U.S. patent application number 11/183412 was filed with the patent office on 2007-01-18 for thick film getter paste compositions with pre-hydrated desiccant for use in atmosphere control.
Invention is credited to Fong-Pei Chang, Christopher J. Roach, Carl B. Wang.
Application Number | 20070013305 11/183412 |
Document ID | / |
Family ID | 36928580 |
Filed Date | 2007-01-18 |
United States Patent
Application |
20070013305 |
Kind Code |
A1 |
Wang; Carl B. ; et
al. |
January 18, 2007 |
Thick film getter paste compositions with pre-hydrated desiccant
for use in atmosphere control
Abstract
The invention is directed to a screen-printable getter
composition comprising: (a) glass frit; and (b) pre-hydrated
desiccant material; dispersed in (c) organic medium. The present
invention further relates to a getter composition utilizing
low-softening temperature glasses comprising, based on weight %,
1-50% SiO.sub.2, 0-80% B.sub.2O.sub.3, 0-90% Bi.sub.2O.sub.3, 0-90%
PbO, 0-90% P.sub.2O.sub.5, 0-60% Li.sub.2O, 0-30% Al.sub.2O.sub.3,
0-10% K.sub.2O, 0-10% Na.sub.2O, and 0-30% MO where M is selected
from Ba, Sr, Ca, Zn, Cu, Mg and mixtures thereof. The glasses
described herein may contain several other oxide constituents that
can substitute glass network-forming elements or modify glass
structure. The desiccant material is pre-hydrated to reach its
saturation level of moisture absorption. The process of
pre-hydration can be done by exposing the desiccant in a normal
temperature/humidity environment of for example, 25.degree. C. and
50-60% RH. for 24 to 48 hours or up to the time when weight gain
(due to moisture absorption) stops increasing. An accelerated
hydration process in a chamber with higher than normal humidity
level (i.e. 50% Relative Humidity) is also applicable to shorten
the time of exposure to fully hydrate the desiccant material.
Inventors: |
Wang; Carl B.; (Raleigh,
NC) ; Roach; Christopher J.; (Apex, NC) ;
Chang; Fong-Pei; (Tadyuan City, TW) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
36928580 |
Appl. No.: |
11/183412 |
Filed: |
July 18, 2005 |
Current U.S.
Class: |
313/553 ;
252/181.1; 257/E23.137; 313/554 |
Current CPC
Class: |
C03C 27/06 20130101;
B01J 20/2803 20130101; H01L 2924/00 20130101; H01L 2924/09701
20130101; C03C 8/04 20130101; C03C 8/10 20130101; B01J 20/28026
20130101; C03C 8/02 20130101; H01L 2924/0002 20130101; H01L 23/26
20130101; C03C 8/245 20130101; H01L 51/5259 20130101; C03C 8/24
20130101; H01L 2924/12044 20130101; B01J 20/28033 20130101; B01J
20/183 20130101; H01L 2924/0002 20130101 |
Class at
Publication: |
313/553 ;
313/554; 252/181.1 |
International
Class: |
H01J 17/24 20060101
H01J017/24; H01J 19/70 20060101 H01J019/70; H01J 61/26 20060101
H01J061/26 |
Claims
1. A screen-printable getter composition comprising: a) glass frit;
and b) desiccant material; dispersed in c) organic medium. wherein
the desiccant material is fully hydrated prior to its use in the
preparation of the getter composition.
2. The composition of claim 1 wherein said desiccant material is
hydrated by exposing said material in an ordinary paste production
environment to reach pseudo steady state weight gain due to the
moisture absorption.
3. The composition of claim 1 wherein said desiccant is hydrated by
exposing the material in a controlled temperature and humidity
environment to reach pseudo steady state weight gain due to the
moisture absorption.
4. The composition of claim 1 wherein said organic medium is
comprised of organic polymeric binder and volatile organic
solvent.
5. The composition of claim 1 wherein said organic polymeric binder
is selected from the group comprising ethyl cellulose, ethyl
hydroxyethy cellulose, wood rosin, mixtures of ethyl cellulose and
phenolic resins, polymethacrylates of lower alcohols, monobutyl
ether of ethylene glycol monoacetate, and mixtures thereof.
6. The composition of claim 1 wherein said volatile organic solvent
is selected from the group comprising ethyl acetate, terpene,
kerosene, dibutyl phthalate, butyl carbitol, butyl carbitol
acetate, hexylene glycol, high boiling alcohols, alcohol esters,
and mixture thereof.
7. The composition of claim 1 comprising 5 to 55 weight percent
glass frit, based on total composition.
8. The composition of claim 1 comprising 5 to 65 weight percent of
said desiccant material, based on total composition.
9. The composition of claim 1 comprising 30 to 55 weight percent of
said organic medium, based on total composition.
10. The composition of claim 1 wherein said glass frit comprises:
1-50% SiO2, 0-80% B2O3, 0-90% Bi2O3, 0-90% PbO, 0-90% P2O5, 0-60%
Li2O, 0-30% Al2O3, 0-10% K2O, 0-10% Na2O, and 0-30% MO where M is
selected from Ba, Sr, Ca, Zn, Mg and mixtures thereof.
11. A method of moisture control comprising: a) providing the
getter composition of claim 1; b) applying said getter composition
to a device; and c) processing said getter composition and device
to volatilize the absorbed moisture in the desiccant material and
organic polymeric binder and sinter the glass frit with the
desiccant.
12. The method of claim 11 further comprising sealing said
device.
13. A device made by the method of claim 12.
14. The device of claim 13 wherein said device is selected from a
light emitting diode device, electroluminescent device, field
emission display device, surface emission display device, plasma
display panel device, cathode ray tube device, and liquid crystal
display device.
Description
FIELD OF THE INVENTION
[0001] The invention is directed to thick film getter paste
compositions with pre-hydrated desiccant for use in controlling
moisture, oxygen, and small organic molecule levels in sensitive
electronic and photonic devices.
