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Surface and Thin Film Compositional Analysis: Charles A. Evans, Jr. \ Materials Research Laboratory •^ and t'". School of Chemical Sciences T^L Λ University of Illinois *· Urbana, III. 61801
The materials analyst today has available to him techniques for char acterizing the three-dimensional ele mental composition of a sample, some of which give a lateral spatial resolu tion on the order of 1 μπι and all of which give a depth resolution of ~200 Â or less. The development of these powerful techniques has been stimulated by the move to thin-film technology in the semiconductor industry and by the growing technological importance of such phenomena as corrosion, embrittlement, and catalysis. In all these areas the composition and properties of surfaces determine many of the properties of materials. The earliest microanalytical technique was the electron microprobe, which made possible analyses with lateral (x, y) and depth (z) resolutions of about 1 (im. The 1960's saw the development of electron spectroscopies (ESCA, Auger) and secondary ion mass spectrometry (SIMS) which are sensitive to the outer 10-20 Â of a sample (i.e., have a shallow escape depth for the analytical signal). The coupling of these spectroscopies with sputtering (sputtering is, of course, intrinsic in the SIMS technique) allows the characterization of successive 10-20 Â layers as sputtering erodes the sample. Lateral (x, y) resolution is attained in the charged particle-induced spectroscopies by using a microfocused primary beam (Auger, ion microprobe) or by ion optically locating the origin of the secondary signal (ion microscope).
While the capability for x, y, and ζ microanalysis is important in many applications (integrated circuit tech nology is an obvious example), fre quently samples are obtained which are laterally homogeneous and which require characterization only in the zdirection. Chemical analyses with zresolution are referred to as depth profiles. (The investigation of thinfilm systems is a case in point.) Tech niques which are not microanalytical in the three-dimensional sense can then be employed. ESCA has been mentioned already. Low-energy ion scattering spectrometry (ISS) and MeV ion backscattering spectrometry (BS) are two others. In such tech niques the exciting beam is usually unfocused owing to the lack of focus ing systems (ESCA), signal loss associ ated with small beam size (ISS), or simply the expense of focusing sys tems (BS). As with the secondary ion and electron techniques, ISS exhibits a very shallow escape depth and thus must be combined with ion sputtering for depth profiling. Sputtering is not required in the use of MeV ion backscattering spectrometry. The techniques introduced here have the common advantage of broad elemental sensitivity (H to U for SIMS, Li to U for the others). They differ widely in other aspects—ease of quantitation, cost, convenience, and sample throughput capability. This article will discuss the basic concepts of the more important techniques
818 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
commonly used for surface and thinfilm analysis and will compare them in their analytical features and capabili ties. Those techniques which are com mercially available will be empha sized. The I N S T R U M E N T A T I O N article in this issue of ANALYTICAL C H E M I S
TRY will discuss the instrumental fea tures of these surface analytical tech niques and the commercially available instruments. Ion Sputtering As mentioned above, almost all techniques for the determination of concentration variations with depth require the use of ion sputtering. If an atomic particle with a kinetic energy in the range of 100 eV to 100 keV im pinges on a material surface, energy and momentum transferred to the sur face atoms cause some of them to be ejected from the surface. Continued particle bombardment will cause suc cessive layers of atoms to be removed and the underlying layers to be ex posed. In almost all applications of the sputtering technique, ions are used as the bombarding species since they are more easily accelerated, focused, and deflected than are neutrals. Ion sputtering is generally accom plished either by ion beam or diode techniques. In the ion beam method the ions are produced in a separate ion source, extracted and accelerated to their final kinetic energy, and directed against the sample surface. In this
Report
Description and Comparison of Techniques
manner the ion beam can be directed to a specific region of the sample. The sample is in a high-vacuum (300 keV), monoenergetic ion beams as the probing species. These techniques depend on three principles. The first is that ions in this energy range can penetrate deep into the sample surface (>1000 A) while causing little sputtering or damage. Second, the deceleration of these ions as they penetrate the sample results from the interaction of the ion with the electrons associated with the sam-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975 · 819 A
