Title: Spectrally tunable detector
Abstract: A spectrally tunable optical detector and methods of manufacture therefore are provided. In one illustrative embodiment, the tunable optical detector includes a tunable bandpass filter, a detector and readout electronics, each supported by a different substrate. The substrates are secured relative to one another to form the spectrally tunable optical detector.
Patent Number: 7,015,457 Issued on 03/21/2006 to Cole, et al.
| Inventors:
|
Cole; Barrett E. (Bloomington, MN);
Higashi; Robert E. (Shorewood, MN);
Subramanian; Arunkumar (Plymouth, MN);
Krishnankutty; Subash (North Haven, CT)
|
| Assignee:
|
Honeywell International Inc. (Morristown, NJ)
|
| Appl. No.:
|
100298 |
| Filed:
|
March 18, 2002 |
| Current U.S. Class: |
250/226; 356/519; 359/578; 359/589 |
| Current Intern'l Class: |
H01J 40/14 (20060101) |
| Field of Search: |
250/226,227.23
356/519,505-507,480,451-454
359/578,579,584,589,847,872,857
257/432,433,436
|
References Cited [Referenced By]
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| 5040895 | Aug., 1991 | Laurent et al.
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| 5146465 | Sep., 1992 | Khan et al.
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| 5278435 | Jan., 1994 | Van Hove.
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| 5550373 | Aug., 1996 | Cole et al.
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| 5677538 | Oct., 1997 | Moustakas et al.
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| 5679965 | Oct., 1997 | Schetzina.
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| 5739554 | Apr., 1998 | Edmond et al.
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| 5834331 | Nov., 1998 | Razeghi.
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| 5847397 | Dec., 1998 | Moustakas.
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| 5909280 | Jun., 1999 | Zavracky.
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| 6080988 | Jun., 2000 | Ishizuya et al.
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| 6147756 | Nov., 2000 | Zavracky et al.
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| 6287940 | Sep., 2001 | Cole et al.
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| 6295130 | Sep., 2001 | Sun et al.
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| 6296779 | Oct., 2001 | Clark et al.
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| 6324192 | Nov., 2001 | Tayebati.
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| 6380531 | Apr., 2002 | Sugihwo et al.
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| 6590710 | Jul., 2003 | Hara et al.
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| 2002/0018385 | Feb., 2002 | Flanders et al.
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| 2002/0031155 | Mar., 2002 | Tayebati et al.
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| Foreign Patent Documents |
| 0177918 | Apr., 1986 | EP.
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| 0667548 | Aug., 1995 | EP.
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| 03-252172 | Nov., 1991 | JP.
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| 05-095130 | Apr., 1993 | JP.
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| 07-288334 | Oct., 1995 | JP.
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| 9326049 | Dec., 1993 | WO.
| |
Other References
Yang W. et al., "Black-Illuminated GAN/AIGAN Heterojunction Photodiodes With
High Quantum Efficiency and Low Noise," Applied Physics Letters, vol. 73, No. 8,
Aug. 24, 1998, pp. 1086-1088, XP000777678.
Brown, J. et al., "Visible-Blind UV Digital Camera Based on a 32*32 Array of
GAN/AIGAN P-I-N Photodiodes", MRS Internet Journal of Nitride Semiconductor Research,
vol. 4S1, Sep. 1999, XP000949328 ISSN: 1092-5783.
Sze. "Physics of Semiconductor Devices." pp. 763-765, John Wiley & Sons, N.Y., 1982.
Cole, et al., "Microscopic Spectroscopy of Optical MEMS Devices," Topic 2 (Materials
and Technology), Honeywell Laboratories, 2 page abstract, submitted on or around
Dec. 11, 2000.
U.S. Appl. No. 09/275,632, filed Mar. 24, 1999, entitled "Back Illuminated Heterojunction Photodiode."
Chung, S. W. et al., "Design and fabrication of 10×10 micro-spatial light
modulator array for phase and amplitude modulation," Sensors and Actuators, vol.
78 No. 1, pp. 63-70, Jan. 1999.
Chitica, J., et al., "Monolithic InP-Based Tunable Filter with 10-nm Bandwidth
for Optical Data Interconnects in the 1550-nm Band," IEEE Photonics Technology
Letters, vol. 11, No. 5, pp. 584-586, May 1999.
Jerman, J.H., et al., "A miniature Fabry-Perot interferometer with a corrugated
silicon diaphragm support," Sensors and Actuators, vol. 129, No. 2, pp. 151-158,
Nov. 1991.
Tayebati, P., et al., "Microelectromechanical tunable filter with stable half
symmetric cavity," Electronics Letters, IEE Stevenage, GB, vol., 34, No. 20, pp.
1967-1968, Oct. 1998.
Tayebati, P., et al., "Widely Tunable Fabry-Perot Filters using High Index-contrast
DBRs," Design and Manufacturing of WDM Devices, Dallas, Texas, Nov. 4-5, 1997,
SPIE vol., 3234, pp. 206-218, 1998.
|
Primary Examiner: Porta; David
Assistant Examiner: Yam; Stephen
Attorney, Agent or Firm: Fredrick; Kris T.
Claims
What is claimed is:
1. A spectrally tunable detector, comprising:
a tunable bandpass filter having a first at least partially reflective plate
and a second at least partially reflective plate separated by a separation gap,
the tunable bandpass filter is selectively tuned to a bandpass wavelength that
is less than 390 nm by moving the first plate and/or the second plate relative
to one another to change the separation gap using an electrostatic force; and
a detector positioned adjacent the tunable bandpass filter to receive one or
more wavelengths that are passed by the tunable bandpass filter and to provide
an output signal in response thereto.
