Surface roughness can tell a lot about the material something is made from and its past processing history. If the surface roughness for a part needs to be taken but making physical contact with the surface is not advised (to avoid surface damage or altering a feature of interest) than white light interferometry is a great solution. The basic principle of interferometry is that light is reflected from a reference mirror and combined with light reflected from the sample surface to produce interference fringes, where the best contrast fringe occurs at the best focus. Ebatco possesses a Wyko NT3300 white light interferometer manufactured by Veeco. The NT3300 has two measurement modes, phase shift interferometry (PSI) and vertical scanning interferometry (VSI).  PSI mode uses variations in phase of a monochromatic reference light compared to light reflected off of the sample to calculate small height variations on the sample surface. This measurement mode is useful for surfaces with step-heights of less than a quarter of the wavelength used and provides sub-nanometer vertical resolution. VSI mode uses a multi-wavelength white light source to measure the degree of fringe modulation, or coherence, between the reference and sample beams. As the beam is moved vertically through the sample focus, the maximum fringe contrast is extracted at each point on the sample surface, and forms a topographical map. This mode of measurement can be used to measure features up to 2 mm in height.

In addition to surface roughness analysis, The Wyko NT3300 optical profilometer can also be used to acquire contour and morphology of parts in three-dimensions. Once the data is acquired, critical dimensions such as distance, depth, thickness, and other geometrical parameters can be obtained. These capabilities have found usefulness in characterizing thin films and coatings for thickness measurement through step height determination across coated and uncoated areas.

Typical Experimental Results

Surface contour image of a South Dakota state quarter (left) with a height map image (right).
3D height map of precision machine marks on a steel substrate.
Vertical line profile of a square pillar grid specimen with a specified step height of 112 nm.
Surface roughness measurement of a piece of SS304 weld specimen.

Tabulated results of 2D surface roughness measured at different locations on a SS304 weld specimen

Test Area Sa (µm) Sq (µm) Sz (µm)
A 0.38 0.48 2.46
B 0.42 0.55 2.51
C 0.27 0.34 1.78
D 0.24 0.29 1.55

Applications

2D Line Profiles 3D Profiles Aeronautical components Automotive Parts Coating Degradation
Coating Failures Critical Dimensions Electronics Embossed Structures Failure Analysis
Lubricants Metallic Coatings Packaging Paint Coatings Particle Contaminants
Personal Care Products Reflective Paint Scratch Analysis Semiconductors Step Height
Surface Finishes Surface Morphology Surface Roughness Surface Topography Thin Films
Voids Wafers Wear Analysis Wear Volume Wear Tracks

For more information please read our Application Notes.

Contouring

Optical Inspection and Profiling of Defects on a Coated Wafer Surface

Surface Roughness Measurements through Optical Profilometry

The Wyko NT3300 Optical Profilometer

Instrument Key Specifications

Vertical Measurement Range 0.1 nm – 2 mm
Vertical Resolution < 1 Angstrom
Lateral Spatial Sampling 0.08 μm – 13.1 μm
Field of View 0.05 mm – 8.24 mm
Reflectivity 1% – 100%
Operation Mode PSI, VSI
Scan Speed Up to 7.2 μm/sec
Maximum Slope 1.8º – 25º
ASTM Title Link
E2244-11(2018) Standard Test Method for In-Plane Length Measurements of Thin, Reflecting Films Using an Optical Interferometer https://www.astm.org/Standards/E2244.htm
E2245-11(2018) Standard Test Method for Residual Strain Measurements of Thin, Reflecting Films Using an Optical Interferometer https://www.astm.org/Standards/E2245.htm
E2246-11(2018) Standard Test Method for Strain Gradient Measurements of Thin, Reflecting Films Using an Optical Interferometer https://www.astm.org/Standards/E2246.htm
ISO Title Link
10110-8 Preparation of drawings for optical elements and systems– Part 8: Surface texture, roughness and waviness https://www.iso.org/standard/46553.html
1302 Geometrical Product Specifications (GPS)– Indication of surface texture in technical product documentation https://www.iso.org/standard/28089.html
4288 Geometrical Product Specifications (GPS)– Surface texture: Profile method– Rules and procedures for the assessment of surface texture https://www.iso.org/standard/2096.html