Mark K. Mondol

Scanning-helium-ion-beam lithography with hydrogen silsesquioxane resist

A scanning-helium-ion-beam microscope is now commercially available. This microscope can be used to perform lithography similar to, but of potentially higher resolution than, scanning electron-beam lithography. This article describes the control of this microscope for lithography via beam steering/blanking electronics and evaluates the high-resolution performance of scanning helium-ion-beam lithography. The authors found that sub-10nm-half-pitch patterning is feasible. They also measured a point-spread function that indicates a reduction in the micrometer-range proximity effect typical in electron-beam lithography.

Interferometric-spatial-phase imaging for six-axis mask control

We describe a unified approach to measuring alignment and gap with nanometer detectivity between two planar objects (e.g., a mask and a substrate) in close proximity. The method encodes lateral position in the phase of interference fringes, formed by diffraction from grating and checkerboard alignment marks, designed to enable a wide acquisition range. For gapping, the method incorporates, in the same mark, coarse-gap detection (30–300 μm) and absolute-gap detection at sub-30 μm using a chromatic Fabry–Perot scheme. Fine detection of sub-30 μm gaps is inferred from the frequency and phase of fringes, calibrated using the chromatic Fabry–Perot. Illumination with a variable-bandwidth source enables either “achromatic” aligning or “chromatic” gapping. Sub-nanometer detection and feedback control of mask position is demonstrated in X, Y, and θ. Overlay of exposed patterns is demonstrated to be <3 nm.

Maskless, parallel patterning with zone-plate array lithography

Zone-plate array lithography (ZPAL) is a maskless lithography scheme that uses an array of shuttered zone plates to print arbitrary patterns on a substrate. An experimental ultraviolet ZPAL system has been constructed and used to simultaneously expose nine different patterns with a 3×3 array of zone plates in a quasidot-matrix fashion. We present exposed patterns, describe the system design and construction, and discuss issues essential to a functional ZPAL system. We also discuss another ZPAL system which operates with 4.5 nm x radiation from a point source. We present simulations which show that, with our existing x-ray zone plates and this system, we should be able to achieve 55 nm resolution.

Nanostructure fabrication by direct electron-beam writing of nanoparticles

Direct additive-layer fabrication of nanostructures is a widely sought goal, which is not possible using traditional layered resist optical and electron-beam lithographic techniques. However, recently, it has been shown that certain metallic and semiconducting nanoparticles capped with protective organic groups are promising “inklike” resist materials for patterning a variety of electronic and mechanical structures [C. A. Bulthaup et al., Appl. Phys. Lett. 79, 1525 (2001)]. Several groups have successfully patterned single-layer gold nanoparticle films by means of direct electron-beam writing [X. M. Lin, R. Parthasarathy, and H. M. Jaeger, Appl. Phys. Lett. 78, 1915 (2001); T. R. Bedson, R. E. Palmer, T. E. Jenkins, D. J. Hayton, and J. P. Wilcoxon, Appl. Phys. Lett. 78, 1921 (2001); L. Clarke et al., Appl. Phys. Lett. 71, 617 (1997)]. In this work, we apply these materials in a new lithographic mode, using an electron beam to cause direct sintering of these 2–10 nm nanoparticles, building structures of m...

Novel mask-wafer gap measurement scheme with nanometer-level detectivity

We describe a means of measuring the gap between mask and substrate in an x-ray lithography system. The method does not require that the gap be scanned. The method encodes the gap in the spatial phase, spatial frequency, and separation of sets of interference fringes. The fringes result from the diffraction from a checkerboard on the mask, with constant period in one direction and varying period in the transverse direction. The separation of fringe sets gives an unambiguous measure of gap when the mask is approaching the substrate, from 400 to 30 μm. At the smaller gaps used for exposure, checkerboards with different chirp periods are utilized to indicate the gap without ambiguity. The phases of the fringes as a function of gap were calibrated with a Fabry-Perot interferometer. The repeatability of the phases between consecutive scans of gap was found to have a 5 nm standard deviation. This method of measuring gap may prove useful in a variety of applications that require a controlled gap between two plates.

Photonic band-gap waveguide microcavities: Monorails and air bridges

Photonic band-gap monorail and air-bridge waveguide microcavities, operating at the wavelength regime of 1550 nm, are fabricated using GaAs-based compound semiconductors. The fabrication process involves gas-source molecular beam epitaxy, electron-beam lithography, reactive ion etching, and thermal wet oxidation of Al0.93Ga0.07As. The fabrication of the air-bridge microcavity, in particular, also entails the sacrificial wet etch of AlxOy to suspend the micromechanical structure. The monorail and air-bridge microcavities have been optically characterized and the transmission spectra exhibit resonances in the 1550 nm wavelength regime. Tunability of the resonant wavelength is demonstrated through changing the defect size in the one-dimensional photonic crystal. The quality factors (Q) of the microcavities are about 140 for the monorail and 230 for the air bridge, respectively.

Dynamic alignment control for fluid-immersion lithographies using interferometric-spatial-phase imaging

We demonstrate the application of a high-sensitivity alignment method called interferometric-spatial-phase imaging (ISPI) to a nanometer-level overlay in fluid-immersion lithography, using step-and-flash imprint lithography as the test vehicle. As a stringent test we used alignment marks that consist of pure phase gratings in a fused silica template, immersed in a fluid of similar refractive index, resulting in a low-contrast alignment signal. Feedback control of alignment is demonstrated with mean=0.0nm and σ=0.1nm using an immersed template. Overlay results, with UV-exposed imprint fluid, were limited to ∼4nm, due to a mechanical disturbance. Because ISPI enables continuous monitoring of the alignment signal, we were able to identify the origin of the mechanical disturbance and can eliminate it in future experiments. In addition, we demonstrate the ability to actively reduce misalignment during the progression of crosslinking in the imprint fluid.

Scanning-spatial-phase alignment for zone-plate-array lithography

In this article, we describe a technique for level-to-level alignment in zone-plate-array lithography that does not require an external microscope, yet provides overlay superior to conventional microscopes.

Nanometer gap measurement and verification via the chirped-Talbot effect

We describe a noncontact, optical method of measuring, with nanometer-level sensitivity, the gap between two planar objects in close proximity, such as a substrate and either a proximity-lithography mask or an imprint template. Interference fringes from a chirped-checkerboard mark on one object are observed using a nonexposing wavelength with long-working-distance, oblique-incidence microscopes. The gap is determined from the spatial frequency and phase of the fringes. We verify the gap measurement using a variation of the Talbot effect with the chirped-checkerboard mark. The two forms of gap measurement are complementary since one is suited to measuring and setting gap prior to exposure, and the other is ideal for confirmation of the gap that existed during exposure.

Experimental study of interactions in the nanostructured Ni pillar arrays

In this work, the experimental way of measuring the interaction fields in nanoscale single domain magnet arrays is presented. This method uses a magnetic force microscopy (MFM) and an in situ electromagnet. The experiments were performed on the arrays of nanoscale nickel pillars with a period, ranging from 200 to 300 nm and a diameter of 90 nm. The up and down switching fields of the pillar were measured by applying a known field to the target pillar and raising the field until the pillar reverses the magnetization state. We used a scheme in which the MFM tip stray field and the bias field from the electromagnet are used to magnetize the individual pillars. The exact strength of the field concentrated near the tip was found experimentally by comparing the switching fields measured with and without the tip in contact with the pillars, having different switching fields. The numerical results were compared with the experimental data.