U.S. patent application number 10/233470 was filed with the patent office on 2008-09-25 for process and apparatus for organic vapor jet deposition.
Invention is credited to Stephen R. Forrest, Max Shtein.
Application Number | 20080233287 10/233470 |
Document ID | / |
Family ID | 39774985 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080233287 |
Kind Code |
A1 |
Shtein; Max ; et
al. |
September 25, 2008 |
PROCESS AND APPARATUS FOR ORGANIC VAPOR JET DEPOSITION
Abstract
A method of fabricating an organic film is provided. A
non-reactive carrier gas is used to transport an organic vapor. The
organic vapor is ejected through a nozzle block onto a cooled
substrate, to form a patterned organic film. A device for carrying
out the method is also provided. The device includes a source of
organic vapors, a source of carrier gas and a vacuum chamber. A
heated nozzle block attached to the source of organic vapors and
the source of carrier gas has at least one nozzle adapted to eject
carrier gas and organic vapors onto a cooled substrate disposed
within the vacuum chamber.
Inventors: |
Shtein; Max; (Princeton,
NJ) ; Forrest; Stephen R.; (Princeton, NJ) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Family ID: |
39774985 |
Appl. No.: |
10/233470 |
Filed: |
September 4, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60317215 |
Sep 4, 2001 |
|
|
|
60316264 |
Sep 4, 2001 |
|
|
|
60316968 |
Sep 5, 2001 |
|
|
|
60332090 |
Nov 21, 2001 |
|
|
|
Current U.S.
Class: |
427/255.6 ;
118/715; 427/255.23 |
Current CPC
Class: |
C23C 14/12 20130101;
C23C 14/24 20130101 |
Class at
Publication: |
427/255.6 ;
427/255.23; 118/715 |
International
Class: |
C23C 16/00 20060101
C23C016/00 |
Goverment Interests
STATEMENT REGARDING GOVERNMENT RIGHTS
[0002] This invention was made with Government support under
Contract No. F49620-92-J-05 24 (Princeton University), awarded by
the U.S. Air Force OSR (Office of Scientific Research). The
Government has certain rights in this invention.
Claims
1-11. (canceled)
12. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate separated from the nozzle block by not more than
about 1500 microns, to form a patterned organic film, wherein the
method is used to fabricate a patterned organic film without the
use of a mask, and wherein the patterned organic film has a
resolution of about 1 micron or less.
13. (canceled)
14. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor, b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate separated from the nozzle block by not more than
about 1500 microns, to form a patterned organic film; wherein the
method is used to fabricate a patterned organic film without the
use of a mask; wherein the pattern is controlled by the separation
between the nozzle block and the substrate, the size of a nozzle in
the nozzle block, and the gas velocity; and wherein the distance
between the substrate and the nozzle block is less than about 2.5
microns.
15-35. (canceled)
36. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at east as great as the thermal velocity of the molecules, onto a
cooled substrate separated from the nozzle block by not more than
about 20 microns, to form a patterned organic film, wherein the
method is used to fabricate a patterned organic film without the
use of a mask.
37. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate separated from the nozzle block by not more than
about 15 microns, to form a patterned organic film.
38. The method of claim 37 wherein the substrate is separated from
the nozzle block by not more than about 2.5 microns.
39. (canceled)
40. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate separated from the nozzle block by not more than
about 15 microns, to form a patterned organic film.
41. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate, to form a patterned organic film comprising a
plurality of pixels, wherein the patterned organic film is a
non-polymeric organic film.
42. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate to form a peed organic film comprising plurality
of pixels, wherein the pixels are arranged in an array.
43. A method of depositing organic film, comprising: a) providing a
heated non-reactive carrier gas transporting an organic vapor; b)
ejecting the heated non-reactive carrier gas transporting an
organic vapor through a nozzle block, with a bulk flow velocity at
least as great as the thermal velocity of the molecules, onto a
cooled substrate, to form a patterned organic film comprising a
plurality of pixels, wherein the nozzle block comprises a plurality
of nozzles, each nozzle to deposit an organic material that emits
light of a different color.
44. The method of claim 43, wherein each nozzle is in communication
with a separate source cell containing the organic material.
45. A method of depositing an organic film, comprising: a)
providing a heated non-reactive carrier gas transporting an organic
vapor; b) ejecting the heated non-reactive carrier gas transporting
an organic vapor through a nozzle block, with a bulk flow velocity
at least as great as the thermal velocity of the molecules, onto a
cooled substrate, to form a patterned organic film comprising a
plurality of pixels, wherein the nozzle block comprises a plurality
of nozzles, each nozzle in communication with a plurality of source
cells.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority benefits to the
following U.S. patent applications: 60,317,215 (filed Sep. 4,
2601), 60/316,264 (filed on Sep. 4, 2001), 60/316,968 (filed on
Sep. 5, 2001), and 60/332,090 (filed Nov. 21, 2001). These patent
applications are incorporated by reference in their entireties.
This patent application is related to simultaneously filed patent
application Ser. No. ______, attorney docket no. 10020/21904, which
is incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0003] The present invention is directed to a process of patterned
deposition of organic materials onto substrates which utilizes the
vapor transport mechanisms of organic vapor phase deposition.
BACKGROUND OF THE INVENTION
[0004] Molecular organic compounds are employed as active materials
in a variety of applications, including organic light emitting
diodes (OLEDs), photovoltaic cells, and thin films. Typically,
these thin (.about.100 nm) film devices are grown by thermal
evaporation in high vacuum, permitting the high degree of purity
and structural control needed for reliable and efficient operation
(see S. R. Forrest, Chem. Rev. 97, 1793 (1997)). However, control
of film thickness uniformity and dopant concentrations over large
areas needed for manufactured products can be difficult when using
vacuum evaporation (see S. Wolf and R. N. Tauber, Silicon
Processing for the VLSI Era (Lattice, 1986)). In addition, a
considerable fraction of the evaporant coats the cold walls of the
deposition chamber. Over time, inefficient use of materials results
in a thick coating which can flake off, leading to particulate
contamination of the system and substrate. The potential throughput
for vacuum evaporated organic thin film devices is low, resulting
in high production costs. Low-pressure organic vapor phase
deposition (LP-OVPD) has been demonstrated recently as a superior
alternative technique to vacuum thermal evaporation (VTE), in that
OVPD improves control over dopant concentration of the deposited
film, and is adaptable to rapid, particle-free, uniform deposition
of organics on large-area substrates (see M. A. Baldo, M. Deutsch,
P. E. Burrows, H. Gossenberger, M. Gerstenberg, V. S. Ban, and S.
R. Forrest, Adv. Mater. 10, 1505 (1998)).
[0005] Organic vapor phase deposition (OVPD) is inherently
different from the widely used vacuum thermal evaporation (VTE), in
that it uses a carrier gas to transport organic vapors into a
deposition chamber, where the molecules diffuse across a boundary
layer and physisorb on the substrate. This method of film
deposition is most similar to hydride vapor phase epitaxy used in
the growth of III-V semiconductors (see G. B. Stringfellow,
Organometallic Vapor-Phase Epitaxy (Academic, London, 1989); G. H.
