U.S. patent application number 09/105399 was filed with the patent office on 2002-03-14 for microelectromechanically tunable, confocal, vertical cavity surface emitting laser and fabry-perot filter.
Invention is credited to AZIMI, MASUD, TAYEBATI, PARVIZ, VAKHSHOORI, DARYOOSH, WANG, PEIDONG.
Application Number | 20020031155 09/105399 |
Document ID | / |
Family ID | 22305620 |
Filed Date | 2002-03-14 |
United States Patent
Application |
20020031155 |
Kind Code |
A1 |
TAYEBATI, PARVIZ ; et
al. |
March 14, 2002 |
MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE
EMITTING LASER AND FABRY-PEROT FILTER
Abstract
A method is provided for fabricating microelectromechanically
tunable vertical-cavity surface-emitting lasers and
microelectromechanically tunable Fabry-Perot filters with precise
lateral and vertical dimensional control. Strained reflective
dielectric film(s) are applied to a multiple quantum well structure
to electronically band-gap-engineer the quantum wells. Appropriate
strain in the reflective dielectric film layers is also used to
create appropriate curvature in one of the reflective dielectric
film stacks so as to form a confocal cavity between a planar
reflective dielectric film layer and the curved reflective
dielectric film layer in the vertical cavity surface emitting laser
or filter. Microelectromechanical tunable vertical cavity surface
emitting lasers and filter structures are also provided which
include a suspended membrane structure made of a dielectric/metal
membrane or metal film that supports a cavity-tuning reflective
dielectric film stack while being anchored at the perimeter by
metal support post(s). Precise air-cavity length and lateral
dimensions are achieved by micro-die-casting using a micro-machined
sacrificial polyimide or aluminum disk. Further, tuning is achieved
by translational movement of the cavity-tuning reflective
dielectric film stack in a controlled electrostatic field.
Inventors: |
TAYEBATI, PARVIZ;
(WATERTOWN, MA) ; AZIMI, MASUD; (NASHUA, NH)
; WANG, PEIDONG; (WOBURN, MA) ; VAKHSHOORI,
DARYOOSH; (CAMBRIDGE, MA) |
Correspondence
Address: |
PANDISCIO & PANDISCIO
470 TOTTEN POND ROAD
WALTHAM
MA
024511914
|
Family ID: |
22305620 |
Appl. No.: |
09/105399 |
Filed: |
June 26, 1998 |
Current U.S.
Class: |
372/50.1 |
Current CPC
Class: |
G01J 3/26 20130101; H01S
5/18388 20130101; G02B 2006/12121 20130101; H01S 5/0614 20130101;
G02B 26/0816 20130101; G02B 26/001 20130101; H01S 5/3201 20130101;
G02B 2006/12104 20130101; H01S 5/02251 20210101; H01S 5/041
20130101; H01S 5/18366 20130101; G02B 6/4203 20130101; H01S 5/18369
20130101 |
Class at
Publication: |
372/50 |
International
Class: |
H01S 005/00 |
Claims
What is claimed is:
1. A method for introducing a pre-selected amount, and type, of
strain into the quantum wells of a pre-grown crystalline
semiconductor material, said method comprising the steps of:
providing a member formed of said crystalline semiconductor
material, said member having an upper surface and defining multiple
quantum wells; and depositing at least one thin film layer on said
upper surface of said member, said at least one thin film layer
containing a pre-selected amount, and type, of strain, said type of
strain in said at least one thin film layer being the opposite type
to that desired to be introduced into said member.
2. A method according to claim 1 wherein said at least one thin
film layer comprises a dielectric material.
3. A method according to claim 2 wherein said dielectric material
is selected from the group consisting of silicon nitride, silicon
dioxide and aluminum oxide.
4. A method according to claim 1 wherein said at least one thin
film layer comprises a non-dielectric material.
5. A method according to claim 4 wherein said non-dielectric
material is selected from the group consisting of chromium, nickel
chromium, titanium gold, titanium platinum, titanium dioxide and
magnesium oxide.
6. A method according to claim 1 wherein said step of depositing
said at least one thin film layer is achieved by means of ion-beam
assisted electron-beam evaporation.
7. A method according to claim 6 wherein the amount, and type, of
strain in said at least one thin film layer is determined by
controlling the ion-beam voltage and current of the "sputtering" or
"assisting" ions.
8. A method according to claim 6 wherein the amount, and type, of
strain in said at least one thin film layer is determined by
controlling the temperature of said member.
9. A method according to claim 1 wherein said step of depositing
said at least one thin film layer is achieved by means of ion-beam
sputtering.
10. A method according to claim 9 wherein the amount, and type, of
strain in said at least one thin film layer is determined by
controlling the ion-beam voltage and current.
11. A method according to claim 9 wherein the amount, and type, of
strain in said at least one thin film layer is determined by
controlling the temperature of said m ember.
12. A method according to claim 1 wherein said at least one thin
film layer comprises one of the group consisting of Si;
Al.sub.2O.sub.3; SiO.sub.2; TiO.sub.2; MgO; Ta.sub.2O.sub.5;
zirconium oxide; or any combination thereof.
13. A method according to claim 1 wherein said quantum wells are
unstrained prior to the application of said at least one thin film
layer.
14. A method according to claim 1 wherein said quantum wells are
under tensile strain prior to the application of said at least one
thin film layer.
15. A method according to claim 1 wherein said quantum wells are
under compressive strain prior to the application of said at least
one thin film layer.
16. A method according to claim 1 wherein the amount, and type, of
strain in said at least one thin film layer is selected so as to
maximize the compressive strain in said quantum wells without
damage to said member.
17. A method according to claim 1 wherein the amount, and type, of
strain in said at least one thin film layer is selected so as to
maximize the tensile strain in said quantum wells without damage to
said member.
18. A method according to claim 1 wherein said material is
incorporated into an edge-emitting laser.
19. A method according to claim 18 wherein the amount, and type, of
strain is selected so as to optimize the optical gain coefficient
of said laser.
20. A method according to claim 1 wherein said material is
incorporated into a vertical cavity surface emitting laser.
21. A method according to claim 20 wherein the amount, and type, of
strain is selected so as to optimize the optical gain coefficient
of said laser.
