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Au-SiO2 thin resistive films
Thin Solid Films, 164 (1988) 129-133
129 INVESTIGATION OF SLIDING CONTACT RESISTANCE OF Ni-Cr/Au A N D Ni~Cr/Au-SiO 2 T H I N R E S I S T I V E F I L M S * A. BANOVEC AND A. ZALAR
lnktitut za elektroniko in vakuumsko tehniko, Teslova 30, 61111 Ljubljana (Yugoslavia)
The contact resistance of N i - C r / A u and N i - C r / A u - S i O 2 thin resistive films with gold-plated metal sliding contact surface have been investigated. The thin resistive films were prepared by r.f. sputtering through a mechanical mask onto polished glass substrates. The interaction at the interface of thin resistive film and the contact slider determines the contact resistance and electrical noise of the system. Contact resistance is influenced by the composition of the thin film, sheet resistance, contact pressure, surface morphology, and diffusion process occurring during the annealing of thin films. The surfaces and depth profiles of thin resistive films were analysed by the Auger electron spectroscopy method. The lowest contact resistance was obtained with the N i - C r / A u - S i O 2 thin resistive film deposited onto polished glass substrates contacted with a wire brush made of N i - P d alloy.
1. INTRODUCTION C o m p a r e d with bulk materials, the electrical contact of vacuum-evaporated and sputtered thin resistive films requires a special treatment. Some phenomena which are characteristic of the contact with bulk materials are also characteristic of thin films systems or of systems where at least one of the partners is a thin film 1'2. The contact performance of thin films is characterized by the fact that the contact properties are, to a large extent, dependent on numerous factors such as the quality of the substrates surface, the method of deposition and conditions during the deposition of the thin film 3'4. Various phenomena which subsequently take place on the surface itself or in the structure of the thin film may have a considerable influence on the contact resistance 5'6. These may be various chemical reactions, diffusion and structural rearrangement, particularly at various annealing temperatures in different residual gases 7'8. Since we are dealing with thin films, tunnel conduction at the surface of the film also takes part in the conductivity mechanism in addition to the metallic contact 2. The action of a contact slider on thin films is direct or indirect 9'~°. We attempted to establish a correlation between the contact resistance Rc and chemical composition of thin films obtained by the Auger electron spectroscopy (AES) analysis. * Paper presented at the 7th International Conference on Thin Films, New Delhi, December 7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printed in The Netherlands
130
A. BANOVEC, A. ZALAR
2. EXPERIMENT
Two types of thin films were investigated: an r.f.-sputtered 80wt.% Ni-20wt.~o Cr thin film with a very thin evaporated gold layer. The top gold layer 1 nm thick is discontinuous and has an island structure. The second type of thin film structure was an 80wt.~oNi-20wt.~oCr base layer covered with a 70wt.~oAu-30wt.~oSiO2 cermet thin film. The surface layer of Au-SiO2 was deposited by r.f. sputtering from an SiO2 target with the diameter of 200 m m and with a narrow gold strip placed across it. The vacuum chamber was evacuated to a pressure of 2 x 1 0 - 4 p a . The deposition was carried out in pure argon at a pressure of 8-10 Pa. The thin films were sputtered through mechanical masks onto heated glass substrates (Schott type 8250). The temperature of the rotated substrates during sputtering was 250 °C. After the deposition, the thin films were annealed in air for 48 h at 200 °C. Some samples were heat treated at 320 °C. The mean values of contact resistance /~ were measured with two types of contact mechanisms according to the IEC standard 1~. The indirect contact consisted of a gold-plated nickel foil 40 ram thick, which was pressed by a small steel ball with a force of 30 N exerted by a steel spring. The direct sliding contact consisted of a wire brush made of N i - P d alloy with a wire diameter of 80 ram. Figure 1 shows both mechanisms and the wear of the thin films. In the indirect contact mechanism, a contact between the partners is created with the pressing of the stationary contact plate onto the surface of the thin film. There is one circular contact surface with a surface area of several tenths of a square millimetre on which a contact load of 30 N exerts a contact pressure of(3-10) x 107 Pa. Direct sliding contact occurs directly on the thin resistive film. The friction during the sliding of numerous wire elements caused the wear of both contact partners. The contact force is the same in both mechanisms, but, owing to the greater number of contact points of the wire brush, the pressure on the thin film is reduced to 1 x 106 Pa. The results of electrical measurements of contact resistance have been correlated with the results of the surface analysis of thin films using the AES method. The AES analysis of the goldplated nickel foil and the N i - P d wire brush indicated that the surface was pure without substantial carbon and oxygen contamination. 3.
