1-s2.0-0039602879904072-main

Surface Science
0 North-Ilokmd

86 (1979) 300.-307
Publishing Cornpan)

SPECIAL FEATURES OF THIN COMPOUND FILMS PREPARED BY
MAGNETRON SPUTTERING

Kiyotaka WASA and Shigeru HAYAKAWA
hlatatczrialslicsrard!
Manuscript

received

I.ahoratoric~s.

Matsushita

in final form 5 Novcmbcr

Electric

Irdustrial

Co.. Isd..

Radoma,

Japrr

1978

Magnetron
sputtering
is now widely used for depositing
thin metal films. The m;lpnetron
sputlering
system, in contrast to the conventional
diode sputtering
system, is characterized
h?
high sputtering
rate and low worhing pressure. The low working pressure causes the impinpemcnt of high speed sputtered
atoms on substrates.
This may induce peculiar propcrtics
in the
deposited
films. Some interesting
phenomena,
e.g. nn abnormal
crystal growth and low temperature synthesis of a high temperature
compound.
are found when WC deposit thin compound
o\idc films.

1. Introduction

Increasing interest has been slmwn in the use of low pressure sputtering systems
of the magnetron type for depositing thin solid films. Penning first proposed the
use of magnetic fields in sputtering systems [ 11. Gill and Kay proposed a sputtering

system with inverted magnetron geometry for depositing high purity thin films [I?].
We previously studied magnetron sputtering for depositing elemental or compound
thin films [3] and proposed improved systems with cylindrical or planar geometry
[41.
Recent technological progress in sputtering enables this kind of system to be ot
industrial use [S]. The system is now an important alternative to electron beam,
induction heating or flash evaporation as a method of metallizing semiconductor
devices [6].
The low working pressure, lo-” to lo-” T err. in the magnetron sputtering system affects the nature of the sputtered films. and the films frequently show peculiar properties in contrast to the films prepared in a conventional diode sputtering
system which operates at gas pressure of 10e2 to 10-l Torr. This report describes
the fundamental operation of the magnetron sputtering system and the features of
the resultant sputtered films.


K. Wasa, S. Ilayakawa / Special jkatlrres of compound
2.

Operation

of magnetron

2.1. Comtnictiorl

films

30

I

sputtering

of the sputtering system

Fig. 1 shows the construction of the dc magnetron sputtering system used in this
experiment. The sputtering system is similar to the cold cathode magnetron of a
coaxial cylinder type. An important aspect of this sputtering system is the application of a strong magnetic field (>lOOO G), enabling the sputtering to operate in the
low gas pressure region down to lo-’ Torr [4].
2.2. Sprltteriilg profile

In the magnetron sputtering system the working pressure is so low that the scattering of sputtered atoms by gas molecules can be neglected. The sputtered atoms
arrive at the substrates by a direct line of flight. When we place the substrates near
the anode and assume that the current distribution is uniform over the cathode surface, the deposition rate R is given by R = (w/pt) (rc/ra), where w is the mass of the
material sputtered off unit cathode area, p the density of the deposited films, t the
sputtering time, rc and r, the radius of the cathode and anode, respectively. For the
sputtering in a rare gas such as argon, IV is governed by w = (i+/e)St(A/N), so that R
is expressed by
R z (i-,/e) SGI/N)~~/PW&J .

(1)

where i, is the ion current density at the cathode, S the sputtering rate, A the
atomic weight of sputtered materials, and N Avogadro’s number. In dc magnetron
sputtering the energy of the impact ion eVi is of the order of the discharge voltage
eVs and ranges from 300 to 1500 eV. Further, S = kVi, where k is a constant [4] ;
1, 71i,, since the secondary electron emission coefficient is much smaller than one.

MAGNETIC

POLES
S

N

CYLINDRICAL
CATt-DDE

CVLI NDRlCAL

SU BSTR ‘ATE

ANODE
--i

vip. 1. Construction ofdc

m;ignetron sputtering system.


