U.S. patent application number 11/513262 was filed with the patent office on 2006-12-28 for layered microlens structures and devices.
Invention is credited to Ulrich Boettiger, Loriston Ford, Jin Li, Jiutao Li.
Application Number | 20060289723 11/513262 |
Document ID | / |
Family ID | 34677916 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060289723 |
Kind Code |
A1 |
Li; Jin ; et al. |
December 28, 2006 |
Layered microlens structures and devices
Abstract
A microlens structure includes lower lens layers on a substrate.
A sputtered layer of glass, such as silicon oxide, is applied over
the lower lens layers at an angle away from normal to form upper
lens layers that increase the effective focal length of the
microlens structure. The upper lens layers can be deposited in an
aspherical shape with radii of curvature longer than the lower lens
layers. As a result, small microlenses can be provided with longer
focal lengths. The microlenses are arranged in arrays for use in
imaging devices.
Inventors: |
Li; Jin; (Boise, ID)
; Li; Jiutao; (Boise, ID) ; Boettiger; Ulrich;
(Boise, ID) ; Ford; Loriston; (Nampa, ID) |
Correspondence
Address: |
DICKSTEIN SHAPIRO LLP
1825 EYE STREET NW
Washington
DC
20006-5403
US
|
Family ID: |
34677916 |
Appl. No.: |
11/513262 |
Filed: |
August 31, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11244101 |
Oct 6, 2005 |
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11513262 |
Aug 31, 2006 |
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10740597 |
Dec 22, 2003 |
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11244101 |
Oct 6, 2005 |
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Current U.S.
Class: |
250/216 |
Current CPC
Class: |
G02B 3/0018 20130101;
G02B 3/0056 20130101; H01L 27/14627 20130101; G02B 3/0037
20130101 |
Class at
Publication: |
250/216 |
International
Class: |
H01J 3/14 20060101
H01J003/14; H01J 40/14 20060101 H01J040/14; H01J 5/16 20060101
H01J005/16 |
Claims
1-17. (canceled)
18. A microlens array comprising: an array of lower lens layers;
and an array of upper lens layers over the array of lower lens
layers, wherein adjacent lower lens layers are separated by gaps,
the gaps being at least partially filled by the respective upper
lens layers.
19. A microlens array according to claim 18, wherein the upper lens
layers are formed by deposition directly on respective ones of the
lower lens layers.
20. A microlens array according to claim 18, wherein the upper lens
layers are aspherical.
21. A microlens array according to claim 18, wherein the upper lens
layers have a thickness that is greater near a bottom portion of
the lower lens layers than at a top portion of respective ones of
the lower lens layers.
22. A microlens array according to claim 18, in which the lower
lens layers have a spherical shape
23. (canceled)
24. An imaging integrated circuit (IC) comprising: an imaging array
including a plurality of photosensitive regions; and a plurality of
lens structures provided on the imaging array, each lens structure
being over a respective one of the photosensitive regions, each
lens structure including upper and lower lens layers, wherein
adjacent lower lens layers are separated by gaps, the gaps being at
least partially filled by the respective upper lens layers.
25. An imaging IC as in claim 24, wherein each of the
photosensitive regions includes a photodiode.
26. An imaging IC as in claim 24, further comprising circuitry for
row and column selection.
27. An imaging IC as in claim 24, wherein the imaging IC is a CMOS
imager.
28. An imaging IC as in claim 24, wherein the imaging IC is a CCD
imager.
29-39. (canceled)
40. A microlens array comprising: an array of lower lens layers,
each lower lens layer having a spherical cross-sectional shape; and
an array of upper lens layers over the array of lower lens layers,
each upper lens layer having an aspherical cross-sectional shape
different from the spherical cross-sectional shape.
41. A microlens array according to claim 40, wherein the upper lens
layers have a thickness that is greater near a bottom portion of
the lower lens layers than at a top portion of the lower lens
layers.
