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In-situ spectroscopic ellipsometry for monitoring the TiSi multilayers during growth and annealing
ELSEVIER
Thin Solid
Films 275 ( 1996) 4447
In-situ spectroscopic ellipsometry for monitoring the Ti-Si multilayers during growth and annealing S. Logothetidis,
I. Alexandrou, N. Vouroutzis
Aristotle University of Thessaloniki. Department of Physics, GR-54006 Thessaloniki, Greece
Abstract The TiSi, formation and properties depend strongly on the deposition and annealing process. We employ spectroscopic ellipsometry (SE) to monitor both processes as well as transmission electron microscopy (XTEM) to verify the above results. The Ti and Si layers were deposited by magnetron sputtering on ( 1OO)Si.The multilayer thickness is about 930 A and the Si/Ti ratio = 2.2, while the thickness of each Ti-Si bilayer = 120 A. After deposition of each Ti or Si layer and during annealing of Ti-Si multilayers up to 680 “C we obtain the SE spectra in the energy region 1S-5.5 eV. Their analysis and XTEM observations show that during deposition an intermixing of 20 A width at each Ti-Si interface occurred. By using SE we found that during annealing below 150 “C there is no drastic intermixing or reaction between Ti and Si. A fast interdiffusion of Ti and a-Si and their reaction is observed between 200 and 580 “C. A phase transition occurs at 580 “C from the amorphous TiSSi3 compound to the C49 TiSiz structure and above this temperature XTEM depicts that only structural modification within the interdiffused layers of TiSi is observed. Keywords:
Ellipsometry;Growth mechanism;Annealing; Multilayers
1. Introduction
mation of an amorphous TiSi compound TiS& phase formation at 590 “C.
TiSi, and related materials are widely used as contact films and as local interconnects in complementary metal-oxidesemiconductor (CMOS) device fabrication. Both pure Ti film and Ti-Si multilayers deposited on c-Si are commonly used for their conversion to TiSi, by thermal annealing. The formation of TiSi, through the reaction of Ti with c-Si has been studied from different aspects [ 1,2]. It has been found that after an amorphous silicide formation, a high-resistivity metastable TiSi, (C49 structure) always forms before a lowresistivity stable TiSi* (C54 structure), and is inevitable during conventional furnace annealing [3] as well as rapid thermal annealing [ 41. On the other hand, reactions upon rapid thermal [ 51 and furnace annealing [ 61 of TiSi multilayers have shown significant interdiffusion to occur and the formation of an amorphous Ti-Si alloy by an interfacial reaction. In this work, we report our studies on the deposition and annealing process of Ti-Si multilayers on silicon (c-Si) substrates using in-situ spectroscopic ellipsometry (SE) and transmission electron microscopy and focus mainly on the interdiffusion and final reaction between Ti and Si layers. The results show that interdiffusion between Si and Ti layers takes place even during the sputter deposition process and the for0040-6090/96/$15.00 SSD10040-6090(95)07016-8
Q 1996 Elsevier Science S.A. All rights reserved
2. Experimental
leads up to the C49
details
The Ti and Si layers were deposited on c-Si( 100) substrates in an Alcatel SCM 600 d.c. and r.f. magnetron sputtering system with base pressure 1 X 10e7. An in-situ ultra-fast spectroscopic ellipsometer (WISEL of JobinYvon) was mounted on the sputtering system in order to monitor the deposition of Ti and Si layers and the Ti-Si multilayer reactions during annealing. Alternating layers of elemental Ti (99.999%) and Si (99.999%) were deposited at room temperature (RT) onto ( 100) Si wafers by rotating the substrates directly under a d.c.-powered 6 inch diameter Ti and a r.f.-powered 6 inch diameter Si target, respectively. The substrates were cleaned using the standard chemical procedure before entered in the deposition chamber through a load-lock system. Additional dry etching by low-energy Ar ions is performed prior to deposition for removing the native oxide from the substrate surface. The Si to Ti layers thickness ratio for each bilayer was varied from 2 to 2.5, the number of bilayers from 8 to 13 with the total thickness of Ti-Si multilayers being about 1 000 A. However, for the sake of clarity, we present here only results for Si/Ti = 2.2 and Ti-Si multilayers deposited at RT and
S. Logothetidis et ul. /Thin Solid Films 275 (1996) 4447
annealed up to 680 “C. After each layer was deposited we obtained the Ti-Si multilayers/Si substrate dielectric function E( o) by SE in the energy region 1.5-5.5 eV. The annealing procedure was performed after deposition in the same system under high vacuum conditions in various steps. The first one was from RT to 200 “C and up to 600 “C, steps of 100 “C were performed. The last one was from 600 to 680 “C keeping the heating rate at about 6 “C min- ‘. During each annealing step in-situ ellipsometry data were obtained at a fixed energy and at the end of each step SE measurements of the film were obtained keeping the temperature constant. Finally, transmission electron microscopy in cross-sectional geometry (XTEM), under 120 KV were performed in the as-grown Ti-Si multilayers and those annealed in various temperatures.
