Growth Related Film Properties

Films deposited by PVD processes invariably have properties that differ from those of the bulk materials (Ref 63). For this reason the specific film properties should be determined for films made with controlled and reproducible materials and processes. The columnar morphology and the residual film stress developed during film growth are important to a number of the film properties and the stability of the deposited film structure, including:

• Density—mechanical deformation, electrical resistivity

• Porosity—corrosion rate, etch rate

• Surface area and morphology—contaminant adsorption, optical reflectivity, electrical resistivity Residual Film Stress

Invariably, atomistically deposited films have a residual film stress that can be tensile or compressive and that can approach the yield or fracture strength of the materials involved. Generally, vacuum-deposited films and sputter-deposited films prepared at a high sputtering gas pressure have a tensile stress that can be anisotropic with off-normal angle-of-incidence depositions. Compressive stresses are generally encountered in films deposited under conditions where there is concurrent high-energy particle bombardment, such as in ion plating and in low-pressure sputtering where high-energy reflected neutrals from the target bombard the growing film (Ref 39). Figure 7 shows the residual film stresses generated in one direction in the film during postcathode magnetron sputter deposition of molybdenum. The film stress is anisotropic and is related to the configuration of the sputtering cathode. At low pressures, where there is bombardment by high-energy reflected neutrals, the stress is compressive. At higher pressures the stress is tensile, and at even higher pressures, where the morphology is more columnar and the film density is lower, the stress decreases. The origin of these stresses is poorly understood, though several phenomenological models have been proposed (Ref 64).

0.50 0.60 0.60 0.60 0.75 0.60 0.53 0.75 0.6Û 0.60 0.74

Molybdenum thickness, «m

0.50 0.60 0.60 0.60 0.75 0.60 0.53 0.75 0.6Û 0.60 0.74

Molybdenum thickness, «m

Fig. 7 Film stress as a function of gas pressure in postcathode magnetron sputter-deposited molybdenum films (Ref 39)

Tensile stresses can be developed when the growth mechanism does not allow the depositing atoms to attain their lowest energy positions. The Klockholm-Berry model is based on a constrained shrinkage of deposited material resulting in lattice parameters greater than normal. The grain boundary model attributes the stress to the development of grain boundary material. It has been proposed that the coalescence of lattice defects into "microvoids" causes the tensile stresses. It has been shown that the mechanical properties of the grain material are important to the resulting stresses and that impurity incorporation and reaction with the depositing film material can also be important factors in the stress generation. Tensile stresses can also be generated by phase changes and recrystallization, which result in volumetric shrinkage.

For high-temperature deposition conditions, the differences in the coefficients of thermal expansion of the substrate and film material can produce thermal (shrinkage) stresses that put the film in tension or in compression, depending on which material has the greater thermal expansion.

Film stress can change with film thickness. Stress gradients can exist in the deposited film due to the growth mode and differing thermal histories of the various layers of the film. These stresses can give the outermost layer of the film a tensile stress compared to the rest of the film. This film stress profile leads to "curling" of a film when it is detached from the substrate. If the adhesion failure is such that some of the substrate material remains attached to the film, the film can curl because of the constrained surface. Local stresses can be found in films where there is nonhomogeneous growth, such as over steps and defects in the substrate.

On a thin substrate that is in the form of a thin, long beam, the sum of all these stresses in the film causes the "beam" to bend. From the degree of bending and the material properties, the film stress can be calculated. The force on a substrate due to the film stress is a function of the film thickness. Film stress can also be determined by x-ray diffraction measurements of lattice strain, but this does not fully take into account boundary effects. The film morphology affects the stress buildup with a columnar morphology resulting in a low total stress.

Film stress and the resulting force and shear stresses are important factors in the adhesion and stability of films. High isotropic compressive film stresses produce "blistering" of the film from the surface in "worm-track" patterns. High isotropic tensile film stresses produce microcracking of the film. The cracks tend to meet orthogonally and form polygon "islands" or "chips" such as are seen in dried "mudflats" (Ref 65). Because the interface is constrained, the mudflat islands will tend to curl at the edges. If the compressive stresses are highly anisotropic, the "worm-track" pattern changes to line-shape blisters. If the tensile stresses are highly anisotropic, the "mud-flatting" pattern changes to linear cracks. If the adhesion between the film and the substrate is high, the stress can cause fracture in the film or substrate material. The film buckling or cracking can be time-dependent and can also depend on the moisture available in the ambient environment. This time/environment-related failure is called static fatigue (Ref 66). Fractures and fracture patterns in films can be detected by the use of fluorescent tracers. Generally, residual film stress should be minimized to prevent failure.

Modification of Film Stress. There are several methods of modifying the mechanical stresses developed in films during growth:

• Limiting the thickness of the stressed film

• Using concurrent energetic particle bombardment during deposition to maintain a near-zero-stress condition (Ref 67)

• Periodically alternating the concurrent bombardment conditions (Ref 39)

• Periodically adding alloying or reacting materials

• Mixing materials

• Deliberately generating an open columnar morphology that cannot transmit a stress

Limiting the film thickness is generally the most easily accomplished approach. As a "rule-of-thumb," the thickness of high-modulus materials such as chromium and tungsten should be limited to less than 50 nm to avoid excessive residual stress. If the film thickness is to exceed that value, some technique for stress monitoring and control should be developed.

One technique to control film stress is to use concurrent ion bombardment during deposition to create compressive stress to offset the tensile stress (Ref 67). By carefully controlling the bombardment parameters it is possible to find a zero-stress condition. Unfortunately this condition is usually very dependent on the process parameters and the proper conditions are hard to control and maintain. A more flexible technique is to alternately deposit layers having tensile and compressive stresses that offset each other (Ref 39). This can be done by varying the concurrent bombardment from the reflected high-energy neutrals in low-pressure sputter deposition, bombardment from a plasma or bombardment from an ion gun.

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