Method of making stoichiometric lithium niobate

#This invention relates to lithium niobate elements. More particularly, the invention relates to stoichiometric lithium niobate devices such as waveguides and modulators and methods for their manufacture.

#Electro-optic modulators have become one of the key components for high speed optical transmission systems. Lithium niobate single crystal materials are used in these applications because of the high electrooptic coefficient of lithium niobate materials. Single crystal lithium niobate wafers having a thickness of about 1-2 mm are generally used as substrate materials for these applications. Waveguide are written into these substrates via a variety of techniques, including but not limited to Ti thermal diffusion, Ti ion implantation, and proton exchange. Thermal diffusion of Ti ions is the most efficient and generally accepted practice. The waveguide is written to a depth of about 10 microns. The in-diffusion of Ti is performed at temperatures near 1000° C.

#The properties of the lithium niobate crystals depend on the composition of the crystals. The most widely industrially produced crystals have a composition that is referred to as the congruent composition. This congruent composition has a lithium to niobium ratio of about 48 to 52 or 0.94. The congruent composition crystals are produced by Czochralski method from lithium niobate melts at a very high crystal growth rate, typically around 8 to 10 mm/hr.

#FIG. 1 is a graph showing the dynamic SIMS depth profile of lithium niobate wafers treated according to one embodiment of the invention and showing the variation of 6Li+/7Li+;

#FIG. 2 is a graph showing the dynamic SIMS depth profile of lithium niobate wafers treated according to one embodiment of the invention and showing the variation of Li+/Nb+;

#FIG. 3 is a graph showing the time of flight SIMS depth profile of lithium niobate wafers treated according to one embodiment of the invention and showing the variation of 6Li+/Nb+; and

#The present invention is based on the need to provide a more economical process to produce crystals having stoichiometric compositions. In addition, there is a need to provide a method that is compatible with the Ti in-diffusion waveguide writing processes. It would be particularly desirable to provide a process that can be carried out simultaneously with the Ti waveguide writing process. The various embodiments of the present invention meet these needs.

#Since waveguide fabrication processes involve writing or diffusing only in the uppermost 10 micron thickness of the lithium niobate wafer substrates, the stoichiometric composition is needed only in about a 15 to 30 microns surface layer of the wafer. The entire crystal substrate thus does not need to be of stoichiometric composition. The process of the present invention is compatible with the Ti waveguide writing process that forms waveguides via thermal diffusion. The Ti thermal diffusion is carried out at about 1050° C. in air or an excess oxygen atmosphere. According to certain embodiments of the present invention, the stoichiometry of a portion of lithium niobate substrates can be adjusted while simultaneously writing waveguides utilizing Ti diffusion.

#The present invention utilizes a simple and economical process to generate the stoichiometric lithium niobate composition in a portion of the crystal substrate, preferably in a surface layer. In certain embodiments, lithium-containing powder and niobium-containing powder are mixed together and sintered into a monolithic solid mass at an appropriate temperature. For example, lithium carbonate and niobium oxide powders (or lithium oxide and niobium oxide powders) (99.99% purity) are mixed in appropriate proportions to obtain a lithium rich (i.e. greater than about 50:50 Li:Nb ratio) stoichiometry after sintering. The excess lithium-containing powders are then sintered at a low temperature, for example, between about 500° and 1200° C. to consolidate the powders and form a monolithic solid piece. A lithium niobate substrate such as a planar wafer of congruent composition, which typically has a ratio of lithium to niobium of about 0.94, is then placed in close proximity, preferably within less than about 1 centimeter, preferably less than about 5 millimeters and more preferably within 1-2 millimeters distance of the monolithic sintered solid piece with the surface intended for waveguide writing facing the monolith sintered solid piece. In preferred embodiments, the Li/Nb ratio in the monolithic solid is greater than one, and more preferably the ratio of Li/Nb in the monolithic solid is greater than 1.5. The lithium niobate substrate and the monolithic sintered solid samples are then heated to process temperatures of between about 800° and 1100° C. so that lithium ions diffuse takes place into the lithium niobate substrate. Factors such as the composition of the monolithic solid piece, the firing temperature, the length of time of the diffusion process, each contribute to the lithium concentration profile in the wafer. Therefore they are critical in the control of lithium in the surface portion of the substrate/wafer.