RESULTS AND DISCUSSION
Contact resistance was measured continuously along the resistance path 0.8 m m wide and 13 m m long. The difference between both contacting mechanisms is in the contact pressure exerted on the contact surface of the thin film. The results indicate that the mean measured contact resistance/~c depends on the thrust F~ of the contact element and is expressed by the empirical e q u a t i o n / ~ = kFc- a/2. This relationship does not depend on the type of thin resistive film or on the contact mechanism. There is a linear relationship between the sheet resistance of the thin resistive film and the contact resistance, since the contact resistance increased with increasing sheet resistance in the range of 10-250 fL as shown in Fig. 2. In Fig. 2, the contact resistance is given in the function of the sheet resistance for the N i - C r / A u and Ni-Cr/Au-SiO2 thin films using the indirect and direct contact mechanisms. The contact resistance depends on the deposition, annealing and measurement con-
INVESTIGATION OF SLIDING CONTACT RESISTANCE
131
eel b a l l
contact sheet thin film
~
/// /Y/ /J ~ _ ~ s u b s t r a t e
(a)
(b)
Fig. 1. (a) Schematic diagram of the indirect contact mechanism; (b) Scanning electron micrograph of direct wire contact mechanism together with the thin resistive film.
af
u7
d
IIZ~ 2
i
i0 8
~e 8 2
8
----O-- N i C r / A u - - + - - NiCr/Au
6 4
t indirect
NiCr/Au SiO2 "-b-'-- NiCr/Au SiO2
)
direct
I
L
h i i
i
~
i
t [
t
2
4
6
2
4
6
8
2
8
10 2
Sheet
10 :~
Resistance
R ~ (~'~)
Fig. 2. Contact resistance as a function of the sheet resistance for the N i - C r / A u thin film (curves a and b) and N i - C r / A u - S i O 2 (curves c and d) for the indirect contact mechanism (curves a, b, c) and the direct contact m e c h a n i s m (curve d). Curves b, c and d are obtained on samples annealed in the air at 200 °C and the sample of curve a was annealed at 320 °C.
132
A. B A N O V E C , A. Z A L A R
ditions. The highest contact resistance was obtained on the N i - C r / A u layer annealed at 320 °C in the air and measured with the indirect contact mechanism. During annealing at higher temperature, a thicker oxide layer grows which increases the contact resistance. The smaller values of contact resistance are noticeable in the case of the direct contact mechanism where the contact resistance is lower by almost one order of magnitude in comparison with that for the same thin resistive film but obtained with the indirect contact mechanism. Thin discontinuous gold layer did not prevent the oxidation of the surface part of N i - C r base layer. The thin contact Au-SiO z cermet film deposited onto the N i - C r base resistive layer has no influence on the sheet resistance, but the contact resistance is reduced in comparison with that obtained for N i - C r / A u thin films. Comparison of the surface chemical compositions of fresh and annealed N i - C r / A u and Ni-Cr/Au-SiO2 thin resistive films determined using the AES method shows that the surfaces are contaminated with a carbon layer after annealing (Table I). The carbon contamination of the Ni-Cr/Au-SiO2 thin film does not have a major influence on the contact resistance which is mainly determined with the contact Au-SiO2 layer. Figure 3 shows the typical chemical composition of the surface layer of annealed N i - C r / A u - S i O 2 thin resistive film after 48 h at 200 °C. TABLE I COMPARISON OF CHEMICAL COMPOSITION AT THE SURFACE OF FRESH AND THERMAL ANI'qEALEDNi-Cr/Au AND Ni-Cr/Au-SiO2 THIN RESISTIVE FILMS OBTAINED BY AUGER ELECTRON SPECTROSCOPY
