U.S. patent application number 10/354289 was filed with the patent office on 2003-07-17 for modification of infrared reflectivity using silicon dioxide thin films derived from silsesquixane resins.
This patent application is currently assigned to The Regents of the University of Michigan. Invention is credited to Banaszak Holl, Mark Monroe, Biscotto, Mark Angelo, Orr, Bradford Grant, Pernisz, Udo C..
Application Number | 20030134136 10/354289 |
Document ID | / |
Family ID | 23808728 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030134136 |
Kind Code |
A1 |
Biscotto, Mark Angelo ; et
al. |
July 17, 2003 |
Modification of infrared reflectivity using silicon dioxide thin
films derived from silsesquixane resins
Abstract
Changes in the infrared reflection spectrum of a thin film of
silica-like resinous material sandwiched between metal electrodes
can be induced by applying an electric potential to a top electrode
which is semitransparent. Characteristic infrared absorption lines
change in proportion to a small electric current flowing through
the material. These changes occur with response times of the order
of seconds, and show time constants of the order of minutes to
reach stationary values.
Inventors: |
Biscotto, Mark Angelo; (Ann
Arbor, MI) ; Banaszak Holl, Mark Monroe; (Ann Arbor,
MI) ; Orr, Bradford Grant; (Ann Arbor, MI) ;
Pernisz, Udo C.; (Midland, MI) |
Correspondence
Address: |
BROOKS & KUSHMAN
1000 TOWN CENTER 22ND FL
SOUTHFIELD
MI
48075
|
Assignee: |
The Regents of the University of
Michigan
Ann Arbor
MI
|
Family ID: |
23808728 |
Appl. No.: |
10/354289 |
Filed: |
January 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10354289 |
Jan 30, 2003 |
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09455420 |
Dec 6, 1999 |
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6572974 |
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Current U.S.
Class: |
428/472 ;
257/E21.008; 257/E21.262; 257/E21.271; 257/E45.001; 428/209;
428/336 |
Current CPC
Class: |
H01L 21/02236 20130101;
H01L 28/40 20130101; Y10T 428/12611 20150115; Y10T 428/12493
20150115; Y10T 428/12736 20150115; Y10T 428/24917 20150115; H01L
21/02345 20130101; Y10T 428/12535 20150115; Y10T 428/12674
20150115; Y10T 428/31678 20150401; Y10T 428/12861 20150115; Y10T
428/12771 20150115; Y10T 428/12889 20150115; Y10T 428/12944
20150115; Y10T 428/31663 20150401; G02F 1/19 20130101; H01L 21/3124
20130101; Y10T 428/12896 20150115; Y10T 428/12903 20150115; H01L
21/02164 20130101; Y10T 428/265 20150115; Y10T 428/12806 20150115;
H01L 21/316 20130101; H01L 45/00 20130101 |
Class at
Publication: |
428/472 ;
428/209; 428/336 |
International
Class: |
B32B 009/00 |
Claims
1. A method of varying the infrared spectrum of a silicon dioxide
thin film derived from a silsesquioxane resin comprising (i)
directing a beam of infrared radiation to a metal-insulator-metal
device containing a silicon dioxide thin film derived from a
hydrogen silsesquioxane resin, an alkyl silsesquioxane resin, or an
aryl silsesquioxane resin, (ii) applying an electric potential
difference across the metal-insulator-metal device containing the
thin film, and (iii) monitoring the variation in the infrared
reflection or transmission spectrum of the thin film in response to
the electric current flowing through the thin film.
2. A method according to claim 1 in which the thin film is arranged
in the metal-insulator-metal device between an upper layer of metal
and a lower layer of metal, the metal of each layer being selected
from the group consisting of gold, palladium, platinum, silver,
chromium, aluminum, copper, nickel, titanium, and tin; and alloys
such as titanium-tungsten, titanium nitride, nickel-chromium,
indium tin oxide, and gallium arsenide.
3. A method according to claim 2 in which the upper layer of metal
has a thickness such that it is transparent or semitransparent to
the passage of a beam of infrared radiation, and the lower layer of
metal has a thickness such that the beam of infrared radiation can
be reflected, whereby when the beam of infrared radiation is
directed at the metal-insulator-metal device, the beam of infrared
radiation is able to pass through the upper layer of metal and be
reflected back by the lower layer of metal.
4. A method according to claim 3 in which the upper layer of metal
has a thickness in the range of about 0.005 .mu.m to 0.080 .mu.m (5
to 80 nanometer); the thin film has a thickness in the range of
about 0.1 .mu.m (100 nanometer) to 1.5 .mu.m (1,500 nanometer); and
the lower layer of metal has a thickness of at least about 0.15
.mu.m (150 nanometer).
