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Characteristics of zinc oxide thin films prepared by r.f. magnetron-mode electron cyclotron resonance sputtering
Thin Solid Films 257 (1995) 22-27
Characteristics of zinc oxide thin films prepared by r.f. magnetron-mode electron cyclotron resonance sputtering T. Inukai”,
M. Matsuoka”,
K. Onob
“Opto-electronics Laboratories, NTT, Tokai-mum, Nnka-gun, Ibaraki 319-I I, Jupun bInterdi.~ciplinury Research Laboratories, NTT, Midori-cho. Mumshim-shi, Tokyo 180, Jupan Received
23 March
1994; accepted
16 August
1994
Abstract oxide thin films were reactively deposited on glass substrates by r.f. magnetron-mode sputtering employing electron cyclotron resonance (ECR) plasma, and their crystallographic characteristics, electrical resistivity and optical properties were characterized. Single-phase zinc oxide thin films were deposited at very low gas pressures of 10M2 Pa in an O2 and Ar mixed gas atmosphere. They exhibited a c-axis orientation of below 3” full width at half maximum (FWHM) for X-ray rocking curves, an extremely high resistivity of 107- 1O’OR cm, and a low optical attenuation of 3.4 dB cm ’ at a wavelength of 0.63 pm. These results indicate that irradiation of ECR plasma during deposition plays an important role in preparing high-quality zinc oxide film. Zinc
Keywords:
Electrical
properties
and measurements;
Optical
properties;
1. Introduction Zinc oxide with a wurtzite structure is well known to be a useful material for surface acoustic wave (SAW) devices and acousto-optic devices. Films of this material also exhibit high potential for application to optical integrated circuits because they can be formed on oxidized silicon at low temperatures. The characteristics required for such applications are good piezoelectric properties, high electrical resistivity and high optical transparency. Good piezoelectric properties [l] and high optical transparency [2] are yielded in films with a good c-axis orientation. The full width at half maximum (FWHM) of the (002) X-ray rocking curve is known to be suppressed below about 7” for obtaining effective electromechanical coupling [ 11. When zinc oxide thin films are deposited on glass substrates by conventional sputtering, the c-axis orientation is strongly influenced by the sputtering conditions and the substrate location. Usually, the film surface is bombarded by high-energy particles during the deposition, and highly oriented films cannot be obtained [3]. Consequently, sputtering used to be carried out in unusual ways, such as sputtering under high gas pressures at which high-energy particles are ther0040-6090/95/$9.50 c 1995 ~-- Elsevier SSDI 0040~6090(94)06325-7
Science S.A. All rights
reserved
Plasma
processing
and deposition;
Zinc oxide
malized [4], or placing the substrate outside the plasma so as to avoid the bombardment of high-energy particles [2]. The electrical resistivity of zinc oxide depends on its content of excess zinc which occupies interstitial positions. When zinc oxide is heated at 300 “C in vacuum, the excess zinc content increases, reducing the resistivity by more than four orders of magnitude [5]. Therefore, zinc oxide thin films should be sufficiently oxidized at the deposition temperature if they are to have high resistivity. Electron cyclotron resonance (ECR) microwave plasma achieves high activity and appropriate ion en30 eV) for film deposition [6], while enabling ergy ( generation at low gas pressures of IO-* Pa [7]. Consequently, the substrate is not bombarded by high-energy particles, and well-oriented, highly-resistive and highlytransparent zinc oxide films should be easy to deposit by reactive sputtering employing ECR plasma irradiation (ECR sputtering). Zinc oxide thin films have already been prepared by this method, and their crystal characteristics [ 81 and SAW characteristics [9] were reported. However, well-oriented thin films that combine high electrical resistivity and high optical transparency have not yet been reported.
This paper focuses on obtaining well-oriented zinc oxide thin films with both high electrical resistivity and high optical transparency. The thin films are reactively deposited by r.f. magnetron-mode ECR sputtering, and their C.-axis orientation, electrical resistivity, and optical attenuation are examined.
2. Experimental
procedures
2.1. .FCR .sputtering
technique
The r.f. magnetron-mode ECR sputtering apparatus is shown in Fig. 1. This apparatus consists of a cylindrical plasma generation chamber and a deposition chamber. The plasma generation chamber is surrounded by an electromagnet. A cylindrical r.f. magnetron cathode with a target is placed at the side of the plasma extraction hole. The substrate, which is electrically floated from the ground shield, is set in the deposition chamber. The distance between the target and the substrate is 10 cm. The electromagnet generates a magnetic flux density of 875 G in the plasma generation chamber. This magnet also generates a flared magnetic field which intensity becomes gradually weaker from the plasma generation chamber to the deposition chamber. ECR plasma is generated by introducing gas into the plasma generation chamber and supplying microwave power at a frequency of 2.45 GHz. This plasma is transferred from the plasma chamber to the deposition chamber along the flared magnetic field. Thi: ECR plasma also affects sputtering discharge characteristics [lo]. When the ECR plasma is irradiated during sputtering, the electric field of the power supplied to a target is effectively shielded inside the sheath
PLASMA
EXTRACTION
HOLE
GAS
INLET ITION
AGNETRON
MATCHING
CHAMBER
CATHODE
NETWORKS
Fig. 1 Schematic diagram of r.f. magnetron-mode apparatus installed a cylindrical target.
