Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION ~ `
Field of the Invention
:: '
This invention relates in general to optical waveguides
and :in more particular to an optical film waveguide suitable as -
an optical integrated circuit, a laser light guide or an integrated
optical circuit.
Description of the Prior Art
Optical fibers have been widely used as optical trans-
mission lines and studies have been made to form optical integrated
circuits with optical fibers. A clad type and a focusing type
are well known for arrangement~of the optical fibers. In the clad
type, a core material is clad with a hollow tube formed of material
having a lower refractive index than the refractive index of the
core material. In the focusing type optical fibers are manu~
factured by a sintering method or a CVD (Chemical Vapor Deposition)
method so that the refractive index is distributed in nearly a
quadratic curve in the section of optical fibers. Optical
integrated circuits have been proposed in which optical wave- `
guides are formed on or in substrates by these above-mentioned
met'hods or by an ion implantation method.
Silicon dioxides or various kinds of glass materials
are used for optical fibers. The refractive index of SiO2 is
about 1.4 and that of the different glass materials is in the
range between 1.4 to 1.6. Large variations of the refractive index
cannot be obtained although the refractive index can be reduced
to a certain extent by the addition of B203 or similar substances
into the glass materials. Thus, the conventional optical wave-
guide has the disadvantage that loss of optical transmission is
very great. Also, high temperatures are required to manufacture
these devices and the manufacturing processes are complicated.
Furthermore, the refractive index cannot be accurately controlled.
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SUMMARY OF THE INVENTION
. . . _
So as to overcome the disadvantages of the prior art,
the present invention provides a novel waveguide in which poly-
crystalline silicon containing oxygen and/or nitrogen is utilized.
In this prior art the grain size of the polycrystalline silicon
was in the range from 50 ~ to 1000 ~ but in the present invention
there is no upper limit for the concentration of oxygen in the
polycrystalline silicon. Oxygen may be contained in the poly-
crystalline silicon at the same concentration as in the silicon
10 dioxide. Various shapes of the polycrystalline layer are also -
proposed in the invention wherein the edges may be concave or
convex, for example.
The present invention provides:
an optical waveguide comprising a substrate and a
polycrystalline silicon layer formed on said substrate with a pre-
determined pattern, and containing atoms selected from the group
consisting of oxygen and nitrogen, the concentration of said
atoms in said polycrystalline silicon layer being varied in the
direction of thickness of said polycrystalline silicon layer.
Other objects, features and advantages of the invention
will be readily apparent from the following description of cextain
preferred embodiments thereof taken in conjunction with the ac-
companying drawings although variations and modifications may
be effected without departing from the spirit and scope of the
novel concepts of the
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disclosure and in which
BRIEF DESCRIPIION OF TIlE DRAWINGS
Figure 1 is a perspective view of a partially cutaway
optical waveguide;
Figure 2 is a plot of refractive index versus oxygen content
and N20/SiH4;
Figure 3 is a graph showing the transmissivity percentage
versus wave length;
Figure 4 is a plot of oxygen content and refractive index
relative to the height or thickness; :
Figure 5 is a sectional view through an optical waveguide;
Figure 6 is a plot of oxygen content and refractive index -
as a function of thickness or height;
Figure 7A is a top plan view of a further embodiment of
the invention;
Figure 7B is a sectional view taken on line VIIB-VIIB in
Figure 7A;
Figure 8 illustrates a modification of the invention;
Figure 9 illustrates a further modification of the invention;
Figure 10A is a top view of another embodiment of the
invention; and
Figure 10B iS a sectional view taken on line XB-~3 from
Figure 10A.
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DESCRIPIION OF THE PREFER~ED EMBODIMENTS
An optical waveguide according to one embodiment of this
invention will be described with reference to Figure 1 and to Figure 3.
In Figure 1, a part of an optical waveguide is shown. A
polycrystalline silicon layer 2 is formed on a selected surface of a
SiO2 layer 1 which is the uppermost layer in a substrate. The SiO2
layer 1 is formed, for example, by thermal oxidation of a silicon substrate.
In the formation of the polycrystalline silicon layer 2, the silicon
substrate including the top layer of SiO2 layer 1 is heated to a tempera-
ture range between 600 to 650C in a reaction furnace, while mono silane
SiH4, dinitrogen mono oxide N2O and carrier gas are fed into the
reaction furnace at predetermined flow rates. SiH4 and N20 thermally
decompose on the top SiO2 layer 1 of the heated silicon substrate and a
polycrystalline silicon layer 2 is formed on the SiO2 layer 1. The
polycrystalline silicon layer 2 may be formed by other CVD methods.
The concentration of oxygen in the polycrystalline silicon
layer 2 as well as the refractive index of the polycrystalline silicon layer
varies with the gas flow ratio of N2O/SiH4. Figure 2 shows the
relationships among the gas flow ratio of N2O/SiH4, the concentration of
oxygen (oxygen content), and the refractive index.