22. A microelectromechanically tunable vertical cavity surface
emitting laser, said laser comprising: (a) a substrate having an
upper surface; (b) a first mirror having a first upper surface
located on the upper surface of said substrate; (c) a layer of gain
material having a second upper surface and defining multiple
quantum wells, said layer of gain material being located on said
first upper surface; (d) a first electrode deposited on said second
upper surface; (e) a membrane having an upper surface and a lower
surface, at least part of said membrane carrying a second electrode
on its lower surface in spaced relation to said first electrode,
said membrane defining the length and lateral dimensions of an air
cavity located between said first and second electrodes; (f) a
thick support structure, said support structure connecting said
second upper surface to the periphery of said at least a part of
said membrane carrying said second electrode, said support
structure being adapted to stabilize said membrane; and (g) a
second mirror located centrally of said at least a part of said
membrane carrying said second electrode, said second mirror being
disposed on said upper surface of said membrane, wherein said
second mirror is translatable relative to said first mirror in
response to an electric field applied between said electrodes.
23. A laser according to claim 22 wherein said substrate comprises
a semiconductor material.
24. A laser according to claim 22 wherein said multiple quantum
wells are compressively stressed.
25. A laser according to claim 22 wherein said mirrors comprise
distributed Bragg reflectors.
26. A laser according to claim 25 wherein said distributed Bragg
reflectors comprise alternating layers of quarter-wavelength thick
deposited dielectric films.
27. A laser according to claim 26 wherein said dielectric films are
fabricated from the group consisting of Si; Al.sub.2O.sub.3;
SiO.sub.2; TiO.sub.2; MgO; Ta.sub.2O; zirconium oxide; or any
combination thereof.
28. A laser according to claim 22 wherein said membrane comprises a
plurality of openings therethrough, between said second mirror and
said support structure.
29. A laser according to claim 28 wherein said openings comprise
radial slits.
30. A laser according to claim 28 wherein said openings comprise
holes.
31. A laser according to claim 28 wherein said openings comprise a
spiral slit.
32. A laser according to claim 22 wherein said support structure
comprises an annulus formed from a material selected from the group
consisting of silicon nitride and titanium tungsten.
33. A laser according to claim 22 wherein said membrane is
fabricated from the group consisting of Si.sub.3N.sub.4 and
TiW.
34. A laser according to claim 22 wherein said first mirror is
planar and said second mirror is curved, so as to form, between
said mirrors, a confocal stable resonator.
35. A laser according to claim 34 wherein said confocal stable
resonator has a well-defined near-Gaussian mode structure.
36. A laser according to claim 34 wherein said second mirror has a
radius of curvature, said radius of curvature being optimized such
that said mode size of said cavity matches the size of the core of
an optical fiber.
37. A microelectromechanically tunable Fabry-Perot filter, said
filter comprising: (a) a substrate having an upper surface; (b) a
first mirror having a first upper surface located on the upper
surface of said substrate; (c) a first electrode deposited on said
second upper surface; (d) a membrane having an upper surface and a
lower surface, at least part of said membrane carrying a second
electrode on its lower surface in spaced relation to said first
electrode, said membrane defining the length and lateral dimensions
of an air cavity located between said first and second electrodes;
(e) a thick support structure, said support structure connecting
said second upper surface to the periphery of said at least a part
of said membrane carrying said second electrode, said support
structure being adapted to stabilize said membrane; and (f) a
second mirror located centrally of said at least a part of said
membrane carrying said second electrode, said second mirror being
disposed on said upper surface of said membrane, wherein said
second mirror is translatable relative to said first mirror in
response to an electric field applied between said electrodes.
38. A Fabry-Perot filter according to claim 37 wherein said
substrate comprises a semiconductor material.
39. A Fabry-Perot filter according to claim 37 wherein said mirrors
comprise distributed Bragg reflectors.
40. A Fabry-Perot filter according to claim 39 wherein said
distributed Bragg reflectors comprise alternating layers of
quarter-wavelength thick deposited dielectric films.
41. A Fabry-Perot filter according to claim 40 wherein said
dielectric films are fabricated from the group consisting of Si;
Al.sub.2O.sub.3; SiO.sub.2; TiO.sub.2; MgO; Ta.sub.2O.sub.5;
zironium oxide; or any combination thereof.
42. A Fabry-Perot filter according to claim 37 wherein said
membrane comprises a plurality of openings therethrough, between
said second mirror and said support structure.
43. A Fabry-Perot filter according to claim 42 wherein said
openings comprise radial slits.
44. A Fabry-Perot filter according to claim 42 wherein said
openings comprise holes.
45. A Fabry-Perot filter according to claim 42 wherein said
openings comprise a spiral slit.
46. A Fabry-Perot filter according to claim 37 wherein said support
structure comprises an annulus formed from a material selected from
the group consisting of silicon nitride and titanium tungsten.
47. A Fabry-Perot filter according to claim 37 wherein said
membrane is fabricated from the group consisting of Si.sub.3N.sub.4
and TiW.
48. A Fabry-Perot filter according to claim 37 wherein said first
mirror is planar and said second mirror is curved, so as to form,
between said mirrors, a confocal stable resonator.
49. A Fabry-Perot filter according to claim 48 wherein said
confocal stable resonator has a well-defined near-Gaussian mode
structure.
50. A Fabry-Perot filter according to claim 48 wherein said second
mirror has a radius of curvature, said radius of curvature being
optimized such that said mode size of said cavity matches the size
of the core of an optical fiber.
51. A method for making a microelectromechanically tunable,
vertical cavity surface emitting laser, said method comprising the
steps of: (a) providing a member having a substrate, a first mirror
disposed on said substrate, and a layer of gain material disposed
on said first mirror; (b) depositing a first electrode onto said
gain material; (c) depositing a calibrated thickness of a
sacrificial material on top of said gain material and said first
electrode; (d) etch-masking said sacrificial material so as to
create a central structure having an inwardly sloped perimeter edge
on said gain material/first electrode structure; (e) depositing a
second electrode on said central structure; (f) depositing a thin
layer of a second material on top of the gain material/central
structure/second electrode structure; (g) depositing a thick
annulus of support material onto said layer of second material such
that said annulus covers the sloped perimeter edge of said support
structure and extends inwardly thereof, adjacent to and parallel to
the top surface of said central structure, and outwardly thereof,
adjacent to and parallel to the upper surface of said gain
material; (h) etch-masking openings through said layer of second
material adjacent to the top of said central structure, between a
substantially circular center portion thereof and the inner edge of
said annulus; (i) selectively depositing a second mirror onto said
substantially circular center portion; and (j) selectively removing
said central structure through said openings using an etching
technique.