42. A microlens array according to claim 40, in which the lower
lens layers'upper surfaces have a smaller radius of curvature than
the upper lens layers'upper surfaces.
43. An imaging integrated circuit (IC) comprising: an imaging array
including a plurality of photosensitive regions; and a plurality of
lens structures provided on the imaging array, each lens structure
being over a respective one of the photosensitive regions, the lens
structures including upper and lower lens layers, each lower lens
layer having a spherical cross-sectional shape, and each upper lens
layer having an aspherical cross-sectional shape different from the
spherical cross-sectional shape.
44. An imaging IC as in claim 43, wherein each of the
photosensitive regions includes a photodiode.
45. An imaging IC as in claim 43, further comprising circuitry for
row and column selection.
46. An imaging IC as in claim 43, wherein the imaging IC is a CMOS
imager.
47. An imaging IC as in claim 43, wherein the imaging IC is a CCD
imager.
48. An image processing system comprising: a processor coupled to a
bus; and an imaging integrated circuit (IC) coupled to the bus, the
imaging IC comprising: an imaging array containing a plurality of
photosensitive regions; and a plurality of lens structures provided
on the imaging array, each lens structure being over a respective
one of the photosensitive regions, the lens structures including
upper and lower lens layers, wherein adjacent lower lens layers are
separated by gaps, the gaps being at least partially filled by the
respective upper lens layers.
49-67. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to improved lens
structures, and in particular to a microlens system for an imager
or display array.
BACKGROUND OF THE INVENTION
[0002] The semiconductor industry currently uses different types of
semiconductor-based imagers, such as charge coupled devices (CCDs),
complementary metal-oxide semiconductor (CMOS) active pixel sensors
(APS), photodiode arrays, charge injection devices and hybrid focal
plane arrays, among others, in which an array of microlenses causes
incident light to converge toward each of an array of pixel
elements. Semiconductor displays using microlenses have also been
developed.
[0003] Microlenses are manufactured using subtractive processes and
additive processes. In an additive process, lens material is formed
on a substrate, patterned and subsequently formed into microlens
shapes.
[0004] In conventional additive microlens fabrication, an
intermediate material is patterned on a substrate to form a
nicrolens array using a reflow process. Each microlens is separated
by a minimum distance from adjacent microlenses, typically no less
than 0.3 micrometers. Distances less than 0.3 micrometers may cause
unwanted bridging of neighboring microlenses during reflow. In the
known process, each microlens is patterned as a single shape,
typically square, with gaps around it. Heat is applied during the
subsequent step of reflowing, which causes the patterned microlens
material to form a gel drop in a partially spherical shape, driven
by the force equilibrium of surface tension and gravity. The
microlenses then harden in this shape. If the gap between two
adjacent gel drops is too narrow, they may touch and merge, or
bridge, into one larger drop. The effect of bridging is that it
changes the shape of the lenses, which leads to a change in focal
length, or more precisely the energy distribution in the focal
range. A change in the energy distribution in the focal range leads
to a loss in quantum efficiency of, and enhanced cross-talk
between, pixels. The gaps also allow unfocused photons through the
empty spaces in the microlens array, leading to increased
cross-talk between respective photosensors of adjacent pixel
cells.
[0005] In addition, as the size of imager arrays and photosensitive
regions of pixels decreases, it becomes increasingly difficult to
provide a microlens capable of focusing incident light rays onto a
photosensitive region. This problem is due in part to the increased
difficulty in constructing a smaller microlens that has the optimal
focal length for the imager device process and that optimally
adjusts for optical aberrations introduced as the light passes
through the various device layers. Also, it is difficult to correct
the distortion created by multiple layered regions above the
photosensitive area, for example, color filter regions, which
results in increased crosstalk between adjacent pixels.
Consequently, smaller imagers with untuned or nonoptimized
microlenses do not achieve optimal color fidelity and signal/noise
ratios.
[0006] It would be advantageous to have improved microlens
structures and techniques for producing them.