Figure 1. Relative number of backscattering events vs. atomic number for 2-MeV 4 He+
ple atoms. The magnitude of the in teraction, called the electronic stop ping power, is well known for each ele ment; thus, the energy at any depth can be accurately predicted. The third principle is that the energetic probing ion can interact with a sample atom to produce one of three secondary pro cesses: a Rutherford backscattering collision, an X-ray, or a nuclear reac tion. Backscattering Spectrometry (BS). When a surface is bombarded with H + or H e + ions in the 300-keV to 3-MeV energy range, a small fraction (~10 - 6 ) of the incoming particles undergoes a Rutherford collision and is backscattered. The energy, E, of the ions scat tered through an angle of 180° is relat ed to the initial energy E0 by
2-MeV 4 H e + incident on several dif ferent surface atoms. Because of the energy loss of the probing particle ( 4 He + is generally used) as it traverses successively deep er atomic layers, the Ea for scattering from an internal atom will be reduced according to the relationship: dx where t is the sample thickness tra versed, and dE/dx, the stopping cross section or stopping power, is the ener gy loss per unit length traversed in the material under analysis. The stopping cross section can be calculated or is available in empirical tables (8). The backscattered ion energy is deter mined by the relationship: jp ^final
where m is the mass of the scattered particle, and M the mass of the target atom. Thus, energy analysis of the pri mary ions backscattered from a sam ple surface provides a mass analysis of the surface atoms. The relative sensi tivity of different surface atoms is de termined by the differential cross sec tion, σ, according to the relationship:
for 180° backscattering. Ζλ and Z 2 are the atomic numbers of the incident and target atoms. Figure 1 shows the mass scale and relative sensitivities for
—
t1 *~300 keV) are employed, as in backscattering spectrometry, the highly penetrat-
ing nature of the ions minimizes sputtering during the analysis, and the analytical step becomes independent of the material removal step. Honig (11) and Honig and Harrington (12) provide detailed discussions on the ISS technique as well as illustrative examples of its use. Secondary Ion Mass Spectrometry (SIMS). In addition to the removal of neutral atoms and molecules, sputtering of a material with 1-30-keV ions produces ions of the material under bombardment. Although only a small fraction of the ejected species is ionized (0.1-10%), these secondary ions can be analyzed and detected by a mass spectrometer to provide a sensitive surface and depth profiling analysis. As with the AES, ESCA, and ISS techniques, the escape depth for the analytical signal (the secondary ions in this case) is of the order of 10-20 Â, thus permitting localization of the analysis to a shallow depth. Since sputtering of the surface atomic layers accompanies the production of the secondary ions, one or more elements can be depth profiled by monitoring the appropriate masses vs. time. Secondary ion images of lateral elemental distributions can be obtained either directly by the use of stigmatic secondary ion optics as in the ion microscope or by rastering a finely focused (1-3 μπι) primary ion beam as in the ion microprobe. References 4, 11, and 13-17 provide detailed discussions of the SIMS technique and the ion pro duction process. Related Techniques. There are three additional techniques presently under development in research labora tories which promise to be valuable surface and thin-film profiling tech niques. Surface Composition by Analysis of Neutral and Ion Impact Radiation (SCANIIR). In addition to the sput tering of neutral and ionized sample atoms, energetic ion bombardment of a surface also produces photon emis sion in the 100-1000-nm region (18, 19). By combining a primary ion source and ion transport optics with a light optical spectrometer and read out, this light emission can be charac terized and used for surface and depth profiling analysis. As in SIMS, the production of the analytical signal by the ion beam is accompanied by sput tering of the surface layers; hence, a depth profiling analysis is obtainable. To date, work in the SCANIIR field has not used microfocused ion beams for high lateral resolution analysis, but there appears to be no fundamental limitation to this mode of operation. Although the detection limits demon strated by SCANIIR to date appear to be an order or two in magnitude poor er than with SIMS, they seem to be
822 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