ECR
sputtering
on the target surface because the density of ECR plasma is high. This cause that the electric field does not heat electrons in the bulk plasma. A static electric field is generated in the ECR plasma along the flared plasma stream, resulting in a low floating potential at the substrate plate ( - 30 V). On the other hand, a high-density plasma is formed on the target surface by the magnetron-mode discharge, and a high-rate sputtering is accomplished [ lo,1 11. The interaction between the bulk ECR plasma and the plasma localized on the target surface is weak because the local magnetic field extends only the target surface. Consequently, the substrate is not bombarded by high-energy particles. but irradiated by the ECR plasma.
A cylindrical zinc with 99.99% purity was used as a target. ECR sputtering was carried out in an 0, and Ar mixed gas atmosphere by supplying r.f.power at a frequency of 13.56 MHz through matching networks. Magnetron-mode discharge occurred at gas pressures higher than 0.04 Pa. Thin films were deposited on glass substrates under conditions listed in Table 1. The standard deposition condition in Table 1 was fixed on the basis of facts that (i) well-oriented zinc oxide films with good crystallographic characteristics were obtained at a substrate temperature of 300 C by sputtering [4]; (ii) the deposition rate became high when high r.f. power was supplied; (iii) magnetron-mode discharge in the ECR sputtering occurred at O2 and Ar mixed gas pressures higher than 0.04 Pa; and (iv) when zinc oxide films were deposited by ECR sputtering. c-axis orientation of the films improved as the O1 gas pressure increased [8]. Each deposition parameter was changed from the standard condition in turn. The substrate temperature was measured with a thermocouple in contact with the substrate surface. The gas pressure was measured by using an ionization vacuum gage. The O2 and Ar gas pressures were corrected by checking with an N, gas pressure. The crystallographic characteristics were evaluated from the lattice spacing and FWHM of the (002) X-ray Table I Deposition
conditions
Deposition
parameter
Substrate temperature Microwave power R.f. power Pressure of Oz and Ar gas mixture O1 partial pressure
of zinc oxide thin films Standard condition
depositlon
Varied
conditions
300 C 400 w 500 w 0.072 Pa
I so - 400 c so - 500 w 200 . 600 W 0.015 - 0.072 Pa
0.063 Pa
0.005 . 0.063 Pa
T. Inukai, M. Mcltsuoka / Thin Solid Films 257 (1995) 22X27
24
10
rocking curve. The electrical resistivity was measured either by the four-probe method for thin films with a resistance lower than about 7 x 10’ Q, or by the two-probe method for thin films with a greater resistance. The optical attenuation was measured for slabwaveguide structure by the scattering detection method [ 121. Here, 0.63~urn wavelength light was coupled into the thin films by using a SrTiO, prism, and the TE, mode light was detected. The refractive index was also measured at 0.63 urn by the prism-coupling method [ 131.
Ts:
2-
0
100
200
300
MICROWAVE
3. Results and discussion
3.1. Crystullographic
churucteristics
Zinc oxide thin films with wurtzite structure were obtained at deposition conditions listed in Table 1, except conditions of 0, partial pressures less than 0.009 Pa under the 0, and Ar mixed gas pressure of 0.072 Pa, the substrate temperature of 300 “C, the microwave power of 400 W and the r.f. power of 500 W. When thin films were deposited at the 0, partial pressures less than 0.009 Pa, they were composed of mixtures of metal zinc and zinc oxide, Zinc oxide films are conventionally deposited by sputtering at gas pressures as high as l- 10 Pa. It was reported that films deposited at the lower pressure of 0.67 Pa by r.f. sputtering in a 500/O,-50%Ar atmosphere had an abnormal crystal structure [ 141. It is confirmed from the above results that zinc oxide film with wurtzite structure can be prepared at very low gas pressures by ECR sputtering. The X-ray diffraction patterns of the zinc oxide thin films exhibited preferential c-axis orientation. When thin films with a thickness of 700-l 1 000 8, were deposited under the standard condition, the FWHM became minimum for thin films with a thickness of 5000-8000 A. Consequently, the characteristics of 8000 A-thick films were evaluated below. Fig. 2 shows FWHMs of thin films deposited at microwave powers of 50-500 W under 300 “C, the r.f. power of 500 W, the mixed gas pressure of 0.072 Pa and the 0, partial pressure of 0.063 Pa. The FWHM narrows from 8” to about 3’ as the microwave power increases from 50 to 200 W, and remains in a narrow range of 2.9-3.9” in the 200-500 W range, showing that the c-axis orientation is good. A larger microwave power generates a higher density of ECR plasma. These results indicate that ECR plasma is effective for obtaining well-oriented films. The FWHM depends largely on the mixed gas pressure, as shown in Fig. 3. It narrows as the pressure increases from 0.015 to 0.052 Pa, and becomes constant at about 2.9^ in the 0.052-0.072 Pa range. In the latter pressure range, magnetron-mode discharge occurs. The
300°C
400
500
POWER
600
(W)
Fig. 2. Full width at half maximum (FWHM) of (002) curves for thin films deposited at microwave powers under a constant substrate temperature of 300 “C, an 500 W and a mixed Ar and OL mixed gas pressure of
X-ray rocking of 50- 500 W r.f. power of 0.072 Pa.
30 Ts: 8 %
300°C
Pp:4OOW
G5 20
%
4
IO
2 -I\
.
