SIMS: Secondary Ion Mass Spectrometry

Transcription

1 SIMS: Secondary Ion Mass Spectrometry SIMS is based on the emission of ions (secondary ions) from the first monolayers of a solid surface after bombardment by high energy primary ions. The cascade collision model is used to explain how the emission of secondary ions (along with neutrals, photons and electrons) occurs: The process was discovered by J.J. Thomson already in 1910, yet only in the 1940s-1950s the first two prototypes of SIMS instruments were developed independently (University of Wien, RCA Laboratories at Princeton). Further instruments (used also by NASA for the analysis of moon rocks) were developed in the 1960s, when the commercial production was started by CAMECA (France).

2 Sputter yield (number of emitted particles per primary ion hitting the sample surface): 1-10 particles/primary ion SIMS main features Secondary ion yield (number of ions emitted per primary ion): ions/primary ion Positive ions, ref. Silicon Negative ions, ref. Silicon

3 Dependence of secondary ion charge on the type of the primary ion: After implanting into the sample surface Cs + primary ions lower the sample work function (due to the low Cs electronegativity), thus the emission of secondary electrons is increased; their capture by sputtered atoms (or clusters) with high electron affinity leads to the generation of negative ions. O 2+ ions may lead to the formation of metal-oxygen bonds at the surface of metal samples. During sputtering such bonds can be broken, thus favoring the generation of positively charged metal ions.

4 Energy distribution of secondary ions / ev Elemental ions are characterized only by translational energy. Molecular ions also have vibrational and rotational energies. Selecting a narrow range of secondary ion energies is very important to assure a good resolution during the acquisition of their mass spectra.

5 General layout of a typical SIMS instrument Cameca TM SIMS spectrometer Adapted from: Valley et al., Review in Econ. Geology, 7, 1998, 73-97

6 Dynamic vs Static SIMS Dynamic SIMS: primary ions with KeV energies and A-mA/cm 2 fluxes (at least ions/cm 2 ). Under these conditions the sample surface is continuously sputtered and destructive depth profiles can be easily obtained. Static SIMS: primary ions with KeV energies and na/cm 2 fluxes (less than ions/cm 2 ). In this case the time required for the erosion of a sample monolayer (ca 1 nm) can be quite long (minutes/a few hours), due to the high probability that a new area of the sample surface is striked each time by a primary ion: low dose ( ions/cm 2 ) high dose (10 14 ions/cm 2 )

7 Dynamic SIMS: peculiar aspects Concentration profiles. Due to the high ion doses adopted, that lead to a fast sputtering of the sample surface, Dynamic SIMS can be easily exploited to obtain concentration profiles: In this case sputtering and analysis occur contemporarily, since secondary ions included among sputtered particles bear directly chemical information on the sample. Due to the higher sensitivity of SIMS the presence of very thin interfaces can be unveiled.

8 Quantification. Dynamic SIMS is frequently used for the elemental quantitative analysis of surfaces. As in XPS, the intensity of a SIMS signal (I M ) can be related to the atomic concentration of the corresponding element (c M ) through several parameters: I M = J A S M T D c M where: J p = primary ion beam current A = analyzed surface S = sputtering yield M = secondary ion yield for element M T = SIMS spectrometer transmission (the ratio between the number of ion reaching the detector and that of ions leaving the sample) D = detector conversion factor

9 Since many of the described parameters are usually unknown a relative quantitation, based on the approach of relative sensitivity factors (RSF), like in XPS, is usually achieved. The RSF for an element E, referred to a reference element R, is expressed as follows: RSF E = (J A S R T R D R )/(J A S E T E D E ) and can be evaluated experimentally on a sample containing both elements E and R at a fixed atomic concentration, using the following formula: I R /c R = RSF E I E /c E When a real sample is analyzed, provided c R is known, c E can be estimated once the signal intensities I E e I R are measured: c E = RSF E I E /I R c R

10 Dynamic SIMS: primary ion sources In a duoplasmatron source an oxygen plasma is generated by the interaction between O 2 molecules and electrons emitted from a cathode and accelerated towards an anode. The electron path length is increased by applying a magnetic field, thus increasing the probability of electronmolecule interaction. An appropriate choice of the extraction electrode potential determines if O 2+, adopted as primary ions, or O - ions, used to neutralize the positive charge accumulating on the sample surface during SIMS analysis, are emitted from the duoplasmatron source.

