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Fig. 1 Smal spot measurment with large numerical aperture (NA) 

 Fig.2 The effect of large NA on reflectance spectrum for 200nm oxide measurement. Lines represent: 0, +-30deg(NA=0.5), +-50deg(NA=0.76),+-70 deg(NA=94) and +-90 deg(NA=1) 

Fig.3 The effect of large NA on reflectance spectrum for 2000nm oxide measurement. Lines represent: 0, +-30deg(NA=0.5), +-50deg(NA=0.76),+-70 deg(NA=94) and +-90 deg(NA=1) 

Thickness measurement with microscope             

            Small spot film thickness  measurement is, typically, done using microscope objectives with low Numerical Apertures (NA). Most standard microscopes are using high -NA objectives i.e. a larger cone angle of light is striking the sample. High-NA objectives have many advantages: higher resolution, better light collection, etc.  But they have some significant downsides when the goal is to measure film thickness.

High-NA objectives problem

              The most serious problem is reduced contrast of interferometric signal that is used to determine film thickness. In a high-NA objective, light rays are refracted in the film at different angles (see Fig. 1), as a result, they have paths of different length in the film material. This means that they have different phase differences. Once different light rays are combined and phases are superimposed on the detector – the contrast between constructive and distructive interference  peaks/valleys is diminished. The severity of this effect depends on the specific filmstack and NA. But, in general, effect increases with the thickness.

How NA affects the thickness measurement

               The effect  is easy to see on an example of Si oxide measurement. Simulation of the UVVis reflectance spectrum (200-1000nm) for 200nm oxide is shown on Fig.2 It shows gradual degradation of the spectrum with the increase of the NA. But the changes are small and can be easily corrected. However, the same simulation for 2000nm oxide (Fig. 3 ) shows a more significant effect of the NA.

What is the solution ? 

               TFCompanion software has an option to correct for high NA effects (Fig. 4). However, if the interference signal is already weakened due to other artifacts (scattering, thickness non-uniformity, etc.) – the NA effect can make interence fringes practically disappear and, in this case, software correction cannot help. Use of objectives needs to be evaluated based on application requirements. If possible, it is always better to use low NA objectives. Low NA (ideally NA< 0.5) has a very small effect on measured spectra, practically, in all applications. 

Fig. 4 Correcting NA effects in TFCompanion.   TFCompanion has an option to set NA of the measurement in Advance Tab (main screen) – this will include NA correction in the calculations during the curve fitting (for FFT correction is normally not needed)

MProbe 40 MSP system uses long-working distance objectives with NA<0.55. This minimizes degradation of the measured signal for, practically, all filmstacks. In addition, long working distance makes it more comfortable to navigate to measurement location, even without switching to a low magnification objective. 

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