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Si multilayer structures
Thin Solid Films, 199 (1991) 343 350
PREPARATION AND CHARACTERIZATION
343
Q U A N T I T A T I V E X-RAY ANALYSIS OF I N T E R D I F F U S I N G Ta/Si MULTILAYER STRUCTURES H. L. MEYERHEIM* AND H. E. GOBEL Siemens A G, Corp, Research and Technology, Applied Materials Research, D-8000 Miinchen 83 ( F. R. G. ) (Received August 22, 1990; accepted October 25, 1990)
Multilayers of amorphous tantalum and silicon form a synthetic onedimensional lattice which can be observed by a sequence of intense low angle X-ray diffraction peaks. Annealing a Ta/Si multilayer structure of 20 layer pairs, each layer pair consisting of nominally 4 nm thick tantalum and 7 nm thick silicon layers, leads to rapid diffusion of silicon into the tantalum layers forming a homogeneous TaSi x solution. The thickness of the pure silicon layers decreases linearly with increasing annealing temperature. The width At of the transition layers between the silicon and the TaSix layers is analysed by comparing the measured absolute reflectivity of the multilayer structure with calculated data up to the 10th diffraction order. The computational approach is based on the Fresnel equations and assumes a linear composition profile at the interfaces leading to a trapezoidal lattice structure. An upper limit value of 0.7nm for the interface width is estimated. The interdiffusion is complete at T = 540°C, indicated by the breakdown of the multilayer structure.
I. INTRODUCTION
Silicides of refractory metals such as TaSi 2 have found widespread applications as materials for interconnects and gate electrodes in very-large-scale integration (VLSI) technology 1'2. One of the preparation methods used to form stoichiometric disilicide is the repeated sequential sputter deposition of the metal and silicon from pure elemental targets followed by subsequent annealing of the multilayer. In this context multilayers are well defined models for the study of interdiffusion, here applied to the process of silicide formation. Typical periods of such multilayers are between 5 and 15 nm and can be used as Bragg-reflectors for X-rays. As an example, multilayer structures are of interest for the construction of X-ray optical elements such as analyser crystals for soft X-rays *Present address: lnstitut ffir Kristallographie und Mineralogie der Universitfit Mfinchen, Theresienstrasse 41, D-8000 Miinchen2 (F.R.G.) 0040-6090/91/$3.50
~) ElsevierSequoia/Printedin The Netherlands
344
FI. 1.. MEYERttEIM, It. E. G{)I~E[
or m o n o c h r o m a t o r s in vacuum UV (VUV) storage ring beamlines -~ ~ Long-term stability against heat load is one of the most severe problems m this field. The structural characterization o1" multilayers, Focusing on temperature stability, is therefore an important problem for the development of optimized materials. On the basis o f X-ray diffraction ( X R D ) mcasurements, we investigate in this paper the behaviour o f T a / S i multilayers against heat treatment. Interest is concentrated on a quantitative analysis ot" the low angle Bragg maxima providing information about the difl'usion process and the stability oF the Ta Si interfaces. 2. EXPERIMI'INTALI)EfAII S The multilayers were deposited onto uncooled silicon wafers by d.c. magnetron sputtering from two pure elemental targets on an Electrotec MS 4200 system at deposition rates of about 0.5 nm s ~. The thickness o f the layer pairs was controlled by the scan speed of the carousel at an optimized sputter rate monitored by quartz thickness resonators. Figure 1 shows a cross-sectional transmission electron microscopy (TEM) p h o t o g r a p h of an as-deposited Ta/Si multilayer above a monocrystallinc silicon wafer. The bright and dark regions corrcspond to silicon and tantalum layers respectively. Within a resolution of about 1 nm, the T E M p h o t o g r a p h indicates abrupt Ta Si interfaces. Near the surface, an increasingly wavy layer m o r p h o l o g y is observed. The samples were annealed |\)r 10 rain in slightly reducing inert (Ar H_,)
Fig. I. Cross-sectional TEM pholograph taken ffon~till as-deposited TaSi muhilayer structure. Bright and dark regions correspond Io silicon and Ianlalurn layers rcspecti~cl_x.In the upper region, a v,:.txy layer morpholog~ is observable,
X-RAY ANALYSIS OF INTERDIFFUSION IN
Ta/Si
345
gas atmosphere at constant temperature and subsequently quenched to room temperature (RT). For additional in situ X R D measurements, the multilayers were heated in steps of 20 K and kept constant at a given temperature for 1 rain during data collection. It was proven quantitatively that both annealing procedures led to indentical diffusion paths. This demonstrates that the reaction rapidly reaches an equilibrium at a given temperature. Experimental details are described in ref. 6. 3. RESULTS AND DISCUSSION The short-range atomic ordering during the annealing process has been studied previously by TEM, extended X-ray absorption fine structure (EXAFS) and X R D measurements 7. It could be shown that during annealing of the samples, consisting of 20 layer pairs (7 nm silicon and 4 nm tantalum), silicon diffuses rapidly into the tantalum layers forming a homogeneous TaSi x solution, where the tantalum atoms are locally neighbored by silicon atoms at a distance of R = 0.256(5) nm. This is very close to the TaSi a bulk value o f R = 0.258 rim. Even after deposition no Ta Ta short-range order could be detected in the EXAFS spectra. This is in agreement with results reported by Lamble et al. s on with W/C multilayers. The authors report that the crystallization of tungsten needs a critical thickness (about 6 nm in their case) below which the metal layer is highly disordered. Furthermore, within about 0.2-0.3 nm resolution, T E M photographs did not show the presence of crystalline tantalum metal. Figure 2 shows a temperature-resolved low angle diffraction pattern of the Ta/Si multilayer structure which was measured in situ by Cu K~ radiation using a position-sensitive detector to record different diffraction orders h simultaneously ~. A lattice periodicity of d = 10.7(2) nm was determined from the positions of the high order diffraction peaks. This is in close agreement with the nominal value of d = 11 nm. Although the measured peak intensities are apparently distorted relative to each other, it is evident that the detected Bragg maxima between h = 3 and h = 12 exhibit strong intensity oscillations with temperature. For the data collection, the sample is rocked around the 6th diffraction order to excite all reflections in the low
"3" zJ
J
0 Fig. 2. Temperature-resolved diffraction pattern of a 20 period (4nm/7nm) Ta/Si multilayer structure in the low angle regime using Cu K s radiation. The diffraction orders h exhibit intensity oscillations with temperature. Above T ~ 540 C , the synthetic lattice structure disappears.
