Impurity concentrations on metal surfaces by auger electron spectroscopy

SURFACE

SCIENCE 36 (1973) 789-796 o North-Holland

IMPURITY CONCENTRATIONS

Publishing Co.

ON METAL

SURFACES BY AUGER ELECTRON SPECTROSCOPY Received 21 August 1972; revised manuscript

received 4 January 1973

Recently quantitative measurements of impurities on metal surfaces by Auger electron spectroscopy (AES) have been made by several authors. Among several approaches the following techniques are commonly used: (1) a standard sample method in which a pure specimen is used as a standard sample to determine the magnitudes of the various Auger peaksr:*), and (2) an adsorption method in which the Auger peak height arising from an alkali or a radio-active material deposited on the surface at a known rate is related to the amount of material depositeds-7). However, even the same element may show a different cross-section for Auger electron production when deposited on substrates which are crystallographically different because of the variation in secondary (backscattered) electron yields within the substrate. An absolute calibration cannot be performed correctly in such a case. Also, when impurities in the bulk having concentrations of the order of hundreds of ppm diffuse to the surface due to some treatment (e.g. heating), it is difficult to know the quantity which has segregated. The present authors have published the results of work function measurements on both pure iron (100)s) and silicon-iron alloy (100)9), in which P, S, Cl and C appeared at each step of the cleaning processes on the sample surfaces as impurities. Hence there was some difficulty in determining a precise value of the work function. Thus it is required to find a method of quantitative measurement of elemental concentrations on the metal surface to clarify the contribution of the impurity to the work function. The purpose of this paper is to estimate quantitatively, using a tentative method, the surface concentration of several impurities which were initially within the bulk of the silicon-iron alloy single crystal used previously and to apply the method to the impurities in both silicon-iron alloy polycrystals and iron single crystals. Since silicon is located just in front of P, S and Cl in the periodic table, their Auger transitions are of the same LMM type. Accordingly, if a concentration of Si were known, quantities of these impurities might be roughly estimated from a ratio of cross sections for ionization of each element. Furthermore it should be noticed that silicon-iron alloy forms a single 789

K. UEDA

790

AND R.SHlMIZU

crystal between 2.15 wt% and lSwt% of silicon, in which the spatial distribution of silicon atoms in an iron matrix might be considered to be homogeneous. First of all, it is of importance in an assessment of quantitative measurements to determine the “detected volume” as discussed by Palmberg and Rhodinlo), and Changl). Once an Auger electron is ejected from an atom, it should be detected as an electron with a characteristic energy without signiticant energy loss (5 eV). Unfortunately there is very little precise knowledge of the mean free path of an electron in the energy range of interest in a material. However, Palmberg and Rhodinro) found by a semi-empirical method that the mean escape depths for 72 eV and 362 eV Auger electrons in silver were 4 A and 8 A respectively. Further in LEED experiments Haque and Farnsworthll) showed that the diffraction pattern of Cu (I I I) disappeared when using an electron beam of energy less than 150 eV when the copper substrate was covered with nickel film 2 monolayers thick and the 125 eV diffraction pattern of W(l1 I) disappears on covering the surface with a Cu film of 3 monolayerslZ). From these results it is reasonable to assume that an electron within the energy range 50-200 eV can escape from only the first few atomic layers of a specimen. In the present quantitative study we have used the theoretical formula described by Bishop and Rivierels) as follows. The number, Iz, of Auger electrons emitted per incident electron is given by tt = Nr cosec 4 (I - W) @(E,/E,),

(1)

where N is the number of atoms in the detected volume, Y is a correction for additional ionization caused by backscattered electrons and 4 is the incident angle of the primary beam. (1 -0) is the mean Auger yield calculated by the X-ray fluorescent yield, III~~). We may assume that (I -co) is nearly equal to unity in this study because the atomic number of carbon is very small and the Auger transitions of iron and the other elements (P, S and Cl) are of MMM and LMM type respectively. The cross section for ionization @(E,/E,) of an atom by electrons is @(E,IE,)

= -;:-;; P

4EplEc

b I,,---mmL

1.65

+ 2.35

exp( I - ED/E,)’

(2)

when E, is the primary energy, E, the critical potential, 6=0.35 for K-shell, 0.25 for L-shell and 0.2 for M-shelll”). The cross sections for ionization of each element are shown in table I for the case of 1200 eV which is the constant primary beam energy used throughout this experiment. Therefore, in a quantitative estimation the dominant factor is the cross section for ionization. Thus, as the Auger current is roughly proportional

791

IMPLJRITYCONCENTRATlONSONMETALSURFACESBYAES TABLE

Cross-section z

6 8 14 1.5 16 17 18 26

Element

C 0

si P S Cl Ar Fe

1

for ionization of several elements at &Transition

KLz,aLz,z KLzLz L3Ml.zM1.2 LyMaMa L8M3M3 LaMaM: L3MaM3 MIM~Ms

to NQi, we can determine

Auger peak

1200 eV

02

EC w

@ x LO-‘*(cm’)

273 510 83 115 148 175 221 44

284 532 99 135 164 200 245 56

3.03 0.97 9.25 6.16 4.66 3.61 2.71 15.3

of atoms in the detected volume from the Auger current. That is, the concentration ratio of silicon to iron is taken as @re Zsi/@si ZFe (Zis the Auger peak-to-peak height for each element). If the above treatment is admitted to be vafid, another impurity composition (Xi) might be determined by taking I,, as a standard as follows the number

xi = ~~~Isi~~s~z~=.