45
Fig. 1. The deposition rate calculated from this figure for both films was found to be 4 A s -’ and 4.1 A s- ’ for a-Si and poly-Ti, respectively. These results were used in order to calculate the deposition time of the a-Si and poly-Ti layers to form the Ti-Si multilayers deposited on c-Si. In Fig. 3 we present the dielectric function of c-Si obtained after etching with very low-energy Ar ions in order to remove the native Si02 and to avoid the amorphization and roughening of Si wafer. In the same figure are also shown representative imaginary parts of the pseudodielectric function (E(W) ) of the first Ti layer/substrate and Ti-Si bilayers/ substrate measured just after deposition of Ti and Si layers. 50
,
, LO Thin films c
Poly-Ti
+304I I 30
3. Results and discussion SE is a non-destructive technique which measures the complex reflection ratio p = rp/rs at a given angle of incidence 0 and energy o, where r,, and r, are the Fresnel reflection coefficients for light polarized parallel (p) and perpendicular (s) to the plane of incidence. From each measurement of fi the dielectric function E(W) ( = E, f ieZ) of a bulk material in the two-phase (ambient/substrate) model is obtained by using the expression [ 71
( 1
1-p* 19+ l+P
E(W) =~,+i+=sin*
tan’ 8 sin*
8.
The dielectric function of a thick, about 1 Frn, amorphous silicon (a-Si) and a Ti film, about 60 nm, grown on c-Si substrates under the same conditions as described above are shown in Fig. 1. The latter film as found from XTEM observations exhibits a polycrystalline structure and thereafter will be referred to as poly-Ti. The dielectric function of this two films will be used later as the reference dielectric function in order to analyse the SE measurements obtained during growth and annealing of Ti-Si multilayers. Fig. 2 shows the thickness of an a-Si film grown by sequential layer versus deposition time and deposited under the same conditions as the a-Si layers in the Ti-Si multilayers. In the inset of Fig. 2 is also shown the corresponding thickness of a poly-Ti film deposited in a similar way. These results were found by analysing the dielectric function spectra obtained after deposition of each a-Si and poly-Ti layer with the effective medium approximation (EMA) [ 81. According to this a composite film has an effective dielectric function ( lerr) depending on the constituent volume fractions,h, given by , I, (y;;;~;f)f, c I
=0
(2)
where E, is the dielectric function of the ith component. For the poly-Ti and a-Si film we assumed that they consist of Ti and voids and a-Si and voids, respectively, and applied EMA, using as reference data those of a-Si and poly-Ti shown in
Photon
Fig. 1. The dielectric c-Si( 100)
function
energy
(eV)
of a-Si and poly-Ti
films deposited
on
7-77T-i Deposition
time
bed
Fig. 2. The thickness of the a-Si layers versus the deposition time calculated through the dielectric spectra of each layer and the EMA. Inset shows the same for the poly-Ti layers.
