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Structural and electrical properties of Ni films grown on Si(100) and SiO2 by d.c. bias sputtering
Thin Solid Films, 229 (1993) 107-112
107
Structural and electrical properties of Ni films grown on Si(100) and SiO2 by d.c. bias sputtering* Hong Qiu a, Gyorgy Sfifrfin b, Bela Pecz b, Peter B. Barna b, Akio Kosuge a, Hisashi N a k a i a, Shigemi Yugo a and Mituru Hashimoto a, ** aThe University of Electro-Communications, I-5-1 Chofugaoka, Chofu-shi, Tokyo I82 (Japan) bResearch Institute of Technical Physics of the Hungarian Academy of Science, PO Box 76, H-1325 Budapest (Hungary)
(Received November 15, 1992; accepted January 25, 1993)
Abstract Ni films are deposited on both pure and SiO2-covered Si(100) substrates at 190 °C by d.c. diode sputtering at 2.5 kV * in pure Ar. A d.c. bias voltage Vs (0 to - 8 0 V) is applied on the substrates during the deposition. A study is made mainly of the effect of Vs on the structural and electrical properties of the films by reflection high energy electron diffraction, transmission electron microscopy and resistance measurements from 30 to 135 °C. Film-substrate interdiffusion is observed in Ni/Si but not in Ni/SiO2. Ni adatoms diffuse preferentially along Si(lll) with the formation of Ni2Si in the Si crystal. The grain size and diffusion depth of the Ni film increase with Vs. A non-columnar structure with voids along the interface is induced by Ni diffusion into Si as Vs ranges from 0 to - 2 0 V in Ni/Si whereas a slightly inclined columnar structure is induced at V~= - 2 0 V in Ni/SiO2. Thick columns grow at V~= --80 V in both systems. The temperature coefficient of resistance, r/, is positive for both Ni/Si and Ni/SiO2. The dependences of r/ on V~ can be understood in terms of the above-mentioned structural changes with Vs.
1. Introduction The Ni/Si (Ni on Si) system has been studied by m a n y researchers. Schaffer et al. [1] reported that the structures of Ni films r.f. sputter deposited on both Si(I00) and Si( I 1 I) substrates depend on the film thickness d: an a m o r p h o u s phase is induced at d = 1-2.8 nm whereas a polycrystalline Ni2Si c o m p o u n d is formed at d = 3 nm. Tung et al. [2] obtained ultrathin NiSi2 films grown epitaxially on both Si(100) and Si(111) at 4 5 0 500 °C. Van Loenen et al. [3] observed the growth of Ni2Si islands at the N i - S i interface at r o o m temperature. A bias-sputtering teehnique has been used to modify the physical and chemical properties of the films. Maissel and Schaible [4] reported that highly pure films are produced by sputtering when a negative bias voltage Vs is applied to the substrate during the deposition. Usami et al. [5] reported that the electrical properties of a d.c. sputter-deposited Ge film are modified at an appropriate value of Vs. Ohmi et al. [6]" obtained a high quality Si(100) film by r.f. sputtering with an optimal value of Vs at 350 °C. *An outline of this paper was presented at the 6th International Conference on Solid Films and Surfaces (ICSFS-6), Paris, June 29-July 3, 1992. **Author to whom correspondence should be adressed.
0040-6090/93/$6.00
We have prepared polycrystalline Ni films about 90-180 nm thick on both pure and SiO2-covered Si(I00) substrates by d.c. diode sputtering at 2.5 kV in pure Ar gas. A negative bias voltage Vs is applied to the substrates during the deposition in order to modify the structures of the film and/or the film-substrate interface. The present work a i m s to study the effect of Vs on the structural and electrical properties of the films by reflection high energy electron diffraction ( R H E E D ) , P t - C surface replica transmission electron microscopy (TEM), cross-sectional T E M ( X - T E M ) and measurements of electrical resistance as a function of temperature.
2. Experimental procedures A sketch of the sputter deposition system is shown in Fig. 1. Both the target (c) and substrate (e), which are 50 m m distant from each other, are closely surrounded by a cylindrical liquid nitrogen trap ( 140 m m in diameter) (d) inside the chamber (b). The chamber was evacuated to a pressure lower than 3 x 10 -5 Pa by a silicone oil diffusion p u m p equipped with a liquid nitrogen trap (j). The substrate, the temperature of which was measured using a c o p p e r - c o n s t a n t a n thermocouple (n), was baked out at 200 °C for 1 h with the use of a Nichrome heater (f) and the t e m p e r a t u r e was
@ 1993- Elsevier Sequoia. All rights reserved
108
H. Qiu et al. / Properties o f Ni films grown on Si(lO0) and SiO 2
k.---o
O-
line(s) of NiO are detected in addition to the main strong lines of Ni. These patterns do not change on changing Vs. Thus it can be said that the films deposited under the present conditions retain a polycrystalline structure of Ni which may be partially oxidized at the film surface.
if------ a
-
f
g
Pump
Fig. 1. Schematic diagram of d.c. bias sputtering: a, guide for water cooling the target; b, chamber; c, Ni target; d, cylindrical liquid nitrogen trap; e, substrate; f, Nichrome heater; g, substrate holder; h, insulator layer; i, bias supply; j, liquid nitrogen trap; k, to target voltage supply; t, guide for argon gas; m, inlets of liquid nitrogen; n, copper-constantan thermocouple; o, vacuum gauge.
