Conformal method of fabricating an optical waveguide on a semiconductor substrate

A method for fabricating a conformal optical waveguide on a semiconductor bstrate which results in an improved conformal processing method of producing ferroelectic ceramic waveguides that is integratable with conventional electronic and optoelectronic devices. First, a patterning of a desired waveguide configuration is made on a desired semiconductor substrate. A conformal confinement layer is fabricated in the pattern of the desired waveguide configuration on the semiconductor substrate. The conformal confinement layer has an index of refraction. Next, the method calls for a placing of a sol-gel waveguide precursor in the conformal confinement layer. Next the spin casting of a sol-gel waveguide precursor shapes a sol-gel conformal waveguide layer in the conformal confinement layer on the semiconductor substrate. The annealing of the spin cast sol-gel conformal waveguide layer forms the conformal optical waveguide which has an index of refraction that is greater than the refractive index of the conformal confinement layer.

BACKGROUND OF THE INVENTION 
Ferroelectrics have received considerable interest due to their 
applicability for a number of potential technologies. In bulk form they 
have high permitivities (dielectric constant), have large 
electromechanical coupling coefficients, exhibit pyroelectric behavior, 
and have electro-optic effects. In thin-film form they have potential 
applications for optical waveguides, optical modulators and shutters, 
optical displays and memories, piezoelectric transducers, decoupling 
capacitors, pyroelectric detectors and ferroelectric memories. Of 
particular interest is the integration of passive optical devices (e.g. 
waveguides) with active electro-optic devices (e.g. modulators) and 
conventional microelectronic devices for optoelectronic integrated 
circuits (OEICs). 
In recent years, researchers have fabricated various thin-film waveguides 
for integrated optical devices. Titanium-diffused lithium niobate 
waveguides have been studied; however, problems with optical damage and 
weak electro-optic effects have limited their practical use, as noted in 
"A New Waveguide Switch/Modulator for Integrated Optics", by W. E. Martin, 
Appl. Phys. Vol. 26 (1975), p. 562; "Optical Channel Waveguide Switch and 
Coupler Using Total Internal Reflection", by C. S. Tsai et al., IEEE J. 
Quantum Electron., Vol. QE-14, (1978), p. 513; and "Optical Damage 
Resistance of Lithium Niobate Waveguides", by R. L. Holman et al. Opt. 
Eng., Vol. 21 (1982), p. 1025. Fabrication of thin-film 
lanthanum-modified-lead-zirconate-titanate (PLZT) ferroelectric ceramics 
have been proposed to overcome the limitations exhibited by lithium 
niobate. Such thin-films have been formed by rf planar magnetron 
sputtering of a powder target onto a sapphire substrate, see, for example, 
"Electro-optic Effects of (Pb,La)(Br,Ti)O.sub.3 Thin Films Prepared by rf 
Planar Magnetron Sputtering", by H. Adachi et al., Appl. Phys. Lett., Vol. 
42 (983), p. 867; and "PLZT Thin-film Waveguides", by T. Kawaguchi et al., 
Appl. Optics., Vol. 23 (1984), pp. 2187-2191. Ridge-type channel 
waveguides have been fabricated using ion-beam etching techniques. These 
techniques are necessary since there is no suitable conventional etchant 
for PLZT films, and high temperature processes can result in out-diffusion 
of lead from the thin film, as noted in above reference by Kawaguchi. Ion 
beam etching techniques have a number of limitations, however: (1) etching 
selectivity between the thin-film and the photoresist mask is poor 
(typically 1.2:1), (2) etch rates are low (typically 13 nm/min), (3) 
typical etch non-uniformities are large (.+-.10%), and (4) there is 
limited control of the resulting surface quality. These limitations 
inhibit low cost, high yield fabrication of integrated devices and also 
affect device performance. For example, optical propagation losses from 
surface scattering from roughened top surface or sidewalls in waveguides 
must be controlled to obtain useful optical structures, note the article 
by D. Marcuse, "Mode Conversion Caused by Surface Imperfection of a 
Dielectric Slab Waveguide", Bell Syst. Tech. J., Vol. 48 (1969), p. 3187. 
