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Tungsten phase transformation induced by low-fluence Ar irradiation in CuW multilayers
MALmtms
Materials Letters 12 ( 1992) 419-423 North-Holland
Tungsten phase transformation induced by low-fluence Ar irradiation in Cu-W multilayers G. Gladyszewski
‘, Ph. Goudeau,
A. Naudon,
Ch. Jaouen
and J. Pacaud
Laboratoire de M&allurgie Physique, AssociP au CNRS (U.R.A. 131), 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received 18 July 199 1; in final form 8 October I99 1
Changes occurring in equiatomic Cu-W superlattices, prepared by direct sputtering, after low-fluence 300 keV A?+ ion irradiation were investigated by X-ray diffraction. The profiles were measured in O-2Ogeometry. A shift of satellite superlattice peaks to higher 20 angles was observed for ion doses up to 2 x 1014ions/cm*. There were no further changes in peak position for higher doses. This effect cannot be interpreted as being caused by an ion mixing process in the copper-tungsten interface regions nor by stress relaxation effects. X-ray diffraction measurements performed on a pure tungsten layer, 1000 A thick and prepared in the same conditions as the superlattices, revealed the presence of the B-W phase. After 300 keV Ar 2+ ion irradiation with low dose (2 x lOI ions/cm’), the 110 peak of the a-W structure appeared whereas the 2 10 peak of the B-W phase disappeared in the X-ray diffraction profile. Thus, during the first stages of irradiation, the transformation from Bto a phase occurred in tungsten sublayers.
1. Introduction Ion irradiation is a powerful procedure to obtain supersaturated solid solutions, metastable crystalline and amorphous phases in immiscible layered systems [ l-3 1. Such systems are characterized by large and positive enthalpies of formation AH‘ and usually involve atomic size differences of more than 10%. In the system we studied, Cu-W, predictions of Miedema [ 41 give AHf= + 36 kJ/g at for equiatomic composition 1 : 1 and the atomic size difference is about 7%. Recently, Gladyszewski and Mikolajczak [ 5 ] proposed to apply X-ray diffraction measurement of irradiated superlattices as a new method to study the ion beam mixing (IBM) process. With this technique, it is possible to observe changes in the interface regions with good accuracy. The number of interface regions must be great enough, such as in multilayered samples, in order to optimize the X-ray diffraction measurements. These measurements are relatively simple and fast but the analysis of changes ’ On leave from Institute of Physics, MCS University, 20-03 1 Lublin, Poland. 0167-577x/92/$
occurring in the profiles is sometimes difficult for at least two reasons: (i) calculations of simulation programs involve a lot of structural parameters concerning the as-prepared state of the studied sample which are often not easy to obtain and (ii) modifications in the profiles after ion irradiation can originate from effects such as strain relaxation [ 61 which do not account for mixing. Thus, knowledge of the initial state is very important and absolutely necessary to achieve a fine analysis of the early stages of mixing. In this paper, we focus on initial stages of the ion irradiation process, i.e. at low fluence, in the Cu-W multilayers. We unambiguously interprete modifications appearing in the diffraction profiles after irradiation: effects due to mixing and those related to the nature of the as-prepared state of the samples are clearly distinguished.
2. Experimental The Cu-W multilayers of equiatomic composition were prepared on silicon wafers using a sputtering system specially developed in our laboratory for dy-
05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
419
Volume 12, number 6
January 1992
MATERIALS LETTERS
namic ion beam mixing (DIBM) [ 71. Deposition was obtained by sputtering of a solid target using a broad beam ion source of the Kauffman type. Vacuum before and during the deposition was about 2 x 10e6 and 10v4 Torr, respectively. A water cooled target holder with four positions offered the possibility to elaborate multilayers. The substrate holder was also water cooled. Instead of using the thickness controller with a quartz oscillator as a monitor, we chose the sputtering time of each element, calculated for equiatomic composition, in order to have a good reproducibility of Cu and W layer thicknesses for each period. The deposition rate, which was four times smaller for copper than for tungsten, was less than 1 A/s. Samples with different periods .4 (50 to 126 A) and numbers (15 to 8) of Cu/W bilayers were elaborated; the total film thickness was fixed at 1000 A. Ion irradiations were performed with 300 keV ArZ+ ions at low dose ( 10’4-1015 ions/cm*). The ion beam energy was fixed in order to adapt the projected range (R,) plus projected range-straggling (AR,) of the impinging ions to the total thickness (D) of the films (D=R, + A&). Samples were irradiated at liquidnitrogen temperature under lo-’ Torr vacuum. The X-ray diffraction profiles were measured using a classical 0-28 diffractometer equipped with a monochromator and controlled by a microcomputer. In the analysis of the profiles of superlattice targets, we applied the computer program IM-SL [ 81 which makes it possible to simulate the profiles taking into account imperfections of the superlattice structure (e.g. presence of grains, fluctuations of the unit cell size, interdiffused interface character, etc. ) as well as to calculate the profile assuming ideal structure of a superlattice.
