Evaluation of the X-ray/EUV Nanolithography Facility at AS through wavefront propagation simulations

Abstract

Synchrotron light sources can provide the required spatial coherence, stability and control to support the development of advanced lithography at the extreme ultraviolet and soft X-ray wavelengths that are relevant to current and future fabricating technologies. Here an evaluation of the optical performance of the soft X-ray (SXR) beamline of the Australian Synchrotron (AS) and its suitability for developing interference lithography using radiation in the 91.8 eV (13.5 nm) to 300 eV (4.13 nm) range are presented. A comprehensive physical optics model of the APPLE-II undulator source and SXR beamline was constructed to simulate the properties of the illumination at the proposed location of a photomask, as a function of photon energy, collimation and monochromator parameters. The model is validated using a combination of experimental measurements of the photon intensity distribution of the undulator harmonics. It is shown that the undulator harmonics intensity ratio can be accurately measured using an imaging detector and controlled using beamline optics. Finally, the photomask geometric constraints and achievable performance for the limiting case of fully spatially coherent illumination are evaluated.

Keywords: extreme ultraviolet lithography; interference lithography; soft X-ray lithography; synchrotron beamline; wavefront propagation.

Figure 1

Schematic of an interference lithography setup using a binary grating mask. The first-order (m = ±1) beams form an aerial image at the image plane, a distance z ₁ from the mask, which is then transferred onto a photoresist. The zero-order (m = 0) and second-order (m = 2) diffracted beams are also shown.

Figure 2

Schematic illustration of the SXR beamline. The distance of each element from the source is indicated (not to scale). Proposed IL optics and an imaging detector are included in branch B. Branch A was used to obtain measurements of the undulator harmonic intensity using photoelectron spectroscopy.

Figure 3

(a) Total photon flux (red), horizontal coherence length (blue), and (b) resolving power at the mask plane of beamline branch B for different SSA sizes. The resolving power is shown as a function of SSA height because the energy resolution of the PGM is defined only by the vertical SSA size. The grey dotted lines correspond to the maximum SSA size that provides coherent illumination of a representative grating mask (see Fig. 7 ▸).

Figure 4

The measured horizontal intensity profile, 50 cm from the BDA with a fundamental photon energy of 185 eV and C _ff = 2. The horizontal line profile through the beam, overlaid with a fitting of two Gaussians representing the fundamental (harmonic n = 1) and the third harmonic (n = 3).

Figure 5

The intensity FWHM measured in the horizontal (red) and vertical (black) direction for photon energies from 90 eV to 270 eV and C _ff = 1.4. The calculated FWHM of intensity profiles generated through partially coherent wavefront propagation through the beamline model are also shown for photon energies of 90.44 eV, 135 eV, 184.76 eV and 250 eV.

Figure 6

Higher harmonic content (ζ) measurements using direct beam measurements (black) and XPS measurements (red). A single calculation from simulation has been included at 184.76 eV (green).

Figure 7

(a) The simulated and measured total flux (Φ _n ) and (b) the total intensity over the area of a four-grating mask ( ) consisting of 50 µm × 50 µm gratings arranged symmetrically with a 50 µm gap in the centre (shown in inset, where the gratings are shown as black squares, the n = 1 harmonic in blue, and the n = 3 harmonic in green). Flux and intensity measurements were taken at branch B, while flux measurements were also taken on branch A using an AXUV100 photodiode. C _ff = 1.4 for all measurements shown.