decreased to 100 °C prior to the deposition. Then, after increasing the pressure to 4.0 Pa by supplying Ar gas (at least 99.9995% purity), a diode sputtering voltage of - 2 . 5 kV was applied to the Ni target (99.99% purity, 63.5 mm in diameter) in order to deposit the Ni film on the substrate. A d.c. bias voltage Vs (0 to - 8 0 V) was applied to the substrates during the deposition. The Si(100) substrates were precleaned by chemical processes: boiled in trichloroethylene solution, rinsed in distilled water, etched in 5% H F acid, ultrasonically rinsed in acetone and then dried by an ultrapure N 2 gas jet. The sample for X-TEM was prepared as follows [7]: it was first thinned by mechanical grinding to about 50 ~tm thickness and finally thinned by ion milling. The mean thickness of the film was measured directly from the microphotograph of the X - T E M image and also examined by the Tolansky technique. The temperature coefficient of resistance (TCR), ~I = R - I d R / d T , was measured from 30 to 135 °C by the four-point probe technique. An SiO2 film 10 nm thick was predeposited on the Ni film in another vacuum chamber in order to prevent it from oxidation during the measurement of q.
3. R e s u l t s a n d d i s c u s s i o n
3.1. Structural properties 3. I. 1. R H E E D R H E E D patterns of Ni films grown on Si(100) and SiO2 at V s = - 2 0 V are shown in Figs. 2a and 2b respectively. Figure 2 shows that very weak diffraction
42O) 311) !20) 220) Fig. 2. R H E E D patterns of Ni films deposited at V~ = - 2 0 V on (a) Si(100) substrate and (b) SiO2 substrate.
3.1.2. P t - C surface replica T E M Electron micrographs of P t - C surface replicas of Ni films grown on Si(100) and SiO2 at Vs=0, - 2 0 and - 8 0 V are shown in Figs. 3a-3f. On increasing Vs to - 8 0 V, the grain sizes of Ni films on both substrates apparently increase. This effect of Vs on the grain size is more evident on Si(100) than on SiO2. 3.1.3. Cross-sectional T E M Figure 4 shows bright and dark field X-TEM images of Ni films grown o n S i O 2 at Vs = 0, - 2 0 and - 8 0 V and Fig. 5 shows bright and dark field images of an Ni film grown on a naturally oxidized surface of Si(100) at Vs = - 2 0 V. The dark field images were obtained from diffraction lines of N i ( l l l ) and Ni(200). Atomic interdiffusion at the interface does not occur for the Ni/SiO2 specimen as shown in Fig. 4 and even a naturally formed oxide layer of Si tends to prevent .Ni atoms from diffusing into Si as shown in Fig. 5. As seen from the dark field images in Fig. 4, Ni films o n S i O 2 generally have a columnar structure which is well established with thicker grain growth particularly at V s = - 8 0 V but is less prominent at V s = - 2 0 V . Under the present experimental conditions the grain growth of the Ni film grown on SiO2 may be mainly determined by the shadowing effect due to the low mobility of Ni adatoms and by the bombarding effect of Ar ions accelerated at Vs, which increase the mobility of Ni adatoms and sputter off or incorporate residual impurities, as reported previously [4, 8, 9]. With respect to the effect on the mobility of Ni adatoms, an increase in Vs and an increase in substrate temperature have nearly the same effect but cannot be considered as identical, because the bombarding effect is based on momentum transfer and the thermal effect on heat transfer [9]. Therefore the grain growth is more prominent with increasing Vs as shown in Figs. 4d-4f. However, at some appropriate value of Vs the bombarding effect may incidentally oppose the shadowing effect and induce an inclined columnar structure as shown in Fig. 4e. When V~ increases up to - 8 0 V, the bombarding effect becomes dominant in inducing thick columns in the grain structure as a result of increasing the mobility of Ni adatoms as shown in Fig. 4f. Bright and dark field X-TEM images of Ni films grown on Si(100) at Vs = 0, - 2 0 and 80 V are shown in Fig. 6, where the Si(110) plane is parallel to the surface of the photograph. It can be seen from these X-TEM
H. Qiu et al. / Properties o f Ni filrns grown on Si(lO0) and SiO,
109
+
1 Fig. 3. P t - C surface replica T E M images of Ni films deposited on Si(100) at V~ = (a) 0, (b) - 2 0 and (c) - 8 0 V and on SiO 2 at V~ = (d) 0, (e) - 2 0 and (f) - 8 0 V. The bar at the top of the figure represents a c o m m o n scale of 100 nm.
a
b
• J'
~
d
J
i
•
Fig. 4. Cross-sectional T E M images of Ni films grown on SiO 2 under bright field at V~ = (a) 0, (b) - 2 0 and (c) - 8 0 V and under dark field at Vs = (d) 0, (e) - 2 0 and (f) - 8 0 V. The dark field images, including those in Fig. 5b and Figs. 6 d - 6 f , were taken from parts of diffraction lines of N i ( l l I) and Ni(200). The bar at the top of the figure represents a c o m m o n scale of 100 nm.
