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Influence of the annealing atmosphere on the chemical and thermal stability of amorphous WSi2
Journal of the Less-Common
Metals, 145 (1988)
197
197 - 207
INFLUENCE OF THE ANNEALING ATMOSPHERE ON THE CHEMICAL AND THERMAL STABILITY OF AMORPHOUS WSiz * A. DENEUVILLE
and M. BENYAHYA
Laboratoire d%tudes des Proprietb Grenoble Cedex (France)
Electroniques
des Solides,
CNRS, B.P. 166X, 38042
M. BRUNEL Laboratoire
de Cristallographie,
CNRS, B.P. 166X, 38042
Grenoble
Ckdex (France)
B. CANUT Universite’ Claude Bernard, Lyon I, Department Physique 11 Novembre 1918, 69622 Villeurbanne Cedex (France)
des Matdriaux,
Boulevard
du
(Received May 31, 1988)
Summary Amorphous WSi, was formed by tungsten ion mixing of an a-Si:H/W/ a-Si:H (where a-Si:H is hydrogenated amorphous silicon) sandwich. Annealing above 350 “C under Hz gives crystalline WSi2. Annealing under NH3 between 700 and 950 “C decomposes the WSiz, first in the “surface” layer (TA = 700 and 800 “C!), then in the whole structure (TA = 850 “C), with an increasingly preferential attachment of nitrogen to silicon.
1. Introduction
The interest in amorphous metallic films is growing because of the large field of their possible applications (e.g. ref. 1). Here, we use amorphous WSi, as the starting phase for the preparation at “low” temperatures of crystalline WSiz [2]. Further, we attempt to cover it with a diffusion barrier of WN,. Crystalline thin films of WSi, can be used as metallic contacts on the source and the drain in silicon integrated circuits [ 31. Their formation at low temperature (900 “C or less) is desired to prevent or to limit the diffusion of doping atoms from the source and drain in WSiz or in the silicon underneath. The top contact on WSi, is usually aluminium, which exhibits strong interdiffusion and the formation of several WA1 compounds when it is annealed. Therefore thin films acting as diffusion barriers are needed between WSi?
*Paper presented at the Symposium on the Preparation and Properties of Metastahle Alloys at the E-MRS Spring Meeting, Strasbourg, May 31 - June 2, 1988. 0022-5088/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
198
and aluminium. Amorphous or crystalline WN, films can be used for this purpose [ 41. Amorphous WSi, can be deposited directly by co-evaporation or cosputtering. However, to take advantage of the opportunity of using selfaligned techniques, it is usually preferable to deposit the metal on the silicon and then to form the crystalline WSiz directly by thermal interdiffusion, or by ion mixing of amorphous WSiz which is then thermally crystallized [5]. We use here a bottom layer of amorphous silicon (a-Si) to increase the W-Si interdiffusion [6] and a top layer of a-Si to prevent the oxidation of the tungsten by minute traces of oxygen or H,O in the annealing ambient (oxygen prevents or delays the Si-W interd~fusion). Then amorphous WSi, is prepared by ion mixing of a-Si:H/W/a-Si:H (where a-Si:H is hydrogenated a-Si) sandwiches. These films are annealed under H, to study their thermal stability, and then tungsten-rich WSi, films are annealed under NH3 to examine the chemical stability of WSiz and the formation of WSiz covered by WN,. The variation in the composition of the film is checked by Rutherford backscattering, the formation of crystalline compounds by X-rays and that of amorphous SiN, by IR absorption.
2. Preparation
and experimental
apparatus
The elemental layers of silicon, tungsten and silicon were deposited by cathodic sputtering on crystalline silicon (&if (with a base vacuum of less than 10e6 Torr in a 20%H,-80%Ar mixture at about 4 X low3 Torr) in separate deposition runs. The first layer of silicon was deposited at 150 a min-’ and 250 “C with a thickness of 220 or 540 a. 110 or 220 a tungsten films were deposited at 45 a min and 200 “C on c-Si covered with the first silicon layer, and the second silicon layer was deposited on tungsten at 30 a min-’ and room temperature with a thickness of 100 - 150 A. The ion mixing was perfortied with tungsten ions to increase its efficiency and to avoid the incorporation of foreign atoms in the sandwich. We used 180 keV ions at a fluence of 1Oz6 atoms crnv2 with a current of 0.2 PA cm-*. The most efficient ion mixing is obtained when the gaussian distribution of the implanted ions is just centred at the interface between the materials to be mixed. Here, from the low projected range RP of tungsten in silicon (about 620 8) and still lower in tungsten (about 150 A), efficient mixing is expected only for very thin films of tungsten. With 150 A of encapsulating silicon, the optimum thickness of tungsten is 110 A. The deposition of the sandwich was performed on a c-Si slice of 2 in diameter which was cut into several identical samples which were then annealed at various temperatures in the same atmosphere. Two deposition runs were used: 540 A a-Si:H/llO a W/150 A a-Si:H for annealing under Hz; 220 A a-Si:H/220 A W/150 a a-Si:H for annealing under NH3 in an attempt to obtain easier formation of WN, on WSiz.
199
Annealing was performed in a classical Joule oven after ~p~ntation at 350, 450, 550, 650 (1 h in Hz), 700,800,850 and 950 (2 h in NH,) “C. The X-ray diffraction was performed at glancing incidence CY= 0.5” to increase the sensitivity. The IR spectrum was recorded with a Perkin-Elmer 683 double-beam spectrophotometer between 2.5 and 50 pm. The Rutherford backscattering measurements were carried out at the University of Lyon with helium ions at an incident energy of 2 MeV and a detection angle of 160”.
