On the formation of copper-rich copper silicides

Thin Solid Films, 200 ( 1991 ) 147-156

147

PREPARATION AND CHARACTERIZATION

ON T H E F O R M A T I O N O F C O P P E R - R I C H C O P P E R SILICIDES L. STOLT*, F. M. D'HEURLE t AND J. M. E. HARPER IBM Research Center, PO 218, Yorktown Heights, N Y 10598 (U.S.A.) (Received August 24, 1990; accepted November 15, 1990)

The reaction of copper with silicon in Cu-Si bilayers with overall compositions between copper and the most silicon-rich compound Cu3Si, was monitored by means of several analytical tools, in situ resistance measurements during controlled heating, backscattering, and X-ray diffraction. The order of phase formation was established to be first q" Cu3Si, followed by the formation of a phase called y, with about 17 at.y0 Si. With a sample of intermediate composition, one then observes the reaction o f q " with ~,, resulting in the formation of the e phase with about 20 at.~ Si. The resistivity of the various phases was estimated.

1. INTRODUCTION

The increasing demands of silicon device technology for high performance has led to some considerations of the use of copper rather than aluminum for interconnections. A paper 1 on the formation of Schottky barrier diodes from Si-Cu contacts gives an indication of the current interest in copper. Quite naturally, this leads to some concern with respect to the formation 2'3 and properties of the silicides of copper. The reaction of copper with silicon results in the direct formation 2 of Cu3Si which forms by the diffusion of copper atoms through the silicide to the Cu3Si-Si interface. This has been shown in bulk materials by Veer et al. 4 and recently in a thin film study 5. The observation concurs with the anticipations of the "ordered Cu3Au" rule 6, which states that in compounds the majority atoms should be more mobile than the minority atoms. Once formed and exposed to air, this silicide displays very peculiar oxidation 7 properties. From the point of view assumed in the present paper, the important consideration is the direct formation of Cu3Si and the absence of formation of other copper-rich silicides whose existence is clearly seen in the Cu-Si equilibrium diagram, part of which is reproduced in enlarged scale in Fig. 1 (from refs. 8 and 9). In order to investigate the possibility of forming these copper-rich silicides, use was made in the present * Permanent address: Institute for Microelectronics, Box 1084, 164 21 Kista, Sweden. t Also at the Institute for Solid State Electronics, Royal Institute of Technology, Box 1298, 164 28 Kista, Sweden. 0040-6090/91/$3.50

© Elsevier Sequoia/Printed in The Netherlands

148

L. STOLT, F. M. D'HEURLE, J. M. E. HARPER

700

600

oo

555° a (Cu)

i,t (Y

ee" m 13-

500

1

-

467 °

y~

w

- ~'5'"

400

J

I I I I

300 0

1 10

20

30

Si IN Cu (Gt. %)

Fig. 1. Part of the equilibrium phase diagram of Cu-Si (redrawn from refs. 8 and 9).

investigation of a technique that was initially reported10 for the formation of nickelrich silicides and later used, for example, for the study 11 of nucleation phenomena. The principle is relatively straightforward: on an inert substrate one deposits in sequence the two elements in the compound with thicknesses corresponding to its chemical formula. When reacted, the first compound to form is usually the same as is otherwise observed, then on complete formation of that compound, here Cu3Si, one anticipates that the reaction of Cu3Si with the remaining copper will eventually lead to the compound initially selected. 2. EXPERIMENTAL PROCEDURES

The substrates used were thermally oxidized silicon wafers with an oxide thickness of the order of 500 nm. On these, bilayers of silicon and copper were deposited, in that order, via vacuum evaporation from electron-beam heated sources. The pressure was in the 10- 7 Torr range; there was no deliberate attempt at heating the substrates. Oxidation or contamination of the interface was minimized by carrying out the two depositions (silicon and copper) in sequence, in one p u m p down. Three bilayers corresponding to different compositions were prepared, in all of these the thickness of the silicon layer was kept about constant at 100 nm, and the thickness of the copper layers was adjusted to the desired compositions. The wafers were then cut into squares of sides 1 cm which received a variety of heat treatments, always in an atmosphere of helium which had been at least purified on a bed of titanium sponge at 900 °C. Most of the samples used for X-ray diffraction (copper target and post sample monochromator) and backscattering analysis received isochronal annealings for 30 min. Other samples were annealed at constant heating rates while their sheet resistance was monitored by means of a Van der Pauw pattern.

