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Room temperature interactions in Pd-metal thin film couples
MATERIALS CHEMISTRYAND PHYSICS ELSEVIER
Materials Chemistryand Physics47 (1997) 246-248
Room temperature interactions in Pd-metal thin film couples V. Simid, Z. Marinkovi6 Institute of Physics, Maksima Gorkog 118, Zemun, Belgrade, Yugoslavia
Received2 January 1996;accepted 15 April 1996
Abstract Results of investigations of the room temperature interaction of palladium with Bi, Cd, Ga, In, Pb, Sb, Sn, Te and Zn in thin films are presented. It has been found that palladium reacts with Bi, Ga, In, Pb, Sb and Sn to form compounds PdBi2, PdGas, PdIns, PdPbz, PdSb, PdT%, PdSn4 and PdSns respectively, but it does not react with Cd and Zn under the same conditions. Keywords: Thin films;Roomtemperatureinteractions;Palladium
1. Introduction Palladium is frequently used in thin film couples with other metals in microelectronics devices. According to published data, both the couples of Pd (a high m.p. metal) with other high m.p. metals, such as Si [1], Cu [2], Ge [3], A1 [4], and those with low m.p. metals, such as Pb [5] and Sn [5,6], were examined. Our experimental results [7-9] show that if in a thin-film couple both metals have high m.p., no compound is formed at room temperature, tf one metal had high m.p. and the other one had low m.p., compounds frequently formed at room temperature [7-9]. According to the papers mentioned, in the thin-film couples made of Pd with Si, Cu, Ge and A1, compounds were formed only at an elevated temperature, but the compound formation did occur at room temperature in the couples of Pd with Pb or Sn. Formation of compounds in thin-film couples at room temperature, at which microelectronics devices work, may influence their characteristics. This was the motive of our study of a number of, until now, uninvestigated metals of low m.p. in thin-fiIm couples with palladium. It was possible to expect formation of compounds at room temperature in these couples.
2. Experimental The thin film couples were prepared by vacuum evaporation applying a previously described procedure [ 7-9 ]. Purity of the metals used for evaporation was: 99.9999% (Bi, Cd, In, Sb, Sn, Zn), 99.999% (Ga, Pb, Te) and 99.9% (Pd). The thicknesses of metal layers were chosen according to the 0254-0584/97/$17.00 © 1997 ElsevierScience S.A. All rights reserved PIIS0254-0584(96)01830-5
concentrations of the elements in the expected compounds known from the literature [ 10]. X-ray diffraction was used to examine the samples for compound formation. Identification of the compounds formed was made using the ASTM standards [ 11 ].
3. Results and discussion Table 1 presents the results of the investigation. In six out of nine analyzed couples (Pb-Bi, Pd--Ga, PdIn, Pd-Pb, Pd-Sb, Pd-Te) one compound per couple was formed, but in one couple (Pd-Sn), two compounds were formed in a sequence. In two couples (Pd-Cd, Pd-Zn) no compounds was formed. We expected, however, Pd to react with Cd and Zn because these metals form compounds at room temperature with Ag [7], Au [8] and Cu [9]. The reaction had been followed for about 4.5 years. It was relatively rapid: the compounds were usually observed within 1-3 days after evaporation, but their formation was finished after long time. The characteristics of the reaction progress are as follows. In the couples of Pd with Ga and Pb, compounds were noticed during the first day for all the concentrations studied. In the Pd-Sb couples, a compound was formed in the samples of both concentrations but much later because antimony is on the upper boundary of the low-melting metals. This could be expected because, as we have shown earlier for the P d Me (Me = Ga, In, bi, Pd) couples, the higher the low-melting metal m.p., the lower the interdiffusion coefficient, i.e. the smaller the reaction rate [ 12]. Concentrations of the compounds formed in the couples Pd with Ga, Pb and Sb increase
247
V. Simid, 5. Marinkovid / Materials Chemistry and Physics 47 (1997) 246-248
Table 1 Formation of compounds in Pd-metal thin film couples at room temperature Investigated couples Metal
Compounds formed
Thickn. of both layers (nm)
Wt.% Me II
Pd-Bi
150-550
Pd-Cd
217-284
Pd--Ga
200-450
Pd-In
125-300
Pd-Pb
195-520
Pd-Sb
222-317
Pd-Sn
170-800
37.1 66.2 79.7 51.0 61.3 26.0 55,0 60.0 26.4 51.8 65.0 46.6 56.0 60.0 79.5 40.0 54.0 27.0 42.0 76.0 81.7 38.1 51.0 55.0 70.5 55.0 72.3
I
II
Pd-Te
185-580
Pd-Zn
312-554
Formula [ 10]
m.p. [ 10l
Time of investigation (months) ASTM card [ 11]
Noticed
(°C) (day) c~-PdBi2~
380
27-0436
2
for wt.% Me II 79.7
52.0
0 0 PdGa5 a,b
200
15-0577
1
All conc.
