U.S. patent application number 14/942940 was filed with the patent office on 2016-05-26 for use of dark mirror coating to suppress stray light in an optical sensor assembly.
The applicant listed for this patent is Viavi Solutions Inc.. Invention is credited to Richard A. Bradley, Jr., Karen Denise Hendrix, Jeffrey James Kuna, Georg J. Ockenfuss.
Application Number | 20160149058 14/942940 |
Document ID | / |
Family ID | 56011041 |
Filed Date | 2016-05-26 |
United States Patent
Application |
20160149058 |
Kind Code |
A1 |
Bradley, Jr.; Richard A. ;
et al. |
May 26, 2016 |
USE OF DARK MIRROR COATING TO SUPPRESS STRAY LIGHT IN AN OPTICAL
SENSOR ASSEMBLY
Abstract
An optical sensor assembly is provided in which a dark mirror
coating is used to suppress stray light in the form of both
unwanted reflections from non-optically active regions of the
sensor assembly surface and unwanted transmission of light into the
surface region of the sensor assembly. The sensor assembly includes
an image sensor positioned in a substrate adjacent to substrate
surface areas that are not optically active. A dark mirror coating
covering those surface areas significantly reduces reflections from
non-optically active surface regions and improves image sensor
performance in terms of signal-to-noise ratio and reduction in the
appearance of "ghost" images, in turn enhancing the accuracy and
precision of the sensor. The dark mirror coating may in the
alternative, or in addition, be positioned underneath an optical
filter, depending on the structure, material, and requirements of a
particular sensor assembly.
Inventors: |
Bradley, Jr.; Richard A.;
(Santa Rosa, CA) ; Hendrix; Karen Denise; (Santa
Rosa, CA) ; Kuna; Jeffrey James; (San Francisco,
CA) ; Ockenfuss; Georg J.; (Santa Rosa, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Viavi Solutions Inc. |
Milpitas |
CA |
US |
|
|
Family ID: |
56011041 |
Appl. No.: |
14/942940 |
Filed: |
November 16, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62079684 |
Nov 14, 2014 |
|
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|
Current U.S.
Class: |
257/432 ;
257/437 |
Current CPC
Class: |
H01L 27/14625
20130101 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216; H01L 31/0232 20060101 H01L031/0232 |
Claims
1. An optical sensor assembly comprising: a substrate having a
surface; an optical image sensor at the substrate surface having an
aperture for detecting and receiving light and providing a signal
in response thereto, wherein the aperture defines an optically
active surface region, with the remainder of the substrate surface
defining a non-optically active surface region; a dark mirror
coating disposed over at least a portion of the substrate surface
but not substantially in the region of the aperture, wherein the
dark mirror coating is configured as an absorptive anti-reflective
coating to reduce reflections from the non-optically active surface
region and to reduce transmission through the non-optically active
surface region and into the substrate, and comprises an initial
absorbing layer provided on the substrate surface and an outwardly
facing dielectric layer on the initial absorbing layer.
2. The OSA of claim 1, wherein the initial absorbing layer
comprises a metallic material selected from aluminum, gray metals,
and alloys thereof.
3. The OSA of claim 2, wherein the initial absorbing layer
comprises two or more absorbing sublayers.
4. The OSA of claim 1, wherein the outwardly facing dielectric
layer comprises two or more dielectric sublayers.
5. The OSA of claim 1, further including an optical thin film
filter disposed over the optical image sensor and configured to
filter light so that only light in a selected wavelength range
passes through the filter and reaches the sensor.
6. The OSA of claim 5, wherein the dark mirror coating is disposed
over at least a portion of the optical thin film filter.
7. The OSA of claim 5, wherein the dark mirror coating is disposed
under at least a portion of the optical thin film filter.
8. The OSA of claim 5, wherein a first dark mirror coating is
disposed over at least a portion of the optical thin film filter
and a second dark mirror coating is disposed under at least a
portion of the optical thin film filter.
9. The OSA of claim 1, wherein the optical sensor assembly
comprises two or more optical sensors at the substrate surface each
having an aperture for detecting and receiving light and providing
a signal in response thereto.
10. The OSA of claim 9, wherein an optical thin film filter is
disposed over each optical sensor.
11. The optical sensor assembly of claim 1, wherein the optical
thin film filter includes terminal tapering regions and the dark
mirror coating is formed over the tapering regions so as to cover
both a portion of the optical thin film filter and a portion of
uncoated substrate surface.
12. An optical sensor assembly comprising: a substrate having a
surface; an optical image sensor at the substrate surface having an
aperture for detecting and receiving light and providing a signal
in response thereto, wherein the aperture defines an optically
active surface region, with the remainder of the substrate surface
defining a non-optically active surface region; a dark mirror
coating disposed over at least a portion of the substrate surface
but not substantially in the region of the aperture, wherein the
dark mirror coating is configured as an absorptive anti-reflective
coating to reduce reflections from the non-optically active surface
region and to reduce transmission through the non-optically active
surface region and into the substrate, and further wherein the dark
mirror coating comprises a first pair of layers provided on the
substrate surface, the first pair of layers including a first
substantially non-absorbing layer disposed directly on the
substrate surface and a first absorbing layer adjacent to and
overlying the first substantially non-absorbing layer, optionally
at least one additional pair of layers including a substantially
non-absorbing layer and an absorbing layer configured such that the
absorbing layers and the substantially non-absorbing layers
alternate, and an outwardly facing dielectric layer serving as the
surface of the dark mirror coating.
