Anal. Chem. 1987, 5 9 , 2081-2087 I
Day 0
Day 7
49
Day 9
0 46
2081
number of substitutionally inert coordination complexes which we studied behaved in a straightforward manner, the same cannot be said of the more prevalent substitutionally labile coordination complexes. As we have shown, this phenomenon can prove to be both a help and a hindrance. While speciation information for certain metal-containing species might be possible, any identification based solely on retention time comparisons is risky at best given the possibility of (a) irreversible adsorption, (b) time-dependent peak intensity, (c) retention data which depend on the amount injected, and (d) the tendency for ligand exchange in certain systems.
LITERATURE CITED
,
0
I
2
4
Retention Time (rnin )
-re 0, Three replicate SFC separations (2 mL/min) of CHCI, solution (5 mi5 p l ) of NNacac), on a C,,column at 99 O C and an inlet pressure of 5000 psi: mobile phase, CO,/MeOH (80/20); detector, 280 nm.
material in the solution which elutes in the broader peak is being converted to material which elutes as the sharp peak. As expected, the increasing amount of reversibly adsorbed material does not affect the retention time, whereas the irreversibly adsorbed species (since its concentration is decreasing) elutes at progressively longer retention times. This observation is in agreement with data earlier presented for Fe(acac)3 which eluted in a large peak volume. As was the case with Cu(acac)z,the chemistry taking place with Ni(acac)z in the injection solution can only be conjectured. In this case, the nickel compound is known to be a trimer in the solid state. The hypotheses given for the copper case may also apply here. A nickel-specific detector would be desirable in order to determine whether the early eluting peak contains nickel. In summary, bonded-phase packed-analytical columns provide an adequate separation of some metal-containing materials. The interpretation of SFC data regarding neutral inorganic systems must, however, be cautiously performed. This is especially true for coordination complexes rather than for true organometallics such as ferrocenes. While the limited
(1) Johnson, C. C.; Jordan, J. W.; Taylor, L. T.; Vidrine, D. W. Chromafographia 1985, 2 0 , 717. (2) Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1984, 5 6 , 619A. (3) Chester, T. L. J . Chromatogr. 1984, 299, 424. (4) Holzer. G.; DeLuca, S.; Voorhees, K. J. HRC CC, J . High Resoluf. Chromafogr. Chromafogr. Commun. 1985, 8 , 528. (5) Gere, D. R.; Board, R.; McManigill, D. Anal. Chem. 1983, 5 4 , 740. (6) Kobayashi. M.; Saitoh, K.; Suzuki, N. Chromafographia 1985, 20, 72. (7) Smith, R. L.; Iskandaranl, Z.; Pietrzyk, D. J. J . Liq. Chromafogr. 1984, 7, 1935. (8) Fish, R. H.; Komlenlc, J. J.; Wines, 8. K. Anal. Chem. 1984, 5 6 , 2452. (9) Dilli, S.; PatsalMes, E. J . Chromatogr. 1983, 2 7 0 , 354. (10) Estes, S. A.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1982, 5 4 , 2402. (11) Willeford, B. R.; Veening, H. J . Chromatogr. 1982, 257, 61. (12) Klesper, E.; Corwin, A. H.; Turner, D. A. J . Org. Chem. 1982, 2 7 , 700. (13) Karayannis, N. M.; Corwin, A. H.; Baker, E. W.; Klesper, E.; Walter, J. A. Anal. Chem. 1968, 4 0 , 1736. (14) Karayannis, N. M.; Corwin, A. H. Anal. Blochem. 1988, 2 6 , 34. (15) Karayannis, N. M.; Corwin, A. H. J . Chromafogr. 1970, 4 7 , 247. (16) Karayannis, N. M.; Corwln, A. H. J . Chromatogr. Sci. 1970, 8 , 251. (17) Jentoft, R. E.; Gouw, T. H. Anal. Chem. 1972, 4 4 , 681. (18) Wenckwiak, B.; Blckmann, F. fresenius' 2.Anal. Chem. 1984, 379, 305. (19) Bickmann, F.; Wencbwiak, B. fresenius' 2.Anal. Chem. 1985, 257, 61. (20) Semonian, B. P.; Rogers, L. B. J . Chromatogr. Scl. 1978, 16, 49. (21) Crowther, J. B.; Henion. J. D. Anal. Chem. 1985, 5 7 , 2711. (22) Wright, R. W.; private communication. (23) Chueh, P. L; Prausnitz, J. M. AICHE J . 1967, 73, 1099. (24) Laver, H. H.; McManigill, D.; Board, R. D. Anal. Chem. 1983, 5 5 , 1370.
RECEIVED for review December 8, 1986. Accepted May 29, 1987. The financial assistance of the Commonwealth of Virginia and Department of Energy Grant DE-FG2284PC70799 is greatly appreciated.