3. Results and ~te~~~tion 3.1. Annealing under Hz of the ion mixed sandwich The formation of crystalline WSiz by annealing under Hz has been described in detail elsewhere [7]. We recall here the main results. 3.1.1. Rutherford backscattering By tilting the sample by 60” with respect to the incident beam of energy 2 MeV, the effect of ion mixing is readily seen (Fig. 1). In the virgin sample (full line), the tungsten signal appears as a peak beginning at an energy below 1.84 MeV (so there is no tungsten on the surface); then there is a peak corresponding to the signal of the silicon in the a-Si:H encapsulating layer which begins at an energy of 1.14 MeV (corresponding to silicon on the surface) and then a step corresponding to the silicon in the bottom a-Si:H layer and in the c-Si substrate. After the tungsten ion mixing (dotted line), the peak corresponding to the tungsten signal decreases in height and widens. There is now tungsten on the surface (1.84 MeV) with a nearly constant concentration and then a tungsten tail. There is also a most obvious modification of the silicon
Fig. 1. Rutherford bsckscattering spectra at an incidence angle of 60” of the virgin (-) and impanted (- - - - ) sandwiches (2 MeV incident helium).
200
signal. Silicon is still on the surface, with a plateau corresponding to a compound of constant composition, then a smooth silicon signal increases up to the second silicon step. From the values for the tungsten and silicon plateaux our previous detailed analysis [7] shows that about 300 A of WSiz has been formed at the surface on a graded layer with tungsten decreasing towards 0% and silicon increasing towards 100% when the c-Si substrate is reached. Annealing up to 650 “C induces crystallization (see Section 3.1.2) with only little variation in the composition through the film [ 71. 3.1.2, X-rcry diffraction The intensities of the X-rays diffracted by the sandwich, in the virgin state, after tungsten ion mixing, and then after annealing for 1 h at various temperatures are shown in .Fig. 2. The virgin sandwich exhibits mainly the diffraction peaks of crystalline a-W around 8 = 20.12” and 29.12”. After tungsten ion mixing, the tungsten peaks disappear and there are only large structures around 0 = 21” and 13.5” indicating an amorphous compound, WSiz according to Rutherford backscattering. When the ion-mixed sandwich is annealed at increasing temperatures sharp peaks around 11.27”, 13.15”, 17.9”, 20.6”, 22.9” and 24.3” corresponding to the occurrence of hexagonal crystalline WSi, with a small gram size of less than 100 A mixed with amorphous WSi, appear for TA > 350 “C. The proportion of the crystalline phase and the size of the grains increase as the annealing temperature increases. At TA 2 550 “C the structure of the crystalline phase becomes tetragonal, with sharp peaks around 11.35*, 15.02”, 19.82”, 22.40”, 23.11” and 28.35”. According to the X-ray diffraction, the proportion of amorphous phase appears negligible at TA 2 550 “C.
Fig. 2. X-ray diffraction curves of the sandwich as prepared (curve 1), just after implantation (curve 2), and with 1 h of subsequent annealing under Hz at 350 “C {curve 3), 450 “C (curve 4) and 550 “C (curve 5).
201
3.2. Annealer under NH, of the ion-mixed ~ndwich By comparison with the previous sandwich, the increase in the tungsten thickness allows a better definition of the tungsten signal by Rutherford backscattering, but from its thickness, which is larger than the R, value of 180 keV tungsten in tungsten, the ion mixing is expected to be less efficient, leading to a-WSi,-a:W mixtures. The decrease in the thickness of the bottom silicon layer allows us to check its effect on the tungsten and silicon tail in part of the film near the silicon substrate. 3.2.1. X-ray dif~act~n The variation in the d~fra~tion intensity with the d~fra~tion angle after various treatments is given in Fig. 3. For the virgin sandwich, we obtain mainly as in Section 3.1 the diffraction peak of crystalline ct+W around 0 =
THETA
Fig. 3. X-ray diffraction curves of the sandwich as prepared, just after implantation (IMP), and with 2 h of subsequent annealing under NH3 at 700, 800, 850 and 960 “C.