46
S. Logothetidis et al. /Thin Solid Films 275 (1996) 44-47
Note that (E) represents the combined dielectric response from the layer or the bilayer( s) and substrate. In particular, in Fig. 3 the (E) of the system Ti layer/substrate, one, two and eight bilayers/substrate are shown, respectively. The thickness of poly-Ti and a-Si layers was chosen to be about 38 and 80 A, respectively. The XTEM micrograph of the same as-grown sample as Fig. 3, consisting of eight sequential Ti-Si bilayers (with the last layer being Si) on c-Si substrate is depicted in Fig. 4. The Si layers (lighter areas in the micrograph) have a typical amorphous appearance while those of Ti are within the darker areas and are polycrystalline with columnar grains of about 20 A. Selected area electron diffraction (SAD) patterns taken from the same specimen shows the poly-Ti rings and an inner diffuse ring corresponding to amorphous silicon. The Ti crystallites have a preferred orientation to that of Si [ 2001 direction which is perpendicular to the substrate. From these patterns the Ti-Si bilayer periodicity is calculated to be 120 A, in agreement with the sum of the thickness of a dark and
I
Xii multilayers as-grown on (1OO)Si -$ixTi -+ + -B-
lx(Ti-Si) Zx(Ti-Si) 8x(Ti-Sil
J”
-40
w” -30
5 z 5 .u
-2oi
a sequential light area. However, the thickness of a lighter (a-Si) and a darker area is not coincident with the one calculated during deposition assuming sequential Ti and Si layers. Analysis of the (E) spectra shown in Fig. 3, assuming the first Ti layer has initially intermixed with the Si substrate, the sequentially deposited a-Si layer with the Ti layer and the next deposited Ti layer with the a-Si layer shows that a thin, about 20 A, mixed Ti-Si interfacial region is formed in the Ti/a-Si and Ti/c-Si interfaces. These results together with those from XTEM observations, that provide a mixed Ti-Si interfacial region of about 18 A, suggest that interdiffusion and probably a reaction at every TilSi interface started during the sputter process. We note here that the substrate temperature during deposition is less than 50 “C because of both the deposition of very thin layers and the sequential rotation of substrate from the d.c. to the r.f. target. Fig. 5 shows the pseudodielectric function of as-grown TiSi multilayers/Si sample and those obtained by sequential thermal annealing at several temperatures. The main feature from this figure is the modification that appears in the dielectric response of the material in the temperature range between 500 and 600 “C. This modification in (E) corresponds to a phase transition in the TiSi alloy system. We have found by in-situ ellipsometry measurements at a fixed energy during annealing this phase transition to occur in Ti-Si multilayers with a Si/Ti ratio close to 2.2 at about 590 “C [ 91 and denotes the formation of the metastable C49 TiSi, structure. We would like to note here that the material annealed at 600 “C and above is highly absorbing in the whole energy region 1S-5.5 eV and the measured dielectric function corresponds
.z D n-51 -10
I
1
I 2
’
I Phokn
’ eneriy
I
’ (eV)
I
multilayers
as-grown
0nSi
+-RT
35
annealedat
+
300°C
+
400%
30
-II-II-II-
+ f
5oooc 600°C
10 6
Fig. 3. The l2 of the dielectric function of c-Si substrate and a number of representative spectra obtained after deposition of poly-Ti and a-Si layers and Ti-Si multilayers.
-25 w .-s t; 520 .uL z ;15 0 10
5 10
I
I
2.0 Photon
Fig. 4. An XTEM micrograph revealing the as-grown Ti-Si multilayers on the ( 1OO)Si substrate. The Si layers (lighter) are amorphous whereas those of Ti, within the darker areas exhibit polycrystalline structure.
I
1
4.0 3.0 energy (eV)
I
I
5.0
’
Fig. 5. The l2 of the dielectric function of Ti-Si multilayers on ( 1OO)Si substrate obtained in the as-grown stage (RT) and at several other temperatures. A phase transition is observed between 500 and 600 “C. The arrow denotes the main absorption peak of a-Si and A and B the regions where the changes in Ti or a-Si layers mostly affects Go.
