The interdiffusion of platimum and gold films, a couple utilized in beam lead microcircuits, has been studied for temperatures up to 550 C. Gold-on-platinum couples and separate platimum and gold films 80-450 nm thick, were deposited by electron beam evaporation onto oxidized (111) silicon substrates. Diffusion was monitored by means of spectral reflectance versus wavelength in the band 500-1000 nm. The separate metal films showed good adhesion and stable reflectances (after an initial change) for at least 6 h at diffusion temperatures, in contrast to the couples. Analysis of platinum diffusion through the gold films yielded an activation energy about 38 kcal/g-atom and a pre-exponential factor of the order 0.001 sq cm/sec, values close to those for volume diffusion. The pre-exponential factor especially is dependent upon film deposition conditions

Nowak, W. B.

English

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FINAL REPORT
NASA RESEARCH GRANT NGR-22-011-060
DIFFUSION AND STRUCTURAL CHANGES IN MICROCIRCUIT INTERCONNECTIONS
Welville B. Nowak
(NASA-CR-132890) DIFFUSION AND STRUCTURAL N74-1390CHANGES IN MICROCIRCUIT INTERCONNECTIONS
Final Report, 1 Dec 1969 3 0 :Jun1971 (Northeastern Univ ) I p HC $3 0 Unclas
CSCL 09C G3/10 15703
Northeastern University
Boston, Massachusetts
7 September 1973
https://ntrs.nasa.gov/search.jsp?R=19740005792 2020-03-23T12:05:52+00:00Z
The research reported here was sponsored by the National Aeronautics and
Space Administration under Research Grant NGR-22-011-060 for the period 1 Dec-
ember 1969 through 30 June 1971. This report is published for information
purposes only and does not represent recommendations or conclusions of the spon-
soring agency.
RESEARCH STAFF
Welville B. Nowak Principal Investigator
Professor of Mechanical Engineering
Richard N. Dyer Graduate Student
James Surette Technician
Diffusion and Structural Changes in Microcircuit Interconnections
Welville B. Nowak
Abstract
The interdiffusion of platinum and gold films, a couple utilized in beam-lead
microcircuits, has been studied for temperatures up to 550 0C, Gold-on-platinum
couples and separate platinum and gold films, 80-450 nm thick, were deposited
by electron beam evaporation onto oxidized (111) silicon substrates. Diffusion
was monitored by means of spectral reflectance versus wavelength in the band
500-1000 nm. The separate metal films showed good adhesion and stable reflect-
ances ( after an initial change) for at least 6 h at diffusion temperatures, in
contrast to the couples. Analysis of platinum diffusion through the gold films
yielded an activation energy about 38 kcal/g-atom and a pre-exponential factor
-3 2
of the order 10 cm /sec, values close to those for volume diffusion. The
pre-exponential factor expecially is dependent upon film deposition conditions.
The work presented here was a continuation of the research previously
reported in the document "Final Report, NASA Research Grant NGR-22-011-007,
Inter-relations Between Advanced Processing Techniques, Integrated Circuits,
Materials Development and Analysis" dated 1 June 1971, by W.B. Nowak, B.L.
Cochrun, W. Carlson, A. Grabel, of Northeastern University, Boston, Massa-
chusetts.
Substantially the same techniques were used here as detailed in the above
referenced report. Better defined and more accurate data were taken system-
atically. An analysis technique was developed that permitted an evaluation of-
both the activation energy for diffusion and the diffusion coefficient for the
case of platinum diffusing through gold films.
The results of this work were presented in a paper delivered at the Fifth
International Vacuum Congress, Boston, Massachusetts, October 13, 1971, and
have also been published as the following paper: "Diffusion in Thin Film Couples
of Platinum-Gold" by Welville B. Nowak and Richard N. Dyer, The Journal of
Vacuum Science and Technology, Vol. 9, pages 279-283, 1972. A copy of this
paper is attached.
yL
Diffusion in Thin Film Couples of Platinum-Gold*
Welville B. Nowak and Richard N. Dyer
Northeastern University, Boston, Massachusetts 02116
(Received 28 July 1971)
The interdiffusion of platinum and gold films, a couple utilized in beam-lead microcircuits, has
been studied for temperatures up to 550 0 C, Gold-on-platinum couples and separate platinum and
gold films, 80-450 nm thick, were deposited by electron beam evaporation onto oxidized (111)
silicon substrates. Diffusion was monitored by means of spectral reflectance versus wavelength in
the band 500-1000 nm. The separate metal films showed good adhesion and stable reflectances
(after an initial change) for at least 6 h at diffusion temperatures, in contrast to the couples.