Thin film
Ni C r / A u , fresh Ni C r / A u , a n n e a l e d N i - C r / A u - S i O 2, fresh N i - C r / A u SiO2, a n n e a l e d
Chemical composition (at.5;) Au
Si
32.0 1.0 38.5 25.0
0.5 0.5 6.5 9.5
S
CI
1.5
5.0
1.5
4.0
C 18.0 58.0 5.5 42.0
N
!.5 1.0
O
Cr
Ni
8.0 9.5 20.5 8.0
-9.0
41.0 23.0 18.5 7.5
AES analysis of the sputter-deposited N i - C r base layer showed a homogenous composition throughout the depth profile. The contact layer of Au-SiO 2 cermet thin film on the N i - C r base layer has no bearing on the sheet resistance, but the contact resistance is reduced in comparison with the results obtained on N i - C r / A u thin resistive films. The presence of the SiO2 matrix did not cause an increase in contact resistance; instead, it prevented a chemical reaction between the N i - C r and the atmosphere. The gold islands in the SiOz matrix enable the lower contact resistance between the contact slider and the base N i - C r resistive layer. 4. CONCLUSIONS The contact resistance of N i - C r / A u and Ni-Cr/Au-SiO2 thin film depends on the contact mechanism. Under direct contact it is one order of magnitude lower than it is under indirect contact. In all cases, the higher contact pressure of the slider reduces the contact resistance. The contact resistance after annealing of N i - C r / A u thin resistive film at a temperature of 320 °C increased as a result of oxidation. The reason for the lower contact resistance ofNi Cr/Au-SiO2 on the annealed thin films
133
INVESTIGATION OF SLIDING CONTACT RESISTANCE ~ I00
z 8oj
4O
q
20
0
2
4
6
_
8
10
12
14
16
18
20
S p u t t e r i n g time (min}
Fig. 3. AES sputter depth profile of the surface part of the r.f.-sputtered Ni-Cr/Au-SiO 2 thin resistive film after annealing at 200 °C for 48 h.
is t h e p r e s e n c e o f g o l d i s l a n d s a t t h e c o n t a c t s u r f a c e a n d o n t h a t p a r t o f S i O 2 m a t r i x w h i c h p r e v e n t s s u r f a c e r e a c t i o n s o f t h e N i C r t h i n film. T h e c a r b o n c o n t a m i n a t i o n which took place during annealing has no marked influence on the contact r e s i s t a n c e o f t h e s e t h i n r e s i s t i v e films. REFERENCES 1 2 3 4 5 6 7 8 9 10 ll
S. Hofmann and A. Zalar, Thin Solid Films, 39 (1978) 219. M. Antler, IEEE Circ. Devices Mag., 3 (16,2) (1978) 8. L. Toth, Vacuum, 37(1987) 102. A. Zalar, A. Banovec and S. Hofmann, in R. Dobrozemsky et al. (eds.), Proc. 7th Int. Vacuum Congr. and 3rd lnt. Conf. on Solid Surfaces, Vol. III, Vienna, 1977, p. 2303. F. Warkusz, Thin Solid Films, 148 (1987) 343. R.W. Balluffi and J. M. Blakely, Thin Solid Films, 25 (1975) 363. F. Warkusz, Vacuum, 37 (1987) 115. J.F. Smith and D. C, Hinson, Solid State Technol., 29 (11) (1986) 135. A. Zalar, M.Sc. Thesis, Univerza Edvarda Kardelja v Ljubljani FNT-VTO Montanistika, 1981. A. Banovec, R. Tavzes, M. Kern, K. Po~un and L. Koller, Proc. 22nd. Yugoslav Symp. on Electronic Components and Materials SD-86, OtoOee na Krki, 1986 p. 157. International Electrotechnical Commission, IEC Recommendation, Publication 391-1, Potentiometers Part 1 : Terms and Methods of Tests, 1973.
