Effects of Bombardment on Film Formation

Film Properties. The properties of a film of a material formed by any physical vapor deposition (PVD) process depend on four factors, namely:

• Substrate surface condition: morphology (roughness, inclusions, particulate contamination), surface chemistry (surface composition, contaminants), surface flaws, outgassing, and so on

• Details of the deposition process and system geometry: angle-of-incidence distribution of the depositing adatom flux, substrate temperature, deposition rate, gaseous contamination, and so on

• Details of film growth on the substrate surface: nucleation, interface formation, interfacial flaw generation, energy input to the growing film, surface mobility of the depositing adatoms, growth morphology of the film (i.e., roughness), gas entrapment, reaction with deposition ambient (including reactive deposition processes), lattice defects produced, grain size and orientation, recoil implantation (atomic peening), and so on

• Postdeposition processing and reactions: reaction of film surface with the ambient, thermal, or mechanical cycling; corrosion; interfacial degradation; burnishing of soft surfaces; shot peening; encapsulation ("topcoat"); and so on

In order to have reproducible film properties, each of these factors must be controlled. Figure 3 depicts the effect of energetic particle bombardment on surfaces and the near-surface region (Ref 14). The near-surface region is defined as the region of physical penetration by the bombarding species and is about 1 nm/keV (10 A/keV). These effects include:

• Reflection of some of the impinging high-energy particles as high-energy neutrals

• Generation of collision cascades in the near-surface region

• Physical sputtering

• Generation of lattice defects

• Trapping of the bombarding species

• Stuffing of atoms into the lattice by recoil processes

• Recoil implantation of surface species

• Enhanced chemical reactivity (bombardment-enhanced chemical reactivity)

• Enhanced diffusion in the surface region

• Heating of the near-surface region

Fig. 3 Schematic showing interactions in the near-surface region and on a surface during massive energetic

particle bombardment. Source: Ref 14

Most of the bombarding energy is given up in the near-surface region in the form of heat. In a growing film that is being concurrently bombarded by energetic particles, the surface and near-surface region is continually being buried and bombardment effects are trapped in the growing film (Ref 14, 15).

Surface Preparation. Bombardment of the substrate surface by energetic particles prior to the deposition of the film material allows in situ cleaning of the surface (Ref 16). Any surface placed in contact with a plasma will assume a negative potential with respect to the plasma (self-bias), due to the more rapid loss of electrons to the surface compared to the loss of ions to the surface. This sheath potential will accelerate ions across the sheath to bombard the surface. The sheath potential that is developed depends on the flux and energy of the electrons striking the surface. For a weakly ionized direct current (dc) plasma, the sheath potential will be several volts. For a system where electrons are accelerated to the surface, the self-bias can be many tens of electron volts. When the surface is conductive, a dc potential can be applied directly to the surface (applied bias). If the surface is nonconductive, a radio frequency (rf) potential can be applied to the surface to give a periodic high negative potential (applied bias) to the surface.

For inert gas ions with less than about 25 eV (4 aJ) energy, cleaning by bombardment is in the form of desorption of volatile materials ("ion scrubbing"). For reactive gases such as hydrogen or oxygen, the cleaning is in the form of reaction with contaminants such as hydrocarbons and desorption of volatile reaction products such as CO or CH3. Energetic reactive ions produce etching of the surface by reacting with the substrate surface material and producing a volatile compound ("plasma etching") (for example, SiCl4 from bombardment of silicon with an energetic Cl-containing ion from a vapor such as CCl4) (Ref 17, 18). More energetic inert particles produce physical sputtering ("sputter cleaning").

This in situ cleaning or surface preparation allows good interfacial contact for adhesion (Ref 19) and the generation of ohmic contacts to semiconductor materials. If done at low bombarding energies, the cleaning of semiconductor materials can be done without introducing surface defects that affect the electronic properties of the surface/interface (Ref 20). Bombardment can also make the surface more "active" by the generation of reactive sites and defects. For example, unbombarded silicon surfaces metallized with aluminum show no interdiffusion, but the bombarded surface gives rapid diffusion (Ref 21).

Bombardment can also be used to change the surface properties such as morphology (roughening) or chemical composition. For example, bombardment of a carbide surface by hydrogen ions results in the decarburization of a thin surface layer to produce a metallic surface on the carbide (Ref 22), and bombardment from a nitrogen plasma can be used to plasma nitride a steel surface prior to the deposition of a titanium nitride film (Ref 23, 24).

