Thin-film growth of Sn On Cu(100)

Applied Surface Science 44 (1990) 17-27

North-Holland

17

THIN-FILM GROWTH OF Sn ON Cu(100) *

F. ABEL, C. COHEN, J.A. DAVIES **. J. MOULIN and D. SCHMAUS

Groupe de Ph.wique des Solides de I’Ecole Normale Suptkieure, LininicersitP Purrs 7. Tour 13. 2 Place Jussieu.

7525I Paris Cedex 05. France

Received 5 August 1989: accepted for publication 13 October 1989

The deposition of Sn on a Cu(100) surface has been investigated as a function of layer thickness. deposition temperature and

subsequent anneal treatment. using a combmation of RBS/channeling, LEED and Auger spectroscopy measurements. No evidence

of pseudomorphic Sn growth could be detected. At Sn coverages up to about one monolayer, RBS/channeling data indicate that Sn atoms displace a comparable number of Cu

atoms from their regular lattice sites, even at the lowest (170 K) deposition temperature studied. LEED studies show evidence of

two-dimensional commensurable phase formation whose structure depends strongly on anneal temperature and Sn coverage.

Copper-rich phases such as the c-phase (Cu,Sn) are not observed at submonolayer Sn coverage. even after a 525 K anneal treatment.

Thicker Sn layers exhibit two major anneal stages: formation of the p’-phase (Cu,Sn,) around 265 K and its subsequent

conversion into the c-phase (Cu,Sn) around 420 K. An isochronal anneal measurement yielded activation energies of 0.65 f 0.05 and

1.07 + 0.08 eV for interdiffusion of Sn and Cu in the n’- and r-phases, respectively. RBS/channeling data indicate that the resulting

q’-phase is partially (lo%-20%) aligned with the Cu(100) substrate. No preferential orientation was observed for the c-phase.

1. Introduction

Low-temperature deposition of tin on a mono- crystalline (100) copper surface and its subsequent thermal annealing behaviour offers the interesting possibility of studying the thin-film growth kinetics of Sn-rich bronze phases, such as the q’- and c-phases in fig. 1 [l]. Because of the large difference in the melting points of Sn and Cu. diffusion of Sn into the underlying Cu lattice to form a solid solution (the a-phase in fig. 1) should be completely negligible at room temperature: even at 510 K (the melting point of Sn). its diffusion constant in the underlying Cu lattice is less than 10P” cm2/s [2]. On the other hand, diffusion of Cu into the Sn overlayer occurs via an interstitial mechanism [3] with very small activation energy (0.2-0.3 eV) and hence may still be significant at

* Work partially supported by the Centre National de la Recherche Scientifique under GDR86.

* * Visiting scientist. Permanent address: Institute for Materi- als Research. McMaster University, Hamilton. Canada LXS

4Ml.

0169-4332/90/$03.50 cp Elsevier Science Publishers B.V.

(North-Holland)

300 K and below. Consequently, formation of the q’-phase should be readily observable. In fact. in an earlier study of room temperature deposition of Sn on Cu(lOO), Argile and Rhead [4] obtained Auger spectroscopy evidence for the formation of several monolayers of an alloy with composition Cu,Sn, (i.e. either the v- or _rl’-phase). Further- more, for depositions up to about one monolayer of Sn, they reported that a distinct copper LEED pattern still existed and that the Auger signals indicating alloy formation had not yet occurred.

This latter observation suggested to us an ad- ditional reason for initiating the present channeling/ Rutherford backscattering study of the Sn/Cu(lOO) system: namely the possibility of ob- taining (at low T) a pseudomorphic Sn overlayer and observing its subsequent “surface melting” behaviour as the substrate temperature is raised. At first glance. such pseudomorphic growth might seem highly unlikely because the normal cubic ((Y) and tetragonal (/?) phases of tin are poorly lattice-matched with respect to a copper (100) substrate. Nevertheless, at high pressure (39 kbar)

WEIGHT PER CENT TIN 10 20 30 40 50 60

I I I I I /

232O

200

CU ATdMlC P<i CENT -T-IN .- -- Sn

Fig. 1. Cu-Sn phase diagram (taken from ref. [I], p. 634: fig. 360).

a second tetragonal phase of tin (Sn(I1)) can form [5], whose interatomic distances along the a- and c-axes (3.81 and 3.48 A, respectively) are within 5% of the Cu(100) value of 3.62 A. Hence, a slight tetragonal compression along the u-axis and a corresponding expansion along the c-direction (similar to that occurring, for example, in the GaInAs/GaAs superlattice structure) would result in an almost perfect lattice match. The pressure of 39 kbar, required to form this Sn(I1) phase, is not excessively large and, for our proposed one-

monolayer experiment, it might be available through the Cu-Sn bonding at the interface.