Recent advances in polymeric solution-gelation (sol-gel) processing of 
ferroelectric ceramics offers new hope for integrated waveguides. Research 
in sol-gel processing has addressed the requirements of fabricating 
ferroelectric ceramics for electronic applications such as high 
permitivity dielectrics, non-volatile memory elements or optical image 
storage such as that shown in the articles "Sol-Gel-Derived PbTiO.sub.3 " 
by Blum et al., J. Mater. Sci., Vol. 20 (1985), pp. 4479-4483; 
"PbTiO.sub.3 Films from Metalloorganic Precursors" by R. W. Vest et al., 
IEEE Trans. UFFC, Vol. 35 (1988), pp. 711-717; "Integrated Sol-Gel PZT 
Thin-Films on Pt, Si, and GaAs for Non-Volatile Memory Applications" by S. 
K. Dey et al., Ferroelectrics, Vol. 108 (1990), pp. 37-46; and "Thin-Film 
Ferroelectrics of PZT by Sol-Gel Processing" by S. K. Dey et al., IEEE 
Trans. UFFC, Vol. 35 (1988), pp. 80-81. The sol-gel procedure involves the 
synthesizing of precursor complexes by vacuum distillation of 
metalloorganic compounds. The precursor complexes are subsequently 
hydrolyzed and condensed to form stable polymeric solutions which can be 
spin cast on substrates using conventional techniques. The resulting 
thin-film precursors undergo a low temperature annealing to volatilize 
organics, and are annealed at higher temperatures to crystallize and 
densify the film, see the first cited by Dey et al. article above, and 
pending USPTO application Ser. No. 07/709,671 by S. D. Russell et al. 
"Method of Laser Processing Ferroelectric Materials". Extensions of the 
existing chemistry can be envisioned by one skilled in the art to include 
the addition of a lanthanum-based compound in the formation of a PLZT 
sol-gel precursor film or other useful optical ceramics. 
Thus, in accordance with this inventive concept a need has been recognized 
for a method using the sol-gel process described above in operative 
association with a predetermined sequence of process steps for fabricating 
a "waveguide mold" which results in an improved conformal processing 
method of producing ferroelectic ceramic waveguides that is integratable 
with conventional electronic and optoelectronic devices. 
SUMMARY OF THE INVENTION 
The present invention is directed to providing a method for fabrication of 
an integratable conformal optical waveguide on a semiconductor substrate. 
First, a patterning of a desired waveguide configuration is made on the 
semiconductor substrate. A fabricating of a conformal confinement layer in 
the pattern of the desired waveguide configuration is made on the 
semiconductor substrate, the conformal confinement layer having an index 
of refraction. Next, the method calls for a placing of a sol-gel waveguide 
precursor in the conformal confinement layer. A spin casting of the 
sol-gel waveguide precursor and the semiconductor substrate is effected 
suitably to shape a sol-gel conformal waveguide layer in the conformal 
confinement layer on the semiconductor substrate. A two step annealing 
procedure transforms the spin cast sol-gel conformal waveguide layer to 
the conformal optical waveguide for the optical transmission of data. The 
conformal optical waveguide has an index of refraction that is greater 
than the refractive index of the conformal confinement layer. 
An object of the invention is to provide an improved method for delineating 
a waveguide structure using conventional semiconductor processing 
techniques with conventional materials. 
Another object is to provide a method for delineating a waveguide structure 
that reduces fabrication costs with an increased yield. 
Another object is to provide a conformal waveguide fabrication process to 
provide a crytallographically smooth surface that reduces surface 
scattering from the waveguide to thereby improve performance. 
Yet another object is to provide a waveguide delineating technique which is 
compatible with conventional VLSI fabrication techniques and devices to 
make the technique integratable with electronic devices on the same chip 
for opto-electronic integrated circuits. 
Another object is to provide a method for delineating a waveguide structure 
that allows the use of novel optical ceramics in an improved 
manufacturable process. 
Still another object is to provide a method for delineating a waveguide 
structure relying on the use of a metallic buffer layer (for example 
aluminum on silicon) to provide an optically confining material that can 
be suitably patterned for the fabrication of active opto-electronic 
devices, such as an optical modulator or optical switch, for example. 
Another object of the invention is to provide a conformal process that 
allows extensions to multilayer waveguides without changes to the overall 
process. 
Another object is to provide a method for fabricating a conformal optical 
waveguide that may be monolithically integrated with other electro-optic 
devices and conventional electronics. 
