Fabrication method and structure of optical waveguides

An optical waveguide fabrication technique utilizing a self-aligned cladding, in which an efficient and reliable optical waveguide can be fabricated with minimum proton exchange; the guided mode property of the optical waveguide can be widely adjusted; and the optical waveguides can be fabricated in various types.

FIELD OF THE INVENTION 
The present invention relates to a structure and a fabrication method for 
an optical waveguide which, as a basic structure of integrated optics, is 
proton-diffused after forming a self-aligned SiO.sub.2 - cladding on a 
lithium niobate (LiNbO.sub.3) single crystal substrate. The present 
invention is also applicable to the fabrication of ion-exchanged glass 
optical waveguides in which glass is used as a substrate material. 
BACKGROUND OF THE INVENTION 
Integrated optics is the technique of manipulating light (photons) by 
miniature optical elements formed on a thin layer, like integrated 
circuits is the operating of electrons by numerous electronic elements 
formed on a thin layer. 
The thin film optical waveguide is the basic structure of optical elements 
used in integrated optics. Among optical waveguides, the optical waveguide 
using a lithium niobate (ferroelectric) (LiNbO.sub.3) single crystal 
substrate has been extensively studied since the 1970s, because from 
LiNbO.sub.3 various optical elements can be fabricated e.g., the optical 
modulator/switch which uses the electro-optic effect. 
The most extensively used and most developed technique for fabricating an 
optical waveguide, particularly having a LiNbO.sub.3 single crystal is the 
titanium (Ti) indiffusion method. This method is carried out at a high 
temperature (about 1000.degree. C.), with the result that the lithium ions 
are outdiffused to the surface of the crystal, thereby increasing its 
refractive index. In waveguides fabricated by this method, when an 
extraordinary wave is guided which uses a large electro-optic effect, 
there is a disadvantage that a surface guiding phenomenon occurs. Further, 
in such waveguides optical damages due to the photorefractive effect is 
produced from light in the visible light region, thereby making them 
difficult to be practical. 
In the 1980s, came the development of the proton exchange method for 
fabricating an optical waveguide such as from LiNbO.sub.3 in which a 
chemical reaction is performed at a temperature lower than that used in 
the Ti indiffusion method (i.e. about 200.degree. C.) 
In the proton exchange method a substrate is immersed in a benzoic acid 
(C.sub.6 H.sub.5 COOH) solution to exchange the lithium ions with protons. 
In this method, a large variation of the refractive index can be obtained, 
and the total time for the whole process can be shorter than the time in 
titanium indiffusion method. However, in the proton exchange method, the 
proton exchange reaction occurs abruptly; and, the resultant products have 
non-uniform characteristics. From the proton exchange fabrication process, 
optical waveguides have instabilities which become apparent and become a 
problem in the use thereof shortly after the fabrication. Therefore, this 
method is too fastidious to fabricate a single mode optical waveguide for 
practical use in integrated optics. The proton exchange method also 
suffers from a further problem in that the electro-optic effect which is 
characteristic of the lithium niobate is markedly lowered in waveguides 
from this method. 
Recently, there have been proposals to overcome the above-described 
disadvantages of the proton exchange method. The first calls for a proton 
exchange method carried out such that benzoic acid solution is buffered 
with lithium ions, and is characterized in that the proton exchange 
reactions are moderated by means of the lithium ions. However, this 
proposal does not have practicality because the reaction time therein is 
too long to form the optical waveguide, and the reaction is sensitive to 
the lithium ion concentration in the solution. 
A second proposal calls for a proton exchange method carried out such that, 
after the proton exchanges, the protons that have exchanged into the 
lithium niobate crystal are annealed therein by applying a temperature of 
over 350.degree. C. In this proposal, due to abrupt proton exchange 
reactions, the excessively exchanged protons gain heat energy which causes 
them to be outdiffused, with the result that the internal stress of the 
crystal structure is relaxed. However, in this proposal, the lithium ions, 
up to as many as the number of the exchanged protons during the exchange 
process, are transferred into the solution and cannot return to their 
original positions. In an attempt to overcome this phenomenon, only as 
many protons as are required for forming the optical waveguide are 
exchanged; and then, the protons have to be diffused only as deep as 
required, with the outdiffusion of the protons inhibited. However, this 
modification to the second proposal provides a process which is a very 
fastidious one. 
SUMMARY OF THE INVENTION 
The present invention is intended to overcome the above-described 
disadvantages of the conventional techniques. 
