Ralf K. Heilmann

Fabrication of ultrahigh aspect ratio freestanding gratings on silicon-on-insulator wafers

The authors report a silicon-on-insulator (SOI) process for the fabrication of ultrahigh aspect ratio freestanding gratings for high efficiency x-ray and extreme ultraviolet spectroscopy. This new grating design will lead to blazed transmission gratings via total external reflection on the grating sidewalls for x rays incident at graze angles below their critical angle (about 1°–2°). This critical-angle transmission (CAT) grating combines the alignment and figure insensitivity of transmission gratings with high broadband diffraction efficiency, which traditionally has been the domain of blazed reflection gratings. The required straight and ultrahigh aspect ratio freestanding structures are achieved by anisotropic etching of ⟨110⟩ SOI wafers in potassium hydroxide (KOH) solution. To overcome structural weakness, chromium is patterned as a reactive ion etch mask to form a support mesh. The grating with period of 574nm is written by scanning-beam interference lithography (SBIL) which is based on the interfer...

Efficiency of a grazing-incidence off-plane grating in the soft-x-ray region.

Efficiency measurements of a grazing-incidence diffraction grating in the off-plane mount were performed using polarized synchrotron radiation. The grating had 5000 grooves/mm, an effective blaze angle of 14 degrees, and was gold coated. The efficiencies in the two polarization orientations (TM and TE) were measured in the 1.5-5.0 nm wavelength range and were compared with the efficiencies calculated using the PCGrate-SX code. The TM and TE efficiencies differ, offering the possibility of performing unique science studies of astrophysical, solar, and laboratory sources by exploiting the polarization sensitivity of the off-plane grating.

Nanometer-level repeatable metrology using the Nanoruler

We report on the measurement of the fringe-to-substrate phase error in our Nanoruler system. This system utilizes scanning beam interference lithography to pattern and measure large-area, nanometer-accuracy gratings that are appropriate for semiconductor and integrated opto-electronic metrology. We present the Nanonruler’s metrology system that is based on digital frequency synthesizers, acousto-optics, and heterodyne phase sensing. It is used to assess the fringe-to-substrate placement stability and the accuracy of the feedback signals. The metrology system can perform measurements in real time, on the fly, and at arbitrary locations on the substrate. Experimental measurements are presented that demonstrate the nanometer-level repeatability of the system. Dominant error sources are highlighted.

High-efficiency 5000 lines/mm multilayer-coated blazed grating for extreme ultraviolet wavelengths

Volume x-ray gratings consisting of a multilayer coating deposited on a blazed substrate can diffract with very high efficiency, even in high orders if diffraction conditions in-plane (grating) and out-of-plane (Bragg multilayer) are met simultaneously. This remarkable property, however, depends critically on the ability to create a structure with near atomic perfection. In this Letter we report on a method to produce these structures. We report measurements that show, for a 5000l/mm grating diffracting in the third order, a diffraction efficiency of 37.6% at a wavelength of 13.6nm. This work now shows a direct route to achieving high diffraction efficiency in high order at wavelengths throughout the soft x-ray energy range.

Image metrology and system controls for scanning beam interference lithography

We are developing scanning beam interference lithography (SBIL) for writing and reading large gratings with nanometer level distortion. Our distortion goals require fringe locking to a moving substrate with subnanometer spatial phase error while measuring and controlling the fringe period to approximately one part per million. In this article, we describe the SBIL optical system design along with some major subsystems. The design incorporates measurements and controls of the parameters that limit the accuracy of our system. We describe in detail a novel image metrology scheme, which uses interferometry to measure in situ both the period and the phase of the grating image formed by the interference of two laser beams. For a grating period of approximately 2 μm, experiments demonstrate a period measurement repeatability of three parts per ten thousand, one sigma. Phase measurement indicates a slow fringe drift at 0.25 mrad/s. Both the repeatability error and the phase drift are expected to improve by about three orders of magnitude after several improvements including the installation of an environmental enclosure and thermally stable metrology frames.

