TOF‐SIMS: Spectrometers - mikee9265/SIMS-Wiki GitHub Wiki

Once the primary ions strike the sample, secondary ions are formed. The instrument needs to gather, focus, separate, and measure these ions. The ions are emitted from the surface with a variety of energies, and they fly off in varied directions.

Fortunately, their energy range is reasonably low; most have much less than 100 eV. Atomic ions have a larger energy distribution than inorganic clusters, molecular ions, and their fragments.

Differences in the energies of these different ion types can lead to slight differences in their mass calibration in the spectra due to the limitations of TOF-SIMS spectrometers energy/angular emission angle compensation.

The ideal scenario is that the secondary ion trajectory begins speeding along the normal to the surface, directly into the aperture through which it is being extracted.

From a flat surface, the emission tends to be in something like a cosine distribution relative to the sample normal.

That is, most ions will be emitted nearly normal to the surface, with an ever-decreasing number being emitted at higher angles. However, in the presence of the sample topography ion emission can end up being forced away from the sample normal to the surface.

Capturing these ions for measurement is a bigger problem.

The ideal spectrometer in a TOF-SIMS instrument would manage to capture all of the secondary ions produced at the sample, transmit them without losing one to the detector where every secondary ion is counted and perfectly resolved by mass. One of the strengths of the TOF-SIMS method is that the transmission and detection of secondary ions is very efficient. As a rule of thumb, modern spectrometers will typically transmit and detect on the order of 50 percent of the secondary ions formed at the sample, an impressive number, indeed. The tuning of the instrument can be varied to maximize transmission at the expense of other parameters such as mass resolution, and conversely, transmission can be sacrificed to improve mass resolution.

Mass resolution in the TOF-SIMS is not solely a function of the spectrometer, nor is it constant across the mass range. Mass resolution is defined as the ratio of the mass/charge ratio to the resolving power or resolution at that mass, m/∆m. This latter ∆m is defined in various ways by various people, as the width of the peak at the half way intensity (Full Width at Half Max or FWHM) or at some other named percentage of the peak maximum or the distance between 2 barely separated peaks when the valley between them is sufficiently low (10 or 50 percent). However one defines it, the TOF-SIMS mass resolution varies across the spectrum. At low masses, the mass resolution is primarily affected by the primary pulse width in time, a function of the ion gun and not the spectrometer. At high masses, the effect of the primary pulse diminishes as the absolute time of travel increases. At the highest masses, the spectrometer’s characteristics dominate the mass resolution.

Extraction of secondary ions into the spectrometer proper is done with the use of an electrostatic field, the result of a potential difference that is created between the sample and the extraction optics. The extraction optics are, in effect, a part of the spectrometer design, and so they are discussed, along with the rest of the time of flight mass spectrometers described in this section.

As noted in previously, once secondary ions have been accelerated so that they have approximately the same energy, all that is needed to measure their mass is to wait. Their times of arrival at the end of an evacuated flight tube can be converted to a mass scale. Unfortunately, as noted above, neither their energies nor their trajectories are exactly the same, and as a result, the mass resolution possible with a linear spectrometer is poor.

The commercial TOF-SIMS systems all have unique designs and differing characteristics that affect experiment design and usage. There is also a hybrid system with an add on Orbitrap spectrometer which, while not being a TOF instrument is nonetheless used in conjunction with one and thus is described below.

IonTof manufactures Reflectron type TOF-SIMS instruments (Niehuis et al. 1987). In concept, the Reflectron is the simplest TOF spectrometer next to the linear TOF. To improve mass resolution, the ions are reflected back using an electrostatic field; nearly directly back at the sample. Ions carrying more energy penetrate the field further before turning back, thus taking a longer path and a little more time. With the right tuning of fields, this extra time nearly exactly makes up for the fact that these ions are travelling faster than lower energy ions with the same mass to charge ratio, and thus all the ions with the same m/e arrive at the detector at the same time. A schematic of such a spectrometer is shown below.

One characteristic of a Reflectron is that most ions produced at the sample or that result from a secondary ion’s decomposition in flight will reach the detector. Ions that are the result of a secondary ion’s decomposition are known as metastable ions. Such ions may have reduced energy (since they share their energy with a neutral fragment of the original ion). Most of these ions will nonetheless reach the detector, producing peaks in the spectrum far from their proper m/e position, and thus such peaks can be difficult to interpret. Further, they may end up with a range of energies, so they may make broad peaks and extended features in the spectra. Ions that are so energetic that they penetrate the ion mirror far enough to hit the far wall inside the Reflectron are not detected. Elastically scattered and most inelastically scattered primary ions will fall into this category.

