Surface microanalysis

Analytica Chimica Acta, 283 (1993) 19-34 Elsevier Science Publishers B.V., Amsterdam

19

Surface microanalysis F. Adams, A. Adriaens, P. Berghmans and K. Janssens Department of Chemistry, University of Antwerp @LA), B-2610 Wdrijk (Belgium) (Received 10th September 1992)

Abstract Methods are discussed which combine surface, interface and thin-film measurements with the analysis of limited spatial domains. The progress which is now occurring rapidly is illustrated. The methods treated are selected from the beam-imaging methods, based on electron, ion, x-ray beam and laser interaction with the solid sample. Two examples are given concerned on one side with the ion microprobe analysis of micro structures in electronic devices and on the other side with the characterization of surface-modified asbestos by electron energy loss spectrometry and electron spectroscopic imaging. &yworu!r~ Electron probe methods; Mass spectrometty; spectrometry; Microsurfaces; Surface microanalysis

Surface characterization of materials is presently amongst the most active, dynamic and rapidly expanding areas in analytical chemistry. It is of increasing importance in materials science and microelectronics and vital for the study of the environment and biological systems. The recognition in many practical problems that surfaces are often spatially inhomogeneous, leads to the increased need for exploitation of spatially resolved surface analytical techniques. In materials science, increasingly small structures must be fabricated with high material uniformity and interface smoothness. This requirement demands appropriate device fabrication techniques, such as cold vapour deposition and molecular beam epitaxy, impractical until a short time ago, since it is essential in some cases to have control of composition and structure down to the atomic level [ll. Also, many of the most critical properties of materials involve their surCorrespondence to: F. Adams, Department of Chemistry, University of Antwerp (UIA), B-2610 Wihijk (Belgium). 0003-2670/93/$06.00

Surface techniques; X-ray diffraction; X-ray fluorescence

.

faces. Surface-modification chemistry provides an answer to many dilemmas in materials science by decoupling the surface properties of materials from those of the bulk solid. Monolayers or thin surface layers of organic and inorganic surfaces increasingly determine electroactive, optical and catalytic properties of many materials. Electronic materials require an ever more detailed understanding of the deposition of extremely small amounts of matter in precise physical arrangements and locations. Many catalysts consist of highly dispersed support particles whose size, morphology, structure and nature of metal-support and metal-promoter interfaces need to be studied more and more in detail. In environmental chemistry, examples of microscopical surface characterization are abundant. The composition or structure of microscopically sized environmental particles are important parameters for their persistence and fate in the environment, their toxicology and for inferring the assignment of particles to specific sources of pollution. Sometimes microsurface analysis (or

0 1993 - Elsevier Science Publishers B.V. All rights reserved

F. Adams et al. /AnaL 0th.

surface enrichment) provides insight information mechanisms and heterogeneous surface reactions

El. Microsurface analysis plays an important role in some of the newest and most exciting discoveries: high-temperature superconductors, fullerenes, biomaterials and high-performance composites, to give only a few examples. In general, there is a tremendous drive for the analytical developments of the enormous range of applications in fields such as the environment, biomedical research and health care, and electronic and structural materials, which serve as the fabric of modern technology. Hence, during the past decade there has been a substantial and continuing

Acta 283 (1993) 19-34

growth of interest among researchers for the development of truly microscopical surface-characterization techniques. Actually, today the number of surface analytical techniques with some kind of microscopical potential has grown so large that it has become a difficult task for the user to select an appropriate technique to solve a given problem. Over a hundred different techniques exist which aim at identifying one or more specific properties at the microscopical or surface level in condensed matter. Together they often provide a reasonably good understanding of a material’s properties, structure and composition. The main application areas and a number of these methods are shown in Table 1. This, by all

TABLE 1 Characterization

of solid materials

Main application area

Analysis method

Abbreviation

Layer thickness/composition

Transmission electron microscopy Auger emission spectrometry X-ray photon spectrometry Secondary-ion mass spectrometry Spectroscopic elipsometry Rutherford backscattering spectrometry Laser mass spectrometry Glow discharge mass spectrometry Instrumental neutron activation SIMS X-ray photon spectrometry Fourier-transform infrared spectrometty Fourier-transform mass spectrometry Infrared spectrometry Raman spectrometry Electron spin resonance spectrometty Nuclear magnetic resonance Laser spectrometry Ion microscopy/microprobe imaging Scanning electron microscopy Electron energy loss spectrometry LAMMS, FTIR, laser RAMAN Imaging NMR Spreading resistance, C-V, Hall measurements Deep level transition spectrometry Time resolved luminescence / transient optical measurements X-ray diffraction High-resolution EM Scanning transmission EM RBS, Raman, ESR, ab initio calculations