129 INVESTIGATION OF SLIDING CONTACT RESISTANCE OF Ni-Cr/Au A N D Ni~Cr/Au-SiO 2 T H I N R E S I S T I V E F I L M S * A. BANOVEC AND A. ZALAR
lnktitut za elektroniko in vakuumsko tehniko, Teslova 30, 61111 Ljubljana (Yugoslavia)
The contact resistance of N i - C r / A u and N i - C r / A u - S i O 2 thin resistive films with gold-plated metal sliding contact surface have been investigated. The thin resistive films were prepared by r.f. sputtering through a mechanical mask onto polished glass substrates. The interaction at the interface of thin resistive film and the contact slider determines the contact resistance and electrical noise of the system. Contact resistance is influenced by the composition of the thin film, sheet resistance, contact pressure, surface morphology, and diffusion process occurring during the annealing of thin films. The surfaces and depth profiles of thin resistive films were analysed by the Auger electron spectroscopy method. The lowest contact resistance was obtained with the N i - C r / A u - S i O 2 thin resistive film deposited onto polished glass substrates contacted with a wire brush made of N i - P d alloy.
1. INTRODUCTION C o m p a r e d with bulk materials, the electrical contact of vacuum-evaporated and sputtered thin resistive films requires a special treatment. Some phenomena which are characteristic of the contact with bulk materials are also characteristic of thin films systems or of systems where at least one of the partners is a thin film 1'2. The contact performance of thin films is characterized by the fact that the contact properties are, to a large extent, dependent on numerous factors such as the quality of the substrates surface, the method of deposition and conditions during the deposition of the thin film 3'4. Various phenomena which subsequently take place on the surface itself or in the structure of the thin film may have a considerable influence on the contact resistance 5'6. These may be various chemical reactions, diffusion and structural rearrangement, particularly at various annealing temperatures in different residual gases 7'8. Since we are dealing with thin films, tunnel conduction at the surface of the film also takes part in the conductivity mechanism in addition to the metallic contact 2. The action of a contact slider on thin films is direct or indirect 9'~°. We attempted to establish a correlation between the contact resistance Rc and chemical composition of thin films obtained by the Auger electron spectroscopy (AES) analysis. * Paper presented at the 7th International Conference on Thin Films, New Delhi, December 7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printed in The Netherlands
130
A. BANOVEC, A. ZALAR
2. EXPERIMENT
Two types of thin films were investigated: an r.f.-sputtered 80wt.% Ni-20wt.~o Cr thin film with a very thin evaporated gold layer. The top gold layer 1 nm thick is discontinuous and has an island structure. The second type of thin film structure was an 80wt.~oNi-20wt.~oCr base layer covered with a 70wt.~oAu-30wt.~oSiO2 cermet thin film. The surface layer of Au-SiO2 was deposited by r.f. sputtering from an SiO2 target with the diameter of 200 m m and with a narrow gold strip placed across it. The vacuum chamber was evacuated to a pressure of 2 x 1 0 - 4 p a . The deposition was carried out in pure argon at a pressure of 8-10 Pa. The thin films were sputtered through mechanical masks onto heated glass substrates (Schott type 8250). The temperature of the rotated substrates during sputtering was 250 °C. After the deposition, the thin films were annealed in air for 48 h at 200 °C. Some samples were heat treated at 320 °C. The mean values of contact resistance /~ were measured with two types of contact mechanisms according to the IEC standard 1~. The indirect contact consisted of a gold-plated nickel foil 40 ram thick, which was pressed by a small steel ball with a force of 30 N exerted by a steel spring. The direct sliding contact consisted of a wire brush made of N i - P d alloy with a wire diameter of 80 ram. Figure 1 shows both mechanisms and the wear of the thin films. In the indirect contact mechanism, a contact between the partners is created with the pressing of the stationary contact plate onto the surface of the thin film. There is one circular contact surface with a surface area of several tenths of a square millimetre on which a contact load of 30 N exerts a contact pressure of(3-10) x 107 Pa. Direct sliding contact occurs directly on the thin resistive film. The friction during the sliding of numerous wire elements caused the wear of both contact partners. The contact force is the same in both mechanisms, but, owing to the greater number of contact points of the wire brush, the pressure on the thin film is reduced to 1 x 106 Pa. The results of electrical measurements of contact resistance have been correlated with the results of the surface analysis of thin films using the AES method. The AES analysis of the goldplated nickel foil and the N i - P d wire brush indicated that the surface was pure without substantial carbon and oxygen contamination. 3.
