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Investigation of interdiffusion in thin film couples of aluminum and copper by Auger electron spectroscopy
Thin Solid Films, 61 (1979) 273-279 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
273
INVESTIGATION OF INTERDIFFUSION IN THIN FILM COUPLES OF A L U M I N U M AND COPPER BY A U G E R ELECTRON SPECTROSCOPY M. P. SHEARER*, C. L. BAUER AND A. G. JORDAN
Center for the Joining of Materials, Carnegie-Mellon University, Pittsburgh, Pa. 15213 (U.S.A.) (Received November 28, 1978; accepted January 31, 1979)
Interdiffusion in thin film couples of aluminum and copper was investigated at relatively low temperatures (100-200 °C) and short annealing times (5-60 min) using Auger electron spectroscopy. Analysis of the results reveals two distinct processes, each characterized by a unique set of diffusion coefficients: (1) diffusion in the vicinity of the original interface and (2) diffusion extending beyond the interfacial region. These processes are identified as defect-induced interfacial diffusion and grain boundary diffusion respectively. Measured values of the activation energy are consistent with those expected for short-circuit diffusion and effectively obliterate, under the imposed conditions of annealing time and temperature, meager effects expected from volume diffusion. In addition, the formation and growth of A12Cu along grain boundaries was detected by transmission electron microscopy.
1. INTRODUCTION
Multiple layers of thin films are often incorporated in electronic devices in order to utilize specific properties of each individual layer. Unfortunately, the reliability of such devices is affected adversely by interdiffusion and the formation of intermetallic phases, which commence at the original interface separating these layers and then expand outward. Moreover, these processes are especially pronounced under typical operating conditions of low temperature, i.e. at temperatures less than half the absolute melting point, and with high densities of short-circuit diffusion paths. Thus, information pertaining to volume interdiffusion is virtually worthless when it is desired to assess the reliability of thin film devices. The investigation of interdiffusion in thin film couples, however, requires resolution of compositional variations over distances much less than film thicknesses 1. One technique which provides such resolution is Auger electron spectroscopy (AES), where the composition is determined as a function of depth below an external surface by simultaneous measurement of characteristic Auger electron energies and ion milling (sputtering). The purpose of this research is to investigate interdiffusion in thin film couples of aluminum and copper by this technique. Previous investigations have concentrated on the diffusion of copper in * Present address: National Semiconductor Corporation, Santa Clara, Calif. 95051, U.S.A.
274
M. P. SHEARER, C. L. BAUER, A. G. JORDAN
aluminum because of its relevance to the microelectronics industry2' 3. However, potential applications also exist for thin films of copper because of its lower electrical resistivity and greater resistance to electromigration. 2. SPECIMEN PREPARATION
Bimetallic thin film couples of aluminum and copper were produced by consecutive evaporation of 99.999 wt.~o pure aluminum and copper from resistanceheated tungsten coils and molybdenum boats respectively, followed by deposition on glass microscope slides (maintained at ambient temperature) at a rate of 2.5 and 4.0 nm s-1 for aluminum and copper respectively. Consecutive depositions were effected, without breaking vacuum, at a pressure of less than 130 ~tPa and the resulting thicknesses were measured by a quartz crystal thickness monitor precalibrated with the aid of a microprofilometer. The specimens were then annealed in a continuous flow of argon at temperatures ranging from 100 to 200-+ 1 °C. Annealing times t were selected so that the grain boundary diffusion lengths were always less than the film thickness. 3.