It is an object of the present invention to provide an optical waveguide 
having a superior quality. It is also an object of the present invention 
to provide an improved method for forming an optical waveguide. 
The present invention thus provides a method for forming an optical 
waveguide wherein, proton exchange is weakly carried out at a relatively 
low temperature (e.g., 120-180.degree. C., typically 140-160.degree. C., 
preferably about 145-155.degree. C., more preferably 150.degree. C.), and 
then, proton diffusion is carried out at a high temperature (e.g., 
370-430.degree. C., typically 390-410.degree. C., preferably about 
395-405.degree. C., more preferably 400.degree. C.) after forming a 
self-aligned dielectric (SiO.sub.2) cladding. While ranges are provided 
for performing steps in the method of this invention, it is preferred that 
exchange and diffusion each be carried out at a constant temperature 
within the herein provided ranges. 
The present invention provides a method for manufacturing an optical 
waveguide comprising: 
forming a substrate having a top face and a bottom face, forming mask 
channel patterns on said top face of said substrate to form a channelled 
substrate, subjecting said channelled substrate having a mask thereon to 
minimum, low temperature proton exchange for a desired period of time to 
form an exchanged substrate, 
coating a photoresist onto the top face of said exchange of substrate to 
form a photoresist substrate, subjecting the bottom face of said 
photoresist substrate to exposure to achieve an aligned pattern 
corresponding to said mask channel pattern, 
depositing on said top face of said exposed substrate a dielectric to form 
a substrate having a self-aligned dielectric cladding, and, 
subjecting said substrate having a self-aligned dielectric cladding to 
proton diffusion for a desired time at a desired temperature. 
The present invention further provides an optical waveguide of proton 
diffusion type comprising a substrate having an optical waveguide pattern 
thereon; and 
a self-aligned dielectric cladding formed on said waveguide pattern; 
wherein said dielectric cladding serves as a mask for inhibiting the 
outdiffusion of protons and promoting the indiffusion of said protons when 
said waveguide pattern is proton-diffused.

DETAILED DESCRIPTION 
Referring to FIG. 1A, metallic mask channel patterns 2A are formed upon 
lithium niobate (LiNbO.sub.3) substrate 1 by the lift-off technique at 
surface 2. The channelled substrate and benzoic acid powder (or crystals) 
are respectively inserted into the upper and lower parts of a glass tube 
which also has a narrowed portion in the middle such as a sandglass (hour 
glass), and then the glass tube is sealed. 
The sealed glass tube is put into a furnace and is heated to a temperature 
of about 150.degree. C. Upon reaching the desired temperature, the glass 
tube is inverted so that the substrate is immersed into the melted benzoic 
acid, causing proton exchange to occur. Dotted line 3 in FIG. 1A denotes 
the protons exchanged in the melted benzoic acid at about 150.degree. C. 
By altering the pressure, one can alter the temperature for melting the 
benzoic acid. 
After carrying out proton exchange for a desired period of time (e.g., 1-4 
hours), the glass tube is removed from the furnace, and restored to its 
original position, from the upside down position, and then, it is cooled. 
Cooling can occur by allowing the glass tube to sit and achieve a lower 
temperature. After cooling, a photoresist 4 is coated onto the substrate 1 
(FIG. 1B) self-aligned pattern 4A is formed on the substrate 1. In 
particular, bottom face 1B of substrate 1 is exposed, e.g., by 
illumination or exposure to ultra violet (UV) light. Arrows 5 in FIG. 1C 
illustrates UV light exposure to surface 1B which achieves alignment of 
proton exchanged region 6 (FIG. 1B). Self-aligned pattern 4A is formed on 
substrate 1 when substrate 1 has the metallic mask used for the proton 
exchange kept thereon (i.e., a single mask is used twice). 
After developing photoresist 4 (which causes pattern 4A to be removed from 
substrate 1), dielectric (SiO.sub.2) 7 is deposited, e.g., by radio 
frequency sputtering (RF sputtering) (FIG. 1D), with the position 7A 
sitting against substrate 1 where pattern 4A was removed to form 
dielectric (SiO.sub.2) cladding 8 by the lift-off technique (FIG. 1E). 
Dielectric (SiO.sub.2) cladding 8 is used as a mask for inhibiting the 
outdiffusion of the protons, and for promoting the indiffusion of the 
protons. To carry out the proton diffusion step, substrate 1 having 
dielectric (SiO.sub.2) cladding pattern 8 is put into a furnace, for a 
desired period of time (e.g., 10-100 minutes) and at a desired temperature 
(e.g., 400.degree. C.) to thereby obtain optical waveguide 9 (FIG. 1F). 
