Segregation of tin on (111) and (100) surfaces of copper

Surface Science 0 North-Holland

68 (1977) 71-78 Publishing Company

SEGREGATION

OF TIN ON (111) AND (100) SURFACES OF COPPER

J. ERLEWEIN and S. HOFMANN Max-Planck-lnstitut fiir Metallforschung, D- 7000 Stuttgart-I, Fed. Rep. Germany

Institut fir

Werkstoffwissenschaften,

Seestrasse 92.

Surface segregation of Sn in Cu is measured at (111) and (100) surfaces by means of AES and LEED. In the case of at temperature measurements and no cosegregation of impurities occurring, equilibrium segregation is accomplished for Sn bulk concentrations between 40 and 4300 at ppm and temperatures of 800 to 1230 K. The maximum segregation level of Sn corresponds to a (d/3 X ,/3)R30” structure for the (111) surface and a ~(2 X 2) structure for the (100) surface. For theoretical analysis, the Langmuir-McLean equation has to be modified. No difference in segregation enthalpies for both surface orientations is found within the experimental error. The mean segregation enthalpy is determined to AH = -(53 + 5) kJ/g-atom.

1. Introduction

Segregation of solute atoms is a generally observed phenomenon in the study of interface or surface composition of alloys after temperature treatment. Many of the segregation experiments at surfaces have shown that complications in theoretical treatment may arise from analysis after quenching to room temperature [ l ,S--81, from contaminations [2,6,7,9] or from absence of true equilibrium composition due to the limiting effects of bulk diffusion and evaporation [8-l 11. For a direct quantitive evaluation of equilibrium surface segregation, Auger electron spectroscopy (AES) has shown to be one of the most powerful techniques. To avoid any possible disturbation by structural imperfections like grain boundaries, experiments on single crystalline surfaces are expected to be most reliable. Using low index planes, auxiliary LEED studies have already shown the occurrence of definite segregation structures of foreign atoms on metal surfaces [1,12,13]. The quantitative thermodynamic data derived from carefully conducted segregation measurements can be readily compared with existing theories [15-l 8,241. In this paper we report on equilibrium segregation studies of tin on (111) and (100) surfaces of copper by AES, which showed maximum segregation levels at ordered structures of tin revealed by LEED. Preliminary results on surface segregation kinetics have been published elsewhere [ 111 and will be discussed in detail in a subsequent paper [ 191. 71

J. .Erkwein, S. Hofmann /Segregation

72 2.

of tin on copper

Experimental

(111) and (100) surfaces of high purity (99.999%) copper single crystals doped with tin (O-O.5 at%, atom absorption analysis) were studied. Each sample of about 200 pm thickness was spot-welded to a NiCr/Ni thermocouple. Segregation experiments were performed in an UHV of <30 nPa using CMA Auger analyzer and a threegrid LEED system (Physical Electronics lnd.). Due to the low residual gas pressure, it was possible to keep the sample surface free from any detectable contamination for a period of about one day. The surface of the sample, which could be heated by electron bombardment from the rear was repeatedly cleaned by bombardment with 1 keV argon ions followed by annealing. A typical Auger spectrum is shown in fig. 1. Only Cu and Sn signals are above the noise level. This proved to be essential for the reliability of segregation experiments. Since the Auger signals were changed during electron bombardment heating in an unpredictable way, direct at temperature measurements gave no valuable results. Therefore, an indirect at temperature measurement was performed which is demonstrated in fig. 2. After heating the sample for a time long enough to establish equilibrium at the (constant) measured temperature, the electron beam heating was interrupted and the true Auger signal of Sn was obtained. By continuous monitoring of the Sn signal, the Auger peak to peak height (APPH) at temperature was

500

00

E KeVJ

Fig. I. Auger

spectrum of a (111)Cu surface with Xsmax = 0.33 of Sn segregated

on the surface.

J. Erlewein, S. Hofmann /Segregation

of tin on copper

1

0 annealmg

Fig.