Secondary Ion Emission from Metal Targets under CF,' 0 , ' Bombardment
and
Wilhad Reuter ZBM T. J. Watson Research Center, Yorktown Heights, New York 10598
CF,' has been extracted from a cold cathode Ion gun supplied wlth a CF,/N, gas mixture. A CF,' beam current of about 200 nA is obtained, whlch Is about a factor of 5 smaller than the - 1 pA 0,' beam currents obtalnable from conventlonai oxygen Ion sources. This decrease k partially offset by the hlgher sputter yields obtalned with CF,'. Ionlzatlon probabliltles are hlgher or equal to those found for 0,' bombardment. Composition and chemical state analyses were performed after saturation bombardment. Generally metal carbldes are found to have formed wlth a fluorlne uptake of about 10-20 atom %. The fluorlne is bonded to the metal. Matrlx effects are smaller under 0,' bombardment due to the low fluorlne concentratlon.
It has long been recognized in secondary ion mass spectrometry (SIMS) that the sensitive detection of electropositive elements in a metallic target requires bombardment with primary ions which form strong ionic bonds with the target atoms. Traditionally, ion sources are operated in an oxygen ions. Excellent ambient to produce abundant amounts of 02+ secondary ion yields are obtained for those elements that can be completely oxidized and which form strong ionic bonds with oxygen. It has been shown in a static SIMS study of positive metal ion (M+)emission that for such elements as Mg, Al, Cr, Si, and Fe, more than 10% of all sputtered particles are emitted in the M+ state ( I ) . Secondary ion yields, however, may be less by up to several
0003-2700/87/0359-2081$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
orders of magnitude for those elements (e.g., Cu, Ni, Zn, Pd, Cd, Ag) that form only weak bonds with oxygen or that are only partially oxidized under 0,' bombardment (2). For example, the yield of Fe+ per primary ion (Ar+) from an oxygenated Fe surface ( I ) is a factor of 8 larger than the Ni+ yield from an oxygenated Ni surface. Nearly the same enhancement was found for the Fe+ vs. the Ni+ yield for the pure bombardment (2),although the first ionimetals under 02+ zation potential of Fe is even higher (7.83) than that of nickel (7.61) and their sputter yields are identical to within h20% ( 3 ) . Furthermore, since both elements have about the same mass, the respective instrumental transmissions for Fe+ and Ni+ are nearly identical. Similar masses and our observation (3)that Fe+ and Ni+ have overlapping secondary ion energy distributions also implies that their escape velocities are nearly the same, a condition required in the comparison of ion yields because of their strong dependence on the escape velocity ( 4 ) . From our earlier work (3)we know that both metals are nearly fully oxidized under normal incidence bombardment (Le., to NiO and Fe203,respectively). These differences in ion yield can be attributed to differences in the ionic character of the NiO and Fe203bonds. Using the empirical expression ( 5 )
with the electronegativities x from the Pauling scale and our first-order approximation that qM0, = x q M O
(2)
we find that the charges q for NiO and FeOl,5 are 0.47 and 0.70, respectively, with x being the atomic ratio of the anion. Experimentally, the stronger ionic character of FeOl vs. NiO is verified by the larger chemical shift found by X-ray photoemission spectroscopy (XPS) for Fe3+- Feo = 2.8 eV vs. the shift Ni2+ - Nio = 1.7 eV ( 3 ) . One would expect a dramatic increase in the Ni+ yield under F2+bombardment if the target is converted to NiF,. The charge on the Ni2+ site is 1.16 with the respective x values of 4.0 and 1.8 for F and Ni. The much larger ionicity of the NiF, compared to the NiO bond has been verified experimentally (6) by an XPS study of a series of simple Ni(I1) compounds. Practically full ionization would be expected since the ionic character of NiF, is considerably stronger than that of iron oxides (FeO or Fe203) for which an Fee yield per primary projectile of 0.35 has been reported (1). Implicit in the correlation of binding energy shifts with changes in the ionicity is the assumption that changes in the crystal potentials are proportional to changes in ionicity for compounds of similar structures. Alternatively, such a correlation is valid if the ionicity term is much larger than the crystal potential term. At least in a first-order approximation one of the two conditions or both prevail in the cases studied in this paper. Problems associated with the high reactivity of F, all but preclude the use of this gas in an ion source. In this paper we have circumvented this problem by the use of fluorocarbons which predominantly generate CF3+ in a cold cathode ion source. Some preliminary results have been published previously (7). After a brief description of the operating characteristics of this ion source, we will systematically compare the performance of the CF3+and 02+ sources with respect to sputter yields and sensitivities. We will apply XPS in order to understand a t least qualitatively the observed differences in relative ionization probabilities under Os+ and CF3+bombardment. Finally, matrix effects (i.e., the influence of A in a binary system AB on the ionization probability of B+) will be compared for the two ion sources. Work is presently in progress to ascertain both the merits and disadvantages of CF3+bom-
bardment in some specific depth-profiling applications.
