TOF‐SIMS: Primary Ion Sources - mikee9265/SIMS-Wiki GitHub Wiki

Ion guns are used in many types of apparatus including sputter systems, focused ion beam (FIB) tools, and in surface analysis equipment. The use in TOF-SIMS as the source of primary ions is the most demanding application of an ion gun. The ideal primary ion source for TOF-SIMS produces quick repeatable pulses of ions that arrive at the sample focused both in time (all ions in the pulse striking the sample at nearly the same moment, at a repeatable time relative to the pulse captured by the electronics) and in space (with a minimum spot size). There is an exception to this, in the case of an instrument that pulses the secondary ions rather than pulsing the primary ions. That exception is dealt with separately.

Additionally, the primary ion should be a heavy atomic ion, or better still, a cluster ion, or even better, a large cluster ion (Kersting et al. 2004). Recent work suggests that the chemical nature of the large cluster ion can be chosen to give large secondary ion enhancement (Sheraz et al. 2015). Finally, the source should be bright, producing pulses with plenty of ion current. It should come as no surprise that the ideal primary ion source is yet to be built. As a result, multiple ion sources may often be present on a single TOF-SIMS instrument.

The liquid metal ion gun (LMIG) is the ion gun of choice for FIB tools, and it is also the most important of the primary ion guns used in most of the TOF-SIMS instruments shipped today.

• The gun can produce ion pulses that arrive at the sample in a very narrow time window, allowing for high mass resolution.

• The gun can alternatively produce very small spots on the sample, the smallest of any ion gun available, spots < 50 nm in width.

• The gun can simultaneously produce submicron spots with short pulses that allow for high mass resolution (<600 nm, the so called “compromise mode”).

• High mass resolution, high lateral resolution, and compromise mode operation can be achieved with decent primary ion flux and therefore reasonable data rates for the instrument.

• With a Bi alloy as the metal, LMIGs can produce cluster ions with reasonable brightness. The Bi₃⁺ cluster in particular is a very practical ion to use in TOF-SIMS analyses.

The chief disadvantage of the LMIG is that the ions it can produce are limited to metals, and it cannot produce the sort of large clusters that have enabled exceeding the static SIMS limit without degrading the sample allowing an analyst to perform organic depth profiling.

The liquid metal ion source (LMIS) consists of a reservoir of metal with an attendant heater attached to a needle, which is coated with the same metal that forms the actual ion source (Komuro 1982; Komuro and Kawakatsu 1981).

The heater keeps the metal liquid. A metal extraction plate with an aperture is placed near the tip of the needle. When a high potential difference is placed between the needle and the extractor, a very high electric field is formed at the tip of the needle. The action of the field is to charge the metal positively near the tip. When the tip “lights,” the metal surrounding the tip elongates, is driven into a cone shape (known as the Taylor cone) by the intense field, and ions begin to be extracted from the tip. One of the most ingenious features of this device is the fact that ultimately the needle itself is buried under the distorted liquid metal and the shape of the cone becomes almost entirely a function of the electric field. The exact shape of the needle itself is not so important and, therefore, the engineering effort needed to consistently create these sources is not so demanding. The source of the ions is very sharp, an emission point of 10 nm or so in diameter. A dynamic equilibrium is set up, driving liquid from the reservoir up the needle where the electrons are stripped from the metal and driven back down the needle, while ions fly out into the ion gun column.

LMIS are commonly manufactured with one of three metals (Swanson 1994). Gallium is the metal of choice for FIB tools. FIB instruments use LMIG sources in DC mode to slice and dice samples, to open up cross sections for analysis, and to produce thin sections for transmission electron microscopy (TEM) analysis. The species formed at the tip are mostly atomic ions. The FIB tools are used to cut, so the direction of the ion beam is parallel to the FIB exposed surfaces. This means that damage caused by penetration of Ga into the material being cut is minimized naturally by the geometry of the cut. In contrast, in the TOF-SIMS tool the ion beam comes in at a much higher angle, and the penetration into the sample along with the subsurface damage that it causes, along with the reduced sputter rates and ion yields that result from much of the energy of the ion beam being deposited well below the surface, makes Ga⁺ a poor primary ion for TOF-SIMS.