Analysis of platinum diffusion through the gold films yielded an activation energy about 38
kcal/g-atom and a pre-exponential factor of the order 10-' cm'/sec, values close to those for volume
diffusion. The pre-exponential factor especially is dependent upon film deposition conditions.
Introduction Au and Pt films were deposited on three such
In recent years a demand has arisen for thin film wafers (labeled A, B, and C) simultaneously by elec-
metal systems that will perform reliably as micro- tron beam evaporation of the respective metals from
circuit interconnections and that are compatible with individual pyrolytic graphite crucibles. First the Pt
the circuit fabrication processes. Aluminum is success- and then the Au was evaporated in a single pump-
fully and conveniently used in many cases. In other down cycle. Pressures were about 1 X 10- Torr in an
cases, aluminum film interconnections have failed oil-pumped vacuum system with liquid nitrogen cold
because of interactions with gold, SiO 2, silicon, oxygen, traps. The Si wafer substrates were heated to 100
0 C
or water vapor, or because of mass migration at high for the Pt deposition and then were cooled to ambient
current densities. Consequently, multilayers of differ- temperature in about 10 min by thermal conduction
ent metals have been investigated and utilized.'- 6  to water-cooled posts. The Au was deposited at
Failure mechanisms in multilayers include' diffusion, ambient temperature. During evaporation, the radiant
chemical reaction, electromigration, and stress relief, heat raised substrate temperatures by 100 C. A shutter
Reported here is an investigation of the inter- covered the substrates until the Pt or Au charges were
diffusion of evaporated Au-on-Pt films, a combination molten and evaporating. Evaporation rates were about
used for "beam-lead" microcircuit connections. 6  1000 A/min.
Spectral reflectance over the visible band at the film A stainless steel sliding mask, remotely moved
surfaces provided an index of the concentration at between the Pt and Au depositions, defined three
(or near to) the surface. areas (approximately 2.0X0.7 cm) on each substrate.
The center area was the Au-on-Pt couple, and the
Experimental Details adjacent areas were Pt and Au single metal monitor
In our early experiments, the Au and Pt films were films, respectively. The monitors were used for judging
deposited on ordinary microscope slides or on Corning thickness, reflectance, and adhesion of the separate
7059 aluminoborosilicate glass microscope slides. The films.
slides are advantageous in that both sides of the Because of fixture geometry in the evaporator, the
couple can be examined optically, but they have the three substrates were tilted and displaced with re-
disadvantage that film behavior (adhesion and solu- spect to a perpendicular from the center of the
bility in substrate) differs from the practical situation. crucibles. This resulted in source-substrate distances
Therefore, the data reported here was obtained onoxidizTherefore  reporsustrates.d ere as1 obtained on of 10, 11, and 12 cm, and in angles of incidence (withoxidized Si wafer substrates. About 1 Jm of Si0 2 was
thermally grown on polished, (111) Si wafers, 2.54 cm respect to substrate normals) of 80, 220, and 290 for
in diameter, by oxidizing them for nine hours at wafer positions A, B, and C, respectively. Conse-
1000 0C in wet oxygen. quences of this geometry are (1) a film thickness
* Work partially supported by the National Aeronautics and variation from position A to C, and (2) a structural
Space Administration. NGR-22-011-060 variation as evidenced in the diffusion results.
Reprinted from: THE JOURNAL OF VACUUM SCIENCE AND TECHNOLOGY VOL. 9 NO. 1 279
5
280 W. B. Nowak and R. N. Dyer
For each deposition, the three substrates A, B, and Co., Rochester, N. Y., catalog No. 6086) as standards
C were simultaneously heat treated in air in a tem- to correct for the spectral response of the optical
perature controlled furnace (+-5 0C) for about 5 min system and detector. Agreement with literature values
at 4500, 475', 5000, 5250, or 550 0 C. Spectral reflectance to within a few percent was obtained for various metal
measurements were then taken at room temperature. films over the range 500-1000 nm. For most measure-
This procedure was repeated for heat treatment in- ments, however, it was more convenient to use an
tervals generally doubling the previous time, up to a aluminum film as a secondary standard.
total of about 6 h.