EXPERIMENTAL SECTION The analytical system combines XPS-Auger electron spectroscopy (AES)-SIMS capabilities under ultrahigh vacuum (UHV) conditions and has been described earlier (8). The gas inlet system of the cold cathode ion gun (9) was modified by the addition of a second leak valve permitting independent gas flow controls of CF, and of the carrier gas (N2or 02).No modifications were made to the ion source or to its power supply. With a clean ion souice and with pure CF, at a source pressure of 1.3 Pa, the initial discharge conditions with respect to discharge voltage and current are identical with O2operation with no changes made in the power supply setting. Within a few minutes the discharge current will drop from a typical value of -10 mA to about 2 mA, stay at this level for about 20 min, and will then rapidly drop to zero. This time is sufficient to perform the primary beam analysis shown in Figure 1,but is far from adequate for a usable ion source for SIMS. The breakdown is caused by the formation of an insulating layer on the anode cylinder of the ion source. Photoemission spectroscopy done on the anode surface after source breakdown gave an F/C ratio of 1.04 with Teflon as a standard. The binding energy difference of the F 1s and C 1s was 400.3 eV, in reasonable agreement with the energy difference of 399.6 eV reported (IO) for the C-F configuration. A second low-intensity (- 10% of C-F) component in the C 1s region is found, shifted by 3.5 eV toward higher binding energy consistent with the CF3assignment made in ref 10. If the formation of the polyfluorocarbon is a surfacecatalyzed reaction, one would expect the life of the source to change with different anode materials. We did not observe any lifetime changes when the inside of the A1 anode was lined with Au, Pd, or Ta metal sheets, from which we conclude indirectly that the rate-determining step in the polyfluorocarbon formation is the formation of unsaturated fluorocarbon precursors in the plasma. There is evidence, however, from reactive ion etching (11) that polymer formation can be substantially reduced by raising the substrate temperature to 80 "C or higher. External heating of the anode cylinder may for this reason greatly increase the lifetime of the source. In this study we have taken an alternative route to increase the life of the source based on the assumption that the addition of a chemically reactive carrier gas will reduce the abundance of unsaturated fluorocarbon precursors in the polymer formation in the discharge. Nitrogen and oxygen were used, both being about equally effective in increasing the source life from a few minutes to about 15 h. Such a life, while acceptable in the context of this study, is still too short for routine applications in SIMS analysis. The optimum partial pressure ratio C,F2n+2/02,N2 in respect to life and extractable CF3+current is about unity. The mass separation of the primary beam is accomplished with a Wien filter in the primary beam line. The data in Figure 1were taken under typical analytical conditions; Le., the primary beam optics were adjusted for best ion focusing conditions thereby introducing divergency of the beam and hence relatively poor mass resolution. The results of the primary beam analysis are shown in Figure 1. At a constant magnetic field of the Wien filter, mass dispersion is achieved by varying the electrostaticplate potential. The target currents were measured on silicon without the application of a bias potential. At a bias potential of +40 V, target currents saturate at values about 40% lower due to the suppression of secondary electron emission. We suspect that the CzFS+peak is due to the ionization of products formed in the plasma rather than due to the presence of a C2F6 impurity in the gas. This is supported by the mass analysis of the primary beam extracted from a C2F6 discharge (not shown here) yielding a C2F6+component only 40% of the intensity of CF3+;i.e., the C2F5+signal in CF4 cannot be explained by a small impurity concentration of C2Fs. Also shown in Figure 1 is the primary beam analysis of a 21 mixture of N2 and CF,. About 200 nA of CF3+can be extracted from the ion source under the focused beam condition. CNF+ is formed abundantly, indicating a plasma reaction of CF, and N2 and thus decreasing the rate of polymer formation and increasing the life of the source. Finally we tried C3F8as a higher member in the homologous series of C,F,,+,. Cracking patterns reported in ref 12 give a C3F7+ component with a 25% abundance relative to CF,'. In terms of
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
I
I
10 keV N:
CF;I
IO -6
--a I-
z
Ag
w
a
Table I. Sputter Yields, S, in Atoms per Primary Ion Determined Experimentally or Calculated with the Formalism Given in Ref 15
Si Cr Ni cu Ge
10
Ta
10
L K W
F 10
100
200 PLATE POTENTIAL ( V )
3 00
Figure 1. Primary beam mass spectra for the respective ion source gases obtained with a low mass resolution Wien filter.