S. Logothetidis et al. /Thin Solid Films 275 (1996) 44-47
to that of bulk C49 TiS& structure, whereas below 580 “C the measured dielectric response corresponds to the last 3-7 TiSi bilayers, as will be discussed below. We can distinguish two energy regions in the spectra shown in Fig. 5. The energy region A up to 3 eV and the energy region B above 3 eV. The peak that appears in region B at about 3.7 eV and shifted to higher energy by raising the temperature corresponds to the a-Si absorption peak (see, for example, Fig. 1) . At this energy the penetration depth of light in a-Si is about 120 A whereas in poly-Ti about 130 A (in the whole energy range 1.5-5.5 eV). Thus, the information involved in ( E( o) ) in region B comes from a depth of about 20&300 A or from the last 2-3 Ti-Si bilayers. In region A the light penetrates in a-Si more than 1 000 A and about 130 A in poly-Ti, so that the information in (E(W) ) comes from a depth that corresponds to about 7 Ti-Si bilayers. Therefore, by following the reduction of the absolute value of (E?) at 3.7 eV we can conclude on the reduction of the a-Si in the last 2-3 bilayers. This reduction means either intermixing of a-Si with poly-Ti at every Ti/Si interface or the formation of a Ti-Si alloy. It is expected that intermixing induces smaller changes in E? value than the formation of Ti-Si compound. Thus, large changes in E* correspond to reaction of Ti with Si. Below 3 eV, in region A, the main information provided by the (E) spectra comes from Ti layers, however by raising the annealing temperature the changes that are observed in (Q) correspond to formation of a more absorbing material than a-Si. This again is explained mainly with the formation of a new Ti-Si alloy. Therefore, it is clear from both, A and B, energy regions in Fig. 5 that the larger modifications in the material occur during annealing from RT to 300 “C. Actually, small changes in (E) was found by in-situ ellipsometry measurements at a fixed energy during annealing below 150 “C [9]. Relatively small changes are observed in the Ti-Si multilayers in the next two annealing steps shown in Fig. 5. However, we can see that larger changes in Ti layers are observed between 400 and 500 “C than between 300 and 400 “C. Finally, small changes are observed between 600 and 675 “C, since the C49 TiSi, structure has already been formed and the transition to the C54 TiSi, phase takes place above 700 “C [ 91. Fig. 6 depicts the XTEM micrograph of the sample annealed at 500 “C. It is clear that a-Si layers are much thinner ( = 18 A) than in the as-grown multilayers (see Fig. 4)) the darker areas have became thicker and their polycrystalline structure, due to initially deposited poly-Ti, disappears. This indicates that interdiffusion has occurred and an amorphous Ti-Si alloys has formed. More precisely, from the SAD pattern taken from the same area and shown in the inset of Fig. 6, a diffuse ring with d = 2.2 A is appeared. This ring corresponds to the formation of the amorphous Ti-Si compound Ti,Si,. Moreover, by comparing the thickness of the as-grown multilayer with that in Fig. 6 and annealed at 500 “C a reduction of about 10% is observed. Results from XTEM
41
Fig. 6. XTEM micrograph showing the Ti-Si multilayers after annealing at 500 “C. Both Ti-Si and Si areas are amorphous, the a-Si layers are thinner than those in Fig. 4 and the total thickness of the multilayer is reduced by 10% in comparison with the as-grown. The SAD pattern shows a diffuse ring that corresponds to an amorphous Ti,Si, compound.
observations from the specimen annealed at 675 “C show that a further 10% reduction is observed. 4. Conclusions Ti and Si layers of 38 and 80 A thick, respectively, deposited by magnetron sputtering on c-Si substrates and their annealing after deposition were monitored with in-situ SE and studied by XTEM. It is found that during deposition interdiffusion and reaction take place in every Ti/Si interface and fast partial interdiffusion and reaction during annealing between 200 and 580°C. A phase transition of the amorphous Ti,Si, compound to the C49 TiSi, structure is observed at 580°C. XTEM observations and SE show that annealing from 600 to 680 “C causes small structural changes within the initial Ti-Si bilayers. Therefore, by means of XTEM verification we show that SE provides reliably the deposition rate and interdiffusion during deposition and the interdiffusion and formation of Ti-Si alloys and compounds during annealing. Acknowledgements The work was supported in part by the EC EPET-II HELLAS 333 project. References [ 11 R. Beyers and R. Sinclair, J. Appl. Phys., 57 ( 1985) 5240. [2] J.M.M. De Nijs and Van Silfhout. Appl. Surf Sk, 40 ( 1990) 333. [3] R.D. Thompson, H. Takai, P.A. Psaras and K.N. Tu. J. Appl. Phys., 61 (1987) 540. [4] L.A. Clevenger, J.M.E. Harper, C. Cabral, Jr., C. Nobilli. G. Ottaviani and R. Mann, J. Appl. Phys., 72 ( 1992) 4978. [5] R.D. Thompson, H. Takai. P.A. Prasas and K.N. Tu, J. Appl. Phys.. 61 (1987) 540. [ 61 L.A. Clevenger, J.M.E. Haarper, C. CabraJ, Jr., C. Nobilli, G. Otta Viani and R. Mann, J. Appl. Phys., 72 ( 1992) 4978. [ 71 R.M.A. Azzam and Bashara, EUipsomerry and Polarized Lighf. North Holland, Amsterdam, 1977. [ 81 D.E. Aspnes, Thin So/id Films. 89 (1982) 249, and references cited therein. [9] S. Logothetidis et al., unpublished work.
