Light absorbing coating with high absorption capacity

Details Light absorbing coating with high absorption capacity Light absorbing coating with high absorption capacity

1998-11-16

2002-01-01

Baxter, Janet (Department: 1752)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Radiation sensitive composition or product or process of making

C430S275100, C430S495100, C430S525000, C430S271100

Reexamination Certificate

active

06335142

ABSTRACT:

TECHNICAL FIELD
This invention relates to a light absorbing coating.
It is a thin multi-layer coating, with an absorption function in the visible and/or near infrared spectral range.
The invention is particularly applicable to imagery, to limit parasite reflections and improve the separation between different detection channels, display screens and optical disks in which the function of the coating is to improve the contrast.
The invention is particularly applicable to:
optical systems designed for space applications,
high precision imagery (telemetry),
flat liquid crystal display or microtip display, and
high definition television.
STATE OF PRIOR ART
One known method of making an absorbent coating (in the visible-near infrared range) consists of trapping a maximum amount of light that penetrates into the coating, and if possible to limit the amount of reflected light.
In most cases, parasite reflections are restricted by depositing an anti-reflection layer (AR) with a wide spectral range, on an absorbent layer.
The global structure thus defined may be made symmetric if necessary in order to limit light reflections from the substrate side or from the air side.
A distinction is made between three main categories of structures, depending on the required performances:
1) substrate/absorbent layer/air type structures,
2) substrate/absorbent layer/AR layer/air type structures, and
3) substrate/AR layer/absorbent layer/AR layer/air type structures.
A highly reflecting opaque metal layer may be deposited on the substrate beforehand, to limit optical transmission through these structures.
A very wide variety of techniques and materials are used to make absorbent layers, often called “black matrices”.
The most frequent manufacturing method is to use a base layer (of polymer, resin or glass), into which absorbent pigments are added.
These absorbent pigments may for example be compounds based on iron oxide, cobalt aluminate or graphite.
The techniques used to deposit this type of composite absorbent layer very frequently include rolling.
However, there are other methods such as electrodeposition, vacuum deposition, or even anodic oxidation, calcination or laser annealing to improve surface absorption.
One known technique is to use absorbent layers made of black chromium obtained by electrodeposition making use of an electrolytic bath with a composition that is adjusted so that the color obtained is black.
This subject is described in document 1, which like other documents referenced later, is mentioned in the references at the end of this description.
We will consider vacuum deposition techniques in particular.
In this case, absorbent layers are generally inserted in an interference structure with thin stacked layers based on dielectric materials, particularly oxides.
There are then structures type 2) and type 3).
The antireflection function may be provided conventionally by multiple layers, with alternating high and low optical indexes n.
This is done for example using TiO
2
layers (n is approximately equal to 2.4) and SiO
2
layers (n is approximately equal to 1.5), or MgF
2
layers (n is approximately equal to 1.39).
The thickness of these layers is of the order of a few tenths of a micrometer, and the way in which they are stacked controls the spectral width and the central wave length of the band or antireflection function.
Several solutions have been proposed for the manufacture of the actual absorbent layers.
Absorbent layers frequently consist of oxides which are naturally absorbent or which are made absorbent by creating an oxygen deficiency at the time of deposition (sub-stoechiometric oxides).
For example, inherently absorbent compounds may include indium oxide, tungsten oxide, chromium oxide, tin oxide (see document 2), or even vanadium oxide.
Among these various compounds, note in particular that CrO
2
, In
2
O and SnO oxides are black.
Concerning sub-stoechiometric oxides, most compounds that are transparent in their stable form, may become absorbent if an oxygen depletion is created in them.
For example, this is the case for NiO
x
nickel oxides recommended in document 3, which are inserted in a double TiO
2
/SiO
2
/NiO
x
/TiO
2
/SiO
2
type of anti-reflection structure.
Note also the structure proposed in document 4, which is composed of a multiple layer stack consisting of Cr/absorbent dielectric/AR layer.
The absorbent layer is then preferably composed of manganese oxide, or chromium or iron oxide, or possibly silica containing dispersed chromium.
Other materials such as nitrides (TiN and ZrN mentioned in document 3) may also form useful absorbent layers.
In particular, it is well known that titanium nitride layers TiN
x
offer a wide range of green-bronze to golden yellow colors, depending on the value of x.
Note also the use of more exotic absorbent materials such as the SiGe compound (see document 5).
Note also the absorbent stack Cr/Cr
2
O
3
/Cr/Cr
2
O
3
that is used to improve the contrast of liquid crystal screens (see document 6), and which includes layers with a thickness varying from 10 nm to a few tens of nanometers.
Light is generally absorbed using more or less absorbent materials, in which the extinguishing coefficient k normally remains less than or equal to about 10
−1
in the visible-near infrared range (k being of the order of 10
−4
to 10
−5
for a transparent dielectric).
In this case, if it is required to make a layer with quasi-total absorption, a layer must be used with a thickness equal to at least 1 &mgr;m, and it must be provided with anti-reflection layers to limit reflections at the air/layer interface.
A better way of maximizing absorption is to use an absorbent structure formed of N layers of Cr alternating with N layers of Cr
2
O
3
, given the high extinguishing coefficient k of chromium (k is equal to 4 in the visible range).
Absorption of at least 95% of the incident light in the visible range then requires that several Cr/Cr
2
O
3
pairs are stacked (N>2), hence the total thickness of chromium and chromium oxide is at least 100 nm to 200 nm.
The thickness of this structure must increase as the required absorption gets closer to 100%.
The performance is then limited due to mechanical strength problems.
It is well known that thin layers of chromium are some of the most highly stressed metals, which typically limits its thickness to 200 nm (see document 7).
Above this thickness the layer crazes and separates.
Therefore, guaranteeing mechanical stability of the absorber requires that the thinnest possible multilayer structure should be used, and using materials with low stresses.
DESCRIPTION OF THE INVENTION
The purpose of this invention is to design a multilayer coating with a high absorption capacity (absorption of more than 95% of light) in the visible near infrared range, while minimizing risks of mechanical instability related to stresses in the layers in this absorbent coating.
The invention achieves this using an absorbent multi-layer structure that includes at least one thin metal layer as an absorption element, with the specific feature that it is optically discontinuous.
In particular, as we will see in the examples given later, this restricts the total thickness of the absorber to less than 200 nm, while providing absorption of between 97% and 99% within the visible-near infrared range.
Specifically, the purpose of the invention is a light absorbing coating in a given spectral range within the visible-near infrared range, this coating being formed on a substrate and characterized in that it comprises:
at least one layer of thin metal which is absorbent in this determined spectral range, and
at least one dielectric layer which is transparent in this determined spectral range, this dielectric layer being formed on this thin metal layer,
and in that this thin metal layer is optically discontinuous, its refraction index being greater than the refraction index of the metal in the bulk state, and its extinguishing coefficient being less than the extinguishing coefficient of the metal in the bulk state, within the

Affiliated with

Also associated with

Rating

Light absorbing coating with high absorption capacity does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Light absorbing coating with high absorption capacity, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Light absorbing coating with high absorption capacity will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2821122