the number of atoms incident on the target, C3F7+would thus be almost equally effective. The use of such a large molecular ion has considerable appeal in SIMS applications. Outside of the limits of space charge broadening dictated by the available beam current, beam focusing can be accomplished at 5 keV with the actual energy per particle reduced to 500 eV due to equal energy partitioning: Beam induced cascade mixing (13) should extend only over a few monolayers, resulting in much better depth resolution in SIMS analysis. The results as shown in Figure 1 are disappointing. CF3+is again the predominant species with a C3F7+component of only about 5%. For this reason and the faster polymer deposition rate experienced with the C3F8/N2source, we have used in this study a CF4/N, gas mixture of about a 1:l ratio and CF3+as the primary ion. The secondary ion mass spectrometer is described in ref 8. The primary ion beam was focused and raster scanned and the secondary ion signal electronically gated to accept signals only from the central 5% fraction of the raster scanned area. Secondary ion yields were integrated over a 36-eV energy range since the width of the energy distributions changed by about a factor of 3 for a given projectile over the range of elements selected. Target current measurements were made with a +40 V bias potential on the sample. By this method, secondary electron emission is supressed giving a reasonable estimate of the primary beam current. The XPS work was done in a Hewlett-Packard 5950 B XPS spectrometer. The instrument is equipped with a monochromatized A1 Kcu source. The energy resolution was determined on Au 4f and was 0.9 eV. The binding energy scale was calibrated against the Fermi edge of argon sputter cleaned chromium and nickel. For good statistics, the Fermi edge region was run overnight at maximum X-ray power. Charging was not observed except for silicon forming a fully oxidized layer of at least 110 8, after saturation bombardment with 10-keV 02+ at normal incidence.
RESULTS A N D
DISCUSSIONS
Sputter Yields. The sputter yields S (atoms per primary ion) summarized in Table I were determined at normal incidence for both 02+ and CF3+beams a t 10 keV. The error in the data is about f 3 0 % , primarily due to the uncertainty in the primary beam current. The relative accuracy is about f 1 0 % since target current measurements were made on only one standard element (Si). Our sputter yield values for silicon
2083
S(Oz+, 10 keV, exptl)
2S(O+, 5 keV, calcd)
0.7 0.60 0.96
1.5 3.2 2.7 4.4 2.5 5.6 0.96
1.3 2.3 5.0 0.55
S(CF,+, 10 keV, exptl) 1.9 2.3 9.2
13.3 5.0 7.6 1.8
4S(F+, 2.5 keV, calcd)
3.0 6.7 5.6 9.1 5.2 11.2 2.0
and germanium, 0.7 and 2.3, are in reasonable agreement with those reported by Wittmaack (14),0.52 and 2.8, respectively. For comparison we have also listed the sputter yield data for 5-keV O+ bombardment using the empirical equation developed by Yamamuri et al. (15) and the compilation of sublimation energies in ref 16. The agreement is nearly perfect for Ge and Ag. We know from earlier work (17) using XPS that under only a small concentration of oxygen normal incidence of 02+ (GeO,,) can be incorporated into the Ge target. Unpublished data by us show that in Ag even less oxygen (Ag0o.J accumulates in the target. It appears that, in the absence of large changes in the target composition under reactive ion bombardment, the formalism used gives fairly reliable estimates for sputter yields. For Si, Cr, Ni, and Ta, our sputter yield values are much smaller than those calculated by using the equation given in ref 13. It is known that Si (17,18) and Cr ( 3 ) are fully and Ni ( 3 ) is nearly completely oxidized under O,+ bombardment; i.e., sputter yields in the oxygen-diluted matrixes are smaller than the calculated values. The experimentally determined sputter yields under 10-keV CF3+bombardment are larger than those found for 10-keV 02+.Excluding Ni, the ratio of S C F 3 + / s O 2 + is 2.8 with a standard deviation of 0.84. In this energy range, 2.5 keV per projectile atom for CF3+ and 5 keV for OZ+, the energy dependence of S can be neglected. By use of the formalism in ref 15, So+calculated for 2.5-keV O+ is only 11% smaller as averaged for all elements in Table I than the value of So+a t 5 keV. The observed ratio S C F 3 + / s O 2 + can be rationalized in terms of the increased number of projectile atoms and a small increase in average atomic mass of the CF3+ vs. O,+. The important consequence for analytical applications of CF3+is that the decrease in available beam current is partially compensated by the increase in sputter yields. The calculated yields for four primary F ions at 2.5 keV are in reasonable agreement with the experimental data SCF3+ at 10 keV, whereas for Ni the calculated value is lower and for Si and Cr significantly higher. We do not understand these deviations. It should be noted, however, that the sputter yields found are consistent with physical sputtering. Chemical etching, the prevailing mechanism in CF, rf sputtering systems (19),can be ruled out here since for this mechanism yields are expected which far exceed those predicted by physical sputtering. This point has been previously made for the case of Si (20). Relative Ionization Probabilities. The relative ionization probability pi+for the formation of a metal ion (M+) under CF3+bombardment normalized to pi+ under either 02+ or N2+ bombardment was calculated from Pi+,M+,CF3+ Pi+rM+,0zf,N2+