5. A method according to claim 1 in which the beam of infrared
radiation has a wavelength in the range of about 2.5 .mu.m to 25
.mu.m.
6. A method according to claim 5 in which the beam of infrared
radiation is essentially monochromatic.
7. A method of modifying the spectral signature of a surface
coating of an object which is subject to remote observation and is
identifiable by its spectral reflectance comprising: (i) mounting
on the surface of the object a metal-insulator-metal device
containing a silicon dioxide thin film derived from a hydrogen
silsesquioxane resin, an alkyl silsesquioxane resin, or an aryl
silsesquioxane resin, (ii) applying an electric potential
difference across the metal-insulator-metal device containing the
thin film, and (iii) varying the infrared reflection spectrum of
the thin film in response to the electric current flowing through
the thin film.
8. A method according to claim 7 in which the object is an aircraft
or a watercraft.
9. In the processing or transmission of data in a communication or
computational device containing an optical switch, the improvement
comprising an optical switch which is a metal-insulator-metal
device containing a silicon dioxide thin film derived from a
hydrogen silsesquioxane resin, an alkyl silsesquioxane resin, or an
aryl silsesquioxane resin, the metal-insulator-metal device being
so constructed and arranged whereby (i) a beam of infrared
radiation can be directed to the metal-insulator-metal device
containing the thin film, so that when (ii) an electric potential
is applied to the metal-insulator-metal device containing the thin
film, (iii) the infrared reflection spectrum of the thin film is
varied in response to the electric current flowing through the thin
film.
10. An optical switch according to claim 9 in which the beam of
infrared radiation is a monochromatic beam having a wavelength
which coincides with a vibrational absorption of the thin film.
11. A coating for an object exposed to a beam of infrared radiation
comprising a metal-insulator-metal device containing a silicon
dioxide thin film derived from a hydrogen silsesquioxane resin, an
alkyl silsesquioxane resin, or an aryl silsesquioxane resin, means
for applying an electric potential difference to the
metal-insulator-metal device for varying the infrared reflection
spectrum of the thin film in response to electric current flowing
through the thin film, the metal-insulator-metal device including
an upper layer of metal having a thickness such that it is
transparent or semitransparent to the passage of a beam of infrared
radiation, and a lower layer of metal having a thickness such that
a beam of infrared radiation can be reflected, whereby when a beam
of infrared radiation is directed at the metal-insulator-metal
device, the beam of infrared radiation is able to pass through the
upper layer of metal and be reflected back by the lower layer of
metal.
12. A coating according to claim 11 in which the upper layer of
metal has a thickness in the range of about 0.005 .mu.m to 0.080
.mu.m (5 to 80 nanometer), the thin film has a thickness in the
range of about 0.1 .mu.m (100 nanometer) to 1.5 .mu.m (1,500
nanometer), and the lower layer of metal has a thickness of at
least about 0.15 .mu.m (150 nanometer).
13. A coating according to claim 11 in which the object is a window
pane.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
REFERENCE TO A MICROFICHE APPENDIX
[0003] Not applicable.
FIELD OF THE INVENTION
[0004] This invention is directed to silicon dioxide thin films
derived from certain silsesquioxane resins, in particular,
hydridosilsesquioxane resins, i.e., hydrogen silsesquioxane resins,
alkyl silsesquioxane resins, or aryl silsesquioxane resins, in
which the infrared reflection of a surface covered by the thin film
can be varied. BACKGROUND OF THE INVENTION
[0005] Silicon dioxide (SiO.sub.2) plays a crucial role in the
microelectronics industry as an insulator. Typically, the oxide is
generated by direct oxidation of silicon using oxygen. A variety of
plasma-based methods have also been developed to allow for lower
temperature processing. These methods, however, although being
capable of forming highly ordered oxides, do not generally lend
themselves to the tailoring of the dielectric properties of the
silicon dioxide for specific applications, and at the same time,
place considerable chemical restrictions on when such oxides can be
incorporated into an electrical device.
[0006] Thus, the generation of silicon oxides using certain
hydridosilsesquioxane (HSQ) resins has emerged as an important
alternative to such thermal and plasma-based methodologies. The use
of these HSQ resins, for example, allows the properties of the
derived silicon oxide to be tailored for specific applications
which run the gamut from computer chips to sensors.
[0007] However, despite the apparent chemical simplicity of these
systems, i.e., H, Si, and O, a detailed understanding of the
structure and the reactivity of such materials, particularly under
conditions experienced by an operating device, has yet to be fully
realized.
[0008] In this regard, while metal-insulator-metal (MIM) devices
exhibiting negative differential resistance (NDR) electrical
properties are generally known, for example, as shown in U.S. Pat.
