Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
2002-04-29
2003-11-11
Picardat, Kevin M. (Department: 2822)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S022000, C438S039000, C438S047000
Reexamination Certificate
active
06645784
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor optoelectronic devices in general and, more particularly, to wavelength tunable surface emitting semiconductor lasers and filters.
BACKGROUND OF THE INVENTION
Tunable vertical cavity surface emitting lasers (VCSEL's) and filters have recently generated considerable interest in the art. This is because these devices show great promise not only for increasing bandwidth during wavelength division multiplexing (WDM) in fiber-optic communications, but also for use in switches, routers, highly compact spectroscopic interferometers, optical trans-receivers and numerous other applications.
More particularly, VCSEL's are extremely attractive for integrated optoelectronic circuits. For one thing, they operate at a single longitudinal mode with a circular aperture, thereby providing efficient coupling to fibers. In addition, they are compact, and can be monolithically fabricated in large, dense arrays on a wafer-scale.
As a fixed wavelength light source, VCSEL's have demonstrated limited application and functionality.
Some past effort has been directed towards achieving wavelength tuning in VCSEL's by introducing refractive index changes with (1) temperature (see, for example, Berger, P. R., Dutta, N. K., Choquette, K. D., Hasnain, G., and Chand, N., “Monolithically Peltier-cooled vertical-cavity surface-emitting lasers”, Applied Physics Letters, Vol. 59, No. 1, pp. 117-119, 1991; and Chang-Hasnain, C. J., Harbison, J. P., Zah, C. E., Florez, L. T., and Andreadakis, N. C., “Continuous wavelength tuning of two-electrode vertical cavity surface emitting lasers”, Electron. Lett., Vol. 27, No. 11, pp. 1002-1003, 1991); or (2) carrier injection (see, for example, Gmachi, C., Kock, A., Rosenberger, M., Gornik, E., Micovic, M., and Walker, J. F., “Frequency tuning of a double-heterojunction AlGaAs/GaAs-vertical-cavity surface-emitting laser by a serial integrated in-cavity modulator diode”, Applied Physics Letters, Vol. 62, No. 3, pp. 219-221, 1993).
Both of these techniques provide a tuning range of roughly 10 nm; however, this is still considerably short of the several tens of nanometer tuning range which is necessary for bandwidth-hungry WDM and dense WDM applications.
In contrast, variation of the length of a Fabry-Perot cavity has been shown to be a viable technique for accomplishing wavelength tuning in VCSEL's without affecting the laser gain medium. This can be achieved in surface emitting devices by the provision of a top mirror that can be translated relative to the bottom mirror by the application of an electrostatic field. This technique has been implemented in tunable Fabry-Perot devices such as (1) filters (see, for example, Larson, M. C., Pezeshki, B., and Harris, J. S., “Vertical coupled-cavity microinterferometer on GaAs with deformable-membrane top mirror”, IEEE Photonics Technology Letters, Vol. 7, pp. 382-384, 1995; and Tran, A. T. T. T., Lo, Y. H., Zhu, Z. H., Haronian, D., and Mozdy, E., “Surface Micromachined Fabry-Perot Tunable Filter”, IEEE Photonics Technology Letters, Vol. 8, No. 3, pp. 393-395, 1996); (2) light emitting diodes (see, for example, Larson, M. C., and Harris, J. S., “Broadly-tunable resonant-cavity light emission”, Applied Physics Letters, Vol. 67, No. 5, pp. 590-592, 1995); and (3) VCSEL's (see, for example, Wu, M. S., Vail, E. E., Li, G. S., Yuen, W., and Chang-Hasnain, C. J., “Tunable micromachined vertical-cavity surface emitting laser”, Electronic Letters, Vol. 31, No. 4, pp. 1671-1672, 1995; and Larson, M. C., Massengale, A. R., and Harris, J. S., “Continuously tunable micromachined vertical-cavity surface emitting laser with 18 nm wavelength range”, Electronic Letters, Vol. 32, No. 19, pp. 330-332, 1996).