ellite peaks in the X-ray diffraction profile of a superlattice target, whereas the peak positions do not change. In the case of the Cu-W system, we observed that the satellite peak positions also changed considerably and this effect could not be interpreted as being caused by an ion mixing process in the copper-tungsten interface regions. Fig. 1 shows X-ray diffraction profiles of one of the Cu-W superlattice samples before and after 300 keV Ar*+ ion irradiation with a dose of 1 X 1014 ions/ cm*. The shift of the peaks to higher 20 angles is clearly visible. These changes versus ion dose are presented for -2nd, + 1st and + 2nd satellite peaks in fig. 2. It is seen that there were no further changes in peak position for doses higher than 2 x 1014 ions/ cm*. The same effect was also observed for all other samples. In order to interpret peak positions and intensities without performing detailed analysis of their halfwidth, it is enough to apply the model of ideal superlattice structure [ 91. Fig. 3 shows experimental profiles of the Cu-W superlattice sample and those calculated assuming ideal structure. The comparison between the profile of an unirradiated sample and the profile calculated assuming that the interplanar distances in Cu and W layers are the same as those observed in the bulk materials, i.e. dc,= 2.087 A for (111) copper texture and d,=2.238 A for (110) tungsten texture, is shown in fig. 3a. In this case, it is possible to obtain a composition of the number of monolayers in the superlattice unit cell that gives the correct satellite peak positions. However, the peak 80 [
1
cu -w co
3. Results and discussion In our investigations, we applied the Cu-W superlattices as targets irradiated sequentially with 300 keV Ar2+ ions. After each dose, we performed an analysis of the changes which occurred in the largeangle X-ray diffraction profiles of the superlattices. Usually, these changes correspond to an initial stage of a mixing process in interfacial regions, and are reflected in a variation of the relative intensities of sat420
2 THETA
[deg]
Fig. 1. X-ray diffraction profiles of the equiatomic Cu-W superlattice (8 periods of 126 A thick) before and after 300 keV A?+ ion irradiation with a dose of Ix lOI ions/cm’.
Volume 12, number 6
MATERIALS LETTERS
January I992
a
2 THETA
[deg]
b
2 THETA
012
5
[deg]
IO
C
DOSE [ x IO” ions/cm-2]
Fig. 2. 28 positions of -2nd, + 1st and + 2nd satellite peaks of the Cu-W superlattice sample (A = 126 A) versus ion dose.
intensities completely differ from those experimentally obtained. Nevertheless, after ion irradiation with a dose of 2 x 1Ot4ions/cm2, the profile can be easily interpreted assuming the abovementioned interplanar distances. As shown in fig. 3b, the agreement between peak positions as well as their intensities is quite satisfactory. The first interpretation of these effects which should be considered is the possible presence of stresses which could lead to changes in the interplanar distances in the growth direction. Such effects are often observed in multilayered structures and have been described in numerous papers [ 10,111. Since Young’s modulus, in bulk materials, is more than 2.5 times larger for tungsten than for copper [ 12 1, the changes in interplanar distances should occur in the copper layers. Obtainment of good agreement between peak positions is then also possible. however, disproportions of peak intensities become stronger. To obtain correct profiles, we had to assume in the calculations a change in the interplanar distance in tungsten to dw = 2.268 A. The profile calculated in this way is shown in fig. 3c.
37
2 THETA
[deg]
47
Fig. 3. Comparison between the profiles of u&radiated (a) and irradiated (b) sample and the profiles calculated assuming interplanar distances of bulk copper and tungsten, and (c) the profile of unirradiated sample and the one calculated assuming a larger interplanar distance in tungsten.
