- Email: [email protected]
Al multilayer structures
Surface
and Coatings
Technology
83 (1996)
45-48
H + processing of Ni/Al multilayer structures M. MozetiC a, A. Zalar a, I. ArEon b, B. PraCek a, M. DrobniC b, P. Panjan b a Institute of Swface Engineering and Optoelectronics, Teslovn 30, 1001 Ljubljana, b Institut Joief Stefan, Jamova 39, 1001 Ljubljana, Slovenia
Slovenia
Abstract The method of heat transfer in low pressure weakly ionized hydrogen plasma was used for the thermal processing of samples of an Ni/& multilayer structure on well polished silicon substrates. The samples were exposed to hydrogen plasma under different conditions. The sample temperature increased typically at a rate of 10 “C s-l, remained constant for several minutes, and decreased at a rate of 10 “C s-l. The steady temperature was between 300 “C and 1000 “C. The samples were analyzed using Auger electron spectroscopyand extendedX-ray absorptionfine structure spectroscopy.The resultsshow that the coating obtained after thermal processingis a mixture of nickel, aluminum and nickel aluminides,different from NiAl binary alloy. At high temperatures,
evaporation of aluminum from the coating was observed. Keywo&: Nickel aluminide;Multilayer structure; Depth profile; Hydrogen plasma
1. Introduction A promising method for the preparation of thin coatings is the thermal treatment of a multilayer structure. Elements forming the coating are deposited onto the
substrate in thin layers with a thickness of the order of 10 nm. The multilayer structure which is formed during deposition is then heated to the appropriate temperature in order to obtain a uniform coating. One such coating is nickel aluminide (NiAl). It has been shown that bulk nickel aluminide is a binary alloy consisting of 44-58 at.% Ni. Its melting point depends slightly on the concentration of nickel, but is always higher than 1400 ‘C [ 11. Because of its excellent properties, it may be used in microelectronics as a diffusion barrier, to suppress hillock formation, and to increase electromigration resistance [2-41. Thermal treatment of the multilayer structure can be performed by different means, including the common heating in a furnace, electron or ion beam heating, laser treatment or r.f. heating. Since the sample tends to oxidize before interdiffusion of nickel and aluminum is completed, the treatment should be performed in an inert atmosphere, or under ultrahigh vacuum conditions. In a common furnace, thermal treatment often leads to oxidation of the sample even though the processing takes place in an “inert” atmosphere. This is due to the fact that a variety of impurities desorb from the inner walls of the furnace. It is therefore more convenient to
heat only the sample, while the walls of the preparation chamber are kept at room temperature. In a common furnace this is not possible, so a more sophisticated method of thermal treatment should be used. Thermal treatment of oxidized samples in a reductive atmosphere leads to reduction of metal oxide, so it seems promising to heat samples in a hydrogen atmosphere. However, Hz does not react with nickel oxide at room temperature. It becomes reductive at a temperature of about 300 “C. If the temperature of a sample is increased slowly in the hydrogen furnace, the oxide layer will form at a low temperature and will be reduced at a high temperature. This procedure could lead to irreversible change in the surface morphology, as was shown for the case of copper [S]. In order to avoid this complication, the sample should either be heated to the appropriate temperature in a short time, or it should be treated in an atmosphere which is highly reductive even at room temperature. An atmosphere which is highly reductive at room temperature is low pressure weakly ionized hydrogen plasma [6]. In the present paper we show that it also allows a high heating rate and direct heating of the sample. 2. Experimental
details
For the experimental study of the thermal treatment of multilayer structures in a hydrogen plasma, a vacuum
46
hf. Mozetic’et al. JSwface and Coatings Technology 83 (1996) 45-43
