Energy analyzed secondary ion mass spectroscopy and simultaneous Auger and XPS measurements of ion bombarded surfaces

NUCLEAR INSTRUMENTS

AND METHODS

149 ( 1 9 7 8 )

547-552

; ©

NORTH-HOLLAND

P U B L I S H I N G CO.

ENERGY ANALYZED SECONDARY ION MASS SPECTROSCOPY AND SIMULTANEOUS AUGER AND X P S MEASUREMENTS OF ION BOMBARDED SURFACES* A. R, KRAUSS and D. M. GRUEN

Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A.

Secondary ion yields and the energy distribution of sputtered ions have been measured using secondary ion mass spectrometry (SIMS) with simultaneous Auger and in situ XPS measurements to characterize the chemical composition and state of oxidation of the surface being sputtered. The SIMS instrument consists of a quadrupole mass filter and a retarding-dispersive energy analyzer and is capable of high resolution energy analysis to several keV. Experimental results are reported for secondary ion yields and molecular/atomic ion ratios as a function of surface oxygen concentration. These results are compared with published values for Ti, AI and K. On clean and oxidized metal surfaces, the mean energy and shape of the energy distribution vary substantially as a function of primary ion energy and angle of emission. The observed behavior indicates an inability, in some cases substantial, of the random collision cascade model of sputtering to account for either the secondary ion yield or the details of the secondary ion energy distribution. This failure becomes most pronounced when there is structure in the high energy tail of the energy distribution, such as would be caused by channeling and surface recoils. Consequently, the kinetic energy limitations of the conventional energy filter-mass quadrupole SIMS arrangement results in a small, and in some instances, very unrepresentative sampling of the secondary ion yield. The extent of the resulting discrepancy will be investigated for various primary energies and angles of emission.

1. Introduction Quantitative determination of secondary ion sputtering yields for well-characterized clean, gas covered, and compound metal surfaces is essential to a more thorough understanding both of the data obtained by means of secondary ion mass spectroscopy (SIMS) and of the secondary ion emission process which provides the physical basis of the analytical technique. A number of measurements have been made of the secondary ion yields of various materials 1-3) but most of these suffer from one of two shortcomings arising from the design characteristics of the mass filters employed for SIMS use. Many of the magnetic sector instruments are capable of collecting the secondary ions regardless of their initial kinetic energy 4) by applying an accelerating potential to the collection electrode. In order to mass analyze the resulting high energy ions (3-10 keV) with good resolution, the instrument must be rather large. Good mass resolution also requires that the secondary ions be well focused and accurately aligned as they enter the analyzer. Consequently, the instrument cannot be readily installed in a conventional multi-purpose surface analysis vacuum chamber, and it is * Work performed under the auspices of the Division of Physical Research of the U.S. Energy Research and Development Administration.

therefore difficult to perform independent in-situ characterization of the surface being sputtered. On the other hand, the electric quadrupole mass analyzer is compact and physically compatible with a wide variety of surface analytical instruments but is capable of sampling only a portion of the kinetic energy distribution of the secondary ions. It has been shown recently that this sampling varies from element to element, with degree of surface oxidation, and from atomic to molecular species of the sputtered metaP). Consequently, the entire energy distribution must be considered if quantitatively meaningful measurements of secondary ion yields are to be made. In this paper we discuss the effect of the primary energy and angle of emission on the measured secondary ion yield and energy distribution. The method of energy integration will be used as a means of circumventing the kinetic energy limitations of the quadrupole mass analyzer and the results of the procedure will be applied to secondary ion emission from metals with substantially different energy distributions. It will be seen that in polycrystalline samples, both the details of the energy distribution and the total secondary ion yield may be at variance with the random collision cascade model of sputtering. In particular, surface recoils and channeling give rise to energy distributions in which the bulk of the secondary ion yield VII. ION I N D U C E D

