Comparison of Secondary Ion Mass Spectrometry (Sims) with Electron Microprobe Analysis (Epma) and Other Thin Film Analytical Methods H

COMPARISON OF SECONDARY ION MASS SPECTROMETRY (SIMS) WITH ELECTRON MICROPROBE ANALYSIS (EPMA) AND OTHER THIN FILM ANALYTICAL METHODS H. Werner, A. von Rosenstiel

To cite this version:

H. Werner, A. von Rosenstiel. COMPARISON OF SECONDARY ION MASS SPECTROME- TRY (SIMS) WITH ELECTRON MICROPROBE ANALYSIS (EPMA) AND OTHER THIN FILM ANALYTICAL METHODS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-103-C2-113. 10.1051/jphyscol:1984225. jpa-00223938

HAL Id: jpa-00223938 https://hal.archives-ouvertes.fr/jpa-00223938 Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL DE PHYSIQUE Colloque C2, supplément au n°2, Tome 45, février 198* page C2-103

COMPARISON OF SECONDARY ION MASS SPECTROMETRY (SIMS) WITH ELECTRON MICROPROBE ANALYSIS (EPMA) AND OTHER THIN FILM ANALYTICAL METHODS

H.W. Werner and A.P. von Rosenstiel*

Philips Research Laboratories, NL-5600 JA Eindhoven, The Netherlands Metaalinstituut TNO, NL-7300 AM Apeldoorn, The Netherlands

Résumé - On décrit différents modes de SIMS pour l'analyse de couches minces. Le principe de la technique est suivi d'une discussion de certains aspects relatifs à l'instrumentation : le type de sources ioniques et leurs caracté ristiques, les avantages comparés de la microsonde ionique et du microscope ionique, les modes spéciaux de SIMS, la spectrométrie de masse a pulvérisa tion par particules neutres (SNMS) et le bombardement par atomes rapides (FAB). La discussion des caractéristiques analytiques porte notamment sur la gamme d'éléments détectables, l'analyse quantitative, l'analyse en profondeur, la répartition bi-dimensionnelle et tri-dimensionnelle des éléments. Des exemples d'application du SIMS dans différents domaines illustrent ces divers aspects. Les auteurs concluent par une comparaison entre le SIMS et d'autres méthodes d'analyse de couches minces, notamment l'analyse par mîcrosonde électronique.-

Abstract - Different modes of SIMS for thin film analysis and the principle of SIMS will be discussed; this will be followed by a discussion of some features related to instrumentation: types of ion sources and their character istics; ion microprobe versus ion microscope; special modes of SIMS: sputter neutral mass spectrometry (SNMS) and fast atom bombardment. (FAB). The discussion of analytical features will include: element range, quantitat ive analysis, depth profiling, two-dimensional and three-dimensional element mapping. Examples from different fields of application of SIMS will serve to illustrate these different features. In conclusion a comparison of SIMS with other thin film analytical methods will be given with the emphasis on electron microprobe analysis.

1. INTRODUCTION Electron microprobe analysis (EPMA), introduced by P. Castaing, was the first beam technique which allowed to carry out elemental micro analysis i.e.: to determine the chemical composition of a volume typically a few micrometer large. SIMS, the main topic of this paper, is another method for elemental micro analysis. The term micro analysis in SIMS, however, is used when the dimensions of the probe volume in at least one direction are smaller than about one ym. In both cases, however, the sampled volume (103 um3) is much smaller than the volume sampled in a bulk analytical method; there the probed volume is between 10° - 10'0 ym^). [1]

Different modes used in SIMS for micro analysis: - Analysis of small preselected volumes (microspot analysis or monolayer analysis). - One-dimensional analysis: depth-analysis and line scans. - Two-dimensional analysis: mapping of the element distribution. - Three-dimensional analysis, as a combination of the latter two modes.

2. PRINCIPLE, INSTRUMENTATION AND SOME FEATURES OF SIMS 2. 1. The principle of SIMS The principle of SIMS is shown in Fig. 1: the sample is bombarded with a beam of

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I I, for mass MI primary

/ 'sec ions ... Is sample Computer (target) T-l

Fig. 1 - Principle of SIMS primary ions having energies of several keV. Target articles are sputtered due to this ion impact as atoms (ground state or excited state) and ions (positive or neg- tive) either in the ground or the excited state. These ions are extracted from the target region and pass a mass analyzer, where they are separated according to their mass-to-charge ratio. These ions are detected by suitable means and the information is fed to a recorder/computer.