BACKGROUND OF THE INVENTION
[0002] The invention describes thick film getter compositions for
use in controlling moisture, oxygen, and small organic molecule
levels. Many packaged devices are susceptible to damage from
undesired levels of moisture, oxygen, and small organic molecules
and this is especially so in the case of various electronic,
display, and light sensing or emitting devices. Examples of the
types of damage caused or aggravated by the gas or condensed phase
impurities include; hydration, oxidation, corrosion, and ion
migration. In the case of FED (Field Emission Display) or SED
(Surface Emission Display), the residue gasses may cause the
undesirable modification of the emitter tip surface work function
and degrade the performance. The concept of using a getter to
control the atmosphere within packaged devices has been known for
many years. Getters are substances that are added to a system or
mixture to consume or deactivate traces of undesirable gas phase
impurities including moisture.
[0003] One approach to minimizing the deleterious effects of gas
phase impurities has involved the enclosure of sensitive devices
with inorganic or organic barriers to physically separate the
active materials from external atmospheric oxygen and moisture.
This approach has had some success, but it does not always
adequately address the problems caused by even those small amounts
of gas phase impurities trapped within the enclosure or diffusing
into the enclosure over time.
[0004] The use of getters for controlling gas phase impurities
within a sealed enclosure for an electronic component or device is
well known. These sealed enclosures are designed to protect
sensitive electronic components and devices from gas phase
impurities including moisture that come from outside environmental
contaminants and/or internally created outgassing or desorption
from various functional components. For example, a great many of
display devices such as PDP (Plasma Display Panel), CRT (Cathode
Ray Tube), FED (Field Emission Display), LCD (Liquid Crystal
Display), LED (Light Emitting Diode) are highly sensitive and can
only tolerate very low levels of gas phase impurities. At the same
time, mass production of these devices dictates that the getter
composition be easily processed while providing sufficient adhesion
to any chosen substrate. The present invention provides a superior
composition and process to accommodate these requirements.
[0005] Prior art getters and their use in electronic applications
are described below. Typically, the getter materials are not
screen-printable, and normally are comprised of a desiccant
material (i.e., zeolite, silica gel, etc.) and a binder. The binder
may be organic or inorganic. The following illustrates the
specifics of the prior art.
[0006] U.S. Pat. No. 5,244,707 to Shores discloses a sealed
enclosure of an electronic device which incorporates a coating or
adhesive with desiccant properties. The coating or adhesive
comprises a protonated aluminum silicate powder dispersed in
polymer.
[0007] U.S. Pat. No. 5,591,379 to Shores teaches a composition of
matter useful as a desiccant in a hermetic electronic device,
comprising a powder dispersed in a binder, wherein said powder is
selected from the group consisting of zeolite molecular sieves,
activated alumina, silica gel, alkaline earth oxide, and alkali
metal carbonate; said binder is a continuous matrix of porous glass
or porous ceramic; and said powder to binder volume ratio is
0.001-2. The glasses disclosed for use as a binder must be made
porous by creating channels for water vapor to penetrate. This may
be done by various techniques known in the art, such as the use of
blowing agents, fast evaporation of water or other gases during
formation, fast decomposition of organometallic polymers and low
temperature or incomplete sintering.
[0008] U.S. Pat. No. 1,626,682 to MacRae discloses a multi-pixel
flat panel display which includes spaced apart first and second
electrodes, with a patterned solid material layer in contact with
at least one of the electrodes, and preferably placed between the
two electrodes. The patterned layer (the web) includes a
multiplicity of apertures, with at least one aperture associated
with any given pixel. In the aperture is disposed a quantity of a
second material, exemplarily, a phosphor in the case of a flat
panel field emission display, or a color filter material in the
case of a liquid crystal display. The web can facilitate second
material deposition by means of screen-printing. The web also can
facilitate provision of spacer structure between two electrodes,
and can include getter or hygroscopic material.
[0009] U.S. Pat. No. 5,401,706 to Fischer teaches a process for
making a desiccant-coated substrate capable of being used at
temperatures above 150 degrees Fahrenheit, the desiccant being in
the form of particles and the particles having pores and being
adhered to the substrate by a binder, the coated substrate being
sufficiently flexible and the coating having sufficient adherence
to the substrate so that the coated substrate can be formed into
corrugated shapes, the desiccant particles in the coated substrate
having at least 60% of their original adsorption capacity and the
binder having good breathability; said process comprising the
steps: (a) forming an aqueous suspension comprising particles of
one or more desiccants, a water-based organic binder, a suspending
agent to help maintain the desiccant particles in suspension, and
an organic pore-clearing agent at least some of which enters at
least some of the pores of the desiccant particles; (b) depositing
the suspension on the substrate; and (c) causing the binder of the
deposited suspension to set so that the deposited desiccant
particles adhere to the substrate and causing at least some of the
pore-clearing agent to leave the pores of the desiccant particles
to prevent the binder from occluding at least some of the pores of
the adhered desiccant particles, thereby to form a desiccant-coated
substrate capable of being used at temperatures above 150 degrees
Fahrenheit and of sufficient flexibility and having a coating
having sufficient adherence to the substrate so that the
desiccant-coated substrate can be formed into corrugated shapes and
in which the desiccant particles in the coated substrate have at
least 60% of their original adsorption capacity and in which the
binder has good breathability.
[0010] Commonly assigned, U.S. National patent application Ser. No.
10/712,670 (Attorney Docket No. EL-0519) to Cho et al provides a
screen-printable thick film getter composition comprising glass
which may be used to promote densification at relatively low
sintering temperatures of 400-650.degree. C. and to provide
increased adhesion between the thick film and the substrate.
Furthermore, the composition of Cho et al may be utilized in
display devices which often require humidity control to levels
below about 1000 ppm and in specific cases below 100 ppm. However,
the thick film compositions of Cho et al may become unstable during
generation of the thick film composition itself, due to the
absorption of moisture from the atmosphere during processing. This
absorption of moisture may lead to the generation of heat and
possible safety concerns during composition processing.
Furthermore, this absorption of moisture during thick film
composition formation may modify the composition rheology, which is
problematic during application.
[0011] The present invention provides a screen-printable thick film
getter composition comprising glass which may be used to promote
densification at low densification temperatures in the range of
400-650.degree. C. and to provide increased adhesion between the
thick film and the substrate. Another key ingredient in the getter
composition is a pre-hydrated desiccant wherein the process of
pre-hydration provides stability of (1) the desiccant under normal
temperature/humidity environment and (2) the rheological behavior
of the getter composition. Furthermore, the composition of the
present invention may be utilized in electronic and photonic
devices which often require humidity control to levels below about
1000 ppm and in specific cases below 100 ppm.