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A. R. K R A U S S

A N D D. N. G R U E N

may come from ions with energies of several hundred eV. Finally, a preliminary calibration will be used to relate the integrated energy distribution to the absolute secondary ion yield in the absence of channeling and surface recoils of an oxidized Ti surface characterized as to oxygen concentration by Auger and X-ray photoemission spectroscopy (XPS). 2. Theoretical For clean metal surfaces, sputtering is largely in the form of isolated atoms and the total sputtering yield is adequately described by the random collision cascade model6). The kinetic energy distribution of the sputtered atoms derived on the basis of this model can be expressed in slightly generalized form 7) as CE N(E) - (~+E0.+,

cos ~b~

(1)

cos q~'

where C is a material-dependent parameter, Eb is the atomic surface binding energy, 0e is the angle of emission, and 0~ is the angle of incidence of the primary beam, both with respect to the normal to sample surface, and l < n < 2 is a fitting parameter. The energy distribution peaks at an energy Ep = Eb/n and, at large energies falls off as E -n. Only a fraction of the sputtered atoms are emitted as ions and it is covvenient to relate the ionic energy distribution to the total energy distribution by means of an ionization coefficient R +(E) such that N - (E)= R+ (E) N(E). By definition, the secondary ion yield is equal to S+ =

N + (E) dEd(2,

(2)

0

where the angular integration is over the 2n Sr into which emission occurs. A detailed theory of secondary ion emission has not yet been established and the ionization coefficient is not even experimentally well known. However, most theories predict, and experiments generally confirm 8) that R+(E) is an increasing function of energy. If R + N E m 9), then the ion energy distribution will fall off with energy more slowly than that of the total (ion+ neutral) distribution, namely as E ~-n at large E. The peak of the ion energy distribution is shifted to the value + _ m +____~1Eb " Ep - n - m

(3)

If rn and n are close in value, this shift may be

quite large. More generally, R + (E) increasing with energy will broaden the ion energy distribution and shift it to higher energies. The behavior of R* (E)depends on properties pertaining to the free electron concentration at the surface and it is therefore to be expected that N+(E) is quite sensitive to the chemical state of the surface. This dependence has been reported by several authors 5,10). The random collision cascade model assumes that there are sufficiently many collisions in the cascade that all " m e m o r y " of the primary ion's energy and direction of incidence are erased by the time a collision event occurs which transfers sufficient normal momentum to a surface atom to cause ejection into the vacuum. Consequently eq. (1) exhibits no dependence on the primary energy and, although the intensity depends on the angles of incidence and emission, the shape of the energy distribution is predicted to be independent of these quantities. However, a number of phenomena such as channelingT), surface recoilsll), and faceting ~2) have been observed to produce deviations in N(E) resulting in higher energies, the introduction of additional structure in the energy distribution, and dependence on the primary energy and angle of emission. The apparent importance of such effects depends on the experimental geometry: they are minimized for emission along the sample normal and become increasingly important as the observed angle of emission approaches 90° . Because of the energy dependence of R+(E), they can become important much more rapidly for secondary ion emission than for the total emission. For analytical SIMS purposes, the geometry is usually chosen to suppress high energy ions2). However, the secondary ions are distributed into 2n sr and a measurement of the yield must include both normal and off-normal emission. Because of the physical and chemical effects discussed above, it is necessary that this measurement be made with an instrument which is able to monitor secondary ions with kinetic energies up to a substantial fraction of the primary energy.

3. Experimental The ion analyzer has been described in detail elsewhere5). Briefly, it consists of a retarding-dispersive energy analyzer of the Staib type j3) coupled to a quadrupole mass analyzer. The energy resolution can be varied electronically to func-

