Outermost Zone Width Research Articles

13 nm high-efficiency nickel-germanium soft x-ray zone plates

Zone plates are used as objectives for high-resolution x-ray microscopy. Both high resolution and high diffraction efficiency are crucial parameters for the performance of the lens. In this article, the authors demonstrate the fabrication of high-resolution soft x-ray zone plates with improved diffraction efficiency by combining a nanofabrication process for high resolution with a process for high diffraction efficiency. High-resolution Ni zone plates are fabricated by applying cold development of electron-beam-patterned ZEP 7000 in a trilayer-resist process combined with Ni-electroplating. High-diffraction-efficiency Ni–Ge zone plates are realized by fabricating the Ni zone plate on a Ge film and then using the finished zone plate as etch mask for anisotropic CHF3 reactive ion etching into the underlying Ge, resulting in a Ni–Ge zone plate with improved aspect ratio and zone plate efficiency. Ni–Ge zone plates with 13 nm outermost zone width composed of 35 nm Ni on top of 45 nm Ge were fabricated. For co...

A Simplified Fabrication Process of Fresnel Zone Plates with Controlling Proximity Effect Correction

Fresnel zone plates (FZPs) for soft X-ray microscopy with an energy range of 284 eV to 540 eV are designed and fabricated in a simple method. An adequate aspect ratio of the resist mold for electroplating was obtained by the proximity effect correction technology for an incident electron beam on a single thick layer resist. Without additional complicated reactive ion etching, a sufficient electro plating mold for nickel structures was fabricated. The overall fabrication procedures which involve a mix-and-match overlay technique for electron beam lithography and an optic exposure system that centers the membrane on the nanostructures, and hybrid silicon etching technology in junction with deep anisotropy and a KOH wet method in order to release the backside Si substrates of the Si3N4 membranes with no deformation of FZPs are introduced. High quality nanostructures with minimum outermost zone widths of 50 nm and diameters of 120 microm were fabricated with simplified fabrication process and with cost-effective.

Fabrication and Performance Test of Fresnel Zone Plate with 35 nm Outermost Zone Width in Hard X-Ray Region

A Fresnel zone plate (FZP) with 35 nm outermost zone width has been fabricated and tested in the hard X-ray region. The FZP was made by electron beam lithography and reactive ion etching technique. The performance test of the FZP was carried out by measuring the focused beam profile for coherent hard X-ray beam at the beamline 20XU of SPring-8. The full width at half maximum of the focused beam profile measured by knife-edge scan method is 34.9±2.7 nm, that agrees well with the theoretical value of diffraction-limited resolution. Applications to scanning microscopy were also carried out.

Design and Fabrication of Hard X-ray Zone Plate

Spatial resolution is determined by the performance of x-ray optics used in the x-ray imaging system. A zone plate was designed for obtaining a high spatial resolution image at x-ray energy of 8.5keV. A spatial resolution of 80 nm was estimated by the ray tracing when an x-ray tube of tungsten targe was used instead of synchrotron radiation. The designed zone plate of outermost zone width of 40nm was successfully fabricated by the electron-beam lithography.

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.

Fabrication of X-ray imaging zone plates by e-beam and X-ray lithography

X-ray imaging and microscopy techniques have been developed in worldwide due to their capabilities of large penetration power and high spatial resolution. Fresnel zone plates is considered to be one of the most convenient optic devices for X-ray imaging and microscopy system. The zone plates with aspect ratio of 7 and 13 have been fabricated by e-beam lithography combined with X-ray lithography in this paper. Firstly, the X-ray lithography mask of zone plates with outermost zone width of 100 nm was fabricated by e-beam lithography and gold electroplating techniques. Secondly, the zone plates with gold profile thickness of 700 and 1,300 nm were replicated by X-ray lithography and gold electroplating techniques. X-ray imaging and microscopy techniques were introduced to characterize the high-aspect-ratio zone plates’ inner structures. At the X-ray energy of 7.5 keV, the first-order focusing efficiency of zone plates with gold profile thickness of 700 nm is about 8.63%.

Cold-developed electron-beam-patterned ZEP 7000 for fabrication of 13 nm nickel zone plates

Cold development was applied to improve the resolution in a trilayer resist that is used for the fabrication of state-of-the-art soft x-ray microscopy zone plates. By decreasing the temperature of the hexyl acetate developer to −50 °C, 11 nm half-pitch gratings have been resolved in the electron-beam resist ZEP 7000. 12 nm half-pitch gratings have been successfully transferred, via the intermediate SiO2 hardmask, into the bottom polyimide layer by CHF3 and O2 reactive ion etching. The trilayer resist, including optimized cold development, has finally been used in an electroplating-based process for the fabrication of nickel zone plates. Zone plates with down to 13 nm outermost zone width have been fabricated and 2.4% average groove diffraction efficiency has been measured for zone plates with 15 nm outermost zone width and a nickel height of 55 nm.

