Deposition processes control the properties of thin films; they can also introduce high residual stresses, which can be relieved by delamination and fracture. Tungsten films with high 1–2 GPa compressive residual stresses were sputter deposited on top of thin (below 100 nm) copper and diamond-like carbon (DLC) films. Highly stressed films store large amounts of strain energy. When the strain energy release rate exceeds the films' interfacial toughness, delamination occurs. Compressive residual stresses cause film buckling and debonding, forming open channels. Profiles of the buckling delaminations were used to calculate the films' interfacial toughness and then were compared to the adhesion results obtained from the superlayer indentation test. Tests were conducted in both dry and wet environments and a significant drop in film adhesion, up to 100 times was noticed due to the presence of moisture at the film/substrate interface.

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Stress and Moisture Effects on Thin Film Buckling Delamination

Thin film delamination can occur when the stored elastic energy per unit area in the film due to the residual stress exceeds the interfacial toughness. Telephone cord morphology is commonly observed in delaminating thin films under compressive stresses. Here, the biaxial film stress is partially relieved by film buckling in the direction perpendicular to the telephone cord propagation, and by “secondary” blister buckling in the direction of telephone cord propagation, which results in the sinusoidal fracture patterns. A superlayer indentation test, in which additional stress is supplied to the crack tip using a nanoindenter, can be used to measure the interfacial toughness. Estimates of the energy release rate for diamond-like carbon (DLC) films on magnetic media were obtained using the superlayer indentation test, as well as the delaminated buckling profiles. The results obtained by these two independent methods are in good agreement with each other. We find the average adhesion energy to be 6 J/m2 for DLC films on magnetic media. Normally telephone cord blisters “run out of steam” and stop once the interfacial toughness exceeds the strain energy release rate. It is possible to make blisters propagate further by either putting mechanical energy into the system, or by introducing liquids at the crack tip, thus reducing the film interfacial toughness. Environmental species can assist cracking and contribute to thin film delamination, which is readily observed in vintage mirrors. Crack propagation rates on the order of microns per minute were measured for DLC films in different fluid environments. We identify how telephone cord buckling delamination can be used as a test vehicle for studying crack propagation rates and environmentally assisted cracking in thin films.

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Sub-critical telephone cord delamination propagation and adhesion measurements

This chapter describes the method of manufacturing microfluidic microchannels formed by delaminated buckled thin films. Thin films under compression tend to delaminate and buckle. Microchannel geometry can be controlled by tailoring film residual stress and placing patterned adhesion-weakening layers utilizing photolithographic techniques. Results based on the photoresist as the adhesion weakening layer and compressed tungsten thin films are described along with the corresponding thin film mechanics.

Delaminated Film Buckling Microchannels

The effects of moisture on copper thin film adhesion have been investigated using a modified version of the superlayer indentation test. Copper films up to 100 nm thick were sputter deposited on thermally oxidized silicon wafers. A 1 μm tungsten superlayer with 1 GPa compressive residual stress was sputter deposited on top of Cu films, providing extra energy for interfacial debonding upon indentation. The samples were first indented to a depth just greater than the thickness of the W/Cu bi-layer and then unloaded. Water was introduced to the indent, and a second higher load indent was made in the exact same location as the first one to drive crack propagation in the wet environment. For the indents made to the same maximum load, there was a dramatic increase in blister size in the wet environment compared to the blister size in the dry environment. Copper film adhesion in the presence of water was up to 20 times lower than in the dry environment (0.1 J/m2 vs. 2 J/m2 ). Chemical reactions at the crack tip along with the surface energy minimization by water are thought to be the causes for the adhesion reduction.Copyright © 2005 by ASME

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Moisture Effects on Copper Thin Film Adhesion

Argon pressure significantly affects the residual stress in sputter deposited thin films and coatings. In case of W thin films, high residual stresses on the order of 1-2 GPa are quite common. With the rest of sputtering parameters being equal, argon pressure determines the sign and the value of residual stress. When the amount of stored elastic energy in the film due to the residual stress exceeds the interfacial toughness, fracture normally occurs. Telephone cord buckling delamination blisters are commonly observed in compressed thin films. These mechanically active features form by a loss of adhesion between the film and the substrate due to residual stress relief, and exhibit directional growth under certain conditions. This paper considers telephone cord delamination channels for micro-fluidics applications, as this could to be a valuable, reliable, and inexpensive method of forming open channels.

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Gregory Wright

Papers

Micro-fluidic applications of telephone cord delamination blisters