Another object is to provide a method of fabricating a passive or active 
waveguide structure for monolithic electro-optic, photonic and electronic 
integration using conformal processing techniques; 
Another object is to provide a fabrication method beginning with the step 
of fabricating a form or mold using well-established photolithographic and 
etching processes on the substrate, buffer layer and/or electrode layer or 
alternately the laser-assisted etching techniques as opposed to the 
conventional difficult steps of patterning waveguides in ceramic 
materials. 
Another object is to provide fabrication steps following the creating of 
the form or mold whereby the waveguide material (or its appropriate 
precursor) is spin cast or deposited in a form filling manner 
(conformally) followed by processing required to achieve the desired 
optical, electro-optical, photonic, mechanical and/or electrical 
properties. 
Another object is to provide a method of fabricating waveguide structures 
using solution-gelation (sol-gel) processing techniques which includes the 
spin casting on a room temperature substrate of a precursor film, and 
subsequent low temperature processing by furnace, rapid thermal annealing, 
or laser annealing that is compatible with monolithically integrated 
electronic devices. 
Another object is to provide a method of fabricating waveguide devices with 
lower losses (due to improved surface quality and fewer voids), higher 
electro-optic coefficients (due to better stoichiometry), allowing 
efficient and novel device integration (due to low temperatures amenable 
with electronic devices), and by a more reliable and inexpensive 
fabrication process (using the sol-gel processing). 
These and other objects of the invention will become more readily apparent 
from the ensuing specification and claims when taken in conjunction with 
the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The method for fabricating a conformal optical waveguide may be 
monolithically integrated with other electro-optic devices and 
conventional electronics on a substrate. The substrate material, typically 
a semiconductor, such as silicon, germanium, gallium arsenide, indium 
phosphide, and related compounds and alloys, will be used for the creation 
of associated electronic and opto-electronic integrated circuitry and is 
patterned using well-established conventional lithography and etching 
processes. The patterning delineates the waveguide structure, and the 
patterned semiconductor substrate essentially is used as a mold for the 
fabrication of the waveguide itself. 
For proper confinement of the traveling light within the waveguide, the 
surrounding refractive indices of the substrate and environment must be 
less than the refractive index of the waveguide itself. The refractive 
index, in general, is a function of wavelength, temperature, 
crystallographic orientation, and inherently, the composition of the 
waveguide. These parameters are taken into consideration with specific 
design requirements for the job at hand and will be suitably tailored by 
one skilled in the art of waveguide design. Some optical designs may find 
it advantageous to provide a conformal buffer layer with appropriate 
optical confining characteristics between the substrate and the waveguide 
to form a "waveguide mold" (see, for example, the preferred embodiment 
below). Following the completion of this "waveguide mold", the sol-gel 
process referred to above may be called on to form the optical waveguide 
to conformably fill the mold. One or more layers of the optical material 
may be spin cast and annealed to create a desired thickness and structure. 
The final structure may undergo the subsequent additional processing 
associated with optically interconnected electro-optical components if 
desired. 
Referring to FIGS. 1 (a) through 1 (g), a fabrication of a PLZT waveguide 
15 is depicted in a cross-sectional view. A top planar view is not shown, 
but may be suitably designed as a single waveguide, waveguide splitter, 
waveguide combiner, star coupler, tapered and chirped waveguides, and 
other designs used in the art. The substrate 10 is selected, FIG. 1 (a) 
and it may be a (100)-oriented silicon (bulk or 
silicon-on-insulator/sapphire) substrate 10 that is patterned using a 
photoresist 10a subjected to a conventional lithography and etching 
processes, see FIG. 1 (b). For example, a potassium hydroxide etch of 
(100)-oriented silicon 10 anisotropically etches to a crystal plane 11 
leaving a mirror smooth surface at an angle of 54.7 degrees with respect 
to the horizontal, as shown in FIG. 1 (c). This etch may be performed as 
detailed in the articles, "Chemical Etching and Slice Cleanup of Silicon" 
by K. E. Bean, Ch. 4, in G. E. McGuire, ed., Semiconductor Materials and 
Processing Technology Handbook. Noyes Publications, NJ (1988), pp. 
126-190; and "Micromachining of Silicon Mechanical Structures" by G. 
