXM) results taken with a zone plate with 25 nm outermost zone width.

Fresnel zone plates made by holography in the extreme ultraviolet region

We present the fabrication of Fresnel zone plates (FZP) by holography that yields FZPs completely free from imperfections such as zone placement errors and finite pixel size effects found in serial writing techniques such as e-beam lithography (EBL). Our holographic scheme is based on the interference of a spherical wave and a plane wave. A partially transparent membrane containing a pinhole is illuminated by a fully coherent plane wave (λ = 13.4 nm). The spherical wave, obtained by diffraction from the pinhole, interferes with the plane reference wave that is transmitted by the membrane. The interference pattern, which is a hologram of the pinhole, is recorded on a photoresist film and used to fabricate a FZP that has resolution similar to the size of the pinhole.

Evaluation of the dynamic range and spatial resolution of nonadiabatic optical near-field lithography through fabrication of Fresnel zone plates

Fresnel zone plates (FZPs) were fabricated in order to evaluate the performance of nonadiabatic photolithography by exploiting the localized nature of optical near fields. This novel photolithography scheme could realize FZPs with structures smaller than the wavelength of the light source used for exposure. The FZP for 325-nm-wavelength UV light could focus the incident light to a spot size of 590 nm. An FZP for focusing soft X-rays was also fabricated and, compared to conventional adiabatic photolithography, showed higher-contrast zones over the whole area of the FZP. This method exhibits a high dynamic range and good spatial resolution, and it was free from artifacts due to the interference of the residual propagating exposure light transmitted through the aperture of the photomask.

Optical Chemical Vapor Deposition and Photolithography using Nonadiabatic Photochemical Reaction by an Optical Near Field

This article explains unique photochemical reaction named nonadiabatic photochemical process. First, the experimental results of the deposition of Zn nano-dots using the nonadiabatic optical chemical vapor deposition are reviewed. Their deposition rate dependences on the incident optical power are explained by the excitation of the molecular vibrational state by the dressed photon. Second application of the nonadiabatic photochemical reaction to the photolithography is reviewed using fabrication of Fresnel zone plates as an experimental example.

Fabrication of Fresnel zone plates by holography in the extreme ultraviolet region

Fabrication of Fresnel zone plates (FZPs) with nanometer scale resolution is one of the major challenges in lithographic fabrication. The authors present a holographic approach to create FZPs where the interference pattern between a spherical beam and a plane wave is recorded to obtain the FZP structure. The spherical beam is obtained by diffraction from a circular aperture (pinhole) in a semiopaque membrane which is illuminated by a spatially coherent extreme ultraviolet beam. The beam transmitted by the membrane serves as the reference plane wave. The resulting pattern gives rise to a FZP with outermost zone width comparable to the dimension of the circular aperture. Fabrication and test results of a FZP obtained with this method are presented. The technique offers a solution to the pattern-placement problem encountered in serial ZP writing techniques as well as high resolution potential.

Fabrication of Fresnel zone plates with high aspect ratio by soft X-ray lithography

High focusing efficiency Fresnel zone plates for hard X-ray imaging is fabricated by electron beam lithography, soft X-ray lithography, and gold electroplating techniques. Using the electron beam lithography, Fresnel zone plates which has an outermost zone width of 100 nm and thickness of 250 nm has been fabricated. Fresnel zone plates with outermost zone width of 150 nm and thickness of 660 nm has been fabricated by using soft X-ray lithography.

Fabrication of Fresnel zone plates for hard X-rays

A method to fabricate gold structures with high aspect ratio is presented. Fresnel zone plates with an outermost zone width of 100 nm and structures of 1 μm height are fabricated. Preliminary focusing results at an X-ray energy of 8 keV are presented and ways to improve the zone plate parameters are discussed.

