U.S. patent application number 14/587068 was filed with the patent office on 2016-06-30 for photodetector focal plane array systems and methods.
This patent application is currently assigned to THE UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE. The applicant listed for this patent is Kenneth W. ALLEN, JR., Vasily N. ASTRATOV, Joshua M. DURAN, Nicholaos I. LIMBEROPOULOS, Augustine URBAS. Invention is credited to Kenneth W. ALLEN, JR., Vasily N. ASTRATOV, Joshua M. DURAN, Nicholaos I. LIMBEROPOULOS, Augustine URBAS.
Application Number | 20160190194 14/587068 |
Document ID | / |
Family ID | 56083233 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160190194 |
Kind Code |
A1 |
ASTRATOV; Vasily N. ; et
al. |
June 30, 2016 |
PHOTODETECTOR FOCAL PLANE ARRAY SYSTEMS AND METHODS
Abstract
A photodetector focal plane array system, comprising: a
substrate comprising a plurality of photosensitive regions; and a
microcomponent disposed adjacent to each of the plurality of
photosensitive regions operable for receiving incident radiation
and directing a photonic nanojet into the associated photosensitive
region. Optionally, each of the microcomponents comprises one of a
microsphere and a microcylinder. Each of the microcomponents has a
diameter of between .about..lamda. and .about.100.lamda., where
.lamda. is the wavelength of the incident radiation. Each of the
microcomponents is manufactured from a dielectric or semiconductor
material. Each of the microcomponents has an index of refraction of
between .about.1.4 and .about.3.5. Optionally, high-index
components can be embedded in a lower index material. The
microcomponents form an array of microcomponents disposed adjacent
to the substrate.
Inventors: |
ASTRATOV; Vasily N.;
(Charlotte, NC) ; ALLEN, JR.; Kenneth W.; (Shelby,
NC) ; LIMBEROPOULOS; Nicholaos I.; (Beavercreek,
OH) ; URBAS; Augustine; (Cincinnati, OH) ;
DURAN; Joshua M.; (Charlotte, NC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASTRATOV; Vasily N.
ALLEN, JR.; Kenneth W.
LIMBEROPOULOS; Nicholaos I.
URBAS; Augustine
DURAN; Joshua M. |
Charlotte
Shelby
Beavercreek
Cincinnati
Charlotte |
NC
NC
OH
OH
NC |
US
US
US
US
US |
|
|
Assignee: |
THE UNIVERSITY OF NORTH CAROLINA AT
CHARLOTTE
Charlotte
NC
|
Family ID: |
56083233 |
Appl. No.: |
14/587068 |
Filed: |
December 31, 2014 |
Current U.S.
Class: |
257/432 ;
438/69 |
Current CPC
Class: |
G02B 13/0085 20130101;
H01L 27/14627 20130101; H01L 27/14685 20130101; H01L 27/14649
20130101; H01L 27/14625 20130101 |
International
Class: |
H01L 27/146 20060101
H01L027/146 |
Claims
1. A photodetector focal plane array system, comprising: a
substrate comprising a plurality of photosensitive regions; and a
microcomponent disposed adjacent to each of the plurality of
photosensitive regions operable for receiving incident radiation
and directing a photonic nanojet into the associated photosensitive
region; wherein each of the microcomponents has a width that is
larger than a width of a mesa of the associated photosensitive
region and is centered with the mesa of the associated
photosensitive region.
2. The photodetector focal plane array system of claim 1, wherein
the plurality of photosensitive regions are disposed in a
photosensitive layer of the substrate.
3. The photodetector focal plane array system of claim 1, wherein
each of the microcomponents comprises one of a microsphere and a
microcylinder, and wherein each of the microcomponents has a
diameter that is larger than a diameter of the mesa of the
associated photosensitive region.
4. The photodetector focal plane array system of claim 1, wherein
each of the microcomponents has a diameter of between about .lamda.
and about 100.lamda., wherein .lamda. is the wavelength of the
incident radiation.
5. The photodetector focal plane array system of claim 1, wherein
each of the microcomponents is manufactured from one of a
dielectric material and a semiconductor material.
6. The photodetector focal plane array system of claim 5, wherein
the dielectric material comprises one or more of barium titanate
glass, titanium dioxide, sapphire, ruby, polystyrene, soda-lime
glass, silica, borosilicate glass, calcium fluoride, and magnesium
fluoride.
7. The photodetector focal plane array system of claim 1, wherein
each of the microcomponents has an index of refraction of between
about 1.4 and about 3.5.
8. The photodetector focal plane array system of claim 5, wherein
the semiconductor material comprises one or more of silicon,
germanium, and GaAs.