TEM AES XPS SIMS

Low level impurities

Chemical information/bond

form

Two-dimensional/three-dimensional

Electrical information

Structural/crystallographic

impurity distribution

RBS LAMMS GDMS INAA XPS FT-IR IT-MS IR ESR NMR SIMS EPMA/SEM EELS

DLTS

XRD HREM STEM

F. Aahs

et al. /Anal. Chim. Acta 283 (1993) 19-34

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means incomplete, list of techniques nevertheless represents a large and expensive collection of instrumentation. Moreover, it requires experienced scientists with diverse interests and backgrounds in analytical chemistry, physics, computer science and instrument engineering, which can be made available only in very large institutes or through the establishment of a collaborating network. The University of Antwerp, for example, has assembled the infrastructure and knowhow of 15 laboratories belonging to the departments of physics and chemistry in an Institute of Materials Science (IMS) with the purpose of joining resources for materials research, especially with the aim of materials characterization. Even then, in order to cover the range of instrumentation required for materials characterization in Belgium, it is necessary to pool resources with other Belgian Institutes, the Interuniversity Microelectronic Centre (IMEC, Leuven) and another Institute of Materials Research @MO) in Diepenbeek. Only then access is gained over a more or less full range of analytical and physical instrumentation for the purpose. As far as their application relates to the characterization of microelectronic materials and devices most of these methods (and indeed many more) are covered in the recent book edited by Grasserbauer and Werner 111. In this paper we will focus the attention on a few methods concerning the combination of sur-

face, interface and thin-film analysis on limited spatial areas and the progress which is now rapidly occurring in these methods. They were selected from the group of techniques termed “beamimaging methods”, based on electron, ion or x-ray beam interactions with the solid sample.

METHODS OF SURFACE MICROANALYSIS

Table 2 summarizes different types of interactions of particle beams or radiation with a solid material, and the main phenomena which give rise to analytical information confined to small spatial domains coupled to a more or less well defined surface discrimination capability. Spatial resolution and imaging are either obtained via the source (such as in the scanning electron microscope or the ion microprobe) or via the detector (as in the transmission electron microscope or the ion microscope). The surface was defined here in a rather general, generic manner as this part of the sample, which by composition or properties, is analytically different from the bulk material. This definition differs from that specified by surface chemists who define the surface as nothing more than the outermost atomic layer 131. Elemental composition, molecular information or structural inhomogeneity may be of interest in the surface layer. Figure 1 shows a graph comparing the spatial

TABLE 2 Beam methods for microsurface analysis and state-of-the-art Excitation

Electrons

X-ray photons KeV ions Low energy ions UV photons (laser) X-rays (synchrotron)

resolution

Analytical signal

Resolution Lateral

Surface

Auger electrons (SAM) Transmitted electrons (EELS)

50 nm 10 nm

Reflected electrons (HREELS) Photoelectrons (XPS) Auger electrons (XAES) Secondary ions (SIMS) Secondary ions (SSIMS) Backscattered ions (ISS) Molecular fragments (LMMS) (LAMMA/LIMA, FT-LMMS) Fluorescence x-rays (SRXRF)

SO-500 pm 10 pm (tube) pm (synchrotron) 0.1 pm lpm none 5pm

l-2 nm thin sample 0.25-3 nm nm nm 3nm nm nm 50 nm

1Ccm

none

22

F. Adams et al /Anal. Chin. Acta 283 (1993) 19-34

lODO !

I

I

Fig. 1. Spatial and surface resolution of a number of surface microanalytical techniques [1,39].

and surface resolution of a number of techniques. Generally speaking the spread in lateral resolution of the methods illustrates the progress realized over time since the original introduction of the methodology. The lateral resolution of the beam methods is primarily related to the confinement of the impinging radiation on the sample surface, although various diffusion effects in the interaction may contribute in limiting the lateral resolution practically achievable. Hence, progress in ion beam focalization is responsible for the steady gain in lateral resolving power of e.g. ion microprobe analysis. Laser beam interaction methods are not included in Fig. 1. Their optimum lateral resolution is 5-10 pm, their depth resolution s 0.1 pm. In Fig. 1, only scanning tunnelling microscopy @TM) and atomic force microscopy (AFlU) approach a resolution at the atomic dimensions, necessary in many examples of nano-fabrication technology. Tremendous progress has occurred in the lateral resolution of methods based on x-ray interaction due to the developments in x-ray photon-focusing technology which was realized recently. Techniques bused on the measurement of electrons Three techniques will be briefly reviewed as far as their surface microscopical potential is concerned: Auger electron spectroscopy @ES), x-ray photoelectron spectroscopy (XPS, or ESCA), and electron energy loss spectroscopy (EELS). Both AES and XPS involve the ejection

of secondary electrons, possessing an energy characteristic of the target’s elemental composition, while EELS studies the primary electrons which have impinged and interacted with the sample. AES is based on the detection of Auger electrons induced in the bombardment with an electron or (see later) an x-ray photon beam. The emitted Auger electrons have energies unique to each atom and are accordingly analyzed as a function of their energy, yielding both qualitative and quantitative information on the sample. AES has proven to be a powerful technique in scanning Auger microprobing (SAM), allowing localized analyses, element mapping, and, when used with a sputtering ion gun for etching away layers of material, depth profiling. The characterization of semiconductor devices 141, superlattice structures [S] and metallurgical specimens [6] are important fields of application of SAM. Electron ejection may also be induced by photons of a characteristic energy, as in XPS. The core electrons, emitted at bombardment by x-rays, obtain discrete kinetic energies. Apart from the capability to identify elements, the advantage of XPS is situated in the possibility to determine the chemical state of the element studied. The main applications of XPS are located in corrosion studies [7], catalysis [8], and polymer science [9]. Theory and applications of both XPS and AES are discussed extensively in the recent edition of the handbook of Briggs and Seah [3]. Both methods involve the study of the energy distribution of the emitted electrons, which implies that the information depth or surface resolution is determined by the electron mean free path. This is a function of the electron energy and the composition of the matrix. The low escape depth (OS-5 nm) of Auger- and photoelectrons, up to a few hundred eV, provides surface analysis within the first few monolayers, which classifies the method as a very surface-sensitive technique. The lateral resolution of SAM is dominated by the diameter of the primary-electron beam and is well into the sub-micron domain. There are advantages in using continuous xrays for excitation of the Auger electrons (x-rayinduced AES or XAES) as near-threshold pho-

