Optical Fluorescence Method For Film Thickness Measurement

INTRODUCTION

Film thickness measurement is an extremely popular application and is widely used in many fields today. Quantifying industrial coatings with small thicknesses is one of the challenges for manufacturers. However, we have developed a film thickness measurement system from fluorescence spectroscopy that can measure very thin, about 20 nm.

Fluorescence spectroscopy is an investigative method based on the fluorescence properties of the sample under study and is used for quantitative measurements of chemical products. Fluorescence is a type of luminescence caused by photons exciting a molecule, raising it to an electronic excited state. and causes them to emit light; typically, but not necessarily, visible light. That light is directed towards a filter and onto a detector for measurement and identification of the molecule or changes in the molecule.

FLUORESCENCE SPECTROSCOPY METHOD

Fluorescence spectroscopy is a non-invasive method of measuring thin film thickness. Based on the fluorescence properties of the measured samples, the significance of the film thickness will be included as a premise for this method.

Theoretically, the fluorescence signal over a wavelength range λ1 <λ <λ2 is chosen to be distinct from the background light and integrates the detected spectrum over a time τ. Therefore, for the excitation spot area A, the fluorescence signal will be given by 

With thickness z is doped with a uniform concentration D of fluorescent substance, resulting in an absorption coefficient BD at the excitation wavelength, and is exposed to an excitation beam of intensity I0, denotes the quantum efficiency of the fluorescent substance.

This means that the fluorescence response is linear with respect to film thickness.

From there, this method will use a beam of light with a certain wavelength to pass through a film that has been mixed with a specific type of fluorescent substance, from there, the incident light will excite fluorescence with a specific wavelength. The amount of fluorescent light emitted will carry information of thickness with high linearity. Therefore, this method will allow the measurement of highly localized thickness information.

That is, the change in thickness will be able to be determined instantaneously over a specific area.

Besides, it is necessary to own an absolute calibration was required in order to allow the thickness of the film to be established. It will be presented below.

CALIBRATION MEASUREMENT

Calibration data are important to this method. The more accurate the calibration set, the more reliable the results obtained.

To determine the properties of the film sample that need to be measured for the calibration data set, we first need to pay attention to the parameters that directly affect the fluorescence spectrum as presented above. They are the thickness z of the film, the concentration D of dye in the layer, the optical excitation power P (=AI0) and the spectrometer integration time τ. So, calibration collects integrated fluorescence signal SF for a known set of conditions for D, P and τ fixed for different thickness. 

And the calibration samples will also have their fluorescence signals measured using a spectrometer. The fluorescence spectrum will be considered for all signals of the emission wavelength region according to the integration time of CCD array. 

The figure below is an example of a calibration data set examining fluorescence signal at different thicknesses.

Figure 1. The fluorescence probe output signal against the maximum thickness of the calibration sample.

APPLICATION

Fluorescence spectroscopy method is applied to vast field applications including mechanical engineering, environments, paint and coating industries, automobile industries, and electronic industries…

According to our research, fluorescence is commonly used to measure liquid and wax films. Typically, measuring the thickness of very thin lubricant films on the surface of water and paint droplets. Or in the automobile industry, reducing emissions from fuel engines helps reduce engine friction losses. Our application can be applied to estimate the appropriate lubricating oil for those machine parts.

In addition, during the production of hot stamping foil. Hot stamping foil is used to print metallic text and images on packaging, this wax coating is only about 10 nm thick, it allows the carrier to be completely separated from the metallic print left on the product. Therefore, it is necessary to ensure the thickness of this wax layer and our application can be used to monitor the thickness of the wax coating to improve the quality control of the final product. Besides, this application is also used with solid films that have fluorescent properties.

recommended EQUIPMENT

INTINS provides many series of spectrometers capable of collecting fluorescence spectra such as Ocean Optics SR, Ocean Optics QEPro, Ocean Optics HDX…

Ocean HDX spectrometer uses a robust optical bench design, optimized components and precision engineering to maximize optical resolution, increase throughput, reduce stray light and maintain thermal stability for integrated, industrial and research applications.

Figure 2: Ocean Optics HDX Spectrometer

Our fluorescence spectroscopy system is user-friendly, accompanied by easy-to-use software that helps users easily observe thickness results with just simple steps.