13. The optical sensor assembly of claim 12, wherein at least one
absorbing layer is comprised of two or more absorbing
sublayers.
14. The optical sensor assembly of claim 12, wherein at least one
substantially non-absorbing layer is comprised of two or more
substantially non-absorbing sublayers.
15. The optical sensor assembly of claim 12, wherein each absorbing
layer is comprised of a metallic absorbing material selected from
aluminum, gray metals, and alloys thereof.
16. The optical sensor assembly of claim 12, wherein the absorbing
material is selected from tantalum, niobium, titanium, nickel,
chromium, silicon, and alloys thereof.
17. The optical sensor assembly of claim 12, wherein the absorbing
material is selected from tantalum, niobium, and alloys
thereof.
18. The optical sensor assembly of claim 12, wherein each
substantially non-absorbing layer is comprised of a dielectric
material.
19. The optical sensor assembly of claim 18, wherein the dielectric
material is selected from SiO.sub.2, Ta.sub.2O.sub.5, NbTaO.sub.5,
Nb.sub.2O.sub.5, TiO.sub.2, NbTiO.sub.x, Al.sub.2O.sub.3,
Si.sub.3N.sub.4, Cr.sub.2O.sub.3, MoO.sub.3, and combinations
thereof.
20. The optical sensor assembly of claim 12, wherein when the dark
mirror coating comprises at least one additional pair of layers,
the absorbing layers may comprise the same material or different
materials and the substantially non-absorbing layers may comprise
the same material or different materials.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority under 35 U.S.C.
.sctn.119(e)(1) to provisional U.S. Patent Application Ser.
62/079,684, filed Nov. 14, 2014, the disclosure of which is
incorporated by reference herein.
TECHNICAL FIELD
[0002] The present invention relates generally to optical sensors,
and, more particularly to optical sensor assemblies and methods for
suppressing stray light in optical sensor assemblies, including
both unwanted reflection and unwanted transmission.
BACKGROUND
[0003] The exposed surface of an optical sensor chip includes
regions that are optically active, i.e., optically sensitive, as
well as regions that are not optically active ("non-active areas").
Ideally, the optical path directs light only to the optically
active surface regions of the sensor chip. The geometry of the
sensor, however, is often dictated by electronics rather than
optics. Thus, in many instances, a significant portion of the
incoming light in the optical path falls onto non-active areas. The
surface reflectance from non-active areas is typically
uncontrolled, and therefore these areas can reflect light back into
the optical system. The reflected light can be inadvertently
collected by the sensor, creating noise in the sensed optical
signal and resulting in ghost reflections and scatter. Typically,
these non-active areas are coated with layers of oxides and
nitrides, as well as layers of metals, and the reflectance of these
materials can be substantial, on the order of 50% or even higher.
The ratio of signal to noise is, of course, an important attribute
of any sensor, and the signal-to-noise ratio should be maximized to
provide an optimal system. Suppressing reflections from
non-optically active surfaces would produce higher quality images,
because ghost reflections are lessened.
[0004] Anti-reflective or anti-reflection ("AR") coatings are known
in the art as coatings that can be applied to a substrate surface
to reduce the reflectance of the surface or of a region on the
surface. The earliest and simplest AR coatings involved refractive
index matching wherein the refractive index of the selected coating
would fall between the refractive index of the underlying substrate
and the refractive index of the external medium, in turn reducing
reflection at the coating-external medium and coating-substrate
interfaces. More recently developed AR coatings involve
single-layer or multi-layer interference systems; while these
coatings are more versatile, the optical properties of the
underlying substrate must still be taken into account in their
construction. The use of conventional AR coatings is, therefore,
generally limited to substrates having consistent optical
properties across the substrate surface. There is, accordingly, an
ongoing need in the art for technology that substantially reduces
unwanted reflections from non-optically active areas on optical
sensor chips in which optical properties vary across the substrate
surface. An ideal such method would also enable prevention of light
transmission into one or more surface regions of the substrate.
SUMMARY OF THE INVENTION
[0005] Accordingly, the invention is directed to the aforementioned
need in the art and provides a system for minimizing reflections
from non-optically active regions of a substrate surface, i.e., a
substrate surface on which there is at least one optical property
that varies across that surface. The system is generally in the
form of an optical sensor assembly comprising:
[0006] a substrate having a surface;
[0007] an optical image sensor at the substrate surface having an
aperture for detecting and receiving light and providing a signal
in response thereto, wherein the aperture defines an optically
active surface region, with the remainder of the substrate surface
defining a non-optically active surface region;
[0008] a dark mirror coating disposed over at least a portion of
the substrate surface but not substantially in the region of the
aperture, wherein the dark mirror coating is configured as an
absorptive anti-reflective coating to reduce reflections from the
non-optically active surface region and to reduce transmission
through the non-optically active surface region and into the
substrate, and comprises an initial absorbing layer provided on the
substrate surface and an outwardly facing dielectric layer on the
initial absorbing layer.
[0009] The reduction in stray light is achieved by virtue of the
fact that the dark mirror coating both reduces unwanted reflections
from non-optically active surface regions and reduces unwanted
transmission of light through those regions and into the
substrate.