Fabrication of 200nm period blazed transmission gratings on silicon-on-insulator wafers

The authors report on the fabrication of 200nm period blazed transmission gratings on silicon-on-insulator (SOI) wafers. These critical angle transmission (CAT) gratings require 3–5μm tall freestanding grating bars with a very high aspect ratio (>100) and smooth sidewalls. In order to meet the challenging geometrical requirements, they modified and improved our previously reported process for the fabrication of a CAT grating prototype with 574nm period. They have used potassium hydroxide (KOH) solutions to fabricate high aspect ratio gratings on ⟨110⟩ SOI wafers. The KOH etching process was improved to minimize the lateral undercut through precise grating alignment to ⟨111⟩ planes within ±0.05° and a room temperature etch process with 50wt% KOH. In addition, an image-reversal technique with a high silicon content spin-on polymer was applied to increase process latitude with a high duty cycle nitride mask. A surfactant was also added to the KOH solution to promote hydrogen bubble release. With the improved...

Monolayer/Bilayer Transition in Langmuir Films of Derivatized Gold Nanoparticles at the Gas/Water Interface: An X-Ray Scattering Study

The microscopic structure of Langmuir films of derivatized gold nanoparticles has been studied as a function of area/particle on the water surface. The molecules (AuSHDA) consist of gold particles of mean core diameter D approximately 22 angstroms that have been stabilized by attachment of carboxylic acid terminated alkylthiols, HS-(CH2)15-COOH. Compression of the film results in a broad plateau of finite pressure in the surface pressure versus area/particle isotherm that is consistent with a first-order monolayer/bilayer transition. X-ray specular reflectivity (XR) and grazing incidence diffraction show that when first spread at large area/particle, AuSHDA particles aggregate two dimensionally to form hexagonally packed monolayer domains at a nearest-neighbor distance of a = 34 angstroms. The lateral positional correlations associated with the two-dimensional (2D) hexagonal order are of short range and extend over only a few interparticle distances; this appears to be a result of the polydispersity in particle size. Subsequent compression of the film increases the surface coverage by the monolayer but has little effect on the interparticle distance in the close-packed domains. The XR and off-specular diffuse scattering (XOSDS) results near the onset of the monolayer/bilayer coexistence plateau are consistent with complete surface coverage by a laterally homogeneous monolayer of AuSHDA particles. On the high-density side of the plateau, the electron-density profile extracted from XR clearly shows the formation of a bilayer in which the newly formed second layer on top is slightly less dense than the first layer. In contrast to the case of the homogeneous monolayer, the XOSDS intensities observed from the bilayer are higher than the prediction based on the capillary wave model and the assumption of homogeneity, indicating the presence of lateral density inhomogeneities in the bilayer. According to the results of Bragg rod measurements, the 2D hexagonal order in the two layers of the bilayer are only partially correlated.

Nanometer-accurate grating fabrication with scanning beam interference lithography

We are developing a Scanning Beam Interference Lithography (SBIL) system. SBIL represents a new paradigm in semiconductor metrology, capable of patterning large-area linear gratings and grids with nanometer overall phase accuracy. Realizing our accuracy goal is a major challenge because the interference fringes have to be locked to a moving substrate with nanometer spatial phase errors while the period of the fringes has to be stabilized to approximately one part per million. In this paper, we present a review of the SBIL design, and report recent progress towards prototyping the first-ever SBIL tool.

High fidelity blazed grating replication using nanoimprint lithography

We report progress in using nanoimprint lithography to fabricate high fidelity blazed diffraction gratings. Anisotropically etched silicon gratings with 200nm period and 7.5° blaze angle were successfully replicated onto 100mm diameter wafers with subnanometer roughness and excellent profile conformity. Out-of-plane distortion induced by residual stress from polymer films was also analyzed and found to be extremely low. The replicated blazed gratings were tested and demonstrated high x-ray diffraction efficiencies. This process was developed for fabricating blazed diffraction gratings for the NASA Constellation-X x-ray telescope.

Digital heterodyne interference fringe control system

In traditional interference lithography, interference fringes are typically phase locked to a stationary substrate using analog homodyne photodiode signals that are fed back to control a phase-shifting device such as an electro-optic modulator or a piezoelectrically transduced mirror. Commercially available fringe-locking systems based on this approach often achieve stability of the interference fringes to within a small fraction of the fringe period p (typically ±p/20 peak-to-peak). We describe the performance of a heterodyne fringe control system utilizing acousto-optic phase shifters and digital controls that is designed to satisfy the much more stringent fringe control requirements for scanning beam interference lithography. We demonstrate locking to ±p/100, and expect further significant improvements. This versatile system can also be used to lock the phase of moving fringes in almost arbitrary fashion at fringe velocities up to 2.5×107 periods/s and to measure the phase of gratings.