Abstrict An effective desiccant for use in a package which has a material
including at least in part solid particles of one or more materials,
at least one of such materials having an average particle size range
0.001 to 0.1 micrometers to provide a high rate of water absorption
and to provide an equilibrium minimum humidity level lower than
a humidity level to which a highly moisture sensitive electronic
device is sensitive within a sealed enclosure.
Claims What is claimed is:
1. A desiccant package useable for protecting highly moisture-sensitive
electronic devices sealed within an enclosure, comprising: a) a
moisture-permeable container which can be positioned in the sealed
enclosure; b) solid water absorbing particles of one or more materials
disposed in the moisture-permeable container; c) said solid water
absorbing particles including solid particles of one or more materials,
at least one of such materials having an average particle size range
0.001 to 0.1 micrometers to provide a high rate of water absorption
and to provide an equilibrium minimum humidity level lower than
a humidity level to which the device is sensitive within the sealed
enclosure; and d) said moisture-permeable container essentially
maintains the moisture absorption rate of the solid water absorbing
particles contained therein, the moisture-permeable container acting
to separate the solid water absorbing particles from the highly
moisture-sensitive device.
2. The desiccant package in accordance with claim 1 wherein one
or more of the materials of the solid water absorbing particles
are selected from the group consisting of alkaline metal oxides,
alkaline earth metal oxides, sulfates, metal halides, perchlorates
and metals with work functions less than 4.5 eV and capable of being
oxidized in the presence of moisture, or combinations thereof.
3. The desiccant package in accordance with claim 1 which provides
a humidity level less than 2500 ppm.
4. The desiccant package in accordance with claim 1 which provides
a humidity level less than 100 ppm.
5. A desiccant package useable for protecting highly moisture-sensitive
electronic devices sealed within an enclosure, comprising: a) solid
water absorbing particles of one or more materials in a moisture-permeable
binder; b) said solid water absorbing particles including solid
particles of one or more materials, at least one of such materials
having an average particle size range 0.001 to 0.1 micrometers to
provide a high rate of water absorption and to provide an equilibrium
minimum humidity level lower than a humidity level to which the
device is sensitive within the sealed enclosure; and c) said binder
being adapted to essentially maintain or enhance the moisture absorption
rate of the solid water absorbing particles contained therein, the
binder being in solid or liquid phase or dissolved in a liquid.
6. The desiccant package in accordance with claim 5 wherein one
or more of the materials of the solid water absorbing particles
are selected from the group consisting of alkaline metal oxides,
alkaline earth metal oxides, sulfates, metal halides, perchlorates
and metals with work functions less than 4.5 eV and capable of being
oxidized in the presence of moisture, or combinations thereof.
7. The desiccant package in accordance with claim 5 wherein the
binder is selected from the group consisting of cellulose acetates,
epoxies, phenoxies, siloxanes, methacrylates, sulfones, phthalates,
and amides or combinations thereof.
8. The desiccant package in accordance with claim 5 wherein the
solid water absorbing particles comprise 10 wt % to 90 wt % of the
solid water absorbing particles and the binder.
9. The desiccant package in accordance with claim 5 which provides
a humidity level less than 2500 ppm.
10. The desiccant package in accordance with claim 5 which provides
a humidity level less than 100 ppm.
11. The desiccant package in accordance with claim 5 wherein water
vapor transmission rate of the binder is greater than 3.5 gm-mil/100
in.sup.2/day.
12. The desiccant package in accordance with claim 5 wherein the
binder is radiation curable.
13. The desiccant package in accordance with claim 5 wherein the
binder is radiation-curable photoresist compositions.
14. The desiccant package in accordance with claim 5 wherein the
binder is selected from the group consisting of acrylates, methacrylates,
cyclized polyisoprenes, polyvinyl cinnamnates, epoxies, silicones,
and adhesives or combinations thereof.
15. A desiccant useable for protecting highly moisture-sensitive
electronic devices scaled within an enclosure, comprising: a) solid
water absorbing particles of one or more materials in a moisture-permeable
binder on a support; b) said solid water absorbing particles including
solid particles of one or more materials, at least one of such materials
having an average particle size range 0.001 to 0.1 micrometers to
provide a high rate of water absorption and to provide an equilibrium
minimum humidity level lower than a humidity level to which the
device is sensitive within the sealed enclosure; and c) said binder
being adapted to reduce degradation of or enhance the moisture absorption
rate of the solid water absorbing particles contained therein, the
binder being in solid or liquid phase or dissolved in a liquid.
16. The desiccant in accordance with claim 15 wherein one or more
of the materials of the solid water absorbing particles are selected
from the group consisting of alkaline metal oxides, alkaline earth
metal oxides, sulfates, metal halides, perchlorates and metals with
work functions less than 4.5 eV and capable of being oxidized in
the presence of moisture, or combinations thereof.
17. The desiccant in accordance with claim 15 wherein the binder
is selected from the group consisting of cellulose acetates, epoxies,
phenoxies, siloxanes, methacrylates, sulfones, phthalates, and amides
or combinations thereof.
18. The desiccant in accordance with claim 15 wherein the solid
water absorbing particles comprise 10 wt % to 90 wt % of the solid
water absorbing particles and the binder.
19. The desiccant in accordance with claim 15 which provides a
humidity level less than 2500 ppm.
20. The desiccant in accordance with claim 15 which provides a
humidity level less than 100 ppm.
21. The desiccant in accordance with claim 15 wherein water vapor
transmission rate of the binder is greater than 3.5 gm-mil/100 in.sup.2/day.
22. The desiccant in accordance with claim 15 wherein the binder
is radiation curable.
23. The desiccant in accordance with claim 15 wherein the binder
is radiation-curable photoresist compositions.
24. The desiccant in accordance with claim 15 wherein the binder
is selected from the group consisting of acrylates, methacrylates,
cyclized polyisoprenes, polyvinyl cinnamates, epoxies, silicones,
and adhesives or combinations thereof.
25. A desiccant, comprising material including at least in part
solid particles of one or more materials, at least one of such materials
having an average particle size range 0.001 to 0.1 micrometers to
provide a high rate of water absorption and to provide an equilibrium
minimum humidity level lower than a humidity level to which a highly
moisture sensitive electronic device is sensitive within a sealed
enclosure.