1 Fig. 5. Cross-sectional T E M images of an Ni film deposited on a naturally oxidized surface of Si(100) at V~ = - 2 0 V under (a) bright field and (b) dark field. The bar at the top of the figure represents a c o m m o n scale of 100 nm.
images that a diffusion layer 60-90 nm thick is formed in the Si substrate rather than in the Ni film, i.e. the dominant diffusing species are clearly Ni atoms rather than Si atoms, and in addition the diffusion front of Ni atoms appears as a series of isosceles triangles. Figure 7 shows a close-up photograph of the X - T E M image of the diffusion front. From this photograph the vertex angle of the isosceles triangle is measured as about 70 °. As is well known, for a diamond (cubic) lattice structure, shadows of two mutually intersecting (111 ) direction (or lines) projected on a (110) plane construct vertically opposite angles of 70.5 °. The microdiffraction
H. Qiu et al. / Properties o f Ni films grown on Si(100) and SiO:
I I0
dg
.
.
.
.
.
.
.
4 N Fig. 6. Cross-sectional TEM images of Ni films grown on Si(100) under bright field at Vs = (a) 0, (b) - 2 0 and (c) - 8 0 V and under dark field at ~(, = (d) 0, (e) - 2 0 and (f) - 8 0 V. The bar at the top of the figure represents a common scale of 100 nm.
Fig. 8. A microdiffraction pattern (right) of X-TEM image (left) of Ni-Si diffusion layer at Vs = - 2 0 V. The bar at the top of the figure represents a scale of 100 nm.
Fig. 7. A close-up photograph of X-TEM image of diffusion front presented in Fig. 6a. The bar at the top of the figure represents a scale of 25 ran.
p a t t e r n o f the diffusion layer o f a specimen p r e p a r e d at Iz~ = - 2 0 V is shown with the X - T E M i m a g e in Fig. 8. F r o m b o t h the m i c r o d i f f r a c t i o n p a t t e r n in Fig. 8 a n d the d a r k field i m a g e in Fig. 6 it is c o n f i r m e d that the diffusion layer consists o f Ni2Si, N i a n d Si. Thus, in conclusion, Ni a t o m s diffuse preferentially a l o n g S i ( l l l ) planes a n d interstitially into the Si crystal to f o r m the silicide Ni2Si inside the diffusion layer. This result is consistent with p r e v i o u s reports: C h u et al. [ 10] r e p o r t e d f r o m a g r o w t h study o f silicides that N i a t o m s are the d o m i n a n t diffusion species which f o r m Ni2Si in the Si crystal, while T u [11] p r o p o s e d in his review p a p e r that at 1 0 0 - 2 0 0 °C N i a t o m s diffuse interstitially into Si to f o r m the silicide Ni2Si whose structure facilitates the diffusion o f Ni a t o m s .
As shown in Figs. 6 d - 6 f , for Ni/Si(100), as Vs increases, the grain g r o w t h ts c o n s t a n t l y p r o m o t e d b u t the c o l u m n a r g r o w t h is r a t h e r suppressed at Vs = 0 V (Fig. 6d) as well as at Vs = - 2 0 V (Fig. 6e). Besides, voids are i n d u c e d a l o n g the interface in the film when V~ ranges f r o m 0 to - 2 0 V. The c o l u m n a r structure a p p e a r s at higher values o f Vs, i.e. at V s = - 8 0 V a p p r e c i a b l y thick c o l u m n s g r o w inside the film w i t h o u t voids near the interface as shown in Fig. 6f. It should be n o t e d t h a t fine grains are a c c u m u l a t e d a l o n g the interface for a n y value o f Vs. Thus, in c o n t r a s t to the case o f Ni/SiO2, the grain g r o w t h m e c h a n i s m o f the Ni film on Si(100) m u s t be influenced also by the N i a t o m i c diffusion into Si. T h e n o n - c o l u m n a r v o i d structure i n d u c e d at low values o f Vs (0 to - 2 0 V) a n d the a c c u m u l a t i o n o f fine grains a l o n g the interface m a y originate m a i n l y f r o m the diffusion p h e n o m e n o n . The c o l u m n a r grains i n d u c e d at Vs = - 8 0 V are a p p a r e n t l y thicker on Si(100) ( F i g . 6f) t h a n on SiOz (Fig. 40. This m a y be due to an epitaxial effect i n d u c e d by the
H. Qiu et al. / Properties of Ni films grown on Si(lO0) and Si02
monocrystalline surface of the substrate (pure Si or Ni2Si ), in addition to the bombarding effect. Finally, with respect to the grain size, the results from X-TEM (Figs. 4 and 6) are consistent with those from P t - C surface replica T E M (Fig. 3). 3.2. Temperature coefficient of electrical resistance If the effect of electron scattering at the film surfaces is negligible, the TCR r/for a polycrystaIline film can be expressed as [ 12] =
Poqo Po + Pi + Pg
(1)
15
I
o IO: x
$ o F--
' -,C'o
15
°i0 x
g
'4o'-~o'4o '-~o' Vs (v)
where P0 is the reference resistivity of bulk material at 0 °C, and q0 is the TCR of bulk material, also referred to 0 °C, and Pi and pg are the temperature-independent resistivities induced by electron scattering at impurities and grain boundaries respectively. Figure 9 shows the relationships between r/(Ni/SiO2) and V~ for two thicknesses. It is seen that r/ (Ni/SiO2) initially decreases until V~ reaches about - 2 0 V and then increases with further increase in V~ but is almost independent of film thickness. The shallow minimum in q at V~ = - 2 0 V may come from an apparent increase in pg due to the slight deterioration of columnar grain growth of the Ni film at Vs = - 2 0 V as mentioned above (Fig. 4e). The insensitivity of r/to film thickness suggests that r/(Ni/SiO2) is dominated by electron scattering at grain boundaries rather than at film surfaces. Figure I0 shows the relationship between r/ (Ni/ Si(100)) and V~ for two film thicknesses. It is seen that q (Ni/Si(100)) increases monotonically with increasing V~ for each film thickness while it decreases with a decrease in film thickness. As shown in Figs. 6d-6f, the grain growth is constantly enhanced as V~ increases from 0 V. Thus, as V~ increases, pg in eqn. (1) decreases continuously, with the result that I/ increases. On the other hand, as the film thickness decreases, the fine
' -io
111
' 4o
' -e~o
'
Vs(V) Fig. 9. Variation in TCR t/with substrate bias voltage Vs for Ni films grown on SiO2. Film thicknesses are ( A ) 180 nm and ( O ) 90 nm.
