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A TEM study of the structure in PtSi ultrathin layers obtained on Si(100) by Pt sputtering and annealing
Surface
150
A TEM study of the structure in PtSi ultrathin on Si( 100) by Pt sputtering and annealing P. Ruterana ‘, K. Salt b and P.-A. Buffat ” EC& Pofytechnique Ft?d&ale, 12M, CH-1015 Lmsanne, A Landis & Gvr Betriebs AG, Corp. R.&D. Received
1 October
1990; accepted
Gubetstrasse,
for publication
Science 251/252
(1991) 150-154 North-Holland
layers obtained
a Swttzerland CH-6301 Zug, Swirzerland
23 November
1990
A structural analysis of 3-150 nm thick PtSi layers on Si(100) has been carried out using HREM and diffraction on plan view and cross section samples. The films were prepared by magnetron sputtering of Pt onto clean Si surfaces and the silicide formed by sub-eutectic solid phase reaction in vacuum. It was found that, in these very thin layers, local epitaxy can occur on the Si substrate. The diffraction results suggest also the presence of an amorphous matrix in which PtSi crystallites are included. The interface with the Si substrate is found to be abrupt in the thinnest films and the silicide top is rich in oxygen. At thicknesses above 10 r.m. the epitaxial growth appears to break down and it is absent altogether in thicker films.
1. Introduction Platinum silicides with thickness I 10 nm are widely used as semi transparent Schottky electrodes in photodetectors as well as shallow contacts in integrated circuits due to their high temperature chemical stability and excellent electrical properties. PtSi makes high barrier Schottky contact to silicon, has low electrical resistivity and is one of these silicides which can be patterned by lift off processes [l]. As thin as 2 nm PtSi films have been used in IR detectors [2] made through the common wet-and-dry device technology. Obviously, the microscopic structure can strongly influence the electrical properties of these thin silicide layers. A large amount of work [3-51 has been performed on the structure of PtSi films of thicknesses between 0.2 and a few ten nm, formed by evaporation of Pt and annealing. It appears that in this thickness range, PtSi grains tend to grow epitaxially on Si(fO0) and Si(ll1) substrates. From the application point of view, films made by plasma processes like sputtering or ion beam assisted deposition have gained a steadily growing importance during the last years. Plasma deposit~O39-6028/91/$03.50
C 1991 - Eisevier Science
Publishers
tion takes place far out of equilibrium as compared to evaporation; the depositing metallic particles are more energetic and can react with the substrate [f&7]. Moreover, it is well known that the pinhole density in the deposited layers is very low which is an important advantage for technological application. The crystallographic growth of the silicide may depend on the structure of the interface between the deposited Pt and the Si substrate. This work was carried out in order to determine the layer structure and the crystallographic relationships at the interface with the Si(100) substrates.
2. Experimental
procedure
Prior to loading into the vacuum chamber. the Si(100) substrates (n-type 23 Q cm ) were chemically cleaned and etched in diluted HF for 60 s in order to remove the native oxide. The wafers were set on a rotating vertical carouse1 and heated from the back side with halogen lamps. After evacuating the chamber to 5 X lo-’ Torr. the substrate was heated to 165°C for - 30 min and kept at this temperature during the Pt sputter-deposition
B.V. (North-HoIland)
P. Ruterana et al. / Structural ana~sis of ultrathin PiSi layers on Si(liM)
151
as shown in the background of the diffraction pattern. The Fig. 1. (a) Chemically thinned sample, strong amorphous contribution, epitaxial spots are very weak (arrows). (b) Plane view bright field, low density of epitaxially grown grains, as shown by moire fringes. (c) Micrograph of [lo01 wedge from the same wafer, abrupt interface and composition change in the silicide film, see whiter contrast (arrow) towards the top of the silicide layer.
process. Prior to the deposition, a mild sputter cleaning in argon was carried out for 2 min. This cleaning does not seem to generate much damage in the substrate surface layer. The formation of PtSi was completed by annealing the samples at 550°C for 30 rnin in vacuum. Thereby a complete formation of PtSi c0ufd be obtained as described previously [S]. Observations were carried out in a high resolution microscope (Philips EM430ST) which has a point resolution of better than 0.20 nm. For sample preparation, we chose wedge cleavage 19,101 for cross section samples and chemical thinning for plan view analysis in order to have more faithful results. Indeed, these latter methods would not modify the structure of the samples [11,12].