I
0’ 0.01
0.05 GAS PRESSURE
0.1
(Pa)
Fig. 3. Mixed-gas-pressure dependence of FWHM for thin films deposited at a microwave power of 400 W and an r.f. power of 500 W. The OJAr partial pressure ratio is constant at 7.0.
peak-to-peak voltage of the r.f. power applied to the target abruptly decreased from 1430 V to 650 V as the mixed gas pressure increased in the 0.015 to 0.052 Pa range, and gently decreased from 650 V to 500 V in the 0.052-0.072 Pa range. The floating potential of the substrate plate decreased at the same time form - 21 V to - 4 V as the mixed gas pressure increased in the 0.015-0.052 Pa range, and gently decreased from - 4 V to - 2 V in the 0.052-0.072 Pa range. These results suggest that the target voltage or the floating potential also affects the orientation. The FWHM of thin films deposited at substrate temperatures of 150 - 400 “C under the microwave power of 400 W, the r.f. power of 500 W, the mixed gas pressure of 0.072 Pa and the 0, partial pressure of 0.063 Pa is shown in Fig. 4. It reaches a minimum (about 3.0’) at 300 “C. The lattice spacing of these films narrows as the temperature decreases as shown in Fig. 5; that at 400 “C is 2.603 A, which coincides with that of zinc oxide powder. These results indicate that the crystal quality of grains in the thin films deposited at temperatures above 300 ‘C becomes high, but their orientation is poor.
12
10"
Pp 400 w
MIXED GAS PRESSURE:
-f? IO -. ::
8
lo8 g
6
f = 2
4 i
g
0
I
I
I
100
200
300
SUBSTRATE Fig. 4. Substrate deposited
I
dependence
.
400
TEMPERATURE
temperature
at x nncro&rve
.
500
:
lo* ' 10' .
(“C)
of FWHM
lo5
2 lo4 L5 v, lo3 .
2 0
10'
c: lo6
. .
u
.
log
.
%
0.072 Pa
10’0
.
loo
for thin films
10'
power bf 400 W
lo*
F
‘o-3o.oo1
2.66
0,
z a
2.64
.
z 2 $
Fig.
.
6. Electrical
resistivity
pressures of 0.005~ 0.063 0.072
2.62
C
0.01 PARTIAL PRESSURE of thin
(Pa)
tilms deposited
at O2 partial
Pa. The mixed gas pressure is constant
at
Pd.
. Y
\,
2.58
-
0
I
,
100 SUBSTRATE
Fig. S.
(002) lattice
I
I
200
.
300
TEMPERATURE
I
400
500
(“C)
spacing of the thin films in Fig. 4.
The above results reveal that a typical FWHM of 3 is easily obtained when thin films are deposited at 300 C, with a high microwave power ( 3 200 W) under a mixed gas pressure at which the magnetron-mode discharge occurs ( >, 0.05 Pa). This value is substantially lower- than the upper limit of 7’ that is needed for the large effective coupling factor [I], and it is comparable to the minimum reported FWHM (about 2-3“) of thin films deposited on glass substrates [4,8,15,16].
Fig. 6 shows the electrical resistivity of thin films deposited at 0, partial pressures of 0.005-0.063 Pa under a constant mixed-gas pressure of 0.072 Pa. It increases abruptly in the 0.007-0.009 Pa range as the parti‘ll pressure increases. This is attributed to a change in the crystalline composition whereby only zinc oxide is formed at pressures around or above 0.009 Pa. Abolre 0.01 Pa, the resistivity reaches lo’-10” R cm. These values are higher than the reported resistivity of non-doped thin films ( d IO’ Sz cm) [4,17,18]. It has been reported that about I wt.% of Cu or Li is usually doped into zinc oxide in order to increase its resistivity [ 171. Therefore, impurities in the thin films were analyzed by X-ray fluorescence analysis. The concentrations of Cu. Ni, Fe, Mn and Cr were below detectable
levels ( < 0.0220.2 wt.‘)A,), and the total amount of impurities was less than 1 wt.‘% Consequently, the high resistivity cannot be attributed to impurities. The above results indicate that the irradiation of oxygen ECR plasma during film deposition enables the formation of highly resistive zinc oxide films, suggesting that the excess zinc content in the films is small in spite of sputtering in a very low-pressure gas atmosphere.
The optical attenuation of the thin films largely depended on the microwave power and the substrate temperature. The attenuation of thin films deposited at microwave powers of 50-400 W is shown in Fig. 7. It decreases abruptly in the 50- 100 W range as the microwave power increases, and reaches 3.4 dB cm ’ at 400 W. The attenuation of thin films deposited at substrate temperatures of 150-400 C is shown in Fig. 8. It
50
c I
h: 0.63 urn
40
z 30 p s
20
E
10
Z
0
100
200
MICROWAVE Fig.
7. Optical
powers of 50
attenuation 400 W.
of thin
300 POWER
400
500
(W)
films deposited
at mtcrownve
26
T. Inukai.
M. Mursuoka
1 Thin Solid Films 257 (1995) 22-27
120 100
g looY % 80-
80
p < 1
60-
6@
40-
40
!
20-
20
0
I 0
100
.
I
SUBSTRATE Fig. 8. Optical attenuation peratures of 150-400 ‘C.
.