COMPOSITION-DEPTH PROFILING BY AES
In theory composition~lepth profiles can be studied in fine detail on both sides of the original interface. However, because of uncertainties associated with sputtering and sputtering rate calibration, the following complementary sample configurations were adopted to extract grain boundary diffusion coefficients from such profiles: bimetallic couples were produced with a relatively thick (1 ~m) film of either copper or aluminum followed by deposition of a thin (40-100 nm) film of the other metal. In this way only diffusion of the underlying component through the top film is measured. The advantages of this geometry are that (1) the outer layer is thin, thus facilitating depth profiling, (2) the outer layer is well defined, thus facilitating calibration of the sputtering rate and (3) provided that the underlying component is not detected on the free surface, semi-infinite boundary conditions for the analysis of grain boundary diffusion are satisfied4' 5. Therefore two complementary specimen configurations of reverse deposition sequence were used to study interdiffusion in bimetallic couples of aluminum and copper by AES. Composition v e r s u s depth profiles were measured prior and subsequent to annealing with a Phi 540 Auger electron spectrometer with an ion sputtering (milling) attachment. The compositions of aluminum, copper and oxygen were measured from the heights of the 1396, 920 and 510 eV characteristic peaks respectively, under operating conditions of 3 kV and 50 ~A with an angle between the incident electron beam and the surface normal of 60 °. The relatively high characteristic energies associated with these peaks correspond to large escape depths, which reduce the possibility of spurious surface effects. Sputtering was effected at an argon pressure of 6.7 mPa under operating conditions of 1 kV for aluminum and 2 kV for copper with an angle between the incident ion beam and the surface normal of 30°, which corresponds to a current density of 30 ~tA cm- 2 at the point of analysis. Sputtering times t S were converted to depths (below the top surface) according to the time necessary to reach the aluminum-copper interface,
INTERDIFFUSION IN THIN FILM COUPLES OF
A1 AND
275
CU
defined by the depth corresponding to equal concentrations of aluminum and copper. This interface was usually marked by trace amounts of oxygen. The following experimental procedures were maintained throughout. About 10 nm of the outer surface was removed by sputtering for 15 s with an accelerating voltage of 1 kV and a current of 15 mA in order to eliminate adsorbed impurities. An AES spectrum was then measured, which corresponds to the composition of the asdeposited top (outer) film. Finally, after sputtering well beyond the aluminumcopper interface, a second spectrum was measured, which corresponds to the composition of the as-deposited bottom (inner) film. In this manner, the relative signal heights of the principal elements IJIol at a depth y below the top surface are converted to composition by the aforementioned internal standards and the expression
ci(y) = (Ii/Ioi)/~,(Ik/Iok)
(1)
where q(y) denotes the composition of the ith element averaged over the beam excitation area at a depth y and lk/Iok denotes the relative signal heights of pure aluminum and copper, as obtained from the AES spectra 6. The composition is then plotted as a function of ts 6/5 according to the analysis of Whipple 4 in order to extract quantitative measures of interdiffusion. y(nm) 5
I0
15
y(nm) 20
25
5
30
50
IO
15 I
20
25
30
50
40
40
30
30 20 20
d o o
I0
5
40
(a)
31.5
3 .0
t 6s/ a ( l O a S st5 }
2 .5
40
Z .0
(b)
31.5
:5.0
Z5
20
te/~(lOSs6tSs
Fig. 1. Thevariati~n~fc~mp~siti~ncwiththe6p~wer~fsputteringtimets(~wersca~e)~rdepthyb~w the external surface (upper scale). (a) AI/Cu thin film couples: ~ , as-deposited; annealed for O, 5 min at 200 °C; F-l,15 min at 175 °C; ©, 300 min at 100 °C. (b) Cu/A1 thin film couples: &, as-deposited; annealed for e , 5 rain at 200 °C;1, 10 rain at 175 °C; I , 60 min at 100 °C. In each case the thicknesses of the outer and the underlying films are 50 nm and 1 ~tm respectively.
4. EXPERIMENTAL RESULTS
Composition ci(y ) versus the 6 power of sputtering time t s profiles are presented in Fig. 1 for (a) aluminum on copper (A1/Cu) and (b) copper on aluminum (Cu/Al) according to procedures outlined by Whipple 4. In general, these profiles are characterized by two distinct linear segments corresponding to (1) diffusion in the vicinity of the original interface and (2) diffusion extending beyond the interfacial
276
M.P. SHEARER, C. L. BAUER, A. G. JORDAN
region. Since the general form of these profiles corresponds closely to that anticipated from grain boundary diffusion, the two segments are tentatively identified as interfacial and grain boundary diffusion respectively and analyzed according to the following procedures. Since the diffusing element is not detected on the outer surface of thin film couples for all the imposed combinations of annealing times and temperatures, it is assumed that the boundary conditions for the semi-infinite diffusion problem are satisfied 7. Thus interfacial diffusion coefficients D i can be extracted from a complementary error function solution, expressed as
ci(y) = (c0/2) erfc{y/2(Dit) 1/2}
(2)
where c o denotes ci(y ) at t s = 0. The depth to which diffusion is controlled by interfacial effects is approximately (Dit) 1/2, which corresponds to a diffusion length in Fig. 1 of approximately 5 nm. The temperature dependence of interfacial diffusion is determined from plots of l n D i versus 1IT as depicted in Fig. 2. D i is computed to be 4.1 × 10 -7 cm z s -1 e x p ( - 0 . 7 8 e V / k T ) for aluminum in copper and 3.3 × 10 -8 cm 2 s -1 e x p ( - 0 . 8 0 e V / k T ) for copper in aluminum, where k is Boltzmann's constant and T is the absolute temperature. T IO"'
200
(*C)
170
IO0
°
D
,0"; (
2!2
21.4 I/T (IO-~K -I}
216
Fig. 2. The variation of the interfacial diffusion coefficients D i with reciprocal temperature 1/T for Cu/AI and A1/Cu couples. The temperature dependences are 4.1 x 10 7 cm z s - t exp(-0.78 eV/kT) and 3.3 x 10-8 cm 2 s-1 exp( -0.80 eV/kT) for diffusion of aluminum in copper (©) and copper in aluminum ([q) respectively.