Hydrogen silsesquioxane double patterning process for 12nm resolution x-ray zone plates

Soft x-ray zone plate microscopy is a powerful nanoanalytic technique used for a wide variety of scientific and technological studies. Pushing its spatial resolution to 10nm and below is highly desired and feasible due to the short wavelength of soft x rays. Instruments using Fresnel zone plate lenses achieve a spatial resolution approximately equal to the smallest, outermost zone width. In this work, a double patterning zone plate fabrication process is developed. based on a high resolution resist, hydrogen silsesquioxane (HSQ), to bypass the limitations of conventional single exposure fabrication to pattern density, such as finite beam size, scattering in resist, and modest intrinsic resist contrast. To fabricate HSQ structures with zone widths on the order of 10nm on gold plating base, a surface conditioning process with (3-mercaptopropyl) trimethoxysilane, 3-MPT, is used, which forms a homogeneous hydroxylation surface on gold surface and provides good anchoring for the desired HSQ structures. Using the new HSQ double patterning process, coupled with an internally developed, subpixel alignment algorithm, the authors have successfully fabricated in-house gold zone plates of 12nm outer zones. Promising results for 10nm zone plates have also been obtained. With the 12nm zone plates, they have achieved a resolution of 12nm using the full-field soft x-ray microscope, XM-1.

Ultrahigh-Resolution Soft-X-Ray Microscopy with Zone Plates in High Orders of Diffraction

We present an x-ray optical approach to overcome the current limitations in spatial resolution of x-ray microscopes. Our new BESSY full-field x-ray microscope operates with an energy resolution up to E/DeltaE=10(4). We demonstrate that under these conditions it is possible to employ high orders of diffraction for imaging. Using the third order of diffraction of a zone plate objective with 25 nm outermost zone width, 14 nm lines and spaces of a multilayer test structure were clearly resolved. We believe that high-order imaging paves the way towards sub-10-nm real space x-ray imaging.

Fresnel zone-plate based X-ray microscopy in Zernike phase contrast with sub-50 nm resolution at NSRL

A transmission X-ray microscope using Fresnel zone-plates (FZPs) has been installed at U7A beamline of National Synchrotron Radiation Laboratory (NSRL). The objective FZP with 45 nm outermost zone width delivers a sub-50 nm resolution. A gold phase ring with 2.5 μm thickness and 4 μm width was placed at the focal plane of the objective FZP at 8 keV to produce a negative Zernike phase contrast. A series of samples were used to test the performance of the Zernike phase contrast X-ray microscopy.

Development of a soft x-ray plasma camera with a Fresnel zone plate to image laser produced plasmas

A soft x-ray plasma camera operated at 3.35nm in the water window x-ray region is developed and demonstrated imaging gas jet plasmas of several spices produced with a 10TW Ti: sapphire laser. The plasma camera consists of a 300nm thick Ag/Ti/Si3N4 x-ray band pass filter with bandwidth of 1.43nm to cut visible light and also to reduce colour aberration of the Fresnel zone plate, a Fresnel zone plate with diameter of 1mm and outermost zone width of 300nm, and a soft x-ray CCD camera. The magnification of the plasma camera is 10. The soft x-ray plasma camera powered by a Fresnel zone plate is a very powerful tool to observe laser produced plasmas since it is 1000 times brighter and has 5 times higher spatial resolution comparing ordinary x-ray pinhole camera. The soft x-ray images of helium, nitrogen, argon, krypton, and xenon gas jet plasmas are obtained changing gas pressure from 0.01MPa to 1MPa.

Soft x-ray zone plate fabrication at KTH, Stockholm

We present the status of our zone plate and test object fabrication processes along with the latest fabricated components. With our nickel process, zone plates with outermost zone width of 20 nm and zone height of 90 nm have been fabricated. A gold electroplating process has recently been introduced for the fabrication of test objects. The first result for gold gratings with 70 nm period and 135 nm height is shown.