In Reflectron based instruments, the sample is kept at a ground potential, and the acceleration voltage is provided by the difference in potential between the sample and an extractor nozzle 1-2 mm above the surface. The primary ion beam skirts the extractor nozzle on its way to the sample, and thus the extractor affects the primary ions’ flight path, but not its energy. The extractor voltage is reversed when between positive and negative ion modes (as are most of the voltages on the ion beam optics within the spectrometer). The primary ion source is largely unaffected by the change in instrument polarity, except that the ion beam deflection needs to be shifted to make up for the reversed effect the extractor has on the primary ion beam.

Secondary ions, produced by primary ion impact, begin life directly beneath the extractor, and are accelerated by the voltage they see. For conductive samples where the surface potential is indeed at the ground level of the sample holder, the extraction voltage is the same as the acceleration voltage, as long as the spectrometer as a whole has that same voltage. IonTof reflections can be run in a variety of modes with different settings and their consequences.

The major tradeoffs are between mass resolution and transmission. If all ions had a low enough energy such that their energy differences could be compensated, and if they all left the sample exactly normal to it straight into the extraction nozzle, virtually all the ions would be detected. However, as noted above, secondary ions do not always leave normal to the sample. Most have some velocity that will take them out of the center line of secondary ion flight paths leading through the spectrometer, either due to their natural angular distribution, or due to sample topography. There is then a race between the secondary ions' divergence from the center path and their progress towards the detector. The slower the progress to the detector, the higher the probability that secondary ions will move so far out of the main flight path that they hit something and do not make it to the detector. Higher spectrometer voltage speeds the secondary ions along, reducing the chances of their being lost and increasing their chance of detection. Higher speeds, though, lead to lower mass resolution, since the secondary ions arrive at the detector less dispersed in time. The collection of ions that have strayed from the center line also reduces mass resolution, since any ion that takes a longer fight path to the detector will arrive later, broadening that species peak.

In summary, tradeoffs can be made with the combination of the extraction voltage and spectrometer (acceleration) voltage to preferentially improve mass resolution, lateral resolution, or transmission, just not all three at the same time.

Changes in the extraction voltage also affect the field of view, the maximum area from which ions can be extracted. Modern instruments are equipped with stage raster capability, and the software seamlessly stitches the data acquired at each stage position into a single data set. The adjustment of the extraction potential is therefore not needed for the analysis of larger sample areas.

Samples with significant topography are problems for TOF-SIMS instruments in general. There are several approaches to solving this problem.

• Discrete samples should be mounted in such a way as to produce an essentially flat surface from which to extract ions. When mounting magnetic recording heads for failure analysis, for example, a head of interest may be mounted adjoining a set of heads with the same height. In this way, the drop off at the head’s edge is replaced with an effectively continuous surface.

• Sample tilt can be employed to effectively extract ions from an edge or a topographical feature. This will tend to reduce the signal from the overall sample, so it is best employed when the signals to be obtained from a sample feature are the main points of the analysis.

• A pulsed late extraction field can be used to effectively eliminate the effects of topography. The trick is to have no extraction potential when the primary ions strike the sample. By allowing a delay before the extraction field is applied, secondary ions have time to drift away from the sample surface with the few eV they naturally have from the sputter process. There is also no need to use short primary ion pulses in this mode. Thus, one can use modes of the primary ion source that produce small spots and therefore high lateral resolution without bunching and the accompanying chromatic aberrations. When the field is finally applied, the secondary ions are not close to any sample topography and see the straighter field leading into the spectrometer exclusively. Setting up pulsed late extraction requires adjustment of the spectrometer since the ions will have moved some distance towards the extractor while there is no extraction field. When the voltage is switched on, the ions will see a smaller voltage being part way between sample and extractor. They will receive less energy unless the acceleration voltage is changed to compensate. They will also be starting later. Low mass peaks (< 40 amu or so) are lost when using this method. Mass resolution is intermediate, much reduced compared to the regular capabilities of the Reflectron, but much better than would be obtained normally with an un-bunched beam. There is also some loss of transmission in general.