-
(i~,M+/ipSf)CF3+
(3) (iS,M+/iPSf)02+,N~+
where is,M+ are the measured secondary ion currents integrated over the energy distribution of the selected isotope of fractional abundance f , S values are the sputter yields from Table I, and i, is the primary beam current. After an initial optimization
2084
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
Table 11. Ionization Probabilities (eq 3) for Seven Elements for Normal Incidence 10-keV CF,' Bombardment Relative to the Ionization Probability of Either N2+or 02+ 10-keV Normal Incidence
Ni cu Ag Ta Cr Ge Si
CF3+/N2+
CF3+/02+
135 267 41 4.6 150 20
9 17 10 0.4 1.8 1.0 0.18
0.6
of the transmission of the secondary ion optics and a spot check that this setting is independent of the primary projectile, no changes were made in the ion optics or the sample position in order to ensure equivalence in conditions for the three series of measurements. A primary beam energy of 10 keV was used for all projectiles. The results are summarized in Table 11. We included data for N2+ bombardment to give a more quantified assessment of the long recognized observation that the substitution of Oz+by N2+leads to large reductions in the ionization probabilities because the ionic character of nitrides formed in the target is much weaker than of oxides. More important are the ionization probabilities with CF3+ bombardment relative to those with Oz+bombardment. The error is estimated to be about f30% primarily due to uncertainties in the primary beam current measurement affecting the reliability of the sputter yield data. The results can be roughly grouped into three sections. The first section (Ni, Cu, Ag) contains elements that give low secondary ion currents (2) under 02+ bombardment. The data in ref 2 are secondary ion currents taken a t constant primary beam current. About a 10-fold increase in the ionization probability relative to OZ+ is found under CF3+bombardment consistent with our initial expectation in this study. The elements in the second section give ionization probabilities for CF3+not much different from Two of these elements (Ta, Cr) give high those found for 02+. secondary ion currents (2) with 02+.Most of the sputtered particles are emitted as Me+ and the use of CF3+will not lead to a further enhancement of the ionization probability. CF3+ bombardment did not increase the ionization probability for Ge, although the secondary ion current under 0,' operation is low due to insufficient oxygen incorporation ( 1 4 , 1 7 ) in the target. For reasons not understood, the yield remained unchanged with CF3+. This suggests that only trace levels of fluorine are incorporated in the target. For silicon we find CF3+less effective than Oz+in the production of Si+. X-ray Photoemission Spectroscopy. We used XPS after saturation bombardment of pure element targets with CF3+ and Ozf under normal incidence to gain a better understanding of the chemical state changes induced by reactive ion bombardment and the resulting changes in the ionization probabilities. The primary beam energy was 10 keV except for Ni and Cr where we used data obtained previously (3) with 15keV Oz+.For 5-keV Of, the projected range in Si calculated with the range equation of ref 21 is 130 A. We previously experimentally established (17) a lower limit of 110 A. For 2.5-keV F+,the calculated range decreases to 80 A. From this we conclude that the probed depth in XPS even at the highest photoelectron energy (1400 eV) used here is smaller than the converted layer thickness. The generation of a target for the XPS study is not trivial. The acceptance area for the XPS signal is about 1.5 x 6 mm and is much larger than the raster scanned 1 mm2 ion bombarded area. This required 18 overlapped areas bombarded to saturation as judged by the simultaneously monitored SIMS signal. For this reason only Ni, Cr, and Si were selected for the XPS study. After satu-
Table 111. Composition in Atomic Percent of a Si, Cr, and Ni Target after Normal Incidence 10-keV CF3+Saturation Bombardment
atom 70 target atom % C atom % F C 1s (eVY F 1s (eV)a
Si
Cr
Ni
62 29 9 283.0 686.3
50 31 19 283.0 685.5
62 27 11 284.0 684.8
a Also shown are the binding energies of the respective C 1s and F 1s core levels.