202
20.12” and 29.12”. After the mixing (IMP), we see a large peak centred around 0 = 20.5”, a lower value than that obtained in Section 3.1. After annealing at 700 and 800 “C, we obtain a composite curve, with sharp diffraction peaks around 11.35”, 15.02”, 19.82”, 22.40”, 23.11” and 28.35” corresponding to tetragonal WSiz and a smooth structure around 19”. As will be seen in Section 3.2.2, there is a significant amount of nitrogen in the film at this temperature. The large structure may originate from silicon or tungsten nitrides. Silicon nitride gives very weak signals by X-ray diffraction centred around 12” when it is amorphous and around 11.5” and 13.2” in this 8 range when it is crystallized at high temperature. Tungsten nitrides have strong diffraction lines around 18.9” and then 21.9”. Therefore we assign this structure to amorphous tungsten nitride. After annealing at 850 and 950 “C, the sharp peaks of tetragonal WS& have completely disappeared. Large peaks appear around l&6”, 20.1” and 21.8” with a small wide structure around 29.1”. Here we assign the structures at 8 = 18.6” and 21.8” to amorphous or microcrystalline tungsten nitride and those at 0 = 20.1” and 29.1” to amorphous or microcrystalline tungsten (amorphous tungsten can be obtained by tungsten ion implantation at 180 keV of tungsten films thinner than about 200 A). 3.2.2. Rutherford backscattering The effect of ion mixing is seen in Fig. 4, where the samples are tilted by about 60’ with respect to the incident helium beam of energy 2 MeV. The signals from the virgin sample (full line), are similar to those found in Section 3.1. After the tungsten ion mixing (dotted line) the peak co~esponding to the tungsten widens with a decreasing height. There is now tungsten at the surface, with a nearly constant concentration, and then a tungsten tail. However, a quick comparison of Figs. 1 and 4 shows that the tungsten and
ENERGY
(MeV)
Fig. 4. Rutherford back~attering spectra at an incidence angle of 60” of the virgin and implanted (- - - -) sandwiches (2 MeV incident helium). ( -)
203
silicon plateaux remain respectively higher and lower than those in Section 3.1. From the heights of the tungsten and silicon plateaux we deduce an approximate W:Si atomic ratio of unity instead of 0.5 as in Section 3.1. Annealing in NH3 at T, > 700 “C results in incorporation of nitrogen in the film, For instance, Fig. 5 shows the Rutherford backscattering spectrum at normal incidence after annealing at 850 “C. A signal from nitrogen beginning around 0.63 MeV (surface of the film) appears on the silicon step from the silicon substrate. An enlargement of the nitrogen signal at various annealing temperatures is also shown. Nitrogen always appears at the film surface, down to similar energies for T, = 850 and 950 “C, which are lower than those for TA = 700 “C. The maximum values of the signal from nitrogen are similar for TA = 850 and 950 “C and these are higher than that for T, = 700 “C. There are also important variations in the tungsten and silicon profiles which are shown in Fig. 6. Here the Rutherford backscattering signals are recorded with a tilt angle of 60” just after the ion mixing, and then for ion mixing followed by thermal annealing under NH, at 700, 850 and 950 “C. In all cases tungsten remains at the film surface. After the annealing step at 700 “C, the tungsten peak decreases s~n~icantly while it widens with a diffusion-like low energy shape. After annealing at 850 and 950 “C, the evolution of the tungsten peak compared with that at TA = 700 “C keeps the same trend but is slower with an increasing appearance of a shoulder on the low energy side of the peak. In all cases there is also silicon at the film surface with two steps. After annealing at 700 “C the height of the first step decreases and this step has better definition while the second step is joined to the first at a significantly lower energy. After annealing at 850 and 950 “C, this effect becomes more and more marked.
06
10
08 ENERGY
12
WeW
Fig. 5. Rutherford backscattering spectra at normal incidence after ion mixing and 950 “C (- - - -). then annealing under NW3 at 700 “C (- - -), 850 “C ( -)
and
204
ENERGY W&V)
Fig. 6. Rutherford backscattering spectra at an incidence angle of 60” of the sandwich after ion mixing () and then annealing under NH3 at 700 “C (- - -), 850 “c (- - - -) and 950 “c (--).
As neither X-rays nor Rutherford backscattering can give information on SiN, formation, we have carried out an IR absorption analysis, which is very sensitive around 840 cm-l to the polar Si-N bond (Fig. 7).
IMP 2oy ,“.**..,*‘*.‘..*.‘. 600
900 WAVENUMBERCcm~‘1
1:
0
Fig. 7. JR transmission of the sandwich after implantation (IMP) and then annealing 2 h under NH3 at 700,850 and 950 “c.
205
The mean transmission of the film continuously increases from 20% after ion mixing to 56% after annealing at 950 “C. There is no new absorption band after ion mixing, and a band of increasing intensity forms around 840 cm-l as the annealing temperature of these layers increases. Unfortunately, as the films are obviously non-uniform from the Rutherford backscattering analysis, we have to remain with the rough transmission curves. If the oscillator strength of the Si-N bond remains constant we can nevertheless deduce from these curves the relative variation in the nitrogen concentration on the whole film. It increases from 1 at ‘700 “C to 2.37 at 850 “C and 3.15 at 950 “c. 4. Discussion The comp~i~n of the results obtained by annealing amorphous “WSi2” under Hz and NH3 allows us to distinguish between the thermal and the chemical effects of the annealing. Annealing under H, of amorphous WSiZ simply results in its crystallization when TA > 350 “C. There is only crystalline WSi, near the surface layer and then a mixture of WSi:, and silicon, with a higher and higher ratio of silicon to WSiZ as the silicon substrate is approached. The chemical interaction of nitrogen with WSi, may give tungsten nitride or/and silicon nitride. There is not much information on tungsten nitride. Two crystalline phases, “p-WN” and “T-WN”, have been reported [8]. &WN has its main diffraction lines around 18.9” and 21.9”. y-WN presents additional lines at 10.78*, 15.41”, 24.73”, 27.25” and 31.87”. The atomic composition of these compounds is still not clear [ 4,8]. From our parallel studies of direct nitridation of tungsten films [9] where we have successively found @-WN and y-WN diffraction at increasing temperature with a continuously increasing nitrogen content, it seems that they are formed by increasing insertion of nitrogen atoms in a tungsten lattice. This will explain also the variation in the positions of the d~fraction lines in the literature [lo]. Silicon nitride crystals are only obtained at very high temperature. They are usually amorphous. A continuous range of amorphous SiN, compounds exists, which give IR absorption bands around 845 cm-’ [ 11). We have seen in Section 3 that both WN, and a-Si:N, are formed, and we try here to see what their relative amounts are and where they appear in the film. The sandwich used for annealing under NH, contains a thicker tungsten film and a thinner a-Si:H bottom film. This results in an amorphous phase after tungsten ion mixing with a higher W:Si ratio, 1 instead of 0.5 according to Rutherford backscattering. X-ray diffraction clearly shows that the centre of the large peak obtained after ion mixing is moved toward lower 6’ values (20.5’ instead of 21’) by the admixture of the amorphous tungsten signal “hexagonal” WSiz signal centred around around 20.1” to the amorphous 21” [7].
