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.
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,