2.

time

13

*

30

60

I Csecl after

90

quenching

Evaluation of true at temperature Sn APPH’s. For details see text U’a,,neal = 1050 K).

determined from extrapolation of the quenched value (at room temperature) as shown in fig. 2. To get the optimum time resolution, maximum and minimum of the Sn 430 eV Auger peak were monitored separately. A crude test for the attainment of segregation equilibrium is a constant Sn signal with increasing annealing time. At low temperatures, however, this conclusion may be erroneous due to the suppressed foreign atom diffusion [8,11,20]. At high temperatures the surface concentration of the segregated species can be decreased by evaporation [8,20] and is likely to go through a maximum with time as shown recently by Lea and Seal-r [21]. In the system Cu-Sn no change of the Sn Auger signal with annealing time was detected up to 1230 K. Within the experimental errors, reversibility of Sn coverage with temperature was obtained between 800 and 1230 K.

3. Results and discussion Only those measurements were taken for the evaluation of Sn segregation, where it was possible to completely avoid contamination of the specimen surface from residual gas atmosphere or from cosegregation of impurities [ 18,221. Sulfur proved as the most obstinate impurity. An example of its influence on Sn segregation is given in fig. 3. At temperature measurements show a decrease of the Sn Auger signal with increasing S coverage (fig. 3, lower curve). A similar but more pro-

74

J. Erlewein,

S. Hofmann

/Segregation

0 quenched

of tin on copper

from

lfZ3K \

.

\ \

\

o,

\

1123K

\

I 0 S

Fig. 3. Relation between room temperature.

I 1

tin and sulfur

APPH

coverages

2

3

Carb. units1 (in APPH’s)

at temperature

and quenched

to

nounced influence is obtained after quenching (upper curve). At temperature, an increasing number of Sn atoms in equilibrium surface concentration is replaced by the segregating sulfur. During quenching, the sulfur coverage remains constant because of its very low bulk concentration, which was estimated from segregation kinetics with the formalism given in ref. [l l] to be < 1 atppm. In contrary, the tin coverage increases towards the equilibrium value at a lower temperature. If a certain amount of surface sites suitable for Sn segregation has been already taken by S atoms at temperature, the Sn segregation during quenching is impeded. Therefore, no simple correction seems to be possible by normalizing the Sn Auger signal at temperature with the Sri/// signal ratio in the quenched state [22]. Auger peak to peak heights (APPH’s) of Sn on (111) and (100) surface of Cu with Sn bulk concentrations (in the solid solution region) between Xb = 4 X 10V5 and Xt, = 4.3 X 10h3 (atom concentration ratios) were measured at temperatures from 800 to 1230 K. An example for the (111) surface is shown in fig. 4. For Xt, = 4.3 X 1OW3a measureable deviation from the maximum attainable value Xrax is obtained only above 1000 K. For Xb = 2 X 10M4, the decrease of the Sn signal is shifted by about 200 K to lower temperatures and XF” is reached at 800 K. LEED (fig. 5) show a (43 X J3)R30” supersturcture. Since it is measurements at Xr” known sputtering experiments that enrichment of Sn is confined to the first surface layer [11,23], the Sn/Cu atom number ratio at the (1ll)Cu surface is readily determined to XpaX = 0.33 [24]. Corresponding experiments were performed at (100) surfaces which show comparable concentration and temperature dependence of X,. In this case, however, LEED experiments reveal a p(2 X 2) structure for XF”‘, which points to XFax = 0.25. The XF”” values obtained by LEED are in accordance with quantitative

J. Erlewein, S. Hofmann / Segregation of tin on copper

15

Fig. 4. LEED patterns of (a) Cu(l11) surface with Sn segregation of Xs < 0.01 ((1 X 1) structure) and (b) Cu(ll1) surface with Sn segregation of X,max = 0.33 ((43 X J3)R30” structure) (Low quality of the pictures is mainly due to the multi-specimen sample holder with electron heating gun and thermocouples in front of the screen).

