U.S. patent application number 10/624912 was filed with the patent office on 2004-07-15 for electrically tunable fabry-perot structure utilizing a deformable multi-layer mirror and method of making the same.
Invention is credited to Tayebati, Parviz.
Application Number | 20040136076 10/624912 |
Document ID | / |
Family ID | 27357671 |
Filed Date | 2004-07-15 |
United States Patent
Application |
20040136076 |
Kind Code |
A1 |
Tayebati, Parviz |
July 15, 2004 |
Electrically tunable fabry-perot structure utilizing a deformable
multi-layer mirror and method of making the same
Abstract
An electrically tunable Fabry-Perot structure using a deformable
multi-layer mirror construction wherein Ga.sub.1-aAl.sub.aAs, where
a<0.1, is used as the sacrificial layer which may be selectively
removed using a citric acid enchant. The multi-layer mirrors
consist of N and M period of quarter wavelength layers where N and
M are integers, or integers plus 1/2. Further, the mirrors are made
from alternating layers of Ga.sub.1-xAl.sub.xAs, where x>0.96,
and a material selected from the group consisting of either
Ga.sub.1-zAl.sub.zAs, where 0.7>Z>0, or
Ga.sub.1-yAl.sub.yAs/Ga.sub.1-zAl.sub.zAs/Ga.sub.1-yAl.sub.yAs
where 0.7>Z>0 and y>0.5. The Ga.sub.1-xAl.sub.xAs is wet
oxidized by exposing its edge to water in a nitrogen or helium
atmosphere at a temperature of between about 360.degree. C. and
450.degree. C. so as to transform it to AlO.sub.x. The resulting
AlO.sub.x layers abut the sacrificial layer and act as etch stops
during the formation of a cantilever Fabry-Perot structure by
etching of the sacrificial layer.
Inventors: |
Tayebati, Parviz;
(Watertown, MA) |
Correspondence
Address: |
Mark J. Pandiscio
Pandiscio & Pandiscio
470 Totten Pond Road
Waltham
MA
02154
US
|
Family ID: |
27357671 |
Appl. No.: |
10/624912 |
Filed: |
July 22, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10624912 |
Jul 22, 2003 |
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09994572 |
Nov 27, 2001 |
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6597490 |
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09994572 |
Nov 27, 2001 |
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09059877 |
Apr 14, 1998 |
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6324192 |
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09059877 |
Apr 14, 1998 |
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08726050 |
Sep 27, 1996 |
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5739945 |
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60004619 |
Sep 29, 1995 |
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Current U.S.
Class: |
359/578 |
Current CPC
Class: |
G02B 26/001 20130101;
H01S 5/18372 20130101; H01S 5/18366 20130101; H01S 5/18363
20130101; G02B 26/0841 20130101; H01S 5/0614 20130101; G02B 26/002
20130101 |
Class at
Publication: |
359/578 |
International
Class: |
G02B 027/00 |
Claims
What is claimed is:
1. An electrically tunable vertical cavity surface emitting laser
comprising: a laterally-extending base comprising an
optically-transparent semi-conductor material; a first
laterally-extending mirror comprising alternating layers of (i)
said optically-transparent semiconductor material, and (ii) air; a
second laterally-extending mirror comprising alternating layers of
(i) said optically-transparent semiconductor material, and (ii)
air; a laterally-extending layer of multiple quantum well material
defining a laterally-extending P-I-N junction therein; said first
laterally extending mirror being fixedly mounted to said laterally
extending base; said laterally-extending layer of multiple quantum
well material being fixedly mounted to said first laterally
extending mirror; said second laterally extending mirror being
movably mounted to said laterally-extending layer of multiple
quantum well material such that an air gap extends between said
first laterally-extending mirror and said layer of multiple quantum
well material; a first electrode electrically connected to said
first laterally-extending mirror; and a second electrode
electrically connected to said base; whereby when a voltage
difference is applied across said first and second electrodes, the
electrically tunable, vertical cavity, surface emitting laser will
change its lasing wavelength in response to the electrostatically
induced movement of said first and second laterally-extending
mirrors relative to one another.
2. An electrically tunable vertical cavity surface emitting laser
according to claim 1 wherein said second laterally-extending mirror
is mounted to said laterally-extending multiple quantum well
material with a cantilever construction.
3. An electrically tunable vertical cavity surface emitting laser
according to claim 1 wherein said second laterally-extending mirror
is mounted to said laterally-extending multiple quantum well
material with a trampoline construction.