200
300
400
TEMPERATURE
0 1.96
500 (“C)
of thin films deposited
1.97
1.98
REFRACTIVE
at substrate
tem-
reaches its minimum value (3.4 dB cm - ‘) at 300 “C. This substrate temperature coincides with that of the minimum FWHM. The dependence of r.f. power or O2 partial pressure on the optical attenuation was small (3.4- 10 dB cm-’ and 3.4-6.4 dB cm-‘, respectively). The minimum attenuation of 3.4 dB cm-’ is close to the lowest reported attenuation of thin films deposited on glass substrates (3.4-6 dB cm ‘) [2,15,19]. It has been reported that the optical waveguide attenuation of thin films deposited by r.f. magnetron sputtering decreases linearly as the FWHM narrows [2]. In the same way, the attenuation of films deposited at microwave powers of 50-400 W (Fig. 7) is plotted against the FWHM in Fig. 9. It is found from this figure that the attenuation correlates with the FWHM. The refractive index of the thin films with an FWHM of 3” converges at about 1.990, which is the highest refractive index we obtained. This value is comparable to the highest refractive index of thin films reported so far (1.990-2.002) [20,21]. All the obtained attenuation values are plotted against refractive index in Fig. 10. The attenuation reaches a minimum at about 1.990. The refractive index of the zinc oxide film, which was
Fig. 10. Relationship dex.
between
optical
1.99
2.00
INDEX attenuation
and refractive
in-
measured by the prism-coupling method, depends on the crystal perfection and optical axis (film orientation). Consequently, it is reasonable to assume that the optical attenuation is also intimately related to the refractive index.
4. Conclusions Zinc oxide thin films were reactively deposited on glass substrates by r.f. magnetron-mode ECR sputtering in an O,-Ar mixed gas atmosphere using a zinc target. Single-phase zinc oxide films were deposited at very low mixed gas pressures of 10 ~’ Pa. The zinc oxide thin films grew preferentially along the [OOl] direction. A typical FWHM of about 3” for the (002) X-ray rocking curve was easily obtained for thin films deposited at a substrate temperature of 300 “C and microwave powers higher than 200 W. These films exhibit extremely high electrical resistivity of 1071O’O0 cm without any dopant, a low optical attenuation of 3.4 dB cm - ’ at 0.63 urn and a refractive index of 1.990. The optical attenuation correlated with the FWHM and the refractive index. Consequently, welloriented zinc oxide thin films with both high electrical resistivity and high optical transparency could be successfully deposited by r.f. magnetron-mode ECR sputtering. This is attributed to the irradiation of oxygen ECR plasma with active species except high-energy particles.
Acknowledgment
0’0
’ 2
’ 4 FWHM,
I
6
.
’
8
’
10
A850 (deg)
Fig. 9. Relationship between optical attenuation films re-plotted from Fig. 2 and Fig. 7.
and FWHM
for the
The authors express their gratitude for the continuous encouragement received from Drs. K. Sugii and S. Tohno, and thank Y. Hirukawa (Advanced Film Technology Inc.) for discussing the ECR sputtering apparatus and K. Namekawa (NTT Advanced Technology) for the thin film deposition and the characterization.
T. Inukui. M. Mutsuokcr / Thin Solid Films 257 (1995) 22-27
References [I] N. Chubachi, M. Minakata and Y. Kiuchi. Jpn. J. Appl. Phys.. I.? suppl. 2. Pt. I (1974) 737. [2] N. Chubachi, Proc,. IEEE. 64 ( 1976) 772. [3] K. Tominaga. N. Ueshima, Y. Shintani and 0. Tada, Jpn. J. Appl. Ph,vv.. 20 (1981) 519. [4] hl. Matsuoka. Y. Hoshi and M. Naoe. .I. Appl. Phy.7.. 63 (1988) 209x. [S] k. Kobayashi. Semiconducfors, Iwanami, Tokyo, 1968, p. 23. [6] S Matsuo and M. Kiuchi. Jpn. J. Appl. Phys.. 22 (1983) LZIO. [7] W.M. Holber. in J. Cumo. S.M. Rossnagel and H.R. Kaufman ( L.ds. ). tfmdhook of Ion Ptwessing Tcchnolog~. Noyes Pub.. Pdrk Ridge. 1989, p. 21. [8] hl. Matsuoka and K. Ono, J. Vuc. Sci. Technol.. A7 (1989) 2975. [9] hl. Kadota. T. Kasanami and M. Minakata, Elecrrorz Lett.. 28 (!092) 2315.
17
[IO] M. Matsuoka. Mater. Sci. Forum. 140 142. (1993) 55. [II] C. Takahashi. M. Kiuchi. T. Ono and S. Matsuo. J. Vat-. Sri. Techno/., A6 ( 1988) 2348. [I21 Y. Okamura. S. Sato and S. Yamamoto. Appl. 0pt.. 22 (1983) 3892. [I31 P. K. Tien and R. Ulrich. J. 0pr. Sot. ,4/n.. 60 ( 1970) 1325. [I41 M. Miurd. Jpn. J. App/. Ph~.v.. 21 (1982) 264. [IS] T. Yamamoto. T. Shiosaki and A. Kawahata. J. A/J@. P/IJs.. Cl ( 1980) 3113. [ 161 B.T. Khuri-Yakub, J.G. Smits and T. Barbee, J. Appl. Phrs., _5I (1981) 4772. [ 171 T. Shiosaki, Serumikku Kinousei Muku 10 Sono 0~0, Gakken Sha. Tokyo. 1988. p. 69 (in Japanese). [I81 T. Mitsyu. S. Ono and K. Wasa, J. App/. Ph~x.. 51 (1980) 2464. [ 191 A. Valentini, F. Quarantd and L. Vasanelli. SPIF //IS, Thin Film in 0p/icx ( 1989) 36. [20] T. Shiosaki. S. Ohnishi. Y. Hirokawa and A. Kawabata. Appl. Phys. f.ctl.. 33 ( 1978) 406. [2l] F.S. Hickernell. Proc,. /.tt /n/. S~wtp. WI /SSP ‘01. (1991) 57.