9. The photodetector focal plane array of claim 1, wherein the
microcomponents are assembled with the substrate using one or more
of the following techniques: i) self-assembly under wet conditions
due to meniscus forces, ii) the use of a patterned substrate, iii)
the use of an electric field, iv) the use of shear force, v) the
use of conventional or optoelectronic tweezers, vi) the use of a
magnetic field, vii) self-assembly under dry conditions, viii) the
use of vacuum tweezers, ix) the use of capillary grippers, and x)
the use of suction arrays.
10. The photodetector focal plane array of claim 1, wherein the
microcomponents are affixed to the substrate using one or more of a
glue, an epoxy, a polymeric material, a photocurable material, and
partial or complete melting.
11. The photodetector focal plane array of claim 1, wherein the
microcomponents form an array of microcomponents disposed adjacent
to the substrate.
12. A photodetector focal plane array method, comprising: providing
a substrate comprising a plurality of photosensitive regions; and
disposing a microcomponent adjacent to each of the plurality of
photosensitive regions operable for receiving incident radiation
and directing a photonic nanojet into the associated photosensitive
region; wherein each of the microcomponents has a width that is
larger than a width of a mesa of the associated photosensitive
region and is centered with the mesa of the associated
photosensitive region.
13. The photodetector focal plane array method of claim 12, wherein
the plurality of photosensitive regions are disposed in a
photosensitive layer of the substrate.
14. The photodetector focal plane array method of claim 12, wherein
each of the microcomponents comprises one of a microsphere and a
microcylinder, and wherein each of the microcomponents has a
diameter that is larger than a diameter of the mesa of the
associated photosensitive region.
15. The photodetector focal plane array method of claim 12, wherein
each of the microcomponents has a diameter of between about .lamda.
and about 100.lamda., wherein .lamda. is the wavelength of the
incident radiation.
16. The photodetector focal plane array method of claim 12, wherein
each of the microcomponents is manufactured from one of a
dielectric material and a semiconductor material.
17. The photodetector focal plane array method of claim 15, wherein
the dielectric material comprises one or more of barium titanate
glass, titanium dioxide, sapphire, ruby, polystyrene, soda-lime
glass, silica, borosilicate glass, calcium fluoride, and magnesium
fluoride.
18. The photodetector focal plane array method of claim 12, wherein
each of the microcomponents has an index of refraction of between
about 1.4 and about 3.5.
19. The photodetector focal plane array method of claim 16, wherein
the semiconductor material comprises one or more of silicon,
germanium, and GaAs.
20. The photodetector focal plane array method of claim 12, further
comprising assembling the microcomponents with the substrate using
one or more of the following techniques: i) self-assembly under wet
conditions due to meniscus forces, ii) the use of a patterned
substrate, iii) the use of an electric field, iv) the use of shear
force, v) the use of conventional or optoelectronic tweezers, vi)
the use of a magnetic field, vii) self-assembly under dry
conditions, viii) the use of vacuum tweezers, ix) the use of
capillary grippers, and x) the use of suction arrays.
21. The photodetector focal plane array method of claim 12, further
comprising affixing the microcomponents to the substrate using one
or more of a glue, an epoxy, a polymeric material, a photocurable
material, and partial or complete melting.
22. The photodetector focal plane array method of claim 12, wherein
the microcomponents form an array of microcomponents disposed
adjacent to the substrate.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to imaging systems
and methods, such as military and civil infrared (IR) imaging
systems and methods and the like. More specifically, the present
invention relates to photodetector focal plane array (FPA) systems
and methods for use with such imaging systems and methods.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to FPAs. FPAs are
widely used in military and civil IR imaging systems and the like,
such as systems for guidance and control, target acquisition,
surveillance, laser range-finding, fiber-optic and free-space
communications, thermal imaging, and other applications. More
specifically, the present invention addresses the problem of
designing FPAs that are capable of detecting weak optical images
with a sufficiently large angle-of-view (AOV).
[0003] For IR applications, the photosensitive material of FPAs is
typically fabricated from narrow-band gap semiconductors, such as
Hg.sub.1-xCd.sub.xTe (mercury-cadmium-telluride (MCT)), or from
intersubband-absorbing layered quantum structures, such as
strained-layer superlattices or quantum dots. The pixels of such
FPAs are represented by a semiconductor pin structure operating in
a photovoltaic or photoconductive mode. Referring to FIG. 1(a), the
structure of the individual photodetector (or a "pixel") of the
FPAs 10 includes a substrate 12, contact layers formed by highly
doped semiconductor regions 13, ohmic (metallic) contacts 14,
barrier layers 15, and a photosensitive layer 16. The structure of
the individual photodetector can also contain additional layers,
such as antireflection coatings, etc. As shown in FIG. 1(b), the
individual pixels are fabricated as a two-dimensional array in the
focal plane of the imaging system forming so-called "focal plane
array" 10. As illustrated in FIG. 1(c), the metallic electrodes 20
connecting each pixel with an electronic circuitry are a part of
the design of such FPA assemblies 10. The location of the array of
dielectric microspheres 30 of the present invention, described in
greater detail herein below, is also illustrated, but is not part
of the conventional FPA assembly 10. The dielectric microsphere 30
is positioned just above the photodetector mesa allowing efficient
coupling of light into the photosensitive layer.