The furnace can be present in accordance with proton exchange conditions. 
In FIG. 1F, the semi-circles 9A denote protons diffused into substrate 1 
and the diffusion pattern schematically. As mentioned earlier, this method 
can also be applied to not only LiNbO.sub.3 single crystal substrate but 
also to glass substrate to form ion-exchanged glass optical waveguides. 
The present invention constituted as above will now be described as to its 
effects. 
First, as the proton exchange is carried out at a low temperature (e.g., 
150.degree. C.), the reaction due to the benzoic acid can be moderated 
without requiring the solution to be buffered by the lithium ions. Thus, 
in the present invention, control of the degree of the proton exchange is 
simpler than in past procedures. Further, bottom face 1B of the substrate 
is exposed for dielectric (SiO.sub.2) cladding 8 to be formed only on the 
portions of the substrate which are proton-exchanged and to be 
proton-diffused. Therefore, in the present invention the indiffusion of 
the protons is efficiently promoted and the outdiffusion of the protons is 
effectively inhibited. 
That is, in the present invention with minimum proton exchange, optical 
waveguides can be produced. Further, if laterally diffused protons depart 
from the region masked by dielectric (SiO.sub.2) cladding 8, they are 
outdiffused from substrate 1, thereby providing an advantage of the 
present invention: In the present invention the proton concentration 
distribution (9A, FIG. 1F) is confined within the region of dielectric 
(SiO.sub.2) cladding 8. 
Therefore, the lateral mode is well confined, and accordingly, the 
radiation loss can be structurally reduced, thereby increasing the degree 
of integration. 
Dielectric (SiO.sub.2) cladding 8 lies between the optical waveguide and 
the air layer, and therefore, any steep variation of the refractive index 
over the face of the optical waveguide can be buffered, thereby 
contributing to keeping the waveguide mode symmetrical in depth direction. 
As a result, any coupling loss between a waveguide of the present 
invention and an optical fiber which has a circular symmetric mode profile 
can be reduced. 
Based on these effects disclosed herein, the proton exchange time, the 
width of the dielectric (SiO.sub.2) cladding and the proton diffusion time 
features of the present invention can be adjusted by the skilled artisan 
to control as desired a guided mode property (such as shape and size) of 
an optical waveguide herein. Thus, the present invention makes it possible 
to fabricate optical waveguides having various characteristics. 
Further, according to the present invention, if the substrate material is 
able to transmit through it light from a light source, a self-aligned 
pattern can be fabricated. Therefore, the present invention is applicable 
to the fabrication of widely used ion-exchanged glass optical waveguides 
in which glass is used as a substrate material. 
As described above, the present invention provides a reliable method for 
fabrication of an optical waveguide which will be a necessity in the 
coming optoelectronic era. In a commercial embodiment of the method of the 
present invention to produce optical waveguides according to the present 
invention, mass production is employed to perform the proton exchange. In 
such mass production the proton exchange is carried out in a large 
quantity of benzoic acid solution or molten benzoic acid which is used to 
apply a photoresist (4, FIGS. 1B, 1D) and pattern (4A, FIG. 1B) on 
substrates of a wafer scale, and the formation of the dielectric cladding 
is by a photolithography process such as those used in the mass production 
of semiconductors. In mass production of the present invention, the proton 
diffusion is carried out in a furnace which can hold substrates of wafer 
scale. Thus, mass production is an embodiment of the present invention in 
addition to the one by one process described above. 
As used herein: The outdiffusion of protons means that the protons in the 
substrate are diffused out of the substrate by proton exchange during the 
proton diffusion step. The indiffusion of protons means that the protons 
in the substrate are diffused into the substrate by proton exchange in the 
depth direction during the proton diffusion step. Indiffusion and 
outdiffusion occur simultaneously during the proton diffusion step. 
Having thus described in detail preferred embodiments of the present 
invention, it is to be understood that the invention defined by the 
appended claims is not to be limited by particular details set forth in 
the above description as many apparent variations thereof are possible 
without departing from the spirit or scope of the present invention. 
From the foregoing, it will be apparent that the present invention provides 
advantages such that it is a simpler manufacturing process when compared 
with the currently well developed semiconductor manufacturing process, and 
when compared with present techniques for making optical waveguides. 
Therefore, the present invention is a pioneer innovation because it is 
forecastingly capable of being adopted as the basic practical technique in 
this field.