After annealing (500°C. 20 min) aI1 of the layer would normally transform to PtSi [13]. Our observations of the thin films aimed at determining the epitaxial relationships with the Si substrates as well as the layer and interface structure. We find a complete tr~sformation to a silicide when examining samples prepared by ion milling. However, the structure was found t0 be less simple when we examined wedges, which agrees quite well with the results obtained on chemically thinned plan view samples and Auger spectroscopy analysis.
In chemically thinned samples, we find mostly a very strong amorphous contribution as shown by the diffraction pattern of fig. la. This agrees well with the low density of crystallites as shown by the moire fringes in fig. lb. Therefore, it seems that a large part of the film is in the amorphous state and that the crystallites show only the afready known epitaxial relationships (3) with the Si(100) substrate (fig. la) which are [110] and [123]. In this diffraction pattern one sees mostly the broad ring which comes from the amorphous
phase. The contribution of the crystallites to the diffraction is very weak as shown by the arrows. However, it is clear that there are no additional spots to those already seen by Ben Ghozlene et al, [3]. This means that, even in these sputtered layers, althougth the growth of silicide crystahites may be difficult, the appearing epitaxial relationships are similar to those obtained in evaporated films. This complicated structure is confirmed when we examine a wedge from the same wafer. The silicide film appears to be made of two layers (fig. lc), and no lattice fringes show up in this figure, but the interface between the silicide and the Si substrate is clearly delineated, Preliminary Auger spectroscopy results indicate that this contrast change is certainly due to a strong enrichment in oxygen towards the top of the silicide film (AES estimates of atomic concentration for oxygen are - 40% tawards the top of the silicide layer).
Independent of the sample preparation method, the crystalline contribution to diffraction patterns is very strong. There is still some reinforcement in the main spots indicating that some PtSi grains are in epitaxial relationships with the SiflOO) surface. Again, only the two main epitaxial relationships (31 seem to be represented. However, it appears that as the silicide thickness is increased, grains are more and more misoriented from the substrate as shown by the diffraction pattern of a 12 nm thick film (rings are almost continuous), which is slightly tiited from the [too] Si zone a,xis (fig. 2a). The film is also made of two layers, the one located at the top of Si appears to be entirely polycrystalline, with larger PtSi grains. The nature of the other layer is still under study, but it is a mixture of crystalline and amorphous material (fig. 2b). The interface with the Si substrate is more complex than in the thinner films (fig. 2b). This rather random growth is even clearer when thicker layers are deposited and regrown. Fig. 2c shows a diffraction pattern taken from a plane view sample. In a 150 nm thick silicide layer, there is no more epitaxial relationship between the film and the Si substrate. This fact may also be
P. Ruterana et al. / Structural analysis of ultrathin PtSi layers on Si(lO0)
accounted for in the conventional growth theory of mismatched lattice materials, which states the disruption of the epitaxy above some critical thickness.
153
4. Conclusion In these PtSi films obtained from Pt sputtering, it is shown that the already known epitaxial rela-
Fig. 2. (a) Diffraction pattern of 12 nm thick PtSi film, slightly tilted from the [loo] Si zone axis. misoriented crystallites (rings) appear together with epitaxial ones (arrows). (b) The silicide film is made of two layers (boundary is arrowed). the interface is complex and lattice fringes are found in both layers. (c) The epitaxial growth is disrupted in thicker films, there are no lower spots which would come from lattice planes parallel to those of the silicon substrate.