No. 5,312,684 (May 17, 1994), and U.S. Pat. No. 5,403,748 (Apr. 4,
1995), the chemical basis for the striking changes in resistance
which occur as a function of an applied voltage or current is not
fully understood.
[0009] In probing the nature of changes in the dielectrics of such
devices, it was unexpectedly discovered that an MIM device could be
designed to allow for its simultaneous electrical and spectroscopic
characterization, and these characteristics of the device are
considered unique.
BRIEF SUMMARY OF THE INVENTION
[0010] The invention relates to varying the infrared reflection of
a surface covered by a silicon dioxide thin film derived from a
silsesquioxane resin. The resin can be, for example, a
hydridosilsesquioxane resin, an alkyl silsesquioxane resin, or an
aryl silsesquioxane resin. Hydridosilsesquioxane resins are
representative, and used hereafter in referring to silsesquioxane
resins in general. The concept involves:
[0011] (i) directing a beam of infrared radiation to a
metal-insulator-metal device containing the silicon dioxide thin
film derived from hydridosilsesquioxane resin,
[0012] (ii) applying an electric potential difference across the
metal-insulator-metal (MIM) device containing the silicon dioxide
thin film derived from hydridosilsesquioxane resin, and
[0013] (iii) monitoring variation in the infrared reflection
spectrum of the MIM device in response to the electric current
flowing through the silicon dioxide thin film as the frequency of
incident radiation is varied.
[0014] These and other features of the invention will become
apparent from a consideration of the detailed description.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0015] FIG. 1 is a top view and a pictorial representation of a
device according to the present invention, including a schematic
wiring diagram of the circuit including the power supply for the
device.
[0016] FIG. 2 is a side view and a pictorial representation of a
device as shown in FIG. 1, including a reflected beam of infrared
radiation.
[0017] In FIGS. 1 and 2, it can be seen that the MIM device
consists of a substrate such as a glass slide upon which there is
applied a gold bottom electrode, a dielectric layer consisting of a
thin film derived from hydridosilsesquioxane polymer, and a thin
palladium top electrode. A beam of infrared radiation, i.e., an
infrared light beam, passes through the palladium and HSQ-derived
thin film layers, and is reflected off the bottom gold electrode.
The thin palladium metal top electrode allows transmission of the
beam of infrared radiation and voltage ramping of the MIM
device.
[0018] FIG. 3 is a graphical representation of the spectroscopic
effect exhibited by devices according to the present invention,
showing various spectra and the relationship between the energy of
an infrared beam passing through the HSQ-derived thin film, and
changes in relative transmittance in percent exhibited by the thin
film. In particular, FIG. 3 shows measured infrared relative
reflection spectra relative to the initial spectrum of an
HSQ-derived thin film before application of an electric potential
difference as a function of the current flowing through the
HSQ-derived thin film.
[0019] The spectra depicted in FIG. 3 are the result of first
obtaining a background scan without electric tension V applied, so
that no current is flowing through the device. At the top is this
spectrum relative to itself, appearing as a straight line.
Subsequent spectra in FIG. 3 are the result of increasing the
applied electric tension V, taking another set of scans, and then
ratioing the results against the background scan.
[0020] It should be noted, in particular, that as the electric
tension V is applied, increases in the ratio (positive features)
can be clearly observed in the v (Si--H) and .delta. (Si--H)
regions, i.e., 2260 cm.sup.-1 and 898 cm.sup.-1 , respectively.
Both positive and negative features are also clearly apparent in
the v.sup.a (Si--O--Si) region between about 1200 cm.sup.-1 and
1000 cm.sup.-1. Additional features of interest, most likely
resulting from top electrode formation, can be pointed out at about
1500-1600 cm.sup.-1, 3663 cm.sup.-1 (SiOH), and at about 2200
cm.sup.-1 (O.sub.2SiH.sub.2). It is apparent, and the spectra in
FIG. 3 indicate, that dramatic changes occur in HSQ-derived
dielectric thin films as a function of current flowing through the
device.
[0021] FIG. 4 is a graphical representation showing the
relationship between the current flowing through the device of the
present invention, and the resultant area under the Si--H
vibrational stretch line at about 2,260 cm.sup.-1 of thin films
derived from HSQ resin.
[0022] FIG. 5 is similar to FIG. 4, but is a corresponding
graphical representation showing the relationship in terms of the
square root of the electrical tension V applied to the device,
i.e., V.sup.1/2.
[0023] FIG. 6 is an alternate embodiment of a device according to
the present invention which is similar to the device depicted in
FIG. 1, except that the device shown in FIG. 6 has a construction
embodying two devices rather than the single device shown in FIG.