The information obtained from such mass spectra [2] is illustrated in Figure 2: one can see atomic ions of the target Al+, ~l++,molecular ions (AIDHC), and ions of the electropositive elements ~a+,K+. Note that H+ is also seen in this spectrum, a unique capability of SIMS. Ions of electronegative elements usually do not appear in great abundance in the positive ion spectrum but in the negative secondary ion spectrum.

' 20 ' 40 ' 60 Mass numberlcharge - Mass number1 charge, [Mle) - Fig. 2 - Positive and negative secundary ion spectra of pure aluminium (~r+primary ions)

2.2. Instrumentation Table 1 gives some criteria to classify the different types of SIMS instruments [I]. As primary ions one uses ~r+,02+, N~+or CS+.

2.2.1. &on sources a) Duoplasmatrons with beam diameter down to I ym. b) Liquid metal ion sources (LMIS) also called electrohydrodynamic sources (EHD) [3]: with beam diameters down to about 300 g. Table 1 - Some criteria used to classify the different types of secondary ion mass spectrometers

Primary ions

Current density : high (Dynamic SIMS) low (Static SIMS) Beam diameter : mm (macroprobe SIMS) pm (microprobe SIMS)

Secondary ions

Mass separation : quadrupole or sectortype (occassionally time of flight (TOF) ) Energy selection : electrostatic designs Mass resolution : low (300), with single focussing instruments; high (up to 10000), with double focussing instruments Element mapping : by scanning beam (pm diameter): Ion Microprobe by ion optical imaging : Ion Microscope Vacuum sys tem : UHV (bakeable) :p = mbar or HV - mbar Modular design : compatible with other thin film analytical methods

2.2.2. Mass separation Mass separation can be carried out by quadrupole mass spectrometers, magnetic sector type mass spectrometers or time of flight mass spectrometer. The high mass resolution (up to 10000) obtained with sector type mass spectrometers is extremely useful when the analytical ion is masked by interference with another mol cu ar ion. Exam- ples are the separation of the doublets 75~s/29~i30~i160,or 31F'/3pSiA. Figure 3 [*] shows how only high resolution SIMS can reveal the actual concentration profiles of aluminium surface layers.

Estimated depth (A) 0 200 LOO 600 800 1000 1200 \loo5

? 2 - 10 *01 5 AIOH + Ca + SiO 4 >- ," 1 C 5 0 + .-2 2 $ lo' c 5 U a, 2 l,-j2 -0 E " 2

'"0 200 LOO 600 800 Erosion time (secl - Fig. 3 - Depth profiles obtained with a mass resolution of 4000, when bombarding an aluminium target with 02+, The curve which would have been obtained with low resolution mass spectrometry is indicated as AlOH+Ca+SiO (DegrZve [4]). C2-106 JOURNAL DE PHYSIQUE

2.2.3. Energy selection Discrimination between atomic ions and molecular ions can also be achieved by proper adjustment of the energy window in the mass spectrometer. For a review see 155. 2.2.4. Ion microprobe versus i6n microscope In the ion microprobe elemental images are obtained in an analogue way as in the electron microprobe: the mass spectrometer is tuned to a given mass number and a fine ion beam (typical I pm in diameter) scans across the surface. The lateral resolution here is typical 1 ym with a duoplasmatron and is about 300 8 with a liquid metal ionsource. Both magnetic sector types and quadrupole instruments can be used as ion microprobes.

In the ion microscope an elemental (ion) image of a small area (typical 100 pm diameter) is obtained by means of suitable ion optics. The lateral resolution, typical 0.5 pm here is limited by aberrations of the ion optical system. 2.2.5. special modes and derivatives of SIMS - Sputter neutral mass spectrometry (SNMS),[6]: here the sputtered neutral particles are postionized in the region between sample and mass separation and are further treated as in SIMS.

