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        "result": "unsupported"
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<reference>
Surface precipitates on single crystal LiNbO3 after dry-etching
by CHF3 plasma
Kaori Shima
Advanced Materials Research Division, New Technology Research Laboratories, Sumitomo Osaka Cement
Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274, Japan
Naoki Mitsugi and Hirotoshi Nagata
Optoelectronics Research Division, New Technology Research Laboratories, Sumitomo Osaka Cement
Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274, Japan
(Received 19 February 1997; accepted 5 August 1997)

The CHF3 electron cyclotron resonance (ECR) plasma etched LiNbO3 (LN) surface
was analyzed chemically and crystallographically to investigate the dry-etch machining
process for LN crystal, which was recently needed to obtain broader-band optical
modulators. The etched surface was entirely covered with amorphous-like precipitates
having Å‘70 nm diameter. These precipitates (or a part of them) were thought to be
LiF from Auger electron and x-ray photoelectron spectroscopy. The results indicated
that the LiF was formed and remained on the etched surface while the Nb was almost
completely removed.

In order to expand the optical bandwidth of LiNbO3
(LN) based modulators, the plasma dry-etching technique is applied to fabricate a trench structure on the
LN substrate surface, in which the LN is removed to
a depth of about 3É m along the optical waveguide.1,2
Such a structure effectively reduces the permittivity of
the substrate, broadening the bandwidth while reducing
the driving voltage. Dry-etching of the LN is performed
fluorocarbons such as CF4 and CHF3 rather than Ar,
in order to prevent surface damage by high energy
ions.1,2 Jackel et al. reported originally the reactive
ion etching of the LN and suggested the possibility
of the formation of a thermally stable compound of
Li with halogen during the chemical etching process.3
They recommended mixing Ar into the chemical etching
gases to physically remove the Li-halides. However,
to our knowledge, there has been no report to show
any evidence for the existence of the Li-halide layer
due to the dry-etching. Here, we investigated the LN
surface etched by an electron cyclotron resonance (ECR)
plasma of CHF3 and confirmed the formation of LiF on
the surface. The CHF3 was chosen in this experiment
because the coexistence of F and H was commonly
known to be suitable for the etching of oxide materials.4
Commercial X-cut (crystallographic a-face) and
Z-cut (c-face) LN substrates with an optically fiat
surface were treated by an ANELVA L-310R ECR
plasma etcher. After the substrate was inserted, the
etching chamber was evacuated to about 3Å~10-5 Pa
by a cryopump. Then, CHF3 was introduced into the
chamber at a rate of 3 sccm, and the pressure was kept
at about 0.01 Pa. Microwave power and high voltages
applied to the magnet and bias electrode were chosen to
J. Mater. Res., Vol. 13, No. 3, Mar 1998

generate a stable CHF3 plasma and to obtain a higher
etching rate. The substrate holder, made of a sintered
SiC, was cooled by water during the etching. The typical
etching rate in this experiment was 500 to 600 nm/h for
both the X-cut and the Z-cut LN substrates.
The etched LN surface became hazy. Figures 1 (a)
and 1( b) show atomic force microscopic (AFM) images
of the substrate surfaces before and after the CHF3
ECR plasma etching for 4 h, respectively. The image in
Fig. 1(a) was a typical morphology for a mechanochemically polished LN surface.5-7 However, due to the CHF3
plasma etching, the surface was covered entirely with
Å‘70nm diameter precipitates. The peak-to-peak surface
roughness in the observed 2É mÅ~2É m area was
2.2nm for Fig. 1(a) before the etching and 43nm for
1(b) after the etching. On the other surface etched for
2 h, similar precipitates were observed, including many
smaller particles. On such a surface, x-ray microanalysis
was used to detect a small amount of F in addition to
Nb and O.
In order to confirm the existence of F on the surface,
Auger electron spectroscopy was carried out for the
1-h-CHF3 plasma etched sample accompanied with Ar
ion etching. The Ar ion etching rate for this LN sample
was calculated to be 8 nm/min from the depth of the
Ar ion etched crater measured by a stylus method.
To prevent a charge-up of the sample, the acceleration
energy, current, and size of the incident electron beam
had been set at 3kV, 1Å~10 -8 A, and 10É m,
respectively. In the measurement, the windows for the
energy detector were fixed at Li KLL, Nb MNN, C KLL,
O KLL, and F KLL peaks which were clearly detected
from the surface before the Ar ion etching. It was noted
© 1998 Materials Research Society

Communications

[µm]

[µm]

(b)

(a)

FIG. 1. AFM images for the LiNbO3 substrate surface (a) and the CHF3 plasma etched LiNbO3 substrate surface ( b). The 2µm × 2µm
area was observed for both samples.

