ABSTRACT

Measurement system of film structure by interferometry and ellipsometry


By Young Ho Yun
Advisor : Prof. Ki-Nam Joo, Ph.D.
Department of Photonic Engineering
Graduate School of Chosun University

The importance of thin film structures has significantly increased in semiconductor and display industries according to the demand of multi-functional and highly efficient products with very tiny sizes. Moreover, complicated dimensional structures and the usages of various materials are essential for the fabrication. In the manufacturing process, the process condition should be optimized for high throughput and the measurement or inspection technologies cannot be avoided to be involved. In science fields, film structures are also indispensable to enhance the quantum efficiency of optical devices such as LEDs and OLEDs, develop new functional components and integrate optical or electrical circuits.
Film structures have been analyzed mainly by two principles; reflectometry and ellipsometry except for the destructive observation technologies such as SEM and TEM. By analyzing spectral density of reflected light from a specimen, reflectometry can obtain film thicknesses on the assumption of known refractive indices of films. Ellipsometry has become a standard tool, where the ration of reflection coefficients can be extracted and compared with the theoretical one, and it is more reliable to measure thin film thicknesses compared to reflectometry because of high sensitivity of the measurement parameters to thin films. However, these two methods have the limitation not to focus on surface profiling and they only provide the information of film layers. When the substrate has specific surface shapes or textures, this important features cannot be characterized. In order to overcome this limitation, several researchers have approached to combine surface profiling techniques with reflectometry. By the similarity of the optical configuration between reflectometry and interferometry, the combined method can obtain surface and film thickness profiles in a single configuration at once. However, reflectometry is not adequate for the analysis of thin film structures due to low sensitivity, and therefore relatively thick film structures with each film thickness around an order of a few hundred nanometers have been analyzed. Another attempt to measure surface and film thickness profiles has been investigated in low coherence interferometry by temporal scanning and spectral resolving techniques. The nonlinear spectral phase containing the information of film layers in low coherence interferometry can be extracted from the measured phase by the Fourier analysis and the optimization, but it is not enough to measure thin film thicknesses under approximately 100 nm because of inherent measurement and calculation errors.
In this thesis, I propose a novel measurement system for multi-layered film structures named as MYFIELM (Measurement sYstem of Film structure by Interferometry and ELlipsoMetry). MYFIELM has the capability of simultaneously measuring the surface and thickness line profiles of thin film structures based on the theoretical models of the interferometric spectral phase and ellipsometric film analysis.
MYFIELM consists of two operating modes, i.e. interferometry and ellipsometry modes according to the existence of a reference beam by a beam shutter. In interferometry mode, the broadband light from the optical source is split into two, a reference and a measurement beams, by a beam splitter and they are interfered after the reflection from a specimen and mirrors. Unlike the typical reflective interferometric configuration, the specimen is located in the middle of the measurement path and the incident angle of measurement beam is not orthogonal to the specimen because of the consideration of ellipsometry mode. In this case, polarizing optical components don’t have any roles to generate the interference. Then, the recombined beam goes toward an imaging spectrometer and becomes spectrally resolved in order to obtain the spectral interferogram by an imaging device. It is noted that the measured image contains two kinds of information such as spatial and spectral density because MYFIELM measures a line profile of a specimen at once. On the other hand, only the measurement beam is used in ellipsometry mode and the spectroscopic ellipsometric principle of P45CRSA-45 type is adopted to analyze the film structure of the specimen.A quarter-wave plate as the compensator is rotated to change the polarization state of the incident 45° linearly polarized light, and an image stack of the spectral and spatial intensity variations corresponding to polarization changes are recorded by the imaging device after passing through a -45°rotated analyzer and the imaging spectrometer. In this case, the measurement beam is reflected off on the specimen twice and the theoretical model of ρ should be modified from the typical model of ellipsometry.
The film thicknesses measured by ellipsometry were transferred to interferometry and only the surface height was extracted from the spectral phase of the interferometry containing both of surface height and film thicknesses. For the verification, each performance of the operating mode was confirmed and 4 film layered specimen was measured in comparison of the reference values. MYFIELM is the most suitable solution for characterizing thin film structures with a textured substrate without any damage on the specimen compared with the previous techniques even though it needs further improvements for stability and reliability.