Since, according to earlier mentioned reason, the change in interplanar distance in tungsten cannot be interpreted by stress effects, it was necessary to find another explanation. Bulk tungsten is well known to crystallize with the body-centred cubic structure of A2 type, denoted uW. However, two further modifications were reported [ 13 ] : a face-centred cubic, A 1 type, occurring in evaporated tungsten films deposited in vacua and a P-W phase with Al 5 structure, for which a controversy existed concerning its nature [ 14,15 1. It was suggested that P-W could be either an oxide of the form W&I (6 W atoms and 2 0 atoms per unit cell) or a metallic phase. Basavaiah and Pollack [ 161, Kuriyama and Ohfuji [ 171 and Haghiri-Gosnet et al. [ 181 confirmed that 8-W is not an oxide but a 421
Volume 12, number 6
MATERIALS LETTERS
2 THETA
[deg]
Fig. 4. X-ray diffraction profiles of a pure tungsten layer 1000 A thick before and after ion irradiation with a dose of 2 x 1014ions/ cm2. After the irradiation, the Si 002 “forbidden” peak became visible. This effect was also reported for Xe, Cu and Al direct irradiation into silicon [ 201.
metallic phase stabilized by the presence of impurity oxygen (up to 3 atoh). The phase structure type of evaporated tungsten films depends on the conditions under which the films are deposited: vacuum [ 141, substrate temperature and film thickness [ 15 1, etc. Basavaiah and Pollack [ 141 showed that the crystal structure is strongly influenced by the background pressure during deposition. From X-ray diffraction measurements, the presence of the P-W was evident for pressure above lo-’ Torr. Furthermore, Petroff et al. [ 15 ] showed that the a-W phase appears when the film thickness exceeds a critical value, but also after annealing at a temperature of 135 ‘C. The thickness effect was confirmed by Haghiri-Gosnet et al. [ 181, as was the annealing effect [ 17 ] but for higher temperature of about 650°C. Petroff et al. [ 151 found that the activation energy associated with the phase transformation j3-W-ML-Wis close to that for W atom migration. They concluded that the phase transformation in this case would be diffusion controlled.
January 1992
Thus, according to our sputtering conditions, the phase structure of tungsten in as-prepared multilayers could be of P-W type. The shift of satellite peaks in the diffraction pattern after irradiation could result from a phase transition, P-W to a-W. In order to verify this possibility, we performed at the same conditions ion irradiation with a dose of 2 x 1Or4 ions/cm’ to 1000 A tungsten layer. Before irradiation, the X-ray diffraction profile of the layer revealed the 200 and 2 10 peaks characteristic of ptungsten. The interplanar distance dw determined from the 2 10 tungsten peak position using the Bragg equation, 2dwsin8=1, was the same as that obtained for the virgin Cu-W superlattice samples, i.e. d,=2.267 A. The average grain size in the growth direction calculated with the Scherrer formula [ 191 was equal to 125 A. After ion irradiation the profile changed radically. The peaks of P-tungsten disappeared, while the 110 peak typical of u-tungsten appeared. As is usually observed after the /3-W to a-W transformation [ 17,181, the average grain size was larger and equal to 250 A. These changes are shown in fig. 4, and peak positions are listed in table 1. Some divergences between values of the P-W lattice constant taken from the literature and that obtained in the present work may be caused by argon ions which were trapped in the film during sputtering [ 2 11.
4. Concluding remarks The results obtained lead to the conclusion that virgin superlattice samples consisted of (2 IO) j3-W and ( 111) Cu layers. In the initial stage of the ion irradiation process, the transition from p to a phase occurred in tungsten layers. For an ion dose of 2 x 1Or4ions/cm*, the transition was completed and
Table 1 Interplanar distances d (A) and lattice constants a (A) determined from the X-ray diffraction profile of a pure tungsten layer, 1000 A thick, before (g-W) and after (a-W) irradiation W phase
422
(h kl)
From ref. [ 141
Measured
d(A)
a (A)
d(A)
a (A)
a-W (bee) A2 type
(110)
2.238
3.165
2.234
3.160
g-WA15 type
(200) (210)
2.52 2.25
5.04 5.03
2.546 2.261
5.092 5.069
Volume 12, number
6
MATERIALS
then, the superlattice unit cells consisted of ( 110) aW and ( 111) Cu layers. It is worth emphasizing that low-temperature irradiation induced here a transformation from metastable to equilibrium state. The energy deposited inside the cascade could effectively induce short-range atomic rearrangements necessary for the transformation. Our further studies on this system have shown that for higher doses obtainment of limited miscibility in this “immiscible” system is possible. These results are under elaboration now and will be published soon [ 22 1.