Thin Solid
Films 275 ( 1996) 4447
In-situ spectroscopic ellipsometry for monitoring the Ti-Si multilayers during growth and annealing S. Logothetidis,
I. Alexandrou, N. Vouroutzis
Aristotle University of Thessaloniki. Department of Physics, GR-54006 Thessaloniki, Greece
Abstract The TiSi, formation and properties depend strongly on the deposition and annealing process. We employ spectroscopic ellipsometry (SE) to monitor both processes as well as transmission electron microscopy (XTEM) to verify the above results. The Ti and Si layers were deposited by magnetron sputtering on ( 1OO)Si.The multilayer thickness is about 930 A and the Si/Ti ratio = 2.2, while the thickness of each Ti-Si bilayer = 120 A. After deposition of each Ti or Si layer and during annealing of Ti-Si multilayers up to 680 “C we obtain the SE spectra in the energy region 1S-5.5 eV. Their analysis and XTEM observations show that during deposition an intermixing of 20 A width at each Ti-Si interface occurred. By using SE we found that during annealing below 150 “C there is no drastic intermixing or reaction between Ti and Si. A fast interdiffusion of Ti and a-Si and their reaction is observed between 200 and 580 “C. A phase transition occurs at 580 “C from the amorphous TiSSi3 compound to the C49 TiSiz structure and above this temperature XTEM depicts that only structural modification within the interdiffused layers of TiSi is observed. Keywords:
Ellipsometry;Growth mechanism;Annealing; Multilayers
1. Introduction
mation of an amorphous TiSi compound TiS& phase formation at 590 “C.
TiSi, and related materials are widely used as contact films and as local interconnects in complementary metal-oxidesemiconductor (CMOS) device fabrication. Both pure Ti film and Ti-Si multilayers deposited on c-Si are commonly used for their conversion to TiSi, by thermal annealing. The formation of TiSi, through the reaction of Ti with c-Si has been studied from different aspects [ 1,2]. It has been found that after an amorphous silicide formation, a high-resistivity metastable TiSi, (C49 structure) always forms before a lowresistivity stable TiSi* (C54 structure), and is inevitable during conventional furnace annealing [3] as well as rapid thermal annealing [ 41. On the other hand, reactions upon rapid thermal [ 51 and furnace annealing [ 61 of TiSi multilayers have shown significant interdiffusion to occur and the formation of an amorphous Ti-Si alloy by an interfacial reaction. In this work, we report our studies on the deposition and annealing process of Ti-Si multilayers on silicon (c-Si) substrates using in-situ spectroscopic ellipsometry (SE) and transmission electron microscopy and focus mainly on the interdiffusion and final reaction between Ti and Si layers. The results show that interdiffusion between Si and Ti layers takes place even during the sputter deposition process and the for0040-6090/96/$15.00 SSD10040-6090(95)07016-8
Q 1996 Elsevier Science S.A. All rights reserved
2. Experimental
leads up to the C49
details
The Ti and Si layers were deposited on c-Si( 100) substrates in an Alcatel SCM 600 d.c. and r.f. magnetron sputtering system with base pressure 1 X 10e7. An in-situ ultra-fast spectroscopic ellipsometer (WISEL of JobinYvon) was mounted on the sputtering system in order to monitor the deposition of Ti and Si layers and the Ti-Si multilayer reactions during annealing. Alternating layers of elemental Ti (99.999%) and Si (99.999%) were deposited at room temperature (RT) onto ( 100) Si wafers by rotating the substrates directly under a d.c.-powered 6 inch diameter Ti and a r.f.-powered 6 inch diameter Si target, respectively. The substrates were cleaned using the standard chemical procedure before entered in the deposition chamber through a load-lock system. Additional dry etching by low-energy Ar ions is performed prior to deposition for removing the native oxide from the substrate surface. The Si to Ti layers thickness ratio for each bilayer was varied from 2 to 2.5, the number of bilayers from 8 to 13 with the total thickness of Ti-Si multilayers being about 1 000 A. However, for the sake of clarity, we present here only results for Si/Ti = 2.2 and Ti-Si multilayers deposited at RT and
S. Logothetidis et ul. /Thin Solid Films 275 (1996) 4447
annealed up to 680 “C. After each layer was deposited we obtained the Ti-Si multilayers/Si substrate dielectric function E( o) by SE in the energy region 1.5-5.5 eV. The annealing procedure was performed after deposition in the same system under high vacuum conditions in various steps. The first one was from RT to 200 “C and up to 600 “C, steps of 100 “C were performed. The last one was from 600 to 680 “C keeping the heating rate at about 6 “C min- ‘. During each annealing step in-situ ellipsometry data were obtained at a fixed energy and at the end of each step SE measurements of the film were obtained keeping the temperature constant. Finally, transmission electron microscopy in cross-sectional geometry (XTEM), under 120 KV were performed in the as-grown Ti-Si multilayers and those annealed in various temperatures.
