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Diffusion of copper in thin TiN films
Thin Solid Films, 91 (1982) 155-162
155
METALLURGICAL AND PROTECTIVE COATINGS
D I F F U S I O N OF C O P P E R IN THIN TiN FILMS M. B. CHAMBERLAIN Sandia National Laboratories, AIbuquerque, N M 87185 (U.S.A.) (Received December 9, 1981 ; accepted December 29, 1981 )
The diffusivity of copper in thin TiN layers was determined in specimens prepared by r.f. sputtering a copper (80 nm) layer onto a TiN (200 nm) layer on sapphire and silicon substrates. Specimens were isothermally heat treated at 608, 635 and 700°C at pressures lower than 2 x 10 -6 Pa; they were compositionally analyzed by Rutherford backscattering spectroscopy and Auger sputter profiling; and they were microstructurally characterized by transmission electron microscopy and electron diffraction. The diffusivity D = 9 x 107 cm 2 s -1 exp(-427 kJ mol-1/R T) from 608 to 700 °C. The mechanisms of copper diffusion were not bulk processes, but they were probably processes involving primarily grain boundaries in the TiN. This very low diffusivity at these temperatures makes TiN/Cu an excellent candidate for a high temperature metallization for silicon solar concentrator cells.
1. INTRODUCTION A reliable thin film metallization for electrical conductors and contacts must be developed for photovoltaic solar concentrator cells planned for use in terrestrial power plants j. The metallization of a 1000x solar concentrator cell must not interdiffuse when operating at about 100 °C during a 10-20 year life 1. It must also withstand a 10 min 600 °C glazing process which is required to seal hermetically and to protect the cell surface from corrosion by atmospheric exposure 2. Evaporated Ti/Pd/Ag has been a reliable metallization in non-concentrating photovoltaic cells for space power systems 3. However, during a 10 min 600 °C heat treatment, this laminate structure interdiffuses with a silicon cell and degrades 4 the cell output by 50~. The integrated circuit metallizations employing Ti/Mo/Au 5, Ti/Pd/Au 6, Ti/Pt/Au 7, Ti/Rh/Au a or Ti/W/Au 9, lo also interdiffuse during exposures at less than 600°C or extended exposures at 100°C. However, the diffusion barrier properties of molybdenum or titanium thin films are significantly improved when the films are reactively sputter deposited in Ar-20~N2 11,12. These improved barrier properties have been attributed to the formation of nitrides 1~' 13. Thin
0040-6090/82/0000-0000/~02.75
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M.B. CHAMBERLAIN
nitride layers have been successfully used as diffusion barriers in Mo/Au ~1 Ti/Pd/Au ~4 and Cr/Ni/Ag ~5 metallizations to improve the reliability of integrated circuits. To establish whether a thin TiN layer could be a reliable diffusion barrier in metallizations for solar concentrator cells, the contact resistance ~, electrical properties 4. ~7, deposition process ~7 and diffusion barrier properties ~s of reactively sputter-deposited TiN are being investigated. The TiN/Ag system on silicon solar cells withstands the 10 min 600 C heat treatment without degrading the cell output power ~'~. At 7 0 0 C the TiN/Cu metallization on silicon is more stable than the TiN/Ag system ~s. Thus on a TiN barrier a copper conductor may be more reliable than a silver one. To establish the high temperature failure mechanisms of the TiN/Cu metallization, we need to know the diffusivity and mechanisms of copper diffusion in TiN thin films. In this study the diffusivity of copper in TiN was determined from 608 to 700 ~'C by compositional and microstructural analysis using Auger electron spectroscopy (AES), Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). 2. SAMPLE PREPARATION AND CHARACTERIZATION
The diffusion of copper in TiN thin films was investigated in 15 specimens that were simultaneously prepared by r.f. sputtering first a TiN (200 nm) layer and then a copper (80 nm) layer on polished sapphire substrates 1.25 cm in diameter and 1.3 mm thick. The TiN was reactively sputter deposited in A r 20";IN 2 at a rate of 6 nm rain l, and the copper was sputter deposited in argon at 42 nm rain ~. To minimize the oxygen concentration in the TiN layer, the substrates were biased to 50 V during the TiN deposition l 7 and the base pressure of the sputtering chamber w a s 7 x l 0 Spa. Diffusion couples were vacuum heat treated in a Perkin Elmer model 548 ESCA-AES spectrometer. Samples were mountea on a hot stage and under two tantalum sheets which formed an isothermal cavity enclosing the specimens. The temperature was monitored with a chromel alumel thermocouple clamped to the sample surface. Isothermal heat treatments were performed from 608 to 700 ~C in a total pressure lower than 2 x 10 ~' Pa. The carbon, copper, oxygen, nitrogen and titanium concentrations as functions of the distance perpendicular to the copper layer surface were calculated from measurements determined by combining AES and ion beam sputtering. The concentration C.i(t ) atomic per cent of element .j at sputtering time t was calculated by -
~Aj(t) C i(t) - Z:~kAk(t)
(It
where :~i is the inverse Auger sensitivity factor of A j(t)is the peak-to-peak deflection in the first derivative of the Auger signal ofelementj measured at time t. The distance X(t) from the specimen surface at time t was calculated from =
(c,,(sh R jiN]d~
(2)
DIFFUSION OF C u IN T i N
157