Acknowledgement
The authors are pleased to acknowledge J. GrilhC for useful discussions and C. Templier for his help during sample irradiation. One of us (GG) also acknowledges the French Ministry of Research and Technology (MRT) and the Polish Committee of Scientific Research (Grant No. II. 1.7/P/04/069) for support.
References [ 1] J. Meissner, K. Kopitzki,
G. Mertler and E. Peiner, Nucl. Instr. Methods B 19/20 (1987) 669. [2] W. Hiller, M. Buchgeister, P. Eitner, K. Kopitzki, V. Lilienthal and E. Peiner, Mater. Sci. Eng. A 115 ( 1989 ) 15 1.
LETTERS
January
1992
[3] B.V. King, S.G. Puranik, M.A. Sobhan and R.J. MacDonald, Nucl. Instr. Methods B 39 ( 1989) 153. [4] A.R. Miedema, PhilipsTech. Rev. 36 (1976) 217. [5] G. Gladyszewski and P. Mikotajczak, Appl. Phys. A 48 (1989) 521. [ 61 S. Mantl, D.B. Poker and K. Reichelt, Nucl. Instr. Methods B 19/20 (1987) 677. [ 71 M. Jaulin, G. Laplanche, J. Delafond and S. PimbertMichaux, Surface Coat. Tech. 37 (1989) 225. [ 8 ] G. Gladyszewski and Z. Mitura, to be published. [9] G. Gladyszewski and Z. Mitura, Surface Sci. 231 ( 1990) 90. [lo] L.L. Chang and B.C. Giessen, eds., Synthetic modulated structures (Academic Press, New York, 1985). [ 111 J. Grilhe, Mater. Sci. Forum 59/60 ( 1990) 48 1. [ 121 R.B. Bruce, in: American Institute of Physics Handbook, Vol. 2, ed. D.E. Gray (1972) p. 51. [ 131 R.L. Moss and I. Woodward, Acta Cryst. 12 (1959) 255. [ 141 S. Basavaiah and S.R. Pollack, J. Appl. Phys. 39 (1968) 5548. [ 15 ] P. Petroff, T.T. Sheng, A.K. Sinha, G.A. Rozgonyi and F.B. Alexander, J. Appl. Phys. 44 (1973) 2545. [ 161 S. Basavaiah and S.R. Pollack, Appl. Phys. Letters 12 ( 1968) 259. [ 171 Y. Kmiyama and S. Ohfuji, J. Appl. Phys. 66 ( 1989) 2446. [ 181 A.M. Haghiri-Gosnet, F.R. Ladan, C. Mayeux, H. Launois and M.C. Joncour, J. Vacuum Sci. Technol. A 7 ( 1989) 2663. [ 19) A. Guinier, X-ray diffraction in crystals, imperfect crystals and amorphous bodies (Freeman, San Fransisco, 1963 ) . [20] C. Templier, B. Boubeker, H. Garem, E.L. Matht and J.C. Desoyer, Phys. Stat. Sol. 92a ( 1985) 5 11. [ 2 1 ] M. Itoh, M. Hori and S. Nadahara, J. Vacuum Sci. Technol. B 9 (1991) 149. [22] G. Gladyszewski, Ph. Goudeau, A. Naudon, Ch. Jaouen and J. Pacaud, to be published.
423
Materials Letters 12 ( 1992) 419-423 North-Holland
Tungsten phase transformation induced by low-fluence Ar irradiation in Cu-W multilayers G. Gladyszewski
‘, Ph. Goudeau,
A. Naudon,
Ch. Jaouen
and J. Pacaud
Laboratoire de M&allurgie Physique, AssociP au CNRS (U.R.A. 131), 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Received 18 July 199 1; in final form 8 October I99 1
Changes occurring in equiatomic Cu-W superlattices, prepared by direct sputtering, after low-fluence 300 keV A?+ ion irradiation were investigated by X-ray diffraction. The profiles were measured in O-2Ogeometry. A shift of satellite superlattice peaks to higher 20 angles was observed for ion doses up to 2 x 1014ions/cm*. There were no further changes in peak position for higher doses. This effect cannot be interpreted as being caused by an ion mixing process in the copper-tungsten interface regions nor by stress relaxation effects. X-ray diffraction measurements performed on a pure tungsten layer, 1000 A thick and prepared in the same conditions as the superlattices, revealed the presence of the B-W phase. After 300 keV Ar 2+ ion irradiation with low dose (2 x lOI ions/cm’), the 110 peak of the a-W structure appeared whereas the 2 10 peak of the B-W phase disappeared in the X-ray diffraction profile. Thus, during the first stages of irradiation, the transformation from Bto a phase occurred in tungsten sublayers.