45
Fig. 1. The deposition rate calculated from this figure for both films was found to be 4 A s -’ and 4.1 A s- ’ for a-Si and poly-Ti, respectively. These results were used in order to calculate the deposition time of the a-Si and poly-Ti layers to form the Ti-Si multilayers deposited on c-Si. In Fig. 3 we present the dielectric function of c-Si obtained after etching with very low-energy Ar ions in order to remove the native Si02 and to avoid the amorphization and roughening of Si wafer. In the same figure are also shown representative imaginary parts of the pseudodielectric function (E(W) ) of the first Ti layer/substrate and Ti-Si bilayers/ substrate measured just after deposition of Ti and Si layers. 50
,
, LO Thin films c
Poly-Ti
+304I I 30
3. Results and discussion SE is a non-destructive technique which measures the complex reflection ratio p = rp/rs at a given angle of incidence 0 and energy o, where r,, and r, are the Fresnel reflection coefficients for light polarized parallel (p) and perpendicular (s) to the plane of incidence. From each measurement of fi the dielectric function E(W) ( = E, f ieZ) of a bulk material in the two-phase (ambient/substrate) model is obtained by using the expression [ 71
( 1
1-p* 19+ l+P
E(W) =~,+i+=sin*
tan’ 8 sin*
8.
The dielectric function of a thick, about 1 Frn, amorphous silicon (a-Si) and a Ti film, about 60 nm, grown on c-Si substrates under the same conditions as described above are shown in Fig. 1. The latter film as found from XTEM observations exhibits a polycrystalline structure and thereafter will be referred to as poly-Ti. The dielectric function of this two films will be used later as the reference dielectric function in order to analyse the SE measurements obtained during growth and annealing of Ti-Si multilayers. Fig. 2 shows the thickness of an a-Si film grown by sequential layer versus deposition time and deposited under the same conditions as the a-Si layers in the Ti-Si multilayers. In the inset of Fig. 2 is also shown the corresponding thickness of a poly-Ti film deposited in a similar way. These results were found by analysing the dielectric function spectra obtained after deposition of each a-Si and poly-Ti layer with the effective medium approximation (EMA) [ 81. According to this a composite film has an effective dielectric function ( lerr) depending on the constituent volume fractions,h, given by , I, (y;;;~;f)f, c I
=0
(2)
where E, is the dielectric function of the ith component. For the poly-Ti and a-Si film we assumed that they consist of Ti and voids and a-Si and voids, respectively, and applied EMA, using as reference data those of a-Si and poly-Ti shown in
Photon
Fig. 1. The dielectric c-Si( 100)
function
energy
(eV)
of a-Si and poly-Ti
films deposited
on
7-77T-i Deposition
time
bed
Fig. 2. The thickness of the a-Si layers versus the deposition time calculated through the dielectric spectra of each layer and the EMA. Inset shows the same for the poly-Ti layers.