RESULTS AND DISCUSSION
Contact resistance was measured continuously along the resistance path 0.8 m m wide and 13 m m long. The difference between both contacting mechanisms is in the contact pressure exerted on the contact surface of the thin film. The results indicate that the mean measured contact resistance/~c depends on the thrust F~ of the contact element and is expressed by the empirical e q u a t i o n / ~ = kFc- a/2. This relationship does not depend on the type of thin resistive film or on the contact mechanism. There is a linear relationship between the sheet resistance of the thin resistive film and the contact resistance, since the contact resistance increased with increasing sheet resistance in the range of 10-250 fL as shown in Fig. 2. In Fig. 2, the contact resistance is given in the function of the sheet resistance for the N i - C r / A u and Ni-Cr/Au-SiO2 thin films using the indirect and direct contact mechanisms. The contact resistance depends on the deposition, annealing and measurement con-
INVESTIGATION OF SLIDING CONTACT RESISTANCE
131
eel b a l l
contact sheet thin film
~
/// /Y/ /J ~ _ ~ s u b s t r a t e
(a)
(b)
Fig. 1. (a) Schematic diagram of the indirect contact mechanism; (b) Scanning electron micrograph of direct wire contact mechanism together with the thin resistive film.
af
u7
d
IIZ~ 2
i
i0 8
~e 8 2
8
----O-- N i C r / A u - - + - - NiCr/Au
6 4
t indirect
NiCr/Au SiO2 "-b-'-- NiCr/Au SiO2
)
direct
I
L
h i i
i
~
i
t [
t
2
4
6
2
4
6
8
2
8
10 2
Sheet
10 :~
Resistance
R ~ (~'~)
Fig. 2. Contact resistance as a function of the sheet resistance for the N i - C r / A u thin film (curves a and b) and N i - C r / A u - S i O 2 (curves c and d) for the indirect contact mechanism (curves a, b, c) and the direct contact m e c h a n i s m (curve d). Curves b, c and d are obtained on samples annealed in the air at 200 °C and the sample of curve a was annealed at 320 °C.
132
A. B A N O V E C , A. Z A L A R
ditions. The highest contact resistance was obtained on the N i - C r / A u layer annealed at 320 °C in the air and measured with the indirect contact mechanism. During annealing at higher temperature, a thicker oxide layer grows which increases the contact resistance. The smaller values of contact resistance are noticeable in the case of the direct contact mechanism where the contact resistance is lower by almost one order of magnitude in comparison with that for the same thin resistive film but obtained with the indirect contact mechanism. Thin discontinuous gold layer did not prevent the oxidation of the surface part of N i - C r base layer. The thin contact Au-SiO z cermet film deposited onto the N i - C r base resistive layer has no influence on the sheet resistance, but the contact resistance is reduced in comparison with that obtained for N i - C r / A u thin films. Comparison of the surface chemical compositions of fresh and annealed N i - C r / A u and Ni-Cr/Au-SiO2 thin resistive films determined using the AES method shows that the surfaces are contaminated with a carbon layer after annealing (Table I). The carbon contamination of the Ni-Cr/Au-SiO2 thin film does not have a major influence on the contact resistance which is mainly determined with the contact Au-SiO2 layer. Figure 3 shows the typical chemical composition of the surface layer of annealed N i - C r / A u - S i O 2 thin resistive film after 48 h at 200 °C. TABLE I COMPARISON OF CHEMICAL COMPOSITION AT THE SURFACE OF FRESH AND THERMAL ANI'qEALEDNi-Cr/Au AND Ni-Cr/Au-SiO2 THIN RESISTIVE FILMS OBTAINED BY AUGER ELECTRON SPECTROSCOPY