SUMMARY OF THE INVENTION
[0012] The invention is directed to a screen-printable getter
composition comprising: (a) glass frit; dispersed in (b) organic
medium. The invention is further directed to a screen-printable
getter composition comprising: (a) glass frit; and (b) pre-hydrated
desiccant material; dispersed in (c) organic medium.
[0013] The present invention further relates to a getter
composition utilizing low-softening temperature glasses comprising,
based on weight %, 1-50% SiO.sub.2, 0-80% B.sub.2O.sub.3, 0-90%
Bi.sub.2O.sub.3, 0-90% PbO, 0-90% P.sub.2O.sub.5, 0-60% Li.sub.2O,
0-30% Al.sub.2O.sub.3, 0-10% K.sub.2O, 0-10% Na.sub.2O, and 0-30%
MO where M is selected from Ba, Sr, Ca, Zn, Cu, Mg and mixtures
thereof. The glasses described herein may contain several other
oxide constituents that can substitute glass network-forming
elements or modify glass structure.
DETAILED DESCRIPTION OF INVENTION
[0014] The thick film getter composition of the present invention
is a screen-printable ceramic composition, which may be formed on a
substrate, for example, a glass substrate acting as a lid in
various electronic display devices. The thick film getter acts as a
gas phase absorbent at various environmental conditions of a few
thousands ppm to a few ppm levels. The present invention is not
limited to display device applications, but can be used for any
other microelectronic device applications where the device lifetime
can be degraded by or sensitive to the presence of gas phase
impurities.
[0015] Furthermore, the thick film getter paste is primarily
intended to be used for various display applications including
organic light emitting diodes (OLED) and electroluminescence (EL),
where lifetime of the display devices are strongly related to the
control of humidity and other harmful gases. The organic light
emitting materials in OLED and EL devices are degraded by high
temperatures, so organic sealing materials are typically used
instead of the more hermetic fired glass or glass-to-metal seals.
The organic light emitting applications differ from other hermetic
microelectronic devices in that the moisture gettering is working
at highly moisture-sensitive or gas-sensitive environments. The
display devices often require humidity control to levels below
about 1000 ppm and in specific cases below 100 ppm.
[0016] Many of the other display and light emitting packages are
inorganic with the more hermetic type of fired seals that are
designed to maintain vacuum. In these packages, the thick film
getter has particular value for absorbing oxygen and trace organic
residue. CNT (carbon nanotube) and FED (field emission displays)
are examples of a sensitive fired package device where oxygen and
outgassing from trace organic residue have both been demonstrated
to degrade performance. In a FED, the nanotubes act as low work
function electron guns. When they are hot, they can be oxidized by
traces of oxygen within the sealed glass panel structure. Outgassed
organic residuals also tend to deposit on and coat the CNT's, thus
decreasing their electron emission efficiency. CNT's are
susceptible to damage from gas phase impurities regardless of
whether they are deposited/ oriented by thick film methods or grown
directly by CVD. Other materials in the various types of displays
also have a recognized sensitivity to various gas phase impurities.
Examples include the various R-G-B light emitting inorganic
phosphor materials that are common to all of the emissive and
plasma displays (FED, SED, PDP, & CRT). All of the phosphor
materials and some of the electrode materials have recognized
sensitivities to various gas phase impurities. The compositions of
the present invention may be utilized in each of the applications
listed above.
[0017] The thick film getter composition is comprised of a mixture
of glass powder and desiccant material dispersed in an organic
medium. The desiccant materials useful in the present invention
include any materials or mixtures of materials which are capable of
absorbing impurities and are compatible with the required substrate
processing conditions (i.e., the desiccant material must be able to
withstand the firing conditions required for processing. Typically,
the substrate is a glass or ceramic material and processing
conditions include a firing temperature of up to 600.degree.
C.).
[0018] The organic medium is comprised of organic polymeric binder
and volatile organic solvent. In one embodiment, zeolite, a
desiccant material or other material that absorbs impurities, such
as hydrogen or other gases can be used alone or as a mixture.
Usually, the desiccant material is the main functional part of the
thick film getter, which determines its performance of gas phase
impurity absorption. Generally, the capacity of gas phase
impurities absorption proportionally depends on the relative
content of the desiccant material along with the retained level of
open porosity after firing. The glass component is primarily
selected based on its ability to provide a low densification
temperature in the range of 400-650.degree. C., good adhesion
between the sintered thick film and substrate, and maintenance of a
reasonable level of open porosity in the fired Zeolite particles.
Firing temperature is also limited by the choice of the lid and
substrate materials and TCE stresses. For example, a firing
temperature below 650.degree. C. is necessary for the typical soda
lime glass substrate to prevent warpage. In case of ceramic or
certain metal-based substrates, a temperature above 650.degree. C.
may be allowed.
[0019] Adhesion between the getter film and substrate is improved
by the existence of a low softening point glass. Glass can help
relieve interface stress by facilitating penetration of glass into
voids of substrates via viscous flow during firing. Mechanical
locking is likely to be the dominant mechanism for adhesion between
thick film and substrate. However, the actual adhesion mechanism
may vary and depends on the choice of substrate and thick film
composition.
[0020] In the present invention, the glass does not necessarily
form a continuous matrix in unfired or fired structure. Before
firing, the glass and desiccant are dispersed in the organic media
to form a viscous paste. In the early stages of sintering the
moisture, that was intentionally added to hydrate the Zeolite,
evaporates. The evaporation and decomposition of the organic media
also occurs before the actual consolidation of solid particles. In
the later stages of sintering the glass begins to soften and wet
the surface of desiccant particles and the surface of the
substrate. The sintered getter layer is densified by the viscous
glass flow. The final fired structure depends on the type of solids
and their relative content.
[0021] The glass itself is not porous and it is not necessary to
produce the glass by a special fabrication process, such as fast
evaporation of water or glasses, the use of blowing or bubbling
agents, any chemical hollow process and low temperature firing, to
intentionally generate pores in the glass. Rather, it is important
to achieve a highly dense or essentially void-free macrostructure
with the proper choice of glass chemistry and sintering process of
the resultant getter paste composition.
