Adhesion

Adhesion is a fundamental requirement of almost all film systems. It is determined by the nature of the stresses that appear at the interface and the energy needed to propagate a fracture and/or cause deformation. Film adhesion is intimately connected with the film and substrate properties as well as the properties of the interfacial (interphase) materials (Ref 26). Good adhesion is promoted by high fracture toughness of the interface and the near-surface material, presence of fracture blunting and deflecting features, low stresses and stress gradients, absence of fracture-initiating features, and an absence of operational adhesion-degradation mechanisms.

Poor adhesion can be attributable to low degree of chemical bonding, poor interfacial contact, low fracture toughness (brittle materials, flaws), high residual film stresses, fracture-initiating features, and/or operational adhesion-degradation mechanisms. Poor adhesion may be localized so as to give local failure (i.e., pinholes). In many systems where direct adhesion is difficult to attain, an intermediate material is introduced onto the substrate surface to bond to both the substrate and the film material. Substrate surface roughness can improve or degrade the adhesion, depending on the ability of the deposition technique to fill in the surface roughness and the film morphology generated. The generation of a good interface is also important to other properties such as thermal transport and electrical contact resistance. The lack or loss of adhesion is often called deadhesion.

Modeling of Adhesion. The principal models used for explaining adhesion are:

• Surface energy reduction, wettability, and spreading--commonly used with polymer bonding

• Interfacial fracture and deformation--used in inorganic systems (Ref 26)

In the latter case the fracture toughness or fracture energy is the relevant physical parameter. This depends on the stress at the interface and the properties of the interface, the "interphase material," and the nearby material of the film and substrate. The failure modes for ductile materials will be quite different from those of brittle materials.

Causes of Deadhesion. The stresses that appear at the interface and can cause adhesion failure (deadhesion) include:

• Mechanical--tensile, shear, compressive, shock, fatigue

• Chemical and electrochemical—corrosion, solution

• Thermal/time--diffusion, reaction

These stresses can cause loss of adhesion at or near the interface by:

• Fracture and deformation at the interface--mechanical

• Generation or propagation of flaws in the interface region--mechanical, static fatigue

• Corrosion at the interface--chemical

• Dissolution of interfacial material--chemical

• Diffusion of material away from the interface—thermal (Ref 21)

• Diffusion of species to the interface—thermal

• Phase change of material at the interface giving flaws and stress—thermal, diffusion

These stresses can originate externally to the film from the environment, subsequent processing, storage, or use, or they can be internal to the film-substrate structure. In addition, residual ionic species in the film can cause interfacial corrosion if exposed to humidity, or dissolved mobile species, such as gases, can migrate to the interface, causing deadhesion.

Deadhesion due to Fracture. The loss of adhesion under mechanical stress (tensile, compressive, shear) occurs by deformation and fracture of material at or near the interface. The fracture mode (brittle or ductile) depends on the properties of the materials. The fracture path depends on the applied tensor stress, the presence of flaws, the interface configuration, "easy fracture paths," and the properties of the materials involved.

The fracture toughness (Kc) of a material is a measure of the energy necessary for fracture propagation and is thus an important adhesion parameter. In fracture, energy is adsorbed in the material and at the propagating crack tip by elastic deformation, plastic deformation, generation of defects, phase changes, and the generation of new surfaces. If this fracture occurs at an interface or in the nearby material, then loss of adhesion (deadhesion) occurs. Fracture mechanics approaches to measuring, describing, modeling, and/or predicting thin film (or any interface) adhesion are few. Some work has been published on the fracture of thick film and thin film systems. Thouless (Ref 68) has described the problem of critical and subcritical crack growth in thin film systems. Very little has been done to elucidate the effects of environment (subcritical crack growth) and film properties (Ref 69) on fracture and adhesion of thin film systems.

The fracture toughness of a material depends on the material composition, the microstructure, the flaw concentration, and the nature of the applied stresses. If an interphase material has been formed in the interfacial region it will be involved in the fracture process. Such interphase material is formed by diffusion, by diffusion plus compound formation, and by physical processes such as physical mixing and recoil implantation. The interphase material may be weaker or stronger than the nearby film and/or substrate material. For example, carbon lost from high-carbon steel substrates by diffusion into the film material during high-temperature processing can weaken the substrate and strengthen the film material. A National Science Foundation workshop in 1987 determined that the properties of the "interphase" (interfacial) material are some of the critical concerns in quantifying, measuring, and modeling the adhesion failure process (Ref 31). At present there are few if any good characterization techniques for determining the properties of interfacial materials that affect adhesion, such as fracture toughness, deformation properties, interfacial stress, presence of microscopic flaws, or effects of degradation mechanisms.

When a fracture surface (crack) advances, energy is needed for the creation of new crack surfaces and the deformation processes around the crack tip. This energy is supplied by the applied stress and the internal strain energy stored in the film-substrate system (residual film stress). The path of crack propagation is determined by the mechanical properties of the materials and by the resolved tensor stresses (tensile and shear) on the crack tip. The crack may progress along a plane of weakness, through weak material, or it can be diverted into stronger materials by the resolved stress. The fracture path is also determined by the presence of features that can blunt or change the fracture propagation direction.

Interfacial Morphology Effects on Fracture. In atomistic film deposition, the nucleation of depositing atoms on a smooth surface is controlled by various factors such as surface chemistry and nucleation sites. If the film-substrate interface is smooth, then any interfacial growth defects, such as interfacial voids, will lie in a plane that will then be a plane of weakness or "easy fracture path" along which fracture will easily propagate.

