Marvin L. Bernt

Design review of a multiwavelength, three-dimensional scatterometer

A design review of a five wavelength, laser-based, full reflection hemispherical BRDF instrument is given. A source box design which provides full polarization control of incident radiation is described. The receiver design uses a periscope mirror which allows near (< 1 .5°) retroreflection measurements with full received polarization control. Replacement of the periscope mirror with a beam splitter pellicle allows 0° retroreflection measurements. The goniometer design shown allows the measurement of full reflection hemisphere scatter with transmission measurements possible. A new software control approach which allows a user to define scans in scatter space without being concerned with mechanical axes motion is described.

Measurement and analysis of scatter from rough surfaces

The technique of characterizing the surface roughness of smooth, clean, front surface reflectors from BRDF measurements is in common use and has been well documented in the literature. It relies on using the Rayleigh-Rice vector perturbation theory to obtain the surface power spectral density function (or PSD) from which surface statistics may be found. These calculations are restricted to smooth surfaces only, as defined by the Rayleigh criterion. A number of potential industrial applications that would benefit from the fast, non-contact aspects of scatter/roughness characterization have not been implemented because the smooth surface restriction is clearly violated. This paper offers a brief arm-waving explanation of why BRDF cannot be used to obtain the PSD for rough surfaces, explores increasing the wavelength and/or the incident angle as measurement techniques to extend the PSD (from BRDF) calculation to rougher surfaces, and presents data showing that TIS measurements (which do not require that the PSD be calculation) can be used to find the rms roughness of surfaces are less severely limited by the impact of the smooth surface criterion.

Wavelength scaling investigation of several materials

A logical approach to investigating materials that scatter non-topographically is to coat them with a thin layer that scatters only topographically (i.e., a layer that wavelength scales) so that the two scatter sources can be separated. This technique has been used to study scatter from beryllium mirrors using layers of aluminum and gold. In the course of these investigations it was learned that many aluminum surfaces do not wavelength scale (no information was available on gold). This was discovered fairly late in the study and caused a lot of extra expense and frustration. This paper presents wavelength scaling data from the uv to the mid-IR for several materials. The intention is to provide guidance for future studies of this nature.

An Investigation Of Anomalous Scatter From Beryllium Mirrors

Scatter from Beryllium mirrors often seems to be higher than would be expected based on mirror surface roughness data. This paper verifies the effect and shows that it is more severe in the IR than in the visible. The implication is that the excess scatter is caused by mirror defects that are non-topographic in nature. The conclusion is that profile, or roughness specifications should not be used when low scatter is the requirement of concern.

Multiple particle technique for determination of differential scattering cross section of very small surface bound particles

Measurement of small particle contamination, via laser particle scanners, is an important tool for maximizing throughput in semiconductor production lines. As line widths shrink, the required minimum observable particle diameter drops in proportion. Unfortunately, the scatter signal from a small particle on a surface falls off very quickly with decreased diameter, even faster than would be predicted by a simple Rayleigh scatter model of an isolated particle. This paper reviews a technique to measure the differential scattering cross-section of very small surface bound particles. The objective is to provide a means of obtaining data for producing the next generation of particle scanners.

Mueller matrix measurements of several optical components

The Stokes/Mueller approach provides complete characterization of sample induced polarization changes to both specular and scattered light that is reflected or transmitted from optical components. Ellipsometry is a subset of this more complete approach to polarization analysis. In this paper, Mueller matrices of several samples are found and compared to the matrix of ideal elements. Samples include optical mirrors that scatter both topographically and non-topographically, waveplates, and optical windows.

TIS uniformity maps of wafers, disks, and other samples

Total integrated scatter measurements, the earliest well defined measurement relating light scatter to surface roughness, is now being used in modern production facilities as a means of monitoring product surface roughness. The two applications reviewed here are computer disks, where the issue is a well defined roughness (as opposed to the smoothest possible surface) and an emerging issue with roughness specifications for the backsides of silicon wafers. The paper describes a scanning instrument that allows sample uniformity to be revealed and thus the manufacturing process investigated.

Roughness measurement of dielectrics with light scatter

The literature now includes a number of examples where light scatter (BRDF) measurements have been used to determine the surface power spectral density function of smooth, clean reflectors. But most of this data is for front surface metal mirrors and semiconductors. Black glass has been considered for use as a BRDF standard, and there are industry applications (computer disks and front panel displays) that could benefit from the same type of characterization from glass and ceramic surfaces. This paper addresses some of the issues involved with making surface roughness measurements on these surfaces. For example, clear glass will scatter a visible beam from its bulk and second surface as well as its front surface. In addition, calculation of the polarization constant Q must be handled in a more accurate manner. Data from several samples will be analyzed.

Near specular measurements of integrated scatter

Measurement of integrated scatter near the specular beam is an excellent way to qualify optics and optical coatings that are used in imaging systems. This paper reviews an instrument design to measure Total Integrated Scatter (TIS) over the range from 0.05 degree(s) to 3.0 degree(s) from specular. The measurement scheme utilizes an integrating plate instead of an integrating sphere.

IR and visible BSDF measurements of several materials

BSDF measurements are made on a large variety of materials at TMA’s measurement facility. Typical BSDF for many of these materials is not available in literature. This paper gives scatter data at several wavelengths for samples that we believe will be of interest to the scatter community and outlines the techniques and considerations required for some of the more difficult measurements. Included are some very low scatter optics, an off axis parabola and several different window materials.