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Brief Introduction to Surface Metrology

This report briefly discusses several important metrology techniques and standard definitions commonly used to assess the topography of surfaces, also known as surface texture or surface finish. With the advent of nanotechnology, thin coatings, and the miniaturization of circuits and devices, surface metrology has become an extremely important field of science and engineering. It deals with precise, representative characterization of surface topography in terms of its microscale and sub-microscale features. These features make up the waviness, roughness, and lay of the surface. Topography plays a crucial role in determining the mechanical, thermal, optical, and electrical properties of materials which are used for many modern technologies, components, parts, and products, such as motors, coatings, electronic devices, etc.

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What is Surface Metrology and Why is it Useful?

Surface metrology is the measurement of the features (regular patterns, irregularities, roughness, waviness, critical dimensions, etc.) of a surface. The topography of a surface, also called surface texture or finish, determines, to a large extent, its mechanical and physical properties, e.g., friction, adhesion, oxidation, thermal and electrical conductivity, etc. The topography is important for materials used in advanced technologies and devices (advanced coatings, bearings, thermal, optical, and electronic/semiconducting devices). For example, greater surface roughness often increases the friction between two parts in contact which may not be desirable if smooth motion is required. Greater friction between parts can also lead to faster wear and shorter lifetimes. The formation of small irregularities in a semiconducting surface can induce charge localization and non-homogenous electrical properties.

The properties of the surface region, which can be roughly defined as the first 100 atomic layers of a material, will usually differ from the bulk region due to oxidation, surface tension, contamination or processing. For example, material preparation methods like mechanical or chemical polishing or etching can induce surface defects and roughness. Because most processes (mechanical or chemical) used to prepare surfaces induce defects and irregularities, metrology instruments and methods are needed to assess surface topography and determine how it affects the device’s properties, including performance, reliability, and lifetime.

Surface metrology methods are used to examine and measure the topography at different length scales and spatial frequencies of a surface. Roughness is typically determined by measuring the height, width and periodicity/frequency of surface patterns or irregularities. Waviness is defined by surface irregularities on a larger scale (lower frequency range) than the roughness. A uniform surface is isotropic. Lay refers to the directionality of surface features (anisotropic) which is often due to material manufacturing or treatment. These standard topography or texture parameters (roughness, waviness, lay, and flaws) are discussed below.

Surface Characterization Methods

Gross features and large defects often can be quickly assessed by the naked eye, finger tips, and low-resolution optical microscopes. However, detailed measurements of finer surface profiles and topographies require advanced surface characterization techniques.

The topography of the surface can be determined with either 2 or 3 dimensional (2D or 3D) measurements, using a variety of high-resolution techniques. It is important to choose the right technique for a specific purpose, because they all have their advantages and limitations. Here, we focus only on some of the most widely used methods in materials science, such as surface probes (stylus, AFM), optical and interferometric methods, and electron beam methods.

The vertical (z) height of points falling on a surface line profile or an area are measured and a 2D contour or 3D map of the surface is displayed. The data is analyzed using defined statistical analysis methods and the resulting values serves as parameters to characterize the surface topography, more specifically, the surface roughness, waviness, lay, and flaws.

Images of the surface topography in 2D or 3D can be acquired using a variety of methods. The most common ones are [1-3]:

  • Contact/non-contact profilometry and probe microscopy where the topography data is gathered by a fine probe scanning across the surface;
  • Optical profilometry with interferometry, focus and phase detection, or confocal microscopy, using light; and
  • Scanning electron microscopy (SEM) using electrons where normally a special software is needed to display 3D topography.

A common probe imaging method is atomic force microscopy (AFM). Although it can achieve very high lateral (XY) and vertical (Z) resolution, acquiring topography data is quite slow and there is a risk that the surface can be altered or damaged. Furthermore, the shape and the size of the probe may change during scanning due to wear and contamination. This phenomenon can influence the appearance of features in the acquired surface topography as the probe and feature geometry are mixed together, a type of convolution [4]​​​​​​​. An example is shown in figure 1 below. Obtaining good results with the AFM also requires users to have a certain level of experience.

