Definition
An optical profiler is a non-contact surface topography measurement instrument based on the principle of optical interference or confocal. It realizes the quantitative analysis of surface roughness, step height, three-dimensional topography and other characteristics at the microscopic scale by obtaining the height information of the sample surface. This instrument provides key metrology methods for surface quality assessment and process control in materials science, precision manufacturing, semiconductor and other industries.
Principle
The core working principle of optical profilers mainly relies on optical interferometry technology or confocal scanning technology. In the interferometry method, the instrument divides the beam emitted by the light source into reference light and measurement light, and the measured light interferes with the reference light after being reflected on the surface of the sample. Its basic interference light intensity formula can be expressed as:
I = I₁ + I₂ + 2√(I₁I₂)cos(φ)
where I is the detection light intensity, I₁ and I₂ are the light intensity of the reference light and the measured light, respectively, and φ is the phase difference between the two. In the confocal method, the instrument uses the pinhole in the confocal optical path to achieve axial tomography scanning, and determines the height of the corresponding point by detecting the peak of the reflected light intensity at the focal point, and scans the three-dimensional contour point by point.
Measurement method
According to the measurement principle and configuration, the measurement methods of optical profiler mainly include phase scanning interferometry, vertical scanning interferometry and white light interferometry. Phase scanning interferometry introduces phase changes through a precision moving reference mirror to solve continuous phase information, and is suitable for high-precision measurement of smooth surfaces. Vertical scanning interferometry scans the sample or interferometric lens along the axial direction under a broadband light source, and determines the height by analyzing the peak position of the white light interference envelope, which is suitable for measuring surfaces with steps or large roughness. During the measurement process, the appropriate objective magnification, scanning range, and interpretation algorithm should be selected according to the characteristics of the sample to ensure data accuracy and repeatability.
Influencing factors
The accuracy of the measurement results is influenced by several factors. Ambient vibration and air turbulence can introduce interference noise and are typically operated on vibration isolation platforms or in a stable environment. The optical properties of the sample surface, such as low reflectivity or transparent materials, can lead to signal attenuation or subsurface reflection interference. The numerical aperture and depth of field of the objective lens determine the lateral resolution and measurable height range, which need to be matched according to the degree of surface undulation. In addition, the calibration status of the instrument, the scan speed setting, and the selection of data filtering algorithms will also affect the authenticity of the final topography data. Operators should reduce system errors by standardizing calibration processes and parameter optimization.
Applications
Optical profilers are widely used in industrial and scientific research scenarios that require accurate surface topography characterization. In semiconductor manufacturing, it is used to measure wafer etch depth, film thickness, and surface flatness after the CMP process. In the field of precision optical components, it is possible to evaluate the surface shape error and roughness of lenses and mirrors. In materials research, it can analyze the microscopic morphology and wear evolution of coatings, metal polishing surfaces, polymer materials, etc. In addition, it is also used as a routine testing tool in the fields of microelectromechanical systems, electronic packaging, and tribology experiments, providing data support for process development and quality control.
Selection considerations
Measurement requirements and technical parameters should be comprehensively considered when selecting. First, it is necessary to clarify the size range, surface roughness expectations, material reflection characteristics and measurement environmental conditions of the sample to be tested. For sub-nanometer high-precision measurements, the horizontal and vertical resolution of phase noise of the interferometer system can be paid attention to. If the sample has large steps or complex topography, it is necessary to confirm the longitudinal scanning range and step measurement ability of the instrument. The lateral resolution depends on the objective lens configuration and camera pixels, and the appropriate magnification should be selected based on the feature size. In addition, the data processing capabilities, compatibility and compliance with relevant international standards such as ISO 25178 of the instrument software are also important points of evaluation. It is recommended to verify the applicability of the instrument in specific applications through the comparison of measured samples.