CHEMICAL VAPOR DEPOSITION OF THIN FILM MATERIALS FOR ...

samples. The IR absorption behavior is related to the various mechanical vibrational modes in
molecules. A classical vibrational frequency formula for a diatomic molecule containing two
atoms with mass m1 and m2 is illustrated as Eqa.(1.7). In this equation, ν and F represent the
vibrational frequency and the force constant respectively. The number of vibration modes in a
ν 1
2
1
F +
m1 1
m2 molecular system with N atoms is determined by the number of internal degree of freedom,
which means for linear molecules the number of vibration modes is 3N-5 and for nonlinear
molecules the number is 3N-6.[19] Among these vibration modes, usually some of them might
be not IR active and this is determined by the IR selection rule. This rule is that the electric
dipole moment of the molecule must change during vibration in order for a particular vibration
mode to be detected by IR absorption. For example, the 1388 cm -1 symmetric stretch mode for
carbon dioxide gas cannot be detected by IR spectroscopy, while the 2349 cm -1 asymmetric
stretch mode is IR active. A special phenomenon for IR spectroscopy worth to be remembered is
that if a vibration mode of frequency ν is detected then its overtones 2ν, 3ν might also be
detected by IR absorption.[20]
19
(1.7)
ATR-FTIR is the conjunction of ATR sampling technique into the traditional FTIR
instrument. By doing so, certain samples can be measured more conveniently without complex
sample preparation and moreover it enables the in situ monitoring of certain thin film growth
behavior. For example, the precursor adsorption, reaction with substrate surface and the initial
thin film nucleation can be detected instantaneously without affecting the processing conditions.
ATR technique is based on the principle of total reflection, which happens when the light wave
transmitted from a high refractive index medium into a low refractive index medium and the