Japanese Journal of Applied Physics
Vol. 43, No. 1, 2004, pp. 267–272
#2004 The Japan Society of Applied Physics
The Characteristics of Reactively Sputtered AgOx Films Prepared at Different Oxygen
Flow Ratios and Its Effect on Super-Resolution Near-Field Properties
Yung-Chiun H ER , Yuh-Chang L AN, Wei-Chih H SU1 and Song-Yeu T SAI1
Department of Materials Engineering, National Chung Hsing University, Taichung, Taiwan
1
Materials Research Laboratory, ITRI, Hsinchiu, Taiwan
(Received July 30, 2003; revised August 25, 2003; accepted September 10, 2003; published January 13, 2004)
The characteristics of several reactively sputtered AgOx films, prepared at different oxygen flow ratios, with and without ZnSSiO2 protective layers have been examined. For the as-deposited AgOx films, the amount and size of Ag clusters decreased,
and the constituent phase of AgOx gradually transferred from pure Ag2 O, to a mixture of Ag2 O and AgO, then to pure AgO, as
the oxygen flow ratio was increased. After annealing, the reduction of AgO into Ag2 O and decomposition of Ag2 O into Ag
and O2 took place, and the decomposed Ag elements would diffuse outward and precipitate small silver particles on the
surface of ZnS–SiO2 protective layers. The chemical decomposition of AgOx film confined by ZnS–SiO2 protective layers
was confirmed to be an irreversible process. The super-resolution near field effect becomes significant only when the superresolution near-field structure (super-RENS) disk with an AgOx mask layer prepared at oxygen flow ratios above a threshold
value, where AgOx film consists of Ag2 O or AgO phase. [DOI: 10.1143/JJAP.43.267]
KEYWORDS: silver oxide, super-RENS disk, chemical decomposition, irreversibility, structural phase transition
1.
prepared by RF reactive magnetron sputtering of a pure
Ag target in Ar/O2 plasma, where the oxygen flow ratios of
O2 /(O2 +Ar) were controlled at 0.2, 0.5 and 0.7. The ZnSSiO2 protective layer with a thickness of 20 nm was prepared
by RF magnetron sputtering of a (ZnS)80 (SiO2 )20 target. The
chemical decomposition process of the AgOx film was
monitored by a reflectivity-temperature measuring system,
where the change of reflectivity with temperature was
recorded in time during a heating and cooling cycle. The
specimen was mounted on a Linkam THMS 600 heating
stage in an argon protective atmosphere, and heated to
300 C at a heating rate of 50 C/min and then cooled down
to room temperature. An UV-Visible spectrophotometer
(Hitachi U-3010) was used to measure the transmittance of
the as-deposited and annealed AgOx films. The structural
phase transition of AgOx films before and after isothermal
annealing was identified by field-emission scanning electron
microscope (FE-SEM), grazing incident X-ray diffractometer (GIXD), and X-ray photoelectron spectroscopy (XPS).
The elemental distributions in the ZnS–SiO2 /AgOx /ZnS–
SiO2 multilayer before and after isothermal annealing at
200 C for 3 min were also analyzed by the depth profile of
secondary ion mass spectroscopy (SIMS). A Super-RENS
disk with a layer structure of ZnS–SiO2 (170 nm)/AgOx (15
nm)/ZnS–SiO2 (20 nm)/Ge2 Sb2Te5(20 nm)/ZnS–SiO2 (40 nm)
was prepared and tested by an optical disk drive tester
(DDU-1000, Pulstec Industrial Co.) with a wavelength of
635 nm and a numerical aperture of 0.6, where the
theoretical resolution limit was about 270 nm.
Introduction
The super-resolution near-field structure (super-RENS)
disk, making use of AgOx film as a mask layer, has been
proposed for terabyte optical storage system.1–5) Small
recording marks beyond the resolution limit have been
successfully recorded and retrieved. It was suggested that
nano-sized Ag particles are precipitated in a small area
heated above the threshold temperature, through chemical
decomposition of AgOx during readout by a laser of
adequate power, and a localized surface plasmon coupling
effect occurs between precipitated Ag particles and subwavelength marks in a closely spaced recording layer that
yields strong near-field intensity and improves the signal
intensity.6,7) After the laser beam is removed, Ag and oxygen
form the AgOx compound again. Based upon this working
mechanism, only the readout process was taken into
consideration, and the chemical decomposition of AgOx
layer was assumed to be a reversible process. Obviously, the
structural phase transition and reversibility of chemical
decomposition of AgOx film in the super-RENS disk
definitely play important roles during the readout process.
