Characterization of Surface Modifications by White Light Interferometry: Applications in Ion Sputtering, Laser Ablation, and Tribology Experiments

Optical Profilometry is a non-contact method for measuring the surface heights of large or small objects to sub-micron accuracy. The overall goal of the following experiment is to use a white light interferometric microscope as a quick method for measuring the topography of small areas, which enables the measurement of the amount of material lost during mechanical wear processes or during material etching processes such as ion sputtering craters, or laser ablation. This measurement is achieved by first obtaining three dimensional profiles of the test surfaces using a white light interference microscope.

Incoherent white light will form distinct interference patterns. Only when the path length delays are the same, then widely available. Software measuring tools are employed to determine changes from the original surface caused, for example, by intense laser irradiation or energetic ions.

This is done by subtraction of the altered surface from the original flat smooth surface. When doing this, it may be necessary to use software tools to remove curvature from the surface to render it a flat plane, such as for a wear scar on a curved surface. It is shown how the method can be applied in two areas.

First, mechanical wear analysis, wear, wear scars on tri logical test samples are analyzed, and second in material science to determine ion beam sputtering or laser ablation volumes and depths. The main advantage of this technique over other methods like stylus, profilometry, or an approximation based on size is that it's fast and accurate Because the method produces detail three-dimensional picture. It is very useful for measuring irregular or regular craters, such as from laser ablation or iron beam spattering.

Secondarily, in the field of triology, the amounts of wear may be quite small and approximations based on simple microscope estimates may be misleading. It is necessary to obtain the actual shape of the deformed region if correct results are to be reported. The same holds true, for instance, in spattering experiments where depth removed maybe of the order of only 10 nanometers Begin by securing use of an optical profilometer and topographic software.

This demonstration makes use of a micro exam, 100 white light interference microscope and scanning probe image processor software. The following steps demonstrate how to measure the volume of a small wear scar on a ball as would be done in triology or lubrication science. To make this presentation as generic as possible, no automated processing is used.

The first step is to position the sample on the measuring stage. Place the ball on the profilometer stage using any convenient and stable pedestal. With the feature of interest facing upward, use a low magnification objective and position the ball directly beneath the lens.

Adjust the vertical position of the specimen so that the interference fringes appear near the center of the screen. For a curved surface, orient the specimen so the fringes are centered. Rotate the ball by hand or tilt the stage so that the wear scar comes into view and is also horizontal if available.

Use intermediate magnification lenses to obtain an image in which the worn area of interest largely fills the screen. Doing so, improves resolution, adjust illumination and scan height in order to obtain the best topographic map. When collecting data, scan the specimen according to instrument instructions.

Fill in any bad or missing data using the interpolate function, and then save the map using a 3D isometric view. This image shows an AB braided area of the ball. The analysis of this surface requires the removal of the curvature of the image so that the original surface of the ball appears flat.

The volume of the depression can then be measured on the 2D view, select an area of interest that excludes the wear scar. Here, the green shade denotes the excluded region. Be sure that the image analysis program applies the surface correction to the entire area, but that the fitting is done using only the area of interest that you have marked.

Select the software curve fitting tool that will remove the curvature. For example, polynomial fifth order. Choose the option to operate on the included area so that the scar does not influence the curvature removal.

It may be necessary to run the fit several times to ensure the area is flat to good accuracy. Set the mean level to zero. The darker circular region is the depression.

The volume of the wear scar is measured in the image processing software using the measuring tool. Any shape can be used here. A blue elliptical measuring tool is used to circle the wear scar.

The software measuring tool should have a feature that totals the amount of material above the level plane and the amount of material lost below the level. In this particular example, the inset shows that the material volume is 136 cubic microns. The void volume is 2, 733 cubic microns giving a net wear of 2, 597 cubic microns.

An estimate of any systematic error can be made by moving the measuring region away from the wear scar and noting that the measured wear volume, which should be zero, is indeed very small. The measurement of the volume of a wear scar on a flat surface is more straightforward than for a ball. To begin the analysis, obtain an image of the trench groove or scar.

It is generally good practice to remove any specimen tilt, and the interference fringes will spread apart. When the tilt is removed, scan the sample. A groove should be imaged.

This image is an isometric view. The surface should be horizontal. If it isn't.

Mask the scar area and apply a plane tilt correction to the rest of the surface. Also set the mean height of the unmasked surface to zero. In this example, the initial orientation was nearly perfect.

Next, use the measuring tool to determine the volume of the trench. The numerical results of this example are avoid volume of 47, 018 cubic microns, a material volume of 68 cubic microns above the surface, giving a net loss of 46, 950 cubic microns. This is the amount of material that is lost in the length of wear scar.

In practice, it is often the case that the surface is only roughened. In a second example shown here, the void and material volumes are nearly equal to each other and little material has been actually removed. The volume analysis of an ion sputter or laser ablated crater is straightforward to begin the analysis, obtain an image with the crater near the center and acquire the scan data.

Some white light interferometers may have more than one mode of operation for shallow features. The phase shifting interferometry mode of scanning should be used. That is the mode that is used in this very shallow example.

In this example, it is seen that the area surrounding the crater is not perfectly flat due to prior processing steps, and also the Z axis is offset to eliminate the influence of the uneven surrounding area. A cropping tool may be used to restrict the area to that shown by the white box. The image should be offset so that the undisturbed area around the perimeter is at z equals zero.

This can be done using a frame or Z offset tool if desired. The proper alignment of the crater can be verified using a 3D view. The crater can now be measured using the standard measuring tool.

Again, the blue highlighted region will have its material volume and void volume measured. The net volume of the crater is 86, 146 cubic microns. For time to depth analysis, various line profile tools may be used to measure the depth, the asymmetry, the wall incline and so forth.

Optical profilometry has been used in these examples to measure test samples for engineering and material science. The method can be used in other fields, also even biomedical for the study of cartilage Surfaces. We first had the idea for this method when we found out that other techniques just were not suited for our tasks.

For example, atomic force microscopy is just too limited in its scanning ranges. Mechanical styles. Peripheral imagery is only one dimensional, and scanning electromicroscopy in many cases gives one essentially only flat images.

This visual demonstration of the method is useful because it gives people unfamiliar with the technique an idea of how it works. Hopefully, it will lead others to apply white light interferometry to their own field of study.