46
S. Logothetidis et al. /Thin Solid Films 275 (1996) 44-47
Note that (E) represents the combined dielectric response from the layer or the bilayer( s) and substrate. In particular, in Fig. 3 the (E) of the system Ti layer/substrate, one, two and eight bilayers/substrate are shown, respectively. The thickness of poly-Ti and a-Si layers was chosen to be about 38 and 80 A, respectively. The XTEM micrograph of the same as-grown sample as Fig. 3, consisting of eight sequential Ti-Si bilayers (with the last layer being Si) on c-Si substrate is depicted in Fig. 4. The Si layers (lighter areas in the micrograph) have a typical amorphous appearance while those of Ti are within the darker areas and are polycrystalline with columnar grains of about 20 A. Selected area electron diffraction (SAD) patterns taken from the same specimen shows the poly-Ti rings and an inner diffuse ring corresponding to amorphous silicon. The Ti crystallites have a preferred orientation to that of Si [ 2001 direction which is perpendicular to the substrate. From these patterns the Ti-Si bilayer periodicity is calculated to be 120 A, in agreement with the sum of the thickness of a dark and
I
Xii multilayers as-grown on (1OO)Si -$ixTi -+ + -B-
lx(Ti-Si) Zx(Ti-Si) 8x(Ti-Sil
J”
-40
w” -30
5 z 5 .u
-2oi
a sequential light area. However, the thickness of a lighter (a-Si) and a darker area is not coincident with the one calculated during deposition assuming sequential Ti and Si layers. Analysis of the (E) spectra shown in Fig. 3, assuming the first Ti layer has initially intermixed with the Si substrate, the sequentially deposited a-Si layer with the Ti layer and the next deposited Ti layer with the a-Si layer shows that a thin, about 20 A, mixed Ti-Si interfacial region is formed in the Ti/a-Si and Ti/c-Si interfaces. These results together with those from XTEM observations, that provide a mixed Ti-Si interfacial region of about 18 A, suggest that interdiffusion and probably a reaction at every TilSi interface started during the sputter process. We note here that the substrate temperature during deposition is less than 50 “C because of both the deposition of very thin layers and the sequential rotation of substrate from the d.c. to the r.f. target. Fig. 5 shows the pseudodielectric function of as-grown TiSi multilayers/Si sample and those obtained by sequential thermal annealing at several temperatures. The main feature from this figure is the modification that appears in the dielectric response of the material in the temperature range between 500 and 600 “C. This modification in (E) corresponds to a phase transition in the TiSi alloy system. We have found by in-situ ellipsometry measurements at a fixed energy during annealing this phase transition to occur in Ti-Si multilayers with a Si/Ti ratio close to 2.2 at about 590 “C [ 91 and denotes the formation of the metastable C49 TiSi, structure. We would like to note here that the material annealed at 600 “C and above is highly absorbing in the whole energy region 1S-5.5 eV and the measured dielectric function corresponds
.z D n-51 -10
I
1
I 2
’
I Phokn
’ eneriy
I
’ (eV)
I
multilayers
as-grown
0nSi
+-RT
35
annealedat
+
300°C
+
400%
30
-II-II-II-
+ f
5oooc 600°C
10 6
Fig. 3. The l2 of the dielectric function of c-Si substrate and a number of representative spectra obtained after deposition of poly-Ti and a-Si layers and Ti-Si multilayers.
-25 w .-s t; 520 .uL z ;15 0 10
5 10
I
I
2.0 Photon
Fig. 4. An XTEM micrograph revealing the as-grown Ti-Si multilayers on the ( 1OO)Si substrate. The Si layers (lighter) are amorphous whereas those of Ti, within the darker areas exhibit polycrystalline structure.
I
1
4.0 3.0 energy (eV)
I
I
5.0
’
Fig. 5. The l2 of the dielectric function of Ti-Si multilayers on ( 1OO)Si substrate obtained in the as-grown stage (RT) and at several other temperatures. A phase transition is observed between 500 and 600 “C. The arrow denotes the main absorption peak of a-Si and A and B the regions where the changes in Ti or a-Si layers mostly affects Go.