Grain boundary diffusion coefficients D b c a n be determined from the second linear region of the composition versus depth profiles displayed in Fig. 1 by subtracting the interfacial contribution and applying the Whipple solution to the grain boundary diffusion problem, expressed as ~D b = 0.661{~1n c(y)/c~y 6Is } - 5/3(4Di/t)x/2
(3)
where 6 denotes the effective grain boundary width. In this case, the region over
I N T E R D I F F U S I O N IN T H I N F I L M C O U P L E S O F m l A N D C u
277
which discernible diffusion occurs extends beyond 30 nm, corresponding to about half the total (outer) film thickness. The temperature dependence of grain boundary diffusion is also determined from plots of In cSDb versus 1/T as depicted in Fig. 3, where c~Db is computed to be 5.1 x 10 -7 cm 3 s -1 e x p ( - 0 . 9 4 eV/kT) for diffusion of aluminum along grain boundaries in copper and 1.8 x 10-lo cm a s-1 e x p ( - 0 . 8 7 eV/kT) for diffusion of copper along grain boundaries in aluminum. The results of Chamberlain and Lehoczky 2 for the diffusion of copper along grain boundaries in aluminum, 4.5 x 10 - s cm 3 s -1 e x p ( - 1 . 0 eV/kT), and the single result of Wildman et al. 3 at 175 °C for the diffusion of copper along grain boundaries in aluminum, 4.5 x 10-20 cm 3 s - 1, are included in Fig. 3 for comparison.
i0-,, L
10"2.0
T (*C) 150
200
I 0"l
~
oo
I O0
~
21.2
2!4 I/T (lO'SK "')
21.6
Fig. 3. The variation of the product of the grain boundary width ~ and the diffusion coefficientDb with the reciprocal temperature 1/T for Cu/AI and AI/Cu couples. The temperature dependences are 5.1 x 10-7 cm3 s -1 exp(-0.94 eV/kT) and 1.8 x 10-1° cm3 s -1 exp(-0.87 eV/kT) for diffusion of aluminum in copper (©) and copper in aluminum (I-q)respectively. The results of Chamberlain and Lehoczky2 for diffusionof copper along grain boundaries in aluminum (broken line)and the singleresult of Wildman et al? also for diffusionof copper along grain boundaries in aluminum (A) are included for comparison. The thin film couples were also characterized by transmission electron microscopy, where the grain sizes for aluminum and copper are determined to be about 125 and 50 nm respectively, irrespective of the deposition sequence. Although intermetallic phases cannot be detected from either bright field images or selectedarea electron diffraction patterns in the as-deposited condition, A12Cu is detected as heterogeneous particles situated along grain boundaries in both aluminum and copper for moderate combinations of annealing times and temperatures, e.g. 1 h at 2130 °C. Under the conditions imposed in this investigation, however, the existing fraction of A12Cu is deemed to be small. Further details concerning the formation and growth of intermetallic phases in thin film couples of aluminum and copper will be presented elsewhere s.
278
M. P. SHEARER, C. L. BAUER, A. G. JORDAN
5. DISCUSSION OF RESULTS
Composition versus depth profiles resulting from interdiffusion in thin film couples are generally characterized by three distinct regions. These regions, extending progressively outward from the original interface, are attributed to (1) volume diffusion, (2) defect diffusion and (3) grain boundary diffusion 9. In this particular study, however, combinations of annealing times and temperatures result in estimated volume diffusion lengths of less than 1 nm lO, ~1, which are below the resolution of composition-depth profiling. Therefore only two distinct regions can be resolved, as verified experimentally. The regions closest to the original interface are characterized by activation energies (0.78 and 0.80 eV) corresponding to short-circuit diffusion, indicating that enhanced diffusion near this interface occurs due to an abnormally high density of defects. These defects could have been produced either during the deposition process or by stresses generated b y differential thermal expansion during subsequent annealing. In contrast, Chamberlain and Lehoczky 2 have measured an activation energy of 1.28 eV for diffusion of copper along grain boundaries in aluminum, which corresponds more closely to that for volume diffusion. The lower value of the activation energy for diffusion in the interracial region in this investigation is attributed to shorter diffusion lengths and to the stressed state of the films. In previous investigations aluminum films were annealed prior to the deposition o f copper in order to relieve internal stresses produced during the deposition process, whilst in this investigation thin film couples were deposited sequentially without subsequent annealing treatments. The region extending beyond the interfacial region is also characterized by activation energies (0.94 and 0.87 eV) corresponding to short-circuit diffusion, indicating that grain boundary diffusion occurs at relatively large