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Structural transformations induced during the annealing of thin Ni-Cr films
Thin Solid Films, 57 (I 979) 337-341 fi Elsevier Sequoia S.A., Lausanne-
STRUCTURAL ANNEALING
Printed
TRANSFORMATIONS OF THIN Ni-Cr FILMS*
M. I. BPRJEGA, N. POPESCU-POGRION,
337
in the Netherlands
INDUCED
DURING
THE
C. SARBU AND I. A. TEODORESCU
Central Institute of Physics, Institute of Physics and Technology of Materials, Bucharest-Mlfgurele, Bou 5206 (Romania) (Received July 20, 1978: accepted
September
P.O.
15, 1978)
We present curves describing the variation ofthe electrical resistance during the in situ annealing of Ni-Cr thin films containing various amounts of chromium and grown at various rates. The results of electron microscopy and electron diffraction investigations of the annealed Ni-Cr thin films are also reported and the structural transformations taking place during the annealing treatment are determined.
1. INTRODUCTION
Ni-Cr thin film resistors are widely used in microelectronic circuits because of their high resistivity and low temperature coefficient of resistivity. In general the NiCr thin films are subjected to various annealing treatments in order to obtain good stability of their electrical parameters. In previous work’ we have established experimentally an in situ annealing treatment that ensures a stability of better than 0.1% for the electrical resistance of thin Ni-Cr films. In this work we present curves describing the variation of the electrical resistance during the annealing treatment of Ni-Cr films containing various amounts of chromium and grown at various deposition rates. The results of electron microscopy and electron diffraction investigations of the annealed films are also reported and the structural transformations taking place during the annealing treatment are determined. 2. EXPERIMENTAL
PROCEDURE
The thin Ni-Cr films were deposited on microscope glass substrates and freshly air-cleaved NaCl substrates heated to 400 “C. Ni-Cr (80: 20) wires were evaporated in a high vacuum unit either directly by sublimation (slow deposition rates) or indirectly from a heated tungsten spiral (higher deposition rates)‘. A movable screen fitted between the evaporation source and the substrates allowed us to start the deposition at various periods after the beginning of the alloy sublimation or melting. The growth of films with compositions that were different from that of the source material was possible because of the different vapour pressures of chromium ‘and nickel’. The chromium content of the films was determined by spectrophotometric techniques3. *Paper presented September 1 l-15,
at the Fourth International 1978: Paper 259.
Congress
on Thin Films,
Loughborough,
Ct. Britain,
338
M. I. BiRJECA
rt d.
After deposition the Ni-Cr samples were annealed in vacuum at 400 “C for 2 h and during this time the value of the electrical resistance was measured stepwise with a Wheatstone bridge. For the structural investigations the thin films were removed from the NaCl substrates by floating off in distilled water and were examined using a JEM- 120 electron microscope working at 100 kV. 3.
RESULTS
AND DISCUSSION
3.1. Indirect evaporation (higher deposition rates) The behaviour of the electrical resistance during the annealing of films grown at a deposition rate of 2 8, s-l depended on their chromium content and the films showed correspondingly distinct structures (Fig. 1). Ni-Cr thin films with 20 wt.% Cr showed a pronounced decrease in electrical resistance during annealing (Fig. l(a)). The electron microscopy and electron diffraction analyses revealed the presence of a mosaic of well-defined grains with the f.c.c. y-Ni structure in which were embedded contrastless islands with a disordered structure (Fig. l(b)). Pitt4 also has reported negative resistance changes on aging Ni-Cr thin films with 75 wt.:/, Ni in vacuum at 300°C. Bicknel15 has obtained similar electron micrographs and electron diffraction patterns from flash-evaporated Ni-Cr (80:20) thin films grown at room temperature and vacuum annealed at 350 “C. We assume that the processes responsible for the pronounced decrease in the electrical resistance are recrystallization (which determines the y-Ni grain growth) and segregation of the disordered phase. The films with 30 wt.