Microroughness

Microroughness is the fine-scale texture of a surface, arising from features that remain after larger geometric form and waviness have been removed. It typically exists on the nanometer to sub-micrometer scale, though in some applications it can extend into the low micrometers. Microroughness is distinguished from macro-roughness by its much smaller characteristic wavelengths and its dominant influence on optical, electronic, and tribological interactions. It is commonly described using roughness parameters such as Ra (average roughness) and Rq (root-mean-square roughness), sometimes complemented by Rz or by a power spectral density to capture the distribution of spatial frequencies.

Measurement and analysis: Microroughness is measured with contact stylus profilometry, optical profilometry, or atomic force microscopy (AFM). Each method yields parameters such as Ra and Rq and can be extended with PSD analysis to reveal how roughness varies with spatial frequency. International standards, for example ISO 4287 for 2D roughness and ISO 25178 for 3D roughness, guide sampling and reporting.

Applications and effects: Microroughness influences light scattering, reflectivity, and haze in optical components; it affects diffusion barriers and interface quality in microelectronics; and it governs real contact area, friction, and wear in tribological contacts. It also impacts coating adhesion, epitaxial growth, and wetting behavior. The relative importance of microroughness depends on the wavelength of interest and the specific physical process.

Control and fabrication: Reducing microroughness is achieved through finishing and planarization techniques such as mechanical polishing, chemical mechanical polishing (CMP), etching, annealing, or optimized deposition strategies. In some cases, deliberate roughening at the micrometer or nanometer scale is engineered to tune optical scattering or adhesion properties.