One necessary condition for such pseudomorphic growth of Sn on Cu(100) is the existence of a temperature range high enough to permit ade- quate surface mobility for surface organization and yet low enough that alloy formation (i.e. diffusion of Cu in the Sn layer) does not occur. Argile and Rhead’s Auger study [4] suggested that indeed such a temperature region might exist around room temperature.

F. Abel et 01. / Thrn-film growth of% on Cu(100) 19

Our objective in the present investigation is, therefore, two-fold. Firstly, we deposit 0.3-1.5 monolayers of Sn, using Rutherford scattering (and Auger analysis) to establish quantitatively the ex- act coverage level in each case, and using LEED and ion channeling to investigate the surface structure as a function of deposition conditions and subsequent anneal treatment. Secondly, we deposit thicker (20-40 monolayers) of Sn at low temperature and apply the channeling/ Rutherford backscattering technique (RBS) to follow the kinetics of Sn/Cu alloy formation during subsequent isochronal anneal cycles.

2. Experimental

2. I. set-up

The UHV target chamber consists of a 30 cm diameter stainless steel cylinder, equipped with 4-grid LEED-Auger optics, sputter ion gun, bakeable solid-state detectors for Rutherford backscattering/channeling studies and a simple two-crucible evaporation system heated by electron bombardment. The entire system, developed by RIBER, has been described in a previous publication [6]. The ultimate pressure achieved in the chamber is - 1 x lo-’ Pa. The chamber is inter- faced to a 2.5 MV Van der Graaff accelerator at - 3 x 10e4 Pa, via a two-stage differentially pumped beam line.

The target holder is mounted on a goniometer with four independent motions: linear (x-axis) displacement and three orthogonal axes of ro- tation that intersect on the target surface. An angular precision of 0.05 O, required for MeV ion channeling, is obtainable on all three angular motions.

When the goniometer is translated out of the beam, it can be replaced by a sample holder with standards for RBS and NRA calibration. In the present experiment, the RBS standard was a Bi- implanted Si sample. The Bi amount was known with a 2% precision [7]. The backscattered particles were analysed by a bakeable ENERTEC surface barrier detector. The energy resolution of this 70 mm2 detector was about 14 keV and the

detection solid angle was about 4 X 10-j sterad. The detection angle is adjustable from outside the chamber, between 80 o and 170 O. The ion beam dose is measured on a mechanical chopper, con- stituted by two rotating blades which intercept the beam during = 25% of the time.

The copper substrate is a single crystal disc, about 2 cm in diameter and 3-4 mm thick. The original surface orientation of the disc was (100). However, the highly polished front surface had been bevelled in such a way that one half retains the original (100) orientation while the other half is a vicinal (510) stepped surface, inclined at approximately 11 o to the original surface. A similar stepped crystal was used in the earlier channeling/ blocking experiments of ref. [6]. In the present RBS/channeling investigations. all measurements were performed on the (100) region of the copper surface.

A standard surface-cleaning procedure was used immediately before each Sn-deposition run. First, the entire crystal surface was bombarded with 600 eV Ar+ for lo-15 min (longer, whenever more than one monolayer of Sn was present) at a current density of - 2 PA/cm’: the crystal was then annealed briefly at - 900 K and allowed to cool slowly to the desired temperature. This 2-step cleaning procedure was repeated (if necessary) until the Sn Auger signal was immeasurable and the Cu peak-to-peak height had returned to its maximum value. This Cu signal height was found to be particularly sensitive to the presence of trace surface contaminants: subsequent calibration by RBS confirmed that the Sn content after such a cleaning procedure had fallen below the RBS de- tectability limit of - 1 X 1O’j Sn atoms cm-‘.