11 In cesium ionization sources cesium is vaporized in a reservoir and its vapors are pushed towards a porous tungsten plug. Due to Cs low ionization potential electrons are transferred from Cs atoms to the tungsten conduction band. The resulting Cs + ions are subsequently ejected from the source using an extraction electrode.

12 Dynamic SIMS: some applications Depth profile of a silicon oxynitride (an antireflecting material) layer on silicon. Quantitative analysis in a concentration profile: quantification of Cd in HgCdTe layers having different composition.

13 Static SIMS: peculiar aspects Static SIMS was developed in 1969 by Benninghoven at the University of Munster (Germany) and was originally based on quadrupole mass analyzers, yet ToF analyzers soon emerged as the most efficient analyzers for this technique nm Considering maximum primary ion dose densities around ions/cm 2 (the so-called static limit), a sputter yield equal to 1 and monolayer densities of atoms or molecules/cm 2 erosion times of s can be estimated for single monolayers. Due to its high transmission a ToF analyzer enables a better detection of the low numbers of secondary ions generated during Static SIMS experiments.

14 Static SIMS with a ToF mass analyzer In order to reduce the surface damage a pulsed ion beam, raster scanned on the area of interest, is usually adopted in ToF-SIMS instruments for operation under static conditions:

15 The energy released by multiple recoils following the primary ion impact on the sample surface decreases quite rapidly with the distance from the point of impact: The nature of neutrals/ions emitted around the point of impact is then related to the distance. Large molecular ions will be emitted relatively far from the point of impact:

16 Application of Static SIMS to polymer analysis Cluster of peaks related to monomeric units or their fragments can be detected in the low m/z range of ToF-SIMS spectra for polymers, often providing useful chemical fingerprints. J. Lausmaa, Dep. Chemistry and Mat. Tech. Swedish National Testing and Research Institute

17 The cationization effect due to Ag + can be exploited to enhance the desorption of entire polymeric chains and even of additives that can be hard to ionize: Polystyrene 2200 dissolved in chloroform and deposited as monolayer on silver foil Tris-(2,4-di-tert-butylphenyl)-phosphite (an antioxidant) J. Lausmaa, Dep. Chemistry and Mat. Tech. Swedish National Testing and Research Institute

18 A calculation of the number of monomeric units and even of the mass of endgroups can be easily made starting from the experimental m/z values. J. Lausmaa, Dep. Chemistry and Mat. Tech. Swedish National Testing and Research Institute

19 The Ag + -mediated cationization can be also exploited to detect hardly ionizable organic molecules, deposited as thin films on a Ag substrate: J. Lausmaa, Dep. Chemistry and Mat. Tech. Swedish National Testing and Research Institute

20 The high resolving power provided by the ToF analyzer can be very useful to confirm the identity of peaks: J. Lausmaa, Dep. Chemistry and Mat. Tech. Swedish National Testing and Research Institute

21 Cluster ion sources for SIMS After impacting on the sample surface a cluster (polyatomic) primary ion breaks apart and each of its atoms retains a fraction of the ion initial energy. This results in a significant reduction of the penetration depth, since the latter is proportional to impact energy. Moreover, the sputtering yield (SY) is significantly enhanced, since there are more atoms bombarding the sample simultaneously:

22 Several cluster ions have been tested as primary ions in Static-SIMS in the last two decades: SF 5+, C m H n+, C m F n+, Au n+, Bi n+, Ar n+, C 60+. The influence on the secondary ionization yield of the number of atoms included in the primary ion structure can be appreciated in this example: 10-2 C 10 H 8 + C 6 F 6 + C 10 F 8 + Au 2 + Yield (219 u) CO 2 + O 2 O + C 7 H 7 + Ar + SF 5 + Xe + Au Primary ion mass (u)

23 SF 5+ ions can be easily generated using a source with an electron impact design, where gaseous SF 6 is leaked into an ionization chamber and bombarded with electrons so that SF n+ ions are created, where n = 1 5; SF 5+ are the most abundant ones. C 60+ ions are generated according to a similar principle, in sources like that shown in the figure, starting from C 60 vapour.