206
The expected effect of this non-stoichiome~ic ratio is only to increase the temperature needed for complete formation of crystalline WSi2 (the fomation of WSiz by simple interdiffusion of tungsten and silicon begins at 550 “C [73). Annealing under NH3 at 700 “C gives, according to the X-ray results, amorphous WN, compounds, no crystalline or amorphous tungsten, and tebagonal WSiz resulting from the crystallization of amorphous WSi*, and from IR absorption an amorphous SiN, compound. From Rutherford backscattering both the silicon and the tungsten concentrations in the surface film decrease and for both elements there is a large increase in the indepth tails as the silicon substrate is approach~. This suggests as for annealing under Hz a thermal interd~fusion (greatly enhanced by the defects introduced by tungsten ion mixing) of tungsten and silicon, resulting in WSiz in the surface region and then a WSi,-Si mixture as the silicon substrate is approached, but with a decomposition of WSi2 by the formation of WN, and SiN, compounds at least in the “surface” layer and WSiz remaining at least in the graded layer to join the c-Si substrate. The lower values of both the silicon and the tungsten signals in the “surface” layer than those expected for WSiz support this picture. Annealing at 850 “C gives only WN, and tungsten signals from X-rays and SiN, from IR absorption. From Rutherford backscattering, the nitrogen is spread deeper in the films and its m~imum concentration is higher than at 700 “C. The tungsten tail goes deeper into the silicon, and the heights of the silicon and tungsten plateaux near the surface have small variations. This implies that WSiz has been completely decomposed so that nitrogen. has now reached the c-Si substrate edge. From the existence of tungsten, the nitrogen concentration is not sufficient to saturate both silicon and tungsten, and nitrogen is preferentially attached to silicon, mainly in the graded layer (little evolution of the surface layer). By comparison with the results of annealing at 700 “C the increase in the SiN concentration by a factor of 2.37, higher than that of the nitrogen area (2.07), supports this picture. Annealing at 950 “C gives the same WN,, tungsten and SiN, signals; nitrogen and tungsten are spread deeper in the film. The increase in the SiN bond concentration (3.15) is greater than that of the nitrogen area (2.26) with little evolution of the “surface” layer. This suggests as previously that nitrogen is more and more preferentially attached to the silicon as SW, with increasing x in the graded layer.
5. Conclusion As for annealing under Hz, annealing under NI-Is keeps a surface layer and then a graded layer structure as after the tungsten ion mixing of an a-Si:HfW/a-Si:H sandwich. However, WSiz, which is stable under H,, is decomposed under NH, for 600 “C < TA < 950 “C. Annealing under NH3 at 700 and 800 “C seems to decompose mainly the WSiz *‘surface” layer
207
(with nitrogen reaction with both the tungsten and silicon freed) above a mixed WSiz + Si graded layer. After annealing at higher temperatures (850 -950 "C), no WSiz remained, so nitrogen has reached the silicon substrate edge. There is in the graded layer a more and more preferential attachment of nitrogen to silicon, as SiN, with increasing x, as TA increases. Therefore annealing of amorphous “WS&” under NH3 cannot be used to form an atomic diffusion barrier of WN, on crystalline WSi2 even when starting with tungsten-rich WSi, films.
References 1 C. N. I. Wagner and W. L. Johnson (eds.), Proc. 5th Int. Conf. on Liquid and Amorphous Metals, in J. Non-Cryst. Solids, 61 - 62 (1984). 2 J. B. Canut, M. Kadri, J. Pivot and A. Deneuville, V. T. Nguyen and A. Golanski (eds.), Proc. European Material Research Symp., 198 7, Les Editions de Physique, Les Ulis. 3 B. L. Crowder and S. Zirinsky, IEEE J. Solid State Circuits, 14 (2) (1978) 291. 4 H. P. Kattelus, E. Kolawa, K. Affolter and M. A. Nicolet, J. Vat. Sci. Technot, A, 3 (6) (1986) 2246. 5 A. Deneuville, Ann. Chim. Fr., 2 (1986) 603. 6 M. Kadri, A. Deneuville and M. Brunel, Proc. Confi on Zon Plating and Allied Techniques, Brighton, 198 7, Conferences Exhibition Publication Consultants Ltd., Edinburgh, 1987, p. 366. 7 A. Deneuville, M. Kadri and M. Brunel, Proc. Eur. Workshop on Refractory Metals and their Silicides, Aussois, 1987, in Vide, Couches Minces, 42 (236) (1987) 37. 8 R. Kiessling and Y. H. Liu, Trans. AZME, (1951) 639. 9 A. Deneuville, M. Benyahya, M. Brunel and B. Canut, J. P. Nougier and D. Gasquet (eds.), Proc. European Solid State Devices Research Conf. 88, in J. Phys. (Paris), 49 (C4) (Suppl. 9) (1988) 499. 10 V. I. Knitrova and Z. G. Pinsker, Sou. Phys. Crystallogr., 6 (6) (1962) 712. 11 E. Bustarret, M. Bensouda, M. C. Habrard, J. C. BruyPre, S. Poulin and S. C. Gujrathi, Phys. Rev. B, 38, (1988).
