Yong-Ju Zhang

THICKNESS DEPENDENCE OF BUCKLING PATTERNS OF Ta FILMS SPUTTERED ON GLASS SUBSTRATES

Tantalum (Ta) films deposited on glass substrates have been prepared by a direct current magnetron sputtering method, and buckling patterns induced by residual compressive stress are investigated in detail. When the film thickness increases, the buckling morphologies evolve from straight-sided buckle network to wavy or wormlike wrinkles gradually, and finally change into telephone cord buckles. The geometrical parameters of the buckling patterns are found to increase linearly with the film thickness. Based on the geometrical parameters of the buckling patterns, the mechanical properties of the Ta films are also discussed in the frame of continuum elastic theory.

COMPARISON OF STRESS RELIEF MECHANISMS OF METAL FILMS DEPOSITED ON LIQUID SUBSTRATES BY THERMAL EVAPORATING AND SPUTTERING

Various metal film systems, deposited on liquid (silicone oil) substrates by thermal evaporating and DC-magnetron sputtering methods, have been successfully fabricated and the stress relief mechanisms are systematically studied by analyzing the characteristic surface morphologies. The experiment shows that the evaporating metal films can move on silicone oil surfaces freely due to the nearly zero adhesion of solid–liquid interface, which results in spontaneous formation of ordered surface patterns with a characteristic sandwiched structure driven by the internal stress. For the sputtering metal film system, however, the top surface of silicone oil can be modified to form an elastomeric polymer layer on the liquid substrate during deposition. Subsequent cooling of the system creates a higher compressive stress in the film, which is relieved by buckling of the film to form periodic wavy structures because the adhesion of solid–elastomer interface is quite strong.

Mechanical Behaviors Of Quenched Iron Film Sputtering Deposited On Glass Substrate

A characteristic wedge-shaped iron (Fe) film system, deposited on glass substrate by a DC-magnetron sputtering method and quenched by silicone oil during deposition, has been successfully fabricated. The mechanical behaviors of the quenched and non-quenched Fe films have been compared and analyzed. The internal stress is found to transform from tension to compression in nature after quenching, which results in the formation of buckling pattern in the quenched Fe film while crack pattern in the non-quenched film. It is proposed that the origin of the compressive stress is mainly due to freezing of the Fe atoms (or clusters) with high kinetic energy and doping of the oil molecules into the Fe film defects.

Impurity induced wrinkling patterns in metal films deposited on soft elastic substrates

Metal (iron and nickel) films have been deposited on soft elastic polydimethylsiloxane (PDMS) substrates by direct current sputtering technique and the impurity induced wrinkling patterns are investigated by using optical microscopy and atomic force microscopy. It is found that the metal films can spontaneously form disordered wrinkles due to the isotropic compressive stress. In the vicinity of film impurities such as extraneous particles, linear defects, cracks and thickness-gradient film edges, the stress field becomes anisotropic owing to symmetry breaking and thus complex wrinkling patterns including straight stripes, herringbones, crossings, labyrinths and their transitions can be observed. The morphological evolutions, structural characteristics and physical mechanisms of the impurity induced wrinkles have been discussed and analyzed based on the continuum elastic theory.

THE INFLUENCE OF EXTERNAL DISTURBANCE ON BUCKLING PATTERNS IN WEDGE-SHAPED Fe FILMS

A wedge-shaped iron (Fe) film system, quenched by silicone oil during deposition, has been prepared on glass substrates by direct current magnetron sputtering, and the influence of external disturbance on buckling patterns is investigated. The experiment shows that the wedge-shaped Fe film possesses a high compressive stress, which is relieved by spontaneous formation of telephone cord buckles in the atmosphere condition. The growth process can last several days to more than a month. When the film is disturbed by an external force, the stored stress energy will release drastically to form straight-sided buckles within several seconds. The morphological characteristics, edge effect and formation mechanism of the straight-sided buckles are discussed in detail.

In situ electric properties of Ag films deposited on rough substrates

Silver (Ag) films have been deposited on rough substrates (including frosted glass and silicone grease), and for comparison on flat glass, by DC-magnetron sputtering, and their sheet resistances measured in situ during deposition. It is found that the growth of Ag films proceeds through three distinct stages: discontinuous, semi-continuous, and continuous regimes. The sheet resistance on rough substrates jumps in the vicinity of the percolation threshold, whereas the resistance on flat substrates decreases monotonically during deposition. The abnormal in situ electric properties on rough substrates are well explained based on the differences of the growth mechanism and microstructure of Ag films on different substrates.

FRACTURE AND WRINKLE PATTERNS IN METAL FILMS DEPOSITED ON LIQUID MENISCUSES

Liquid meniscuses were prepared by spraying silicone oil drops with different sizes onto clean glass slides. Metal (iron) films with varied thicknesses were then deposited on the liquid meniscuses by direct current magnetron sputtering. The fracture and wrinkle behaviors of the films resulting from residual stresses are investigated in detail. It is found that cracks nucleate and propagate in the film during deposition owing to the thermal expansion of the liquid substrate. Subsequent cooling of the system creates a high compressive stress, resulting in the formation of various wrinkles in the film. The initiation and shape of the cracks are closely related to the film thickness and oil drop size. The wrinkle morphologies are dependent on the stress anisotropy induced by the liquid meniscus and crack edge.