Sputtering: What is it? How to control it….

Table of Contents

Background: What is it? #

For our purposes, sputtering results when a focused beam of ions ejects material from the sample surface. There are two parameters to adjust in relation to sputtering; your sample and beam conditions.

Thoroughly understanding sputter characteristics makes achieving your microscopy goals a smoother process. Which can be especially helpful when a smooth surface is your end goal. 🤣

How to Control: #

Sample Parameters #

Sputtering happens when an atom is displaced by a collision with:

  1. a incident ion
  2. a recoiling atom

The sputtered atom needs to have sufficient energy to overcome the binding energy of the substrate in order to leave the surface. Only atoms near the surface are able to be maintain enough energy after the collisions to exit the material.

Sputter yield is the average number of atoms ejected from the substrate per incident ion. The sputter yield varies depending on the ion species (xenon, gallium, helium, etc.) and the type of material you are trying to sputter. Learn more about the stopping range of ions in matter (here) or by visiting SRIM.org.

Beam Parameters: #

The physics of sputtering depends on many factors including:

Incident Angle #

Changing the incidence angle of the primary ion beam impacts the sputtering yield, as seen the graph below. The incidence angle (θ) is 0° when the ion beam is normal to the sample surface. The sputtering yield increases significantly when increasing the incident beam angle. This general behavior is true for all ion species in FIBs. Different ion species produce different yields. Learn more about the different ion species here.

Figure 1. Plot of the sputter yield as a function of incident ion beam angle
for 30 keV Ne (black plus signs) and 30keV Ga (blue x’s) on Fe.

Changing the angle of the incident beam changes the shape of the interaction volume. This moves the collision cascade closer to the surface. This is visible in the simulated ion trajectories plot below calculated using SRIM (Figure 2). Increasing the number of near-surface collisions increases material removal speed. Maximum sputter yield occurs at incident angles between 80°-89°, as shown in Figure 1.

Figure 2. SRIM simulations of ion trajectories with the primary ion beam incident angle of 0 and 80°.

Incident Energy #

In general, increasing the incident ion energy will increase the sputtering yield. More energy is transferred from the primary ion to the substrate atoms. The table below shows the sputter yield at two incident angles for a range of voltages. In this example, higher beam energy results in a 2 to 5 fold increase in the sputtered atom yields (Table 1).

Acceleration Voltage
/ kV		Yield 0°
Incident Angle 		Yield 85°
Incident Angle
22.55.6
53.910.5
305.727.7
Table 1 : Sputtering yield Ga ions in Fe (SRIM simulations)

The trade off with increasing the accelerating voltage is the increase in penetration depth. This results in less overall collisions with surface atoms and more beam damage to the substrate. Choosing optimal accelerating voltage is linked with choosing the incident angle.

Incident Ion Mass, Charge, and Atomic Number #

The incident ion species also has a significant influence on the sputter yield. The larger the ion size (atomic number) the more collisions the primary ion has with the substrate….

Have more to say about Mass Charge and Atomic Number? Become a contributor.

Figure 3. Sputter yields in Fe with a He and Ga primary ion beam
as a function of accelerating voltages at a 0° incident angle.
Theory
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