1. Introduction Ion irradiation is a powerful procedure to obtain supersaturated solid solutions, metastable crystalline and amorphous phases in immiscible layered systems [ l-3 1. Such systems are characterized by large and positive enthalpies of formation AH‘ and usually involve atomic size differences of more than 10%. In the system we studied, Cu-W, predictions of Miedema [ 41 give AHf= + 36 kJ/g at for equiatomic composition 1 : 1 and the atomic size difference is about 7%. Recently, Gladyszewski and Mikolajczak [ 5 ] proposed to apply X-ray diffraction measurement of irradiated superlattices as a new method to study the ion beam mixing (IBM) process. With this technique, it is possible to observe changes in the interface regions with good accuracy. The number of interface regions must be great enough, such as in multilayered samples, in order to optimize the X-ray diffraction measurements. These measurements are relatively simple and fast but the analysis of changes ’ On leave from Institute of Physics, MCS University, 20-03 1 Lublin, Poland. 0167-577x/92/$
occurring in the profiles is sometimes difficult for at least two reasons: (i) calculations of simulation programs involve a lot of structural parameters concerning the as-prepared state of the studied sample which are often not easy to obtain and (ii) modifications in the profiles after ion irradiation can originate from effects such as strain relaxation [ 61 which do not account for mixing. Thus, knowledge of the initial state is very important and absolutely necessary to achieve a fine analysis of the early stages of mixing. In this paper, we focus on initial stages of the ion irradiation process, i.e. at low fluence, in the Cu-W multilayers. We unambiguously interprete modifications appearing in the diffraction profiles after irradiation: effects due to mixing and those related to the nature of the as-prepared state of the samples are clearly distinguished.
2. Experimental The Cu-W multilayers of equiatomic composition were prepared on silicon wafers using a sputtering system specially developed in our laboratory for dy-
05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
419
Volume 12, number 6
January 1992
MATERIALS LETTERS
namic ion beam mixing (DIBM) [ 71. Deposition was obtained by sputtering of a solid target using a broad beam ion source of the Kauffman type. Vacuum before and during the deposition was about 2 x 10e6 and 10v4 Torr, respectively. A water cooled target holder with four positions offered the possibility to elaborate multilayers. The substrate holder was also water cooled. Instead of using the thickness controller with a quartz oscillator as a monitor, we chose the sputtering time of each element, calculated for equiatomic composition, in order to have a good reproducibility of Cu and W layer thicknesses for each period. The deposition rate, which was four times smaller for copper than for tungsten, was less than 1 A/s. Samples with different periods .4 (50 to 126 A) and numbers (15 to 8) of Cu/W bilayers were elaborated; the total film thickness was fixed at 1000 A. Ion irradiations were performed with 300 keV ArZ+ ions at low dose ( 10’4-1015 ions/cm*). The ion beam energy was fixed in order to adapt the projected range (R,) plus projected range-straggling (AR,) of the impinging ions to the total thickness (D) of the films (D=R, + A&). Samples were irradiated at liquidnitrogen temperature under lo-’ Torr vacuum. The X-ray diffraction profiles were measured using a classical 0-28 diffractometer equipped with a monochromator and controlled by a microcomputer. In the analysis of the profiles of superlattice targets, we applied the computer program IM-SL [ 81 which makes it possible to simulate the profiles taking into account imperfections of the superlattice structure (e.g. presence of grains, fluctuations of the unit cell size, interdiffused interface character, etc. ) as well as to calculate the profile assuming ideal structure of a superlattice.