[0004] The layered quantum structures are represented by quantum
well (QWIP), quantum dot (QDIP), or strained-layer superlattice
(SLSIP) IR photodetectors, which are usually fabricated on GaAs or
InP substrates. The electronic processing circuitry can be created
by using wire-bonding or by using flip-chip bonding with a
silicon-based readout integrated circuit (IC).
[0005] It is important to note that one of the main trends in FPA
design is the reduction of pixel size. Smaller pixels provide
better resolution and increased spatial sampling. The reduction of
pixel size also results in reduced FPA dark current. In addition,
the frequency response of the individual pixels can be enhanced due
to reduced capacitance. Pixel sizes in FPAs used in mid-wave IR
(MWIR, .lamda.=3-5 .mu.m) and long-wave IR (LWIR, .lamda.=8-12
.mu.m) systems are now being reduced below 20 .mu.m, approaching
the diffraction limit of conventional imaging optics.
[0006] The reduction of pixel sizes and the need to allocate a
significant fraction of the FPA area for electronic processing
circuitry reduces the sensitivity of the arrays. Such arrays can be
characterized by the pixels' area fill factor, which can be on the
order of few percent in QWIPs or QDIPs.
[0007] The sensitivity of individual pixels can in principle be
enhanced by increasing their absorbance. This can be achieved by
structuring the surface of the pixels or by fabricating plasmonic
nanogratings, for example. It has been demonstrated that the
photoresponse of QWIP and QDIP structures can be enhanced through
the use of metal nanoparticles, antennas, and metal gratings
fabricated on the surface of the devices. The idea behind these
designs is to couple incident light to surface plasmons bound to
the metal/semiconductor interface, leading to enhanced responsivity
and detectivity. Recently, a notable advance in this field has been
made based on the use of substrate-side illumination, as opposed to
air-side illumination.
[0008] It should be noted, however, that all of these designs do
not address the problem related to the pixels' area fill factor
being limited at the level of a few percent, which leads to
relatively inefficient collection of light in conventional QWIP,
QDIP, and SLSIP structures.
[0009] It should also be noted that a standard solution of the
problem of increasing collection of light by FPAs is based on using
microlens arrays fabricated by standard fabrication technologies
such as photolithography followed by etching or by other methods.
Speaking about the light collection only, microlens arrays provide
very high light collection efficiency, something on the order of
70-80% depending on the fine features of the design. However, this
advantage of such commercial off-the-shelf (COTS) microlenses
should be considered along with their AOV for developing practical
imaging devices. The AOVs of the COTS microlenses are too small for
developing practical mid-IR imagers. The angle of view (a) can be
calculated from the chosen diameter of the photosensitive area
(pixel diameter) (d), and effective focal length (f) as
follows:
.alpha. - 2 arctan d 2 f , ( 1 ) ##EQU00001##
For d=30 .mu.m photodetector mesas, assuming f=200 .mu.m, one can
estimated AOV=8.6 deg. For d=12 .mu.m mesas, AOV=3.4 deg. Such AOVs
are too small for most of mid-IR imaging applications.
[0010] The way of increasing AOV is connected with decreasing the
effective focal length f. However, COTS microlenses are limited in
this regard because of their limited refractive index contrast and
their dome shape. The present invention addresses this and other
issues suggesting a solution of the problem of high efficiency of
collection of light combined with large AOV.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention is devoted to new FPA systems and
methods providing enhanced sensitivity, reduced dark current,
increased speed, and improved angular characteristics. The proposed
systems and methods are based on the assembly of an array of
dielectric microspheres at the top of the FPA in such a way that
individual microspheres are positioned above the photosensitive
regions of the FPA. These regions can be represented by pin
junctions containing quantum wells, quantum dots, strained-layer
superlattices or other materials with light absorption properties
in the desired spectral range. Dielectric microspheres provide
strong concentration of electromagnetic power, sometimes termed the
"photonic nanojet" effect, directly into the photosensitive regions
of the FPA. This is provided through the mesas fabricated at the
surface of FPA. This leads to improved efficiency in the collection
of light in such structures. The subwavelength width of the
photonic nanojets allows using mesas with wavelength-scale
dimensions, which results in reduced dark current and increased
frequency response of the FPAs. The parameters of the microspheres
are optimized for a given FPA to achieve the best focusing
properties at the optimal depth inside the structure. The typical
values of the index of refraction (n) and the diameter (D) of the
microspheres are within 1.4<n<2.0 and
2.lamda.<D<100.lamda. ranges, by way of example only. The
light collection efficiency is improved due to the fact that the
sphere diameter, D, can be much larger than the pixel diameter, d.