2. A spectrally tunable detector according to claim 1 wherein the tunable bandpass
filter and the detector are fixed relative to one another.
3. A spectrally tunable detector according to claim 1 wherein the tunable bandpass
filter is provided on a first substrate and the detector is provided on a second
substrate, the first substrate and the second substrate being substantially transparent
to the one or more wavelengths that are passed by the tunable bandpass filter.
4. A spectrally tunable detector according to claim 3 wherein first substrate
is fixed relative to the second substrate.
5. A spectrally tunable detector according to claim 3 wherein an incoming light
beam enters the tunable bandpass filter and subsequently passes through the first substrate.
6. A spectrally tunable detector according to claim 5 wherein the incoming light
beam subsequently enters the detector through the second substrate.
7. A spectrally tunable detector according to claim 6 wherein the first substrate
has a first major surface and an opposite second major surface, the first plate
and the second plate of the tunable bandpass filter are positioned adjacent the
first surface of the first substrate, and the second surface of the first substrate
is positioned adjacent the second substrate.
8. A spectrally tunable detector according to claim 7 further comprising a read
out circuit provided on a third substrate.
9. A spectrally tunable detector according to claim 8 wherein the detector is
electrically connected to the read out circuit.
10. A spectrally tunable detector according to claim 8 wherein the third substrate
is mounted to a package.
11. A spectrally tunable detector according to claim 3 wherein a lens is positioned
adjacent the first substrate.
12. A spectrally tunable detector, comprising:
a tunable bandpass filter for selectively passing a band of wavelengths from
a predetermined spectral range of wavelengths, the tunable bandpass filter having
an etalon with a top plate and a bottom plate that are separated by a separation
gap, both the top plate and the bottom plate having a reflective portion that is
at least partially reflective, the top plate and/or bottom plate adapted to move
relative to the other plate to vary the separation gap across a predetermined range
of separation gaps, the separation gap at least partially determining the band
of wavelengths that are passed by the tunable bandpass filter and the predetermined
range of separation gaps at least partially determining the predetermined spectral
range of wavelengths; and
a detector positioned adjacent the tunable bandpass filter, the detector being
sensitive to the predetermined spectral range of wavelengths but insensitive to
wavelengths substantially outside of the predetermined spectral range of wavelengths;
wherein a normalized response of the detector is greater than 90% across a frequency
range, and the band of wavelengths passed by the tunable bandpass filter substantially
fills said frequency range.
13. A spectrally tunable detector according to claim 12 wherein the etalon is
a Micro Electro Optical Mechanical System (MEOMS) etalon.
14. A spectrally tunable detector according to claim 13 wherein the bottom plate
is positioned atop a substrate.
15. A spectrally tunable detector according to claim 14 wherein the top plate
is positioned above the substrate, and mechanically connected to the substrate
by one or more support legs.
16. A spectrally Unable detector according to claim 15 further including one
or more top electrodes coupled to the top plate.
17. A spectrally tunable detector according to claim 16 further including one
or more bottom electrodes coupled to the substrate.
18. A spectrally tunable detector according to claim 17 further comprising a
control circuit for applying a voltage between one or more of the top electrodes
end one or more of the bottom electrodes to pull at least part of the top plate
closer to at least part of the bottom plate via an electrostatic force to thereby
reduce the separation gap between the reflective portion of the top plate and the
reflective portion of the bottom plate.
19. A spectrally tunable detector according to claim 18 wherein the substrate
is Pyrex.
20. A spectrally tunable detector according to claim 12 wherein the detector
is an AlGaN diode.
21. A spectrally tunable detector according to claim 12 further comprising a
lens for directing an incoming light beam to the tunable bandpass filter.
22. A spectrally tunable detector according to claim 12 wherein the tunable bandpass
filter is adapted to allow only a single band of wavelengths from the predetermined
spectral range of wavelengths for each separation gap in the predetermined range
of separation gaps.
23. A spectrally tunable detector, comprising:
a tunable bandpass filter having a first at least partially reflective plate
and a second at least partially reflective plate separated by a separation gap,
the tunable bandpass filter is selectively tuned to a bandpass wavelength by moving
the first plate and/or the second plate relative to one another to change the separation gap;
a detector fixed relative to a first substrate, the detector positioned adjacent
the tunable bandpass filter to receive one or more wavelengths that are passed
by the tunable bandpass filter and to provide an output signal in response thereto,
the detector providing and/or receiving at least one electrical signal to/from
a conductive pad that is accessible from a first side of the first substrate;
control electronics fixed relative a second substrate, the control electronics
coupled to a conductive pad that is accessible from a first side of the second
substrate; and
the first side of the first substrate is positioned adjacent to the first side
of the second substrate such that the conductive pad of the detector is electrically
coupled and physically bonded to the conductive pad of the control electronics
to pass the at least one electrical signal between the detector and the control electronics.
24. A spectrally tunable detector according to claim 23 Wherein the tunable bandpass
filter is fixed relative to a third substrate, the tunable bandpass filter having
one or more control signals fir controlling the separation gap, wherein the one
or more control signals are electrically coupled to one or more conductive pads
that are accessible from a first side of the third substrate.
25. A spectrally tunable detector according to claim 24 wherein the first substrate
has a second side opposite to the first side, the second side of the first substrate
having one or more conductive pads, the first side of the third substrate is positioned
adjacent to the second side of the first substrate such that the one or more conductive
pads of the tunable bandpass filter are electrically coupled and physically bonded
to the one or more of the conductive pads of the second side of the first substrate.