In the embodiment shown on Figure 1, the refractive index
of the SiO2 layer 1 is about 1. 4, and the gas flow ratio of N2O/SiH4 is
selected so that the oxygen content of the polycrystalline silicon layer 2
is 30 wt~, so that the refracti~e index of the polycrystalline silicon
layer 2 is about 2.1.- The lower surface of the polycrystalline silicon
layer 2 rnakes contact with the upper surface of the SiO2 layer 1 having
a refractive index of 1. 4, and the upper surface of the polycrystalline
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silicon layer 2 makes contact with air having a refractive index of 1.
A light emitting diode or laser (not shown) is coupled to a light inlet
(not shown) of the polycrystalline silicon layer 2. Light from the light
emitting diode or laser is radiated into the polycrystalline silicon layer
2 through the light inlet. The light is totally reflected by the contact
surfaces between the polycrystalline silicon layer 2 and the SiO2 layer
1, and between the polycrystalline silicon layer 2 and air because the
refractive index of the polycrystalline layer 2 is greater than the
refractive indexes of the SiO2 layer l and air. The light is totally
reflected and transmitted to a light outlet (not shown) of the polycrystalline
silicon layer 2. Thus, an optical waveguide is formed. ~ photo-
coupler may be formed by coupling a photo-diode to the light outlet of
the polycrystalline silicon layer 2.
When the polycrystalline silicon layer 2 is formed on a
layer of material other than SiO2, the oxygen content of the polycrystalline
silicon layer 2 is controlled so that the refractive index of the poly-
crystall~ne silicon layer 2 at least near the contact surface with the
layer of the other material is larger than the refractive index of the
layer o~ the other material. With such control, the above-described
total reflection can be obtained.
As shown in Figure 3, the light transmissivity of the poly-
crystalline silicon layer 2 varies with the oxygen content of the poly-
crystalline silicon layer 2. As apparent from Figure 2 and Figure 3,
light of shorter wave length cannot always be used for the whole range
of refractive indexes from l. 4 to 4. lIowever, even light of shorter
wave leng~h can be used for the range of refractive indexes from 1.4 to
2. 2. Moreover, the polycrystalline silicon layer has the advantage that
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it can be formed at relatively low temperat-lres, according to the CVD
method.
According to this invention, as shown on Figure 4, the
refractive index can be varied in the thickness direction of the poly-
crystalline silicon layer 2. In such a case, the gas flow ratio of
N20/SiH4 is varied during the formation of the polycrystalline silicon
layer 2 to obtain the gradient oxygen content, and thus, the gradient
refractive index. In Figure 4, the solid curve shows the variation of
the oxygen content, the chain line "n" the variation of the refractive
index, and x represents the height or thickness from the contact sur-
face between the polycrystalline silicon layer 2 and the SiO2 layer 1.
Figure 5 is a sectional view of a part of an optical wave-
guide according to a further embodiment of this invention. In this
embodiment, a polycrystalline silicon layer 4 having a gradient oxygen
content, and a gradient refractive index n, as shown in Figure 6, is
formed on a silicon substrate 3. The refractive index of the silicon
substrate 3 is relatively as large as layer 4. However, the refractive
index of the polycrystalline silicon layer 4 as a light propagating body
decreases from its center axis towards both its contact surfaces.
Accordingly, the light advancing in that body will be confined within the
central portion of the light propagating body.
Since the oxygen content varies in the thickness direction
of the polycrystalline silicon layer 4 in such a manner as shown in
Figure 6, the property of the central portion of the polycrystalline silicon
layer 4 is nearer to that of a pure polycrystalline silicon layer, while
the property of the upper and lower portions of the polycrystalline
silicon layer 4 is nearer to that of a SiO2 layer. Accordingly, when
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the polycrystalline silicon layer 4 is etched to form a predetermined
pattern of the polycrystalline silicon by a mixed etching solution of HF
and NH4 which is an etchant for SiO2 in a conventional photo-process,
the side surfaces of the pattern become convex, since the etching rate
for SiO2 is higher than that for pure polycrystalline silicon.
Figure 7A and Figure 7B show an optical directional coupler
according to a further embodiment of this invention. Polycrystalline
silicon layers 4a and 4b formed as patterns are formed by the etching
operation of the polycrystalline silicon layer 4 shown in Figure 5. For
the above-described reason, the side surfaces of the polycrystalline
silicon layers 4a and 4b are convex, and they function as convex lenses.
Accordingly, the polycrystalline silicon layer 4a can be optically and
partially coupled with the polycrystalline silicon layer 4b in such a
manner when a radius of the convex surface and the distance between
the polycrystalline silicon layers 4a and 4b are suitably selected.