4. An electrically tunable vertical cavity surface emitting laser
comprising: a laterally-extending base comprising an
optically-transparent semiconductor material; a first
laterally-extending mirror comprising alternating layers of (i)
said optically-transparent semiconductor material, and (ii)
Al0.sub.x; a second laterally-extending mirror comprising
alternating layers of (i) said optically-transparent semiconductor
material, and (ii) Al0.sub.x; a laterally-extending layer of GaAlAs
or GaInAs based multiple quantum well material defining a
laterially-extending P-I-N junction therein; said first
laterally-extending mirror being fixedly mounted to said
laterally-extending base; said laterally-extending layer of GaAlAs
or GaInAs based multiple quantum well material being fixedly
mounted to said first laterally-extending mirror; said second
laterally-extending mirror being movably mounted to said
laterally-extending layer such that an air gap extends between said
laterally-extending layer and said second laterally-extending
mirror; a first electrode electrically connected to said first
laterally-extending mirror; and a second electrode electrically
connected to said base; whereby when a voltage difference is
applied across said first and second electrodes, the electrically
tunable vertical cavity surface omitting laser will change its
lasing wavelength in response to the electrostatically induced
movement of said first and second laterally-extending mirrors
relative to one another.
5. An electrically tunable vertical cavity surface emitting laser
according to claim 4 wherein said second laterally-extending mirror
is mounted to said laterally-extending quantum well material with a
cantilever construction.
6. An electrically tunable vertical cavity surface emitting laser
according to claim 4 wherein said second laterally-extending mirror
is mounted to said laterally-extending quantum well material with a
trampoline construction.
7. An electrically tunable vertical cavity surface emitting laser
according to claim 4 wherein said base comprises GaAs, and said
first and second mirrors comprise alternating layers of (i) a
material selected from the group consisting GaAs; GaAlAs; and
Ga.sub.1-yAl.sub.yAs/Ga.sub.1- -zAl.sub.zAs/Ga.sub.1-yAl.sub.yAs
where y>0.5 and where Z<0.7, and (ii) AlO.sub.x.
8. An electrically tunable optical filter comprising: a
laterally-extending base comprising an optically-transparent GaAs
material; a first laterally-extending mirror comprising alternating
layers of (i) a material selected from the group consisting of
GaAs; GaAlAs; and
Ga.sub.1-yAl.sub.yAs/Ga.sub.1-zAl.sub.zAs/Ga.sub.1-yAl.sub.yA- s
where y>0.5 and Z<0.7, and (ii) Al0.sub.x; a second
laterally-extending mirror comprising alternating layers of (i) a
material selected from the group consisting of GaAs; GaAlAs; and
Ga.sub.1-yAl.sub.yAs/Ga.sub.1-zAl.sub.zAs/Ga.sub.1-yAl.sub.yAs
where y>0.5 and Z<0.7, and (ii) Al0.sub.x; said first
laterally-extending mirror being fixedly mounted to said
laterally-extending base; said second laterally-extending mirror
being movably mounted to said first laterally-extending mirror such
that an air gap extends between said first laterally-extending
mirror and said second laterally-extending mirror; a first
electrode electrically connected to said first laterally-extending
mirror; and a second electrode electrically connected to said
second laterally-extending mirror; whereby when a voltage
difference is applied across said first and second electrodes, the
electrically tunable optical filter will change its spectral
response.
9. A method for making an electrically tunable Fabry-Perot
structure, said method comprising the steps of: (1) providing a
GaAs substrate having two epitaxially grown distributed Bragg
reflectors separated by a sacrificial layer on one surface thereof;
(2) lithographically defining craters on said substrate; (3) using
a photoresist as a masking layer, etching the distributed Bragg
reflectors until the sidewalls of the bottom distributed Bragg
reflector are exposed; (4) lithographically defining cantilever
structures in said distributed Bragg reflector/sacrificial layer
structure and metal electrodes at the base of said crater and on
the top distributed Bragg reflector; (5) depositing metal
electrodes on the lithographically defined locations; (6)
lithographically defining a protection layer for use in the
selective removal of portions of the distributed Bragg reflector
layers; and (7) removing selected portions of the distributed Bragg
reflectors and the sacrificial layer by etching, whereby the top
distributed Bragg reflector forms a cantilever relative to the
bottom distributed Bragg reflector and the outer tips of the
distributed Bragg reflectors each define a series of semiconductor
fingers separated by air gaps.
10. A method according to claim 9 wherein said selected portions of
said distributed Bragg reflectors are oxidized prior to the final
etching step such that only said selected portion of said
sacrificial layer is removed by said final etching step.
11. A method according to claim 9 wherein said sacrificial layer is
AlAs and said distributed Bragg reflectors comprise alternating
layers of (1) GaAs and (2) a material selected from the group
consisting of AlAs and GaAlAs.
12. A method according to claim 10 wherein said sacrificial layer
is AlAs and said distributed Bragg reflectors comprise alternating
layers of (1) GaAs and (2) a material selected from the group
consisting of AlAs and GaAlAs.
13. A method according to claim 10 wherein said sacrificial layer
is Ga.sub.1-aAl.sub.aAs where a<0.1, and wherein said
distributed Bragg reflectors comprise alternating layers of (1)
Ga.sub.1-aAl.sub.aAs where x>0.96 and (2) a material selected
from the group consisting of Ga.sub.1-zAl.sub.zAs and
Ga.sub.1-yAl.sub.yAs/Ga.sub.1-zAl.sub.zAs/Ga.sub-
.1-yAl.sub.yAs.
14. A method according to claim 13 wherein 0.7>Z>0 and
y>0.5.
