Ian E. Barry

Microstructuring of lithium niobate using differential etch-rate between inverted and non-inverted ferroelectric domains

Single crystal samples of lithium niobate have been spatially patterned with photoresist, and subsequently domain inverted using electric field poling, to produce a range of two dimensional spatial domain structures. Differential etching has subsequently been carried out using mixtures of hydrofluoric and nitric acids, at a range of temperatures between room temperature and the boiling point. The structures produced show very smooth, well defined, deep features, which have a range of applications in optical ridge waveguides, alignment structures, V-grooves, and micro-tips. Details are given of the fabrication procedures, and examples of structures are shown.

Ridge waveguides in lithium niobate fabricated by differential etching following spatially selective domain inversion

Ridge structures have been fabricated in z-cut LiNbO3 using the technique of differential etching following spatially selective domain inversion. Waveguides within these ridges have been achieved using the techniques of ion beam implantation, proton exchange, and titanium indiffusion. Using this last method, guides with losses <0.8dB/cm have been realised for light at a wavelength of 1.3µm. We briefly discuss applications for these structures.

Fabrication of piezoelectric micro-cantilevers in domain-engineered LiNbO3 single crystals

We report on a novel route for fabrication of micro-cantilevers in ferroelectric single-crystal lithium niobate (LiNbO3). Using the sequential techniques of photolithographic patterning, electric field poling, direct bonding and domain-oriented differential etching, free-standing cantilevers of dimensions 50 μm × 50 μm × 5 mm in the x, z and y crystallographic directions, respectively, have been fabricated.

Latency effects and periodic structures in light-induced frustrated etching of Fe:doped LiNbO3

Single crystals of z-cut 0.05% Fe:doped lithium niobate (Fe:LiNbO3), have been etched in a mixture of HF and HNO3 acids, under simultaneous illumination from a ∼100 mW 488 nm wavelength Ar ion laser light source, focused to power densities of ∼50 W cm−2 at the crystal surface exposed to the etchant. Etching is partially inhibited in illuminated regions, and the degree of inhibition shows a systematic latency: sites illuminated early in the etch run resist further etching even after the light is removed. Etched structures additionally exhibit regular periodic features of ∼0.5 μm scale length. Details of these structures are shown, and the latent etching effect is discussed.

Light-induced frustration of etching in Fe-doped LiNbO3

Abstract We report the results of light-induced frustration of the normal etching behaviour observed when LiNbO 3 is immersed in a solution of HF and HNO 3 acids. Light of wavelength 488 nm, from an air-cooled 100 mW Ar ion laser, is incident on the rear surface (+ z -face) of a thin Fe-doped LiNbO 3 sample, whose front face (− z -face) is in contact with the etchant solution. At power densities of >100 W cm −2 , etching is suppressed through light-induced charge migration. Below this power density, partial suppression occurs, leading to submicron scale features, whose orientation follows the crystal symmetry.

Microstructuring in LiNbO3: a route to MEMS devices in piezoelectric crystal media

Ferroelectric materials such as LiNbO3 and LiTaO3 offer many potential advantages over silicon for MEMS structures and self-actuating miniature devices. These materials possess numerous useful intrinsic properties such as piezoelectricity, pyroelectric and electro-optic coefficients, enabling the construction of micro-scale cantilevers, membranes, tips and switches. So far however, reliable and accurate methods for machining and microstructuring LiNbO3 single crystals have been lacking. We have recently been exploring several such methods, which are sensitive to ferroelectric domain orientation. A sample that has been domain-engineered shows a large difference in etch characteristics: the +z face does not etch at all, whereas the -z face etches normally. Microstructured devices can be fabricated therefore, via spatially selective domain poling followed by etching. The extreme sensitivity of the etch process to domain orientation has enabled us to fabricate ridge waveguides for electro-optic modulator applications, alignment grooves for efficient fibre pig-tailing to LiNbO3 modulators, and micro-cantilevers using a novel technique of contact bonding of dissimilar ferroelectric hosts.

Microstructuring of ferroelectric crystal media using light poling and etching techniques

LiNbO3 and LiTaO3 are commonly used ferroelectric crystal materials. Since the first reports of successful single domain crystal growth in 1965, these materials have found increasing use in optoelectronics, laser systems, Q- switching and frequency conversion, holographic data storage, surface acoustic wave devices, integrated optics and modulator use, and most recently, microwave telecommunications. In single domain format these ferroelectrics are photorefractive, pyroelectric and piezoelectric, and possess usefully large nonlinear optical and electro-optical coefficients. If domain engineering or micron/nano-scale bulk or surface modification is performed however, greater functionality is introduced, leading to additional uses such as phase-matched frequency conversion, grating and photonic structures, and the recently proposed use in MEMS and MOEMS devices. We discuss here a range of techniques for domain engineering and domain selective etching, as well as the use of light in poling and etching modification, and illustrate this potential with several devices that we have constructed by these routes.

Optically assisted chemical surface microstructuring of LiNbO/sub 3/

Summary form only given. Preferential wet etching of LiNbO/sub 3/ surface is reported, based on two distinct methods of optical treatment of the materials surface. (a) Excimer laser-induced surface damage followed by wet etching with a mixture of HF-HNO/sub 3/ acids, where an excimer laser is used for printing an interference pattern on the surface of LiNbO/sub 3/ in the form of periodic damage on the high intensity areas of the interference fringes. Wet etching, following the exposure of the materials surface, preferentially attacks the affected areas leading to higher etch rates, thus revealing the grating and also removing the debris produced by the ablation procedure. The method can be applied to any crystallographic orientation. (b) The second method is applied to the z face of iron doped LiNbO/sub 3/ niobate which has been thermally fixed to produce an ionic charge distribution created by two-beam interference. Again, preferential etching occurs revealing the space charge field pattern. Clearly the etching procedure is affected by the fixed ionic space charge distribution since the grating has been revealed in both developed and undeveloped crystals. The methods have been applied to fabricate grating structures onto titanium in-diffused LiNbO/sub 3/ channel waveguides and preliminary spectral filtering results are presented.

Light induced frustration of etching in Fe doped LiNbO/sub 3/

Summary form only given. There is considerable interest in etching, structuring, and patterning a wide range of insulators, dielectrics, and semiconductors, for microelectronic and microphotonic device applications. The authors report a method of optical control of photoelectrochemical etching in lithium niobate doped with iron.

Fabrication and Optimisation of Waveguides in Lithium Niobate by Differential Etching Following Spatially Selective Domain Inversion

Lithium niobate is extensively used in a wide range of applications, due to its favourable electro-optic, acousto-optic, piezoelectric, elastic and photorefractive properties. The ability to engineer precisely controlled and very smooth structures has applications in lateral guidance such as waveguides and modulators, for multiple arrays in photonic bandgap structures and for surface acoustic wave devices such as filters and time delays.