26. The desiccant of claim 25 wherein the material includes a binder
adapted to reduce degradation of or enhance the moisture absorption
rate of the solid water absorbing particles contained therein, the
binder being in solid or liquid phase or dissolved in a liquid.
27. The desiccant of claim 25 wherein one or more of the materials
of the solid water absorbing particles are selected from the group
consisting of alkaline metal oxides, alkaline earth metal oxides,
sulfates, metal halides, perchlorates and metals with work functions
less than 4.5 eV and capable of being oxidized in the presence of
moisture, or combinations thereof.
28. The desiccant in accordance with claim 26 wherein the binder
is selected from the group consisting of cellulose acetates, epoxies,
phenoxies, siloxanes, methacrylates, sulfones, phthalates, and amides
or combinations thereof.
29. The desiccant in accordance with claim 26 wherein the solid
water absorbing particles comprise 10 wt % to 90 wt % of the solid
water absorbing particles and the binder.
30. The desiccant in accordance with claim 25 which provides a
humidity level less than 2500 ppm.
31. The desiccant in accordance with claim 25 which provides a
humidity level less than 100 ppm.
32. The desiccant in accordance with claim 26 wherein water vapor
transmission rate of the binder is greater than 3.5 gm-mil/100 in.sup.2/day.
33. The desiccant in accordance with claim 26 wherein the binder
is radiation curable.
34. The desiccant in accordance with claim 26 wherein the binder
is radiation-curable photoresist compositions.
35. The desiccant in accordance with claim 26 wherein the binder
is selected from the group consisting of acrylates, methacrylates,
cyclized polyisoprenes, polyvinyl cinnamates, epoxies, silicones,
and adhesives or combinations thereof.
Description FIELD OF THE INVENTION
[0001] The present invention relates to control of moisture inside
a packaged electronic device and relates particularly to an improved
desiccant and desiccant package which desiccates highly moisture-sensitive
electronic devices to prevent premature device failure or premature
degradation of device performance.
BACKGROUND OF THE INVENTION
[0002] Various microelectronic devices require humidity levels
in a range of about 2500 to below 5000 parts per million (ppm) to
prevent premature degradation of device performance within a specified
operating and/or storage life of the device. Control of the environment
to this range of humidity levels within a packaged device is typically
achieved by encapsulating the device or by sealing the device and
a desiccant package within a cover. Desiccant packages include a
container for receiving solid water absorbing particles (a desiccant)
or providing such particles into a binder. Examples of solid water
absorbing particles include molecular sieve materials, silica gel
materials, and materials commonly referred to as Drierite materials
which are used to maintain the humidity level within the above range.
[0003] Particular microelectronic devices, for example, organic
light-emitting devices (OLED) or panels, polymer light-emitting
devices, charge-coupled device (CCD) sensors, and micro-electro-mechanical
sensors (MEMS) require humidity control to levels below about 1000
ppm and some require humidity control below even 100 ppm. Such low
levels are not achievable with desiccants of silica gel materials
and of Drierite materials. Molecular sieve materials can achieve
humidity levels below 1000 ppm within an enclosure if dried at a
relatively high temperature. However, molecular sieve materials
have a relatively low moisture capacity at humidity levels at or
below 1000 ppm, and the minimum achievable humidity level of molecular
sieve materials is a function of temperature within an enclosure:
moisture absorbed, for example, at room temperature, can be released
into the enclosure or package during temperature cycling to higher
temperature, such, as, for example, to a temperature of 100.degree.
C. Solid water absorbing particles used within such packaged devices
include 0.2 to 200 .mu.m particle size powders of metal oxides,
alkaline earth metal oxides, sulfates, metal halides, or perchlorates,
i.e. materials having desirably relatively low values of equilibrium
minimum humidity and high moisture capacity. However, such materials
even when finely divided into powders of 0.2 to 200 .mu.m particle
size often chemically absorb moisture relatively slowly compared
to the above-mentioned molecular sieve, silica gel, or Drierite
materials. Such relatively slow reaction with water vapor leads
to a measurable degree of device degradation of performance following
the sealing of the desiccant inside a device cover due to, for example,
moisture absorbed on the inside of a device, moisture vapor present
within the sealed device, and moisture permeating through the seal
between the device and the cover from the outside ambient.
[0004] Some solid water absorbing particles, particularly molecular
sieve materials which entrain moisture by physical absorption within
microscopic pores, require a dehydrating step at substantially elevated
temperature prior to use within a device enclosure, thus increasing
the number of process steps and calling for additional apparatus,
such as, for example, a controllable furnace to achieve substantial
dehydration.
[0005] Selection of solid water absorbing particles and the method
of applying selected particles to an inner portion of a device enclosure
prior to sealing the device within or by the enclosure is governed
by the type of device to be protected from moisture. For example,
highly moisture-sensitive organic light-emitting devices or polymer
light-emitting devices require the selection of particular solid
water absorbing particles and methods of application, since organic
materials or organic layers are integral constituents of such devices.
The presence of organic materials or layers may, for example, preclude
the use of certain solvents or fluids in the application of a solid
water absorbing particles dispersed in a fluid to organic-based
devices. Furthermore, a thermal treatment, if required, of a desiccant
contained within a sealed device enclosure, needs to be tailored
to the constraints imposed by thermal properties of the organic
constituents or layers of the device. At any rate, release of solvent
vapors during a thermal treatment of a desiccant disposed within
a sealed device enclosure must be avoided or minimized if solvent
vapors can adversely affect organic constituents of organic-based
electronic devices. The aforementioned considerations pertaining
to organic-based electronic devices may not be as important if the
electronic device to be desiccated is strictly an inorganic or metallic
device such as, for example, a MEMS device or a CCD sensor without
an organic color filter overlay.