systemwas constructed. The systemis pumped with a mechanical rotary pump and a liquid nitrogen trap. The base pressure is 2 x 10m2mbar when the system is pumped only by the rotary pump, and 4 x low3 mbar when the trap is cooled with liquid nitrogen. The experimental chamber is made of glass with a rather low recombination coefficient for the reaction H + H --f Hz. Commercially available hydrogen is leaked into the system so that the pressure during experiments is between 0.1 and 10 mbar. Weakly ionized plasma is created by an inductively coupled r.f. generator. Plasma parameters are measuredwith a double Langmuir probe [7,8] and a catalytic probe [9,10]. The neutral gas temperature is 50 “C and the electron temperature is about 6 x 104”C. The plasma density is of the order of iO16 mU3, the Debye length is of the order of 10d4 m, and the degree of dissociation of hydrogen molecules is of the order of 10m2. A multilayer structure of nickel and aluminum was prepared as follows. Highly polished silicon substrates were mounted in a vacuum systemfor sputter deposition of thin layers. First, a layer of aluminum was deposited onto the silicon substrate in order to prevent migration of nickel into the silicon substrate.The thicknessof the aluminum layer was determined by a quartz microbalante and was 38 nm. The aluminum layer was then covered with a nickel layer with a thickness of 25 nm. The procedure was repeated five times in order to obtain a multilayer structure. The composition of the sample was then determined by Auger electron spectroscopy (AES) depth profile analysis.We used a scanning Auger -~miEZj?Zjb?(Physical Electronics Ind SAM 545 A). A static primary electron beam with an energy of 5 keV, a beam current of 0.5 pm, and beam diameter of about 10 pm was used. The incidence angle of the electron beam with respect to the normal to the surface plain was 30”. The samples were ion sputtered with one or two symmetrically inclined beams of 1 keV Art ions, rastered on a surface area larger than 10 x 10 mm2. The atomic concentration was calculated by meansof relative sensitivity factors according to the literature [ 111. A depth profile of an as-depositedsample is shown in Fig. 1. The atomic concentration in the structure is plotted vs. sputtering time instead of depth. It is noticeable that the thicknessof both the nickel and aluminum layers is fairly constant. Becauseof the different sputtering rates of nickel and aluminum, the ratio between the thicknessesof the nickel and aluminum layers is not that which might be derived from Fig, 1. The whole multilayer structure consistsof 50% of both nickel and aluminum, as was determined by the microbalance. A sample was then cut into small pieces which were connected to a thermocouple and mounted in the center of the discharge vessel. Samples were always kept at floating potential, which was about 20 V. They were exposedto hydrogen plasma at different pressures.
b - Xl
0 - Ni
*-Si
0-O
100
80
GO
0
40
20
0 0
10 20 30 10 50 GO 70 so DO 100 sputter time [mill]
Fig.1. Depthprofileof as-deposited Ni/AI multilayerstructure.
After the treatment, the sampleswere analyzed using AES and extended X-ray absorption fine structure spectroscopy (EXAFS). EXAFS spectra at the Ni K edge were measured at the EXAFS II station in the HASYLAB at DESY (Hamburg, Germany). Synchrotron radiation from the DORIS storage ring running at 44.4 GeV and 80mA was focused by a golden-coated mirror on an Si( 111) double-crystal monochromator with an energy resolution of 1.5eV at 8 keV. Harmonics were eliminated effectively by detuning the monochromator crystal using a stabilization feedback control. With four foils of the Ni/Al sample superimposed, the absorption thickness of$*O.5 at the Ni K edgewas reached.Referencespectra without the sample were taken under identical conditions. A standard stepping progression within the 1000eV region above the edge was adopted. The EXAFS spectrum of a standard nickel foil was measured for comparison. 3. Results During the thermal treatment in plasma the sample temperature was measured. A typical curve and its derivative are plotted in Fig. 2(a) and (b) respectively.It is noticeable that the sample temperature increasesat a rate of more than 10 “C s-l (about 1000“C min-I). After less than 1 min the sample reaches the steady temperature of about 400 “C. When the r.f. generator was turned off, the valve between the experimental chamber and the liquid nitrogen trap was closed so that the pressurebegan to rise. As a consequence,the sample was cooled down to 50 “C in lessthan 1 min. Fig. 2(b) shows that the cooling rate is more than 1000“C min-‘. After plasma processing, the sample was analyzed
M. MozetiE et al./Swface and Coatings
Technology
0 - Al
L[min]
6oo0 , 2 / 4, I 6/
83 (1996)
41
45-48
0-N
*-Si
a-c
o-o
81 I,10 / 12I1 14 / I,16 118 I I 20I< 221 I
c e n
60
1 f a t
00 I 200(I”““” 400
600
800
1000 1200 1400
+I (4 20
0
2 4 6 8 1 / I I / I I
10
12
i
40
[%I
2.
