In the fields of semiconductor, precision optics, and display panel manufacturing, film thickness control is the core link that determines product yield. With the continuous advancement of process technology towards the micro nano scale, traditional contact measurement methods are no longer able to meet the high-precision and non-destructive detection requirements. The breakthrough in the performance of confocal profilometer provides a reliable solution to this problem. Based on the principle of confocal optics, measurement technology can accurately capture the reflected signals on the surface of the sample, enabling nanoscale measurement of the thickness of transparent films, metal coatings, and multilayer structures. This article will delve into the technical principles, key performance indicators, and practical application performance of confocal profilometers in film thickness measurement.
1、 Principle of film thickness measurement for confocal profilometer
The confocal profilometer uses point light source illumination and pinhole filtering technology to image only at the focal plane, effectively filtering out stray light outside the focal plane. This characteristic is particularly critical in the context of film thickness measurement. When the laser beam passes through the transparent film, reflection signals will be generated on the upper surface and lower interface of the film, respectively. Through precise Z-scan, the system can capture the positions of two reflection peaks, and the distance between them is the physical thickness of the thin film.
Compared to traditional optical thickness measurement methods, the advantage of confocal technology lies in its excellent tomographic ability. Confocal profilometers typically use high numerical aperture objectives and short wavelength light sources, with axial resolution up to the nanometer level. This means that even ultra-thin films with a thickness of only tens of nanometers can be clearly distinguished.Performance of confocal profilometerAt this stage, the reliability of the measurement data is directly determined - only when the axial resolution is high enough, the reflection peaks of the upper and lower interfaces will not overlap, and the thickness calculation can be accurate and error free.
2、 Core indicators determining the accuracy of film thickness measurement
To evaluate the performance of a confocal profilometer in film thickness measurement, the following key performance parameters need to be considered:
Axial resolution
Axial resolution is one of the core indicators of confocal profilometers, which determines the minimum height difference that the system can resolve. For film thickness measurement, axial resolution directly corresponds to the lower limit of the minimum measurable thickness. High resolution confocal microscope combined with piezoelectric ceramic scanner can achieve nanometer or even sub nanometer step accuracy in the Z direction, and can stably measure thin films on the order of 10nm.
Horizontal resolution and spot size
In addition to the thickness value itself, the uniformity of the film layer and the smoothness of the interface are also important evaluation parameters. The spot size of the confocal profilometer is usually in the micrometer range, and when combined with a high-precision scanning platform, it can obtain two-dimensional and three-dimensional morphology information of the film surface and interface. Through these data, not only can the average thickness be calculated, but the spatial distribution uniformity of thickness can also be evaluated.
Maximum measurable inclination angle
For samples with curved or irregular structures, the maximum measurable inclination angle of the confocal profilometer determines whether the measurement can be completed smoothly. Thanks to the pinhole filtering design of confocal technology, this type of instrument can measure steep surfaces with slopes of up to 70 ° or more. This means that even if the thin film is deposited on the micro lens array or V-shaped groove structure, the performance of the confocal profilometer can still be stably utilized to achieve accurate measurements.
Repeatability and measurement uncertainty
In actual production testing, measurement repeatability is often more important than single absolute accuracy. Excellent confocal profilometers should measure the same position repeatedly under the same conditions, and the standard deviation of thickness data should be controlled at the nanometer level. According to relevant research reports, the measurement uncertainty of the thin film measurement system based on confocal spectroscopy principle can be controlled within 0.12 μ m, which can meet the quality control requirements of the vast majority of high-end manufacturing scenarios.
3、 Measurement capability of complex membrane structure
Transparent film measurement
Transparent film is one of the best measurement objects for confocal profilometers. When a light beam passes through a transparent medium, it will reflect at different refractive index interfaces. The confocal system, with its excellent optical tomography capability, can clearly capture the position signals of various reflective interfaces. For single-layer transparent films, the measured thickness can be as low as several hundred nanometers; Combined with short wavelength light sources and high numerical aperture objective lenses, the thickness measurement limit of some high-end equipment can be further extended to the 10nm level.
Multilayer thin film structure
Multilayer film structures are very common in the fields of semiconductor and optical coating. The confocal profilometer can obtain the reflection signal of each interface by scanning layer by layer, thereby calculating the thickness of each film layer. It should be noted that the more layers and the smaller the difference in refractive index between each layer, the higher the requirements for the performance of the confocal profilometer. Excellent signal processing algorithms and high dynamic range detectors are key to ensuring successful measurement of multilayer films.
High reflectivity and low contrast surfaces
For metal coatings or highly reflective surfaces, traditional optical measurement methods often face problems of signal saturation or insufficient signal-to-noise ratio. Confocal technology, through pinhole filtering and signal gain adjustment, can effectively suppress stray light interference and ensure stable measurement results in high reflective areas.
4、 Data reference in practical applications
According to existing literature reports and industry application cases, confocal technology has accumulated a large amount of validation data in the field of film thickness measurement:
Thickness measurement range: Mainstream equipment can cover cross scale measurement needs from tens of nanometers to hundreds of micrometers
Repeatability index: Under optimized conditions, the standard deviation of thickness data from 10 repeated measurements can reach within 0.6 μ m
Measurement uncertainty: The comprehensive uncertainty of customized measurement systems can be controlled at around 0.12 μ m
These data indicate that the performance of confocal profilometer can meet the stringent requirements of high-end industrial fields such as semiconductor wafer manufacturing and optical thin film coating.
5、 How to choose a suitable confocal profilometer
In actual procurement and application, when evaluating the performance of confocal profilometers, the following points should be focused on:
Clarify your own measurement requirements: What is the minimum thickness that needs to be measured? Is the sample a transparent film or a metal film? Is there a curved surface or aspect ratio structure?
Pay attention to the axial resolution index: This is the core parameter that determines the lower limit of film thickness measurement and needs to be evaluated in conjunction with the measured sample material.
Assessing the environmental adaptability of the system: In the production workshop environment, factors such as temperature drift and vibration have a significant impact on measurement repeatability, and stable and reliable optical and mechanical design equipment should be selected.
Emphasize software analysis functionality: Film thickness calculation involves processes such as reflection peak recognition and fitting algorithms. Software with complete functionality can significantly improve data processing efficiency and accuracy.
The continuous improvement of the performance of confocal profilometer provides strong technical support for precision thin film measurement. From the principle of confocal optics to high-precision scanning actuators, and then to intelligent data analysis algorithms, the progress of each link is driving the evolution of film thickness measurement towards thinner, more accurate, and faster directions. Whether it is gate oxide layer detection in semiconductor manufacturing or film thickness monitoring in optical coating processes, confocal profilometer has become an indispensable measuring tool. When selecting equipment, it is recommended that users evaluate various performance indicators comprehensively based on the actual sample characteristics and process requirements, in order to find the most suitable solution for their own application scenarios.
About Us:
Since its establishment, Younikon Technology Co., Ltd. (Yiying Technology) has been focusing on precision thin film measurement and laboratory environment optimization. It is the general agent of Filmetrics film thickness gauge in China. We are committed to providing professional products and attentive services to safeguard your precision instruments.