[0010] In another embodiment, an optical sensor assembly is
provided as above in which an optical thin film filter is disposed
over the optical image sensor. The thin film filter is configured
to filter light so that only light in a selected wavelength range
passes through the filter and reaches the sensor. The dark mirror
coating may be disposed under the optical thin film filter or over
the thin film filter and form an aperture. In a variation on this
embodiment, there are two dark mirror coatings, one disposed under
the optical thin film filter and the other over the thin film
filter. The configuration of the optical sensor assembly in this
regard, i.e., with respect to the positioning of the dark mirror
coating, will depend on the structure, materials, and requirements
of a particular sensor assembly.
[0011] In an additional embodiment, the optical sensor assembly of
the invention is provided with two or more optical sensors at the
substrate surface, each having an aperture for detecting and
receiving light and providing a signal in response thereto.
[0012] Another embodiment of the invention pertains to an optical
sensor assembly in which the dark mirror coating comprises at least
one pair of a light absorbing layer and a substantially
non-absorbing layer, such that in some cases the dark mirror
coating will comprise a stack of alternating absorbing and
substantially non-absorbing layers. In this embodiment, the optical
sensor assembly comprises:
[0013] a substrate having a surface;
[0014] an optical image sensor at the substrate surface having an
aperture for detecting and receiving light and providing a signal
in response thereto, wherein the aperture defines an optically
active surface region, with the remainder of the substrate surface
defining a non-optically active surface region;
[0015] a dark mirror coating disposed over at least a portion of
the substrate surface but not substantially in the region of the
aperture, wherein the dark mirror coating is configured as an
absorptive anti-reflective coating to reduce reflections from the
non-optically active surface region and to reduce transmission
through the non-optically active surface region and into the
substrate, and further wherein the dark mirror coating
comprises
[0016] a first pair of layers provided on the substrate surface,
the first pair of layers including a first substantially
non-absorbing layer disposed directly on the substrate surface and
a first absorbing layer adjacent to and overlying the first
substantially non-absorbing layer, optionally at least one
additional pair of layers including a substantially non-absorbing
layer and an absorbing layer configured such that the absorbing
layers and the substantially non-absorbing layers alternate, and an
outwardly facing dielectric layer serving as the surface of the
dark mirror coating.
[0017] Any one absorbing layer may actually be comprised of two or
more individual absorbing layers, or "sublayers," and, similarly,
any one substantially non-absorbing layer may be comprised of two
or more individual substantially non-absorbing sublayers. Further,
different absorbing materials may be selected for each of the
absorbing layers in an optical sensor assembly that comprises two
or more absorbing layers; it is not essential that each absorbing
layer be composed of a material identical to that used for all
other absorbing layer. The same is true for the substantially
non-absorbing layers, i.e., different substantially non-absorbing
materials may or may not be selected for each of the substantially
non-absorbing layers in an optical sensor assembly that comprises
two or more substantially non-absorbing layers.
[0018] Additional objects, advantages, aspects, and novel features
of the invention will be set forth in part in the description which
follows, and in part will become apparent to those skilled in the
art upon examination of the following, or may be learned by
practice of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a visible light reflectance spectrum at near
normal incidence of a dark mirror coating of the invention, and
indicates the percent reflectance from the coated side of the
substrate, the percent reflectance from inside the substrate, and
the optical density.
[0020] FIG. 2 is a schematic cross-sectional view of an optical
sensor assembly of the invention, in which a dark mirror coating is
provided on the non-optically active areas of the substrate
surface, extending from a central optical thin film coating
covering the image sensor to the bond pads.
[0021] FIG. 3 is a schematic cross-sectional view of another
embodiment of an optical sensor assembly of the invention, in which
a dark mirror coating is provided on the non-optically active areas
of the substrate surface over an optical thin film coating that
covers the sensor and extends to the bond pads.
[0022] FIG. 4 is a schematic cross-sectional view of an additional
embodiment of an optical sensor assembly of the invention, in which
a dark mirror coating overlies a substrate in which there are two
image sensors.
[0023] FIG. 5 is a schematic cross-sectional view of an alternative
embodiment of an optical sensor assembly of the invention, in which
a dark mirror coating overlies non-optically active areas of the
substrate surface, and an optical thin film filter is then provided
that partially or completely covers the dark mirror-coated
regions.
[0024] FIG. 6 is a schematic cross-sectional view of an additional
alternative embodiment of an optical sensor assembly of the
invention, in which a first dark mirror coating is disposed on
non-optically active areas of the substrate surface, an optical
thin film filter is provided that covers the dark mirror-coated
regions as well as the sensor aperture, and a second dark mirror
coating overlies the optical thin film filter.
[0025] FIGS. 7A through 7D (collectively FIG. 7) are
cross-sectional scanning electron microscope (SEM) photos of an
optical sensor assembly of the invention. The optical image sensors
are photodiodes, shown in FIG. 7A (at 200X), with two image sensors
shown in FIG. 7B at a higher magnification (700X). Two regions are
identified in FIG. 7B that are focused on at higher magnification
in FIGS. 7C and 7D (both at 15 k.times.). The dark mirror coating
is shown disposed over a surface defect (FIG. 7C) and over a
tapered region of an optical thin film filter (FIG. 7D).