Olsen, in GaInAsP, edited by T. P. Pearsall (Wiley, New York,
1982)). In LP-OVPD, the organic compound is thermally evaporated
and then transported in a hot-walled reactor by an inert carrier
gas toward a cooled substrate where condensation occurs. Flow
patterns may be engineered to achieve a substrate-selective,
uniform distribution of organic vapors, resulting in a very uniform
coating thickness and minimized materials waste.
[0006] Using atmospheric pressure OVPD, Burrows et al. (see P. E.
Burrows, S. R. Forrest, L. S. Sapochak, J. Schwartz, P. Fenter, T.
Buma, V. S. Ban, and J. L. Forrest, J. Cryst. Growth 156, 91
(1995)) first synthesized a nonlinear optical organic salt
4'-dimethylamino-N-methyl-4-stilbazolium tosylate. In a variation
on this method, Vaeth and Jensen (see K. M. Vaeth and K. Jensen,
Appl. Phys. Lett. 71, 2091 (1997)) used nitrogen to transport
vapors of an aromatic precursor, which was polymerized on the
substrate to yield films of poly (s-phenylene vinylene), a
light-emitting polymer. Recently, Baldo and co-workers (see M. A.
Baldo, V. G. Kozlov, P. E. Burrows, S. R. Forrest, V. S. Ban, B.
Koene, and M. E. Thompson, Appl. Phys. Lett. 71, 3033 (1997)) have
demonstrated apparently the first LP-OVPD growth of a
heterostructure OLED consisting of N,N-di-(3-methylphenyl)-N,N
diphenyl-4,4-diaminobiphenyl and aluminum tris(8-hydroxyquinoline)
(Alq.sub.3), as well as an optically pumped organic laser
consisting of rhodamine 6G doped into Alq.sub.3. More recently,
Shtein et al. have determined the physical mechanisms controlling
the growth of amorphous organic thin films by the process of
LP-OVPD (see M. Shtein, H. F. Gossenberger, J. B. Benziger, and
S.R. Forrest, J. Appl. Phys. 89:2, 1470 (2001)).
[0007] Virtually all of the organic materials used in thin film
devices have sufficiently high vapor pressures to be evaporated at
temperatures below 400.degree. C. and to then be transported in the
vapor phase by a carrier gas such as argon or nitrogen. This allows
for positioning of evaporation sources outside of the reactor tube
(as in the case of metalorganic chemical vapor deposition (see S.
Wolf and R. N. Tauber, Silicon Processing for the VLSI Era
(Lattice, 1986); G. B. Stringfellow, Organometallic Vapor-Phase
Epitaxy (Academic, London, 1989)), spatially separating the
functions of evaporation and transport, thus leading to precise
control over the deposition process.
[0008] Though these examples demonstrate that OVPD has certain
advantages over VTE in the deposition of organic films, especially
over large substrate areas, the prior art has not addressed the
special problems that arise when depositing an array of organic
material. Recent successes in fabricating organic light emitting
diodes (OLEDs) have driven the development of OLED displays (see S.
R. Forrest, Chem. Rev. 97, 1793 (1997)). OLEDs makes use of thin
organic films that emit light when voltage is applied across the
device. OLEDs are becoming an increasingly popular technology for
applications such as flat panel displays, illumination, and
backlighting. OLED configurations include double heterostructure,
single heterostructure, and single layer, and a wide variety of
organic materials may be used to fabricate OLEDs. Several OLED
materials and configurations are described in U.S. Pat. No.
5,707,745, which is incorporated herein by reference in its
entirety.
[0009] As is the case for fabrication of arrays using VTE, to adapt
OVPD to OLED technology, a shadow mask delineating the shape of the
desired pixel grid is placed close to the substrate to define the
pattern of deposition on the substrate. Control of the shadow mask
patterning is a critical step, for example, in the fabrication of
full-color OLED-based displays (see U.S. Pat. No. 6,048,630,
Burrows, et al.). Ideally, the resultant pattern on a substrate is
identical to that cut into the shadow mask, with minimal lateral
dispersion and optimal thickness uniformity of the deposited
material. However, despite the overall advantages of OVPD in
depositing organic layers, the use of the shadow mask in OVPD has
certain disadvantages including: significant lateral dispersion
compared to VTE; material waste; potential for dust contamination
on the film from the mask; and difficulty in controlling the
mask-substrate separation for large area applications.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to provide a
process of patterned deposition of organic materials onto
substrates that utilizes the vapor transport mechanisms of organic
vapor phase deposition. It is also an object of the present
invention to provide an apparatus for performing this process of
patterned deposition of organic materials onto substrates, without
the need for a shadow mask.
[0011] A method of fabricating an organic film is provided. A
non-reactive carrier gas is used to transport an organic vapor. The
organic vapor is ejected through a nozzle block onto a cooled
substrate, to form a patterned organic film. A device for carrying
out the method is also provided. The device includes a source of
organic vapors, a source of carrier gas and a vacuum chamber. A
heated nozzle block attached to the source of organic vapors and
the source of carrier gas has at least one nozzle adapted to eject
carrier gas and organic vapors onto a cooled substrate disposed
within the vacuum chamber.
[0012] In an embodiment of the present invention, by organic vapor
jet deposition ("OVJD"), organic vapors are carried by an inert gas
from the source cell, through a timed valve and into a nozzle
block, from which they are ejected onto a substrate. Preferably,
the substrate is cooled and the nozzle block is heated. Preferably,
the substrate is translated at a rate v synchronization with the
valve timing to achieve the desired pattern of deposition. By
controlling the gas flowrate, V, width of the nozzle, z, the
distance to the substrate, d, the rate of substrate translation, v,
the source temperature, T, and the valve timing, a, a uniform
thickness profile, t, may be achieved for multiple pixels of
desired width. The process is preferably carried out at a reduced
pressure to minimize the dispersion in l. Decreasing s and
increasing V can also minimize the dispersion even at ambient
pressures.