Kaminsky, J. Vac. Sci. Technol. B, vol. 3 (1985), pp. 1015-1024. The etch 
results in a delineation of waveguide mold 20 for fabrication of waveguide 
15. 
Next, a conventional deposition process, e.g. low temperature oxidation 
(LTO) is used to form a conformal SiO.sub.2 buffer layer 12 for optical 
confinement of the conformal waveguide or metalized layer, if desired, to 
be produced, see FIG. 1 (d). Conformal buffer layer 12 that is deposited 
in the etched waveguide mold 20 provides a conformal smooth layer with a 
refractive index less than a PLZT waveguide material to be applied as the 
waveguide. Refractive indices of some useful semiconductor substrate, 
conformal insulator buffer layer, ceramic waveguide and metalizing layer 
materials that can be used within the scope of this inventive concept are 
listed in Table I, (also see the articles by E. W. Palik, ed., Handbook of 
Optical Constants of Solids, Academic Press, San Diego, Calif. (1985) pp. 
341, 398-0400, 438-439, 473-474, 565-566, 700, 760, 774; and by C. E. Land 
et al., "Electrooptic Ceramics", in R. Wolfe, Ed., Applied Solid State 
Science: Advances in Materials and Device Research, Vol. 4, Academic 
Press, NY (1974), pp. 191-193, 198-205. 
TABLE I 
______________________________________ 
Class Material Index (n) Wavelength 
______________________________________ 
Semiconductor 
Si 3.882 0.6326 
3.5007 1.372 
3.4784 1.532 
Ge 5.5 0.6358 
4.285 1.378 
4.275 1.550 
GaAs 3.856 0.6326 
3.3965 1.378 
3.3737 1.550 
Metal Al 1.39 0.6358 
1.26 1.378 
1.44 1.550 
Pt 2.38 0.6525 
4.50 1.305 
5.31 1.550 
Insulator SiO.sub.2 1.45671 0.643847 
1.44621 1.3622 
1.44427 1.52952 
Navy Case No. 
Si.sub.3 N.sub.4 
2.022 0.6199 
73,650 1.998 1.240 
Ceramic PLZT/PZT .about.2 - .about.100 depending on 
composition, temperature, 
structure and wavelength 
LiNbO.sub.3 
n.sub.o (.perp.) = 2.2835 
0.64385 
n.sub.e (.parallel.) = 2.2002 
n.sub.o (.perp.) = 2.2211 
1.29770 
n.sub.e (.parallel.) = 2.1464 
n.sub.o (.perp.) = 2.2113 
1.60 
n.sub.e (.parallel.) = 2.1361 
______________________________________ 
References: E. W. Palik, ed., and C. E. Land et al., as cited above. 
An alternative to the LTO process may be a thermally grown oxide or nitride 
on silicon; however, these processes and materials may impose additional 
constraints for variations in growth conditions and waveguide design in 
order to achieve a smooth conformal buffer layer. In some instances, 
optical transmission losses from non-conformal buffer layers may be within 
acceptable limits so that the addition of alternative buffer layers may be 
appropriate. Note, depending on the job at hand, one may omit buffer layer 
12 if substrate 10 provides a suitable confining of the optical wave in 
waveguide 15. 
Sol-gel preparation of the PLZT waveguide material 14 follows in accordance 
with practices established in the art, noting the articles by Blum et al., 
R. W. Vest et al. and the two articles by S. K. Dey et al. The sol-gel 
precursor is applied to waveguide mold 20 and spin cast forming a 
conformal layer 13 in waveguide mold 20, formed in buffer layer 12 and 
substrate 10, see FIG. 1 (e). The spin casting typically is done at 2000 
rpm and forms a layer .about.0.2 .mu.m thick. The thickness of conformal 
precursor layer 13 is a function of the sol-gel viscosity and the spin 
cast speed and time to provide variation in the desired configuration. 
