Theodore V. Vorburger

Regimes of surface roughness measurable with light scattering.

In this paper we summarize a number of previous experiments on the measurement of the roughness of metallic surfaces by light scattering. We identify several regimes that permit measurement of different surface parameters and functions, and we establish approximate limits for each regime. Using a straightforward criterion, we calculate that the smooth-surface regime, in which the angular distribution of scattered light is closely related to the power spectral density of the roughness, ranges over 0 < σ/λ ≲ 0.05, where σ is the rms roughness and λ is the opitcal wavelength. Above that the surfaceautocorrelation function may be calculated from a Fourier transform of the angular distribution over 0 < σ/λ ≲ 0.14. Then comes the specular regime where the specular beam can still be identified andmeasured over 0 < σ/λ ≲ 0.3. For all these regimes and for rougher surfaces too, the rms width of thescatter distribution is proportional to the rms slope of the surface.

Direct and inverse problems for light scattered by rough surfaces

Calculations are performed to relate the stylus profile of a one-dimensionally rough surface to the angular distribution of the light scattered by such a surface. In the direct problem, the angular distribution of the scattered light calculated from the profile is shown to agree with the measured one. In the inverse problem, the rms roughness and the autocorrelation function are found by a least-squares fit to the measured angular distribution. For the smoother surfaces, the rms roughness is mostly determined by the ratio between the power of the specular beam and the total power of the scattered light; the computed values are proportional to those calculated directly from the stylus profiles. The values of the parameters obtained by the least-squares fit are affected by a variety of errors and agree only partially with those obtained from the stylus profile.

Determination of optimal parameters for CD-SEM measurement of line-edge roughness

The measurement of line-edge roughness (LER) has recently become a topic of concern in the litho-metrology community and the semiconductor industry as a whole. The Advanced Metrology Advisory Group (AMAG), a council composed of the chief metrologists from the International SEMATECH (ISMT) consortium’s Member Companies and from the National Institute of Standards and Technology (NIST), has a project to investigate LER metrics and to direct the critical dimension scanning electron microscope (CD-SEM) supplier community towards a semiconductor industry-backed, standardized solution for implementation. The 2003 International Technology Roadmap for Semiconductors (ITRS) has included a new definition for roughness. The ITRS envisions root mean square measurements of edge and width roughness. There are other possible metrics, some of which are surveyed here. The ITRS envisions the root mean square measurements restricted to roughness wavelengths falling within a specified process-relevant range and with measurement repeatability better than a specified tolerance. This study addresses the measurement choices required to meet those specifications. An expression for the length of line that must be measured and the spacing of measurement positions along that length is derived. Noise in the image is shown to produce roughness measurement errors that have both random and nonrandom (i.e., bias) components. Measurements are reported on both UV resist and polycrystalline silicon in special test patterns with roughness typical for those materials. These measurements indicate that the sensitivity of a roughness measurement to noise depends importantly both on the choice of edge detection algorithm and the quality of the focus. Measurements are less sensitive to noise when a model-based or sigmoidal fit algorithm is used and when the images are in good focus. Using the measured roughness characteristics for UV resist lines and applying the ITRS requirements for the 90 nm technology node, the derived expression for sampling length and sampling interval implies that a length at least 8 times the node (i.e., 720 nm) must be measured at intervals of 7.5 nm or less.

Photoelectron spectra of the decomposition of ethylene on (110) tungsten

Abstract Photoelectron spectra, LEED patterns, and work function changes were obtained for ethylene adsorbed on (110) tungsten at room temperature, and with subsequent heat treatment. For saturated adsorption of C 2 H 4 on (110) W at room temperature, features in the photoelectron spectrum were observed which are believed to be due to the C, HCC, and Cmetal bonds in an adsorbed species of the form C 2 H 2 . The work function decreased by 1.2 eV at saturation, but LEED showed no change from the clean surface pattern. Upon heating to ≈ 500 K, where hydrogen is known to desorb, the CH bond was broken, whereas the CC and Cmetal bonds remained. The work function increased, from saturation, by ≈ 0.6 eV and the LEED pattern exhibited a large diffuse background with no new spots. Upon heating to ≈ 1100 K the CC bond broke and the LEED pattern ordered into the characteristics carbon contamination pattern.

Stylus profiling at high resolution and low force

This paper describes experimental work to improve the lateral resolution of stylus instruments. Our efforts involve (1) use of a fine stylus, (2) low stylus load, (3) high magnification in the lateral direction, and (4) specimens with fine surface structure by which the lateral resolution of stylus instruments could be detected. By using styli with tip widths between 0.05 and 0.15-microm, a stylus load of 0.6-1.2 x 10(-6)-N (0.06-0.12-mgf), and a piezostage for lateral displacement, we detected 0.05-0.15-microm lateral resolution on the surfaces of different kinds of specimens. To get a high lateral resolution, the most important consideration is a fine stylus with small tip size.

