The thickness of a deposited thin film during evaporation can be measured using mechanical methods such as stylus profilometry and interferometry. These methods rely on the presence of a groove or step between the film surface and the substrate, which is created either by masking parts of the substrate or by removing parts of the deposited film. The film thickness is measured at specific points, and the uniformity of the film is crucial for accurate measurements.
Stylus Profilometry: Stylus profilometry involves using a stylus that moves across the surface of the film. The stylus detects the vertical movement as it encounters the groove or step, which corresponds to the thickness of the film. This method is relatively straightforward and can provide detailed surface profiles, but it requires physical contact with the film, which might damage delicate surfaces.
Interferometry: Interferometry, on the other hand, uses light waves to measure the thickness. When light is reflected off the film and the substrate, interference patterns are created due to the difference in optical path lengths. These interference fringes can be analyzed to determine the thickness of the film. This method requires a highly reflective surface and is non-invasive, making it suitable for delicate films. However, it can be more complex to interpret the interference patterns compared to stylus profilometry.
Both methods are effective but have limitations based on the film's uniformity and the presence of a suitable groove or step. The choice between these methods depends on the specific requirements of the film, such as its sensitivity to physical contact and the need for non-destructive testing.
Optimization and Considerations: The accuracy of these measurements is influenced by several factors, including the purity of the deposited film, which depends on the quality of the vacuum and the purity of the source material. Higher deposition rates under a given vacuum pressure can lead to higher film purity by minimizing the inclusion of gaseous impurities. The geometry of the evaporation chamber and collisions with residual gases can affect the uniformity of the film thickness.
For thicker films, methods like thermal evaporation using evaporation boats and crucibles are preferred over wire filaments, which are limited by the size of the filament. Electron-beam evaporation allows for tight control of the evaporation rate, making it suitable for depositing complex materials or compounds.
In summary, measuring the thickness of thin films during evaporation involves careful selection of measurement techniques based on the film's properties and the requirements of the application. Both stylus profilometry and interferometry provide viable options, each with its own advantages and limitations.
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