Quinn J. Leonard

Demagnification in proximity x-ray lithography and extensibility to 25 nm by optimizing Fresnel diffraction

This new understanding and demonstration of features printed by proximity x-ray lithography allows a revolutionary extension and simplification of otherwise established processes for microfabrication. The ability to produce fine features is controlled predominantly by diffraction and photoelectron blur. The diffraction manifests itself as feature bias. In the classical approach the bias is minimized. Bias optimization in terms of mask/wafer gap and resist processing allows the formation, on a wafer, of features smaller than those on the mask: thus producing local demagnification. This demagnification ( ? 3- ? 6) is achieved without lenses or mirrors, but it offers the same advantages as projection optical lithography in terms of critical dimension control. The photoelectron blur is more or less pronounced depending on exposure dose and development conditions. Resist exposure and process can be optimized to utilize a ~ 50% photoelectron energy loss range. In consequence proximity x-ray lithography is extensible to feature sizes below 25 nm, taking advantage of comparatively large mask features (> 100 nm) and large gaps (30-15 ? m). The method is demonstrated for demagnification values down to ? 3.5. To produce DRAM half-pitch fine features, techniques such as multiple exposures with a single development step are proposed.

Progress in the fabrication of high-aspect-ratio zone plates by soft x-ray lithography

Fabrication of Fresnel zone plates for the hard x-ray spectral region combines the challenge of high lateral resolution (~100 nm) with a large thickness requirement for the phase-shifting material (0.5-3 μm). For achieving a high resolution, the initial mask was fabricated by e-beam lithography and gold electroforming. To prevent the collapse of the structures between the developing and electroforming processes, drying was completely eliminated. Fabrication errors, such as nonuniform gold electroplating and collapse of structures, were analyzed and systematically eliminated. We optimized the exposure and developing processes for 950k and 2200k polymethylmethacrylate of different thicknesses and various adhesion promoters. We discuss the effects of these fabrication steps on the zone plates resolution and aspect ratio. Fresnel zone plates with 110 nm outermost zone width, 150 μm diameter, and 1.3 μm gold thickness were fabricated. Preliminary evaluation of the FZPs was done by scanning electron microscopy and atomic force microscopy. The FZP focusing performance was characterized at the Advanced Photon Source at Argonne National Laboratory.

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 appro...

Experimental and theoretical study of image bias in x‐ray lithography

The successful application of x‐ray lithography to a manufacturing process requires a detailed understanding of the image formation process. In a series of articles, a theoretical study of the optical processes involved in the definition of the image in proximity printing has been presented. In addition to the diffraction process, it is included here that the image formation in the very near field (microgaps) due to the guiding effects in the absorber which alters the boundary condition for diffraction and thus changes the final image pattern. Also presented here is an experimental study designed to verify the predictions of the models of image formation. Preliminary result shows a very large depth‐of‐focus for 0.25 μm features.

3D microstructures fabricated by partially opaque X-ray lithography masks

X-ray lithography has several advantages over optical or e-beam techniques, since x-ray proximity printing can overcome typical limitations of such techniques, enabling the imaging of ultra thick resist layers (micro-mechanical application). In this work we will discuss the fabrication of a multilevel x-ray mask capable of producing a radiation dose modulation when used in a typical proximity x-ray lithography set-up. The aim of our investigation is to develop a process that in a single x-ray exposure can replicate a multilevel mask profile. The mask was realised by e-beam lithography and made by a number of overlapped gold layers, so producing a modulation of the x-ray dose absorption. We describe in some detail the process for the fabrication of the mask and the first result of the x-ray exposure.

An optimization design method for chemically amplified resist process control

A novel optimization method called chemically-amplified resist process optimization design (CARPOD) applicable to the chemically amplified resist (CAR) process development is described. The method finds the optimal process conditions and the design center (maximum process tolerance space) of a CAR process with minimum experimental runs. First a modified response surface method is used to form the numerical response surface of a CAR, and its most sensitive point, which is the minimum requirement of X-ray dose, is located as an optimal process condition by an optimization method called POSM under the constraint of the contrast of the photoresist. Second, the design center is found to maximize the process tolerance space around the optimal process condition. Third, verifications are made on the optimal design as well as the design center. The process optimization of AZ PF-514 has been used as an example to show that the CARPOD method can identify the optimal process condition as well as the maximum tolerable parameter space with minimum experimental runs. >

Metrology of high-resolution resist structures on insulating substrates

One of the main problems in the field of ultra‐large‐scale integration is the accurate use of metrological tools to provide information for critical dimension control of insulating material structures. In particular, for the inspection of high‐resolution resist structures with a scanning electron microscope, it is necessary to choose the relevant working parameters, such as voltage and current, to take into account the electrical charging of the resist. Charging may influence the secondary electron (SE) signal that is used for metrology. In this article, the SE signal, collected while inspecting a resist structure on silicon substrates with an electron beam, is studied both theoretically and experimentally. Cold‐cathode tungsten field emission microscopes, Hitachi S6100 and Hitachi S900, have been used for this study. Low‐voltage signals are computed by direct simulation of low energy SE emission, using Monte Carlo (MC) calculations. The charging effect is evaluated as a function of beam energy and beam c...

FIB repair of clear and opaque defects in x-ray masks

Abstract The repair of a gold on silicon nitride X-ray mask containing simulated defects was carried out using FIB methods. Both opaque and clear defects were repaired by micromachining and tungsten deposition, respectively. PMMA images of the repaired features were produced by X-ray lithography and examined microscopically to determine the properties and effectiveness of the repair process.

Modeling, fabrication, and experimental application of clear x-ray phase masks

Phase-shifting masks are widely used in optical lithography, and the question is whether the technology can be extended to much shorter wavelengths. We have extensively modeled the use of clear phase-mask materials as absorbers for x-ray exposures. Similar to the optical domain, the imaging from x-ray phase masks is highly nonlinear, and thus can be used to produce a feature reduction of 3–5× compared to the mask pattern. This observation suggests that mask fabrication might be easier, but the technique is most suited for less dense pattern requirements. In this article we review some of the salient modeling, examine the requirements for effective fabrication of the masks, and provide some experimental verification of this approach to reach the sub-50nm region. It is not essential to have exactly a 180° phase shift in the clear material, because mask feature reductions can be achieved with 45, 90, 180, and 270° shifts by choosing the appropriate thicknesses of the clear material. A mask with a 235nm featu...

Experimental study of aerial images in x‐ray lithography

Depth‐of‐focus (gap latitude in proximity lithography) and exposure latitude are the two most important figures‐of‐merit for linewidth control in lithography. In a recent article, the preliminary result of the experimental verification of this theoretical study was presented—the linewidth variation as a function of gap. In this article, the experimental results of the exposure latitude are presented and discussed. Excellent agreement between experimental and simulation results are found. Optimized parameters based on verified model are presented here.