ration bombardment the sample was transferred into the XPS chamber via a vacuum interlock. Some of the XPS results for CF3+ bombardment are summarized in Table 111. The atomic concentration CA of the element A was calculated from
(4)
where I is the background corrected integrated photoelectron intensity and u, A, and Tare the photoionization cross sections, the mean free path, and the instrument transmission for the respective elements and photoelectron energies. On the basis of experience with a wide variety of targets of known composition, we expect that concentration ratios should be accurate within *30%. The most striking result is the small F/C ratio of about 0.5; i.e., F is rejected by the target. Almost the identical value of 0.6 for the F/C ratio was reported by Coburn et al. (20)for silicon after 2-keV CF3+bombardment. Possible explanations were suggested (20)to be the following: (1)a large ratio of the sputtering of F when compared to C; (2) the recombination of two fluorine atoms to form Fzwhich is subsequently desorbed; (3) a low sticking probability for fluorine atoms from energetic CF3+as compared to carbon atoms; and (4) possible reduction of the fluorine surface concentration through desorption of SiF,. The fact that we find such a low ratio for Cr and Ni targets (which do not form volatile fluorides) should allow the exclusion of (4) as a possible bombardment (14,17) mechanism. We know that under 02+ full SiOz stoichiometry can be generated whereas under CF3+ bombardment the highest F/M ratio found (Cr) is only 0.38. It appears that carbon plays a major role either through mechanism 1 or 3 in lowering the implanted fluorine concentration. Silicon. The F 1s binding energy (BE) after CF3+ bombardment is found to be 686.3 eV, in good agreement with the value of 686.5 (22)for F 1s found for SiF4 absorbed on Si at -150 "C. Bonding of fluorine to carbon can be excluded in view of results (23) found for partially fluorinated graphite 689.2 eV (CF) and 687.2 eV (C4F). Carbon 1s is found at 283.0 eV in reasonable agreement with the BE of 282.5 eV reported (24)for Sic. Carbon bonded to fluorine in a polymeric chain of (CHFCH2), results in a large shift to 287.6 eV (25),giving additional evidence that fluorine is not bonded to carbon. The Si 2p spectrum is shown in Figure 2 for Ar+ sputter cleaned silicon and after saturation bombardment with 10-keV 02+and CF3+. Under 02+bombardment silicon is totally converted to SiOz over a range of at least 110 A ( 1 7 ) resulting in a charging effect (not observed under CF3+bombardment) which shifted the peak to higher BE. The large B E shift toward higher BEs from Si to SiOz is indicative of the formation of a strong ionic bond consistent with the high Si+ intensities found for silicon. CF3+bombardment gives a broad peak at much lower BE (100.0 eV) with a trailing shoulder toward higher BEs. The BE of 100.0 eV is in agreement with the BE reported for silicon
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
,
106
1 1 ,
t
,
I
104 102 IO0 BINDING ENERGY (eV)
98
2085
/ \
15 keV ' 2 0
Flgure 2. Si 2p XPS spectra after lOkeV normal incidence saturation bombardment of silicon with Ar', CF,' and 02+,respectively.
BINDING ENERGY (eV) BINDING ENERGY (eV1
Figure 3. Cr 2p,,, XPS spectra after normal incidence saturation bombardment of Cr with lOkeV Ar', 5- and 10keV CF,', and 15-keV 0*+.
carbide (24). The presence of some unreacted silicon cannot be ruled out particularly in view of the compositional data. The trailing edge toward higher BE indicates the presence of SiF, not resolvable from S i c with the available energy resolution. Fluorine absorption on silicon has been studied by McFeely et al. (26)with a spectrometer of better energy resolution which resolved the Si 2p components with the BE positions as shown in Figure 2 for SiF, SiF,, SiF3, and SiF,, respectively. At the low concentration of fluorine in our target, only SiF is formed with detectable abundance. The general conclusion to be drawn from Figure 2 is that the charge shift with respect to Si is small for both S i c and SiF and the ionization probability should be lower under CF3+than under Oz+bombardment in agreement with our results. Chromium. Since no data were found for the BE of F 1s in chromium fluorides, we can only infer indirectly that fluorine is bonded to chromium from the fact that the BE found here (685.5) is typical of the BEs of many metal fluorides which form strong ionic bonds. Bonding to carbon can be excluded by the argument presented for silicon above. The BE of C 1s is in very good agreement with the BE of 282.8 eV reported (27) for the carbides Cr3C2and Cr7C3and a -2 eV shift (28) in respect to hydrocarbon (284.5 eV) for Cr3C2. In Figure 3 the Cr 2p3jzspectra are shown after saturation bombardment with Ar+, CF3+,and 02+.Chromium is fully oxidized under Oz+ bombardment to the most stable form (Cr203),a strongly ionic compound resulting in very high ionization probabilities. BEs marked for Cr 2p3jzfor CrzO3 and Cr02 are from ref 29. Under CF3+bombardment a peak is found invariant from the BE of pure Ar+ sputter cleaned Cr with a long trailing shoulder extending over 6 eV toward higher BEs. The Cr 2p,/, BEs given in ref 27 for Cr,C3 (574.1 eV) and Cr3C2(573.9 eV) are sufficiently close to explain the coincidence with CrO. The long trailing shoulder toward higher BEs is consistent with the formation of chromium-fluoride bonds of the type CrF, CrF,, and CrF, in decreasing abundance. The BE for CrF3 was obtained from ref 30. This is the only element studied by us also a t 5-keV CF3+yielding