[0026] FIGS. 8A through 8D (collectively FIG. 8) are additional
cross-sectional SEM photos of the optical sensor assembly in which
two different regions of the substrate surface are identified for
enlarging at 15 k.times., with FIG. 8C illustrating the dark mirror
coating disposed over a surface defect in the form of a "foot," and
FIG. 8D illustrating the dark mirror coating tapering over the
optical thin film filter.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Unless defined otherwise, all technical and scientific terms
used herein have the meaning commonly understood by one of ordinary
skill in the art to which the invention pertains. Specific
terminology of particular importance to the description of the
present invention is defined below.
[0028] In this specification and the appended claims, the singular
forms "a," "an" and "the" include plural referents unless the
context clearly dictates otherwise. Thus, for example, "an
absorbing layer" refers not only a single layer but also to a
combination of two or more absorbing layers, "an absorbing
material" refers to a single absorbing materials as well as to a
combination (e.g., mixture) of two or more absorbing materials, a
"gray metal" refers to a single gray metal or to a mixture of
different gray metals, and the like.
[0029] By "absorbing" as that term is used herein to describe
certain layers and materials is meant that a particular layer or
material exhibits an absorbance of greater than about 0.4,
typically greater than about 0.6, and more typically greater than
about 0.7 (which in terms of percent transmittance, or % T,
corresponds to an approximate % T of less than 40%, more typically
less than 25%, and most typically less than 20%) at a particular
wavelength or within a particular wavelength range.
[0030] By "substantially non-absorbing" as that term is used herein
to describe certain layers and materials is meant that a particular
layer or material exhibits an absorbance of less than about 0.2,
typically less than about 0.13, most typically less than about 0.1
(which, again, in terms of percent transmittance, corresponds to an
approximate % T of greater than 60%, typically greater than 75%,
and more typically greater than about 80%) at a particular
wavelength or within a particular wavelength range.
[0031] The term "external medium" is used in conjunction with the
description of a coated substrate in which the coating lies between
the substrate and the "external medium," which may be, for
instance, air, water, oil, epoxy, or any number of other
materials.
[0032] As noted above, the invention in one aspect provides an
optical sensor assembly in which reflections from non-optically
active regions of a substrate surface as well as light transmission
through the non-optically active regions into the substrate are
both minimized. The invention is effective in this regard even in
the instance where at least one optical property varies across the
substrate surface. By "optical property" is meant a property
relating to the interaction of the surface material with light. By
"at least one optical property" is generally meant at least one of
absorption, transmission, reflection, and scatter.
[0033] For instance, tapered regions of a coating or layer, such as
an optical thin film filter, can cause optical properties to vary
in the location of the taper. As another example, a drop in the
height of a layer or series of layers (such as in a "foot" defect")
can cause optical properties to vary in that location. By
appropriate placement of a dark mirror coating as described infra,
unwanted reflection from non-optically active surface regions can
be eliminated or at least significantly reduced, even in those
instances wherein optical properties vary across the substrate
surface as just described. Placement of a dark mirror coating as
provided herein simultaneously reduces light transmission through
the non-optically active surface regions into the substrate. This
is an important consideration as well, for example in preventing
light from reaching the electronic circuitry associated with the
image sensor and being converted to an unwanted electronic signal,
and in preventing light for transmitting into an underlying optical
filter structure.
[0034] The optical sensor assembly of the invention includes a
substrate housing an optical image sensor and associated electronic
circuitry. The optical image sensor has an aperture for detecting
and receiving light and provides an optical signal in response
thereto, wherein the aperture defines an optically active surface
region, with the remainder of the substrate surface defining a
non-optically active surface region. When the optical sensor
assembly contains two or more optical image sensors, it is to be
understood that each aperture defines an optically active surface
region, with the remainder of the substrate surface defining a
non-optically active surface region.
[0035] The optical sensor may comprise any type of optical image
sensor suited to a particular application, including a
single-channel discrete detector or a photodetector array-type
sensor (for example linear 1-D or areal 2-D array), wherein the
photodetectors may be photodiodes, phototransistors, or the like.
Generally, the optical sensor is a CCD (charge coupled device) or
CMOS (complementary metal oxide semiconductor) image sensor, both
of which, as is well known in the art, depend on the photoelectric
effect to create an electronic signal from light. In a CCD, an
image is projected through a lens onto the optically active region,
which, in the case of the CCD, is a capacitor array, in turn
causing each capacitor to accumulate an electrical charge
proportional to the intensity of light at that location. The
associated electronic control circuitry causes a cascade of charge
transfer ultimately directed into a charge amplifier, which then
converts the charge into a voltage. Repetition of the process
results in conversion of the entire contents of the capacitor array
to a sequence of voltages. In a digital device, these voltages are
then sampled, digitized, and usually stored in memory; in an analog
device (such as an analog video camera), they are processed into a
continuous analog signal (e.g. by feeding the output of the charge
amplifier into a low-pass filter), which is then processed and fed
out to other circuits for transmission, recording, or other
processing. In a CMOS sensor, each pixel within the optically
active region undergoes its own charge-to-voltage conversion, a
massively parallel process that provides for high speed imaging and
a higher signal-to-noise ratio. Front and back illuminated sensors
are also useful in conjunction with the present invention.