154
P. Ruierana et al. / Structural analysis
tionships [3] exist for very thin layers. Moreover, as it can be expected from the large misfits between Si and PtSi, the epitaxial growth breaks down for thicker silicide layers. In the thinnest layers, the interface to the Si substrate appears to be smooth and there seems to remain a large amount of amorphous material, whether this can be due to incomplete crystallization or reoxidation is still under study. The top layer of the silicide film contains a lot of oxygen. Acknowledgment The help of Nicolas Xanthopoulos of EPFLLMCH for AES measurements is gratefully acknowledged. References [l] M.P. Lepselter 195.
and SM. Sze, Bell Sys. Techn. .I. 47 (1968)
of ultrathin PtSi layers on Si(IOOj 121W.F. Kosonocky,
F.V. Shallercross, T.S. Villani and J.V. Groppe, IEEE Trans. Electron Dev. ED-32 (1985) 8. and A. Authier, J. Appl. 131 H. Ben Ghozlene, P. Beaufrtre Phys. 49 (1978) 3998. ]41 H. Ishiwara, K. Hikosaka and S. Furukawa, J. Appl. Phys. SO (1979) 5302. (51 A.N. Larsen, J. Chevallier and AS. Pedersen, Mater. Lett. 3 (1985) 242. 161J.R. Abelson, K.B. Kim, D. Mercier, CR. Helms. R. Sinclair and T.W. Sigmond. J. Appl. Phys. 63 (1987) 689. [71 P. Ruterana, P. Houdy and P. Boher, J. Appl. Phys. 68 (1990) 1033. PI K. Salt, Vacuum 38 (1988) 703. and F. Nagata, Jpn. J. Appl. Phys. 25 [91 H. Kakibayashi (1986) 1644. 1101 P. Ruterana and P.A Buffat. Inst. Phys. Conf. Ser. 100 (1989) 677. [Ill J.M. Philips, J.L. Batstone, J.C. Hensel. M. Cerullo and F.C. Untervald, Mater. Res. Bul. 4 (1989) 144. v-1 P. Ruterana, P. Houdy and J.P. Chevalier, J. Appl. Phys. 65 (1989) 3907. [13] R.M. Walser and R.W. Bent, Appl. Phys. Lett. 28 (1976) 624.
150
A TEM study of the structure in PtSi ultrathin on Si( 100) by Pt sputtering and annealing P. Ruterana ‘, K. Salt b and P.-A. Buffat ” EC& Pofytechnique Ft?d&ale, 12M, CH-1015 Lmsanne, A Landis & Gvr Betriebs AG, Corp. R.&D. Received
1 October
1990; accepted
Gubetstrasse,
for publication
Science 251/252
(1991) 150-154 North-Holland
layers obtained
a Swttzerland CH-6301 Zug, Swirzerland
23 November
1990
A structural analysis of 3-150 nm thick PtSi layers on Si(100) has been carried out using HREM and diffraction on plan view and cross section samples. The films were prepared by magnetron sputtering of Pt onto clean Si surfaces and the silicide formed by sub-eutectic solid phase reaction in vacuum. It was found that, in these very thin layers, local epitaxy can occur on the Si substrate. The diffraction results suggest also the presence of an amorphous matrix in which PtSi crystallites are included. The interface with the Si substrate is found to be abrupt in the thinnest films and the silicide top is rich in oxygen. At thicknesses above 10 r.m. the epitaxial growth appears to break down and it is absent altogether in thicker films.
1. Introduction Platinum silicides with thickness I 10 nm are widely used as semi transparent Schottky electrodes in photodetectors as well as shallow contacts in integrated circuits due to their high temperature chemical stability and excellent electrical properties. PtSi makes high barrier Schottky contact to silicon, has low electrical resistivity and is one of these silicides which can be patterned by lift off processes [l]. As thin as 2 nm PtSi films have been used in IR detectors [2] made through the common wet-and-dry device technology. Obviously, the microscopic structure can strongly influence the electrical properties of these thin silicide layers. A large amount of work [3-51 has been performed on the structure of PtSi films of thicknesses between 0.2 and a few ten nm, formed by evaporation of Pt and annealing. It appears that in this thickness range, PtSi grains tend to grow epitaxially on Si(fO0) and Si(ll1) substrates. From the application point of view, films made by plasma processes like sputtering or ion beam assisted deposition have gained a steadily growing importance during the last years. Plasma deposit~O39-6028/91/$03.50
C 1991 - Eisevier Science
Publishers
tion takes place far out of equilibrium as compared to evaporation; the depositing metallic particles are more energetic and can react with the substrate [f&7]. Moreover, it is well known that the pinhole density in the deposited layers is very low which is an important advantage for technological application. The crystallographic growth of the silicide may depend on the structure of the interface between the deposited Pt and the Si substrate. This work was carried out in order to determine the layer structure and the crystallographic relationships at the interface with the Si(100) substrates.