[0006] For highly moisture sensitive electronic devices, such as
organic light-emitting devices or polymer light-emitting devices,
VanSlyke, U.S. Pat. No. 5047687 teaches the use of a protective
layer comprised of a mixture of at least one organic component of
the organic electroluminescent medium and at least one metal having
a work function in the range of from 4.0 to 4.5 eV capable of being
oxidized in the presence of ambient moisture. The metal in the protective
layer is described by VanSlyke as being sufficiently reactive to
be oxidized by ambient atmospheric moisture over an extended period
of time when incorporated into the organic EL device. In this use
the metal is used as solid water absorbing particles for moisture
in the protective layer. That neither a coated layer of metal film
alone nor successively coated layers of the metal and organic films
were effective in preventing the dark spot growth due to ambient
moisture was attributed to the slow oxidation of the bulk metal.
VanSlyke, therefore, teaches that the oxidation susceptibility of
reactive metals that can be oxidized by ambient moisture is enhanced
by the higher surface to volume ratios achieved by co-deposition
of the metal into a mixed layer of metal and an organic medium.
However, VanSlyke does not teach the required metal desiccant particle
size for optimal moisture absorption protection nor does he teach
the effect of metal particle size on performance in protecting organic
EL devices.
[0007] Numerous publications describe methods and/or materials
for controlling humidity levels within enclosed or encapsulated
electronic devices. For example, Kawami et al., European Patent
Application EP 0 776 147 A1 disclose an organic EL element enclosed
in an airtight container which contains a drying substance comprised
of a solid compound for chemically absorbing moisture. The drying
substance is spaced from the organic EL element, and the drying
substance is consolidated in a predetermined shape by vacuum vapor
deposition, sputtering, or spin-coating. Kawami et al. teach the
use of the following solid water absorbing particles: alkali metal
oxides, alkali earth metal oxides, sulfates, metal halides, and
perchlorates. Kawami et al., however, do not teach the effect of
particle size of these solid water absorbing particles on their
performance.
[0008] Shores, U.S. Pat. No. 5304419 discloses a moisture and
particle getter for enclosures which enclose an electronic device.
A portion of an inner surface of the enclosure is coated with a
pressure sensitive adhesive containing a solid desiccant with average
particle size usually 0.2 to 100 .mu.m and preferably 0.5 to 10
.mu.m.
[0009] Shores, U.S. Pat. No. 5401536 describes a method of providing
a moisture-free enclosure for an electronic device, the enclosure
containing a coating or adhesive with desiccant properties. The
coating or adhesive comprises a protonated alumina silicate powder
with average particle size 0.2 to 100 .mu.m, preferably 1 to 10
.mu.m, dispersed in a polymer.
[0010] Shores, U.S. Pat. No. 5591379 discloses a moisture gettering
composition for hermetic electronic devices. The composition is
applied as a coating or adhesive to the interior surface of a device
packaging, and the composition comprises a water vapor permeable
binder which has dispersed therein a desiccant with average particle
size of 0.2-100 .mu.m, preferably 0.3-50 .mu.m, which is preferably
a molecular sieve material.
[0011] Many of the desiccants disclosed by Shores will not function
effectively with highly moisture-sensitive devices at a humidity
level lower than 1000 ppm. In addition, Shores does not teach why
the particle sizes disclosed are chosen or the effect of particle
size on the performance of the desiccants.
[0012] Similarly, binders, such as polyethylene disclosed by Shores,
that have low moisture absorption rates compared to the absorption
rate of the pure selected desiccants would not function effectively
to achieve and to maintain a humidity level below 1000 ppm during
a projected operational lifetime of a highly moisture-sensitive
device.
[0013] Deffeyes, U.S. Pat. No. 4036360 describes a desiccating
material that is useful as a package insert or on the interior walls
of packaging boxes for applications requiring only moderate moisture
protection, such as film or cameras. The material comprises a desiccant
and a resin having a high moisture vapor transmission rate.
[0014] The desiccants disclosed by Deffeyes are alumina, bauxite,
calcium sulfate, clay, silica gel, and zeolite, but Deffeyes does
not describe the particle size of any of the desiccants. None of
these desiccants will function effectively with highly moisture-sensitive
devices at a humidity level lower than 1000 ppm. In addition the
moisture vapor transmission rate requirement for the resin is not
adequately defined since there is no reference to the thickness
of the measured resins. A material that transmits 40 grams per 24
hrs per 100 in.sup.2 at a thickness of 1 mil would be very different
than one that transmits 40 grams per 24 hrs per 100 in.sup.2 at
a thickness of 100 mils. It is therefore not possible to determine
if the moisture vapor transmission rates disclosed by Deffeyes are
sufficient for highly moisture-sensitive devices.
[0015] Taylor, U.S. Pat. No. 4013566 describes solid desiccant
bodies that are useful as drier materials in refrigerant fluid systems.
The solid desiccant body comprises finely divided particles of desiccant
material bound in a moisture transmissive aliphatic epoxy polymer
matrix.
[0016] The desiccants disclosed by Taylor are molecular sieves,
activated alumina, and silica gel. Taylor teaches the use of particle
sizes 1 to 10 .mu.m, but does not teach the impact of particle size
on desiccant performance. None of these desiccants will function
effectively with highly moisture-sensitive devices at a humidity
level lower than 1000 ppm. In addition the moisture vapor transmission
rate requirement for the resin is not adequately defined; stating
only that the solid desiccant bodies have rates of adsorption of
absorption comparable to the desiccant materials alone. It is therefore
not possible to determine if the resins disclosed by Taylor are
sufficient for highly moisture-sensitive devices.
[0017] Booe, U.S. Pat. No. 4081397 describes a composition used
for stabilizing the electrical and electronic properties of electrical
and electronic devices. The composition comprises alkaline earth
oxides in an elastomeric matrix.
[0018] The desiccants disclosed by Booe are barium oxide, strontium
oxide, and calcium oxide. Booe teaches the use of particle sizes
less than 80 mesh (177 .mu.m) to minimize the settling of oxides
within the suspension. Booe does not teach the impact of particle
size on desiccant performance. These desiccants will function effectively
with highly moisture-sensitive devices at humidity levels lower
than 1000 ppm; however, Booe claims the elastomeric matrix has the
property of retarding the rate of fluid absorption of the alkaline
earth particles. In the examples the water absorption rate of the
compositions are 5 to 10 times slower than the alkaline earth particles
alone. This decrease in absorption rate is disclosed as a desirable
feature that improves the handling of the highly reactive alkaline
earth oxides. In highly moisture-sensitive devices, however, any
decrease in the absorption rate of moisture will increase the likelihood
of device degradation, and identification of resins that will increase
the absorption rate of moisture would be highly desirable. For highly
moisture-sensitive devices, therefore, it is important to determine
the minimum allowable water vapor transmission rate of the binders
used in combination with effective desiccant materials.
