Secondary Ion Mass Spectrometry

Anal. Chem. 2008, 80, 7194–7197 Secondary Ion Mass Spectrometry Jennifer Griffiths An old dog with some nifty new tricks. Six years ago, things were looking bleak in the secondary ion MS (SIMS) community. According to researchers, the field had reached maturity, and not much was new. The AC SIMS Product Review that was published around that time (2002, 74, 335 A-341 A) even began with the line, “If it ain’t broke, don’t fix it.” Joe Gardella of the University at Buffalo, State University of New York, says that shortly thereafter, at the 15th International Conference on SIMS in 2003, many in the field wondered if the technique had run out of novel applications. “There was a palpable sense at that point that maybe the conference couldn’t stand alone anymore,” he says. “That is, maybe the technique was routine enough that you didn’t need a separate conference to talk about it.” Then, suddenly, everything turned around. “The field has certainly undergone a seismic shift, both in intensity and direction,” says Nick Winograd of Pennsylvania State University. “The seeds of it were there in 2002, but I don’t think many people were quite prepared for the rapidity of the change.” Two advances that were just beginning to be widely adopted that year have taken over and rejuvenated the field: cluster ion beams and TOFMS analyzers. SIMS traditionally has been limited to atomic analyses of surfaces, but these two techniques in combination provide a route to a long-sought-after goal in the SIMS community: analysis of intact complex organic molecules. In light of these changes, a Product Review update is in order. The tables that accompany this article are divided into two categories: instruments offering quadrupole or magnetic-sector MS (Table 1) and those offering TOFMS (Table 2). These tables are not meant to be comprehensive sources of product information; please contact the manufacturers for more details. THE BASICS SIMS is a surface analysis technique in which a beam of “primary ions”straditionally Ar+, Ga+, or alkali metal ionssare shot at a sample’s surface; the primary ions transfer their energy to molecules on the surface and dislodge “secondary ions”. (Secondary ions are either positive or negative, depending on the primary ions’ identity; neutral species are also ejected, but these cannot be analyzed by MS unless more convoluted measures are taken.) 7194 Analytical Chemistry, Vol. 80, No. 19, October 1, 2008 “It’s rather like playing molecular pool,” says Fraser Reich of Kore Technology (U.K.). “The cue ball goes into the material, and it sputterssit lifts off material that’s characteristic of the top surface.” Once the particles are in flight, an electric field directs them into an MS analyzer. These come in a variety of flavors: at first, quadrupole and magnetic-sector instruments dominated the field, but more recently, TOF instruments have started to take over. Each type of mass spectrometer has advantages and disadvantages. For example, according to John Eccles of Millbrook (U.K.), a big advantage of a TOF system is that you can acquire a full mass spectrum in one operation. “So, rather than having to scan through the spectrum to build it up, you can acquire it simultaneously, which means it’s much more efficient than a quadrupole [or magnetic-sector] system for analyzing small areas.” Parallel detection also means the sample needs to be hit less often with the destructive primary ion beam; therefore, TOF SIMS is much better suited to analyzing fragile complex organic molecules than are the other techniques. Quadrupole and magnetic-sector instruments, on the other hand, are very good for looking at individual atoms. These 10.1021/ac801528u  2008 American Chemical Society Published on Web 08/29/2008 Table 1. Selected quadrupole and magnetic-sector instruments.a Product Company Approximate price (U.S.D.) Applications IMS 7f/NanoSIMS50 Cameca Instruments 208-442-6559 www.cameca.com Contact vendor Depth profiling, imaging, layer characterization, isotope ratios, trace elements Primary ion source Vacuum chamber features O2, Cs Mass analyzer Magnetic sector Mass range Mass resolving power Detector 0-360 u Titanium sublimation pump with ion or turbomolecular pumps MAXIM SIMS Workstation MiniSIMS alpha Hiden Analytical www.hidenanalytical.com Physical Electronics 952-828-6100 www.phi.com Contact vendor Depth profiling, imaging, static SIMS, sputtered neutral MS (SNMS), desorbed gas analysis, thin films, glass coatings, hard coatings, semiconductors, cleanliness monitoring, failure analysis, research, and production control O2, Cs, Ar, Xe Static, imaging, or dynamic SIMS Semiconductors, glass, photovoltaics, thin films Ga+ Spherical, bakeable, ultrahigh-vacuum (UHV) chamber using industry-standard components; 15 ports with spare ports for customization or addition of optional detectors; ion or electron guns MAXIM SIMS/SNMS analyzer fitted with 9-mm-triple-filter Hiden quadrupole Optional: 1 to 300, 510, or 1000 u High-vacuum, automated sample