Below are the factors influence the local thickness of metal and oxide coatings by the microscopical examination of cross sections using an optical microscope.
Under good conditions, when using an optical microscope, it is capable of giving an absolute measuring accuracy of 0.8 μm. This will determine the suitability of the method for measuring the thickness of thin coatings.
1 Surface Roughness—If the coating or its substrate has a rough surface, one or both of the interfaces bounding the coating cross section may be too irregular to permit accurate measurement.
2 Taper of Cross Section—If the plane of the cross section is not perpendicular to the plane of the coating, the measured thickness will be greater than the true thickness. For example, an inclination of 10° to the perpendicular will contribute a 1.5 % error.
3 Deformation of the Coating—Detrimental deformation of the coating can be caused by excessive temperature or pressure during mounting and preparation of cross sections of
soft coatings or coatings melting at low temperatures, and also by excessive abrasion of brittle materials during preparation of cross sections.
4 Rounding of Edge of Coating—If the edge of the coating cross section is rounded, that is, if the coating cross section is not completely flat up to its edges, the true thickness cannot be observed microscopically. Edge rounding can be caused by improper mounting, grinding, polishing, or etching. It is usually minimized by overplating the test specimen before mounting.
5 Overplating—Overplating of the test specimen serves to protect the coating edges during preparation of cross sections and thus to prevent an erroneous measurement. Removal of coating material during surface preparation for overplating can cause a low-thickness measurement.
6 Etching—Optimum etching will produce a clearly defined and narrow dark line at the interface of two metals. Excessive etching produces a poorly defined or wide line which may result in an erroneous measurement.
7 Smearing—Improper polishing may leave one metal smeared over the other metal so as to obscure the true boundary between the two metals. The apparent boundary may be poorly defined or very irregular instead of straight and well defined.
To verify the absence of smearing, the coating thickness should be measured and the polishing, etching, and thickness measurement repeated. A significant change in apparent thickness indicates that smearing was probably present during one of the measurements.
8 Magnification—For any given coating thickness, measurement errors generally increase with decreasing magnification.
If possible, the magnification should be chosen so that the field of view is between 1.5 and 3 3 the coating thickness.
9 Calibration of Stage Micrometer—Any error in calibration of the stage micrometer will be reflected in the measurement of the specimen. Errors of several percent are not unrealistic unless the scale has been calibrated or has been certified by a responsible supplier. The distance between two lines of a stage micrometer used for the calibration shall be known to within 0.2 μm or 0.1 %, whichever is the greater. If a stage micrometer is not certified for accuracy, it should be calibrated. A generally satisfactory means of calibration is to assume that the stated length of the full scale is correct, to measure each subdivision with a filar micrometer, and to calculate the length of each subdivision by simple proportion.
10 Calibration of Micrometer Eyepiece:
10.1 A filar micrometer eyepiece generally provides the most satisfactory means of making the measurement of the specimen. The measurement will be no more accurate than the calibration of the eyepiece. As calibration is operator dependent, the eyepiece shall be calibrated by the person making the measurement.
10.2 Repeated calibrations of the micrometer eyepiece can be reasonably expected to have a spread of less than 1 %.
10.3 Some image-splitting micrometer eyepieces have a nonlinearity that introduces an error of up to 1 % for short measurement distances.
11 Alignment—Errors can be introduced by backlash in the movement of the micrometer eyepiece. If the final motion during alignment of the hairline is always made in the same direction, this error will be eliminated.
12 Uniformity of Magnification—Because the magnification may not be uniform over the entire field, errors can occur if both the calibration and the measurement are not made over the same portion of the field with the measured boundaries centered about the optical axis.
13 Lens Quality—Lack of sharpness of the image contributes to the uncertainty of the measurement. Poor quality lenses could preclude accurate measurements. Sometimes image sharpness can be improved by using monochromatic light.
14 Orientation of Eyepiece—The movement of the hairline of the eyepiece for alignment has to be perpendicular to the boundaries of the coating cross section. For example, 10° misalignment will contribute a 1.5 % error.
15 Tube Length—A change in the tube length of the microscope causes a change in magnification and if this change occurs between the time of calibration and the time of measurement, the measurement will be in error.
A change in tube length may occur when the eyepiece is repositioned within the tube, when the focus of the eyepiece tube is changed, and, for some microscopes, when the fine focus is adjusted or the
interpupillary distance for binoculars is changed.
Contact Person: Mr. Raymond Chung