[0019] Boroson et al., U.S. Pat. No. 6226890 describes a method
of desiccating an environment surrounding a moisture-sensitive electronic
device sealed within an enclosure, including selecting a desiccant
comprised of solid particles having a particle size range 0.1 to
200 micrometers. The desiccant is selected to provide an equilibrium
minimum humidity level lower than a humidity level to which the
device is sensitive within the sealed enclosure. A binder is chosen
that maintains or enhances the moisture absorption rate of the desiccant
for blending the selected desiccant therein. The binder may be in
liquid phase or dissolved in a liquid. A castable blend is formed
including at least the desiccant particles and the binder, the blend
having the solid water absorbing particles comprise 10 wt % to 90
wt % of the solid water absorbing particles and the binder.
[0020] The blend is cast in a measured amount onto a portion of
an interior surface of an enclosure to form a desiccant layer thereover,
the enclosure having a sealing flange. The blend is solidified to
form a solid desiccant layer, and the electronic device is sealed
with the enclosure along the sealing flange. Boroson et al., however,
do not teach the effect of particle size of these solid particle
desiccants on their performance, nor do they teach any benefit of
particles smaller than 0.1 micrometers.
SUMMARY OF THE INVENTION
[0021] It is an object of the present invention to provide a desiccant
package which includes a desiccant for protecting highly moisture-sensitive
electronic devices sealed within an enclosure.
[0022] It is another object of the present invention to provide
a desiccant for protecting highly moisture-sensitive electronic
devices sealed within an enclosure. This object is achieved by a
desiccant comprising solid water absorbing particles of one or more
materials, at least one of such materials having an average particle
size range 0.001 to 0.1 micrometers to provide a high rate of water
absorption and to provide an equilibrium minimum humidity level
lower than a humidity level to which the device is sensitive within
the sealed enclosure.
[0023] These objects are achieved by a desiccant package useable
for protecting highly moisture-sensitive electronic devices sealed
within an enclosure, comprising:
[0024] a) a moisture-permeable container which can be positioned
in the sealed enclosure;
[0025] b) solid water absorbing particles of one or more materials
disposed in the moisture-permeable container;
[0026] c) said solid water absorbing particles including solid
particles of one or more materials, at least one of such materials
having an average particle size range 0.001 to 0.1 micrometers to
provide a high rate of water absorption and to provide an equilibrium
minimum humidity level lower than a humidity level to which the
device is sensitive within the sealed enclosure; and
[0027] d) said moisture-permeable container essentially maintains
the moisture absorption rate of the solid water absorbing particles
contained therein, the moisture-permeable container acting to separate
the solid water absorbing particles from the highly moisture-sensitive
device.
[0028] The present invention provides the following advantages:
a moisture absorption rate that enhances the moisture absorption
rate of a solid material capable of providing a low equilibrium
minimum humidity within the enclosure, by the reduction of said
solid material particle size to an average less than 0.1 micrometers;
a moisture absorption rate that essentially maintains or enhances
the moisture absorption rate of said less than 0.1 micrometer solid
water absorbing particles capable of providing a low equilibrium
minimum humidity within the enclosure, by a container or binder
in which the solid water absorbing particles are contained; simple,
fast, and reliable placement of a desiccant layer on an interior
surface of a device enclosure; containment of solid desiccant particles
by a container or binder within the device enclosure; thermally
curable binders provide for removal of moisture trapped in an uncured
desiccant layer by thermal curing of the layer; radiation-curable
binders provide for fast curing of a desiccant layer by exposure
to radiation; forming a desiccant layer on a separate adhesively
bondable support or between a water permeable membrane and a separate
adhesively bondable support provides for high speed, roll-to-roll
manufacturing of a desiccant layer supply; and providing a desiccant
layer having relatively low sensitivity of its desiccation efficacy
to temperature cycling at elevated temperature up to 150.degree.
C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a schematic perspective view of an organic light-emitting
device (OLED) test structure useful for testing the efficacy of
a desiccant package;
[0030] FIG. 2 is a schematic sectional view of the OLED test structure
taken along section lines 2-2 of FIG. 1 and depicting emitted light
upon application of an electrical potential across the test structure
from a drive voltage supply;
[0031] FIG. 3A shows the test structure of FIG. 2 in an inverted
position, and defines a total thickness of the layers comprising
the test structure;
[0032] FIG. 3B depicts a sectional view of an enclosure for the
test structure, the enclosure having a desiccant package formed
on a portion of an interior surface of the enclosure;
[0033] FIG. 4 shows in sectional view the OLED test structure of
FIG. 3A, with the enclosure of FIG. 3B sealed to the test structure
by a seal;
[0034] FIG. 5A depicts a desiccant package comprised of solid water
absorbing particles in a moisture-permeable binder formed over one
surface of a support;
[0035] FIG. 5B shows an enclosure having a piece of the desiccant
package of FIG. 5A adhesively bonded to a portion of an interior
surface of the enclosure;
[0036] FIG. 6A shows an enclosure having a desiccant package comprised
of solid water absorbing particles and a moisture-permeable radiation-curable
binder;
[0037] FIG. 6B indicates schematically curing radiation directed
at the desiccant package to cure the moisture-permeable radiation-curable
binder; and
[0038] FIG. 7 shows an enclosure having a desiccant package comprised
of solid water absorbing particles and a moisture-permeable container
attached to an interior surface of the enclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0039] Throughout the specification and claims, the term "desiccant
package" will be understood to mean a desiccant made of solid
water absorbing particles and either a package which contains such
solid water absorbing particles or the solid water absorbing particles
being placed in a binder which can be provided on a surface of an
enclosure.
