Adhesion is the mechanical force joining two different objects or materials, and is a fundamental requirement of most deposited-film systems. "Apparent adhesion" is the adhesion as determined by applying external mechanical stress. Failure of adhesion, or "deadhesion", can be brought on by mechanical failure by fracture or deformation; chemical causes, such as corrosion or dissolution; or diffusion processes, such as the diffusion of material to or away from the substrate interface. Poor adhesion may be localized, causing local failure and pinholes. In PVD processing, film adhesion is intimately connected with the nucleation, interface formation, film growth and surface coverage, as well as the properties of the materials in contact and the environmental stresses (mechanical, chemical, thermal, fatigue) to which the system is exposed.

Good adhesion is promoted by a high-fracture-toughness of the interface and nearby materials; absence of fracture--initiating features; presence of fracture-blunting and deflecting features; low residual film stress; and no operational adhesion degradation mechanisms, such as diffusion or corrosion. Poor adhesion can be attributed to a low degree of chemical bonding, poor interfacial contact, low-fracture-toughness of the interfacial (or nearby) material, high residual film stresses, fracture initiating features, and/or operational adhesion degradation mechanisms. In many systems where film adhesion is difficult to attain, an intermediate material is introduced into the interfacial region, to bond to the substrate and the film material. In the gold metallization of oxides, for example, a layer of oxygen-active material, such as titanium, which has a solid solubility with gold, is used in the interracial region, to react chemically with the oxide surface and to alloy with the desired gold film.

An Early Indicator

The nucleation density of the deposited adatoms is an early indication of good or poor adhesion. A high nucleation density indicates strong chemical interaction of the deposit adatoms with the substrate surface and is desirable for good adhesion. A low nucleation density indicates poor interaction and the development of poor interfacial contact and the formation of interfacial flaws, which will lead to poor adhesion .

The nature of the interfacial region is important to developing a fracture-resistant interfacial material. A diffusion-type or compound-type interfacial region is good for adhesion, provided excessive diffusion and reaction do not introduce voids, stresses and fractures in the interfacial region (P&SF, Jan. and Feb.,1994). Roughening the substrate surface 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.

Is Residual Film Stress Correct?

An important factor in apparent adhesion is the residual film stress. Invariably, PVD 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 arise from differences in the thermal coefficient of expansion between the film and substrate in high-temperature depositions, thermal gradients formed in the depositing film, and/or stresses caused by the growth of the film. In some cases, the stress level can change with film thickness. The total stress that appears at the interface from residual film stress will depend on the film thickness and material. High-modulus materials, such as chromium, tungsten and compound materials, generate the highest stresses. These will be added to any applied stress, but can be capable of causing spontaneous deadhesion of the film.

High-residual film stress can cause blistering of the film from the surface, in the case of compressive stress; or by microcracking and flaking, in the case of high-tensile stress. If the compressive stresses are isotropic, the blistering will be in the form of "wormtracks." If the tensile stresses are isotropic, the microcracking will be in the form of a "dried-mudflat" cracking pattern, often with the edges curled away from the substrate. If the film adhesion is high or the fracture strength of the surface is low, the actual fracture path may be in the substrate and not at the interface. Localized regions of high-intrinsic stress may be found in films because of growth discontinuities. These stressed areas can lead to localized adhesion failure, creating pinholes.

If high-residual film stresses are generated, they can often be limited by restricting the film thickness, changing the film materials, changing the film structure, or by changing the deposition technique or parameters. When depositing an electrically conductive layer of chromium on glass, for instance, it is often found that if the chromium thickness exceeds several thousand A, the residual film stress will peel up a layer of the glass. To avoid the problem, limit the chromium thickness to less than 500 A, as well as the electrical conductivity obtained via a top layer of gold or copper, which typically does not develop high stresses. If this is not done, the stress in the thick chromium films deposited must be carefully controlled. Another commonly encountered problem is the high compressive stresses that can be developed in low-pressure sputter deposition, where high-energy reflected neutrals from the sputtering target bombard the growing film. The compressive stresses can be lowered by increasing the deposition pressure, "thermalizing" the high-energy reflected neutrals before they reach the growing film surface.

Film composition and growth morphology can also be important to adhesion. Generally, a dense film is desirable. Such a film, however, will transmit stresses more easily than a less-dense, or porous, film. In some cases, a porous film, formed by the columnar growth morphology can be used as a "compliant" film, or layer in the film structure. When there is a large difference in the physical and mechanical properties of the film and substrate, it may be advantageous to grade the properties through the interfacial region, rather than to have a sharp discontinuity in properties. In coating tool steel with TiN, for example, it may be desirable to first deposit a thin layer of titanium on the steel and then grade the Ti-N composition gradually to the stoichiometric composition TiN. This can be done by controlling the nitrogen availability in the plasma during deposition.

Adhesion can decrease with time, if there are operational degradations. If a chromium-gold metallization system is exposed to a temperature higher than 200 °C in air, for instance, the chromium will diffuse through the gold and react with the oxygen, to form an oxide. In the extreme, all the chromium will diffuse away from the interface, causing deadhesion. In titanium-gold metallization, the Ti-Au forms a galvanic corrosion couple, and the interface will corrode if an electrolyte is present. To prevent this corrosion, a layer of palladium is added, to give a Ti-Pd-Au film structure. When the film has a high residual stress, "static fatigue" can allow the propagation of a fracture, resulting, over time, in long-term failure with no externally applied stress.

There are many film adhesion tests, and some of the most common are the tape-peel, the stud-pull and the scratch methods. In general, adhesion tests are used as comparatives and not to give absolute values of adhesion strength. Film adhesion is generally tested immediately after deposition. Because of the possibility of time/environment adhesion degradation, however, an adhesion testing program should be designed to expose the film system to the subsequent fabrication, storage and service environment, and times that it will endure after fabrication. In the Au-AI metallization system, for example, prolonged exposure to a temperature above 200 °C in service will cause progressive interfacial diffusion and the formation of pores, fractures and the Al-Au inter-metallic phase in the interfacial region. The degraded interfacial region will easily fracture and exhibit the purple color of the inter-metallic phase

(AuAl2), and this failure mode is called the "purple plague."

A Problem-solving Checklist

If film adhesion is a problem, consideration must be given to:

• Are the materials in contact chemically reactive, in order to form strong chemical bonds?
• Are there contaminant layers that prevent intimate contact?
• Is the substrate surface mechanically strong and defect-free, so that fracture does not occur in a weak substrate surface region?
• Is there a high nucleation density of the adatoms on the substrate surface?
• Do the interfacial region and interfacial material have a high-fracture toughness?
• Are there high-residual film stresses?
• Are there operational degradation mechanisms?
  

In many cases, adhesion problems that occur in production can be traced to variable substrate surfaces, poor surface preparation, or uncontrolled deposition parameters.