In devices of this sort, the amount of deflection of the top mirror depends on a number of parameters, e.g., the length, width, thickness and Young's modulus of the mirror-supporting arm. Although the aforementioned width, thickness and Young's modulus of the mirror-supporting arm are generally fairly precisely controllable, the current fabrication techniques used in such devices generally provide very limited control over the exact length of the supporting arms. This results in significant performance variations from device-to-device and batch-to-batch.
The present invention provides the precise dimensional control necessary for realizing reproducible, tunable Fabry-Perot devices that are necessary for producing commercially usable tunable filters and VCSEL's.
SOME ASPECTS OF THE PRESENT INVENTION
This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 60/068,931 filed Dec. 12, 1997 for MICROELECTROMECHANICALLY TUNABLE CONFOCAL VERTICAL CAVITY SURFACE EMITTING LASER VCSEL AND FABRY PEROT FILTER, which document is hereby incorporated herein by reference.
The present invention comprises a novel, microelectromechanically (MEM) tunable, confocal filter.
The present invention also comprises a novel, MEM tunable, confocal vertical cavity surface emitting laser (VCSEL).
The laser preferably utilizes post-growth control of strain in the quantum wells.
In addition, the present invention also comprises a novel technique for VCSEL/filter fabrication which provides the precise dimensional control necessary for mass producing reliable devices having predictable performance characteristics.
More particularly, the present invention provides a new technique for introducing appropriate strain into a thin, lattice-matched layer of laser active medium, i.e., in the quantum wells, after crystal growth has been effected. This is achieved by depositing distributed Bragg reflectors (DBR's) on the laser active medium, wherein the distributed Bragg reflectors comprise carefully engineered, strained, dielectric multi-layer films. By carefully modifying the strain in the deposited DBR films, the strain and the gain properties of the quantum well regions can be optimized. In VCSEL's, when quantum wells are under compressive strain, the differential gain of the laser increases, and threshold current density decreases, thereby dramatically improving the performance of the VCSEL's. Tensile strain, on the other hand, has adverse effects on the lasing properties of VCSEL's. Dielectric multi-layer combinations, such as silicon (Si) and aluminum-oxide (Al
2
O
3
), or Si and silicon-dioxide (SiO
2
), or Si and magnesium-oxide (MgO), or TiO
2
and SiO
2
, can be deposited by means of ion-beam assisted electron-beam evaporation or ion-beam assisted ion-beam sputtering, with a controlled strain in the deposited films. By carefully controlling the ion-beam voltage and current, dielectric films with either tensile or compressive strain can be deposited, with the magnitude of the strain ranging from a few Kilo Pascal (KPa) to a few Giga Pascal (Gpa). These multi-layer dielectric films provide a multi-purpose function, i.e., they induce strain in the quantum wells, they provide optical feedback to the gain medium, and they efficiently remove heat from the active region, all of which are important aspects of creating commercially useful VCSEL's, especially in the wavelength range of between about 1300 nm and about 1500 nm.
The present invention also includes another innovation for producing, via micromachining, a confocal cavity VCSEL that comprises a tunable cavity formed between a set of planar DBR's and a set of curved DBR's. Curvature in the DBR's is achieved by the judicious introduction of an appropriate magnitude of strain in the deposited layers. By the creation of a confocal microcavity, the spatial mode and divergence of the laser mode can be controlled precisely so as to (a) produce single spatial modes by optically restricting the lasing domain in the gain region, and (b) manipulate the divergence angle of the VCSEL so as to optimize the coupling of generated light into a single mode fiber.
The fabrication techniques of the present invention provide extremely precise control of t
Azimi Masud
Tayebati Parviz
Vakhshoori Daryoosh
Wang Peidong
CoreTek, Inc.
Pandiscio & Pandiscio P.C.
Picardat Kevin M.
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