Characteristics of zinc oxide thin films prepared by r.f. magnetron-mode electron cyclotron resonance sputtering T. Inukai”,
M. Matsuoka”,
K. Onob
“Opto-electronics Laboratories, NTT, Tokai-mum, Nnka-gun, Ibaraki 319-I I, Jupun bInterdi.~ciplinury Research Laboratories, NTT, Midori-cho. Mumshim-shi, Tokyo 180, Jupan Received
23 March
1994; accepted
16 August
1994
Abstract oxide thin films were reactively deposited on glass substrates by r.f. magnetron-mode sputtering employing electron cyclotron resonance (ECR) plasma, and their crystallographic characteristics, electrical resistivity and optical properties were characterized. Single-phase zinc oxide thin films were deposited at very low gas pressures of 10M2 Pa in an O2 and Ar mixed gas atmosphere. They exhibited a c-axis orientation of below 3” full width at half maximum (FWHM) for X-ray rocking curves, an extremely high resistivity of 107- 1O’OR cm, and a low optical attenuation of 3.4 dB cm ’ at a wavelength of 0.63 pm. These results indicate that irradiation of ECR plasma during deposition plays an important role in preparing high-quality zinc oxide film. Zinc
Keywords:
Electrical
properties
and measurements;
Optical
properties;
1. Introduction Zinc oxide with a wurtzite structure is well known to be a useful material for surface acoustic wave (SAW) devices and acousto-optic devices. Films of this material also exhibit high potential for application to optical integrated circuits because they can be formed on oxidized silicon at low temperatures. The characteristics required for such applications are good piezoelectric properties, high electrical resistivity and high optical transparency. Good piezoelectric properties [l] and high optical transparency [2] are yielded in films with a good c-axis orientation. The full width at half maximum (FWHM) of the (002) X-ray rocking curve is known to be suppressed below about 7” for obtaining effective electromechanical coupling [ 11. When zinc oxide thin films are deposited on glass substrates by conventional sputtering, the c-axis orientation is strongly influenced by the sputtering conditions and the substrate location. Usually, the film surface is bombarded by high-energy particles during the deposition, and highly oriented films cannot be obtained [3]. Consequently, sputtering used to be carried out in unusual ways, such as sputtering under high gas pressures at which high-energy particles are ther0040-6090/95/$9.50 c 1995 ~-- Elsevier SSDI 0040~6090(94)06325-7
Science S.A. All rights
reserved
Plasma
processing
and deposition;
Zinc oxide
malized [4], or placing the substrate outside the plasma so as to avoid the bombardment of high-energy particles [2]. The electrical resistivity of zinc oxide depends on its content of excess zinc which occupies interstitial positions. When zinc oxide is heated at 300 “C in vacuum, the excess zinc content increases, reducing the resistivity by more than four orders of magnitude [5]. Therefore, zinc oxide thin films should be sufficiently oxidized at the deposition temperature if they are to have high resistivity. Electron cyclotron resonance (ECR) microwave plasma achieves high activity and appropriate ion en30 eV) for film deposition [6], while enabling ergy ( generation at low gas pressures of IO-* Pa [7]. Consequently, the substrate is not bombarded by high-energy particles, and well-oriented, highly-resistive and highlytransparent zinc oxide films should be easy to deposit by reactive sputtering employing ECR plasma irradiation (ECR sputtering). Zinc oxide thin films have already been prepared by this method, and their crystal characteristics [ 81 and SAW characteristics [9] were reported. However, well-oriented thin films that combine high electrical resistivity and high optical transparency have not yet been reported.
This paper focuses on obtaining well-oriented zinc oxide thin films with both high electrical resistivity and high optical transparency. The thin films are reactively deposited by r.f. magnetron-mode ECR sputtering, and their C.-axis orientation, electrical resistivity, and optical attenuation are examined.
2. Experimental
procedures
2.1. .FCR .sputtering
technique
The r.f. magnetron-mode ECR sputtering apparatus is shown in Fig. 1. This apparatus consists of a cylindrical plasma generation chamber and a deposition chamber. The plasma generation chamber is surrounded by an electromagnet. A cylindrical r.f. magnetron cathode with a target is placed at the side of the plasma extraction hole. The substrate, which is electrically floated from the ground shield, is set in the deposition chamber. The distance between the target and the substrate is 10 cm. The electromagnet generates a magnetic flux density of 875 G in the plasma generation chamber. This magnet also generates a flared magnetic field which intensity becomes gradually weaker from the plasma generation chamber to the deposition chamber. ECR plasma is generated by introducing gas into the plasma generation chamber and supplying microwave power at a frequency of 2.45 GHz. This plasma is transferred from the plasma chamber to the deposition chamber along the flared magnetic field. Thi: ECR plasma also affects sputtering discharge characteristics [lo]. When the ECR plasma is irradiated during sputtering, the electric field of the power supplied to a target is effectively shielded inside the sheath
PLASMA
EXTRACTION
HOLE
GAS
INLET ITION
AGNETRON
MATCHING
CHAMBER
CATHODE
NETWORKS
Fig. 1 Schematic diagram of r.f. magnetron-mode apparatus installed a cylindrical target.