346
u, L. MEYERHEIM, H. E. G()BEL
angle regime. Therefore, the relative intensities cannot be compared quantitatively. Over a wide temperature range, neighbouring diffraction orders h and h + 1 are in opposite phase relation. All Bragg peaks vanish above about 540 'C, indicating the breakdown of the multilayer structure. The oscillatory intensity behaviour is explained by a shift of the Si TaSi~ interfaces induced by silicon diffusion into the tantalum layers (TaSi x layers at higher temperatures). Normal to the sample surface (z-axis) the multilayer structure is characterized by a periodically varying electron density p(z) (Fig. 3). Ta(TaSi~a.) and silicon layers correspond to regions of high and low electron density respectively. In a simple model, the multilayer can be regarded as a one-dimensional crystal, where the unit cell contains one silicon and one Ta(TaSi~a,) layer. Only two unit cells are shown. lattice profiles: rectangular
trapezoidal [.,-&t W
pTa - ~ d s i pSi ,,l 7nm
,,
:, ~ 3
'l 4nm
~ PTaSix-"~ pSi
T .i.
~I
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1.. [
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i
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Fig. 3. Schematic representation ofa Ta Si multilayer structure. Tantalum and silicon layers correspond to regions of high and low electron density p(z) respectively. The multflayer can be regarded as a onedimensional crystal, where the unit cell contains one silicon and one Ta(TaSi,a.) layer. The thickness of the silicon la~,er dsi is related to the normalized coordinate zo of the Si Ta(TaSi~.,) interface within the unit cell Idsx = -,d, where d is the lanice periodicityL The second interface is located at -% 0. In the rectangular lattice model shown on the left. the width At of the interface between the silicon and Ta(TaSi,) layers is infinitcl,, narrow. In the trapezoidal lattice prolilc (upper panel, right side) u linear transition between the layers is assumed. With increasing temperature (steps 1,2,3) the Si Ta(TaSi,) interfaces shift to lower _%values. Some arbitrary values are indicated (see e.A'. ref. 7).
The thickness of the silicon layer (dsi) is related to the normalized coordinate :o of the Si Ta(TaSixa.) interface within the unit cell by dsi = zod, where dis the lattice periodicity. The second interface is located at c o = 0. It is assumed that with increasing temperature (steps 1,2,3), silicon atoms diffuse into the Ta(TaSi.~,y) layers, thereby continuously increasing the thickness of the Ta(TaSi~,a, ) layers at the expense of the pure silicon layers. This corresponds to the shift of the interface to lower z o values and the scattered intensity I varies by I ~ cos(2rthz) (ref. 9) as observed in Fig. 2. In the "rectangular profile model" shown on the left side of Fig. 3. no special assumptions about the interface structure are rnade and the width of the transition
X-RAY ANALYSIS OF INTERDIFFUSION IN
Ta/Si
347
zone between the silicon and the TaSi x layers is infinitely sharp (At = 0). Additionally, this model neglects the continuously decreasing electron density contrast within the multilayer resulting from the "dilution" of the TaSix layers by indiffusing silicon atoms. The detailed analysis of the experimental data 7, based only on the phase relations between neighbouring maxima and minima of the Bragg peaks, indicates the linear dependence of dsi on the annealing temperature as shown in Fig. 4. Reasonable agreement with cross-sectional TEM is obtained. From the deviation between the nominal (7 nm) and the measured silicon layer thickness at RT (sample after deposition without annealing), we conclude that the diffusion process already begins during deposition. This is supposed to be induced by the energy transferred to the sample during the sputter deposition, during which the sample is heated to 300 °C 1°.
• rectangul ar le modelXRDprofi
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Fig. 4. Silicon layer thickness dsi vs. annealing temperature as determined from the intensity oscillations. Reasonable agreement with cross-sectional TEM is obtained.