[0040] Referring to FIG. 1 and FIG. 2 there are shown schematic
perspective and sectional views of an organic light-emitting device
(OLED) test structure 10 which has been found to be useful for testing
the efficacy of a desiccant package, since OLED devices in general
are highly moisture-sensitive as evidenced by degradation of a cathode
18 when such a device is operated under ambient conditions of humidity.
Details of the construction of the OLED test structure 10 are described
under the heading "Experimental Procedures", Section I.
Other test structures and test methods can be used, for example,
so called "time-to-failure" testing of MEMS devices.
[0041] The OLED test structure 10 has a light-transmissive substrate
12 on which is disposed a light-transmissive anode 14. Over the
anode 14 and over a portion of the substrate 12 is formed an organic
light-emitting layer 16 which comprises at least two layers, namely,
an organic hole-transporting layer in contact with the anode 14
and an organic electron-transporting layer in contact with the hole-transporting
layer. A cathode 18 is formed in a direction orthogonal to the direction
of the anode 14 and having a width dimension w which is a physical
width dimension as well as representing an initial electrical width
dimension, i.e. an electrical width dimension substantially identical
with the physical width dimension w after fabrication and short-duration
operation of the OLED test structure 10. The operation of an unprotected
OLED test structure 10 is schematically indicated in FIG. 2 wherein
a drive voltage supply 20 depicted here as a battery, has its negative
terminal connected to the cathode 18 via a lead 22 at a contact
23. The positive terminal of the drive voltage supply 20 is connected
to the anode 14 via a lead 24 at a contact 25. The drive voltage
supply 20 causes a current to flow through the OLED test structure
10 by injecting electrons into the organic light-emitting layer
from the cathode 18 and by injecting holes from the anode 14. The
electrons and the holes are transported across the electron-transporting
layer (not shown) and the hole-transporting layer (not shown), and
these charge carriers recombine at an interface between the electron-
and hole-transporting layers, thereby generating emitted light 30
from an area of the OLED test structure which is congruent with
the area of intersection between the anode 14 and the cathode 18
during initial operation of the test structure.
[0042] Turning to FIG. 3A, the OLED test structure 10 is depicted
in an inverted position, and a total thickness d of the anode 14
the organic light-emitting layer 16 and the cathode 18 is indicated.
The thickness d may be in a range of 200 to 2000 nm.
[0043] Turning to FIG. 3B, there is depicted a sectional view of
an enclosure 40 dimensioned to sealingly enclose at least the light-emitting
portions of the OLED test structure 10. The enclosure 40 may be
a metallic structure, a glass structure, a ceramic structure, or
a plastic structure having a height dimension h. The enclosure can
also comprise a combination of materials, such as, for example,
a metallized plastic structure. The enclosure 40 has an exterior
surface 42 comprising generally sloped side surfaces and a bottom
surface not further identified. The enclosure 40 has a sealing flange
44 which will be used to seal the enclosure 40 against the substrate
12 of the test structure 10 to effectively enclose the test structure.
[0044] A desiccant package generally indicated at 50 has a thickness
t, and is shown disposed within the enclosure 40 and extending generally
along a bottom portion of the surface 42. The desiccant package
50 is comprised of a moisture-permeable binder 52 and solid water
absorbing particles 54 dispersed or contained within the moisture-permeable
binder 52 as will be described in greater detail hereinafter.
[0045] Turning to FIG. 4 the enclosure 40 containing the desiccant
package 50 is shown in sealing relationship with the OLED test structure
10 via a seal 60 which is formed between the sealing flange 44 of
the enclosure and a perimeter portion of the substrate 12 so that
the OLED test structure is enclosed, and the humidity level present
in the OLED test structure 10 or present in the environment surrounding
the OLED test structure (i.e. the environment between the OLED test
structure and an upper surface of the desiccant package 50) can
be controlled by the desiccant package 50.
[0046] The desiccant package 50 comprises:
[0047] a) solid water absorbing particles 54 of one or more materials
in a moisture-permeable binder 52;
[0048] b) said solid water absorbing particles 54 including solid
particles of one or more materials, at least one of such materials
having an average particle size range 0.001 to 0.1 micrometers to
provide a high rate of water absorption and to provide an equilibrium
minimum humidity level lower than a humidity level to which the
OLED test structure 10 is sensitive within the sealed enclosure
40; and
[0049] c) said binder 52 being adapted to reduce degradation of
or enhance the moisture absorption rate of the solid water absorbing
particles 54 contained therein, the binder 52 being in solid or
liquid phase or dissolved in a liquid.
[0050] Currently one or more of the materials of the solid water
absorbing particles 54 is preferably selected from the group consisting
of alkaline metal oxides, alkaline earth metal oxides, sulfates,
metal halides, perchlorates and metals with work functions less
than 4.5 eV and capable of being oxidized in the presence of moisture,
or combinations thereof.
[0051] Currently the preferred the water vapor transmission rate
of the moisture-permeable binder 52 is greater than 3.5 gm-mil/100
in.sup.2/day. The currently preferred moisture-permeable binder
52 may be selected from the group consisting of cellulose acetates,
epoxies, phenoxies, siloxanes, methacrylates, sulfones, phthalates,
and amides or combinations thereof.
[0052] Currently it is preferred that the solid water absorbing
particles 54 comprise 10 wt % to 90 wt % of the solid water absorbing
particles 54 and the moisture-permeable binder. Currently it is
preferred that the desiccant package 50 provides a humidity level
less than 2500 ppm and most preferably less than 100 ppm.
[0053] A measured amount of the blend of solid water absorbing
particles 54 dispersed in a moisture-permeable binder 52 is then
cast on a portion of the lower interior surface of the enclosure
40 for example, by dispensing a measured amount of the blend from
an automated dispensing nozzle until the dispensed blend has spread
along the lower surface of the enclosure to form a desiccant package
50. The desiccant package 50 is then cured to a solid, for example,
by heating the desiccant package 50 under controlled conditions
so as to remove residual solvents, until the cured desiccant package
50 has the desired desiccant package thickness t.
