Optics: measuring and testing – For light transmission or absorption
Reexamination Certificate
2000-04-28
2003-09-02
Smith, Zandra V (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
C356S630000
Reexamination Certificate
active
06614532
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a technique of depth dependent analysis in thin films using optical or other radiation, on depth scales of ca. 1 to several thousand micrometers.
BACKGROUND OF THE INVENTION
The capacity to reliably analyze and to recover images of the depth dependent properties of a thin film, which the invention specified in this application discloses, is of fundamental importance to both industrial film processing, materials science and to medicine.
Many processes employed in the fabrication of thin films for industrial applications involve or produce a depth variation of the material composition on the length scale of a few micrometers to a few millimeters. Many of the present day coatings systems used by industry consist of four or five layers or more, for example. The migration of additives such as plasticizers and stabilizers, through thin films is a commonly encountered problem, as is the problem of thermal and optical degradation, which directly affect film performance and lifetime.
In histology, the depth variation of tissue properties on micrometer length scales may be fundamental in understanding tissue function, assessing drug delivery, or in diagnosing disease.
While the problem of analyzing material composition with depth on these length scales is of great interest to a number of fields, relatively few methods exist to achieve this analysis both easily and reliably. This is true regardless of whether or not the analysis method is destructive of the material under study.
Past destructive depth dependent analysis methods for films have involved lateral (orthogonal to the depth axis) stripping or microtomy of thin layers from the original test material followed by chemical or optical analysis of the sampled layers. See the article by A. P. Aleksandrov, V. N. Genkin and V. V. Sokolov, Polym. Sci. USSR 27, 1188 (1985).
Primary difficulties with the above destructive sampling methods are the time and labor required by the stripping or microtomy procedure. The depth sampling is not always reliable: it is not always possible to ensure that layers of precisely equal thickness have been sampled, leading to calibration difficulties in expressing material composition as a function of depth. A strict conformity of the sample to a solely one dimensional (depth) variation of properties must usually apply. The number of depth samples that can be recovered by these methods is usually relatively small. Finally, the microsampling procedure itself may modify the sample itself or contribute depth dependent contamination of the sampled material.
A common destructive analytical method described in the article by J. L. Gardette, S. Gaumet, and J. L. Phillippart, J. Appl. Polym. Sci. 48, 1885 (1993) is associated with conventional light microscopy, and has been used for the depth analysis of polymeric materials. This latter preparation procedure involves embedding the test material in a matrix of resin which acts as a substrate for the cutting of thin cross-sectional slices (orthogonal to the depth axis) of said material, using a microtome apparatus. The thin cross-sectional slices which are cut from the material are then analyzed by transmission or reflectance microscopy.
The difficulties encountered with this procedure are numerous. The primary setting of the sample in the matrix is time consuming, and the, use of a microtome apparatus, while routine in many laboratories, is an expensive requirement of the sample preparation procedure. Many materials have weak adhesion to the resin substrate in which the test material is embedded. Individual layers comprising the material tend to easily delaminate under slicing by the microtome blade. This produces an obvious violation of the mechanical integrity of the original sample, and may seriously complicate the interpretation of the experimental micrographs.
As a result of the above complications, there may be many practical situations in which a destructive depth resolved analysis of a test material on the 1-100 micrometer length scale is not possible.
The above difficulties with destructive sampling methods have led to the more recent development of non-destructive methods of depth profile analysis, usually based on the interaction of optical radiation with the test material. An effectively comprehensive list of these methods consists of the following methods: (i) photoacoustic and photothermal spectroscopy (see the article by R. J. W. Hodgson, J. Appl. Phys. 76, 7524 (1994)); (ii) attenuated total reflectance (ATR) (see the article by R. Shick, J. L. Koenig, and H. Ishida, App. Spec. 50, 1082 (1996)) and variable angle reflectance methods; (iii) optical computed tomography (see the article by S. Kawata, O. Nakamura and S. Minami, J. Opt. Soc. Am. A 4. 292 (1987)); (iv) methods which integrate the material under analysis into the cladding of an optical waveguide (see the article by P. W. Bohn, Anal. Chem. 57, 1203 (1985)); and (v) techniques of confocal microscopy (see the article by T. Wilson and C. Sheppard,
Theory and Practice of Scanning Optical Microscopy
, Academic Press, London, 1984).
With the exception of confocal microscopy (as discussed in more detail below), the above methods are based on indirect depth detection mechanisms. In these cases, the experimental detector response is measured as a function of some depth sensitive parameter or condition in the experiment, and a depth profile of the sample properties is then recovered from a mathematical analysis of the detector data. The mathematical problem of reconstructing a depth profile of the sample properties from the experimental data in most of these cases, requires application of an inverse scattering theory. The reconstruction problem is usually very ill posed, which means that the experimentally measured signals have only a weak dependence on the depth of an optically interacting feature. In practical terms, ill posedness requires that the data being analyzed must be highly free of both systematic and random errors if the reconstructed depth profile is to be reliable.
For example, optical depth profiling methods based on photoacoustic and photothermal spectroscopy measure signals arising from transient or modulated heat flow in the test material. This heat flow in turn arises from light absorption as a function of depth in the sample, caused by irradiation of the sample with a pulsed or modulated optical beam. The measured photothermal or photoacoustic signal derives its depth sensitivity from the signal's dependence on the optical beam's modulation frequency (or, in the case of pulsed irradiation, on the delay time past application of a short irradiating impulse). This signal dependence is mathematically related to the depth of an absorbing feature below the surface. Reconstruction of a depth profile of optical absorption from photoacoustic or photothermal signals has been experimentally demonstrated, but to date, this can only be done if the sample is substantially planar, having a variation in structure along the thinnest dimension, which are called herein the depth dimension, and substantial homogeneity along all directions transverse thereto. Materials for optical photoacoustic or photothermal depth profile analysis must furthermore be substantially homogeneous in their thermal properties, and measurements must be carried out under conditions of a precise knowledge of the sample's detection geometry. Relative errors in the experimental data must be less than 1% of the full scale signal, typically, for a reliable depth profile reconstruction.
A related set of depth profiling techniques based on attenuated total reflectance (ATR) of an optical beam, measure depth dependent optical absorption in the test material by launching evanescent optical waves into the material. This is accomplished by means of a slab or guide of optical material of large refractive index which is physically contacted to the material under test. By varying the launch angle of radiation entering the slab, the depth of penetration of the evanesce
Fu Shao-wei
Power Joan F.
(Ogilvy Renault)
Anglehart James
McGill University
Smith Zandra V
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