[0022] The hardness and mechanical strength of the film primarily
depend on the content and type of glass. The mechanical integrity
is assumed to be important because weak film structure can be
damaged by an external mechanical shock. It is not necessary to
maintain high levels of porosity in the glass itself for better
performance of moisture absorption, since microstructure porosity
is provided by the other key desiccant ingredient(s) in the getter
paste composition.
[0023] The thick film getter composition, as described herein, is
bonded to substrate by depositing the thick film composition onto
the substrate and firing the substrate. Depending on the device
structure and its required performance, thickness of the getter
film may be adjusted by sequentially printing and drying the paste.
A typical thickness of the thick film getter after single printing
and subsequent firing ranges from 10 to 40 microns. An excessively
thick getter film is unnecessary as long as strong absorption of
moisture is provided by a getter film of normal thickness.
[0024] Although screen-printing is used as the common method for
the deposition of thick film getter paste, any other methods
including stencil printing, syringe dispensing, or other deposition
or casting techniques may be utilized.
[0025] This invention is based on the finding that thick film
getter compositions (sometimes referred to as "pastes") can be made
by combining low softening temperature glasses with desiccant
material in an organic medium. More specifically, the desiccant
material is pre-hydrated to reach its saturation level of moisture
absorption. The composition and pre-hydration process and results
are described in further details below.
Inorganic Composition
[0026] The inorganic composition of thick film getter paste is
comprised of a glass frit powder and a desiccant material.
[0027] The glass frit powder is essential to the composition to
promote adhesion to the substrate and densification of the
composition upon firing process. It has been determined that the
adequate levels of glass must be within the range of 1 0 wt % to 30
wt % and preferably within 1 0 and 20 weight %, based on the total
solid weight % including frit and pre-hydrated desiccant.
[0028] The present invention relates to low-softening temperature
glasses comprising, based on weight %, 1-50% SiO.sub.2, 0-80%
B.sub.2O.sub.3, 0-90% Bi.sub.2O.sub.3, 0-90% PbO, 0-90%
P.sub.2O.sub.5, 0-60% Li.sub.2O, 0-30% Al.sub.2O.sub.3, 0-10%
K.sub.2O, 0-10% Na.sub.2O, and 0-30% MO where M is selected from
Ba, Sr, Ca, Zn, Cu, Mg and mixtures thereof. The glasses described
herein may contain several other oxide constituents. For instance,
ZrO.sub.2 and GeO.sub.2 may be partially incorporated into the
glass structure. Hygroscopic lithium-based silicates or phosphate
glasses may also be applied for the getter purpose. Furthermore,
the glass compositions of the present invention are not required to
be made porous by creating channels for water vapor to
penetrate.
[0029] High content of Pb, Bi or P in glass provides a very low
softening point that allows thick film pastes to become dense below
650.degree. C. These glasses are not crystallized during firing,
since these additional elements tend to provide good stability of
glass and a high solid solubility for other constituents in the
glass recipe. In addition, the existence of a large content of
phosphorous may be useful in gettering moisture without affecting
the degree of densification. Li-based silicate glasses may also be
used in this application, since they are sensitive to moisture and
can be densified at low temperature below 650.degree. C. However,
the content of Li is to be adjusted to reach full densification
before crystallization happens.
[0030] Other glass modifiers or additives may be added to modify
glass properties for better compatibility with a given substrate.
For example, Temperature Coefficient of Expansion (TCE) of the
glass may be adjusted by the relative content of other glass
constituents in the low-softening temperature glasses.
[0031] The glasses described herein are produced by conventional
glass making techniques. More specifically, the glasses may be
prepared as follows. The glasses were prepared in the batch size of
500-2000 gram. Typically, the ingredients were weighed then mixed
in the desired proportions and heated in a bottom-loading furnace
to form a melt in platinum alloy crucibles. Heat was applied to
reach a peak temperature of 1100-1400.degree. C. and for a time
duration for the melt to become a homogeneous liquid. The glass
melt was quenched by a counter rotating stainless steel roller to
form a 10-20 mil thick platelet of glass. The resulting glass
platelet was then milled to form a powder with its 50% volume
distribution set between 2-5 microns. The glass powders were then
formulated with desiccant and/or other filler and organic medium
into a thick film composition (or "paste"). The glass content of
the present invention in the amount of about 8 to 10 weight %,
based on total composition comprising, glass, pre-hydrated
desiccant material, and organic medium.
[0032] Various desiccant materials such as molecular sieves (or
zeolites), alkaline earth metal oxides, metal oxides, sulfates,
chlorides, bromides may be selected for getter paste applications.
Various types of zeolites are known to be the best candidates in
terms of capacity of water absorption due to their intrinsic nature
of porous structure.
[0033] Zeolites are materials that absorb moisture by physical
absorption and may be naturally or synthetically derived. Natural
zeolites are hydrated silicate of aluminum and either sodium or
calcium or both, of the type Na.sub.2O, Al.sub.2O.sub.3, xH.sub.2O,
and xSiO.sub.2. Synthetic zeolites are made either by a gel process
or a clay process, which forms a matrix to which the zeolite is
added. Both natural and synthetic zeolites may be used in the
present invention. Well known zeolites include chabazite (also
referred to as zeolite D), clinoptilolite, erionite, faujasite
(also referred to as zeolite X and zeolite Y), ferrierite,
mordenite, zeolite A, and zeolite P. Detailed descriptions of the
aforementioned zeolites, as well as others, may be found in D. W.
Breck, Zeolite Molecular Sieves, John Wiley and Sons, New York,
1974, hereby incorporated by reference. For example, type 3A, 4A
and 13X zeolites all have the ability to adsorb water molecules and
are presently used as the preferred adsorbent molecular sieve for
making the new moisture getter compositions. Such zeolites comprise
Na.sub.2O, Al.sub.2O.sub.3 and SiO.sub.2. Certain adsorbent getters
can adsorb gaseous contaminants in addition to moisture, such as
gaseous H.sub.2 and O.sub.2. The 3A & 4A zeolites have
relatively narrow pore diameters of 3 & 4 angstroms
respectively. The narrower pores only allow very small molecules to
enter. The 13X zeolite nominally has 10 angstrom pore diameters
that are less selective and allow somewhat larger molecules to
enter. The 13X zeolite is effective for removing a broader range of
molecules including: moisture, oxygen, and small organic molecules.
