Variation in stress with background pressure in sputtered Mo/Si multilayer films

Abstract

We have measured the stress in Mo/Si multilayer films deposited by magnetron sputtering, using the wafer-curvature technique, and find a strong dependence on background pressure. We find that for multilayers containing 40 bilayers of ∼4.3 nm Si layers and ∼2.6 nm Mo layers, the stress increases from approximately −280 MPa (compressive) to −450 MPa as the background pressure in the deposition chamber (i.e., measured just prior to deposition) decreases from 1.0×10−5 to 6.0×10−8 Torr. For multilayers of the same period but with thicker Mo layers, the dependence on background pressure is even stronger. X-ray (λ=0.154 nm) diffraction measurements reveal only a slight increase in interfacial roughness for films deposited at high background pressure, but no evidence was found for any differences in the microstructure of the polycrystalline Mo layers that comprise these structures. The peak soft x-ray (λ=13 nm) reflectance, which is sensitive to interfacial roughness at longer spatial wavelengths, also shows no correlation with background pressure or stress. Atomic concentrations of incorporated oxygen and carbon, measured with Auger electron spectroscopy, were found to be less than ∼0.5 at. % for all samples. However, the average hydrogen concentration, as determined from forward-recoil-scattering measurements made using a 2.6 MeV He beam, was found to increase linearly with background pressure. We discuss possible mechanisms for the observed dependence of film stress on background pressure, including gas incorporation and the affect of residual gas atoms on adatom mobility.