Materials Science and Engineering A336 (2002) 75 – 80
www.elsevier.com/locate/msea
Influence of temperature and ion kinetic energy on surface
morphology of CeO2 films prepared by dual plasma deposition
L.P. Wang a,b, K.Y. Fu a, X.B. Tian a, B.Y. Tang a,b, P.K. Chu a,*
a
Department of Physics and Materials Science, City Uni6ersity of Hong Kong, 83 Tat Chee A6enue, Kowloon, Hong Kong
Ad6anced Welding Production and Technology National Key Laboratory, Harbin Institute of Technology, Harbin, China
b
Received 25 April 2001; received in revised form 31 October 2001
Abstract
The effects of the substrate temperature and ion impact energy on the surface roughness and morphology of insulating cerium
oxide films fabricated by dual plasma deposition were investigated. Cerium oxide films were synthesized on Si(1 0 0) substrates,
and the influence of the substrate temperature and applied voltage on the surface roughness was systematically studied. Our results
indicate that the roughness of the deposited films decreased when the substrate temperature was increased, probably because of
enhanced surface diffusion of the adatoms. The ion kinetic energy had a similar effect as the substrate temperature, and it is
believed that a higher ion kinetic energy also improves the surface diffusion of adatoms. It is found that a direct-current voltage
applied to the substrate could lead to a rougher surface, whereas an appropriate alternating-current voltage gave rise to a
smoother topography. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Cerium; Surface roughness; Plasma processing and deposition
1. Introduction
Low-energy ion bombardment is common in thin
film growth and deposition processes and in many cases
plays an important role in the fabrication mechanism
and properties of the films. From the perspective of
surface diffusion, ion bombardment creates direct
transfer of energy and momentum to the surface atoms
via ion –atom collisions, and in fact, the magnitude of
the lateral momentum transfer can be calculated using
the Molecular Dynamics (MD) method [1]. The probability of an atom to diffuse is changed from P :exp(−
Ed/kBt) to P: exp[− (Ed −Ei)/kBt], where Ed and Ei
are the activation energy for surface diffusion of the
* Corresponding author. Tel.: + 852-27887724; fax:
27889549/87830
E-mail address: [email protected] (P.K. Chu).
+ 852-
adatoms and the energy transferred from the ion to the
surface atom, respectively. It has been reported that
when the kinetic energy of the incident ions exceeds an
effective value, the ion-induced diffusion of the
adatoms can be prominent, and the temperature, t, of
the substrate has little influence on the surface diffusion
constant [2]. Energetic ion bombardment can, however,
induce structural damage. A more serious and critical
issue is that some microdefects can be formed and
injected into the layer [3]. To minimize this effect, the
kinetic energy of the ions is typically set to several to
several hundred electron volts in practice. In addition,
sputtering by incoming ions can lead to different surface morphologies. Under the appropriate low-energy
ion bombardment conditions, ion irradiation creates
fluxes of recoiled atoms parallel to the surface during
the collisional cascade, and so the surface topography
can be smoothed out. Besides, preferentially sputtering
tends to erode developing surface asperities faster than
troughs and smooth [4].
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 9 2 3 - 2
L.P. Wang et al. / Materials Science and Engineering A336 (2002) 75–80
76
Dual plasma deposition employing metal vacuum
arc and gaseous plasma sources is a novel technique
derived from plasma immersion ion implantation
(PIII) and possesses many advantages as a thin film
fabrication technique [5– 10]. The obvious one is the
easily adjustable ion kinetic energy. In general, for
electrically conducting thin films and substrates, the
ion incident energy is determined by the voltage applied to the samples. The same is true for the synthesis of insulating films if charging can be effectively
compensated using the pulsing mode or by applying
an alternating-current (AC) potential onto the sample
[11,12]. However, if surface charging cannot be effectively eliminated, for instance, when the insulating
Table 1
Process parameters and roughness of different samples (scanned area is 4×4 mm2)
Sample ID
Substrate temperature (°C)
Type of applied voltage
Applied voltage (V)
RMS roughness (nm)
1-1
1-2
1-3
1-4
2-1
2-2
2-3
3-1
3-2
3-3
200
300
400
500
200
200
200
200
200
200
None
None
None
None
DC
DC
DC
AC
AC
AC
None
None
None
None
100
200
300
100
200
300
1.96
1.12
1.43
0.67
1.84
14.36
5.29
1.42
1.75
2.14
Fig. 1. Surface morphologies of samples deposited with different substrate temperatures: (a) Sample 1-1; (b) Sample 1-2; (c) Sample 1-3; and (d)
Sample 1-4.