loading Cs+ surface ionization source, O2+ duoplasmatron source Five-axis stage, three ion gun ports, charge neutralization Quadrupole Quadrupole 3-300 u 1-340 u Unit mass resolution Unit mass resolution Faraday cup for analog and channel electron multiplier for pulse counting Windows-based PC instrument control with dedicated SIMetric SIMS depth-profiling software 390,000 >20,000 Electron multiplier, Faraday cup Channeltron Channeltron Hardware/ software PC with MS Windows-based control and data treatment software; imaging software capabilities Hiden MasSoft running on PC provides visual build of experiment flow; data are easily exported to standard processing packages PC- and Windows-based software for data acquisition, display, and processing Options Charge compensation for insulating samples; rotating stage for rough analysis; low-energy kit for excellent depth resolution Positive or negative secondary ion analysis; low depth resolution at high sensitivity High sensitivity for light elements; excellent lateral resolution Special features a ADEPT-1010 Millbrook Instruments +44-1254-699606 www.minisims.com 200,000 Highly flexible design to permit a very wide range of samples Electron gun for charge neutralization; enhanced sample handling for multiple samples; spectral library Tabletop design; automated operation Gallium liquid-metal ion gun (LMIG) Extremely high vacuum (<8 × 1011 torr) enables low detection limits for atmospherics (C, O, H) Low extraction field enables easy charge neutralization Contact vendors for full product lines. instruments were developed mainly for use in the semiconductor industry to detect small amounts of dopant in silicon chips. That industry required high sensitivity and an instrument capable not only of analyzing the top surface (static SIMS) but also of acquiring a “depth profile” down into the sample (dynamic SIMS). Graham Cooke of Hiden (U.K.) says that when used in industry, magnetic-sector or quadrupole SIMS is usually one of an array of surface techniques used to analyze a sample. “It’s not often that you only use SIMS by itself to solve a problem,” he says. “Gone are the days where people used to have one analysis technique in each lab, and they wouldn’t talk to each other.” Researchers now combine SIMS with X-ray photoelectron spectroscopy, electron microscopy, and perhaps transmission electron microscopy. “No one technique has [all of] the depth resolution, the sensitivity, and the lateral resolution that you could possibly want,” he adds. Some of these surface analysis techniques are now being combined into one instrument, and Cooke points out that quadrupole analyzers are the best choice for this purpose. That’s because they use a relatively weak electric field to funnel ions into the mass spectrometer. (The stronger electric field of a TOF instrument could, for example, deflect incoming electrons from other techniques away from the target surface.) WHAT’S NEW? Although magnetic-sector and quadrupole instruments still definitely have important roles to play in industry, researchers in the field say that the current “hot” SIMS research areas are cluster ion sources and TOFMSsusually used in combination to look at complex organic molecules. For example, the two technologies have caused an explosion in biological SIMS research, says Nathan Havercroft of ION-TOF. “Six years ago it was never even Analytical Chemistry, Vol. 80, No. 19, October 1, 2008 7195 Table 2. Selected TOF instruments.a Product J105 Chemical Imager TOF.SIMS 5 Series SurfaceSeer Ionoptika ION-TOF GmbH Kore Technology +44-2380-270-735 +49-251-1622140 +44-01353-653030 www.ionoptika.com www.iontof.com www.kore.co.uk Approximate 1,500,000 (depending on Contact vendor 290,000 price configuration) (U.S.D.) Applications Static and dynamic SIMS, Static or dynamic SIMS; Surface MS; ideal for 3D analysis, identification surface spectroscopy; quality control or and mapping of organic ion mapping; depth profiling troubleshooting for the molecules and 3D analysis in performance of materials fields such as materials that will be coated, characterization, polymers, printed, or bonded microelectronics, together, where generally catalysis, biomaterials, the condition of the and failure analysis; surface of the items basic research is of critical importance Pulsed 5 keV inert gas Bismuth field emission Primary ion Gold LMIG, C60 (Ar+) cluster ion source, source oxygen source, cesium source, C60 Vacuum Precision three-axis stage Oil- and cooling-waterMultiple-port UHV chamber with heating and cooling; free pumping system; analysis vacuum chamber; features ports available for horizontal and grounded high-vacuum gauge add-on techniques sample stage configuration; monitors the pressure in sample size up to 300 mm; the analytical chamber and up to three ion columns shuts down high voltages with up to five sources can if the pressure be fitted rises beyond a set point Mass TOF with dual time Nonlinear reflectron TOF (reflectron) analyzer focus analyzer, full mass resolution at full