[0013] Typical deposition pressures for organic vapor jet
deposition (OVJD) range from 1 to 10 Torr. Both amorphous and
crystalline films may be grown by OVJD.
[0014] In an embodiment of the present invention, the carrier gas
rate V is increased so that the bulk flow velocity is at least on
the order of the thermal velocity of the molecules, about 100-1,000
m/s, creating a "jet" of material that is unidirectional. In
mathematical terms, this condition may be met when the mean
velocity in the direction of the axis of the nozzle (the bulk flow
velocity) is at least on the order of the mean absolute velocity in
directions perpendicular to the axis of the nozzle (the thermal
velocity). Preferably, the mean velocity in the direction of the
axis of the nozzle is at least as great as the mean absolute
velocity in directions perpendicular to the axis of the nozzle. The
term "absolute" velocity is used with respect to mean velocity in
directions perpendicular to the axis of the nozzle, because the
mean velocity in those directions may be about zero--for every
molecule moving to the left at a particular velocity, there may be
another molecule moving to the right at the same velocity.
[0015] An embodiment of the present invention further provides that
under the appropriate conditions of substrate temperature, reactor
pressure, and nozzle geometry, an array of sharp-edged pixels with
a resolution of about 1 .mu.m is achievable with jet deposition if
the nozzle-substrate separation, s, is within the molecular mean
free path of the carrier gas, A. In addition, because of the
unidirectional flow, use of a heavier carrier gas can provide
better directionality of deposition and subsequently sharper
pixels.
[0016] One advantage of certain embodiments of the present
invention is that material waste is minimized due to the heating of
the nozzle and the directional flow. For example, the nozzle may be
heated to a temperature sufficient to avoid physisorbtion
(condensation) of organic material on nozzle surfaces, thereby
reducing waste, and also reducing the need to clean the nozzle. The
substrate may be cooled to enhance deposition characteristics, and
avoid a situation where the carrier gas heats the substrate to the
point that organic material will not deposit. Another advantage is
the absence of the masking step, resulting in an increased rate of
production, a more compact deposition apparatus design, and the
elimination of contamination from a shadow mask. In high-resolution
deposition requiring a separation distance s typically less than 1
mm, contamination from a shadow mask using OVPD is especially
problematic. An additional problem arises in maintaining that small
mask-substrate separation over a large substrate area, particularly
since the mask would normally be thin and flexible.
[0017] Another advantage of certain embodiments of the present
invention is that the process may be used in manufacturing
full-color organic light emitting diode ("OLED") displays by
patterning the multiple color pixels on the same substrate without
the need to use a separate shadow mask. The apparatus incorporates
an array of nozzles arranged and operated synchronously much like
the print-head of an ink-jet printer. Each nozzle may incorporate
three source cells, for red, green and blue luminophores, with
valve control for sequentially layering the materials, without
having to move a shadow mask. For example, each nozzle may be
connected to multiple source cells through different valves, such
that the deposition from each nozzle may be alternated between
different colors at different locations on the substrate. Or, each
nozzle may be connected to only one of multiple source cells, where
each nozzle has its own valve, or different groups of nozzles may
be connected to different sources, with each group having its own
valve, such that the nozzle block deposits a predetermined pattern
of different organic materials.
[0018] Embodiments of the present invention provide a process of
patterned deposition of organic materials onto a substrate, said
process comprising: transporting organic vapors via an inert
carrier gas moving at a flowrate V from a source cell, through a
timed valve, and into a nozzle block, wherein said transport occurs
at a pressure P and wherein said flowrate V of the inert carrier
gas is increased so that the bulk flow velocity is at least on the
order of the thermal velocity of the molecules; and ejecting the
organic vapors via the inert carrier gas moving at the flowrate V
from the nozzle block onto a cooled substrate, wherein the cooled
substrate is maintained at a temperature T and at a distance s from
the nozzle block.
[0019] Embodiments of the present invention further provides a
process comprising: maintaining the cooled substrate at the
distance s from the nozzle block while laterally translating one of
said cooled substrate or said nozzle block at a rate v, wherein the
rate v is synchronized with the timed valve to create the patterned
deposition of organic materials.
[0020] Embodiments of the present invention further provides this
process for patterned deposition at a pressure P between 0.01 and
10 torr.
[0021] Embodiments of the present invention further provides this
process for patterned deposition wherein the distance between
substrate and nozzle block s is within the molecular mean free path
of the carrier gas.
[0022] Embodiments of the present invention further provides an
apparatus for patterned deposition of organic materials onto
substrates, said apparatus comprising: at least one nozzle jet,
wherein each of said one nozzle jet comprises one or more source
cells; one timed valve connected to each of the one or more source
cells; and a heated nozzle block connected to the one timed
valve.
[0023] Embodiments of the present invention further provides an
apparatus for patterned deposition of organic materials onto
substrates wherein the patterned deposition is a full-color organic
light emitting diode display, and wherein at least one nozzle jet
is a rectangular array of n.times.m nozzle jets, and wherein one or
more source cells is three source cells for red, green, and blue
luminophores.
[0024] Embodiments of the present invention further provides an
apparatus for patterned deposition of organic materials onto
substrates comprising a variable-aperture at the output of the
heated nozzle block.
DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 shows a vacuum thermal evaporation system.
[0026] FIG. 2 shows a vacuum thermal evaporation system.
[0027] FIG. 3 shows an organic vapor phase deposition system.
[0028] FIG. 4 shows an organic vapor phase deposition system.
[0029] FIG. 5 shows simulated results for deposition through a
shadow mask, showing the effect of varying deposition pressure.
[0030] FIG. 6 shows simulated results for deposition through a
shadow mask, showing the effect of varying the separation between
mask and substrate.
[0031] FIG. 7 shows simulated results for deposition through a
shadow mask, showing the effect of varying mask thickness.
[0032] FIG. 8 shows simulated results for deposition through a
shadow mask, showing the effect of varying the effective boundary
layer thickness.
[0033] FIG. 9 shows an embodiment of an organic vapor jet
deposition apparatus.
[0034] FIG. 10 shows scanning electron micrographs of some
representative Alq.sub.3 patterns formed on silver-coated glass
substrates after deposition through shadow masks.
[0035] FIG. 11 shows a plot of the dimensionless dispersion
parameter, R=d/s, versus de position pressure, P.sub.dep.
[0036] FIG. 12 shows a material concentration map.