Deposition of Sn on the Cu crystal surface was carried out at a controlled and reproducible rate by using a tungsten crucible surrounded by a metallic wire of = 1 Q resistance which can be heated by an adjustable current up to 8 A. The crucible can be heated either radiatively by the hot wire alone, or also by supplementary electron bombardment through the application of an adjustable polarization voltage with an upper value of 1000 V. A set of diaphragms defined the selected direction and solid angle of emission. The crucible-to-target distance was sufficiently large (20 cm)

to give uniform coverage: the Sn deposition rates were typically 0.1 monolayer per minute for submonolayer deposits and 1 monolayer per minute for thicker deposits. The e--bombardment power could be maintained constant to +0.5% and, for a given power level. the resulting deposition rate was reproducible to better than 5%. In all cases, a constant power level to the crucible was maintained for at least 30 min prior to the start of Sn deposition in order to establish a reproducible evaporation rate. A manually controlled shutter between the crucible and the copper surface was used to start and stop the Sn deposition. No tungsten signal, due to a contamination of the deposit by the crucible. was ever observed on the RBS spectra.

1 ) Procedures _._.

Quantitative calibration ( f 2%) of the resulting amount of deposited Sn was carried out in situ by RBS analysis. We also systematically registered Auger spectra at various steps of the deposition, measuring the peak-to-peak heights of the Sn and Cu signals at 430 and 63 eV, respectively. The RBS data allow us to calibrate the evolution of these peaks as a function of Sn coverage.

The target temperature was monitored continu- ously to +0.5 K, via a thermocouple embedded in the target mounting block on the goniometer. For low temperature (< 300 K) deposition, cooling was provided by a long flexible copper braid between the target mounting block and a liquid-N, cooled reservoir. Controllable resistive heating (O-50 W) in the target mounting block enabled any desired temperature between 170 and 800 K to be maintained. not only during deposition. but also during the subsequent LEED/Auger and RBS/channeling measurements. During the preliminary cleaning or annealing procedures, considerably higher temperatures (up to 1100 K) can be achieved.

For the isochronal kinetic studies reported in section 3. the desired heating rate of - 2 K/min was maintained constant to *lOR over the temperature range of interest (250-470 K). This was accomplished by manually increasing the input power to the resistive heater every 10 K in order

to compensate for the gradually increasing thermal losses with increasing target temperature. A preliminary thermal calibration run in which the heating rate versus temperature was recorded at a constant (and much higher) input power provided the necessary calibration curve of thermal losses versus temperature. At the lowest temperatures (i.e. 190-240 K). after removing the liquid-N1 reservoir. the observed heating rate (even with zero resistive heating) was slightly faster than the optimum 2 K/min value for the RBS/channeling measurements in section 3. However. by the onset of the first anneal stage (250-270 K), the desired isochronal condition of c~nstunt heating rate had become established. Typically. a RBS spectrum was registered in 2 min. thus the overall temperature variation during one spectrum did not exceed 4 K.

The channeling experiments were performed along the (100) axis. normal to the crystal surface. The aim of these measurements is to determine the amount of copper atoms either displaced from regular lattice sites by interaction with tin or shadowed by epitaxial tin. This amount is obtained from the Cu surface-peak areas in the aligned RBS spectra.

For submonolayer experiments. one has to minimize the number of crystal layers contrib- uting to the Cu surface peak in order to improve the sensitivity for studying the effect of Sn on Cu atoms. The experiments should thus he done with low 4He beam energies. However, we also want to get well separated signals from Sn and Cu. and the mass separation is proportional to the beam energy. For thin layers of Sn, the best compromise is to use a 500 keV 4He beam. For a perfect crystal at room temperature, the Cu surface peak then corresponds to about 2.1 atom per row. i.e. 6.5 X

lOI Cu cm- ‘. For “thick” deposits (20-40 monolayers) one

has to ensure that, even after alloy formation, the Cu and Sn peaks will be fully separated; hence, a 2.4 MeV 4He beam had to be chosen. In this case, the underlying crystal contribution to the Cu peak is always small with respect to that from the c‘u atoms in the alloyed phase. The main problem is to obtain precise information on the depth concentration profile in the alloy. Therefore, we im-

F. Abel et al. / Thin-film growth of% on Cu(100) 21

proved the depth resolution by detecting the backscattered particles at 0,,, = 100 O, i.e. using a graz- ing emergence geometry. Some control spectra were also taken at alar, = 156 O.