Fig. 10. Variation in TCR q with substrate bias voltage Vs for Ni films grown on Si(100). Film thicknesses are ( O ) 180 nm and (I1) 90 nm.
grains accumulated along the interface (Fig. 6) should cause an increase in pg, i.e. a decrease in q. Thus it may still be said for this case that the effect of electron scattering at film surfaces can be negligible.
4. Summary The structural and electrical properties of both Ni/ Si(100) and Ni/SiO2 were comparatively investigated as a function of VS from 0 to - 8 0 V. (1) Ni adatoms diffuse into Si(100) preferentially and interstitially along S i ( l l l ) planes to form the silicide Ni2Si in the Si crystal. Such interdiffusion does not occur in Ni/SiO 2. As Vs increases, the grain growth of the Ni films is more enhanced and the diffusion depth of Ni into Si increases. The columnar grain growth characteristic of the sputter-deposited film is znore or less disturbed when Vs ranges around - 2 0 V: a noncolumnar or nearly isotropic grain structure with voids along the interface is induced in Ni/Si(100) while slightly inclined columns are induced in Ni/SiO2. Very thick columns are induced at Vs -- - 8 0 V in both systems, but the columns are thicker on Si than on SiO2. Fine grains of Ni are accumulated along the interface for any value of Vs. These phenomena in the physical structure may be understood via the structure zone model [8, 9], except for the active diffusion phenomena in Ni/Si. (2) The TCR t/ is positive for both Ni/Si and Ni/ SiOz, i.e. the films are metallic, q (Ni/SiO2) changes with Vs, taking a shallow minimum at Vs = - 2 0 V, but is almost independent of film thickness. ~/ (Ni/Si(100)) increases monotonically with V~ but decreases with film thickness. The dependences of q on V~can be understood in terms of the above-mentioned structural changes with V~. (3) The Ni films prepared under the present conditions are only partially oxidized at the surface.
112
H. Qiu et al. [ Properties of Ni films grown on Si(lO0) and Si02
In conclusion, the structural and electrical properties o f N i f i l m s d.c. s p u t t e r d e p o s i t e d o n S i ( 1 0 0 ) a n d SiO2 can be controlled by applying a negative bias voltage to the substrates.
Acknowledgments The authors wish to thank Dr. G. Radnoczi d i s c u s s i o n o n t h e r e s u l t s a n d M r s . E. H a j m a s y Pt-C surface replica preparation.
4 5
6 for for
7 8
References 1 W. J. Schaffer, R. W. Bene and R. W. Walser, Structural studies of thin nickel films on silicon surfaces, J. Vac. Sci. Technol., 15 (1978) 1325-1331. 2 R. T. Tung, J. M. Gibson and J. M. Poate, Formation of ultrathin single-crystal silicide films on Si: surface and interfacial stabilization of Si-NiSi 2 epitaxial structures, Phys. Rev. Lett., 50 (1983) 429 -432. 3 E. J. van Loenen, J. W. M. Frenken and J. F. van der Veen,
9
10
11 12
N i - S i ( l l l ) interface: growth of Ni2Si islands at room temperature, Appl. Phys. Lett., 45 (1984) 41-43. L. I. Maissel and P. M. Schaible, Thin films deposited by bias sputtering, J. Appl. Phys., 36 (1965) 237-242. K. Usami, S. Gotou and B. Yamanaka, The energetic species effects on electrical properties of Ge films at sputtering deposition, Ovo Buturi (J. Appl. Phys. Jpn.), 56 (1987) 1527-1534 (in Japanese). T. Ohmi, K. Matsudo, T. Shibata, T. Ichikawa and H. Iwabuchi, Low-temperature silicon epitaxy by low energy bias sputtering, Appl. Phys. Lett., 53 (1988) 364-366. A. Barna, Topographic kinetics and practice of low angle ion beam thinning, M R S Symp. Proc., 254 (1991) 3-22. J. A. Thornton, The microstructure of sputter-deposited coatings, J. Vac. Sci. Technol. A, 4 (1986) 3059-3065. R. Messier, A. P. Giri and R. A. Roy, Revised structure zone model for thin film physical structure, J. Vac. Sci. Technol. A, 2 (1983) 500-503. W. K. Chu, H. Krautle, J. W. Mayer, H. Muller, M.-A. Nicolet and K. N. Tu, Identification of the dominant diffusing species in silicide formation, Appl. Phys. Lett., 25 (1974) 454-457. K. N. Tu, Selective growth of metal-rich silicide of near-noble metals, Appl. Phys. Lett., 27 (1975) 221-224. G. L. Baldini and A. Scorzoni, Electromigration and Matthiessen's rule: experiments on non-passivated A1-l%Si films, Thin Solid Films, 191 (1990) 31-36.