Description
FIELD OF THE INVENTION
The present invention relates to tunable detectors, and more specifically, to
spectrally tunable detectors and methods of manufacture therefore.
BACKGROUND OF THE INVENTION
Optical filters are commonly used in a wide variety of applications. For
example, optical filters are used to provide separate optical "channels" in optical
fiber networks. Optical filters are also used to monitor the spectral emission
from the power plants and engines to provide a level of combustion monitoring and
control. Optical filters can also be used in biological particle identification
systems to provide spectral resolution of the fluorescence needed for high levels
of discrimination of biological materials. These are just a few of the many applications
for optical filters.
Many optical filters are formed from thin films that reflect or transmit a narrow
band of wavelengths. In many cases, such filters are constructed from several hundred
layers of stacked narrow band filters, which collectively reflect or transmit a
narrow band of wavelengths. Arrayed waveguide gratings are also commonly used.
A limitation of many of these filters is that they are not wavelength tunable.
That is, the operative wavelength cannot be dynamically changed during operation
to select a different optical wavelength.
SUMMARY OF THE INVENTION
The present invention relates to spectrally tunable optical detectors and methods
of manufacture therefore. In one illustrative embodiment, the tunable optical detector
includes a tunable bandpass filter, a detector, and readout electronics, each supported
by a different substrate. The substrates are secured relative to one another to
form the spectrally tunable optical detector.
The tunable bandpass filter may include a top plate and a bottom plate. Both
the top plate and the bottom plate may be adapted to include a reflective region,
and may be separated by a separation gap to form a Fabry-Perot cavity. When so
provided, the tunable bandpass filter may be selectively tuned to a desired bandpass
wavelength by moving the top plate and/or bottom plate relative to one another
to change the separation gap. This movement can be driven by an electrostatic force.
The range of movement of the top and/or bottom plate can determine the spectral
range of the selected wavelengths. In some embodiments, a lens is positioned adjacent
the tunable bandpass filter to help direct and/or shape the incoming light beam.
In one illustrative embodiment, the top plate is suspended above the bottom plate
by one or more supporting legs and/or posts. One or more top electrodes are mechanically
coupled to the top plate, and one or more bottom electrodes are mechanically coupled
to the bottom plate. The one or more bottom electrodes are preferably in registration
with the one or more top electrodes. When an electric potential is applied between
corresponding top and bottom electrodes, an electrostatic force is generated to
pull the top plate toward the bottom plate, which changes the separation gap of
the Fabry-Perot cavity. In some embodiments, the movement to the top plate is provided
by the temporary deformation of one or more of the supporting legs that suspend
the top plate above the bottom plate.
A detector is preferably disposed adjacent the tunable bandpass filter. The detector
receives the one or more wavelengths that are passed through the tunable bandpass
filter. Preferably, the detector is sensitive to the entire spectral range of wavelengths
that can be selected by the tunable bandpass filter, but this is not required.
In one embodiment, the tunable bandpass filter is supported by a first substrate,
and the detector is supported by a second substrate. The first and second substrates
are preferably substantially transparent to the expected spectral range of wavelengths.
On some embodiments, the first and second substrates are secured together in a
back-to-back fashion. When arranged in this manner, the wavelengths of interest
pass, in sequence, through the tunable bandpass filter, the first substrate, and
the second substrate, before reaching the detector. Alternatively, and in other
embodiments, the first and second substrates are secured together in a front-to-back
fashion. When arranged in this manner, the wavelengths of interest pass, in sequence,
through the first substrate, the bandpass filter, and the second substrate, before
reaching the detector. Other arrangements of the first and second substrates are
also contemplated, including a back-to-front arrangement and a front-to-front arrangement,
as desired.
In some embodiments, readout electronics are provided on a third substrate. The
readout electronics may be electrically connected to one or more electrodes of
the detector through, for example, one or more bump bonds, one or more wire bonds,
a common carrier or package, etc. Alternatively, the readout electronics may be
provided on the first and/or second substrates, if desired.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic cross-sectional side view of an illustrative tunable bandpass
detector in accordance with the present invention;
FIG. 2 is a schematic cross-sectional side view of another illustrative tunable
bandpass detector in accordance with the present invention;
FIG. 3 is a schematic cross-sectional side view of another illustrative tunable
bandpass filter in accordance with the present invention;
FIG. 4 is a layout of an illustrative bandpass filter in accordance with the
present invention;
FIG. 5 is a layout showing a support leg, posts and top and bottom electrodes
of another illustrative bandpass filter in accordance with the present invention;
FIG. 6 is a layout showing a support leg, posts and top and bottom electrodes
of yet another illustrative bandpass filter in accordance with the present invention;
FIG. 7 is a layout showing a support leg, posts and top and bottom electrodes
of another illustrative bandpass filter in accordance with the present invention;
FIG. 8 is a schematic diagraph showing an illustrative control circuit for controlling
a bandpass filter in accordance with the present invention;
FIG. 9 is a graph showing the calculated percent transmission of the tunable
filter of FIG. 3 versus wavelength and gap;
FIG. 10 is a graph showing the calculated normalized response of the tunable
bandpass detector of FIG. 3 versus wavelength;
FIGS. 11A-11F are schematic cross-sectional side views showing an illustrative
method for making a tunable bandpass filter in accordance with the present invention;
FIGS. 12A-12I are schematic cross-sectional side views showing another illustrative
method for making a tunable bandpass filter in accordance with the present invention;
FIGS. 13A-13H are schematic cross-sectional side views showing another illustrative
method for making a tunable bandpass filter in accordance with the present invention;
FIGS. 14A-14K are schematic cross-sectional side views showing yet another
illustrative method for making a tunable bandpass filter in accordance with the
present invention;
FIGS. 15A-15C are perspective views of an illustrative assembly of a tunable
bandpass filter in accordance with the present invention; and
FIG. 16 is a perspective view of another illustrative assembly of a tunable
bandpass filter in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following description should be read with reference to the drawings wherein
like reference numerals indicate like elements throughout the several views. The
detailed description and drawings are presented to show embodiments that are illustrative
of the claimed invention.