Nonetheless there are labs that still use Ga⁺, principally for inorganic analysis. Ga requires little heating, and therefore is more monoenergetic. Until recently, Ga⁺ beams had a superior spot size. Ga is also available only as the ⁶⁹Ga isotope, and a Ga source produces atomic ions only with no clusters. As a result, no separation in the column is required when using a Ga source (see below). Finally, when a set of analytical protocols have been developed in a lab using a specific source, it is difficult to make the transition to a new primary ion.

Au sources still see some use (Hill and Blenkinsopp 2004). To bring the temperature of these sources down, a AuGe eutectic is used, and the Au and Au cluster ions are separated from ions containing Ge. However, only a small percentage of the ions produced from the Au source are Au clusters, which represents a significant disadvantage for this source in comparison to Bi.

Finally, there is the Bi source, which is the preferred source in use today for TOF-SIMS. The percentage of cluster ions, in particular Bi₃⁺, is so high that with the enhanced secondary ion yield from the cluster, the ion produces both faster and richer results.

The small source of the LMIS is what makes it possible to focus a small spot onto the sample. Large sources can be focused into small spots (think of a magnifying glass in the sun being used to burn paper), but those sources need to be collimated, not an easy thing to achieve in an ion source. The ions flying out from the LMIS point source can be manipulated and focused back into the image of its source, a point.

The art of manipulating ions is analogous to the manipulation of light. Instead of clear lenses, electric fields act on the charge of the ions to change their flight paths. Electric potentials between shaped metals in the ion path act as lenses and deflectors. Different from light, ion beams can be accelerated and decelerated. Ions with different masses can be separated from each other. Pulses of ions can be made and focused onto the sample.

Neutral species are not affected by the ion optics of the column. Since ions can be neutralized by collision in the column and neutrals will continue heading in the direction they were going when they lost their charge until they run into something, it is important that the ion column should not be perfectly straight and unimpeded. Ions accelerated near the source and neutralized should not be able to reach the sample. Such neutrals essentially add a DC noise into the spectrum, since their arrival at the sample would not be timed with the pulse of primary ions. LMIG guns shipped today are not sources of neutrals.

For use in a FIB tool, the LMIG tuning and usage is straightforward, far more so than when an LMIG is used in a TOF-SIMS instrument. There is a steady stream of ions emitted from the tip. All the ions emitted are given the same energy and are focused onto the sample. There are three major differences between this and what is needed for use in a TOF-SIMS operation when the more common instrument is used, one that cannot use DC primary ion beams unless you are pulsing the secondary ions instead.

1. The ion stream needs to be pulsed. To produce mass spectra, tightly spaced packets of ions are the ideal. The tighter the spacing in the time of arrival at the sample, the better the mass resolution of the instrument will be, at least at low mass. Typically, a voltage on one deflection plate is kept static so that without an additional voltage, the ion beam is deflected and will not reach the sample. A voltage pulse sent to a plate opposite the plate with the DC voltage swings the ion beam onto the path to the sample. An edge placed just by the ion path helps make the pulses crisp.

2. To produce short pulses that have a reasonable number of ions in each, the ion beam needs to be bunched. This way, a long pulse can be produced initially with a larger number of ions, with bunching used to shorten the time window in which they strike the sample. Bunching is accomplished by applying a pulsed voltage difference between two plates, one behind the pulse of ions and one in front. The voltage accelerates the ions at the back of the pulse more than the ions at the front. Tuned properly, the buncher arranges the ions in such a way that the ions at the back just catch up with the ions at the front when they all reach the sample. The down side is that the ions now have a distribution of energies, making it harder to focus them. This is known as a chromatic aberation.

3. For sources that produce more than one species of ions, it is necessary to separate these. If they are not separated, then ions of higher mass will arrive at the sample at a later time than the lighter ones, and you will have multiple peaks in the spectrum for each secondary ion species. This is, of course, another time of flight effect. The primary ions all have the same energy, so like all the secondary ions with nominally the same energy, their flight times to the sample will be distinct. In the LMIG, this time of flight difference for the primary ion species is what aids separation of the species. All that is needed is a pair of pulses. Ions are pulsed far up the column, and then pulsed again as described earlier. The difference in time between the two pulses is adjusted to match the flight time of the species wanted in the ion beam. All other species will arrive too early or too late for the second pulse and, therefore, will never make it through to the sample, impacting instead on a surface within the gun. As long as the elements in the source are monoisotopic, it will not take great mass resolution in the LMIG column to separate the different species.

