New Ellipsometric Approach to Critical Dimension Metrology Utilizing Form Birefringence Inherent in a Submicron Line-and-Space Pattern

A new ellipsometric approach to critical dimension (CD) metrology is proposed, which utilizes the form birefringence of the submicron line-and-space (L&S) patterns of a photoresist. With the continued requirement of further narrowing of the period of the L&S to dimensions less than the probing wavelengths, higher-order diffraction will no longer occur and the 0th-order diffraction beam will be the only tool for CD metrology. Since ellipsometry uses this 0th-order diffraction beam, the method is promising for use in the evaluation of L&S patterns in the subwavelength region. We have been able to relate the large variation in the ellipsometric parameter to the form birefringence of the submicron pattern. Using the higher-order effective medium equations for the ordinary and extraordinary refractive indices, the ellipsometric parameter changes due to the linewidth changes are calculated. The calculated results agree well with the experimental results. Furthermore, the linewidths are determined from the best fit between the calculated and experimental data. The same is done for the latent images of the undeveloped resist patterns.