Tomas D. Milster

Effects of mask roughness and condenser scattering in EUVL systems

The wavefront reflected from extreme UV lithography mirror and mask surfaces can contain a non-negligible amount of phase variation due to roughness of the mirror surface and variations in multilayer thin-film coatings. We examine the characteristics of image and pattern formation as a function of phase variations originating at the mask surface and at condenser mirrors. A theoretical development and a Monte- Carlo simulation are used to show relationships between statistics of the phase variations and the mask pattern, coherence factor, and numerical aperture of the projection camera. Results indicate that it is possible to produce nearly 1 percent line-edge roughness in a photoresist pattern from moderate values of phase variations.

Near-field optical data storage: avenues for improved performance

Because they produce small spot size, near-field techniques are applied to optical data storage systems to increase recording density. Two practical near-field implementations are aperture probes and solid immersion lenses (SILs). The basic signal modulation characteristics of these systems are reviewed, and some considerations for improving performance are discussed. Combinations of SILs and apertures could produce data storage systems with ultra-fine resolution and good detection characteristics.

Objective lens design for multiple-layer optical data storage

We discuss methods of compensating aberrations in multiple-layer storage with common objective lens designs and introduce a new class of objective lenses that could be useful for this technology. An optical system is designed based on a Galilean telescope and a Burch-type objective. The front negative lens is moved to compensate for thickness variations. Results are shown for the residual Strehl ratio for a Burch-type lens made from Corning C05-50 glass.

Measurement of a convex secondary mirror using a holographic test plate

A 26-cm diameter aspheric convex secondary mirror was successfully measured using a holographic test plate. This measurement demonstrates the viability of the holographic test plate method for measuring convex aspheres. An opticai writer was built and used to demonstrate the ability to write precise holograms on large, curved substrates. The hologram written for this test was fabricated using a thermochemical method that does not require the use of photoresist. The accuracy of the holographic test is demonstrated with a comparison with data from an independent Hindle test.

Beam shaping for optical data storage

The basic instrument used for optical data storage is a scanning laser microscope. Each device contains a beam from a laser diode that is collimated, shaped and focused with an objective lens to produce a microscopic spot on the recording media. The reflected light is collected by the objective lens and directed to data and servo detectors with a beam splitter. Data density on the storage medium is primarily defined by the size and shape of the focused laser beam used to scan the data. Several interesting techniques have been used to shape the focused spot in a way that decreases the primary feature of the spot, thus increasing density. For example, both amplitude and phase filters have been used to decrease the central lobe, at the expense of increased sidelobe levels. Effects of the sidelobes can be minimized with special electronic circuits. The configuration of the readout optics also can influence density. That is, optical filters can be placed in the collection pupil to improve the system transfer function. When combined with electronic shaping circuits, the optical filters significantly improve device performance. This paper reviews the techniques used for beam shaping in optical data storage with an explanation of each technique and its success or failure.

Scattering and coherence in EUVL

We illustrate the importance of considering scattering from the illuminator in extreme UV lithography systems. Our results indicate that a significant amount of amplitude modulation noise is present in the aerial image if scatter is present in a Koehler illuminator. The effect depends on the spatial frequency of the pattern on the mask, the numerical aperture of the projection camera, the coherence factor, and placement of the plane in the illuminator where the scattering occurs.

Design aspects of waveguide hybrid advance MEMS (WHAM)

Three different versions of high numerical aperture 0.3 degree field diffraction limited WHAM lens systems are designed for hi-density data storage by using over-hemi- cylinder surfaces. A prototype WHAM lens with NAi equals 1.23 is also discussed. Detail of unique tolerance issue is presented.

Technique for aligning optical and mechanical axes based on a rotating linear grating

We describe a technique for aligning the axis of an optical system with the axis of a rotating mechanical system. The optical system is focused onto a linear grating that rotates in conjunction with the mechanical system. Light reflected from the grating produces an interference pattern in the pupil of the optical system. Temporal modulation of the fringes in the interference pattern is observed as the mechanical system rotates if the optical and mechanical axes are misaligned. Experimentally we demonstrate alignment to better than 0.1 μm.

Three-dimensional modeling of high-numerical-aperture imaging in thin films

This paper describes a modelling technique used to explore three dimensional (3D) image irradiance distributions formed by high numerical aperture (NA > 0.5) lenses in homogeneous, linear films. This work uses a 3D modelling approach that is based on a plane- wave decomposition in the exit pupil. Each plane wave component is weighted by factors due to polarization, aberration, and input amplitude and phase terms. This is combined with a modified thin-film matrix technique to derive the total field amplitude at each point in a film by a coherent vector sum over all plane waves. Then the total irradiance is calculated. The model is used to show how asymmetries present in the polarized image change with the influence of a thin film through varying degrees of focus.

Detector patterns from optical disks

The irradiance in the exit pupil of a typical optical data storage system (i.e., the detector pattern) consists of fringes formed by the inter- ference of fields reflected from the optical disk. The reflected fields are decomposed using Babinets principle into desired and undesired com- ponents. The behavior of the fringes depends on the interaction of the focused light spot with the features on the optical disk and media param- eters, such as mark and land reflectivities, groove depth, and mark pat- tern. The behavior of the desired and of the undesired fringes are deter- mined for both write-bright media (bright data marks written on a dark background) and write-dark media (dark marks written on a bright back- ground). Both a theoretical development and a scalar diffraction simula- tion tool are used in the analysis.