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 Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984225 JOURNAL DE PHYSIQUE 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 resol- ution (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 elec- tron 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 diam- eter) 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 en- ergy 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.