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    },
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        "idx": 2,
        "result": "unsupported"
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<reference>
WeD3

Eindhoven, The Netherlands, June 11-13, 2008

Etching of Lithium Niobate: From Ridge Waveguides
to Photonic Crystal Structures
H. Hui, R. Ricken and W. Sohler
Angewandte Physik,, Universität Paderborn, 33098 Paderborn, Germany
sol_hu@physik.upb.de

Abstract. Recent progress of wet etching of Z-cut LN, of inductively coupled plasma
(ICP-) etching of X-cut LN, and of ICP-etching of proton-exchanged X-cut LN is
reported to fabricate low loss ridge guides, micromechanical, and photonic crystal
structures.

Introduction
The development of lithium niobate (LiNbO3, LN) integrated optical devices requires
etching techniques for a reliable fabrication of deep (sub-) micrometer structures.
Examples are ridge guides, Bragg gratings, and photonic crystal structures. The existing
etching methods can be classified into two categories: wet (chemical) etching and dry
(ion) etching. Wet etching is generally performed in a mixture of HF and HNO3, which
attacks the –Z-face of the crystal, whereas the +Z-face is hardly affected. Therefore, selective chemical etching can either be achieved by depositing a metallic mask of the
structure to be fabricated on the –Z-face [1] or by defining the structure first by a corresponding domain inversion [2]. Wet etching can also be applied to proton-exchanged
or ion-implanted LN to form (sub-) micrometer structures in the surface of the crystal
[3, 4]. In dry etching, plasma etching, ion beam milling and focussed ion beam etching
are generally used [5, 6, 7]. In this contribution we report our recent progress of wet
etching of Z-cut LN to fabricate low loss ridge guide and micromechanical structures,
of inductively coupled plasma (ICP-) etching of X-cut LN to get ridges and other
microstructures, and of ICP-etching of proton-exchanged X-cut LN to develop photonic
crystal structures.

Wet etching of Z-cut LN: ridge waveguides and micromechanics
Wet etching of Z-cut LN with HF/HNO3 proved to be a simple and reliable method
to fabricate low loss Ti-doped ridge waveguides with TE propagation losses of 0.3
dB/cm only [4]. To get optical guiding, the ridges were defined in a planar Ti:LiNbO3
waveguide. Cr-stripes have been used as etch masks.
Here we report a modified procedure yielding ridge guides of propagation losses nearly
one order of magnitude lower (TE). They were fabricated in three steps using undoped
(congruent) Z-cut LN as substrate: 1. Ridge fabrication: Using a Cr-mask on the –Z surface of the LN substrate 4 to 12 µm wide ridges were fabricated by wet chemical
etching in a mixture of 21 ml HF (40% concentration), 14 ml HNO3 (100% concentration) and 5 ml ethanol, following the procedure described in [1]. A ridge of 6.5 µm (10
µm) height (top width) is shown in Fig. 1 (left); it is aligned parallel to the X-axis. 2. Tistripe definition: A novel photolithographic process was developed, which allows
coating the surface of a ridge selectively. Spin-coating of the sample with photo-resist
results in an inhomogeneous thickness distribution with a thinner layer on top of the
ridge. This is exploited by the following flood exposure (5 seconds), which leads to full
exposure only on top of the ridge. By the subsequent development the photo-resist on

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ECIO ‘08 Eindhoven

the top of the ridge is totally removed, while a thin layer remains besides the ridge and
on the ridge walls. After lift-off a Ti-stripe is precisely defined on top of the ridge only
(see Fig. 1-middle). 3. Ti-indiffusion: The Ti-stripe can now be indiffused using
conventional parameters (1060 oC @ 8.5 hrs in Ar (7.5 hrs) and O2 (1 hr) atmosphere,
respectively). The result is a Ti-doped ridge waveguide as shown in Fig. 1 (right); the
edges are rounded and the surface roughness is reduced (see Fig. 1-right).

Fig. 1: Wet etched ridge on Z-cut LN, (left), Ti-coated ridge before indiffusion (middle), and after
indiffusion (right). The height (top width) of the ridge is 6.5 µm (10 µm), aligned parallel X.