52. A method according to claim 51 wherein said sacrificial
material is selected from the group consisting of polyimide,
aluminum and photoresist.
53. A method according to claim 51 wherein said central structure
is in the form of a disk.
54. A method according to claim 51 wherein said central structure
is in the form of a polygon.
55. A method according to claim 51 wherein said second material is
selected from the group consisting of silicon nitride and metals
other than aluminum.
56. A method according to claim 51 wherein said support material is
selected from the group consisting of silicon nitride or a
metal.
57. A method according to claim 51 wherein said etching technique
comprises dry etching.
58. A method according to claim 51 wherein said etching technique
comprises wet etching.
59. A method according to claim 51 wherein said first and second
mirrors comprise distributed Bragg reflectors.
60. A method according to claim 51 wherein the material of said
mirrors is selected from the group consisting of Si;
Al.sub.2O.sub.3; SiO.sub.2; TiO.sub.2; MgO; Ta.sub.2O.sub.5;
zirconium oxide; or any combination thereof.
61. A method according to claim 51 wherein said thick metal annulus
is formed of TiW.
62. A method according to claim 51 wherein said central structure
is formed of polyimide and said etching is accomplished with oxygen
plasma.
63. A method according to claim 51 wherein said central structure
comprises aluminum, and said etching is accomplished using CF.sub.4
plasma.
64. A method according to claim 51 wherein said first mirror is
planar and, subsequent to step (j), said second mirror is curved,
so as to form, between said mirrors, a confocal stable
resonator.
65. A method according to claim 64 wherein said confocal stable
resonator has a well-defined near-Gaussian mode structure.
66. A method according to claim 64 wherein the curvature of said
second mirror is controlled by introducing a stress gradient within
said second mirror.
67. A method according to claim 66 wherein said stress gradient is
created by controlled changes in deposition temperatures or
deposition voltage.
68. A method for making a microelectromechanically tunable
Fabry-Perot filter, said method comprising the steps of: (a)
depositing a first mirror onto the upper surface of a substrate;
(b) depositing a first electrode onto said first mirror; (c)
depositing a calibrated thickness of a sacrificial material on top
of the substrate/first mirror/first electrode structure; (d)
etch-masking said sacrificial material so as to leave a central
structure having an inwardly sloped perimeter edge on said first
mirror/first electrode structure; (e) depositing a second electrode
on top of said central structure; (f) depositing a thin layer of a
second material on top of the central structure/second
electrode/substrate structure; (g) depositing a thick annulus of
support material onto said layer of second material such that said
annulus covers the sloped perimeter edge of said central structure
and extends inwardly thereof, adjacent to and parallel to the top
surface of said central structure, and outwardly thereof, adjacent
to and parallel to the upper surface of said first mirror; (h)
etch-masking openings through the layer of second material adjacent
to the top of said central structure, between a substantially
circular center portion thereof and the inner edge of said annulus;
(i) selectively depositing a second mirror onto said substantially
circular center portion; and (j) selectively removing said central
structure through said openings using an etching technique.
69. A method according to claim 68 wherein said first mirror is
planar and, subsequent to step (j), said second mirror is curved,
so as to form, between said mirrors, a confocal stable
resonator.
70. A method according to claim 68 wherein said sacrificial
material is selected from the group consisting of polyimide,
aluminum and photoresist.
71. A method according to claim 68 wherein said central structure
is in the form of a disk.
72. A method according to claim 68 wherein said central structure
is in the form of a polygon.
73. A method according to claim 68 wherein said second material is
selected from a group consisting of silicon nitride and metals
other than aluminum.
74. A method according to claim 68 wherein said support material is
selected from the group consisting of silicon nitride or a
metal.
75. A method according to claim 68 wherein said etching technique
comprises dry etching.
76. A method according to claim 68 wherein said etching technique
comprises wet etching.
77. A method according to claim 68 wherein said first and second
mirrors comprise distributed Bragg reflectors.
78. A method according to claim 68 wherein the material of said
mirrors is selected from the group consisting of Si;
Al.sub.2O.sub.3; SiO.sub.2; TiO.sub.2; MgO; Ta.sub.2O.sub.5;
zirconium oxide; or any combination thereof.
79. A method according to claim 68 wherein said thick metal annulus
is formed of TiW.
80. A method according to claim 68 wherein said central structure
is formed of polyimide and said etching is accomplished with oxygen
plasma.
81. A method according to claim 68 wherein said central structure
comprises aluminum, and said etching is accomplished using CF.sub.4
plasma.
82. A method according to claim 9 wherein said step of depositing
said at least one thin film layer is achieved by means of ion-beam
assisted ion-beam sputtering.
Description
REFERENCE TO PENDING PRIOR PROVISIONAL PATENT APPLICATION
[0001] This patent application claims benefit of pending prior U.S.
Provisional Patent Application Ser. No. 60/068,931 filed Dec. 29,
1997 for MICROELECTROMECHANICALLY TUNABLE CONFOCAL VERTICAL CAVITY
SURFACE EMITTING LASER VCSEL AND FABRY PEROT FILTER.
FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] 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.
[0005] As a fixed wavelength light source, VCSEL's have
demonstrated limited application and functionality.
[0006] 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).
[0007] 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.
[0008] 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).
[0009] 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.
[0010] 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
[0011] This patent application claims benefit of pending prior U.S.
Provisional Patent Application Ser. No. 60/068,931 filed Dec. 29,
1997 for MICROELECTROMECHANICALLY TUNABLE CONFOCAL VERTICAL CAVITY
SURFACE EMITTING LASER VCSEL AND FABRY PEROT FILTER, which document
is hereby incorporated herein by reference.
[0012] The present invention comprises a novel,
microelectromechanically (MEM) tunable, confocal filter.
[0013] The present invention also comprises a novel, MEM tunable,
confocal vertical cavity surface emitting laser (VCSEL).
[0014] The laser preferably utilizes post-growth control of strain
in the quantum wells.
[0015] 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.
[0016] 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.sub.2O.sub.3), or Si and silicon-dioxide (SiO.sub.2), or Si and
magnesium-oxide (MgO), or TiO.sub.2 and SiO.sub.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.
[0017] 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
[0018] (a) produce single spatial modes by optically restricting
the lasing domain in the gain region, and
[0019] (b) manipulate the divergence angle of the VCSEL so as to
optimize the coupling of generated light into a single mode
fiber.
[0020] The fabrication techniques of the present invention provide
extremely precise control of the physical dimensions of both the
top DBR structure and the supporting structure, which is
indispensable for achieving highly reproducible performance with
inconsequential device-to-device variation.
