Metroscope Joseph A. Gardella, Jr. Department of Chemistry and Surface Science Center State University of New York, Buffalo Buffalo, NY 14214
X-ray photoelectron spectroscopy (XPS or ESC A) is one of the many electron spectroscopies particularly useful for the chemical analysis of surfaces. The technique, based on the photoemission of electrons induced by soft X-rays, is widely used for detailed surface analytical problem solving because it allows multiple-element detection, provides chemical bonding and state information from chemical shifts, and provides quantitative information.
optics, and multichannel solid-state techniques for charged-particle detection, however, the technique is still evolving, leading into new areas of application. Of particular importance to the surface analytical chemist are the combinations of instrumentation improvements that lead to better signal-tonoise (S/N) ratio and speed. One result is more sensitivity (i.e., a higher slope of the calibration curve) and lower detection limits for quantitative analysis. In addition, the precision of quantitative analysis is improved because the error caused by instrumental noise is lower. These results allow analysis of increasingly complex samples and detection of a greater dynamic range of
INSTRUMENTATION Many scientists believe that of the methods of electron spectroscopy used for surface analysis, both ESCA and Auger electron spectroscopy are fairly mature in terms of development of information content and applicability in chemical analysis. They are widely used for problem solving in a variety of technological fields, including electronic materials and devices, metallurgical and materials engineering, catalysis, corrosion, and adhesion. Because of rapidly improving instrumentation in X-ray and electron sources, electron 0003-2700/89/0361 -589A/$01.50/0 © 1989 American Chemical Society
concentrations. These advances, combined with ESCA's capability to determine multiple elements in a single sample and provide chemical information, have extended the technique to new areas of interfacial science for ultrahigh-vacuum (UHV) analysis. This article will emphasize recent developments in ESCA instrumentation and methodology that have allowed this technique to be more widely used. All of the methods discussed here have been available to photoelectron spectroscopists for some time, but only
in limited fields of application. We will focus on three areas of instrumentation development: improvements in the source, electron energy analyzer, and detector technology. We will not cover advances in data acquisition speed and analysis methodology that have been driven by advances in computer hardware and software. Developments in electron spectroscopy leading to new quantitative capabilities were described in a previous article (1). Specifically, this I N S T R U M E N T A -
TION article will focus on the added capabilities gained by the use of multichannel electron detectors, highthroughput energy analyzers, and high- (energy and spatial) resolution X-ray sources. Improved efficiency in all of these units has led to higher S/N ratio and greater speed of analysis. Hercules (2) first noted that, of the most widely used applied UHV surface analytical methods, ESCA was slow and had a lower S/N ratio. Thus improvements in ESCA instrumentation that gain speed are especially desirable. The instrumental improvements that will be discussed have led to the following advances: • • • • •
lower X-ray damage, use of lower flux X-ray sources, higher resolution conditions, smaller analysis areas, ease of application of low S/N ratio experiments, and • faster data acquisition. In considering each of these ad-
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Figure 1. Block diagram of an ESCA experiment.
varices, we can compare ESCA instru mentation growth with that of Fourier transform (FT) instrumentation in IR spectroscopy. In the case of FT-IR in strumentation, the S/N ratio and speed increased, allowing far broader appli cation of the method, especially in the area of quantitative analysis. In the examples discussed here, the lower X-ray damage means more appli cations to previously sensitive samples such as polymers and biological materi als. Lower flux X-ray sources, such as those emitting higher energy photons or those that are focused, allow analysis of microstructure in three dimensions without destructive sputtering, or in conjunction with it. The application of higher energy resolution conditions, where losses in total signal previously precluded application, has opened up more chemical state information from chemical shifts. Using focused X-rays or lensing to analyze small areas can be accomplished routinely for 150-μπι spot sizes, and newer instrumentation promises 10 μπι, with 1 μηι easily on the horizon. In addition, low S/N ratio ex periments such as shallow takeoff an gle-dependent measurements for shal low sampling depths, or rapid data ac quisition for time-dependent mea surements, now are more routine; pre viously they were limited because of damage or noise considerations. In general, we can now acquire data faster. In the mid-1970s, data acquisi tion for a-single sample took at least 2-4 h and sometimes longer. Currently data acquisition takes only a few min utes, and better resolution is achieved. Moreover, recent developments prom ise times of less than a few seconds.