50.0
Pdin3 ~.b
664
21-407
2 3
51.8 65.0
51.8
PdPba a
474
8-365
1
All conc.
42.0
PdSb
805
26-0888
All conc.
49.5
PdSn4 a.b
295
36-137
1
52.0
PdSn3
345
15-575
13
81.7 76.0 76.0
PdTea ~
752
29-0970
2 3
55.0 70.5
51.0
51.5
c
0 0
50.5
a The compound has maximal concentration of low melting metal in the couple. b The compound has lowest m.p. in the couple. ¢ The compound was identified after long ageing. considerably with time. The compound PdPb2 was found to transform to PbCO 3 in the course of time, because lead is very sensitive to the influence of the atmosphere. In the couples of Pd with Bi, In and Te, compounds were noticed during the second and third day for the samples with concentrations at or near the stoichiometric value of the compound formed. This happens because in the couples with stoichiometric ratios, a greater part of the sample is simultaneously involved in the reaction than for any other concentration ratio. The compound rate of formation in such specimens is, roughly speaking, higher than in those with non-stoichiometric ratios. Consequently, the compounds can be noticed earlier by the X-ray diffraction. In the specimens having other concentrations, the compounds were formed somewhat later. Concentrations o f the compounds formed increased with time. The P d - S n couple is the only one in the P d - m e t a l system in which, sequentially, two compounds were formed. The first compound, PdSn4, was noticed during the first day in the samples with more than 50 wt% Sn and in the course of the first week in the samples with less than 50 wt.% Sn. The second compound, PdSn3, was noticed after two weeks, in
the samples with 76% Sn (stoichiometric value of PdSn3), but in the samples with 81.7% Sn (stoichiometric value of PdSn4), it was noticed only after three months. The PdSn 3 compound was formed from PdSn4 according to the reaction: 3PdSn4 + Pd = 4PdSn3
( 1)
After long ageing, both compounds were identified in the specimens of all concentrations. Tu and Rosenberg [5] have not observed the transformation of PdSn4 to PdSn3 because they followed the process for too short a time. The same is valid for the work of Nakahara and M c C o y [6]. The results o f the present work show that our earlier conclusion is also valid for the P d - m e t a l couples: in a thin-film metal couple, compounds are formed at room temperature if one metal has a high m.p. and the other metal has a low m.p., but the compounds are not formed if both metals have high m.p. [ 7 - 9 ] . In the P d - m e t a l thin-film couples, it was confirmed (Table 1) that the compound formed at room temperature mainly (6 of 7) has the lowest m.p. in the respective couple (similar to the A g - m e t a l [ 7 ], A u - m e t a l [ 8 ] and C u metal [9] systems), as well as the m a x i m u m concentration
248
v. Simid, ~. Marinkovid / Materials Chemistry and Physics 47 (1997) 246-248
of the l o w - m e l t i n g metal (as in the P d - P b and P d - S n couples [5] ). A n analysis of all the 35 thin-film couples shows that in 90% of these the first-formed c o m p o u n d has either a m i n i m u m m e l t i n g point or a m a x i m u m concentration of the lowm e l t i n g metal for a given couple [ 13].
