Etching a substrate: processes – Nongaseous phase etching of substrate – Irradiating – ion implanting – alloying – diffusing – or...
Reexamination Certificate
2001-01-16
2003-04-08
Mills, Gregory (Department: 1763)
Etching a substrate: processes
Nongaseous phase etching of substrate
Irradiating, ion implanting, alloying, diffusing, or...
C438S406000, C438S407000, C438S458000, C117S003000, C117S004000, C117S915000, C216S062000, C216S024000
Reexamination Certificate
active
06544431
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thin film lithium niobate structure and, more particularly, to using an ion implanted etch stop to form a film of virtually any desired thickness with excellent uniformity.
BACKGROUND OF THE INVENTION
Lithium niobate (LiNbO
3
) and other ferroelectric materials are often used as waveguiding layers in various optical devices such as, for example, optical switches, electro-optic modulators and the like. In these applications, it is particularly advantageous to be able to form relatively thin (i.e., <1 &mgr;m) layers of such films, due to their large optical confinement properties and strong optical nonlinearities.
Many techniques have been used in the past to form thin ferroelectric oxide films. In most cases, for example with LiNbO
3
, a liquid phase epitaxy (LPE) process is used. In an exemplary LPE method, Li
2
O—V
2
O
5
is used as an LPE growing flux, and the raw materials are weighed and mixed in such a way that the melt composition becomes LiNbO
3
:Li
0.7
Na
0.3
VO
3
=20:80 (mol %), and the mixture, placed in a platinum crucible, is set in a furnace. The mixture is melted at 1000° to 1100° C. to have an even composition, and is then over-cooled to or below a saturating temperature. Next, a suitable substrate (such as LiTaO
3
), attached to a platinum substrate holder with the +z face of the substrate facing downward, is inserted in the furnace and is sufficiently preheated on the flux. The resultant structure is then isothermally grown by, for example, a one-side dipping system. In one conventional arrangement, the growing temperature is between 930° and 950° C., the number of rotations of the substrate at the time of growth is 10 to 100 rpm, and the growing speed is about 1.0 &mgr;m per minute.
Although this method is suitable for forming waveguiding structures in lithium niobate, the quality of the material in the waveguide-formed region is often less than desirable. In a best case, it would be preferred to fabricate a thin film of lithium niobate from a bulk grown LiNbO
3
substrate, since bulk grown LiNbO
3
is of much higher quality than materials previously used, thus improving the quality of the grown film.
Thus, a need remains in the art for a method of providing high quality, thin film lithium niobate structures.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to a thin film lithium niobate structure and, more particularly, to using an ion implanted etch stop to form a film of virtually any desired thickness (e.g., ≦15 &mgr;m).
In accordance with the present invention, a single crystal bulk lithium niobate substrate is subjected to ion bombardment so as to create a “damaged” layer at a predetermined distance below the substrate surface. The implant energy determines the depth of this damaged layer below the surface. Following the ion implant, a heat treatment process is performed, where the heat treatment serves the dual purpose of “healing” some of the damage in the lithium niobate material between the damaged layer and the surface, and modifies the etch rate of the damaged layer. Hereinafter, the “damaged layer” will be referred to as the “etch stop layer”. Subsequent to the heat treatment process, the etch properties of the lithium niobate bulk material and etch stop layer are sufficiently different that a conventional wet chemical etch may be used to form the desired thin film lithium niobate structure.
In one embodiment of the present invention, the ion implant process is performed to yield an etch stop layer at a relatively shallow depth (e.g., 2 &mgr;m) below the substrate surface. Subsequent to the heattreatment step, the substrate is bonded to a “handle” wafer, where the substrate surface that had been subjected to the ion bombardment is bonded to the handle wafer (i.e., the substrate is turned “upside down” and bonded to the handle wafer). The exposed bulk of the lithium niobate substrate is then removed by a conventional wet chemical etch and will stop, in accordance with the present invention, at the etch stop layer created by ion bombardment. The remaining lithium niobate material, therefore, will be the relatively thin, 2 &mgr;m top surface region of the original substrate. Therefore, in accordance with the present invention, a thin lithium niobate film is formed from the original bulk substrate material.
In another embodiment, a ridge waveguide structure (or any other patterned structure) may be formed by first masking the surface of the lithium niobate bulk crystal substrate prior to the ion implantation. As before, a heat treatment process is used to modify the etch rate characteristics of the ion implanted regions with respect to the remaining substrate material. A following wet chemical etch will then preferentially remove the original lithium niobate substrate material with respect to the ion bombarded layer.
Various and other embodiments of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
REFERENCES:
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patent: 5209917 (1993-05-01), Ohno et al.
patent: 5315432 (1994-05-01), Ohno
patent: 5512383 (1996-04-01), Chikuma et al.
patent: 5763055 (1998-06-01), Kawaguchi et al.
patent: 6120597 (2000-09-01), Levy et al.
patent: 6172791 (2001-01-01), Gill et al.
M. Levy and R.M. Osgood, Jr. “Fabrication of single-crystal lithium niobate films by crystal ion slicing” Applied Physics Letters vol. 73. No. 16. Oct. 19, 1998.*
P. Rejmankova, J. Baruchel, P. Moretti, “Investigation of hydrogen implanted LiNb03crystals under DC electric field by synchroton radiation topography” Physica B 226 (1996) 293-303.
Gill Douglas M.
Jacobson Dale Conrad
Culbert Roberts P.
Koba Wendy W.
Mills Gregory
TriQuint Technology Holding Co.
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