However, whereas FT methods of IR analysis decreased data acquisition time to the point of introducing rapid kinetics and flow experiments, this ad vance has not yet occurred for ESCA, although slow kinetics can be moni tored (3). Instrumentation developments Three areas of instrumentation devel opment can be considered as three ma jor experimental components in the typical block diagram of an ESCA ex periment (Figure 1). In this section we will describe the advances in the devel opment and application of multichan nel detection, high-throughput kinetic energy analysis, and high- (energy and spatial) resolution X-ray sources. Multichannel detection. Histori cally, the development of multichannel detection in ESCA analysis was driven by the need to use efficient signal col lection from low X-ray flux monochromatized sources. Siegbahn and FellnerFeldegg's ESCA design, including a fo cusing X-ray monochromator, was realized in the Hewlett Packard (HP) Model 5950 instruments (4, 5), which were outfitted with Vidicon detectors coupled with TV camera detection. Al though the instruments were efficient, quantitative analysis was severely lim ited in dynamic range because of signal saturation effects in the Vidicon. In the mid-1970s Surface Science Laborato ries (now Surface Science Instru ments), the offspring of the HP ESCA development group, developed an ad vanced multichannel detector (MCD) based on a Chevron type design, with a resistive anode strip position computer (6). This detector could be used for en
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ergy or spatial resolution of energetic particles. Other types of MCDs were also de veloped, including the so-called fast re sponse ESCA (or FRESCA) developed at Xerox Webster Research Laborato ries in the late 1970s (7,8). Placing the MCD in the exit plane of the hemi spherical analyzer replaces a slit and single-channel electron multiplier (SCD), which looks at a single channel or bandpass of electron kinetic energies from the analyzer. The MCD simulta neously collects 2n channels of electron energies, gaining speed or S/N ratio from the traditional Fellgett's advan tage (9) by a factor based on the num ber of resolution elements. With wider use of monochromatized X-ray sources, and now with any appli cation where small spots are analyzed, MCDs increasingly are being used to resolve energy and position. Future de velopments involving an increase in re solvable energy/spatial channels are expected. Advances are also being made in single-channel detection effi ciency and gain (10), so that there are reasons for retaining the simpler and more rugged SCDs, which still allow more gain in S/N ratio than does older instrumentation. Kinetic energy analysis. In ESCA, the kinetic energy analysis of electrons takes place using two components: a lens/retardation unit and a kinetic en ergy filter. The two standard means of scanning electron kinetic energy, fixed analyzer transmission (FAT) and fixed retarding ratio (FRR), involve the com bination of both units, discussed by Armstrong and co-workers (1). The ba sic aspects of various kinetic energy fil ter designs have been described by Barrie (11) and Rivière (12) in books edited by Briggs and Briggs and Seah. Although various designs were initially developed for the energy filters, two have been widely used: the cylindrical mirror analyzer (CMA) and the hemispherical sector analyzer. Generally, the CMA is best suited when high throughput is desired at lower energy resolution. Thus it is widely used in Auger analysis, where chemical shift information is generally not used (and therefore higher energy resolution is not needed), because it is so complex (13). (Note that high-resolution Auger spectrometers using hemispherical analyzers are being developed by VG Scientific.) It can be shown (11) that the hemisphere is the most efficient filter for charged particles where high throughput at high-energy resolution is required. Whereas CMAs have been used—especially in double-pass configurations for ESCA instruments com-
INSTRUMENTATION bined with Auger—all ESCA manufac turers except Shimadzu currently mar ket instruments based on the hemispherical analyzer. The need to acquire data and resolve chemical shifts of a few tenths of an electron volt in ESCA makes the hemisphere the op timum choice. Recently, Scienta Analytica intro duced an extremely large hemispheri cal analyzer (60-cm diameter, as op posed to the 30-cm diameter typically found in most commercial instru ments) to obtain higher performance. When compared with smaller diameter hemispheres, this yields either higher throughput at an equivalent resolution or higher resolution at equivalent throughput. However, it is the former capability that will improve the S/N ratio or speed, because most current applications are not limited in resolu tion by instrumentation. Major developments