evaluation [19,23] by means of the measured 10% decrease of the Cu 920 eV Auger signal due to the segregated Sn at X~“x(lll). Assuming an exponential decay of the Auger signal output with depth [23], an attenuation length XT cos 42” = 13 A [29] for 920 eV gives a coverage of 0.5 at xax, which by division with the ratio of atomic coverage per unit area of Cu and Sn (~1.6) yields cax(l 11) 4.3. Only absolute values of APPH of Sn can be set linearly proportional to Sn coverage [11,23] and not the often used signal ratios like APPH(Sn)/APPH(Cu). Small discrepancies between quantative Auger signal evaluation and LEED information at

0

’ 800

900

1000

0

xb =q0002

.

xb = 0.0043

1200

!I00

1.30 0

TCKI

Fig. 5. Equilibrium surface coverage Xs of Sn at Cu(ll1) for Sn bulk concentrations xb = 2 X 10e4 and xb = 4.3

X

surfaces 1 0W3.

as a function

of temperature

16

J. Erlewein, S. Hofmann 1 Segregation of tin on copper

substantial monolayer fractions of the segregant ]13] can be explained by this erroneous assumption [ 19,23,25]. The experimental results cannot be described adequately by the simple Langmuir-Mc~an equation [10,13,14]. In studies of the kinetics of surface se~egation of Sn in Cu [l 1 ] a marked discontinuity of the X,(t) curves was observed at Xr”. This behaviour is theoretically deduced [19] from the occurrence of an ordered segregation structure according to the LEED results. With the implication of site occupancy probab~ities, the following mo~~cation of the ~n~uir-McLe~ equation must be used for a proper description of X,(7’, Xt,) in thermodynamic equilibrium: max XJ( 1 - xSlx~>XS

=X,/(1

- Xt,)] exp(-AG/R~.

(1)

Xt, must be small compared to the solubility limit of the segregant (-0.06 1261). AG = AH - TAS is the free enthalpy (in terms of enthalpy AH and entropy AS) of surface segregation. Eq. (l} reduces to the simple ~n~~r-~c~an equation for tax -+ 1 [lo]. Arrhenius plots of eq. (1) with the measured values of X, and T for different Xt, and surface orientations are shown in fig. 6, from which the segregation enthalpies and entropies are deduced and compiled in table 1. For comparison, the values obtained by applying the simple Langmuir-McLean equation are also included. Ob-

T fK.7

0.21 0.8

l0

0.9 1000/T

J.J

r,z

I 1.3

Cl/K3

Fig. 6. Plot of e/f1 - B)X~ax versus 103jT (e = XS/X~ax). Straight lines according to eq. (1).

11

J. Erlewein, S. Hofmann / Segregation of tin on copper

Table 1 Comparison of segregation McLean equation .%&ace orientation

parameters

obtained

by eq. (1) and by the simple Langmuir-

After Langmuir-McLean

According to eq. (1):

xb

-AH (kJ/g atom)

-AS/R

eq.

--AH’ (kJ/g atom)

-AS/R

117 80 96 82

I

(111)

(Ill) (100)

(100)

4.3 2 6 4

x x x x

10-S 1O-4 1O-4 10-s

55 48 48 53

1 2 1 1.5

1

5 1

viously this simplified evaluation leads to a considerable scatter in segregation enthalpy and to entropy values with unreasonable magnitude of &‘/I? for the higher bulk concentrations of Sn. In the contrary, evaluation according to eq. (1) gives considerable agreement in the AH values and a reasonable order of magnitude for aS in all cases. The deviations of the measured values form the straight lines in fig. 6 at low bulk concentrations and low temperatures are due to the fact, that in this case large mean diffusion lengths and hence too long annealing times will be requested for the attainment of equilibrium surface concentration [11,19]. A limiting experimental uncertainty of the measured data comes from the temperature determination which has been estimated to about +lO I(, resulting in a mean error for AH of +3 kJ/g atom. Together with a small uncertainty in the measured APPH’s, a mean value for the segregation enthalpy of Sn on Cu of AH= -53 + 5 kJ/g-atom (-12.5 + 1.5 kcal/g-atom) is determined including both surface orientations. It is expected, that the dependence of segregation enthalpy on surface orientation is of the order of the difference in the surface energies [17]. For copper, this difference between (111) and (100) orientations is about 5% [27] which lies within the mean error of AH. Errors in the dete~nation of the segre~tion entropy comprise those of the enthafpy. Additionally, local variations of bulk concentration in the sample are important. (The integral bulk concentration obtained by atom absorption analysis is a mean value over a relatively large volume of low2 cm3.) The mean segregation entropy (in units of R) is determined to ~/~ = -1 f 2. Such a low value IS expected from estimations of Ewing and Chalmers [28].