1.
[0024] In this alternate embodiment, there are a pair of palladium
top electrodes located above the single bottom gold electrode which
extends beneath each top electrode. The cured HSQ thin film
functions as insulator strip, and covers only selected portions of
the bottom electrode as indicated in the drawing. Otherwise, this
embodiment of a device operates in similar fashion as the device
shown in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0025] In FIGS. 1 and 2, the MIM device is seen to include as one
of its component parts a dielectric layer consisting of a cured HSQ
thin film. The cured HSQ thin film can be produced, for example, by
applying a composition comprising hydridosilsesquioxane resin onto
a suitable reflective substrate such as a layer of gold.
[0026] The hydridosilsesquioxane resin includes hydridosiloxane
resins consisting of units of the formula
HSi(OH).sub.x(OR).sub.yO.sub.z/2 in which each R is independently
an organic group or a substituted organic group, which when bonded
to silicon through the oxygen atom, forms a hydrolyzable
substituent. In the formula, x has a value of 0 to 2; y has a value
of 0 to 2; z has a value of 1 to 3; and the sum of x+v+is equal to
3.
[0027] Examples of suitable R groups include alkyl such as method,
ethyl, propyl, and butyl; aryls such as phenyl; and alkenyls such
as allyl or vinyl. These resins may be essentially fully condensed
(HSiO.sub.{fraction (3/2)}).sub.n wherein n is 8 or greater or they
may be only partially hydrolyzed, i.e., containing some Si--OR,
and/or partially condensed, i.e., containing some Si--OH. Although
not represented by this structure, the resins may also contain a
small number, e.g., less than about 10 percent, of silicon atoms
which have either 0 or 2 hydrogen atoms attached thereto, or a
small number of SiC bonds due to various factors involved in their
formation or handling.
[0028] Structurally, such hydridosilsesquioxane resins are
essentially ladder or cage polymers of the type: 1
[0029] wherein n typically has a value of four or more. For
instance, when n has a value of four, the result is a bond
arrangement for the silsesquioxane cubic octamer depicted below:
2
[0030] As the value of n is increased, i.e., n being ten or more,
double-stranded polysiloxanes of indefinitely higher molecular
weight can be formed containing regular and repeated cross-ties in
their extended structure.
[0031] HSQ resins and methods for their production are known in the
art. For example, U.S. Pat. No. 3,615,272, which is incorporated
herein by reference, teaches the production of a nearly fully
condensed HSQ resin which may contain up to 100-300 parts per
million (ppm) silanol, i.e., .ident.SiOH, by a process of
hydrolyzing trichlorosilane in a benzenesulfonic acid hydrate
hydrolysis medium, and then washing the resultant resin with water
or aqueous sulfuric acid. Similarly, U.S. Pat. No. 5,010,159, which
is hereby incorporated by reference, teaches an alternative method
of hydrolyzing hydridosilanes in an arylsulfonic acid hydrate
hydrolysis medium to form a resin, which is then contacted with a
neutralizing agent.
[0032] Other HSQ resins include those described in U.S. Pat. No.
4,999,397; those produced by hydrolyzing an alkoxy or acyloxy
silane in an acidic alcoholic hydrolysis medium; those described in
Kokai Patents 59-178749, 60-86017 and 63-107122; will also function
herein.
[0033] Specific molecular weight fractions of the above HSQ resins
may also be used. Such fractions and methods for their preparation
are taught in U.S. Pat. Nos. 5,063,267 and 5,416,190, which are
hereby incorporated by reference. A preferred fraction is a
material where at least 75 percent of the polymeric species have a
molecular weight above about 1200, and a more preferred fraction is
a material where at least 75 percent of the polymeric species have
a number average molecular weight between about 1200 and about
100,000.
[0034] The hydridosilsesquioxane resin may contain a platinum,
rhodium or copper catalyst to increase the rate and extent of cure
of the resin. Generally, any platinum, rhodium or copper compound
or complex which can be solubilized will be useful. For instance,
platinum acetylacetonate, a rhodium catalyst such as
RhCl.sub.3[Si(CH.sub.2CH.sub.2CH.sub.2CH.sub.3).- sub.2].sub.3
available from Dow Corning Corporation, Midland, Mich., or cupric
naphthenate, are all representative and suitable materials. These
catalysts are generally added in an amount of between about 5 to
1000 ppm platinum, rhodium or copper, based on the weight of
hydridosilsesquioxane resin. Platinum and rhodium catalysts useful
herein are also described in U.S. Pat. No. 4,822,697, herein
incorporated by reference.