Conventional annealing of precursor layer 13 forms the polycrystalline 
ceramic layer 14, see FIG. 1 (f). The annealing is typically done in two 
steps, first, a low temperature annealing at 250.degree. C. for 1 hour to 
volatilize organics, and, second, a high temperature annealing at 
550.degree. C. for 30 min to densify and crystallize the film. Repetition 
of the spin-on process and annealings can be performed to achieve the 
integrable conformal waveguide 15 of the desired thickness and structure, 
note FIG. 1 (g). Typical values that may be desired for a waveguide 
geometry are about 20 .mu.m wide and 1 .mu.m thick although other 
dimensions may be required for specific device designs. If desired, 
additional material layers may be deposited and/or patterned for confining 
or modulating effects. The conformality of this process allows for design 
of multiple alternating layers with no modifications to the process. 
Similarly, patterning of the ceramic using the techniques described in the 
pending S. D. Russell et al. patent application, vide supra, or other 
techniques may be desired. 
Referring now to FIG. 2 of the drawings, the method of fabricating a 
conformal waveguide for optical and electronic integration is set forth in 
a functionally interrelated sequence of steps. First, there is the 
providing 50 of a suitable semiconductor substrate 10 such as silicon, 
germanium, gallium arsenide, etc., which has a discrete refractive index 
at a wavelength of interest, to establish a substrate on which 
optoelectronic as well as electronic components can be built up in 
accordance with well-known integrated circuit fabrication techniques. The 
substrate material is selected for a particular application so that its 
refractive index is less than the refractive index of the particular 
waveguide material selected, such as PLZT, PZT or LiNbO.sub.3, for 
example. Patterning 51 of the substrate using conventional techniques 
delineates a waveguide structure to be formed. The configuration of the 
pattern itself may essentially function as a mold for the waveguide. 
Fabricating 52 a conformal buffer or confinement layer 12 of an 
appropriate material in the pattern on the substrate forms a waveguide 
mold 20. The fabrication of the buffer or conformal confinement layer can 
be done by a conventional low temperature oxidation, deposition process, 
of silicon dioxide or silicon nitride, for example, that has a refractive 
index less than the waveguide material such as PLZT or LiNbO.sub.3, for 
example. Optionally, metalized layers of aluminum or platinum, for 
example, could be appropriately deposited for interconnection of other 
circuit components either before or after the conformal confinement layer 
is placed. Placing and spin casting 53 the sol-gel precursor material in 
the waveguide mold established by the conformal buffer confinement layer 
(or metalized layer) creates a precursor layer 13 that conforms to the 
conformal layer. The spin casting of a typical sol-gel precursor may be 
done at about 2000 rpms to form a layer approximately 2 microns thick. 
Since the thickness of the conformal precursor layer is a function of the 
sol-gel viscosity as well as the spin cast speed and time, the thickness 
and configuration may be changed to accommodate a particular need. A 
subsequent annealing 54 of the sol-gel precursor cures the polycrystalline 
ceramic layer 14 and may be a two-step operation. First, a low temperature 
annealing volatizes organics, typically at about 250.degree. C. for one 
hour and, second, a high temperature annealing densifies and crystallizes 
the ceramic layer 14, typically at about 550.degree. C. for 30 minutes. 
After spin casting and annealing, a repeating 56 of the spin casting and 
annealing steps will allow a designer to arrive at a desired conformal 
waveguide configuration 15. The foregoing procedure allows a subsequent 
processing 57, such as metalization, or completion of other desired 
electro-optic devices or electronic circuitry devices as desired. 
This method of fabrication provides for an uncomplicated method to 
delineate the waveguide structure using conventional semiconductor 
processing techniques in conventional materials as opposed to processing 
the ceramics themselves to thereby reduce fabrication costs and increasing 
yield. Furthermore, the conformal nature of the fabrication process 
provides a crystallographically smooth surface eliminating surface 
scattering from the waveguide, thereby improving performance. This method 
of fabrication also is compatible with conventional VLSI fabrication 
techniques and devices, making it integratable with electronic devices on 
the same chip for optoelectronic integrated circuits. It also allows the 
use of novel optical ceramics in an improved manufacturable process. The 
use of a metallic buffer layer (e.g. aluminum on silicon) provides an 
optically confining material that can be suitably patterned within the 
scope of this invention for the fabrication of active optoelectronic 
devices, for example an optical modulator or switch. Also, this conformal 
process easily allows extensions to multilayer waveguides without changes 
to the overall process. 
As described above, variations in materials parameters and geometry as 
required for the job at hand can be easily accommodated by this process. 
Obviously, many other modifications and variations of the present invention 
are possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.