Stress evolution in molybdenum/silicon multilayer mirrors for extreme ultraviolet lithography

The continued shrinking of microelectronic device size necessitates advances in lithography, including possibly using extreme ultraviolet (EUV) light. The Mo/Si multilayer system is a promising candidate for reflective optics at a wavelength of roughly 135 A. However, these multilayers manifest high compressive stresses of approximately -350 MPa, which cause unacceptable distortion of the optical element. The goal of this project was to develop fundamental understanding of the origins of stress during growth of Mo/Si multilayers.; A 40-bilayer structure deposited by DC-magnetron sputtering yielded a peak reflectivity of 65.7% at a wavelength of 136 A. We collected the stress data during deposition by in situ substrate curvature measurements using a multiple parallel laser beam technique. We measured large tensile and compressive curvature transients during initial growth of Mo on Si and Si on Mo. However, by sputtering with Kr rather than conventional Ar, it is possible to suppress the compressive transient upon Si deposition and thereby redress the compressive stress. Evidence implies that intermixing and alloying at the Mo-Si interfaces by asymmetric Si diffusion cause the transients. Indeed, Mo/Si multilayers sputtered with Kr exhibit less intermixing and high EUV reflectivity. However, the roughness of the multilayer may limit reflectivity and we therefore compare the roughness of Kr- and Ar-sputtered multilayers.; Roughness, which leads to nonspecular scattering is problematic for EUV imaging systems because it decreases the useful throughput of a lithography system. We used x-ray diffraction to characterize the evolution of roughness with increasing number of bilayers in Mo/Si multilayers sputtered by Ar and Kr. By fitting a self-affine model of roughness to the diffuse spectra, we extracted the roughness and in-plane correlation lengths. We find that the lateral length scale of the roughness increases with the number of bilayers; however, the magnitude of the roughness, approximately 2 A or Ar- and Kr-sputtered samples, remains constant, contrary to the scaling law prediction. This discrepancy is ascribed to the smoothing effect of the amorphous interfaces which tends to flatten only the high-frequency components of roughness and preserves the conformal, layer to layer roughness replication.