[0054] Alternatively, if the binder is chosen to be radiation-curable
or to be radiation-polymerizable, the curing step of the desiccant
package 50 may include heating the desiccant package 50 while exposing
the desiccant package 50 to curing radiation. Upon completion of
the radiation-curing step to cure the desiccant package 50 to a
solid, the enclosure 40 containing the cured desiccant package 50
is sealed against the test structure 10 as described previously
with reference to FIG. 4.
[0055] Turning to FIG. 5A, there is depicted a desiccant package
50 comprised of solid water absorbing particles 54 said solid water
absorbing particles 54 including solid particles of one or more
materials, at least one of such materials having an average particle
size range 0.001 to 0.1 micrometers in a moisture-permeable binder
52 formed over one surface of a support 72 having on an opposing
surface an adhesively bondable surface 74. The adhesively bondable
surface 74 in practice is covered with a peelable protective layer
(not shown). The desiccant package 50 can be prepared and cured
in a so-called in-line coating and curing facility to provide an
extensive supply of a desiccant package 50 having a uniform layer
thickness achievable by automated coating processes.
[0056] Turning to FIG. 5B, there is shown an enclosure 40 having
a piece of the desiccant package 50 of FIG. 5A cut to an appropriate
dimension and adhesively bonded to the lower portion of the interior
surface of the enclosure.
[0057] FIG. 6A shows an enclosure 40 having a desiccant package
50 formed along a lower interior portion of the enclosure, the desiccant
package 50 comprised of selected solid water absorbing particles
54 said solid water absorbing particles 54 including solid particles
of one or more materials, at least one of such materials having
an average particle size range 0.001 to 0.1 micrometers dispersed
in a radiation-curable binder 55. The binder 55 is selected to be
moisture-permeable. Preferred moisture-permeable radiation-curable
binders are radiation-curable photoresist compositions, or radiation-curable
acrylates, methacrylates, cyclized polyisoprenes, polyvinyl cinnamates,
epoxies, silicones, and adhesives or combinations thereof.
[0058] In FIG. 6B curing radiation 90 is shown directed to be incident
on the desiccant package 50 to provide for the curing or polymerization
of the radiation-curable binder 55 in FIG. 6A, now designated as
a radiation-cured binder 55c in FIG. 6B.
[0059] It will be appreciated that the desiccant package 50 having
the radiation-curable binder 55 can be formed as part of a desiccant
package 50 as described with reference to FIG. 5A.
[0060] With the exception of choosing an organic radiation-curable
binder, and with the exception of curing the binder by exposure
to curing radiation, the preparation of a castable blend, and casting
the blend to form a desiccant package 50 are process steps substantially
identical to the preparation steps described with reference to the
description of FIG. 3B and FIG. 4 including the step of sealing
the OLED test structure 10 with the enclosure 40 by the seal 60
(see FIG. 4).
[0061] As a general rule of practice in preparing a castable blend
comprised of at least solid water absorbing particles dispersed
in a moisture-permeable binder, care is taken to avoid the introduction
of bubbles or other discontinuities in the blend. For example, it
is well known that manufacturers of paints introduce into the paint
blend various so-called leveling agents, surfactants, antifoam agents,
and other addenda in order to provide a stable dispersion of paint
pigment at a high paint pigment loading factor in a relatively viscous
binder such as, for example, an acrylic binder. Such agents and
addenda can be used advantageously in the preparation of castable
blends of solid water absorbing particles in a moisture-permeable
binder.
[0062] FIG. 7 shows an enclosure 40 having a desiccant package
50 formed along a lower interior portion of the enclosure, the desiccant
package comprised of selected solid water absorbing particles 54
said solid water absorbing particles 54 including solid particles
of one or more materials, at least one of such materials having
an average particle size range 0.001 to 0.1 micrometers and a moisture-permeable
container 56 attached to an interior surface of the enclosure. The
moisture-permeable container 56 being adapted to reduce degradation
of the moisture absorption rate of the solid water absorbing particles
54 contained therein, while separating the solid water absorbing
particles 54 from the moisture sensitive device (not shown). The
moisture-permeable container can be made from any materials that
allow moisture permeation while still containing the solid water
absorbing particles 54. Examples of materials used to form moisture-permeable
containers include polymer films, woven or non-woven fabrics or
tapes, perforated metal foils, and combinations thereof. In addition
only a portion of the moisture-permeable container 56 is required
to be permeable to moisture, such that parts of the moisture-permeable
container 56 can be made of materials that are impermeable to moisture.
For example one side of the moisture-permeable container 56 can
be a solid metal foil or low permeability polymer film or one side
of the moisture-permeable container 56 can be the enclosure 40 itself.
[0063] Currently one or more of the materials of the solid water
absorbing particles 54 is preferably selected from the group consisting
of alkaline metal oxides, alkaline earth metal oxides, sulfates,
metal halides, perchlorates and metals with work functions less
than 4.5 eV and capable of being oxidized in the presence of moisture,
or combinations thereof.
[0064] Currently it is preferred that the desiccant package 50
provides a humidity level less than 2500 ppm and most preferably
less than 100 ppm.
[0065] The moisture-permeable container 56 including a measured
amount of the solid water absorbing particles 54 is then positioned
on a portion of the lower interior surface of the enclosure 40 or
the enclosure 40 is used as an integral part of the moisture-permeable
container 56 for example, by bonding a moisture-permeable component
along the lower surface of the enclosure with an adhesive to form
a desiccant package 50.
[0066] It will be appreciated that the desiccant package 50 having
the moisture-permeable container 56 can be formed as part of a desiccant
package 50 as described with reference to FIG. 5A.
[0067] With the exception of choosing a moisture-permeable container
process steps substantially identical to the preparation steps described
with reference to the description of FIG. 3B and FIG. 4 including
the step of sealing the OLED test structure 10 with the enclosure
40 by the seal 60 (see FIG. 4).