have occurred in the design of the lens/retardation unit. Originally, lenses were placed in front of the energy analyzer to allow longer focal distances to be used for electron collection. This allowed multi ple-probe and analyzer units to focus on the same sample for multitechnique instruments. Currently the role of the lens/retardation unit is dual: to focus electrons collected at the entrance plane/slit of the energy analyzer and to retard the electron energies to a range where higher overall energy resolution can be gained. Advanced methods of electron optics design have yielded commercially available multiple lens units with apertures. These additional lenses have allowed the focusing role to be expanded to one where high throughput of electrons collected from a small area or spot can be analyzed (14-17). Thus, even without the use of focused X-ray sources (discussed be low), small areas can be analyzed or microscopically viewed with spatial resolution (commercially available) to spot sizes of 200 μνα with a standard Perkin-Elmer (PE) Omnifocus lens system (15) and the newly introduced Vacuum Generators's ESCAscope and Surface Science Instruments's MicroESCA (which is based on entirely different focusing/collection methods (18)), each specifying 10 μνα. Using a modification of Wannberg's basic de sign approach that produced the Om nifocus lens, Scienta's ESCA 300 also specifies areas to a few micrometers. This is accomplished with both a focus ing lens and the focused source dis cussed below. High-resolution X-ray sources. X-ray source capabilities have been ex tended to high(er) energy and spatial resolution. Again, it is interesting to
Table 1. Characteristics of X- •ray sources for ESCA Anode material MgKa AIKa a Si Κα ZrLa Au Ma Ag La 0 TiKa CrKa
Energy (eV)
Line width (eV)
Relative intensity
1253.6 1486.6 1739.5 2042.4 2122.9 2984.3 4510.9 5417.0
0.7 0.85 1.0 1.7 2.4 Not available 2.0 2.1
1.0 0.5 0.19 0.05 0.03 0.02 0.05 Not available
* Often used with monochromator.
consider the historical development based on the Siegbahn/Fellner-Feldegg/HP X-ray monochromator (4, 5), which was developed to decrease line width of the X-ray source and to filter unwanted Bremstrahllung and X-ray satellite background. This approach has reached the stage where natural Xray line widths are hardly broadened by the source; line widths of 0.34 eV are measured on the Si 2p core levels with the X-ray monochromator available from PE. However, in the HP design (4, 5), focusing considerations of the Row land Circle monochromator mounting of the X-ray crystals led to commer cially available focused X-ray mono chromator output available on Surface Science Instruments's ESCAs (15). Us ing a focused electron beam source to the anode allows an initial point source of X-rays before monochromatization. This has permitted the routine excita tion and data collection from small spots to a size of 150 μία. Thus the higher energy resolution gained from the monochromatized source (tradi tionally ΑΙ Κα radiation) using bent crystals is also spatially focused. Three other advances are likely to be important in the near future with the application of monochromatized X-ray sources. The first is the use of alterna tive anode materials. As discussed be low, many other X-ray sources have been available to supplement informa tion from the typical Mg Κα or ΑΙ Κα sources (Table I). Two other sources, Ag La (2984 eV) (17) and Cr Κα (5417 eV), are particularly appropriate for monochromatization, even though they are limited to low fluxes relative to Mg Κα. Both sources have near inte gral multiples of energy relative to Al Κα (Ag La = 2.0075 Al Ka (17,19); Cr K a = 3.64 Al Ka). With reasonable ad justment of alignment of X-ray mono chromator crystals (in the case of Ag, the second-order reflection can be used directly), the same spacing and curva ture as those for the Al Κα can be used
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to narrow the X-ray line, eliminate Bremstrahllung background, and filter X-ray satellites with higher energies. This accesses more ESCA signals and allows sampling to deeper depths (see below). Secondly, high flux output rotating anode designs, although complex and bulky, promise to increase data yield to the point where data acquisition in the newly introduced Scienta ESCA, using the rotating anode source, can take place in less than 1 s at extremely high resolution. Further in the future is the develop ment of smaller spot size focusing op tics. Golijanin and Wittry (20) have published encouraging results of X-ray focusing crystal experiments using plastically deformed focusing crystals that promise l-μιη spot sizes, and they also have proposed using this for X-ray microprobe use. The application to ESCA source design is natural and will allow microstructural analysis of small er and smaller features with very high resolution.