better than can be attained by AES, BS, ISS, or ESCA. In addition, the in strumentation is less expensive and less complicated than in SIMS since a light, rather than a mass, spectrome ter is used for signal detection. Glow Discharge Mass Spectrometry (GDMS) and Glow Discharge Optical Spectrometry (GDOS). These two techniques which employ high-pres sure ( 1 0 - 1 - 1 0 - 3 torr) diode or dis charge sputtering rather than ion beam sputtering are somewhat analo gous to the SIMS and SCANIIR tech niques. In GDMS and GDOS the sam ple is part of a diode sputtering sys tem (either dc or rf). The sample atoms are sputtered into the highpressure region immediately above the sample surface. In this region the atoms are ionized or excited to optical transitions by interaction with the metastable sputtering gas (generally argon). In GDMS the resultant ions exit the high-pressure region through an orifice into a higher vacuum ( 1 0 - s - 1 0 - 6 torr) where they are mass analyzed generally by a quadrupole mass filter. Full mass scans permit qualitative and quantitative analysis (20, 21 ) . Repetitive monitoring of one or more masses as sputtering exposes successively deeper layers is used for depth profiling analysis. The GDOS technique analyzes the light emission from the excited atoms in the dis charge for qualitative and quantitative analysis. Depth profiles are performed by monitoring the characteristic wave lengths of the elements of interest vs. time (22, 23). The detection limits demonstrated by GDOS and GDMS are similar to those of SCANIIR, i.e., between those of SIMS and AES, ISS, or ESCA. Since diode or discharge sputtering cannot be localized as ion beam sputtering can be, GDMS and GDOS provide little lateral resolution capability. An interesting relationship exists among the SIMS, SCANIIR, GDOS, and GDMS surface analytical meth ods. SIMS and SCANIIR employ ion beam excitation at the sample surface (with 1 0 - 5 - 1 0 - 9 torr pressure in the sample region) with ion and photon detection, respectively. GDMS and GDOS monitor the same secondary processes, ions and photons, respec tively, but with low-enërgy ion sputtering of the sample surface and excitation away from the sample surface by interactions in a high-pressure region. Comparison of Analytical Features Since the above surface analytical techniques utilize a wide diversity of excitation and secondary processes, there are major differences in the analytical characteristics of each. The
Table I. Elemental Sensitivity Technique
Coverage
Specificity
Sensitivity variation
Auger electron Li-U spectrometry (AES) MeV ion backLi-U scattering (ω/2-MeV *He+) spectrometry (BS) Electron Li-U spectrometry for chemical analysis (ESC A)
Good
Ion scattering spectrometry (ISS)
Li-U
Small
Secondary ion mass spectrometry (SIMS)
H-U
good Ai Large - — m poor Good Depends on (also ionization provides efficiency isotopic 104-10s detec tion)
"best" technique for each given ana lytical system will differ depending on the information or characterization desired. Moreover, it is becoming in creasingly obvious that several tech niques must be used to fully charac terize a "sample". The analyst must be familiar with or even expert in the many available techniques if he is to provide a complete sample character ization. This section provides a brief overview of the analytical features of some of the techniques discussed above to give the analyst an idea where to turn when trying to meet the demands of a given analytical system. Only those techniques which are most widely used and more fully evaluated are compared. Therefore, nuclear mi croanalysis, ion-induced X-ray emis sion, SCANIIR, and the glow dis charge optical and mass spectrometry techniques are omitted. Elemental Sensitivity. The first question generally asked of a surface analytical technique is whether it can detect this element of interest at this estimated concentration in the pres ence of this or these matrix elements. Several aspects of elemental sensitivi ty dictate the answer to this question. • Coverage—What elements in the periodic table can each technique de tect based on the fundamental princi ples involved? • Specificity—How well do the fun damental processes and instrumenta tion employed permit the detection of
Lo/good HiZpoor Good
M
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Less than a factor of 10 Sensitivity increases with 7 Bi/O ^ 100 Less than a factor of 10
Sensitivity increases with 7. Bi/O * 5
Detection limits (atomic fraction)