0 n at.
t[min] 14
16
1s
20
I / I , / , I / 1 1
22
I /
1200 900
0
0
10
20
dT/dt
0
-5 -
50
60
70
SO
90
100
time [min]
Fig. 3. Depth profile of Ni/Al multilayer structure after treatment in plasma.
300
0 --
40
sputter
6ooWlmin i
30
l
- Al
0 - Ni
* - Si
o-o
-300
A-C I .:-:-
100
-600 -15 -
-900
-20 -10 --,,1,,,7,:
-1200
.‘L5 0
400
800
+I
1200
-1500
(‘4 Fig. 2. Temperature of the sample vs. time (a) and first derivative curve (b). The sample was treated in plasma at the pressure of 2.5 x 10-l mbar.
using AES again. The structure of the sample treated at the temperature of 400 “C is shown in Fig. 3. We can see that a rather uniform coating was formed on the silicon substrate. The concentration of both nickel and aluminum ,is close to 50% near the substrate, while a little shortage of aluminum is observed near the surface. The higher the temperature, the less aluminum is observed near the surface. In Figs. 4 and 5 we show the depth profile of samples treated in plasma at the temperatures of 300 “C and 1000 “C respectively. At the temperature of 1000 “C, the layer near the substrate still consists of 50% of both nickel and aluminum, while near the surface the aluminum concentration is much smaller. Depth profiles of the samples treated in plasma at the temperature of 300 “C! show that in the original aluminum layer, a material with approximately 45% Ni and 55% Al was formed, which indicates that the alloy Ni,Al, might have formed, while in the original nickel layer, the distribution of elements is not flat. The samples treated in plasma at 300 “C were also
0 n c e n
60
i
40
at. PI
2.
t r a t
0 n
sputter
time [min]
Fig. 4. Depth profile of NY.4 multilayer structure after treatment in plasma for 5 min at 300 “C.
analyzed by EXAFS in order to determine the chemical strhcture of the coating. Fig. 6 shows the magnitude of the Fourier transform of the EXAFS spectrum of the Ni K edge measured on a standard nickel foil and on the coating of the sample treated in plasma. It is noticeable that nickel atoms in the coating heated at 300 “C in hydrogen plasma are bonded mainly to nickel, and only about 20% of them are bonded to aluminum.
4. Discussion and conclusion A method of heat treatment in hydrogen plasma of samples of an Ni/Al multilayer structure was described.
M. Mozeti? et al. JSurface and Coatings Technology 83 (1996) 45-48 l
*- Si
0 - Ni
- Al
o-o
100
c 0 n c e n 1. r a t. i 0 n at.
WI
60
40
20
0
0
10
20
30 sputter
40'
so
60
70
time [min]
Fig. 5. Depth profile of Ni/Al multilayer structure after treatment in plasma for 10 s at 1000 “C. In this case, the sample was sputtered with two identical ion guns.
5ol-----l
The distribution of aluminum and nickel in the sample treated at the temperature of 300 “C shows that, at this temperature, a rather uniform layer with 45 at.% Ni and 55 at.% Al is formed, which indicates that an N&Al3 compound might have formed. However, the EXAFS spectrum (Fig. 6) of the samples shows that only few nickel atoms are bonded to aluminum, indicating that a mixture of nickel and aluminum was obtained rather than a chemical compound. At the temperature of 400 “C, the AES analysisshows about 50% of both nickel and aluminum throughout the coating. SinceNiAl alloy has not yet beenproved to exist in thin films [2], it may not necessarilyhave been obtained by us. According to the EXAFS results it is more likely that we obtained a mixture of nickel, aluminum and different nickel aluminides according to the binary phase diagram. This assumption is supported by the results for the sample treated at a temperature of 1000“C. In Fig. 5, a shortage of aluminum on the surface is clearly observed.If the binary alloy NiAl were formed at a lower temperature,there would be no reason for this shortage of aluminum, since the melting point of NiAl is at the much higher temperature of 1440“C, Therefore, the coating obtained from the original multilayer structure is far from being a unique chemicalcompound. Acknowledgements
J. Feldhaus and R. Frahm made possible the realization of experiments at the EXAFS II station in HASYLAB at DESY in Hamburg, Germany. The project was sponsored by the Ministry of Science and Technology of the Republic of Slovenia. Fig. 6. Magnitude of the Fourier transform of the Ni K edge of the EXAFS spectrum measured on standard nickel foil (thick solid line) and NijAl multilayer (dotted line).