2. Experimental
procedure
Prior to loading into the vacuum chamber. the Si(100) substrates (n-type 23 Q cm ) were chemically cleaned and etched in diluted HF for 60 s in order to remove the native oxide. The wafers were set on a rotating vertical carouse1 and heated from the back side with halogen lamps. After evacuating the chamber to 5 X lo-’ Torr. the substrate was heated to 165°C for - 30 min and kept at this temperature during the Pt sputter-deposition
B.V. (North-HoIland)
P. Ruterana et al. / Structural ana~sis of ultrathin PiSi layers on Si(liM)
151
as shown in the background of the diffraction pattern. The Fig. 1. (a) Chemically thinned sample, strong amorphous contribution, epitaxial spots are very weak (arrows). (b) Plane view bright field, low density of epitaxially grown grains, as shown by moire fringes. (c) Micrograph of [lo01 wedge from the same wafer, abrupt interface and composition change in the silicide film, see whiter contrast (arrow) towards the top of the silicide layer.
process. Prior to the deposition, a mild sputter cleaning in argon was carried out for 2 min. This cleaning does not seem to generate much damage in the substrate surface layer. The formation of PtSi was completed by annealing the samples at 550°C for 30 rnin in vacuum. Thereby a complete formation of PtSi c0ufd be obtained as described previously [S]. Observations were carried out in a high resolution microscope (Philips EM430ST) which has a point resolution of better than 0.20 nm. For sample preparation, we chose wedge cleavage 19,101 for cross section samples and chemical thinning for plan view analysis in order to have more faithful results. Indeed, these latter methods would not modify the structure of the samples [11,12].
After annealing (500°C. 20 min) aI1 of the layer would normally transform to PtSi [13]. Our observations of the thin films aimed at determining the epitaxial relationships with the Si substrates as well as the layer and interface structure. We find a complete tr~sformation to a silicide when examining samples prepared by ion milling. However, the structure was found t0 be less simple when we examined wedges, which agrees quite well with the results obtained on chemically thinned plan view samples and Auger spectroscopy analysis.
In chemically thinned samples, we find mostly a very strong amorphous contribution as shown by the diffraction pattern of fig. la. This agrees well with the low density of crystallites as shown by the moire fringes in fig. lb. Therefore, it seems that a large part of the film is in the amorphous state and that the crystallites show only the afready known epitaxial relationships (3) with the Si(100) substrate (fig. la) which are [110] and [123]. In this diffraction pattern one sees mostly the broad ring which comes from the amorphous
phase. The contribution of the crystallites to the diffraction is very weak as shown by the arrows. However, it is clear that there are no additional spots to those already seen by Ben Ghozlene et al, [3]. This means that, even in these sputtered layers, althougth the growth of silicide crystahites may be difficult, the appearing epitaxial relationships are similar to those obtained in evaporated films. This complicated structure is confirmed when we examine a wedge from the same wafer. The silicide film appears to be made of two layers (fig. lc), and no lattice fringes show up in this figure, but the interface between the silicide and the Si substrate is clearly delineated, Preliminary Auger spectroscopy results indicate that this contrast change is certainly due to a strong enrichment in oxygen towards the top of the silicide film (AES estimates of atomic concentration for oxygen are - 40% tawards the top of the silicide layer).