AOV is improved due to the fact that microspheres with spherical
shape have stronger focusing capability compared to dome-shaped
COTs microlenses. Another important factor is that the microspheres
can be easily fabricated using relatively high-index materials
(n>1.6). Such microspheres are particularly efficient for
reducing f. In fact, the spheres with n.about.1.8 focuses the
collimated beam close to the back (not illuminated) surface of the
sphere which means that the condition f.about.D/2 can be approached
in such structures. Simple estimation based on Eq. (1) shows that
d=12 .mu.m mesas integrated with D=50 .mu.m spheres should possess
AOV=27 deg which is significantly larger angle compared to AOV
provided by COT microlenses. Two times larger spheres with D=100
.mu.m should still have sufficiently large AOV=13.7 deg. In
addition to AOV advantage, such spheres would also provide
significantly higher efficiency of collection of light compared to
the same structures without spheres. The efficiency advantage can
be estimated as (1-k).sup.2.times.(D/d).sup.2, where k is the total
amplitude reflection coefficient of the microspherical surface. The
efficiency advantage on the order of 50-60 can be obtained for d=12
.mu.m mesas integrated with D=100 .mu.m spheres. In the proposed
designs there is a trade-off between the light collection
efficiency advantage over bare structures (no spheres) and AOV
advantage over structures integrated with COT microlenses.
Generally speaking, larger spheres favor higher light collection
efficiencies by the expense of AOV. However, in terms of the
parameters required for imaging applications, the proposed
structures over perform bare structures and structures equipped
with COT microlenses. The positioning of a large number of
microspheres can be performed by various self-assembly and
micro-manipulation techniques. After that, the microspheres are
fixed using glues, epoxies, or, more generally, materials with the
ability to solidify, photocurable materials, temperature-curable
materials, etc., or by other such techniques. In particular, a
deliberate temperature treatment can be used to slightly melt of
soften the material of the spheres or material of the adjacent
layers to fix the spheres exactly above the detector mesas.
[0012] Similar mechanisms of the enhancement of light collection
efficiency and improvement of AOV can be realized by using a
microcylindrical lens arrays assembled at the top of the
photodetector arrays. In this case, the focusing is provided in
only one direction perpendicular to the microcylinder axis. This
means that the enhancement of light collection can be smaller than
that for spheres, but it can still be significantly improved
compared to that in bare photodetector arrays. The advantage of
microcylindrical arrays is that they can be obtained from
microfibers with relatively well reproducible diameters. In fact,
standard single-mode telecom fibers with extremely well preserved
diameter 125 .mu.m can be used for this purpose. Another advantage
of such structures is connected with their potentially simple
manufacturability (close-packed array of microfibers) and their
extremely large area fill factor which can reach unity and exceeds
the area fill factor for close-packed arrays of microspheres.
[0013] In one exemplary embodiment, the present invention provides
a photodetector focal plane array system, comprising: a substrate
comprising a plurality of photosensitive regions; and a
microcomponent disposed adjacent to each of the plurality of
photosensitive regions operable for receiving incident radiation
and directing a photonic nanojet into the associated photosensitive
region. Optionally, the plurality of photosensitive regions are
disposed in a photosensitive layer of the substrate. Optionally,
each of the microcomponents comprises one of a microsphere or a
microcylinder. Each of the microcomponents has a diameter of
between 2.lamda.<D<100.lamda., so that in mid-IR range of
operation the typical diameters are between 8 and 400 micron. Each
of the microcomponents is manufactured from a dielectric or
semiconductor material. Optionally, the dielectric material
comprises one of barium titanate glass, titanium dioxide, sapphire,
ruby, polystyrene, soda-lime glass, silica, borosilicate glass,
calcium fluoride, magnesium fluoride, or other materials. Each of
the dielectric microcomponents has an index of refraction of
between 1.4 and 2.2. Semiconductor materials usually have higher
indices of refraction than needed for optimal focusing near the
surface of the spheres. However, they can be used for these
applications if they are embedded in the materials with an
intermediate index of refraction. Semiconductor spheres can be made
from silicon, germanium, GaAs, or other materials. The refractive
index of microspheres can be smaller than the bulk values. The
semiconductor spheres can have indices from 2.2 to 3.5 in the
optical range. They can be embedded in PDMS, photoresist or other
materials which typically have indices in 1.4-1.6 range. The
microcomponents are assembled with the substrate using one or more
of the following techniques: i) self-assembly under wet conditions
due to meniscus forces, ii) the use of a patterned substrate, iii)
the use of an electric field, iv) the use of shear force, v) the
use of conventional or optoelectronic tweezers, vi) the use of a
magnetic field, vii) self-assembly under dry conditions, viii) the
use of vacuum tweezers, ix) the use of capillary grippers, and x)
the use of suction arrays. Optionally, the microcomponents are
affixed to the substrate using one or more of a glue, an epoxy, a
polymeric material, a photocurable material, and partial or
complete melting. The microcomponents form an array of
microcomponents disposed adjacent to the substrate.