15. A method according to claim 13 wherein said oxidation comprises
exposing the edges said Ga.sub.1-xAl.sub.xAs layers to water at a
temperature between about 360.degree. C. and 450.degree. C. in an
atmosphere selected from the group consisting of nitrogen and
helium.
16. A method according to claim 9 wherein said sacrificial layer is
GaAs and said distributed Bragg reflectors comprise alternating
layers of (1) GaAs and (2) a material selected from the group
consisting of AlAs and GaAlAs.
17. A method according to claim 10 wherein said sacrificial layer
is GaAs and said distributed Bragg reflectors comprise alternating
layers of (1) GaAs and (2) a material selected from the group
consisting of AlAs and GaAlAs.
Description
REFERENCE TO PENDING PRIOR PATENT APPLICATION
[0001] This patent application is a continuation-in-part of pending
prior U.S. patent application Ser. No. 08/726,050, filed Sep. 27,
1996 by Parviz Tayebati for NOVEL MICROELECTROMECHANICAL GAALAS
OPTOELECTRONIC DEVICES, which in turn claimed benefit of
then-pending prior U.S. Provisional Patent Application Serial No.
60/004,619, filed Sep. 29, 1995 by Parviz Tayebati for NOVEL
MICROELECTROMECHANICAL GAALAS OPTOELECTRONIC DEVICES, both of which
documents are hereby incorporated herein by reference.
THE PRESENT INVENTION
[0002] I have developed a novel micromachined GaAlAs/air mirror
technology for optoelectronic applications.
[0003] More particularly, I have developed a series of advanced
micro-mechanical optoelectronic devices based on a novel broad-band
multilayer GaAlAs/air mirror technology (see FIGS. 1A-1D). My new
micro-mirror technology is fabricated by epitaxial growth of
GaAs/GaAlAs structures, followed by highly selective lateral
etching of the high-aluminum-content GaAlAs layers. Because of the
large index difference between the GaAs and air layers (3.5 and 1),
the resulting multilayer GaAlAs/air structure is an extremely
efficient multilayer mirror, with very broad bandwidth. It can be
shown that with only three (3) periods of my mirror structure,
reflectivity of over 99.990% can be achieved with 700 nm
(nanometer) bandwidth. By comparison, over twenty (20) periods of
standard GaAs/AlAs structure are needed to achieve 99.900%
reflectivity, at the cost of limited bandwidth of less than 25
nm.
[0004] My simple but powerful micromachined structure can be
applied to solve a variety of technological problems, and allows
the fabrication of new devices where broadband mirrors are
required.
[0005] A short list of technological breakthroughs resulting from
this concept are as follows:
[0006] Broadly Tunable Fabry-Perot Filters
[0007] A novel tunable Fabry-Perot filter based on my GaAlAs/air
mirror is shown in FIG. 1B. The device consists of top and bottom
GaAlAs/air mirrors and an AlAs cavity spacing which is etched away
to allow formation of a cantilever. Application of an applied field
will change the cavity length and shift the transmission peak of
the Fabry-Perot filter. The broad (700 nm) bandwidth of the mirror
allows a tuning range of over 500 nm. The high reflectivity and
high quality of the mirrors will allow better than 1 angstrom
linewidth.
[0008] However, due to the bending of the cantilever, the
Fabry-Perot interfaces shown in FIG. 1B will not remain parallel,
causing a slight broadening of the linewidth. For many
applications--such as switching--this will not be important. For
other applications where the narrow linewidth is critical, a
"trampoline" structure has been designed and is presented.
[0009] The device can further be integrated with laterally grown
detectors for a wide range of spectroscopic applications such as
environmental monitoring (e.g., toxic gases such as methane or
acetylene, with absorption lines at 1330 nm and 1770 nm), or
biomedical applications such as the measuring of blood sugar levels
requiring spectroscopy near 2100 nm, etc.
[0010] Furthermore, the small size, and the compatibility of the
devices with multimode fibers, is very attractive for such
commercial applications.
[0011] Fixed Wavelength VCSEL's
[0012] A critical parameter in fabricating vertical cavity surface
emitting laser (VCSEL) devices is that the top and bottom mirror
reflectance "peaks", and the laser "exciton peak", must correspond.
Considering the narrow spectral response (25 nm) of GaAs/AlAs, and
the inhomogeneous growth of GaAlAs across a wafer (more than 3%
composition variations resulting in over 30 nm variation), makes
the growth of such devices low yield and costly. My GaAlAs/air
mirror technology shown in FIG. 1C will impact this technology in
two ways:
[0013] (i) The broad bandwidth of the GaAlAs/air mirrors will relax
the restrictive growth conditions since the mirrors are broadband,
allowing perfect overlap with the exciton peak throughout a wafer.
This will allow fabrication of working VCSEL devices with
GaAlAs/air mirrors from any part of the wafer.
[0014] (ii) The higher mirror reflectivities will reduce the laser
threshold conditions by increasing the Q-factor of the laser
cavity.