6000
calhode
diameter
20mm
60mm
anode dsameter

;sooo
I-

,E
o;154000

ii

a3000

5
i=

0

1

0
POWER

I(J-~ Torr of Ar + Nz (Ar/Nl

2
INPUT

4

3

v, I$ ( volt~anp.km2

)

= 7/3) for TIN.

Thus, the deposition rate is proportional

tu the discharge power input V, i, In a
reactive gas such as oxygen and nitrogeli, thin films of oxides or nitrides are
deposited similar to the conventional diode sputtering. These oxides or nitrides are
chiefly formed at the cathode surface and/or at the substrates [7]. The dep~~siti~~~~
rates are usually reduced in the reactive gas.
Fig. 9 shows typical deposition rates measured in the dc magnetron system for
various sputtered films.

3. Phenomena

of some interests

3.1. Abnormal crystal growth
P~)lyc~stal~ille ZnO films of a hexagonal structure are prepared on a glass substrate by dc or rf sputtering from Zn or ZnO target in an oxidizing atmosphere.
0 c~~ldit~~ns and the crystallograp~lic structure
Table 1 shows the typical s~~~ltterill~
of the ZnO films prepared in the de conventional sputtering system and in the do
sputtering system where the
magnetron sputtering system. In the conventional
working pressure is lo-’ to 10 -’ Torr, it is seen that the (z-axis is preferentially
oriented normal to the film surface, i.e. the (001 ) plane is parallel to the film sur-


K. Wasa, S. HaJ’akawa

/ Special features

of comportr~d

Table 1

Crystallogaphic

orientation

of polycrystalline

Sputtering
sy ctem Gl

Sputtering

Substrate

pressure

tetnp.

Ikposition
rate

I:ilm
thickness

( 10e3 Torr)

CC)

(!Jlll/hJ

o.ll11)

35 b

40
100
200
200
‘00
300
300
300

0.03
0.15
0.03
0.3
0.03
0.15
0.3

0.1
0.3
0.1
0.3
0.3
0.1
0.3
0.3

40

40
70
150
150
200
270
‘70

0.03
0.12
0.7
0.1
0.7
0.7
0.07
0.6

0.1
0.36
0.35
0.3
0.3
II.3
0.2
0.3

Cr)nventional
tic diode

dc magnetron

a
b
’
d

1’

fZlms

303

ZnO films

0.15

Crystallographic
d
orientation

Cl

Cl
CJ.
CL
CL
CL
Cl
CJ~
Cl

Cl1+ Cl
Cll
Cl1

Cl1
Cl1

Cll
Cll

l’urc Zn cathode, 7059 $ISS substrates.
02.
Ar+02.50%
Ar+02.30::
Oz.
Cl. c-axis normal to film surface; Cfl, c-skis in film surface.

face. While, when the ZnO films are prepared at a low working pressure of 10m3
Torr or less by magnetron sputtering, the c-axis is predominantly
parallel to the
film surface, i.e. the (1 IO) or (100) plane is parallel to the film surface [8].
The c-axis orientation obtained in conventional
sputtering is explained by the
fact that the surface mobility of adatoms is high during film growth and the sputtered films obey the empirical law of Bravais. The change of crystallographic orientation with the sputering system may be related to the difference in the working
pressure. In the low working pressure the oxidation of the Zn cathode during sputtering will be not completed. Moreover, the low working pressure causes the impingement of high energetic sputtered Zn atoms and/or negative oxygen ions on the
substrates. This may lead in the nucleation and film growth process to an unusual
state, so that the familiar empirical Bravais law becomes inapplicable.
3.3. 12~1 tmperature

synthesis of high terliperatrwe cot~pour~ds

Previously, Bickly and Champbell deposited mixed films of PbO and TiOz by
reactive sputtering from a composite lead&titanium cathode in an oxidizing atmos-


304
RATIO

OF

Pb/T,

6

l8CCI

l600

l400

l200

0
.
2
::
::

0

+

-200

-400
0

5
%AREA

10

15
OF

Pb

l’ip. 3. Dielectric properties of Pb-Ti-0
magnetron
sputtering
(measured at 1 Ml17,

20

25

100

ON CATHODE
films of 3UOO ,4 thick

RT).