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one element in the presence of anoth er? In other words, is the technique subject to spectral interferences? • Sensitivity Variations—Are the excitation and detection of the analyt ical signal uniform from element to el ement? Sensitivity variations from el ement to element complicate quanti tative analysis and can make it diffi cult or impossible if the sensitivity variations are not predictable or vary owing to matrix changes, exciting con ditions, etc. • Detection Limits—What is the minimum concentration of an element that can be detected in the absence of a spectral interference? Elemental de tection limits can be modified by vari ations in sensitivity and a specific spectral interference. Table I provides a summary of these four aspects of elemental sensitivity for the five techniques to be discussed in this comparison section. The com ments and values of Table I are as generally practiced and in an average situation. Specific analytical situa tions can improve or degrade any or all of the elemental sensitivity fea tures of Table I. Some general trends, observations, and groupings can be made. • All the techniques have the same general coverage, Li to U, with SIMS also being able to detect hydrogen. • The two electron spectroscopy techniques (AES and ESCA) are not highly subject to spectral interferen
824 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
ces. The variation in sensitivity from the most sensitive to the least sensi tive element is about a factor of 10, and the detection limits are in the range of 0.1-1% atomic. • The specificity or elemental reso lution capabilities of the high- and low-energy ion backscattering tech niques are highly dependent on the relative masses of the probing and sample atoms. In both techniques, mass resolution decreases dramatical ly as the target atom's mass greatly exceeds that of the probing or incident particle. The use of probing ions of higher mass improves mass perception with both the backscattering tech niques and is commonly employed in the low-energy technique (ISS). Since radiation damage to the solid-state detector used in the BS technique oc curs very rapidly (a few hours) when ions with a mass above 4 H e + are used, the practitioner of MeV ion backscat tering spectrometry rarely uses prob ing ions of higher mass. He must ac cept the mass resolution illustrated in Figure 1. The ISS detection limits vary by only a factor of five from oxy gen to bismuth, but the average detec tion limit of the ISS technique is of the order of 1% atomic. The major contribution to this poor average de tection limit is that greater than 99% of the incident ions undergo charge neutralization during scattering and cannot be detected by the electrostatic analyzer/electron multiplier spectrom eter system. Ionic charge state is of no concern to the surface barrier detec tors used for detection and energy analysis of MeV backscattered ions in BS; hence, this technique can general ly realize better average detection lim its than ISS. • Intrinsically, the SIMS technique provides excellent elemental specifici ty as well as isotopic resolution. How ever, the presence of spectral interfer ences can prevent the detection of a specific element in a specific matrix (24). As with any analytical technique, the potential for spectral interferences in SIMS increases as the analysis re quires better detection limits. At the 0.1% level (the ultimate detection limit of most of the techniques under discussion), SIMS spectral interferen ces do not occur very often, and the use of high-resolution mass spectrom etry can reduce their occurrence (25). As the concentration of the sought-for element approaches the ultimate de tection of the technique (sub-ppm on the average), the possibility for inter ferences increases. Thus, SIMS does not encounter significant interference problems until one is analyzing at lev els well below the detection limits of most surface techniques. As can be seen from Table I, the SIMS tech nique exhibits large variations in sen-
H
Ne Co Zn Zr
Sn Nd
Figure 2. Bulk detection limits in interference-free situation vs. atomic number for AES, BS, ESCA, ISS, and SIMS
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sitivity from element to element and consequently large variations in ele mental detection limits. These varia tions result from the relative ion yields of the different elements under energetic ion bombardment. Figure 2 graphically illustrates the relative detection limits of these five techniques and the extent of varia tions which can be encountered. This figure shows the bulk detection limits realizable under interference-free con ditions for AES, ESCA, BS, ISS, and SIMS. In-Depth Analysis. All of the five techniques being discussed provide a depth profiling capability. Backscat tering analysis with MeV 4 H e + ions samples all depths to about a microm eter simultaneously. The depth scale (i.e., the depth of a subsurface feature) is established by the energy loss pro cess as the probing 4 H e + ion traverses the sample atomic layers before and after the backscattering event. The energy loss per atomic layer traversed is well established in the published lit erature and by physical principles. Thus, depth scale assignments can be made without direct comparative standards. The parameters controlling depth assignment accuracy are the quality of the literature values (gener ally ±5%), energy straggling effects, and detector resolution. The depth resolutions attainable are of the order of 100-300 Â. However, this is a complex situation, and the reader is referred to the literature for more detailed discussions (9, 26). All of the other techniques (AES, ISS, ESCA, and SIMS) combine a shallow escape depth for the analytical signal (3-20 A) and ion beam sput-