Metals, 145 (1988)
197
197 - 207
INFLUENCE OF THE ANNEALING ATMOSPHERE ON THE CHEMICAL AND THERMAL STABILITY OF AMORPHOUS WSiz * A. DENEUVILLE
and M. BENYAHYA
Laboratoire d%tudes des Proprietb Grenoble Cedex (France)
Electroniques
des Solides,
CNRS, B.P. 166X, 38042
M. BRUNEL Laboratoire
de Cristallographie,
CNRS, B.P. 166X, 38042
Grenoble
Ckdex (France)
B. CANUT Universite’ Claude Bernard, Lyon I, Department Physique 11 Novembre 1918, 69622 Villeurbanne Cedex (France)
des Matdriaux,
Boulevard
du
(Received May 31, 1988)
Summary Amorphous WSi, was formed by tungsten ion mixing of an a-Si:H/W/ a-Si:H (where a-Si:H is hydrogenated amorphous silicon) sandwich. Annealing above 350 “C under Hz gives crystalline WSi2. Annealing under NH3 between 700 and 950 “C decomposes the WSiz, first in the “surface” layer (TA = 700 and 800 “C!), then in the whole structure (TA = 850 “C), with an increasingly preferential attachment of nitrogen to silicon.
1. Introduction
The interest in amorphous metallic films is growing because of the large field of their possible applications (e.g. ref. 1). Here, we use amorphous WSi, as the starting phase for the preparation at “low” temperatures of crystalline WSiz [2]. Further, we attempt to cover it with a diffusion barrier of WN,. Crystalline thin films of WSi, can be used as metallic contacts on the source and the drain in silicon integrated circuits [ 31. Their formation at low temperature (900 “C or less) is desired to prevent or to limit the diffusion of doping atoms from the source and drain in WSiz or in the silicon underneath. The top contact on WSi, is usually aluminium, which exhibits strong interdiffusion and the formation of several WA1 compounds when it is annealed. Therefore thin films acting as diffusion barriers are needed between WSi?
*Paper presented at the Symposium on the Preparation and Properties of Metastahle Alloys at the E-MRS Spring Meeting, Strasbourg, May 31 - June 2, 1988. 0022-5088/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
198
and aluminium. Amorphous or crystalline WN, films can be used for this purpose [ 41. Amorphous WSi, can be deposited directly by co-evaporation or cosputtering. However, to take advantage of the opportunity of using selfaligned techniques, it is usually preferable to deposit the metal on the silicon and then to form the crystalline WSiz directly by thermal interdiffusion, or by ion mixing of amorphous WSiz which is then thermally crystallized [5]. We use here a bottom layer of amorphous silicon (a-Si) to increase the W-Si interdiffusion [6] and a top layer of a-Si to prevent the oxidation of the tungsten by minute traces of oxygen or H,O in the annealing ambient (oxygen prevents or delays the Si-W interd~fusion). Then amorphous WSi, is prepared by ion mixing of a-Si:H/W/a-Si:H (where a-Si:H is hydrogenated a-Si) sandwiches. These films are annealed under H, to study their thermal stability, and then tungsten-rich WSi, films are annealed under NH3 to examine the chemical stability of WSiz and the formation of WSiz covered by WN,. The variation in the composition of the film is checked by Rutherford backscattering, the formation of crystalline compounds by X-rays and that of amorphous SiN, by IR absorption.
2. Preparation
and experimental
apparatus
The elemental layers of silicon, tungsten and silicon were deposited by cathodic sputtering on crystalline silicon (&if (with a base vacuum of less than 10e6 Torr in a 20%H,-80%Ar mixture at about 4 X low3 Torr) in separate deposition runs. The first layer of silicon was deposited at 150 a min-’ and 250 “C with a thickness of 220 or 540 a. 110 or 220 a tungsten films were deposited at 45 a min and 200 “C on c-Si covered with the first silicon layer, and the second silicon layer was deposited on tungsten at 30 a min-’ and room temperature with a thickness of 100 - 150 A. The ion mixing was perfortied with tungsten ions to increase its efficiency and to avoid the incorporation of foreign atoms in the sandwich. We used 180 keV ions at a fluence of 1Oz6 atoms crnv2 with a current of 0.2 PA cm-*. The most efficient ion mixing is obtained when the gaussian distribution of the implanted ions is just centred at the interface between the materials to be mixed. Here, from the low projected range RP of tungsten in silicon (about 620 8) and still lower in tungsten (about 150 A), efficient mixing is expected only for very thin films of tungsten. With 150 A of encapsulating silicon, the optimum thickness of tungsten is 110 A. The deposition of the sandwich was performed on a c-Si slice of 2 in diameter which was cut into several identical samples which were then annealed at various temperatures in the same atmosphere. Two deposition runs were used: 540 A a-Si:H/llO a W/150 A a-Si:H for annealing under Hz; 220 A a-Si:H/220 A W/150 a a-Si:H for annealing under NH3 in an attempt to obtain easier formation of WN, on WSiz.
199
Annealing was performed in a classical Joule oven after ~p~ntation at 350, 450, 550, 650 (1 h in Hz), 700,800,850 and 950 (2 h in NH,) “C. The X-ray diffraction was performed at glancing incidence CY= 0.5” to increase the sensitivity. The IR spectrum was recorded with a Perkin-Elmer 683 double-beam spectrophotometer between 2.5 and 50 pm. The Rutherford backscattering measurements were carried out at the University of Lyon with helium ions at an incident energy of 2 MeV and a detection angle of 160”.