24 The use of cluster ions leads to a dramatic Improvement in the quality of SIMS depth profiles on organic layers, compared to monoatomic ions: Depth profiles obtained under Dynamic SIMS conditions from a 180 nm thick glutamate film vapor-deposited on silicon. About ions/cm 2 were required to reach the substrate. G. Gillen, S. Robertson, Rapid Commun. Mass Spectrom. 12, 1998,

25 In the following example the effect of C 60+ primary ions fluence on the signals detected from a 352 nm film obtained by depositing a solution of the GGYR peptide in trehalose (1:100 ratio) on silicon is evidenced: trehalose J. Cheng, N. Winograd, Anal. Chem. 77, 2005,

26 More recently, special Ar n+ cluster ions, with n = , have been generated through electron ionization of Ar neutral clusters produced by cooling during supersonic expansion: flight Neutral Ar clusters are ionized by electron ionization in the ionization chamber and accelerated by 5-10 kv voltages. The magnet removes Ar + ions and small ion components. Cluster ion sizes are selected by exploiting the time of flight between the two deflectors.

27 Using Ar ions the first ToF-SIMS spectra of intact proteins ever reported have been obtained: Cytochrome C Lysozyme Chymotripsin K. Mochiji, J. Anal. Bioanal. Techniques, S2, 2011, 1-5

28 Using Ar cluster ions ToF-SIMS spectra of peptides exhibiting the typical ion series observed during CID-MS/MS of such molecules, have been obtained, as shown in the figure for [Val 5 ]-Angiotensin I: S. Aoyagi et al., Anal. Bioanal. Chem. 405, 2013,

29 Chemical imaging by SIMS (ion microprobe) A focused ion beam is scanned over a chemically heterogenous surface The mass spectrum from total area shows which species are present Chemical images can be constructed from raw data file, showing where each species is located:

30 MS-based imaging: a comparison between DESI, MALDI and SIMS SIMS provides the best lateral resolution but its application to protein imaging is difficult, due to structural damage occurring even under static conditions.

31 Ion microprobe Dynamic SIMS: the CAMECA NanoSIMS 50L

32 A more detailed picture of the CAMECA NanoSIMS instrument.

33 The NanoSIMS is equipped with peculiar co-linear optics, capable of focusing the primary ions with high quality and collecting most of the secondary ions simultaneously : The shorter distance between the probe forming and the extraction lenses provides a smaller spot size, a higher collection efficiency and reduction of the secondary ion beam broadening. The normal incidence of primary ions also minimizes shadowing effects. The only constrain is due to the opposite polarity of primary and secondary ions (Cs + /negative ions, O - /positive ions).

34 NanoSIMS 50L: an application SIMS images of the section of an arthery at different magnifications.

35 Liquid metal ion guns (LMIGs) for Imaging SIMS In LMIGs primary ions are emitted from the surface of a thin liquid metal film covering a needle with a tip radius of 5-10 m. A potential difference of some kv between the needle and an extractor can lead to electrical fields, at the liquid apex, as high as 10 8 V/cm. Metal atoms near the needle apex become ionized and their ions are released from the tip of a Taylor cone.

36 Gallium has been the first metal used in LMI sources due to its low melting point (30 C) and vapor pressure and to the low [Ga 2+ ]/[Ga + ] ratio (10-4 ) observed. Experiments have also shown that: a threshold extraction voltage (ca. 2 kv) needs to be overcome to trigger ion emission the angular distribution of emission current is rather uniform the energy spread of emitted ions is large (ca. 15 ev), potentially leading to aberrations in the ion optical systems.