If the surface is rough and the deposited film material "fills in" the roughness, the propagating fracture must take a circuitous path with the likelihood that the fracture will be arrested and have to be reinitiated, as in the case of fiber-reinforced composite materials. If the roughness is not "filled in," there will be weakness (voids and low contact area) built into the interfacial region. Therefore the nature of the substrate surface roughness and the ability of the deposition process to fill in this roughness is important to the development of good adhesion.

The energy necessary for fracture propagation (fracture energy) can be lessened by mechanisms that weaken the material at the crack tip or reduce the elastic-plastic deformation in the vicinity of the crack tip. These mechanisms can be dependent on the environment in the case of ionically bonded materials. If time is involved in reducing the strength of the crack tip, the loss of strength is called static fatigue (Ref 66). Static fatigue depends strongly on mechanical (stress) and environmental (chemical) effects, particularly moisture and hydrogen (Ref 70).

Brittle surfaces and interfaces can be strengthened by placing them in compressive stress. This can be done by chemically replacing some surface ions with larger ions (chemical strengthening), by ion implantation, by putting the interior of the bulk material into tensile stress, or by placing the film in a state of compressive stress.

Residual Film Stress Effects on Adhesion. Invariably, atomistically deposited films have a residual stress that can be either tensile or compressive and can approach the yield or fracture strength of the materials involved. These stresses can arise from high-temperature deposition when there are differences in the thermal coefficients of expansion between the film and substrate, thermal gradients formed in the depositing film, and/or stresses due to the growth of the film. These stresses can enhance or retard fracture propagation.

In thin-film systems, high residual stress can be relieved by plastic deformation, blistering of the film from the surface in the case of compressive stress, or by microcracking and flaking in the case of high tensile stresses. If the film adhesion is high or the strength of the surface or film is low, the actual fracture path can be in the substrate or film and not at the interface. In many cases, in order to obtain good adhesion the residual film stress must be minimized and controlled. In some cases, film adhesion can increase with time due to the relief of high residual film stress.

Localized regions of high intrinsic stress can be found in films due to growth discontinuities or defects such as pinholes, nodules, or surface features such as edges or inclusions. These stressed areas can lead to localized adhesion failure under applied stress, producing pinholes.

Static fatigue is the slow growth of a crack under ambient stress and environmental conditions (Ref 66). The static fatigue failure mode due to moisture can be accelerated by breathing on the films to condense moisture at the crack tip. This moisture condensation method is an easy method of quickly determining if the residual film stresses are high, if the adhesion is poor, and whether the stresses in a failed film are compressive or tensile.

Corrosion. Interfacial corrosion/dissolution occurs when chemical or electrochemical (galvanic) effects create a solid or soluble corrosion product at the interface. An example of the loss of adhesion due to corrosion effects is the degradation of Ti-Au metallization in an HCl environment. This electrochemical degradation can be prevented by the addition of a thin intermediate layer of palladium between the titanium and the gold. The presence of chloride ions is generally to be avoided; they are often present as residues from cleaning and processing steps. Corrosion products can aggravate failure by a "wedging" action at the crack tip, either by solid corrosion products or by gas accumulation.

The diffusion of material away from the interface can weaken the interface by producing voids or in the extreme the complete removal of a bonding layer. For example, in the case of Cr-Au metallization, heating the system to higher than 200 °C in air will cause the chromium to diffuse from the interface to the surface, where it is tied up as the oxide (Ref 21). If all of the chromium diffuses from the interface, the gold film will not adhere. This "out-diffusion" of the interfacial material is dependent on the gaseous ambient, and a nonoxidizing ambient reduces the diffusion. The incorporation of a small amount of oxygen in the gold during deposition (by deposition in an oxygen plasma) reduces the chromium diffusion rate and gives a more thermally stable metallization.

Diffusion to the Interface. Interfaces generally act as preferential condensation regions for diffusion species. Diffusion of species to the interface can weaken or strengthen the interface. The deposition process may have influence on this effect. For instance, plasma cleaning of glass surfaces prior to silver deposition has been shown to give a time-dependent improvement in the adhesion of the silver films after deposition. Precipitation of gas at the interface to form voids will reduce adhesion. The diffusion of hydrogen through a film to an interface where it precipitates has been used by the electroplating community as an adhesion test. Diffusion and precipitation of vacancies form voids at interfaces that can cause adhesion loss.

Diffusion of water vapor through a polymer film to the interface can lead to the degradation of metal-polymer adhesion (Ref 71). Interfacial mixing can improve the moisture degradation properties of polymer-metal film systems.

Interdiffusion and reaction at the interface can generate an undesirable interphase material that results in a loss of adhesion. For example, in Au-Al metallization, interdiffusion and reaction form both Kirkendall voids and a brittle intermetallic phase (AuAl2), termed "purple plague," that causes loss of adhesion (Ref 28).

Film Morphology Effects on Adhesion. Film properties can influence the apparent adhesion of a film-substrate couple (Ref 72). The mechanical, microstructural, and morphological properties of the film material determine the ability of the material to transmit mechanical stress and to sustain internal stresses. For example, a columnar film morphology can exhibit good adhesion because each column is separately bonded to the substrate and the columns are poorly bonded to each other. The columnar morphology is generally not desirable because of its low density and high surface area.

Deliberate Nonadhering Coatings. In some situations, adhesion is not desirable. For example, one technique to form freestanding films or shapes is to deposit a coating on a mandrel and then separate the coating from the mandrel. The coating can be deposited on a substrate to which it will not adhere, or a "parting layer" (release layer) can be used (Ref 73). In the electrodeposition of freestanding copper or gold structures, stainless steel or carbon is often used as a mandrel, because the chromium oxide and carbon are good electrical conductors but copper and gold will not adhere to the oxide or carbon surface. Easily dissolved materials such as sodium chloride or polymers can be used as parting layer materials. In some cases, particularly for complex shapes, the mandrel must be completely dissolved to release the structure.

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