Flgure 4. Ni 2 ~ , ,XPS ~ spectra after normal incidence saturation bombardment of Ni with 10-keV Ar' and CF,' and 1bkeV 02+. higher intensities a t the trailing shoulder. The target composition a t 5-keV CF3+ is 30, 46, and 24 atom 70 for Cr, C, and F, respectively. Again the F/C ratio is low (0.5), but the incorporated fluorine concentration is higher than for 10-keV CF3+. Similarity in the ionization probabilities of pif for Cr' formation under CF3+and Ozf bombardment implies in accordance with the work of Yu (31) that (Pi+Y)Cr203
-
(Pi+CY)CrF3
+ (Pi+CY)CrF, + ( P i + C Y ) C r F + bi+CY)Cr-carbide
(5)
where C and Y are the concentrations and partial sputtering yields of Cr in its respective chemical state. Husinsky (32) reports a value of 25% for the partial yield of Cr sputtered as ions and ionized molecules relative to the total Cr yield for oxidized Cr. The large BE of Cr 2p in CrF3 suggests complete ionization of chromium emitted from such sites. This may also apply for CrFz sites, if we assume that the BE for Cr 2p is at 578 eV, i.e., at two-thirds the BE shift from chromium metal to CrF,. The other two constituents, Cr bound to one fluoride atom, and chromium carbide, have ionization probabilities smaller than the ionization probability from Crz03. Nickel, The F 1s BE is in perfect agreement with the BE of 684.9 eV reported (33) for NiF,. For C 1s excellent agreement is found with the value of 284 eV reported (34) for Ni&. Oxygen bombardment leads to almost complete oxidation to NiO (Figure 4). The charge shift Nio to NiO is only 1.7 eV with the broad shoulder a t higher BE due to multiple electron processes (35). Secondary ion yields are lower by a factor of 8 than those obtained (2)from Fe, although the first ionization potential of Fe is higher (7.83) than that of Ni (7.61). The Ni 2p,,, spectrum after CF3+bombardment has a major peak with a BE invariant to Nio as consistent with the observation in ref 34 for Ni3C. The C Is BE proves, however, the formation of Ni3C, although the presence of a small fraction of unreacted Ni cannot be excluded. The low-intensity peak at a BE of 858 eV coincides with the Ni2p3jzBE in NiF, (7) although there is an interference a t least in part with this peak by a multielectron excitation component (35) in the 2p3i2peak. Note the absence of a weak intensity
2086
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
A
9-
8-
3
t
r
5
Iza
2
J
W
7-
m
5
0
E
0
F 5
2
4
$
6-
0
5
5-
0
3
8
4-
E
A
15keV
A
IbkeV
X IOkeV
1
a
2
z
3-
0 20
'10
30
40
50
60
70
z
80
MASS/CHARGE
2-
Flgure 5. Secondary ion mass spectra obtained from a silicon target under normal incidence bombardment with 10-keV CF,' and 0,'.
component a t about 855 eV, which we would have expected at a low implant concentration of fluorine with only one fluorine atom bonded to Ni. This suggests that in nickel, in contrast to Si and Cr, fluorine forms strongly ionically bonded NiFz islands in the carbide matrix. Nickel sputtered from these sites has a very high probability of ionization, probably the cause of the higher relative ionization probability under bombardment. CF3+ compared to 02+ Mass Interferences. In Figure 5 the mass spectra for silicon are shown under 02+and CF3+ bombardment, respectively. In addition to Si,F,+, Si,$,+ components are also found for CF3+bombardments; Le., interference problems are more likely to occur with this ion source. Note that, for the common dopant arsenic, a strong interference by 2sSi19Fwill limit the detection sensitivity for arsenic. High mass resolution capabilities and/or off-sets in the secondary ion energy acceptance will be required more frequently than for 02+operation. Mass spectra taken on 18 pure element standards give a relative intensity distribution of M/M,F,+ very similar bombardment. to those observed for M+/M,O,+ under 02+ Matrix Effect. For two series of binary alloys, Ni-Cr and Ni-Fe, we determined the normalized ionization probability a of the elements A in the binary AB from aA+
&,A(AB) S A 1 = Pi,A(AB) -= -Pi,A(A)
is,*(*)
SAB CA
(6)
where is is the secondary ion currents for element A in the alloy AB and the pure element standard A, respectively, S values are the sputter yields for A and AB, and CA is the atomic concentration of A in AB. If the ionization probability of A is not affected by the presence of the other alloy constituent, Le., in the absence of a matrix effect, the normalized ionization probability should be independent of the alloy concentration and should be equal to unity. The results are shown in Figure 6 for the Ni-Fe alloy under normal incidence bombardment by CF3+and 02+ and also for 02+a t 4 5 O incidence. The error in a can be of the order of f50% due to which we obtained by linear interthe uncertainty in SAB, polation from SAand SB.The data for 02+ bombardment were taken from previous publications (3, 36). Note that a large matrix effect is observed under normal incidence bombardment by 02+, resulting in a nearly 10-fold increase of the relative ionization probability of Ni+ in the presence of 60 atom % Fe. We have shown (3)using XPS that the bonding
"Fe
20
60
40
80
Ni
a t % Ni
Figure 6. Normalized ionization probabilities for Ni+ for binary alloys of Fe,Ni,-, for normal incidence bombardment with 15-keV 0 , ' and 15-keV CF,' and for 45' incidence of 15-keV 0,'.