80
JiPPb 100
tering for sequential removal of atom ic layers to perform depth profiling analyses over 10-10,000 Â thick layers. Since all the techniques have essentially the same escape depth, the attainable depth resolution depends on the quality of the sputtering process and its calibration and control in each instrument. Since depth assignment accuracy is dependent on the conversion of sputtering time to material removed, the rate of material removal (A/sec) must be calibrated for the sample matrix under study. This requires that the ion beam characteristics (ion species, energy, and angle of impact) and the sample characteristics (matrix atoms, chemistry, and structure) be identical in the standard and unknown to obtain depth assignment accuracies of ±5%. The need for an accurately known sputtering rate is the main limiting feature in depth profiles employing ion sputtering. If differential sputtering of the different elements or crystalline phases in the sample is not encountered and if all the analytical signal is taken from the same depth into the sample (i.e., crater edge effects are avoided), depth resolutions of 50-200 A can be realized from these techniques. Lateral (x, y) Analysis. For an analytical technique to provide lO-^m lateral microanalyses, some method must be used to localize the emission of the analytical signal. As generally practiced, only AES and SIMS can be considered microanalytical techniques. In the Auger microprobe and ion microprobe, this is accomplished by a microfocused electron or ion beam. Point-to-point analysis is accomplished by positioning this micro-
826 A · ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
focused beam on the area of interest. Images of the lateral distribution of an element or elements is accomplished by synchronously rastering the probing beam and using an oscilloscope whose intensity (or ζ axis) is modu lated by either the Auger transition or ion intensity of interest. The ion mi croscope has the same capabilities through the use of stigmatic secondary ion optics (see the accompanying IN STRUMENTATION article for details). For the two techniques which can provide a localized analysis, there is an important interaction between the area analyzed and the detection limits which are obtained from this area. The detection limits of the AES tech nique are controlled by the number of atoms present in the volume bounded by the area bombarded and the elec tron escape depth, the number of pri mary electrons that can be delivered to the area of analysis (current densi ty), signal-to-background, and instru mental parameters. The detection limits of SIMS are controlled by the number of atoms in the volume of ma terial consumed (determined by the area bombarded and the depth of the sputtered crater), the efficiency with which these atoms are ionized, and the efficiency with which the mass spec trometer can extract, transport, and analyze these ions. Figure 3 illustrates this concept by plotting the concen tration required for 10% statistics vs. the area bombarded by the primary electron beam (AES) or the primary ion beam (SIMS). Since this relation ship is a complex function of many pa rameters, the relationships of Figure 3 were calculated for a given set of con ditions. As can be seen, below ~30 μια2 the attainable AES detection lim its depend on beam diameter. This is due to the low bombarding absolute current at ΙΟ - 7 Α/μπι 2 current density and the limitation of usable time con stants in the lock-in amplifier. How ever, increasing the primary electron current and, hence, the number of Auger electrons by going above ~30 μηι 2 does not improve the detection limits. At this point, fundamental and instrumental effects come into play and do not permit improved detection limits. In the SIMS technique two factors are seen to control detection limits. First, the number of atoms available is determined by the area bombarded since the sampling depth is con strained to 50 A, as might be the case in a depth profiling analysis. Second, the ionization efficiency of a given ele-
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