[0037] FIG. 13 shows simulated profile of material deposited by
organic vapor jet deposition.
[0038] FIG. 14 shows simulated OVPD deposition results where the
carrier gas has a bulk flow velocity.
DETAILED DESCRIPTION
[0039] Embodiments of the present invention are directed to a
process of patterned deposition of organic materials onto
substrates utilizing the vapor transport mechanisms of organic
vapor phase deposition, and to an apparatus for performing this
process of patterned deposition. In one embodiment, the process
comprises: transporting organic vapors via an inert carrier gas
moving at a flowrate V from a source cell, through a timed valve,
and into a nozzle block, wherein the transport occurs at low
pressure P; ejecting the organic vapors from the nozzle block onto
a cooled substrate via the inert carrier gas moving at a flowrate
V; and laterally translating the cooled substrate, which is
maintained at a distance s from the ejection end of the nozzle
block, at a rate v. The rate of translation is synchronized with
the timed valve to create the desired patterned deposition of
organic materials.
[0040] FIG. 1 shows a vacuum thermal evaporation (VTE) system 100.
A source 110 is heated such that material evaporates into a vacuum
chamber 120. The material diffuses through vacuum to substrate 130,
where it may be deposited.
[0041] FIG. 2 shows a more detailed view of a VTE system 200 having
a mask 220. A source 210 provides organic material that diffuses
into a vacuum, on the order of 10.sup.-6 to 10.sup.-7 Torr. The
organic material diffuses through the vacuum and through a shadow
mask 320. Shadow mask 220, which has apertures 222, is disposed a
distance s away from a substrate 230. After the organic material
passes through the shadow mask, it deposits on substrate 230 to
form patterned organic layer 240.
[0042] Because of the low pressures typically used for VTE, the
molecular mean free path, .lamda., (also referred to as mfp) may be
quite large. For example, at 10.sup.-7 Torr, .lamda. is about 1 m.
As a result, for example, a mask-substrate separation of less than
50 .mu.m can yield pixels of up to .about.100 .mu.m with
well-defined edges, where the source-substrate distance in the
chamber is on the order of 10-100 cm. Preferably, the distance
between substrate 230 and source 210 is less than the molecular
mean free path .lamda., such that collisions between molecules in
the vacuum are minimal, and patterned layer 240 is deposited where
there is a clear line of sight from substrate 230 to source 210,
unblocked by mask 220. Using VTE, a pixel profile that is trapezoid
with a well-defined, finite base may be obtained. 10.sup.-3 to
10.sup.-13 Pa is a preferred range of pressures for VTE.
[0043] Because source 210 is not a single point, patterned layer
240 may be slightly larger than aperture 222. With reference to
FIG. 2, the length of the base of patterned layer 240, l.sub.3, is
given by:
l 3 = 1 2 ( s + t ) ( l 1 + l 2 ) h . ##EQU00001##
where s=mask-substrate separation, t=mask thickness, l.sub.1=source
width, l.sub.2=aperture width, and h=source-mask distance. This
formula gives very close d values to those observed
experimentally.
[0044] FIG. 3 shows a organic vapor phase deposition (OVPD) system
300. A carrier gas is passed over a source cell 310, from which an
organic material is evaporated into the carrier gas. Multiple
source cells (not shown) may be used to provide a mixture of
organic materials, and/or to provide different organic materials at
different times. The carrier gas then passes through a mask 320
located a distance .delta. from a substrate 330. The carrier gas
then impinges on substrate 330, where the organic material
physisorbs onto the substrate surface. Substrate 330 may be cooled.
Walls 340 of system 300 may be heated to reduce or prevent organic
material from depositing on walls 340. The organic material may be
a small molecule material, or it may be a polymer material.
[0045] FIG. 4 shows an OVPD system 400. A carrier gas is used to
transport organic molecules from a source (not shown in FIG. 4,
see, for example, FIG. 3). The molecules have an average mean free
path .lamda.. A mask 410 is disposed a distance s above a substrate
420. Organic layer 430 is deposited on substrate 420 through
apertures 412 in mask 410. Because of collisions between molecules
in the carrier gas, significant deposition of organic material may
occur to a distance d under the mask, in regions that are not
directly over apertures 412. The deposition is preferably carried
out in at the lower end of the pressure range, such that the mean
free path is greater than it would be at higher pressures, and d is
correspondingly less, so that the micron-scale resolution preferred
for full-color display applications may be achieved.
[0046] FIG. 5 shows simulated results for deposition through a
shadow mask in the diffusive regime. For a nominal s=10 .mu.m and
mask thickness of 18 .mu.m, the deposition patterns for
.lamda.=8.25, 82.5, and 825 .mu.m (P.sub.dep=0.01, 0.1, 1.0 Torr)
are shown in FIG. 5. Molecules were launched from 2000 .mu.m away
from the mask at random angles having average molecular thermal
velocities and allowed to diffuse throughout the simulated space
volume. The concentration profile in the vicinity of the substrate
was found to be linear, indicating that transport is purely
diffusive. This is why, in FIG. 5, no difference in d is observed
for different values of .lamda.. Also in agreement with the
continuum model, the fraction of molecules, which deposit on the
substrate and the mask, i.e. the deposition efficiency, is lower
for small .lamda., which correspond to small D.sub.org. The
simulation was performed with 30 .mu.m wide mask openings; a mask
thickness of 18 .mu.m, and a mask separation, s=10 .mu.m. Plots
510, 520 and 520 show deposition thickness profiles for the mask
(higher) and the substrate (lower) for .lamda.=8.25, 82.5, 825 with
P.sub.dep=1.0, 0.1, 0.01 Torr. There is no noticeable difference in
the pixel shape between pots 510, 520 and 530, indicating that in
the purely diffusive regime pressure has little effect on edge
dispersion; the efficiency of deposition, as expected, drops for
lower values of .lamda..
[0047] FIG. 6 shows simulated results for deposition through a
shadow mask in the diffusive regime. The mask openings remain 30
.mu.m wide, with t=18 .mu.m and .lamda.=82.5 .mu.m, while s=3, 10,
20 .mu.m, respectively, for plots 610, 620 and 630. Smaller values
of s result in sharper pixels. As long as s.about..lamda.,
trapezoidal pixel shapes may be obtained, similar to vacuum
deposition. Pixel overlap starts to occur when s.about.t. Keeping
the purely diffusive framework for the simulation, FIG. 6 shows how
variation in s affects pixel edge dispersion. Since .lamda. does
not affect d in this regime, we use .lamda.=82.5 .mu.m; for t=18
.mu.m pixel cross-talk starts to occur when s=20 .mu.m, i.e. as s
approaches t, there is overlap of the neighboring pixels.
