Ion energy, measured in electron volts [eV], is an important detail in the application of ion beam technology. At certain energies, an ion striking a surface will have little or no effect on it, while at higher energies, the ion may erode or damage the surface. Balancing the appropriate ion energy for the process desired is critical to achieving a high quality coating.
Sputtering vs. Densifying
The ion energy distribution (number of ions at a given energy) of an ion source is an important parameter that will determine what the ion achieves when it strikes the surface. For a given ion specie and substrate material combination, the energy at which sputtering starts to occur is roughly constant and does not start near 0 eV. There will be a minimum energy at which sputtering occurs, and a “probability” of sputtering that increases with increasing ion energy.
Figure 1: Sputter rate of Argon ions striking Silicon. Calculated from Yamamura, Y and Tawara, H, ADNDT 62 149-253 (1996)
In Figure 1, the sputter rate dependence of ion energy is shown. While these particular data are specific to the combination of Argon ion and Silicon target, the curve shape is typical of most ion / target combinations. In particular, the interest is in the low end of ion energies, where Ion Assist applications usually occur. Figure 2 is a closer look at this ion energy range.
Figure 2: Sputter rate of Argon ions striking a Silicon surface: focus on low energy.
From Figure 2, sputtering only begins above about 50 eV. Indeed, up to about 90 eV, only two out of every 100 ions that strike the surface will cause a sputtered atom to come off the surface (essentially, a 2% chance of sputtering). In contrast, an ion with an energy of 200 eV has a 13% chance of sputtering – much more likely. Sputtering causes the newly coated surface to wear away, loosens adhesion, and can introduce defects into the structure of the coating. Thus, for Ion Assist applications, minimizing sputtering is ideal.
Ion Energy Distributions
To minimize sputtering, control of ion energies is important. The ion energy distribution of an ion source shows both the range of energies emitted ions can have, and the probability that a given ion will (or fraction of ions which) have a given energy. As an example, Figure 3 shows the ion energy distribution for a typical gridless Hall-Effect source.
Figure 3: Ion Energy Distribution for a Mark-2 ion source. Nominal voltage = 100V. Zhurin, Surf. Eng. 27 5 311-319 (2011)
From Figure 3, it is apparent that setting the emission voltage of a gridless ion source to 100V does not mean that the emitted ions have 100 eV of energy. In fact, only 75% of the ions fall between about 40 eV and 140 eV, and 25% are outside this range, and can be as low as 10 eV. About 1/3 of the ions are being emitted with very low voltage (<35 eV), and are therefore doing almost nothing for the process. This can be offset by requesting a higher voltage - for example, requesting 140V brings the minimum voltage up to around 50 eV. However, this also brings the maximum energy up to over 180 eV. Referring back to Figure 2, at this energy a significant fraction of the ions would now have a more than 10% chance of sputtering. About 60% of the ions would now have over a 5% chance of sputtering. This means many ions would now be damaging the coating instead of improving it.
In contrast, a typical ion energy distribution for a gridded source is shown in Figure 4.
Figure 4: Ion Energy Distribution for a gridded ion source. Nominal voltage = 150V. Rubin, et. Al. RSI 80 103506 (2009). Edited with permission.
From Figure 4, the entire energy range of the ion source is 20 eV. While not exact, if 125 eV ions are requested from a gridded ion source, the ions emitted will be between 115 eV and 135 eV. As such, to minimize sputtering a request of 90 eV could be made, with 100% of ions still contributing to the Ion Assist application, and 98% of the ions having a 3% or less chance of sputtering.
Based on the above ion energy distributions, it is clear that a gridded ion source presents the most efficient and effective method of executing an Ion Assist application, with a minimum of sputtering damage. Furthermore, it is clear that a gridded ion source presents far more consistent, reliable, and predictable sputtering rates due to the narrower Ion Energy Distribution. Variation in ion energy can and does present
significant challenges in ion applications, and only gridded ion sources can offer the control needed to produce the highest quality coatings, repeatedly and with reliability.