References [1] F. Nava, G. Majni, G. Ottaviani and E. Galli, Thin Solid Films, 77 (1981) 319. [2] J. Li, J.W. Strane, S.W. Russell, S.Q. Hong, J.W. Mayer, T.K. Marais, C.C. Theron and R. Pretorius, 3".Appl. Phys., 72 (1992) 2810.
[3] G. Ottaviani, C. Kanali, G. Ferrari, R. Ferrari, G. Majni, M. Prudenziati and S.S. Lau, Thin Solid Fihns, 47 (1977) 187. [4] E.G. Colgan, Mater. Sci. Rep., 5 (1990) i. [5] K.N. Tu and R. Rosenberg, Proc. 6th Int. Vacuum Congr., 1974, in Jpn. J. Appl. Phys., Suppl. 2, Part I (1974) 633. [6] S. Nakahara and R.J. McCoy, Tt,in Solid Fihns, 88 (1982) 285. [7] V. Simi6 and 7.. Marinckovi6, Thir, solid Fihns, 61 (1979) 149. [8] 2;. Marinkovi6 and V. Simi6, J. Less-Common Met., 115 (1986) 225. [9] V. Simid and 7,. Marinkovi6, J. Less-Common Met., 72 (1980) 133. [10] T.B. Massalski (ed.), Binary Alloy Phase Diagrams, American Society for Metals, Matals Park, OH, 1990. [ 11] Joint Committee on Powder Diffraction Standards, ASTM Powder Diffraction File Swarthmore, PA. [ 12] ~,. Marinkovi6 and V. Simi6, Thin Solid Fihns, 217 (1992) 26. [ 13] V. Simi6 and Z. Marinkovi6, to be published.
Materials Chemistryand Physics47 (1997) 246-248
Room temperature interactions in Pd-metal thin film couples V. Simid, Z. Marinkovi6 Institute of Physics, Maksima Gorkog 118, Zemun, Belgrade, Yugoslavia
Received2 January 1996;accepted 15 April 1996
Abstract Results of investigations of the room temperature interaction of palladium with Bi, Cd, Ga, In, Pb, Sb, Sn, Te and Zn in thin films are presented. It has been found that palladium reacts with Bi, Ga, In, Pb, Sb and Sn to form compounds PdBi2, PdGas, PdIns, PdPbz, PdSb, PdT%, PdSn4 and PdSns respectively, but it does not react with Cd and Zn under the same conditions. Keywords: Thin films;Roomtemperatureinteractions;Palladium
1. Introduction Palladium is frequently used in thin film couples with other metals in microelectronics devices. According to published data, both the couples of Pd (a high m.p. metal) with other high m.p. metals, such as Si [1], Cu [2], Ge [3], A1 [4], and those with low m.p. metals, such as Pb [5] and Sn [5,6], were examined. Our experimental results [7-9] show that if in a thin-film couple both metals have high m.p., no compound is formed at room temperature, tf one metal had high m.p. and the other one had low m.p., compounds frequently formed at room temperature [7-9]. According to the papers mentioned, in the thin-film couples made of Pd with Si, Cu, Ge and A1, compounds were formed only at an elevated temperature, but the compound formation did occur at room temperature in the couples of Pd with Pb or Sn. Formation of compounds in thin-film couples at room temperature, at which microelectronics devices work, may influence their characteristics. This was the motive of our study of a number of, until now, uninvestigated metals of low m.p. in thin-fiIm couples with palladium. It was possible to expect formation of compounds at room temperature in these couples.