[0015] Tunable VCSEL's
[0016] The new tunable filter technology shown in FIG. 1B, and the
new VCSEL technology shown in FIG. 1C, can be combined to yield a
new tunable VCSEL technology as shown in FIG. 1D. In this new
tunable VCSEL, the top GaAlAs/air mirror is formed into a
cantilever which can be moved up and down electrostatically to tune
the lasing wavelength. Again, the high reflectivity and the wide
bandwidth of the mirrors will allow the laser to emit continuously
and over a wide wavelength range. The tuning range will, of course,
be limited to the gain bandwidth of the diode, i.e., approximately
50 nm.
[0017] My new tunable filter device consists of a GaAs
micromachined tunable filter chip and can be as small as 500
microns by 500 microns, with a response time of several
microseconds. The cost of this device is low, since thousands of
GaAs chips are mass-manufactured using conventional semiconductor
processing techniques. One of the most important features of my new
device is its improved spectral resolution and bandwidth, due to my
approach for fabrication of very high quality mirrors with minimum
complexity. As a result, the resolution and the bandwidth of this
device is about an order of magnitude better than other
micromachining-based technologies.
[0018] Competing Technologies
[0019] FIG. 2 illustrates the evolution of tunable filters over the
last ten years. Current state-of-the-art filters are typically
hybrid, making it difficult to fabricate them in large volumes.
They generally either rely on the use of piezoelectric drivers with
complex feedback systems (Queensgate Instruments, England), or on
the use of birefringent materials placed between crossed polarizers
(for example, Cambridge Research Instruments, Massachusetts, using
liquid crystals).
[0020] Piezoelectric tunable filters generally have a resolution of
0.1 nm, with 50 nm tuning range (bandwidth).
[0021] Liquid crystal (LC)-based filters can exhibit better
resolutions, but at the expense of very low efficiency, e.g., as
low as 99.0%.
[0022] The fabrication of these hybrid systems is a labor-intensive
process, thus increasing the cost of these devices. For example,
the top-of-the-line model sold by Queensgate Instruments costs
above $10,000. Such high costs make them unrealistic for most
applications. Specifically, in upcoming communications networks, it
is anticipated that all of the information delivered to each
household (500 channel TV, telephone, etc.) will be transmitted
over a fiber optic line using wavelength division multiplexing.
This will necessitate the use of a tunable filter in every home. In
order for such a system to be feasible, the cost of a tunable
filter should be in the tens-of-dollars range, at the most.
[0023] In attempting to adopt micromachined technologies for
fabrication of low cost tunable filters.sup.1,2,3, a number of
approaches have been initiated.
[0024] Larson and et al. built a GaAs-based interferometer.sup.1.
They used a GaAs/AlAs stack as the bottom mirror and a gold-coated
silicon nitride membrane as the top mirror.
[0025] Jerman and et al..sup.2 bonded two different wafers to build
their micromachined membranes. They used dielectric mirrors with
97.5% refractivity at 1.55 mm.sup.2.
[0026] In the work of Reference 3, a silicon nitride membrane is
suspended over a silicon substrate. The device is used as a light
modulator based on the interference effect between the substrate
and the suspended membrane.
[0027] All prior micromachined filter technology tends to suffer
from the limited reflectivity and bandwidth of the cavity
mirrors.
[0028] The bandwidth .DELTA..lambda. of a periodic layered
structure (with indices n.sub.1 and n.sub.2) is given by the well
know formula.sup.4:
.DELTA..lambda./.lambda.=(4/.pi.)
sin.sup.-1((n.sub.2-n.sub.1)/(n.sub.2+n.- sub.1)
[0029] This shows a direct link between the index difference
between layers and the bandwidth.
[0030] Similarly, the peak reflectivity is a function of the number
of layers in the dielectric stack and the index difference between
the layers.
[0031] When used in a tunable Fabry-Perot device (e.g., filter or
VCSEL), both the reflectivity and the bandwidth of the mirrors play
key roles: the reflectivity determines the spectral resolutions of
the Fabry-Perot device and the bandwidth limits the tunability
range. Therefore, it is desirable to have as large of an index
difference as possible in order to achieve highly reflective and
broadband mirrors.
[0032] As mentioned above, a conventional mirror stack consists of
GaAs/AlAs layers with closely matched indexes of refraction (3.5
vs. 3.0). It is therefore difficult to make high quality
Fabry-Perot structures using these layers.
[0033] I substitute the low index AlAs material with the
following:
[0034] (i) Air Gaps: By selectively etching AlAs (or high aluminum
content GaAlAs) layers (using HF or HCl based solutions) from the
original GaAs/AlAs stack, one can achieve a mirror stack consisting
of GaAs/air gaps. As air has an index of refraction of 1, this
results in the highest possible index difference using GaAs
technology. The selective etching region can also be GaAs, in which
case it can be removed by conventional citric acid solutions.