ECR
sputtering
on the target surface because the density of ECR plasma is high. This cause that the electric field does not heat electrons in the bulk plasma. A static electric field is generated in the ECR plasma along the flared plasma stream, resulting in a low floating potential at the substrate plate ( - 30 V). On the other hand, a high-density plasma is formed on the target surface by the magnetron-mode discharge, and a high-rate sputtering is accomplished [ lo,1 11. The interaction between the bulk ECR plasma and the plasma localized on the target surface is weak because the local magnetic field extends only the target surface. Consequently, the substrate is not bombarded by high-energy particles. but irradiated by the ECR plasma.
A cylindrical zinc with 99.99% purity was used as a target. ECR sputtering was carried out in an 0, and Ar mixed gas atmosphere by supplying r.f.power at a frequency of 13.56 MHz through matching networks. Magnetron-mode discharge occurred at gas pressures higher than 0.04 Pa. Thin films were deposited on glass substrates under conditions listed in Table 1. The standard deposition condition in Table 1 was fixed on the basis of facts that (i) well-oriented zinc oxide films with good crystallographic characteristics were obtained at a substrate temperature of 300 C by sputtering [4]; (ii) the deposition rate became high when high r.f. power was supplied; (iii) magnetron-mode discharge in the ECR sputtering occurred at O2 and Ar mixed gas pressures higher than 0.04 Pa; and (iv) when zinc oxide films were deposited by ECR sputtering. c-axis orientation of the films improved as the O1 gas pressure increased [8]. Each deposition parameter was changed from the standard condition in turn. The substrate temperature was measured with a thermocouple in contact with the substrate surface. The gas pressure was measured by using an ionization vacuum gage. The O2 and Ar gas pressures were corrected by checking with an N, gas pressure. The crystallographic characteristics were evaluated from the lattice spacing and FWHM of the (002) X-ray Table I Deposition
conditions
Deposition
parameter
Substrate temperature Microwave power R.f. power Pressure of Oz and Ar gas mixture O1 partial pressure
of zinc oxide thin films Standard condition
depositlon
Varied
conditions
300 C 400 w 500 w 0.072 Pa
I so - 400 c so - 500 w 200 . 600 W 0.015 - 0.072 Pa
0.063 Pa
0.005 . 0.063 Pa
T. Inukai, M. Mcltsuoka / Thin Solid Films 257 (1995) 22X27
24
10
rocking curve. The electrical resistivity was measured either by the four-probe method for thin films with a resistance lower than about 7 x 10’ Q, or by the two-probe method for thin films with a greater resistance. The optical attenuation was measured for slabwaveguide structure by the scattering detection method [ 121. Here, 0.63~urn wavelength light was coupled into the thin films by using a SrTiO, prism, and the TE, mode light was detected. The refractive index was also measured at 0.63 urn by the prism-coupling method [ 131.
Ts:
2-
0
100
200
300
MICROWAVE
3. Results and discussion
3.1. Crystullographic
churucteristics
Zinc oxide thin films with wurtzite structure were obtained at deposition conditions listed in Table 1, except conditions of 0, partial pressures less than 0.009 Pa under the 0, and Ar mixed gas pressure of 0.072 Pa, the substrate temperature of 300 “C, the microwave power of 400 W and the r.f. power of 500 W. When thin films were deposited at the 0, partial pressures less than 0.009 Pa, they were composed of mixtures of metal zinc and zinc oxide, Zinc oxide films are conventionally deposited by sputtering at gas pressures as high as l- 10 Pa. It was reported that films deposited at the lower pressure of 0.67 Pa by r.f. sputtering in a 500/O,-50%Ar atmosphere had an abnormal crystal structure [ 141. It is confirmed from the above results that zinc oxide film with wurtzite structure can be prepared at very low gas pressures by ECR sputtering. The X-ray diffraction patterns of the zinc oxide thin films exhibited preferential c-axis orientation. When thin films with a thickness of 700-l 1 000 8, were deposited under the standard condition, the FWHM became minimum for thin films with a thickness of 5000-8000 A. Consequently, the characteristics of 8000 A-thick films were evaluated below. Fig. 2 shows FWHMs of thin films deposited at microwave powers of 50-500 W under 300 “C, the r.f. power of 500 W, the mixed gas pressure of 0.072 Pa and the 0, partial pressure of 0.063 Pa. The FWHM narrows from 8” to about 3’ as the microwave power increases from 50 to 200 W, and remains in a narrow range of 2.9-3.9” in the 200-500 W range, showing that the c-axis orientation is good. A larger microwave power generates a higher density of ECR plasma. These results indicate that ECR plasma is effective for obtaining well-oriented films. The FWHM depends largely on the mixed gas pressure, as shown in Fig. 3. It narrows as the pressure increases from 0.015 to 0.052 Pa, and becomes constant at about 2.9^ in the 0.052-0.072 Pa range. In the latter pressure range, magnetron-mode discharge occurs. The
300°C
400
500
POWER
600
(W)
Fig. 2. Full width at half maximum (FWHM) of (002) curves for thin films deposited at microwave powers under a constant substrate temperature of 300 “C, an 500 W and a mixed Ar and OL mixed gas pressure of
X-ray rocking of 50- 500 W r.f. power of 0.072 Pa.