Normally, AgOx film is prepared by reactive sputtering with
an Ag target and mixed gas of Ar and O2 . The oxygen flow
ratio of O2 /(O2 +Ar) is expected to affect the characteristics
of AgOx film that may lead to the change of super-resolution
and near-field properties of the super-RENS disk where
AgOx film is adopted as a mask layer.
In this article, we carefully investigated the characteristics
of several reactively sputtered AgOx films, prepared at
different oxygen flow ratios, with and without ZnS-SiO2
protective layers. The microstructural change of AgOx film
after isothermal annealing was examined to verify the
reversibility of chemical decomposition of AgOx film. A
super-RENS disk was prepared and tested by an optical disk
drive tester.
2.
3.
Results and Discussion
Figure 1 shows the FE-SEM images of the as-deposited
AgOx films prepared at various oxygen flow ratios. At the
oxygen flow ratio of 0.2, silver clusters with sizes of
100 nm were found to be dispersed in the AgOx matrix. As
the oxygen flow ratio increased to 0.5, nano-sized silver
particles sporadically distributed in the AgOx matrix, were
observed. As the oxygen flow ratio further increased to 0.7,
silver particles could hardly be found in the AgOx matrix.
The results clearly showed that the as-deposited AgOx films
prepared at low oxygen flow ratios would compose of
Experimental Procedure
Single-layered AgOx and multi-layered ZnS-SiO2 /AgOx /
ZnS-SiO2 films were deposited on glass and silicon
substrates. The AgOx film of 15 nm in thickness was
267
268
Jpn. J. Appl. Phys., Vol. 43, No. 1 (2004)
Y.-C. HER et al.
: Ag2O
: AgO
: Ag
Intensity
0.7
0.5
0.2
(a)
20
30
40
50
60
70
80
2θ
Fig. 2. GIXD diffraction patterns of the as-deposited AgOx films prepared
at various oxygen flow ratios.
Ag 3d5/2
25000
Ag
0.2
0.5
0.7
(b)
Intensity
20000
Ag 3d3/2
15000
10000
5000
0
384
382
380
378
376
374
372
370
368
366
364
Binding energy (eV)
Fig. 3. Ag 3d5=2 and 3d3=2 XPS for pure Ag and as-deposited AgOx films
prepared at various oxygen flow ratios.
(c)
Fig. 1. FE-SEM images of the as-deposited AgOx films prepared at
oxygen flow ratios of (a) 0.2, (b) 0.5, and (c) 0.7.
metallic silver particles and AgOx because the oxygen
injection rate was not high enough to react with all the
sputtered silver atoms, and the amount and size of metallic
silver particles decreased with increasing oxygen flow ratio.
Once the oxygen flow ratio reached a threshold value, AgOx
film composed of pure AgOx would be obtained. It is well
known that AgOx has two different phases – AgO and Ag2 O.
With the help of GIXRD and XPS, the phase of AgOx in
each as-deposited film can be identified. Figure 2 shows the
GIXD diffraction patterns of the as-deposited AgOx films
prepared at various oxygen flow ratios. It was found that the
constituent phase of AgOx in the as-deposited film was
Ag2 O when the oxygen flow ratio of 0.2 was applied, and
became a mixture of Ag2 O and AgO, and AgO when the
oxygen flow ratio increased to 0.5 and 0.7, respectively. Ag
was also detected by GIXD in the sample prepared at the
oxygen flow ratio of 0.2. Clearly, the constituent phase of
AgOx would gradually transform from pure Ag2 O, to a
mixture of Ag2 O and AgO, then to pure AgO as the oxygen
flow ratio increased. The same constituent phases were also
detected in AgOx layers prepared at various flow ratios and
sandwiched between ZnS–SiO2 protective layers. Fuji et al.