BRIEF SUMMARY OF THE INVENTION.
[0007] Exemplary embodiments of the invention provide a microlens
structure having at least two differing layers which together
produce a desired microlens characteristic. In a two-layer
exemplary embodiment, for example, the top layer can have a
different shape than the bottom layer, thus obtaining a desired
focal property. The top layer can be formed by off-angle
deposition, e.g., sputtering, of a transparent glassy material,
such as a silicon oxide, over a pre-formed lower layer.
[0008] The invention also provides methods of producing
microlenses. An exemplary method embodiment includes forming a
bottom layer with precursor microlens material such as by
photoresist reflow. A top layer is deposited over the precursor
microlens material using a glass-forming oxide, for example.
Deposition takes place by sputtering the oxide at an angle off
normal by about 45.degree.-60.degree.. As a result of depositing
the glass at an angle off normal, glass is deposited in greater
amounts around the peripheral edges of the precursor material,
thereby changing the shape and increasing the effective focal
length of the lenses. According to one exemplary two-layer
embodiment the resulting shape is aspherical.
[0009] These and other features and advantages of various
embodiments of the invention will be better understood from the
following detailed description, which is provided in connection
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 illustrates a cross sectional view of a portion of a
microlens structure in accordance with an exemplary embodiment of
the invention, and represents a cross-section taken along line I-I
in FIG. 6;
[0011] FIG. 2 illustrates a top view of a portion of the FIG. 1
embodiment;
[0012] FIG. 3 illustrates the focal lengths of lower and upper lens
regions of the microlens structure of FIG. 1;
[0013] FIG. 4 is a schematic illustration of an apparatus for
manufacturing a microlens structure according to an exemplary
embodiment of the present invention;
[0014] FIG. 5 is a cross-section illustrating a method of
manufacturing a microlens structure with the apparatus of FIG.
4;
[0015] FIG. 6 shows a block diagram of an imager integrated circuit
(IC) in accordance with an exemplary embodiment of the
invention
[0016] FIG. 7 is a schematic diagram of a processor system with an
imager IC as in FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In the following detailed description, reference is made to
the accompanying drawings, which form a part hereof and show by way
of illustration specific embodiments in which the invention may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to practice the invention, and it
is to be understood that other embodiments may be utilized, and
that structural, logical, and electrical changes may be made
without departing from the spirit and scope of the present
invention. The progression of processing steps described is
exemplary of embodiments of the invention; however, the sequence of
steps is not limited to that set forth herein and may be changed,
with the exception of steps necessarily occurring in a certain
order.
[0018] The term "wafer" or "substrate," as used herein, is to be
understood as including silicon, silicon-on-insulator (SOI) or
silicon-on-sapphire (SOS) technology, doped and undoped
semiconductors, epitaxial layers of silicon supported by a base
semiconductor foundation, and other semiconductor or insulating
structures in, on, or at a surface of which circuitry or optical or
electrical devices can be formed. Furthermore, when reference is
made to a "wafer" or "substrate" in the following description,
previous processing steps may have been utilized to form regions,
junctions, or material layers in or over the base semiconductor
structure or foundation. In addition, a semiconductor wafer or
substrate need not be silicon-based, but could be based on
silicon-germranium, germanium, gallium arsenide or other
semiconductors.
[0019] The term "pixel," as used herein, refers to a picture
element unit cell containing a photosensor and other components for
converting electromagnetic radiation to an electrical signal. For
purposes of illustration, a representative CMOS imager pixel cell
is illustrated in the figures and description herein. However, this
is just one example of the types of imagers and pixel cells with
which the invention may be used. The invention may also be used to
create microlens arrays for display devices.
[0020] The term "microlens" refers herein to one of an array of
optical components over an array of photosensors or photoemitters.