Thin film
Ni C r / A u , fresh Ni C r / A u , a n n e a l e d N i - C r / A u - S i O 2, fresh N i - C r / A u SiO2, a n n e a l e d
Chemical composition (at.5;) Au
Si
32.0 1.0 38.5 25.0
0.5 0.5 6.5 9.5
S
CI
1.5
5.0
1.5
4.0
C 18.0 58.0 5.5 42.0
N
!.5 1.0
O
Cr
Ni
8.0 9.5 20.5 8.0
-9.0
41.0 23.0 18.5 7.5
AES analysis of the sputter-deposited N i - C r base layer showed a homogenous composition throughout the depth profile. The contact layer of Au-SiO 2 cermet thin film on the N i - C r base layer has no bearing on the sheet resistance, but the contact resistance is reduced in comparison with the results obtained on N i - C r / A u thin resistive films. The presence of the SiO2 matrix did not cause an increase in contact resistance; instead, it prevented a chemical reaction between the N i - C r and the atmosphere. The gold islands in the SiOz matrix enable the lower contact resistance between the contact slider and the base N i - C r resistive layer. 4. CONCLUSIONS The contact resistance of N i - C r / A u and Ni-Cr/Au-SiO2 thin film depends on the contact mechanism. Under direct contact it is one order of magnitude lower than it is under indirect contact. In all cases, the higher contact pressure of the slider reduces the contact resistance. The contact resistance after annealing of N i - C r / A u thin resistive film at a temperature of 320 °C increased as a result of oxidation. The reason for the lower contact resistance ofNi Cr/Au-SiO2 on the annealed thin films
133
INVESTIGATION OF SLIDING CONTACT RESISTANCE ~ I00
z 8oj
4O
q
20
0
2
4
6
_
8
10
12
14
16
18
20
S p u t t e r i n g time (min}
Fig. 3. AES sputter depth profile of the surface part of the r.f.-sputtered Ni-Cr/Au-SiO 2 thin resistive film after annealing at 200 °C for 48 h.
is t h e p r e s e n c e o f g o l d i s l a n d s a t t h e c o n t a c t s u r f a c e a n d o n t h a t p a r t o f S i O 2 m a t r i x w h i c h p r e v e n t s s u r f a c e r e a c t i o n s o f t h e N i C r t h i n film. T h e c a r b o n c o n t a m i n a t i o n which took place during annealing has no marked influence on the contact r e s i s t a n c e o f t h e s e t h i n r e s i s t i v e films. REFERENCES 1 2 3 4 5 6 7 8 9 10 ll
S. Hofmann and A. Zalar, Thin Solid Films, 39 (1978) 219. M. Antler, IEEE Circ. Devices Mag., 3 (16,2) (1978) 8. L. Toth, Vacuum, 37(1987) 102. A. Zalar, A. Banovec and S. Hofmann, in R. Dobrozemsky et al. (eds.), Proc. 7th Int. Vacuum Congr. and 3rd lnt. Conf. on Solid Surfaces, Vol. III, Vienna, 1977, p. 2303. F. Warkusz, Thin Solid Films, 148 (1987) 343. R.W. Balluffi and J. M. Blakely, Thin Solid Films, 25 (1975) 363. F. Warkusz, Vacuum, 37 (1987) 115. J.F. Smith and D. C, Hinson, Solid State Technol., 29 (11) (1986) 135. A. Zalar, M.Sc. Thesis, Univerza Edvarda Kardelja v Ljubljani FNT-VTO Montanistika, 1981. A. Banovec, R. Tavzes, M. Kern, K. Po~un and L. Koller, Proc. 22nd. Yugoslav Symp. on Electronic Components and Materials SD-86, OtoOee na Krki, 1986 p. 157. International Electrotechnical Commission, IEC Recommendation, Publication 391-1, Potentiometers Part 1 : Terms and Methods of Tests, 1973.