[0048] FIG. 7 shows simulated results for deposition through a
shadow mask in the diffusive regime. The mask openings remain 30
.mu.m wide, with .lamda.=82.5 .mu.m and s=10 .mu.m. Here, mask
thickness, t, is varied to 18, 36, and 54 .mu.m, respectively, for
plots 710, 720 and 730. Thicker masks result in sharper pixels,
albeit at the expense of cutting off material flux to the substrate
and reducing deposition efficiency, as can be seen by the low
mask-to-substrate deposition ratio. As t approaches .lamda., the
collimated molecular flux results in trapezoidal pixels, similar to
vacuum deposition. The dome-shaped profiles become increasingly
like the vacuum-thermal deposited trapezoids as t approaches
.lamda..
[0049] FIG. 8 shows simulated results for deposition through a
shadow mask in the diffusive regime. The mask openings are 30 .mu.m
wide, with .lamda.=82.5 .mu.m and s=10 .mu.m. Plots 810 and 820
show results for .delta.=410 and .delta.=2060 .mu.m, respectively.
Here, the effective boundary layer thickness is decreased from 2060
to 410 to 80 .mu.m by adjusting the launching point to be closer to
the substrate. As .delta. approaches .lamda., the deposition
efficiency increases, in agreement with the continuum model for
diffusion-limited transport to the substrate. Here, the effective
.delta. was varied by launching the molecules closer to the
substrate.
[0050] Changing the mass of the carrier gas in the purely diffusive
deposition regime was found to have no first order effect on the
deposition profile, as expected from the discussion of the previous
sections.
[0051] FIG. 9 shows an embodiment of an organic vapor jet
deposition apparatus. The process of patterned deposition of
organic materials onto substrates according to embodiments of the
present invention will now be described in reference to FIG. 9.
[0052] In one embodiment, organic vapors are carried by an inert
carrier gas from the source cell 910 into a timed valve 920. The
source cell 910 is preferably kept at temperature T, and the inert
carrier gas is moving at a flowrate V. The opening and closing of
the timed valve 920 (i.e., the valve timing, .tau.,) is preferably
regulated throughout the process of patterned deposition. When the
timed valve 920 is open, the inert gas carrying the organic vapors
moves through the timed valve 920 and into the nozzle block 930.
The nozzle block 930 preferably contains heating/cooling units 940
which are used to control the temperature of the inert gas carrying
the organic vapors through the nozzle block 930. One difference
between OVJD and OVPD is that the benefit of heated walls, such as
heated walls 340, may be significant in OVPD, but less significant
in OVJD. In particular, where nozzle block 930 includes heating
units 940, the benefit of additional separate heating units that
heat the walls of a vacuum chamber (not shown in FIG. 9) may not be
needed. However, heating units that heat the walls of the OVJD
vacuum chamber may nevertheless be used. The nozzle block 930
preferably has a nozzle with a width z. From the nozzle block 930,
the organic vapors in the inert carrier gas are ejected out through
the nozzle onto a substrate 950, preferably a cooled substrate,
whereon the organic vapors condense to form a patterned layer 960.
Preferably, the organic vapors must travel a distance s from the
nozzle block 930 to the substrate 950. Substrate 950 may be moved
at a rate of translation v, in between the deposition of material,
during the deposition of material, or both. The substrate is
preferably translated using a motorized stage, and both the stage
and valve timing are operated and synchronized by computer control.
The apparatus can be repeated in series for multi-layer deposition
and multi-color display deposition.
[0053] By controlling the aforementioned process variables, a
desired patterned deposition can be achieved. Specifically, a
uniform thickness profile, t, can be achieved for a patterned layer
960 of a desired width l. By conducting the process at a reduced
pressure, the dispersion in width l can be minimized. Furthermore,
even at ambient pressures, decreasing the distance s from the
nozzle block 930 to the substrate 950, and/or increasing the
carrier gas flowrate V will minimize the dispersion in width l.
[0054] If the distance s from the nozzle block 930 to the substrate
950 in FIG. 9 is of comparable magnitude to the several microns
separating the substrate from the shadow mask, and the gas flow
rate V is sufficiently high, then the dispersion in l will be
minimized, with resolutions expected on the order of 1 micron.
[0055] While it is relatively easy to achieve sharply defined
pixels using vacuum thermal evaporation at pressures <10.sup.-6
because the molecular mean free path, .lamda., is typically >30
cm, (see FIGS. 1 and 2), the situation is more complicated in OVPD.
Because OVPD typically proceeds at pressures >0.01 Torr, with
0.1 .mu.m.ltoreq..lamda..ltoreq.1 cm, the increased frequency of
intermolecular collisions in the vicinity of the mask plane leads
to pixels with comparatively more diffuse edges (see FIG. 4).
Nevertheless, we have demonstrated organic film deposition through
a shadow mask with a pattern definition on the order of microns
(see FIG. 10).
[0056] FIG. 10 shows scanning electron micrographs of the patterns
resulting from OVPD through a shadow-mask at P.sub.dep ranging from
2.times.10.sup.-6 to 2 Torr. As the deposition pressure increases,
both simulations and experimental data indicate the loss of edge
sharpness. Images 1010, 1020 and 1030 show results for
P.sub.dep=210.sup.-6, P.sub.dep=0.2 Torr, and P.sub.dep=2 Torr,
respectively. The separation s=5 and 2.5 .mu.m for the left and
right column respectively. As predicted by the model, the pixels
become more diffuse as pressure and mask-substrate separation
increase. It was found that at pressures of 0.2 Torr and separation
of up to 15 .mu.m it is possible to achieve a pixel resolution on
the order of several microns, which is sufficient for full color
display applications.
[0057] Co-pending patent application Attorney/Docket No. 10010/37
describes the basis for organic vapor phase deposition ("OVPD") and
is incorporated herein by reference. Co-pending provisional
application, Attorney/Docket # 10020/21901, (herein "'901
application") is also incorporated herein by reference. The '901
application is directed to a hybrid technique for the fabrication
of organic devices, whereby organic materials are deposited using
organic vapor phase deposition (OVPD) through a shadow mask, and
metals are sequentially deposited via vacuum evaporation through
the same shadow mask. In the '901 application, the theory behind
OVPD deposition is developed fully and models used for simulations
are described. Using these same models developed for vapor phase
transport, we have further determined that a resolution of 1 micron
is achievable if the bulk flow velocity is increased to create a
gas jet and the substrate to nozzle distance is within the
molecular mean free path. The model is described below.
