Ultraviolet surface plasmons on aluminium and a non-contact technique for characterizing short-pitch gratings

Abstract

In this work we present grating-coupled surface plasmon characterization of aluminium in the ultraviolet. Light of wavelength 325 nm from a helium-cadmium laser was used to excite surface plasmon-polaritons both at a buried aluminium-silica interface and on oxidized aluminium. By fitting the angle-dependent reflectivity data to model grating theory, the optical permittivity of uncontaminated aluminium at 325 nm is deduced. These results are compared with those extrapolated from prism-coupled surface plasmon excitation in the visible. The further characterization of the oxidized aluminium allows optical non-contact profilometry to be performed on short-pitch gratings that are opaque or are comprised of materials with ill-defined optical properties. This profile characterization is demonstrated for a short-pitch grating manufactured in photoresist.     

Reduction of the zero-order intensity in binary Dammann gratings

The source of the reduction in the zero-order intensity in binary Dammann gratings is described as an error in the areas in the phase areas within the unit cell of the grating. Equations for determining the amount of error required to produce a specific reduction ratio are given. A two-dimensional, N = 1, Dammann grating that creates a 3 × 3 beam fan-out with a 24% reduction of the zero order provides an example of such an effect. The calculation shows agreement with the measured error.     

Antireflection structures with use of multilevel subwavelength zero-order gratings

It is known that the zero-order two-phase-level gratings with a period much smaller and a thickness much larger than the wavelength may have antireflection properties the same as appropriate dielectric layers under normal incidence. On the basis of the rigorous coupled-wave analysis method formulation, it is shown that multilevel unidimensional phase gratings, for both TE and TM polarization, are functionally equivalent to antireflection structures of multilevel dielectric layers, even if the period is close to the wavelength.     

A computational approach to determining the depth of surface flaws by the ACPD technique

The boundary element method is used to analyze the ACPD (alternating current potential drop) field perturbed by a semi-elliptical surface flaw in a metallic sheet. The depth of the penetration of the alternating current is assumed to be small as compared to the crack depth. This allows us to adopt the “unfolding” technique developed by Dover et al. [ASTM STP 722, 401–427 (1981)], which reduces the problem to a two-dimensional surface Laplacian field. This surface potential field is determined for different surface cracks with crack aspect ratios ranging from 0.2 to 2. To facilitate the use of these results, an empirical formula is developed based on the numerical results. A simple inverse analysis methodology is also developed for determining the crack depth for semi-elliptical surface flaws of known surface length with measured potential drop readings.     

Methods for determining the depth of near-surface defects

In previous work by the authors,^(1,6) it was demonstrated that the presence of near-surface defects could be detected reliably, even though the defect echo was contained within the near-surface echo. The algorithm consists of examining the variation in the composite (near-surface plus defect) response after it has been deconvolved from a near-surface response known to be defect-free. This paper presents two algorithms that have been developed subsequent to the work presented in ref. (6) for estimating thedepth of a near-surface defect, given that its presence has already been detected. One algorithm uses complex frequency domain techniques, and the other uses time domain analysis. Both procedures operate on the surface-plus-defect signal, using reference signals containing surface-only and defect-only responses. The defect signal is extracted from the composite signal. Defect depth is then computed from the time difference between the centers of the front-surface and extracted defect responses. A mean absolute depth error of 0.015 in. was obtained by applying the algorithms to experimental data containing depths from 0.020 to 0.130 in. below the near-surface.     

Encoding technique for design of zero-order (on-axis) Fraunhofer computer-generated holograms

Diffractive optical elements able to generate zero-order (on-axis) distributions with phase as well as amplitude distributions are described. The proposed elements are surface relief plates, i.e., phase-only elements, that are based on the concept of computer-generated masks followed by common etching processes. The encoding method assumes fixed spatial partitioning of the cell and a phase-only value allocated to each subelement. The reconstructed amplitude and phase distributions contain imperfections (noise) resulting from the encoding process. Methods of error reduction and improvements are provided.     