[0021] Another aspect of the present invention is a confocal
microelectromechanical tunable Fabry-Perot structure. When the gain
region is left out of the foregoing confocal VCSEL structure, only
the optical cavity remains, and the device acts as a confocal
Fabry-Perot filter. The confocal nature of the short cavity (e.g.,
0.2-10 micron) device allows efficient coupling of light (i) from
an input single mode fiber to the device, and (ii) back out to a
single mode output fiber.
[0022] Confocal tunable filter and VCSEL devices are depicted in
FIGS. 1 and 2, respectively. These devices operate at a single
longitudinal mode over the entire bandwidth (e.g., 30-120 nm) of
the gain medium, in the case of a VCSEL; and over a 100 nm tuning
range, in the case of a filter.
[0023] As depicted in FIG. 1, the tunable Fabry-Perot filter device
comprises (i) a distributed Bragg reflector (DBR) with a curvature
R, formed by high index contrast multi-layers atop a thin membrane
(or tethers) of silicon nitride (Si.sub.3N.sub.4) or a thin metal
film such as titanium-tungsten (TiW), wherein the membrane is
supported at its perimeter by thicker metal posts, (ii) an air
cavity formed by selective removal of a sacrificial layer, and
(iii) a bottom set of dielectric DBR's deposited in the substrate
facing the top DBR.
[0024] In the case of a VCSEL, a gain medium, consisting of
multiple quantum wells, is inserted in the air cavity as shown in
FIG. 2. These VCSEL's can be photo-pumped, or intra-cavity
electrical interconnections can be made for current injection.
[0025] Of course, it is also to be appreciated that the tunable
filter, and/or the tunable VCSEL, can be formed with a top
distributed Bragg reflector having a planar configuration, without
departing from the scope of the present invention.
[0026] The following is a list of some technological breakthroughs
resulting from the present invention.
Strain-Optimized Fixed-Wavelength VCSEL's
[0027] Since, in a VCSEL, the resonant optical mode interacts with
an extremely small volume of gain medium, it is imperative that the
gain medium provide maximum differential gain, while the DBR's
provide maximum feedback and the least thermal resistance
possible.
[0028] Although using compressively strained multiple quantum wells
can provide the maximum possible gain, there is a limitation on the
maximum number of strained quantum wells that can be grown without
generating crystalline defects.
[0029] A solution to this problem is to grow strain-compensated
multiple quantum wells. In practice, however, this is difficult and
costly. Although GaAs/GaAlAs-based VCSEL's show maturity, the
presently preferred material system for long wavelength (e.g., 1300
nm and 1500 nm) lasers is InP/InGaAsP-based, and much improvement
upon this material system remains to be made.
[0030] Since the maximum index contrast between InP and InGaAsP is
only about 0.2, a large number of quarter-wave stacks are required
in order to provide sufficient feedback. This, however, causes
significant resistive losses in the device, and an unacceptable
degree of thermal bottleneck. In addition, the low index-contrast
also results in relatively narrow bandwidth mirrors, thereby
placing a severe constraint on the accuracy of the mirror
thickness.
[0031] The present device may consist of an InGaAsP/InGaAs multiple
quantum well (MQW), with a dielectric DBR on top and a dielectric
DBR between the MQW and the substrate. See, for example, FIG. 5,
which illustrates a fixed wavelength VCSEL. If desired, the fixed
wavelength VCSEL may have a curved top distributed Braff reflector.
Both DBR's are deposited by vacuum deposition techniques.
[0032] As summarized below, the present invention provides a
solution to all of the foregoing problems, simultaneously, with
implications of significant cost reduction and high manufacturing
yield.
[0033] 1. Deposited DBR's can externally alter the strain in the
quantum well regions of the MQW, thereby modifying the gain
coefficient, threshold current and slope-efficiency. By controlling
the strain of the deposited DBR, the strain in the quantum wells,
and therefore the lasing properties of the VCSEL, can be optimized.
Post crystal-growth modification of strain in the quantum wells
relieves the constraint of devising expensive and difficult
strain-compensation techniques employed in epitaxial crystal growth
(MBE or MOCVD).
[0034] 2. In epitaxially grown monolithic VCSEL's, thickness
variations across the wafer cause lasing wavelength variations,
resulting in poor yield. Since the index contrast between Si and
Al.sub.2O.sub.3 is relatively high (i.e., about 2.8), with only
four pairs of DBR's, over 99.9% reflectivity can be achieved over a
large bandwidth (e.g., 500 nm). As a result, a Fabry-Perot cavity
formed by these DBR's creates a high-Q cavity with sufficient
feedback. Since the mirrors are broad band, the stopgap of the
mirror can easily straddle over 100 nm of the gain spectrum. Any
thickness variations in the active layer, and/or in the cladding
thickness over the wafer, can be compensated for by depositing
phase-compensating layers of dielectric films before depositing the
final DBR mirror. This allows harvesting most of the wafer for the
desired wavelength of emission. The high reflectivity of the DBR
mirrors also helps to lower threshold conditions.
[0035] 3. Dielectric mirrors formed by materials such as Si and
Al.sub.2O.sub.3 or MgO have a very high thermal conductivity,
thereby providing for efficient removal of heat from the active
region. In addition, only a few pairs of DBR's are required; hence,
the thermal path to the heat sink is shorter in the present
invention than in traditional semiconductor DBR's, thereby adding
to the efficient heat removal process.
Wavelength Tunable VCSEL's
[0036] A schematic diagram of the steps used in fabricating a novel
wavelength tunable VCSEL based on the present invention is shown in
FIG. 4. The device comprises bottom DBR's consisting of high
index-contrast dielectric pairs such as Si/Al.sub.2O.sub.3,
Si/SiO.sub.2, Si/MgO, or TiO.sub.2/SiO.sub.2, along with
selectively-deposited top DBR mirrors, with an air-cavity and an
active medium embedded in the Fabry-Perot cavity formed by the two
DBR's.
[0037] The present invention also accommodates a hybrid mirror
system such as bottom epitaxially grown DBR's and top deposited
DBR's.
[0038] The top DBR resides on a thin, supporting membrane or
multiple tether structure made of Si.sub.3N.sub.4 or metal (TiW)
that is supported at its perimeter by a thicker metal support (see
FIGS. 6A-6C). This forms a trampoline type of structure. In the
case of a circular membrane structure, radially extending openings
in the Si.sub.3N.sub.4 or metal film (TiW) are used for selectively
removing an underlying sacrificial layer during the top DBR release
process, as will be discussed further below.
