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<reference>
Plasma etching of proton-exchanged lithium niobate
H. Hu
Department of Physics, University of Paderborn, D-33095 Paderborn, Germany

A. P. Milenin
Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany

R. B. Wehrspohn,a兲 H. Hermann, and W. Sohler
Department of Physics, University of Paderborn, D-33095 Paderborn, Germany

共Received 25 January 2006; accepted 26 April 2006; published 9 June 2006兲
Plasma etching of lithium niobate with fluorine gases is limited by the redeposition LiF. This results
in a low etch rate and nonvertically etched walls. Etching of proton-exchanged lithium niobate can
prevent the LiF deposition to a large extent because of the greatly reduced lithium concentration in
lithium niobate. We performed different inductively coupled plasma etching processes using SF6 or
CHF3 / Ar on proton-exchanged lithium niobate. Negligible underetching and nearly vertically
etched walls on proton-exchanged lithium niobate samples were obtained by CHF3 / Ar gas at
chamber pressure of 6 mTorr and 130 V dc bias. © 2006 American Vacuum Society.
关DOI: 10.1116/1.2207150兴

I. INTRODUCTION
Lithium niobate 共LiNbO3, LN兲 is a crystalline dielectric
material being of particular interest for integrated optics due
to its excellent electro-optical, acousto-optical, and nonlinear
optical properties.1 The realizations of LN ridge waveguide
and LN photonic crystal waveguides require anisotropic
etching techniques of the LN substrate. Wet etch and dry
etching methods are widely used in shaping LN crystals.2,3
Dry etching is a very controllable process and has the advantage of being highly anisotropic.4 Plasmas based on fluorine
gases are generally used for plasma etching of LN due to the
good volatility of fully fluorinated niobium species at temperature about 200 ° C. However, one of the main problems
of dry etching of LN with fluorine based plasmas is the redeposition of LiF.5 During the plasma etching of LN, LiF is
formed, which has a melting temperature of more than
800 ° C. The LiF will redeposit on the surface of the substrate and lower the etching rate. Therefore, it is difficult to
obtain vertical etching profiles if redeposition becomes the
dominating process. To avoid redeposition of LiF, it is possible to enhance the sputtering process during the etching,
for instance, by adding argon in the etching gas.6 Another
possible way is to reduce the lithium concentration in lithium
niobate. By a proton exchange 共PE兲 process, lithium ions can
be replaced by protons in LN. This is believed to be one-toone substitution. Depending on the PE parameters, up to
85% of Li ions can be substituted.7 Thus, plasma etching of
PE-LN will have a significant smaller amount of LiF redeposition than that of pure LN, increasing the etching rate and
improving the etch profiles. By using the reverse proton exchange process, protons can be substituted by lithium ions.8
In principle, the plasma-etched PE-LN sample can be reverse
proton exchanged to restore the optical properties of LN.
a兲

Author to whom correspondence should be addressed; electronic mail:
wehrspohn@physik.upb.de

1012

J. Vac. Sci. Technol. A 24„4…, Jul/Aug 2006

Parallel plate reactive ion etching of proton-exchanged LN
waveguide was recently performed by Fogeletti et al. to fabricate linear gratings.9 The etching rate was about 0.6 ␮m / h
and the etching depth was about 0.25– 0.4 ␮m. We carried
out a detailed study using also inductively coupled plasmas
and different fluorine-containing gas mixtures. First photonic
crystal waveguide structures have been successfully fabricated with an etch depth of 1.5 ␮m.

II. EXPERIMENT
Proton exchange of Z-cut LN samples was performed in
pure benzoic acid at 240 ° C for 5 h. The PE depth was
2.8 ␮m as determined by optical microscopy of a polished
end face. The PE-LN surface was then covered by a
120– 400 nm thick Cr layer using sputter deposition. The Cr
layer serves as etch mask for the successive plasma-chemical
etching process of LN. For the ridge waveguides, Cr stripes
of 9.5– 19.5 ␮m width were defined by photolithography.
For the photonic crystal waveguides, electron beam lithography has been used with pore diameter of 340 nm and an
interpore distance of about 500 nm. The Cr layer was opened
for the ridge waveguides by wet etching and for the photonic
crystal waveguides by plasma etching using the parameters
described in Ref. 10. The etching of LN was performed either in a standard reactive ion etching equipment, Tepla
100-E, or in an Oxford Plasmalab System 100 with ICP 380
source installed in a clean room of class 1. Table I give a
summary of the plasma parameters and the etch rate. In experiments 1 and 2, the Tepla parallel plate reactor was used,
and in the experiments 3, 4, and 5 the Oxford Plasmalab
machine was used in the inductively coupled mode. Aspurchased, pure LN samples with Cr stripes having the same
parameters were etched together with PE-LN for comparison. After etching, the Cr structure was removed by wet

0734-2101/2006/24„4…/1012/4/$23.00

©2006 American Vacuum Society

1012

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Hu et al.: Plasma etching of proton-exchanged lithium niobate

1013

TABLE I. Dry etching parameters, etching rate, and average selectivity.

