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<reference>
Challenges in electron cyclotron resonance plasma etching of LiNbO3
Surface for fabrication of ridge optical waveguides
Naoki Mitsugi and Hirotoshi Nagata a)
Optoelectronics Research Division, New Technology Research Laboratories,
Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi, Chiba 274-8601, Japan

Kaori Shima and Masumi Tamai
Advanced Material Research Division,New Technology Research Laboratories,
Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi, Chiba 274-8601, Japan

(Received 5 February 1998; accepted 24 April 1998)
Ridge-shaped optical waveguide are promising structures to provide broadband characteristics for
LiNbO3 (LN) base optoelectronic devices. In this regard, electron cyclotron resonance plasma
etching with fluorocarbons have been commonly applied in the preparation of ridge waveguides,
3-5 µm in height, but certain fabrication problems need investigation and clarification. The etched
LN surface was found to be covered with LiF leading to a weak adhesive strength for the overlayer,
such as a SiO2 film, over the ridge waveguides. When CF4 etchant was used, a notch appeared
along the foot of the ridge waveguides. The notch was a possible origin for the chipping of the
waveguides. Such an undesirable notch was found to be prevented by the use of a CHF3 etchant.
Here, these inevitable problems during the plasma etching of the LN were presented and discussed.
© 1998 American Vacuum Society. [S0734-2101 (98) 11204-6]

Ⅰ. INTRODUCTION
LiNbO3 (LN) based optical waveguide devices are commonly used in optical fiber communication systems as an
external optical intensity modulator and an optical polarization scrambler. Current LN devices are designed to be operated at 2.5-5 Gb / s speed, and the fabrication technology for
them is simple. Over the z-cut LN substrates with Tiindiffused waveguides, an approximately 1 ∝m thick SiO2
buffer layer, a thin Si layer, and approximately 15 ∝m thick
gold electrodes are formed using a conventional film deposition technology. By increasing the thickness of the buffer
layer and the electrodes, the operation speed of the device
can be increased. However, in order to realize a LN modulator faster than 40 Gb /s, the distribution of the electric field
applied to the device must be adjusted to concentrate on the
waveguides by machining the LN surface. Fabrication of
ridge shaped waveguides is an example of such technology
and recently, Noguchi et al. reported 100 Gb / s LN modulators with 3-4 ∝m high ridge waveguides. 1-3
As a method for fabrication ridge waveguides, both wet
and dry etching techniques have previously been demonstrated. Wet etching was performed on the +z face of the
LN substrates using a HF / HNO3 mixture following a protonexchange treatment of the substrate surface.4,5 This method
was reported to produce a smooth etched surface compared
with dry etching. However, use of the +z face is not favorable for the preparation of conventional Ti-indiffused
waveguides because of the occurrence of domain inversion
during the thermal diffusion process of Ti.6,7 The alternative
dry etching can be applied irrespective of the substrate orientation. Chemically assisted etching with fluorocarbon
a)

Author to whom correspondence should be addressed; electronic mail:
LDT00246@niftyserve.or.jp
2245 J. Vac. Sci. Technol. A 16 (4), Jul / Aug 1998