However, within this theoretical and experimental framework no information can be obtained about the width At of the interfacial regions between the silicon and the TaSi x layers. In order to analyse quantitatively the abruptness of the interfaces, X-ray measurements were performed on a modified powder diffractometer (Siemens D 500) equipped with a diffracted beam monochromator. The beampath (arm length and beam apertures) was optimized for low angle measurements in the range of total external reflection. A diffraction pattern using Cu Kct radiation is shown in the centre of Fig. 5 for a sample annealed at 370°C for 10 min. Starting from an angle of almost 20 -= 0 °, the total reflection curve and the sequence of ! 0 multilayer reflections is observed up to 20 ~ 8 °. The dynamical intensity range required for a quantitative interpretation has to cover about six orders of magnitude. The experimental intensity data were analysed by comparison with calculated reflection curves based on the Fresnel equations 11 for reflectivity at the boundary of
348
H . i . . MEYERHEIM, H. E. G/:)BEI.
two h o m o g e n e o u s media. W e a p p l i e d P a r r a t ' s recursion f o r m u l a 1-~ l\)r the specularly reflected intensity from a thin fihn system consisting o f ,%' media, The p a r a l n e t e r s D and ft. which characterize tile c o m p l e x refraction index, n = 1 - D - i f i , were e v a l u a t e d for silicon and t a n t a l u m oil the basis o f t a b u l a t e d d a t a i ,~.~ . F o r the TaSi., layers, we calculated a p w o x i m a t e p a r a m e t e r s by linear i n t e r p o l a t i o n between the d a t a for pure t a n t a l u m and s t o i c h i o m e t r i c TaSi 2 using dsi as the p a r a m e t e r describing the degree o f indiflusion. F o r ds~ = 7 nm. the t a n t a l u m layers exist in their pure form and ds~ = 0 nm c o r r e s p o n d s to c o m p l e t e reaction, where stoichiometric TaSi 2 has been formed. Tile incident r a d i a t i o n was a s s u m e d to be strictly parallel arid m o n o c h r o n l a t i c . In the u p p e r and lower panels o f Fig. 5 we show calculated diffraction p a t t e r n s allowing f'or a finite transition width At between the h o m o g e n e o u s T a S i , and the pure silicon layers. The profile within the t r a n s i t i o n zone has been a s s u m e d to be linear ~5 leading to a "'trapezoidal hittite profile" as shown in the u p p e r panel on the right side of Fig. 3. F o r the calculation, the t r a n s i t i o n zone is s u b d i v i d e d into then lametlae o f equal thickness dl, where 61
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2 0 ~ I:'ig. 5. Comparison between cxperin~cnlal (ccnlrc cur\ c) and simulated spectra ( upper and Io,a cr curves) for a ~70 C annealed l a Si mu]tilaycr. Rcasomib]c agreemcnl ~ i l h the cxperimcnial data can bc obtained assuming a linear Iransition ;'~idth ,'M ().7 nm between the pure silicon and the l'aSi~ layers I:or A/ 1.3 nm a strong Dcbyc Waller like altcnuation o f the dilt'raction orders h 7,H.9 and 10 i~ observed.
R e a s o n a b l e a g r e e m e n t can be achieved lbr At = 0 . 7 n m ( u p p e r panel) in c o n t r a s t to Al = 1.3 nm (lower panel), where the high o r d e r reflections h = 7 . 8 . 9 , and 10 in p a r t i c u l a r are strongly d a m p e d similar to a Debye W a l l e r a t t e n u a t i o n . At a given transition width AI. different interface profiles (e.~. cosine, gaussian or e r r o r function) did not lead to differences in the calculated spectra which were larger than the e x p e r i m e n t a l uncertaint>.
X-RAY ANALYSIS OF INTERDIFFUSION IN
Ta/Si
349
However, in the experimental spectrum, the subsidiary maxima between two Bragg peaks are smeared out and small shoulders (logarithmic scale!) at the high angle side of the main maxima are observable. This is due to small variations in the double-layer periodicity within the multilayer structure. Additionally, the wavy layer morphology observed by T E M attenuates the reflectivity of the multilayer and could be taken into account by a Debye-Waller factor 16. As can be seen in Fig. 1, the wavy layer morphology continuously develops with film thickness, resulting from slowly growing perturbations during deposition. Both kinds of sample imperfections, deviations from the exact lattice periodicity and lateral layer roughness, were neglected in the present analysis. It is not possible to include them in the calculation without considerably increasing the number of parameters characterizing the sample structure. In this case it is not possible to deconvolute the different contributions affecting the reflected intensity unambiguously in order to determine the "true" interface width. Consequently, the evaluated transition width At = 0.7nm assuming an unstrained and perfectly flat multilayer structure represents an upper limit value, since both deviations from the ideal lattice structure lead to an attenuation of the Bragg peaks. For theory of the analysis of the lateral interface roughness and interdiffusion, we refer to ref. 17. The sharp transition between the silicon and the TaSi x layers is stable up to 540 °C, where the diffraction pattern vanishes indicating the breakdown of the multilayer structure. The formation of a locally ordered TaSix solution even during multilayer deposition 7, thepresence of sharp interfaces within a few atomic layers and their temperature stability, is explained by the diffusion properties of the Ta/Si couple. Recent radiotracer measurements indicated a diffusion constant for tantalum in silicon of at most D = 8 × 10-19 m 2 s 1 or a maximum solubility of 2 × 10 ~8 m 3 at T = 1000 ~C 18. In contrast, Oppolzer et al. 19 estimated an approximate diffusion constant o f D ~ 5 × 10 13 m 2 s ~ at T = 900 °C for silicon diffusion in TaSi 2. From our experiments, we can estimate a lower limit ofD = 3.4 × 10-19 m 2 s - 1 for silicon diffusion in the tantalum rich silicide layers in the temperature regime between T = 300 °C and 370 °C. Correspondingly, the interdiffusion process mainly proceeds in one way leaving the silicon layers in their pure state and leading to a steep composition gradient at the interfaces. Our conclusion is supported by structural investigations by Lamble et al. 8 After annealing a multilayer structure of 10 layer pairs, each layer pair composed of 4.3 nm tungsten and 4.0 nm carbon, for several hours at 350 °C, Lamble et al. observed a general decrease in the scattered intensity indicating the formation of diffuse interfaces. Only Bragg maxima up to the 4th diffraction order could be observed. In agreement, EXAFS measurements on as-deposited samples did not show any formation of locally ordered carbon neighbours around the tungsten atoms. This is explained by the limited diffusion of carbon in tungsten. Using thicker tungsten layers (dw = 6.5 nm), only the local crystallization of b.c.c, tungsten was found. In contrast, for the Ta/Si system, only locally ordered silicon atoms could be detected around the tantalum atoms at any temperature 7.