distances from the interface. The excellent agreement with the results of Chamberlain and Lehoczky 2 and Wildman et al. 3 for diffusion of copper along grain boundaries in aluminum (Fig. 3) strongly suggests that the second region is indeed due to grain boundary diffusion. Apparently, the rapid diffusion of aluminum in copper more than offsets the effect of the smaller grain size. The formation and growth of AI2Cu along the grain boundaries in both aluminum and copper were revealed by transmission electron microscopy. Previous investigations of bimetallic couples of aluminum and copper have also revealed preferential formation and growth of A12Cu along grain boundaries 12'13 Nevertheless, it appears that the formation of AI2Cu neither noticeably retards grain boundary diffusion nor affects diffusion coefficients as extracted by the methods described here, at least for the early stages of interdiffusion. Brailsford and Aaron 14, studying the growth of A12Cu resulting from copper diffusing along grain boundaries in aluminum, have measured a value of 4 x 10 . 9 cm 3 s -~ exp(-0.8 e V / k T ) for 6Db, in good agreement with values reported here. More surprising is the fact that, although A12Cu would be expected to form for copper diffusing in aluminum, an intermetallic phase richer in copper, e.g. AI4Cu 9, would be expected to form first for aluminum diffusing in copper. Of course, once grain boundaries become saturated with the complementary metal, diffusion proceeds more slowly into the grain interiors and should eventually result in the formation of intermetallic
INTERDIFFUSION IN THIN FILM COUPLES OF A1 AND C u
279
phases corresponding to existing weight fractions of aluminum and copper a5. This process, however, requires much longer annealing times and higher temperatures. 6. SUMMARIZING REMARKS
Interdiffusion in thin film couples of aluminum and copper was investigated at relatively low temperatures (100-200 °C) and short annealing times (5-60 min) by Auger electron spectroscopy. Analysis of the results reveals two distinct processes, each characterized by a unique set of diffusion coefficients: (1) diffusion in the vicinity of the original interface and (2) diffusion extending beyond the interfacial region. Corresponding diffusion coefficients for the diffusion of aluminum in copper and copper in aluminum are 4.1 x 10 -v cm 2 s- t exp(-0.78 eV/kT) and 3.3 x 10 -8 cm 2 s-a exp(- 0.80 eV/kT) respectively. The products of the grain boundary width and the diffusion coefficient for the diffusion of aluminum in copper and copper in aluminum are 5.1 x 10 -7 cm 3 s -a exp(-0.94 eV/kT) and 1.8 x 10 - l ° cm 3 s -1 exp(-0.87 eV/kT) respectively. These processes are identified as defect-induced interfacial diffusion and grain boundary diffusion respectively. Measured values of the activation energy are consistent with those expected for short-circuit diffusion and effectively obliterate, under the imposed conditions of annealing time and temperature, the meager effects expected from volume diffusion. In addition, the formation and growth of the intermetallic phase A12Cu along grain boundaries in both aluminum and copper was detected by transmission electron microscopy. The presence of this phase, however, neither noticeably retards grain boundary diffusion nor affects extracted diffusion coefficients, at least for the early stages of interdiffusion. ACKNOWLEDGMENT
Support of this research by the Division of Materials Research, National Science Foundation, under grant DMR76-81561-A01 is gratefully acknowledged. REFERENCES 1 J. E. E. Baglin and J. M. Poate, in J. M. Poate, K.-N. Tu and J. W. Mayer (eds.), Thin Films--lnterdiffusion and Reactions, Wiley Interscienee, New York, 1978, Chap. 9. 2 M.B. Chamberlain and S. L. Lehoczky, Thin Solid Films, 45 (1977) 189. 3 H.S. Wildman, J. K. Howard and P. S. Ho, J. Vac. Sci. Technol., 12 (1975) 75. 4 R.T.P. Whipple, Philos. Mag., 45 (1954) 1225. 5 J.C. Fisher, J. Appl. Phys., 22 (1951) 74. 6 L.E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riaeh and R. E. Weber (eds.), Handbook of Auger Electron Spectroscopy, Physical Electronics Industry, Eden Prairie, Minn., 1976. 7 J. Crank, The Mathematics of Diffusion, 2rid edn., Clarendon Press, Oxford, 1975, p. 13. 8 M.P. Shearer, J. A. Rayne and C. L. Bauer, Thin Solid Films, in the press. 9 D. Gupta, Metall. Trans., 8A (1977) 1431. 10 Y. Funamizu and K. Watanabe, Trans. Jpn. Inst. Met., 12 ( 1971) 147. 11 N.L. Peterson and S. J. Rothman, Phys. Rev., 81 (1970) 3264. 12 G.A. Walter and C. C. Goldsmith, J. Appl. Phys., 44 (1974) 2452. 13 B.N. Agarwala, L. Berenbaum and P. Peressini, J. Electron. Mater., 3 (1974) 137. 14 A.D. Brailsford and J. B. Aaron, J. Appl. Phys., 40 (1969) 1702. 15 M.P. Shearer, C. L. Bauer and A. G. Jordan, Proc. 13th Annu. Meet. Soc. Eng. Sci., 1 (1976) 3.
