Profilometry - An Overview
PROFILOMETRY
Surface texture, also referred to as surface roughness, is an important parameter for many manufactured items. It can be measured on ground or machined surfaces and is especially significant for polished surfaces. The performance of optical elements and the processing from shaping, to grinding, to final polishing is largely influenced by surface roughness. The surface roughness at each step of fabrication is a result of the operation, the substrate properties, and the conditions of the machine to part interactions, with a little chemistry thrown in for good measure.
The Texture of a Surface is Described by Roughness Measurements
Machined and ground surfaces display a characteristic appearance. There may be a very consistent finish, typical of a loose abrasive grinding process. If the surface has been machined or has been surface ground on a Blanchard type grinder, there is usually a repeating pattern called a lay. This pattern typically has a direction of straight lines or arcs, depending on the relative motion between the cutting tool of the machine and the part. There may be patches of evenly rough material between deeper flaws. Occasionally there are random scratches or small craters. In such cases there will be a marked difference between the peak-to-valley versus the average roughness. It is common to specify or report the root-mean-squared (RMS) average.
For optical elements, loose abrasive grinding typically follows a machining operation, although modern diamond pellet laps can perform the equivalent process. It refines the previous texture and leaves a very uniform, less rough surface. There may be more than one grinding operation. Each step leaves a finer surface than the one before. It is important that the grinding removes all visible traces of the previous operation. Less obviously, it must remove chipping, scratching, and especially subsurface damage resulting from the prior step.
Once the surface is sufficiently refined by grinding, an optic is ready to polish. Polishing results in a very smooth surface with no obviously visible flaws. Polishing permits transmission through the optic if it is transparent material, or a specular reflective surface if the optic is intended to be a mirror. Scratches or digs detract from the performance of an optical element. They have an especially ill effect on laser damage resistance. There usually is micro ripple on a polished surface. That will cause scattering of the light, which can be a problem for laser optics especially. The ability to detect and quantify this condition aids in developing improved processes that will reduce the roughness to less than the Angstrom level. Continuing profilometry during production guarantees that quality is maintained, and that processes remain “in control.”
How Do You Measure Surface Roughness?
Depending on the requirements for characterizing surface roughness, visual comparison to a standard, mechanical stylus instruments, or optical instruments are available, in ascending order of discrimination. Metal or plastic cards can be replicated with a variety of surface textures and lays. A side-by-side judgement allows a quick inexpensive evaluation. Of course, it is an approximation and subject to interpretation, thus not adequate for precision requirements.
A better method uses a stylus with a smooth radius diamond tip. That is traversed across a sampling length of the part. The scan is a narrow linear measurement over a relatively short length. Measurements over a few regions are often necessary, because the surface texture can vary from place to place. Those instruments may output values including peak-to-valley, arithmetical average (Ra), and Rq, or RMS (root mean squared) roughness, plus a graphical image of the trace. A raster scan can yield approximations of a three-dimensional surface. Generally, the probe won’t damage any but the most delicate surfaces. A very well-known stylus instrument is the Taylor-Hobson Talysurf. The atomic force microscope (AFM) is a very precise and delicate example of a stylus device. Stylus instruments can measure ground or machined surfaces, and even polished surfaces may be evaluated safely, depending on the measured substrate properties.
For gaging precision polished optics, especially laser elements, white light interferometer profilometers are the preferred choice. They are a non-contact device, so they will not impart damage. Schematically they resemble a microscope. The equipment gathers interference data from light reflected by the surface of an optic and interferes it with a reference leg. The continuous spectrum of colors of white light interference reveals finer detail than a monochromatic device such as a laser. However, rough surfaces that do not reflect light cannot be measured this way.
Such contraptions are capable of discriminating as little as one Angstrom height difference, or even better. They are so sensitive that they are ordinarily mounted on vibration isolation tables, in temperature controlled rooms, and shielded from air currents. A sensor provides intensity data that is processed to yield peak-to-valley error, RMS roughness, spatial frequency, and power spectral density among others. It is a three-dimensional measurement. Of course, the scan area is small because of the high-power microscope objective lenses used. Stitching algorithms can closely approximate what a larger area might measure.
There are other ways to image surface roughness at the finest scale, such as a Nomarski differential interference contrast (DIC) microscope, but they only yield qualitative information, not quantitative values.
There are few sources of white light interference apparatus. Among them are Zygo Corporation, who leveraged their interferometer expertise to develop the line of NewView optical profilers. Veeco, which purchased Wyko and gained access to their knowledge base, continued to produce optical profilometers for a few years. Bruker Corporation continued this heritage after obtaining the technology from Veeco. Other vendors are Sensofar and Filmetrics, a division of KLA.
GAMDAN Optics, Innovators
GAMDAN Optics uses a white light interference tool to monitor surface conditions of their super-polished surfaces, and continuously improve the process. We pride ourselves on continued growth and innovation through partnerships and cutting edge projects all over the world.
DENNIS J. GARRITY, AUTHOR
Dennis is an engineer with over 45 years of experience in fabrication, testing, and material evaluation for high precision optics, with extensive hands-on experience. More on the author can be found here.
Contact us today to see if we can be of help in your development efforts.