2. Experimental The thin film couples were prepared by vacuum evaporation applying a previously described procedure [ 7-9 ]. Purity of the metals used for evaporation was: 99.9999% (Bi, Cd, In, Sb, Sn, Zn), 99.999% (Ga, Pb, Te) and 99.9% (Pd). The thicknesses of metal layers were chosen according to the 0254-0584/97/$17.00 © 1997 ElsevierScience S.A. All rights reserved PIIS0254-0584(96)01830-5
concentrations of the elements in the expected compounds known from the literature [ 10]. X-ray diffraction was used to examine the samples for compound formation. Identification of the compounds formed was made using the ASTM standards [ 11 ].
3. Results and discussion Table 1 presents the results of the investigation. In six out of nine analyzed couples (Pb-Bi, Pd--Ga, PdIn, Pd-Pb, Pd-Sb, Pd-Te) one compound per couple was formed, but in one couple (Pd-Sn), two compounds were formed in a sequence. In two couples (Pd-Cd, Pd-Zn) no compounds was formed. We expected, however, Pd to react with Cd and Zn because these metals form compounds at room temperature with Ag [7], Au [8] and Cu [9]. The reaction had been followed for about 4.5 years. It was relatively rapid: the compounds were usually observed within 1-3 days after evaporation, but their formation was finished after long time. The characteristics of the reaction progress are as follows. In the couples of Pd with Ga and Pb, compounds were noticed during the first day for all the concentrations studied. In the Pd-Sb couples, a compound was formed in the samples of both concentrations but much later because antimony is on the upper boundary of the low-melting metals. This could be expected because, as we have shown earlier for the P d Me (Me = Ga, In, bi, Pd) couples, the higher the low-melting metal m.p., the lower the interdiffusion coefficient, i.e. the smaller the reaction rate [ 12]. Concentrations of the compounds formed in the couples Pd with Ga, Pb and Sb increase
247
V. Simid, 5. Marinkovid / Materials Chemistry and Physics 47 (1997) 246-248
Table 1 Formation of compounds in Pd-metal thin film couples at room temperature Investigated couples Metal
Compounds formed
Thickn. of both layers (nm)
Wt.% Me II
Pd-Bi
150-550
Pd-Cd
217-284
Pd--Ga
200-450
Pd-In
125-300
Pd-Pb
195-520
Pd-Sb
222-317
Pd-Sn
170-800
37.1 66.2 79.7 51.0 61.3 26.0 55,0 60.0 26.4 51.8 65.0 46.6 56.0 60.0 79.5 40.0 54.0 27.0 42.0 76.0 81.7 38.1 51.0 55.0 70.5 55.0 72.3
I
II
Pd-Te
185-580
Pd-Zn
312-554
Formula [ 10]
m.p. [ 10l
Time of investigation (months) ASTM card [ 11]
Noticed
(°C) (day) c~-PdBi2~
380
27-0436
2
for wt.% Me II 79.7
52.0
0 0 PdGa5 a,b
200
15-0577
1
All conc.
50.0
Pdin3 ~.b
664
21-407
2 3
51.8 65.0
51.8
PdPba a
474
8-365
1
All conc.
42.0
PdSb
805
26-0888
All conc.