L.P. Wang et al. / Materials Science and Engineering A336 (2002) 75–80
77
ms. The growth rate of CeO2 film was close to 0.1
nm s − 1, and the deposition time was 1 h. A cerium
seed layer of several nanometers thick was first deposited using only the Ce plasma. Oxygen was then
introduced into the chamber and the oxygen plasma
was generated using RF glow discharge to oxidize the
Ce seed layer. The rest of the CeO2 film was then
deposited in the presence of both the cerium and oxygen plasmas. During deposition, the substrate temperature and the applied voltage were varied, and the
process parameters are displayed in Table 1. For AC
deposition, the voltage was 14 V, the pulse frequency
was 44 kHz, and the negative pulse width was 15 ms.
The root-mean-square (RMS) roughness was derived
from AFM results using
RRMS = %(Zi − Zav)2/N
Fig. 2. Surface roughness line-scan profiles (height versus scanned
distance) of samples deposited with different substrate temperature.
n
1/2
,
where Zav is the average of the Z height within a
selected area (4× 4 mm2), Zi is the current Z value,
and N is the number of points in that area.
3. Results and discussion
film is thick or a long AC pulse or direct-current
(DC) voltage is used, the surface morphology may be
affected by the bias voltage [13].
In order to investigate the influence of the substrate
temperature and applied voltage on the film roughness, we deposited CeO2 film on Si(1 0 0) substrate at
different substrate temperature and monitored the surface roughness quantitatively using atomic force microscopy (AFM). The impact of DC and AC applied
voltages was also studied.
2. Experimental
Pieces of p-type Si(1 0 0) 20× 10 mm2 in size were
ultrasonically cleaned in acetone, rinsed in deionized
water, and dried by nitrogen. After the substrates
were loaded into the multi-purpose plasma immersion
ion implanter, the vacuum chamber was evacuated to
a
base pressure of 5.0×10 − 6 Torr. Cerium and oxygen
plasmas were generated simultaneously in the vacuum
chamber by means of a metal vacuum arc plasma
source and radio frequency (RF) source, respectively.
During deposition, the flow of the oxygen gas was set
to 16 sccm and the pressure in the vacuum chamber
was 3.0×10 − 4 Torr. The power of the RF source
was 500 W, and the reflected power was adjusted to
zero. The frequency of the metal vacuum arc plasma
source was 40 Hz, and its pulse width was about 250
Figs. 1 and 2 depict the surface morphologies and
surface line-scan profiles of the samples fabricated
with different substrate temperatures. It can be observed that the RMS values in the line-scans decrease
when the substrate temperature is increased (Table 1).
The result suggests that diffusion plays an important
role in the mechanism. As the substrate temperature
goes up, diffusion of adatoms increases and the
roughness of the substrate decreases.
Figs. 3 and 4 exhibit the surface morphologies and
line scans of samples deposited using different DC
voltages at a constant substrate temperature of
200 °C. When the applied voltage was 200 V, columnar growth was dominant; but when the applied
voltage was 100 V, fewer columns could be observed
on the surface. At a higher applied voltage of 300 V,
the size of the columns is smaller than those on the
sample deposited at 200 V, and the density decreases
as well. In general, the surface of these three samples
is rougher than that of the sample deposited at a zero
bias voltage.
The surface morphology and line scans of the samples deposited using different AC voltages are shown
in Figs. 5 and 6, respectively. The columnar structures
observed in the DC process cannot be detected and
the surface is relatively smooth. The surface roughness
of sample 3-1, 3-2 is lower than that of sample 1-1,
and among the three samples prepared using AC,
sample 3-3 shows the highest surface roughness.
78
L.P. Wang et al. / Materials Science and Engineering A336 (2002) 75–80
In dual plasma deposition, the surface morphology
of the film is determined by surface diffusion of the
adatoms, the ion incident angle and flux, as well as
sputtering or sublimation loss of the deposited film
[14]. When a DC voltage is applied to the substrate
during deposition, since CeO2 is electrically insulating,
charge accumulation will occur on the surface of the
film and the surface potential will eventually evolve to
an equilibrium value. Since the electric field tends to
be in a direction normal to the film surface and the
surface always shows a hill-and-valley structure during
kinetically controlled crystal growth [15], the electric
field is more dispersed at ridges [16]. Consequently,
since the ions impacting the surface follow the direc-
tion of the electrical field, preferential deposition
caused by this dispersed electric field will take place
at these asperities. Our study reveals a reversal trend
with regard to the voltage dependence (samples 2-1,
2-2, and 2-3) indicating that the process is quite complex and the overall effect is a combination of the
three phenomena. As the applied voltage is increased,
the sputtering yield goes up. In addition, surface diffusion of the adatoms is enhanced by ion irradiation.
Our results indicate that the surface roughening mechanism changes at around 200 V. As the applied
voltage is increased from 100 to 200 V, the dispersion
of the electric field appears to play a key role to yield
the columnar structures. When the voltage is in-
Fig. 3. Surface morphologies of samples deposited using different DC voltages: (a) Sample 2-1; (b) Sample 2-2; and (c) Sample 2-3.