[0014] In another exemplary embodiment, the present invention
provides a photodetector focal plane array method, comprising:
providing a substrate comprising a plurality of photosensitive
regions; and disposing a microcomponent adjacent to each of the
plurality of photosensitive regions operable for receiving incident
radiation and directing a photonic nanojet into the associated
photosensitive region. Optionally, the plurality of photosensitive
regions are disposed in a photosensitive layer of the substrate.
Optionally, each of the microcomponents comprises one of a
microsphere or a microcylinder. Each of the microcomponents has a
diameter of between 2.lamda.<D<100.lamda., so that in mid-IR
range of operation the typical diameters are between 8 and 400
micron. Each of the microcomponents is manufactured from a
dielectric or semiconductor material. Optionally, the dielectric
material comprises one of barium titanate glass, titanium dioxide,
sapphire, ruby, polystyrene, soda-lime glass, silica, borosilicate
glass, calcium fluoride, magnesium fluoride, or other materials.
Each of the dielectric microcomponents has an index of refraction
of between 1.4 and 2.2. Semiconductor materials usually have higher
indices of refraction than needed for optimal focusing near the
surface of the spheres. However, they can be used for these
applications if they are embedded in the materials with an
intermediate index of refraction. Semiconductor spheres can be made
from silicon, germanium, GaAs, or other materials. The refractive
index of microspheres can be smaller than the bulk values. The
semiconductor spheres can have indices from 2.2 to 3.5 in the
optical range. They can be embedded in PDMS, photoresist or other
materials which typically have indices in 1.4-1.6 range. The
microcomponents are assembled with the substrate using one or more
of the following techniques: i) self-assembly under wet conditions
due to meniscus forces, ii) the use of a patterned substrate, iii)
the use of an electric field, iv) the use of shear force, v) the
use of conventional or optoelectronic tweezers, vi) the use of a
magnetic field, vii) self-assembly under dry conditions, viii) the
use of vacuum tweezers, ix) the use of capillary grippers, and x)
the use of suction arrays. Optionally, the microcomponents are
affixed to the substrate using one or more of a glue, an epoxy, a
polymeric material, a photocurable material, and partial or
complete melting. The microcomponents form an array of
microcomponents disposed adjacent to the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The present invention is illustrated and described herein
with reference to the various drawings, in which like reference
numbers are used to denote like system components/method steps, as
appropriate, and in which:
[0016] FIG. 1 illustrates (a) a structure of individual
photodetector (or a "pixel"), (b) a typical FPA represented by an
array of pixels, and (c) an individual photodetector with metallic
contacts before and after integration with 90 .mu.m polystyrene
microsphere;
[0017] FIG. 2 illustrates (a) an electric field map calculated for
the plane wave illumination of a 125 .mu.m cylinder with n=1.80,
(b) a longitudinal line profile of the irradiance, showing the beam
waist in the near-surface region of the slab, and (c) a transverse
line profile through the peak of the longitudinal line profile;
[0018] FIG. 3 illustrates a direct comparison of photocurrent
measured from the same pixel, before and after integration with
microspheres made from different dielectric materials and having
different diameters. The photocurrent spectrum obtained from bare
mesa is shown by black line. The photocurrent spectra measured
after integration with spheres are illustrated using different
colors: (red) sapphire sphere with D=300 .mu.m, (green) sapphire
sphere with D=200 .mu.m, (violet) soda lime glass (SLG) sphere with
D=300 .mu.m, and (cyan) polystyrene sphere with D=90 .mu.m. The
plot illustrate significant photocurrent enhancement (up to 100
times) achieved as a result of integration with microspheres;
[0019] FIG. 4 illustrates the massive-scale integration of
microspheres with a 2D array of pixels; and
[0020] FIG. 5 illustrates the self-assembly of 90 .mu.m polystyrene
microspheres in 38 .mu.m dents with 10 .mu.m depth and 100 .mu.m
pitch-ordering was achieved in a layer of isopropanol with a
thickness on the order of a sphere diameter.
