A further increase in depth penetration by light-interference microscopy can be accomplished with the immersion method. This consists of observing the specimen surface through a medium with a high refractive index, such as oil, into which the object, or its replica is being immersed. With the immersion method, the depth of penetration of light-interference microscopy can be increased to about 0.001 inch. Figures 15-21 and 15-22 illustrate the advantages of the immersion technique when inspecting surfaces on which the departures of certain features exceed the discriminating capacity of regular light-interference microscopes.

Fig. 15-21. (Top) Interference micrograph of a generally flat surface, interrupted by a deep groove in the center of the observed area. The abrupt change in level causes the compression of the fringes beyond the limit of resolution.

Fig. 15-22. (Bottom) The specimen area shown in Fig. 15-21, as it appears when prepared by immersion technique. A replica made from the surface is immersed into a medium with high refractive index, the value of which is considered as a factor when computing the depth of the now distinctly appearing groove.

While monochromatic light is preferred to obtain a reliable scale in the form of known fringe separation, most interference microscopes offer the alternative use of white light, as well, for producing a single dark fringe, often termed zero fringe. This latter is easier to follow in the assessment of the interference image when variations of the object surface are causing fringe shifts that amount to several band widths.

Multiple-beam interference microscopes are sometimes preferred for the examination of very fine surfaces where the clear definition of the fringe boundaries offered by the multiple-beam system is required. In this system, the mutually interfering light beams are forced to pass the interference space several times, resulting in particularly sharp fringe lines, permitting measurements in the submicroinch range. The multiple-beam system, however, requires focusing very close to the objective, which usually limits its application to flat surfaces of excellent geometry, unless replicas of the test piece surface are used.

In conclusion, it can be stated that the interference microscopy, although subjected to certain practical limitations, offers many technical advantages for the analysis and measurement of surface texture. A few characteristic properties are mentioned in the following, to permit an evaluation of the suitability of the interference method for the surface examination of specific workpieces.

The following are the limitations of interference microscopy:

a. The depth range that can be covered by direct measurements excludes its use for rough surfaces, unless an immersion technique is applied;

b. It is adaptable only to external surfaces, which are accessible by the microscope, and of substantially regular form, unless replicas are made and inspected; and

c. The assessment of surface conditions, as presented by the interference image, is not directly correlated with the average-roughness values specified by current standards; however, good correlations exist with the stylus type, peak-to-valley measurements.

The following are the advantages of interference microscopy:

a. It is an area sampling method, which can be visualized as simultaneously presenting a series of closely spaced cross-sections of the inspected area;

b. The direction of these imaginary cross-sectional planes, as well as their spacing, can be selected to provide the most informative fringe pattern;

c. It offers its own standard of measurement of the highest accuracy and never requires recalibration;

d. Being a noncontact method, it is nondestructive and applicable to materials that, because of their softness, could not be inspected by the stylus method; and

e. Even minute variations of the surface that defeat faithful scanning with a stylus of finite dimensions can be clearly shown.

Sub-Angstrom, Non-Contact 3D Surface Profiling

Somewhere at the intersection of non-contact electronic gages, engineering microscopes, contour and form scanning we find a metrology tool capable surface profiling at the sub-angstrom level (see Fig. 15-23). An angstrom is one hundred-millionth of a centimeter and given that a nanometer is one-billionth of a meter, an angstrom is 0.1 (or one-tenth) of a nanometer; the angstrom is a truly small unit of measure. Coherence correlation interferometry (CCI) is able to complete a measurement with over one million data points in less than ten seconds with a resolution of 0.1 angstroms (0.01 nm). Surface inspection at this level must be accomplished by noncontact means. Surface quality at nanometer scale is becoming commonplace in optical, semiconductor and orthopedic component manufacturing. The instrument shown in Fig. 15-23 can provide critical information related to manufacturing processes working in the nanometer realm.

Fig. 15-23. “Talysurf CCI” advanced 3-dimensional non-contact metrology tool used for advanced surface characterization connected to computers and monitors for analysis and graphical display.

Because the surface features being discussed here are so small, sophisticated computer software that allows for the analysis and graphical display of surface images was developed. Taylor Hobson Precision has its proprietary Talymap software for use with its non-contact 3D surface profiling products. One company, TrueGage Surface Metrology has developed a series of software analysis and imaging products that can be used with a variety of surface measurement products and by their manufacturers. An example of a topographical “weather map” is shown in Fig. 15-24.

On a computer monitor or in a color copy of the same, the shades of gray in Fig. 15-24 would be colors of the spectrum making the image even more vivid and meaningful.

Fig. 15-24. “Truemap” surface topology visualization and analysis software digital output display.