Therefore, also the propagation losses of the ridge guides, monomode up to a top width
of 9 µm, are significantly lower than previous results [1]. They were measured using the
Fabry-Perot resonance method at 1.55 µm wavelength. Both, the TE and TM losses
decrease with increasing width. For TE-polarization the loss drops from 0.22 dB/cm at
5 µm width to 0.05 dB/cm at 7 µm width. For TM-polarization the losses are 1.3 and
0.36 dB/cm at 5 µm and 9 µm widths, respectively. They are significantly larger than
TE-losses, though the TM mode is smaller than the TE-mode with lower field strength
at the ridge walls. An explanation for the strongly polarization-dependent losses might
be that due to the growth of Cr2O3 under the mask and due to outdiffusion of Li2O the
concentration of Li and O might vary near the surface leading to corresponding
fluctuations of the extraordinary index of refraction (the ordinary index would remain
nearly unaffected). Such index fluctuation would lead to scattering losses of the TMmode alone.
It was even possible to control the slope of the ridge walls by adjusting the etching
temperature. This allows getting even steeper walls and to fabricate micromechanical
structures such as the interdigital fingers given as example in Fig. 2. It was etched at a
temperature of 8 oC. The lamellas have a width of 2.6 µm with a separation of 2.8 µm.
The etching depth is 7.7 µm.

Fig. 2: Wet etched interdigital lamellas of 2.6 µm width and of 2.8 µm separation on
Z-cut LN. The etching depth is 7.7 µm.

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Eindhoven, The Netherlands, June 11-13, 2008

ICP-etching of X-cut LN: ridges and microstructures

Plasma etching is a very controllable process and has the advantage of being highly
anisotropic. Plasmas based on fluorine gases are generally used for plasma etching of
LN due to the good volatility of fully fluorinated niobium species at temperatures
around 200 °C. However, a problem is the formation and re-deposition of LiF, which
has a melting temperature of more than 800 oC. It will be deposited on all surfaces and
will lower in this way the etching rate. Therefore, if re-deposition dominates the process, vertical side walls, which are necessary for the definition of small structures, can
hardly be obtained. Our solution for this problem is as follows: at first, the sample with
a Cr layer defining the structure to be fabricated is ICP-etched for several minutes in a
C4F8/He (1:1) plasma. Then the etching process is stopped and the sample is cleaned in
SC-1 solution (70% H2O, 20% H2O2, 10% NH4OH) for 1 minute to remove the deposition, before ICP-etching is continued. These two steps are repeated several times until
the desired etching depth is reached. Fig. 3 shows as an example etched ridges in X-cut
LN, aligned along the Y-direction; the ridges have a height (width) of 5.8 µm (8 µm).
On the right of Fig. 3 more complicated microstructures of the same height are shown
demonstrating that even small connections between the squares can be fabricated in a
reproducible way with nearly vertical walls.

Fig. 3: ICP-etched ridges on X-cut LN of 5.8 µm height and 8 µm width (left and middle). Further
microstructures of the same height with nearly vertical side walls (right).

ICP-etching of H+-exchanged X-cut LN: photonic crystal structures
Another way to reduce the problem of LiF re-deposition is to lower the Li concentration in LN by a proton exchange (PE) process [8]. Thus, the rate of LiF re-deposition
will be significantly reduced in comparison to etching of pure LN. As a consequence,
the etching rate will be increased and the etch profiles will be improved. We demonstrated the realization of this concept be performing first a PE of congruent X-cut LN to
a depth of 1.4 µm. Then the surface of the PE-LN was covered by a 110 nm thick Cr
film deposited by sputtering and coated with a 120 nm thick photo-resist layer. Using
conventional optical contact lithography, photonic crystal structures were defined in the

Fig. 4: Holes of a photonic crystal structure defined in photo resist (left), transferred by wet etching in
the Cr film (middle) and by ICP-etching into the LN substrate.