transmission Mass e5000 Da 1->10,000 u >1000 m/z range Mass >10,000 >12,000 at 29 u 1000 m/∆m resolving >18,000 at 300 u power Detector Microchannel plate with Secondary electron Dual microchannel plate preamplifier; secondary detector and secondary detector electron detector ion detectors: microchannel plate/scintillator/ photomultiplier tube type Hardware/ Full software control of PC control using GRAMS/AI software by software instrument systems; Windows XP; Thermo Scientific with data acquisition data-reduction Kore Technology mass and display software for imaging, spectral extensions profiling, and 3D analysis; high-massresolution spectral library Options Bright duoplasmatron ion Temperature-controlled Multiple sample holder to source, 30 kV or floating heating and cooling in permit analysis of more low-energy ion gun load-lock and analysis than one sample per chamber sample load; optical (-150 to +600 °C); microscope/camera; G-SIMS with dedicated additional gun for Bi/Mn emitter; depth profiling customized sample preparation chambers Special 3D imaging of All-purpose Bi source Compact, very simple to features biomolecules inside with high cluster use; sample turnaround cells, MS/MS currents; modular design 5 min; positive and open to retrofits; negative SIMS; sputter live sample viewing by cleaning facility two cameras; raw data stored for retrospective analysis Company a MiniSIMS ToF nanoTOF Millbrook Instruments +44-1254-699606 www.minisims.com 400,000 Physical Electronics 952-828-6100 www.phi.com Contact vendor Static, imaging, and dynamic SIMS Polymers, semiconductors, biomedical devices, pharmaceuticals, biological tissues, minerals Ga+ LMIG High vacuum, automated sample loading Five-axis stage, four ion gun ports; charge neutralization; air shock vibration isolation TOF reflectron (secondary ion beam pulsing) TOF 1-1200 1-10,000 u 650 m/∆m >10,000 m/∆m Channel plate detector Dual channel plate PC- and Windows-based software for data acquisition, display, and processing Windows-based PC instrument control with dedicated WinCadence TOF SIMS software for acquiring spectra, 2D and 3D images, and depth profiles Electron gun for charge Hot/cold stage, LMIG neutralization; enhanced gold cluster emitter, sample handling C60 ion gun, gas ion gun, oxygen leak, Cs+ ion for multiple samples; spectral library gun, 300 mm wafer stage Tabletop design, automated operation Stage design shuttles between intro chamber and analysis position automatically; air shock vibration isolation for high-magnification imaging Contact vendors for full product lines. really talked about. There were a couple of people who were just starting, but now we are seeing many groups that are involved in SIMS on biological samplessit’s great!” 7196 Analytical Chemistry, Vol. 80, No. 19, October 1, 2008 Cluster ion sources are exactly what their name implies: rather than a single atom as a primary ion, these charged particles are made up of multiple atoms. “The reason that these clusters are so important... is that [with] the other [monatomic] projectiles, if you take a molecular surfacesa polymer, biomolecules, any kind of organic moleculesand you hit it with these energetic beams, it blows the molecules apart,” says Winograd. “You get some of them desorbed that you can see in your mass spectrometer, but it leaves a great deal of chemical damage behind. And eventually your surface is converted into junk, and the signal goes away.” The cluster ion beams, on the other hand, can lift molecules with molecular weights of several thousand daltons off of the surface intact. Currently, researchers can choose from four types of cluster ion beams: several different types of Bi or Au polyatomic clusters, C60+, and SF5+. Each type has advantages and disadvantages, and some are used more commonly than others. For example, the metal polyatomic ions are important for the emerging field of SIMS imaging. SIMS has been adapted for imaging in much the same way that MALDI has been. “You can raster scan [the incoming primary ion beam] over the surface, and in that way you can build up an image of the distribution of a particular secondary ion generated from the surface, which is essentially a chemical map,” explains Eccles. “One of the advantages of SIMS is that you have excellent S/N in the data, so you can do that process very quickly, and you can generate images in literally just 5 or 10 seconds.” And with a TOF analyzer, a single acquired image actually stores a complete mass spectrum at each pixel, so multiple chemical maps each showing an individual species can be generated retrospectively. SIMS and MALDI imaging are quite complementary. For example, using SIMS, “our lateral resolution is much higher. We’re down in the hundred nanometer range for imaging, whereas MALDI is still stuck in the tens or twenties of microns,” says Havercroft. “But we’re never going to have the same upper mass range as MALDI.” (SIMS can analyze molecules up to ∼10,000 Da, whereas MALDI’s mass limit is in the hundreds of thousands of daltons.) As