273
INVESTIGATION OF INTERDIFFUSION IN THIN FILM COUPLES OF A L U M I N U M AND COPPER BY A U G E R ELECTRON SPECTROSCOPY M. P. SHEARER*, C. L. BAUER AND A. G. JORDAN
Center for the Joining of Materials, Carnegie-Mellon University, Pittsburgh, Pa. 15213 (U.S.A.) (Received November 28, 1978; accepted January 31, 1979)
Interdiffusion in thin film couples of aluminum and copper was investigated at relatively low temperatures (100-200 °C) and short annealing times (5-60 min) using Auger electron spectroscopy. Analysis of the results reveals two distinct processes, each characterized by a unique set of diffusion coefficients: (1) diffusion in the vicinity of the original interface and (2) diffusion extending beyond the interfacial region. These processes are identified as defect-induced interfacial diffusion and grain boundary diffusion respectively. Measured values of the activation energy are consistent with those expected for short-circuit diffusion and effectively obliterate, under the imposed conditions of annealing time and temperature, meager effects expected from volume diffusion. In addition, the formation and growth of A12Cu along grain boundaries was detected by transmission electron microscopy.
1. INTRODUCTION
Multiple layers of thin films are often incorporated in electronic devices in order to utilize specific properties of each individual layer. Unfortunately, the reliability of such devices is affected adversely by interdiffusion and the formation of intermetallic phases, which commence at the original interface separating these layers and then expand outward. Moreover, these processes are especially pronounced under typical operating conditions of low temperature, i.e. at temperatures less than half the absolute melting point, and with high densities of short-circuit diffusion paths. Thus, information pertaining to volume interdiffusion is virtually worthless when it is desired to assess the reliability of thin film devices. The investigation of interdiffusion in thin film couples, however, requires resolution of compositional variations over distances much less than film thicknesses 1. One technique which provides such resolution is Auger electron spectroscopy (AES), where the composition is determined as a function of depth below an external surface by simultaneous measurement of characteristic Auger electron energies and ion milling (sputtering). The purpose of this research is to investigate interdiffusion in thin film couples of aluminum and copper by this technique. Previous investigations have concentrated on the diffusion of copper in * Present address: National Semiconductor Corporation, Santa Clara, Calif. 95051, U.S.A.
274
M. P. SHEARER, C. L. BAUER, A. G. JORDAN
aluminum because of its relevance to the microelectronics industry2' 3. However, potential applications also exist for thin films of copper because of its lower electrical resistivity and greater resistance to electromigration. 2. SPECIMEN PREPARATION
Bimetallic thin film couples of aluminum and copper were produced by consecutive evaporation of 99.999 wt.~o pure aluminum and copper from resistanceheated tungsten coils and molybdenum boats respectively, followed by deposition on glass microscope slides (maintained at ambient temperature) at a rate of 2.5 and 4.0 nm s-1 for aluminum and copper respectively. Consecutive depositions were effected, without breaking vacuum, at a pressure of less than 130 ~tPa and the resulting thicknesses were measured by a quartz crystal thickness monitor precalibrated with the aid of a microprofilometer. The specimens were then annealed in a continuous flow of argon at temperatures ranging from 100 to 200-+ 1 °C. Annealing times t were selected so that the grain boundary diffusion lengths were always less than the film thickness. 3.
COMPOSITION-DEPTH PROFILING BY AES
In theory composition~lepth profiles can be studied in fine detail on both sides of the original interface. However, because of uncertainties associated with sputtering and sputtering rate calibration, the following complementary sample configurations were adopted to extract grain boundary diffusion coefficients from such profiles: bimetallic couples were produced with a relatively thick (1 ~m) film of either copper or aluminum followed by deposition of a thin (40-100 nm) film of the other metal. In this way only diffusion of the underlying component through the top film is measured. The advantages of this geometry are that (1) the outer layer is thin, thus facilitating depth profiling, (2) the outer layer is well defined, thus facilitating calibration of the sputtering rate and (3) provided that the underlying component is not detected on the free surface, semi-infinite boundary conditions for the analysis of grain boundary diffusion are satisfied4' 5. Therefore two complementary specimen configurations of reverse deposition sequence were used to study interdiffusion in bimetallic couples of aluminum and copper by AES. Composition v e r s u s depth profiles were measured prior and subsequent