[0035] (ii) oxidized AlAs: AlAs (or high aluminum content GaAlAs)
layers in epitaxially grown GaAlAs materials can be oxidized
laterally using conventional wet oxidation techniques. Oxidized
AlAs has an index of refraction of 1.5, as opposed to 3.0 of
non-oxidized AlAs. This is a significant index difference, and
mirrors made of oxidized AlAs/GaAs stacks approach the quality of
GaAs/air gap stacks in terms of bandwidth and reflectivity.
[0036] FIG. 3 illustrates the number of layers in a dielectric
stack required to achieve a reflectivity approaching 100%. In the
case of GaAs/AlAs, 15 layers are needed to obtain 99% reflectivity,
whereas with GaAs/air mirrors, only 4 layers are needed to achieve
99.999% reflectivity.
[0037] Although oxidized AlAs/GaAs based mirrors are inferior to
GaAs/air gap mirrors, they may have a higher fabrication yield due
to the inherent mechanical stability. However, as shown below,
their fabrication as a whole does require more photolithographic
steps.
[0038] The use of these low index air gap or oxidized AlAs layers
increases the bandwidth of my devices by an order of magnitude
compared to other known devices. Similarly, my devices can achieve
a resolution in the 0.1-0.3 nm range. This is comparable to the
best filters available with the expensive hybrid technology, and
one order of magnitude better than with other micromachining-based
approaches.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIGS. 1A-1D are representations of broad band multi-layer
mirror technology;
[0040] FIG. 2 is a diagram of the evolution of tunable filters over
the last ten years;
[0041] FIG. 3 is a reflectivity diagram of a multi-layer stack
reflector;
[0042] FIG. 4 is a diagram of the frequency spectrum of a typical
Fabry-Perot structure;
[0043] FIGS. 5A-5D are illustrations of the steps to produce a
cantilever-shaped tunable filter;
[0044] FIG. 6 shows the cross-section of a multi-layer device
before oxidation;
[0045] FIG. 7 is a representation of a "trampoline" platform
embodiment of the present invention;
[0046] FIG. 7A shows the cross-section of another multi-layer
device before oxidation;
[0047] FIG. 7B is a top view of the device of FIG. 7A wherein the
direction of oxidation is shown by arrows and the oxidized and
undercut areas are shown in gray;
[0048] FIGS. 8A and 8B are graphs describing the amount of bending
of the reflective device under an applied field; and
[0049] FIG. 9 is a graph of the spectral response of the present
invention.
DETAILED DISCUSSION
[0050] A key to fabrication of the "high aspect" ratio features in
the GaAlAs/air mirror structure shown in FIG. 1A is the high
selective etching of Ga.sub.(1-x)Al.sub.xAs materials as a function
of the composition factor x. HF:H.sub.20 etches
Ga.sub.(1-x)Al.sub.xAs materials with x>0.45 at a rate of over
10.sup.7 times faster than those with x<0.45. This technology
has been the basis of the so-called "epitaxial liftoff" technology
whereby a layer of AlAs grown underneath a GaAlAs device (e.g.,
lasers and detectors) can be etched laterally to allow the liftoff
of the entire structure for follow-up deposition on other
substrates such as glass or silicon. Up to 2 cm (centimeter) films
have been lifted from the GaAs substrate using this technique,
indicating the extreme selectivity of the etching technique. I have
designed a complete procedure which allows using this property for
fabrication of reliable GaAlAs/air micro-mirrors.
[0051] Why GaAs Micromachining?
[0052] I chose the micromachining of GaAs over Si for several
important reasons:
[0053] (i) Use of GaAs technology for the proposed tunable filters
opens up the possibility of integration with other optoelectronic
devices such as laser diodes and detectors which are not possible
with silicon-based micromachining.
[0054] (ii) GaAs/AlAs crystal growth technologies such as Molecular
Beam Epitaxy (MBE) and Metal Oxide Chemical Vapor Deposition
(MOCVD) make it possible to grow layers accurately down to a single
monolayer. This is a key requirement for high quality Fabry-Perot
cavities where small thickness variations can cause large
fluctuations in the frequency response.
[0055] (iii) GaAs/AlGaAs based filters cover a broader wavelength
range than silicon based filters (0.65-1.6 mm vs. 1.1-1.6 mm),
rendering them more versatile devices for many applications.
[0056] (iv) The availability of highly selective etching solutions
(up to 10.sup.7:1 using HF:H.sub.2O) in III-V systems makes it
possible to easily fabricate high aspect ratio devices such as the
present devices. This is a key technology factor in the present
invention.
[0057] (v) Recent studies show that GaAs cantilevers can withstand
forces which approach 10 G's..sup.6 This means that the present
devices can be rugged and suitable for field applications.
Background
[0058] In its simplest form, a Fabry-Perot structure consists of
two parallel, flat, transparent surfaces coated with high
reflectivity layers. The spacing between the surfaces forms the
resonant cavity. When light is incident on such a structure, it is
subject to multiple reflections between the two coated surfaces. At
the resonant wavelengths, these reflected beams interfere
destructively and all of the incident light is transmitted through
the structure.
