What Is Sputtering of Alloys?
Thin film alloys or composites of various compositions can be deposited via Physical Vapor Deposition (PVD) processing in a variety of different ways. Probably the most common practice is to simply sputter the alloy or composite material from a homogeneous alloy or composite of the material composition which is to be desired in the resultant film. Of course there are any number of other methods with which to deposit multi constituent compound materials onto a substrate. Others include co-deposition, segmented targets, dual sources, etc. These can be discussed in subsequent blogs, but the basic concept of how the various species of atoms transfer from the target to the substrate while maintaining the same chemical composition will be discussed here.
As discussed previously, sputtering is essentially just the momentum transfer between charged particles (ions) in a plasma and atoms contained within a target material. In the plasma, an outer electron (negative charge) in the working gas is stripped away, leaving a net positive charge on the molecular structure of the gas. This creates an ion with a positive charge that can then be directed by an electrical and/or magnetic field of negative charge potential on the target surface that is to be sputtered. When the kinetic energy of the impinging ions exceeds the thermal energy (binding energy) of the atomic bonds within the crystalline structure of the material being sputtered, individual atoms of the target composition are ejected and propelled toward the substrate which is to be coated.
Incident ions collide with the atoms of the target material setting off collision cascades within the target. Similar to shooting pool, or billiards, when the cue ball hits the rack of object balls, the leading contact balls strike adjacent balls and then they, in turn, strike the next row of balls and so on and so forth until all the energy from the initial cue ball reaction subsides. So, from the initial collision of an ion (cue ball), a great number of additional atoms absorb the energy and react accordingly, bouncing to and fro within the target matrix. These cascading atoms will somewhat randomly alter their positions based on a number of factors such as the initial arrival energy of the ion, the incident angle of arrival at the target surface, the mass of the impinging ion, the mass of the atoms being sputtered, the energy of the atomic bonds and the binding surface energy of the atoms within the target. The target atoms recoil from the initial ion collision with some of the atoms reaching the target surface. Just like some of the object balls do in shooting pool when they end up beyond the initial position of the cue ball placement. If these atoms arriving at the target surface have sufficient energies greater than that of the surface binding energy, the atoms will be ejected from the surface and continue on into the plasma. Statistically some of these host atoms will arrive at the substrate surface. The statistical average of the number of atoms being ejected from the target surface per impinging ion is referred to as the “Sputter Yield” for any given set of variables described above. The sputter yield is a fixed constant for each element or compound in a given energy field.
If all these variables are held constant, there would be a constant rate of atomic deposition. However, in an alloy or composite material, it stands to reason that the energy of the atomic bonds for each constituent are not likely to be the same. This attraction between atoms, or bonding energy, may be due to electrostatic forces between individual atoms with opposite charges or covalent bonding energy from atoms with shared electrons. The atomic bonding energy is unique to any given element based on the individual atomic structure. There are stronger bonds in certain materials, such as covalent or ionic bonds, as well as weaker bonds such as dipolar interactions. So if the physical geometry of the deposition system is held constant and the arrival energy of the impinging ions are held constant, it would stand to reason that more weaker bonded atoms would be ejected from the surface than would be stronger bonded atoms. How will these individual sputter yields then affect the deposition rates for alloys or composite materials that may be comprised of elements with vastly different atomic bonding energies creating different sputter yields for each constituent?
It would be expected that a material with weaker atomic bonds would have its atoms ejected from the target surface faster than materials with stronger atomic bonds, based on sputter yield calculations. This is, in fact, what happens. So how can the resultant films maintain the same chemical composition as that of the host target material? Simple. Yes, the constituent with the weakest atomic bonds does sputter away from the target surface at a higher rate. Accordingly, this depletes the number of host atoms of the weaker bonded material from the target surface. So there are fewer of these weaker bonded atoms available to be ejected from the target surface compared to the atoms with the higher strength atomic bonds. Within a very short period of time there reaches an equilibrium proportion of chemical composition on and near the target surface. This new equilibrium composition is in accordance with the atomic binding energies of the available surface species. There are fewer weaker bonded atoms than stronger bonded atoms available to be sputtered. This equilibrium ratio is in direct proportion to the reciprocal of the sputter yields. Once this equilibrium composition is reached on the target surface, the proportion of each species available statistically enables the resultant film composition to be exactly the same as the original homogeneous chemical composition of the initial starting target composition. Thus is the magic of sputtering.