No.

Gas and
flow rate
共SCCMa兲

Power
共W兲

dc
bias
共V兲

Chamber
pressure
共mTorr兲

1

CF4

300

¯

2

SF6

300

3

SF6
50

4

5

Sample
cooling

Average etching
rate 共nm/h兲

Average
selectivity

500

No
cooling

Pure LN/ Cr= 1.12
PE-LN/ Cr= 3.43

¯

500

No
cooling

1500

153

6

CHF3 / Ar
50/ 50

1500

122

6

CHF3 / Ar
50/ 50

1500

130

6

He
cooling
backside
He
cooling
backside
No
cooling

Pure LN: 157
PE-LN: 480
Cr: 140
Pure LN: cannot be
etched
PE-LN: 1530
Cr: 33
Pure-LN: 450
PE-LN: 2740
Cr⬃ 90
Pure LN: 650
PE LN: 2880
Cr⬃ 60
PE-LN: 5760
Cr⬃ 180

PE-LN/ Cr= 46

Pure LN/ Cr⬃ 5
PE-LN/ Cr⬃ 30
Pure LN/ Cr⬃ 11
PE-LN/ Cr⬃ 48
PE-LN/ Cr⬃ 32

a

SCCM denotes standard cubic centimer per minute.

etching. Scanning electron microscopy and optical microscopy were performed to analyze the etch rate and the side
walls.
III. RESULTS AND DISCUSSION
The dry etching parameters, etching rates, and selectivity
are shown in Table I. In experiments 1 and 2, a high gas
pressure 共p = 500 mTorr兲 was used to test if plasma-chemical
etching processes occur on PE-LN. Two different gases were
studied: CF4 and SF6. With CF4 at p = 500 mTorr, the etching rate of PE-LN is 480 nm/ h. The etching rate ratio of
PE-LN to pure LN is about 3. On the sample surface, there
are cracks for the CF4-etched PE-LN sample 关Fig. 1共a兲兴.
With SF6 gas at p = 500 mTorr, the etching rate of PE-LN is
1.53 ␮m / h. A relatively smooth etched surface was observed
by optical microscope 关Fig. 1共b兲兴. The etching ratio of
PE-LN to Cr is 46. The pure LN was not etched at all, probably due to the redeposition of LiF species.
Parallel plate reactive ion etching at a p = 500 mTorr is
basically an isotropic chemical etching process with almost
no physical component. The large etching rate difference between PE-LN and pure LN shows that proton exchange of
LN can overcome the formation of LiF to a large extent, in

FIG. 1. Optical microscope images of the etched surface of PE-LN in different gases. The straight stripe was not etched because of Cr mask. 共a兲
Etched with CF4 共1兲. 共b兲 Etched with SF6 共2兲.
JVST A - Vacuum, Surfaces, and Films

particular, with SF6, which exhibits an etching ratio between
PE-LN and Cr of more than 40. We further investigated
sample 2 by optical microscope and scanning electron microscopy 共SEM兲. Figure 2 shows the PE-LN surface etched
by SF6. The Cr stripe covered initially the PE-LN but a part
of the Cr layer was taken away to show the underlying nonetched LN stripe 共right part of Fig. 2兲. Due to the highly
isotropic etching, a significant underetching of the Cr stripe
occurred. On each edge of the stripe, the lateral underetching rates were 3.5 and 4.0 ␮m / h, respectively. Figure 3
shows the SEM picture of the end facet of PE-LN etched in
SF6. The strongly nonvertical side walls are another indication for a highly isotropic etching process.
In order to obtain vertical side walls and deeper etched
structures, we changed to an inductively coupled plasmareactive ion etching 共ICP-RIE兲 process. We used SF6 and
also CHF3 based on our results above and since it is reported
that CHF3 is superior to CF4 in dry etching of LN due to
suppressed deterioration of the surface.3 SF6 gas was used in
experiment 3. CHF3 gas was used in experiments 4 and 5,
and argon was added in the chamber to enhance the physical

FIG. 2. PE-LN surface etched by SF6 gas. On each side of the stripe, the
under-etching rates are 3.5 and 4.0 ␮m / h, respectively.