[0059] The resonant frequencies of the Fabry-Perot structure are
given by the equation:
m.lambda.=2nt
[0060] where m is an integer known as the order of interference, 1
is the wavelength, n is the index of refraction and t is the
thickness of the structure. The spectral response of such a
structure consists of a comb-like series of bandpass peaks (see
FIG. 4). The width of each resonant peak determines the resolution
of the device. The full width of the peaks at the half-maximum
value is given by the equation.sup.4:
.DELTA.v.sub.(1/2)=(c/21)((1-R)(.pi.{square root}R)
[0061] where c is the speed of light, l is the cavity length and R
is the reflectivity of the mirrors.
[0062] A very high reflectivity is desired to achieve high
resolutions. The general equation for the transmission
.tau..sub..DELTA. of a Fabry-Perot device is given by the
equation.sup.7:
.tau..sub..DELTA.(T.sup.2/((1-R).sup.2))[1+(((4R)/(1-R).sup.2)
sin.sup.2)(2.pi.nl cos .theta./.lambda.)]
[0063] where R and T are the reflection and transmission
coefficients, and .theta. is the angle of incidence within the
cavity. A examination of this equation shows that one can change
the transmission wavelength by changing the index of refraction in
the cavity, by tilting the structure with respect to the incident
beam, or by mechanically changing the spacing between the two
surfaces. This last approach previously proved itself to be most
viable due to the availability of high quality piezoelectric
transducers. However, the accuracy with which the plates must be
moved is still very demanding and requires complicated mechanisms
to actively control the plate spacing.
[0064] A more simple approach can be introduced by using tunable
birefringent filters such as Lyot, partial polarizing and Solc
filters..sup.8, 9, 10 These techniques rely on the coupling of
light from the fast to slow axis of birefringent crystals placed
between crossed polarizers. These techniques require the use of at
least one polarizer and an analyzer, therefore reducing the
efficiency of the system by at least 50%. These devices are also
far from being compact. In some cases, they consist of multiple
stages of analyzer/polarizers with birefringent crystals.
[0065] Device Fabrication
[0066] There are two important factors effecting the quality of a
Fabry-Perot structure: the thickness of the Fabry-Perot cavity
which determines the resonant wavelengths, and the properties of
the mirrors which determine the resolution and the bandwidth of the
Fabry-Perot structure.
[0067] GaAs/Air Gap Mirror Based
[0068] Fabry-Perot Structure
[0069] FIG. 5A shows the cross-section of my starting substrate. My
structure consist of two mirror stacks separated by a sacrificial
layer, GaAlAs. The layers for the proposed structure can be grown
with great accuracy using MBE or MOVCD techniques. By selectively
removing the sacrificial layer, the top mirror can be machined into
a cantilever or a platform. In this case, the cantilever or the
platform contains the top distributed Bragg reflector (DBR),
whereas the substrate contains the bottom DBR of the Fabry-Perot
structure. The two mirrors are now separated by an air gap, and
this gap can be changed by applying an electric field to the top
and bottom electrodes of the device which makes it possible to tune
the resonant frequencies of the device.
[0070] The removal of the sacrificial layer is due to the fact that
AlAs (or AlGaAs) etches at a much faster rate than GaAs when it is
exposed to hydrofluoric based solutions. The selectivity can be as
high as 1000:1. As the sacrificial layer and the low index material
of the mirror stack are made of the same material (AlAs or AlGaAs),
they can be selectively removed during the same fabrication step.
The removal of the AlAs layer from the initial mirror stack results
in a new mirror stack which consists of alternating air gap/GaAs
layers. The index difference for air/GaAs stacks is 3.5, and very
large reflectivity can be achieved with very few stacks of air/GaAs
pairs.
[0071] The sequence of the fabrication process for
cantilever-shaped top mirrors is illustrated in FIGS. 5A-5D.
[0072] The starting substrate is a GaAs structure with two
epitaxially grown DBR's separated by an AlAs sacrificial layer. The
sample is then subject to the following fabrication steps:
[0073] 1. Lithography: Definition of "craters" on the
substrate.
[0074] 2. Etching: Using photoresist as the masking layer, the
sample is etched until the sidewalls of the bottom DBR is exposed.
The exposure of sidewalls will make it possible to selectively
remove the AlGaAs layers of the bottom DBR mirror during the
subsequent steps to replace them with air gaps. The etch stop point
is not critical and deep craters can be etched to ensure the
exposure of the sidewalls (FIG. 5B).
[0075] 3. Lithography: Definition of cantilever structures.
[0076] 4. Etching: Using the photoresist mask of the second
photolithography step, the top DBR mirror is removed. In this case,
it is very critical to stop at the sacrificial layer. This is
accomplished by using different selective etching solutions for
each layer of the top DBR mirror until the sacrificial layer has
been exposed. For example, HF and NH.sub.4OH based solutions can be
used to remove AlAs and GaAlAs layers respectively.
[0077] 5. Lithography: To define the metal electrodes which are
required to electrostatically deflect the metal cantilever
structures.
[0078] 6. Metallization: Deposition of metal electrodes (FIG.
5C).