have also reported similar results where they deduced the
phases of AgOx films from changes of optical constants.4)
The constituent phases of AgOx in the as-deposited film
prepared at various oxygen flow ratios can be further
confirmed by the core-level spectra of silver, as displayed in
Fig. 3. For a pure Ag metallic film, the Ag 3d5=2 and 3d3=2
peaks occurred at 368.2 eV and 374.2 eV, respectively. For
AgOx films prepared at various oxygen flow ratios, the Ag
3d5=2 and 3d3=2 peaks were found to have shifted to 367.6
and 373.6 eV at the flow ratio of 0.2, and shifted to 367.4 and
373.4 eV, and 367.2 and 373.2 eV at the flow ratios of 0.5
and 0.7, respectively. According to the reported data,8–10) the
Ag 3d5=2 and 3d3=2 peaks of Ag, Ag2 O and AgO powders are
located at 368.1 and 373.2 eV, 367.7 and 373.7 eV, and
367.3 and 373.2 eV, respectively. It is evident that the Ag
3d5=2 and 3d3=2 peaks of AgOx films gradually shifted from
those of Ag2 O to those of AgO as the oxygen flow ratio
increased from 0.2 to 0.7, indicating the constituent phase of
AgOx progressively transformed from Ag2 O to AgO when
more oxygen was supplied, which was consistent with the
Jpn. J. Appl. Phys., Vol. 43, No. 1 (2004)
Y.-C. HER et al.
269
100
O2/(Ar+O2)=0.7
O2/(Ar+O2)=0.5
O2/(Ar+O2)=0.2
Transmittance (%)
Reflectivity (a.u)
80
60
40
0.2 (as-deposited)
0.2 (annealed)
0.5 (as-deposited)
0.5 (annealed)
0.7 (as-deposited)
0.7 (annealed)
20
100
150
200
250
300
o
Temperature ( C)
0
400
Fig. 4. Reflectivity changes of ZnS–SiO2 /AgOx /ZnS–SiO2 films with
temperature.
500
600
700
800
Wavelength (nm)
Fig. 5. Wavelength dependence of the transmittance of the as-deposited
and annealed AgOx films prepared at various oxygen flow ratios.
result found in the GIXD diffraction experiment. It can be
concluded that the as-deposited AgOx film was composed of
Ag clusters with size of 100 nm and Ag2 O when prepared
at an oxygen flow ratio of 0.2, and changed to nano-sized Ag
particles and a mixture of Ag2 O and AgO, and pure AgO, as
the oxygen flow ratios were increased to 0.5 and 0.7,
respectively.
To characterize the chemical decomposition process of
the AgOx film protected by two ZnS-SiO2 dielectric layers,
the reflectivity changes of the multi-layered films, prepared
at various oxygen flow ratios, with temperature during a
heating and cooling cycle were monitored, as shown in
Fig. 4. It was found that all the reflectivity versus temperature curves exhibited a reflectivity drop at the temperature
range between 140 and 180 C during the heating process,
corresponding to the occurrence of chemical decomposition
of AgOx into Ag and O2 . The chemical decomposition
temperature, which was defined as the temperature at the
midpoint of reflectivity change, of the AgOx film was
determined to be 150 C when the oxygen flow ratio was
controlled at 0.2, and increased to a constant value of 170 C
as the oxygen flow ratios were increased to 0.5 and 0.7.
Seemingly the AgOx film composed of either a mixture of
Ag2 O and AgO or pure AgO would show a constant
chemical decomposition temperature, and the existence of
Ag clusters in the AgOx film would promote the occurrence
of chemical decomposition and lower the chemical decomposition temperature. As the heated samples were cooled
down back to room temperature, the reflectivity of all the
multi-layered films remained unchanged, indicating the
chemical decomposition of AgOx film sandwiched between
ZnS-SiO2 protective layers was an irreversible process.
Figure 5 shows the wavelength dependence of the
transmittance of AgOx films prepared at various oxygen
flow ratios before and after annealing at 200 C for 3 min.
Apparently, all the AgOx films exhibited a substantial
increase in transmittance after chemical decomposition, and
the transmittance of AgOx film prepared at a lower oxygen
flow ratio was higher than that prepared at a higher oxygen
flow ratio. At the wavelength of 635 nm, the transmittance of
AgOx films, prepared at oxygen flow ratios of 0.2, 0.5, and
0.7, in the as-deposited state were about 55, 45, and 31%,
respectively, and increased to 81, 69, and 39%, respectively,
after chemical decomposition. That means in the asdeposited state, 0:3Rr , 0:2Rr , and 0:1Rr of the incident
light intensity, where Rr is the reflectivity of the recording
layer, would be reflected from the recording layers as the
AgOx mask layers were prepared at oxygen flow ratios of
0.2, 0.5, and 0.7, respectively. The reflection light intensity
would be increased to 0:66Rr , 0:48Rr , and 0:15Rr when
the AgOx mask layers prepared at oxygen flow ratios of 0.2,
0.5, and 0.7, respectively, were chemically decomposed into
Ag and O2 . As a result, the optical contrasts of super-RENS
disks with AgOx mask layers prepared at oxygen flow ratios
of 0.2, 0.5, and 0.7, before and after chemical decomposition, were estimated to be 54.5, 58.3, and 33.3%, respectively.