An example of a commercially available, solid getter tablet based
on zeolite technology that can be made to adsorb organics, as well
as moisture is described in European Patent Application No. WO
02/430098 A1 by Synetix.
[0034] For this invention, the desiccant material is pre-hydrated
to reach its saturation level of moisture absorption. The process
of pre-hydration can be done by exposing the desiccant in a normal
temperature/humidity environment of 25.degree. C. and 50-60% RH.
for 24 to 48 hours or up to the time when weight gain (due to
moisture absorption) stops to occur. An accelerated hydration
process in a chamber with higher than normal humidity level (i.e.
50% Relative Humidity) is also applicable to shorten the time of
exposure till weight gain stops to occur.
[0035] Table 1 illustrates the % weight gain as a function of
normal temperature/humidity exposure of commonly used desiccant
materials such as zeolite 3A and 13X, with two test specimen for
each type. The weight of these powders was monitored to determine
the rate of weight gain and the time when the weight of these
pre-hydrated powders reached a pseudo steady state. It appears that
the weight of both zeolite reached a steady state upon exposure for
46 hours and both samples for each type of zeolite showed
consistent weight gain with less than 3% variation for 3A and less
than 2% variation for 13X. It was noted, however, that the pseudo
steady state weight gain of 3A differed from that of 13X due to the
in size distribution and morphology of the given zeolite in the
zeolite particles. TABLE-US-00001 TABLE 1 % Weight Gain of Zeolite
Exposure Time, % Weight Gain of Zeolite 3A or 13X hours 3A-1 3A-2
13X-1 13X-2 0.16 9.3 9.3 11.5 11.5 4 22.9 21.2 29.4 25.7 6 23.3
22.0 29.8 27.4 8 23.3 22.3 29.5 28.4 22 23.9 23.2 30.7 30.2 32 24.0
23.1 30.8 30.2 46 24.3 23.6 31.4 30.8 70 24.2 23.6 31.2 30.8 80
24.0 23.2 30.8 30.2 94 23.9 23.2 30.8 30.3
The pre-hydrated zeolite processed according to the above method
was used as the desiccant ingredient in the getter paste
compositions. A Thermal Gravitational Analysis (TGA) was used to
study the dehydration of the above treated zeolite 13X-1. 35.72 mg
of the hydrated 13X-1 was used in the TGA sample holder and the
test was carried out in air atmosphere at 10.degree. C./minute
heating rate from room temperature to 550.degree. C. The final
weight of 27.34 mg reflected a weight loss of 23.47% and was
calculated to show 30.67% moisture or small gaseous organic
molecule absorption (per 100% anhydrous zeolite) due to the 94-hour
exposure treatment. The perspective weight loss percentage of the
absorbed moisture and/or other small gaseous organic molecule was
22.62% at 100.degree. C., 76.74% at 200.degree. C., 95.34% at
300.degree. C., 99.72% at 400.degree. C., and 100% at 500.degree.
C. and beyond. This confirmed the dehydration and hence activation
of zeolite after the firing of the getter paste composition in a
furnace profile with a peak temperature of 550.degree. C. whereas a
range of 400 to 650.degree. C. is applicable depending on the
substrate type and available furnace conditions.
[0036] The moisture and/or small gaseous organic molecule
absorption by desiccant materials, such as zeolite, is through
physical instead of chemical interaction between water molecule and
the pores in the desiccant materials. Therefore, a reversible water
absorption-desorption process continues until a pseudo steady state
is reached wherein the rate of water absorption by the zeolite is
equivalent to that of water desorption from the zeolite, depending
on the specific environmental temperature and relative humidity
condition. "Pseudo steady state" means that there may be slight
variations in weight due to the reversible water
absorption-desorption process. This pseudo steady state weight gain
due to the moisture absorption-desorption process may be reached by
exposing the dessicant material in an ordinary paste production
environment or by exposing the dessicant material in a controlled
temperature and humidity environment. Those skilled in the art will
understand that an ordinary (thick film) paste production
environment can encompass a multitude of temperature and humidity
conditions, such as those typically present in a thick film paste
production environment.
[0037] Without the step of pre-hydration, moisture absorption on
the as-received dehydrated zeolite resulted in heat dissipation
that raised the temperature of the mixture of paste ingredients and
its container prior to, during, or after the paste production
process by typically that of 3-roll milling. The extent of moisture
absorption depends on the environmental temperature/relative
humidity condition and the paste material handling process. Some of
the smaller organic molecules from various solvents can also
penetrate into the .about.10 angstrom diameter pores of the 13X
zeolite. Hydration of the zeolite pores effectively blocks most of
the organic molecules in the paste from penetrating. Typically a
process that creates freshly exposed zeolite such as ingredient
mixing or 3-roll milling results in higher rate of moisture
absorption. However, the organic medium used in the getter paste is
able to passivate the pores in the desiccant, which lowers the rate
of absorption. Therefore, the above co-existing mechanisms make the
process and material property less controllable. Since stability
and consistency of process (such as temperature of material under
mixing or 3-roll milling) and material properties (such as paste
rheology and its actual moisture content) are both critical, it is
necessary to stabilize the water absorption-desorption process as
disclosed by this invention.
[0038] The above pre-hydrated zeolite powders were used as
ingredients in the getter paste composition and found to provide
consistent temperature in the paste preparation process wherein the
rheology of the resultant paste samples remained stable upon room
temperature storage for more than three months.
[0039] The amount of pre-hydrated desiccant material added to the
composition should be determined based on the necessary capacity to
absorb moisture and other small gaseous organic molecules, as
dictated by the application. The approximate volume % ratio of
desiccant to glass frit ranges from 0 to about 15. The preferred
volume % ratio of desiccant to glass frit ranges from 2.1 to
10.
Organic Medium
[0040] The organic medium in which the glass and desiccant
materials are dispersed is comprised of an organic polymeric binder
or mixture of binders dissolved in a volatile organic solvent
which, optionally, was added other soluble or dispersible materials
such as plasticizers, release agents, dispersing agents, stripping
agents, antifoaming agents or wetting agents.