2.3. Analytical features of SIMS (see also table 2) 2.3. I. Element range All elements and their isotopes ranging from hydrogen up to uranium can be analysed with SIMS. 2.3.2. Detection limits Under given conditions (see below) SIMS is the most sensitive method for surface analysis. Concentrations down to 1 o1 atoms/cm3 can be detected under the optimum conditions (see Fig. 4a and 4b). Typical values for the detection limits are - atoms/cm3. 2.3.3. Quantitative analysis It has been mentioned already that SIMS suffers from matrix dependent ion yield. This is in part caused by the relation Y+ a exp(-Ei/Em), where E is the ionization energy and Em is a matrix dependent parameter. i

A number of first principle methods, fitting parameter methods and calibration curve methods have been reported in the literature (for a review see [71). From all these methods the calibration curve method gives the best results (accuracy 5 lo%, 86% con- fidence level) provided that sufficient oxygen is present in the sample. However, this accuracy is by far worse than the accuracy obtained by EPMA. 2.3.4. Depth profiling 2.3.4.1. Sputter profiling One of the main fields of application of SIMS is elemental in-depth profiling. In this mode a surface Ab is continuously eroded by bombardment with a constant beam of primary ions. The secondary ion current I of one (or several) elements is recorded as a function of time t(az). From the relation I(t) one can determine c(z), where c is the concentration of the element under consideration. It is evident that this mode of depth profiling is essentially destructive in the sense that it consumes material. SIMS is used in this destructive depth profiling mode for the following reasons: a) Low limit of detection (down to about 1013 at/cm3). b) Large dynamic range (ratio of intensity in the top of the depth profile to the one in the background) of 7 decades (cf. Fig. 4a and 4b [8,9]). c) Good depth resolution. The depth resolution indicates to which extend an ideal step function profile or a delta function (width Az= 0) is smeared out. A depth resolution is indicated as the depth Az across which the intensity of a step function profile drops from 84% down to 16% [lo]. Values for the depth resolution obtained with SIMS are between 30 2 and several hundred 2, depending on the target and the depth in which the profile is to be measured. The depth resolution can deteriorate due to several artefacts: - Primary beam inhomogeneity - Deposition effects - Mixing effects; by which the original depth profile is changed: a) primary recoil mixing, when a target atom is knocked into the targed (mixed) by collisions with primary ions. b) cascade mixing, when target atoms are hit by target (recoil) atoms. - Miscellaneous effects . Interference effects: when two ions with the same nominal mass (mass doublet) have different dependence in depth (see Fig. 3). . Charging of insulating materials: this may cause migration of mobile ions as e.g. ~a+in the target.

SIMS: : 12keV 0; (2pA) . Raster 1.5mmx 1% : Analysis area 0.35mm x 0.35mrn

Depth (em)

Fig. 4a - Boron depth profile with Fig. 4b - Manganese depth ~rofilein increased sputtering rate in silicon. the tail region. (Le Goux [8]) (Clegg [91)

2.3.4.2. Tapered section method The artefacts, inherent to sputter depth profilingcan be avoided in the tapered- section or angle lapping method, well known in EPMA. This method can be succesfully used and recommended for depth profiling of layers larger than 1.5 vm.

3. FIELD OF APPLICATIONS Due to its high sensitivity SIMS is used both for surface analysis and for the analysis of bulk specimens, thus often extending and complementing EPMA. As high sensitivity can be achieved already with low specimen currents per unit area also beam sensitivie materials as glass, plastic, biological samples and organic materials can be succesfully analyzed. The field of SIMS applications is very brood and still expanding. Main areas are, treated in several review articles : - electronic industry [11,12] (implantation and diffusion, thin films, sandwich layers, interfaces, corrosion, inclusions, analysis and distribution of trace and minor elements homogeneity, isotope analysis etc.). C2-108 JOURNAL DE PHYSIQUE

- metallurgy [13,14,151 (oxydation, corrosion, adhesion, diffusion, homogeneity, inclusions and grain boundaries., nucleation studies, thin films, coatings, analysis and distribution of minor and trace elements, phase analysis of trace constituens in small areas, isotope analysis etc.). - geology [16,17] (analysis of minor and trace elements, inclusions, zoning phenomena, diffusion, geochronical age dating in microdomains etc.). - glass and ceramic [18,19,20] (corrosion phenomena, leaching, diffusion, interfaces, thin films, inclusions, analysis and distribution of minor and trace elements etc.). - biology [21,22,23] (analysis and distribution of light and trace elements, inclusions, diffusion, isotope analysis etc.). - environmental analysis [24,25,26] (analysis and distribution of trace elements, single- particle- analysis, surface enrichmont of trace elements, diffusion and leaching, isotope analysis etc.). - organic materials [27,28] (compound identification, surface reactions. analysis and distribution of minor and trace elements, additives, pigments etc.) . 3.1. SIMS modes of analysis SIMS modes of analysis used in these above-mentioned fields of applications are mainly: - qualitative analysis - quantitative analysis - comparison of "good" and "bad" product - point analysis - line scan analysis - elemental mapping - three dimensional analysis [3] - depth profiling 129,301 - isotope analysis [31,32,33] - "fingerprint", viz. phase analysis [34,35]