3. Results and discussion

3.1. Submonola_yer coverage

In these experiments, we have studied the in- fluence of three parameters: (i) the amount of deposited Sn (between 0.5 and 1.5 x 1015 per cm2), (ii) the deposition temperature (190-350 K), (iii) the annealing temperature (up to 525 K). The information obtained by all three experimental techniques (LEED, Auger and RBS/channeling) was necessary to obtain a clear picture of the physical situation.

Energy (MeV) J;i4 0.;'" 0.:" O.fO 0 I" 0,;" 0.f6 /

Fig. 2. (100) aligned RBS spectra before (W) and after (0) 1.2x1015 Sn cm-2 deposition: 488 keV 4He+, tila,, = 100°;

0.95 keV/channel. The spectra were registered at the deposition temperature: T = 200 K.

Table 1

Summary of RBS/channeling data

Deposition

temperature

(K)

Annealing

temperature

(K)

Deposited

Sn content

(10” atoms cm-‘)

Cu peak area after

subtraction of

crystal contribution

(lOI atoms cm-‘)

Cu : Sn ratio

As

deposited

After

annealing

Monolayer experrmenls

350 525 0.75 0.5 * 0.2 _ 0.7

200 525 0.9 1.0 f 0.2 _ 1.1

200 1.2 1 .o f 0.2 0.8 0.8

200 300 1.2 1.0 + 0.2 0.8

525 1.2 1.0 + 0.2 0.8

350 525 1.35 2.3 f 0.3 _ 1.7

350 19 19

Thicker deposrts

29 64

1.5 1.5

3.4

” 350 350 38 68

525 38 128 3.4

190 39 9 0.2 0.2 190 350 39 59 1.5

525 39 134 3.4

‘) An extra 19 X lOI Sn cmm2 were deposited at 350 K on top of the previous annealed deposit to produce a total Sn content of

38 X lOI Sn cme2. This extra deposit only displaces 4 x 10” Cu cm- 2; the r-phase already formed thus prevents reaction

between Cu and redeposited Sn at 350 K. After 525 K annealing all the deposited Sn has reacted, to form the c-phase.

The channeling results clearly demonstrate that, whatever the experimental conditions, the formation of a pseudomorphic Sn layer is never observed. The results are summarized in table 1 and typical spectra, before and after Sn deposition, are shown in fig. 2. If the deposited Sn was located in four-fold sites one would expect a lowering of the Cu surface peak, proportional to the number of deposited Sn atoms. On the contrary, we observed that Sn deposition always leads to an increase of the Cu surface peak. If this increase is interpreted, as in table 1, by assuming that a certain amount of Cu atoms are displaced from regular lattice sites and randomly distributed, we find that each Sn atom displaces roughly one Cu atom, whatever the deposition temperature. Furthermore, there is no increase or lowering of the number of displaced Cu atoms when annealing up to 525 K. Note also that no Sn loss was found during annealing.

The most simple interpretation of the channeling results would then be that the deposited Sn layer is not pseudomorphic and that it introduces disordering in the first Cu crystal plane via Cu-Sn interactions. As the amount of displaced Cu does not depend on deposition or annealing temperature, these results are not inconsistent with the conclusions of Argile and Rhead [4]: up to about one monolayer of Sn, channeling shows no clear

evidence of CuSn alloy formation. At least, the amount of displaced Cu is small enough to ex- clude completely the possibility of c-phase formation (i.e. Cu,Sn stoichiometry). However, as dem- onstrated by our LEED and Auger experiments (discussed below), channeling gives only a very crude picture and identical channeling results may correspond to very different situations.

