Ar sputter cleaning of gold mounts

Here are some calculations that I used to determine how long to sputter clean gold foils with an Ar ion beam, depending on current and beam size. These gold foils were subsequently used to mount presolar SiC grains.

Used equipment:

Hiden Analytical gas gun, loaded with Ar

Some constants for gold

Density $\rho = 19.3 \ \mathrm{g} \ \mathrm{cm}^{-3}$
Molar molar weight $M = 197 \ \mathrm{g} \ \mathrm{mol}^{-1}$

Gold atoms per monolayer

Firt, let us calculate how many atoms of gold we expect in a cubic centimeter of material:

$$ n_V = \frac{N_A \times \rho}{M} = 5.9 \times 10^{22} \ \mathrm{cm}^3$$

Assuming a homogeneous distribution, we can now calculate how many atoms of gold there are in one monolayer per square centimeter:

$$n_A = n_V^{2/3} = 1.5 \times 10^{15} \ \mathrm{cm}^{-2}$$

Some assumptions

Here, we assume a sputter rate $s = 10$, i.e., each Ar atom (incident energy 3 kV) removes 10 Au atoms. This seems to be a reasonable value.

Second, we have to know the current of the Ar sputtering. Back in 2021-05-26 using the Hiden Analytical gun, I measured a beam current of 195 nA. However, only a metal plate (not even a simple Faraday cup) was used. Thus, and to be conservative, let's assume a current of $I = 150 \ \mathrm{nA}$ from here on.

Calculation of time per monolayer

We can first calculate the flux of Ar atoms per second:

$$ f = \frac{I}{e} = 9.4 \times 10^{11} \ \mathrm{s}^{-1}$$

Here, $e ~ 1.6 \times 10^{-19} C$ is the elementary charge. Using the sputter rate per incident Ar atom $s$, we can now calcualte the total sputter rate per time:

$$S = s \cdot f = 9.4 \times 10^{12} \ \mathrm{s}^{-1}$$

Finally, this allows us to calculate the required sputter time per monolayer assuming a $1\ \mathrm{cm}^{2}$ beam:

$$t = \frac{n_A}{S} = 2.7 \ \mathrm{min}$$

Presolar grain cleaning

For the Hiden Analytical gun mounted on LION at LLNL, a rastering span of 70% x 35% resulted in a beam area of about $(9.4 \times 9.4)$ mm². This requires around 2.5 min per monolayer to remove. Note that the gun is however also firing under an angle.

Thus, I put all mounts into the instrument and sputtered off 5 monolayers per mount. Then I turned the cassette around and sputtered off an additional 5 monolayers per mount from the other side. In case there is any shadowing effects going on, these should be minimized thanks to the sputtering from either side.