FIG. 1 is a schematic cross-sectional side view of an illustrative tunable bandpass
detector
10 in accordance with the present invention. The illustrative tunable
bandpass detector
10 includes a tunable bandpass filter
12, a detector
14 and readout electronics
16, each supported by a different substrate.
For example, the tunable bandpass filter
12 is supported by a first substrate
18, the detector
14 is supported by a second substrate
20,
and the readout electronics
16 are supported by a third substrate
22.
In the illustrative embodiment, the tunable bandpass filter
12 includes
a Micro Electro Optical Mechanical System (MEOMS) etalon. The MEOMS includes a
top plate
24 and a bottom plate
26. The bottom plate
26 may
correspond to the first substrate
18, or other layers provided on the first
substrate
18, as desired. Both the top plate
24 and the bottom plate
26 may be adapted to include a reflective region. In FIG. 1, the top plate
includes a reflective region
28, which may include for example a Distributed
Bragg reflector that includes a semiconductor and/or dielectric mirror stack. Alternatively,
the reflective region
28 may simply include one or more metal layers, such
as an Aluminum layer. It should be recognized that these are only illustrative,
and that the reflective region
28 may be made from any suitable material
or material system that provides the desired reflectivity. Like the top plate,
the bottom plate
26 may include a reflective region
30, which like
above, may be made from any suitable material or material system that provides
the desired reflectivity.
The top plate
24 and the bottom plate
26 are preferably separated
by a separation gap
32 to form a Fabry-Perot cavity. To selectively tune
the tunable bandpass filter
12 to a desired bandpass wavelength, the top
plate is preferably pulled toward the bottom plate
26, which changes the
separation gap
32. The range of movement of the top plate
24 relative
to the bottom plate
26 determines the spectral range of the wavelengths
that can be selected. In some embodiments, a lens
34 is positioned adjacent
the tunable bandpass filter
12 to help direct and/or shape the incoming
light beam.
In a preferred embodiment, the top plate
24 is suspended above the bottom
plate
26 by one or more supporting legs and/or posts
36. In addition,
one or more top electrodes
38 may be mechanically coupled to the top plate
24, and one or more bottom electrodes
40 may be mechanically coupled
to the bottom plate
26. When an electric potential is applied between corresponding
top electrodes
38 and bottom electrodes
40, an electrostatic force
is generated to pull the top plate
24 toward the bottom plate
26.
This changes the separation gap
32 of the Fabry-Perot cavity. In some embodiments,
the electrostatic force causes one or more supporting legs
36 to deform,
which provides the movement of the reflective region
28 of the top plate
24 relative to the bottom plate
26. In a preferred embodiment, the
reflective region
28 is relatively rigid to help prevent curvature across
the reflective region
28 when actuated.
The detector
14 is preferably disposed adjacent the tunable bandpass filter
12, and receives the one or more wavelengths that are passed through the
tunable bandpass filter
12. Preferably, the detector
14 is sensitive
to the entire spectral range of wavelengths that can be passed through the tunable
bandpass filter
12. In an illustrative embodiment, the detector
14
is an AlGaN PIN photodiode, such as described in co-pending commonly assigned U.S.
patent application Ser. No. 09/275,632, to Wei Yang et al., filed Mar. 24, 1999,
and entitled "BACK-ILLUMINATED HETEROJUNCTION PHOTODIODE".
In the illustrative embodiment shown in FIG. 1, the tunable bandpass filter
12
is supported by the first substrate
18, and the detector
14 is supported
by a second substrate
20. The first and second substrates are preferably
substantially transparent to the expected spectral range of wavelengths. The first
substrate can be selected for its transmission properties allowing only the proper
range of wavelengths to be transmitted. In one illustrative embodiment, the first
substrate is Pyrex and the second substrate is sapphire. The first and second substrates
may be secured together in a front-to-back fashion, as shown in FIG. 1. That is,
the front side of the first substrate
18 is provided adjacent to the back
side of the second substrate
20. Bump bonds
44 or the like may be
used to secure the first substrate
18 to the second substrate
20,
and to make any electrical connection therebetween, as desired. A dielectric seal
54 may be provided as shown to protect the tunable bandpass filter
12.
In some embodiments, the dielectric seal
54 provides a vacuum seal. Arranged
in this manner, the wavelengths of interest pass, in sequence, through the first
substrate
18, the bandpass filter
12, and the second substrate
20,
before reaching the detector
14.
Alternatively, and as shown in FIG. 2, the first and second substrates
may be secured together in a back-to-back fashion. That is, the back side of the
first substrate
18 may be secured to the back side of the second substrate
20. Arranged in this manner, the wavelengths of interest may pass, in sequence,
through the tunable bandpass filter
12, the first substrate
18, and
the second substrate
20, before reaching the detector or detectors
14.