• The latest M6 reflection spectrometer from IonTof has an added lens inside the spectrometer, placed just before the reflector. When activated, this "topography" lens acts to drive secondary ions that have strayed from the center line through the spectrometer back towards the center. This has the effect of improving transmission, especially in circumstances where surface topography has acted to drive secondary ion emission trajectories away from the sample normal. Capturing ions that stray is bad for mass resolution, however, since these ions arrive later and broaden peaks in the spectra.

The primary ion beam is pulsed at intervals that allow time for the slowest ion in the mass range desired to reach the detector before the next pulse is initiated. The smaller the mass range desired in the analysis, the faster the system can pulse, and, therefore, the greater the data rate and the quicker the analysis will be completed at any given primary ion flux. The mass range the analyst chooses may be less than what the sample will produce. To eliminate secondary ions with masses greater than the range chosen by the analyst, secondary ions with larger masses are deflected out of the flight path. Without this deflection, higher mass ions would appear in the next spectrum at a false position, creating false peaks in the spectrum.

A secondary electron detector is also present in the system. Because the sample is at ground potential, secondary electrons can leave the sample in either instrument polarity, but in negative ion polarity the extractor cone will attract them, and foil detection by the secondary electron detector. Electron images (most often used for tuning) are thus best obtained in positive ion mode.

The raster of the primary ion beam across the surface changes the origin of the secondary ions relative to the extractor ion optics. The resulting variation in the flight path can change the relative flight times. Deflection plates placed after the extractor optics in the flight path can be used to guide the secondary ion back down the center of the spectrometer. The voltages on those plates are varied with the predictable location of the primary ions within their raster. This trick, known as dynamic emittance matching (DEM), significantly improves the spectrometer’s mass resolution.

Once they are accelerated and lined up inside the spectrometer, the secondary ions are allowed to fly unaffected until they reach the electrostatic mirror that is the heart of the Reflectron TOF spectrometer. The modern Reflectron has more than one ring acting as an ion optic component, allowing for some fine-tuning of the energy compensation that the Reflectron provides. The Reflectron also needs some adjustment for use with the delayed extraction method described earlier and for analysis of insulators. The Reflectron is slightly canted so that instead of the ions being directly reflected back at the sample, their flight is shifted a few degrees, so that after traveling through another flight tube in which they pass unhindered, they reach the detector.

Before the detector, the secondary ions receive an added boost in energy known as postacceleration. The reason for this is that higher mass ions can end up reaching the detector with much lower velocities, and so even though their energies are the same as those with small masses, with their lower momentums they do not always create a pulse at the detector that is sufficient to be registered as a count in the spectrum. By applying anywhere from 5 to 20 KeV of extra energy (but most typically 10 KeV), the higher mass ions are not discriminated against at the detector, and thus the end up being detected as sensitively as lower mass ions.

The detector in the IonTof instruments is a channel plate and scintillator assembly that works in conjunction with a photomultiplier (Niehuis et al. 1987). The secondary ions hit the channel plate producing secondary electrons. The potential across the plate drives the electrons down the channels, accelerating the electrons as they go. The electrons cannot help but strike the walls of the channels, producing more electrons in a cascade that amplifies the signal. A pulse of electrons exits the plate and strikes the scintillator, producing a burst of light. The light exits the vacuum to reach the photomultiplier tube. The photomultiplier further amplifies the signal, transforming it into an electrical pulse that is subsequently captured by the electronics.

While improvements have been made to this system over the years, it is still true that there is a low but quite discernable background noise produced at this detector. For measurements requiring trace analysis and the best possible signal to noise ratio, this background noise can be significant. In these cases, a high primary ion current, which increases the signal leaving the noise unchanged, can be used to improve the signal to noise ratio. There is no upper limit on the secondary ion current this system will tolerate; so high primary ion currents can be used safely, although this will produce issues with instrument dead time (see below).

The detector is used in a pulse counting mode. The electrical pulse is first sent to a constant fraction discriminator (CFD), which determines whether a signal is intense enough to be counted (to represent a real secondary ion). Beyond checking to see if the signal meets a predetermined threshold, the CFD does not use the intensity of the signal. This is why whether one secondary ion reaches the detector or many do at the same time, the instrument will register only one pulse.