EXPERIMENTAL PROCEDURES
I. CONSTRUCTION OF THE OLED TEST STRUCTURE OF FIG. 1 AND FIG. 2
[0068] A plurality of identical OLED test structures were fabricated
by the following process sequence:
[0069] (1) a glass substrate having a light-transmissive anode
of indium-tin-oxide (ITO) on one surface thereof was cleaned by
ultrasonicating in a commercial detergent, rinsing in deionized
water, degreasing in toluene vapor, and contacting by a strong oxidizing
agent;
[0070] (2) a 150 nm thick organic hole-transporting layer of 44'-bis-[N-(1-naphthyl)-N-phenylamino]biphenyl
(NPB) was formed over the substrate and over the anode by conventional
vacuum evaporation;
[0071] (3) a 375 nm thick organic emission layer of tris(8-quinolinato-N108)-aluminum
(Alq) doped with 0.5 vol % (C545T) was formed over the NPB hole-transporting
layer by conventional vacuum evaporation;
[0072] (3) a 375 nm thick organic electron-transport layer of tris(8-quinolinato-N108)-aluminum
(Alq) was formed over the NPB hole-transporting layer by conventional
vacuum evaporation;
[0073] (4) a cathode was formed over the Alq electron-transporting
layer by vacuum evaporation of 0.5 nm of LiF and 100 nm of aluminum,
through a shadow mask, so that the cathode was formed in a direction
orthogonal to the anode, thereby defining an area of intersection
between the anode and the cathode, and thus defining an area from
which light would initially be emitted via electron-hole recombination
at or near an interface between the Alq electron-transporting layer
and the NPB hole-transporting layer.
II. PREPARATION AND SEALING OF ENCLOSURES TO OLED TEST STRUCTURES
[0074] (1) each one of a plurality of suitably dimensioned enclosures
(40; see, for example, FIGS. 3B, 5B, 6A, 6B) was cleaned, prior
to forming a desiccant package, by a cleaning process substantially
identical to the substrate cleaning process described in Section
I above, except that the step of contacting by a strong oxidizing
agent was deleted;
[0075] (2) a desiccant package was formed and cured along a lower
interior surface of the enclosure;
[0076] (3) a sealing material was used to form a bead of sealing
material along an upper surface of the sealing flange of the enclosure;
[0077] (4) the flange with the bead was positioned, and pressed
against, a peripheral sealing area of the OLED's substrate, and
the seal was cured by either cooling of a hot melt adhesive or UV
exposure for 2 minutes of UV radiation curable adhesives, thus enclosing
the OLED test structure (see FIG. 4).
III. TESTING OF ENCLOSED OLED TEST STRUCTURES
[0078] (1) three to five nominally identical OLED test structures,
each sealed with an enclosure containing one and the same nominally
identical desiccant package, were tested to provide data related
to performance and variance of performance of such sets of enclosed
test structures;
[0079] (2) the initial physical and electrical width dimension
w of the cathode was measured as follows:
[0080] (i) the physical width dimension was determined by microscopy
in a calibrated optical microscope;
[0081] (ii) the initial effective electrical width dimension was
determined during test structure in operation by applying a voltage
from a drive voltage source between the cathode and the anode, and
adjusted such that a current density of 20 mA/cm.sup.2 was achieved
in the area defined by the intersection of the anode with the cathode.
The width dimension of the emitted light in a direction across the
cathode provided a measure of an initial effective electrical cathode
width dimension which was also measured by the calibrated microscope;
[0082] (3) initial values of physical and electrical cathode width
dimensions coincided within a deviation of about .+-.5 micrometer;
[0083] (4) each set of enclosed OLED test structures was then stored
under dark conditions in an environment test chamber at 85.degree.
C. and 85% RH for at least 100 hours;
[0084] (5) the test structures were operated under conditions described
in paragraph (ii) above to measure the effective electrical width
dimension of the cathode(s) following storage in the environmental
test chamber;
[0085] (6) depending on the efficacy of the desiccant package formed
within the enclosure of the OLED test structure, an effective electrical
width dimension of the cathode was reduced more or less with respect
to the initial cathode width dimension. The difference between the
initial electrical (and physical) width dimension and the effective
electrical cathode width dimension following storage in the environmental
test chamber was tabulated for each set of test structures, and
was taken as a measure of effectiveness of a particular desiccant
package within an enclosure.
IV. EXAMPLES
Example 1
[0086] Pure desiccant powders (particle size 1-10 .mu.m) in a moisture-permeable
container were tested to determine their relative effectiveness
in preventing cathode width shrinkage in the OLED test structure.
The procedure described in the application was used to prepare the
test structures except that the desiccant layers were the pure powders
and they were held in place by a moisture-permeable non-woven fabric
membrane that was coated on one side with a pressure sensitive adhesive.
The sealing material was a hot melt polyolefin that was melted at
150.degree. C. on the 5 mm flange of the enclosure and then pressed
against the substrate while it cooled. The results of testing after
500 hrs at 85.degree. C./85%RH were:
1 Cathode Width Shrinkage Desiccant Type (.mu.m) Phosphorous Pentoxide
0 Calcium Oxide 0-5 Barium Oxide 0-15 Magnesium Perchlorate 5-10
Calcium Sulfate 45-50 Molecular Sieves 50 No Desiccant 80-150
[0087] The results show that phosphorous pentoxide, calcium oxide,
barium oxide, and magnesium perchlorate are the best performing
desiccants as predicted by their very low equilibrium minimum humidity
capabilities. The results also show that the non-woven fabric container
allows moisture to permeate quickly to the contained desiccant since
all desiccant containing devices were better than the no desiccant
control.
Example 2
[0088] Calcium oxide particles with an average size of 1.17 .mu.m
were compared to calcium oxide particles with an average size of
0.07 .mu.m. A mixture of 25 wt % calcium oxide was dispersed in
a solution of 17 wt % polyethylmethacrylate (Water Vapor Transmission
Rate of 28 gm-mil/100 in.sup.2/day) dissolved in ethyl acetate.
The desiccant package was formed on the interior surface of the
enclosure and baked at 150.degree. C. for 2 hrs to remove the ethyl
acetate. The sealing material was a UV curable epoxy on the 1.5
mm flange that was exposed to 15 J/cm.sup.2 of UV energy. The results
after 593 hrs at 85.degree. C./85%RH were:
2 Cathode Width Shrinkage Desiccant Type (.mu.m) 1.17 .mu.m CaO
with PEMA binder 64 0.07 .mu.m CaO with PEMA binder 43
[0089] The results show that particle sizes less than 0.1 .mu.m
of calcium oxide dispersed in polyethylmethacrylate and cast into
a film outperform the film of 1.17 .mu.m calcium oxide dispersed
in polyethylmethacrylate. This demonstrates the importance of increasing
the water vapor absorption rate by decreasing the particle size
of low equilibrium minimum humidity desiccants to provide sufficient
protection for highly moisture sensitive electronic devices.