Nucleation. In ion plating, it is important that bombardment of the substrate surface during the surface preparation stage be continued into the deposition stage, where adatoms (atoms adsorbed on a surface so they will migrate over the surface) are continually being added to the surface. This prevents the surface from being recontaminated. Nucleation of adatoms on the surface is modified by concurrent energetic particle bombardment. This modification can be due to a number of factors, including cleaning of the surface, the formation of defects and reactive sites on the surface, recoil implantation of surface species, and the introduction of heat into the near-surface region (Ref 14, 25). Generally, this modification of nucleation increases the nucleation density. In addition, where there is high energy bombardment, sputtering and redeposition allow nucleation and film formation in areas that would not otherwise be reached by the depositing adatoms.

Interface Formation. Bombardment enhances the formation of a diffusion- or compound-type interface on the "clean" surface if the materials are mutually soluble, or it enhances the formation of a pseudodiffusion-type of interface, due to the energetic particle bombardment, if the materials are insoluble (Ref 14). Interface formation is aided by defect formation and the deposition of energy (heat) directly into the surface without the necessity for bulk heating (Ref 26, 27). In some cases, the temperature of the bulk of the material can be kept very low while the surface region is heated by the bombardment. This allows the development of a very high temperature gradient in the surface region, which limits diffusion into the surface (Ref 28). Ion bombardment along with a high surface temperature can cause all of the depositing material to be diffused into the surface, producing an alloy or compound coating.

Film Growth. Energetic particle bombardment during the growth of the film can modify a number of film properties, including (Ref 14):

Density

• Bulk morphology

• Surface morphology

• Crystallographic orientation

• Electrical resistivity

The changes in film properties are due to a number of factors, including

• Input of energy into the surface region during deposition

• Forward sputtering and redeposition of deposited atoms that densify the film

• Bombardment-enhanced chemical reaction

• Sputtering of loosely bonded contaminants and unreacted reactive species

Surface Coverage. The macroscopic and microscopic surface coverage of a deposited film on a substrate surface can be improved by the use of concurrent bombardment during film deposition. The macroscopic ability to cover complex geometries depends mostly on scattering of the depositing material in the gas phase (Ref 29, 30). On a more microscopic scale, sputtering and redeposition of the depositing film material will lead to better coverage on micron-sized and submicron-sized features (Ref 31, 32, 33, 34, 35, 36) and to reduced pinhole formation. On the atomic scale, the increased surface mobility, increased nucleation density, and erosion/redeposition of the depositing adatoms will disrupt the columnar microstructure and eliminate the porosity along the columns. As a result, the use of gas scattering, along with concurrent bombardment, increases the surface-covering ability and decreases the microscopic porosity of the deposited film material as long as gas incorporation does not generate voids.

Reactive Deposition. In reactive ion plating, codepositing species, or depositing species and gaseous species, react to form a nonvolatile compound film material (Ref 37). For example, depositing titanium atoms can react with "activated" gaseous nitrogen to form TiN, with codeposited carbon (Ref 38, 39) to form TiC, or with a combination to form TiCN,. In plasma-based ion plating, the plasma activates reactive species and creates new species in the gas phase. The concurrent bombardment of the surface enhances chemical reaction (bombardment-enhanced chemical reactions) (Ref 40, 41, 42, 43), desorbs unreacted adsorbed species (Ref 44), and densifies the film (Ref 45). In general, it has been found necessary to have concurrent bombardment in order to deposit hard and dense coatings of materials. Figure 4 shows the relative effects of heating and concurrent bombardment on the resistivity of ion plated and non-ion plated TiN films (Ref 46, 47).

Fig. 4 Relative effect of deposition temperature and bias on reactively sputter-deposited titanium nitride. A lower resistivity rating indicates that the titanium film is more dense (that is, hard) and stoichiometric. Source:

Ref 46

In vacuum-based ion plating, the bombardment of the depositing film by energetic reactive gas ions enhances the chemical reaction (Ref 48, 49). In reactive deposition, the extent of the reaction depends on the plasma conditions, bombardment conditions, and the availability of the reactive species. By limiting the availability, the composition of a deposit can be varied. For example, in the reactive ion plating of TiN, by reducing the availability of the nitrogen in the plasma at the beginning of the deposition, an initial layer of titanium is deposited. The composition can then be graded to TiN by increasing the availability of nitrogen in the plasma, thus forming a "graded" interface.

Properties of Films Deposited by Ion Plating. The properties of films formed by processes depend on a number of factors. Because the ion plating process has more deposition parameter variables than other PVD processes, the film properties can be varied over a wide range, depending on the process parameters.