[0036] A variety of different materials are utilized in the
manufacture of sensors, each of which has its own surface
properties and reflectance. A non-limiting list of sensor materials
includes silicon, germanium, indium gallium arsenide (InGaAs),
platinum silicide (PtSi), mercury cadmium telluride (MCT, HgCdTe),
lead sulfide (PbS), indium antimonide (InSb), mercury zinc
telluride (MZT, HgZnTe), lead selenide (PbSe), lithium tantalate
(LiTaO3), indium antimonide (InSb), triglycine sulfate (TGS and
DTGS), vanadium pentoxide, and indium arsenide (InAs).
[0037] The coatings used in connection with the present invention
are "dark mirror" coatings, i.e., coatings that reduce both
reflection and transmission and are thus absorptive anti-reflective
(AAR) coatings. These coatings are configured to reduce reflection
and transmission of light in a specific spectral bandwidth, e.g., a
wavelength range within the visible light spectrum, a wavelength
range within the near-infrared (near-IR) or IR spectrum, and the
like. In some instances, the dark mirror coating is deposited
directly on the sensor material (such as silicon, as would be the
case for a back illuminated sensor). In other instances, the dark
mirror coating can be deposited onto an oxide or nitride layer that
has been previously deposited or formed on the surface of the
sensor material (for example SiOx or SiNx). In still other
instances, the dark mirror coating can be deposited onto a single
or multi-layer optical coating or optical filter. In each case, the
dark mirror affects the spectral reflectance and transmittance of
the previously uncoated non-optically active surface.
[0038] The absorbing, anti-reflective dark mirror coating is
designed to provide the optimum suppression of unwanted reflections
from non-optically active areas on the surface of the optical
sensor assembly as well as unwanted transmission through those
non-optically active areas into the substrate. As will be
understood by those of ordinary skill in the art, the dark mirror
can be optimized to suppress reflection and transmission over a
narrow wavelength range or a more broad range of wavelengths,
depending on the electromagnetic radiation that the sensor is
capable of detecting and receiving. A visible light sensor may be
paired with a dark mirror designed to suppress wavelengths near the
visible spectrum, for example in the range of about 300 nm to 800
nm. A red-green-blue (RGB) light sensor may be paired with a dark
mirror designed to suppress wavelengths in both the visible and
near-infrared spectral regions, for example in the range of about
300 nm to 2500 nm. An ultraviolet sensor may be paired with a dark
mirror designed to suppress wavelengths near the ultraviolet, for
example in the range of about 100 nm to 450 nm. A near-infrared
sensor may be paired with a dark mirror customized to suppress
near-infrared wavelengths, for example in the range of about 700 nm
to 2500 nm. Dark mirrors can be designed for use with mid-infrared
sensors by suppressing reflections at wavelengths up to
approximately 7 micrometers. Dark mirrors can be designed for use
with far-infrared sensors by suppressing reflections at wavelengths
up to at least about 11 .mu.m.
[0039] The dark mirror can also be optimized to suppress reflection
and transmission over a range of incident light angles using
techniques and/or technology that is commercially available and/or
known to those of ordinary skill in the art. Commercially available
optimization software may be advantageously employed in this
context, and include various products available from OptiLayer GmbH
as well as TFCalc from Software Spectra, Inc. (Portland Oreg.). It
is generally desired that the dark mirror be able to suppress
reflections over a broad range of incident angles, to avoid direct
and/or scattered light from causing an increased amount of noise in
the sensor signal(s).
[0040] Absorbing anti-reflecting dark mirror coatings of the
present invention operate under the principles of constructive and
destructive interference of electromagnetic waves. For this reason,
the layers of coating material should have interfaces that are
approximately parallel to each other over the scale of the relevant
wavelength range. Dark mirror coatings and manufacture thereof have
been described. See, e.g., U.S. Pat. No. 4,898,435 to Jungkman et
al., U.S. Pat. No. 5,808,714 to Rowlands et al., and U.S. Patent
Publication No. 2004/0247906 A1 to Gasloli. Reference may also be
had to Philip W. Baumeister, Optical Coating Technology
(Bellingham, Wash.: SPIE--The International Society for Optical
Engineering, 2004), in Chapter 8, and to Physics of Thin Films:
Advances in Research and Development, Eds. George Haas et al. (New
York: Academic Press, Inc., 1982). The pertinent sections of the
foregoing patent documents and text are incorporated by reference
herein in their entireties.
[0041] In a first embodiment, the dark mirror coating used in
conjunction with the present invention is composed of an initial
absorbing layer on the substrate surface and an outwardly facing
dielectric layer disposed thereon. In context, this embodiment
provides an optical sensor assembly that includes the
aforementioned substrate, i.e., an active device wafer surface, an
optical image sensor at the substrate surface that has an aperture
for detecting and receiving light and providing a signal in
response thereto, where the aperture defines an optically active
surface region, the remainder of the substrate surface defining a
non-optically active surface region. The dark mirror coating is
disposed over at least a portion of the substrate surface but not
substantially in the region of the aperture, meaning that the
coating blocks less than 20%, typically less than 10%, and
optimally less than 5% of the aperture. The dark mirror coating, as
noted earlier herein, is configured as an absorptive
anti-reflective coating to reduce reflections from the
non-optically active surface region and to reduce transmission
through the non-optically active surface region into the substrate.