107
Structural and electrical properties of Ni films grown on Si(100) and SiO2 by d.c. bias sputtering* Hong Qiu a, Gyorgy Sfifrfin b, Bela Pecz b, Peter B. Barna b, Akio Kosuge a, Hisashi N a k a i a, Shigemi Yugo a and Mituru Hashimoto a, ** aThe University of Electro-Communications, I-5-1 Chofugaoka, Chofu-shi, Tokyo I82 (Japan) bResearch Institute of Technical Physics of the Hungarian Academy of Science, PO Box 76, H-1325 Budapest (Hungary)
(Received November 15, 1992; accepted January 25, 1993)
Abstract Ni films are deposited on both pure and SiO2-covered Si(100) substrates at 190 °C by d.c. diode sputtering at 2.5 kV * in pure Ar. A d.c. bias voltage Vs (0 to - 8 0 V) is applied on the substrates during the deposition. A study is made mainly of the effect of Vs on the structural and electrical properties of the films by reflection high energy electron diffraction, transmission electron microscopy and resistance measurements from 30 to 135 °C. Film-substrate interdiffusion is observed in Ni/Si but not in Ni/SiO2. Ni adatoms diffuse preferentially along Si(lll) with the formation of Ni2Si in the Si crystal. The grain size and diffusion depth of the Ni film increase with Vs. A non-columnar structure with voids along the interface is induced by Ni diffusion into Si as Vs ranges from 0 to - 2 0 V in Ni/Si whereas a slightly inclined columnar structure is induced at V~= - 2 0 V in Ni/SiO2. Thick columns grow at V~= --80 V in both systems. The temperature coefficient of resistance, r/, is positive for both Ni/Si and Ni/SiO2. The dependences of r/ on V~ can be understood in terms of the above-mentioned structural changes with Vs.
1. Introduction The Ni/Si (Ni on Si) system has been studied by m a n y researchers. Schaffer et al. [1] reported that the structures of Ni films r.f. sputter deposited on both Si(I00) and Si( I 1 I) substrates depend on the film thickness d: an a m o r p h o u s phase is induced at d = 1-2.8 nm whereas a polycrystalline Ni2Si c o m p o u n d is formed at d = 3 nm. Tung et al. [2] obtained ultrathin NiSi2 films grown epitaxially on both Si(100) and Si(111) at 4 5 0 500 °C. Van Loenen et al. [3] observed the growth of Ni2Si islands at the N i - S i interface at r o o m temperature. A bias-sputtering teehnique has been used to modify the physical and chemical properties of the films. Maissel and Schaible [4] reported that highly pure films are produced by sputtering when a negative bias voltage Vs is applied to the substrate during the deposition. Usami et al. [5] reported that the electrical properties of a d.c. sputter-deposited Ge film are modified at an appropriate value of Vs. Ohmi et al. [6]" obtained a high quality Si(100) film by r.f. sputtering with an optimal value of Vs at 350 °C. *An outline of this paper was presented at the 6th International Conference on Solid Films and Surfaces (ICSFS-6), Paris, June 29-July 3, 1992. **Author to whom correspondence should be adressed.
0040-6090/93/$6.00
We have prepared polycrystalline Ni films about 90-180 nm thick on both pure and SiO2-covered Si(I00) substrates by d.c. diode sputtering at 2.5 kV in pure Ar gas. A negative bias voltage Vs is applied to the substrates during the deposition in order to modify the structures of the film and/or the film-substrate interface. The present work a i m s to study the effect of Vs on the structural and electrical properties of the films by reflection high energy electron diffraction ( R H E E D ) , P t - C surface replica transmission electron microscopy (TEM), cross-sectional T E M ( X - T E M ) and measurements of electrical resistance as a function of temperature.
2. Experimental procedures A sketch of the sputter deposition system is shown in Fig. 1. Both the target (c) and substrate (e), which are 50 m m distant from each other, are closely surrounded by a cylindrical liquid nitrogen trap ( 140 m m in diameter) (d) inside the chamber (b). The chamber was evacuated to a pressure lower than 3 x 10 -5 Pa by a silicone oil diffusion p u m p equipped with a liquid nitrogen trap (j). The substrate, the temperature of which was measured using a c o p p e r - c o n s t a n t a n thermocouple (n), was baked out at 200 °C for 1 h with the use of a Nichrome heater (f) and the t e m p e r a t u r e was
@ 1993- Elsevier Sequoia. All rights reserved
108
H. Qiu et al. / Properties o f Ni films grown on Si(lO0) and SiO 2
k.---o
O-
line(s) of NiO are detected in addition to the main strong lines of Ni. These patterns do not change on changing Vs. Thus it can be said that the films deposited under the present conditions retain a polycrystalline structure of Ni which may be partially oxidized at the film surface.
if------ a
-
f
g
Pump
Fig. 1. Schematic diagram of d.c. bias sputtering: a, guide for water cooling the target; b, chamber; c, Ni target; d, cylindrical liquid nitrogen trap; e, substrate; f, Nichrome heater; g, substrate holder; h, insulator layer; i, bias supply; j, liquid nitrogen trap; k, to target voltage supply; t, guide for argon gas; m, inlets of liquid nitrogen; n, copper-constantan thermocouple; o, vacuum gauge.