Spectral reflectance was measured at normal inci- Results and Discussion
dence by directing monochromatic light into a The Au monitor films remained stable in reflectance
metallurgical microscope (with the film specimen on and resisted physical deterioration, such as loss of
the stage) and detecting the light output of the eye- adhesion, throughout the experiments. The Pt monitor
piece with a silicon solar cell. The solar cell was films, after an initial rapid change (to be reported
loaded with a 1000- resistor, and the voltage across elsewhere), also exhibited stable properties. The Au/Pt
the resistor (5-500/ V) provided a measure of the couples, however, often blistered, lost adhesion, and
reflected light. The combination of monochromator peeled off the substrate. When this happened, they
slit dimensions and microscope magnification resulted always curled with the Au film inside, giving evidence
in a specimen effective area of 0.35 XO.72 mm. This of a tensile stress in the Au film. The Pt film may have
is large enough to average out small inhomogeneities a compressive stress, since there was a marked
in the film, yet small enough to allow various parts of tendency for buckling or "crinkling" of the Pt films
a specimen to be sampled. on the glass slides.
Absolute reflectivities were obtained using fresh Electron microscope examination of 1000-A films
MgO or "White Reflectance Paint" (Eastman Kodak from specimens heat treated at 4750 C for 10 h re-
vealed an average grain size of about 104 A in the gold
films of both couple and monitor, with grains larger
00TIME (MIN) and smaller by a factor of 2. In some areas the Au
o grains had a substructure with cells about 10
3 A on a
side. Occasional pin holes appeared in the couple Au
90 film, but not in the monitor, and the Pt film structure
5 could be seen through the hole. The Pt structure
80 appeared as small dark dots and ill-defined dark
regions in a uniform light matrix. The average size
of the dark portions was about 200 A.
70o As a result of the heat treatments, the couple Au
surface was somewhat silvery, similar to the appear-
S60 0 ance of Au-Pt alloys. Typical spectral reflectivity
Z 4curves of a couple Au film after annealing in air for
40_ various times are shown in Fig. 1. For clarity, the
so o data points are not shown in the figure; they were
0 taken every 0.1 /m from 0.5 to 1.0 Im, inclusive, and
are within ±-1% of the curves except for a few stray
4o tpoints. Since only the curve shapes are taken as indi-
cators of composition (thus removing extraneous
30 ' factors that might uniformly affect the reflectance
magnitude), each curve was normalized to its value
at 1.0 pm. Figure 2 shows plots of a normalized
20 reflectance vs wavelength for the curves of Fig. 1.
The slopes of the normalized curves between 0.6
o and 1.0 um show a fairly steady increase with time,
which we interpret as indicating an increase in Pt
concentration. An average measure of the slope at
0.5 0.6 0.7 0.8 0.9 1.0 any time was taken as the sum of the change in
normalized reflectance from time zero at = 0.6 and
WAVELENGTH (j&) 0.7 Mm, i.e.,
FIGURE 1. Normalized reflectance vs wavelength curves from
the data of Fig. 1. zANR = [NR (0)- NR(t)] 0 .6+[NR(0) - NR(t)] 0.7.
Diffusion in Thin Film Couples of Platinum-Gold 281
Typical plots of ;ANR vs time are shown in Fig. 3. so
Each curve may be roughly divided into two regions: A
an extremely rapid increase followed by a very slow
increase or plateau. A transition point between these
regions was defined by the intersection of two straight
lines approximating the curves in the two regions. 6o
The time at this transition point was chosen as an
indicator of the diffusion rate. That is, when the
character of the spectral reflectance changes abruptly so
it is assumed that a certain concentration of Pt is at
the Au surface. The slight dip in the curves of Fig. 3 " 40
for the thicker specimens was a general occurrence.