“/, Cr exhibited an increase followed by a decrease and stabilization of the electrical resistance (Fig. l(c)). Selected area electron diffraction analysis6 revealed the presence of ordering processes and long range order corresponding to the Ni,Cr superlattice 7-9. The presence of the ordered domains was identified in the electron micrographs (Fig. l(d)) by the mottled and striated contrast effects. This electrical resistance behaviour is characteristic for ordering processes7-9. 3.2. Direct evaporation (slow deposition rates) Ni-Cr films grown at a deposition rate of 0.2 A s- ’ with a chromium content ranging from 24 to 32 wt.% exhibited an increase in electrical resistance during the annealing treatment (Fig. 2(a), (c)). The corresponding electron micrographs revealed a worm-like structure (Fig. 2(b)) or a periodic structure (Fig. 2(d)) depending on the chromium content. The electron diffraction patterns (Figs. 2(b) and 2(d)) showed the presence of metastable disordered phases”. Rastogi and Duwez r1 have studied the rate and the mode of crystallization of alloy by thermal analysis, X-ray diffraction, electron amorphous Fe,,P,,C,, microscopy and electrical resistivity measurements. They have established that at lower rates of heating it is possible to stop the crystallization process so that the specimen consists of clusters of microcrystals embedded in an amorphous matrix. In this temperature range the resistivity increases faster than linearly with temperature. This behaviour has also been observed in other amorphous alloys’2q ’ 3. Rastogi and Duwez’ ’ have suggested that the anomalous resistance increase is due to the
STKUCTUKALTKANSFOKMATIONSANDTHEANNEALINGOF
Ni-Cr
FILMS
212
196
0
’
10
“1
20
30
LO
11 50 60
I 90
I I I 100 110 120
Time lmmutes)
11 70 80
(4
' 10
' 20
(133000 x
200. Cc) 0
Fig. 2. (a), (c)The electrical resistance us. time of annealing and(b),(d) electron micrographs 24 wt.% Cr; (c),(d) 32 wt.% Cr) grown at a deposition rate ofO.2 A s-‘.
(b)
(4
z 202
;201. L
2 2206
0210 E a,208
1 LO
1 _.I 1 _~~I_L~p,li_ 50 60 70 80 90 100 110 120 Time lm~rwtes)
) and electron diffraction patterns of thin NiCr
1 30
films ((a), (b)
a z .-
STRUCTURAL TRANSFORMATIONS AND THE ANNEALING OF
Ni-Cr
FILMS
341
fact that microcrystalline precipitates act as scattering centres for the conduction electrons in a manner similar to that found in the early stage of precipitation (Guinier-Preston zones) in supersaturated crystalline solid solutions. According to Mottr4 the critical radius of Guinier-Preston zones for resonant scattering is close to the wavelength of the conduction electrons. Hardy and Murti” have also found a worm-like structure (Fig. 2(b)) in sputtered Ni-Cr thin films. Mader and Nowick16 have observed periodic structures in some amorphous Cu-Ag and Co-Au thin films deposited at a substrate temperature of 70 K on annealing at higher temperatures. We assume that the processes responsible for the resistance increase in Ni-Cr thin films during annealing are the nucleation and growth of microcrystalline particles in the amorphous matrix. Depending on the chromium content this process can lead to a spinodal decomposition. 4. CONCLUSIONS Our results demonstrate the complex dependences of annealing-induced structural transformations in Ni-Cr thin films on the composition and growth conditions. REFERENCES 1 2 3
M. I. Birjega and C. A. Constantin, Thin Solid Films, !(I 967/68) 396. P. Huijer, W. T. Langendam and J. A. Lely, Phi&s Tech. Rev., 24 (1962/63) 144. M. I. Birjega, C. A. Constantin, M. M. Paraschiv and N. G. Popescu-Pogrion, Rev. Roum. Phys., 16 (1971) 1229.
4 5 6
8 9 10 II 12 13 14 15 16
K. E. G. Pitt, J. R. Inst. Chem., 88 (1964) 38 1. R. W. Bicknell, Er. J. Appl. Phys., 16 (1965) 17. M. I Birjega, N. Popescu-Pogrion and C. Sarbu, Rev. Roum. Phys., 24 (1979) 107. H. cr. Baer, Z. Meta/lkd.,49(1958)614. H. J. Klein, C. R. Brooks and E. E. Stansbury, Phys. Sfarus Solidi, 38 (1970) 831. A.,Lasserre, F. Reynaud and P. Coulomb, Phi/ox Mag., 29 (1974) 665. M. 1. Birjega, N. Popescu-Pogrion, C. SPrbu and S. RQu, Thin Solid Films, 34 (1976) 153. P. K. Rastogi and P. Duwez, J. Non-Cryst. So/ids, 5 (1970) 1. H. S. Chen and D. Turnbull, J. Chem. Phys., 48 (I 968) 2560. P. L. Maitrepierre, J. Appl. Phys., 41 (1970) 52. N. F. Mott, J. Inst. Me!., 60 (1937) 267. W. R. Hardy and D. K. Murti, Thin SolidFilms, 20 (1974) 345. S. Mader and A. S. Nowick, Acra Metal/., I5 (1967) 215.