Extended areas of application The six advantages yielded by the new instrumentation developments listed at the beginning of this article can be illustrated by a number of examples. Interest in our laboratories (and many others) has extended into materials that are sensitive to X-ray damage (e.g., organic, polymeric, and biological surfaces). Applications to all of these areas are important in biomedical ma terials, biological adhesion and corro sion problems, and general composite materials science and engineering. Unique challenges in these fields, some outlined recently by Hercules (21), are posed for the surface analytical chem ist. One specific need is the ability to describe composition, structure, and bonding heterogeneity near or at the surface of materials in the lateral and transverse planes. X-ray damage is a key aspect of anal-
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ysis in ESCA, which, as Hercules ob served, is the least "damaging/destruc tive" of the major surface analytical tools (2). Because X-ray sources are used, there is little limitation to con ducting or semiconducting samples. In sulators are routinely analyzed by ESCA. However, one still must be con cerned with damage to sensitive mate rials. Damage encompasses many mechanisms, including the primary Xray induced chemical changes and the secondary factors of heating in vacuo, secondary electrons, Bremstrahllung, etc. (22, 23). A monochromatized source (4,5) was first used to determine that most damage observed in ESCA experiments was not a result of prima ry X-ray flux. Rather, it resulted from a combination of the secondary factors listed, most of which involve instru mental factors not related to X-ray damage (22). Although most solids are stable under the X-rays in a typical ESCA experiment, many materials de grade with heating or secondary elec trons in vacuo before spectral data can be acquired. A typical example is polyvinyl chlo ride) (PVC). PVC is very sensitive to radiation of any sort, and the primary degradation process involves dehydrochlorination. Thomas and Chang (24) showed that this can be evaluated in ESCA using both the ratio of carbon to chlorine and the ratio of the two equal intensity peaks in the carbon Is enve lope. Their results show that some of the sensitivity to damage is caused by oxygen-containing additives, which ac celerate the kinetics of degradation. This concern is exemplified in our work to perform nondestructive ESCA depth profile measurements on multicomponent polymers, to determine the distribution and extent of surface ex cesses of one component in the mix ture, or to determine microdomain het erogeneity near the air interface. Blends of various homopolymers with PVC have been studied by using angledependent ESCA to provide the depth profile (25, 26) of surface excess of one component. With modern collection and filtering and multichannel detec tion—even with a standard X-ray source—what took Thomas and Chang a minimum of 15 min of acquisition time for a single analysis angle can be accomplished in 2-3 min (25, 26). This has allowed the collection of data for three separate angle-dependent mea surements in less than 10 min total, ensuring against detecting chemical ef fects induced by damage. Other groups have studied the dam age induced by focused X-rays from the small spot source (27). They con cluded that the filtered Bremstrahl-
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lung, the lack of secondary electrons, and the separation of the X-ray source from the sample (thus eliminating heat transfer) drastically reduce damage even in very sensitive polymers. Even with the X-rays focused into a small spot, damage is limited to that region of a sample so that analysis of adjacent regions is affected minimally. Thus the added S/N ratio can be used to de crease total analysis time and to limit damage to a variety of samples previ ously considered too delicate for ESCA analysis. The added S/N ratio can obviously be used to address the use of weaker sources. This was the initial reason for the multichannel Vidicon detector in the HP instrument (4,5). However, one burgeoning area is that of higher ener gy photon sources (17,19, 28). The de sire to use other sources of X-rays is twofold. The higher energy X-ray pro duces electrons with higher kinetic en ergy, which can travel from deeper re gions because of attenuation effects. This produces deeper sampling depths, the so-called energy-dependent mea surements. Additionally, higher energy core-level orbitale can be excited, al lowing use of Auger parameter analysis (28, 29) not accessible with a standard Mg