Film Adhesion. The adhesion of a deposited film to a surface depends on the deformation and fracture modes associated with the failure (Ref 19). Energetic particle bombardment prior to and during the initial stages of film formation can enhance adhesion by:

• Removing contaminant layers

• Changing the surface chemistry

• Generating a microscopically rough surface

• Increasing the nucleation density by forming nucleation sites (defects, implanted species, and recoil-implanted species)

• Increasing the surface mobility of adatoms

• Decreasing the formation of interfacial voids

• Introducing thermal energy and defects directly into the near-surface region, thereby promoting reaction and diffusion

Film adhesion can be degraded by the diffusion and precipitation of gaseous species to the interface. The adhesion can also be degraded by differences in the coefficient of thermal expansion of the film and substrate material in high-temperature processing, or the residual film growth stresses developed in low-temperature processing.

Residual Film Stress. Invariably, atomistically deposited films have a residual stress that may be tensile or compressive in nature and may approach the yield or fracture strength of the materials involved. Generally, vacuum-deposited films and sputter-deposited films prepared at high pressures (>0.7 Pa, or 5 mtorr) have tensile stresses that can be anisotropic, with off-normal angle of incidence depositions. In low-pressure sputter deposition and ion plating, energetic particle bombardment can give rise to high compressive film stresses due to the recoil implantation of surface atoms (Ref 50, 51, 52, 53, 54). This effect is sometimes called atomic peening and generally requires 20 to 30 eV (3 to 5 aJ) per deposited atom of additional energy from bombardment. Studies of vacuum-evaporated films with concurrent bombardment have shown that the conversion of tensile stress to compressive stress is very dependent on the ratio of bombarding species to depositing species. The residual film stress anisotropy can be very sensitive to geometry and gas pressure (Ref 55, 56) during sputter deposition, due to bombardment of high-energy reflected neutrals and the effect of gas-phase and surface collisions at higher pressures. Figure 5 shows the effect of gas pressure on residual film stress in postcathode magnetron sputter deposition of molybdenum.

Fig. 5 Effect of gas pressure on residual stress in molybdenum films formed by postcathode magnetron sputter deposition. High compressive stress at low pressures is a result of reflected high-energy neutral bombardment. The low stress at high pressures is a result of columnar growth in a low-density film. Bars on points indicate range of values. Source: Ref 56

Fig. 5 Effect of gas pressure on residual stress in molybdenum films formed by postcathode magnetron sputter deposition. High compressive stress at low pressures is a result of reflected high-energy neutral bombardment. The low stress at high pressures is a result of columnar growth in a low-density film. Bars on points indicate range of values. Source: Ref 56

The lattice strain associated with the residual film stress represents stored energy, and this energy, together with a high concentration of lattice defects, can lead to:

• Lowering of the recrystallization temperature in crystalline materials

• A lowered strain point in glassy materials

• A high chemical etch rate

• Electromigration enhancement

• Room-temperature void growth in films

• Other such mass transport effects

Film Density. Under nonbombardment conditions at low temperature, the morphology of the deposited material is determined by geometrical effects with the film density being a function of the angle of incidence of the depositing particles. Under ion plating conditions, forward sputtering, sputtering and redeposition, increased nucleation density, and increased surface mobilities of adatoms on the surface under bombardment conditions can be important in disrupting the columnar microstructure and thereby increasing the film density and modifying film properties (Ref 57, 58). The bombardment also improves the surface coverage and decreases the pinhole porosity in a deposited film. This increased density and better surface coverage is reflected in film properties such as (Ref 59, 60, 61):

Better corrosion resistance

• Lower chemical etch rate

• Higher hardness

• Lowered electrical resistivity of metal films

• Lowered gaseous and water vapor permeation through the film

• Increased index of refraction of dielectric coatings

However, it has been found that if the bombarding species is too energetic and the substrate temperature is low, high gas incorporation, defect concentration, residual stress, and the formation of voids can lead to poor-quality films.

Film Porosity. The porosity in atomistically deposited films results from:

• Incomplete surface coverage

• Deposition on particulate contamination that is subsequently dislodged

• Formation of a columnar film morphology

• Precipitation of voids at grain boundaries

• Precipitation of incorporated gases in the film

The increased surface-covering ability, densification of the film material, and disruption of the columnar morphology in ion plating decrease the film porosity unless bombardment gases are incorporated in the depositing film.

For more information about bombardment effects on film formation, see the article "Growth and Growth-Related Properties of Films Formed by Physical Vapor Deposition Processes" in this Volume.

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