[0039] By applying an appropriate voltage across this membrane and
the bottom DBR's, the trampoline structure, along with the top
mirror, can be translated toward, and away from, the bottom DBR so
as to tune the laser emission. Since the DBR's are broad band,
tuning is possible over the entire bandwidth of the laser gain
spectrum, which is nominally about 60 nm.
[0040] One of the important features of the present invention is
that the new fabrication process provides precise control over the
lateral dimensions of the trampoline structure and the air-cavity
length, both of which are important for the consistent
manufacturing of substantially identical devices. This is made
possible in the present invention by allowing the sacrificial layer
to act as a die in order to define the lateral dimensions of the
trampoline structure and the vertical dimension of the air-cavity.
As a result, the possible ill effects of uncontrolled dimensions,
ensuing during the selective removal of the sacrificial layer, are
effectively eliminated.
[0041] In addition, the new devices are small and compact
(approximately 500 .mu.m.times.500 .mu.m), thereby allowing arrays
thereof to be manufactured and coupled to fibers.
Wavelength Tunable Filter
[0042] As shown in FIG. 3, the tunable Fabry-Perot filter is
obtained by omitting the quantum well gain material from the
above-described VCSEL structure.
[0043] High index-contrast DBR stacks provide broad bandwidth
(e.g., 500 nm); hence, for a lambda-cavity, the Fabry-Perot
resonance can be tuned over the entire bandwidth of the DBR's.
Since the reflectivity of the DBR's is high, an extremely narrow
(sub-Angstrom) linewidth is attainable.
[0044] Tuning speed in these devices is on the order of
microseconds, making them one of the fastest tunable filters, with
extremely high spectral resolution. These devices are also easily
mass-produced using standard semiconductor fabrication techniques,
thereby making them affordable for consumer products.
Tunable VCSEL/Filter With Confocal Cavity
[0045] The MEM tunable Fabry-Perot filter with confocal cavity is a
highly innovative resonator design that comprises an air cavity
between a first set of distributed Bragg reflectors (DBR's) that
are planar and a second set of DBR's having a finite radius of
curvature. These two sets of DBR's form a confocal cavity as shown
in FIG. 1.
[0046] One innovation of this design is the fact that the curvature
of one of the mirrors creates a micro-resonator that can sustain
Hermite Gaussian modes. As described below, by introduction of
appropriate curvature in the top mirror, coupling of light from a
standard single mode fiber into and out of the device can be
simplified by avoiding the use of lenses that are otherwise
necessary.
[0047] It is well known that the Rayleigh range, z.sub.0, which
defines the distance at which the wave front is most curved, is
related to mirror curvature, R, and cavity length, d, by the
equation z.sub.0=[(R-d)/d].sup.1/2 ("Equation 1"). For instance, a
resonator with a cavity length of 1.5 microns, and a radius of
curvature of 1.5 millimeter for the curved DBR's, leads to a
z.sub.0 value of 150 microns, and to a fundamental mode beam waist,
W.sub.C, of 8.5 microns at a wavelength, .lambda., of 1.5 microns,
according to the relationship
W.sub.0=(z.sub.0.lambda./.pi.).sup.1/2 ("Equation 2"). Since the
value of the mode size at position z is given by the equation
W(z)=W.sub.0[1+(z/z.sub.0).sup.2]{fraction (1/2)} ("Equation 3"),
and since z.sub.0 is approximately a hundred times larger than the
cavity length, the mode size remains virtually the same over the
length of the cavity. Consequently, light from a 9-micron-core,
single-mode fiber on the input side can excite this fundamental
mode, and the transmitted single mode beam can be efficiently
coupled to a single-mode fiber. As such, by curving the mirror, the
mode spot size can be adjusted to match that of a single mode fiber
without requiring a lens. The trade off is, however, that in this
case the fiber has to be positioned within 0.5 micron (in the
lateral direction) with respect to the optical axis of the cavity
in order to avoid exciting undesirable higher order
Hermite-Gaussian modes. In order to improve the alignment tolerance
of the coupling fiber, a thermally expanded core fiber with mode
size of 20-50 microns can be used in conjunction with mirrors with
appropriately reduced curvature. The curvature R of the mirror is
adjusted based on Equations 1-3 above to match the mode size
W.sub.0 of the thermally expanded core fiber. Because of the larger
size of the Gaussian mode, the lateral positioning of the fiber is
relaxed.
[0048] This design is distinctly different from the single-crystal,
parallel mirror resonator design disclosed in U.S. Pat. No.
4,825,262, issued Apr. 25, 1989 to Stephen R. Mallinson.
[0049] The processing steps for the fabrication of a novel MEM
tunable filter with a confocal cavity of the present invention are
similar to those utilized in the fabrication of a novel planar
cavity tunable filter/VCSEL of the present invention. A significant
difference is in the deposition of the curved DBR's. Control of the
magnitude and type of strain in the deposited multilayer dielectric
stack of DBR's, and the supporting thin silicon-nitride membrane,
is carefully engineered so as to achieve the desired mirror
curvature. The magnitude and the type of strain (tensile or
compressive) is introduced in these films by the judicious choice
of deposition parameters, such as the ratio of the gas mixtures of
silane (SiH.sub.4) and ammonia (NH.sub.4), the total pressure of
the gases used, and the magnitude of RF power used. The resulting
stress gradient between the tensile strain silicon-nitride membrane
and the compressively strained dielectric mirror stacks results in
a concave DBR. Further control of the curvature of the top DBR can
be achieved by introducing a stress gradient within the mirror
layers by a gradual change of temperature and/or deposition
voltage. Alternative methods for introducing the desired stress
gradient within the mirror layers include the use of a secondary
ion source to selectively modify the stress within each layer of
the mirror by varying the current or voltage. In one example, a
silicon nitride layer of 0.5 micron thickness, with 100 MPa of
tensile stress, was deposited by PECVD, and the top mirror was
deposited at 100.degree. C. using ion-beam sputtering at 700V. The
resulting mirror curvature of approximately 1 mm was achieved
following removal of the sacrificial layer. Furthermore, varying
the temperature of the substrate during the mirror deposition from
room temperature to 120.degree. C. resulted in a further stress
gradient in the mirror layers, decreasing the mirror curvature to
0.75 mm.