DETAILED DESCRIPTION OF THE INVENTION
[0021] By way of enabling background, prior work has taken place in
three main areas: i) developing the general concept of photonic
nanojets; ii) developing techniques for the self-assembly of
microspheres; and iii) developing micro-assembly technologies, such
as vacuum or suction tweezers and grippers.
[0022] Photonic nanojets. It has been proposed that dielectric
spheres can be used for obtaining tightly focused beams with
lateral dimensions which can be smaller than the diffraction limit.
Such tightly focused beams have been termed "photonic nanojets."
These photonic nanojets appear for a wide range of diameters of
microspheres, typically in a 2.lamda.<D<100.lamda. range,
with the refractive index contrast relative to the background
typically in a 1.4<n<2.0 range. Many applications of photonic
nanojets have been proposed, including polarization filters based
on chains of spheres and focusing single-mode and multi-mode
microprobes. More recently, an application of photonic nanojets for
focusing electromagnetic energy into a photodiode has been
proposed. However, the enhancement of the performance of
photodetector FPAs based on using an ordered array of spheres has
not been proposed or contemplated.
[0023] Self-assembly of microspheres. Many methods for the
self-assembly of microspheres have been developed in the material
science community. Most of these studies have been focused on
self-assembly under wet conditions, where capillary forces between
components exist due to menisci forming between the components.
Such capillary forces bring the components together to minimize the
interfacial free energy of the system. These methods allow the
fabrication of extremely long and straight chains of touching
microspheres and two-dimensional (2D) arrays of spheres. An
additional control of self-assembly is provided by the patterned
substrate, which allows obtaining ordered clusters of spheres.
Another example is given by the self-assembly of microspheres on
patterned electrodes by an applied electric field. Ordered 2D
arrays of 100 .mu.m glass microspheres with a 1% defect rate have
been obtained by this method. Another example is represented by a
method using a shear force in the course of drying the suspension.
Another example is given by the self-assembly of microspheres in a
magnetic field. In the magnetic field methods, micron to
millimeter-sized spheres can be manipulated by immersing them in a
dispersion of colloidal, magnetic nanoparticles. Another example is
given by synthetic opals, where silica spheres with submicron
diameters are packed in relatively ordered three-dimensional (3D)
structures. In addition to wet fabrication techniques, the template
self-assembly of microspheres into ordered 2D arrays has been
developed under dry conditions.
[0024] Optical tweezers. Microspherical arrays can be assembled
using parallel manipulation of microspheres by conventional or
optoelectronic tweezers.
[0025] Micro-assembly technologies. A whole class of manipulation
and gripping technologies has been developed based on using vacuum
tweezers, capillary grippers, and other similar methods. These
techniques represent a more "deterministic" way of assembling
arrays of microspheres as compared to self-assembly approaches. An
example of these techniques is represented by a suction array. Its
fabrication can be performed in such a way that each cavity can
hold exactly one micro part in a defined position. When the array
is connected to a suction gripper and positioned over an incoherent
batch of micro components, the air flow sucks the micro components
into the cavities, which are then plugged and the air flow is cut
off. When the array is filled up, excess micro components fall off
or are not even grasped in the first place. In this way, a defined
quantity of micro components can be picked up and aligned with a
defined and constant pitch simultaneously.
[0026] The three developments described above, i) the concept of
photonic nanojets, ii) the methods of self-assembly of
microspherical arrays, and iii) the micro-assembly technologies,
took place independently in three different research communities.
The present invention combines the advantages of the efficient
collection of light provided by the individual spheres with the
advantages of large-scale self-assembly and micro-assembly
techniques to produce photodetector FPAs with enhanced
performance.
[0027] In many modern photodetector devices, QWIPs, QDIPs, and
SLSIPs have mesas with lateral sizes below 20 .mu.m. There are
different designs of photodetector structures including front
surface-illuminated and back-illuminated devices. In the case of
front surface-illuminated structures, the photosensitive regions
are located at a very small, micron-scale, depth below the surface
of the structure. Using two-dimensional modeling by COMSOL
Multiphysics for a wavelength .lamda.=4 .mu.m, we studied the
focusing of light by a cylinder placed at the surface of a
high-index (n=3.3) semiconductor slab, as illustrated in FIG. 2a.
We demonstrated that the optimal cylinder index for focusing light
in near-surface regions of the slab is n=1.8, as illustrated in
FIG. 2b. We also showed that the transverse width of the beam at
its waist is about 2/3, as illustrated in FIG. 2c. Thus, the
photonic nanojet produced by a cylinder has a much smaller size
than the size of the device mesa. Similar focusing effects are
expected for dielectric microspheres. It can be concluded that for
applications in MWIR detectors, .lamda.=3-5 .mu.m, one of the
suitable materials for spheres is barium titanate glass, which has
index around n.apprxeq.1.8 at these wavelengths. However, there are
many other materials which are slightly sub-optimal in terms of
their index, but can be still used in these applications. The
examples include sapphire and ruby (n.apprxeq.1.73), polystyrene
(n.apprxeq.1.56), etc.