To understand the structural phase transition of AgOx film
with and without ZnS–SiO2 protective layers after chemical
decomposition, the single-layered and multi-layered samples, after being annealed at 200 C for 3 min were examined
by FE-SEM, GIXRD, and XPS. Figure 6 shows the FE-SEM
images of the single-layered AgOx films prepared at various
oxygen flow ratios after annealing. It was found that silver
clusters were formed in all samples. The core-level spectra
of silver, as shown in Fig. 7, also indicated that only the Ag
3d5=2 and 3d3=2 peaks corresponding to pure Ag were
detected in the single-layered AgOx films after being
annealed. For the annealed ZnS–SiO2 /AgOx /ZnS–SiO2
films, only Ag was detected by GIXRD in the films prepared
at oxygen flow ratios of 0.2 and 0.5, while both Ag and
Ag2 O were found in the film prepared at an oxygen flow
ratio of 0.7, as shown in Figure 8. These results evidently
suggested that the reduction of AgO into Ag2 O, the
decomposition of Ag2 O into Ag and O2 , and the aggregation
of decomposed Ag were taking place successively during
annealing, and the chemical decomposition of AgOx with or
without ZnS–SiO2 protective layers was an irreversible
process.
To further verify the irreversibility of chemical decomposition of AgOx film sandwiched between protective layers,
the room temperature depth profiles of Ag, O, Zn, S and Si
for the as-deposited and annealed ZnS–SiO2 /AgOx /ZnS–
SiO2 samples were analyzed by SIMS. Figure 9(a) shows the
results for the multi-layered sample prepared at an oxygen
270
Jpn. J. Appl. Phys., Vol. 43, No. 1 (2004)
Y.-C. HER et al.
: Ag2O
: Ag
Intensity
0.7
0.5
0.2
(a)
20
30
40
50
60
70
80
2θ
Fig. 8. GIXRD diffraction patterns of the multi-layered ZnS–SiO2 /AgOx /
ZnS–SiO2 films after being annealed.
(b)
(c)
Fig. 6. FE-SEM images of the annealed AgOx films prepared at oxygen
flow ratios of (a) 0.2, (b) 0.5, and (c) 0.7.
Ag 3d5/2
Ag
O2/(Ar+O2)=0.7
O2/(Ar+O2)=0.5
O2/(Ar+O2)=0.2
Intensity (a.u)
Ag 3d3/2
384
380
376
372
368
364
Binding energy (eV)
Fig. 7. Ag 3d5=2 and 3d3=2 XPS for pure Ag and the annealed AgOx films
prepared at various oxygen flow ratios.
flow ratio of 0.5. In the as-deposited state, AgOx layer was
clearly sandwiched between two ZnS–SiO2 protective layers
according to the depth profiles of Zn and Ag. The depth
profile of Zn in the outer ZnS–SiO2 protective layer was not
as uniform as we expected that might be due to the radiationenhanced diffusion effect as samples were bombarded with
O . After being annealed at 200 C for 3 min, Ag element
was found to diffuse outward to the protective layers and the
secondary ion counts of O could hardly be detected
throughout the whole sample. When we closely examined
the annealed ZnS–SiO2 /AgOx /ZnS–SiO2 sample by FESEM, metallic Ag particles with tens of nanometers in size
due to out diffusion of Ag element, was clearly observed on
the surface of the ZnS–SiO2 protective layer, as shown in
Fig. 9(b). The observation of metallic Ag particles was
consistent with the result found in GIXRD diffraction
patterns. These evidences again strongly support that the
chemical decomposition of AgOx film sandwiched between
protective layers is an irreversible process. The irreversible
precipitation of Ag particles in the AgOx mask layer of a
super-RENS disk after high readout power irradiation at a
linear velocity of 6 m/s was also reported by Kikukawa et
al.11) Therefore, the readout principle of the super-RENS
disk, based on the reversible precipitation of Ag particles in
the AgOx film, may be in doubt.