In this embodiment, the dark mirror coating comprises the initial
absorbing layer on the substrate surface with an outwardly facing
dielectric layer disposed on the initial absorbing layer, the
outwardly facing dielectric layer serving as the surface of the
dark mirror coating. This embodiment of the optical sensor assembly
with the dark mirror coating can be represented as follows:
[0042] Active device wafer surface/M/D-ext
where M is the initial absorbing layer and D is the outwardly
facing dielectric layer. The material selected for M is generally
aluminum, a gray metal, or an alloy thereof, i.e., an alloy of two
or more gray metals, an alloy of at least one gray metal and at
least one other metal, an alloy of aluminum and at least one other
metal that may or may not include a gray metal. Examples of gray
metals useful herein include, without limitation, tantalum,
niobium, titanium, nickel, chromium, silicon, and alloys thereof,
particularly tantalum, niobium, and tantalum-niobium alloys such as
Ta.sub.80Nb.sub.20, Ta.sub.60Nb.sub.40, Ta.sub.40Nb.sub.60, and
Ta.sub.20Nb.sub.80. The dielectric surface layer is composed of a
dielectric material or a combination or mixture of two or more
dielectric materials, examples of which are provided infra. For
optimal anti-reflective performance, the outermost dielectric layer
should have a low refractive index. Selection of dielectric
materials for use herein at a particular wavelength or within a
particular wavelength range may also be carried out with reference
to the n(.lamda.) and k(.lamda.) spectra of any candidate
material.
[0043] The initial absorbing layer M may be composed of two or more
distinct absorbing layers, or "sublayers," such that a stack of
absorbing sublayers essentially serves as a single absorbing layer.
The sublayers within the layer may be composed of identical
materials, similar materials, different materials, alternating
materials, or randomly distributed materials. Analogously, the
dielectric surface layer may be composed of two or more distinct
dielectric sublayers, which, similarly, may be the same or
different.
[0044] In another embodiment, the dark mirror coating includes a
first pair of layers, i.e., a first absorbing layer and a first
substantially non-absorbing layer, disposed on the substrate
surface, with, again, an outwardly facing dielectric layer serving
as the surface of the dark mirror coating. In this case, the first
substantially non-absorbing layer is disposed directly on the
substrate surface and the first absorbing layer is adjacent to and
overlies the first substantially non-absorbing layer. This
embodiment can be represented as follows:
[0045] Active device wafer surface/D1/M1/D-ext
wherein D1 represents the first substantially non-absorbing layer,
M1 represents the first absorbing layer, and D-ext represents the
surface dielectric layer.
[0046] The "DM pair" can define a period of a multilayer structure
in which the dark mirror coating is composed of two or more pairs
of a substantially non-absorbing layer and an absorbing layer. That
is, the dark mirror coating may contain, in addition to the first
"DM" pair, at least one additional pair of layers each including a
substantially non-absorbing layer and an absorbing layer configured
so that the absorbing layers and the substantially non-absorbing
layers alternate. One such embodiment may be represented as:
[0047] Active device wafer surface/D1/M1/D2/M2/D-ext
where D1 and D2 are the substantially non-absorbing layers and M1
and M2 are the absorbing layers.
[0048] A two-period dark mirror coating of the invention can be
represented as:
[0049] Active device wafer surface/D1/M1/D2/M2/D3/M3/D-ext where
D1, D2, and D3 are the substantially non-absorbing layers and M1,
M2, and M3 are the absorbing layers. It will be appreciated that
dark mirror coatings with additional DM pairs can also be used in
conjunction with the optical sensor assembly of the invention.
[0050] In the foregoing embodiments, each D1, D2, D3, etc.
comprises a substantially non-absorbing material, e.g., a
dielectric material, and can be composed of a single layer or two
or more sublayers, where the sublayers, similarly, may be the same
or different. Analogously, each M1, M2, M3, etc. comprises an
absorbing material and can be composed of a single layer or two or
more sublayers, where the sublayers, again, may be the same or
different.
[0051] Examples of absorbing materials suitable for the absorbing
layers in the embodiments wherein the dark mirror coating includes
at least one "DM" pair are metallic materials, including aluminum,
gray metals, and alloys thereof, as explained with respect to the
initial absorbing layer M, and include, for purposes of
illustration, tantalum, niobium, titanium, nickel, chromium,
silicon, and alloys thereof, particularly tantalum, niobium, and
tantalum-niobium alloys such as Ta.sub.80Nb.sub.20,
Ta.sub.60Nb.sub.40, Ta.sub.40Nb.sub.60, and Ta.sub.20Nb.sub.80.
Examples of substantially non-absorbing materials suitable for the
substantially non-absorbing layers are dielectrics such as
SiO.sub.2, Ta.sub.2O.sub.5, NbTaO.sub.5, Nb.sub.2O.sub.5,
TiO.sub.2, NbTiO.sub.x, Al.sub.2O.sub.3, Si.sub.3N.sub.4,
Cr.sub.2O.sub.3, MoO.sub.3, and combinations thereof.
[0052] The material pairs, i.e., the DM pairs, can include, without
limitation, alternating layers of metals and their oxides, such as
silicon and silicon oxide; titanium and titanium oxide; tantalum
and tantalum oxide; and chromium and chromium oxide.