F. Adams et al. /Anal. Chim. Acta 283 (1993) 19-34

toionization cross sections can be several orders of magnitude higher than the maximum electron cross sections. Moreover, there is a gain in signalto-noise ratio as the high secondary-electron backgrounds are significantly reduced. High-resolution spectrometry of high-energy (several keV) Auger electrons could furthermore provide information on chemical species [lo]. ‘All this should indicate that conventional electron-induced AES and SAM is a choice resulting from the ease with which electron beams can be produced and focused. Siegbahn coined the acronym ESCA (electron spectroscopy for chemical analysis, as opposed to XPS), to underline the fact that both photoelectron and Auger electron peaks appear in the electron spectrum obtained under x-ray irradiation [ll]. The progress in x-ray optics gave rise to increasingly small x-ray photon beams, while synchrotron radiation sources deliver extremely intense beams in the UV to the hard x-ray energy region. Both technologies now tend to change the electron spectroscopy situation completely from what it has been only recently. Spatial resolution of XPS has improved from areas of millimetre dimensions in “large-area” XPS systems to nowadays lateral resolutions of 5-20 pm in imaging systems based on rotatinganode x-ray tubes [12]. Spatial resolutions of 1 pm may eventually be possible in routine XPS systems while a considerably better resolution, approaching 0.1 pm, has recently been reached in synchrotron-based instruments [3,13,14]. In addition to evolving into a microscopical technique, XPS has developed into a widely applicable method for the analysis of the outermost atomic layers of solid samples. Recently, attention has been focused on the use of angledependent methods to extract information as a function of depth in the top few nanometres below the surface of the sample. Indeed, measurements with a tilted sample, from almost grazing ejection angle to the normal of the surface, give rise to the observation of different surface layers. These surface layers change with the normal effective depth times the sine of the angle, hence from the top atomic surface layer at angles near grazing incidence to the full depth corresponding to the complete electron mean free

23

path. Many depth-profile deconvolution algorithms have been reported for acquiring data at a number of angles and for computing a depth profile throughout the film analyzed [15]. This concept of angle-dependent measurements is not new; it has been applied in many other methods. Nevertheless, despite its obvious shortcomings, the angle-resolved application of XPS avoids a major drawback of the conventional sputter-assisted depth-profile measurements, where ionbombardment-induced modifications (ion mixing, ion-induced surface roughening, projectile implantation) can otherwise falsify results. Concomitantly with the progress described till now, there is the continuing development of detection systems which increase both the AES and XPS imaging capability. Progress in the quantization of surface analysis with the electron spectroscopies have been rapid over the last ten years. In well characterized and simple systems errors are now of the order of 5%. Accuracy becomes more limited for materials lacking homogeneity in the surface region or showing surface roughness [161 and for UV-induced photoelectrons (UPS). The topic is of industrial importance and international organizations have been formed to follow and direct the quantification effort [171. The energy distribution of bombarding electrons, which have interacted with the atoms of a specimen, is studied in electron energy loss spectroscopy (EELS). This method provides elemental information on minute locations of the sample [18]. In transmission EELS, the spectrometer, which separates the energy of the electrons, is integrated in a transmission electron microscope. Lateral resolution is limited by aberrations of the optical system and operation parameters of the microscope. The information depth of the method is determined by the sample thickness and ranges typically between 10 and 100 nm. We will illustrate the surface microanalytical potential of this method with an example in a later section of this paper. Reflected EELS, which is also understood as high-resolution EELS (HREELS), studies the energy loss of reflected low-energy bombarding electrons. Promising applications of HREELS are situated in semiconductor research and polymer technology 1191. The information depth depends

24

F. Adams et al. /Anal. Chim Acta 283 (1993) 19-34

on the energy of the bombarding electron beam and the angle of incidence. The surface sensitivity of this method can go down to probing a single monolayer adsorbed on well defined surfaces. Lateral resolution is limited by the incident beam spot size and ranges between 50 and 500 pm [20]. The understanding of the interaction mechanism between monochromatic electrons and the vibrations of the solid are not yet fully understood and hamper at present quantization [21]. Techniques based on the measurement of ions Secondary-ion mass spectrometry (SIMS) and ion scattering spectroscopy (ISS) are two techniques which are based on ion detection. SIMS is an analytical method in which bombardment of energetic primary ions on a solid specimen removes particles by sputtering [ll]. These emitted particles are partially ionized and are accelerated into a mass spectrometer. The so-called secondary ions are characteristic for the elemental and isotopic composition of the sample. SIMS analyses allow localized depth profiling, line scans, elemental micromapping and isotopic analyses. SIMS has achieved great success as a materials characterization tool because of its pg ml-’ to ng ml-r sensitivity, excellent depth and lateral resolution, and even for its capability for threedimensional analyses. It is best known for its applications in the microelectronics industry, but is now increasingly used in many other fields.

