In recent years, the demand for measuring surface profiles of transparent thin films deposited on patterned structures has grown in various fields such as chemical mechanical planarization (CMP) and micro opto-electro-mechanical systems (MOEMS). So far, it was difficult to measure the film surface profile accurately by using non-destructive methods like optical measurement when a thin film is deposited on a patterned sample. The available and common methods for such measurements are destructive methods like scanning electron microscope (SEM) or mechanical stylus profilometer. However, although such destructive approaches can produce accurate measurements, they are undesirable due to long measurement times and destruction of samples. Thus, novel non-destructive methods for volumetric thickness profile measurement of a transparent film, thinner than the white light coherence length of 3~4 ㎛, that is deposited on a patterned sample is described in this thesis as an attempt to solve such shortcomings.
The proposed AOTF based volumetric thickness profile measurement system consists of a visible acousto-optic tunable filter (AOTF) for wavelength scanning, a CCD sensor for 2-dimensional imaging, and a Michelson interferometer with a specially designed reference beam blocking mechanism. This thesis presents new three approaches for measuring the volumetric thickness profile as follows.
The first one is peak detection method in the spectral domain. The key idea is to divide the measurement into two states using a beam blocking mechanism to separately obtain the two unknowns of thickness and surface profile. Such separate measurements are required to compensate for the phase change effect caused by the multi-reflected beams from the thin film. The final thin film volumetric thickness profile information is measured by obtaining the number of peaks and phase deviations from the two separately scanned spectral intensity values.
The second is phase model fitting method with the capability of simultaneous volumetric thickness profile measurement. The thickness profile information is obtained through least square fitting with a phase model $Φ_m (k) = 2kh + ψ(k,d) + offset$, which has three unknowns of surface profile h, thickness d, and indeterminate initial phase offset. The accurate phase information in the spectral domain can be obtained by introducing a concept of spectral carrier frequency. Experimental results for a metal patterned sample shows that the thickness profile can be determined with an error range of around 10 nm.
The third method called phase calculation method has fast and accurate volumetric thickness profile measurement capability. This method combines the benefits of the previously mentioned two methods. In this approach, the two phase information are obtained separately with the help of beam blocking plate. The first phase function compensates for the phase change effect caused by the multi-reflected beams from the thin film and the other is a total phase function for the interference between a reference mirror plane and the film deposited patterned sample. Compensation for the phase change effect was achieved by measuring the 3 dimensional film thickness information separately prior to surface profile measurement. Then the final thin film surface profile information was measured by using the total phase function which is obtained through spectral frequency domain signal processing. This total phase calculation algorithm is based on the newly proposed spectral carrier frequency concept.