on 7059

glac

dcp~~sited

h!,

phere [9]. They used a conventional dc diode sputtering system. In this case, the
mean permittivity of the resultant films with the nominal chemical composition
PbTiOa was 33, which is much smaller than that of bulk PbTi03. The as-grown
films scarcely showed any indication of formation of PbTi03. In order to synthesize PbTiOs, the substrate temperature should be more than 600°C.
Contrary to their experiments, we have found that when dc magnetron sputtering is used, the resultant films exhibit several features of the PbTi03 compound,
even at a low substrate temperature of 200°C or less during film deposition [lo].
The composite lead-titanium
cathode was sputtered at 6 X 10m4 Torr of a mixed
gas of Ar and O2 (Ar/O, = 1). The deposition rates and substrate temperatures were
30 to 600 A min and 150 to 3OO”C, respectively. Experiments with pure lead
cathodes [ 1 l] and titanium cathodes suggest that in these sputtering conditions.
PbO and TiOz are co-deposited onto the substrate and thus mixed films of PbO and
TiOz will be formed. Fig. 3 shows the dielectric properties of the mixed films for
various chemical compositions. It is seen that the permittivity maximum is observed
at the chemical composition of PbTi03. The maximum permittivity is higher than
that of PbO or TiOz. It is also found that the temperature variation of the permittivity shows a maximum at about 490°C as indicated in fig. 4, which is expected in
the PbTi03 compound. These electrical properties suggest that the PbTi03 is synthesized in the mixed films prepared by magnetron sputtering.


K. Wasa,S. Hayakawa/ Special features of compound firms

305

100

,001
0

100

200

3cO

uKJ

500

600

TEMPERATURE (“C)
F’ig. 4. Temperature

deposited

variation of permittivity
by magnetron
sputtering
(measured

of PbbTi-0
at 1 MHz).

films of 3000 Ii thick on 7059 glass

The permittivity of the sputtered PbTi03 films is, however, still lower than that
of bulk PbTiOj. Thus the PbTi03 films will be composed of a mixture of PbO,
TiOz and PbTiO,. The contents of PbTi03, XpT, are estimated by the relation
XPT

=

1og(EM/XTi02XPbo)

(2)

10dEPT/XTi02XPb0)’

if we assume that Lichteneker’s empirical logarithmic mixing rule will be established between fpbo, ETiOZ and EpT, where EpbO, ETiOZ, epT and EM are the permittivity of PbO, TiOz, PbTi03 and the sputtered mixed fdms, respectively, and XpbO,
XTiOZ and XpT are the proportions by volume of PbO, TiOz and PbTi03, respectively,
such that Xpbo + XTiOz + XpT = 1. Putting EM 2 120, EpT ‘” 200, EpbO ‘c 25
and ETiOZ = 60, we have XpT * 0.7. This estimation suggests that 70% of the sputtered films are composed of PbTi03 compound.
3.3. Low temperature dophg

of foreign atoms into semiconducting

films

Co-sputtering of foreign atoms seems to be useful for controlling electrical conductivity of semiconducting
films during sputtering deposition. Table 2 shows typical experiments for polycrystalline ZnO thin films in various sputtering systems. In

the experiments Al or Cu auxiliary cathodes were co-sputtered with the Zn main
cathode in an oxidizing atmosphere [8].
It is seen that in the conventional sputtering system the co-sputtering of Al or
Cu scarcely affects the conductivity of the resultant films. While, in the magnetron
sputtering system the co-sputtering of Al or Cu strongly affects the conductivity:
Al increases the conductivity by over three orders of magnitude and Cu decreases it


Sputtcrin;
pressure
( IO-BTorr)