[0079] 7. Lithography: Definition of "protection" layer for
subsequent etching process. The protection layer is required to
selectively remove AlAs layers from the DBR mirrors at the tip of
the cantilever structures.
[0080] 8. Etching: The removal of the sacrificial AlAs layer and
other AlAs layers of the top and bottom DBR mirrors using HCl or HF
solutions. This step completes the fabrication of cantilevers.
These cantilevers consist of GaAs/AlAs layers except for at the tip
of the cantilever structure, where we have GaAs/air gap layers
instead (FIG. 5D). The sacrificial layer can also be GaAs, in which
case citric acid solutions are used to remove the sacrificial
layer.
[0081] GaAs/Oxidized AlAs Based
[0082] Fabry-Perot Structures
[0083] The use of oxidized AlAs layers can provide more mechanical
strength and higher fabrication yields.
[0084] The fabrication sequence is very similar to the one
described in the preceding section. The only difference is that the
AlAs layers in the top and bottom DBR mirrors are subject to an
oxidation process before the sacrificial layer is removed.
[0085] FIG. 6 shows the cross-section of the device before the
oxidation. The most critical issue here is to prevent the
sacrificial AlAs layer from being oxidized as it will be later
removed to achieve the tunable cavity. This can be achieved by
encapsulating the sacrificial AlAs layer with protective layers
from every side during the oxidation process. As shown in FIG. 6,
the GaAs layer on the top of the AlAs sacrificial layer prevents it
from being oxidized in the vertical direction. A second trench
opened around this sacrificial layer and filled with metal will
stop its oxidation in the vertical layer. The opening of the second
trench and its filling with metal can be done with one lithography
mask and does not introduce major complications. The oxidized AlAs
has also a low etch rate against the AlAs etching solution and they
do not get removed during the sacrificial layer etch.
[0086] "Trampoline" Platforms vs. Cantilevers as Mirrors
[0087] Using a cantilever mirror in a Fabry-Perot cavity will
result in departures from parallelism between the top and bottom
mirrors, which will broaden the interferometer passband. To solve
this problem, the top mirror can be fabricated in the shape of a
platform resting on two polyimide beams (FIG. 7). When the platform
is attracted towards the substrate through electrostatic forces,
the polyimide beams will elastically deform, lowering the platform
parallel to the substrate. The optimum polyimide material will
provide good adhesion to the top mirror, durability in the
selective etching solution, and a good enough elasticity to allow
lowering and raising of the top mirror.
[0088] Additional Construction
[0089] FIG. 7A shows the cross-section of a similar device prior to
oxidation, and FIG. 7B shows the directions of oxidation by arrows
and the oxidized and undercut regions in gray. In this embodiment
of the invention, the reference character L designates
quarter-wavelength thick Ga.sub.1-xAl.sub.xAs where x>0.96. This
material will transform, during a wet oxidation process, to what is
commonly referred to in the industry as AlO.sub.x, so as to create
a low index (n=1.5) layer. By a "wet oxidation" process, I mean
exposing the edge of the Ga.sub.1-xAl.sub.xAs layers to water in a
nitrogen or helium atmosphere at a temperature between about
360.degree. C. and about 450.degree. C.
[0090] In this case, the alternate layers of the DBR, designated by
the reference character H, are a quarter-wavelength thick, high
index layer, or a combination of layers with an effective thickness
of one quarter-wavelength. By way of example but not limitation,
these layers may be composed of either (i) Ga.sub.1-zAl.sub.zAs,
with Z typically being, but not limited to, 0.7>Z>0, or (ii)
Ga.sub.1-yAl.sub.yAs/Ga- .sub.1-zAl.sub.zAs/Ga.sub.1-yAl.sub.yAs
with y typically being greater than 0.5. In the latter case, the
addition of the Ga.sub.1-yAl.sub.yAs to the H layers is for the
purpose of buffering the stress induced in the AlO.sub.x layer
during the high temperature oxidation process.
[0091] The sacrificial layer is contemplated to be
Ga.sub.1-aAl.sub.aAs where a<0.1. This material may be
selectively removed using a citric acid enchant.
[0092] During the wet oxidation period, the Ga.sub.1-xAl.sub.xAs
layer is oxidized, but the Ga.sub.1-zAl.sub.zAs layers are not
oxidized. Further, the layers adjacent to the sacrificial
Ga.sub.1-aAl.sub.aAs layer will be AlO.sub.x with an extremely high
selectivity. The top and bottom DBR's consist of N and M period of
quarter-wavelength layers, respectively, where N and M are either
an integer, or an integer plus 1/2, as determined by the narrowest
possible Fabry-Perot linewidth and transmittance.
[0093] In this example, the layers of the top and bottom DBR's have
very broad bandwidths of up to 700 nm and reflectivities of up to
99.9999% using only four or five periods. The theoretical result is
Fabry-Perot linewidths of less than 0.1 nm. To date devices have
been built by this process which achieve a 0.5 nm linewidth with
only four or five periods.