Furthermore, there is a brief period following the registration of a pulse during which the detector will be unable to register the next pulse. This is what is known as “dead time”. Up to a point, the data can be corrected for simultaneous arrivals of secondary ions with the same mass using statistical methods (Stephan 1994), although when the signals get so intense that a particular secondary ion is detected with most every pulse, no correction is possible. In such cases it is often possible to use a less intense secondary ion containing a minor isotope for the analysis. Alternatively, there is a hardware option that can solve this problem. Called “extended dynamic range” (EDR), the method involves the shunting of select secondary ions in the drift tube after the Reflectron so that they pass through a grid of known transparency. The secondary ions in a narrow mass range can then be attenuated by an exactly known factor of 10–100X. In practice, EDR is more useful in depth profile analyses, but it can also be used in situations where a major component needs to be monitored during trace analyses.

The characteristics of this detection system have implications for how the system tends to be used. The primary ion guns the instrument is paired with are optimized in many configurations for current, allowing rapid data collection. Because of the potentially significant background noise level, this also leads to the best signal to noise when using the system. The exceptions to this tendency towards running the system with high primary ion currents are when decent lateral resolution and high mass resolution are wanted simultaneously, or when the ultimate lateral resolution is needed. Running the system “hot” means planning acquisitions properly in advance. At high primary ion currents, the static SIMS limit is reached rather quickly. Dead time correction of the data is essential for almost any analysis.

Instead of an ion mirror, the triple focusing time of flight analyzer (TRIFT) spectrometer, used by Physical Electronics in their TOF-SIMS instruments has three electrostatic sector analyzers (ESAs) (Schueler, Sander, and Reed 1990a, 1990b). Although no longer used as an ion microscope in a commercial instrument, the TRIFT spectrometer has the characteristic that it projects an image of the sample onto the detector, thus allowing it to be used in an ion microscope. Like the Reflectron, it is used in Physical Electronics instruments as a microprobe (where the image information comes from the position of the primary ion raster instead). A schematic of the TRIFT spectrometer is shown below.

It provides energy compensation in a fashion similar to the Reflectron, with higher energy ions penetrating each 90 degree curve farther and thus taking a longer flight path than lower energy ions with the same m/e ratio, nearly exactly compensating for the higher velocities of the higher energy ions and thus producing identical flight times for identical secondary ions. A diagram showing how this energy compensation works is shown here.

Unlike the Reflectron, the TRIFT will not pass ions outside of a 240 eV or so range for which energy compensation is possible. This means that metastable ions are rarely detected in the TRIFT instrument. Since metastable peaks are more often a nuisance than not, this can be an advantage.

In the TRIFT instrument, the spectrometer is kept at ground potential, and it is the sample that is placed at a potential (usually 3 KeV) relative to the grounded extractor so as to provide for secondary ion acceleration into the spectrometer. The difference matters mainly with regard to situations in which that voltage needs to be pulsed. The time constant for the change in potential at the sample surface can vary from sample to sample, and in any case it involves charging up the entire sample holder, an object larger than the extraction element. The most common case in which such pulsing is required is for charge compensation. Variations in voltage have the effects noted earlier with the Reflectron, namely, altered flight times, mass resolution, transmission, and field of view.

When the TRIFT spectrometer is used as an ion microscope, the ions travel through the extractor directly into the grounded field free flight path leading to the first ESA. However, such a flight path length varies somewhat with the exact origin of the ions at the sample. Better mass resolution is obtained by using scanning plates to center the ions on a single path headed into the spectrometer, the same dynamic emittance trick (DEM) used in the Reflectron. This results in improved mass resolution. DEM can be turned off for instrument alignment and troubleshooting.

Secondary ions and electrons can be shunted into a detector by a voltage pulse placed on a deflection plate placed part way down the first part of the secondary ion flight path. The primary use for this detector is in tuning the primary ion source, but it can also be useful in efficiently capturing secondary electron images when the system is in negative ion mode. In positive ion mode, electrons are brought right back to the positively charged sample. Note that the polarity in which secondary electron images can be obtained is different for the two main spectrometer types (Reflectron and TRIFT).

While the TRIFT spectrometer has the advantage of better acceptance of ions with momentum in the plane of the sample as described above, the acceptance of such ions degrades the mass resolution. The user can choose to block these ions with more divergent flight paths by moving an aperture known as the contrast diagram (CD) into the flight path ahead of the first ESA. With the CD in place, mass resolution is improved with some transmission loss. On flat samples, this has a larger effect on atomic ions, which tend to have higher energy ranges to start with, and, therefore, represent a higher percentage of ions leaving the sample with significant velocities in the sample plane. Molecular species tend to be much less affected. Consequently it is also true that the CD improves mass resolution for atomic species more than it does for molecular fragments. As a corollary to this, the transmission loss when using the CD tends to be greater for atomic ions.