[0053] When deposited onto the non-optically active surface regions
of the present optical sensor assembly, the dark mirror coating can
reduce the level of reflectance from uncoated values such as 9%,
23%, or 60% to the coated level of less than 1% (see FIG. 1). This
reduction of reflectance by one or two orders of magnitude greatly
suppresses the presence of ghost images and unwanted
noise-generating light reaching the sensor from the non-sensor
areas. By suppressing unwanted noise, the signal-to-noise ratio is
increased, thereby increasing both the accuracy and precision of
the sensor. The optical sensor assembly of the present invention
provides for reflectance in a wavelength band of the optical signal
that is less than about 10%, generally less than about 2%, more
typically less than about 1%, and optimally less than about 0.5% of
the optical signal itself, the reduction dependent in part on
wavelength range and the particular design vis-a-vis relative
positioning of aperture, optical filter, and dark mirror
coating.
[0054] The geometry of the present optical sensor assembly can be
important to its function. When dark mirror coatings are
incorporated into the sensor geometry, their placement can be
selected to improve the system performance while maintaining the
desired functionalities of the sensor, as will be evident from the
description, infra, regarding the optical sensor assemblies
depicted in the figures. For example, in some instances it is
desirable to coat the dark mirror across a patterned edge of an
optical filter to prevent unwanted light from entering the filter
at its edge, where it could scatter sideways and become trapped
either in the optical filter or between the substrate and the
filter surface, and eventually scattered towards the detector.
[0055] Dark mirror coatings can be deposited onto non-optically
active areas of the surface of the optical sensor assembly using a
variety of techniques. Non-limiting examples include physical vapor
deposition, chemical vapor deposition, spin coating, reactive
sputtering, or other techniques as will be recognized by those of
ordinary skill in the art. Combinations of techniques may also be
used.
[0056] Several well-known techniques such as photolithography and
physical masking can be used to allow deposition of the dark mirror
in selected areas, i.e., the non-optically active areas, while
avoiding deposition in other areas.
[0057] FIG. 2 illustrates one embodiment of the invention in
schematic cross-sectional view. The sensor assembly is shown
generally at 10 with the image sensor 12 centrally positioned in
substrate 14. Optical thin film filter 16 covers the surface of the
sensor and extends outwardly, beyond the edges of the sensor, to
overlie regions of the substrate surface 18 that are directly
adjacent to the sensor. Dark mirror coating 20 extends from each
tapering region 22 and 22' of the optical thin film filter to each
bond pad 24 and 24', which serve as electrical connection points to
the associated electronic circuitry (shown as 26 and 26', which may
be in separate regions of the substrate, as shown, or combined and
present in a single region of the substrate) and are thus in
electrical communication therewith. The optical thin film filter
covers the sensor and directly adjacent regions, as indicated. If
the sensor is intended to measure wavelengths of green light, the
optical thin film filter may function as a green filter;
analogously, if the sensor is intended to measure wavelengths of
red light, the optical thin film filter may function as a red
filter. Optionally, the dark mirror can be made larger such that
the outer regions of the optical thin film are physically covered
by a portion 28 and 28' (i.e., the regions under the dotted lines
shown in the figure) of the dark mirror. This is a desirable
outcome in some instances, as the dark mirror in these cases can
prevent light from leaking into the sensor from the edges of the
optical thin film region. It is important that the distance between
the perimeter of the dark mirror and the sensor perimeter be
sufficient to prevent blocking of too great an area of the sensor
by the dark mirror, as this would unnecessarily reduce the amount
of light reaching the sensor and potentially degrade sensor
performance.
[0058] FIG. 3 illustrates another embodiment of the invention in
schematic cross-sectional view, with the optical sensor assembly
shown generally at 30. The image sensor 32 is again centrally
positioned in substrate 34. In this embodiment, an optical thin
film filter 36 covers both the sensor 32 and the non-optically
active adjacent substrate surface regions 38. In this embodiment,
the dark mirror coating 40 is applied to non-optically active areas
adjacent to the sensor, overlying the optical thin film filter 36
except in the region of aperture 42 approximately vertically
aligned with the image sensor. Dark mirror coating 40 extends from
each edge of aperture 42 toward each bond pad 44 and 44',
terminating in the region thereof; typically, the distance between
the perimeter of the dark mirror and the bond pad is in the range
of about 30 .mu.m to about 100 .mu.m. The bond pads, again, serve
as electrical connection points to the associated electronic
circuitry (shown as 46 and 46', which, as in the embodiment of FIG.
2, may be in separate regions of the substrate, as shown, or
combined and present in a single region of the substrate) and are
thus in electrical communication therewith. Again, there is a
trade-off, and it is important that the dark mirror not be so large
as to block too large a fraction of the sensor, thereby
unnecessarily reducing the amount of light reaching the sensor.
[0059] FIG. 4 illustrates an alternative embodiment of the
invention in schematic cross-sectional view. In FIG. 4, optical
sensor assembly 48 contains two image sensors, a first image sensor
50 and a second image sensor 52, both positioned in the surface 54
of substrate 56. It will be appreciated that this is a
representative embodiment, and that additional alternative
embodiments can be envisioned containing three or more image
sensors in a substrate surface. Optical thin film filters 58 and
58' cover the surface of first image sensor 50 and second image
sensor 52, respectively. Three regions of dark mirror coating are
shown at 60, 62, and 64, with coating regions 60 and 64 overlying
the substrate surface 54 and extending from each outer tapering
region 66 and 68 of the first and second image sensors 50 and 52,
respectively, to each bond pad 70 and 70', which, as above, serve
as electrical connection points to the associated electronic
circuitry (shown as 72, 74, and 74', which, again, may be in
separate regions of the substrate, as shown, or combined and
present in a single region of the substrate) and are thus in
electrical communication therewith.