[0050] Akin to the confocal-cavity tunable filter (FIG. 1), the
innovative micro-cavity design using the confocal resonator scheme
also provides a stable fundamental spatial mode in a MEM tunable
VCSEL. More particularly, in the case of tunable VCSEL's, the gain
medium resides inside the Fabry-Perot cavity defined by a set of
planar DBR's and a set of movable curved DBR's, as shown in FIG. 2.
Excitation of the gain medium by the fundamental mode leads to
laser emission of a single, circular spatial mode. As a result,
lateral optical mode confinement arises naturally, without having
to form a lateral waveguide. This results in highly efficient
VCSEL's.
[0051] The confocal cavity scheme is equally applicable to a fixed
wavelength VCSEL, as shown in FIG. 5. As explained previously, the
fundamental spatial mode in the fixed wavelength VCSEL provides a
stable lateral optical confinement, and leads to a single mode
laser emission.
Competing Technologies
[0052] Currently, there are fixed-wavelength VCSEL's commercially
available below 1.0 .mu.m.
[0053] There are no tunable VCSEL's or MEM tunable Fabry-Perot
filters commercially available at this time.
[0054] The only commercially available tunable filters rely on
piezoelectric drivers with a complex feedback system (Queensgate
Instruments, England), or on the use of bi-refringent materials
sandwiched between cross-polarizers (Cambridge Research
Instruments, Massachusetts, using liquid crystals).
[0055] Piezoelectric tunable filters have a resolution of about 0.1
nm, with a 50 nm tuning range, but they also require high voltage
for operation.
[0056] Liquid crystal-based filters can exhibit better resolution,
but at the expense of low efficiency, e.g., as low as 99.0%.
[0057] The fabrication of the two aforementioned systems is
labor-intensive, and thus costly. For example, the top-of-the-line
model offered by Queensgate Instruments costs more than $10,000.
Such high cost, combined with the bulkiness of these systems, make
them unrealistic for most applications. Specifically, in
applications such as future communications networks that will
deliver voice, video, data and upstream communications to
consumers, all through a single optical fiber cable, very low cost,
compact filters and laser sources are needed. It is believed that
the feasibility of harnessing optical fiber cable bandwidths on a
broad scale will hinge upon the availability of compact, low cost
devices such as the devices of the present invention.
[0058] There has been some past effort towards realizing this goal.
This effort has been confined to a small research community, and
has resulted in reports of tunable LED's, VCSEL's and MEM tunable
Fabry-Perot filters.
[0059] For example, Larson et al. have published results on (1) a
GaAs-based tunable filter (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); (2)
an LED (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) a VCSEL (see,
for example, 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).
[0060] These results indicate that Larson et al. used GaAs/AlAs for
bottom DBR's, and a gold-coated silicon-nitride membrane as the top
mirror. In all of the foregoing Larson et al. devices, the top
mirror release is accomplished by selectively wet-etching an
underlying sacrificial layer of GaAlAs with hydrochloric acid.
Since this technique provides no controlled way of undercutting,
the length of the support structure for the top mirror is not well
defined from device to device. Furthermore, since the top mirror in
Larson et al. has lower bandwidth and reflectivity than the
dielectric DBR's of the present invention, the tuning range of the
devices of Larson et al. is limited, and their spectral linewidth
is broader than that provided by the present invention.
[0061] Similarly, Tran et al. have shown (1) a tunable Fabry-Perot
filter (see, for example, 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); and (2) an LED (see, for example, Christenson,
G. L., Tran, A. T. T., Zhu, Z. H., Lo, Y. H., Hong, M., Mannaerts,
J. P., and Bhat, R., "Long-Wavelength Resonant Vertical-Cavity
LED/Photodetector with a 75-nm Tuning Range", IEEE Photonics
Technology Letters, Vol. 9, No. 6, pp. 725-727, 1997); the
aforementioned filter and LED using polyimide as the sacrificial
layer. This method suffers from the same lack of control over
precise length fabrication. In addition, polyimide is not a stable
material for making a robust device, because aging tends to degrade
the stability of the cavity's length.
[0062] A tunable filter (see, for example, Vail, E. C., Wu, M. S.,
Li G. S., Eng, L. and Chang-Hasnain, C. J., "GaAs micromachined
widely tunable Fabry-Perot filters", Electronic Letters, Vol. 31,
pp. 228-229, 1995) and a VCSEL (see, for example, Vail, E. C., Li,
G. S., Yuen, W. and Chang-Hasnain, C. J., "High performance
micromechanical tunable vertical-cavity surface-emitting lasers",
Electronic Letters, Vol. 32, No. 20, pp. 1888-1889, 1996) also have
been reported by Vail et al. The Vail et al. devices use GaAs/AlAs
for the top and bottom DBR's, with a GaAs sacrificial layer for top
DBR release. Although Vail et al. use a dry-etching technique to
selectively remove the sacrificial GaAs layer, precise control of
the top mirror length is still not feasible.
[0063] The present invention is distinct from the aforementioned
devices in the following aspects, among others:
[0064] 1. the present invention provides a precise method for
defining the lateral dimensions of the top mirror support and the
cavity length by deposited supporting posts;
[0065] 2. the present invention provides an optimized control of
the VCSEL gain properties by control of the strain in the deposited
DBR; and
[0066] 3. the confocal design of the VCSEL structure allows single
spatial modes, lower threshold and efficient coupling into a single
mode fiber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] The description of the present invention is intended to be
considered in conjunction with the drawings, wherein like elements
are referred to by like reference numerals throughout, and further
wherein:
[0068] FIG. 1 is a side elevational view, in cross-section,
schematically and diagrammatically illustrating a
microelectromechanical tunable filter having a confocal cavity;
[0069] FIG. 2 is a side elevational view, in cross-section,
schematically and diagrammatically illustrating a tunable vertical
cavity surface emitting laser having a confocal cavity;
[0070] FIG. 3 (i.e., FIGS. 3A-3F) comprises a series of side
elevational, diagrammatic, cross-sectional views schematically
illustrating the fabrication procedure for making a
microelectromechanical tunable filter;
[0071] FIG. 4 (i.e., FIGS. 4A-4G) comprises a series of side
elevational, diagrammatic, cross-sectional views schematically
illustrating the fabrication procedure for making a
microelectromechanical, tunable, vertical cavity surface emitting
laser;
[0072] FIG. 5 is a side elevational view, in cross-section,
schematically and diagrammatically illustrating a fixed-wavelength
vertical cavity surface emitting laser having a confocal
cavity;
[0073] FIGS. 6A-6C show top elevational diagrammatic views of three
different forms of the tunable filter/VCSEL device, with FIG. 6A
showing a membrane type structure, FIG. 6B showing a four tether
device structure, and FIG. 6C showing a three tether device
structure; and
[0074] FIG. 7 is a schematic diagram illustrating compressive
strain induced in a multiple quantum well structure by strained
dielectric distributed Bragg reflectors.