where R= (z = Cu and TiN) are the sputtering rates for a 2.0 keV argon ion beam in the respective pure layer. The sputtering rate in copper was calculated as 2.88 nm m i n - 1 using a measurement by RBS of the areal density (atoms per square centimeter) of this layer. The copper-to-TiN ratio of the sputtering rates was 3.40. This ratio was calculated from measurements performed with a H o b s o n - T a y l o r profilometer of the heights of steps that were sputtered into the two layers. After the specimens had been sputter profiled, the crater depths were measured to corroborate the depths calculated by eqn. (2). The RBS technique 2° was used to measure the number of atoms in the copper layer. In the Sandia backscattering spectrometer, 4He ions were accelerated to 2.0 MeV, were backscattered from TiN/Cu targets at a laboratory scattering angle of 170 ° and were detected with a surface barrier detector. Because the kinematic factors, differential scattering cross sections and stopping cross sections can be calculated or have been measured to an accuracy of a few per cent 2°, RBS is an excellent technique to determine the number of atoms in the copper layer. The microstructure and crystalline structure of the copper and TiN layers were determined in specimens consisting of a copper (50 nm) layer sputtered over a TiN (50 nm) layer on polished silicon substrates. These specimens were prepared for T E M and selected-area electron diffraction by dissolving holes through the silicon substrate with 5 % H F - 9 5 % H N O 3. The copper and TiN layers were then thinned using ion beam sputtering. In a specimen that was vacuum aged at 655 °C for 11 h, the grains in both layers were equiaxed and the average grain diameters in the copper and TiN layers were 100 nm and 30 nm respectively. A faint ring with a lattice parameter of 348 pm was present in an electron diffraction pattern of the C u TiN interface. This ring was attributed to titanium oxides. Although there was some interdiffusion of the copper and TiN, we did not observe any phases from reaction products in the interdiffusion zone. 3. RESULTS AND DISCUSSION The kinetics of copper diffusion in TiN were investigated in specimens that were vacuum heat treated at temperatures up to 700 °C. The interdiffusion was slow, even at these elevated temperatures, as revealed by the RBS spectra in Fig. 1. These spectra were recorded of an as-deposited sample and of a sample aged at 700 °C for 260 min. The difference in the masses of the copper and titanium nuclei leads to the separation of the peak at about 1.5 MeV, which was formed by 4He ions backscattered from copper atoms, and the broad peak centered at 1.2 MeV, which was produced by ions backscattered from titanium atoms in the TiN layer. The step at about 0.8 MeV was produced by ions backscattered from aluminum atoms in the sapphire substrate. F r o m the data in these spectra, the number of copper atoms in the samples can be calculated. These calculations show that only 20~o of the copper atoms in the copper layer diffused into the TiN layer or sublimed from the copper surface. Thus, compared with the integrated circuit metallizations discussed above, which interdiffuse significantly more than this during shorter exposure times below 600 °C, the TiN/Cu system is a more stable metallization at 700 °C. To determine the diffusivity of copper in the TiN layer, we aged diffusion couples at 608, 635 and 700 °C (Table I) and compositionally analyzed them with
158
M.B. CHAMBERLAIN I
I
I
I
Cu (surface) ~,~sa pphire :~,Ti N~/J Cu I
1.5k
-
.&.\\\\\\\\\~l////'1\fl ++
2 o
o c o o
:,o ~
I.Ok
•
o
O,5k
_1
~-
0
,I I.O
I
0
0,5
ENERGY
,~,~o.~.
1.5
I
2.0
(MeV)
Fig. 1. 2.0 MeV '*He- Rutherford b a c k s c a n e r i n g spectra of as-deposited (C)) and heat-treated ( + , 700' C, 260 min) T i N / C u metallizations on s a p p h i r e substrates. The heat t r e a t m e n t produced a small decrease in the area under the c o p p e r p e a k : this is a t t r i b u t e d to c o p p e r diffusing into the TiN and s u b l i m i n g into the vacuum. TABLE I ISOTHERMAL
}{EAT TREATMENT
PARAMETERS
Temperature ( C )
Time ( x 104 s)
M(t ),, ,~d(e.vperin (n tal
D(cm2 s 1I
2(Dr) t 2{nm)
608 635 635 700 700
25.3 2.16 5.76 (1.6 1.56
(I.(1282 0.00188 ().0466 0.0151 t1.120
1.7× 1.5x 1.5 x 4.4x 4.4x
4.1 3.6 5.9 10 17
10 10 10 10 10
*~ ~s TM
~' *
ALES. The Auger depth profiles of an as-deposited specimen and a specimen aged at 635 C for 16 h are shown in Fig. 2. The profiles of nitrogen (about 46 at.?{;) and a carbon contaminant (about 8 at.{:o) in the TiN layer are not plotted in this figure. The oxygen at a depth of 75 nm was combined in titanium oxides that probably formed on the TiN surface during specimen preparation when the Ar 20','~;N2 mixture was replaced with argon. The center of this oxygen peak was used as an inert marker locating the C u - T i N interface. The difference in the areas under the copper profiles at depths greater than the position of the oxygen peak equals the amount of copper that diffused into the TiN during the 635 ' C aging. This difference area is labeled "diffused Cu" in Fig. 2, and it was measured for specimens aged at 608, 635 and 700 'C. Before using these areas to determine a copper diffusivity, the diffusion mechanisms must be established in order to select a model for the calculation. Possible mechanisms of copper diffusion in the TiN layer include the following: bulk diffusion in TiN grains, diffusion in grain boundaries, surface diffusion in pinholes or voids, and diffusion along dislocations in grains. Below the T a m m a n n temperature of about one-half to two-thirds of the absolute melting temperature of a polycrystalline solid, diffusion is not controlled by bulk mechanisms but is controlled by mechanisms associated with grain boundaries, pores, pinholes and dislocations 13. The Cu/TiN specimens were aged below 973 K, which is 3130of the TiN melting temperature (3170 K): therefore, bulk diffusion should not control the
DIFFUSION OF C U IN T i N
159
transport of copper in the TiN layer. Furthermore, below about 0.3Tm the vacancy mobility in crystalline solids is so small that impurity diffusion and other diffusional processes which require vacancy-assisted bulk diffusion are practically negligible 21. Thus copper undoubtedly diffused along grain boundaries in the TiN layer and it did not diffuse into the lattice of grains. This type of diffusion has been called type C kinetics by Harrison 22. With these kinetics the copper concentration in the TiN layer should be vanishingly small in the lattice of grains, but it could be finite in grain boundaries, pores, pinholes and dislocations. [
o
80
E
60
~.~ ox_ -I-E
I
i
L_
i
I
i
I
i
\cu
T1 40--
-- . . . . .
i O
0 0
50
100 Depth
150
P00
(nm)
Fig. 2. Auger depth profiles of as-deposited ( ) and heat-treated ( - - - , 908 K, 960 miD) TiN/Cu metallizations showing copper diffusion into the TiN layer. The difference in the areas of the copper profiles in the TiN layer, labeled "diffused Cu", is used to calculate the amount of copper which diffused into the TiN during the annealing.