[0060] Another embodiment of the invention is illustrated
schematically in cross-section in FIG. 5. The optical sensor
assembly 76 is shown with a single image sensor 78 positioned
centrally in the surface 80 of substrate 82. In this embodiment,
the dark mirror coating 84 and 86 are underneath the optical thin
film filter 88. The optical filter 88 and dark mirror coating 84
can be coterminal as shown at 90. In this case, the arrangement
does not prevent reflection from the thin film filter surface 94,
but the dark mirror coating 84 does block light into the substrate
82 and the electronic circuitry 96 contained therein, as well as
preventing reflections between the substrate 82 and the dark mirror
coating 84. Alternatively, and as shown at 92, the optical thin
film filter 88 overlying dark mirror coating 86 may extend only
partway across that coating, leaving a region 98 of the dark mirror
coating that is exposed as a surface. The inverted embodiment
wherein the dark mirror coating is under the optical filter may be
advantageously employed when the optical filters are relatively
thick. This inverted embodiment may also be advantageously employed
when two or more image sensors are present in a single optical
sensor assembly and two adjacent sensors are in close proximity, so
as to prevent cross-talk between the adjacent sensors.
[0061] FIG. 6 illustrates an additional embodiment of the invention
in schematic cross-sectional view, wherein the dark mirror coating
is provided on both sides of an optical thin film filter. The
optical sensor assembly is shown generally at 100, with the optical
image sensor 102 positioned centrally in the surface 104 of
substrate 106. Inner dark mirror coatings 108 and 110 are
underneath the optical thin film filter 112 and outer dark mirror
coatings 114 and 116 disposed on the opposing side of the filter.
The bond pads 118 and 118' again serve as electrical connection
points to the associated electronic circuitry (shown as 120 and
122, which, again, may be in separate regions of the substrate, as
shown, or combined and present in a single region of the
substrate).
[0062] In all of the above-described embodiments, it is generally
desired to avoid depositing the dark mirror over the electric
connection points, sometimes referred to as bond pads, to allow
electrical contact to be made at the electrical connection points.
As noted earlier herein, the dark mirror coating terminates in the
region of the bond pad, but does not cover it. Typically, the dark
mirror coating terminates between the edge of the sensor and the
bond pad, with a distance between the two in the range of about 30
.mu.m to about 100 .mu.m.
[0063] The SEM photos in FIG. 7 illustrate one function of the dark
mirror coating herein, in terms of masking surface defects at an
edge of the aperture (FIGS. 7B and 7C) and in the tapering region
of an optical thin film filter, where the individual layers of the
filter become narrower and appear to merge, potentially altering
the function of the filter (FIG. 7D). In FIG. 8, the ability of the
dark mirror coating to essentially smooth over a physical defect at
the opposing aperture edge is shown in the SEM photos of FIGS. 8B
and 8C. A tapering region of the dark mirror coating overlying the
thin film optical filter may be seen in the SEM photo of FIG.
8D.
[0064] A visible dark mirror coating of the invention was prepared
using a sputtering deposition technique as described in U.S. Pat.
No. 8,163,144. The individual layers of the coating were
successively deposited on a fused silica substrate with the
following layers, ordered from innermost, i.e., on the substrate,
to outermost:
TABLE-US-00001 n k Phys. Th., Layer # Material @ 550 nm @ 550 nm nm
1 Ta.sub.2O.sub.5 2.178 0 9.2 2 SiO.sub.2 1.475 0 36.9 3
Ta.sub.2O.sub.5 2.178 0 45.2 4 Ta 3.227 2.927 6.9 5 SiO.sub.2 1.475
0 75.4 6 Ta 3.227 2.927 7.0 7 SiO.sub.2 1.475 0 66.7 8 Ta 3.227
2.927 5.7 9 SiO.sub.2 1.475 0 148.2 10 Ta 3.227 2.927 6.2 11
SiO.sub.2 1.475 0 332.8 12 Ta 3.227 2.927 9.8 13 SiO.sub.2 1.475 0
228.3 14 Ta 3.227 2.927 16.6 15 SiO.sub.2 1.475 0 81.7 16 Ta 3.227
2.927 6.2 17 SiO.sub.2 1.475 0 81.2
[0065] FIG. 1 is a reflectance spectrum obtained at wavelengths in
the range of 300 nm to 800 nm, and indicates the percent
reflectance from the coated side of the substrate, the percent
reflectance from inside the substrate, and the optical density. The
dark mirror coating was found to reduce the level of reflectance
from (uncoated) values such as 9%, 23%, or 60% to less than 1%.
[0066] It is to be understood that while the invention has been
described in conjunction with a number of specific embodiments, the
foregoing description is intended to illustrate and not limit the
scope of the invention. Other aspects, advantages and modifications
will be apparent to those skilled in the art. All patents, patent
applications, and publications mentioned here are hereby
incorporated by reference in their entireties.
* * * * *