Quantification with an accuracy of lo-20% is now commonplace for simple, conductive materials [22,23]. The recent evolution in one microanalytical SIMS system, the CAMECA ion microscope-microanalyzer, is shown in Table 3. The gradual improvements in the lateral resolution show the remarkable progress achieved in SIMS microsurface analysis. The lateral resolution obtained in SIMS is determined by its mode of operation. The ion microprobe mode rasters a focused ion beam over the surface of the sample, and the resultant lateral resolution is determined by the beam diameter (0. 1 km, even better in some ion microprobe systems). In the ion microscope mode, on the other hand, a magnified image of the surface is produced using a relatively large beam (up to several hundreds of pm) in combination with an ion optical system. The lateral resolution in this case is determined by the ion optical system and its aberrations. In dynamic SIMS instruments, such as those of Table 3, material is quickly sputtered from the exposed surface and in a dynamic way sub-surface layers are analyzed leading eventually to three-dimensional data stacks of elemental analytical data [24]. In static SIMS systems @SIMS) the primaryion beam current density is lowered to values corresponding with sputtering rates as long as several hours [25,11]. Under such conditions SIMS becomes a true surface analytical technique of

TABLE 3 Main characteristics of the Cameca range of ion microscopes/ion microprobes Characteristics

IMS 3f

Year of introduction Mode of operation Lateral resolution Information depth Mass range Maximum mass resolution Transmission Sensitivity Charge compensation Vacuum sample chamber Detection

1979 Microscope 1000 nm 0.3-l nm l-280 ‘I

IMS 4f

1985 Microscope Microprobe 1000 nm 200 nm 0.3-I nm l-500 104 ::% (MRP = 800) 30% (MRP = 800) ng ml-‘-pg ml-’ ng ml-‘-j.tg ml-’ No Yes 8 x lo-’ torr 5 X lOWi torr Sequential

IMS Sf

IMS 1270

1991 Microscope Microprobe 500 nm 150 nm 0.3-l nm l-2000 4 x 104 40% (MRP = 800) ng ml-‘-pg ml-’ Yes 5 x 10-1s torr Sequential

1991 Microscope Microprobe 50 nm 350 nm 0.3-l nm l-2000 10s 50% (MRP = HHIO) ng ml-*+g ml-’ Yes 5 x 10-m torr Simultaneous (up to 5 detectors)

F. Adams et al. /Anal. Chim. Acta 283 (1993) 19-34

the top surface layer of the sample with high detection sensitivity. The method has evolved over the last two decades from a large-area technique into a microanalytical technique. It is a scanning microprobe configuration where a focused and pulsed liquid metal ion source is used as primary radiation and a high-transmission reflection type time-of-flight (TOF) tube as the mass analyzer in a III-IV environment (lo-” torr) [26,27]. This configuration combines parallel mass registration with high sensitivity, high mass resolution and a high mass range. It allows elemental and molecular surface analysis in small surface areas of a few pm in imaging mode and extremely high (submonolayer) sensitivity for inorganic as well as organic materials at high mass resolution. In particular the potential of this kind of mass spectrometric surface analysis is documented for the analysis of microelectronic materials, processes and devices. Applications in this area range from the detection of metal contaminants and organic molecules [28] to the control of surface reactions on silicon wafers [29]. Ion scattering spectrometry (ISS) is based on the energy analysis of backscattered ions from the target surface atoms [301. An important aspect of ISS is its high sensitivity for the outermost surface layer. This is due to the low-energy primary

25

ions (0.1 keV to a few keV). Lateral resolution is limited to 10 pm. The technique is frequently used in adsorption and reaction studies of surfaces. High-energy equivalents of ISS range up to the Rutherford backscattering spectrometer (RBS) using MeV ions. Micro-RBS is available on a number of nuclear microprobes in combination with proton-induced x-ray emission (PIXE) and scanning transmission ion microscopy @TIM). Resolutions down to 50 nm have been achieved with high-energy and low-current ion beams [31,32]. RBS mapping and cross-sectional (in-depth) RBS measurements can be combined when a proton microprobe is scanned over a sample. The depth information is derived from the RBS spectra at each pixel point and is also called RBS tomography [331. The method provides information on the atomic composition and distribution of matrix elements and impurities beneath insulating layers in semiconductors. Both SIMS and ISS show a strong dependence of their information depth upon the kinetic energy and the mass of the primary ions. It ranges between 0.3 and 1 nm. In SIMS the attainable depth resolution is determined by the initial surface roughness and by ion beam mixing. The minimum achievable depth resolution for the easiest matrices (semiconductors) at 2-3 atomic lay-