3.2. Illustrative examples The by far most important applications of SIMS are in the semiconductor and electronic industry. Some brief examples are listed below. Magnetic and optical properties of garnets used in microware and magnet optical shorage divices are strongly dependant on small changes in composition and have been systematically investigated by Willich and Tolksdorf 136,371. In general such devices consist of a substrate of Gd3Ga5012 or (Y,Pb)3Fe5012 covered by an epilayer of Y3Fe5012 (Yttrium Iron Garnet, YIG) or (Y,Pb)3Fe5012. Analysis of growth induced defects in flux grown YIG single crystal wafers were performed by EPMA. Epilayers of GaAs deposited by MBE and VPE were investigated by Clegg by means of spark source mass spectrometry (SSMS) and secondary ion mass spectrometry (SIMS). The relevant concentrations of doping element and contaminants in GaAs introduced during material preparation and processing range between 1013 and at/cm3 (5 x 1016 at/ca 2 1 ppm in GaAs). Similar analyses were carried out by Huber [381, see Fig. 5. A combination 05 SIMS, SAM, X-ray topography and X-ray diffraction, and electron microscopy have been used to find an ifficient technhlogy for solid state laser (GaAlAs) production. Aluminium sheet material of high purity 99,999 % or even 99,9999 % is widely used for cathode foils of electric capacitors. A large number of trace elements, although within the acceptance criteria of bulk analysis may cause serious problems if present in complex inclusions. Fig. 6 shows ion microprobe images of such inclusions - a low melting complex Ga, Ca, K-hydride causing perforation and subsequent breakdown of DEPTH (pm) Fig. 5 - Chromium depth profile in a Fig. 6 - Ion microprobe images of complex GaAs epitaxial layer. hydride inclusions in an alumin- (Huber [38]) ium foil. electric properties. Within the production processes of milling, etching and heat treatment of aluminium cathode foil a number of trace elements in the bulk material are diffusing in the outmost surface layer. The electronic properties are determined not by the bulk material but by this surface compostion, where a strong enrichment of specific trace elements as Mg, Ti etc. was found bij SIMS depth profiling. In addition to this surface enrichment the homogeneous distribution of these elements in this surface layer is of great importance. Fig. 7 shows the secondary electron image, indicating the

Fig. 7 - Ion microprobe and electron microprobe images of trace elements in an aluminium cathode foil. rolling direction and the corresponding electron and ion microprobe images, clearly depicting the inhomogeneous bandlike distribution of Mg and Ti in the surface layer by SIMS which could not be detected at all by EPMA. JOURNAL DE PHYSIQUE

Table 2 - Features of SIMS,EPMAand other analytical methods Anal. SIMS EPMA PIXE TEM ESCA AES LEIS HEIS LMP LOES (EDX) SAM (RBS) Element R 00 00 00 00 00 00 o R 00 Range

Detection % 00 000 00 00 00 00 00 00 00 Limit Quantitative oo I * 000 000 000 000 X 00 000 Analysis Depth Resol. oo 00 oo ST') ooo ooo * 00 o o Dynamic Range h 00 00 00 00 00 00 00 00 00

~estructivity~)e eee eee w ee ee ee *me l l

Lateral 000 00 00 X o 000 o 00 00 00 Resolution

Compound 000 o - o % 00 o o 00 - Information - Atom location ooo - - 00 - - * * - in lattice Isotope separ. * - - - - - 0 0 h - Organic 000 o o o 000 o o o 000 - samples

') ST = limited by sample thickness Performance indicator for destructivity: l Intrinsic, me Moderate, eee Weak Performance indicator: - Not reported, Not possible, o Poor, oo Reasonable, ooo Good, X Excellent.