to annealing with a Phi 540 Auger electron spectrometer with an ion sputtering (milling) attachment. The compositions of aluminum, copper and oxygen were measured from the heights of the 1396, 920 and 510 eV characteristic peaks respectively, under operating conditions of 3 kV and 50 ~A with an angle between the incident electron beam and the surface normal of 60 °. The relatively high characteristic energies associated with these peaks correspond to large escape depths, which reduce the possibility of spurious surface effects. Sputtering was effected at an argon pressure of 6.7 mPa under operating conditions of 1 kV for aluminum and 2 kV for copper with an angle between the incident ion beam and the surface normal of 30°, which corresponds to a current density of 30 ~tA cm- 2 at the point of analysis. Sputtering times t S were converted to depths (below the top surface) according to the time necessary to reach the aluminum-copper interface,
INTERDIFFUSION IN THIN FILM COUPLES OF
A1 AND
275
CU
defined by the depth corresponding to equal concentrations of aluminum and copper. This interface was usually marked by trace amounts of oxygen. The following experimental procedures were maintained throughout. About 10 nm of the outer surface was removed by sputtering for 15 s with an accelerating voltage of 1 kV and a current of 15 mA in order to eliminate adsorbed impurities. An AES spectrum was then measured, which corresponds to the composition of the asdeposited top (outer) film. Finally, after sputtering well beyond the aluminumcopper interface, a second spectrum was measured, which corresponds to the composition of the as-deposited bottom (inner) film. In this manner, the relative signal heights of the principal elements IJIol at a depth y below the top surface are converted to composition by the aforementioned internal standards and the expression
ci(y) = (Ii/Ioi)/~,(Ik/Iok)
(1)
where q(y) denotes the composition of the ith element averaged over the beam excitation area at a depth y and lk/Iok denotes the relative signal heights of pure aluminum and copper, as obtained from the AES spectra 6. The composition is then plotted as a function of ts 6/5 according to the analysis of Whipple 4 in order to extract quantitative measures of interdiffusion. y(nm) 5
I0
15
y(nm) 20
25
5
30
50
IO
15 I
20
25
30
50
40
40
30
30 20 20
d o o
I0
5
40
(a)
31.5
3 .0
t 6s/ a ( l O a S st5 }
2 .5
40
Z .0
(b)
31.5
:5.0
Z5
20
te/~(lOSs6tSs
Fig. 1. Thevariati~n~fc~mp~siti~ncwiththe6p~wer~fsputteringtimets(~wersca~e)~rdepthyb~w the external surface (upper scale). (a) AI/Cu thin film couples: ~ , as-deposited; annealed for O, 5 min at 200 °C; F-l,15 min at 175 °C; ©, 300 min at 100 °C. (b) Cu/A1 thin film couples: &, as-deposited; annealed for e , 5 rain at 200 °C;1, 10 rain at 175 °C; I , 60 min at 100 °C. In each case the thicknesses of the outer and the underlying films are 50 nm and 1 ~tm respectively.
4. EXPERIMENTAL RESULTS
Composition ci(y ) versus the 6 power of sputtering time t s profiles are presented in Fig. 1 for (a) aluminum on copper (A1/Cu) and (b) copper on aluminum (Cu/Al) according to procedures outlined by Whipple 4. In general, these profiles are characterized by two distinct linear segments corresponding to (1) diffusion in the vicinity of the original interface and (2) diffusion extending beyond the interfacial
276
M.P. SHEARER, C. L. BAUER, A. G. JORDAN
region. Since the general form of these profiles corresponds closely to that anticipated from grain boundary diffusion, the two segments are tentatively identified as interfacial and grain boundary diffusion respectively and analyzed according to the following procedures. Since the diffusing element is not detected on the outer surface of thin film couples for all the imposed combinations of annealing times and temperatures, it is assumed that the boundary conditions for the semi-infinite diffusion problem are satisfied 7. Thus interfacial diffusion coefficients D i can be extracted from a complementary error function solution, expressed as
ci(y) = (c0/2) erfc{y/2(Dit) 1/2}
(2)
where c o denotes ci(y ) at t s = 0. The depth to which diffusion is controlled by interfacial effects is approximately (Dit) 1/2, which corresponds to a diffusion length in Fig. 1 of approximately 5 nm. The temperature dependence of interfacial diffusion is determined from plots of l n D i versus 1IT as depicted in Fig. 2. D i is computed to be 4.1 × 10 -7 cm z s -1 e x p ( - 0 . 7 8 e V / k T ) for aluminum in copper and 3.3 × 10 -8 cm 2 s -1 e x p ( - 0 . 8 0 e V / k T ) for copper in aluminum, where k is Boltzmann's constant and T is the absolute temperature. T IO"'
200
(*C)
170
IO0
°
D
,0"; (
2!2
21.4 I/T (IO-~K -I}
216
Fig. 2. The variation of the interfacial diffusion coefficients D i with reciprocal temperature 1/T for Cu/AI and A1/Cu couples. The temperature dependences are 4.1 x 10 7 cm z s - t exp(-0.78 eV/kT) and 3.3 x 10-8 cm 2 s-1 exp( -0.80 eV/kT) for diffusion of aluminum in copper (©) and copper in aluminum ([q) respectively.