TABLE 4 Comparison of characteristics of TOF-LMMS and FT-LMMS Characteristics

TOF-LMMS

FT-LMMS

Mass resolution Mass range Sensitivity Geometry Lateral resolution Information depth Measured species

850

10s 1-15 x 10s lO-“-lo-‘2 g Reflection 5-10 pm 100 nm Prompt ions and post-laser ionization ions(+/-1 Post ionization

Special remarks Applications

l-unlimited 10-1s g Transmission Reflection >lpm IFm 100 nm Promptly generated ions (+ / - 1 Post-ionization is not possible in commercially available instrumentation Organic: detailed structure characterization Inorganic: speciation

Organic: detailed structure characterization Inorganic: speciation

F. Adams et al. /Anal. Chim. Acta 283 (1993) 19-34

26

ers ties in nicely with the stochastic nature of the sputtering process and the depth of origin of the ejected particles 1341.

tion (CID) or electron impact post-ionization. Structural specific fragmentation also facilitates identification.

Laser microprobe analysis as a microscopical surface tool There are numerous examples [2,35] proving the utility of laser microprobe mass spectrometry (LMMS) for microsurface analysis with one of two instruments which became available around 1980, the laser microprobe mass analyzer (LAMMA) [36] and the laser ionization mass analyzer (LIMA) [37]. Roth instruments are based on low-resolution (m/z = 800) TOF mass spectrometers to analyze the ionized species released upon impact by a focused and pulsed UV radiation of a Nd-YAG laser. The analytical characteristics are compared in Table 4 with those of a recent contender, in which Fourier-transform laser mass spectrometry @‘I-LMMS) is used for high-resolution mass spectrometric detection [38]. The tremendously high mass resolution and the other features offered by IT-MS indicate that this quite new method for microsurface analysis may provide an unpresented specificity for inorganic and organic structural surface analysis. In Table 5 its characteristics for organic surface analysis are compared with those of TOFSSIMS. The possibilities for structural identification are superior to those of TOF-SSIMS due to the inherent possibilities of the IT-MS technique, directly through the high mass resolution and indirectly, e.g. by collision-activated dissocia-

Atomic resolution imaging techniques Scanning tunnelling microscopy WI%0 and atomic force microscopy (ARM) are two techniques which do not fit into the classical particlein-particle-out scheme of other analytical surface methods. The principle of STM is based on the quantum mechanical tunnelling of electrons through the region between the sample and a sharp metallic tip, which is brought very close (1 nm) to the specimen surface. A voltage applied across the sample and the tip, causes the electrons to cross the region in a narrow channel. The measured intensity is very sensitive to the distance betwe.en sample and tip. Once the interaction is established, the surface is scanned by the probe, and the measured intensity will be used to create an image. The lateral resolution of the surface is limited by the sharpness of the tip (0.2-1.2 nm) [39]. Aside from measuring the surface topography, STM provides information on the atomic composition, since the measured current depends on the electronic structure of the surface. Applications are reviewed by Ray et al. [40]. Considerable effort has recently been directed towards using STM to characterize, under close to physiological conditions, the structure of biological macromolecules deposited on a clean surface. Despite the atomic resolution, the method is not yet capable of providing a reproducible and unambiguous visualization of the structure or to identify bases [41]. Atomic force microscopy is a surface technique which is closely related to STM [42]. It consists of measuring the atomic forces between a tip and the surface of a sample, the tip now being attached to a cantilever. When the tip approaches the surface, the cantilever will tilt due to the impact of interatomic forces. Optical methods are then used to measure these deflections. The advantage of this method over STM is the possibility to analyze non-conducting samples. Applications of both STM and AFM are situated in several fields: surface physics and chemistry, electrochemistry, biology and the study of electronic

TABLE 5 Comparison of TOF-SSIMS and FT-LMMS for organic surface analysis Characteristics

TOF-SSIMS

FT-LMMS

Mode of operation Lateral resolution (pm) Information depth (nm) Detection limit Mass resolution

Imaging 5 1 10-12 loo00

Spot analysis 5 50 10-10 100000 (UP to m/

Mass range Structural discrimination

Limited by mass resolution Good

z=looo) 15000 Excellent

F. Adams et aL/AnaL

Chim. Acta 283 (1993) 19-34

Fig. 2. HREM image showing a surface located at the level of the plane between SrO and PbO layers in (Pb, Bi)zSr2-x superconductor. Black dots at the surface can LaxcuZo6+, be identified as Pb atom configurations [43].

devices. High-resolution electron microscopy (HREM) has also reached the level of atomic imaging on a more or less routine basis. HREM images are obtained by allowing the transmitted and one or more diffracted beams through the objective aperture of a point-resolution TEM. Image formation occurs by the interference of these selected electron beams. However, the interpretation of the images which contain the characteristic periodicities of the crystalline specimen in terms of atomic identity and structure remains tedious. Image simulation must be used to compare simulated (calculated) and experimental images so as to obtain unambiguous atomic images. An example of the application of this methodology is shown in Fig. 2 [43]. The presence of particular atoms at the top surface of a superconducting sample is directly evidenced. Synchrotron radiation in surface microanalysis The high brightness of synchrotron radiation sources has significantly enhanced the research capability for methods relying on UV or x-rays.