Light Scattering from Manufactured Surfaces

We present an evaluation of light scattering theories and experimental techniques for measuring the roughness of manufactured surfaces. Our goal was to define the ultimate capabilities and regions of validity of these methods for deducing surface microtopography. Available theories are evaluated in terms of a heuristic interpretation of direct electromagnetic scattering from surfaces. Experimental methods using specular reflectance, total integrated scatter, angular scattering distribution, and speckle are reviewed in terms of their spatial-bandwidth sampling of surface wavelengths and their roughness amplitude sensitivity and accuracy. Graphs in a slope/relative-wavelength space are used to intercompare the regions of validity of theories, regions of applicability for experimental methods and regions occupied by typical manufactured surfaces. A relatively unknown theory, valid for light scattering from many typical manufactured surfaces but previously unused for this purpose, is briefly described.

Comparison on Nanometrology: Nano 2?Step height

The ability to measure step height and to calibrate step height artefacts is of vital interest in nanometrology. On that score the WGDM7 decided in 1998 to include measurements of step heights in a series of comparisons on the field of nanometrology. The comparison about step height (NANO2) started in September 2000 with the Physikalisch-Technische Bundesanstalt (PTB) as pilot laboratory. Fourteen national metrology institutes worldwide participated in this comparison. A set of five step height standards in the range from 7 nm to 800 nm was used for the comparison. The lateral size of the structures of the step height standards was chosen so that the height could be measured by different types of instruments, for example, interference microscopes, stylus instruments and scanning probe microscopes (SPM). The reference values were calculated as the weighted mean of all measurements that fulfilled the En?<?1 criteria. Most of the results were in good agreement with the reference values. It is noticeable that the results obtained by different types of instruments are quite compatible. Also this comparison is the first comprehensive test of the reliability of SPM and their suitability for traceable measurements of step heights. Further it was shown that today step heights on samples can be measured with uncertainties in the sub-nanometre range. Differences in the calculation of the uncertainty depend on the types of instruments and on the users. For each class of instrument, e.g. SPM, it would be meaningful to homogenise these models. Hence the results of this comparison are of great importance in many respects. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the Mutual Recognition Arrangement (MRA).

Contribution of CIRP to the Development of Metrology and Surface Quality Evaluation during the last fifty years

Abstract An overview of the contribution of the members of the C.I.R.P. community to the progress of Metrology and Surface Roughness Quality Evaluation is given. The following items are included in the part on metrology: brief overview of the existing situation before 1950, contribution to the successive definitions of the unit of length and related reference length standards, traceability, preliminary work to standardization, thermal effects, design and construction of precision machine tools and measuring machines, CMM, large scale metrology. In the field of surface quality evaluation, the following items are reviewed: reference profiles and related definitions of parameters, filtering, surface and subsurface integrity, functional meaning of parameters, instrumentation, scanning probe microscopy, 3D surface evaluation. A comprehensive list of references is provided.

Key-Note-PapersLight Scattering from Manufactured Surfaces†

We present an evaluation of light scattering theories and experimental techniques for measuring the roughness of manufactured surfaces. Our goal was to define the ultimate capabilities and regions of validity of these methods for deducing surface microtopography. Available theories are evaluated in terms of a heuristic interpretation of direct electromagnetic scattering from surfaces. Experimental methods using specular reflectance, total integrated scatter, angular scattering distribution, and speckle are reviewed in terms of their spatial-bandwidth sampling of surface wavelengths and their roughness amplitude sensitivity and accuracy. Graphs in a slope/relative-wavelength space are used to intercompare the regions of validity of theories, regions of applicability for experimental methods and regions occupied by typical manufactured surfaces. A relatively unknown theory, valid for light scattering from many typical manufactured surfaces but previously unused for this purpose, is briefly described.

Ellipsometry of rough surfaces

Ellipsometry measurements on several different kinds of rough surfaces were compared with stylus measurements of the surface texture. For steeply sloped periodic surfaces, the ellipsometric angles Delta and psi varied rapidly as the angle of incidence was varied near a diffraction minimum. This effect is interpreted in terms of the Kirchhoff theory and is ascribed to interference between singly and doubly reflected light waves. For a set of Ni replicas of machined surfaces with random surface profiles, Delta and psi varied systematically with surface texture. These variations persisted even after the surface composition was changed by evaporating first Al then Au on the surfaces. The systematic effects due to surface roughness are in disagreement with those of a previous experiment and are not readily explainable in terms of the Kirchhoff theory. The possible reasons for this are discussed along with the prospects for using ellipsometry as a tool for measuring surface roughness.