Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1998-12-22
2001-04-10
Nguyen, Nam (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C216S066000
Reexamination Certificate
active
06214178
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to optical device fabrication and, more particularly, to the utilization of a focused ion beam technique to form optoelectronic devices with angled facets that exhibit minimal light scattering.
Semiconductor optical devices such as lasers, superluminant diodes, modulators and amplifiers typically use cleaved ends of the semiconductor crystal as substrates upon which coatings are applied to form mirror and/or antireflection coatings. In the case of a Fabry-Perot diode laser, the perpendicular orientation of the facets with respect to the laser waveguide provides a convenient mirror orientation which efficiently couples light back into the resonator cavity. In the case of other types of lasers, as well as amplifiers and superluminant diodes, the smooth cleaved facet produces very little light scattering which, in conjunction with antireflection coatings, produces the very low feedback levels crucial for optimal performance of these devices.
It is difficult to create antireflection coatings with the extremely low reflection coefficients needed for proper laser amplifier and superluminant diode performance through the conventional process of deposition of dielectric layers on facets oriented perpendicular to the device waveguide. In an effort to reduce reflections from an imperfectly coated facet, devices have been fabricated with the waveguide oriented at an angle off-normal with respect to the cleaved facet. With this orientation, the majority of any light reflected from the facet will not re-couple back into the active waveguide. This method is effective in reducing reflectivity, but has the disadvantage that the process results in angling both the front and rear mirrors; thus producing lower reflectivities at both facets. This lower-than-desired reflectivity of the rear facet leads to lower-than-optimal output power from superluminant diodes. In general, it would be desirous to have the rear facet normal to the device beam mode, while having the front mirror angled. To this end, devices have been produced wherein the waveguide was bent so that while both facets are cleaved, one end of the waveguide terminates perpendicular to a cleaved facet while the other end, because of the bent waveguide, terminates at an angle with respect to the waveguide. In these cases the fabrication of the angled or bent waveguides has been problematic. Another approach has been to make devices in which one facet has been etched at an angle using a mask on top of the device and reactive ion beam etching or chemically-assisted ion beam etching the facet as discussed in the article “Superluminescent Diodes with Angled Facet Etched by Chemically Assisted Ion Beam Etching”, by C. F. Lin appearing in
Electronics Letters
, Vol. 27, No. 11, 1991, at p. 968. In particular, the article discusses removal of material in a plane extending vertically downward from the edge of the mask. This process is made difficult by the care needed to establish etching conditions that remove material straight down the desired vertical plane below the mask edge. Furthermore, serrations along the edge of the photolithographically defined mask are replicated down into the etched facet, producing striations and roughness that scatter light back into the active waveguide. This scattering, as mentioned above, reduces the antireflection property of the angled facet, thereby mitigating its usefulness.
It has been suggested by Harriot, Scotti et al. in an article appearing in
Applied Physics Letters
, Vol. 48, No. 25, 1986, at pp. 1704 et seq. that out of-plane light beams may be produced by etching wedge-shaped recesses into the device substrate by making cuts with a focused ion beam directed normal to the wafer surface such that the total dose delivered to each location defines the depth of the cut at that location. One wall of the wedge-shaped recess serves as the laser mirror while the other sloping side of the wedge acts as a turning mirror directing the light out of the plane of the substrate. A demonstration of this method was implemented using a focused ion beam to cut a series of layered rectangular cuts, with the cut in each successive layer being slightly smaller than the previous. In this way, a very finely-divided staircase may be etched into the device, producing an essentially vertical wall and an essentially sloping surface. The disadvantage of this technique is that such surfaces are not particularly smooth and, therefore, scatter light undesirably. In addition, this technique is sensitive to inhomogeneities in the material density, phase, and crystallographic orientation (for polycrystalline samples) which translate into unexpected fluctuations in the depth of the cut. This again results in a rough and uneven mirror surface which scatters light. Backscattered light re-entering the laser or amplifier constitutes unwanted reflectivity and compromises the performance of the device. Likewise, forward scattered light from different positions of the mirror surface will be out of phase with the specularly reflected light from the mirror and therefore contributes to spatial inhomogeneities in the phase of the outgoing wavefront (degrading the reflected beam quality).
Thus, a need remains in the art for improved methods of facet etching that produce minimal reflection of light in unwanted directions.
SUMMARY OF THE INVENTION
The need remaining in the art is addressed by the present invention, which relates to optical device fabrication and, more particularly to the utilization of a focused ion beam technique to form optoelectronic devices with angled facets which exhibit minimal light scattering.
In particular, the present invention relates to the utilization of a focused ion beam (FIB) technique to form angled facet and mirror surfaces that exhibit very low levels of light scattering. Such formations are particularly useful as antireflection facets in certain types of optoelectronic devices such as superluminant diodes, semiconductor laser amplifiers, distributed Bragg reflection (DBR) lasers, and distributed feedback (DFB) lasers. In accordance with the present invention, the substrate to be processed is tilted so that the final ion beam cut is made with the ion beam parallel (or nearly parallel) to the plane of the desired finished surface. With this inventive technique of using focused ion beam etching to create such angled facets, and selectively orienting the substrate, an optical device may be formed with a cleaved rear facet oriented perpendicular to the waveguide while the front facet may be angled with respect to the waveguide. Alternatively, devices may be produced with front and back facets angled at any predetermined angle.
It is an aspect of the present invention that a facet may be angled in either the vertical or horizontal planes, or a combination thereof. For the purposes of the present discussion, a “vertical angled” plane will be defined as any plane with its vector normal lying in the plane of the substrate. A “horizontally tilted” plane will be defined as any plane with its vector normal lying in the plane defined by the device's waveguide axis and the substrate normal vector. A plane at a “compound” angle will be defined as one in which the vector normal does not lie in either a “vertical angled” or “horizontally tilted” plane as defined above. Further, a combined facet and mirror structure, or any other combination of surfaces that exhibits low scattering may be made wherein the substrate is tilted at the necessary angle for the grazing incidence formation of each surface in turn.
These and other aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.
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patent: 4869780 (1989-09-01), Yang et al.
patent: 5430751 (1995-07-01), Weterings
patent: 5492607 (1996-02-01), Yap
patent: 3-292776 (1991-12-01), None
P. Reese Puckett, et al., “Ion Beam Etching”, Thin Film Processes II, pp. 749-782, 1991.*
Y. Ochiai, et al., “Pressure and irradiation an
Chakrabarti Utpal Kumar
Peale David Reese
Cantelmo Gregg
Koba Wendy W.
Lucent Technologies - Inc.
Nguyen Nam
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