Plasma, such as an electron cyclotron resonance (ECR)
Plasma etching, is frequently used for preparation of ridge
Waveguides. Actually, the ECR etching process has been apPlied to achieve ultrahigh speed LN modulators.1-3 HowEver, a problem in the dry etching process of the LN substrates has not clarified yet. Generation of Lif
precipotates on the etched surface. as recently reported by us,
may have been the cause of the problem in the device fabrication processes.8,9 The purpose of this article is to investigate and clarify such problems occurring in the fabrication of
high speed LN modulators with ridge waveguide. A possible solution can be proposed for some of these problems.
For instance, when the SiO2 buffer layer was formed on
the etched LN surface, the adhesive strength of the film decreased and the layer peeled off during the substrate cutting
process. Such weak adhesive strength was considered to
come from the existence of a LiF layer on the LN surface,
which was induced by ECR etching with fluorocarbons. The
second problem was a chipping of the ridge waveguides and
the reason for this phenomenon is discussed.
Ⅱ. EXPERIMENTAL PROCEDURES
The dry etching of the LN substrates was carried out using a commercial ECR etcher, ANELVA model L-310R,
with CF 4 or CHF 3 as an etching gas. A -z-cut LN wafer
having a 3 in. diam and a 0.5mm thickness was placed on a
silicon carbide substrate holder which faced the plasma
chamber The distance between the substrate and the acceleration electrode at the plasma outlet was about 20cm. After
the etching chamber was evacuated to under 4×10-4 Pa, the
etching gas was introduced at a rate of 3 sccm and the pressure was kept at about 4 × 10-2 Pa. The electric power supplied to the microwave generator was 400W, and the magnet
voltage and the ion acceleration voltage were set to 80 and

0734-2101/98/16 (4) / 2245 / 7 / $ 15.00

©1998 American Vacuum Society 2245

Mitsugi et al.: Challenges in ECR plasma etching

2246

(a)

using vacuum evaporation deposited 1.2µm thick Ni film.
Recently, an electroplated Ni film was tested as an etching
mask and found to be just as suitable as the thicker mask.
After ECR etching, the remaining Ni was easily removed
using dilute HNO3 at room temperature.
Inspection of the etched substrates was performed using a
secondary electron microscope (SEM: JEOL model JSM T330A) and an atomic force microscope (AFM: Seiko Instrumentals SPI-3700). The etched depth and surface profile
were measured by a stylus method using a probe with 1.25
µm diam (DekTak). Chemical analyses of the surface were
done with an Auger electron spectrometer (AES:JEOL
model JAMP-10SX) and an electron probe microanalyzer
(JEOL model JXA 8800).

Electrode

LiNbO 3
① Distance Between Electrodes : 25µm
② Etching Width
③ Etching Depth( ○ :3.6µm, ● :2.8µm)

(b)

30
25

Optical Bandwidth[GHz]

2246

20

Ⅲ . RESULTS AND DISCUSSION

15

A. Formation of rldge waveguides
10
5

(c)

6

Harf-wave Voltage(Vπ)[V]

0

5.5

0

2

4

6
8
10
12
Etching Width [µm]

14

16

0

2

4

6
8
10
12
Etching Width [µm]

14

16

5
4.5
4
3.5
3

FiG. 1 (a) Schematic illustration of cross section of the fabricated LN MZ
optical intensity modulator having ridge waveguides. (b) Optical bandwidth
and (c) halfwave voltage Vπ measured for the fabricated devices as a function of the gap distance between the ridge MZ waveguide arms.

Figure 1(a) shows a schematic illustration of the ridge
waveguides fabricated on the z-cut LN substrate. After the
Mach-Zehnder (MZ) type waveguides were formed on the
substrate by a conventional Ti-indiffusion technique, the
rectangular spaces on both side of the MZ waveguide arms
were etched to 2 8 ∝m in depth, 40mm in length along the
waveguide and 5 or 15∝m in width. The etched width corresponded to the gap between the obtained ridge waveguides
in Fig. 1(a). Then, the SiO2 buffer layer was deposited by rf
sputtering from a SiO2 target with an Ar / O2 mixture to a
thickness of 1.2 ∝m, and a 100nm thick Si layer was sputter
deposited [not shown in Fig. l(a)]. Over the ridge
waveguides covered by the SiO2 and Si layers, 17∝m thick
coplanar gold electrodes were formed by electroplating to
give a 25 ∝m gap between the hot and ground electrodes.
The effects of the ridge waveguides on device characteristics are shown in Figs. 1(b) and 1(c) for the optical bandwidth and the drive voltage Vπ, respectively. Increasing the
gap between the MZ waveguide arms, expanded the optical
bandwidth and Vπ was reduced. Results indicated the application of ridge waveguides was effective in realizing broader
band LN waveguide modulators. However, in the fabrication