[0035] Ceramic oxide precursors may also be used in combination
with the hydridosilsesquioxane resin. The ceramic oxide precursors
contemplated include compounds of various metals such as aluminum,
titanium, zirconium, tantalum, niobium and/or vanadium, as well as
various non-metallic compounds such as those of boron or
phosphorus, which may be dissolved in solution, hydrolyzed, and
subsequently pyrolyzed at relatively low temperatures to form
ceramic oxides.
[0036] These ceramic oxide precursors generally have one or more
hydrolyzable groups bonded to the metal or non-metal depending on
the valence -of the metal. The number of hydrolyzable groups in
these compounds is not critical as long as the compound is soluble
or can be dispersed in the solvent. Likewise, selection of the
exact hydrolyzable substituent is not critical, since the
substituents are either hydrolyzed or pyrolyzed out of the system.
Typical hydrolyzable groups include alkoxy such as methoxy,
propoxy, butoxy and hexoxy; acyloxy such as acetoxy; and other
organic groups bonded to the metal or non-metal through an oxygen
such as acetylacetonate or an amino group. Specific compounds
include zirconium tetracetylacetonate, titanium dibutoxy
diacetylacetonate, aluminum triacetylacetonate, tetraisobutoxy
titanium and Ti(N(CH.sub.3).sub.2).sub.4. Such ceramic oxide
precursors are more gully described in U.S. Pat. No. 4,808,653;
5,008,320; and 5,290,354; herein incorporated by reference.
[0037] When a ceramic oxide precursor is combined with the
hydridosilsesquioxane resin, it is generally used in an amount such
that the final coating contains 0.1 to 30 percent by weight of the
ceramic oxide precursor.
[0038] The hydridosilsesquioxane resin is typically applied to the
substrate with solvent. Solvents which may be used include any
agent or mixture of agents which will dissolve the
hydridosilsesquioxane resin to form a homogeneous liquid mixture
without affecting the resulting coating. These solvents include
alcohols such as ethyl alcohol or isopropyl alcohol; aromatic
hydrocarbons such as benzene or toluene; alkanes such as n-heptane,
dodecane or nonane; ketones such as methyl iso-butyl ketone;
esters; glycol ethers; and siloxanes including cyclic
dimethylpolysiloxanes such as octamethylcyclotetrasiloxane, linear
dimethylpolysiloxanes such as hexamethyldisiloxane and
octamethyltrisiloxane, and mixtures thereof. The solvent is present
in an amount sufficient to dissolve the hydridosilsesquioxane resin
to the concentration desired for application as a thin film.
Typically, the solvent is present in an amount of 20 to 99 weight
percent, preferably from 50 to 80 weight percent, and most
preferably about 55 to 75 weight percent.
[0039] Some methods for application of the HSQ Resin include spin
coating, dip coating, spray coating, flow coating, and screen
printing. A preferred method for application is spin coating. When
a solvent is used, the solvent is allowed to evaporate from the
coated substrate, resulting in the deposition of a
hydrLdosilsesquioxane resin thin film. Any suitable means for
evaporation may be used such as simple air drying by exposure to an
ambient environment, by applying a vacuum, or by the application of
mild heat at about 50.degree. C. or less during the early stages of
the curing process. When spin coating is used, drying is minimized
as the spinning drives off the solvent.
[0040] Following application to the substrate, the
hydridosilsesquioxane resin thin film is cured to a preferably
crack-free insoluble coating by heating the deposited
hydridosilsesquioxane thin film for a sufficient time and at a
temperature of about 150.degree. C. to 500.degree. C., preferably
200.degree. C. to 400.degree. C., and more preferably 300.degree.
C. to 380.degree. C. By insoluble coating is meant a coating that
is essentially not soluble in the solvent from which the
hydridosilsesquioxane resin was deposited to form the
hydridosilsesquioxane thin film, or any solvent mentioned
previously as being useful. By crack-free is meant a coating that
does not contain any cracks visible to the human eve when examined
under an optical microscope at about 1000.times. or less
magnification.
[0041] Any method of heating may be used such as a convection oven,
rapid thermal processing, a hot plate, or the radiation absorbed
from microwave energy, but the preferred heating method is the use
of a hot plate. The method used should be capable of rapidly
heating the thin film to the desired temperature.
[0042] The duration of time that the coating is heated to cure will
depend on the environment during the heating, the temperature at
which it is heated, i.e., the soak temperature, the rate at which
it is heated, and the thickness of the hydridosilsesquioxane resin
thin film. For example, at higher soak temperatures and/or higher
concentrations of oxygen in the cure environment, the cure time
will be shorter. Typically, the coatings are heated from 1 second
to 2 hours, preferably from 5 seconds to 30 minutes.