In fig. 3, we report the Cu and Sn Auger signal intensities as a function of the amount of deposited Sn, calibrated by RBS. The deposition and measurement temperature is around 200 K. A slope change in the intensity variation, both for the Sn and Cu signals, is very noticeable when the deposited quantity becomes greater than - 1 x

10” Sn cm-‘. Up to this value there is a linear decrease of the Cu signal and a linear increase of the Sn signal. Above, the signal intensities vary more slowly. tending asymptotically towards zero for Cu and towards a constant value for Sn. These observations suggest that up to 1 x lOI5 Sn cm ‘. the deposit leads to the completion of a flat monolayer of pure Sn on the top of the Cu crystal. without islanding. Throughout the paper, we will consider that one Sn monolayer corresponds to 1 X lOI Sn cm -‘. The formation of such a flat monolayer implies the existence of surface Sn mobility, even at 200 K. As we have already seen

I /

1 2 3 4 5 6 7

-.8

Deposited Sn (1015crn~2 1

Fig. 3. Cu and Sn Auger peak-to-peak signal amplitudes versus amount of deposited Sn, as determined by RBS measurements. For the two signals. the same arbitrary normalization was applied. Deposition and measurement temperature. T= 200 K. Cu: 61 rV

signal: Sn: 430 eV signal. The solid lines are drawn to guide the eyes and to underline the strong change of slope around 1 x IO”

Sn cm- *.

17 Abel et al. / Thin-film growth of Sn on Cu(100) 23

that channeling does not indicate any Cu shadow- ing, we must then conclude that this mobility does not lead to a pseudomorphic growth. The very strong decrease of the Cu signal after the deposition of only 1 X 10” Sn cm-* (down to 11% of its value for bare Cu) is surprising. It would correspond to a mean free path for the corresponding 61 eV electrons of - 1 A, i.e. approximately five times less than what could be expected. A possible explanation is that the MW Auger process pro- ducing the Cu signal is affected by a Cu-Sn interaction which modifies the valence density of states. Note that this interaction has already been evidenced by channeling via disordering the first Cu surface plane. Another noticeable feature is that for one monolayer deposit the Sn signal has reached - 65% of its saturation value. Consider- ing a layer-by-layer growth and a mean free path of - 10 A for the corresponding 430 eV electrons one would expect the saturation intensity to be about five times greater than the signal intensity for one monolayer deposit. We are then led to assume either that the Sn Auger yield near the interface is enhanced by the Cu-Sn interaction, or that, above about two monolayers some islanding takes place, i.e. a Stransky-Krastanov mode of growth. In principle, the Cu-Sn interaction could be evidenced by a change of shape of the Auger signals; this was not observed in the recorded spectra, however the Auger system used in our experiments does not allow a precise determina- tion of line shapes.

We have already indicated that the Cu signal tends asymptotically towards zero when increasing the quantity of deposited Sn. For instance, for 5 X lOi Sn cm-* it has dropped to 1% of the bare Cu value. Th%is is in sharp contrast with the results obtained in ref. [4] for room temperature deposition. In these experiments the Cu signal reaches a constant non-zero value. This difference suggests that at room temperature Cu moves towards the surface through the Sn film, while there is no Cu mobility for deposits below 200 K. To confirm this hypothesis we have performed annealing studies. In fig. 4, we show for a 9 x 1014 Sn cm-* deposit at 170 K the evolution of the ratio of the intensities of the Cu and Sn Auger signals as a function of annealing temperature.

& , , , , , , , , , , I

200 300 400 500

T(K) Fig. 4. Ratio of the Cu and Sn peak-to-peak AES signal amplitudes versus annealing temperature: 9 x lOI Sn cmm2,

deposited at T = 170 K.

This deposition-plus-annealing experiment was performed several times; all the results are reported in the figure. The ratio of the Cu and Sn signals increases steadily from 4 at 170 K to 15 at 525 K, indicating that an increasing number of Cu atoms are not covered by Sn. The increase of this ratio is mainly due to an increase of the Cu signal, but at the same time we also evidenced a slight (25% in the temperature range explored) decrease of the Sn signal. However, even at 525 K, the Cu signal is only 30% of the one observed on a clean Cu sample and thus the Cu surface concentration remains small. It is difficult to deduce from the Auger results whether the increase of the Cu signal (and decrease of the Sn signal) with annealing temperature is due to exchange between the Sn layer and the first Cu crystal plane or to the formation of small Sn islands leaving the Cu surface partially uncovered.

