Restrictions

C method domain of applicability is mainly defined by the ratio of grating depth to its period.
Chandezon method for crossed grating gives reliable results when the full grating depth is less than maximum of X or Y periods. If the grating depth is close to the critical value there are an optimal number of field decomposition orders providing the best result. In practice it was found that the optimal number of decomposition orders for calculating deep gratings does not exceed 30 for both X and Y axes. Most likely this restriction is similar to the Rayleigh method convergence problem¹, and finally because of insufficient calculation accuracy of high order harmonics.

The implemented method has a peculiarity under incident angles close to the normal (see Ref. 5). Around the incident angles less than 0.01 degrees a special processing is implemented. Nevertheless this domain of incident angles needs a special care. The increasing number of decomposition orders usually improves the convergence in this situation.

Calculation of grating structures with long periods needs large number of decomposition orders and has the restriction mentioned above.

The balance (sum of all diffraction orders power) is a good criterion of results reliability. In case of lossless structures this value should be close to unity and less than unity for dissipative structures.