STRUCTURAL ANNEALING
Printed
TRANSFORMATIONS OF THIN Ni-Cr FILMS*
M. I. BPRJEGA, N. POPESCU-POGRION,
337
in the Netherlands
INDUCED
DURING
THE
C. SARBU AND I. A. TEODORESCU
Central Institute of Physics, Institute of Physics and Technology of Materials, Bucharest-Mlfgurele, Bou 5206 (Romania) (Received July 20, 1978: accepted
September
P.O.
15, 1978)
We present curves describing the variation ofthe electrical resistance during the in situ annealing of Ni-Cr thin films containing various amounts of chromium and grown at various rates. The results of electron microscopy and electron diffraction investigations of the annealed Ni-Cr thin films are also reported and the structural transformations taking place during the annealing treatment are determined.
1. INTRODUCTION
Ni-Cr thin film resistors are widely used in microelectronic circuits because of their high resistivity and low temperature coefficient of resistivity. In general the NiCr thin films are subjected to various annealing treatments in order to obtain good stability of their electrical parameters. In previous work’ we have established experimentally an in situ annealing treatment that ensures a stability of better than 0.1% for the electrical resistance of thin Ni-Cr films. In this work we present curves describing the variation of the electrical resistance during the annealing treatment of Ni-Cr films containing various amounts of chromium and grown at various deposition rates. The results of electron microscopy and electron diffraction investigations of the annealed films are also reported and the structural transformations taking place during the annealing treatment are determined. 2. EXPERIMENTAL
PROCEDURE
The thin Ni-Cr films were deposited on microscope glass substrates and freshly air-cleaved NaCl substrates heated to 400 “C. Ni-Cr (80: 20) wires were evaporated in a high vacuum unit either directly by sublimation (slow deposition rates) or indirectly from a heated tungsten spiral (higher deposition rates)‘. A movable screen fitted between the evaporation source and the substrates allowed us to start the deposition at various periods after the beginning of the alloy sublimation or melting. The growth of films with compositions that were different from that of the source material was possible because of the different vapour pressures of chromium ‘and nickel’. The chromium content of the films was determined by spectrophotometric techniques3. *Paper presented September 1 l-15,
at the Fourth International 1978: Paper 259.
Congress
on Thin Films,
Loughborough,
Ct. Britain,
338
M. I. BiRJECA
rt d.
After deposition the Ni-Cr samples were annealed in vacuum at 400 “C for 2 h and during this time the value of the electrical resistance was measured stepwise with a Wheatstone bridge. For the structural investigations the thin films were removed from the NaCl substrates by floating off in distilled water and were examined using a JEM- 120 electron microscope working at 100 kV. 3.
RESULTS
AND DISCUSSION
3.1. Indirect evaporation (higher deposition rates) The behaviour of the electrical resistance during the annealing of films grown at a deposition rate of 2 8, s-l depended on their chromium content and the films showed correspondingly distinct structures (Fig. 1). Ni-Cr thin films with 20 wt.% Cr showed a pronounced decrease in electrical resistance during annealing (Fig. l(a)). The electron microscopy and electron diffraction analyses revealed the presence of a mosaic of well-defined grains with the f.c.c. y-Ni structure in which were embedded contrastless islands with a disordered structure (Fig. l(b)). Pitt4 also has reported negative resistance changes on aging Ni-Cr thin films with 75 wt.:/, Ni in vacuum at 300°C. Bicknel15 has obtained similar electron micrographs and electron diffraction patterns from flash-evaporated Ni-Cr (80:20) thin films grown at room temperature and vacuum annealed at 350 “C. We assume that the processes responsible for the pronounced decrease in the electrical resistance are recrystallization (which determines the y-Ni grain growth) and segregation of the disordered phase. The films with 30 wt.