Κα or ΑΙ Κα source. Application of this approach previously was limited by the lack of flux from these sources, some of which are listed in Table I. Castle's group (19) has worked on the application of alternative X-ray sources, with special emphasis on add ed chemical information from Auger parameter measurements, which is very useful for metallurgical and corro sion problems. In our laboratory we attempted to expand the use of the Ti Κα X-ray source (he = 4510.9 eV) to polymers (30, 31). The Ti Κα source is particu larly useful because of the desire to sample much deeper into polymers, and the sampling depth estimated from empirical electron attenuation length relationships is approximately twice that of the Mg Κα sampling for the carbon Is photoelectron (30, 31). Although use of the Ti Κα source was attempted earlier by Clark (32), the S/ Ν ratio limited the full use of high reso lution and introduced problems with analysis of polymers and other materi als sensitive to damage. Our results, taken with a multichannel detectorbased instrument, have allowed the collection of data with analysis times of less than 1 h, thus minimizing damage in sensitive polymers. A table of acces sible Auger lines for third-row ele ments of importance in high-perfor mance polymers (phosphazenes, siloxanes, sulfur-containing polymers) and
INSTRUMENTATION
Table II. polymer Anode Mg Mg Mg Ti
sis of insulators, where charge-based broadening contributes to overlapping lines from chemical shifts, reducing the utility of functional group analysis from curve resolution. The use of high er energy resolution is illustrated in Figure 3. ESCA sulfur 2p lines for a highly ordered rigid rod polymer, poly(bisbenzothiazole) (PBT), are shown as a function of analysis angle (15°, 45°, and 90°). The high resolution is obvi ously able to partially resolve the spin orbit splitting of the 2p 3 / 2 and 2pi/ 2 lines. At shallow angles of analysis (shallower sampling depths), the line broadens. Without the high resolution, this slight change would not be detect ed, yet this broadening effect can be attributed to the disordered structure near the surface of the polymer, where slight amounts of amorphous material would be situated.
Depth-dependent analysis of poly(ether urethane)
Depth (A)
Carbon/oxygen ratio
Nitrogen/carbon ratio
Nitrogen/oxygen ratio
27 73 103 215
4.0 4.0 4.0 4.0
0.04 0.04 0.05 0.07
0.15 0.17 0.19 0.32
quantitative sensitivity factors is re ported for typical polymer-containing elements (30, 31). One particularly interesting applica tion involves the energy-dependent depth profiling to complement the an gle-dependent measurements on multicomponent polymers. Building a depth profile from both angle- and en ergy-dependent measurements is com plicated by the fact that sampling depths are based on a multiple of the inelastic mean free path, or attenua tion length, of electrons through the material. Thus the sampling depths quoted are in fact integral values, with the signal contribution from the top most attenuation length more impor tant than the second and third attenu ation lengths. However, adding infor mation with greater sampling depths can complement more surface-sensi tive methods, where the profile of con stituents involves species segregated from the surface. Table II shows the results of angledependent and Ti Κα analysis of a seg mented poly(ether urethane) (Figure 2) block copolymer (30, 31) where the nitrogen-containing urethane hard block component is segregated from the topmost interface, and the deter mination of the nitrogen profile is com plicated by low concentrations in the region sampled by the Mg Κα. The val ues of sampling depths quoted are three times the attenuation length, de termined empirically from tabulated data (30, 31). The absolute values of sampling depth are estimated; howev er, the relative values are extremely im portant, especially given the nitrogen concentration at deeper depths. Confi dence in the distribution and detection of the urethane (nitrogen) profile is in creased when the Ti Κα data are con sidered. The use of higher energy resolution conditions usually involves both the monochromatization of X-rays (4, 5, 16) and the proper filtering in the lens/ retardation unit and kinetic energy fil ter (16). The development of X-ray monochromators has continued to pro
vide instruments with narrower and narrower line width specifications. The standard for the fwhm of silver 3d5/2 is now 0.50 eV, as compared with typical specifications for nonmonochromatized radiation for the silver 3ds/2 line at 0.80 eV. The practical ramifications of the narrower lines are important in analy