Grain boundary diffusion coefficients D b c a n be determined from the second linear region of the composition versus depth profiles displayed in Fig. 1 by subtracting the interfacial contribution and applying the Whipple solution to the grain boundary diffusion problem, expressed as ~D b = 0.661{~1n c(y)/c~y 6Is } - 5/3(4Di/t)x/2
(3)
where 6 denotes the effective grain boundary width. In this case, the region over
I N T E R D I F F U S I O N IN T H I N F I L M C O U P L E S O F m l A N D C u
277
which discernible diffusion occurs extends beyond 30 nm, corresponding to about half the total (outer) film thickness. The temperature dependence of grain boundary diffusion is also determined from plots of In cSDb versus 1/T as depicted in Fig. 3, where c~Db is computed to be 5.1 x 10 -7 cm 3 s -1 e x p ( - 0 . 9 4 eV/kT) for diffusion of aluminum along grain boundaries in copper and 1.8 x 10-lo cm a s-1 e x p ( - 0 . 8 7 eV/kT) for diffusion of copper along grain boundaries in aluminum. The results of Chamberlain and Lehoczky 2 for the diffusion of copper along grain boundaries in aluminum, 4.5 x 10 - s cm 3 s -1 e x p ( - 1 . 0 eV/kT), and the single result of Wildman et al. 3 at 175 °C for the diffusion of copper along grain boundaries in aluminum, 4.5 x 10-20 cm 3 s - 1, are included in Fig. 3 for comparison.
i0-,, L
10"2.0
T (*C) 150
200
I 0"l
~
oo
I O0
~
21.2
2!4 I/T (lO'SK "')
21.6
Fig. 3. The variation of the product of the grain boundary width ~ and the diffusion coefficientDb with the reciprocal temperature 1/T for Cu/AI and AI/Cu couples. The temperature dependences are 5.1 x 10-7 cm3 s -1 exp(-0.94 eV/kT) and 1.8 x 10-1° cm3 s -1 exp(-0.87 eV/kT) for diffusion of aluminum in copper (©) and copper in aluminum (I-q)respectively. The results of Chamberlain and Lehoczky2 for diffusionof copper along grain boundaries in aluminum (broken line)and the singleresult of Wildman et al? also for diffusionof copper along grain boundaries in aluminum (A) are included for comparison. The thin film couples were also characterized by transmission electron microscopy, where the grain sizes for aluminum and copper are determined to be about 125 and 50 nm respectively, irrespective of the deposition sequence. Although intermetallic phases cannot be detected from either bright field images or selectedarea electron diffraction patterns in the as-deposited condition, A12Cu is detected as heterogeneous particles situated along grain boundaries in both aluminum and copper for moderate combinations of annealing times and temperatures, e.g. 1 h at 2130 °C. Under the conditions imposed in this investigation, however, the existing fraction of A12Cu is deemed to be small. Further details concerning the formation and growth of intermetallic phases in thin film couples of aluminum and copper will be presented elsewhere s.
278
M. P. SHEARER, C. L. BAUER, A. G. JORDAN
5. DISCUSSION OF RESULTS
Composition versus depth profiles resulting from interdiffusion in thin film couples are generally characterized by three distinct regions. These regions, extending progressively outward from the original interface, are attributed to (1) volume diffusion, (2) defect diffusion and (3) grain boundary diffusion 9. In this particular study, however, combinations of annealing times and temperatures result in estimated volume diffusion lengths of less than 1 nm lO, ~1, which are below the resolution of composition-depth profiling. Therefore only two distinct regions can be resolved, as verified experimentally. The regions closest to the original interface are characterized by activation energies (0.78 and 0.80 eV) corresponding to short-circuit diffusion, indicating that enhanced diffusion near this interface occurs due to an abnormally high density of defects. These defects could have been produced either during the deposition process or by stresses generated b y differential thermal expansion during subsequent annealing. In contrast, Chamberlain and Lehoczky 2 have measured an activation energy of 1.28 eV for diffusion of copper along grain boundaries in aluminum, which corresponds more closely to that for volume diffusion. The lower value of the activation energy for diffusion in the interracial region in this investigation is attributed to shorter diffusion lengths and to the stressed state of the films. In previous investigations aluminum films were annealed prior to the deposition o f copper in order to relieve internal stresses produced during the deposition process, whilst in this investigation thin film couples were deposited sequentially without subsequent annealing treatments. The region extending beyond the interfacial region is also characterized by activation energies (0.94 and 0.87 eV) corresponding to short-circuit diffusion, indicating that grain boundary diffusion occurs at relatively large distances from the