3. Results and ~te~~~tion 3.1. Annealing under Hz of the ion mixed sandwich The formation of crystalline WSiz by annealing under Hz has been described in detail elsewhere [7]. We recall here the main results. 3.1.1. Rutherford backscattering By tilting the sample by 60” with respect to the incident beam of energy 2 MeV, the effect of ion mixing is readily seen (Fig. 1). In the virgin sample (full line), the tungsten signal appears as a peak beginning at an energy below 1.84 MeV (so there is no tungsten on the surface); then there is a peak corresponding to the signal of the silicon in the a-Si:H encapsulating layer which begins at an energy of 1.14 MeV (corresponding to silicon on the surface) and then a step corresponding to the silicon in the bottom a-Si:H layer and in the c-Si substrate. After the tungsten ion mixing (dotted line), the peak corresponding to the tungsten signal decreases in height and widens. There is now tungsten on the surface (1.84 MeV) with a nearly constant concentration and then a tungsten tail. There is also a most obvious modification of the silicon
Fig. 1. Rutherford bsckscattering spectra at an incidence angle of 60” of the virgin (-) and impanted (- - - - ) sandwiches (2 MeV incident helium).
200
signal. Silicon is still on the surface, with a plateau corresponding to a compound of constant composition, then a smooth silicon signal increases up to the second silicon step. From the values for the tungsten and silicon plateaux our previous detailed analysis [7] shows that about 300 A of WSiz has been formed at the surface on a graded layer with tungsten decreasing towards 0% and silicon increasing towards 100% when the c-Si substrate is reached. Annealing up to 650 “C induces crystallization (see Section 3.1.2) with only little variation in the composition through the film [ 71. 3.1.2, X-rcry diffraction The intensities of the X-rays diffracted by the sandwich, in the virgin state, after tungsten ion mixing, and then after annealing for 1 h at various temperatures are shown in .Fig. 2. The virgin sandwich exhibits mainly the diffraction peaks of crystalline a-W around 8 = 20.12” and 29.12”. After tungsten ion mixing, the tungsten peaks disappear and there are only large structures around 0 = 21” and 13.5” indicating an amorphous compound, WSiz according to Rutherford backscattering. When the ion-mixed sandwich is annealed at increasing temperatures sharp peaks around 11.27”, 13.15”, 17.9”, 20.6”, 22.9” and 24.3” corresponding to the occurrence of hexagonal crystalline WSi, with a small gram size of less than 100 A mixed with amorphous WSi, appear for TA > 350 “C. The proportion of the crystalline phase and the size of the grains increase as the annealing temperature increases. At TA 2 550 “C the structure of the crystalline phase becomes tetragonal, with sharp peaks around 11.35*, 15.02”, 19.82”, 22.40”, 23.11” and 28.35”. According to the X-ray diffraction, the proportion of amorphous phase appears negligible at TA 2 550 “C.
Fig. 2. X-ray diffraction curves of the sandwich as prepared (curve 1), just after implantation (curve 2), and with 1 h of subsequent annealing under Hz at 350 “C {curve 3), 450 “C (curve 4) and 550 “C (curve 5).
201
3.2. Annealer under NH, of the ion-mixed ~ndwich By comparison with the previous sandwich, the increase in the tungsten thickness allows a better definition of the tungsten signal by Rutherford backscattering, but from its thickness, which is larger than the R, value of 180 keV tungsten in tungsten, the ion mixing is expected to be less efficient, leading to a-WSi,-a:W mixtures. The decrease in the thickness of the bottom silicon layer allows us to check its effect on the tungsten and silicon tail in part of the film near the silicon substrate. 3.2.1. X-ray dif~act~n The variation in the d~fra~tion intensity with the d~fra~tion angle after various treatments is given in Fig. 3. For the virgin sandwich, we obtain mainly as in Section 3.1 the diffraction peak of crystalline ct+W around 0 =
THETA
Fig. 3. X-ray diffraction curves of the sandwich as prepared, just after implantation (IMP), and with 2 h of subsequent annealing under NH3 at 700, 800, 850 and 960 “C.