After rounding the 90-degree bend produced by the first ESA, the ion flight paths now vary with energy. The TRIFT has an optional slit available that can be moved in to partially block the beam, allowing secondary ions with more limited energy range to pass. You can block only the higher energy ions (which amounts to the same thing as reducing the Reflectron voltage allowing higher energy ions to hit the back wall), or you can move the slit into place selecting ions with higher energies. The energy slit has been used in studies of ion formation (Delcorte and Bertrand 1996), for example.

After the final two ESAs, the secondary ion beam is now pointed directly at the detector. The detector in the Physical Electronics TRIFT system is of a type that will be damaged by high secondary ion currents. A deflector plate placed along this line to the detector can be used as a high mass blanker, selectively removing very intense secondary ions that are of less interest (or which can be effectively replaced by ions containing less common isotopes), and thus allowing analyses with intense primary ion beams that otherwise will not be tolerated by the system.

Postacceleration is as necessary in the TRIFT as it is in the Reflectron. Similarly, it is placed just before the detector. Different though is the addition of additional raster plates in this region. To further reduce consumption of this more fragile detector, the secondary ion beam, which unaltered would be focused to a small spot on the detector, is rastered across the detector with a raster size and frequency that is unrelated to the primary ion beam raster at the sample. The result is that the secondary ions fall on the detector in a square region that can be steered independently by the user. Reduction of the raster size can improve mass resolution, but at the expense of the lifetime of the detector.

The first part of the detector is the same as in the Reflectron, a channel plate. But in the TRIFT, behind the first channel plate is a second channel plate, which magnifies the first pulse into a larger pulse of electrons, a pulse that can be directly evaluated by a constant fraction discriminator (CFD). As in the Reflectron, a pulse is counted as a single secondary ion, so the detector is effectively dead to further arrivals for a short time, secondary ions with the same mass to charge ratio arriving at nearly the same time. It is also the same for the binning of the pulses in time channels by the time to digital converter (TDC). For tuning and alignment purposes, a phosphorescent plate is also part of this detector assembly, along with a camera that can see the light it generates. The image of the secondary ions’ impacts on the detector allows the operator to ensure that the secondary ion raster at the detector is well centered on the “sweet spot” where mass resolution is best.

The double channel plate detector described here has characteristics that make for important differences in the way the TRIFT instrument is used in comparison with the Reflectron and its channel plate and scintillator detector. The detector can be damaged when secondary ion currents exceed 5 × 10⁴ counts per second. Consequently, the system is paired with primary ion sources that produce less current, and analyses tend to take longer than in the Reflectron. Dead time corrections of the data are less necessary and can be avoided entirely in this system where primary ion currents are routinely controlled to avoid high secondary ion currents. Lower data rates encourage real time monitoring of the data, since early hints in the ion images that, for example, the raster could be better centered on the area of interest, can be acted on while the analyst is still far from the static SIMS limit. Higher data rates can be obtained by blanking out the most intense ions. This is more of use when there are only a few of them, for example, the Si⁺ ion from a silicon wafer.

One of the perennial problems with using TOF-SIMS for depth profiling applications is that the pulsed primary ion beams used for analysis etch the sample only very slowly, precisely because they are pulsed. In any case, for inorganic depth profiling applications it is generally better to analyze with a preferred analysis beam while etching with a different beam. However, while the sample is being etched, the material removed is not analyzed, so in fact only a fraction of the material in the sample sees the analysis beam. When cluster beams showed promise for 3D molecular analysis, it still remained true that much of the sample being analyzed would have to be thrown away when a conventional TOF-SIMS instrument performs the analysis. This is particularly frustrating for analysts attempting to study biological samples, where there can be precious few analyte molecules present in the sample to start with.

The design of the J series instruments of Ionoptica, is such that continuous or nearly continuous beams can be used on the sample, and the pulsing needed for mass analysis is provided by the spectrometer itself (Fletcher et al. 2008); Sheraz 2010). This instrument effectively separates the creation and extraction of secondary ions from their analysis. The primary beam need not be pulsed, which can allow for better lateral resolution with the cluster ion guns that, when pulsed, are poor imaging sources. Even though these smaller spot cluster sources (with a specification as of this writing of 1.5 microns) have very low currents, the fact that they do not have to be pulsed still allows them to produce decent data rates. Topography at the sample does not play a role in the mass resolving power of the spectrometer. Ion extraction is optimized simply to extract secondary ions as efficiently as possible. A diagram of the first in the series, the J105 spectrometer, is shown below.