SECONDARY

tion as either a secondary ion mass analyzer with a low resolution energy pre-filter or as a high resolution energy analyzer for ions of a selected mass. The secondary ions are emitted into a fieldfree region and then retarded between two concentric spherical grids. Only those ions within a narrow energy range above the retardation cut off are allowed to enter the quadrupole. All potentials, including the quadrupole rod reference are fixed relative to the retarding potential. The secondary ions are therefore transmitted at a fixed kinetic energy of - 1 0 eV regardless of the emission energy, and the only energy discrimination is at the retardation stage. The major effect of the retardation potential is a decrease in resolution at high energies14). Since sharp features at high energies are neither expected nor observed, the experimental data is considered to be a reasonably accurate representation of the ion energy distributions. For quantitative measurements of the secondary ion yield, the energy distribution is recorded over a range of several hundred to 2000 eV. The angular integration range of eq. (2) is experimentally taken over the acceptance of the energy analyzer, a cone with a 15° half-angle. The total number of counts

,x

4

- ~

ooO~E-I.4

Ti + ~

)2 \ \ :

0

I

2 log E(eV)

549

ION MASS SPECTROSCOPY

3

Fig. 1. Secondary ion energy distributions for K ÷ , AI + and Ti + sputtered by 2 . 8 k e V argon ions. T h e lines indicating slopes corresponding to E -1 and E 2 are t h e limiting slopes for the total (ion + neutral) energy distribution in t h e absence o f c h a n n e l i n g a n d surface recoil effects. ~be = 55 °, 0i = 65°.

accumulated during the scan is, after normalization to the primary current and scan speed, proportional to the secondary ion yield intercepted by the analyzer. The data presented here has not been corrected for the mass dependence of the quadrupole transmission function nor have the energy distributions been corrected for the sampleanalyzer work function difference. The sample is located in a bakeable stainless steel UHV system (base pressure - 1 × 10 -l° torr) and is mounted in such a way as to be able to perform-Auger spectroscopy during ion bombardment and simultaneously to observe the secondary ions via the ion analyzer. By rotating the sample, ins i t u XPS measurements may be made under ultrahigh vacuum conditions. The ion source is a 3 keV ion gun mounted in a differentially pumped appendage coupled to the vacuum chamber through an einzel lens. This arrangement allows the ion beam to be focused onto the sample while full pumping speed for gases desorbing from the chamber walls is maintained. Beam current was measured by biasing the sample positively to suppress secondary electrons. 4. Results and discussion The high energy tails of the secondary ion energy distributions for the atomic metal ions sputtered from K, A1 and Ti surfaces by 2.8 keV argon ions are shown in fig. 1. The aluminum and titanium samples were lightly oxidized poly-crystalline foils, while the potassium was an impurity on the surface of a molybdenum sample. The total (ion+neutral) sputtering yield of potassium has been observed ~5) to behave as E -n with n - 1 . 5 . According to the theory of Sroubek 9) the ionization coefficient for an alkali metal should increase as E m with m = ¼, thereby predicting that the ion energy distribution will fall off as E ~.2s, in excellent, but possibly fortuitous agreement with the observed E 1.2 behavior. The intensity for A1+ also exhibits a power law behavior well within the range of theoretical expectation. The distribution for Ti + however, lies well outside the range of expected behavior and does not appear to follow a power law. The hump appearing between 100 and 1000 eV for Ti + is similar to structures observed in the total energy distribution of sputtered gold 7'll) and attributed to surface recoil and focused collision effects in single and poly-crystalline foils. Almost 70% of the observed titanium secondary ions have VII.

ION

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A. R. KRAUSS AND D. M. G R U E N