Other arrangements of the first and second substrates are also contemplated, including
a back-to-front arrangement and a front-to-front arrangement, as desired.
In some embodiments, readout electronics are provided on a third substrate
22.
The readout electronics are preferably fabricated using conventional integrated
circuit processing techniques. For example, the readout electronics may be fabricated
using a CMOS process on a silicon substrate
22. Metal pads may be provided
to provide electrical connections to the detector
14. In the embodiment
shown in FIG. 1, bump bonds
46 are used to electrically connect one or more
electrodes (usually combinations of each pixel and a common ground terminal) of
the detector
14 to corresponding metal pads of the readout electronics.
The bump bonds may also be used to secure the third substrate
22 relative
to the second substrate
20, as shown. The third substrate may be mounted
to a package
50, if desired. In the illustrative embodiment, bond wires
52 are used to connect selected package pins to the readout electronics
and the electrodes of the tunable bandpass filter
12, as shown.
FIG. 2 is a schematic cross-sectional side view of another illustrative tunable
bandpass detector in accordance with the present invention. The embodiment shown
in FIG. 2 is similar to the embodiment shown in FIG. 1. However, unlike the embodiment
of FIG. 1, the first and second substrates are secured together in a back-to-back
fashion. That is, the back side of the first substrate
18 is secured to
the back side of the second substrate
20. Arranged in this manner, the wavelengths
of interest pass, in sequence, through the tunable bandpass filter
12, the
first substrate
18, and the second substrate
20, before reaching
the detector(s)
14. Another difference is that the detector
14 includes
an array of detectors. Such an array of detectors
14 may be used to capture
an array of pixels to form an image, rather than a single pixel as shown in FIG.
1. While FIGS. 1 and 2 show some illustrative methods to assemble various components
to form a tunable bandpass filter, it should be recognized that any suitable method
may be used, including those further described below.
FIG. 3 is a schematic cross-sectional side view of an illustrative tunable bandpass
filter in accordance with the present invention. The illustrative tunable bandpass
filter
60 includes a top plate
62 and a bottom plate
64. Both
the top plate
62 and the bottom plate
64 may be adapted to include
a reflective region. In the illustrative embodiment, the top plate
62 includes
a reflective region
66, which in the embodiment shown, includes a Distributed
Bragg reflector that has a semiconductor and/or dielectric mirror stack. Likewise,
the bottom plate
64 may include a reflective region
30, which in
the embodiment shown, extends across the entire surface of the bottom plate
64
and may include a Distributed Bragg reflector that has a semiconductor and/or dielectric
mirror stack. Alternatively, the reflective regions
66 and
64 may
simply include one or more metal layers, such as an Aluminum layer. It should be
recognized that these are only illustrative, and that the reflective regions
66
and
64 may be made from any suitable material or material system that provides
the desired reflectivity.
As discussed above, the top plate
62 and the bottom plate
64 are
preferably separated by a separation gap
68 to form a Fabry-Perot cavity.
To selectively tune the tunable bandpass filter
60 to a desired bandpass
wavelength, the top plate
62 is preferably pulled toward the bottom plate
64, which changes the separation gap
68. The range of movement of
the top plate
62 relative to the bottom plate
64 determines the spectral
range of the wavelengths of interest.
As shown in FIG. 3, the top plate
62 is suspended above the bottom plate
64 by one or more supporting legs and/or posts
70. In addition, one
or more top electrodes
72 may be mechanically coupled to the top plate
62,
and one or more bottom electrodes
74 may be mechanically coupled to the
bottom plate
64. The one or more top electrodes
72 are preferably
in registration with the one or more bottom electrodes
74. A dielectric
layer
76 may be provided over the one or more bottom electrodes
74,
and/or a dielectric layer
78 may be provided over the one or more top electrodes
72. These dielectric layers may help protect the top and bottom electrodes
from environmental conditions, and may help prevent a short when the top plate
is fully actuated toward the bottom plate.
When an electric potential is applied between top electrodes
72 and bottom
electrodes
74, an electrostatic force is generated that pulls the reflective
region
66 of the top plate
62 toward the bottom plate
64 to
change the separation gap
68 of the Fabry-Perot cavity. In some embodiments,
the electrostatic force causes at least part of the supporting legs to at least
temporarily deform to provide the necessary movement of the reflective region
66.
FIG. 4 is a layout of an illustrative bandpass filter in accordance with the
present invention. The bottom substrate is not shown. The top plate includes a
reflective region
100, which may include for example a Distributed Bragg
reflector with a semiconductor and/or dielectric mirror stack, one or more metal
layers, or any other material or material system that provides the desired reflectivity.
In one illustrative embodiment, the reflective region
100 includes a Distributed
Bragg reflector that has a number of alternating layers of ZrO
2/SiO
2,
HfO
2/SiO
2, or any other suitable material system. The bottom
plate (not shown) also preferably has a reflective region that is positioned below
the reflective region
100 of the top plate to form a Fabry-Perot cavity therebetween.
In the illustrative embodiment, the reflective region
100 of the top plate
is secured to a top support member
102. The illustrative top support member
102 has a ring that extends around and is secured to the reflective region
100. In the illustrative embodiment, the top support member
102 also
includes four thin supporting legs
106. The thin supporting legs
106
are used to suspend the ring and reflective region
100 above the bottom
plate. In the illustrative embodiment, the thin supporting legs are mechanically
secured to posts
104a-
104d. Posts
104a-
104d
preferably extend upward from the bottom plate and support the top support
member
102. The top support member may be, for example, SiO
2
or any other suitable material or material system.
