This question is of interest to many manufacturers today. Why? Because more of them are making high precision, small-scale components that are critical to the function of a host of end products. A company’s livelihood may mandate that the components they use in their products are uniform, definable, and "high-precision". This is where an understanding of smoothness is helpful.
So how smooth is smooth or super-smooth? For example, is a windowpane smooth? Is a sealing surface in an automobile’s engine smooth? Is the lens in your cellphone camera smooth? And how does one define and quantify smooth? The best way is by taking a three-dimensional areal measurement and by assigning numeric parameters to the surface. Understanding the surface texture characteristics in this way helps ensure that the part is manufactured as designed and performs as intended.
Measurement of Super Smooth Surfaces with Optical Metrology
There are a variety of measurement tools available to measure surfaces, from stylus profilometers to atomic force microscopes, to non-contact optical metrology tools. A stylus profilometer provides roughness results by running a contact probe along a path on the sample. The stylus is relatively inexpensive; however, it only provides 2D results along a pathway or profile, and it may damage the very surface it measures. An atomic force microscope (AFM) uses a very small cantilever probe that is brought into proximity of a sample sensing the force between the tip and sample. While an AFM provides nanoscale resolution, the measurement process is time consuming, the equipment is costly, and the maximum scale of the surface being measured is limited.
The third class of instrument mentioned – non-contact optical metrology tools – avoid the contact that is the hallmark of the stylus and AFM by using all optical techniques to sense the surface. It is important to realize that not all non-contact optical tools provide the level of repeatability and precision required to measure the surface roughness of super smooth surfaces, though. For example, a confocal microscope features an optical technique that uses illumination and detection optics focused on the same diffraction-limited spot, which is moved over the sample to build the complete image on the detector. This confocal technique is suited for rough and highly sloped (angled) surfaces but is not as effective on smooth featureless samples.
Another optical metrology technology, coherence scanning interferometry (CSI), uses the principle of optical interference to compare a measured part to a perfect reference. Reflections from the part surface and the reference surface combine at a detector where they interfere with each other. The interference pattern represents the surface topography of the test surface and when the imaging system’s objective is scanned through focus, surfaces that are rough, structured, and exceptionally smooth can be measured.
CSI provides several advantages over confocal and other non-contact technologies:
- CSI is very precise with nanometer or sub-nanometer height precision at all magnifications, enabling both low magnification, larger field of view imaging and higher magnification surface texture metrology on the same instrument.
- CSI delivers a fast and consistent measurement, producing millions of pixels within a few seconds at all magnifications.
- CSI can be used on super-smooth or optically transparent surfaces, as well as surfaces with transparent optical films.
When smooth must be smooth, look for coherence scanning interferometry solutions for your business. Seek out a company with experience, that adheres to worldwide metrology standards, and has software and hardware that are easy to use.
ZYGO has many years of experience in precision metrology, our CSI Based 3D optical profilers including the NewView 9000, Nexview NX2 and ZeGage Pro, provide parameters compliant with ISO 25178, ISO 4287/4288 and additional standards. Mx software provides numerous 3D areal plots including a non-directional power spectral density (PSD) plot to show frequency content that is insensitive to alignment. Our 3D optical profilers have a smart setup feature to minimize operator and part guess work, all with the press of a single button.