The assertion that copper did not diffuse in the TiN by a bulk process is supported by the Auger measurements. When two semi-infinite single-crystal solids interdiffuse across their flat interface, the concentration depth profile for fickian diffusion is proportional to the complementary error function 23. The slope at the original interface of such a profile decreases with increasing heat treatment time 24. Since the slope of the copper profile between 20 and 80 at.% did not change during the 700°C exposure (Fig. 2), copper did not diffuse into the TiN by a bulk mechanism. The grain boundary diffusivity of copper in the TiN layer cannot be calculated using the Whipple theory 23 for the following reason. The penetration of copper into the TiN layer at concentrations greater than 2 at.% was 60-120 nm or 2-4 times the average grain size, whereas the penetration must be less than one grain diameter to apply the Whipple analysis 25. Although this theory cannot be used, a similar analysis can be used to estimate the grain boundary diffusivity. This similar analysis approximates the microstructure of the TiN layer as follows: the grain boundaries are semi-infinite slabs of a high diffusivity material embedded in a semi-infinite solid (the TiN) with zero diffusivity. This microstructure is the same as that used in the Whipple theory; however, for the present analysis, copper cannot diffuse into the zero diffusivity grains. Since copper diffused and accumulated only in the grain boundaries, the fraction of the TiN layer available for copper was equal to the volume fraction of grain boundaries in the layer. This fraction was 36/d ~ 0.05, where the grain boundary width 6 ~ 0.5 nm and the average grain diameter d ~ 30 nm. In the following discussion the grain boundary diffusivity is estimated by
160
M.B. CHAMBERLAIN
first calculating a diffusivity assuming that the whole TiN layer is a grain boundary. This diffusivity is then multiplied by the inverse of the volume fraction. The multiplication accounts for the fact that copper diffused only in the fraction of the TiN layer occupied by grain boundaries. The b o u n d a r y value problem describing the laterally uniform fickian diffusion of atoms from one layer into an adjacent thin layer during an isothermal aging is reviewed by Crank z3. This analysis predicts that the a m o u n t of copper diffusing into the TiN layer during an isothermal aging at temperature T is given by
M(t)= M 1 - -
_
-
-
t3)
K- n = 0
where t is the aging time, D is the diffusivity of copper in TiN at temperature T, I is the TiN layer thickness (200 nm) and M is the a m o u n t of copper diffused into the TiN layer at t = ~ . The value of M is given by M = ½(C 1 + ( 7 2 ) / z 4 × 101: Cu atoms
(4)
since the copper concentration C~ at the Cu TiN interface is 4.2 × 1022 cm 3, the copper concentration C2 at the T i N - s a p p h i r e interface is zero, and / = 2 x 10 5 cm. The value of M(t) was calculated by multiplying the total n u m b e r of atoms in the copper layer (6.9 × 10 ~7 atoms cm 2) as determined by RBS (Fig. 1) by the ratio of the "diffused Cu" area to the total area under the copper profile (Fig. 2). The ratio M(t)/M was then calculated. 700 I
TEMPERATURE (°C) 675 650 T
1
625 I
=o10 17
e~E I-
~a 1 0 - 1 8
10-19 1.02
1.06
1.10
1.14
IO00/T (K -1)
Fig. 3. A r r h e n i u s plot s h o w i n g the lit o f e q n . (6) to the m e a s u r e d diffusivit~ values listed m T a b l e I.
The diffusion coefficient of copper in the TiN layer at 608, 635 and 7 0 0 C was calculated from least-squares fits of eqn. (3) to the values of M(t)/M measured at these temperatures. These results are presented in Table I. As shown by the plot in
D I F F U S I O N OF C u
IN T i N
161
Fig. 3, the temperature dependence of these calculated values follows the Arrhenius relation D = D Oexp(-- RQT)
(5)
where Q is the activation energy of atomic diffusion. The diffusivity evaluated from the D values of Table I was D = 4.5
x 10 6 cm 2 s -1
exp
427 kJ mol- 1) RT
(6)
The diffusivity of eqn. (6) is for laterally uniform diffusion into the total halfplane but, as discussed above, copper diffused and accumulated only in the fraction of the TiN layer attributed to grain boundaries. To estimate the grain boundary diffusivity, we need to multiply the coefficient in eqn. (6) by the inverse of the volume fraction of the TiN occupied by grain boundaries. Thus an estimate of the grain boundary diffusivity of copper in the TiN layer is Db = 9 x
10 v
cm 2 s- x exp
(- 4 2 7 k J m ° l - 1 ) RT
(7)
4. CONCLUSIONS The objectives of this applied research were to measure near 600°C the diffusivity of copper in a TiN layer 200 nm thick and to establish the mechanism of copper diffusion in this layer. Diffusion couples were prepared by r.f. sputter depositing a copper layer (80 nm) onto a TiN layer (200 nm) on sapphire and silicon substrates. Specimens were vacuum aged at 608,635 and 700 °C for times selected to produce interdiffusion profiles suitable for compositional analysis by RBS and by Auger sputter profiling. The grain boundary diffusivity of copper in the TiN layer was estimated from the RBS and Auger measurements to be Db =
9 x 10 v c m
2 S- 1
exp
_427 kJ mo1-1"] RT J
