Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Coating selected area
Reexamination Certificate
1999-11-30
2001-11-06
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Coating selected area
C205S199000, C205S221000, C205S224000, C205S229000, C385S014000, C385S130000, C438S031000
Reexamination Certificate
active
06312581
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to optical devices, and in particular, planar optical waveguides.
2. Art Background
Optical fiber communication systems are becoming more prevalent. In addition to the optical fiber itself, optical fiber communication systems use a wide variety of optical devices for receiving, transmitting, and using optical signals. One type of integrated optical device is a silica optical circuit formed on silicon substrates. The basic structure of such devices is described in Henry, C. H., et al., “Silica-based optical integrated circuits,” IEE
Proc
.-
Optoelectron
, Vol. 143, No. 5, pp. 263-280 (1996). The waveguide is formed from 3 layers; those layers are a lower cladding (referred to as a base in Henry) core and upper cladding. The lower cladding layer isolates the fundamental mode from the silicon substrate. Such isolation prevents optical signal leakage through the silica-silicon (substrate) interface, which, unlike other waveguide interfaces, is not totally reflecting. The refractive index of the upper cladding layer is chosen to be nearly equal to that of the base layer.
The lower cladding layer is made out of undoped or lightly doped silica. This is the most rigid layer and it keeps the core, which is adhered to it, from moving after it is patterned. The other glasses are more highly doped silica.
A variety of processing strategies for fabricating silica-on-silicon, optical devices have been proposed. Such processing strategies typically require that the base layer be deposited on the surface of the silicon substrate. Current techniques for forming the lower cladding are deposition processes such as low-pressure chemical vapor deposition (LPCVD) and high pressure oxidation (HiPOX). While such deposition techniques provide an acceptable lower cladding, these techniques are somewhat slow. Accordingly, alternative techniques for forming the lower cladding for silica-on-silicon optical devices are sought.
SUMMARY OF THE INVENTION
The present invention is directed to a process for forming an optical device in which the lower cladding is formed in the silicon substrate. The lower cladding is fabricated by first forming a region of porous silicon in the silicon substrate. Expedients for forming regions of porous silicon in a substrate are well known to one skilled in the art. It is advantageous if the porous silicon is formed using an electrolytic process. In the electrolytic process, silicon is anodized in an electrolytic solution. Such a technique is described in Unagmi, T., et al., “An Isolation Technique Using Oxidized Porous Silicon,” Semiconductor
Technologies
, Vol. 8, Chap. 11, pp. 139-154 (OHMSHAT and North Holland Publishing Company 1983) which is hereby incorporated by reference.
The silicon substrate is anodized selectively to form porous silicon regions therein. The substrate is selectively anodized by forming a mask on the silicon substrate prior to anodization. The mask has at least one opening therein. The underlying silicon substrate surface is exposed through such openings.
Similarly the porosity of the porous silicon region is also largely a matter of design choice. The porosity of the material is controlled by the doping level of the substrate (e.g. silicon wafer) and the anodization conditions used to form the porous areas. Anodization conditions such as an applied voltage and the associated current density as well as the concentration and pH values of the HF solutions are selected to obtain the desired porosity. In determining the degree of porosity, the silicon density reduction caused by generating the porous silicon and the swelling caused by the subsequent oxidation of the porous silicon must be considered. Volume expansion or reduction of the porous silicon region is controlled so that the waveguide structure is not overly stressed by the changes in volume caused by the generation of a porous silicon region and the subsequent oxidation of that region.
Volume expansion and reduction is readily controlled because the relative volume of the porous silicon before and after oxidation is known. Specifically, thermal oxidation of the porous silicon increases the porous silicon volume by a factor of 2.2. It is advantageous if the density of the porous silicon (density is the converse of porosity such that density plus porosity equals 1) is no less than about forty-four percent that of silicon. If the density of the porous silicon is not less than forty-four percent that of silicon, then the volume of the oxidized porous silicon will not exceed the volume of the silicon region converted to porous silicon. That is, the volume increase (2.2) of the porous silicon is such that the volume of the oxidized silicon is roughly equal to the volume of the silicon before it was converted to porous silicon. Mathematically, 2.2 times 0.44 (the density of porous silicon with respect to the density of silicon) is about 1. Approximating a one to one relationship between the volume of silicon prior to anodization and the volume of oxidized porous silicon avoids stress and non-planarity due to volume expansion or contraction.
Conversely, if the density of the porous silicon is significantly less than forty-four percent that of silicon, the surface of the porous silicon region, following oxidation and densification, will be lower than the other portions of the substrate surface. Thus, in order to avoid having a region of silica that is significantly above or below the surface of the substrate, it is advantageous if the porous silicon has a density of forty-four percent (which corresponds to a porosity of 56 percent) that of silicon.
The dimensions of the porous silicon region are largely a matter of design choice. The depth and size of a porous silicon region depends upon the size of the waveguide subsequently formed on the region.
After the region of porous silicon is formed in the substrate, the region is oxidized. The porous silicon is oxidized in an oxygen-containing atmosphere at elevated temperature (e.g. about 850° C. to about 1150° C.). The non-porous silicon is also oxidized under such conditions. However, the oxidation rate of the porous silicon is substantially higher than the oxidation rate of the bulk silicon in the substrate. The amount of bulk silicon that is oxidized is negligible. If necessary, the oxidized bulk silicon is subsequently removed. Specifically the dimension of the silicon skeleton in a porous silicon region is on the order of 100 angstroms. Furthermore, the entire porous silicon region (typically 10-20 microns thick) is exposed to the oxidizing ambient simultaneously. Consequently, the oxidation process for converting the entire porous silicon region into bulk SiO
2
is carried out under conditions (e.g. flow rate, H content in O
2
gas, furnace temperature, and oxidation time) that only oxidize about 100 angstroms of bulk Si. The resulting thin layer of SiO
2
in the non-porous region of the wafer is easily removed after the oxidation without any significant impact on the lower cladding layer.
The oxidized porous silicon is then densified to form silica. In the context of the present invention, densification refers to the collapse of the porous structure of the porous silicon. Densification is accomplished by heating the substrate in an oxygen-containing atmosphere. Again, the bulk silicon is also oxidized, but at a slow rate compared to the rate of densification. Thus, the bulk silicon is only oxidized to a small degree, and the oxidized porous silicon is readily removed, if required.
After the cladding is formed on the substrate, the fabrication of the planar optical devices is completed. Conventional processes and materials for forming planar optical devices are contemplated as suitable. Consequently, fabrication techniques for forming planar optical devices on the oxidized and densified porous silicon are not discussed in detail herein. The core is formed on the oxidized and densified porous silicon region and etched to form the desired pattern. An upper cladding is then for
Bruce Allan James
Glebov Alexei
Shmulovich Joseph
Xie Ya-Hong
Agere Systems Optoelectronics Guardian Corp.
Botos Richard J.
Gorgos Kathryn
Leader William T.
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