350
H.L. MEYERHEIM, H. E. GOBEL
4. SUMMARY Using X R D we have shown that 20 period ( 4 n m / 7 n m ) Ta/Si multilayer structures prepared by d.c. sputter deposition exhibit sharp interfaces, where the transition width At between the a m o r p h o u s silicon a n d the h o m o g e n e o u s TaSi.,. layers is 0.7 n m and remains sharp up to temperatures of a b o u t 540 :C. This allows the o b s e r v a t i o n of Bragg peaks up to the 12th order. The a n n e a l i n g process induces a c o n t i n u o u s shift of the Si TaSi~ interfaces within the multilayer structure. The thickness of the pure silicon layers decreases linearly with increasing temperature. These results d e m o n s t r a t e that the Ta/Si multilayer structure represents a model system, where u n d e r the p r e p a r a t i o n c o n d i t i o n s one c o m p o n e n t ( t a n t a l u m ) is nearly i m m o b i l e and insoluble in the other (silicon), which is a prerequisite for o b t a i n i n g sharp interfaces after sputter deposition a n d d u r i n g a n n e a l i n g . The t e m p e r a t u r e stability of the interfaces r e c o m m e n d s such multilayers as favourable materials in cases where high interface quality has to be c o m b i n e d with t e m p e r a t u r e resistivity, e.g. in s y n c h r o t r o n radiation applications. However, the long-term a n n e a l i n g b e h a v i o u r of these multilayers is not k n o w n a n d needs further investigations. ACKNOWLEDGMENT The a u t h o r s would like to t h a n k W. R u d n i c k for the p r e p a r a t i o n of the samples, a n d C. F r u t h for providing the T E M p h o t o g r a p h s . REFERENCES l S.P. Murarka, Silieides.[br VLSI Applications. Academic Press, New York. 1983. 2 G. Rossi, Surf Sci. Rep., 7 (1987) I. 3 T.W. Barbee, Jr., in D. T. Attwood and B. Henke (cds.), Lou'-Ener~y X-Ray Diajznostics, American Institute of Physics Conf. Proc. No. 75, AIP, New York, 198I, p. 170. 4 M. Schuster, L. Mfiller, K. E. Mauser and R. Straub, Thin Solid Films, 157 (1988) 325. 5 E.E. Koch and G. Schmal (eds.), Proe. SPIE Con[i on Sq[? X-Ray Optics and Technology, Berlin, 1986, Vol. 733, SPIE, Bellingham,WA, 1986. 6 W. RuBwurm,H. von Philipsborn, H. E. G6bel and F. Neppl, Aeta Co'stallojzr. A, 40 (1984) C- 188, and references therein. 7 H.L. Meyerheim, B. Lengelerand H. E. G6bel, J. Appl. Phys., 68 (1990) 2694. 8 G.M. Lamble, S. M. Heald, D. Sayers, E. Ziegler and P. J. Viccaro, J. Appl. Phys., 65 (1989) 4250. 9 A.M. Saxena and P. M. Sch6nborn, Acta Crystallok{r. A, 33 (1977) 805. 10 W. Rudnick, personal communication, 1989. I 1 M. Born and E. Wolf, Principles of Optics, Pergamon, Oxford, 1987. 12 L.G. Parrat, Phys. Rev., 95 (1954) 359. 13 C. H. McGillarry, G. D. Rieck and K. Lonsdale (eds.), International Tables ./or X-Ray Co'stallography, Vol. IIl, Reide], Dordrecht, 1983. J. Leroux and T. P. Tinh, in Revised Tabh, s o[" X-Ray Attenuation Coefficients, Corporation ScientifiqueClaisse Inc., Quebec, 1977. 14 W.J. Veigele, At. Data Tables, 5 (1973) 51. 15 R.A. Simpson, IEEE Trans. Antennas Propa¢{., 24 (1976) 17. 16 T.W. Barbee, Opt. Eng., 25 (1986) 898. 17 D.G. Stearns, J. Appl. Phys., 65(1988)491. 18 W. Frank, Inst. Phys am MPI ffir Metallforschung. Stuttgart, personal communication, 1989. 19 H. Oppolzer, F. Neppl, K. Hieber and V. Huber, J. Vac. Sei. Technol. B, 2 (1984) 630.