1014

Hu et al.: Plasma etching of proton-exchanged lithium niobate

FIG. 3. SEM picture of end face of PE-LN etched by SF6 gas. Asymmetric
etching on two sides of the ridge structure is shown.

FIG. 4. PE-LN surface etched by CHF3 / Ar gas. The width of PE-LN stripe
is almost the same as that of Cr, which indicates the reduced under-etching
effect.

1014

FIG. 6. SEM micrograph of a PE-LN photonic crystal waveguide. Interpore
distance of the periodic pattern is 500 nm. Etching parameters are CHF3 / Ar
etching gas, 1500 W ICP power, 130 V dc bias, 6 mTorr chamber pressure,
and no substrate cooling.

component during etching. Since fully fluorinated niobium
will also be formed in the etching process 共NbF5兲, which has
its boiling point at about 234 ° C,11 we expect a substrate
temperature dependence on the etching rate. The etching rate
of PE-LN is 2.88 ␮m / h with He-backside cooling keeping
the sample temperature nominally at room temperature,
while the etching rate was about 5.7 ␮m / h without cooling
共Table I兲. The selectivity of PE-LN to the Cr mask was 32:1.
The etched surface of experiment 5 is shown in Fig. 4. The
Cr stripe covered the PE-LN, and part of the Cr layer was
peeled off to display the fabricated PE-LN stripe, shown in
the left part of the picture. The width of PE-LN stripe is
nearly the same as that of Cr, which means that the under
etching is greatly reduced. In experiment 5, a nearly vertical
etched wall is obtained with an angle of 82° 共Fig. 5兲. In
literature, mainly parallel plate RIE based plasma based on
CHF3 is applied. The reported etching rates are 0.4 ␮m / h
for pure LN and 0.6 ␮m / h for proton-exchanged LN with
strongly rounded side walls of the trenches.9 We also performed etching of complex photonic crystal waveguide
structures using the etching recipe 5. The diameter of the
pore is 340 nm while the periodicity is 500 nm, and the etching depth measured in ridge waveguide region is about
1.5 ␮m. First etching results are shown in Fig. 6.
IV. CONCLUSION

FIG. 5. SEM picture of end face of PE-LN etched by CHF3 / Ar gas; a nearly
vertical etched wall is shown.
J. Vac. Sci. Technol. A, Vol. 24, No. 4, Jul/Aug 2006

In conclusion, we tested different plasma-chemical etching processes to etch proton-exchanged LiNbO3. We obtained for dry etching with SF6 gas at a gas chamber pressure
500 mTorr: PE-LN was etched at about 1.5 ␮m / h while
pure LN could not be etched. Due to the high pressure,
nearly isotropic etching was observed. The selectivity of
PE-LN to the Cr mask was 46:1. Using ICP-RIE etching,
negligible under etching and nearly vertical etched walls on
PE-LN samples were obtained applying CHF3 / Ar gas at a
chamber pressure of 6 mTorr. The etching rate of PE-LN is
about 5.7 ␮m / h, and the selectivity of PE-LN to the Cr mask
was 32:1. This compares to literature values of PE-LN of
about 0.6 ␮m / h. First photonic crystal waveguide structures
with a lattice constant of 500 nm have been successfully fabricated with etching depth of about 1.5 ␮m.

1015

Hu et al.: Plasma etching of proton-exchanged lithium niobate

ACKNOWLEDGMENTS
One of the authors 共H.H.兲 thanks for the financial support
from the Alexander von Humboldt Foundation.
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D. M. Manos and D. L. Flamm, Plasma Etching: An Introduction 共Academic, New York, 1989兲, p. 476.
5
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1
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1015

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8
V. Fedorov, Y. Korkishko, A. Alkaev, and E. Maslennikov, Proceedings
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11
www.webelements.com
6
</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. Reducing the lithium concentration at the surface via proton exchange (PE) markedly suppresses LiF formation during F‑based plasma etching, which increases etch rate and makes it easier to obtain vertical sidewalls and clean profiles. PE‑assisted dry etching has been used to form deep ridges in LN with better morphology than in congruent LN, precisely because the lower Li content reduces the amount of involatile LiF available to redeposit. Surface H₂‑plasma treatments that substitute protons and relax surface stress have also been shown to improve hard‑mask quality and long‑etch fidelity, indirectly mitigating plasma‑induced defects and redeposition issues during extended ICP runs.
</statements>

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