The LEED studies also provide very interesting information. For 170 K deposits, we observed only the (1 x 1) diagram of pure Cu, the spots becoming fainter and broader as a function of the amount of deposited Sn. This is in good agreement with the channeling results which show no evidence of Sn pseudomorphic growth but only Cu disordering. However, during annealing, extra LEED spots appear at well defined temperatures,

Fig. 5. LEED diagrams of Sn-covered Cu(100) crystal after Sn

deposition at 170 K and annealing at 400 K. Incident electron

energy: 142 eV; deposited quantities: (a) 9 x lOI Sn cmm2, (b)

5X1014 Sncm-*.

indicating the formation of commensurable surface structures. These structures are most probably related to the formation of commensurable two-dimensional Cu-Sn alloys which take place at well- defined Cu concentrations. For instance. for 9 X

lOI Sn cm-’ deposited at 170 K, extra spots appear abruptly after annealing at 330 K and correspond to a (2fi X fi)R45 o two-domain structure. When annealing above 400 K, we ob- serve a transformation of the LEED diagram. with formation of a (3fi x &)R45” two-domain structure. This diagram is shown in fig. 5a.

Another striking fact is that the temperature at which commensurable two-dimensional phases are

formed and the very structure of these phases depend on the amount of deposited Sn. This is again consistent with the hypothesis that these structures correspond to two-dimensional alloys of well-defined compositions. For instance, if only 5 x lOI Sn cm2 are deposited at 170 K. extra spots appear on the LEED pattern already at 250 K indicating the formation of a p(2 X 2) structure. On further annealing above 400 K, we ob- serve a much more complex structure, corresponding to the pattern shown in fig. 5b. This structure has obviously a very large two-dimensional unit mesh.

As noted earlier, our Auger measurements indicate an increasing Cu surface concentration with anneal temperature. This also is consistent with the formation of two-dimensional CuSn surface phases, as suggested by the LEED data.

It must be noticed that the very different situations observed as a function of annealing temperature in LEED and Auger experiments are seen as identical in the channeling data. This demon- strates that channeling alone is not a sufficiently powerful tool for the study of complex two-dimensional structures. Channeling is able to provide quantitative information on the number of Cu atoms displaced luteral(v by the deposited Sn atoms. However, Auger and LEED data are needed to indicate how many of these displaced Cu atoms have moved out to the surface plane and whether or not they have formed a commensurable two-dimensional Cu-Sn “alloy” structure.

We have further investigated the kinetics of CuSn alloy formation as a function of deposition temperature and subsequent anneal treatment. For this purpose. considerably larger amounts of Sn (i.e. 20-40 ML) were used.

Again, RRS/channeling measurements have been used to determine accurately the number ol deposited Sn atoms/cm’ and the corresponding number of displaced Cu atoms/cm’. as indicated in section 2.2. For these thicker films. a ‘He beam energy of 2.4 MeV was chosen in order to ensure that, even after alloy formation. the resulting Sn and Cu peaks would be fully separated. There is

F. Abel et al. / ThrQilm growth of& on Cu(100) 25

now a significant background subtraction problem under the Cu peak, due to the enhanced dechan- neling level observed in the underlying (100) substrate. However, after alloying, the amount of displaced Cu atoms is sufficiently large that it can still be estimated with = 5% precision.

The results are summarized in table 1, together with the submonolayer data from section 3.1. The thick film data lead to the following observations: (i) At 200 K, there is a negligible number of displaced Cu atoms and hence negligible CuSn alloy formation. (ii) By 350 K, the Cu: Sn ratio has risen to = 1.5. Furthermore, the RBS measurements show that the upper edge of the Cu spectrum now extends to the outer surface and that the Cu : Sn ratio is fairly constant throughout the film. This is consistent with Tu’s previous work [8,9], showing that the Cu,Sn, alloy forms readily by interdiffusion at room temperature. Our observation of a slightly higher Cu : Sn ratio than that for Cu,Sn, suggests that partial formation of a more Cu-rich bronze alloy (e.g. the r-phase in fig. 1) has begun to occur at 350 K. (iii) Further annealing to 525 K causes the Cu : Sn ratio to increase markedly to = 3.4; this value is slightly higher than the 3.0 value characteristic of r-brass. Again, RBS measurements indicated that the Cu : Sn ratio was essentially constant throughout the film. (iv) Comparison of the Sn peak areas in (100) aligned and in random spectra showed that, after 350 K treatment, the (100) Sn yield was always about 15% lower than the random value, indicating that the q-phase is partially aligned with the underlying Cu( 100) substrate. Following the 525 K anneal, however, this slight degree of epitaxy had completely disappeared.