Independent of the sample preparation method, the crystalline contribution to diffraction patterns is very strong. There is still some reinforcement in the main spots indicating that some PtSi grains are in epitaxial relationships with the SiflOO) surface. Again, only the two main epitaxial relationships (31 seem to be represented. However, it appears that as the silicide thickness is increased, grains are more and more misoriented from the substrate as shown by the diffraction pattern of a 12 nm thick film (rings are almost continuous), which is slightly tiited from the [too] Si zone a,xis (fig. 2a). The film is also made of two layers, the one located at the top of Si appears to be entirely polycrystalline, with larger PtSi grains. The nature of the other layer is still under study, but it is a mixture of crystalline and amorphous material (fig. 2b). The interface with the Si substrate is more complex than in the thinner films (fig. 2b). This rather random growth is even clearer when thicker layers are deposited and regrown. Fig. 2c shows a diffraction pattern taken from a plane view sample. In a 150 nm thick silicide layer, there is no more epitaxial relationship between the film and the Si substrate. This fact may also be
P. Ruterana et al. / Structural analysis of ultrathin PtSi layers on Si(lO0)
accounted for in the conventional growth theory of mismatched lattice materials, which states the disruption of the epitaxy above some critical thickness.
153
4. Conclusion In these PtSi films obtained from Pt sputtering, it is shown that the already known epitaxial rela-
Fig. 2. (a) Diffraction pattern of 12 nm thick PtSi film, slightly tilted from the [loo] Si zone axis. misoriented crystallites (rings) appear together with epitaxial ones (arrows). (b) The silicide film is made of two layers (boundary is arrowed). the interface is complex and lattice fringes are found in both layers. (c) The epitaxial growth is disrupted in thicker films, there are no lower spots which would come from lattice planes parallel to those of the silicon substrate.
154
P. Ruierana et al. / Structural analysis
tionships [3] exist for very thin layers. Moreover, as it can be expected from the large misfits between Si and PtSi, the epitaxial growth breaks down for thicker silicide layers. In the thinnest layers, the interface to the Si substrate appears to be smooth and there seems to remain a large amount of amorphous material, whether this can be due to incomplete crystallization or reoxidation is still under study. The top layer of the silicide film contains a lot of oxygen. Acknowledgment The help of Nicolas Xanthopoulos of EPFLLMCH for AES measurements is gratefully acknowledged. References [l] M.P. Lepselter 195.
and SM. Sze, Bell Sys. Techn. .I. 47 (1968)
of ultrathin PtSi layers on Si(IOOj 121W.F. Kosonocky,
F.V. Shallercross, T.S. Villani and J.V. Groppe, IEEE Trans. Electron Dev. ED-32 (1985) 8. and A. Authier, J. Appl. 131 H. Ben Ghozlene, P. Beaufrtre Phys. 49 (1978) 3998. ]41 H. Ishiwara, K. Hikosaka and S. Furukawa, J. Appl. Phys. SO (1979) 5302. (51 A.N. Larsen, J. Chevallier and AS. Pedersen, Mater. Lett. 3 (1985) 242. 161J.R. Abelson, K.B. Kim, D. Mercier, CR. Helms. R. Sinclair and T.W. Sigmond. J. Appl. Phys. 63 (1987) 689. [71 P. Ruterana, P. Houdy and P. Boher, J. Appl. Phys. 68 (1990) 1033. PI K. Salt, Vacuum 38 (1988) 703. and F. Nagata, Jpn. J. Appl. Phys. 25 [91 H. Kakibayashi (1986) 1644. 1101 P. Ruterana and P.A Buffat. Inst. Phys. Conf. Ser. 100 (1989) 677. [Ill J.M. Philips, J.L. Batstone, J.C. Hensel. M. Cerullo and F.C. Untervald, Mater. Res. Bul. 4 (1989) 144. v-1 P. Ruterana, P. Houdy and J.P. Chevalier, J. Appl. Phys. 65 (1989) 3907. [13] R.M. Walser and R.W. Bent, Appl. Phys. Lett. 28 (1976) 624.