and diffusion did not take place by a lattice mechanism but was probably by dislocation and grain boundary mechanisms. The diameters of the grains in the copper and TiN layers were measured by TEM to be 100 nm and 30 nm respectively. This investigation as well as those of Nicolet and coworkers 16-~9 is demonstrating that a thin TiN film is an excellent high temperature barrier to the diffusion of copper. This property and the mechanical and electrical properties of the Si/TiN/Cu system make the TiN/Cu system a promising candidate for conductor metallizations on silicon solar concentrator cells which must operate for years at about 100 °C. The TiN/Cu metallization will soon be tested on 30x and 100x solar concentrator cells. ACKNOWLEDGMENTS
The author acknowledges J. A. Borders for measuring and interpreting the
162
M.B. CHAMBERLAIN
Rutherford backscattering spectra, C. R. Hills for performing the electron diffraction and TEM analyses, D. Ward and R. S. Nowicki of the Perkin-Elmer Corporation for fabricating the TiN/Cu diffusion specimens and M.-A. Nicolet of Caltech for helpful discussions on the data analysis. This work was supported by the U.S. Department of Energy under Contract DE-AC04-76-DP00789. REFERENCES 1 W.V. McLcvige, in D. E. Sawyer and H. A. Schafl(eds.), Natl. Bureau ~!/Smmhtrds Department O/ E , er,~y Workshop. Slahilit.l" (~/ Thin Fihn Sohtr ('ells and Materials, Au~,s't 1979, p. 95 (available from National Technical Information Service, Springfield, VA 22161 ). L. L. Kazmerski, in D. E. Sawyer and H. A. Schaff(eds.), Natl. Bureau ~!/Stamktrds Department ~?/ Ener~r Workshop, Stability (?[ Thin Film Solar Ck,lls a , d Materials. August 1979, p. 128 (available from National Technical Information Service, Springfield, VA 22161 ). 2 E.N.Sickafus`J.Tabock`J.L.B~mback.S.M.L~eandV.S.S~ndaram`77t#7S~lidFihns`7~(~98~ 49. 3 R.B. Campbell and A. Rohatgi. J, Eh~etrochem. Soe., 12711980) 2702. P. A. lies. in D. E. Sawyer and H. A. Schaff(eds.), Natl. Bureau (~/Stamlar(Is Department ()[EnerL~3' Workshop. Stability ~?/' Thi, l"ihn Sohtr ('ells and Materials. ,4u~,,usl 1979, p. 86 (available from National Technical Information Service, Springfield, VA 22161 ). 4 H. w)n Seefeld, W N. ( h e u n g , M. Miienpiiii and M.-A. Nicolel. I E E k 7)',,s. Ele('lr(m Detiees, 27 ( 19801 873. 5 J . M . Harris. E. Lugujjo, S. U. Campisano, M.-A. Nicolet and R. Shima..I. 1"(,'. 5"ci. 7eelmol.. 12 (1975) 524. 6 J . M . Poate, P. A. Turner. W. J. DeBonte and J. Yahalom, J. App/. Phy,L, 46 ( 19751 4275. 7 S. Kanamori, Th#~SolidFihns, 75(19811 19. 8 W.J.DeB~nte`J.M.P~ate~C.M.Me~iar-SmithandR.A~L~vesqu~`J.Appl.Phy`s.`46(~975)4284. 9 P.B. Ghate, J.C. Blair. C . R . FullerandG. E. McGuirc, ThmSolidklh,.L5311978) l l l . 10 J.M. Harris, S. S. Lau, M.-A. Nicolet and R. S. Nowicki, J. Eh'etrochem. Sot., 12311976) 120. I 1 R.S. Nowicki and 1. Wang, J. Vae. Sci. Teehm)l., 15 (1978) 235. 12 R.S. Nowicki, J. M. Harris, M.-A. Nicolet and 1. V. Mitchell, "llti, Solid f,lh,,s, 53 (1978) 195, 13 M.-A. Nicolet, Thin Solid Fih,s, 52 (19781415. 14 W. J, Garceau, P. R. Fournier and G. K. Herb, Thin Solid l'llms, 60 ( 19791 237. 15 M. Winmcr, Appl. Phys. Lell., 36 (19801456. 16 M. Miienpiiii, M.-A. Nicolet. 1. Suni and E. G. Colgan. So/. Em'rt,,v, 27( 19811 20;3. 17 M. M~ienp/iii, tf. yon Seefeld, H. Cheung, M.-A. Nicolct and A. G. Cullis, in .1. E. Baglin and J. M. Poate (eds.), Proe. Syrup. o, ThiH l:lhtl hltetJ(tces and lnleraelion.L Vo[. 80-2. Electrochemical Society, Princeton. N.I, 1980, p. 316. 18 N. Cheung, H. yon Seefeld and M.-A. Nicolet, in J. F~. Baglin and J. M. Poate (eds.), Proe. 5~w~lp. o, Thi, Fihn hlteJ;[(wes (rod hlteraetions. Vol. 811-2, Electrochemical Society, Princeton. N J, 1981/, p. 323. 19 N.W. Cheung, tl. w)n Seefeld, M.-A. Nicolet, F. Ho and P. A. lies, J. Appl. Phy,L, 52 ( 1981 ) 4297. 20 W.-K. Chu, J. W. Mayer and M.-A. Nicolet, Backscalteri,~,, Spectrometr.l', Academic Press, Ne~ York, 1978, p. 354ff 21 C.R. Barrett, W. D. Nix and A. S. Tetelman, 7he Principles Of EtlL~#teerblL, Material.s, Prentice-Hall. Englewood Clill;, N J, 1973, p. 154, pp. 273 282. 22 L.G. Harrison, Tram'. Faraday Soe.,57 (1961) 1191. 23 J. Crank, The Mathematics o]D(ffusion, Oxford University Press. London, 2nd edn., 1975, p. 50. 24 P.M. Hall and J. M. Morabito, SuUI Sci., 54 ( 1976t 79. 25 A.D. LeClaire, Br. J. Appl. Phys., 14119631351,
155
METALLURGICAL AND PROTECTIVE COATINGS
D I F F U S I O N OF C O P P E R IN THIN TiN FILMS M. B. CHAMBERLAIN Sandia National Laboratories, AIbuquerque, N M 87185 (U.S.A.) (Received December 9, 1981 ; accepted December 29, 1981 )
The diffusivity of copper in thin TiN layers was determined in specimens prepared by r.f. sputtering a copper (80 nm) layer onto a TiN (200 nm) layer on sapphire and silicon substrates. Specimens were isothermally heat treated at 608, 635 and 700°C at pressures lower than 2 x 10 -6 Pa; they were compositionally analyzed by Rutherford backscattering spectroscopy and Auger sputter profiling; and they were microstructurally characterized by transmission electron microscopy and electron diffraction. The diffusivity D = 9 x 107 cm 2 s -1 exp(-427 kJ mol-1/R T) from 608 to 700 °C. The mechanisms of copper diffusion were not bulk processes, but they were probably processes involving primarily grain boundaries in the TiN. This very low diffusivity at these temperatures makes TiN/Cu an excellent candidate for a high temperature metallization for silicon solar concentrator cells.