5000

J. Vac. Sci Technol. A, Vol. 16, No. 4, Jul / Aug 1998

Intensity

4000

500 V, respectively. During the etching, the substrate holder
was water-cooled at 21 ºC and rotated (17 rpm). Under these
etching conditions, typical etching rates of 800 nm / h by CF4
and 700 nm / h by CHF3 were obtained for the LN. The deviation in etched depth for a 3 in. diam substrate was
± 20 nm for 240 nm deep etching by CHF3, for instance.
Patterned etching of the LN for fabrication of ridge
waveguides was performed using a Ni film mask. A conventional resin mask could not endure the several-hours-long
etching for 3-5 µm depth. The etching rate ratio of the LN
to the Ni film was measured at about 3.7 independent of the
etching gas. The Ni film mask was prepared on the LN substrate by conventional photolithography and a lift-off process

Electrode
SiO 2

3000

2000

LiNbO 3
1000

0
0

100
200
Kinetic energy [eV]

300

Fig. 2. Result of AES analyses for the failed device, in which the surface
electrodes and the SiO2 buffer layer peeled off from the etched LN wafer
surface.

2247

Mitsugi et al.: Challenges in ECR plasma etching

2247

process of LN ridge waveguides, problems emerged which
needed to be solved for improvement of the device fabrication yield.

In the fabrication of LN modulators with ridge
waveguides, some of the modulators failed because of a peeling off of the thick ground electrodes during the cutting process of the LN chips from the wafer. The ground electrodes
were formed over the ECR etched LN surface via. the SiO2
buffer layer. In our experience on thousands of conventional
LN modulators made without the ECR etching process, such
failure has rarely been observed. Figure 2 shows the AES
profiles for surfaces of the peeled electrode and the substrate.
The dashed line in the figure denotes the peeled position, and
the profiles over and under the dashed line denote the peeled
electrode underneath and the remaining substrate surface, respectively. The other two profiles were measured after a few
minutes of Ar ion etching of the corresponding surfaces to
exclude the effect of surface contamination. As is seen, the
Auger electron peaks come from Nb were detected only from
the substrate, indicating that the electrode peeled off at the

Intensity

20000

10000

0

200

400
600
Kinetic energy [eV]

800

200

400
600
Kinetic energy [eV]

800

(b)
20000

Intensity

B. Chemical deterioration of etched surface

(a)

10000

0

Fig. 4. AES measurement results for (a) virgin LN surface and for (b) the
ECR etched surface with CF4

Fig. 3. SEM image of the cross-section of the LN wafer with ridge
waveguides and the buffer layer. The ridge waveguides were prepared by
ECR etching using CF4

JUST A – Vacuum, Surfaces, and Films

boundary of the LN and the SiO2 buffer layer. In other
words, the adhesive strength between the ECR etched LN
and the SiO2 was weak.
Figure 3 shows a SEM image of a cross section of the
ridge waveguide with only the 1.2 µm thick SiO2 buffer
layer without the electrodes. The boundary between the
buffer layer and the LN ridge waveguide was smooth and the
buffer layer and waveguides adhered closely to each together. However, the boundary on the etched LN surface was
rough accompanied with small voids, suggesting the inferior
adhesion of the buffer layer.
Results of the AES examination of the ECR etched surface, as shown in Figs. 4(a) for the virgin LN surface and
4(b) for the etched surface, show significant amounts of fluorine and carbon from the ECR etched surface. These contaminants are thought to deteriorate the adhesive strength of
the buffer layer on the etched LN surface. The origin for the
extrinsic contaminants was previously investigated and
found to be precipitation of LiF and polymers, possibly due
to the etching gases (CHF3 and CF4).8,9 In particular, the LiF
precipitates were found to be crystalline in the CF4 etching,
while amorphous in the CHF3 etching.9 Presumably, the ex-