Figure 10(a) shows the dependence of carrier to noise
ratio (CNR) on the readout power at the mark length of
200 nm in the super-RENS disks with AgOx layers prepared
at various oxygen flow ratios. Here, the optimal writing
power was set to be 14 mW. It was found that CNR of all the
super-RENS disks initially increased with the reading
power, and reached a maximum value at the reading power
of 3.5 mW. As the readout power was further increased,
CNRs of the disks started to decrease. Figure 10(b) shows
the dependence of CNR on the mark length in the superRENS disks. Obviously, CNR of the disk prepared at an
oxygen flow ratio of 0.5 was slightly higher than that
prepared at 0.7, and much higher than that prepared at 0.2.
At the mark size of 150 nm, corresponding to 1/7 of the laser
spot diameter, CNRs of 32, 29, and 16 dB were obtained in
the super-RENS disks with AgOx layers prepared at oxygen
flow ratios of 0.5, 0.7, and 0.2, respectively. Seemingly, the
Jpn. J. Appl. Phys., Vol. 43, No. 1 (2004)
Y.-C. HER et al.
600000
271
40
35
as-deposited
O
Si
S
Zn
Ag
400000
300000
mark size=200nm
30
PW=14mW
25
CNR (dB)
Secondary ion counts
500000
200000
20
15
10
O2 /(Ar+O2)=0.7
O2 /(Ar+O2)=0.5
O2 /(Ar+O2)=0.2
5
100000
0
0
-5
1.0
1.5
2.5
3.0
3.5
4.0
(a)
O
Si
S
Zn
Ag
500000
annealed
400000
45
40
300000
35
pw= 14 mW
Pr= 3.5 mW
30
200000
CNR (dB)
Secondary ion counts
2.0
Readout power (mW)
600000
100000
0
25
20
15
10
0
20
40
60
80
Depth (nm)
(a)
O2 /(Ar+O2)=0.7
O2 /(Ar+O2)=0.5
O2 /(Ar+O2)=0.2
5
0
-5
100
150
200
250
300
Mark size (nm)
(b)
Fig. 10. Dependence of CNR (a) on the readout power, (b) on the mark
length in the super-RENS disk.
(b)
Fig. 9. (a) Depth profiles of the as-deposited and annealed ZnS–SiO2 /
AgOx /ZnS–SiO2 prepared at an oxygen flow ratio of 0.5, (b) FE-SEM
image of the annealed ZnS–SiO2 /AgOx /ZnS–SiO2 after being annealed.
super-resolution near field effect becomes significant only
when the as-deposited AgOx mask layer consists of Ag2 O or
AgO phase with minimum precipitated Ag particles. It is
believed that the constituent phases of AgOx mask layer
must play important roles in recording and readout mechanisms of super-RENS disk. Further investigation of the
relationship between the microstructure of AgOx mask layer
and the recording and readout mechanisms of super-RENS
disk is ongoing.
4.
ratios. As the oxygen flow ratio was increased, the amount
and size of Ag clusters decreased, and the constituent phase
of AgOx gradually transferred from pure Ag2 O, to a mixture
Ag2 O of AgO, then to pure AgO. The chemical decomposition of AgOx film sandwiched between ZnS–SiO2
protective layers was confirmed to be an irreversible process.
The existence of Ag clusters in the AgOx film could promote
the occurrence of chemical decomposition and lower the
chemical decomposition temperature. During annealing, the
reduction of AgO into Ag2 O, the decomposition of Ag2 O
into Ag and O2 , and the out diffusion and aggregation of
decomposed Ag were suggested to take place successively.
The AgOx film exhibited a substantial increase in transmittance after chemical decomposition, and its transmittance
decreased as oxygen flow ratio was increased. The superresolution near field effect becomes significant only when
the as-deposited AgOx mask layer consists of Ag2 O or AgO
phase with minimum Ag precipitated particles.
Acknowledgments
This work was sponsored by the National Science Council
of the Republic of China under Grant No. NSC91-2216E005-020. The authors would like to thank Edward Young
for reviewing the manuscript.
Conclusions
Ag clusters were found to co-exist with AgOx matrix in
the as-deposited AgOx films prepared at low oxygen flow
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