In an imager array each microlens tends to focus incident light
toward a respective photosensor. A microlens array may be part of a
layered structure formed over a substrate using photolithographic
techniques. Various processes have been developed for producing
microlenses, including fluid self-assembly, droplet deposition,
selective curing in photopolymer by laser beam energy distribution,
photoresist reflow, direct writing in photoresist, grayscale
photolithography, and modified milling. These processes are
described in more detail in U.S. Pat. No. 6,473,238 to Daniell, the
disclosure of which is incorporated herein by reference.
[0021] While the invention is described with particular reference
to a semiconductor-based imager, such as a CMOS imager, it should
be appreciated that the invention may be applied in any
micro-electronic or micro-optical device that includes a microlens,
especially one that requires high quality microlenses for optimized
performance. Other exemplary micro-optical devices that can include
microlenses include CCDs and display devices, as well as
others.
[0022] Referring initially to FIGS. 1 and 2, an exemplary
embodiment of an imager array 2 is shown illustratively in cross
sectional and top views, respectively. A plurality of microlens
structures is provided, each having a lower lens region 4 and an
upper lens region 6. The microlens structures are provided over
passivation layer 8, intervening layer 10 (e.g., color filter
array, metallization, etc.), and an array of imaging pixels 12,
with one microlens structure over each pixel 12. Each pixel 12
includes a photosensor for converting photons to free electrical
charges, and the array 2 also includes structures that obtain
electrical signals based on charge levels. Each pixel's microlens
is structured in at least two layers, e.g., layers 4 and 6 shown in
FIGS. 1 and 2, to increase the pixel's light collection
efficiency.
[0023] In the illustrated embodiment of FIGS. 1 and 2, the two lens
layers 4, 6 in each microlens structure cause light from a larger
arc to converge onto a light sensitive photosensor of a respective
pixel 12 and to lengthen the effective focal length of each
microlens structure. Lower lens layer 4 covers a smaller area of
substrate 14 than upper lens layer 6 as shown in FIG. 2, so that a
light ray 16 is deflected onto the photosensor of pixel 12 from
outside the area of pixel 12, increasing the percentage of incident
light that reaches a corresponding photosensor of a pixel 12. In
addition, upper layer 6 has a shape that results in different focal
properties than lower layer 4.
[0024] In the illustrated embodiment, the upper surface of upper
layer 6 has radii of curvature longer than the substantially
uniform radius of curvature at the upper surface of lower layer 4.
As a result, the effective (or average) focal length of each
microlens structure is longer than if both layers had the same
shape. FIG. 3 illustrates schematically a focal length A of lower
layer 4 alone, as compared to a longer focal length B of combined
layers 4 and 6. More generally, the upper surface of layer 6 can
have a shape that improves efficiency by distributing light onto
the photosensor of a pixel in a way that improves conversion of
photons to free charge carriers.
[0025] The lens layers 4, 6 can be formed into various symmetrical
geometric shapes, such as circles, squares, etc., and asymmetrical
shapes to provide a path for incident light rays to reach the photo
sensors of the pixels 12. FIG. 2 shows the lower lens layer 4 as
having a substantially circular cross-section (FIG. 1). Lens layer
6 has a rounded, rectangular perimeter. It should be understood,
however, that a variety of shapes for each of layers 4 and 6 may be
used in embodiments of the invention, as discussed below.
[0026] Referring again to FIG. 2, upper lens layer 6 has an
aspherical shape with radial dimensions larger than that of
spherical lens layer 4. In the illustrated embodiment, lens layer 6
extends horizontally to a boundary between adjacent microlens
structures. Because lens layer 6 is aspherical in shape, the radius
of curvature of its upper surface varies with orientation, being
near its minimum in the cross section of FIG. 1 and near its
maximum along a diagonal cross section (not shown). At all
orientations, the radius of curvature is significantly longer than
that of lower lens layer 4. As a result, an effective focal length
of the lens structure, made up of lens layers 4 and 6, is longer
than an effective focal length of lens layer 4 alone.