New method for determining the depth of field of microscope systems

This paper presents new formulas to determine the depth of field (DOF) of optical and digital microscope systems. Unlike the conventional DOF formula, the new methods consider the interplay of geometric and diffraction optics for infinite and finite optical microscopes and for corresponding digital microscope systems. It is shown that in addition to the well understood parameters such as numerical apertures, focal length, and light wavelength, system components such as aperture stops also affect the DOF. For the same objective lens, the DOF is inversely proportional to the size of the aperture stop, and it is proportional to the focal length of the ocular lens. It is also shown that under optimal viewing and operating conditions, the visual accommodation of human observers has no meaningful impact on DOF. The new formulas reported are useful for accurately calculating the DOF of microscopes.     

Very-large-scale-integration fabrication technique for binary-phase gratings on sapphire

An efficient, high-yield process for the production of binary-phase holograms is presented by controlled deposition of silicon nitride over a sapphire substrate with the binary structure formed by plasma etch of the silicon nitride. Optical results are presented for a 16 × 16 transmission fanout element that shows near-optimal performance.     

A new method for determining the sharpness of InGaAs/GaAs heterojunctions by auger depth profiling

It is established for the first time that the phenomenon of ion-stimulated surface segregation can be used to increase the depth resolution of Auger profiling during analysis of the In_x Ga_{1 − x} As/GaAs heterostructures. It is demonstrated that, by varying the energy of the sputtering Ar⁺ ion beam from 1 to 0.5 keV in the region of the GaAs/InGaAs heterojunction, the junction sharpness can be estimated at a resolution on the order of 0.5 nm determined by a difference in the projected range of Ar⁺ ions and independent of the escape depth of the Auger electrons.     

Self-calibrating ultrasonic technique for crack depth measurement

A configuration of transducers together with a self-calibrating measurement technique is proposed to investigate the reflection and transmission of surface waves by a surface-breaking or near surface defect. By means of this technique, the ratio of the reflection and transmission coefficients (R/T and/orT/R) can be obtained in a reliable and accurate manner. The reflection and transmission of surface waves for oblique incidence on a surface breaking crack is investigated in detail. Information onT/R for the latter case can be used to determine the depth of the crack. The experimental measurements ofT/R show excellent agreement with theoretical results.     

Optical measurement of depth and duty cycle for binary diffraction gratings with subwavelength features

We describe a new and unique method for simultaneous determination of the groove depth and duty cycle of binary diffraction gratings. For a near-normal angle of incidence, the +1 and -1 diffracted orders will behave nearly the same as the duty cycle is varied for a fixed grating depth. The difference in their behavior, quantified as the ratio of their respective diffraction efficiencies, is compared to a look-up table generated by rigorous coupled-wave theory, and the duty cycle of the grating is thus obtained as a function of grating depth. Performing the same analysis for the orthogonal probe-light polarization results in a different functional dependence of the duty cycle on the grating depth. By use of both TE and TM polarizations, the depth and duty cycle for the grating are obtained by the intersection of the functions generated by the individual polarizations. These measurements can also be used to assess qualitatively both the uniformity of the grating and the symmetry of the grating profile. Comparison with scanning electron microscope images shows excellent agreement. This method is advantageous since it can be carried out rapidly, is accurate and repeatable, does not damage the sample, and uses low-cost, commonly available equipment. Since this method consists of only four fixed simple measurements, it is highly suitable for quality control in a manufacturing environment.     

Simple measuring technique for the diffraction efficiency of slanted volume gratings at various wavelengths

To measure diffraction efficiencies of gratings as a function of wavelength, it is necessary to have quasi-monochromatic light sources of various wavelengths. We propose a method to measure the wavelength dependence of the grating diffraction efficiency by using a quasi-monochromatic light source. This method of estimating the real diffraction characteristics of the gratings for various wavelengths is very useful and simple. First the diffraction efficiency of the grating as a function of various incident-beam angles of monochromatic light is measured, then, using these data, we can obtain the diffraction efficiencies for various wavelengths of the same incident angle of light by virtue of a mathematical-conversion method. The mathematical-conversion results for two laminated differently slanted angle gratings of the same volume grating period are in good agreement with the experimental ones.     

Review: a review of the recent methods for determining trap depth from glow curves

The paper reviews some of the important methods used in determination of the trap depthE from glow curves. Their suitability, merits and demerits are examined.