149

COPPER-RICH COPPER SILICIDES

For the estimation of resistivities, the thickness of the samples was determined from the backscattering spectra and the available crystal structure information. 3.

RESULTS AND DISCUSSION

One of the three sets of samples was deposited with an excess of silicon with respect to Cu3Si in order to see whether the reaction of copper with "amorphous" (no features in the X-ray diffraction spectra were found to originate from the evaporated silicon layer) as-deposited silicon displayed significantly different characteristics from those that were obtained with crystalline silicon. In situ measurements of the sheet resistance during heating showed a somewhat lower reaction temperature (about 30 °C) than with (100) or (111) silicon wafers, typically Cu3Siis formed at about 200 °C on crystalline silicon 2' 5. Some of this difference may be due to the anticipated higher reactivity of as-deposited silicon (especially to the extent that CuaSiformation might be controlled by nucleation); but the most likely determining factor is the absence of interfacial oxide in the bilayer samples. The X-ray diffraction patterns obtained with samples heated at several temperatures were not significantly different from those obtained on crystalline silicon. The patterns were determined to be characteristic of the rl" phase as formed by Cu-Si reaction, with the predominant line at 22.5 ° (as in ref. 2). The resistivity of the q" phase is high, about 60 ~tf~cm. 3.1. Samples with 16 at.~oo Si The resistance measurements data in Fig. 2, with a heating rate of 3 °C min- 1, show the formation of a distinct phase at about 200 °C, which was identified by further experiments to be the q" phase, and a new reaction proceeding slowly from 300 °C (at which temperature the formation of the rl" phase is complete) up to about 400 °C. Identification of the new phase formed required the assistance of X-ray diffraction (the samples do not correspond entirely to those in Fig. 2 since, as previously mentioned, these received isochronal heat treatments). Characteristic parts of the patterns are reproduced in Fig. 3. The peak at 22.5 °, clearly visible after annealing at 200°C, is characteristic of the q" phase. It is still seen after heat ~E~2 . 5 z: 2.0 0

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200

i

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400

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600

l

800

TEMPERATURE (*C)

Fig. 2. The sheet resistance of Cu-Si bilayer (16 at.% Si) monitored in situ during heating at a rate of 3 °C m i n - 1 in an inert ambient.

150

L. STOLT, F. M. D ' H E U R L E , J. M. E. H A R P E R

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BRAGG A N G L E ( O )

Fig. 3. C h a r a c t e r i s t i c parts of X-ray diffraction p a t t e r n s of C u Si bilayers (16 at.% Si) a n n e a l e d in purified h e l i u m at t e m p e r a t u r e s indicated in the figure.

treatment at 250 °C but disappears for samples annealed at higher temperatures. The three other peaks at about 22.0 °, 23.4 ° and 24.5 ° can be identified 12 as the (300), (310) and (311) peaks of the cubic 7 phase. The data listed in Table I leave no doubt about the proper identification of the y phase since almost all the possible lines are observed. The line at 21.8 ° characteristic of the (111) spacing of copper appears to remain visible throughout, which means that the average composition of the samples is slightly inside the two-phase region of the diagram shown in Fig. 1. For the sample annealed at 750 °C, it is likely that the apparent persistence of copper, which is in violation of the phase diagram (Fig. 1, at 750 °C one expects equilibrium conditions to be rapidly established), is due to some ambiguity in the X-ray diffraction patterns for copper and for the K phase, whose formation will be discussed in more detail further on. Unfortunately, because of the silicon in the SiO2 of the substrate, very precise determination of the composition from backscattering measurements is not possible, particularly not within the degree of accuracy necessary to investigate details of the tight equilibrium phase diagram shown in Fig. 1. The backscattering spectra in Fig. 4 for samples annealed at 230 °C and 400 °C deserve careful examination. The spectrum for the sample annealed at 400 °C would appear to correspond to a layer of uniform composition, however, with the knowledge of the X-ray diffraction results one can see in the area between 1.2 and 1.3 MeV that the sample is somewhat deficient in silicon (or copper rich, but that is less easily seen in the copper part of the spectrum). An unanticipated detail in Fig. 4 is the fact that the sample annealed at 400 °C is more uniform in thickness than that annealed at 230°C (compare the two spectra at about 1.45 MeV). The non-

