Optical waveguides – Planar optical waveguide
Reexamination Certificate
2001-09-04
2003-09-02
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S130000, C385S131000, C385S141000
Reexamination Certificate
active
06614977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the vapor deposition of silicon-containing amorphous thin films, and in particular to vapor deposition of silicon-containing non-polymeric amorphous thin films with significantly reduced optical absorption (reduced optical loss) in the near-infrared optical communications wavelength region of about 1.45 to 1.65 microns for such uses as in the fabrication of optical devices, such as optical waveguides, ring resonators, arrayed waveguide grating multiplexers/demultiplexers, optical add/drop multiplexers, optical switches, variable attenuators, and dispersion compensators.
2. Background of the Technology
Conventional vapor deposition methods used for growing silicon-containing amorphous thin films on substrates for optical device applications typically rely on at least one hydrogen-bearing source gas. For example, silicon-oxide (SiO
2
)
1
or silicon-oxynitride (SiO
x
N
y
)
2
thin films, as shown in
FIG. 1
, are grown on a substrate
3
using low pressure chemical vapor deposition (LPCVD) (see, e.g., J. Yota, J. Hander, and A. A. Saleh, “A comparative study on inductively-coupled plasma high-density plasma, plasma-enhanced, and low pressure chemical vapor deposition silicon nitride films,” J. Vac. Sci. Technol. A 18 (2), 372 (2000)) or plasma enhanced chemical vapor deposition (PECVD) (see, e.g., L. Martinu and D. Poitras, “Plasma deposition of optical films and coatings: A review,” J. Vac. Sci. Technol. A 18 (6), 2619 (2000)). These techniques generally rely on silane (SiH
4
) and ammonia (NH
3
) as source gases for silicon and nitrogen, in combination with an oxygen bearing gas, such as O
2
or N
2
O. The resulting as grown SiO
x
N
y
films contain substantial amounts of hydrogen (2-25%) in the form of Si—H, N—H, and O—H bonds. The presence of atomic hydrogen affects many of the film's physical properties, including density, porosity, optical absorption, index of refraction, hardness, and stress.
One problem with near-infrared optical device applications using these materials is optical absorption, or loss, in the near-infrared optical communication wavelength region from 1.45 to 1.65 microns. The absorption occurs at least partially due to an effect commonly referred to as “stretching mode”—the motion of atoms that occurs in perpendicular directions on the same axis, away from each other. As a result, simple optical waveguides having a waveguide core consisting of vapor deposited SiO
x
N
y
show optical losses of 10 dB/cm and higher for optical wavelengths near 1.51 microns, which results from optical absorption by the overtones of the vibrational stretching modes of Si—H and N—H bonds (see, e.g., G. Grand, J. P. Jadot, H. Denis, S. Valette, A. Fournier, and A. M. Grouillet, “Low-loss PECVD silica channel waveguides for optical communications,” Electronics Letters 26 (25), 2135 (1990)). In addition, there is also significant infrared absorption near 1.4 microns, resulting from the presence of O—H bonds.
One way to eliminate this effect is to remove the hydrogen from the substance through which the light is being transmitted. A technique commonly used to accomplish this removal is to anneal the films at high temperatures (~1140° C.), driving as much of the hydrogen from the film as possible (see, e.g., R. Germann, H. W. M. Salemink, R. Beyeler, G. L. Bona, F. Horst, I Massarek, and B. J. Offrein, “Silicon oxynitride layers for optical waveguide applications,” Journal of the Electrochemical Society 147 (6), 2237 (2000)). This technique can substantially reduce the optical loss in the wavelength region of interest to below 1 dB/cm, but at the expense of an additional process step that can cause shrinkage of the film and introduce significant tensile stress in the film. These effects can create cracks in the film and bowing of the wafers.
These effects occur because the annealing temperature is high enough to drive hydrogen atoms out of the film, but not high enough to melt the film and allow it to flow and reshape. The resulting stretch or bending of the wafer makes the wafer difficult to process using lithography and standard semiconductor processing techniques. Finally, this process results in a number of dangling bonds of silicon atoms remaining in the film, and if the film is later exposed to sources of hydrogen, such as water vapor from humidity, the hydrogen can react and reattach to the silicon, eventually resulting in the same problem with absorption that was present absent annealing.
Low optical losses at near-infrared wavelengths have been achieved in organic polymer devices and optical fibers by making use of deuterated and halogenated materials, but these methods and devices are not useful for integrated and other non-polymeric optical devices. (See, e.g., U.S. Pat. No. 5,062,680 to S. Imamura; U.S. Pat. No. 5,672,672 to M. Amano; T. Watanabe, N. Ooba, S. Hayashida, T. Hurihara, and S. Imamura “Polymeric optical waveguide circuits formed using silicone resin,” Journal of Lightwave Technology 16 (6), 1049 (1998); U.S. Pat. No. 6,233,381 to Borrelli et al.) Deuterated gases have also been applied to the field of semiconductor electronics to create insulating and passivation layers in semiconductor transistor devices for such purposes as to mitigate hot-electron effects in gate oxides, but these methods and devices have no applicability to integrated and other non-organic optical devices, nor are the purposes for which deuterium is used in semiconductor transistor devices generally useful for producing optical devices. (See, e.g., U.S. Pat. No. 5,972,765 to W. F. Clark; U.S. Pat. No. 6,025,280 to D. C. Brady; U.S. Pat. No. 6,023,093 to Gregor et al.; U.S. Pat. No. 6,077,791 to M. A. DeTar.) Each of the references referred to herein is hereby incorporated by reference in its entirety.
There remains an unmet need to provide optical devices, including non-polymeric passive optical devices and integrated optical devices that have low optical losses at selected wavelengths. There is a further need to provide devices and methods of making devices for use with waveguides on wafers, such as planar lightwave circuits, including circuits with multiple devices connected by waveguides on a single wafer, that incorporate other processes than annealing and overcome the problems with this technique.
SUMMARY OF THE INVENTION
The present invention relates to optical devices, including integrated optical devices, and methods for fabrication via vapor deposition of non-polymeric, silicon-containing thin films using vapor sources, such as deuterated liquids, comprised of deuterated species. With embodiments of the present invention, thin films are grown on a substrate to form optical devices or portions thereof that have at least one deuterium containing layer. These devices have significantly reduced optical absorption or loss in the near-infrared optical spectrum, which is the spectrum commonly used for optical communications, compared to the loss in waveguides formed in thin films grown using conventional vapor deposition techniques and hydrogen containing precursors.
The devices produced in accordance with embodiments of the present invention have deuterium in place of hydrogen within bonds for the formed films. Deuterium, which is an isotope of hydrogen that has a neutron in its nucleus, vibrates within bonds with other atoms at frequencies different from hydrogen in the same bonds. This difference in frequency results from the increased mass of deuterium over hydrogen. Because of the different frequency of vibration of the deuterium in these bonds, relative to hydrogen, different wavelengths of energy, including light, are absorbed within the materials formed using deuterium than the wavelengths absorbed by the same materials when hydrogen is present. Deuterium used in the formation of optical devices in accordance with the present invention results in shifts of energy peaks for these materials, such that the primary band of wavelengths to be transmitted, in the 1.45 to 1.65 micron range,
Chu Sai T.
Gill David M.
Hryniewicz John V.
Johnson Frederick G.
Joneckis Lance G.
Kelber Steven B.
Little Optics Inc.
Pak Sung
Piper Rudnick LLP
Sanghavi Hemang
LandOfFree
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