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20.12” and 29.12”. After the mixing (IMP), we see a large peak centred around 0 = 20.5”, a lower value than that obtained in Section 3.1. After annealing at 700 and 800 “C, we obtain a composite curve, with sharp diffraction peaks around 11.35”, 15.02”, 19.82”, 22.40”, 23.11” and 28.35” corresponding to tetragonal WSiz and a smooth structure around 19”. As will be seen in Section 3.2.2, there is a significant amount of nitrogen in the film at this temperature. The large structure may originate from silicon or tungsten nitrides. Silicon nitride gives very weak signals by X-ray diffraction centred around 12” when it is amorphous and around 11.5” and 13.2” in this 8 range when it is crystallized at high temperature. Tungsten nitrides have strong diffraction lines around 18.9” and then 21.9”. Therefore we assign this structure to amorphous tungsten nitride. After annealing at 850 and 950 “C, the sharp peaks of tetragonal WS& have completely disappeared. Large peaks appear around l&6”, 20.1” and 21.8” with a small wide structure around 29.1”. Here we assign the structures at 8 = 18.6” and 21.8” to amorphous or microcrystalline tungsten nitride and those at 0 = 20.1” and 29.1” to amorphous or microcrystalline tungsten (amorphous tungsten can be obtained by tungsten ion implantation at 180 keV of tungsten films thinner than about 200 A). 3.2.2. Rutherford backscattering The effect of ion mixing is seen in Fig. 4, where the samples are tilted by about 60’ with respect to the incident helium beam of energy 2 MeV. The signals from the virgin sample (full line), are similar to those found in Section 3.1. After the tungsten ion mixing (dotted line) the peak co~esponding to the tungsten widens with a decreasing height. There is now tungsten at the surface, with a nearly constant concentration, and then a tungsten tail. However, a quick comparison of Figs. 1 and 4 shows that the tungsten and
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Fig. 4. Rutherford back~attering spectra at an incidence angle of 60” of the virgin and implanted (- - - -) sandwiches (2 MeV incident helium). ( -)
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silicon plateaux remain respectively higher and lower than those in Section 3.1. From the heights of the tungsten and silicon plateaux we deduce an approximate W:Si atomic ratio of unity instead of 0.5 as in Section 3.1. Annealing in NH3 at T, > 700 “C results in incorporation of nitrogen in the film, For instance, Fig. 5 shows the Rutherford backscattering spectrum at normal incidence after annealing at 850 “C. A signal from nitrogen beginning around 0.63 MeV (surface of the film) appears on the silicon step from the silicon substrate. An enlargement of the nitrogen signal at various annealing temperatures is also shown. Nitrogen always appears at the film surface, down to similar energies for T, = 850 and 950 “C, which are lower than those for TA = 700 “C. The maximum values of the signal from nitrogen are similar for TA = 850 and 950 “C and these are higher than that for T, = 700 “C. There are also important variations in the tungsten and silicon profiles which are shown in Fig. 6. Here the Rutherford backscattering signals are recorded with a tilt angle of 60” just after the ion mixing, and then for ion mixing followed by thermal annealing under NH, at 700, 850 and 950 “C. In all cases tungsten remains at the film surface. After the annealing step at 700 “C, the tungsten peak decreases s~n~icantly while it widens with a diffusion-like low energy shape. After annealing at 850 and 950 “C, the evolution of the tungsten peak compared with that at TA = 700 “C keeps the same trend but is slower with an increasing appearance of a shoulder on the low energy side of the peak. In all cases there is also silicon at the film surface with two steps. After annealing at 700 “C the height of the first step decreases and this step has better definition while the second step is joined to the first at a significantly lower energy. After annealing at 850 and 950 “C, this effect becomes more and more marked.
06
10
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WeW
Fig. 5. Rutherford backscattering spectra at normal incidence after ion mixing and 950 “C (- - - -). then annealing under NW3 at 700 “C (- - -), 850 “C ( -)
and
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Fig. 6. Rutherford backscattering spectra at an incidence angle of 60” of the sandwich after ion mixing () and then annealing under NH3 at 700 “C (- - -), 850 “c (- - - -) and 950 “c (--).
As neither X-rays nor Rutherford backscattering can give information on SiN, formation, we have carried out an IR absorption analysis, which is very sensitive around 840 cm-l to the polar Si-N bond (Fig. 7).
IMP 2oy ,“.**..,*‘*.‘..*.‘. 600
900 WAVENUMBERCcm~‘1
1:
0
Fig. 7. JR transmission of the sandwich after implantation (IMP) and then annealing 2 h under NH3 at 700,850 and 950 “c.
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The mean transmission of the film continuously increases from 20% after ion mixing to 56% after annealing at 950 “C. There is no new absorption band after ion mixing, and a band of increasing intensity forms around 840 cm-l as the annealing temperature of these layers increases. Unfortunately, as the films are obviously non-uniform from the Rutherford backscattering analysis, we have to remain with the rough transmission curves. If the oscillator strength of the Si-N bond remains constant we can nevertheless deduce from these curves the relative variation in the nitrogen concentration on the whole film. It increases from 1 at ‘700 “C to 2.37 at 850 “C and 3.15 at 950 “c. 4. Discussion The comp~i~n of the results obtained by annealing amorphous “WSi2” under Hz and NH3 allows us to distinguish between the thermal and the chemical effects of the annealing. Annealing under H, of amorphous WSiZ simply results in its crystallization when TA > 350 “C. There is only crystalline WSi, near the surface layer and then a mixture of WSi:, and silicon, with a higher and higher ratio of silicon to WSiZ as the silicon substrate is approached. The chemical interaction of nitrogen with WSi, may give tungsten nitride or/and silicon nitride. There is not much information on tungsten nitride. Two crystalline phases, “p-WN” and “T-WN”, have been reported [8]. &WN has its main diffraction lines around 18.9” and 21.9”. y-WN presents additional lines at 10.78*, 15.41”, 24.73”, 27.25” and 31.87”. The atomic composition of these compounds is still not clear [ 4,8]. From our parallel studies of direct nitridation of tungsten films [9] where we have successively found @-WN and y-WN diffraction at increasing temperature with a continuously increasing nitrogen content, it seems that they are formed by increasing insertion of nitrogen atoms in a tungsten lattice. This will explain also the variation in the positions of the d~fraction lines in the literature [lo]. Silicon nitride crystals are only obtained at very high temperature. They are usually amorphous. A continuous range of amorphous SiN, compounds exists, which give IR absorption bands around 845 cm-’ [ 11). We have seen in Section 3 that both WN, and a-Si:N, are formed, and we try here to see what their relative amounts are and where they appear in the film. The sandwich used for annealing under NH, contains a thicker tungsten film and a thinner a-Si:H bottom film. This results in an amorphous phase after tungsten ion mixing with a higher W:Si ratio, 1 instead of 0.5 according to Rutherford backscattering. X-ray diffraction clearly shows that the centre of the large peak obtained after ion mixing is moved toward lower 6’ values (20.5’ instead of 21’) by the admixture of the amorphous tungsten signal “hexagonal” WSiz signal centred around around 20.1” to the amorphous 21” [7].