49.5
PdSn4 a.b
295
36-137
1
52.0
PdSn3
345
15-575
13
81.7 76.0 76.0
PdTea ~
752
29-0970
2 3
55.0 70.5
51.0
51.5
c
0 0
50.5
a The compound has maximal concentration of low melting metal in the couple. b The compound has lowest m.p. in the couple. ¢ The compound was identified after long ageing. considerably with time. The compound PdPb2 was found to transform to PbCO 3 in the course of time, because lead is very sensitive to the influence of the atmosphere. In the couples of Pd with Bi, In and Te, compounds were noticed during the second and third day for the samples with concentrations at or near the stoichiometric value of the compound formed. This happens because in the couples with stoichiometric ratios, a greater part of the sample is simultaneously involved in the reaction than for any other concentration ratio. The compound rate of formation in such specimens is, roughly speaking, higher than in those with non-stoichiometric ratios. Consequently, the compounds can be noticed earlier by the X-ray diffraction. In the specimens having other concentrations, the compounds were formed somewhat later. Concentrations o f the compounds formed increased with time. The P d - S n couple is the only one in the P d - m e t a l system in which, sequentially, two compounds were formed. The first compound, PdSn4, was noticed during the first day in the samples with more than 50 wt% Sn and in the course of the first week in the samples with less than 50 wt.% Sn. The second compound, PdSn3, was noticed after two weeks, in
the samples with 76% Sn (stoichiometric value of PdSn3), but in the samples with 81.7% Sn (stoichiometric value of PdSn4), it was noticed only after three months. The PdSn 3 compound was formed from PdSn4 according to the reaction: 3PdSn4 + Pd = 4PdSn3
( 1)
After long ageing, both compounds were identified in the specimens of all concentrations. Tu and Rosenberg [5] have not observed the transformation of PdSn4 to PdSn3 because they followed the process for too short a time. The same is valid for the work of Nakahara and M c C o y [6]. The results o f the present work show that our earlier conclusion is also valid for the P d - m e t a l couples: in a thin-film metal couple, compounds are formed at room temperature if one metal has a high m.p. and the other metal has a low m.p., but the compounds are not formed if both metals have high m.p. [ 7 - 9 ] . In the P d - m e t a l thin-film couples, it was confirmed (Table 1) that the compound formed at room temperature mainly (6 of 7) has the lowest m.p. in the respective couple (similar to the A g - m e t a l [ 7 ], A u - m e t a l [ 8 ] and C u metal [9] systems), as well as the m a x i m u m concentration
248
v. Simid, ~. Marinkovid / Materials Chemistry and Physics 47 (1997) 246-248
of the l o w - m e l t i n g metal (as in the P d - P b and P d - S n couples [5] ). A n analysis of all the 35 thin-film couples shows that in 90% of these the first-formed c o m p o u n d has either a m i n i m u m m e l t i n g point or a m a x i m u m concentration of the lowm e l t i n g metal for a given couple [ 13].
References [1] F. Nava, G. Majni, G. Ottaviani and E. Galli, Thin Solid Films, 77 (1981) 319. [2] J. Li, J.W. Strane, S.W. Russell, S.Q. Hong, J.W. Mayer, T.K. Marais, C.C. Theron and R. Pretorius, 3".Appl. Phys., 72 (1992) 2810.
[3] G. Ottaviani, C. Kanali, G. Ferrari, R. Ferrari, G. Majni, M. Prudenziati and S.S. Lau, Thin Solid Fihns, 47 (1977) 187. [4] E.G. Colgan, Mater. Sci. Rep., 5 (1990) i. [5] K.N. Tu and R. Rosenberg, Proc. 6th Int. Vacuum Congr., 1974, in Jpn. J. Appl. Phys., Suppl. 2, Part I (1974) 633. [6] S. Nakahara and R.J. McCoy, Tt,in Solid Fihns, 88 (1982) 285. [7] V. Simi6 and 7.. Marinckovi6, Thir, solid Fihns, 61 (1979) 149. [8] 2;. Marinkovi6 and V. Simi6, J. Less-Common Met., 115 (1986) 225. [9] V. Simid and 7,. Marinkovi6, J. Less-Common Met., 72 (1980) 133. [10] T.B. Massalski (ed.), Binary Alloy Phase Diagrams, American Society for Metals, Matals Park, OH, 1990. [ 11] Joint Committee on Powder Diffraction Standards, ASTM Powder Diffraction File Swarthmore, PA. [ 12] ~,. Marinkovi6 and V. Simi6, Thin Solid Fihns, 217 (1992) 26. [ 13] V. Simi6 and Z. Marinkovi6, to be published.