decreased to 100 °C prior to the deposition. Then, after increasing the pressure to 4.0 Pa by supplying Ar gas (at least 99.9995% purity), a diode sputtering voltage of - 2 . 5 kV was applied to the Ni target (99.99% purity, 63.5 mm in diameter) in order to deposit the Ni film on the substrate. A d.c. bias voltage Vs (0 to - 8 0 V) was applied to the substrates during the deposition. The Si(100) substrates were precleaned by chemical processes: boiled in trichloroethylene solution, rinsed in distilled water, etched in 5% H F acid, ultrasonically rinsed in acetone and then dried by an ultrapure N 2 gas jet. The sample for X-TEM was prepared as follows [7]: it was first thinned by mechanical grinding to about 50 ~tm thickness and finally thinned by ion milling. The mean thickness of the film was measured directly from the microphotograph of the X - T E M image and also examined by the Tolansky technique. The temperature coefficient of resistance (TCR), ~I = R - I d R / d T , was measured from 30 to 135 °C by the four-point probe technique. An SiO2 film 10 nm thick was predeposited on the Ni film in another vacuum chamber in order to prevent it from oxidation during the measurement of q.
3. R e s u l t s a n d d i s c u s s i o n
3.1. Structural properties 3. I. 1. R H E E D R H E E D patterns of Ni films grown on Si(100) and SiO2 at V s = - 2 0 V are shown in Figs. 2a and 2b respectively. Figure 2 shows that very weak diffraction
42O) 311) !20) 220) Fig. 2. R H E E D patterns of Ni films deposited at V~ = - 2 0 V on (a) Si(100) substrate and (b) SiO2 substrate.
3.1.2. P t - C surface replica T E M Electron micrographs of P t - C surface replicas of Ni films grown on Si(100) and SiO2 at Vs=0, - 2 0 and - 8 0 V are shown in Figs. 3a-3f. On increasing Vs to - 8 0 V, the grain sizes of Ni films on both substrates apparently increase. This effect of Vs on the grain size is more evident on Si(100) than on SiO2. 3.1.3. Cross-sectional T E M Figure 4 shows bright and dark field X-TEM images of Ni films grown o n S i O 2 at Vs = 0, - 2 0 and - 8 0 V and Fig. 5 shows bright and dark field images of an Ni film grown on a naturally oxidized surface of Si(100) at Vs = - 2 0 V. The dark field images were obtained from diffraction lines of N i ( l l l ) and Ni(200). Atomic interdiffusion at the interface does not occur for the Ni/SiO2 specimen as shown in Fig. 4 and even a naturally formed oxide layer of Si tends to prevent .Ni atoms from diffusing into Si as shown in Fig. 5. As seen from the dark field images in Fig. 4, Ni films o n S i O 2 generally have a columnar structure which is well established with thicker grain growth particularly at V s = - 8 0 V but is less prominent at V s = - 2 0 V . Under the present experimental conditions the grain growth of the Ni film grown on SiO2 may be mainly determined by the shadowing effect due to the low mobility of Ni adatoms and by the bombarding effect of Ar ions accelerated at Vs, which increase the mobility of Ni adatoms and sputter off or incorporate residual impurities, as reported previously [4, 8, 9]. With respect to the effect on the mobility of Ni adatoms, an increase in Vs and an increase in substrate temperature have nearly the same effect but cannot be considered as identical, because the bombarding effect is based on momentum transfer and the thermal effect on heat transfer [9]. Therefore the grain growth is more prominent with increasing Vs as shown in Figs. 4d-4f. However, at some appropriate value of Vs the bombarding effect may incidentally oppose the shadowing effect and induce an inclined columnar structure as shown in Fig. 4e. When V~ increases up to - 8 0 V, the bombarding effect becomes dominant in inducing thick columns in the grain structure as a result of increasing the mobility of Ni adatoms as shown in Fig. 4f. Bright and dark field X-TEM images of Ni films grown on Si(100) at Vs = 0, - 2 0 and 80 V are shown in Fig. 6, where the Si(110) plane is parallel to the surface of the photograph. It can be seen from these X-TEM
H. Qiu et al. / Properties o f Ni filrns grown on Si(lO0) and SiO,
109
+
1 Fig. 3. P t - C surface replica T E M images of Ni films deposited on Si(100) at V~ = (a) 0, (b) - 2 0 and (c) - 8 0 V and on SiO 2 at V~ = (d) 0, (e) - 2 0 and (f) - 8 0 V. The bar at the top of the figure represents a c o m m o n scale of 100 nm.
a
b
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~
d
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i
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Fig. 4. Cross-sectional T E M images of Ni films grown on SiO 2 under bright field at V~ = (a) 0, (b) - 2 0 and (c) - 8 0 V and under dark field at Vs = (d) 0, (e) - 2 0 and (f) - 8 0 V. The dark field images, including those in Fig. 5b and Figs. 6 d - 6 f , were taken from parts of diffraction lines of N i ( l l I) and Ni(200). The bar at the top of the figure represents a c o m m o n scale of 100 nm.