30 Ts: 8 %
300°C
Pp:4OOW
G5 20
%
4
IO
2 -I\
.
I
0’ 0.01
0.05 GAS PRESSURE
0.1
(Pa)
Fig. 3. Mixed-gas-pressure dependence of FWHM for thin films deposited at a microwave power of 400 W and an r.f. power of 500 W. The OJAr partial pressure ratio is constant at 7.0.
peak-to-peak voltage of the r.f. power applied to the target abruptly decreased from 1430 V to 650 V as the mixed gas pressure increased in the 0.015 to 0.052 Pa range, and gently decreased from 650 V to 500 V in the 0.052-0.072 Pa range. The floating potential of the substrate plate decreased at the same time form - 21 V to - 4 V as the mixed gas pressure increased in the 0.015-0.052 Pa range, and gently decreased from - 4 V to - 2 V in the 0.052-0.072 Pa range. These results suggest that the target voltage or the floating potential also affects the orientation. The FWHM of thin films deposited at substrate temperatures of 150 - 400 “C under the microwave power of 400 W, the r.f. power of 500 W, the mixed gas pressure of 0.072 Pa and the 0, partial pressure of 0.063 Pa is shown in Fig. 4. It reaches a minimum (about 3.0’) at 300 “C. The lattice spacing of these films narrows as the temperature decreases as shown in Fig. 5; that at 400 “C is 2.603 A, which coincides with that of zinc oxide powder. These results indicate that the crystal quality of grains in the thin films deposited at temperatures above 300 ‘C becomes high, but their orientation is poor.
12
10"
Pp 400 w
MIXED GAS PRESSURE:
-f? IO -. ::
8
lo8 g
6
f = 2
4 i
g
0
I
I
I
100
200
300
SUBSTRATE Fig. 4. Substrate deposited
I
dependence
.
400
TEMPERATURE
temperature
at x nncro&rve
.
500
:
lo* ' 10' .
(“C)
of FWHM
lo5
2 lo4 L5 v, lo3 .
2 0
10'
c: lo6
. .
u
.
log
.
%
0.072 Pa
10’0
.
loo
for thin films
10'
power bf 400 W
lo*
F
‘o-3o.oo1
2.66
0,
z a
2.64
.
z 2 $
Fig.
.
6. Electrical
resistivity
pressures of 0.005~ 0.063 0.072
2.62
C
0.01 PARTIAL PRESSURE of thin
(Pa)
tilms deposited
at O2 partial
Pa. The mixed gas pressure is constant
at
Pd.
. Y
\,
2.58
-
0
I
,
100 SUBSTRATE
Fig. S.
(002) lattice
I
I
200
.
300
TEMPERATURE
I
400
500
(“C)
spacing of the thin films in Fig. 4.
The above results reveal that a typical FWHM of 3 is easily obtained when thin films are deposited at 300 C, with a high microwave power ( 3 200 W) under a mixed gas pressure at which the magnetron-mode discharge occurs ( >, 0.05 Pa). This value is substantially lower- than the upper limit of 7’ that is needed for the large effective coupling factor [I], and it is comparable to the minimum reported FWHM (about 2-3“) of thin films deposited on glass substrates [4,8,15,16].
Fig. 6 shows the electrical resistivity of thin films deposited at 0, partial pressures of 0.005-0.063 Pa under a constant mixed-gas pressure of 0.072 Pa. It increases abruptly in the 0.007-0.009 Pa range as the parti‘ll pressure increases. This is attributed to a change in the crystalline composition whereby only zinc oxide is formed at pressures around or above 0.009 Pa. Abolre 0.01 Pa, the resistivity reaches lo’-10” R cm. These values are higher than the reported resistivity of non-doped thin films ( d IO’ Sz cm) [4,17,18]. It has been reported that about I wt.% of Cu or Li is usually doped into zinc oxide in order to increase its resistivity [ 171. Therefore, impurities in the thin films were analyzed by X-ray fluorescence analysis. The concentrations of Cu. Ni, Fe, Mn and Cr were below detectable
levels ( < 0.0220.2 wt.‘)A,), and the total amount of impurities was less than 1 wt.‘% Consequently, the high resistivity cannot be attributed to impurities. The above results indicate that the irradiation of oxygen ECR plasma during film deposition enables the formation of highly resistive zinc oxide films, suggesting that the excess zinc content in the films is small in spite of sputtering in a very low-pressure gas atmosphere.
The optical attenuation of the thin films largely depended on the microwave power and the substrate temperature. The attenuation of thin films deposited at microwave powers of 50-400 W is shown in Fig. 7. It decreases abruptly in the 50- 100 W range as the microwave power increases, and reaches 3.4 dB cm ’ at 400 W. The attenuation of thin films deposited at substrate temperatures of 150-400 C is shown in Fig. 8. It
50
c I
h: 0.63 urn
40
z 30 p s
20
E
10
Z
0
100
200
MICROWAVE Fig.