COPPER-RICH COPPER S1LICIDES

151

TABLE I PEAKS OBSERVED IN X-RAY DIFFRACTION ANALYSIS OF A C u - S i HEAT TREATED AT 4 0 0 ° C ;

B1LAYER W I T H A COMPOSITION OE

16 at.% Si

THE STANDARD P O W D E R DIFFRACTION PATTERN FOR C u 5 S i , "/ PHASE, IS ALSO

INCLUDED

Experimental d (,~)

Reported" I/Io b

2.320 2.087 2.067 1.957 1.867 1.657 1.458

0.5 800 100 23 6 0.6 1

1.214 1.191 1.151 1.131

1 0.3 2 0.3

Cu d(,~)

d (A)

I/I o

hkl

2.80 2.55

40 40

210 211

2.07 1.97 1.88 1.66 1.47 1.39 1.33 1.27 1.24 1.22 1.20 1.16 1.14 1.10

100 80 80 60 80 80 40 40

2.088 221,300 310 311 321 330,411 420 332 422 430,500 431,510 333,511 432,520 521 440

a Ref. 12. b Ratios of peak intensities, not integrated intensities.

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8ACKSCATTERING ENERGY(MeV) Fig. 4. Backscattering spectra of Cu-Si bilayers (16 at.% Si) heat treated at 230 °C and 400 °C. The low energy portion of the spectrum is magnified four times.

uniformity with respect to thickness of the sample annealed at 230 °C is a likely indication that the formation of the q" phase, Cu3Si, is not entirely controlled by diffusion; nucleation 13 must also play a significant role in the reaction as has been alluded to in ref. 5. The plateau of almost constant resistance in Fig. 2 above 450°C should

152

L. STOLT, F. M. D'HEURLE, J. M. E. HARPER

correspond to the complete formation of the y phase. One would then have the y phase and copper remaining in equilibrium up to 555 °C, at which temperature a third phase is also in equilibrium, the ~cphase (Fig. 1). This, however, cannot nucleate and grow at 555 °C; its formation requires the presence of some excess free energy (for nucleation, see in ref. 13 the discussion about PdSi) and that can occur only with some superheating above the eutectoid temperature. It is likely that the drop in resistance seen in Fig. 2 almost at 700 °C is due to such ~c phase formation. The appearance of an extra line (weak) at 20.6 ° in the diffraction pattern for the sample annealed at 750°C with a spacing of 2.21A fits such an interpretation. It corresponds to the spacing for the (100) line of hexagonal ~; (ref. 14, a = 2.560 A and c = 4.185 A). Further support for such a conclusion is derived from observation of the increased intensity of the line at 21.8 ° (compared with that obtained for the sample annealed at 400 °C). With a spacing of 2.09 A it is as likely to be the (111) line of copper as the (002) line of the ~ phase, but in the light of the present discussion the latter interpretation is more likely to be correct. In Fig. 2 the formation of the 7 phase appears to be much more sluggish than that of the initially formed 11" phase. While one indeed anticipates that the first phase to form and grow should be characterized by higher diffusion coefficients than the phases that follow, the difference here appears remarkably well marked. In the absence of detailed information about diffusion in the y phase, a thorough discussion of this matter is excluded. However, it is most likely that the observed difference in behavior is not due to a particularly sluggish formation of the 7 phase, but to the remarkably rapid formation of Cu 3Si. That phase is known15 to contain a high density of vacancies, which leads to unusually high diffusion coefficients for copper, a matter which is analyzed in ref. 5. Finally, before proceeding to the consideration of other samples with different compositions, one notes that no indications of the formation of the e phase were detected in the present set of experiments. The sequence of phase formation is q" first, as usual, and then ? from the reaction of rl" with the excess remaining copper. Estimating the resistivity of the 7 phase is difficult because of the shortcircuiting effect of any excess copper. In order to account for the measured resistance, the resistivity of the 7 phase should be about 60-80 I ~ cm, of the same order as the ~q" phase but perhaps still higher. Considering that the K phase results from the reaction of about four times as much copper as the ? phase (ignoring changes of volume in all of these copper-rich phases), the decrease in resistance observed on formation of that phase requires that it should have a very low resistivity, close to that of copper and an order of magnitude smaller than that of the other phases encountered here, either y or g (see below). 3.2. Samples with 20 at.~oo Si These samples were prepared with the specific intention of producing the phase (Cu158i4)s. The resistance vs. heating curve for one such sample is shown in Fig. 5, where one immediately sees the abrupt increase in resistance at 140 °C typical of the 1"1"forming reaction. The reaction temperature here is lower than in Fig. 2 because of the slower heating rate (0.5°C min 1). The interpretation of other features in the heating curve requires the assistance of information obtained from