1. INTRODUCTION A reliable thin film metallization for electrical conductors and contacts must be developed for photovoltaic solar concentrator cells planned for use in terrestrial power plants j. The metallization of a 1000x solar concentrator cell must not interdiffuse when operating at about 100 °C during a 10-20 year life 1. It must also withstand a 10 min 600 °C glazing process which is required to seal hermetically and to protect the cell surface from corrosion by atmospheric exposure 2. Evaporated Ti/Pd/Ag has been a reliable metallization in non-concentrating photovoltaic cells for space power systems 3. However, during a 10 min 600 °C heat treatment, this laminate structure interdiffuses with a silicon cell and degrades 4 the cell output by 50~. The integrated circuit metallizations employing Ti/Mo/Au 5, Ti/Pd/Au 6, Ti/Pt/Au 7, Ti/Rh/Au a or Ti/W/Au 9, lo also interdiffuse during exposures at less than 600°C or extended exposures at 100°C. However, the diffusion barrier properties of molybdenum or titanium thin films are significantly improved when the films are reactively sputter deposited in Ar-20~N2 11,12. These improved barrier properties have been attributed to the formation of nitrides 1~' 13. Thin
0040-6090/82/0000-0000/~02.75
© ElsevierSequoia/Printedin The Netherlands
156
M.B. CHAMBERLAIN
nitride layers have been successfully used as diffusion barriers in Mo/Au ~1 Ti/Pd/Au ~4 and Cr/Ni/Ag ~5 metallizations to improve the reliability of integrated circuits. To establish whether a thin TiN layer could be a reliable diffusion barrier in metallizations for solar concentrator cells, the contact resistance ~, electrical properties 4. ~7, deposition process ~7 and diffusion barrier properties ~s of reactively sputter-deposited TiN are being investigated. The TiN/Ag system on silicon solar cells withstands the 10 min 600 C heat treatment without degrading the cell output power ~'~. At 7 0 0 C the TiN/Cu metallization on silicon is more stable than the TiN/Ag system ~s. Thus on a TiN barrier a copper conductor may be more reliable than a silver one. To establish the high temperature failure mechanisms of the TiN/Cu metallization, we need to know the diffusivity and mechanisms of copper diffusion in TiN thin films. In this study the diffusivity of copper in TiN was determined from 608 to 700 ~'C by compositional and microstructural analysis using Auger electron spectroscopy (AES), Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM). 2. SAMPLE PREPARATION AND CHARACTERIZATION
The diffusion of copper in TiN thin films was investigated in 15 specimens that were simultaneously prepared by r.f. sputtering first a TiN (200 nm) layer and then a copper (80 nm) layer on polished sapphire substrates 1.25 cm in diameter and 1.3 mm thick. The TiN was reactively sputter deposited in A r 20";IN 2 at a rate of 6 nm rain l, and the copper was sputter deposited in argon at 42 nm rain ~. To minimize the oxygen concentration in the TiN layer, the substrates were biased to 50 V during the TiN deposition l 7 and the base pressure of the sputtering chamber w a s 7 x l 0 Spa. Diffusion couples were vacuum heat treated in a Perkin Elmer model 548 ESCA-AES spectrometer. Samples were mountea on a hot stage and under two tantalum sheets which formed an isothermal cavity enclosing the specimens. The temperature was monitored with a chromel alumel thermocouple clamped to the sample surface. Isothermal heat treatments were performed from 608 to 700 ~C in a total pressure lower than 2 x 10 ~' Pa. The carbon, copper, oxygen, nitrogen and titanium concentrations as functions of the distance perpendicular to the copper layer surface were calculated from measurements determined by combining AES and ion beam sputtering. The concentration C.i(t ) atomic per cent of element .j at sputtering time t was calculated by -
~Aj(t) C i(t) - Z:~kAk(t)
(It
where :~i is the inverse Auger sensitivity factor of A j(t)is the peak-to-peak deflection in the first derivative of the Auger signal ofelementj measured at time t. The distance X(t) from the specimen surface at time t was calculated from =
(c,,(sh R jiN]d~
(2)
DIFFUSION OF C u IN T i N
157
where R= (z = Cu and TiN) are the sputtering rates for a 2.0 keV argon ion beam in the respective pure layer. The sputtering rate in copper was calculated as 2.88 nm m i n - 1 using a measurement by RBS of the areal density (atoms per square centimeter) of this layer. The copper-to-TiN ratio of the sputtering rates was 3.40. This ratio was calculated from measurements performed with a H o b s o n - T a y l o r profilometer of the heights of steps that were sputtered into the two layers. After the specimens had been sputter profiled, the crater depths were measured to corroborate the depths calculated by eqn. (2). The RBS technique 2° was used to measure the number of atoms in the copper layer. In the Sandia backscattering spectrometer, 4He ions were accelerated to 2.0 MeV, were backscattered from TiN/Cu targets at a laboratory scattering angle of 170 ° and were detected with a surface barrier detector. Because the kinematic factors, differential scattering cross sections and stopping cross sections can be calculated or have been measured to an accuracy of a few per cent 2°, RBS is an excellent technique to determine the number of atoms in the copper layer. The microstructure and crystalline structure of the copper and TiN layers were determined in specimens consisting of a copper (50 nm) layer sputtered over a TiN (50 nm) layer on polished silicon substrates. These specimens were prepared for T E M and selected-area electron diffraction by dissolving holes