sputter

sputter

Ions impacting solid materials may eject atoms of the target material itself. This process is known as “physical sputtering” or simply “sputtering”, and depends on the incident ion energy and angle of incidence, as well as the projectile-target material combination. The sputtered neutral yield is ultimately determined from the nuclear stopping power of the ion/target combination.

VSim provides a method of simulating this behavior using the sputter kind of ParticleSource. Using sputter, you can simulate the emission of sputtered neutral atoms from solid materials when bombarded by energetic ions. If a flux of ions is incident upon a wall on which a sputter emitter is defined, these incident ions deposit energy into the wall, initiating neutral transport to the material surface and subsequent emission back into the simulation domain. The incident ions are absorbed by the wall and will be removed from the simulation.

Only ions impacting a wall are used to calculate the source for sputtered neutrals; electrons cannot be used as impactors. If the incident particles are electrons then new electrons are the only secondary particles that can be emitted.

The yield and angular dependence of emitted atoms is based on the Yamamura sputtering model [YT96]. The model depends on the mass ratio and surface binding energy of target material. The implementation includes enhancements to the yield due to angular dependence of the incoming projectile. The model includes two fit parameters which are \(\theta_{\rm opt} = 1.4486\) and \(f = 1.8\).

The Yamamura model gives a non-integral yield based on the parameters provided. We provide routines that also return an integer yield, chosen with probability such that the ensemble average of many identical trials produces the desired expectation value. We provide implementations of the Yamamura model for both single incident ions, and for an array of incident ions, each with its own energy and angle of incidence. These routines return the total sputtering yield, the energy of each sputtered atom, and velocity components for each sputtered atom. Currently, velocities are assigned as thermal, ie. Gaussian in each direction, but with a non-negative component in the (outward) normal direction.

It is also possible to compute the sputtering yield for alloys using these routines, by specifying the different elemental materials and the mass stoichiometry of the alloy target. Sputtering yields for each individual material element are calculated and then weighted by the stoichiometry.

More details on sputtering can be found in the following papers:

• P. Sigmund, Phys. Rev. Vol 184, 2 “Theory of Sputtering. I. Sputtering Yield of Amorphous and Polycrystalline Targets” p383 (1969).

• Y. Yamamura Nuclear Instruments and Methods in Physics Research B2, “A Simple Analysis of the Angular Dependence of Light-Ion Sputtering” p. 578-582 (1984).

This kind of particle source is available with a VSimPD license.

sputter Parameters

direction

Direction for the source of the outward-facing normal; the algorithm uses this to determine the direction in which sputtered electrons are emitted. If this direction vector is incorrect, it may result in particles being emitted outside the simulation domain, leading to crashes. Sputtered neutrals are always emitted with a positive normal velocity component, i.e., moving away from the emitting surface.

ptclAbsorber

Specifies the absorber that will remove particles generated by this ParticleSource.

sputterAtomType (string)

Defines the material of which the source is composed. Material choices are:

• carbon (C)

• aluminium (Al)

• silicon (Si)

• chromium (Cr)

• iron (Fe)

• nickel (Ni)

• copper (Cu)

• germanium (Ge)

• silver (Ag)

• osmium (Os)

• platinum (Pt)

• gold (Au)

• uranium (U)

sputterAtomSpecies (string)

Name of the neutral atom species that will be sputtered.

secVSig (float)

value defining the velocity spread of the sputtered atoms.

ignoreProb (float)

The fraction of electrons that does not get absorbed. Default value is 0.

Example sputter Block

# The sputter emitter
<ParticleSource rightSputterEmitter>
    kind = sputter
    minDim = 1
    ptclAbsorber = rightAbsorber
    # The 'direction' vector should point along the
    # *outward-facing* normal...be sure to check this!
    direction = [1. 0. 0.]
    # The material that makes up the wall
    sputterAtomType = Cr
    # The species of neutral atoms that is being
    # sputtered off the wall
    sputterAtomSpecies = chromiumAtoms
    # The velocity spread of the sputtered atoms
    secVSig = CHROMIUMVSIG
</ParticleSource>