L.P. Wang et al. / Materials Science and Engineering A336 (2002) 75–80
Fig. 4. Surface roughness line-scan profiles (height versus scanned
distance) of samples deposited using different DC applied voltages.
79
creased from 200 to 300 V, sputtering becomes more
predominant and the columnar structures are suppressed.
To reduce surface charge accumulation during dual
plasma deposition of an insulating film such as CeO2,
an AC voltage can be applied. In this case, since charge
neutralization is quite effective, the electric field in the
hill-and-valley structures is determined by the substrate
and the surface morphology of the film has little influence. Therefore, the ion flux should be laterally uniform
and the film morphology is determined by the combined effects of surface diffusion of the adatoms and
sputtering. As shown in our results, the surface roughness of samples 3-1 and 3-2 is less than that of sample
1-1. Meanwhile, it is known that the sputtering coefficients of different crystalline planes are not the same
[17,18], and so the plane with a large sputtering coefficient will grow more slowly. This can partially explain
why the surface roughness increases with a higher AC
voltage.
Fig. 5. Surface morphologies of samples deposited using different AC voltages: (a) Sample 3-1; (b) Sample 3-2; and (c) Sample 3-3.
80
L.P. Wang et al. / Materials Science and Engineering A336 (2002) 75–80
Acknowledgements
This work is jointly supported by grants from the
Hong Kong Research Grants Council (CERG c
9040498 or CityU 1032/00E) and City University of
Hong Kong (SRG c 7001177).
References
Fig. 6. Surface roughness line-scan profiles (height versus scanned
distance) of samples deposited using different AC voltages.
4. Conclusion
At a zero applied voltage, the surface RMS roughness of CeO2 films synthesized by dual plasma deposition decreased from 1.96 to 0.67 nm when the substrate
temperature was increased from 200 to 500 °C. Under
a DC voltage, the film surface shows more columnar
structures between 100 and 200 V, which are suppressed
at a higher voltage. When an AC voltage was used, the
surface roughness increased with the voltage due to
more severe preferential sputtering.
[1] Z. Insepov, I. Yamada, M. Sosnowske, Mat. Chem. Phys. 54
(1998) 234.
[2] M.A. Makeev, A.-L. Barabas, Appl. Phys. Lett. 71 (1997) 2800.
[3] W.X. Ni, G.V. Hansson, I.A. Buyanova, A. Henry, W.M. Chen,
B. Momemar, Appl. Phys. Lett. 68 (1996) 238.
[4] G. Carter, Thin Solid Films 322 (1998) 177.
[5] P.K. Chu, S. Qin, C. Chan, N.W. Cheung, L.A. Larson, Mat. Sci.
Eng.: Reports R17 (1996) 207.
[6] J.R. Conrad, J.L. Radtke, R.A. Dodd, F.J. Worzala, N.C. Tran,
J. Appl. Phys. 62 (1987) 4591.
[7] L.P. Wang, K.Y. Gan, X.B. Tian, B.Y. Tang, P.K. Chu, Rev. Sci.
Instrum. 71 (2000) 4435.
[8] X.B. Tian, T. Zhang, Z.M. Zeng, B.Y. Tang, P.K. Chu, J. Vac.
Sci. Technol. A 17 (6) (1999) 3255.
[9] Z.M. Zeng, T. Zhang, B.Y. Tang, X.B. Tian, P.K. Chu, Surf.
Coatings Technol. 115 (1999) 234.
[10] P.K. Chu, B.Y. Tang, Y.C. Cheng, P.K. Ko, Rev. Sci. Instrum.
68 (1997) 1866.
[11] L.P. Wang, B.Y. Tang, X.B. Tian, Y.X. Leng, Q.Y. Zhang, P.K.
Chu, J. Mat. Sci. Tech. 17 (2001) 29.
[12] L.P. Wang, B.Y. Tang, N. Huang, X.B. Tian, P.K. Chu, Mat. Sci.
Eng. A308 (2001) 176.
[13] L.P. Wang, B.Y. Tang, K.Y. Gan, X.B. Tian, P.K. Chu, J. Vac.
Sci. Technol. A, 19 (6) (2001) 2851.
[14] A. Pimpinelli, J. Villain, Physics of Crystal Growth, Cambridge
University Press, UK, 1998.
[15] A.A. Golovin, S.H. Davis, A.A. Nepomnyashchy, Phys. Rev. E
99 (1999) 803.
[16] K. Shimizu, H. Habazaki, P. Skeldon, G.E. Thompso, G.C.
Wood, Surf. Interface Anal. 27 (1999) 950.
[17] S.N. Okuno, S. Hashimoto, K. lnomata, J. Appl. Phys. 71 (1992)
5926.
[18] P. Stefanov, A. Stanchev, Mat. Chem. Phys. 50 (1997) 209.