With all these elements in place in the ion column and the extractor and suppressor elements for control of the source, deflectors, lenses, buncher plates, and finally stigmator elements for fine tuning the shape of the primary ion beam, the operator can tune an LMIG in a variety of ways. The beam can be optimized for the highest lateral resolution. Since the pulsing of the gun itself affects the spot size as does bunching, in this case the pulse width would be set to be long (reducing the proportion of the time of operation when the beam is subject to a pulse) and the buncher would be turned off, producing poor (up to 1 amu) mass resolution. Or, at the other extreme, the ion gun can be tuned to produce the optimum mass resolution with shorter pulses and with the buncher turned on. Other modes allow for higher currents with small spot size, or higher currents with short pulses and therefore high mass resolution. Finally, a compromise can be struck that gives decent mass resolution and submicron spot size. The following figure shows a schematic of an LMIG tuned in two quite different ways.

Additionally, the operator gets to choose the primary ion from those emitted by the source. In fact, most of the choices the operator makes when planning an analysis is in the use of the ion gun.

At the other extreme from the atomic and small cluster species produced in an LMIG, are the large clusters that can be produced by condensing Ar in a supersonic jet (Mahoney 2013; Kayser et al. 2013). The expansion of the gas stream leaving the nozzle directly results in a sudden cooling of the gas. The Ar gas gets so cold that it condenses into clusters of Ar atoms. Skimmers placed about the expanding jet allow for differential pumping and the introduction of the clusters into the vacuum system of the instrument. The skimmers also effectively collimate the beam. This continuous beam is then ionized by electron impact. The result is a beam of mixed ionized clusters with a variety of sizes and energies, along with neutral clusters.

The ions then need to be accelerated, separated from the neutrals and from each other by mass, focused, and, if being used analytically in a system that uses pulsed primary ion beams, pulsed. Of these tasks, the one that in particular varies from gun to gun is the mass separation. Unlike in the LMIG, the time of flight trick for separating these ions is impractical given the size of the ions and, therefore, their substantially lower speeds. The application of a Wien filter is one method that is used. A beam passing between two plates with a voltage difference between them will be deflected differently as a function of the mass to charge ratio. Simply placing a slit in the beam path allows some significant filtering. Ion-Tof sends the ions through the Wien filter for initial mass separation, and then when pulsing the beam for use as a primary ion source, adds further mass separation using a 90-degree bend (Kayser et al. 2013). The voltage on the bend is pulsed. The pulse has the effect of giving all the ions the same added momentum (and not energy), which means that the angle of the deflection will depend on the mass of the ion. These guns still end up providing a range of cluster sizes to the sample, but when multiple filters are used, the range is fairly narrow. In this way reasonable mass resolution can be achieved.

Focus is still an issue for these sources. The ability to focus is aided by the fact that the beam is somehwat collimated, and can be further aided by design of the source to reduce the size of the volume in which the electron impact ionization takes place. Spot sizes of a few microns have been achieved. A DC beam that can be used for spectrometry in the J series III can be focused down to 1.5 microns.

If a molecular species is volatile enough to produce sufficient vapor when heated in a vacuum (but nonvolatile enough to wait in the said vacuum system until heated), the vapor can be entrained into an electron beam, and then the ionized molecules can be accelerated and otherwise dealt with as described earlier for the previously discussed ion sources. While a variety of such molecular species have been tried, the C₆₀⁺ Buckminster Fullerine (also known as the “bucky ball”) molecule has been by far the most studied and the most commonly utilized molecule in such an ion source (Hill and Blenkinsopp 2004). C₆₀⁺ sources are in general use, as much for X-ray Photoelectron Spectroscopy (XPS) depth profiling as for Secondary Ion Mass Spectrometry (SIMS).

These sources also produce very different SIMS spectra, with intense higher mass fragments and molecular ions. As with the gas cluster sources, C₆₀⁺ can produce characteristic organic ions well beyond the static SIMS limit, although with C₆₀⁺ the effect is more material dependent than it is for the more massive cluster ions. Success with TOF-SIMS 3D depth profiling using C₆₀⁺ is very material-dependent (Rading et al. 2013).

On the other hand, C₆₀⁺ is a better source for sputtering inorganic materials; so on surfaces with inorganic features (such as in plastics with fillers) the C₆₀⁺ source may be preferred. However, the gas cluster sources can also be used in these situations if smaller cluster sizes are chosen.