3D Surface Measurements

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What is surface profilometry?

Surface profilometry extracts quantitative topographical information—surface roughness, step height, form, and morphology—by measuring surface height (Z) as a function of lateral coordinates X and Y. Common questions that surface profilometry can be used to answer include: 

• How flat is the sample? 

• How high are distinct features?

• What is the surface roughness over an area?
  

• What is the distribution of defects, voids, or particles?

Each input of surface height data can be a single point, a line scan, or a full 3D area scan. Single-point methods are always probe-based and are commonly seen in coordinate-measuring machines (CMM). Line-scan methods can be either stylus- or optically based. Area-scan methods are always optically based.

What are the types of surface profilometry?

Surface profilometry (or surface profiling) methods can be segmented into two main categories: contact and non-contact. 

• Contact methods are based on scanning probe microscopy. They drag or tap a probe or stylus along the surface, collecting data points that can be combined to form line or area scans of height data. Examples of contact methods include stylus profilometry and some atomic force microscopy. Contact methods are inherently destructive since the surface is being touched, though damage can be minimized in many cases.
• Non-contact optical profilometry methods are based on optical microscopy and never contact the surface, making them inherently non-destructive. They can collect data areally (over a 2D area) with a single exposure and use that data to generate 3D height maps. 

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What are the advantages of 3D surface measurement vs 2D surface measurement?

Surfaces with similar or even identical average surface roughness values (Ra) might have vastly different surface topographies. Three-dimensional standards enable better quantification and differentiation of these types of surfaces.

3D surface parameters also have helped to improve communication and allowed a process control that traditional R parameters could not do alone. 

Though there was initially some resistance to using the techniques of 3D surface measurement, this was eventually overcome as R&D engineers took the time to fully understand the advantages of three-dimensional surface analysis and a shift became apparent as these S parameters were employed on more and more drawings and suppliers were being held to those specifications. Modern 3D surface measurement has given engineers, process designers, and quality control professionals a significantly improved toolkit for describing surfaces since three-dimensional measurements uniquely differentiate not only surface shapes but functionalities as well. All of which, ultimately, results in better surface performance.

What are 3D surface measurement parameters?

Worldwide 3D surface measurement parameters (S parameters) were defined in 1991 by the attendees of the first European Consortium Workshop and have since been developed in accordance with ISO standards to complement the traditional 2D metrology R parameters.

3D surface measurement parameters can be divided into four general categories: amplitude, spatial, hybrid and functional.

• Amplitude parameters are based on overall heights and include the root-mean-square of height distribution, skewness (or the degree of asymmetry of a surface height distribution), the degree of peakedness of a surface height distribution (or kurtosis), and an average of the highest and lowest points.
• Spatial parameters are based on frequencies of features and include the texture direction of a surface, texture aspect ratio, and the density of summits.
• Hybrid parameters are based on a combination of height and frequency and include the mean summit curvature, developed surface area ratios, and the root-mean-square of surface slopes.
• Functional parameters include several parameters that are based on applicability of particular functions.

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How are 3D surface measurements collected?

There are a few techniques that provide a 3D surface representation from a microscope image, including the two key techniques of white light interferometry (WLI) and confocal microscopy, also known as laser scanning confocal microscopy (LSCM).

The principle of operation for each method provides different advantages and disadvantages, yet there are critical advantages to using Bruker's WLI-based 3D optical microscopes over confocal microscopes for certain applications. Key to these advantages is the ability to maintain subnanometer vertical resolution and 0.1 nanometer RMS repeatability, regardless of magnification or field of view.