Delamination of LPD Tungsten Films by Residual Stress
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DELAMINATION OF LPD TUNGSTEN FILMS BY RESIDUAL STRESS A. BORER*, B. DERBY* AND I. MAY** *Department of Metallurgy and Science of Materials, Oxford University, Parks Road, Oxford OXi 3PH, U.K. **GEC Research Limited, Hirst Research Centre, East Lane, Wembley, Middlesex HA9 7PP, U.K. ABSTRACT Thin tungsten films have been deposited on sapphire and glass substrates using laser photolytic deposition from a tungsten hexa-fluoride precursor. Failure of the films is caused by the presence of large residual stresses (1-5 GPa). Compressive and tensile stresses are found at different stages of film growth and either may be responsible for delamination. Early compression is caused by oxidation and impurity influenced intrinsic stress. Tensile stress occurs with microstructural changes but is not maintained with film growth. Large stress gradients, indicative of up to 0.75-1.2% lattice strain difference across the film, are found through both compressive and tensile films and are not explained. Introduction Tungsten combines the advantages of a refractory film with low electrical resistivity and high resistance to electromigration. Consequently its use is of great interest in very large scale integration processing. Laser photolytic deposition can be used for direct writing of very fine features at lower temperatures than conventional chemical vapour deposition. In theory this should enable minimisation of thermal stresses and reduce the risk of process induced damage. Tungsten films have been deposited from a tungsten hexafluoride precursor onto sapphire substrates up to a depth of 2000nm. Deposition temperatures ranged between 80 and 450eC. The adhesive failure of these films is often found and has been attributed to the presence of large residual stresses in the films. The mode of failure is neither uniform across a film nor consistent between films. Initially thin films are observed to suffer from a compressive stress. As the film grows a tensile stress is induced which is in turn replaced by compressive stress in the thickest films. Failure of Thin Tungsten Films A compressive stress is apparent in the thinnest films through surface buckling. This occurs in filmd 20 to 4Onm deep (Dektak profilometer measurements) and may be either disc or ridge bucking (Figs 1,2). The biaxial compressive stress residual in the disc buckled film is calculated by measuring the expansion in radius of disc from the height of the buckle. Using elementary elasticity theory a
6a.E (1-v)a
(1)
where a is the radius of the film prior to buckling, 6a is the expansion in radius of the buckled film, E the film elastic modulus and v the film Poisson ratio. The magnitude of the compressive stress present to cause buckling appears to be thickness related (as seen in fig.l). Stress values of =o = 3-5GPa are found for most of this film. However in the Mat. Res. Soc. Symp. Proc. Vol. 130. 41989 Materials Research Society
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Fig.l Disc buckling of 20-30Om amorphous film (optical).
Fig.2 Ridge buckling of 40-5Onm amorphous film (optical).
thicker are
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