Optical methods for surface characterization can have a high vertical (z) resolution, but not as high a lateral (xy) resolution as probe methods or electron microscopy. However, the topography acquisition is much faster. This fact means that optical methods can provide surface topography data over large areas making them more practical for reliable, accurate statistical analysis.

SEM can also achieve very high resolution, but imaging is done in a vacuum chamber. If the material is not conductive enough, then charging will occur in the electron beam, so the sample must be coated with a conductive film. Generally, acquiring the image can be time-consuming.

Common Surface Topography Parameters

Roughness

The surface irregularities that form a predominant pattern of roughness on a surface.  A smaller surface roughness value indicates smaller and/or fewer irregularities. Examples of parameters which give an indication of surface roughness [1,2,5,6] are shown in table 1 below. Refer also to figure 2 and 3.

Fig. 2: Example of how the parameters Rmax (A), Ra (B), and Rq (C) in table 1 are calculated using a contour profile of an imaginary surface. For Ra the |Z| values in B are averaged. For Rq the Z2 values in C are averaged and the square root of the average value is taken.

Waviness

A measure of surface irregularities with a spacing greater (lower spatial frequency range) than that of the predominant pattern measured for roughness [1,2,5,6]. It is illustrated in figure 3 for an arbitrary surface topography.

Lay

Direction of the predominant surface pattern normally determined by the material production method [1,2,5,6]. The texture aspect ratio (Str) [7], a spatial parameter which indicates whether a surface is isotropic or anisotropic, is useful for determining the lay of a surface. A schematic showing the lay of an arbitrary surface topography is shown in figure 3.

Flaws

Infrequent, isolated irregularities on a surface from specific causes, e.g., scratches, cracks, etc. [1,2,5,6].

International and Regional Standards for Surface Topography Characterization

The need to check the quality of a surface, to ensure the reliable performance and lifetime of manufactured parts and components, especially for such things as surface topography (also called texture or finish), has led to the development of international and regional standards [3,5,6]. Early standards were reference surfaces with a known topography which could be compared to other surfaces in a qualitative way. Later, the development of an instrument with a stylus, which could measure peaks and valleys when it was scanned across a surface, [3] led to the first standards based on the measurement of the average roughness (Ra).

Further innovations in electronic circuits and the digitalization of analog outputs led to the development of 2D and 3D measurements of topography which could be acquired and recorded with high resolution. These advances helped develop various standards which are used to define topography/texture parameters. Table 2 shows some examples of surface texture standards currently in use worldwide [5,6].

Standard

Title

Link

ISO 4287

Surface texture: Profile method -- Terms, definitions and surface texture parameters

https://www.iso.org/standard/10132.html

ISO 4288

Surface texture: Profile method -- Rules and procedures for the assessment of surface texture

https://www.iso.org/standard/2096.html

ISO 8785

Surface imperfections -- Terms, definitions and parameters

https://www.iso.org/standard/16210.html

ISO 12085

Surface texture: Profile method -- Motif parameters

https://www.iso.org/standard/20867.html

ISO 12780

Straightness -- Part 2: Specification operators

https://www.iso.org/standard/53623.html

ISO 12781

Flatness -- Part 2: Specification operators

https://www.iso.org/standard/53625.html

ISO 13565

Surface texture: Profile method; Surfaces having stratified functional properties – Parts 1 to 3

https://www.iso.org/standard/22279.html

https://www.iso.org/standard/22280.html

https://www.iso.org/standard/26280.html

ISO 25178

Surface texture: Areal -- Parts 1 to 607

https://www.iso.org

ASME B46.1

Surface Texture (Surface Roughness, Waviness, and Lay)

https://www.asme.org/codes-standards/find-codes-standards/b46-1-surface-texture

Table 2: Some international and regional standards for surface topography/texture and characterization.

Summary

Surface metrology is an important field of science and engineering concerning the precise, representative characterization of surface topography, also known as texture or finish. It involves the measurement of a surface’s microscale and sub-microscale features. Surface topography has a critical influence on the mechanical, thermal, optical and electrical properties of materials used to make components, parts, and products. In this report, several important surface metrology techniques and common topography or texture parameters, such as roughness, waviness, and lay, were discussed. Moreover, an overview of international and regional standards for surface topography or texture was given.