COPPER-RICH

COPPER

153

SILICIDES

X-ray diffraction. That is displayed in Fig. 6. The partial diffraction pattern obtained after heating at 230 °C is characteristic of the formation of the q" phase. H o w e v e r , s o o n afterwards, on heating to 240 °C, one sees that the well defined peaks from the 7 phase have also appeared. After annealing at a higher temperature, 260 °C, the main copper line at 21.8 ° has totally disappeared, but the reaction of 7 with q" has resulted in the formation of the e phase w h o s e presence is revealed by the new peak at 24 °. Heating to 300 °C results in the c o m p l e t e formation of the e phase. As with the previous sets of samples, there is s o m e ambiguity about the proper identification of one line, that at about 22.0 °, which could belong as well to y as to ~. However, the "~2.5 2.0 v z

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400

J

600

TEMPERATURE

800

(°C)

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bilayers ( 2 0 a t . % Si) a n n e a l e d in

154

L. STOLT, F. M. D ' H E U R L E , J. M. E. H A R P E R

persistence of the line at 24.5 ° is a sure sign of the remaining presence of excess 7. This observation indicates that with respect to the composition of 8 in Fig. 1 the samples here are slightly copper rich. This point is further emphasized by reference to Table II where several peaks belonging to the 7 phase are reported. Proper identification 16 of the ~ phase (cubic with a = 9.615 &) from its diffraction pattern is provided in Table II. Thus the drop in resistance in Fig. 5 slightly below 300 °C can be attributed to the formation of the e phase which, according to the backscattering results in Fig. 7, is completed after treatment at 300 °C. Thus it is likely that the plateau and decrease in resistance seen in Fig. 5 in the range 300-400 °C are due partly to the completion of the e phase formation, but mostly to an increased ordering of this structure and grain growth.

I

6

2 0 0 " C ~ 500*C'-~i

0

'

I

1,0 1.4 i.8 BACKSCATTERINGENERGY(MeV) Fig. 7. Backscatteringspectra of Cu Si bilayers (20 at.~oSi) heat treated at 200 C and 300 °C. The low energy p o r t i o n of the s p e c t r u m is magnified four times.

On completion of the heating cycle shown in Fig. 5, the sheet resistance, monitored on cooling, followed the heating curve down to the knee at 400 °C, and then continued linearly down to room temperature. The resistivity at room temperature for the e phase was found to be about 30 Ixf~cm. 4. CONCLUSIONS (1) A study of the reaction of copper and silicon bilayers with overall compositions ranging up to 25 at.% Si and below, confirms that the first phase formed in the reaction of copper with silicon is Cu3Si, q". (2) In the presence of excess copper, once the q" phase is formed, it reacts with the remaining copper to form the 7 phase with a composition between 17 and 18 at.% Cu. (3) Above a eutectoid temperature of 555 °C in the presence of more excess copper another phase still richer in copper, the ~: phase, may be formed, whose nucleation requires superheating above 555 °C. (4) With samples having an overall composition close to 20at.% Cu, one

COPPER-RICH COPPER SILICIDES

155

TABLE II PEAKSOBSERVEDIN X-RAYDIFFRACTIONANALYSISOF A Cu-Si BILAYERWITH A COMPOSITIONOF 20 at.~ Si HEAT TREATEDAT 300 °C; THE STANDARDPOWDERDIFFRACTIONPATTERNFOR Cu 15Si4, g PHASE, IS ALSO INCLUDED Experimental

y~

Reported ~

d(A) d (•) 3.430 3.062 2.773 2.591 2.534 2.421 2.069 2.029 1.978 1.961 1.899 1.870

I/1o b

d (~)