Fig. 6 shows three aligned RBS spectra, taken after deposition at 170 K, and after annealing at 300 K and at 440 K, respectively. They clearly illustrate the phase transformations described above. In fig. 7, we superimpose the experimental random and aligned spectra taken after annealing at 440 K. The aligned and random levels corresponding to the alloyed c-phase are identical in the two spectra, clearly showing that there is no preferential orientation in this phase. We also

Energy (MeV)

60 1.8 1.9 2.0 2.1 2.2 2.3

1 I I I I

360 3io Channel

Fig. 6. Aligned (100) RBS spectra for 39~10’~ Sn cm-’

deposited at 170 K: (m) as-deposited: (+ ) after 300 K anneal-

ing: (0) after 440 K annealing. Incident beam: 2.4 MeV 4He:

0 ,ah = 100 O; 3.11 keV/channel.

display in the figure a fit of the random spectrum obtained with the RUMP program [lo]. For this fit, we have assumed a uniform Cu3,2Sn composi-

tion for the alloyed phase. The decay corresponding to the back edge of the alloyed region is not sharp. An interpretation could be that the thickness of the alloy is not uniform. Indeed, the “RUMP” simulation shown in fig. 7 assumes a

Energy (MeV)

0

200 I I I

250 300 350 400 Channel

Fig. 7. (100) aligned (0) and random (+) RBS spectra for 39 x 10” Sn cmm2 deposited at 170 K and annealed at 440 K.

Same experimental conditions as in fig. 6. The full line corre-

sponds to a RUMP simulation (see text).

I._-.- 200 300 400

T(K) Fig. 8. Composition of the “alloyed” region as a function of annealing temperature, as measured from channeling/KWS experiment\

Deposition temperature: T== 170 K: deposited Sn quantity: 39 x lO’5 cm ‘.

thickness distribution between 60 and 260 X 10” (Cu + Sn) atoms per cm’, with a mean value of 160 X 10”. The large spread in the film thickness is probably related to the grain structure of the alloyed c-phase. We have also registered spectra at B,,, = 156”; they confirm the absolute numbers of alloyed Cu + Sn atoms found in the Blah = 100” experiments; they also reveal the strong non-uni- formity in film thickness.

3.3. Isochronul kinetic .study

In order to obtain quantitative information on the thin film kinetics of Cu-Sn alloy formation, we deposited about 40 monolayers (3.9 X 10lh atoms cm ~~’ ) of Sn on the clean Cu(100) crystal at low temperature (170 K) and then heated the substrate at a controlled constant rate (isochronal anneal). A fairly slow heating rate (- 2 K/min) was chosen in order to permit a series of aligned (100) RBS spectra to be recorded at roughly 10 K intervals. The entire heating run (180-525 K) took about 3 h.

We have extracted from the RBS data the Cu : Sn ratios which are plotted as a function of temperature in fig. 8. Two sharp anneal steps are clearly visible: (i) At 250-265 K. the Cu : Sn ratio rises from its as-deposited value (= 0.2) to 1.2, in excellent

agreement with the expected formation of the Cu,Sn, (71’) phase around room temperature [Xl. (ii) At 4188430 K. the Cu: Sn ratio again rises rapidly to - 3.2, indicating that the n’-phase has now undergone an almost total conversion into the c-phase.