[0027] Lens layers 4 illustratively are substantially spherical and
can be formed using a photo resist reflow technique, as is known to
those of skill in the art for forming microlenses. The lens layers
4 illustratively are formed from a layer of microlens material,
such as photo resist, referred to herein as a "precursor microlens
material." Other inorganic, as well as organic and
organic-inorganic hybrid materials, also could be used. The
precursor microlens material is illustratively coated and patterned
upon the passivation layer 8. After patterning, a portion of the
material over each pixel has a substantially rectangular or
circular configuration and each portion is substantially equal in
size with the others. Upon reflow, the precursor microlens material
hardens and preferably is impervious to subsequent reflow
processes. As a result of the reflow process, the patterned
precursor microlens material is transformed into lens layers 4. The
lens layers 4 each have a substantially circular perimeter
configuration with a spherically curved profile.
[0028] The layer 8 upon which the lens layers 4 are formed can be
any suitable material that is transparent to electromagnetic
radiation in the relevant wavelength range. The lens layers 4,
which are also transparent to electromagnetic radiation in the
relevant wavelength range, will retain their shape even if a
subsequent reflow process is performed. As shown in FIG. 2, there
are spaces between the lens layers 4 of adjacent microlenses.
[0029] After patterning and reflowing the precursor microlens
material to form lower lens layers 4, upper lens layers 6 are
formed. Lens layers 6 are deposited over lens layers 4 by an
off-angle deposition process, illustrated in FIGS. 4 and 5. In an
exemplary embodiment, an SiO.sub.2 beam 20 is supplied from a
sputtering source 22 through a collimator 24. The SiO.sub.2 beam is
directed toward a rotating platform 26. Platform 26 supports the
substrate 14, on which only two of lens layers 4 are shown in
cross-section in FIG. 5, formed on layer 8.
[0030] Platform 26 rotates relative to sputtering source 22 as
indicated by the arrows in FIG. 4. Collimated SiO.sub.2 is directed
toward the rotating platform 26, and condenses as a glass to form
lens layers 6 deposited directly on lens layers 4. The speed of
rotation of platform 26 can be varied to allow for sufficient
deposition of material without disturbing the integrity of the
deposited glass.
[0031] SiO.sub.2 beam 20 is directed at an angle .theta. away from
normal such that most of the glass deposition takes place around
the perimeters or peripheral edges of the lens layers 4 and little
is deposited at the tops or central surfaces. The angle .theta. can
range between about 0.degree. and about 90.degree., and preferably
is between about 45.degree. and about 60.degree.. Accordingly, the
layer of deposited glass on lens layers 6 is thicker toward the
bottoms of lens layers 4, near layer 8, than it is toward the tops
of lens layers 4. To obtain a rectangular shape as in FIG. 2, the
speed of rotation of platform 26 or the rate of emission from
source 22 can be varied as a function of orientation of platform
26. Consequently, the shape of the lens structures is changed from
that of spherical lens layers 4 such that horizontal aspherical
radial dimensions of the lens layers 6 are larger than a horizontal
radius of the lens layers 4. As a result of the off-angle
deposition, the radius of curvature of the upper surfaces is
increased, so that the effective focal length of the lens
structures is increased from that of lens layers 4.
[0032] Various materials can be used for both the lens layers 4 and
6. Exemplary materials for lens layers 6 are those that provide a
substantially transparent layer and are amenable to physical vapor
deposition. In addition to SiO.sub.2, exemplary materials include
nitrides such as Si.sub.3N.sub.4, borophosphosilicate glass (BPSG),
phosphosilicate glass (PSG), and zinc selenide. Advantageously, a
refractive index of the deposited lens layers 6 and the lens layers
4 will be substantially identical to minimize loss of incident
light that otherwise would occur as the result of reflections from
the interface between layers 6 and 4.
[0033] Layers 6 also provide a protective layer for later
processes, and can have excellent optical properties. In particular
layers 6 can have lower absorption than lower layers 4 formed of an
organic microlens material. Further, the layers 6 can protect
organic microlenses 4 to prevent cracking, oxidation, aging during
high temperature baking processes, and physical or chemical attack
in subsequent processes, for example.