[0058] The concept of OVPD is illustrated in FIGS. 3 and 4. The
process consists of three steps outlined below. Vapors of a species
A are generated by heating the source material in a stream of an
inert carrier gas. Gaseous A is subsequently transported into the
deposition chamber by the carrier gas, where the flow forms a
hydrodynamic boundary layer (BL) in the vicinity of the substrate.
In the last step, organic molecules (present in typical
concentrations of <0.01%) diffuse across the BL and physisorb or
adsorb on the substrate. These three stages of transport may be
represented as a series of reactions:
##STR00001##
where A, represents an organic molecular species in the solid or
liquid state. Species A.sub.s and A.sub.g evaporate and recondense
inside the source cell with characteristic rates k.sub.evap and
k.sub.cond, respectively. Evaporation takes place either in the
so-called "kinetic" regime, where k.sub.evap>k.sub.cond, or is
in an equilibrium regime, where k.sub.evap=k.sub.cond. The organic
species is swept out of the source cell by the carrier gas in (1b).
Entrainment by the carrier results in taking A.sub.cg to the
vicinity of the substrate with a characteristic bulk transport
rate, k.sub.t, where it becomes A.sub.cg,s, with an overall
efficiency of=100%, while the remainder is pumped out of the
deposition chamber. Deposition takes place by diffusion of A across
the boundary layer and adsorption with a characteristic rate
k.sub.ads. The overall deposition rate,
r.sub.dep=k.sub.dep-k.sub.des, where k.sub.des is the rate of
desorption from the substrate.
[0059] With qualifications with respect to the highly molecular
nature of OVPD to be discussed below, we can state that, typically,
the ratio of carrier gas velocity to the mean molecular velocity,
v.sub.c/u, is about 0.01-1, i.e. the flow in LP-OVPD is either
below or borders on the sonic regime. Due to the low pressure used,
the Reynolds number, Re, is well within the laminar flow regime
(Re<<2000). The Grashof number, Gr, in the vicinity of the
substrate is also less than 1, implying that natural convection is
not significant in gas mixing near the substrate. For the present
discussion of deposition dynamics, only the steps 2, 3a and 3b are
relevant. Since the efficient deposition of amorphous thin films
requires minimal surface diffusion and desorption, we employ the
lowest practicable substrate temperatures. Two things happen in
this case: k.sub.afd>>k.sub.des, while the crystallization
rate, k.sub.c, is very high, meaning that the surface diffusing
organic molecules become immobilized much faster than they diffuse
to the substrate. Thus, "reaction" 3b is very fast and does not
need to be considered for deposition of amorphous films. The
rate-limiting steps are thus 2 and 3a.
[0060] As shown in previous work, (see M. Shtein, et al., J. of
Appl. Phys., 89: 1470 (2001)), the overall deposition rate,
r.sub.dep, for the combination of steps 2 and 3a can be expressed
as:
r dep = P org RT V . 1 + V . .delta. / D org , ( 4 )
##EQU00002##
where P.sub.org/RT is the concentration of the organic species,
{dot over (V)} is the carrier gas flow rate and is equivalent to
the variable V, which is also used to denote the carrier gas flow
rate throughout this disclosure, .delta. is the BL thickness, and
D.sub.org is the diffusivity of organic molecules in the carrier
gas. The kinematic viscosity itself depends on pressure via:
v=.mu./.rho., where .rho.=P/RT. Increasing the background gas
pressure, P.sub.dep, will result in a sublinear decrease in the
deposition rate, r.sub.dep, due to two opposing factors: a decrease
in the diffusivity, D.sub.org, which lowers r.sub.dep, and a
decrease in .delta., which improves the transport rate. This
equation may be used to predict the overall deposition rate for
given process conditions and, coupled with a surface molecular
diffusion model, to estimate crystallization rate and grain size of
polycrystalline thin films.
[0061] In the vicinity of the substrate the system may be
engineered as a gas jet impinging normal to a flat plate, a uniform
flow coming to stagnation near a flat plate, or flow impinging on a
rotating disk (to improve coating uniformity); in all cases,
.delta. takes the form:
.delta. = 2.4 v a , ( 5 ) ##EQU00003##
where v is the kinematic viscosity of the gas, and a is a quantity
decreases linearly with {dot over (V)} and/or the rate of rotation
in such a way that the formula may be used directly to estimate
.delta. in units of cm when v is in cm.sup.2/s and bulk flow axial
velocity in cm/s is used for a. For typical conditions used in OVPD
and in this work, such as T=275.degree. C., P.sub.dep=0.2 Torr and
{dot over (V)}=15 sccm of nitrogen, .delta. is approximately 1-10
cm. However, since the .delta. is on the order of the axial
dimension of the typical deposition chamber, the term boundary
layer must be applied in OVPD with caution. Patterned Film
Deposition using OVPD
[0062] The preceding discussion relies on the validity of the
continuum assumption due to the use of the uniform bulk
diffusivity, D.sub.org, and the boundary layer thickness, .delta..
This section examines the validity of the continuum assumption when
applied to shadow masking in OVPD.
[0063] A central question in analyzing OPVD through a shadow mask
is to what extent the organic molecules retain their initial bulk
flow velocity when they arrive at the mask plane. First, we assume
the presence of a boundary layer, "BL," where by definition, the
molecules lose memory of bulk transport, and their velocity
distribution is fully thermalized. In this case, it can be seen
qualitatively that the decrease in D.sub.org due to higher
P.sub.dep will not make the patterns less sharp. Since D.sub.org is
isotropic, the longer it takes for a molecule to diffuse
perpendicularly to the substrate, the longer it will take (by the
same amount) for it to diffuse laterally. The mutual cancellation
of these rates will result in identical patterns at different
pressures, which is not the observed experimental trend. A slightly
more realistic model for D.sub.org (see Eq. (8) below, for example)
is where it decreases in the direction of the substrate, along the
decreasing temperature gradient. But once again, because the
decrease is isotropic, the pattern should remain unaffected.