(arbitrary units)

O

Peak Intensity

that, for the virgin LN surface without CHF3 plasma
etching, the F KLL peak was not observed. Figure 2
shows a change of the Auger electron peak intensities
measured after Ar ion etching treatments. The horizontal
axis of Fig. 2 denotes the cycle number of the Ar ion
etching (each 4 s) and the measurement, corresponding
to the depth from the surface. The intensity of the F KLL
peak decreased rapidly after several Ar ion etching
cycles, and then damped gradually within 70-80 etch
cycles, which corresponded to about 50 nm depth from
the surface. The C KLL peak was confirmed to disappear
beyond about 100nm depth, which is not shown in the
figure. These data suggested that the LN surface was
fluoridated during the CHF3 plasma etching treatments.

Nb
Li

C

F
0

20

40

60

80

100

Ar lon Etching Cycles

FIG. 2. Distribution of Li, C, F, O, and Nb along the depth direction
for the CHF3 plasma etched LiNbO3 measured by an Auger electron
spectroscopy. The full horizontal axial length corresponds to about
70 nm depth from the surface.

Next, to investigate the chemical state of the F, a
similar sample was analyzed by an x-ray photoelectron
spectrometer (XPS) using a Mg Kα incident x-ray beam
with about a 1mm diameter. The angles between the
sample surface and analyzer and between the surface and
the source were 45 and 90º, respectively. Ar ion etching
was not performed in this measurement to prevent a
reduction of the surface due to the interaction with high
energy ions in the high vacuum atmosphere. Figure 3
reveals the peak profiles obtained for F 1s, O 1s, C 1s,
Nb 3d, and Li 1s photoelectrons. The binding energy
in the horizontal axes was calibrated by the C 1s peak
for C-C and/or C-H configuration at 284.8 eV. As is
seen, the main peak for F 1s appeared near 685 eV with
the small shoulder peak at about 688 eV. The binding
energy of this main peak corresponded to that for the F
bonded with a metal (685-686 eV for LiF), while the
shoulder peak was attributed to C-F bonding.8 The O 1s
peaks could be identified as a peak from oxidized metals
around 530 eV (e.g., 〜 531 eV for Li2O and 〜 530.3 eV
for LiNbO 3), C=O bonding at about 532 eV, and
C-O-C bonding at about 533 eV.8 The observed C 1s
peaks could be attributed to C-C bonding at 284.8 eV,
C-O and/or C*-CFχ at 285.5-286.5 eV, C=0 and/or
-CF at 287-288 eV, and O=C-O at 289 eV. 8 The
Nb 3d5/2 peak observed at 207 eV seemed to come from
oxidized Nb such as LiNbO3 ( 〜 207.1 eV) and Nb2O5
(206-208 eV). 8 Although the Li 1s peak was very
weak, its binding energy, about 56 eV, corresponded
evidently to that for LiF at 55.7-56.7 eV rather than
for LiNbO3 at about 54.8 eV.8 The additional peak at
60 eV which appeared near Li 1s was due to Nb 4s
for LiNbO3. As a result, the Li on the etched surface
was almost completely combined with F, while the Nb
existed as LiNbO3 . Further, the C due to the etching gas
also remained on the surface, in addition to the carbons
combined with oxygen which were the main part of

J. Mater. Res., Vol. 13, No. 3, Mar 1998

Communications

F 1s

695

Nb 3 d

690

685

680

215

210

205

200

O 1s

195

Li 1 s

542 540 538 536 534 532 530 528 526

65

60

55

Binding Energy

50

[eV]

C 1s

295

290

285

Binding Energy

280

[eV]

FIG. 3. X-ray photoelectron spectroscopic results for the CHF3 plasma etched LiNbO3. No surface cleaning by Ar ion etching was carried
out before the measurement.