Example 3
[0090] Calcium oxide particles with an average size of 0.02 .mu.m,
0.1 .mu.m, and 4 .mu.m were compared for water absorption rate.
Three mixtures of 25 wt % calcium oxide were dispersed in solutions
of 17 wt % polyethylmethacrylate (Water Vapor Transmission Rate
of 28 gm-mil/100 in.sup.2/day) dissolved in ethyl acetate. The desiccant
packages were formed on the interior surface of a pre-weighed aluminum
weighing pan and baked at 150.degree. C. for 2 hrs to remove the
ethyl acetate. The samples were re-weighed to determine initial
desiccant layer mass. The samples were then placed into a humidity
chamber at 73.degree. F. and 70% RH. The samples were removed periodically
and weighed to determine the water absorption rate of the different
desiccant packages. The results were:
[0091] The results show that the rate of water absorption is highly
dependant on the size of the calcium oxide particles. The reduction
of particle size from 4 .mu.m to 0.1 .mu.m increases the water absorption
rate by over two orders of magnitude. The results also show that
reduction of particle size below 0.1 .mu.m provides additional benefits
by further increasing the water absorption rate over the rate for
0.1 .mu.m particles and by eliminating the decrease in water absorption
rate as the calcium oxide approaches complete conversion to calcium
oxide at 32%.
Example 4
[0092] Calcium oxide, barium oxide, calcium sulfate, and calcium
bromide powders were compared to the same desiccants dispersed in
phenoxy resin. 11 vol % desiccant was dispersed in a 21 wt % solution
of phenoxy resin in diethyladipate. The desiccant package was formed
on the interior surface of the enclosure and baked at 160.degree.
C. for 2 hrs to remove the diethyladipate. The powder was placed
in the enclosure by the same method as Example 1. The sealing material
and method was the same as example 2. The results after 146 hrs
at 85.degree. C./85%RH were:
3 Cathode Width Shrinkage Desiccant Type (.mu.m) Barium Oxide 0
Calcium Oxide 26 Calcium Bromide 100 Calcium Sulfate 562 Phenoxy
with Barium Oxide 11 Phenoxy with Calcium Oxide 50 Phenoxy with
Calcium Bromide 133 Phenoxy with Calcium Sulfate 872
[0093] The results show again that calcium oxide and barium oxide
are the best performing desiccants as predicted by their very low
equilibrium minimum humidity capabilities. The results also show
that a low water vapor transmission rate of the resin can decrease
the effectiveness of all desiccants compared to the pure powders.
However, the results also show that barium oxide in phenoxy still
outperforms both the calcium oxide powder in the moisture-permeable
container and the phenoxy with calcium oxide. This result demonstrates
that the minimum resin water vapor transmission rate is dependent
on the desiccant choice. This again demonstrates the importance
of matching the water vapor transmission rate, the desiccant, and
the water absorption rate requirements to provide sufficient protection
for highly moisture sensitive electronic devices, and demonstrates
the inability to predict performance of the desiccant and resin
blend based solely on equilibrium minimum humidity capabilities
and water vapor transmission rates.
Example 5
[0094] Blends of calcium oxide dispersed in polyethylmethacrylate
(PEMA), polydiallylphthalate (PDAP), polysulfone (PSF), phenoxy
and two UV curable acrylates (UV1 and UV2) were compared. 37 vol
% desiccant was dispersed in 63 vol % of each resin after drying
or curing of the mixture. For polyethylmethacrylate, polydiallylphthalate,
polysulfone, and phenoxy mixtures the desiccant package was formed
on the interior surface of the enclosure as in Example 4. For the
UV curable acrylates the desiccant package was formed on the interior
surface of the enclosure and cured by exposure to 15J/cm.sup.2 of
UV energy. The sealing material and method was the same as Example
2. The results after 146 hrs at 85.degree. C./85%RH were:
4 Resin Water Vapor Transmission Rate Cathode Width Desiccant Type
(gm-mil/100 in.sup.2/day) Shrinkage (.mu.m) UV1 with Calcium Oxide
136 26 PEMA with Calcium Oxide 28 23 UV2 with Calcium Oxide 15 20
PDAP with Calcium Oxide Not measured 43 Phenoxy with Calcium Oxide
3.5 50 PSF with Calcium Oxide Not measured 62
[0095] The results show that a resin water vapor transmission rate
of greater than 3.5 gm-mil/100 in.sup.2/day is required to prevent
a decrease in the effectiveness of the calcium oxide compared to
calcium oxide in resins with water vapor transmission rates of 15
to 136 gm-mil/100 in.sup.2/day. This again demonstrates the importance
of matching the water vapor transmission rate, the desiccant, and
the water absorption rate requirements to provide sufficient protection
for highly moisture sensitive electronic devices.
Example 6
[0096] Calcium oxide powder was compared to 45 wt % calcium oxide
dispersed in polyamide. The desiccant package was formed on the
interior surface of the enclosure by melting the desiccant and resin
mixture, dispensing the melt, and cooling to room temperature. The
powder was placed in the enclosure by the same method as Example
1. The sealing material and method was the same as Example 2. The
results after 146 hrs at 85.degree. C./85%RH were:
5 Cathode Width Shrinkage Desiccant Type (.mu.m) Calcium Oxide
24 Polyamide with Calcium 23 Oxide
[0097] The results show that calcium oxide dispersed in a melt
processable polyamide performs as well as the calcium oxide powder
alone.
[0098] The present invention is particularly suitable for use with
microelectronic devices which are highly moisture-sensitive. It
prevents premature device failure or premature degradation of device
performance. With certain microelectronic devices humidity levels
should not be greater than 2500 ppm. With other devices such as
organic light-emitting devices, they typically should have a humidity
level no greater than 100 ppm.
[0099] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will
be understood that variations and modifications can be effected
within the spirit and scope of the invention. |