interface. The excellent agreement with the results of Chamberlain and Lehoczky 2 and Wildman et al. 3 for diffusion of copper along grain boundaries in aluminum (Fig. 3) strongly suggests that the second region is indeed due to grain boundary diffusion. Apparently, the rapid diffusion of aluminum in copper more than offsets the effect of the smaller grain size. The formation and growth of AI2Cu along the grain boundaries in both aluminum and copper were revealed by transmission electron microscopy. Previous investigations of bimetallic couples of aluminum and copper have also revealed preferential formation and growth of A12Cu along grain boundaries 12'13 Nevertheless, it appears that the formation of AI2Cu neither noticeably retards grain boundary diffusion nor affects diffusion coefficients as extracted by the methods described here, at least for the early stages of interdiffusion. Brailsford and Aaron 14, studying the growth of A12Cu resulting from copper diffusing along grain boundaries in aluminum, have measured a value of 4 x 10 . 9 cm 3 s -~ exp(-0.8 e V / k T ) for 6Db, in good agreement with values reported here. More surprising is the fact that, although A12Cu would be expected to form for copper diffusing in aluminum, an intermetallic phase richer in copper, e.g. AI4Cu 9, would be expected to form first for aluminum diffusing in copper. Of course, once grain boundaries become saturated with the complementary metal, diffusion proceeds more slowly into the grain interiors and should eventually result in the formation of intermetallic
INTERDIFFUSION IN THIN FILM COUPLES OF A1 AND C u
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phases corresponding to existing weight fractions of aluminum and copper a5. This process, however, requires much longer annealing times and higher temperatures. 6. SUMMARIZING REMARKS
Interdiffusion in thin film couples of aluminum and copper was investigated at relatively low temperatures (100-200 °C) and short annealing times (5-60 min) by Auger electron spectroscopy. Analysis of the results reveals two distinct processes, each characterized by a unique set of diffusion coefficients: (1) diffusion in the vicinity of the original interface and (2) diffusion extending beyond the interfacial region. Corresponding diffusion coefficients for the diffusion of aluminum in copper and copper in aluminum are 4.1 x 10 -v cm 2 s- t exp(-0.78 eV/kT) and 3.3 x 10 -8 cm 2 s-a exp(- 0.80 eV/kT) respectively. The products of the grain boundary width and the diffusion coefficient for the diffusion of aluminum in copper and copper in aluminum are 5.1 x 10 -7 cm 3 s -a exp(-0.94 eV/kT) and 1.8 x 10 - l ° cm 3 s -1 exp(-0.87 eV/kT) respectively. These processes are identified as defect-induced interfacial diffusion and grain boundary diffusion respectively. Measured values of the activation energy are consistent with those expected for short-circuit diffusion and effectively obliterate, under the imposed conditions of annealing time and temperature, the meager effects expected from volume diffusion. In addition, the formation and growth of the intermetallic phase A12Cu along grain boundaries in both aluminum and copper was detected by transmission electron microscopy. The presence of this phase, however, neither noticeably retards grain boundary diffusion nor affects extracted diffusion coefficients, at least for the early stages of interdiffusion. ACKNOWLEDGMENT
Support of this research by the Division of Materials Research, National Science Foundation, under grant DMR76-81561-A01 is gratefully acknowledged. REFERENCES 1 J. E. E. Baglin and J. M. Poate, in J. M. Poate, K.-N. Tu and J. W. Mayer (eds.), Thin Films--lnterdiffusion and Reactions, Wiley Interscienee, New York, 1978, Chap. 9. 2 M.B. Chamberlain and S. L. Lehoczky, Thin Solid Films, 45 (1977) 189. 3 H.S. Wildman, J. K. Howard and P. S. Ho, J. Vac. Sci. Technol., 12 (1975) 75. 4 R.T.P. Whipple, Philos. Mag., 45 (1954) 1225. 5 J.C. Fisher, J. Appl. Phys., 22 (1951) 74. 6 L.E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riaeh and R. E. Weber (eds.), Handbook of Auger Electron Spectroscopy, Physical Electronics Industry, Eden Prairie, Minn., 1976. 7 J. Crank, The Mathematics of Diffusion, 2rid edn., Clarendon Press, Oxford, 1975, p. 13. 8 M.P. Shearer, J. A. Rayne and C. L. Bauer, Thin Solid Films, in the press. 9 D. Gupta, Metall. Trans., 8A (1977) 1431. 10 Y. Funamizu and K. Watanabe, Trans. Jpn. Inst. Met., 12 ( 1971) 147. 11 N.L. Peterson and S. J. Rothman, Phys. Rev., 81 (1970) 3264. 12 G.A. Walter and C. C. Goldsmith, J. Appl. Phys., 44 (1974) 2452. 13 B.N. Agarwala, L. Berenbaum and P. Peressini, J. Electron. Mater., 3 (1974) 137. 14 A.D. Brailsford and J. B. Aaron, J. Appl. Phys., 40 (1969) 1702. 15 M.P. Shearer, C. L. Bauer and A. G. Jordan, Proc. 13th Annu. Meet. Soc. Eng. Sci., 1 (1976) 3.