Mitsugi et al.: Challenges in ECR plasma etching

2248
20000

2248

(a)

Intensity

15000

[µm]

10000

[µm]
5000

(b)
0

0

200

400

600

800

1000

Kinetic energy [eV]
FIG. 5 AES analysis results for the LN surface etched by CHF3 ECR plasma
measured before (top of the figure) and after the annealing in O2 (a) at
400 ºC, (b) at 600 ºC (c) at 800 ºC, and (d) at 1000 ºC

[µm]
[µm]

(c)
istence of such plasma species as HF in the ECR plasma
affects the structure of the deposits.
In order to improve buffer layer adhesion, a method to
remove surface contaminants was investigated. The LiF precipitates at least, they were easily removed by wet etching
the substrate in dilute HNO3 at room temperature. Alternatively, a postannealing of the substrate in an oxidizing atmosphere was found to be effective in excluding the LiF and
reducing the carbon contaminants. For instance after 3 h of
etching using CHF3, the samples were placed in a platinum
box and annealed for 1 h in flowing O2. Figures 5(a)-5(d)
show the AES results for samples annealed at 400 ºC in (a),
600 ºC in (b), 800 ºC in (c) and 1000 ºC in (d). With the
400 ºC annealing, the LiF precipitates seemed to be completely removed, judging from the absence of the F peak in
the AES profile. Further, the intensity of the C peak decreased with increasing annealing temperature, and the Nb
peaks appeared in the AES profiles.
Figures 6(a)-6(d) show AFM images corresponding to
the samples shown in Figs. 5(a)-5(d). The surface morphology changed significantly, depending on the annealing temperatures. The peak-to-peak surface roughness measured for
the 5µm×5 µm area was (a) 13.6 nm, (b) 22.7 nm, (c) 41.5
nm and (d) 4.5 nm, while 16.6 nm for the as-etched surface.
Although the surface contaminants due to the etching could
be largely reduced, the morphology was not improved by the
postannealing.
The ridge waveguide devices were prepared again, adopting processes for removing the etching deposits (especially
the LiF) before the buffer layer deposition process. The
above described O2 annealing at 600 ºC and the alternative
HNO3 etching were attempted, and no buffer layer or electrodes peeled off, even after machining the wafer. The adnesive strength of the buffer layer was improved by removing
the etching deposits on the LN surface.
J. Vac. Sci Technol. A, Vol. 16, No. 4, Jul / Aug 1998

[µm]
[µm]

(d)

[µm]
[µm]

Fig. 6. AFM images for the LN surface etched by CHF3 ECR plasma observed after the annealing in O2 (a) at 400 ºC, (b) at 600 ºC, (c) at 800 ºC,
and (d) at 1000 ºC.

C. Mechanical problems in etched surfaces
In addition to the chemical problems, a catastrophic mechanical problem occurred in the fabrication of the ridge
waveguide devices. Figure 7 shows the SEM image for the
mechanically broken ridges, in which the waveguide was
also broken and could not transfer any optical signal. Such
failure was found frequently after the buffer layer deposition
on the substrates ECR etched by CF4. With increasing the
buffer layer thickness, the number of broken waveguides in-

2249

Mitsugi et al.: Challenges in ECR plasma etching

2249

(a)

(b)
[µm]
1.5

1.0

0.5

0
0
FIG. 7. SEM images of the broken ridge waveguides after the Si02 Buffer
layer deposition process.