1 Fig. 5. Cross-sectional T E M images of an Ni film deposited on a naturally oxidized surface of Si(100) at V~ = - 2 0 V under (a) bright field and (b) dark field. The bar at the top of the figure represents a c o m m o n scale of 100 nm.
images that a diffusion layer 60-90 nm thick is formed in the Si substrate rather than in the Ni film, i.e. the dominant diffusing species are clearly Ni atoms rather than Si atoms, and in addition the diffusion front of Ni atoms appears as a series of isosceles triangles. Figure 7 shows a close-up photograph of the X - T E M image of the diffusion front. From this photograph the vertex angle of the isosceles triangle is measured as about 70 °. As is well known, for a diamond (cubic) lattice structure, shadows of two mutually intersecting (111 ) direction (or lines) projected on a (110) plane construct vertically opposite angles of 70.5 °. The microdiffraction
H. Qiu et al. / Properties o f Ni films grown on Si(100) and SiO:
I I0
dg
.
.
.
.
.
.
.
4 N Fig. 6. Cross-sectional TEM images of Ni films grown on Si(100) under bright field at Vs = (a) 0, (b) - 2 0 and (c) - 8 0 V and under dark field at ~(, = (d) 0, (e) - 2 0 and (f) - 8 0 V. The bar at the top of the figure represents a common scale of 100 nm.
Fig. 8. A microdiffraction pattern (right) of X-TEM image (left) of Ni-Si diffusion layer at Vs = - 2 0 V. The bar at the top of the figure represents a scale of 100 nm.
Fig. 7. A close-up photograph of X-TEM image of diffusion front presented in Fig. 6a. The bar at the top of the figure represents a scale of 25 ran.
p a t t e r n o f the diffusion layer o f a specimen p r e p a r e d at Iz~ = - 2 0 V is shown with the X - T E M i m a g e in Fig. 8. F r o m b o t h the m i c r o d i f f r a c t i o n p a t t e r n in Fig. 8 a n d the d a r k field i m a g e in Fig. 6 it is c o n f i r m e d that the diffusion layer consists o f Ni2Si, N i a n d Si. Thus, in conclusion, Ni a t o m s diffuse preferentially a l o n g S i ( l l l ) planes a n d interstitially into the Si crystal to f o r m the silicide Ni2Si inside the diffusion layer. This result is consistent with p r e v i o u s reports: C h u et al. [ 10] r e p o r t e d f r o m a g r o w t h study o f silicides that N i a t o m s are the d o m i n a n t diffusion species which f o r m Ni2Si in the Si crystal, while T u [11] p r o p o s e d in his review p a p e r that at 1 0 0 - 2 0 0 °C N i a t o m s diffuse interstitially into Si to f o r m the silicide Ni2Si whose structure facilitates the diffusion o f Ni a t o m s .
As shown in Figs. 6 d - 6 f , for Ni/Si(100), as Vs increases, the grain g r o w t h ts c o n s t a n t l y p r o m o t e d b u t the c o l u m n a r g r o w t h is r a t h e r suppressed at Vs = 0 V (Fig. 6d) as well as at Vs = - 2 0 V (Fig. 6e). Besides, voids are i n d u c e d a l o n g the interface in the film when V~ ranges f r o m 0 to - 2 0 V. The c o l u m n a r structure a p p e a r s at higher values o f Vs, i.e. at V s = - 8 0 V a p p r e c i a b l y thick c o l u m n s g r o w inside the film w i t h o u t voids near the interface as shown in Fig. 6f. It should be n o t e d t h a t fine grains are a c c u m u l a t e d a l o n g the interface for a n y value o f Vs. Thus, in c o n t r a s t to the case o f Ni/SiO2, the grain g r o w t h m e c h a n i s m o f the Ni film on Si(100) m u s t be influenced also by the N i a t o m i c diffusion into Si. T h e n o n - c o l u m n a r v o i d structure i n d u c e d at low values o f Vs (0 to - 2 0 V) a n d the a c c u m u l a t i o n o f fine grains a l o n g the interface m a y originate m a i n l y f r o m the diffusion p h e n o m e n o n . The c o l u m n a r grains i n d u c e d at Vs = - 8 0 V are a p p a r e n t l y thicker on Si(100) ( F i g . 6f) t h a n on SiOz (Fig. 40. This m a y be due to an epitaxial effect i n d u c e d by the
H. Qiu et al. / Properties of Ni films grown on Si(lO0) and Si02
monocrystalline surface of the substrate (pure Si or Ni2Si ), in addition to the bombarding effect. Finally, with respect to the grain size, the results from X-TEM (Figs. 4 and 6) are consistent with those from P t - C surface replica T E M (Fig. 3). 3.2. Temperature coefficient of electrical resistance If the effect of electron scattering at the film surfaces is negligible, the TCR r/for a polycrystaIline film can be expressed as [ 12] =
Poqo Po + Pi + Pg
(1)
15
I
o IO: x
$ o F--
' -,C'o
15
°i0 x
g
'4o'-~o'4o '-~o' Vs (v)
where P0 is the reference resistivity of bulk material at 0 °C, and q0 is the TCR of bulk material, also referred to 0 °C, and Pi and pg are the temperature-independent resistivities induced by electron scattering at impurities and grain boundaries respectively. Figure 9 shows the relationships between r/(Ni/SiO2) and V~ for two thicknesses. It is seen that r/ (Ni/SiO2) initially decreases until V~ reaches about - 2 0 V and then increases with further increase in V~ but is almost independent of film thickness. The shallow minimum in q at V~ = - 2 0 V may come from an apparent increase in pg due to the slight deterioration of columnar grain growth of the Ni film at Vs = - 2 0 V as mentioned above (Fig. 4e). The insensitivity of r/to film thickness suggests that r/(Ni/SiO2) is dominated by electron scattering at grain boundaries rather than at film surfaces. Figure I0 shows the relationship between r/ (Ni/ Si(100)) and V~ for two film thicknesses. It is seen that q (Ni/Si(100)) increases monotonically with increasing V~ for each film thickness while it decreases with a decrease in film thickness. As shown in Figs. 6d-6f, the grain growth is constantly enhanced as V~ increases from 0 V. Thus, as V~ increases, pg in eqn. (1) decreases continuously, with the result that I/ increases. On the other hand, as the film thickness decreases, the fine
' -io
111
' 4o
' -e~o
'
Vs(V) Fig. 9. Variation in TCR t/with substrate bias voltage Vs for Ni films grown on SiO2. Film thicknesses are ( A ) 180 nm and ( O ) 90 nm.