PREPARATION AND CHARACTERIZATION
343
Q U A N T I T A T I V E X-RAY ANALYSIS OF I N T E R D I F F U S I N G Ta/Si MULTILAYER STRUCTURES H. L. MEYERHEIM* AND H. E. GOBEL Siemens A G, Corp, Research and Technology, Applied Materials Research, D-8000 Miinchen 83 ( F. R. G. ) (Received August 22, 1990; accepted October 25, 1990)
Multilayers of amorphous tantalum and silicon form a synthetic onedimensional lattice which can be observed by a sequence of intense low angle X-ray diffraction peaks. Annealing a Ta/Si multilayer structure of 20 layer pairs, each layer pair consisting of nominally 4 nm thick tantalum and 7 nm thick silicon layers, leads to rapid diffusion of silicon into the tantalum layers forming a homogeneous TaSi x solution. The thickness of the pure silicon layers decreases linearly with increasing annealing temperature. The width At of the transition layers between the silicon and the TaSix layers is analysed by comparing the measured absolute reflectivity of the multilayer structure with calculated data up to the 10th diffraction order. The computational approach is based on the Fresnel equations and assumes a linear composition profile at the interfaces leading to a trapezoidal lattice structure. An upper limit value of 0.7nm for the interface width is estimated. The interdiffusion is complete at T = 540°C, indicated by the breakdown of the multilayer structure.
I. INTRODUCTION
Silicides of refractory metals such as TaSi 2 have found widespread applications as materials for interconnects and gate electrodes in very-large-scale integration (VLSI) technology 1'2. One of the preparation methods used to form stoichiometric disilicide is the repeated sequential sputter deposition of the metal and silicon from pure elemental targets followed by subsequent annealing of the multilayer. In this context multilayers are well defined models for the study of interdiffusion, here applied to the process of silicide formation. Typical periods of such multilayers are between 5 and 15 nm and can be used as Bragg-reflectors for X-rays. As an example, multilayer structures are of interest for the construction of X-ray optical elements such as analyser crystals for soft X-rays *Present address: lnstitut ffir Kristallographie und Mineralogie der Universitfit Mfinchen, Theresienstrasse 41, D-8000 Miinchen2 (F.R.G.) 0040-6090/91/$3.50
~) ElsevierSequoia/Printedin The Netherlands
344
FI. 1.. MEYERttEIM, It. E. G{)I~E[
or m o n o c h r o m a t o r s in vacuum UV (VUV) storage ring beamlines -~ ~ Long-term stability against heat load is one of the most severe problems m this field. The structural characterization o1" multilayers, Focusing on temperature stability, is therefore an important problem for the development of optimized materials. On the basis o f X-ray diffraction ( X R D ) mcasurements, we investigate in this paper the behaviour o f T a / S i multilayers against heat treatment. Interest is concentrated on a quantitative analysis ot" the low angle Bragg maxima providing information about the difl'usion process and the stability oF the Ta Si interfaces. 2. EXPERIMI'INTALI)EfAII S The multilayers were deposited onto uncooled silicon wafers by d.c. magnetron sputtering from two pure elemental targets on an Electrotec MS 4200 system at deposition rates of about 0.5 nm s ~. The thickness o f the layer pairs was controlled by the scan speed of the carousel at an optimized sputter rate monitored by quartz thickness resonators. Figure 1 shows a cross-sectional transmission electron microscopy (TEM) p h o t o g r a p h of an as-deposited Ta/Si multilayer above a monocrystallinc silicon wafer. The bright and dark regions corrcspond to silicon and tantalum layers respectively. Within a resolution of about 1 nm, the T E M p h o t o g r a p h indicates abrupt Ta Si interfaces. Near the surface, an increasingly wavy layer m o r p h o l o g y is observed. The samples were annealed |\)r 10 rain in slightly reducing inert (Ar H_,)
Fig. I. Cross-sectional TEM pholograph taken ffon~till as-deposited TaSi muhilayer structure. Bright and dark regions correspond Io silicon and Ianlalurn layers rcspecti~cl_x.In the upper region, a v,:.txy layer morpholog~ is observable,
X-RAY ANALYSIS OF INTERDIFFUSION IN
Ta/Si
345
gas atmosphere at constant temperature and subsequently quenched to room temperature (RT). For additional in situ X R D measurements, the multilayers were heated in steps of 20 K and kept constant at a given temperature for 1 rain during data collection. It was proven quantitatively that both annealing procedures led to indentical diffusion paths. This demonstrates that the reaction rapidly reaches an equilibrium at a given temperature. Experimental details are described in ref. 6. 3. RESULTS AND DISCUSSION The short-range atomic ordering during the annealing process has been studied previously by TEM, extended X-ray absorption fine structure (EXAFS) and X R D measurements 7. It could be shown that during annealing of the samples, consisting of 20 layer pairs (7 nm silicon and 4 nm tantalum), silicon diffuses rapidly into the tantalum layers forming a homogeneous TaSi x solution, where the tantalum atoms are locally neighbored by silicon atoms at a distance of R = 0.256(5) nm. This is very close to the TaSi a bulk value o f R = 0.258 rim. Even after deposition no Ta Ta short-range order could be detected in the EXAFS spectra. This is in agreement with results reported by Lamble et al. s on with W/C multilayers. The authors report that the crystallization of tungsten needs a critical thickness (about 6 nm in their case) below which the metal layer is highly disordered. Furthermore, within about 0.2-0.3 nm resolution, T E M photographs did not show the presence of crystalline tantalum metal. Figure 2 shows a temperature-resolved low angle diffraction pattern of the Ta/Si multilayer structure which was measured in situ by Cu K~ radiation using a position-sensitive detector to record different diffraction orders h simultaneously ~. A lattice periodicity of d = 10.7(2) nm was determined from the positions of the high order diffraction peaks. This is in close agreement with the nominal value of d = 11 nm. Although the measured peak intensities are apparently distorted relative to each other, it is evident that the detected Bragg maxima between h = 3 and h = 12 exhibit strong intensity oscillations with temperature. For the data collection, the sample is rocked around the 6th diffraction order to excite all reflections in the low
"3" zJ
J
0 Fig. 2. Temperature-resolved diffraction pattern of a 20 period (4nm/7nm) Ta/Si multilayer structure in the low angle regime using Cu K s radiation. The diffraction orders h exhibit intensity oscillations with temperature. Above T ~ 540 C , the synthetic lattice structure disappears.