ellite peaks in the X-ray diffraction profile of a superlattice target, whereas the peak positions do not change. In the case of the Cu-W system, we observed that the satellite peak positions also changed considerably and this effect could not be interpreted as being caused by an ion mixing process in the copper-tungsten interface regions. Fig. 1 shows X-ray diffraction profiles of one of the Cu-W superlattice samples before and after 300 keV Ar*+ ion irradiation with a dose of 1 X 1014 ions/ cm*. The shift of the peaks to higher 20 angles is clearly visible. These changes versus ion dose are presented for -2nd, + 1st and + 2nd satellite peaks in fig. 2. It is seen that there were no further changes in peak position for doses higher than 2 x 1014 ions/ cm*. The same effect was also observed for all other samples. In order to interpret peak positions and intensities without performing detailed analysis of their halfwidth, it is enough to apply the model of ideal superlattice structure [ 91. Fig. 3 shows experimental profiles of the Cu-W superlattice sample and those calculated assuming ideal structure. The comparison between the profile of an unirradiated sample and the profile calculated assuming that the interplanar distances in Cu and W layers are the same as those observed in the bulk materials, i.e. dc,= 2.087 A for (111) copper texture and d,=2.238 A for (110) tungsten texture, is shown in fig. 3a. In this case, it is possible to obtain a composition of the number of monolayers in the superlattice unit cell that gives the correct satellite peak positions. However, the peak 80 [
1
cu -w co
3. Results and discussion In our investigations, we applied the Cu-W superlattices as targets irradiated sequentially with 300 keV Ar2+ ions. After each dose, we performed an analysis of the changes which occurred in the largeangle X-ray diffraction profiles of the superlattices. Usually, these changes correspond to an initial stage of a mixing process in interfacial regions, and are reflected in a variation of the relative intensities of sat420
2 THETA
[deg]
Fig. 1. X-ray diffraction profiles of the equiatomic Cu-W superlattice (8 periods of 126 A thick) before and after 300 keV A?+ ion irradiation with a dose of Ix lOI ions/cm’.
Volume 12, number 6
MATERIALS LETTERS
January I992
a
2 THETA
[deg]
b
2 THETA
012
5
[deg]
IO
C
DOSE [ x IO” ions/cm-2]
Fig. 2. 28 positions of -2nd, + 1st and + 2nd satellite peaks of the Cu-W superlattice sample (A = 126 A) versus ion dose.
intensities completely differ from those experimentally obtained. Nevertheless, after ion irradiation with a dose of 2 x 1Ot4ions/cm2, the profile can be easily interpreted assuming the abovementioned interplanar distances. As shown in fig. 3b, the agreement between peak positions as well as their intensities is quite satisfactory. The first interpretation of these effects which should be considered is the possible presence of stresses which could lead to changes in the interplanar distances in the growth direction. Such effects are often observed in multilayered structures and have been described in numerous papers [ 10,111. Since Young’s modulus, in bulk materials, is more than 2.5 times larger for tungsten than for copper [ 12 1, the changes in interplanar distances should occur in the copper layers. Obtainment of good agreement between peak positions is then also possible. however, disproportions of peak intensities become stronger. To obtain correct profiles, we had to assume in the calculations a change in the interplanar distance in tungsten to dw = 2.268 A. The profile calculated in this way is shown in fig. 3c.
37
2 THETA
[deg]
47
Fig. 3. Comparison between the profiles of u&radiated (a) and irradiated (b) sample and the profiles calculated assuming interplanar distances of bulk copper and tungsten, and (c) the profile of unirradiated sample and the one calculated assuming a larger interplanar distance in tungsten.
Since, according to earlier mentioned reason, the change in interplanar distance in tungsten cannot be interpreted by stress effects, it was necessary to find another explanation. Bulk tungsten is well known to crystallize with the body-centred cubic structure of A2 type, denoted uW. However, two further modifications were reported [ 13 ] : a face-centred cubic, A 1 type, occurring in evaporated tungsten films deposited in vacua and a P-W phase with Al 5 structure, for which a controversy existed concerning its nature [ 14,15 1. It was suggested that P-W could be either an oxide of the form W&I (6 W atoms and 2 0 atoms per unit cell) or a metallic phase. Basavaiah and Pollack [ 161, Kuriyama and Ohfuji [ 171 and Haghiri-Gosnet et al. [ 181 confirmed that 8-W is not an oxide but a 421
Volume 12, number 6
MATERIALS LETTERS
2 THETA
[deg]
Fig. 4. X-ray diffraction profiles of a pure tungsten layer 1000 A thick before and after ion irradiation with a dose of 2 x 1014ions/ cm2. After the irradiation, the Si 002 “forbidden” peak became visible. This effect was also reported for Xe, Cu and Al direct irradiation into silicon [ 201.