energies in the 100-1500eV range. This energy range has been largely ignored in experiments which attempt to measure the secondary ion yield by means of quadrupole SIMS3). Titanium differs from the other two metals shown in that the peak of the ion energy distribution is shifted rather far from the Eb/n value expected for the total yield, thereby indicating a sizable variation of the ionization coefficient with energy. In such a case, the high energy features of the total energy distribution will appear in exaggerated form in the ion distribution. For example, if R + - E ~ and two features of equal intensity appear in the total energy distribution at ten and 1000 eV, then the 1000 eV feature will appear three orders of magnitude larger in the ion energy distribution. The multiple collision m o m e n t u m transfer model also fails to describe some of the low energy features of the secondary ion emission. Fig. 2 shows the secondary ion energy distribution of A1+ sputtered by argon ions with energy varying from 500 to 2500 eV. The angle of incidence was 65 ° relative to the surface normal and the angle of emission was 55 ° . The peak in the distribution shifts continuously with increasing primary energy, in contradiction with eq. (3). The primary current density was varied over a range of 10-500 nA/crn 2 with no change in result. A similar effect has been observed in nickel by Sroubek 9) who attributed the shift to " b u l k " vs an energyindependent "surface" sputtering. In the primary energy range of 150-1500 eV Dawson 16) observed little shift in the peak energy of the A1 + distribution. In that experiment, the analyzer was along the sample normal and the angle of Incidence was

~o5~ 55 °

1051001500

io4_/,/

close to oblique, resulting in suppression of surface recoils and more limited penetration, a situation corresponding more closely to Sroubek's "surface" sputtering. A further indication of the difference between ion and neutral emission may be found in the fact that Oechsner and Reichert 17) observed a mean energy of 4.0 eV for the total yield of A1 atoms sputtered by an argon ion beam whereas Castaing and Hennequin 18) reported a mean kinetic energy of 50 eV for A1 + emitted at an angle of 30° under 8 keV argon bombardment. We also observe the peak of the AI + energy distribution to shift with increasing angle of emission, again in contradiction with expectation based on the random cascade emission and the currently available theories of the change transfer process. The data of fig. 3 were taken with the ion source, detector, and sample normal in the horizontal plane as in figs. 1 and 2, while the sample was rotated about the vertical axis. The angle between source and detector was fixed at 120°. The peak shifted continuously from 6 to 25 eV as the angle of emission approached 90°. The behavior of figs. 2 and 3 can be understood as the result of a collision cascade which is short enough to retain some of the momentum and energy information of the primary knock-on. The observed angular dependence of the peak energy is in accord with the findings of Castaing and Hennequin ~8) that the mean energy of sputtered AI- increases with increasing angle of emission. In order to account for this behavior, they postulated an "erratic emission" model in which secondary ions were created

41 / ~ x , 6 5 ° IO ~1,' 7 5 o " ~ "

500eV A r e A l

Ar+~AI I00 na/cm 2

taJ

k-

g 8

.i ' /

90 °

102 Idd

I°lL

50 i~)0 150 KINETIC ENERGY (eV) Fig. 2. Secondary ion energy distribution of AI ~ sputtered by 500, 1500 and 2500eV argon ions. 0e =55°, 0i =65°-

5b

~

'

1oo

. . . .

~o

KINETIC ENERGY (eV) Fig. 3. Secondary ion energy distributions of AI + sputtered by 500 eV argon ions for various angles of emission. The curves are identified by the angle between the sample normal and the axis of the energy analyzer.

SECONDARY

I O N MASS S P E C T R O S C O P Y

by high energy collisions resulting from very short collision chains or direct impact with the primary ion. Even though the random collision cascade model does not correctly describe the behavior of the AI + energy distribution, it does account for the angular variation of the A1+ yield as measured by the integrated distribution. Fig. 4 shows the angular variation of the A1÷ signal as measured three different ways. The solid lines represent the COS 0e/COS/~i variation. The data points are normalized to the solid curves at an emission angle of 55° . The ion yield as measured by the integrated energy distribution (top trace) follows the predicted angular variation quite well. The count rate at the peak of the energy distribution (center trace) is influenced by the change in shape and does not properly represent the secondary ion yield. The bottom trace shows the angular variation of the count rate at a fixed energy of 10 eV, as would be measured in a conventional SIMS experiment in which the pass energy would be fixed and the mass scanned. In this case the data is even less representative of the secondary ion yield. Such scans are however, qualitatively useful. Because of the greater data acquisition time, the