346
u, L. MEYERHEIM, H. E. G()BEL
angle regime. Therefore, the relative intensities cannot be compared quantitatively. Over a wide temperature range, neighbouring diffraction orders h and h + 1 are in opposite phase relation. All Bragg peaks vanish above about 540 'C, indicating the breakdown of the multilayer structure. The oscillatory intensity behaviour is explained by a shift of the Si TaSi~ interfaces induced by silicon diffusion into the tantalum layers (TaSi x layers at higher temperatures). Normal to the sample surface (z-axis) the multilayer structure is characterized by a periodically varying electron density p(z) (Fig. 3). Ta(TaSi~a.) and silicon layers correspond to regions of high and low electron density respectively. In a simple model, the multilayer can be regarded as a one-dimensional crystal, where the unit cell contains one silicon and one Ta(TaSi~a,) layer. Only two unit cells are shown. lattice profiles: rectangular
trapezoidal [.,-&t W
pTa - ~ d s i pSi ,,l 7nm
,,
:, ~ 3
'l 4nm
~ PTaSix-"~ pSi
T .i.
~I
D
g ~ ~.TaSiy-~
1.. [
F i
i
i
O. 27
,63 45 z0
1
Fig. 3. Schematic representation ofa Ta Si multilayer structure. Tantalum and silicon layers correspond to regions of high and low electron density p(z) respectively. The multflayer can be regarded as a onedimensional crystal, where the unit cell contains one silicon and one Ta(TaSi,a.) layer. The thickness of the silicon la~,er dsi is related to the normalized coordinate zo of the Si Ta(TaSi~.,) interface within the unit cell Idsx = -,d, where d is the lanice periodicityL The second interface is located at -% 0. In the rectangular lattice model shown on the left. the width At of the interface between the silicon and Ta(TaSi,) layers is infinitcl,, narrow. In the trapezoidal lattice prolilc (upper panel, right side) u linear transition between the layers is assumed. With increasing temperature (steps 1,2,3) the Si Ta(TaSi,) interfaces shift to lower _%values. Some arbitrary values are indicated (see e.A'. ref. 7).
The thickness of the silicon layer (dsi) is related to the normalized coordinate :o of the Si Ta(TaSixa.) interface within the unit cell by dsi = zod, where dis the lattice periodicity. The second interface is located at c o = 0. It is assumed that with increasing temperature (steps 1,2,3), silicon atoms diffuse into the Ta(TaSi.~,y) layers, thereby continuously increasing the thickness of the Ta(TaSi~,a, ) layers at the expense of the pure silicon layers. This corresponds to the shift of the interface to lower z o values and the scattered intensity I varies by I ~ cos(2rthz) (ref. 9) as observed in Fig. 2. In the "rectangular profile model" shown on the left side of Fig. 3. no special assumptions about the interface structure are rnade and the width of the transition
X-RAY ANALYSIS OF INTERDIFFUSION IN
Ta/Si
347
zone between the silicon and the TaSi x layers is infinitely sharp (At = 0). Additionally, this model neglects the continuously decreasing electron density contrast within the multilayer resulting from the "dilution" of the TaSix layers by indiffusing silicon atoms. The detailed analysis of the experimental data 7, based only on the phase relations between neighbouring maxima and minima of the Bragg peaks, indicates the linear dependence of dsi on the annealing temperature as shown in Fig. 4. Reasonable agreement with cross-sectional TEM is obtained. From the deviation between the nominal (7 nm) and the measured silicon layer thickness at RT (sample after deposition without annealing), we conclude that the diffusion process already begins during deposition. This is supposed to be induced by the energy transferred to the sample during the sputter deposition, during which the sample is heated to 300 °C 1°.