References [l]
A sample exposed to low pressure, weakly ionized hydrogen plasma was heated by recombination of atomic hydrogen, absorption of light quanta, weak bombardment with positive ions, recombination of charged particles, and relaxation of metastables. Samples were treated in hydrogen plasma under different conditions. In each case, high heating and cooling rates were obtained, sincethe sample was actually the only part of the systemwhich was heated. As the hydrogen plasma is a very reductive atmosphere even at room temperature, no oxidation of the sample took place during the treatment. A small amount of oxygen which is observed at the very surface layer of the samplesis probably due to secondary oxidation since the sampleswere exposed to air prior to the AES analyses.
T.M. Massalski, Binary Alloy Phase Diagrams, Vol. 1, ASM International, Materials Park, OH, 1990, 2nd edn,, pq 183. [2] E.G. Colgan, Mater, Sci. Rep., 5 (1990) 1. [S] H.P, Kattelus and M.A. Nicolet, in D, Gupta and P.S. Ho (eds.), Diffirsion Phenorxena in Thin Films nttd Wcroelec~ronic Materials, Noyes Publications, Park Ridge, NJ, 1988. [4] E. Ma, M.A. Nicolet and M. Nathan, J. ,4ppl. Plqx, 65 (1989) [S]
2703.
M. MozetiE, M. Kveder, T. MozetiE and M. DrobniE, C-e& L Phys., 43 (1993)953.
F. Brecelj and M. MozetiE, Vacwm, 40 (1990) 177. [7] J.D. Swift and M.J.R. Schwar, Ekctrical Probes for Plasma Diagnostics, Iliffe Books, London, 1969. [8] F.F. Chen, in R.H. Huddlestone and S.L, Leonard (eds.), Plasma Diagnostic Technipes, Academic Press, New York, 1965. [9] F. Brecelj, M. MozetiE, K. Zupan and M. DrobniE, Vaclclrltl, 44 (1993) 459. [lo] M. MozetiE, M. Kveder, M. DrobniE, A. Paulin and A. Zalar, [6]
Vacuum,
45 (1994)1095.
and Coatings
Technology
83 (1996)
45-48
H + processing of Ni/Al multilayer structures M. MozetiC a, A. Zalar a, I. ArEon b, B. PraCek a, M. DrobniC b, P. Panjan b a Institute of Swface Engineering and Optoelectronics, Teslovn 30, 1001 Ljubljana, b Institut Joief Stefan, Jamova 39, 1001 Ljubljana, Slovenia
Slovenia
Abstract The method of heat transfer in low pressure weakly ionized hydrogen plasma was used for the thermal processing of samples of an Ni/& multilayer structure on well polished silicon substrates. The samples were exposed to hydrogen plasma under different conditions. The sample temperature increased typically at a rate of 10 “C s-l, remained constant for several minutes, and decreased at a rate of 10 “C s-l. The steady temperature was between 300 “C and 1000 “C. The samples were analyzed using Auger electron spectroscopyand extendedX-ray absorptionfine structure spectroscopy.The resultsshow that the coating obtained after thermal processingis a mixture of nickel, aluminum and nickel aluminides,different from NiAl binary alloy. At high temperatures,
evaporation of aluminum from the coating was observed. Keywo&: Nickel aluminide;Multilayer structure; Depth profile; Hydrogen plasma
1. Introduction A promising method for the preparation of thin coatings is the thermal treatment of a multilayer structure. Elements forming the coating are deposited onto the
substrate in thin layers with a thickness of the order of 10 nm. The multilayer structure which is formed during deposition is then heated to the appropriate temperature in order to obtain a uniform coating. One such coating is nickel aluminide (NiAl). It has been shown that bulk nickel aluminide is a binary alloy consisting of 44-58 at.% Ni. Its melting point depends slightly on the concentration of nickel, but is always higher than 1400 ‘C [ 11. Because of its excellent properties, it may be used in microelectronics as a diffusion barrier, to suppress hillock formation, and to increase electromigration resistance [2-41. Thermal treatment of the multilayer structure can be performed by different means, including the common heating in a furnace, electron or ion beam heating, laser treatment or r.f. heating. Since the sample tends to oxidize before interdiffusion of nickel and aluminum is completed, the treatment should be performed in an inert atmosphere, or under ultrahigh vacuum conditions. In a common furnace, thermal treatment often leads to oxidation of the sample even though the processing takes place in an “inert” atmosphere. This is due to the fact that a variety of impurities desorb from the inner walls of the furnace. It is therefore more convenient to