(31

Thus a known composition of silicon-iron alloy can be employed as a calibration standard in this manner. The sample of silicon-iron alloy used here, (100) oriented, and cut to a size of IO x 8 x 1 mm from a Iarge grain, was mechanically polished by progressively finer emery paper and subsequently electropolished to obtain a mirror surface. In the UWV chamber (2 x 10d9 Torr) the sample was cleaned by using standard LEED techniques, i.e. argon ion bombardment and annealing cycles. For AES measurements a conventional four-grid LEED-Auger system described elsewheres,ls) was used. The primary beam in this experiment had an energy of 1200 eV and a current of about 1.5 x x IOe6 A. A modulation frequency of 350 ffz at an amplitude of 3 V (r.m.s.) was superimposed on the two inner grids. An Auger spectrum of the silicon-iron alloy is shown in fig. 1. The energy assigned to an Auger electron peak is the voltage at which the derivative curve passes through a minimum in the spectrum. It should be noted that this silicon spectrum is very different from that of pure siliconr). After annealing the silicon-iron alloy at various annealing temperatures we corrected each Auger peak height in the way mentioned above and then estimated concentrations of the silicon and impurities. These are shown in fig. 2 against annealing temperatures *. In fig. 2 the silicon concentrations agree well with the * The original data are presented in fig. 2h of ref. 9 in which the phosphorus 650°C should be omitted.

point at

K. UEDA AND R. SHIMIZU

792

Fe

Xl

Fe

Fe

,J1 r11 ,; Y

P c.y-

5.59at%Si-Fe(100) After

annealing

at

550°c

IN(EI dE

I I-_LL__I_L_

0

100

200

300

400

I.-

LA

500

600

700

Energy(eV)

Fig. 1. Auger spectrum of a 5.59 at 7: %-Fe alloy (100) surface obtained after annealing for 20 min at 550°C. Primary beam, 1.5 PA, 1200 eV; modulation signal 3V(r.m.s.).

original composition, which is determined as 5.08 kO.9374 Si throughout the experiment. On changing the heating time, the phosphorus concentration changes drastically as shown in fig. 3 without much change in the silicon concentration. It is considered that silicon atoms may diffuse slightly into the bulk after heat treatment for a long time. In figs. 2 and 3, the silicon-iron (100) single crystal shows that the silicon

5.59at%Si-Fe(100)

'

C(273V)

we

’

I .,B. 450

91

500

Annealing

Fig. 2.

Si(B3V)

x,xo>%

,x JY

'\

-A--a 600

x

4 700

temperature("C)

Variation of estimated impurity concentrations versus annealing temperature the 5.59 at “4 Si-Fe(100) single crystal. Each temperature was maintained for 20 min.

on

IMPURITY

CONCENTRATIONS

ON METAL

Heating

SURFACES

BY AES

793

timefminl

Fig. 3. Variation of phosphorus concentration versus heating time, In this case the heating was repeated each time without ion bombardment. Ii represents the corrected Auger peak height for phosphorus and silicon respectively.

concentration

coincides

with

the

original

anaIysis fairly from the surface by heating at

one

in the Auger

well, and impurities, P and C are removed more than 650°C. The same treatment was applied to a silicon-iron alloy polycrystal and a pure iron (I IO) single crystal for comparison. Auger spectra of 3.79 at% Si-Fe alloy obtained after annealing at 55O’C and ion bombardment are shown in fig. 4 and the estimated concentration of impurities are tabulated in table 2. When the sample was annealed, the signal for silicon concentration appeared with a rather high Auger peak height, This, we consider, is because a single crystal cannot be obtained in 3.79 at% (i.e. 1.94 wt’/J silicon-iron alloy as is well known, the spatial distribution of silicon in the matrix might fluctuate considerably. Moreover a polycrystal has many grain boundaries in which silicon and impurities are accumulated in a complicated manner. Finally this approach is applied to a pure iron (I i0) single crystal. The estimated impurity concentrations are shown in fig. 5 for each step of the cleaning processes together with the LEED results. In the above treatment it was assumed that (1) the Auger excitation probability (1 -0) is almost unity, (2) the contribution from backscattered electrons was also assumed to be roughly equal for each Auger line, (3) the cross section for ionization was taken as the theoretically calculated result, and (4) the detected volume for Auger electron production was uncertain to within a few atomic layers. It is unlikely that a Iarge error

794

K.UEIIA AND K.SHlMlZU

3.79atXSi-Fe After SlXI

2.5

s

ion

Cl,

bomb.

c

dN(E) -r

After

annealing

at

55O'C

dN(E) dE __

(b)

0

200

100

300

Energy(eV)

Fig. 4.