The gradual rise in Cu : Sn ratio (from I .2 at 265 K to - 2.2 at 418 K) can probably be attri- buted to grain boundary diffusion of Cu through the n/-phase of Cu,Sn,, with the resulting gradual formation of the c-phase around the grain boundaries. The sharp step at 418--4.10 K then indicates the onset of CuSn interdiffusion within the resulting c-phase material. The Cu excess observed above 430 K. with respect to the Cu,Sn

stoichiometry of the c-phase may again be indica- tive of some Cu grain-boundary diffusion.

Note that the Cu : Sn ratio of 1.5 observed in the earlier 350 K deposition run (table 1) is in excellent agreement with the isochronal anneal curve in fig. 8.

From the mid-points (2588424 K, respectively) of the two anneal stages in fig. 8, one can obtain a reasonably accurate estimate of the activation energies for the CuSn interdiffusion processes in the Cu,Sn, (7’) and r-phases. respectively. In both cases, the thin film involved is less than 100 monolayers (ML) in thickness and hence the total number of activated jumps required per diffusing

17 Abel ct al. / Thhrn-film growth of Sn on Cu(lO0) 21

atom should fall in the range, 102-104, depending on grain size *‘. Assuming a mean value of 10’ jumps and a pre-exponential frequency term of 10” s-‘, the resulting activation energies are 0.65 k 0.05 and 1.07 + 0.08 eV for the n- and e-phases, respectively. The error bars of kO.05 and +0.08 eV correspond to the roughly lo-fold uncertainty in the number of jumps required to complete the 100 ML films growth. Our activation energy value for the c-phase agrees well with Tu and Thomp- son’s value of 0.99 eV in their earlier study [8] of the growth kinetics of much thicker Cu,Sn (i.e. e-phase) films.

4. Conclusions

“Monoluyrr” deposits

(1) Pseudomorphic growth of Sn on Cu(100) does not occur at any temperature, even at Sn coverage less than one monolayer.

(2) c-phase formation (i.e. Cu,Sn) does not occur at submonolayer Sn coverage, even after annealing up to 525 K.

(3) RBS/channeling data demonstrate that the deposited Sn displaces a comparable number of Cu atoms from regular lattice sites, even at 170 K.

(4) Auger measurements show that the surface Cu concentration increases with annealing temperature.

(5) LEED studies give evidence of two-dimensional commensurable alloyed phase formation

*’ Since D/ a I*, a single grain extending throughout the film

could require up to lo4 jumps.

when annealing; their structures depend on annealing temperature and Sn coverage and thus probably on Cu surface concentration.

Thicker deposits

(6) Thicker Sn layers exhibit two major anneal stages: formation of the n’-phase (Cu,Sn,) alloy around 258 K and the c-phase (Cu,Sn) around 424 K, with interdiffusion activation energies of 0.65 k 0.05 and 1.07 i 0.08 eV, respectively.

(7) RBS/channeling data indicate that the Cu,Sn, alloy is partially aligned with the Cu(100) substrate. No preferential orientation was observed for the c-phase.

References

[I] M. Hansen, Ed.. Constitution of Binary Alloys

(McGraw-Hill, New York. 1958) p. 634.

[2] L.C. Correa da Silva and RF. Mehl, Trans. Am. Inst. Min.

Eng. 191 (1951) 155.

[3] B.F. Dyson, T.R. Anthony and D. Turnbull, J. Appl.

Phys. 38 (1967) 3408.

[4] G. Argile and G.E. Rhead. Surf. Sci. 135 (1983) 18.

[S] W.B. Pearson, Ed.. Handbook of Lattice Spacings and

Structure of Metals, Vol. 2 (Pergamon, New York. 1967).

[6] J.C. Boulliard, C. Cohen, J.L. Domange, A.V. Drigo, A.

L’Hoir, J. Moulin and M. Sotto, Phys. Rev. B 30 (1984)

2470.

[7] C. Cohen, J.A. Davies, A.V. Drigo and T.E. Jackman.

Nucl. Instr. Meth. 218 (1983) 147.

[8] K.N. Tu and R.D. Thompson, Acta Met. 30 (1982) 947.

[9] K.N. Tu. Acta Met. 21 (1973) 347.

[lo] L.R. Doolittle. Nucl. Instr. Methods B 9 (19X5) 344.

Thin-film growth of Sn On Cu(100)

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