heat only the sample, while the walls of the preparation chamber are kept at room temperature. In a common furnace this is not possible, so a more sophisticated method of thermal treatment should be used. Thermal treatment of oxidized samples in a reductive atmosphere leads to reduction of metal oxide, so it seems promising to heat samples in a hydrogen atmosphere. However, Hz does not react with nickel oxide at room temperature. It becomes reductive at a temperature of about 300 “C. If the temperature of a sample is increased slowly in the hydrogen furnace, the oxide layer will form at a low temperature and will be reduced at a high temperature. This procedure could lead to irreversible change in the surface morphology, as was shown for the case of copper [S]. In order to avoid this complication, the sample should either be heated to the appropriate temperature in a short time, or it should be treated in an atmosphere which is highly reductive even at room temperature. An atmosphere which is highly reductive at room temperature is low pressure weakly ionized hydrogen plasma [6]. In the present paper we show that it also allows a high heating rate and direct heating of the sample. 2. Experimental
details
For the experimental study of the thermal treatment of multilayer structures in a hydrogen plasma, a vacuum
46
hf. Mozetic’et al. JSwface and Coatings Technology 83 (1996) 45-43
systemwas constructed. The systemis pumped with a mechanical rotary pump and a liquid nitrogen trap. The base pressure is 2 x 10m2mbar when the system is pumped only by the rotary pump, and 4 x low3 mbar when the trap is cooled with liquid nitrogen. The experimental chamber is made of glass with a rather low recombination coefficient for the reaction H + H --f Hz. Commercially available hydrogen is leaked into the system so that the pressure during experiments is between 0.1 and 10 mbar. Weakly ionized plasma is created by an inductively coupled r.f. generator. Plasma parameters are measuredwith a double Langmuir probe [7,8] and a catalytic probe [9,10]. The neutral gas temperature is 50 “C and the electron temperature is about 6 x 104”C. The plasma density is of the order of iO16 mU3, the Debye length is of the order of 10d4 m, and the degree of dissociation of hydrogen molecules is of the order of 10m2. A multilayer structure of nickel and aluminum was prepared as follows. Highly polished silicon substrates were mounted in a vacuum systemfor sputter deposition of thin layers. First, a layer of aluminum was deposited onto the silicon substrate in order to prevent migration of nickel into the silicon substrate.The thicknessof the aluminum layer was determined by a quartz microbalante and was 38 nm. The aluminum layer was then covered with a nickel layer with a thickness of 25 nm. The procedure was repeated five times in order to obtain a multilayer structure. The composition of the sample was then determined by Auger electron spectroscopy (AES) depth profile analysis.We used a scanning Auger -~miEZj?Zjb?(Physical Electronics Ind SAM 545 A). A static primary electron beam with an energy of 5 keV, a beam current of 0.5 pm, and beam diameter of about 10 pm was used. The incidence angle of the electron beam with respect to the normal to the surface plain was 30”. The samples were ion sputtered with one or two symmetrically inclined beams of 1 keV Art ions, rastered on a surface area larger than 10 x 10 mm2. The atomic concentration was calculated by meansof relative sensitivity factors according to the literature [ 111. A depth profile of an as-depositedsample is shown in Fig. 1. The atomic concentration in the structure is plotted vs. sputtering time instead of depth. It is noticeable that the thicknessof both the nickel and aluminum layers is fairly constant. Becauseof the different sputtering rates of nickel and aluminum, the ratio between the thicknessesof the nickel and aluminum layers is not that which might be derived from Fig, 1. The whole multilayer structure consistsof 50% of both nickel and aluminum, as was determined by the microbalance. A sample was then cut into small pieces which were connected to a thermocouple and mounted in the center of the discharge vessel. Samples were always kept at floating potential, which was about 20 V. They were exposedto hydrogen plasma at different pressures.
b - Xl
0 - Ni
*-Si
0-O
100
80
GO
0
40
20
0 0
10 20 30 10 50 GO 70 so DO 100 sputter time [mill]
Fig.1. Depthprofileof as-deposited Ni/AI multilayerstructure.