27

Synchrotron radiation x-ray fluorescence spectrometry (SRXRF) is coming to the fore as a very promising analytical method, complementing other techniques based on x-rays, such as electron probe microanalysis and the nuclear microprobe. SRXRF will mostly be used to attain lower detection limits, to perform dynamic observations for the study of transient phenomena and to obtain high spatial resolution for element mapping. Applications of the micro SRXRF technique are numerous. They range from the analysis of micro-heterogeneities in materials of different composition over sensitive bulk analysis of technologically important materials to the study of distributions of elements across different kinds of interfaces. In x-ray fluorescence analysis under total reflection conditions, monochromatic x-rays are directed onto a sample surface under glancing incident angle. The primary-ion beam then excites a surface layer of about 3 nm in depth. The fluorescent x-radiation, as detected by a Si(Li) detector, provides detection limits of about 108-10’o atoms cm-* for impurities like Fe, Cu, Zn and Ni [44]. With high-intensity synchrotron radiation, the technique can be used for microscopical samples or spot analyses [451. Synchrotron radiation (SRI plays an increasingly important role in microscopical surface analysis in (i) the study of chemical composition, (ii) structural studies and (iii) electronic structure studies. About 50% of the actual SR beam time is used for the study of condensed matter and surface research (about 500000 hours per year now) using diverse techniques. We cannot give a complete overview of the role of SR sources but want to stress a few emerging trends. The role of synchrotron radiation in imaging XPS was already emphasized in the section “Techniques based on the measurement of electrons”. The important advantage of SR in surface studies is that it gives continuously variable photon energies which in photoemission allows one to optimize (i) the sensitivity to particular species, (ii) the surface sensitivity by setting the photoelectron kinetic energy in the energy range of minimum escape depth around 50-100 eV and (iii) energy resolution, state-of-the-art at 100 eV is now 25 meV. In the past few years extended

28

F. Adnms et al. /Anal. Chim. Acta 283 (1993) 19-34

x-ray absorption fine-structure analysis (EXAFS) was widely used for the investigation of the local structural environment of specific atoms. The method is based on the small oscillations in the absorption cross section as a function of energy for several hundred eV above the absorption edge. Surface-specific versions of the technique (surface EXAFS or SEXAFS) have been developed and are widely exploited, e.g. for the study of catalysts. Unique new possibilities will be offered by the new generation of high-energy SR sources (ESRF at Grenoble, France; APS in Argonne, IL; and SPring-8 in Harima, Japan). Fig. 3. Depth profile of the A.&i- signal, measured in the centre of the n+ diode. APPLICATIONS

Zon microprobe study of microstnrctures in electronicdevices

Secondary-ion mass spectrometry (SIMS), using the ion microprobe or ion microscope configuration in combination with image processing, is a technique which couples lateral imaging and dynamic ion beam sputtering. It allows the extension of the common capabilities of SIMS with a number of interesting features such as local area depth profiling, line analysis and the acquisition of three-dimensional image depth profiles. This technique is applied here to locate and analyze micro structures in electronic devices. Similar types of applications have been performed, having analyzed elements of either high concentration or high sensitivity [46,47]. The fact that micron-scale structures of very low concentration are analyzed, makes this study an exception. In these types of analyses, it is necessary to make a compromise between the spatial resolution obtained and the precision acquired. These two aspects are of course linked. Since in the microprobe mode, the probe size is critical for a good spatial resolution, a reduction in the primary beam intensity will improve it. On the other hand the volume analyzed will be reduced, implying an inevitable reduction of the signal and hence the precision of the analysis. The purpose of this study therefore is to develop a procedure

to measure microstructures of very low concentration with an acceptable precision. The sample analyzed consists of an n+ (arsenic) diode, which has an area of a few square microns and a depth of a few hundreds nanometres, located in a silicon wafer. The arsenic has a maximum concentration of the order of 1019-1020 at cmm3 (I 1 at%). The purpose of these analyses was to check the lateral and in-depth distribution of As in the Si matrix. The instrument used to analyze the sample was a Cameca IMS 4f ion microprobe. The surface was scanned with a focused Cs+ beam of 1 nA over an area of 10 pm2. Negative secondary ions were accelerated into the mass spectrometer and were detected with an electron multiplier in pulse-counting mode. The AsSi- signal was measured to prove the presence of arsenic in the sample. This signal was preferred over the As- signal because of its higher sensitivity. The sample was analyzed with a high mass resolution of 3000 to eliminate the isobaric interference of 29Si30-. In Fig. 3, a profile of the A&i- signal, which is measured in the centre of the nf diode, is shown as a function of time (which can be correlated with depth). The Si; signal is measured to have a reference signal. The plot shows a fairly flat signal close to the surface, but then it decreases gradually as the border is reached, showing a smooth bottom interface of the diode, where the pn junction is located, with its environment.