Zernike phase-contrast x-ray microscope with pseudo-Kohler illumination generated by sectored (polygon) condenser plate

Zernike phase contrast x-ray microscope has been developed at the undulator beamline 20XU and 47XU of SPring-8. The system consists of a pseudo-Köhler-illuminating system, a Fresnel zone plate objective with outermost zone width of 100 nm, a Zernike phase plate (0.96-μm-thick tantalum, λ/4 or 3λ/4 phase-shifter at 8 keV) installed at the back-focal plane of the objective, and a visible-light conversion type cooled CCD camera as an image detector. A sectored (polygon) condenser plate is employed as the condenser in order to secure a large and flat field of view. Details and experimental results of the system will be shown.

High-aspect-ratio germanium zone plates fabricated by reactive ion etching in chlorine

This article describes the fabrication of soft x-ray germanium zone plates with a process based on reactive ion etching (RIE) in Cl2. A high degree of anisotropy is achieved by sidewall passivation through cyclic exposure to air. This enables structuring of higher aspect ratios than with earlier reported fabrication processes for germanium zone plates. The results include a zone plate with a 30 nm outermost zone width and a germanium thickness of 310 nm having a first-order diffraction efficiency of 70% of the theoretical value. 25 nm half-pitch gratings were also etched into 310 nm of germanium. Compared to the electroplating process for the commonly used nickel zone plates, the RIE process with Cl2 for germanium is a major improvement in terms of process reproducibility.

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.

Focusing of hard x-rays to 16 nanometers with a multilayer Laue lens

We report improved results for hard x-ray focusing using a multilayer Laue lens (MLL). We have measured a line focus of 16nm width with an efficiency of 31% at a wavelength λ=0.064nm (19.5keV) using a partial MLL structure with an outermost zone width of 5nm. The results are in good agreement with the theoretically predicted performance.

Fabrication of high-aspect-ratio Fresnel zone plates by e-beam lithography and electroplating

The fabrication of gold Fresnel zone plates, by a combination of e-beam lithography and electrodeposition, with a 30 nm outermost zone width and a 450 nm-thick structure is described. The e-beam lithography process was implemented with a careful evaluation of applied dosage, tests of different bake-out temperatures and durations for the photoresist, and the use of a developer without methylisobutylketone. Electrodeposition with a pulsed current mode and with a specially designed apparatus produced the desired high-aspect-ratio nanostructures. The fabricated zone plates were examined by electron microscopy and their performances were assessed using a transmission X-ray microscope. The results specifically demonstrated an image resolution of 40 nm.

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.

Silicon Fresnel zone plates for high heat load X-ray microscopy

A technique to produce diffractive X-ray lenses optimized for high heat load applications is demonstrated. The lenses are made from single crystal silicon membranes, which have uniform thermal conductivity and homogeneous thermal expansion. Silicon Fresnel zone plates with an outermost zone width down to 30nm were produced by means of electron beam lithography and reactive ion etching. They were tested in conventional synchrotron light sources in a full-field transmission X-ray microscope and in a scanning transmission X-ray microscope. Features below 35nm were resolved. High resolution in combination with ability to withstand high heat load radiation make them a potential candidate for the new generation of X-ray sources based on the free electron laser principle.

Direct Images of the Virtual Source in a Supersonic Expansion

Direct images of the virtual source in a supersonic expansion of helium are presented. The images were obtained using a Fresnel zone plate with free-standing zones, 540 microm in diameter and with an outermost zone width of 50 nm. The general method can be extended to other beams, including seeded beams. Measurements were carried out at absolute source pressures ranging from 11 to 171 bar using a 10 microm nozzle with a source temperature of 320 K. The size of the virtual source was found to be strongly dependent on pressure, changing from a diameter of 67+/-6 microm at an absolute nozzle pressure of 11 bar to 180+/-9 microm at 171 bar. The virtual-source brightness displays a maximum at an absolute nozzle pressure of around 30 bar. This phenomenon occurs because of two competing effects: As the pressure increases, the total flux also increases, but at the same time the virtual source broadens. We modeled the expansion process by calculating the velocity distribution with solutions from the Boltzmann equation to estimate the location of the quitting surface where the frequency of interatomic collisions is assumed negligible. Realistic potentials have been used to calculate the cross section for atomic collisions and, for the velocity distribution perpendicular to the center streamline, a proper scaling with distance derived from the continuum expansion model has been introduced. A good agreement between experiments and model has been found and we discuss its approximation limits. For instance, backscattering effects are not included in the calculations and at present we cannot exclude that they also contribute to a broadening of the virtual-source size for the highest pressure regime. The results presented here are important for improving the understanding of the supersonic expansion process. The experimental method might eventually be used as a new way to test molecular and atomic interaction potentials.