37 Use of cluster ion beams in Imaging SIMS Focused cluster ion beams have been obtained using LMIGs loaded with metals different from Gallium: Gold Au n+ ions, with n = 1-3, were obtained in a LMI source already in th early 1990s, yet the first commercial Au cluster source was introduced in Quite higher temperatures (about 1100 C) are required for gold melting, compared to gallium, yet lateral resolutions as low as 150 nm can be achieved using Au 3+ ions (compared to about 400 nm for Ga + ). The melting temperature can be decreased by using special AuSiBe or AuGe alloys. Bismuth LMI sources loaded with Bi were introduced in the 2004; their design is similar to that of liquid Au sources, although Bi can be melted at a quite lower temperature (ca. 270 C). Compared to Au-based sources, Bi has proved to be beneficial due to the emission of larger cluster ions, Bi n+ with n = 1-7, with higher currents. The use of Bi cluster ions results in more intense molecular ion images with lateral resolutions comparable to those achieved with Au cluster ions.

38 Au Sales statistics reported by the IonToF manufacturer in 2005 showed how rapidly the Bi-based source emerged among LMI sources, overcoming Ga- and Au-based sources in just one year. At that time the C 60+ source was still in the early stages of application. Nowadays it is a powerful alternative to LMI sources also in the field of Imaging ToF-SIMS.

39 Increase in Imaging ToF-SIMS applications during the last two decades The combination between cluster ion sources and ToF analyzers has led to a remarkable increase in the number of Imaging SIMS applications in the last 15 years, with biomolecules and cells/tissues representing the most relevant sample types. J.C. Vickerman, D. Briggs, ToF-SIMS: Materials Analysis by Mass Spectrometry, 2013, Chapt. 18

40 Applications of Imaging ToF-SIMS: micropatterned surfaces Ion imaging of micropatterned CH 3 - and COOH-terminated thiols on a gold surface (stripes width: 40 and 60 m, respectively): Au C 2 H 3 (CH 3 -) C 2 H 3 O (COOH-) The signal intensity on each ion channel increases while going from black to yellow, thus a clear separation between the polymer stripes is observed.

41 Applications of Imaging ToF-SIMS: cell imprinting Cell cultures are transferred on modified glass surfaces, enhancing cellular adhesion. A silver block is then deposited onto the cells. Cell membrane/cytoplasma components stick to the silver block surface and once this is removed a layer of cellular material is ready for subsequent Imaging ToF-SIMS analysis. Cationization by Ag + ions reduces fragmentation and enables higher secondary ion yields, thus improving sensitivity.

42 ToF-SIMS spectrum obtained after an experiment of blood cell imprinting on a silver block: ph = phosphocoline ion (m/z 184), arising from the fragmentation of phosphatydilcholine, the major components of eucariotic cell membranes. ch = cholesterol, detected as a monomer or a dimeic cluster after cationization by Ag + P. Sjovall et al., Anal. Chem., 75, 2003,

43 SEM image of a human leucocyte deposited on glass ToF.SIMS images obtained from leucocyte imprints on silver: CH 2 =N + H 2 (proteins/dna) phosphocholine ion cholesterol-ag + P. Sjovall et al., Anal. Chem., 75, 2003,

44 Tissue analysis by ToF-SIMS Freeze-dried tissue/organ slices can be analyzed by Imaging-ToF SIMS (ion microprobe) using cluster ion sources to enhance the generation of molecular secondary ions: As many images of the same sample can be obtained as the number of detectable secondary ions. D. Touboul et al. in J.C. Vickerman and D. Briggs, TOF-SIMS: Materials Analysis by Mass Spectrometry, IM Publications LLD, Chapter 22

45 Impressive images were obtained by ToF-SIMS with Au 3+ primary ions from a mouse brain cross-section: Adapted from: P. Sjovall, J. Lausmaa, B. Johansson, Anal. Chem., 76, 2004,

46 Similar images have been obtained using Bi 3+ primary ions: 255/283 = C16/C18 carboxylates 771 = phospholipid; 892 = triglyceride D. Touboul et al., J. Am. Soc. Mass Spectrom. 16, 2005,

47 The last frontier of ToF-SIMS: 3D imaging Tridimensional reconstructions of the distribution of molecular species can be obtained by using special programs to align 2D images retrieved at different depths from the same sample using Imaging ToF-SIMS with cluster primary ions. Lateral and in-depth distribution of acetaminophen (m/z 152 ion) embedded into a polylactic acid film for drug delivery purposes. Primary ion: SF 5 + Field of view : m 2 G. Gillen et al., Appl. Surf. Sci., 252, 2006,