state of Ni depends on the alloy composition shifting the binding energy of Ni 2p3j2toward higher bonding energy and hence higher ionization probabilities with increasing Cr conreduces the centration. Bombardment under 45O with 02+ oxygen uptake to about 10 atom %, similar to the fluorine concentration found at normal incidence CF3+. Matrix effects are much smaller for normal incidence CF3+ (also true for bombardment. Common Ni-Cr not shown here) and 45' 02+ to these conditions is the low uptake of oxygen and fluorine. The results can be rationalized by the work of Yu (31). In a system of mixed chemical states the secondary ion current is of A is given by Pip
is,A
=
-ccnY&+n f n
(7)
where @ is the instrument transmission of the isotope of abundance f of element A and ip is the primary beam current. C,, Y,, and pi+,,are the concentrations, partial sputtering yields, and ionization probabilities for A in its nth chemical configuration. At low reactive ion implant concentration of fluorine eq 7 reduces to &,A
(8)
CA-FYA-Fpi+A-F
because this is the only term in the summation of ( 7 ) significantly contributing to is,*. If the formation of A-F bonds is determined by the statistics of the implantation process CA-F
0:
(9)
CA'CF
or if in a first-order approximation we assume the fluorine implant concentration CF to be independent of the sample composition is,A
0:
CA
OC
(10)
CA-F
i.e., the normalized ionization probability
cyA+
is independent
ANALYTICAL CHEMISTRY, VOL. 59, NO. 17, SEPTEMBER 1, 1987
of the target composition. The large dependence of aA+on the sample composition for 02+ bombardment under normal incidence (3)is due to a strong dependence of the bombardment induced chemistry on the sample composition.
Ge, 7440-56-4;Ag, 1440-22-4;Ta, 7440-25-7; Ni-Fe alloy, 1876324-3.
LITERATURE CITED
CONCLUSION With few exceptions, ionization probabilities for CF3+ bombardment are higher or equal to those found for 0,' bombardment. The results can be qualitatively understood in terms of stronger ionic bonds formed by fluorides than by oxides. The F/C ratio in metals bombarded to saturation with CF3+ is about 0.5 and is much smaller than the ratio of 3 in the primary projectile either due to preferential sputtering of fluorine or to a low sticking coefficient of fluorine. Consequently the fluorine uptake (10-20 atom ?&) is smaller than normal incidence bombardment, yet ionization that under 02+ probabilities can be larger under CF3+ bombardment. This can be rationalized by a very large ionization probability of M+ from fluorinated metal sites. Under conditions of low reactive ion implant concentration the chemical state of the target is in first approximation invariant with the concentration of A in a alloy AB reducing matrix effects in the ionization probability of A. Sputter yields are larger by about and this is primarily a factor of 2 for CF3+ compared to 02+ attributed to the factor of 2 increase in the number of projectile atoms per primary ion with nearly the same average mass. This increase in sputter yield compensates in part for the lower extractable CF3+current, about a factor of 5 smaller current. Molecular ion interferences are more than the 02+ likely to occur under CF3+bombardment and may frequently require high mass resolution capabilities or energy offsets in the secondary ion energy distribution. Clearly much can be gained in terms of understanding, reduction of interference problems, and sensitivities if the CF3+source can be substituted by a source generating Fz+ions at reasonable abundance. Short of using F,, an alternative candidate may be NF3 which in contrast to CF4 has much lower dissociation energies (NF3 NF, F, 59 kcal/mol; NF, N F F, 58 kcal/mol) with a rapidly occurring disproportion reaction 2NF N2 Fz in contrast to CF, with dissociation energies of 130, 87, and 124 kcal/mol for the respective dissociation steps. Furthermore the reactivity of NF3 and its toxicity are smaller than for F,.
-
+
-
+
-
+
ACKNOWLEDGMENT I am indebted to R. E. Fern for his strong collaboration in the XPS study. Discussions with J. C. Clabes, E. Kay, and M. L. Yu are gratefully acknowledged. Registry No. 02+, 12185-07-8;CF3+,18851-76-8;Ar+, 1479169-6; Si, 7440-21-3;Cr, 7440-47-3;Ni, 7440-02-0; Cu, 7440-50-8;
2087
(32) (33) (34) (35) (36)
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RECEIVED for review September 3, 1986. Accepted May 22, 1987.
