Optical Profiler Basics
Optical profilers like ZeGage and Zescope are interference microscopes, and are used to measure height variations—such as surface roughness—on surfaces with great precision using the wavelength of light as the ruler. Optical interference profiling is a well-established method of obtaining accurate surface measurements.
Optical profiling uses the wave properties of light to compare the optical path difference between a test surface and a reference surface. Inside an optical interference profiler, a light beam is split, reflecting half the beam from a test material which is passed through the focal plane of a microscope objective, and the other half of the split beam is reflected from the reference mirror.
When the distance from the beam splitter to the reference mirror is the same distance as the beam splitter is from the test surface and the split beams are recombined, constructive and destructive interference occurs in the combined beam wherever the length of the light beams vary. This creates the light and dark bands known as interference fringes.
Since the reference mirror is of a known flatness—that is, it is as close to perfect flatness as possible—the optical path differences are due to height variances in the test surface.
This interference beam is focused into a digital camera, which sees the constructive interference areas as lighter, and the destructive interference areas as darker.
In the interference image (an "interferogram") below, each transition from light to dark represents one-half a wavelength of difference between the reference path and the test path.
If the wavelength is known, it is possible to calculate height differences across a surface, in fractions of a wave. From these height differences, a surface measurement—a 3D surface map, if you will—is obtained.
IMAGE CAPTION: Hypothetically, if the wavelength is 500 nanometers, one could estimate the distance of slope over a full wavelength by looking at the light and dark interference bands—known as interference fringes—in an interferogram. Optical profilers like ZeScope are much more accurate in calculating these differences than we can be with our eyes.
Optical profilers obtain amazing accuracy with interferometry
Looking at the interferogram above, you might notice that the light and dark bands near the bottom aren't as bright or dark as the ones near the ruler marks. This is because the lower portion of the interferogram is going out of focus: out of focus means less interference. By carefully calculating the area of greatest contrast, optical profilometers determine the point that has best focus.
In practice, an optical profiler scans the material vertically. As the material in the field of view passes through the focal plane, it creates interference. Each level of height in the test material reaches optimal focus (and therefore greatest interference and contrast) at a different time. With well-calibrated optical profilers, accuracy well below a nanometer is possible. A nanometer is ten Angstroms.
In a Zemetrics profilometer, each data point is monitored to determine its most precise focal point. Every pixel's height is measured relative to every other by comparing its maximum contrast (point of focus) relative to the pixels around it—producing a very sensitive surface measurement. ZeMapper's accuracy is better than ten picometers -- tenths of an Angstrom, or one one-hundredth of a nanometer. An Angstrom is equal to the diameter of a hydrogen atom.
What is the purpose of optical profiling?
Non-contact profilometers are used in many situations where micro-measurement of surface variations are essential. Industries such as optics and data storage use highly polished surfaces that are measured with interference profilometers.
- Optics metrology is focused on lens and mirror surface finish and surface roughness, rate of curvature, and sometimes surface texture. Some binary lenses and diffraction gratings require measurements of volume, slope and radius of curvature.
- Data storage surface metrology concentrates not only on surface finish roughness, but also the surface shape of the disk at the edge, and the geometry of laser-textured bumps for minimizing 'stiction' the adhesive force of the read head to the magnetized surface. Bump spacing, their alignment to a grid, peak-to-valleys, consistency, and general bump shape are measured.
Optical profiling can be used to measure surface finish, roughness and shape on many surfaces, so long as enough light is reflected back into the objective from the surface. Optical profiling can be limited by very high slopes, where the light is reflected away from the objective, unless the slope has enough texture to provide the light.
Optical profiling has been used to measure paper, plastic, epoxies, metals, glass, paint and ink. It is useful in analyzing the fingerprints of materials processing, such as the cumulative effects of sawing, grinding and polishing. It is useful in wear analysis, since it is able to calculate the volume of voids and scratches.
Optical profiling is used in a number of precision-engineering surface measurement situations:
- for large step height measurement, where a stylus profiler may have difficulty reaching each step
- where a three dimensional map of a surface is important, such as:
- in determining the average heights of different areas
- where sampling location selection may lead to varying results
- where volume is a necessary parameter, such as measuring voids
- a soft or fragile surface may be altered by contact measurements
- where surface roughness, as opposed to linear roughness, must be known
- in some cases, transparent films can be measured at their top and bottom surfaces
Optical profiling is quicker than stylus profiling in measuring surface areas. Areas are measured with stylus profilers with a series of parallel linear measurements. Previously, stylus profilers had an advantage of permitting long profiles, using extended stage travel. However, Zemetrics optical profilers have larger fields of view and more pixels per measurement than typical optical profilers: therefore our profilers can measure longer profiles than previously possible in a single optical surface measurement acquisition.
A further advantage of optical profiling is ease of use: the highly automated ZeMapper and ZeScope are loaded with ease-of-use features that permit accurate and repeatable measurements with far less operator training.
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Optical Profiler Advantages:
- Optical profiling, as opposed to stylus profiling, is non-contact.
- Optical profilers are inherently three dimensional: they measure height (the Z-axis) over an area of X and Y lateral dimensions. Stylus profilometers are inherently linear (2 dimensional), as the stylus is dragged across the surface, sampling a continuous line.
- Until Zemetrics introduced surface metrology systems, optical profilers typically had about 0.3 million pixels of sampling resolution. ZeScope Profiler has up to 1.3 million, and ZeMapper Profilometer has a staggering 4 million data points per measurement.
- Every pixel in the imaging camera is a datum: its optical path difference is calculated relative to each adjacent pixel, by comparing the contrast between them. So, the more pixels in the field of view, the more data you get in each measurement.
- Stylii wear out, or need to be changed for varying surface conditions. Because they have no need for a stylus, optical profilers have no expensive 'consumable' parts to replace.
- Accuracy of the optical path difference is essential to good non-contact profilometry, so Zemetrics calculates any flaws in reference mirrors, and subtracts them from the measurement
- Since reliable coupling of focus with interference contrast is essential to repeatable surface metrology, our interference microscopes come with our own athermal objectives: offering better repeatability in variable temperature environments