FURTHER ASPECTS OF THE INVENTION
[0075] A MEM tunable filter 2, and a MEM tunable VCSEL 4, are shown
in FIGS. 3 and 4, respectively.
[0076] More particularly, and looking now at FIG. 4, tunable VCSEL
4 includes a gain medium 6, usually comprising multiple quantum
wells, located in a mechanically tunable high-Q Fabry-Perot cavity
8 formed by a pair of spaced-apart DBR's 10 and 12, respectively.
In the present invention, one of the DBR's 12 can be translated
towards the other of the DBR's 10 by an applied electrostatic
field. This changes the Fabry-Perot cavity length, and provides
tuning in the emission wavelength. The VCSEL can be photo-pumped,
or charge-injection may be accomplished by intra-cavity electrical
interconnections.
[0077] Further, the same basic structure, without the gain medium,
functions as a tunable filter (see FIG. 3).
[0078] If desired, the top DBR 12 can be formed with a curved
configuration, so as to form a confocal filter (FIG. 1) or confocal
VCSEL (see FIG. 2).
[0079] Quantum wells provide the necessary gain during stimulated
emission in semiconductor lasers. In the case of a VCSEL, the
resonant optical mode inside the micro-cavity interacts with an
extremely small volume of this gain medium. As a result, it is
important to provide maximum gain per quantum well over the
interaction volume. The best method for accomplishing this is to
grow the quantum wells under compressive strain. Compressive strain
in the quantum wells leads to anisotropy in the band structure, and
to splitting of the degeneracy of the in-plane heavy-hole and
light-hole bands, resulting in heavy-hole band shifting of several
tens of meV (for 1% compressive strain) above the light-hole band.
While the anisotropy leads to a reduction in the in-plane
heavy-hole effective mass, which reduces the density of states
available for transition near the zone center, the splitting of the
degeneracy leads to a preferential population inversion between the
conduction band and the desired heavy-hole band. Due to these two
effects, the threshold current density decreases, temperature
sensitivity is improved, and differential efficiency increases.
Compressively strained multiple quantum wells are especially
desirable at about 1300 nm and about 1500 nm, since Auger
recombination and inter-valence band absorption contribute to
intrinsic loss at these long wavelengths. Therefore, the importance
of maximizing gain in VCSEL's cannot be over-emphasized.
[0080] Strained multiple quantum wells are difficult to grow, and
are expensive compared to unstrained quantum wells.
[0081] In the present invention, a method is provided for
introducing a desired strain in the unstrained quantum wells of any
semiconductor material after the crystal has been grown. The
process requires depositing dielectric films with a controlled
amount of strain. In the case of VCSEL's, it has been found
preferable to deposit one of the DBR stacks with a judiciously
chosen strain.
[0082] For example, and looking now at FIG. 7, in order to induce
compressive strain in the quantum wells, DBR stacks 10, with
tensile strain, will be deposited on top of a MQW structure 6 (for
example, a InGaAsP/InGaAs MQW), and subsequently flip-chip bonded
to a temporary host substrate 20 such as silicon. Thereafter, the
temporary host substrate 20 will be selectively removed using an
epitaxial lift-off process, and the second set of DBR's will be
deposited.
[0083] The amount of strain introduced into the MQW by the
foregoing process may be approximated by the following equation: 1
a a = - 2 Pl 2 c 11 l 1
[0084] where P is the compressive stress in one of the DBR layers;
c.sub.11 is the average coefficient of stiffness (Young's modulus)
of the MQW layers; l.sub.2 is the thickness of the MQW layers; and
l is the thickness of the dielectric DBR's. From this equation, it
is evident that several Mega Pascals of tensile strain in the DBR's
will induce roughly 1% compressive strain in the MQW that are
initially lattice-matched. Controlled strain in the DBR's can be
introduced by controlling the energy of the ion-beam during
deposition of the DBR's, as discussed above. Typically, a few Kilo
Pascal to a few Giga Pascal of stress (tensile or compressive) can
be Introduced in DBR pairs of Si/Al.sub.2O.sub.3, Si/SiO.sub.2,
Si/MgO or TiO.sub.2/SiO.sub.2 in this manner.
Device Fabrication
[0085] A schematic top view of three embodiments of the tunable
VCSEL or filter structure is depicted in FIGS. 6A-6C.
[0086] In FIGS. 3 and 4, schematic cross-sectional views of the
fabrication steps used in making microelectromechanical, tunable
filters and VCSEL's are shown, respectively. Although the two
structures (i.e., the tunable filters and VSCEL's) resemble each
other in form, there are subtle differences in the fabrication
steps utilized in making the two different devices. The fabrication
steps for each of these devices are discussed in detail below.
MEM Tunable VCSEL/Filter Fabrication Procedure
[0087] 1. When fabricating a tunable VCSEL, DBR's 10 are deposited
on top of the MQW structure 6. The MQW structure 6, with the DBR's
10 deposited thereon, is applied to a suitable temporary substrate
20, such as silicon, GaAs or sapphire. This is accomplished by a
method such as flip-chip bonding, fusion bonding or Van der Waals
bonding (see FIG. 4A). On the other hand, when fabricating a
tunable filter, the DBR's 10 are deposited directly onto a host
substrate of choice 24 (FIG. 3A).
[0088] 2. When fabricating a tunable VCSEL, the structure of FIG.
4A is mounted to a host substrate of choice 24 (FIG. 4B). Next, the
temporary substrate 20, upon which the MQW structure 6 resides, is
selectively removed by an etch-back technique (FIG. 4B). In this
method, a highly selective etchant is used to etch the temporary
substrate 20, and etching is terminated at a strategically located
etch-stop layer 26. It has been found that a one-to-one mixture of
concentrated hydrochloric acid and hypochloric acid removes InP
preferentially over InGaAs. In the case of a GaAs substrate, a
citric acid and hydrogen peroxide mixture can be used for selective
removal of the temporary substrate 20 over AlAs. Another approach,
which has been found to be useful with GaAs substrates, is to grow
a thin layer of AlAs between the temporary substrate 20 and the MQW
structure 6 deposited thereon. The AlAs may then be selectively
etched. This allows the MQW structure 6 to be lifted away from the
GaAs substrate.