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However, within this theoretical and experimental framework no information can be obtained about the width At of the interfacial regions between the silicon and the TaSi x layers. In order to analyse quantitatively the abruptness of the interfaces, X-ray measurements were performed on a modified powder diffractometer (Siemens D 500) equipped with a diffracted beam monochromator. The beampath (arm length and beam apertures) was optimized for low angle measurements in the range of total external reflection. A diffraction pattern using Cu Kct radiation is shown in the centre of Fig. 5 for a sample annealed at 370°C for 10 min. Starting from an angle of almost 20 -= 0 °, the total reflection curve and the sequence of ! 0 multilayer reflections is observed up to 20 ~ 8 °. The dynamical intensity range required for a quantitative interpretation has to cover about six orders of magnitude. The experimental intensity data were analysed by comparison with calculated reflection curves based on the Fresnel equations 11 for reflectivity at the boundary of
348
H . i . . MEYERHEIM, H. E. G/:)BEI.
two h o m o g e n e o u s media. W e a p p l i e d P a r r a t ' s recursion f o r m u l a 1-~ l\)r the specularly reflected intensity from a thin fihn system consisting o f ,%' media, The p a r a l n e t e r s D and ft. which characterize tile c o m p l e x refraction index, n = 1 - D - i f i , were e v a l u a t e d for silicon and t a n t a l u m oil the basis o f t a b u l a t e d d a t a i ,~.~ . F o r the TaSi., layers, we calculated a p w o x i m a t e p a r a m e t e r s by linear i n t e r p o l a t i o n between the d a t a for pure t a n t a l u m and s t o i c h i o m e t r i c TaSi 2 using dsi as the p a r a m e t e r describing the degree o f indiflusion. F o r ds~ = 7 nm. the t a n t a l u m layers exist in their pure form and ds~ = 0 nm c o r r e s p o n d s to c o m p l e t e reaction, where stoichiometric TaSi 2 has been formed. Tile incident r a d i a t i o n was a s s u m e d to be strictly parallel arid m o n o c h r o n l a t i c . In the u p p e r and lower panels o f Fig. 5 we show calculated diffraction p a t t e r n s allowing f'or a finite transition width At between the h o m o g e n e o u s T a S i , and the pure silicon layers. The profile within the t r a n s i t i o n zone has been a s s u m e d to be linear ~5 leading to a "'trapezoidal hittite profile" as shown in the u p p e r panel on the right side of Fig. 3. F o r the calculation, the t r a n s i t i o n zone is s u b d i v i d e d into then lametlae o f equal thickness dl, where 61
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2 0 ~ I:'ig. 5. Comparison between cxperin~cnlal (ccnlrc cur\ c) and simulated spectra ( upper and Io,a cr curves) for a ~70 C annealed l a Si mu]tilaycr. Rcasomib]c agreemcnl ~ i l h the cxperimcnial data can bc obtained assuming a linear Iransition ;'~idth ,'M ().7 nm between the pure silicon and the l'aSi~ layers I:or A/ 1.3 nm a strong Dcbyc Waller like altcnuation o f the dilt'raction orders h 7,H.9 and 10 i~ observed.
R e a s o n a b l e a g r e e m e n t can be achieved lbr At = 0 . 7 n m ( u p p e r panel) in c o n t r a s t to Al = 1.3 nm (lower panel), where the high o r d e r reflections h = 7 . 8 . 9 , and 10 in p a r t i c u l a r are strongly d a m p e d similar to a Debye W a l l e r a t t e n u a t i o n . At a given transition width AI. different interface profiles (e.~. cosine, gaussian or e r r o r function) did not lead to differences in the calculated spectra which were larger than the e x p e r i m e n t a l uncertaint>.
X-RAY ANALYSIS OF INTERDIFFUSION IN
Ta/Si
349
However, in the experimental spectrum, the subsidiary maxima between two Bragg peaks are smeared out and small shoulders (logarithmic scale!) at the high angle side of the main maxima are observable. This is due to small variations in the double-layer periodicity within the multilayer structure. Additionally, the wavy layer morphology observed by T E M attenuates the reflectivity of the multilayer and could be taken into account by a Debye-Waller factor 16. As can be seen in Fig. 1, the wavy layer morphology continuously develops with film thickness, resulting from slowly growing perturbations during deposition. Both kinds of sample imperfections, deviations from the exact lattice periodicity and lateral layer roughness, were neglected in the present analysis. It is not possible to include them in the calculation without considerably increasing the number of parameters characterizing the sample structure. In this case it is not possible to deconvolute the different contributions affecting the reflected intensity unambiguously in order to determine the "true" interface width. Consequently, the evaluated transition width At = 0.7nm assuming an unstrained and perfectly flat multilayer structure represents an upper limit value, since both deviations from the ideal lattice structure lead to an attenuation of the Bragg peaks. For theory of the analysis of the lateral interface roughness and interdiffusion, we refer to ref. 17. The sharp transition between the silicon and the TaSi x layers is stable up to 540 °C, where the diffraction pattern vanishes indicating the breakdown of the multilayer structure. The formation of a locally ordered TaSix solution even during multilayer deposition 7, thepresence of sharp interfaces within a few atomic layers and their temperature stability, is explained by the diffusion properties of the Ta/Si couple. Recent radiotracer measurements indicated a diffusion constant for tantalum in silicon of at most D = 8 × 10-19 m 2 s 1 or a maximum solubility of 2 × 10 ~8 m 3 at T = 1000 ~C 18. In contrast, Oppolzer et al. 19 estimated an approximate diffusion constant o f D ~ 5 × 10 13 m 2 s ~ at T = 900 °C for silicon diffusion in TaSi 2. From our experiments, we can estimate a lower limit ofD = 3.4 × 10-19 m 2 s - 1 for silicon diffusion in the tantalum rich silicide layers in the temperature regime between T = 300 °C and 370 °C. Correspondingly, the interdiffusion process mainly proceeds in one way leaving the silicon layers in their pure state and leading to a steep composition gradient at the interfaces. Our conclusion is supported by structural investigations by Lamble et al. 8 After annealing a multilayer structure of 10 layer pairs, each layer pair composed of 4.3 nm tungsten and 4.0 nm carbon, for several hours at 350 °C, Lamble et al. observed a general decrease in the scattered intensity indicating the formation of diffuse interfaces. Only Bragg maxima up to the 4th diffraction order could be observed. In agreement, EXAFS measurements on as-deposited samples did not show any formation of locally ordered carbon neighbours around the tungsten atoms. This is explained by the limited diffusion of carbon in tungsten. Using thicker tungsten layers (dw = 6.5 nm), only the local crystallization of b.c.c, tungsten was found. In contrast, for the Ta/Si system, only locally ordered silicon atoms could be detected around the tantalum atoms at any temperature 7.