7. Optical
powers of 50
attenuation 400 W.
of thin
300 POWER
400
500
(W)
films deposited
at mtcrownve
26
T. Inukai.
M. Mursuoka
1 Thin Solid Films 257 (1995) 22-27
120 100
g looY % 80-
80
p < 1
60-
6@
40-
40
!
20-
20
0
I 0
100
.
I
SUBSTRATE Fig. 8. Optical attenuation peratures of 150-400 ‘C.
.
200
300
400
TEMPERATURE
0 1.96
500 (“C)
of thin films deposited
1.97
1.98
REFRACTIVE
at substrate
tem-
reaches its minimum value (3.4 dB cm - ‘) at 300 “C. This substrate temperature coincides with that of the minimum FWHM. The dependence of r.f. power or O2 partial pressure on the optical attenuation was small (3.4- 10 dB cm-’ and 3.4-6.4 dB cm-‘, respectively). The minimum attenuation of 3.4 dB cm-’ is close to the lowest reported attenuation of thin films deposited on glass substrates (3.4-6 dB cm ‘) [2,15,19]. It has been reported that the optical waveguide attenuation of thin films deposited by r.f. magnetron sputtering decreases linearly as the FWHM narrows [2]. In the same way, the attenuation of films deposited at microwave powers of 50-400 W (Fig. 7) is plotted against the FWHM in Fig. 9. It is found from this figure that the attenuation correlates with the FWHM. The refractive index of the thin films with an FWHM of 3” converges at about 1.990, which is the highest refractive index we obtained. This value is comparable to the highest refractive index of thin films reported so far (1.990-2.002) [20,21]. All the obtained attenuation values are plotted against refractive index in Fig. 10. The attenuation reaches a minimum at about 1.990. The refractive index of the zinc oxide film, which was
Fig. 10. Relationship dex.
between
optical
1.99
2.00
INDEX attenuation
and refractive
in-
measured by the prism-coupling method, depends on the crystal perfection and optical axis (film orientation). Consequently, it is reasonable to assume that the optical attenuation is also intimately related to the refractive index.
4. Conclusions Zinc oxide thin films were reactively deposited on glass substrates by r.f. magnetron-mode ECR sputtering in an O,-Ar mixed gas atmosphere using a zinc target. Single-phase zinc oxide films were deposited at very low mixed gas pressures of 10 ~’ Pa. The zinc oxide thin films grew preferentially along the [OOl] direction. A typical FWHM of about 3” for the (002) X-ray rocking curve was easily obtained for thin films deposited at a substrate temperature of 300 “C and microwave powers higher than 200 W. These films exhibit extremely high electrical resistivity of 1071O’O0 cm without any dopant, a low optical attenuation of 3.4 dB cm - ’ at 0.63 urn and a refractive index of 1.990. The optical attenuation correlated with the FWHM and the refractive index. Consequently, welloriented zinc oxide thin films with both high electrical resistivity and high optical transparency could be successfully deposited by r.f. magnetron-mode ECR sputtering. This is attributed to the irradiation of oxygen ECR plasma with active species except high-energy particles.
Acknowledgment
0’0
’ 2
’ 4 FWHM,
I
6
.
’
8
’
10
A850 (deg)
Fig. 9. Relationship between optical attenuation films re-plotted from Fig. 2 and Fig. 7.
and FWHM
for the
The authors express their gratitude for the continuous encouragement received from Drs. K. Sugii and S. Tohno, and thank Y. Hirukawa (Advanced Film Technology Inc.) for discussing the ECR sputtering apparatus and K. Namekawa (NTT Advanced Technology) for the thin film deposition and the characterization.
T. Inukui. M. Mutsuokcr / Thin Solid Films 257 (1995) 22-27
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17
[IO] M. Matsuoka. Mater. Sci. Forum. 140 142. (1993) 55. [II] C. Takahashi. M. Kiuchi. T. Ono and S. Matsuo. J. Vat-. Sri. Techno/., A6 ( 1988) 2348. [I21 Y. Okamura. S. Sato and S. Yamamoto. Appl. 0pt.. 22 (1983) 3892. [I31 P. K. Tien and R. Ulrich. J. 0pr. Sot. ,4/n.. 60 ( 1970) 1325. [I41 M. Miurd. Jpn. J. App/. Ph~.v.. 21 (1982) 264. [IS] T. Yamamoto. T. Shiosaki and A. Kawahata. J. A/J@. P/IJs.. Cl ( 1980) 3113. [ 161 B.T. Khuri-Yakub, J.G. Smits and T. Barbee, J. Appl. Phrs., _5I (1981) 4772. [ 171 T. Shiosaki, Serumikku Kinousei Muku 10 Sono 0~0, Gakken Sha. Tokyo. 1988. p. 69 (in Japanese). [I81 T. Mitsyu. S. Ono and K. Wasa, J. App/. Ph~x.. 51 (1980) 2464. [ 191 A. Valentini, F. Quarantd and L. Vasanelli. SPIF //IS, Thin Film in 0p/icx ( 1989) 36. [20] T. Shiosaki. S. Ohnishi. Y. Hirokawa and A. Kawabata. Appl. Phys. f.ctl.. 33 ( 1978) 406. [2l] F.S. Hickernell. Proc,. /.tt /n/. S~wtp. WI /SSP ‘01. (1991) 57.