Each thin supporting leg
106 has an electrode region
108 that
supports a top electrode, as shown. Each top electrode region
108 preferably
has an interconnect line that extends along the corresponding supporting leg to
a corresponding anchor or post
104. Each post
104a-
104d
preferably provides a conductive path that electrical connects the interconnect
lines of the top electrodes to corresponding interconnect lines
110 on the
bottom plate.
In the illustrative embodiment, the interconnect lines
110 on the bottom
plate electrically connect each of the posts
104a-
104d
to a corresponding pad
112a-
112d, respectively.
Rather than connecting the posts to corresponding pads, it is recognized that the
interconnect lines
110 may electrically connect the posts
104a-
104d
to one or more driving circuits, if desired. In addition, it is contemplated
that the interconnect lines may be electrically tied together so that all of the
top electrodes are commonly driven.
Bottom electrodes are preferably positioned below each of the top electrodes.
In the example shown, interconnect lines
120 electrically connect each of
the bottom electrodes to a single pad
114. Thus, in the illustrative embodiment,
all of the bottom electrodes are commonly driven. However, this is not required.
To tune the illustrative bandpass filter to a desired band of wavelengths, an
electrical potential is provided between the bottom electrodes and the top electrodes.
When an electric potential is applied in such a manner, an electrostatic force
is generated that pulls the electrode region
108 of the top plate toward
the bottom plate to change the separation gap of the Fabry-Perot cavity. In some
embodiments, the electrostatic force causes the supporting legs
106 of the
top support plate
102 to deform to provide the necessary movement of the
reflective region
100. Preferably, the top support member
102 is
relatively rigid to help prevent curvature across the reflective region
100
when actuated.
FIG. 5 is a layout showing a support leg
116, posts
128a-
128d
and top and bottom electrodes of another illustrative bandpass filter in accordance
with the present invention. In this illustrative embodiment, support leg
116
is shown with one end attached to the top support member
118 of a top reflective
region, and the other end attached to a bridge portion
124 of a top electrode
120. The illustrative top electrode
120 is "H" shaped with a first
electrode leg portion
122a and a second electrode leg portion
122b
connected by a bridge portion
124. The first electrode leg portion
122a
is suspended above a bottom plate by elongated supporting legs
126a
and
126b, which are connected to posts
128a and
128b, respectively. The second electrode leg portion
122b
is suspended in a similar manner.
When a potential is applied between the first and second electrode leg portions
122a and
122b and a corresponding bottom electrode
130, the elongated supporting legs
126a-
126d deform
at least temporarily down toward the bottom plate
130. Because the supporting
leg
116 is connected to the bridge portion
124, which is situated
at a central location with respect to the first and second electrode leg portions
122a and
122b, the supporting leg
116 may not
substantially deform when providing movement to the top support member
118.
This may help reduce any deformation of the top support member
118 when
the top support member
118 is moving from an upward position toward the
bottom plate.
FIG. 6 is a layout showing a support leg
136, posts and top and bottom
electrodes of yet another illustrative bandpass filter in accordance with the present
invention. In this illustrative embodiment, the top electrode includes a first
electrode portion
132a and a second electrode portion
132b,
which are offset relative to one another as shown. Support leg
136 is shown
with one end attached to the top support member
144 of a top reflective
region, and the other end attached to a bridge portion
134 of a top electrode
132. The bridge portion
134 connects two adjacent ends of the first
electrode portion
132a and the second electrode portion
132b,
as shown.
When a potential is applied between the first and second electrode portions
132a and
132b and a corresponding bottom electrode
138, the elongated supporting legs
140a-
140d deform
at least temporarily down toward the bottom plate. In this embodiment, an intermediate
part of the first and second electrode portions
132a and
132b
preferably snap down, and in some embodiments, actually engage the bottom electrode
138. As more potential is then applied, the first and second electrode portions
132a and
132b may begin to roll down toward the bottom
electrode
138, which lowers the position of the supporting leg
136
and the support member
144. This rolling action may provide greater control
over the movement of the top support member
144 relative to the bottom plate.
FIG. 7 is a layout showing a support leg, posts and top and bottom electrodes
of another illustrative bandpass filter in accordance with the present invention.
FIG. 7 is similar to the embodiment shown in FIG. 6, but has two separate bottom
electrodes
148 and
150. During operation, a relatively high potential
is applied between one of the bottom electrodes, such as electrode
148,
to cause an intermediate portion of the first and second electrode portions
152a
and
152b to snap down, and in some embodiments, to actually engage
the bottom electrode
148. With the first and second electrode portions
152a
and
152b in the snapped down position, the support member
154
is preferably in an upper most position.
Then, smaller potential may be applied between the first and second electrode
portions
152a and
152b and the other bottom electrode
150. This potential may cause the first and second electrode portions
152a
and
152b to begin to roll down toward the bottom electrode
150,
which like above, may cause the supporting leg
154 and support member
156
to move to a lower position. As noted above, this rolling action may provide greater
control over the movement of the top support member
156 relative to the
bottom plate.
FIG. 8 is a schematic diagraph showing an illustrative control circuit for controlling
the bandpass filter of FIG. 4. A microcontroller
160 provides four control
words to a Quad Digital-to-Analog (D/A) converter
162. The Quad D/A converter
162 provides individual analog signals to each of the capacitance sensors
164a-
164d. In one embodiment, the four capacitance
sensors
164a-
164b correspond to the four pairs of top
and bottom electrodes of FIG. 4. Alternatively, separate capacitance sensors may
be provided. The individual analog signals provide the necessary electric potential
to pull the top plate toward the bottom plate by a desired amount to change the
separation gap of the Fabry-Perot cavity. One advantage of providing individual
signals to each of the electrode pairs is to help control the tilt of the top plate.