We are unsure of its origin. 4
Arrhenius plots were made by finding the natural 30
logarithm of the ratio of gold film thickness squared
to the time of the transition point and graphing this 20
logarithm vs the reciprocal absolute temperature, as
shown in Fig. 4. The value 32.2 in the ordinate scale
of Fig. 4 is related to the conversion of (A)/hr to to
cm 2/sec. Data relating to Fig. 4 are presented in
Table I. The data at 5000 C have been omitted because 0
they lay significantly above the lines for unknown 0o 2 3 4 5 6
reasons. For a diffusion coefficient expressed by D = Do TIME (H)
exp(-Q/IRT), values of the activation energy Q FIGURE 3. Changes in the slope of the normalized reflectance
calculated from Fig. 4 are 39, 38, and 34 kcal/g-atom curves at large wavelength vs diffusion time. Specimen 58.
for the A, B, and C series, respectively. These values
are probably accurate to -15%. Since the activation energy for volume diffusion of Pt into Au is 39 kcal/
g-atom,7 we conclude that Pt is moving through the
'TIME (MIN) Au film predominantly by volume diffusion.
To find whether the value of Do also corresponds
to volume diffusion the Pt concentration at the Au100 
_surface must be known. Attempts to produce cali-
brating alloy films by electron beam and flash evapora-
90 I tion from bulk alloys were unsuccessful. Instead, the
Pt concentration was estimated by matching experi-
mental data to theory.
so 0 The Au-Pt phase diagram has a large, solid-state
miscibility gap, extending from 15 to 98 at.% Pt in
7o / / Au at about 5000 C. 8 In such a case, the concentration
0profile contains an interface at which the Pt concen-
w 60 TABLE I. Data used for Arrhenius plots shown in Fig. 4.
L.
Diffusion Transition
o Film thickness (i) temperature timeN 50 Specimen Pt Au (°C) (h)
55A 2730 2160 450 4.0
40 55B 1920 1210 450 2.0o 40 55C 1270 780 450 1.6
57A 4450 3000 475 4.0
30 57B 2730 2300 475 2.2557C 1000 1150 475 1.0
58A 4000 2730 525 1.0
20 I 58B 2730 2400" 525 0.60
0,5 0.6 0.7 0.8 0.9 I.0 58C 1300 770 525 0.21
WAVELENGTH (s) 56A ... 3000 550 0.47
56B ... 2500 550 0.40FIGURE 2. Typical reflectance vs wavelength curves for pro- 56C 1000 550 0.15gressive times of diffusion at a fixed temperature. Specimen 56A.
2
282 W. B. Nowak and R. N. Dyer
tration drops from 0.98 to 0.15 discontinuously.9 For TABLE II., Diffusion coefficient constants of Pt into Au for
tration drops from 0.98 to 0.15 discontinuously. For various film specimens.
our diffusion conditions and with reasonable values
for the diffusion coefficients of Pt in Au and Au in Pt,"' Au D Do Q
one may calculate that the interface motion is very Specimen thickness (cm/sec) (cm/sec) (kcal/
small. Thus, the Pt concentration is effectively held 55A 2200 3.9 22 3955A 2200 3.9 22 39
at 0.15 at the original Pt/Au interface. 57A 3000 7.0 15 39
The concentration C of the Pt at the free surface 58A 2700 24 10 3956A 3000 62 12 39
of a Au film of thickness L is given by
55B 1200 2.3 6.4 38
C 4 (-1) + l  Dt(2n+1)2 r 2 -  57B 2300 7.6 8.1 38
-=I+- IexPL - , 58B 2400 30 6.6 38
Co r n0 2n+1 4L2  56B 2500 51 5.7 38
55C 780 1.2 0.19 34
where Co=0.15 at the other boundary. Solutions of 57C 1150 4.3 0.32 34
this equation were compared with experimental transi- 58C 770 9.0 0.17 34,
tion times and thicknesses, and consistent matching
was found for C/Co~ 0.9, i.e., near the solubility
limit. This would also correspond to the level at diffusion predominates, but is somewhat modified by
which the change of ZANR with concentration be- film structure. This is not too surprising since the
comes small if the reflectivity of Au-Pt alloys were surface area presented to the incident light for grains
similar to that of Au-Pd." Table II presents the is 103 times that for grain boundaries in our films,
values of D obtained by the above matching.. Values and we would not expect to observe the direct effect
of Do, also given in Table II, were calculated from of the grain boundaries, which would be quickly
D and the appropriate Q value. It should be noted saturated to the solubility limit. It can be seen from
that Do-1.2X10- 3 cm 2/sec for volume diffusion.' Table II that reasonable agreement exists for Do and
Therefore, we may again conclude that volume Q among films deposited at the same position (A, B,
or C). This correlation appears much more consistent
than one based upon film thickness, and is probably
TEMP(*C) related to film structure. However, D is approxi-
550 525 500 475 450S500 475 450mately proportional to thickness.