AE S Auger EJectron Spectroscopy EPMA Electron Probe Microanalysis ESCA Electron Bpectroscopy for Chemical Analysis HELS High Energy Ion Scattering = RBS = Rutherford Backscattering LEIS Low Energy Ion Scattering = ISS = Ion Scattering Spectroscopy LMP Lase Microprobe LOES Laser Optical Emission Spectroscopy PIXE Proton Induced X-ray Emission SEM Scanning Electron Microscopy SIMS Secondary Ion Mass Spectrometry SNMS Sputtered-Neutral Mass Spectrometry STEM Scanning-Transmission Electron Microscope TEM Transmission Electron Microscope 4. COMPARISON OF SIMS WITH ELECTRON MICROPROBE AND SEVERAL OTHER METHODS Table 2 gives the highlights of the different methods in relation to some analytical features . Secondary ion mass spectrometry (SIMS) - All elements and their isotopes from hydrogen to uranium can be detected. However, the ion yield is strongly dependant on the element and its chemical state (ionization energy), on the matrix (matrix effeet) and the ex erimental conditions. - The detection limit ranges from 10~~at/cm~to 1017at/cm S . - Depth profiling with SIMS, essentially destructive, is characterized by a depth resolution of typically 30 - 100 2 (information depth 3 - 5 2). - Large dynamic range up to 7 decades. - Low detection limits. - The lateral resolution is typically 0.1 - 0.5 pm for ion microscopes; when using an ion microprobe it is expected to be limited by the beam diameter (1000 2). The fundamental limit is given by the lateral dimensions of the collision cascade of typically 100 2. - Three-dimensional analysis is a unique feature of SIMS. - Compound information and phase analysis down to the ppm range (carbides, oxides, fluorides etc.) can be obtained by means of fingerprint spectra. - Isotope separation is a superior property of SIMS (NLMP). - Organic samples can be analysed. Electron probe microanalysis (EPMA) - All elements from beryllium to uranium can be detected. The X-ray yield is depending on the element, the matrix, and the experimental conditions. - Relative det ctions limits range from 0.002 to 0.1 % and absolute detection limits down to 10-l5 - 10-l6 grams. - EPMA is a straight forward, well established, quantitative technique. High accuracy and relative errors as low as I% relative can be achieved. - The lateral resolution in solid specimens is typically 0.5 - 1 pm and down to 100 2 in thin films. - The depth resolution in solid specimens ranges from about 300 2 to 1 pm depending on the matrix, element line and experimental conditions. - Compound information by valence band spectroscopy is possible in some restricted cases (sulfides, oxydes, carbides). - Non-conductive samples are analyzed after coating with a thin conductive layer. - Organic samples can be analyzed if special precautions are taken in sample preparation and measuring methodology. Analytical electron microscopy (AEM) In the last years anlytical facilities have become available in (S)TEM instruments by energy dispersive detectors and ElectronEnergy Loss Spectrometry (EELS) allowing elemental analysis of areas down to 50 g [39]. The lateral resolution in TEM is determined by the beam diameter and sample thickness. Electron spectroscopy for chemical analysis (ESCA) ESCA is widely used for large area quantitative analysis and chemical valence state determination [40]. ESCA microspot analyses have also been achieved recently [41]. Scanning Auger electron spectroscopy (SAM) In SAM the lateral resolution down to 250 8 is determined by beam diameter and scattering from the primary electron beam. SAM is widely used on fractured surfaces in metallurgy and in semiconductor industry [42]. ] LEIS is famous for its extreme surface sensitivity, but has limited mass resolution and sensitivity [431. High energy ion backscattering (HEIS) In general only elements heavier than those of the bulk composition can be detected. The mass resolution is poor. HEIS is famous for its quantitative analysis. The method C2-112 JOURNAL DE PHYSIQUE

works fast and reliable. Information on the position of the atoms in the lattice can be obtained by channeling experiments under different angles of incidence 1441. Laser microprobe (LME') LPtP analysis is a very fast multielement technique due to the use of time of flight spectrometers. Instruments are available both for transmission and reflection mode [45]. Insulatorsrcanbe analyzed without special precautions. Recently also organic mass spectrometry on single particles has been performed by LMP 1461.

5. CONCLUDING REMARKS The complexity both of the methods and the analytical problems necessitates a close cooperation between the experts in different analytical disciplines and those in technology and material science.

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