[0089] 3. At this stage, one of the tuning electrodes, 28, is
deposited on top of the DBR layer 10 in the case of a tunable
filter 2 (FIG. 3B), and on top of the MQW structure 6 in the case
of tunable VCSEL 4 (FIG. 4C). If appropriate, an isolation layer 29
(i.e., an electrically insulating layer) may then be deposited atop
some or all of the electrodes 28. See, for example, FIG. 4C, where
an isolation layer 29 is shown (in phantom) atop electrodes 28.
[0090] 4. After the DBR layer 10 (in the case of a filter), or the
MQW structure 6 and DBR layer 10 (in the case of a VCSEL), has been
deposited on the substrate 24, a calibrated thickness 30 of
polyimide, or aluminum, or some other sacrificial material, is
deposited on top of the MQW structure 6 in the case of tunable
VCSEL (FIG. 4D), and on top of the DBR layer 10 in the case of a
tunable filter (FIG. 3C). The polyimide or aluminum structure 30
will act as a sacrificial layer later in the method as described in
detail below. It should be appreciated that it is very important to
accurately control the thickness and lateral dimensions of the
polyimide or aluminum structure 30. This is because the thickness
of this deposit will determine the ultimate length of the air
cavity 8 in the tunable Fabry-Perot device and, hence, the unbiased
resonant wavelength of the device. The lateral dimension of the
polyimide or aluminum deposit 30, on the other hand, determines the
voltage response of the device and the resonance frequency.
[0091] 5. Thereafter, an etch-mask is used to pattern the polyimide
or aluminum deposit 30 so as to leave a circular disk-shaped
deposit defining an outwardly slanted edge 32 on its etched
perimeter (FIGS. 3C and 4D). The size and shape of the etched
deposit 30 is carefully designed and controlled because its outer
surface will determine the length of the top mirror support.
Specifically, the disk of polyimide or aluminum acts like a
"micro-die" which precisely controls the lateral dimensions and
shape of the tunable VCSEL or filter. This precise control of the
lateral dimensions of the tunable VCSEL or filter is unparalleled
by any existing techniques employed in existing MEM tunable VCSEL
or filter fabrication. As alluded to above, later in the process,
the polyimide or aluminum layer 30 will be selectively removed
using a suitable dry-etching technique.
[0092] 6. In the case wherein a Si.sub.3N.sub.4 membrane is used
for top mirror support, a thin layer 36 (FIGS. 3D and 4E) of metal
is first deposited on the exposed top surface of the polyimide or
aluminum deposit to form the top tuning electrode.
[0093] 7. Thereafter, either a thin layer of silicon nitride or a
thin layer of another metal other than aluminum, e.g.,
titanium-tungsten (TiW), generally shown at 37, is deposited over
the entire structure, i.e., over the polyimide or aluminum
sacrificial layer 30 and the remaining structure (FIGS. 3D and 4E).
In the case where layer 37 is not transparent, the center portion
is removed (see FIGS. 3D and 3E, and FIG. 4E).
[0094] 8. A thick layer 38 of metal (such as Al or TiW) or hard
dielectrics (such as silicon nitride) forming a rim (in case of a
membrane type device such as is shown in FIG. 6A) or patches of
support forming arms (in the case of a tether device such as is
shown in FIGS. 6B or 6C) is then selectively deposited on the
periphery of the device where the device membrane or tethers meet
the bottom DBR. The width of annulus or support patches 38 is
selected such that a thick metal rim extends from the top of the
bottom DBR 10, over the sloped edge 32 of the sacrificial layer 30
of polyimide or aluminum, and onto the top of the sacrificial disk
30, as indicated in FIGS. 3E and 4F. This is an important
innovation because the thick metal structure 38 provides robust
support to the thin membrane 37 of silicon nitride or TiW after the
underlying sacrificial layer 30 of polyimide or aluminum has been
removed.
[0095] 9. Using an etch-mask, radially emanating openings 40 (FIG.
6A) then are formed by etching through the transparent metal and
silicon nitride or the thin (TiW) film, 37, down to the underlying
sacrificial disk 30. These openings provide gateways for the
etchants to selectively remove the underlying sacrificial disk
30.
[0096] 10. A circular top DBR stack 12, extending tens of
micrometer in diameter, is then selectively deposited only at the
center 42 (FIGS. 3E, 4G and 6A-6C) of the silicon nitride/metal
membrane or TiW film 37. This selective deposition provides an
extremely good quality DBR, and avoids having to etch the top DBR,
which can be a difficult and costly effort. To the extent that top
DBR stack 12 is to assume a curved configuration in the completed
device (e.g., as shown in FIGS. 1 and 2), an appropriate magnitude
and type of strain is introduced into top DBR stack 12 during
deposition of the top DBR stack, in the manner discussed above.
[0097] 11. Finally, an oxygen plasma is used to selectively remove
the polyimide layer 30 (FIGS. 3F and 4G). This releases the silicon
nitride/metal membrane 37 along with the top DBR 12. To the extent
that top DBR stack 12 is formed with an appropriate magnitude and
type of strain to result in the formation of a curved DBR stack,
the release of silicon nitride/metal membrane 37 permits the top
DBR stack 12 to assume its desired curved configuration. CF.sub.4
plasma is used in the case of selective removal of an aluminum
sacrificial layer 30. Since wet chemistry is not involved, there is
no risk of the released silicon nitride/metal membrane or TiW film,
37, collapsing due to surface tension.
[0098] This completes the fabrication of a MEM tunable filter/VCSEL
in which lasing can be accomplished by photo-pumping with a
separate pump laser having a wavelength that is highly absorptive
within the gain spectrum of the MQWs being used. The techniques
discussed in the present invention to achieve wavelength tuning can
be easily adapted to a current-injected MEM tunable VCSEL as well.
In this situation, intra-cavity electrical interconnections have to
be made to the p-i-n junction in the MQW structure after the first
mirror deposition step, which was explained in step 1 of the
tunable VCSEL fabrication procedure.
[0099] It will be understood that the foregoing detailed
description of preferred embodiments of the invention has been
presented by way of illustration, and not limitation. Various
modifications, variations, changes, adaptations and the like will
occur to those skilled in the art in view of the foregoing
specification. Accordingly, the present invention should be
understood as being limited only by the terms of the claims.
* * * * *