I/Io

hkl

1 1 2

3.39 3.04

40 40

320 310

2.683 2.573

5 80

320 221,300

2.410 2.096 2.054

40 50 100

400 421 332

1.969

80

422

1.890

90

510

1.767 1.706 1.656 1.566 1.490

5 5 10 60 50

521 440 530 532 541

1.399 1.343 1.317 1.263 1,229

5 60 60 5 90

444 515 552 730 560

6 1 0.6 100 1 4 19 12 6

1.659 1.571

1 1

1.461

1

1.232 1.217 1.193

2 1 0.6

1.152

3

1.126

1.5

2.80

2.55

2.07

1.97 1.88

1.66

1.47

1.22

1.20 1.173 1.158 1.142 1.126

40 40 50 90

733 742 653 830

1.16

a Ref. 16. b Ratios of peak intensities, not integrated intensities. c Ref. 12. o b s e r v e s in t h i s o r d e r t h e f o r m a t i o n s o f t h e p h a s e s q " , 7, a n d f r o m t h e r e a c t i o n o f t h e s e t w o p h a s e s , a. (5) T h e f o r m a t i o n o f t h e q " p h a s e r e s u l t s in a s h a r p i n c r e a s e i n r e s i s t a n c e b e c a u s e o f t h e h i g h r e s i s t i v i t y o f t h a t p h a s e , a b o u t 60 p.fl cm. W i t h e x c e s s c o p p e r , t h e f o r m a t i o n o f t h e y p h a s e l e a d s t o a f u r t h e r i n c r e a s e in r e s i s t a n c e b e c a u s e o f t h e consumption of the remaining copper and of the high resistivity of the 7 phase, e s t i m a t e d t o b e a l s o in t h e v i c i n i t y o f 6 0 - 8 0 ~t~ cm. I n c o n t r a s t , t h e r e a c t i o n o f v w i t h r I'', r e s u l t i n g in t h e f o r m a t i o n o f e, is a c c o m p a n i e d b y a d e c r e a s e in r e s i s t a n c e . T h e r e s i s t i v i t y o f t h e a p h a s e h a s b e e n e s t i m a t e d a t a b o u t 30 lad cm. T h e f o r m a t i o n o f t h e

156

L. STOLT, F. M. D'HEURLE, J. M. E. HARPER

~; phase from the reaction of 7 with copper results in a lowering of the resistance of the sample, indicating a relatively low resistivity for the ~ phase, comparable with that of copper. REFERENCES 1 T. Ohmi, T. Saito, T. Shibata and T. Nitta, Proc. 1988 VLSI Multilevel lnterconnection ConiC, IEEE, New York, 1989, p. 135. 2 C.-A. Chang, J. Appl. Phys., 67 (1990) 566. 3 A. Cros, M. O. Aboelfotoh and K. N. Tu, J. Appl. Phys., 67 (1990) 3328. 4 F . A . Veer, B. H. Kolster and W. G. Burgers, Trans. Met. Soe. AIME, 242 (1968) 669. 5 L. Stolt and F. M. d'Heurle, Thin Solid Films, 189 (1990). 6 F . M . d'Heurle and P. Gas, J. Mater. Res., 1 (1986) 205. 7 J.M.E.Harper•A.Charai•L.St••t•F.M.d•Heur•eand•.Fryer•Appl.Phys.Lett.•56(•99•)25•9. 8 M. Hansen and K. Anderko, Constitution of Bina O, Alloys, McGraw Hill, New York, 1958, p. 629. 9 R.P. Elliott, Constitution of Binary Alloys, First Supplement, McGraw Hill, New York, 1965, p. 384. 10 G. Majni, M. Costato and F. Panini, Thin Solid Films, 125 (1985) 71. 11 P. Gas, F. M. d'Heurle, F. K. LeGoues and S. J. LaPlaca, J. Appl. Phys., 59 (1986) 3458. 12 Joint Committee on Powder Diffraction Standards, Pattern 4-0841. 13 F . M . d'Heurle, J. Mater. Res., 3 (1988) 167. 14 P. Villars and L. D. Calvert, Pearson's Handbook of Co stallographie Data for Intermetallic Phases, Vol. 2, American Society for Metals, Metals Park, OH, p. 2024. 15 J . K . Solberg, A cta Crystallogr. A, 34 (1978) 6~4. 16 Joint Committee on Powder Diffraction Standards, Pattern 23-222, 1973.