Sputtcrinp
s~stcrll H

C‘ontentc \)I
foreign ni~t3ls
iat ‘Y)
~____

fit-l [’

~~~n~crltion~l1
dc diode

1u

dc rnagnctron

0

I)..! (;\I)
Cf.:! (C’li)
0

1.3 (Al)
--.-

a Pure Zn catIwde.

0.5 (Crr)
---.---.--~._l

Suhstratc
temp.

CC)

Depwiticin
rate

Film
thickness

~pm/h)
_- ________

-.---

lhrk
c(~n[i~lcti~it~

(f2-1 Lx-t 1

(pm)
..___
.__._“_“____.__

3N~
3 r10
300

0.25
0.075
0.1

0.5
(I.1 5
iI ._7

I.6 Y 10-6
4
% 1P
1.9 x 10-6

2ttc1
701)

0.7
I.?.

II.3

0.b

8

x 10-2

200

0.9
.._-.___

O.JS
_--_____

3

x 10-R

1 * 1K4
_

71IS9 .glass substr;ttes,

b Ar + 02, 30:: 02.

c Ar + o-J,swr 02.

2 2

2.4

2.6
103/T

Fig. 5. Temperature
variation of dark c(~nd~ictivity
foreign metals prepared by magwtron
sputtering.

2.0
(Ok-’

30

3.2

3L

)

for ZnO films with and without

.

co-sputtered

._


K. Wasa, S. Ha.vakawa /Special features

of compoundfilms

307

in approximately
the same ratio. This suggests that in magnetron sputtering Al is
probably introduced as donor and Cu is introduced as acceptor or deep trap. In
conventional sputtering the co-sputtered atoms chiefly stay at crystal boundary in
the sputtered films and are scarcely incorporated into the crystal lattice. Fig. 5
shows the typical temperature variation of the dark conductivity of ZnO films prepared by magnetron sputtering. The conductivity is controlled from 10-l to IO-’
R-r cm-’ by doping the foreign atoms in the co-sputtering process.
Optical absorption measurements suggest that the width of the forbidden gap of
these films is 3.3 eV and the acceptor or deep trap level due to Cu is 2.5 eV below
the conduction band at room temperature. The temperature dependence of the carrier concentration
suggests that the donor level to to Al is 0.08 eV below the conduction band. The doping by foreign atoms in the co-sputtering process is possibly
caused by the impingement of high energetic sputtered atoms during film growth.
The highly conductive Al-doped ZnO films are applicable for making ZnO/Si
heterojunction
photo diodes [ 121 and switching diodes [ 131.

4. Conclusions
The magnetron sputtering system is now believed to be available in industry for
the metallization
of semiconductor
devices due to its high deposition rate. However, the system will offer much more attractive features when we deposite films of
compounds such as oxides, nitrides and carbides [ 141. Further study will bring succes in the formation of exotic materials.

References
[l] I:.M. Penn&, US Patent 2146025,

1939.
[2] W.D. Gill and E. Kay, Rev. Sci. In&r., 36 (1965) 277.
[3] K. Wasa and S. Hayakawa,
IEEE Trans. Parts, Mater. Packaging PMP-3 (1967)
K. Wasa and S. Hayakawa,
Proc. IEEE 55 (1967) 2179.
[4] K. Wasa and S. Hayakawa,
Japan. Patent 642012,1972.
K. Wasa and S. Hayakawa,
Rev. Sci. Instr. 40 (1969) 693.
K. Wasa and S. Hayakawa,
US Patent 3528902,197O.
[5] J.S. Chapin, Res. Develop. 25 (1974) 37.
[6] R.W. Wilson and L.E. Terry, J. Vacuum Sci. Technol. 13 (1967) 157.
[7] K. Wasa and S. Hayakawa,
Microelectron.
ReIiab. 6 (1967) 213.
[8] T. Hada, K. Wasa and S. Hayakawa,
Thin Solid Films 7 (1971) 135.
[9] W.P. BickIey and D.S. Champbell,
Vide 99 (1962) 214.
[lo] K. Kusao, K. Wasa and S. Hayakawa,
Japan. J. Appl. Phys. 7 (1968) 437.
[ 11) K. Wasa and S. Hayakawa, Japan. J. Appl. Phys. 8 (1969) 276.
[12] K. Wasa and S. Hayakawa,
Japan. J. Appl. Phys. 10 (1971) 1732.
[13] T. Hada, K. Wasa and S. Hayakawa,
Japan. J. Appl. Phys. 10 (1971) 521.
[14] K. Wasa and S. Hayakawa,
Thin Solid Films 10 (1972) 367.

70.