Source: Adapted from (Fletcher et al. 2008).

After extraction the secondary ions pass through an RF quadrupole filled with N2 gas, then through an electrostatic analyzer (ESA). The gas collisionally cools the secondary ions, reducing the energy spread induced by their extraction. The ESA filters the secondary ions so that their energy spread is reduced to less than 100 eV. This is clearly a step that can reduce the transmission of secondary ions. Further, the transmission effects will be mass dependent. If collisional cooling is optimized to allow transmission of high mass ions, low mass ions may not even make it to the ESA. If the cooling gas pressure is optimized to allow lower mass ions to exit the quadrupole, higher mass ions may not be cooled sufficiently to exit the ESA. In effect, this is one place in the spectrometer where the instrument needs to be optimized for the mass range of greatest interest in a given analysis. Note that the secondary ion beam remains continuous (or nearly so) at this point.

The ions then pass a deflector and enter the buncher. When the buncher is “filled,” it is fired. While it is being fired, the deflector temporarily prevents further ions from entering the buncher. This buncher is constructed with a series of electrodes. Each electrode is given a potential, 7 KeV to the electrode at the entrance to the buncher, down to 1 KeV at the last electrode before the exit. Like the buncher used in primary ion columns to reduce the pulse width of the ion gun, this buncher causes ions that have the same mass to charge ratio, which are caught within the buncher when it is fired, to arrive at a point past the exit of the buncher at a narrow point in time. Naturally, the same energies applied to ions with different mass to charge ratios will have them arrive at different times. The deflector goes off, and the buncher begins to fill again. The cycle repeats every ~100 μs or so, taking ~90 μs for the buncher to fill, and ~10 μs for the buncher to operate. The cycle time is also an adjustable parameter with implications for secondary ion transmission. The secondary ions all had 100 eV or so of energy to start with. Low mass ions will tend to be travelling faster than high mass ions. The first and fastest ions to enter the buncher at each cycle will overshoot the buncher before it is fired and be lost. The instrument could be cycled so fast that no low mass ions are lost, but then the mass range and sensitivity to high mass ions would be affected. Instead, the instrument, by design, is intended to maximize the transmission and detection of the higher mass ions that are more likely to be molecular ions or large fragments. The result is transmission loss of lower mass ions.

The secondary ions are now focused in time just at the entrance to the next element in the spectrometer, a quadratic field Reflectron. In fact, a detector in this position would record a complete mass spectrum, albeit one with less resolution than that obtained using the added Reflectron. Unlike the Reflectron described previously, this portion of the J series instruments are not just free field regions with electrostatic mirrors. Instead, the electrostatic field increases as the square of the distance from this Reflectron’s entrance. The quadratic field Reflectron (also called a Harmonic Field TOF Reflectron) is needed because the energy spread of the secondary ions at the Reflectron’s entrance is now 6 KeV (the difference in potential between the maximum and minimum voltages the secondary ions see inside the buncher).

The detector used in this instrument is similar to what is used with the more common Reflectron-based instruments; a multichannel plate, a scintillator, and a photomultiplier. Pulse counting is still the normal mode in which this detector is used, so in the case where saturation due to intense secondary ion yields is found, the solution is to reduce the primary ion current. The “DC” nature of this instrument increases the chance of such a problem, while the reduced transmission for lower mass ions reduces it.

IonTof in conjunction with the UK's National Physical Labs (NPL) has designed a hybrid instrument with two spectrometers on it, one a Reflectron TOF-SIMS, and the other an Orbitrap. The Orbitrap is not a time of flight instrument, but given that it is measuring secondary ions in an instrument that is otherwise a TOF-SIMS, it must be discussed here. The Orbitrap delivers orders of magnitude higher mass resolution and accuracy than is possible with any TOF analyzer, and since it does not require that the secondary ions be pulsed, it can analyze ions produced by a DC beam such as an otherwise optimized primary ion source can produce. This lends it particularly suited to depth profiling with a GCIB.

The Orbitrap is, however, also orders of magnitude slower, so that imaging in particular will take a good deal of instrument time. The 2 methods can be used in conjunction. An image can be produced with the TOF, and then a DC beam of a cluster beam be used to erode the sample, with the Orbitrap taking data during that phase, then an image can be obtained using the TOF on the next layer. However, if the Orbitrap's resolution is required for proper imaging of ions with similar mass, very long acquisitions will be required.