[0043] If the coating is not heated long enough, or is heated too
long at the soak temperature, then cracking will result. However,
there is a window of time at a given soak temperature and
environment for a given coating thickness that will produce a
crack-free coating. Thus, at lower soak temperatures, the window is
large. As the temperature increases, the window decreases. Further,
as the amount of oxygen present in the environment increases, the
window decreases.
[0044] When coatings are not heated long enough, cracks develop
when the coatings are cooled to room temperature. It is possible to
repair/heal the cracks by further heating the coating for a
sufficient period of time. However, when coatings are heated too
long, again cracks will develop when the coatings are cooled to
room temperature. However, it is not possible to repair those types
of cracks.
[0045] The environment in which the hydridosilsesquioxane resin
thin film is cured is typically an inert environment such as
nitrogen, argon, helium, or an environment containing oxygen such
as air. As the oxygen content in the environment increases, the
minimum time required to cure the hydridosilsesquioxane resin to a
crack-free coating will be reduced. Further, the window of time
during which a crack-free coating will be produced will be
narrowed. When heating at lower temperatures such as about
330.degree. C. or less, it is preferred to have oxygen-present to
accelerate the cure. However, when heating at higher temperatures
such as about 340.degree. C. or more, it is preferred to use an
inert environment.
[0046] The curing may take place at atmospheric, superatmospheric,
or at subatmospheric pressures, but preferably it is carried out at
atmospheric pressure. At lower temperatures, higher pressures may
be used to accelerate the cure. Vacuum, however, may be used at any
temperature.
[0047] The rate at which the coatings are heated to the soak
temperature plays a role to produce a crack-free coating. If the
heating rate is fast, then the window of time in which a crack-free
coating can be produced will be longer, or higher soak temperatures
and/or higher coating thickness may be achieved without cracking.
On the contrary, if the heating rate is slow, then the window of
time in which a crack-free coatings can be produced will be
shorter, or lower soak temperatures and/or reduced coating
thickness will be necessary to achieve the crack-free coating.
[0048] If desired, thick coatings may be produced by forming a
single thick hydridosilsesquioxane film, and thereafter curing it
under controlled conditions. Thick coatings may also be produced by
forming a hydridosilsesquioxane thin film, curing it under
controlled conditions, and repeating the process until a desired
thickness in the coating or film is achieved.
[0049] The HSQ-derived thin films preferred in accordance with the
teaching of this invention generally have a thickness of at least
about 0.1 .mu.m (100 nanometer), and are preferably of a thickness
in the range of 0.1 .mu.m (100 nanometer) to about 1.5 .mu.m (l,500
nanometer).
[0050] In particular, therefore, and according to the concept of
the present invention, a thin film of silicon dioxide derived from
hydridosilsesquioxane resin is deposited onto a reflective
electrode and cured by a heat treatment, and then a
metal-insulator-metal sandwich is formed by depositing a
transparent or a semitransparent top electrode onto the HSQ thin
film. By transparent or semitransparent is meant that the film used
as the top electrode is deposited onto the HSQ thin film in a layer
that is so thin, that the beam of infrared radiation is capable of
shining through the top electrode. A reflective back electrode is
supported on a glass or other type of substrate and completes the
MIM sandwich device.
[0051] Alternatively, the HSQ film can be deposited first onto a
semitransparent electrode on a suitable transparent substrate such
as a transparent conductive oxide or a thin metal film, and the
reflective electrode is deposited last onto the cured HSQ film.
[0052] The spectrally resolved reflectivity of the device is
measured by shining a beam of infrared radiation through the
transparent top electrode, and reflecting it from the back
electrode, thus passing the beam twice through the HSQ thin film.
Upon application of an electric potential difference across the two
electrodes, the spectral characteristics or the HSQ thin film
change while an electric current is flowing through the device,
according to the residual resistance of the HSQ thin film. The
changes occur in the spectral bands characteristic of the Si--H
stretch and bend vibrations, the Si--O--Si manifold, and the
Si--O--H line. These changes are reversible, and their extent can
be controlled by the device current.
[0053] This response of HSQ thin films in device configurations is
not believed to have been observed previously in the spectral
signature, i.e., fingerprint, of such materials. The fact that the
infrared signature of HSQ-derived thin films can be altered over
large areas of a device geometry by the simple application of an
electric potential is therefore unique.