350
H.L. MEYERHEIM, H. E. GOBEL
4. SUMMARY Using X R D we have shown that 20 period ( 4 n m / 7 n m ) Ta/Si multilayer structures prepared by d.c. sputter deposition exhibit sharp interfaces, where the transition width At between the a m o r p h o u s silicon a n d the h o m o g e n e o u s TaSi.,. layers is 0.7 n m and remains sharp up to temperatures of a b o u t 540 :C. This allows the o b s e r v a t i o n of Bragg peaks up to the 12th order. The a n n e a l i n g process induces a c o n t i n u o u s shift of the Si TaSi~ interfaces within the multilayer structure. The thickness of the pure silicon layers decreases linearly with increasing temperature. These results d e m o n s t r a t e that the Ta/Si multilayer structure represents a model system, where u n d e r the p r e p a r a t i o n c o n d i t i o n s one c o m p o n e n t ( t a n t a l u m ) is nearly i m m o b i l e and insoluble in the other (silicon), which is a prerequisite for o b t a i n i n g sharp interfaces after sputter deposition a n d d u r i n g a n n e a l i n g . The t e m p e r a t u r e stability of the interfaces r e c o m m e n d s such multilayers as favourable materials in cases where high interface quality has to be c o m b i n e d with t e m p e r a t u r e resistivity, e.g. in s y n c h r o t r o n radiation applications. However, the long-term a n n e a l i n g b e h a v i o u r of these multilayers is not k n o w n a n d needs further investigations. ACKNOWLEDGMENT The a u t h o r s would like to t h a n k W. R u d n i c k for the p r e p a r a t i o n of the samples, a n d C. F r u t h for providing the T E M p h o t o g r a p h s . REFERENCES l S.P. Murarka, Silieides.[br VLSI Applications. Academic Press, New York. 1983. 2 G. Rossi, Surf Sci. Rep., 7 (1987) I. 3 T.W. Barbee, Jr., in D. T. Attwood and B. Henke (cds.), Lou'-Ener~y X-Ray Diajznostics, American Institute of Physics Conf. Proc. No. 75, AIP, New York, 198I, p. 170. 4 M. Schuster, L. Mfiller, K. E. Mauser and R. Straub, Thin Solid Films, 157 (1988) 325. 5 E.E. Koch and G. Schmal (eds.), Proe. SPIE Con[i on Sq[? X-Ray Optics and Technology, Berlin, 1986, Vol. 733, SPIE, Bellingham,WA, 1986. 6 W. RuBwurm,H. von Philipsborn, H. E. G6bel and F. Neppl, Aeta Co'stallojzr. A, 40 (1984) C- 188, and references therein. 7 H.L. Meyerheim, B. Lengelerand H. E. G6bel, J. Appl. Phys., 68 (1990) 2694. 8 G.M. Lamble, S. M. Heald, D. Sayers, E. Ziegler and P. J. Viccaro, J. Appl. Phys., 65 (1989) 4250. 9 A.M. Saxena and P. M. Sch6nborn, Acta Crystallok{r. A, 33 (1977) 805. 10 W. Rudnick, personal communication, 1989. I 1 M. Born and E. Wolf, Principles of Optics, Pergamon, Oxford, 1987. 12 L.G. Parrat, Phys. Rev., 95 (1954) 359. 13 C. H. McGillarry, G. D. Rieck and K. Lonsdale (eds.), International Tables ./or X-Ray Co'stallography, Vol. IIl, Reide], Dordrecht, 1983. J. Leroux and T. P. Tinh, in Revised Tabh, s o[" X-Ray Attenuation Coefficients, Corporation ScientifiqueClaisse Inc., Quebec, 1977. 14 W.J. Veigele, At. Data Tables, 5 (1973) 51. 15 R.A. Simpson, IEEE Trans. Antennas Propa¢{., 24 (1976) 17. 16 T.W. Barbee, Opt. Eng., 25 (1986) 898. 17 D.G. Stearns, J. Appl. Phys., 65(1988)491. 18 W. Frank, Inst. Phys am MPI ffir Metallforschung. Stuttgart, personal communication, 1989. 19 H. Oppolzer, F. Neppl, K. Hieber and V. Huber, J. Vac. Sei. Technol. B, 2 (1984) 630.