creased and the fabrication yield for the devices deteriorated
significantly (almost all device chips in the wafer failed due
to the brake at somewhere in the waveguides).
As a reason for the damaged ridge waveguides, surface
stress due to the buffer layer formation was considered. Internal stress of the sputtering-deposited SiO2 buffer layer on
the LN was previously found to be compressive stress and
tensile stress for the LN surface. On the other hand, growth
of a notch line along the ridge waveguides, as shown in Fig.
8, was found for the samples ECR etched by CF4. The SEM
image of Fig. 8(a) was observed before the buffer layer
deposition. There was a possibility that the notches were the
origin of the fractured ridge waveguides with the tensile
stress on the LN surface being induced by the buffer layer
deposition.
For instance, the magnitude of the surface stress was calculated from a wafer deflection measured for a sample with a
l.2 µm thick sputtering deposited SiO2 buffer layer. The
wafer deflection was measured by a stylus method for a 50
mm distance along the wafer diameter. The change of the
wafer deflections before and after the buffer layer deposition
was + 16µm per 50mm. The resulting tensile stress was
estimated to be 1.7-4.5 × 108 N/m2 using a Young’s modulus for the LN of 7-7.5 Pa, a Poisson ratio of 0.2-0.5 and a
wafer thickness of 0.5mm This stress was comparable with
JUST A – Vacuum, Surfaces, and Films

100

200

[µm]

FIG. 8. (a) Cross-sectional SEM image of the ridge waveguide prepared by
the CF4 ECR plasma etching, and (b) the corresponding surface profile
measured by the stylus method.

the strength measured for the LN crystal, suggesting the
ridge breaks were due to the buffer layer deposition. Here,
the strength of the LN crystal was measured by a four-point
bending method to be approximately 2 × 10 8N/m 2 (the
63.2% characteristics strength in the Weibull plotting of the
results for 24 test pieces). Because the broadband LN devices
commonly demand thicker and dense SiO2 buffer layers, efforts to reduce the film internal stresses are needed and are
being investigated.
Another concern with the ridge waveguide fabrication
was notches along the ridge waveguide foot formed during
the dry etching (see Fig. 8). This problem was found to be
improved by changing the etching gas from CF4 to CHF3,
although the reason for the improvement is not known at this
time. Figures 9(a) and 9(b) show the SEM image and the
surface profiles, respectively, of the ECR etched ridge
waveguides using CHF 3. The ridges with 4-5µm height
were successfully fabricated over the 3 in. diam LN wafer
without any notches. It was found that size and crystalline
structure of the surface precipitates differed depending on the
etching gas; large particles consisting of crystalline LiF for
the CF4 etching, while small particles consisting of amorphous LiF for the CHF3 etching.9 Further, in the case of CF4
etching, the boundary between the surface precipitates and

Mitsugi et al.: Challenges in ECR plasma etching

2250

2250

(a)

(b) [µm]
4.0
3.0
2.0
1.0
0
0

100

200

[µm]

FIG. 9. (a) Cross-sectional SEM image of the ridge waveguide prepared by
the CHF3 ECR plasma etching, and (b) the corresponding surface profile
measured by the stylus method.

FIG. 10. Cross-sectional SEM image of the ridge waveguide prepared by the
CF4 ECR plasma etching.

the LN wafer was also found to be roughened as shown in
Fig. 10. Even after the removal of the surface precipitates by
the methods described in the previous section, the notches
did not disappear and the rough LN surface appeared. Such
difference in the surface reaction depending on the etching
gas was considered to be one of the reasons for the improvement of etched surface morphology by using CHF3 instead of
CF4.
Some of the ridge waveguide devices were fabricated using a CHF3 etching process, but without any improvement
concerning the buffer layer deposition process; e.g., the devices of Fig. 1. Although the problem of the buffer layer
stress was not eliminated, the breaks in the ridge waveguides
could be largely suppressed and the fabrication yield increased.

D. Other problems due to plasma processing
In the plasma process for LN wafers such as the dry etching described here and the sputtering film deposition process,
generation of pin holes through the wafer was observed although it was not common. In our experience, such failure
was found once or twice per several times of the ECR etching process.
J. Vac. Sci Technol. A, Vol. 16, No. 4, Jul / Aug 1998

FIG. 11. Example of the pin hole observed at the center of the LN wafer
after ECR etching process.

