Why SiO2 Films Show Color?

In semiconductor manufacturing processes, it is often necessary to grow a thin film of silicon dioxide (SiO2) on the surface of a silicon (Si) substrate. PAM-XIAMEN can offer SiO2 thin film, more specifications please read: https://www.powerwaywafer.com/silicon-wafer/300-mm-silicon-wafers-300mm-tox-si-thermal-oxidation-wafer.html . This type of film can be prepared by various methods such as thermal oxidation and chemical vapor deposition (CVD), and has excellent electrical insulation, chemical stability, and mechanical properties. Silicon oxide materials themselves are highly transparent in the visible light spectrum and have extremely low absorption rates. The color observed on the film is not the color of the material itself, but originates from the physical optical phenomenon of film interference.

1. Color Change Principle of SiO2 Film: Film Interference

When light shines on the silicon oxide film deposited on the substrate, a part of the light is reflected on the upper surface of the film (air—SiO2 interface), while another part of the light is refracted into the interior of the SiO2 film, reflected when it propagates to the lower surface (SiO2—substrate interface), and then refracted back into the air. When these two beams of light meet in space, interference effects occur due to the optical path difference, resulting in the following interference results:

1) When the optical path difference is an integer multiple of the wavelength of light, constructive interference occurs, and the intensity of light at that wavelength is enhanced;

2) When the optical path difference is an odd multiple of half a wavelength, destructive interference occurs and the intensity of light at that wavelength decreases;

Due to the fact that white light contains various visible wavelengths, light of specific wavelengths can be enhanced or weakened by interference, ultimately forming the color we observe.

2. Key Factors Affecting Interference Color

1) Film thickness: This is the most significant influencing factor. The thickness of SiO2 directly determines the path length of light propagation inside the film, thereby determining the optical path difference. Even small changes in thickness at the nanometer level can significantly alter the interference conditions, leading to the enhancement or attenuation of light of different wavelengths and resulting in different colors. As the thickness increases, the level of interference increases, and the color will exhibit periodic changes (for example, from yellow to purple, then to blue, green, and then cycling again, but the saturation may gradually decrease). According to the principle of thin film interference, SiO2 thin films of different thicknesses will exhibit specific colors, and Fig. 1 shows typical corresponding relationships. It should be noted that due to the multi-level nature of interference phenomena, the same color may correspond to multiple different thickness values, and the color will change with the observation angle. Therefore, color can only be used as a reference for thickness estimation, and accurate measurement still requires the use of professional equipment such as ellipsometers.

Fig. 1 Colors corresponding to SiO2 films of different thicknesses

2) Incident angle of light: Changing the incident angle can affect the optical path difference, resulting in color changes of the same sample at different observation angles (rainbow effect).

3) The refractive index: The refractive index of SiO2 (about 1.46) affects the propagation speed and wavelength of light in thin films; The refractive index of substrate material affects the reflection behavior of light at the SiO2—Substrate interface.

4) Spectrum of light source: The colors observed under different light sources may vary slightly.

Whether you need Silicon wafers for research or for industrial applications, please contact us email at [email protected] and [email protected].