“/, Cr exhibited an increase followed by a decrease and stabilization of the electrical resistance (Fig. l(c)). Selected area electron diffraction analysis6 revealed the presence of ordering processes and long range order corresponding to the Ni,Cr superlattice 7-9. The presence of the ordered domains was identified in the electron micrographs (Fig. l(d)) by the mottled and striated contrast effects. This electrical resistance behaviour is characteristic for ordering processes7-9. 3.2. Direct evaporation (slow deposition rates) Ni-Cr films grown at a deposition rate of 0.2 A s- ’ with a chromium content ranging from 24 to 32 wt.% exhibited an increase in electrical resistance during the annealing treatment (Fig. 2(a), (c)). The corresponding electron micrographs revealed a worm-like structure (Fig. 2(b)) or a periodic structure (Fig. 2(d)) depending on the chromium content. The electron diffraction patterns (Figs. 2(b) and 2(d)) showed the presence of metastable disordered phases”. Rastogi and Duwez r1 have studied the rate and the mode of crystallization of alloy by thermal analysis, X-ray diffraction, electron amorphous Fe,,P,,C,, microscopy and electrical resistivity measurements. They have established that at lower rates of heating it is possible to stop the crystallization process so that the specimen consists of clusters of microcrystals embedded in an amorphous matrix. In this temperature range the resistivity increases faster than linearly with temperature. This behaviour has also been observed in other amorphous alloys’2q ’ 3. Rastogi and Duwez’ ’ have suggested that the anomalous resistance increase is due to the
STKUCTUKALTKANSFOKMATIONSANDTHEANNEALINGOF
Ni-Cr
FILMS
212
196
0
’
10
“1
20
30
LO
11 50 60
I 90
I I I 100 110 120
Time lmmutes)
11 70 80
(4
' 10
' 20
(133000 x
200. Cc) 0
Fig. 2. (a), (c)The electrical resistance us. time of annealing and(b),(d) electron micrographs 24 wt.% Cr; (c),(d) 32 wt.% Cr) grown at a deposition rate ofO.2 A s-‘.
(b)
(4
z 202
;201. L
2 2206
0210 E a,208
1 LO
1 _.I 1 _~~I_L~p,li_ 50 60 70 80 90 100 110 120 Time lm~rwtes)
) and electron diffraction patterns of thin NiCr
1 30
films ((a), (b)
a z .-
STRUCTURAL TRANSFORMATIONS AND THE ANNEALING OF
Ni-Cr
FILMS
341
fact that microcrystalline precipitates act as scattering centres for the conduction electrons in a manner similar to that found in the early stage of precipitation (Guinier-Preston zones) in supersaturated crystalline solid solutions. According to Mottr4 the critical radius of Guinier-Preston zones for resonant scattering is close to the wavelength of the conduction electrons. Hardy and Murti” have also found a worm-like structure (Fig. 2(b)) in sputtered Ni-Cr thin films. Mader and Nowick16 have observed periodic structures in some amorphous Cu-Ag and Co-Au thin films deposited at a substrate temperature of 70 K on annealing at higher temperatures. We assume that the processes responsible for the resistance increase in Ni-Cr thin films during annealing are the nucleation and growth of microcrystalline particles in the amorphous matrix. Depending on the chromium content this process can lead to a spinodal decomposition. 4. CONCLUSIONS Our results demonstrate the complex dependences of annealing-induced structural transformations in Ni-Cr thin films on the composition and growth conditions. REFERENCES 1 2 3
M. I. Birjega and C. A. Constantin, Thin Solid Films, !(I 967/68) 396. P. Huijer, W. T. Langendam and J. A. Lely, Phi&s Tech. Rev., 24 (1962/63) 144. M. I. Birjega, C. A. Constantin, M. M. Paraschiv and N. G. Popescu-Pogrion, Rev. Roum. Phys., 16 (1971) 1229.
4 5 6
8 9 10 II 12 13 14 15 16
K. E. G. Pitt, J. R. Inst. Chem., 88 (1964) 38 1. R. W. Bicknell, Er. J. Appl. Phys., 16 (1965) 17. M. I Birjega, N. Popescu-Pogrion and C. Sarbu, Rev. Roum. Phys., 24 (1979) 107. H. cr. Baer, Z. Meta/lkd.,49(1958)614. H. J. Klein, C. R. Brooks and E. E. Stansbury, Phys. Sfarus Solidi, 38 (1970) 831. A.,Lasserre, F. Reynaud and P. Coulomb, Phi/ox Mag., 29 (1974) 665. M. 1. Birjega, N. Popescu-Pogrion, C. SPrbu and S. RQu, Thin Solid Films, 34 (1976) 153. P. K. Rastogi and P. Duwez, J. Non-Cryst. So/ids, 5 (1970) 1. H. S. Chen and D. Turnbull, J. Chem. Phys., 48 (I 968) 2560. P. L. Maitrepierre, J. Appl. Phys., 41 (1970) 52. N. F. Mott, J. Inst. Me!., 60 (1937) 267. W. R. Hardy and D. K. Murti, Thin SolidFilms, 20 (1974) 345. S. Mader and A. S. Nowick, Acra Metal/., I5 (1967) 215.