[0028] To demonstrate the advantage of single pixels integrated
with individual microspheres, we used a number of spheres made from
different materials and having different diameters, as illustrated
in FIG. 3. The spheres were fixed into position using a silicone
rubber. The spectral responses were characterized before and after
positioning the microspheres. The results in FIG. 3 illustrate up
to two orders of magnitude enhancement of the sensitivity of the
detector equipped with the focusing microsphere. Additional dips
visible in spectra of detectors integrated with different spheres
are likely due to absorption in the material of spheres. Such dips
have relatively narrow spectral width and, generally, do not
provide a limitation for the performance of the broad band mid-IR
imaging devices. Somewhat reduced response at the wavelengths
longer than 4.5 micron is explained by the increased lateral
dimensions of the focused spot. Due to imperfect alignment with the
mesa center this factor leads to the partial blocking of the beam
by the metallic electrodes surrounding the photodetector mesa. This
factor can be minimized by the optimal structural design and better
alignment of the spheres.
[0029] In the proposed methods and systems for enhancing the
performance of FPAs, the massive number of microspheres needs to be
positioned above the photosensitive mesas of the FPAs, as
illustrated in FIG. 4. For example, making an enhanced
256.times.256 array would require the positioning of 65,536
spheres. This task can be solved by various techniques. The present
invention is not limited to any specific self or micro-assembly
technique. The examples of such methods include, but are not
limited to: i) self-assembly under wet conditions due to meniscus
forces, ii) the use of a patterned substrate, iii) the use of an
electric field, iv) the use of shear force, v) the use of
conventional and optoelectronic tweezers, vi) the use of a magnetic
field, vii) self-assembly under dry conditions, viii) the use of
vacuum tweezers, ix) the use of capillary grippers, and x) the use
of suction arrays.
[0030] In FIG. 5, we illustrate the results of our work on the wet
self-assembly of 90 .mu.m polystyrene microspheres in 38 .mu.m
dents with 10 .mu.m depth fabricated in a photoresist. The choice
of isopropanol as a liquid medium was determined by its fast
evaporation properties and its small surface tension of 21.7
dynes/cm at 200 C. We found that water (surface tension 72.8
dynes/cm at 200 C) is a less favorable medium for this type of
self-assembly. These results illustrate that the ordering of
microspheres takes place when the thickness of the liquid layer is
reduced to the size of individual spheres. Under these conditions,
the surface of the evaporating liquid provides a downward pressure
on spheres which stimulates their ordering in the prefabricated
dents. In principle, the defect rate can be reduced to .about.1% by
using similar self-assembly techniques.
[0031] For applications in military or civil imaging systems, the
spheres need to be fixed in the positions aligned with the mesas of
the FPAs. This can be achieved by using glues, epoxies or, more
generally, liquid or polymer materials with the ability to
solidify, photocurable materials, or by using temperature
treatments or otherwise. In addition, these materials should have
relatively small absorption losses in the spectral range of
interest. As an example, the plastic spheres in many practical
cases can be fixed due to a controllable heating effect, so that
the spheres are slightly melted and attached to the substrate due
to a material reflow. Although this is accompanied by a change of
the spherical shape of the lens, small deformations can be
tolerated by the design of FPAs.
[0032] Placing the microspheres at the top of the photodetector
FPAs can be considered as the "tiling" of corresponding lattices of
pixels with the identical circular elements (as it can be viewed
from the top). In terms of photosensitivity, it would mean that the
size of each pixel is effectively determined by the size of
corresponding microsphere integrated with this pixel. The densest
packing is possible for touching circles packed in a triangular
lattice with 0.9069 area fill factor. For square close-packed
lattices of circles, the area fill factor is 0.7854. We do not
present here detailed analysis of losses of the incident light due
to its reflection at the spherical surface as well as due to light
scattering in such close-packed monolayers of spheres. Calculations
show that in most of the cases these losses are limited at
.about.10%. This means that integration with microspheres should
increase the sensitivity of FPAs by more than an order of magnitude
and in many cases by up to two orders of magnitude. Simultaneously,
using pixels with the wavelength-scale dimensions should reduce the
dark current by more than an order of magnitude. In addition, due
to the decreased capacitance of each pixel, its frequency response
should be significantly increased. Finally, in comparison with COTs
microlenses, the integration with microspheres leads to very large
AOVs. Typically, AOV>10 deg can be realized in most of the
designs, however AOV>20 deg can also be achieved in such
FPAs.