206
The expected effect of this non-stoichiome~ic ratio is only to increase the temperature needed for complete formation of crystalline WSi2 (the fomation of WSiz by simple interdiffusion of tungsten and silicon begins at 550 “C [73). Annealing under NH3 at 700 “C gives, according to the X-ray results, amorphous WN, compounds, no crystalline or amorphous tungsten, and tebagonal WSiz resulting from the crystallization of amorphous WSi*, and from IR absorption an amorphous SiN, compound. From Rutherford backscattering both the silicon and the tungsten concentrations in the surface film decrease and for both elements there is a large increase in the indepth tails as the silicon substrate is approach~. This suggests as for annealing under Hz a thermal interd~fusion (greatly enhanced by the defects introduced by tungsten ion mixing) of tungsten and silicon, resulting in WSiz in the surface region and then a WSi,-Si mixture as the silicon substrate is approached, but with a decomposition of WSi2 by the formation of WN, and SiN, compounds at least in the “surface” layer and WSiz remaining at least in the graded layer to join the c-Si substrate. The lower values of both the silicon and the tungsten signals in the “surface” layer than those expected for WSiz support this picture. Annealing at 850 “C gives only WN, and tungsten signals from X-rays and SiN, from IR absorption. From Rutherford backscattering, the nitrogen is spread deeper in the films and its m~imum concentration is higher than at 700 “C. The tungsten tail goes deeper into the silicon, and the heights of the silicon and tungsten plateaux near the surface have small variations. This implies that WSiz has been completely decomposed so that nitrogen. has now reached the c-Si substrate edge. From the existence of tungsten, the nitrogen concentration is not sufficient to saturate both silicon and tungsten, and nitrogen is preferentially attached to silicon, mainly in the graded layer (little evolution of the surface layer). By comparison with the results of annealing at 700 “C the increase in the SiN concentration by a factor of 2.37, higher than that of the nitrogen area (2.07), supports this picture. Annealing at 950 “C gives the same WN,, tungsten and SiN, signals; nitrogen and tungsten are spread deeper in the film. The increase in the SiN bond concentration (3.15) is greater than that of the nitrogen area (2.26) with little evolution of the “surface” layer. This suggests as previously that nitrogen is more and more preferentially attached to the silicon as SW, with increasing x in the graded layer.
5. Conclusion As for annealing under Hz, annealing under NI-Is keeps a surface layer and then a graded layer structure as after the tungsten ion mixing of an a-Si:HfW/a-Si:H sandwich. However, WSiz, which is stable under H,, is decomposed under NH, for 600 “C < TA < 950 “C. Annealing under NH3 at 700 and 800 “C seems to decompose mainly the WSiz *‘surface” layer
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(with nitrogen reaction with both the tungsten and silicon freed) above a mixed WSiz + Si graded layer. After annealing at higher temperatures (850 -950 "C), no WSiz remained, so nitrogen has reached the silicon substrate edge. There is in the graded layer a more and more preferential attachment of nitrogen to silicon, as SiN, with increasing x, as TA increases. Therefore annealing of amorphous “WS&” under NH3 cannot be used to form an atomic diffusion barrier of WN, on crystalline WSi2 even when starting with tungsten-rich WSi, films.
References 1 C. N. I. Wagner and W. L. Johnson (eds.), Proc. 5th Int. Conf. on Liquid and Amorphous Metals, in J. Non-Cryst. Solids, 61 - 62 (1984). 2 J. B. Canut, M. Kadri, J. Pivot and A. Deneuville, V. T. Nguyen and A. Golanski (eds.), Proc. European Material Research Symp., 198 7, Les Editions de Physique, Les Ulis. 3 B. L. Crowder and S. Zirinsky, IEEE J. Solid State Circuits, 14 (2) (1978) 291. 4 H. P. Kattelus, E. Kolawa, K. Affolter and M. A. Nicolet, J. Vat. Sci. Technot, A, 3 (6) (1986) 2246. 5 A. Deneuville, Ann. Chim. Fr., 2 (1986) 603. 6 M. Kadri, A. Deneuville and M. Brunel, Proc. Confi on Zon Plating and Allied Techniques, Brighton, 198 7, Conferences Exhibition Publication Consultants Ltd., Edinburgh, 1987, p. 366. 7 A. Deneuville, M. Kadri and M. Brunel, Proc. Eur. Workshop on Refractory Metals and their Silicides, Aussois, 1987, in Vide, Couches Minces, 42 (236) (1987) 37. 8 R. Kiessling and Y. H. Liu, Trans. AZME, (1951) 639. 9 A. Deneuville, M. Benyahya, M. Brunel and B. Canut, J. P. Nougier and D. Gasquet (eds.), Proc. European Solid State Devices Research Conf. 88, in J. Phys. (Paris), 49 (C4) (Suppl. 9) (1988) 499. 10 V. I. Knitrova and Z. G. Pinsker, Sou. Phys. Crystallogr., 6 (6) (1962) 712. 11 E. Bustarret, M. Bensouda, M. C. Habrard, J. C. BruyPre, S. Poulin and S. C. Gujrathi, Phys. Rev. B, 38, (1988).