\\

\

2.8 keY A r e A l 20na/cm 2

[150ON(E) dE

'\

integration procedure is inherently more destructive than the fixed-energy mass scan. Each data point on the integral plot in fig. 4 corresponds to the removal of N 10% of a monolayer. If additional noise can be tolerated in the plot of the energy distribution, the integral can still be obtained with good accuracy in a shorter scan time, resulting in the consumption of less than 1% of a monolayer per scan. In order to confirm the procedure of energy integration as a means of determining the secondary ion yield, the integrated distributions of Ti + and A1+ were compared. The high energy Ti + ions of fig. 1 were suppressed (by making the ion source-detector angle 90° and then tipping the ion gun from the horizontal by 53°) in order to make our secondary ion yield values more directly comparable with the published values based on measurements in which the high energy ions were excluded. The integrated A1+ count rate in units of eV. counts//.tA, s was equated to the published value of 2.0 for the secondary ion yield of an oxidized aluminum foil bombarded by 3 keV argon ions2). This calibration was then applied to determine the secondary ion yield of the titanium surface for varying degrees of oxygen coverage. To determine the oxygen concentration, the sample was first argon ion sputtercleaned until the X-ray photoemission spectrum of the Ti L2, 3 transitions indicated the oxidation state corresponding to clean titanium. The sarnple was then progressively oxidized, while the oxygen to titanium ratio was monitored by means of Auger spectroscopy. When the XPS data indicated an oxidation state corresponding to that of TiO2 19) it was observed that I.O

0

"\~ \

O~

I

0.8

" ,OeV

60 70 80 ANGLE of EMISSION (degrees)

°

I

~

0.6

I

t_

o

Fig. 4. Angular variation of the count rate with a fixed l0 eV pass energy (bottom trace), count rate at the peak of the secondary ion energy distribution (middle trace) and integrated distribution (top trace). The solid curves represent cos ~e/COS ~i behavior. The data points are scaled to coincide with the solid curve at 55 ° .

0.2

~

.05

o o oO

oo

~ 90

I~' ~ "' I ~ I *

o o o o

_

. ,. ~ . . , 50

I

,,,,,~ E A K

\ \,.,

551

.04 ~" ,.--

.0,3

o. 2.8 keV 0~. /~•

~'• 2.8 keV Ar+

.02 .01

0

I

I

0.5

1,0 [0] /[Ti]

]

1.5

I

2.0

.00

Fig. 5. Secondary ion yield and oxide to metal secondary ion ratio as a function of surface oxygen concentration on Ti.

VII.

I0N

INDUCED

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A. R. KRAUSS AND D. M. GRUEN

the ratio of the O~:LL to the TiLMM Auger lines reached a value which did not change with fur.ther oxygen exposure. This value was taken to correspond to an oxygen/titanium atomic ratio of 2.0. The ratio of the peak-to-peak signal heights was taken as proportional to the concentration ratio for lower oxygen coverages2°). Variation of the TiLMv line shape with oxidation was ignored, thereby introducing some error in the interpretation of the peak-to-peak signal21). This discrepancy will be removed in future experiments by integration of the Auger peak. The resulting data for the secondary ion yield vs oxygen concentration are shown in fig. 5. Exposure to 1 × 10 -6 torr 02 at room temperature until saturation of the surface was achieved (as monitored by AES) did not result in the + 4 oxidation state of TiO2 as seen by XPS. Saturation occurred at an [O]/[Ti] ratio of 0.8 in fig. 5. Further oxidation required the use of an O f beam. The value of 0.5 for S(Ti +) at [O]/[Ti] = 0.8 is in good agreement with published data 22) for titanium exposed to a saturation exposure of oxygen and sputtered by 3 keV Ar +. The value of S + = 0.8 at high oxygen concentration for 2.8 keV 02+ bombardment is a factor of two higher than the published value for the total Ti sputtering yield23), indicating some error on the part of one or both experiments and a very high (possibly 100%) ion fraction. The ratio of TiO + to Ti + was not observed to exeeed 0.05, in contrast with the value of 1.1 reported by Milllet and Benninghoven22). It should be noted however, that when our analyzer is operated in the conventional SIMS mass scanning mode with the pass energy _<5 eV, we too see the TiO + signal exceeding that of Ti +. This behavior arises as a result of the difference between the Ti + and TiO + energy distributions which have been published elsewhereS). The TiO + energy distribution falls off much more rapidly with energy than that of Ti + and the signals observed in the mass scan are not representative of the actual secondary ion yield.