metallic phase stabilized by the presence of impurity oxygen (up to 3 atoh). The phase structure type of evaporated tungsten films depends on the conditions under which the films are deposited: vacuum [ 141, substrate temperature and film thickness [ 15 1, etc. Basavaiah and Pollack [ 141 showed that the crystal structure is strongly influenced by the background pressure during deposition. From X-ray diffraction measurements, the presence of the P-W was evident for pressure above lo-’ Torr. Furthermore, Petroff et al. [ 15 ] showed that the a-W phase appears when the film thickness exceeds a critical value, but also after annealing at a temperature of 135 ‘C. The thickness effect was confirmed by Haghiri-Gosnet et al. [ 181, as was the annealing effect [ 17 ] but for higher temperature of about 650°C. Petroff et al. [ 151 found that the activation energy associated with the phase transformation j3-W-ML-Wis close to that for W atom migration. They concluded that the phase transformation in this case would be diffusion controlled.
January 1992
Thus, according to our sputtering conditions, the phase structure of tungsten in as-prepared multilayers could be of P-W type. The shift of satellite peaks in the diffraction pattern after irradiation could result from a phase transition, P-W to a-W. In order to verify this possibility, we performed at the same conditions ion irradiation with a dose of 2 x 1Or4 ions/cm’ to 1000 A tungsten layer. Before irradiation, the X-ray diffraction profile of the layer revealed the 200 and 2 10 peaks characteristic of ptungsten. The interplanar distance dw determined from the 2 10 tungsten peak position using the Bragg equation, 2dwsin8=1, was the same as that obtained for the virgin Cu-W superlattice samples, i.e. d,=2.267 A. The average grain size in the growth direction calculated with the Scherrer formula [ 191 was equal to 125 A. After ion irradiation the profile changed radically. The peaks of P-tungsten disappeared, while the 110 peak typical of u-tungsten appeared. As is usually observed after the /3-W to a-W transformation [ 17,181, the average grain size was larger and equal to 250 A. These changes are shown in fig. 4, and peak positions are listed in table 1. Some divergences between values of the P-W lattice constant taken from the literature and that obtained in the present work may be caused by argon ions which were trapped in the film during sputtering [ 2 11.
4. Concluding remarks The results obtained lead to the conclusion that virgin superlattice samples consisted of (2 IO) j3-W and ( 111) Cu layers. In the initial stage of the ion irradiation process, the transition from p to a phase occurred in tungsten layers. For an ion dose of 2 x 1Or4ions/cm*, the transition was completed and
Table 1 Interplanar distances d (A) and lattice constants a (A) determined from the X-ray diffraction profile of a pure tungsten layer, 1000 A thick, before (g-W) and after (a-W) irradiation W phase
422
(h kl)
From ref. [ 141
Measured
d(A)
a (A)
d(A)
a (A)
a-W (bee) A2 type
(110)
2.238
3.165
2.234
3.160
g-WA15 type
(200) (210)
2.52 2.25
5.04 5.03
2.546 2.261
5.092 5.069
Volume 12, number
6
MATERIALS
then, the superlattice unit cells consisted of ( 110) aW and ( 111) Cu layers. It is worth emphasizing that low-temperature irradiation induced here a transformation from metastable to equilibrium state. The energy deposited inside the cascade could effectively induce short-range atomic rearrangements necessary for the transformation. Our further studies on this system have shown that for higher doses obtainment of limited miscibility in this “immiscible” system is possible. These results are under elaboration now and will be published soon [ 22 1.
Acknowledgement
The authors are pleased to acknowledge J. GrilhC for useful discussions and C. Templier for his help during sample irradiation. One of us (GG) also acknowledges the French Ministry of Research and Technology (MRT) and the Polish Committee of Scientific Research (Grant No. II. 1.7/P/04/069) for support.
References [ 1] J. Meissner, K. Kopitzki,
G. Mertler and E. Peiner, Nucl. Instr. Methods B 19/20 (1987) 669. [2] W. Hiller, M. Buchgeister, P. Eitner, K. Kopitzki, V. Lilienthal and E. Peiner, Mater. Sci. Eng. A 115 ( 1989 ) 15 1.
LETTERS
January
1992
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