a result, many researchers envision the two techniques merging into a powerful method to simultaneously obtain information. “MALDI imaging focuses on peptides, and SIMS imaging focuses on lipid drugs, so there’s a great potential combination there,” says Winograd. “I think there’s a great opportunity there for expanding the sphere of influence of SIMS if we can get the MALDI people interested in including our technology.” Another new application of cluster ion beams and TOF SIMS is what is called “molecular” depth profiling. (For this purpose, the C60+and SF5+ ion sources are more often used.) In depth profiling, the intensity of the incoming ion beam is increased to the point that rather than removing just a small amount of material for analysis, a larger quantity is blasted off the surface layer by layer. Each layer is analyzed separately by MS, and a profile of analyte versus depth is constructed. “Elemental depth profiling is well established and used heavily in industry, but organic [molecule] depth profiling is new,” says Scott Bryan of Physical Electronics. “The research efforts now are to optimize these cluster ion beams for minimizing the damage to the organics while you are depth profiling.... You have to be able to maintain enough of the molecular information to monitor the concentration of these molecules as a function of depth and not reduce them down to elemental carbon.” The new ability to depth profile more complex species has many applications. For example, says Havercroft, academic groups recently have done some very exciting work on the depth profiling of polymers. “[This] has been a little bit of a holy grail for SIMS in general,” he says. “We’re now finding you can actually profile through polymers and monitor the chemical changes with depth.” There are also biological applications for molecular depth profiling. “The other thing that people are looking at with SIMS is the possibility of actually depth profiling with these cluster ion sources through tissue,” says Gardella. “So, not only do you get a spatial distribution that’s subcellular but you could actually look at different portions of a tissue section.” Winograd also believes that SIMS imaging and depth profiling could some day be combined. “I have this dream of taking a single biological cell, preparing it without changing it so I can put it in the mass spectrometer, and then lifting it directlysentirely preserving all the x, y, z informationsso you can get a complete chemical analysis of the cell in three dimensions,” he says. LOOKING AHEAD Despite its recent rejuvenation, the SIMS field still has several potential areas for improvement if it is to become a more widely used technique. One big issue for industrial users is the cost and complexity of the instruments. “It’s hard to convince companies that this is a really cool technique [when] you need to spend $2 million, and then you need to have three Ph.D.s and other professionals to keep the infrastructure up,” says Gardella. Several companies offer instruments that address these issues, and each costs a fraction of a typical SIMS system. “We’re trying to do our bit to make SIMS available for customers with less money,” says Reich. “I think otherwise it will remain a fairly niche technique for those 15 or 20 [buyers] a year that can afford the big price.” In academia, where the trend seems to be moving toward biological applications, researchers say that one big challenge in the near future is going to come from the huge mound of data produced by a TOF SIMS imaging experiment. “Normally when you take an image, it’s 256 × 256 pixelssthat’s 65,536 mass spectra per image.... Each mass spectrum is ∼100,000 data points... which is a terabyte [of data], or a fraction thereof,” says Winograd. “There will be quite a bit of need for data compression and capabilities of dealing with all this information.” Inroads are already being made toward solving this problem, which will only grow in the future. One final area in which SIMS has recently taken a great leap forward is tandem MS (which will add to the data-handling challenge). In 2002, no commercially available system offered this capability, but researchers hoped that one would become available. “To do bioanalysis, you’ve got to have the tandem MS capabilities,” Winograd says. Only this year did that dream become a reality with Ionoptika’s new SIMS instrument that includes MS/MS functionality. Why did it take so long? “I think it’s only becoming necessary with the advent of cluster beams,” says Rowland Hill of Ionoptika. “Before 2000, you could only really look at elementals with SIMS, because the atomic primary beams smash all the molecules up. Now that we’ve got cluster beams, there’s a drive to do better mass spectrometry on organics.” The potential of this type of analysis will become clear once more of these instruments are installed in laboratories. Jennifer Griffiths is a senior associate editor of Analytical Chemistry. AC801528U Analytical Chemistry, Vol. 80, No. 19, October 1, 2008 7197