F. Adams et al. /Anal. Chim. Acta 283 (1993) 19-34

In order to have a two- and three-dimensional view of the arsenic distribution in the silicon wafer, a set of images was taken while the surface was gradually being disintegrated during the ion beam bombardment. A Kontron imaging system was applied for the acquisition of the images. This computer is linked with a SEM box, through which signals are sent to a set of deflectors having the primary beam scan the selected area. The imaging system is synchronized with the scanning of the primary beam: it accumulates and stores the measured intensity for each position scanned, after which the images can be reconstructed. About 50 images were taken over a period of an hour. Figure 4 illustrates the selected area over which the ion probe is scanned. The lateral distribution of the A&i- signal taken close to the surface is shown in Fig. 5a, demonstrating a lower As- intensity near the edges of the diode. The image-processing software allows advanced operations such as real-time display of vertical slices in any direction of the image stack. The line drawn in Fig. 5a represents the region were a cross section is made through the 50 images stored underneath each other as a function of depth. The result is demonstrated in Fig. 5b, showing a smooth crossing of the diode with the silicon wafer. The cross section makes it possible to evaluate parameters such as lateral and in-depth diffusion of the arsenic as a function of, e.g. thermal treatment of the sample.

: analyzed area

Skwafer

Fig. 4. The selected area over which the ion

probe is scanned.

29 As a final step, the distribution of arsenic can also be represented in a three-dimensional solid object. In this case a two-dimensional area needs to be selected. This is displayed in Fig. 5c, in which the selected area shows part of an image taken on the surface of the diode. The bottom image represents an image taken near the base of the diode. A three-dimensional cross section is made through the entire stack of images, after which the solid block is rotated in order to have a view indicated by the arrow, as is demonstrated in Fig. 5d. The sample analyzed here was a test sample, which demonstrates the possibility to obtain both a good spatial resolution and still have a good precision while analyzing very low concentrations. The analysis of these types of materials will be a challenge for a further analytical methodology development. The importance of these analyses can be situated in a number of areas. It serves on one hand as a control procedure, in which the shape of the analyzed structure and the element distribution within the structure can be examined. Furthermore, it provides the possibility to prove the presence of contaminants within and around the structure. On the other hand these analyses create a possibility for process engineers to have a means for evaluation or feedback, in order to control the fabricated structure.

Characterization of sur$ace-modified asbestos by EELS The cytotoxic and hemolytic properties of asbestos fibres may be strongly influenced through chemical modification of the mineral surfaces with TiCl, [48]. Biological investigations already have demonstrated a significant reduction in the binding ability of a carcinogen such as benzopyrene to the titanium-treated fibres [49j. Systematic studies of the characterization of the surface and the inside of the fibre tubes are therefore essential in the development of less pathogenic chrysotile fibres. The characterization of the fibers’ condition is not easy. The short-range inhomogeneity of the composition of the asbestos fibres and the small fibre size (typically 20-100 nm diameter) requires the use of analytical techniques providing extremely high lateral resolution. Furthermore, if

30

these additional titanium species are present at the surface, they are present generally as very thin surface layers. In this case a highly sensitive microanalytical technique for investigating the local elemental composition is required. Given these constraints, EELS is an analytical technique very well suited to address this problem. With EELS inner-shell loss edges are studied; these are characteristic of the chemical composition of the sample. Furthermore, there is a minimal lateral electron scatter and so electron spectroscopic imaging (ES11 has the added advantage to monitor elemental images of the sample with spatial resolutions approaching the diameter of

F. Adams et al. /Anal. Chim. Acta 283 (1993) 19-34

the electron beam. The electron-loss near-edge structure (ELNES) can also be used to acquire information on both the spatial distribution and the atomic environment. Even though the origin of the details of the different edge features is not well understood, the technique can be used to probe local bonding and oxidation states. Energy-loss spectra and images were obtained with the Zeiss EM 902 (Oberkochen, Germany) instrument operating at 80 kV. Due to the high electron beam intensity required for high-magnification electron microscopy a cryo-stage cooled with liquid nitrogen at - 150°C was used to reduce the irradiation damage of the hydrous min-

Fig. 5. (a) Lateral distribution of the As.%- signal taken close to the surface. The line represents the region where a cross section is made through the fifty images stored underneath each other as a function of depth. (b) Vertical slice through the image stack. (c) Selected two-dimensional area showing part of an image taken on the surface of the diode. The bottom image represents an image taken near the base of the diode. (d) A three-dimensional cross section through the entire stack of images.

F. Aabns et al. /Anal. Chim. Acta 283 (1993) 19-34

eral fibres. The resolution in the electron spectroscopic images clearly depends on the susceptibility of the asbestos fibres to radiation damage. Although the typical dose was 3 X lo5 e nm-*, it remains important to establish how the electron spectroscopic images are affected by mass loss. With the use of a cooled sample holder it is possible to obtain chemical information from cross-sectioned asbestos fibres at a high spatial resolution, provided that the counting time used to collect the spectra is long enough to ensure adequate statistics. The present work was done with serial EELS acquisition, which means that the different energy-filtered images are acquired successively. For detection limits we estimate the minimum detectable signal-to-background ratio to be about 0.01, corresponding to a titanium concentration of ca. 1 atom percent. Two approaches were considered for the sample observation: as cross sections and with the fibres deposited on holey carbon film. The use of cross-sectioned samples is very useful because it provides structural and morphological information, while the deposition method reveals more local information about the surface of the modified fibres. Figure 6 shows a high-magnification ES1 transmission electron microscope image ( X 140 000) of

Fig. 6. Transmission electron microscope image of a cross-sectioned chrysotile fibre. The entire picture corresponds to 185 nm.