5. Summary Secondary ion emission may be viewed as the result of two processes, one of which results in ejection from the surface regardless of charge state, and a charge exchange process which results in the neutralization of sputtered ions and/or the ionization of sputtered neutrals. There is no generally adequate description of the charge exchange

process although theory does present some limits of expected behavior. Because of the differences between total and ionic energy distributions, phenomena which produce relatively small departures from the total sputtering yield predicted by the random collision cascade model may produce very serious errors when the model is applied to secondary ion emission. Consequently, the usual method of scanning the mass while retaining a fixed pass energy normally employed in quadrupole SIMS is often inadequate for quantitative purposes. The procedure of electronically integrating the energy distribution is shown to give reasonable secondary ion yield values when an extrapolation is made between two secondary ion species with rather dissimilar energy distributions.

References l) C. A. Andersen, Int. J. Mass. Spec. Ion Phys. 3 (1970) 413. 2) A. Benninghoven, Surf. Sci. 35 (1973) 427. 3) A. Benninghoven, C. Plog and N. Treitz, Int. J. Mass. Spec. Ion Phys. 13 (1974) 415. 4) F. G. Rtidenauer and W. Steiger, Jap. J. Appl. Phys. Suppt. 2 (1974) 383., 5) A. Krauss and D. M. Gruen, Appl. Phys., 14 (89)1977. 6) p. Sigmund, Phys. Rev. 184 (1969) 383. 7) M. W. Thompson, Phil. Mag. 18 (1968) 377. Note added in proof. R. S. Nelson (Defects in Crystalline Solids, Vol. 1, Wiley, New York, 1968, p. 245) has modified this model to treat the surface binding energy as an effective index of refraction and obtains an expression for N(E) in which the 2 peak energy increases from 1E b to feb with increasing angle of emission. This shift is in the same direction but rnuch smaller than that observed here for N + (E). 8) M. Yu, these proceedings. 9) Z. Sroubek, Surf. Sci. 44 (1974) 47. 10) A. R. Bayly, P. J. Martin and R. J. MacDonald, Nucl. Instr. and Meth. 132 (1976) 459. 11) I. Reid, B. W. Farmery and M. W. Thompson, Nucl. Instr. and Meth. 132 (1976) 317. 12) I. Reid, P h . D . Thesis (University of Sussex, Sussex, England, 1976) unpublished. t3) p. Staib, Vacuum 22 (1972) 481. 14) D. A. Huchital and J, D. Ridgen, J. Appl. Phys. 43 (1972) 2291. 15) G. P. K~Snnen, J. Grosser, A. Haring, A. E. DeVries and J. Kistemaker, Rad. Eft. 21 (1974) 171. 16) p. H. Dawson, Surf. Sci. 57 (1976) 229. 17) H. Oechsner and L. Reichert, Phys. Lett. 23 (1966) 90. 18) R. Castaing and J-F. Hennequin, Adv. Mass Spec. 5 (1971) 419. 19) L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman and C. Nordling, J. Phys. Chem. Sol. 30 (1969) 1835. 2o) p. W. Palmberg, Anal. Chem. 45 (1973) 549A. 21) j. T. Grant, T. W. Haas and J. E. Houston, J. Vac. Sci. Tech. 11 (1974) 227. 22) A. Miiller and A. Benninghoven, Surf. Sci. 41 (1974) 493. 23) O. K. Kurbatov, Sov. Phys. Tech. Phys. 12 (1968) 1328.