After the treatment, the sampleswere analyzed using AES and extended X-ray absorption fine structure spectroscopy (EXAFS). EXAFS spectra at the Ni K edge were measured at the EXAFS II station in the HASYLAB at DESY (Hamburg, Germany). Synchrotron radiation from the DORIS storage ring running at 44.4 GeV and 80mA was focused by a golden-coated mirror on an Si( 111) double-crystal monochromator with an energy resolution of 1.5eV at 8 keV. Harmonics were eliminated effectively by detuning the monochromator crystal using a stabilization feedback control. With four foils of the Ni/Al sample superimposed, the absorption thickness of$*O.5 at the Ni K edgewas reached.Referencespectra without the sample were taken under identical conditions. A standard stepping progression within the 1000eV region above the edge was adopted. The EXAFS spectrum of a standard nickel foil was measured for comparison. 3. Results During the thermal treatment in plasma the sample temperature was measured. A typical curve and its derivative are plotted in Fig. 2(a) and (b) respectively.It is noticeable that the sample temperature increasesat a rate of more than 10 “C s-l (about 1000“C min-I). After less than 1 min the sample reaches the steady temperature of about 400 “C. When the r.f. generator was turned off, the valve between the experimental chamber and the liquid nitrogen trap was closed so that the pressurebegan to rise. As a consequence,the sample was cooled down to 50 “C in lessthan 1 min. Fig. 2(b) shows that the cooling rate is more than 1000“C min-‘. After plasma processing, the sample was analyzed
M. MozetiE et al./Swface and Coatings
Technology
0 - Al
L[min]
6oo0 , 2 / 4, I 6/
83 (1996)
41
45-48
0-N
*-Si
a-c
o-o
81 I,10 / 12I1 14 / I,16 118 I I 20I< 221 I
c e n
60
1 f a t
00 I 200(I”““” 400
600
800
1000 1200 1400
+I (4 20
0
2 4 6 8 1 / I I / I I
10
12
i
40
[%I
2.
0 n at.
t[min] 14
16
1s
20
I / I , / , I / 1 1
22
I /
1200 900
0
0
10
20
dT/dt
0
-5 -
50
60
70
SO
90
100
time [min]
Fig. 3. Depth profile of Ni/Al multilayer structure after treatment in plasma.
300
0 --
40
sputter
6ooWlmin i
30
l
- Al
0 - Ni
* - Si
o-o
-300
A-C I .:-:-
100
-600 -15 -
-900
-20 -10 --,,1,,,7,:
-1200
.‘L5 0
400
800
+I
1200
-1500
(‘4 Fig. 2. Temperature of the sample vs. time (a) and first derivative curve (b). The sample was treated in plasma at the pressure of 2.5 x 10-l mbar.
using AES again. The structure of the sample treated at the temperature of 400 “C is shown in Fig. 3. We can see that a rather uniform coating was formed on the silicon substrate. The concentration of both nickel and aluminum ,is close to 50% near the substrate, while a little shortage of aluminum is observed near the surface. The higher the temperature, the less aluminum is observed near the surface. In Figs. 4 and 5 we show the depth profile of samples treated in plasma at the temperatures of 300 “C and 1000 “C respectively. At the temperature of 1000 “C, the layer near the substrate still consists of 50% of both nickel and aluminum, while near the surface the aluminum concentration is much smaller. Depth profiles of the samples treated in plasma at the temperature of 300 “C! show that in the original aluminum layer, a material with approximately 45% Ni and 55% Al was formed, which indicates that the alloy Ni,Al, might have formed, while in the original nickel layer, the distribution of elements is not flat. The samples treated in plasma at 300 “C were also
0 n c e n
60
i
40
at. PI
2.
t r a t
0 n
sputter
time [min]
Fig. 4. Depth profile of NY.4 multilayer structure after treatment in plasma for 5 min at 300 “C.
analyzed by EXAFS in order to determine the chemical strhcture of the coating. Fig. 6 shows the magnitude of the Fourier transform of the EXAFS spectrum of the Ni K edge measured on a standard nickel foil and on the coating of the sample treated in plasma. It is noticeable that nickel atoms in the coating heated at 300 “C in hydrogen plasma are bonded mainly to nickel, and only about 20% of them are bonded to aluminum.
