Etching of LiNbO 3 by Laser-Driven Fusion of Salts
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ETCHING OF LiNbO3 BY LASER-DRIVEN FUSION OF SALTS C. I. H. ASHBY AND P. J. BRANNON Sandia National Laboratories, Division 1126, 87185
P. 0. Box 5800, Albuquerque,
NM
ABSTRACT Lithium niobate exhibits low reactivity with most chemical etchants. Consequently,
etching a LiNbO3 surface to produce optical structures such as
ridge waveguides or grooves for fiber coupling normally requires relatively slow processes such as ion milling.
We have developed a laser-driven
chemical etching process for etching highly unreactive ionic solids based on the fusion of salts In the molten phase and show that the etch rate can be more than 100 times faster than ion milling rates. This process involves spatially localized melting of LiNbO3 by high-power density laser pulses with photon energies in excess of the band gap of LiNbO3 .
While molten,
LiNbO3 undergoes reaction with a surface coating of KF to form niobium oxyfluoride anions by fusion of the salts. water soluble.
The resulting solid is highly
The insolubility of LiNbO 3 permits subsequent removal of
only the irradiated area by rinsing in water. determined by laser power density.
Surface morphology is
The process exhibits a wavelength
dependence.
INTRODUCTION Lithium niobate has found important applications in surface acoustic wave (SAW)
devices, waveguides, and photonic devices.
However,
processes
for etching the structures required for these applications, such as holes and grooves,
into LiNbO3 are very slow.
Most current processes for
producing features in LiNbO3 are based on ion-driven techniques such as reactive ion etching and ion beam milling.
These processes are quite slow,
with typical etch rates on the order of 0.01 micrometers/mmn for typical ion current densities of 1 mA/cm2 [1,2].
In contrast,
the laser-driven chemical
etching process described herein can achieve material removal rates in excess of 10 micrometers/min [3]. The laser-based process involves spatially localized melting of KFcoated LiNbO3 by high-power-density laser pulses with photon energies in excess of the band gap of LiNbO3 (4.0 eV,
310 nm).
The photons must also be
sufficiently low in energy that most of them are transmitted through a layer Mat. Res- Soc. Symp. Proc. Vol. 75. 1987 Materials Research Society
420
of potassium fluoride located on the LiNbO surface. While molten, LiNbO3 undergoes reaction with KF to form complex niobium oxyfluoride anions, such as NbOFs, by fusion of the salts. soluble.
The resulting solid is highly water
The insolubility of LiNbO3 permits subsequent removal of only the
irradiated area by rinsing in water.
This approach for rapidly etching
patterns into chemically resistant materials has general applicability to other ionic materials which can undergo fusion reactions to produce soluble products.
EXPERIMENTAL Powdered KF or small crystals of KF are applied directly in air to the surface of the LiNbO3 substrate.
Since KF is extremely deliquescent, water
is rapidly absorbed from the air if the ambient humidity is appreciable.
To
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