Nanofabrication of Fresnel zone plate lenses for X-ray optics

Fresnel zone plate (FZP) lenses are, in many cases, essential elements to obtain X-ray microbeams in synchrotron light (SL) facilities. They have become a common optical device during the latest years. The increasing complexity of the relevant scientific problems is associated with an increasing demand on the optical performances of the X-ray microscopy to the point that the optical elements have to be tailored to fit the characteristics of the optical source and of the sample. This fact justifies a research effort to optimize and simplify the fabrication processes of such devices. However, FZP fabrication for X-ray wavelengths is not straightforward, since the involved dimensions are in the nanometer scale. A joint effort involving CELLS (consortium for the development of a synchrotron light source in Barcelona), the Optics Department of the Autonomous University and the Microelectronics Center in Barcelona has been recently undertaken in order to develop the processes for the fabrication of FZP lenses as X-ray focusing elements. In this paper, we present the first results using a very simple lift-off technique, which up to now allows the fabrication of FZP with sub-100 nm resolution with 110 nm metal rings.

Fabrication of Fresnel zone plate embedded in silica glass by femtosecond laser pulses

We fabricated the Fresnel zone plate by embedding voids in silica glass. We investigated the focusing properties by launching a He-Ne laser beam into the zone plate. The spot size of the primary focal point was 7.0 mum and agreed with the theoretical value of 6.1 mum. The diffraction efficiency was 2.0 %. This technique enables us to make alignment free micro-scale lenses inside bulk materials.

Fabrication of circular optical structures with a 20 nm minimum feature size using nanoimprint lithography

We demonstrated the fabrication of Fresnel zone plates with a 75 nm minimum feature size and circular gratings with a 20 nm minimum linewidth in polymethyl methacrylate using nanoimprint lithography, and in metals by means of a lift-off technique. Observation of sharp Moiré patterns indicated the high fidelity of nanoimprint lithography in pattern duplication. Our results showed that nanoimprint lithography is a promising technology for patterning integrated optics.

Line width control using a defocused low voltage electron beam

We present a method for line width control in electron beam lithography, which is especially suited for the manufacture of diffractive optical elements. By defocusing the spot of our “gaussian beam” electron beam writer, the line width can be controlled continuously from the 100 nm range to many microns. Test exposures at a beam energy of 2.5 keV with varying defocus and line dose values have been evaluated. It is shown that a simple analytical expression can predict the line widths with high precision down to 300 nm. By an empirical modification, we were able to extend our model down to 100 nm line width. As an example of the method's usefulness, the fabrication of Fresnel zone plates for x-ray microfocussing applications is described.

Design and fabrication of Fresnel zone plates with large numbers of zones

The advent of high-brightness x-ray sources in the 10–40 keV region opens new possibilities of experiments with microbeams. Techniques to form these focused beams may be based on glancing mirrors, phase elements, or diffractive optics, in particular Fresnel zone plates (FZPs). Because of the long focal length and large acceptance, FZPs designed to work in the hard x-ray region tend to have quite large diameters and large numbers of zones. For instance, the zone plate described in this article has a 1860 μm diam, a focal length of f=3 m (for 8 keV), and 1860 zones. On a standard pattern generator, circular shapes are always approximated as simpler structures. The tolerance requirement for shape and positions of zones depends on the number of zones, and it is necessary to guarantee that the circular structures are approximated to the required degree of accuracy while keeping the size of the data structure to a reasonable size for processing by the exposure system. For instance, if polygons are used to approximate circular zones, a formula for the minimum acceptable number of polygon sides can be derived. An x-ray mask for a Fresnel phase zone plate (FPZP) with 1860 zones was designed directly in Cambridge source pattern data format and fabricated using the Leica Cambridge e-beam tool installed in the CXrL. The zone plates presented in this article were designed for hard x rays, and multilevel x-ray lithography was employed as a fabrication technique to form absorber thickness sufficient to provide the necessary phase shift. Minimum gold features of 0.25 μm with thicknesses of 1.6 and 3 μm, were formed to be used with 8 and 20 keV photons, respectively. Finally, in order to estimate the quality of the zone plates during fabrication, a scanning electron microscope based moiré method was used.