[0041] The solids are typically blended with an organic medium by
mechanical means to form a cohesive mass, called "premix", and then
further dispersed in an equipment such as 3-roll mill to produce a
viscous liquid called "paste" having proper dispersion, suitable
consistency, and right rheology for screen printing application. A
wide variety of chemicals in liquid or solid form can be used as
ingredients in an organic medium. The organic medium of choice must
be the wherein the solids can be dispersed with a prolonged
stability of dispersion upon room temperature storage. The
rheological properties of the medium must also provide good
patterning capability to the composition by method such as
screen-printing. These properties include: dispersion of solids
with an adequate degree of stability, appropriate viscosity, shear
thinning for ease of screen printing, thixotropy for retention of
pattern resolution, appropriate wettability of the substrate and
the solids, adequate drying rate, a dried film strength sufficient
to withstand rough handling, and adequate properties after firing.
The materials suitable to be used in an organic medium are well
known in the art and an organic medium is normally made of a
solution of polymer dissolved in solvent(s).
[0042] The most frequently used polymer for this purpose is ethyl
cellulose. Other examples of suitable polymers include
ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl
cellulose and phenolic resins, polymethacrylates of lower alcohols,
and monobutyl ether of ethylene glycol monoacetate, as well as
others known to those skilled in the art.
[0043] The most widely used solvents in thick film compositions are
ethyl acetate and terpenes such as alpha- or beta-terpineol or
mixtures thereof with other solvents such as kerosene,
dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene
glycol and high boiling alcohols and alcohol esters. In addition,
volatile liquids for promoting rapid hardening after application on
the substrate can be included in the vehicle. The preferred mediums
are based on ethylcellulose and .beta.-terpineol. Various
combinations of these and other solvents are formulated to obtain
the viscosity, volatility, and screen life necessary to achieve
acceptable paste printability and subsequent drying for mass
production application.
[0044] The ratio of organic medium to the inorganic solids in the
thick film paste composition depends on the method of application
and the kind of organic medium used. Usually, the dispersion will
contain 50-80 weight % of inorganic solids and 20-50 weight % of
vehicle or the organic medium in order to obtain acceptable
patterning. Within these limits, it is desirable to use the least
possible amount of organic binder in order to minimize the amount
of organics that must be removed by pyrolysis and to obtain dense
particle packing that results in small dimensional shrinkage upon
firing. The content of the organic medium is further selected to
provide suitable paste consistency and rheology for casting, screen
printing, ink-jet printing, or coating by spraying, brushing,
syringe-dispensing, doctor blading, and the like.
[0045] In general, the thickness of the fired getter pattern depend
on the solid content in the getter paste composition, the
rheological characteristics such as shear thinning and thixotropy
of the organic medium, the dispersion quality and packing density
of solids in the paste and in the dried patterned film, the
softening point, melt viscosity, and densification behavior of the
frit and desiccant materials.
Application of Thick Films
[0046] The thick film getter is typically deposited on a substrate,
such as glass, low temperature co-fired ceramics (LTCC) or silicone
that is impermeable to gases and moisture. The substrate can also
be a sheet of flexible and impermeable plastic material, such as
polyester, for example polyethylene terephthalate, or a composite
material made up of a combination of plastic sheet with optional
metallic or dielectric layers deposited thereupon. In one
embodiment, the substrate can be transparent (or semi-transparent)
to enable light to enter into the encapsulated region or to enable
light to be emitted from the encapsulated region through the
substrate.
[0047] The deposition of thick film getter is performed preferably
by screen-printing although other deposition techniques such as
stencil printing, syringe dispensing or other deposition or casting
techniques can be utilized. In case of screen-printing, parameters
such as the screen mesh size, the wire diameter, and the emulsion
thickness can be adjusted to obtain desirable thickness of
deposited thick film.
[0048] The deposited thick film is dried at 100-120.degree. C. for
5-10 minutes in an oven to remove volatile organic solvent(s), and
then firing is carried out at 400-650.degree. C. in a standard
thick film conveyor belt furnace or in a box furnace with a
programmed heating cycle. As used herein, the term "firing" means
heating the article in an oxidizing or inert atmosphere such as
air, nitrogen or argon to a temperature and for a time sufficient
to decompose and volatilize (burn-out) the organic material in the
layers of the assemblage and to sinter the remaining glass and
filler (such as desiccant) composite material in the layers thus,
densifying the thick film layers.
[0049] Fired thickness of the film can vary depending on screen
mesh size, screen wire diameter, screen emulsion thickness, content
of glass and solid % in paste, but typically ranges from 10 to 40
microns. To prevent re-adsorption of volatiles that deactivate the
desiccant (for example zeolite), the firing step is often conducted
in a controlled oxidizing atmosphere void of moisture and other
gases, and in extreme case, vacuum condition can be applied. Firing
is usually performed immediately prior to the step of sealing the
device into a hermetic container unless the fired panel with getter
material is stored in an atmosphere void of moisture and/or other
gases. Depending on the specific processing procedure, an
additional firing at 400-550.degree. C. to activate (or de-hydrate)
the getter layer may be required to evaporate re-absorbed moisture
during the storage period.
[0050] The humidity sensitivity of fired getter thick films was
evaluated by exposing the getter composition to changing humidity
conditions, such as different humidity levels, exposure time, and
temperature. A thermal gravitational analysis (TGA) analysis was
used to quantify the weight loss up to a specific temperature. The
evaluation results showed a similar decreasing tendency of weight
loss with increasing the glass content and with a much slower rate
of weight loss when reaching 400.degree. C. and beyond. In
addition, it revealed that most of humidity absorption on a fired
getter film occurred within a relatively short time of less than 1
hour of exposure.
Applications--Use in Electronic Display Devices
[0051] While the composition of the present invention is described
herein below as useful in the formation of a superior PLED device,
it is understood by those in the art that the composition of the
present invention may be utilized in various types of electronic
devices, including, but not limited to the following: PDP (Plasma
Display Panel); CRT (Cathode Ray Tube); FED (Field Emission
Display); LCD (Liquid Crystal Display); and LED (Light Emitting
Diode).
