By illuminating samples with white light, interference spectrums are created as a function of the geometric layer thickness and refractive index of the materials . When white light is incident on optically transparent layers, interference occurs, as the path difference between specific wavelengths is exactly a multiple of the optical layer thickness. The maximum measurable thickness is linked to the spectral resolving power, the minimum thickness to the spectral range to be covered. The measurement of even thinner layers requires that the absolute intensity value is known. A high absolute accuracy of the wavelength ensures an exact measurement result.
Depending on layer condition, the thickness can be calculated using two methods:
Peak method:
The layer thickness is derived from the maxima and minima of the interference spectrum. This method is very accurate and fast, however, noise-sensitive. It is suitable for single layers < 5 µm.
Fast-Fourier-Transformation (FFT) method:
The layer thickness is calculated from the periodicity of the interference spectrum. This method is insensitive to noise and suitable for thick layers, however, requires a large computational effort and is less accurate. It is suitable for single and multi-layer systems from 1-200 µm.
Benefits
- Contact-free and non-destructive
- Highly accurate results
- Suitable for short- and long-term repeatability
- Quick reset of coating parameters, resulting in low cost concerning quality and material consumption
Applications
- Optical Emission Spectroscopy
- Reflectometry
- Ellipsometry
- Process Control
- Examples:
- Wafer Inspection
- Displays
- Photoresists / Dielectric Layers
- Plasma Monitoring
- Wafer Inspection