If tilt is not a concern, a single analog signal may be used to commonly drive
all four electrode pairs of FIG. 4.
Feedback signals may be provided from each of the capacitance sensors
164a-
164b
back to the microcontroller
160 through an Analog-to-Digital (A/D) converter
168. The feedback signals may be used to provide a measure of the capacitance
between each electrode pair of FIG. 4. The measure of capacitance is proportional
to the separation gap between each electrode pair. When so provided, the microcontroller
160 may adjust each of the four control words provided to the Quad D/A converter
162 so that the capacitance between each electrode pair is substantially
equal. This may help reduce and/or control the tilt in the top plate relative to
the bottom plate.
FIG. 9 is a graph showing the calculated percent transmission of the tunable
filter of FIG. 3 alone versus incoming wavelength and separation gap. The separation
gap between the top plate and the bottom plate is shown across the top of the graph.
The wavelength of the incoming light beam is shown across the bottom of the graph.
The percentage of the incoming light that is transmitted through the bandpass filter
is shown along the "y" axis. As can be seen, as the separation gap increases, the
peak wavelength that is transmitted through the bandpass filter also increases.
Thus, the bandpass frequency of the filter can be controlled by simply changing
the separation gap between the top and bottom plates. It is recognized that other
separations of a similar fractional wavelength can produce similar effects.
FIG. 10 is a graph showing the calculated normalized response of the tunable
bandpass detector of FIG. 3 versus wavelength. The wavelength of the incoming light
is shown along the "X" axis, and the normalized response is along the "Y" axis.
A first curve
200 shows the normalized response versus wavelength for a
separation gap of 320 nm. Likewise, a second curve
202 shows the normalized
response versus wavelength for a separation gap of 376 nm. The range of movement
of the top and/or bottom plate determines the spectral range of the wavelengths
of interest. In the example shown, the top and/or bottom plate can be moved between
a separation gap of 320 nm to 376 nm. This produces a spectral range of the bandpass
filter from about 320 nm to about 355 nm.
Preferably, the response of the detector and transmission of the substrate
is set to encompass the entire expected spectral range of bandpass filter. Curve
204 shows such a spectral range. Curve
204 encompasses the entire
spectral range from about 320 nm to about 355 nm of the bandpass filter.
A number of illustrative methods are contemplated for forming a tunable bandpass
filter in accordance with the present invention. FIGS. 11A-11F are schematic cross-sectional
side views showing one such illustrative method. Turning to FIG. 11A, a first substrate
200 and a second substrate
202 are provided. The first substrate
200 is preferably a silicon wafer or some other suitable material. The second
substrate
202 is preferably a silica substrate, glass, Pyrex, sapphire or
some other suitable material. The second substrate
202 is preferably relatively
optically transparent to the desired wavelength of interest (such as UV).
Turning again to FIG. 11A, an etch stop layer
204 is provided on the
first substrate
200. The etch stop layer may be any type of etch stop layer,
but in the illustrative embodiment, is preferably molybdenum. Molybdenum is preferred
because it can be easily removed, such as with hydrogen peroxide, to separate the
first substrate from the remaining structure, as further described below. Next
a support layer
206 is provided. The support layer is preferably polysilicon,
but any suitable material will do. A buffer layer
208 may be provided if
desire to help bond the mirror region to the polysilicon support layer
206,
as further discussed below.
Next, a top mirror
210 is provided and patterned. The top mirror is
preferably a Distributed Bragg reflector that includes a semiconductor and/or dielectric
mirror stack. The Distributed Bragg reflector may include, for example, a number
of alternating layers of ZrO
2/SiO
2, HfO
2/SiO
2,
etc. Alternatively, the top mirror may simply include one or more metal layers,
such as an Aluminum layer. It should be recognized that these are only illustrative,
and that the top mirror may be made from any suitable material or material system
that provides the desired reflectivity.
Once patterned as shown, upper electrodes
212 are provided and patterned.
The upper electrodes
212 are preferably metal, such as aluminum, copper
or some other suitable conductor. Conductive pads
214 are then provided,
as shown. Finally, a layer of polyimide
216 is provided over the top mirror
210, upper electrodes
212 and conductive pads
214, as shown.
(OK w/o deletion).
A bottom mirror
218 is provided and patterned on the second substrate
202,
as shown. The bottom mirror is preferably a Distributed Bragg reflector that includes
a semiconductor and/or dielectric mirror stack. Alternatively the bottom mirror
may not be patterned. Like the top mirror
210, the Distributed Bragg reflector
may include, for example, a number of alternating layers of ZrO
2/SiO
2,
HfO
2/SiO
2, etc. Alternatively, the top mirror may simply
include one or more metal layers, such as one or more Aluminum layers. It should
be recognized that these are only illustrative, and that the top mirror may be
made from any suitable material or material system that provides the desired reflectivity.
In some embodiments, the bottom mirror
218 is not patterned, and is left
to cover the entire surface of the second substrate
202.
Bottom electrodes
222 and bottom pads
220 are then provided
and patterned. Bottom electrodes
222 are preferably arranged to be in registration
with the upper electrodes
212. Likewise, bottom pads
220 are preferably
arranged to be in registration with the upper conductive pads
214. Bottom
conductive pads
226 are preferably provided on top of bottom pads
220,
as shown. Bottom conductive pads
226 and top conductive pads
214
are preferably sized to provide the desired separation between the top mirror
210
and the bottom mirror
218.