[0052] In one embodiment of the present invention, the pre-hydrated
getter composition is utilized in a polymer light-emitting diode
(PLED) display device. This device includes the solid-state
electronic device made up of the anode and cathode with electrical
attaching leads, the layer of electrically active organic polymer,
and, in this embodiment, a substrate. The device also includes an
encapsulating container isolating the electronic device from the
atmosphere. This container comprises the substrate as a base with a
cover or lid affixed to the base with a bonding agent. A thin layer
of activated adsorbent getter for water, oxygen and/or organic
gases is bonded to the interior surface of the lid of the container
so that the getter is sealed within the container along with the
electronic device.
[0053] Other features and advantages of the invention will be
apparent from the following detailed description, and from the
claims.
The Substrate
[0054] The PLED is typically fabricated on a substrate, such as
glass or silicone that is impermeable to gases and moisture. The
substrate can also be a sheet of flexible material, such as an
impermeable plastic such as polyester, for example polyethylene
terephthalate, or a composite material made up of a combination of
plastic sheet with optional metallic or dielectric layers deposited
thereupon. In one embodiment, the substrate can be transparent (or
semitransparent) to enable light to enter into the encapsulated
region or to enable light to be emitted from the encapsulated
region through the substrate.
The Container
[0055] The active area of the device can be encapsulated in a
container to seal out the environment. For example, the active area
can be covered with a covering, such as a lid, that is impermeable
to gases and moisture and which is sealed to a gas impermeable
substrate or container base. In one embodiment, the covering
comprises a metal. In another embodiment, the covering comprises
glass or a ceramic material. In one aspect of the invention, an
interior surface of the container, such as a lid that seals to the
substrate or to a container base, can be used as the surface that
is coated with the adsorbent getter materials and heat treated. In
such a case, the container, or the portion that serves as the
surface to which the getter is bonded, must be selected to
withstand heat treating of the getter as described below.
[0056] The container can be assembled by sealing the covering to
the base of the container (e.g., to the substrate) with a bonding
agent, such as a glue or other adhesive. This bonding agent should
cure at low temperatures such as below 75.degree. C., below
50.degree. C. or even at ambient temperature so that the device is
not exposed to a temperature high enough to damage or degrade the
electronic device. Preferred bonding agents for sealing the
assembled container include epoxies, either cured by exposure to
ultraviolet light or by exposure to moderately elevated
temperatures as just noted (or both). Various primer materials may
be used to assist in the bonding process.
[0057] Electrical leads emanate from the device. These should be
sealed as well in a moisture and gas impermeable fashion.
[0058] Although the invention is illustrated with reference to
PLEDs as the moisture-sensitive electronic device for convenience,
other types of moisture sensitive or gas-sensitive electronic
devices are contemplated to protected by the new methods of
desiccating or degassing an environment surrounding an electronic
device. For example, the construction of, and materials used in,
photodetecting devices and arrays of devices are very similar to
the fabrication of polymer-based LEDs. The main differences between
polymer-based LEDs and photodetectors is that reactive low work
function electrodes need not be used and electrical polarity of the
electrodes is reversed. Nevertheless, hermetically sealed packaging
is required for long lifetime of photodetecting devices fabricated
from conducting polymers.
[0059] The present invention will be discussed in further detail by
giving practical examples. The scope of the present invention,
however, is not limited in any way by these practical examples.
EXAMPLES
Examples 1-3
[0060] A series of bismuth or lead silicate-based glass
compositions that have been found to be suitable in the present
invention for application to thick film getter paste are shown in
Table 2. The examples particularly illustrate silicate glasses
containing more than 69 weight % bismuth.
[0061] All glasses were prepared by mixing raw materials and then
melting in a platinum crucible at 1100-1400.degree. C. The
resulting melt was stirred and quenched by pouring on the surface
of counter rotating stainless steel rollers or into a water tank.
The glass powders prepared for the invention were adjusted to a 2-5
micron mean particle size by wet or dry milling using alumina ball
media prior to being used as ingredients in the formulation of a
paste composition. In the case of wet milling, the slurry after
milling was dried in a hot air oven and the excessively large
particles were removed by a sieving process.
[0062] It is noted that the examples of glass composition given
below only serve as several of the candidates suitable for this
invention; the application of this invention is therefore not
limited to the three glass compositions herein. The glass
compositions were identified based on dilatometer softening point
among other factors that support the densification and adhesion of
the getter paste compositions of this invention. For example, glass
1, 2, or 3 below has respectively a softening point of 453, 453, or
370.degree. C. Furthermore, the intrinsic density (not bulk
density) of the aforementioned is respectively 5.96, 5.16, or 7.20
gram per cubic centimeter. TABLE-US-00002 TABLE 2 Glass composition
in weight % Ex. # 1 2 3 SiO.sub.2 7.1 14.8 3.5 Al.sub.2O.sub.3 2.1
0.8 3.5 Bi.sub.2O.sub.3 69.8 82.0 B.sub.2O.sub.3 8.4 11.8 CaO 0.5
ZnO 12.0 6.6 PbO 66.0 11.0
Examples 4-19
[0063] Thick film getter paste was prepared by mixing glass and
pre-hydrated desiccant (3A or 13X powder) material (molecular
sieve) with organic media based on the mixture of Texanol.RTM.
and/or terpineol solvent and ethyl cellulose resin. Table 3
represents the examples of thick film compositions containing 7-10%
glass and 47-60% pre-hydrated zeolite (3A or 13X). Although
identical level of glass to pre-hydrated zeolite, namely, 1 to 6.1
for glass 1 to pre-hydrated 3A and 1 to 6.5 for glass 1 to
pre-hydrated 13X, it is noted that a glass to pre-hydrated zeolite
ratio other than the aforementioned value can be applicable in the
getter paste compositions depending on the specific requirements in
adhesion strength and the getter effect. Due to the density
differential of glass among the glass compositions 1 to 3, the
relative weight % glass in paste composition containing other than
glass 1 was adjusted to maintain identical volume ratio of glass to
zeolite 3A (Table 3) or 13X (Table 4). Particularly, example 10 or
18 of paste compositions containing glass 2 is equivalent to 6 or
14; and example 11 or 19 of paste compositions containing glass 3
is equivalent to 7 or 15. In order to provide similar rheology of
the paste composition examples, two organic mediums 1 and 2 were
used, wherein medium 1 is a lower viscosity version of ethyl
cellulose in terpineol and medium 2 is a higher viscosity version
of the same type of ingredients. It is noted, for the purpose of
viscosity adjustment that various ethyl cellulose molecular weight,
its distribution, and the ethyl cellulose content in the solvent
can be used in both organic mediums.