2251

Mitsugi et al.: Challenges in ECR plasma etching

Figure 11 shows an example of a pin hole of approxiMately 40µm size which was found almost at the center of
the 0.5mm thick z-cut wafer after the etching. During the
plasma process, a plasma spark occurred sometimes on the
wafer surfer possibly due to pyroelectrically induced
charges on the LN surface as a result of temperature increase
of the surface by the process. Such charges generated largely
for the z-cut LN surface because of the pyroelectric effect
along the crystal z axis. At this moment, we consider that the
observed pin hole were caused by dielectric breakdown of
the material due to the plasma spike. One reason for this
speculation is that such failure was not observed in the similar process for x-cut LN wafers, in which surface an occurrence of the pyroelectric effect can be neglected.

Ⅳ . CONCLUSION
Problems in the fabrication of LN ridge waveguide devices were observed. Weakening of over-layer adhesion
strength and ridge waveguide mechanical strength were
found to be the cause of reduced fabrication yield. These
phenomena were found to come mainly from the chemical
deterioration of the ECR etched LN surface and to be improved by changing the process conditions. Both in situ
(choice of the etching gas, etc.) and ex situ (postannealing,

JUST A – Vacuum, Surfaces, and Films

2251

etc.) methods were effective in reducing the failure. Present
experiments indicated that CHF3 is superior to CF4 for ECR
dry etching of LN wafers due to suppressed deterioration of
the surface.
ACKNOWLEDGMENTS
This study was supported by Special Coordination Funds
for Promoting Science and Technology “Fundamental research on new materials of function-harmonized oxides,”
from the Japanese Science and Technology Agency, to
whom we are deeply indebted.
1

K. Noguchi, O. Mitomi, H. Miyazawa, and S. Seki, J. Lightwave Technol. 13, 1164 (1195).
2
K. Noguchi, O. Mitomi, K. Miyazawa, Optical Fiber Communication, OFC’96 Technical Digest (Optical Society of America, Washington
D.C., 1996), p. 205.
3
K. Noguchi, O. Mitomi, K. Kawano, and M. Yanagibashi, IEEE Photonics Technol. Lett. 5, 52 (1993)
4
F. Lanrell, J. Webjorn, G. Arvidsson, and J. Holmberg, J. Lightwave
Technol. 10, 606 (1992).
5
R. S. Cheng, W. L. Chen, and W. S. Wang, IEEE Photonics Technol.
Lett. 7, 1282 (1995).
6
S. Miyazawa, J. Appl. Phys. 50, 4599 (1979).
7
C. Q. Xu, H. Okayama, and M. Kawahara, Appl. Phys. Lett. 64, 2504
(1994).
8
K. Shima, N. Mitsugi, and H. Nagata, J. Mater. Res. 13 527 (1998).
9
H. Nagata, N. Mitsugi, K. Shima, M. Tamai, and E. M. Haga, J. Cryst.
Growth (to be published).
</reference>

<statements>
1. Ion milling, implantation, and aggressive plasma etching all introduce lattice damage and oxygen vacancies; high‑temperature annealing of LNOI films after these steps is routinely used to repair implantation and ion‑milling damage and reduce propagation loss. In thin‑film LN, combinations of high‑temperature annealing and chemical‑mechanical polishing (CMP) are used to remove the damaged top layer and reduce surface roughness to the sub‑nanometer level, which is critical for high‑Q nonlinear photonic devices. Hybrid processing that uses high‑temperature reduction, RIE dry etching, and subsequent wet etching can achieve clean 90° sidewalls and smooth surfaces, but some studies show that poorly tuned post‑annealing can leave voids and rough morphology, so anneal temperature and ambient need to be carefully optimized.
</statements>

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