through the silicon substrate with 5 % H F - 9 5 % H N O 3. The copper and TiN layers were then thinned using ion beam sputtering. In a specimen that was vacuum aged at 655 °C for 11 h, the grains in both layers were equiaxed and the average grain diameters in the copper and TiN layers were 100 nm and 30 nm respectively. A faint ring with a lattice parameter of 348 pm was present in an electron diffraction pattern of the C u TiN interface. This ring was attributed to titanium oxides. Although there was some interdiffusion of the copper and TiN, we did not observe any phases from reaction products in the interdiffusion zone. 3. RESULTS AND DISCUSSION The kinetics of copper diffusion in TiN were investigated in specimens that were vacuum heat treated at temperatures up to 700 °C. The interdiffusion was slow, even at these elevated temperatures, as revealed by the RBS spectra in Fig. 1. These spectra were recorded of an as-deposited sample and of a sample aged at 700 °C for 260 min. The difference in the masses of the copper and titanium nuclei leads to the separation of the peak at about 1.5 MeV, which was formed by 4He ions backscattered from copper atoms, and the broad peak centered at 1.2 MeV, which was produced by ions backscattered from titanium atoms in the TiN layer. The step at about 0.8 MeV was produced by ions backscattered from aluminum atoms in the sapphire substrate. F r o m the data in these spectra, the number of copper atoms in the samples can be calculated. These calculations show that only 20~o of the copper atoms in the copper layer diffused into the TiN layer or sublimed from the copper surface. Thus, compared with the integrated circuit metallizations discussed above, which interdiffuse significantly more than this during shorter exposure times below 600 °C, the TiN/Cu system is a more stable metallization at 700 °C. To determine the diffusivity of copper in the TiN layer, we aged diffusion couples at 608, 635 and 700 °C (Table I) and compositionally analyzed them with
158
M.B. CHAMBERLAIN I
I
I
I
Cu (surface) ~,~sa pphire :~,Ti N~/J Cu I
1.5k
-
.&.\\\\\\\\\~l////'1\fl ++
2 o
o c o o
:,o ~
I.Ok
•
o
O,5k
_1
~-
0
,I I.O
I
0
0,5
ENERGY
,~,~o.~.
1.5
I
2.0
(MeV)
Fig. 1. 2.0 MeV '*He- Rutherford b a c k s c a n e r i n g spectra of as-deposited (C)) and heat-treated ( + , 700' C, 260 min) T i N / C u metallizations on s a p p h i r e substrates. The heat t r e a t m e n t produced a small decrease in the area under the c o p p e r p e a k : this is a t t r i b u t e d to c o p p e r diffusing into the TiN and s u b l i m i n g into the vacuum. TABLE I ISOTHERMAL
}{EAT TREATMENT
PARAMETERS
Temperature ( C )
Time ( x 104 s)
M(t ),, ,~d(e.vperin (n tal
D(cm2 s 1I
2(Dr) t 2{nm)
608 635 635 700 700
25.3 2.16 5.76 (1.6 1.56
(I.(1282 0.00188 ().0466 0.0151 t1.120
1.7× 1.5x 1.5 x 4.4x 4.4x
4.1 3.6 5.9 10 17
10 10 10 10 10
*~ ~s TM
~' *
ALES. The Auger depth profiles of an as-deposited specimen and a specimen aged at 635 C for 16 h are shown in Fig. 2. The profiles of nitrogen (about 46 at.?{;) and a carbon contaminant (about 8 at.{:o) in the TiN layer are not plotted in this figure. The oxygen at a depth of 75 nm was combined in titanium oxides that probably formed on the TiN surface during specimen preparation when the Ar 20','~;N2 mixture was replaced with argon. The center of this oxygen peak was used as an inert marker locating the C u - T i N interface. The difference in the areas under the copper profiles at depths greater than the position of the oxygen peak equals the amount of copper that diffused into the TiN during the 635 ' C aging. This difference area is labeled "diffused Cu" in Fig. 2, and it was measured for specimens aged at 608, 635 and 700 'C. Before using these areas to determine a copper diffusivity, the diffusion mechanisms must be established in order to select a model for the calculation. Possible mechanisms of copper diffusion in the TiN layer include the following: bulk diffusion in TiN grains, diffusion in grain boundaries, surface diffusion in pinholes or voids, and diffusion along dislocations in grains. Below the T a m m a n n temperature of about one-half to two-thirds of the absolute melting temperature of a polycrystalline solid, diffusion is not controlled by bulk mechanisms but is controlled by mechanisms associated with grain boundaries, pores, pinholes and dislocations 13. The Cu/TiN specimens were aged below 973 K, which is 3130of the TiN melting temperature (3170 K): therefore, bulk diffusion should not control the
DIFFUSION OF C U IN T i N
159
transport of copper in the TiN layer. Furthermore, below about 0.3Tm the vacancy mobility in crystalline solids is so small that impurity diffusion and other diffusional processes which require vacancy-assisted bulk diffusion are practically negligible 21. Thus copper undoubtedly diffused along grain boundaries in the TiN layer and it did not diffuse into the lattice of grains. This type of diffusion has been called type C kinetics by Harrison 22. With these kinetics the copper concentration in the TiN layer should be vanishingly small in the lattice of grains, but it could be finite in grain boundaries, pores, pinholes and dislocations. [
o
80
E
60
~.~ ox_ -I-E
I
i
L_
i
I
i
I
i
\cu
T1 40--
-- . . . . .
i O
0 0
50
100 Depth
150
P00
(nm)
Fig. 2. Auger depth profiles of as-deposited ( ) and heat-treated ( - - - , 908 K, 960 miD) TiN/Cu metallizations showing copper diffusion into the TiN layer. The difference in the areas of the copper profiles in the TiN layer, labeled "diffused Cu", is used to calculate the amount of copper which diffused into the TiN during the annealing.