It is interesting to note that most of the metal
5.0 film difftision studies employing optical reflectivity,'"
-
including this study, led to the conclusion that
volume diffusion predominated. One optical investiga-
A tion, however, resulted in diffusion coefficients many
orders of magnitude higher than the extrapolated
. B volume coefficients.1 6 Other investigators using elec-
Q=39000 CAL/G-ATOM trical resistivity, x-rays, and electron microprobe
cQ -38 000 CAL/G-ATOM techniques reported observations corresponding to
03 grain boundary diffusion;17"" however, it should be
3.0 - noted that the grain sizes in those studies were 10-100
"xltimes smaller than in the work reported here. An inter-
, mediate case appears to be that of Rairden et al.'9:
2.0- Acknowledgments
The authors are indebted to Thomas Hull and
Peter Salmon for many preliminary experiments, to
.0, James Surette for laboratory assistance, to John0 34 000 CAL/G-ATOM Cashman for computer programming, and to Professor
Richard J. Murphy for helpful discussions and aid in
electron microscopy.
I2 r.25 1.3 1.35 1.4 References
1000 1. J. A. Cunningham, Solid-State Electron. 8, 735 (1965).
T 2. W. M. Berger, R. S. Keen, and G. L. Schnable, in Physics
of Failure in Electronics (Clearinghouse for Federal Scientific
FIGURE 4. Arrhenius plots for the diffusion of Pt into Au in thin and Technical Information, Springfield, Va., 1966), Vol. 4,
film couples on oxidized Si substrates. Rep. No. 637529.
Diffusion in Thin Film Couples of Platinum-Gold 283
3. W. H. Gianelle, in Physics of Failure in Electronics (Clearing- in Solids, Liquids, Gases (Academic, New York, 1960), p.
house for Federal Scientific and Technical Information, 71-72.
Springfield, Va., 1966), Vol. 4, Rep. No. 637529. 10. C. J. Smithells, Metals Reference Book (Butterworths,
4. K. D. Kang, R. R. Burgess, M. G. Coleman, and J. G. Keil, Washington, D. C., 1962), 3rd ed., Vol. 2, pp. 590, 597.
IEEE Trans. ED-16, 356 (1969). 11. H. Rbhl, Ann. Physik 18, 155 (1933).
5. G. L. Schnable, R. S. Keen, and L. R. Loewenstern, "Study 12. H. Schopper, Z. Physik 143, 93 (1955).
of Contact Failures in Integrated Circuits," Final Report 13. C. Weaver and L. C. Brown, Phil. Mag. 7, 1 (1962).
by Philco-Ford Corp. for the Rome Air Development Center, 14. C. Weaver and L. C. Brown, Phil. Mag. 8, 1379 (1963).
Griffiss Air Force Base, N. Y., Contract AF30(602)-4034, 15. C. Weaver and L. C. Brown, Phil. Mag. 17, 881 (1968).
1967. 16. P. A. Turner, H. C. Theuerer, and K. L. Lai, J. Vac. Sci.
6. M. P. Lepselter and R. W. MacDonald, Proc. IEEE 53, 405 Technol. 6, 650 (1969).
(1965). 17. J. L. Richards and W. H. McCann, J. Vac. Sci. Technol. 6,
7. C. J. Smithells, Metals Reference Book (Butterworths, 644 (1969).
Washington D. C., 1962), 3rd ed., Vol. 2, p. 590. 18. E. I. Alessandrini and J. D. Kuptsis, J. Vac. Sci., Technol.
8. M. Hansen, Constitution of Binary Alloys (McGraw-Hill, 6, 647 (1969).
New York, 1958), 2nd ed., p. 227. 19. J. R. Rairden, C. A. Neugebauer, and R. A. Sigsbee, Met.
9. C. Wagner, unpublished results quoted in W. Yost, Diffusion Trans. 2, 719 (1971).


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