Radiant energy – Electron energy analysis
Reexamination Certificate
1998-10-13
2001-07-10
Berman, Jack (Department: 2878)
Radiant energy
Electron energy analysis
C250S309000
Reexamination Certificate
active
06259092
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
The present invention relates to methods, apparatuses and systems for determining one or more physical dimensions of an entity, more particularly for determining the thickness of a thin material overlayer on a solid surface.
There are a number of approaches for determining the thickness of a thin carbon overlayer on a sample surface. Several approaches involve the use of x-ray photoelectron spectroscopy (“XPS”).
An example of carbonaceous overlayer thickness determination implementing XPS involves ion beam depth profiling. In accordance therewith, the sample surface is gradually eroded away using an ion beam, and subsequently analyzed with XPS, or Auger electron spectroscopy (“AES”). Performing this process iteratively yields a depth profile of the surface.
Angle-resolved XPS represents another example of XPS-based carbonaceous overlayer thickness determination. According to angle-resolved XPS, photoelectrons are detected at multiple angles with respect to the sample surface. This results in different photoelectron path lengths through the carbon overlayer. From this data, the layer thickness can be determined.
Another XPS-based approach for determining the thickness of a thin carbon overlayer on a sample surface uses two different photoelectron lines with widely differing energies from the same element, in either the substrate or the overlayer. The relative attenuation of the two lines can be used to determine the overlayer thickness.
There are also non-XPS layer thickness techniques, such as ellipsometry.
XPS is a technique by which the elemental composition and chemistry of a solid surface is determined. The surface is illuminated by soft x-rays, resulting in the emission of photoelectrons. XPS is sensitive only to the top few atomic layers of a surface because the photoelectrons have mean free paths of only a few nanometers. XPS can be used to identify the elemental composition of a surface because the photoelectrons have kinetic energies characteristic of the elements in the sample. In addition, XPS can be used to characterize the chemical state of these elements because different chemical states give rise to measurable photoelectron kinetic energy shifts.
The determination of the thickness of thin carbonaceous overlayers on solid surfaces has important technological applications and implications.
For instance, in order to properly determine, using XPS, the composition of a substrate, the presence of an adventitious carbon layer (overlayer) must be accounted for. All samples possess an adventitious carbon layer a few nanometers thick on their surface; however, the adventitious carbon layer on a sample surface can drastically change the quantitation obtained with XPS, because the adventitious carbon layer absorbs photoelectrons from different elements (differing kinetic energies) with different efficiencies. This effect can be corrected for, but only if the thickness of the adventitious carbon layer is known. Since the overlayer thickness is a necessary aspect of this type of substrate composition determination, a speedy and practical method of determining overlayer thickness would be quite beneficial.
Moreover, biofouling of surfaces immersed in seawater is an extremely costly problem for the U.S. Navy. The prevention of such films is of great concern to the U.S. Navy. An effective technique for determining carbonaceous overlayer thickness may help characterize the initial stages of biofilm formation.
Also, there exist certain samples that possess an intentionally formed carbonaceous overlayer on their surface. For example, magnetic disk drives possess a thin lubricating layer on their surface. There is a need in the disk drive industry to measure the thickness of this layer.
Furthermore, layers of small organic molecules on a surface are used as crosslinkers to attach a biological molecule to a surface, as in a biosensor. Layer thickness determination could provide a valuable strategy for characterizing this crosslinking layer.
SUMMARY OF THE INVENTION
In view of the foregoing, it is a principal object of the present invention to provide a methodology for accurately and efficiently determining the thickness of a thin overlayer of a material such as carbon on a solid surface.
The present invention provides a new methodology, based on XPS, for measuring the thickness of a thin material overlayer on an inorganic substrate. The inventive overlayer thickness determination method, apparatus and system are styled herein the “Substrate Effect Model,” as distinguished from the overlayer thickness determination methodology which is known as the “Ebel Model.”
The Ebel Model, a method proposed by Ebel et al. in the 1980's, provides an important scientific underpinning for the inventive Substrate Effect Model. The Ebel Model provides a mathematical regime which makes use of the C1s photoelectron to CKVV Auger peak area ratio for estimating carbonaceous overlayer thickness on inorganic substrates. In principle, the Ebel Model takes advantage of the differential attenuation of two widely differing kinetic energy photoelectron lines from the same element, viz., carbon. As first proposed by Ebel et al., two electron emission peaks from the carbonaceous overlayer—namely, the C1s electron emission peak and the CKKV electron emission peak—are quite suitable for the purpose of determining carbonaceous overlayer thickness, since these two electron emission peaks are characterized by a large difference between their respective kinetic energies.
The Ebel Model was described in the following paper which is hereby incorporated herein by reference: Ebel, Maria F., M. Schmid, H. Ebel and A. Vogel. “Reduced Thickness of Contamination Layers Determined from C1s is and CKVV-Lines.”Journal of Electron Spectroscopy and Related Phenomena 34 (1984): 313-316. Elsevier Science Publishers B. V., Amsterdam, The Netherlands.
The following three papers, each of which is incorporated herein by reference, also discuss use of the C1s photoelectron to CKVV Auger peak area ratio for estimation of carbonaceous overlayer thickness on inorganic substrates:
Reich, T and V. I. Nefedov. “Quantitative XPS Surface Analysis: Correction for Contamination Layer.” Journal of Electron Spectroscopy and Related Phenomena 56 (1991): 33-49. Elsevier Science Publishers B. V., Amsterdam, The Netherlands.
Ebel, Maria F., H. Ebel, C. Puchberger and R. Svagera. “On the Energy Dependence of Attenuation Lengths in Hydrocarbon Contaminations,” Journal of Electron Spectroscopy and Related Phenomena 57 (1991): 357-372. Elsevier Science Publishers B. V., Amsterdam, The Netherlands.
Weng, L. T., G. Vereecke, M. J. Genet, P. F Rouxhaut, J. H. Stone-Masui, P. Bertrand and W. E. E. Stone. “Quantitative XPS. Part II: Comparison Between Quantitative Approaches for Two Different Spectrometers—Determination of the Contamination-reduced Thickness, Application of the Determined Transmission Functions and Accuracy Achieved.” Surface and Interface Analysis 20 (1993): 193-205. John Wiley & Sons, Inc., New York. After introduction of the Ebel Model, experiments by various workers demonstrated that the Ebel Model often significantly underestimated the layer thickness; nevertheless, these experiments were not very accurate. The inventors devised and performed more accurate tests of the Ebel Model; these tests confirmed the previous work demonstrating the Ebel Model to have significant errors.
The inventors subsequently proposed and tested their proposition that a certain physical phenomenon, referred to herein as the “Substrate Effect,” is responsible for the inaccuracy in the Ebel Model. The existence of a physical phenomenon like the Substrate Effect was previously known in other contexts, but the effect of such a physical phenomenon on the Ebel Model was first truly appreciated by the inventor
Beard Bruce C.
Brizzolara Robert A.
Berman Jack
Kaiser Howard
Smith II Johnnie L
The United States of America as represented by the Secretary of
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