The assertion that copper did not diffuse in the TiN by a bulk process is supported by the Auger measurements. When two semi-infinite single-crystal solids interdiffuse across their flat interface, the concentration depth profile for fickian diffusion is proportional to the complementary error function 23. The slope at the original interface of such a profile decreases with increasing heat treatment time 24. Since the slope of the copper profile between 20 and 80 at.% did not change during the 700°C exposure (Fig. 2), copper did not diffuse into the TiN by a bulk mechanism. The grain boundary diffusivity of copper in the TiN layer cannot be calculated using the Whipple theory 23 for the following reason. The penetration of copper into the TiN layer at concentrations greater than 2 at.% was 60-120 nm or 2-4 times the average grain size, whereas the penetration must be less than one grain diameter to apply the Whipple analysis 25. Although this theory cannot be used, a similar analysis can be used to estimate the grain boundary diffusivity. This similar analysis approximates the microstructure of the TiN layer as follows: the grain boundaries are semi-infinite slabs of a high diffusivity material embedded in a semi-infinite solid (the TiN) with zero diffusivity. This microstructure is the same as that used in the Whipple theory; however, for the present analysis, copper cannot diffuse into the zero diffusivity grains. Since copper diffused and accumulated only in the grain boundaries, the fraction of the TiN layer available for copper was equal to the volume fraction of grain boundaries in the layer. This fraction was 36/d ~ 0.05, where the grain boundary width 6 ~ 0.5 nm and the average grain diameter d ~ 30 nm. In the following discussion the grain boundary diffusivity is estimated by
160
M.B. CHAMBERLAIN
first calculating a diffusivity assuming that the whole TiN layer is a grain boundary. This diffusivity is then multiplied by the inverse of the volume fraction. The multiplication accounts for the fact that copper diffused only in the fraction of the TiN layer occupied by grain boundaries. The b o u n d a r y value problem describing the laterally uniform fickian diffusion of atoms from one layer into an adjacent thin layer during an isothermal aging is reviewed by Crank z3. This analysis predicts that the a m o u n t of copper diffusing into the TiN layer during an isothermal aging at temperature T is given by
M(t)= M 1 - -
_
-
-
t3)
K- n = 0
where t is the aging time, D is the diffusivity of copper in TiN at temperature T, I is the TiN layer thickness (200 nm) and M is the a m o u n t of copper diffused into the TiN layer at t = ~ . The value of M is given by M = ½(C 1 + ( 7 2 ) / z 4 × 101: Cu atoms
(4)
since the copper concentration C~ at the Cu TiN interface is 4.2 × 1022 cm 3, the copper concentration C2 at the T i N - s a p p h i r e interface is zero, and / = 2 x 10 5 cm. The value of M(t) was calculated by multiplying the total n u m b e r of atoms in the copper layer (6.9 × 10 ~7 atoms cm 2) as determined by RBS (Fig. 1) by the ratio of the "diffused Cu" area to the total area under the copper profile (Fig. 2). The ratio M(t)/M was then calculated. 700 I
TEMPERATURE (°C) 675 650 T
1
625 I
=o10 17
e~E I-
~a 1 0 - 1 8
10-19 1.02
1.06
1.10
1.14
IO00/T (K -1)
Fig. 3. A r r h e n i u s plot s h o w i n g the lit o f e q n . (6) to the m e a s u r e d diffusivit~ values listed m T a b l e I.
The diffusion coefficient of copper in the TiN layer at 608, 635 and 7 0 0 C was calculated from least-squares fits of eqn. (3) to the values of M(t)/M measured at these temperatures. These results are presented in Table I. As shown by the plot in
D I F F U S I O N OF C u
IN T i N
161
Fig. 3, the temperature dependence of these calculated values follows the Arrhenius relation D = D Oexp(-- RQT)
(5)
where Q is the activation energy of atomic diffusion. The diffusivity evaluated from the D values of Table I was D = 4.5
x 10 6 cm 2 s -1
exp
427 kJ mol- 1) RT
(6)
The diffusivity of eqn. (6) is for laterally uniform diffusion into the total halfplane but, as discussed above, copper diffused and accumulated only in the fraction of the TiN layer attributed to grain boundaries. To estimate the grain boundary diffusivity, we need to multiply the coefficient in eqn. (6) by the inverse of the volume fraction of the TiN occupied by grain boundaries. Thus an estimate of the grain boundary diffusivity of copper in the TiN layer is Db = 9 x
10 v
cm 2 s- x exp
(- 4 2 7 k J m ° l - 1 ) RT
(7)