[0064] The getter paste was printed using a 200 mesh screen on a
glass lid substrate based on soda-lime silicates, dried at
120.degree. C. for 10 minutes to evaporate the solvent(s), and then
fired at a peak temperature of 450-550.degree. C. for 1-2 hours in
a box furnace. Some samples were also processed at 550.degree. C.
for 1 hour using a conveyor furnace with a heating/cooling profile
of about 6 hours. The printing/drying step was repeated to generate
thicker getter films when needed. The fired thickness of
single-printed thick film ranged from 10 microns to 40 microns,
depending on paste viscosity, % solid content, screen mesh size,
screen wire diameter, screen emulsion thickness, and the setup
parameters on the screen printer.
[0065] The thick film fired dense and showed good adhesion with the
glass substrate. No cracking or blistering was observed on the
surface of fired thick films. Good thickness uniformity of fired
film was kept within +/-2 micron regardless of the paste
composition.
[0066] The densification degree of fired samples depended on the
relative content of glass. A higher content of glass was desirable
for better densification of thick films and stronger bonding to the
glass lid substrate. However, a lower glass to desiccant ratio
provides more desiccant effect. Therefore, it is necessary to
evaluate between the two properties and determine the optimal
combination. TABLE-US-00003 TABLE 3 Thick film getter composition
in weight % Example 4 5 6 7 8 9 10 11 Glass Type 1 1 1 1 1 1 2 3
Glass 8.72 8.37 7.77 9.78 9.67 8.95 6.73 11.81 Zeolite 3A 53.81
51.66 47.96 60.34 59.69 55.24 47.96 60.34 Medium 1 6.97 7.44 11.36
7.82 8.59 13.09 11.36 7.82 Medium 2 0.87 0.93 1.94 0.97 1.07 2.23
1.94 0.97 Surfactant 0.87 0.93 0.97 0.97 1.07 1.35 0.97 0.97
Solvent 28.76 30.67 30.00 20.12 19.91 19.14 31.04 18.09
[0067] TABLE-US-00004 TABLE 4 Thick film getter composition in
weight % Example 12 13 14 15 16 17 18 19 Glass Type 1 1 1 1 1 1 2 3
Glass 8.46 8.13 7.68 9.46 9.35 8.70 6.65 11.43 Zeolite 13X 55.18
53.03 50.09 59.98 59.25 55.16 50.09 59.98 Medium 1 6.77 7.23 7.68
7.57 8.30 12.72 7.68 7.57 Medium 2 0.84 0.90 1.92 0.94 1.03 2.17
1.92 0.94 Surfactant 0.84 0.90 0.96 0.94 1.03 1.09 0.96 0.94
Solvent 27.91 29.81 31.67 21.11 21.03 20.16 30.64 19.14
Examples 20-22
[0068] Fired thick film samples were evaluated to quantify the
degree of moisture absorption after exposure to certain humidity
environment. Table 5 shows the absorption performance of selected
getter samples 15 to 17 based on the thermal gravitational analysis
(TGA) results. The moisture desorption and absorption results were
expressed as weight loss and gain of thick films up to a given
temperature. The TGA samples were prepared by firing at a peak
temperature of 550.degree. C. for 30 minutes and exposing to an
85.degree. C./85% RH humidity condition for 2 hours including the
time took to cool from 550.degree. C. to room temperature. No
additional firing for activation (i.e. to remove moisture absorbed
by the zeolite materials) was used for the TGA analysis.
[0069] The weight loss proportionally increased with increasing
temperature during the firing profile as expected, but it tends to
demonstrate a much slower rate of weight loss after 400.degree. C.
Increasing the content of glass decreased the percentage of weight
loss. This tendency is reasonable when considered the fact that the
absorption capability is dominantly determined by the relative
content of the desiccant materials. The glass is believed to
generally act as an inorganic binder and/or an adhesion promoter
and is normally unable to absorb water. Assuming all of the organic
solvent(s) contained in the paste compositions 15 to 17 was
evaporated wherein the organic polymer resin (i.e. ethyl cellulose)
and surfactant remained intact during the initial getter paste
drying step at 120.degree. C. for 10 minutes, the calculated weight
% water retained in the zeolite 13X in the dried getter film was
respectively, 21.26, 21.51, or 20.75% (row E in Table 5), this was
further calculated to represent 33.42, 34.05, or 33.08 weight %
based on the existing 13X in the getter film made from the paste
15, 16, or 17, respectively. These values exceeded the measured
weight gain (26.8%, row F) due to the pre-hydration of the 13X and
were attributed to the retained volatile solvent(s) in the dried
getter film, either brought in by simple solvent(s) addition or
from the organic mediums 1 and 2. Furthermore, the TGA measured
weight gain (row H) due to the exposure of the fired getter film
was used to calculate the water re-absorption on 13X in the fired
getter film as respectively, 29.76, 29.76, or 30.48% based on the
weight of 13X. This, while larger than the measured 13X weight gain
(row G) is not unreasonable due to the degree of a more complete
dehydration (or activation) of zeolite by the 550.degree. C. firing
cycle than the as-received zeolite from the supplier. That is to
say that the as-received 13X may not be totally free from moisture
absorption and/or partial moisture absorption occurred in the
storage period. TABLE-US-00005 TABLE 5 Thick Film getter
composition in weight % (based on solid %) and weight loss in % ID
Description 15 16 17 A Measured % Weight Loss from dried to 23.7
24.2 24.6 fired getterpaste B Wt % frit in the dried getter film
per recipe 12.72 12.63 12.57 C Wt % dehydrated 13X in the dried
getter 63.58 63.17 62.86 film per recipe D Wt % nonvolatile organic
materials in the 2.44 2.69 3.82 dried getter film E Calculated wt %
water absorbed in 13X in 21.26 21.51 20.75 the dried getter film F
Calculated wt % water vs. 13X 33.42 34.05 33.08 G Measured % weight
gain from 13X due to 26.8 26.8 26.8 exposure H Measured % weight
gain from fired getter 24.8 24.8 25.4 film upon treatment I
Calculated % weight gain of 13X due to 29.76 29.76 30.48 moisture
absorption
* * * * *