[0033] This technology results in the low-cost, high-volume
production of photodetector FPAs. Barium titanate glass and
polystyrene microspheres are available in massive quantities, such
for example as .about.10.sup.6-10.sup.8 spheres in a wet or dry
sample which can be obtained from various manufacturers. They
spheres are inexpensive. Sapphire and ruby spheres are not
typically suitable for FPA production because of their high price,
although they are not rules out here since large samples of these
spheres can be obtained. The techniques of self-assembly and
micro-assembly by a suction array are suitable for massive-scale
production and are very inexpensive. Alternatively, low-defect rate
massive-scale fabrication can be achieved by the self-assembly of
microspheres on patterned electrodes by an applied electric field.
The fabrication of FPAs integrated with microspheres seems to be
easier to realize for MWIP and LWIP because of the longer pitch of
the array, which allows using larger spheres.
[0034] Potential markets for this technology include companies and
governmental laboratories working on increasing the sensitivity of
current MWIPs and LWIPs. Current IR multi-spectral imagers are
large and difficult to integrate on small size, weight, and power
(SWaP)-limited platforms, such as Puma, Shadow, and Tube Launched
Expendable UAS (TLEU). The deficiency of these imagers is their
large optical systems, which are needed to simultaneously collect
both the spatial and spectral data. Detecting weak signals requires
large objective diameters, which translate into the large size and
weight of the optical system. Integration with microspheres opens a
unique way of solving this problem by increasing of the response of
the system, increasing AOV and reducing its dark current by orders
of magnitude.
[0035] One of the competing approaches to solving the problems
addressed by this invention is represented by the concept of the
flat metamaterial lens. Due to its planar design and potentially
short focal lengths, this concept attracted significant interest
recently. In particular, a design of such a flat lens has been
proposed based on a stack of strongly coupled waveguides sustaining
backward waves. As a result, such metamaterial exhibits a negative
index of refraction to incoming light regardless of its incidence
angle. It should be noted, however, that the concept of flat
metamaterial lens has some drawbacks which are not totally overcome
at the present time. These include a relatively narrow spectral
range of operation, the inevitable absorption losses in the
metallic layers, and somewhat complicated fabrication. Overall,
this concept still requires significant development before it can
become a practical solution for solving problems addressed by this
invention.
[0036] Another competing approach is based on using standard
photolithography techniques, which allows the fabrication of 2D
arrays of microlenses. The fabrication of microlenses by melting
and reflow of photoresist has some advantages because it is based
on using established planar technologies, such as photolithography,
etching, etc. For this reason, it allows the reproducible
fabrication of 2D arrays of microlenses over wide areas. However,
the dome-shaped microlenses fabricated by this method have a
limited refractive index contrast and they are far from being
completed spheres. As a result, their focal length is much longer
than in methods and systems proposed in this invention. As shown
earlier in this patent application, this results in very small AOVs
of such systems which made them impractical in many mid-IR imaging
applications. In this sense, such arrays cannot compete with the
methods and systems for near-surface focusing proposed in this
invention.
[0037] Another competing approach is based on using microspheres
embedded in thin films. Such microspheres have been used as lenses
for projection photolithography. For photodetector applications,
the spheres need to be aligned with the photosensitive mesas. This
problem has not been stated and solved yet.
[0038] Theoretically, the idea of using an individual microsphere
for more efficient coupling of light into the photosensitive area
of photodetector has been expressed previously. However, this
proposal has not been analyzed in comparison with COTs microlenses
which have been known for a long time as a tool for concentrating
light on the detectors. In this patent, we show that integration
with microspheres allows combining high light collection efficiency
with large AOVs which is a unique advantage of such detectors.
Another important feature which has not been considered in previous
proposal of using photonic nanojets for photodetectors is an
ability to assemble microspheres in a regular array to make
possible fabrication of FPAs.
[0039] As we stated previously, there have been multiple studies of
directed self-assembly and micro-manipulation assembly of
microspheres. In some cases, these studies have been performed with
the goal of positioning microspheres on patterned electrodes or
inside prefabricated dents. However, these studies have not been
intended to be used for enhancing the performance of photodetector
FPAs. For this reason, corresponding analysis of the essential
physical parameters, such as the relationship between the size of
the photonic nanojets and size and depth of the photosensitive
regions, have not been performed. The whole inventive idea of the
proposed methods based on the simultaneous use of light focusing
and manufacturing advantages provided by microspheres have not been
expressed.
[0040] Although the present invention is illustrated and described
herein with reference to preferred embodiments and specific
examples thereof, it will be readily apparent to those of ordinary
skill in the art that other embodiments and examples may perform
similar functions and/or achieve like results. All such equivalent
embodiments and examples are within the spirit and scope of the
present invention, are contemplated thereby, and are intended to be
covered by the following claims.
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