4. Conclusions (1) Reliable equilibrium surface segregation measurements of Sn in Cu can only be performed if no detectable concentration of any other species is present at the surface (e.g., no sulfur co-segregation).

78

J. Erlewein, S. Hofmann /Segregation

of tin on copper

(2) Since segregating Sn forms an ordered structure on well-defined surface orientations, the Langmuir-McLean equation must be modified to evaluate the segregation enthalpies and entropies. This should be valid for all systems Witi ordered segregation structure. (3) No appreciable difference is found between the segregation enthalpies of Sn on Cu(l11) and Cu(lO0) surfaces. Their mean value is determined to AH = -53 _+5 kJ/g atom.

References [l] J. Ferrante, Acta Met. 19 (1971) 743. [2] W.P. Ellis, J. Vacuum Sci. Technol. 9 (1972) 1027. [3] S. Floreen and J.H. Westbrook, Acta Met. 17 (1969) 1175. [4] A. Joshi and D.F. Stein, Met. Trans. 1 (1970) 2543. [S] P. Douglas, J. Mater. Sci. 8 (1973) 1647. [6] J.-P. Servais, H. Graas and V. Leroy, Centre Rech. Met. 44 (1975) 30. [7] H.P. Bonzel and H.B. Aaron, Scripta Met. 5 (1971) 1057. [8] J. Ferrante, Scripta Met. 5 (1971) 1129. [9] M.P. Seah and C. Lea, Phil. Mag. 31 (1976) 627. [ 10) S. Hofmann, G. Blank and H. Schultz, Z. Metallk. 67 (1976) 189. [ll] S. Hofmann and J. Erlewein, Scripta Met. 10 (1976) 857. [12] L.C. Isett and J.M. Blakely, Surface Sci. 47 (1975) 645. [13] H.J. Grabke, G. Tauber and H. Viefhaus, Scripta Met. 9 (1975) 1181. [14] D. McLean, Grain Boundaries in Metals (Clarendon Press, Oxford, 1957). [15] M.P. Seah and E.D. Hondros, Proc. Roy. Sot. (London) A335 (1973) 191. [ 161 F.L. Williams and D. Nason, Surface Sci. 45 (1974) 377. [ 171 G.A. Somorjay and S.H. Overbury, Farady Discussions Chem. Sot. 60 (1975) 279. [18] M. Guttmann, Metal Sci. 10 (1976) 337. [ 191 S. Hofmann and J. Erlewein, to be published. [20] J.J. Burton, C.R. Helms and R.S. Polizotti, J. Vacuum Sci. Technol. 13 (1976) 204. [21] C. Lea and M.P. Seah, Phil. Mag. 35 (1977) 213. [22] C. Lea and M.P. Seah, Surface Sci. 53 (1975) 272. [23] S. Hofmann, Mikrochim. Acta Suppl. 9 (1977) 000. [24] M. Prutton, Met. Rev. 152 (1971) 57. 1251 J.J. Grabke, W. Paulitschke, G. Tauber and H. Viethaus, Surface Sci. 63 (1977) 377. [26] M. Hansen, Constitution of Binary Alloys (McGraw-Hill, New York, 1958). [27] W.L. Winterbottom, in: Surfaces and Interfaces, Vol. I, Eds. J.J. Burke, N.L. Reed and V. Weiss (Syracuse Univ. Press, Syracuse, NY, 1967) p. 133. [28] R.H. Ewing and B. Chalmers, Surface Sci. 31 (1972) 161. [29] C.J. Powell, Surface Sci. 44 (1974) 29.