[0054] With reference to the drawings, in FIGS. 1 and 2 there can
be seen an experimental arrangement for a device which consists of
a suitable substrate such as a glass slide upon which is supported
an evaporated gold back electrode typically having a thickness of
about 0.15 .mu.m (150 nanometer). Deposited on the gold back
electrode is the HSQ-derived thin film which has a thickness in the
range of about 0.1 .mu.m (100 nanometer) to about 1.5 .mu.m (1,500
nanometer). A transparent or semitransparent palladium top
electrode is deposited over the HSQ-derived thin film. The top
electrode is applied over the HSQ-derived thin film as an
evaporated layer having a thickness of about 0.005 .mu.m to 0.08
.mu.m (5 to 80 nanometer).
[0055] The top electrode can be constructed of other suitable
electrically conductive or semiconductive materials such as gold,
platinum, silver, chromium, aluminum, copper, nickel, titanium, and
tin; and alloys such as titanium-tungsten, titanium nitride,
nickel-chromium, indium tin oxide, and gallium arsenide.
Particularly preferred herein is palladium. The back electrode can
also be constructed of other suitable electrically conductive or
semiconductive materials such as palladium, platinum, silver,
chromium, aluminum, copper, nickel, titanium, and tin; and alloys
such as titanium-tungsten, titanium nitride, nickel-chromium,
indium tin oxide, and gallium arsenide. Particularly preferred
herein is gold.
[0056] Contact between the top electrode, the back electrode, and
the HSQ-derived thin film, can be established by any known
technique. For instance,-the top electrode and the back electrode
can be formed on the HSQ-derived thin film by evaporating or
sputtering an appropriate electrode material onto the HSQ-derived
thin film in a vacuum.
[0057] In sputtering, the part to be coated is placed in an
evacuated chamber in close proximity to a flat plate or a coating
material, i.e., a metal. The flat plate functions as a target, and
it is bombarded by a beam of electrons. The electrons essentially
dislodge atoms from the target and sputter them onto the surface of
the part facing the target. Only the portion of the part directly
exposed to the target is coated. The advantage of sputtering
techniques is that the purity of the coating can be controlled, and
parts are not required to be heated during the coating process.
[0058] Techniques involving heat can also be employed such as
processes employing electron beam heating. Other alternate
deposition techniques that can be used include physical vapor
deposition, electroless plating, and electrolytic plating of metals
in addition, the electrodes can be formed as a metal pattern which
is deposited by a photolithographic technique.
[0059] When gold is selected as the material for an electrode, it
can be deposited by methods generally described in U.S. Pat. No.
5,616,202 (Apr. 1, 1997), entitled "Enhanced Adhesion of H-Resin
Derived Silica to Gold". According to this patented method, silica
derived from HSQ resin is adhered to gold by a low temperature
annealing process carried out in an oxidizing atmosphere for about
an hour or more.
[0060] Alternatively, HSQ-derived thin films can be deposited
directly onto performed top electrodes and performed back
electrodes to create the necessary electrical contacts, or
performed top electrodes and performed back electrodes can be
adhered directly to HSQ-derived thin films by known techniques.
[0061] Electric circuitry for supplying power to the MIM device is
shown in FIG. 1, and consists of a power supply, an amperemeter,
and a voltmeter. A beam of infrared radiation is shown in FIG. 2,
and the beam can be incident under an angle to optimize
observability of the spectrum. The beam of infrared radiation can
be generated by any conventional source such as a laser. The beam
of infrared radiation should have a wavelength in the range of
about 2.5 .mu.m to about 25 .mu.m (4,000 to 400 cm.sup.-1),
although other wavelengths within the infrared region of the
electromagnetic spectrum can be employed, i.e., wavelengths in the
range of from about 0.75 .mu.m to about 1,000 .mu.m (13,333 to 10
cm.sup.-1). The spectrum is recorded in the absorption mode with
the infrared beam traversing the HSQ thin film two times.
[0062] FIG. 3 shows a series of absorption spectra which were
obtained from top to bottom at increasing electric bias across the
device illustrated in FIGS. 1 and 2. Each spectrum was plotted as a
ratio to an original trace that was obtained without bias. For that
reason, an absorption feature that decreases as a function of
treatment appears as a line with increased value, while smaller
values of the ratio indicate an increase in absorption, i.e., a
decrease in transmission.
[0063] The abscissa in FIG. 3 represents the energy of the infrared
beam. The location of an infrared absorption band or peak can be
specified in frequency units by a wave number measured in
reciprocal centimeters (cm.sup.-1) or by its wavelength measured in
micrometers (.mu.m). The wavenumber is used in FIG. 3. For example,
a wavenumber having a value of 4000 cm.sup.-1 represents a high
energy beam and corresponds to a beam having a wavelength of 2.5
.mu.m, and a wavenumber having a value of 1000 cm.sup.-1 represents
a low energy beam and corresponds to a beam having a wavelength of
10.0 .mu.m. The current passing through the device of FIGS. 1 and 2
at the time the absorption spectra were obtained is shown as the
ordinate on the right side of FIG. 3.