The bottom conductive pads
226 and top conductive pads
214 are
preferably formed using conventional metal film processing techniques. Since metal
film processing techniques are typically accurate to Angstrom like thickness over
short distances, the desired separation gap may be achieved across the structure.
Standoffs
230 may be provided to help prevent the top mirror
210
from engaging the bottom mirror
218 during actuation of the bandpass filter,
as further described below.
A first layer
232 of polyimide is then provided. The first layer
232
of polyimide is heated and hard cured. A second layer of polyimide
234 is
also provided. Like the layer of polyimide
216 discussed above, the second
layer of polyimide
234 is preferably only soft cured.
Next, the first substrate
200 is preferably brought into engagement
with the second substrate
202, as indicated by arrow
240. The result
is shown in FIG. 11B. This step uses polyimide adhesion. Because the polyimide
layers
216 and
234 are only soft cured, they remain deformable. Preferably,
the two substrates are assembled in a wafer bonding process where heat, pressure
and vacuum are applied. The vacuum helps remove trapped constituents. The pressure
is used to force the two substrates together. The heat (e.g. to 400 degrees C.)
hard cures the polyimide to form a fused substrate sandwich.
Next, and as shown in FIG. 11C, holes are etched through the first substrate
200, preferably down to the etch stop layer
204. Next, the etch stop
layer
204 is removed to release the first substrate
200 from the
structure. When the etch stop layer
204 is molybdenum, a hydrogen peroxide
solution can be used to remove the etch stop layer and release the first substrate.
Next, and as shown in FIG. 11D, holes
240 are etched through the polysilicon
layer, the buffer layer
208, the upper electrodes
212, and into the
upper conductive pads
214. Also, a window
244 is etched through the
polysilicon layer and the buffer layer
208 to expose the top mirror
210.
Next, and as shown in FIG. 11E, metal is deposited into the etched holes
240
to provide plugs
250 that make electrical contact to both the upper electrodes
212 and the conductive pads
214. Besides providing an electrical
connection, the plugs
250 also help pin the polysilicon support layer
206
to the conductive pads
214. A final dry etch (e.g. an oxygen plasma etch)
is used to removes the polyimide sacrificial layers
216,
234 and
232 to release the top structure from the bottom structure, as shown in
FIG. 11F.
FIGS. 12A-12I are schematic cross-sectional side views showing yet another
illustrative method for making a tunable bandpass filter in accordance with the
present invention. Turning first to FIG. 12A, a bottom mirror
300 is grown
on a substrate
302. The bottom mirror
300 is preferably a Distributed
Bragg reflector that includes a semiconductor and/or dielectric mirror stack. The
Distributed Bragg reflector may include, for example, a number of alternating layers
of ZrO
2/SiO
2, HfO
2/SiO
2, etc. Alternatively,
the bottom mirror may simply include one or more metal layers, such as one or more
Aluminum layers. It should be recognized that these are only illustrative, and
that the bottom mirror
300 may be made from any suitable material or material
system that provides the desired reflectivity.
Next, and as shown in FIG. 12B, bottom electrodes
304 and bottom conducting
pads
306 are provided. A dielectric or other protecting layer
310
is then provided over the bottom electrodes
304 and bottom conducting pads
306. The dielectric or other protecting layer
310 is then patterned
to expose the bottom conducting pads
306, as shown.
Next, and as shown in FIG. 12C, a sacrificial layer
312 is provided.
The sacrificial layer
312 is preferably polyimide, but may be any suitable
material. Next, and as shown in FIG. 12D, a top mirror
320 is provided.
The top mirror
320 is preferably a Distributed Bragg reflector that includes
a semiconductor and/or dielectric mirror stack. Like the bottom mirror
300,
the Distributed Bragg reflector may include, for example, a number of alternating
layers of ZrO
2/SiO
2, HfO
2/SiO
2, etc.
Alternatively, the top mirror may simply include one or more metal layers, such
as one or more Aluminum layers. It should be recognized that these are only illustrative,
and that the top mirror may be made from any suitable material or material system
that provides the desired reflectivity. The top mirror
320 is then patterned,
as shown in FIG. 12E.
Next, and as shown in FIG. 12F, holes
324 are etched through the polyimide
layer
312 down to the conductive pads
306. Next, a metal layer is
deposited and pattered to form top electrode regions
330. The metal extends
into holes
324 to form an electrical connection with bottom conducting pads
306, as shown.
Next, and as shown in FIG. 12G, a support layer
340 is provided over
the top surface of the structure. The support layer preferably bonds to the top
mirror
320, and fills the holes
324. A buffer layer may be provided
first to help bond the layers, if desired. In a preferred embodiment, the support
layer
340 is Si
02.
Next, the support layer
340 is patterned to expose the top mirror
320.
Preferably the support layer
340 overlaps the outer perimeter of the top
mirror
320, as shown. This overlap helps form a bond between the support
layer
340 and the top mirror
320. Finally, and as shown in FIG. 12I,
a dry etch is used to remove the polyimide sacrificial layer
312 to releases
the top structure from the bottom structure, as shown. The dry etch is preferably
an oxygen plasma etch. Note, the dielectric or protective layer
310 may
help prevent an electrical short between the top electrodes
330 and the
bottom electrodes
304 if they are drawn together under electrostatic actuation.
An anneal may be performed to help reduce the stress in the structure, including
the SiO
2 support layer
340. The anneal can be per