S. Logothetidis et al. /Thin Solid Films 275 (1996) 44-47
to that of bulk C49 TiS& structure, whereas below 580 “C the measured dielectric response corresponds to the last 3-7 TiSi bilayers, as will be discussed below. We can distinguish two energy regions in the spectra shown in Fig. 5. The energy region A up to 3 eV and the energy region B above 3 eV. The peak that appears in region B at about 3.7 eV and shifted to higher energy by raising the temperature corresponds to the a-Si absorption peak (see, for example, Fig. 1) . At this energy the penetration depth of light in a-Si is about 120 A whereas in poly-Ti about 130 A (in the whole energy range 1.5-5.5 eV). Thus, the information involved in ( E( o) ) in region B comes from a depth of about 20&300 A or from the last 2-3 Ti-Si bilayers. In region A the light penetrates in a-Si more than 1 000 A and about 130 A in poly-Ti, so that the information in (E(W) ) comes from a depth that corresponds to about 7 Ti-Si bilayers. Therefore, by following the reduction of the absolute value of (E?) at 3.7 eV we can conclude on the reduction of the a-Si in the last 2-3 bilayers. This reduction means either intermixing of a-Si with poly-Ti at every Ti/Si interface or the formation of a Ti-Si alloy. It is expected that intermixing induces smaller changes in E? value than the formation of Ti-Si compound. Thus, large changes in E* correspond to reaction of Ti with Si. Below 3 eV, in region A, the main information provided by the (E) spectra comes from Ti layers, however by raising the annealing temperature the changes that are observed in (Q) correspond to formation of a more absorbing material than a-Si. This again is explained mainly with the formation of a new Ti-Si alloy. Therefore, it is clear from both, A and B, energy regions in Fig. 5 that the larger modifications in the material occur during annealing from RT to 300 “C. Actually, small changes in (E) was found by in-situ ellipsometry measurements at a fixed energy during annealing below 150 “C [9]. Relatively small changes are observed in the Ti-Si multilayers in the next two annealing steps shown in Fig. 5. However, we can see that larger changes in Ti layers are observed between 400 and 500 “C than between 300 and 400 “C. Finally, small changes are observed between 600 and 675 “C, since the C49 TiSi, structure has already been formed and the transition to the C54 TiSi, phase takes place above 700 “C [ 91. Fig. 6 depicts the XTEM micrograph of the sample annealed at 500 “C. It is clear that a-Si layers are much thinner ( = 18 A) than in the as-grown multilayers (see Fig. 4)) the darker areas have became thicker and their polycrystalline structure, due to initially deposited poly-Ti, disappears. This indicates that interdiffusion has occurred and an amorphous Ti-Si alloys has formed. More precisely, from the SAD pattern taken from the same area and shown in the inset of Fig. 6, a diffuse ring with d = 2.2 A is appeared. This ring corresponds to the formation of the amorphous Ti-Si compound Ti,Si,. Moreover, by comparing the thickness of the as-grown multilayer with that in Fig. 6 and annealed at 500 “C a reduction of about 10% is observed. Results from XTEM
41
Fig. 6. XTEM micrograph showing the Ti-Si multilayers after annealing at 500 “C. Both Ti-Si and Si areas are amorphous, the a-Si layers are thinner than those in Fig. 4 and the total thickness of the multilayer is reduced by 10% in comparison with the as-grown. The SAD pattern shows a diffuse ring that corresponds to an amorphous Ti,Si, compound.
observations from the specimen annealed at 675 “C show that a further 10% reduction is observed. 4. Conclusions Ti and Si layers of 38 and 80 A thick, respectively, deposited by magnetron sputtering on c-Si substrates and their annealing after deposition were monitored with in-situ SE and studied by XTEM. It is found that during deposition interdiffusion and reaction take place in every Ti/Si interface and fast partial interdiffusion and reaction during annealing between 200 and 580°C. A phase transition of the amorphous Ti,Si, compound to the C49 TiSi, structure is observed at 580°C. XTEM observations and SE show that annealing from 600 to 680 “C causes small structural changes within the initial Ti-Si bilayers. Therefore, by means of XTEM verification we show that SE provides reliably the deposition rate and interdiffusion during deposition and the interdiffusion and formation of Ti-Si alloys and compounds during annealing. Acknowledgements The work was supported in part by the EC EPET-II HELLAS 333 project. References [ 11 R. Beyers and R. Sinclair, J. Appl. Phys., 57 ( 1985) 5240. [2] J.M.M. De Nijs and Van Silfhout. Appl. Surf Sk, 40 ( 1990) 333. [3] R.D. Thompson, H. Takai, P.A. Psaras and K.N. Tu. J. Appl. Phys., 61 (1987) 540. [4] L.A. Clevenger, J.M.E. Harper, C. Cabral, Jr., C. Nobilli. G. Ottaviani and R. Mann, J. Appl. Phys., 72 ( 1992) 4978. [5] R.D. Thompson, H. Takai. P.A. Prasas and K.N. Tu, J. Appl. Phys.. 61 (1987) 540. [ 61 L.A. Clevenger, J.M.E. Haarper, C. CabraJ, Jr., C. Nobilli, G. Otta Viani and R. Mann, J. Appl. Phys., 72 ( 1992) 4978. [ 71 R.M.A. Azzam and Bashara, EUipsomerry and Polarized Lighf. North Holland, Amsterdam, 1977. [ 81 D.E. Aspnes, Thin So/id Films. 89 (1982) 249, and references cited therein. [9] S. Logothetidis et al., unpublished work.