31

Fig. 7. Energy-loss spectrum of chrysotile asbestos modified with TiCl, showing the titanium Lzs and oxygen K edges after background subtraction and Fourier deconvolution (A). The dashed lines display the spectrum of the non-modified standard chrysotile fibres (B).

a cross-sectioned fibre. It shows the smaller fibrils within the fibre. In the experimental curve in Fig. 7A the ELNES above the titanium L,,, edge is indicated as well as the oxygen K edge. The spectrum was processed in the following way. The background absorption curve preceding the edge was fitted to an inverse power law AE-’ where E is the energy loss of the transmitted electron. The curve was extrapolated beyond the edge and subtracted. The remaining contribution of the innershell excitation was deconvoluted with respect to the low-loss spectrum (mainly interband transitions and plasmon loss) using a Fourier log deconvolution method. The spectrum reveals the L 3,2 edges of titanium which are marked by prominent features with a sharp increase in intensity at the threshold of the edge. These features are caused by the excitation of the electrons from the 2p3,* CL,) and 2p,,, CL,) spin-orbit split levels to the unfilled 3d levels [50]. The titanium L,,, threshold peak shows two dissimilar peaks with multiplet structures at both L, and L,. In addition to the above, in comparison with the spectrum of the non-modified standard chrysotile fibre in Fig. 7B, a substantial variation in the near-edge fine structure of the oxygen K edge is observed. The appearance of a shoulder at the onset of the oxygen K edge in the spectrum of the TiCl,-modified asbestos could be assigned to a state arising by hybridization of the oxygen

32

F. Adams et al. /Ad

Chim Acta 283 (1993) 19-34

Fig. 8. (a) Brightfield image of a selected part of the analyzed area of a cross section of TiCl,-treated chrysotile fibres embedded in a Spurr resin (full picture is 95 nm). (b) Net-titanium image obtained by calculating a background image at 460 eV with a least-squares fitting procedure.

2p and the titanium metal 3d level. This might be an indication that TiCl, has chemically reacted with the chrysotile fibres. Around the titanium L,,, absorption edge, a series of ten energy loss images from 380 to 470 eV loss at a lo-eV interval were recorded. By using a data-analysis program, PV N WAVE (Precision Visuals, Boulder, CO>, a least-squares fitting procedure was performed in parallel for all pixels in the series of images. The background counts underlying the characteristic core edge were subtracted from the measured intensity at each pixel by fitting and extrapolating the preedge spectrum. This calculation ensures that the images represent a true titanium distribution and not mass thickness variations. Figure 8a displays the brightfield image of a selected part of the analyzed area at a higher magnification than in Fig. 8b, showing the fine structure of a cross section of a chrysotile fibril. The net titanium distribution image of the analyzed area is shown in Fig. 8b. In Fig. 9 I(AE) versus E is plotted, together with the fitted curve for the background I,,, for one pixel located on the asbestos section. The signal from the titanium L 3,2 edge is seen as the clean rise above the extrapolated background. The procedure can be very easily modified to take the uncertainty on the intensities in the original images into account.

In this way, next to the net image, also a corresponding uncertainty image can be calculated which allows the identification of the area in the image with a net intensity significantly different from zero. By selecting different locations in the net image, the selected part of the EEL spectrum, its background and the cross sections through the net image at that location can be interactively inspected. An example of two orthogonal cross sections starting from one pixel located on the asbestos section is shown in Fig. 10a and b. ”

1 “““‘I

EELS Spa&urn

‘.

8 ‘. + +

+

Fig. 9. Datapoints (+ ) of a selected part of the EEL spectrum at one pixel location near the surface of a fibril, together with the fitted curve of the background.

33

F. Adorns et aL /Anal. Chim. Acta 283 (1993) 19-34

P 5 loEJ '= 4 k. 0 E 5I -c -

-

+

2 EIO-+

+

T s

+

B

+

15-

:

-

+

+ P 5 S

++ + +

0 .,,.1...'I....(,..,I,(.~J,,,, 50 100 150 200 0 Pixel Position

250

300

:

++

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+ +

o‘~...~~.~~~~~~.-~~~~.~~~~~~~.~". 50 -100 ;&I 200 0

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300

Fig. 10. (A) x-section and (B) y-section at this location through the net image, showing the pixels in the image with a net intensity significantly different from zero (+) while the other pixels (. 1 indicate noise.

From these images and other images we could conclude that the titanium atoms are concentrated at the surface of the fibre and form an encapsulation around the material. Some elemental maps display also a contribution of titanium inside the fibre tubes which follow a spiral curvature, typical for chrysotile asbestos. These results indicate that low Z core shell energy loss images from adequately prepared specimens can provide useful chemical information about asbestos fibres. A.A. is a research fellow at NFWO/FNRS (Belgium). This work was funded by FKFO, Brussels and DPWB, Brussels (IUAP-III program).

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