When the polycrystall~ne silicon layer 4 of Figure 5 is
etched to form a predetermined pattern of the polycrystalline silicon by
another mixed etching solution of HF, HNO3 and CH3COOH, which is an
etchant for silicon the side surfaces of the pattern become concave,
since the etching rate for SiO2 is lower than that for pure polycrystalline
silicon.
Figure 8 shows an optical waveguide according to a further
embodiment of this invention, in which a concave side surface is formed
in the polycrystalline silicon layer 4 by the above-described method.
A semiconductor laser chip 5 is arranged adjacent to the polycrystalline
silicon layer 4. The optical waveguide comprising the substrate 3 and
the polycrystalline silicon layer 4 formed on the substrate 3, and the
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semiconductor laser chip 5 are combined with a heat sink 6. Light
from the laser chip 5 can be positively transmitted through the poly-
crystalline silicon layer 4. Generally, light is radiated in a wide angle
from the semiconductor laser chip 5. A large quantity of light is re-
flected by the surface of the light inlet of a conventional waveguide when
the light is transmitted into the waveguide and it is clifficult to guide all
the light into the light inlet in the conventional optical waveguide.
However, since the light inlet is concave in the waveguide of Figure 8,
the light from the laser chip 5 will be effectively incident on the light
inlet of the waveguide without total reflection at the surface of the light
inlet.
In the conventional optical waveguide, it is undesirable to
attach the semiconductor laser chip on the central portion of the heat
sink having a large area, since the light from the semiconductor laser
chip is reflected by the surface of the heat sink. However, according
to the embodiment of Figure 8, the semiconductor laser chip 5 can be
attached on the central portion of the heat sink without the possibility
that the light will be reflec~ed by the surface of the heat sink.
Accordingly, the optical waveguide according to this invention has very
desirable heat dissipation characteristics.
Figure 9 shows an optical waveguide according to a further
embodiment of this invention. In this embodiment, the light inlet
portion of the polycrystalline silicon layer 4 is shaped to be triangular
or fan shaped in form. By such an arrangement, the transmission
efficiency of the light can be improved. The inlet surface may be flat.
Figure lOA and Figure lOB show an optical waveguide
according to a still further embodiment of this invention. In this
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embodiment, an opening is made in the silicon substrate 3 and the
polycrystalline silicon layer 4 is formed on the silicon substrate 3 by
the conventional etching method. At the opening, the semiconductor
laser chip 5 is fixed onto the silicon substrate 3. The depth of the
opening is nearly equal to the level of the light-emitting portion of the
semiconductor laser chip 5. The light inlet portion of the polycrystalline
silicon layer 4 surrounds the semiconductor laser chip 5. The heat
from the laser chip S is transmitted through the silicon substrate 3 to
the heat sink 6. By such an arrangement, the transmission efficiency
of the light can be further improved more than in the embodiment of
Figure 9. The thermal conductivity of the silicon substrate 3 is higher
than that of the material (III-V groups chemical compound) of the laser
chip 5, and the area of the silicon substrate is relatively large. Thus,
this embodiment has very advantageous heat dissipation characteristics.
Although the use of a polycrystalline silicon layer containing
oxygen has been described in the above embodiments, a polycrystalline
silicon layer containing nitrogen may be also used. The refractive
index of the polycrystalline silicon layer can be varied by changing the
nitrogen content. At a nitrogen content of zero, the refractive inde~ is
4. It can be varied from 4 to 2.0 by increasing the nitrogen content.
Accordingly, a polycrystalline silicon layer containing nitrogen can be
used for all of the above embodiments, instead of a polycrystalline
silicon layer containing oxygen. Moreover, a polycrystalline silicon
layer containing a mixture of both oxygen and nitrogen may be used for
the above embodiments.
In the formation of the polycrystalline silicon layer
containing nitrogen, the above described CVD method can be used, and
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N~13 is fed into the reaction furnace, instead of N20. The refractive
index can be controlled by varying the flow rate of NH3.
The polycrystalline silicon layer has a good passivation
effect on a semiconductor device. Also, very little stress due to
thermal expansion of the polycrystalline silicon layer is imposed on the
substra~e. Accordingly, the polycrystalline silicon layer formed on a
normal integrated circuit can be used both as a passivation layer and
as an optical waveguide.
When the polycrystalline silicon layer is formed on the
silicon substrate in the above embodiments, the oxygen content of the
polycrystalline silicon layer varies in the thickness direction as shown
by "x" in ~igure 6. However, when the upper surface of the poly-
crystalline silicon layer contacts air, the refractive index near the
upper surface is not reduced so much as shown in Figure 6. When a
layer of material having a lower refractive index is formed on the
polycrystalline silicon layer, the thickness of the polycrystalline silicon
layer can be reduced.
Although the invention has been described with respect to
preferred embodiments, it is not to be so limited, as changes and
modifications may be made which are within the full intended scope as
defined by the appended claims.
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