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ECIO ‘08 Eindhoven

photo-resist (Fig. 4, left). The smallest lines have a width of 170 nm. By wet etching
these structures were transferred into the Cr film (Fig. 4, middle). Then the sample was
ICP-etched for several minutes, and cleaned afterwards in SC-1 solution to remove all
depositions. This process was repeated several times until the desired etching depth in
the PE-LN was reached (Fig. 4, right). Fig. 5 shows the photonic crystal structure as a
whole together with some details in higher magnification.

Fig. 5: Photonic crystal structure in PE-LN as a whole (left) and with more details in higher
magnification (middle and right).

Proton exchange is also used for the fabrication of single polarization waveguides, as it
increases only the extraordinary index of refraction. A subsequent reverse proton
exchange (RPE) can even increase the Li concentration again to form buried waveguide
profiles.

Conclusions
In conclusion, significant progress has been achieved to improve wet etching of Zcut LN, ICP-etching of X-cut LN, and ICP-etching of proton-exchanged X-cut LN
allowing a reliable fabrication of integrated optical devices with deep (sub-) micrometer
structures. As examples ridge guides of very low propagation losses, micromechanical,
and photonic crystal structures have been demonstrated.

References
[1] H. Hu, R. Ricken, W. Sohler, and R. B. Wehrspohn, “Lithium Niobate Ridge Waveguides
Fabricated by Wet Etching”, IEEE Photon. Technol. Lett., vol. 19, pp. 417–419, Mar. 2007.
[2] I. E. Barry, G. W. Ross, P. G. R. Smith, and R. W. Eason, “Ridge waveguides in lithium niobate
fabricated by differential etching following spatially selective domain inversion,” Appl. Phys. Lett.,
vol. 74, pp. 1487–1488, Mar. 1999.
[3] T.-L. Ting, L. -Y. Chen, and W.-S. Wang, “A novel wet-etching method using joint proton source in
LiNbO3”, IEEE Photon. Technol. Lett., vol. 18, pp. 568–570, Feb. 2006.
[4] D. M. Gill, D. Jacobson, C. S. White, Y. Shi, W. J. Minford, and A. Harris, “Ridged LiNbO3
modulators fabricated by a novel oxygen-ion implant/wet-etch technique”, J. Lightwave Technol.
vol. 22, pp. 887–894, Mar. 2004.
[5] Masashi Tamura, Shinzo Yoshikado, “Etching characteristics of LiNbO3 crystal by fluorine gas
plasma reactive ion etching”, Surface and Coatings Technology, Vol. 169 –170, pp. 203–207, 2003.
[6] P. Rabiei and W. H. Steier, “Lithium niobate ridge waveguides and modulators fabricated using
smart guide,” Appl. Phys. Lett., vol. 86, Art. No. 161115, 2005.
[7] F. Lacour, N. Courjal, M.-P. Bernal, A. Sabac, C. Bainier, M. Spajer, “Nanostructuring lithium
niobate substrates by focused ion beam milling”, Optical Materials, Vol. 27, pp. 1421–1425, 2005.
[8] H. Hu, A. P. Milenin, R. B. Wehrspohn, H. Herrmann, and W. Sohler, “Plasma etching of protonexchanged lithium niobate”, J. Vac. Sci. Technol. A, vol. 24, pp. 1012-1015, Jul./Aug., 2006

78
</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.
4. 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.
5. Robust hard‑mask stacks such as Ti/Al/Cr, combined with periodic etch pauses and chemical cleaning, help avoid overheating and by‑product buildup on the LN surface, minimizing micro‑cracks and mask‑induced roughness. Introducing controlled oxygen during Ar plasma cleaning/etching can re‑oxidize the surface and neutralize the metal‑rich damaged layer produced by pure Ar bombardment, thereby reducing waveguide loss and preserving optical properties. At the device‑design level, techniques such as forming ridges on PE‑LN and then performing reverse proton exchange to bury the guiding region, or overcladding and slightly over‑etching so the optical mode is pushed away from the most damaged surface layer, are used to keep nonlinear photonic modes largely out of the plasma‑damaged region.
</statements>

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