[0064] Changes in the spectral distribution of the infrared
reflectivity of the HSQ-derived thin film were obtained with the
HSQ thin film under vacuum to exclude reactive gases such as
oxygen. It can be seen in FIG. 3, that the stretch vibration at
2260 cm.sup.-1 and the bend vibration at 890 cm.sup.-1, diminished
in the film as the applied electric tension V was increased from
the top trace to the lowest trace. Simultaneously, the Si--O--Si
bands at about 1200 cm.sup.-1 increased with this treatment. Thus,
FIG. 3 demonstrates the loss of the Si--H band with increasing
current and the growing absorption in the Si--O--Si region. Also,
at about 1530 cm.sup.-1, a new band appeared in a spectral region
characteristic for the Si--O--; vibration. Additional changes were
observed in a broad region around 3500 cm.sup.-1 due to the O--H
stretch.
[0065] As an illustration, FIG. 4 shows the area under the Si--H
stretch at 2260 cm.sup.-1, i.e., the decrease in Si--H content in
the film, plotted as a function of the device current. In FIG. 4,
one can observe that the SiH formation was linear with current
after a threshold of 0.7 mA. This point corresponds to an applied
potential, and this can be seen in FIG. 5 at about 5.5 on the
abscissa, where the SiH increase is plotted as a function of the
square root of the electric tension V.
[0066] These features are reversible such that when the electric
bias is removed, the spectral characteristics revert to those
observed before treatment.
[0067] Thus, FIGS. 3-5 indicate that Si--H and the Si--O--Si
vibrational lines can be controlled in devices according to the
present invention, that is, lines at 2260 cm.sup.-1 and below about
1000 cm.sup.-1, and bands falling between about 1050 and 1300
cm.sup.-1, respectively. While the magnitude of the effect was
observed using an HSQ-derived thin film of only 0.15 .mu.m (150
nanometer) in thickness, it can be as high as four percent using an
electric potential difference across the top and back electrodes
not exceeding about 4 millivolt.
[0068] In the use of devices according to the present invention, a
power supply with a variable output is applied to the MIM device,
with one connection being provided to the non-transparent back
electrode, i.e., the gold layer, and the other connection being
provided to the semitransparent top electrode, i.e., the palladium
layer. Then the intensity of a reflected beam of an incident source
of infrared radiation is modulated as a function of the current
flowing through the MIM device. When the reflected beam is
analyzed, its spectral characteristics will be found to vary
accordingly, in that Si--H absorption lines decrease while SiO
absorption lines increase, and vice versa, as the power supply
settings are changed.
[0069] The device is suitable for use as an optical switch in the
processing or transmission of data in communications or
computational devices, i.e., microprocessors. When the device is
used as an optical switch, it should be noted that instead of
measuring the full spectral characteristics of the device, a
monochromatic beam of infrared radiation, i.e., a beam of light of
only one wavelength, is selected such as to coincide with one of
the vibrational absorptions of the HSQ-derived insulator material,
and is switched ON or OFF as it interacts with the thin film
according to the electric tension V applied to the device.
[0070] In another application, the device could be installed or
constitute a portion of a coating applied to a moving object such
as an aircraft or watercraft. In such an application, the device
would be configured as a metal-insulator-metal device in order that
the infrared reflection spectrum, i.e., the infrared signature,
could be randomly modulated for the purpose of scrambling, jamming,
or damaging detection signals emitted by remote observers.
[0071] In a further application, the device could be installed or
constitute a portion of a coating on a window pane. This would
enable one to control the amount of infrared radiation that passes
through the window pane.
[0072] While the present invention has been described in terms of
hydridosilsesquioxane resins, i.e., (HSiO.sub.{fraction
(3/2)}).sub.n, in particular, it should be understood that it is
intended to encompass other types of silsesquioxane resins, such as
alkyl silsesquioxane resins, e.g., (CH.sub.3SiO.sub.{fraction
(3/2)}).sub.n, aryl silsesquioxane resins, e.g.,
(C.sub.6H.sub.5SiO.sub.{fraction (3/2)}).sub.n, as well as mixtures
of such resins. In addition, when resins of the type
(HSiO.sub.{fraction (3/2)}).sub.n are employed, the hydrogen atom
may be substituted in varying degrees by alkyl or aryl groups.
[0073] Other variations may be made in compounds, compositions, and
methods described herein without departing from the essential
features of the invention. The embodiments of the invention
specifically illustrated herein are exemplary only and not intended
as limitations on their scope except as defined in the appended
claims.
* * * * *