In most cases of optical critical dimension (OCD) metrology, when applying rigorous coupled-wave analysis (RCWA) to
optical modeling, a high order of Fourier harmonics is usually set up to guarantee the convergence of the final results.
However, the total number of floating point operations grows dramatically as the truncation order increases. Therefore, it
is critical to choose an appropriate order to obtain high computational efficiency without losing much accuracy in the
meantime. In this paper, the convergence order associated with the structural and optical parameters has been estimated
through simulation. The results indicate that the convergence order is linear with the period of the sample when fixing
the other parameters, both for planar diffraction and conical diffraction. The illuminated wavelength also affects the
convergence of a final result. With further investigations concentrated on the ratio of illuminated wavelength to period, it
is discovered that the convergence order decreases with the growth of the ratio, and when the ratio is fixed, convergence
order jumps slightly, especially in a specific range of wavelength. This characteristic could be applied to estimate the
optimum convergence order of given samples to obtain high computational efficiency.