4. Discussion and conclusion A method of heat treatment in hydrogen plasma of samples of an Ni/Al multilayer structure was described.
M. Mozeti? et al. JSurface and Coatings Technology 83 (1996) 45-48 l
*- Si
0 - Ni
- Al
o-o
100
c 0 n c e n 1. r a t. i 0 n at.
WI
60
40
20
0
0
10
20
30 sputter
40'
so
60
70
time [min]
Fig. 5. Depth profile of Ni/Al multilayer structure after treatment in plasma for 10 s at 1000 “C. In this case, the sample was sputtered with two identical ion guns.
5ol-----l
The distribution of aluminum and nickel in the sample treated at the temperature of 300 “C shows that, at this temperature, a rather uniform layer with 45 at.% Ni and 55 at.% Al is formed, which indicates that an N&Al3 compound might have formed. However, the EXAFS spectrum (Fig. 6) of the samples shows that only few nickel atoms are bonded to aluminum, indicating that a mixture of nickel and aluminum was obtained rather than a chemical compound. At the temperature of 400 “C, the AES analysisshows about 50% of both nickel and aluminum throughout the coating. SinceNiAl alloy has not yet beenproved to exist in thin films [2], it may not necessarilyhave been obtained by us. According to the EXAFS results it is more likely that we obtained a mixture of nickel, aluminum and different nickel aluminides according to the binary phase diagram. This assumption is supported by the results for the sample treated at a temperature of 1000“C. In Fig. 5, a shortage of aluminum on the surface is clearly observed.If the binary alloy NiAl were formed at a lower temperature,there would be no reason for this shortage of aluminum, since the melting point of NiAl is at the much higher temperature of 1440“C, Therefore, the coating obtained from the original multilayer structure is far from being a unique chemicalcompound. Acknowledgements
J. Feldhaus and R. Frahm made possible the realization of experiments at the EXAFS II station in HASYLAB at DESY in Hamburg, Germany. The project was sponsored by the Ministry of Science and Technology of the Republic of Slovenia. Fig. 6. Magnitude of the Fourier transform of the Ni K edge of the EXAFS spectrum measured on standard nickel foil (thick solid line) and NijAl multilayer (dotted line).
References [l]
A sample exposed to low pressure, weakly ionized hydrogen plasma was heated by recombination of atomic hydrogen, absorption of light quanta, weak bombardment with positive ions, recombination of charged particles, and relaxation of metastables. Samples were treated in hydrogen plasma under different conditions. In each case, high heating and cooling rates were obtained, sincethe sample was actually the only part of the systemwhich was heated. As the hydrogen plasma is a very reductive atmosphere even at room temperature, no oxidation of the sample took place during the treatment. A small amount of oxygen which is observed at the very surface layer of the samplesis probably due to secondary oxidation since the sampleswere exposed to air prior to the AES analyses.
T.M. Massalski, Binary Alloy Phase Diagrams, Vol. 1, ASM International, Materials Park, OH, 1990, 2nd edn,, pq 183. [2] E.G. Colgan, Mater, Sci. Rep., 5 (1990) 1. [S] H.P, Kattelus and M.A. Nicolet, in D, Gupta and P.S. Ho (eds.), Diffirsion Phenorxena in Thin Films nttd Wcroelec~ronic Materials, Noyes Publications, Park Ridge, NJ, 1988. [4] E. Ma, M.A. Nicolet and M. Nathan, J. ,4ppl. Plqx, 65 (1989) [S]
2703.
M. MozetiE, M. Kveder, T. MozetiE and M. DrobniE, C-e& L Phys., 43 (1993)953.
F. Brecelj and M. MozetiE, Vacwm, 40 (1990) 177. [7] J.D. Swift and M.J.R. Schwar, Ekctrical Probes for Plasma Diagnostics, Iliffe Books, London, 1969. [8] F.F. Chen, in R.H. Huddlestone and S.L, Leonard (eds.), Plasma Diagnostic Technipes, Academic Press, New York, 1965. [9] F. Brecelj, M. MozetiE, K. Zupan and M. DrobniE, Vaclclrltl, 44 (1993) 459. [lo] M. MozetiE, M. Kveder, M. DrobniE, A. Paulin and A. Zalar, [6]
Vacuum,
45 (1994)1095.