Fig. 10. Variation in TCR q with substrate bias voltage Vs for Ni films grown on Si(100). Film thicknesses are ( O ) 180 nm and (I1) 90 nm.
grains accumulated along the interface (Fig. 6) should cause an increase in pg, i.e. a decrease in q. Thus it may still be said for this case that the effect of electron scattering at film surfaces can be negligible.
4. Summary The structural and electrical properties of both Ni/ Si(100) and Ni/SiO2 were comparatively investigated as a function of VS from 0 to - 8 0 V. (1) Ni adatoms diffuse into Si(100) preferentially and interstitially along S i ( l l l ) planes to form the silicide Ni2Si in the Si crystal. Such interdiffusion does not occur in Ni/SiO 2. As Vs increases, the grain growth of the Ni films is more enhanced and the diffusion depth of Ni into Si increases. The columnar grain growth characteristic of the sputter-deposited film is znore or less disturbed when Vs ranges around - 2 0 V: a noncolumnar or nearly isotropic grain structure with voids along the interface is induced in Ni/Si(100) while slightly inclined columns are induced in Ni/SiO2. Very thick columns are induced at Vs -- - 8 0 V in both systems, but the columns are thicker on Si than on SiO2. Fine grains of Ni are accumulated along the interface for any value of Vs. These phenomena in the physical structure may be understood via the structure zone model [8, 9], except for the active diffusion phenomena in Ni/Si. (2) The TCR t/ is positive for both Ni/Si and Ni/ SiOz, i.e. the films are metallic, q (Ni/SiO2) changes with Vs, taking a shallow minimum at Vs = - 2 0 V, but is almost independent of film thickness. ~/ (Ni/Si(100)) increases monotonically with V~ but decreases with film thickness. The dependences of q on V~can be understood in terms of the above-mentioned structural changes with V~. (3) The Ni films prepared under the present conditions are only partially oxidized at the surface.
112
H. Qiu et al. [ Properties of Ni films grown on Si(lO0) and Si02
In conclusion, the structural and electrical properties o f N i f i l m s d.c. s p u t t e r d e p o s i t e d o n S i ( 1 0 0 ) a n d SiO2 can be controlled by applying a negative bias voltage to the substrates.
Acknowledgments The authors wish to thank Dr. G. Radnoczi d i s c u s s i o n o n t h e r e s u l t s a n d M r s . E. H a j m a s y Pt-C surface replica preparation.
4 5
6 for for
7 8
References 1 W. J. Schaffer, R. W. Bene and R. W. Walser, Structural studies of thin nickel films on silicon surfaces, J. Vac. Sci. Technol., 15 (1978) 1325-1331. 2 R. T. Tung, J. M. Gibson and J. M. Poate, Formation of ultrathin single-crystal silicide films on Si: surface and interfacial stabilization of Si-NiSi 2 epitaxial structures, Phys. Rev. Lett., 50 (1983) 429 -432. 3 E. J. van Loenen, J. W. M. Frenken and J. F. van der Veen,
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10
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N i - S i ( l l l ) interface: growth of Ni2Si islands at room temperature, Appl. Phys. Lett., 45 (1984) 41-43. L. I. Maissel and P. M. Schaible, Thin films deposited by bias sputtering, J. Appl. Phys., 36 (1965) 237-242. K. Usami, S. Gotou and B. Yamanaka, The energetic species effects on electrical properties of Ge films at sputtering deposition, Ovo Buturi (J. Appl. Phys. Jpn.), 56 (1987) 1527-1534 (in Japanese). T. Ohmi, K. Matsudo, T. Shibata, T. Ichikawa and H. Iwabuchi, Low-temperature silicon epitaxy by low energy bias sputtering, Appl. Phys. Lett., 53 (1988) 364-366. A. Barna, Topographic kinetics and practice of low angle ion beam thinning, M R S Symp. Proc., 254 (1991) 3-22. J. A. Thornton, The microstructure of sputter-deposited coatings, J. Vac. Sci. Technol. A, 4 (1986) 3059-3065. R. Messier, A. P. Giri and R. A. Roy, Revised structure zone model for thin film physical structure, J. Vac. Sci. Technol. A, 2 (1983) 500-503. W. K. Chu, H. Krautle, J. W. Mayer, H. Muller, M.-A. Nicolet and K. N. Tu, Identification of the dominant diffusing species in silicide formation, Appl. Phys. Lett., 25 (1974) 454-457. K. N. Tu, Selective growth of metal-rich silicide of near-noble metals, Appl. Phys. Lett., 27 (1975) 221-224. G. L. Baldini and A. Scorzoni, Electromigration and Matthiessen's rule: experiments on non-passivated A1-l%Si films, Thin Solid Films, 191 (1990) 31-36.