Etching of Lithium Niobate: From Ridge Waveguides to Photonic Crystal Structures

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 WeD3