[0034] Advantageously, deposition continues until gaps between the
lens layers 4 are substantially filled with glass, thereby
increasing the area of coverage of each lens. Consequently, a
greater portion of light incident upon the lens structure array is
captured and focused toward pixels 12. The deposition process may
take several minutes, for example, depending on the rate of
deposition, desired thickness, subsequent processing requirements,
etc. Typically, deposition takes place at least until gaps between
individual lens in the lower layers 4 are filled. Exemplary,
non-limiting thicknesses of the resulting lens layers 6 can be in
the range of 0.1-2.0 micrometers, most preferably 0.4-0.8
micrometers, for example.
[0035] FIG. 6 illustrates a block diagram of an imager integrated
circuit (IC) 308 having a pixel array 200 containing a plurality of
pixels arranged in rows and columns. A cross-section taken along
line I-I in array 200 would be the same as the cross-section
illustrated in FIG. 1. In other words, pixel array 200 includes at
least one microlens structure, illustratively with components as in
FIGS. 1 and 2, formed over an associated pixel cell. The pixels of
each row in array 200 are all turned on at the same time by a row
select line, and the pixels of each column are selectively output
by respective column select lines. The row lines are selectively
activated by a row driver 210 in response to row address decoder
220. The column select lines are selectively activated by a column
selector 260 in response to column address decoder 270.
[0036] The pixel array 200 is operated by the timing and control
circuit 250, which controls address decoders 220, 270 for selecting
the appropriate row and column lines for pixel signal readout. The
pixel column signals, which illustratively include a pixel reset
signal (Vrst) and a pixel image signal (Vsig), are read by a sample
and hold circuit 261 associated with the column selector 260. A
differential signal (Vrst-Vsig) is produced by differential
amplifier 262 for each pixel, and the differential signal is
amplified and digitized by analog to digital converter (ADC) 275.
ADC 275 supplies the digitized pixel signals to an image processor
280 which can perform image processing before providing image
output signals.
[0037] Imager IC 308 can be a CMOS imager or CCD imager, or can be
any other type of imager that includes a microlens structure.
[0038] FIG. 7 shows system 300, a typical processor based system
modified to include an imager IC 308 as in FIG. 6. Processor based
systems exemplify systems of digital circuits that could include an
imager IC 308. Examples of processor based systems include, without
limitation, computer systems, camera systems, scanners, machine
vision systems, vehicle navigation systems, video telephones,
surveillance systems, auto focus systems, star tracker systems,
motion detection systems, image stabilization systems, and data
compression systems for high-definition television, any of which
could utilize the invention.
[0039] System 300 includes an imager IC 308 having the overall
configuration depicted in FIG. 6 with array 200 including a
microlens structure in accordance with any of the various
embodiments of the invention. System 300 includes a processor 302
having a central processing unit (CPU) that communicates with
various devices over a bus 304. Some of the devices connected to
the bus 304 provide communication into and out of the system 300;
an input/output (I/O) device 306 and imager IC 308 are examples of
such communication devices. Other devices connected to the bus 304
provide memory, illustratively including a random access memory
(RAM) 310, hard drive 312, and one or more peripheral memory
devices such as a floppy disk drive 314 and compact disk (CD) drive
316. The imager IC 308 may receive control or other data from CPU
302 or other components of system 300. The imager IC 308 may, in
turn, provide signals defining images to processor 302 for image
processing, or other image handling operations.
[0040] While exemplary embodiments of the invention have been
described and illustrated above, it should be understood that these
are exemplary of the invention and are not to be considered as
limiting. Additions, deletions, substitutions, and other
modifications can be made without departing from the spirit or
scope of the present invention. Accordingly, the invention is not
to be considered as limited by the foregoing description but is
only limited by the scope of the appended claims.
* * * * *