[0094] Important Fabrication Steps
[0095] The following steps are important:
[0096] (i) The precise growth of dielectric mirrors: The
fabrication of high quality Fabry-Perot structures requires the
precise growth capability of each layer. Fortunately, due to the
advances in MBE and MOCVD, these layers can be grown within several
atomic layers.
[0097] (ii) The availability of highly selective etching solutions:
The mirror quality can be improved if the low index material is
replaced by an air gap. This necessitates the use of a highly
selective etching solution which would etch the low index material
but would not attack the high index material. Hydrofluoric acid
based solutions do have a selective etch ratio of 1000:1 for
GaAs/AlAs systems and provide the ideal solution for the task.
[0098] (iii) Undercutting of cantilever structures: The most
important step is the fabrication of GaAs/air gap mirrors. It is
important that GaAs layers do not bend and stick to each other.
GaAs has enough mechanical strength and will not bend under
gravitational forces for the proposed physical dimensions.sup.6.
However, during the fabrication, when AlGaAs layers are selectively
removed, water trapped between the GaAs layers can force them
towards each other due its surface tension. As a result of this
bending, two GaAs surfaces can come in contact and permanently
stick to each other due to strong surface forces.
[0099] Some recent publications show encouraging results as to how
to eliminate this problem.
[0100] Namatsu et al. have recently fabricated silicon micro-walls
which are 24 nm wide and 8000 nm long. They found out that these
silicon micro-walls stick to each other when the distance between
the two lines is less than 160 nm. They showed that it is possible
to calculate the amount of bending force and place the micro-walls
far from each other to prevent their collapsing.
[0101] Although the GaAs layers are placed much further from each
other in my constructions, I have chosen a safer approach to
fabricate them.
[0102] Takeshima et al. has developed a t-butyl alcohol
freeze-drying method to prevent microstructures from sticking to
each other t-butyl alcohol freezes at room temperature. Their
technique is based on transferring the sample from the etching
solution to a t-butyl alcohol solution which is kept above the room
temperature and therefore in liquid form. When the t-butyl alcohol
cools back to the room temperature, solution between the air gaps
freezes to the solid state. Next, the sample is evacuated and the
frozen liquid solution is sublimated in the vacuum. Using this
technique, it is possible to dry samples without introducing
surface tension forces. The technique is very simple to use and it
only requires a hot plate, a mechanical pump and a vacuum jar.
[0103] (iv) Use Of "Trampoline" Platform As Top Mirrors:
Fabrication of platform-shaped top mirrors is not as
straightforward as cantilevers, and it requires the use of a
polyimide layer with optimum properties.
[0104] Modeling Of The Structures
[0105] It is possible to vary the cavity length as much as 100%
with very small voltages. Assuming a parallel plate capacitor, the
force between the cantilever and the substrate is given
by.sup.1:
F.sub.electrostatic=.epsilon..sub.0A(V.sup.2/2d.sup.2)
[0106] where .epsilon..sub.0 is the permittivity in vacuum, A is
the area of the cantilever, V is the applied voltage and d is the
spacing between the top and bottom mirrors.
[0107] For a cantilever, the amount of bending can be given
by.sup.5:
F.sub.bend=(3EI/l.sup.3)x
[0108] where F.sub.bend is the force applied to the tip of the
cantilever, E is the Young's modules, I is inertia, l is the length
of the cantilever and x is the amount of bending.
[0109] The beam will break if the applied force exceeds a certain
value.sup.5:
F.sub.break=(wt.sup.2/6l)(.sigma..sub.F/n)
[0110] where w is the width, l is the length, t is the thickness of
the beam and .sigma..sub.F is the fracture stress.
[0111] Based on the above equations, it is possible to compute the
amount of bending under the applied field (FIGS. 8A and 8B). As it
can be seen from FIG. 8A, a 100 micron long cantilever can be moved
by a micron with only 10 V applied across it. The voltage drops
even further when the cantilever length is increased to 300
microns. It can also be shown from the above equations that the
bending is independent of the cantilever width.
[0112] I have also developed a model for multilayer films to
compute the transmission and the reflectivity through such
structures. FIG. 9 shows the spectral response for the proposed
structure. For a 2.5 pairs of GaAs/air gap pairs in each mirror, a
resolution of 0.3 nm can be achieved. Here the GaAs and air gap
layers are 0.01 micron and 0.0325 micron, respectively. This device
has a bandwidth of 750 nm and its resonant cavity can be tuned from
1 micron to 1.75 mm by changing the separation between the two
mirrors.
[0113] Other Applications
[0114] This invention targets a multi-billion dollar fiber-optic
telecommunications industry. Future fiber lines will largely
utilize wavelength division multiplexing, and a low-cost, high
quality tunable filter will find usage in every household. However,
there are many other areas where the present apparatus or
fabrication techniques will find immediate usage. One such field is
optical spectrometry. A PC-compatible, rugged spectrometer is
crucial in many engineering applications. Other important areas are
tunable surface emitting lasers, and resonant cavity diodes. The
fabricated devices can also be used as optical switches and as
spatial light modulators. These devices will also have application
in many other areas including optical computing, optical neural
nets, robotics, etc.
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* * * * *