Auger

spectra after (a) ion bombardment and (b) annealing 3.79 at % Si-Fe alloy, polycrystal.

from

results from (I) in this study as described before. (3) has proved satisfactory for the ionization of inner shell electrons but not accurate enough for the M-shell. (2) is a property of the substrate as described by Bishop and Rivikrel3) who determined the contribution of backscattered electrons, and TABLE 2

The estimated

Element

After Auger

Si P S Cl C

impurity concentrations silicon-iron surface

After

ion bombardment

peak ratio li/lP~ (:G)

on 3.79 at “/;

Estimated concentration (“:,)

2.11 _

3.5 _

0.51 1.22 4.4

1.7 5.2 22.2

Auger

annealing

peak ratio Ii/IF, ( %, 4.1 0.09 2.08 2.9 I 0.19

Estimated concentration ( “‘) ,0 6.8 0.2 6.8 12.3 I .o

IMPURITY

CONCENTRATIONS

ON METAL

SURFACES

BY AES

795

Fe(llO) PfZXl) clan Bomb.

40

60

80

100

TimeChrsl

Fig. 5. Variation of impurity concentrations at various stages of the cleaning treatment performed on the Fe(llO) sample which was investigated in the study of secondary electron emissionls). Annealing was done at about 550°C for 20 min.

showed that an Auger electron can also be excited by those back-scattered electrons which are distributed between EC and Ep’ If the critical potentials for the various elements differ greatly, the contribution from backscattered electrons should not be taken as constant for the elements in question. In this study the main purposes are the calibration of the silicon concentration and an estimation of P, S, CL and C in quantity. Since the critical potentials of these elements are close to one another except for oxygen as may be seen in table I, the errors caused by taking Y as a constant are considered to be within the observational errors of the experiment (the reproducibility of experiments is + 5%). As to (4) as described above, since escape depths are not known it should be considered that the surface region of the crystal means that depth corresponding to the detected volume for the Auger electron transition that has the lowest characteristic energy in this study. In conclusion, the silicon concentration in a silicon-iron (100) alloy was calibrated using calculated cross sections for ionization to estimate the concentrations of some elements (P, S, Cl and C) which have close atomic numbers. Furthermore this approach was applied to a silicon-alloy polycrystal and pure iron (I IO}. It is considered that this approach is useful in estimating the impurity concentrations on the metal surface. Several investigations associated with silicon-iron alloys which have different concentrations of silicon are in progress,

196

K.UEDA

AND R.SHIMIZtI

The authors wish to thank Professor this work, and Professor emeritus G. We also gratefully acknowledge our Department of Metallurgy, University and valuable advice in connection with

H. Hashimoto for his interest in Shinoda for his helpful suggestions. thanks to Dr. C. J. Humphreys, of Oxford, for helpful comments the publication of this paper.

KAZUYUKI UEDA and RYUICHI SHIMIZU

Department of Applied Physics, Faculty of Engineering, Osaka University, Yamadakami, Suita, Osaka, Japan

References I) 2) 3) 4) 5) 6) 7) 8) 9)

IO) 11) 12) 13) 14)

15) 16) 17)

C. C. Chang, Surface Sci. 23 (1970) 283. D. T. Quinto, V. S. Sundaram and W. D. Robertson, Surface Sci. 28 (1971) 504. R. E. Weber and W. T. Peria, J. Appl. Phys. 38 (1967) 4355. R. E. Weber and A. L. Johnson, J. Appl. Phys. 40 (1969) 314. J. M. Charig and D. K. Skinner, Surface Sci. 19 (1970) 283. J. J. Vrakking and F. Meyer, Appl. Phys. Letters 18 (1971) 226. M. Perdereau, Surface Sci. 24 (1971) 239. K. Ueda and R. Shimizu, Japan J. Appl Phys. 11 (1972) 916. K. Ueda and R. Shimizu, Phys. Status Solidi a 12 (1972) K43. P. W. Palmberg and T. N. Rhodin, J. Appl. Phys. 39 (1968) 2425. C. A. Haque and H. E. Farnsworth, Surface Sci. 4 (1966) 195. N. J. Taylor, Surface Sci. 4 (1966) 161. H. E. Bishop and J. C. Riviere, J. Appl. Phys. 40 (1969) 1740. N. C. MacDonald, in: Scanning Electron Microscopy, Proc. Fourth Annual Scanning Electron Microscope Symp., 1971 (IIT Research Institute, Chicago, Illinois, 1971) part I, pp. 88-96. N. F. Mott and H. S. W. Massey, The Theory of Atomic Collisions (2nd ed., Clarendon Press, Oxford, 1961) p. 243. K. Ueda and R. Shimizu, Technol. Rept. Osaka Univ. 22 (1972) 419. T. Koshikawa, K. Ueda and R. Shimizu, Technol. Rept. Osaka Univ. 22 (1972) 51.