the observed C 1s peak and considered to come from
surface contaminants. The atomic percents for F, O, C,
Nb, and Li were estimated to be 8, 34, 48, 4, and 6%,
respectively.
Finally, the structure of the surface precipitates
on the CHF 3 plasma etched LN of Fig. 1(b) was
investigated by a glancing angled x-ray (XRD) and a
transmission electron (TEM) diffractometer. In both
measurements, no diffraction peak specific to the precipitates was detected. It is important to note, the TEM
observation was performed only for the precipitate layers
after a conventional ion-thinning of the sample from the
backside and a halo pattern without any diffraction
spots was obtained, suggesting the precipitates were
amorphous-like.
The above-mentioned results suggested the formation of Li fluoride, LiF, during the CHF3 plasma etching
of the LiNbO3 surface. It is likely that this LiF is
amorphous rather than crystalline. On the other hand, the
Nb existed as an oxide, possibly LiNbO3, on the etched
surface, indicating the Nb was successfully volatilized
by the CHF3 plasma etching. In the dry-etching process
for the LiNbO3 using fluorocarbons, the removal of the
LiF precipitates is a key issue for getting a high etching

rate and a smooth surface without a chemically deteriorated layer.
ACKNOWLEDGMENTS
The authors gratefully thank Mr. Takano of Foundation for Promotion of Materials Science and Technology
of Japan for XPS analyses and Dr. Min of Sumitomo
Osaka Cement for the helpful discussion.
REFERENCES
1. M. Minakata, Electron. Commun. Jpn. Part 2 77, 37 ( 1994).
2. K. Noguchi, O. Mitomi, H. Miyazawa, and S. Seki, J. Lightwave
Technol. 13, 1164 (1995).
3. J. L. Jackel, R. E. Howard, E. L. Hu, and S. P. Lyman, Appl. Phys.
Lett. 38, 907 (1981).
4. Handbook of Plasma Materials Science, edited by K. Akashi (Ohmsha, Tokyo, 1992).
5. H. Nagata and K. Shima, Jpn. J. Appl. Phys. 35, 4040 (1996).
6. H. Nagata, T. Sakamoto, H. Honda, J. Ichikawa, E. M. Haga,
K. Shima, and N. Haga, J. Mater. Res. 11, 2085 (1996).
7. H. Nagata, K. Shima, and J. Ichikawa, J. Am. Ceram. Soc. 80,
1203 (1997).
8. Handbook of Photoelectron Spectroscopy, edited by J. Chastain
(Perkin-Elmer Co., Eden Prairie, MN, 1992).

J. Mater. Res., Vol. 13, No. 3, Mar 1998
</reference>

<statements>
1. Material damage in lithium niobate (LN) after plasma etching can be mitigated mainly by changing the etch chemistry and ion energy to reduce LiF formation and lattice disorder, combining the etch with well‑chosen wet cleans, and using pre‑/post‑treatments such as proton exchange, doping, annealing, and polishing to restore the surface and near‑surface region.
2. Fluorocarbon plasmas (CF₄, CHF₃, C₄F₈) generate LiF, which is involatile and accumulates on the surface and sidewalls, lowering etch rate and roughening profiles, so simply running “harder” often increases damage rather than depth. Ar‑based ICP‑RIE processes avoid LiF formation by relying on physical sputtering; with optimized pressure, low bias, and a post‑wet cleaning step they can produce smooth, vertical sidewalls while minimizing chemical damage. Directional atomic layer etching (ALE) using HBr/BCl₃ for modification followed by a low‑power Ar plasma for removal improves volatility of etch products relative to F‑ and Cl‑based plasmas, reducing surface roughening and premature etch termination and thus limiting damage per cycle.
3. LiF formation during CHF₃ and other F‑based etches has been directly observed as dense nanoscale precipitates that increase peak‑to‑peak roughness by over an order of magnitude compared to the polished starting surface. One successful strategy is cyclic processing: ICP etch in C₄F₈/He or similar chemistry for a few minutes, pause, then use an SC‑1 clean (H₂O/H₂O₂/NH₄OH) or HF to remove LiF and redeposited LN before resuming etching, which yields deeper etches with improved sidewall profiles. Post‑etch RCA cleans (e.g., 5:1:1 H₂O:NH₄OH:H₂O₂ at ~80 °C) after ALE or continuous plasma etching can actually reduce RMS and Ra roughness below the pre‑etch values by removing redeposited compounds and smoothing high‑frequency features.
</statements>

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