[0064] Relaxing the requirement for an isotropic velocity
distribution within the boundary layer and allowing the molecules
to retain the z-component of their initial velocity, it can be
shown that d.sub.max is given approximately by:
d max .apprxeq. const * 6 * D org * s u ##EQU00004##
where d.sub.max is the pixel edge dispersion, as shown in FIG. 4,
and u is the carrier gas velocity in the deposition chamber. Here,
we assumed that .lamda. is small enough to model the process as
diffusion from a series of point sources located along the mask
aperture. The pixel edge dispersion increases with the square root
of the pressure, through D.sub.org as well as the mask-substrate
separation, s. Increasing the bulk flow velocity, naturally for
this model, improves sharpness. However, this formula overestimates
the pixel edge dispersion for moderate pressures (e.g. 0.1 Torr) by
at least an order of magnitude, because the diffusive transport
assumption does not strictly hold for the dimensions and pressures
relevant to this discussion. Experimentally obtained deposition
patterns suggest that the mechanism lies somewhere between the two
diffusive modes.
[0065] Here, it should be noted that the continuum and hence the
diffusion assumptions are incorrect for most of OVPD conditions.
The Knudsen number (.lamda./L, where L=characteristic length) based
on the dimensions of the shadow mask is large, and the mass and
energy conservation equations no longer form a closed set. The VTE
and OVPD mechanisms are shown schematically in FIGS. 2 and 4,
respectively. The random collisions experienced by the organic
molecules near the substrate are responsible for the lateral
spreading of the pixels. Since, by definition, the complete
randomization of molecular velocities takes place within the BL,
the magnitude of .delta. is expected to affect the sharpness of the
pattern. Furthermore, the latter will be limited by the following
factors: molecular mean free path, .lamda., mask-to-substrate
separation, s, and the shape of the mask aperture. In terms of the
process parameters, these factors are controlled via the deposition
pressure, carrier gas flow rate, the type of carrier gas used, and
design of the shadow mask. Since D.sub.org and .lamda. are
intimately related, we next examine how .lamda. varies with
P.sub.dep and its effect on pattern sharpness.
[0066] A Monte-Carlo type simulation may be used to model
deposition through a shadow mask. We now set up the equations
needed to carry out further analysis. From the logic in FIG. 4 it
is evident that a larger .lamda. will result in fewer
intermolecular collisions inside the BL and, coupled with a
laterally uniform concentration distribution above the shadow mask,
less lateral dispersion of the pattern on the substrate. For a
single-component, low-pressure non-polar gas, .lamda. has the
form:
.lamda. = k B T 2 .pi. .sigma. 2 P dep . ( 6 ) ##EQU00005##
Thus, by decreasing the gas pressure, the mean free path increases
and sharper pixels will be obtained. However, pressure cannot be
decreased indefinitely; the in-flow of a carrier gas used to
transport the organic vapors necessarily gives rise to a background
gas pressure. The limit of very low deposition pressure, P.sub.dep,
represents the free molecular transport regime, where .lamda. is
large and the carrier gas flow rate, {dot over (V)}, limits
material transport. Increasing {dot over (V)} results in greater
P.sub.dep and transport becomes diffusion limited as .lamda.
decreases. The trade-off between using a sufficient carrier gas
flow rate and maximizing gas-phase diffusion of organics gives rise
to the 0.01 to 10 Torr optimum pressure range preferably used in
OVPD.
[0067] While Eq. (6) may be used accurately with dilute, non-polar
gases like helium and argon, OVPD deals with a mixture of complex
molecules, e.g. Alq.sub.3, along with the carrier gas, such as
nitrogen or argon. The effective nominal mean free path and
collision cross-section, .lamda. and .sigma., can be determined via
modified expressions for the diffusivity through Eq. 6 and the
relationship:
D org = 1 3 u _ .lamda. ( 7 ) ##EQU00006##
Here, the Chapman-Enskog expression for the diffusivity of
molecules with dipoles or induced-dipoles may be used:
D AB = 1.835 10 8 T 1.5 ( 1 M A + 1 M A ) 0.5 P .sigma. AB 2
.OMEGA. D , AB , ( 8 ) ##EQU00007##
where M.sub.i is the mass of the diffusing species i, T is the gas
temperature, and .sigma..sub.AB is the average collision
cross-section,
.sigma..sub.AB=[1/2(.sigma..sub.A+.sigma..sub.B).sup.2].sup.1/2.
The quantity .OMEGA..sub.DAB is a dimensionless function of the
Lennard-Jones intermolecular potential and temperature.
Unfortunately, for the materials commonly used in OLEDs, reliable
Lennard-Jones parameters are not available, and the Fuller
correlation may be substituted:
D AB = 0.1013 T 1.75 ( 1 M A + 1 M A ) 0.5 P [ ( v A ) 0.5 + ( v B
) 0.5 ] 2 , ( 9 ) ##EQU00008##
where .SIGMA.v is the summed effective volume contribution of the
individual structural components of the diffusing molecule. The
various molecule-specific constants have been calculated using
standard group contribution methods described elsewhere (R. B.
Bird, W. E. S., and E. N. Lightfoot, Momentum, Heat and Mass
Transfer (1996) John Wiley & Sons). As evident from Table 1,
the values of D.sub.AB vary by half an order of magnitude between
the different theories, and it may be necessary to carry out more
detailed experiments and/or molecular dynamics simulations to
determine the binary diffusivities more accurately. However,
approximate values of .lamda. and .sigma. should suffice for
determining trends with pressure.
TABLE-US-00001 TABLE 1 D.sub.org (Kinetic D.sub.org (Chapman -
Theory) D.sub.org (Fuller et al.) Enskog) T (K) (cm.sup.2/s)
(cm.sup.2/s) (cm.sup.2/s) 273 0.0355 (N.sub.2) 0.68 (N.sub.2) 0.105
(Alq.sub.3) 0.0629 (N.sub.2) 548 0.101 (N.sub.2) 2.30 (N.sub.2)
0.356 (Alq.sub.3) 0.179 (N.sub.2)
[0068] The Monte-Carlo simulation incorporating the above analysis
proceeds as follows. The computational space is divided into a
3-dimensional grid with variable cell size. Particles representing
organic molecules are assigned random initial locations inside the
boundary layer and above the mask, and velocities that satisfy the
Maxwell-Boltzmann distribution. After an elapsed time interval and
a short travel distance no greater than 1/10.sup.th of the mean
free path, the molecule is allowed to collide with a locally
generated carrier molecule having a random velocity from a
Maxwell-Boltzmann distribution. The acceptance of collision is
calculated using the following function:
P coll = F N .sigma. T u r .DELTA. t V C ( 10 ) ##EQU00009##
where F.sub.N is the number of real molecules represented by one
simulated molecule, .sigma..sub.T is the total cross-section of the
colliding molecules, u, is their relative speed, .DELTA.t is the
time interval allowed for the collision to take place, while
V.sub.C is the volume of the cell in which the collision occurs.