4. CONCLUSIONS The objectives of this applied research were to measure near 600°C the diffusivity of copper in a TiN layer 200 nm thick and to establish the mechanism of copper diffusion in this layer. Diffusion couples were prepared by r.f. sputter depositing a copper layer (80 nm) onto a TiN layer (200 nm) on sapphire and silicon substrates. Specimens were vacuum aged at 608,635 and 700 °C for times selected to produce interdiffusion profiles suitable for compositional analysis by RBS and by Auger sputter profiling. The grain boundary diffusivity of copper in the TiN layer was estimated from the RBS and Auger measurements to be Db =
9 x 10 v c m
2 S- 1
exp
_427 kJ mo1-1"] RT J
and diffusion did not take place by a lattice mechanism but was probably by dislocation and grain boundary mechanisms. The diameters of the grains in the copper and TiN layers were measured by TEM to be 100 nm and 30 nm respectively. This investigation as well as those of Nicolet and coworkers 16-~9 is demonstrating that a thin TiN film is an excellent high temperature barrier to the diffusion of copper. This property and the mechanical and electrical properties of the Si/TiN/Cu system make the TiN/Cu system a promising candidate for conductor metallizations on silicon solar concentrator cells which must operate for years at about 100 °C. The TiN/Cu metallization will soon be tested on 30x and 100x solar concentrator cells. ACKNOWLEDGMENTS
The author acknowledges J. A. Borders for measuring and interpreting the
162
M.B. CHAMBERLAIN
Rutherford backscattering spectra, C. R. Hills for performing the electron diffraction and TEM analyses, D. Ward and R. S. Nowicki of the Perkin-Elmer Corporation for fabricating the TiN/Cu diffusion specimens and M.-A. Nicolet of Caltech for helpful discussions on the data analysis. This work was supported by the U.S. Department of Energy under Contract DE-AC04-76-DP00789. REFERENCES 1 W.V. McLcvige, in D. E. Sawyer and H. A. Schafl(eds.), Natl. Bureau ~!/Smmhtrds Department O/ E , er,~y Workshop. Slahilit.l" (~/ Thin Fihn Sohtr ('ells and Materials, Au~,s't 1979, p. 95 (available from National Technical Information Service, Springfield, VA 22161 ). L. L. Kazmerski, in D. E. Sawyer and H. A. Schaff(eds.), Natl. Bureau ~!/Stamktrds Department ~?/ Ener~r Workshop, Stability (?[ Thin Film Solar Ck,lls a , d Materials. August 1979, p. 128 (available from National Technical Information Service, Springfield, VA 22161 ). 2 E.N.Sickafus`J.Tabock`J.L.B~mback.S.M.L~eandV.S.S~ndaram`77t#7S~lidFihns`7~(~98~ 49. 3 R.B. Campbell and A. Rohatgi. J, Eh~etrochem. Soe., 12711980) 2702. P. A. lies. in D. E. Sawyer and H. A. Schaff(eds.), Natl. Bureau (~/Stamlar(Is Department ()[EnerL~3' Workshop. Stability ~?/' Thi, l"ihn Sohtr ('ells and Materials. ,4u~,,usl 1979, p. 86 (available from National Technical Information Service, Springfield, VA 22161 ). 4 H. w)n Seefeld, W N. ( h e u n g , M. Miienpiiii and M.-A. Nicolel. I E E k 7)',,s. Ele('lr(m Detiees, 27 ( 19801 873. 5 J . M . Harris. E. Lugujjo, S. U. Campisano, M.-A. Nicolet and R. Shima..I. 1"(,'. 5"ci. 7eelmol.. 12 (1975) 524. 6 J . M . Poate, P. A. Turner. W. J. DeBonte and J. Yahalom, J. App/. Phy,L, 46 ( 19751 4275. 7 S. Kanamori, Th#~SolidFihns, 75(19811 19. 8 W.J.DeB~nte`J.M.P~ate~C.M.Me~iar-SmithandR.A~L~vesqu~`J.Appl.Phy`s.`46(~975)4284. 9 P.B. Ghate, J.C. Blair. C . R . FullerandG. E. McGuirc, ThmSolidklh,.L5311978) l l l . 10 J.M. Harris, S. S. Lau, M.-A. Nicolet and R. S. Nowicki, J. Eh'etrochem. Sot., 12311976) 120. I 1 R.S. Nowicki and 1. Wang, J. Vae. Sci. Teehm)l., 15 (1978) 235. 12 R.S. Nowicki, J. M. Harris, M.-A. Nicolet and 1. V. Mitchell, "llti, Solid f,lh,,s, 53 (1978) 195, 13 M.-A. Nicolet, Thin Solid Fih,s, 52 (19781415. 14 W. J, Garceau, P. R. Fournier and G. K. Herb, Thin Solid l'llms, 60 ( 19791 237. 15 M. Winmcr, Appl. Phys. Lell., 36 (19801456. 16 M. Miienpiiii, M.-A. Nicolet. 1. Suni and E. G. Colgan. So/. Em'rt,,v, 27( 19811 20;3. 17 M. M~ienp/iii, tf. yon Seefeld, H. Cheung, M.-A. Nicolct and A. G. Cullis, in .1. E. Baglin and J. M. Poate (eds.), Proe. Syrup. o, ThiH l:lhtl hltetJ(tces and lnleraelion.L Vo[. 80-2. Electrochemical Society, Princeton. N.I, 1980, p. 316. 18 N. Cheung, H. yon Seefeld and M.-A. Nicolet, in J. F~. Baglin and J. M. Poate (eds.), Proe. 5~w~lp. o, Thi, Fihn hlteJ;[(wes (rod hlteraetions. Vol. 811-2, Electrochemical Society, Princeton. N J, 1981/, p. 323. 19 N.W. Cheung, tl. w)n Seefeld, M.-A. Nicolet, F. Ho and P. A. lies, J. Appl. Phy,L, 52 ( 1981 ) 4297. 20 W.-K. Chu, J. W. Mayer and M.-A. Nicolet, Backscalteri,~,, Spectrometr.l', Academic Press, Ne~ York, 1978, p. 354ff 21 C.R. Barrett, W. D. Nix and A. S. Tetelman, 7he Principles Of EtlL~#teerblL, Material.s, Prentice-Hall. Englewood Clill;, N J, 1973, p. 154, pp. 273 282. 22 L.G. Harrison, Tram'. Faraday Soe.,57 (1961) 1191. 23 J. Crank, The Mathematics o]D(ffusion, Oxford University Press. London, 2nd edn., 1975, p. 50. 24 P.M. Hall and J. M. Morabito, SuUI Sci., 54 ( 1976t 79. 25 A.D. LeClaire, Br. J. Appl. Phys., 14119631351,