The value of .sigma..sub.T can be calculated from d.sub.eff an
effective collision diameter which scales with the relative
particle velocity, v.sub.r:
d eff = d 0 v r v r 0 ( 11 ) ##EQU00010##
The whole process is repeated, while the mega-molecules are tracked
in space. Upon collision with the substrate plane or any side of
the mask, the organic particles are immobilized there. Periodic
boundary conditions are imposed laterally, while a constant
concentration of organics and carrier gas is set at the edge of the
boundary layer. The simulation runs until a desired film thickness
has been formed on the substrate. Tracking mega-molecules
consisting of several individual molecules is done to save
computational costs. The simulation was applied to generate the
results shown in FIGS. 5 through 8.
[0069] FIG. 11 shows plot of the dimensionless dispersion
parameter, R=d/s, versus deposition pressure, P.sub.dep (bottom
axis) and mean free path, .lamda. (top axis), for both experimental
and simulated results. Star symbols 1110, square symbols 1120,
triangle symbols 1130 and circle symbols 1140, show experimental
results with a mask-substrate separation of 2, 5, 15 and 115
microns, respectively. Plots 1150, 1160, 1170 and 1180 show
simulated results for a mask-substrate separation of 2, 5, 15 and
115 microns, respectively. As the pressure decreases, R does not
decrease to zero, but rather saturates at a constant value,
characteristic of the finite size of the source in VTE and the
source-mask and mask-substrate gaps. The points between 10.sup.-6
and 0.2 Torr are not readily accessible with the current
experimental set-up and were filled in using the Monte-Carlo
simulation.
[0070] If the molecules are allowed to keep their original bulk
flow velocity as they enter and propagate through the BL, the
deposition profiles become sharper. They approach the trapezoidal
shape characteristic of vacuum deposition when the bulk flow
velocity, U.sub.bulk, approaches the molecular thermal velocity, u.
This suggests a mode of deposition where organics are "sprayed"
onto the substrate using an ultra-fast jet of carrier gas, similar
to ink-jet printing.
[0071] An example of vapor-jet deposition mode is illustrated in
FIG. 12, with the simulation results in FIG. 13. FIG. 12 is a
material concentration map which shows the jet-like character of
deposition with ultra-fast carrier flow for simulated Alq.sub.3
flowing past a 100 mm thick mask with an initial vertical velocity
of 100 m/s. Vertical dimension is 200 .mu.m, horizontal=60 .mu.m.
The overall deposition efficiency of this process can approach
100%, since pixels are patterned by the directed gas jets and no
material is wasted in coating the shadow mask. A deposition system
with individual nozzles for each color pixel may provide an
efficient, precise, and more portable deposition system.
[0072] FIG. 13 shows a plot of the thickness of material deposited
(simulated) by OVJD. Vertical dimension is 9 .mu.m, horizontal=60
.mu.m.
[0073] In the regime representing the present invention, in which a
gas jet impinges normal to a flat plate, (Equation 5 herein) the
models developed above are applied to ascertain the process
parameters for an organic vapor jet deposition apparatus. Operation
in this regime was first suggested by the observation, both through
the simulations presented here and through experimental
verification, that sharp pixels could be obtained by OVPD using a
shadow mask, if the mask-substrate separation distance was reduced
to the order of the molecular mean free path, .lamda..
Additionally, by increasing the mask thickness, the molecules that
do reach the substrate become effectively collimated, resulting in
sharper patterns, albeit at the expense of deposition efficiency.
If, however, the thick mask is also heated, material losses are
minimized. Increasing the aspect ratio of the mask aperture above
10 and increasing the carrier gas velocity perpendicular to the
substrate results in a gas jet being formed at the exit of the
mask. In the jet deposition regime, therefore, the thick mask
design converges to the heated nozzle of the present invention. The
process parameters for OVJD are now discussed below.
[0074] In one embodiment, the carrier gas flow rate, V, for jet
deposition has to be sufficient to create a "jet" of material, as
suggested by the name. To make the gas flow appear as a
unidirectional jet stream, the bulk flow velocity has to be on the
order of the thermal velocity of the molecules (.about.(8
kT/.pi.m).sup.1,2) or greater. For example, at room temperature,
the thermal velocity of nitrogen, N.sub.2, is approximately 450
m/s. Thus, the rough magnitude of the total volumetric gas flow
rate is 450 m/s*A.sub.cs,tot, where A.sub.cs,tot is the total
cross-sectional area of the nozzles. The nozzle geometry is
selected according to the particular application.
[0075] The source temperature and gas flow rate together control
the concentration of the organic vapors in the gas phase, (see M.
Shtein, H. F. Gossenberger, J. B. Benziger, and S. R. Forrest, J
Appl. Phys. 89:2, 1470 (2001)). Thus, T is set by the required
concentration, which is, in turn, set by how much material needs to
be delivered, M.sub.A.
[0076] Given the amount of material A, M.sub.A, to be deposited for
a particular layer of a particular device within a reasonable time
segment, Dt, the concentration of material A, C.sub.A, is set
by:
C.sub.A=M.sub.A/(V*.DELTA.t)
where V is the volumetric flow rate of the carrier gas (plus the
material, which is usually, but not always, insignificant). The
total amount of material to be delivered, M.sub.A, for one pixel is
given by (pixel area)*(layer thickness). The pixel size of a
typical OLED display is on the order of microns to tens of microns,
and the thickness of individual layers is typically on the order of
0.1 micron. Preferably, the process pressure is ultimately dictated
by the solubility of the organic (or other) compound in the gas
used at the operating pressure and temperature, i.e. the maximum
mole fraction of A, x.sub.A, that can be in the carrier gas without
condensing on the walls or in the gas phase. The sufficient number
of carrier gas molecules present in the flow to entrain all of the
solute vapor molecules in a particular carrier gas can then be
calculated. This and the total gas flow rate and the total pumping
capacity and speed of the OVJD system determine the operating
pressure. Since all the variables are interdependent, an iterative
process is used to develop the proper operating parameters for a
particular deposition and application.
[0077] Preferably, the separation or working distance, s, in the
OVJD apparatus is governed by the hydrodynamics of the jet and the
operating pressure. Typically, but not strictly necessarily, for
minimum pixel edge dispersion, s will be on the order of (or less
than) the molecular mean free path, .lamda., of the gas system at
hand, where (see also equation 6 herein):
.lamda..about.const*T.sub.gas/.sigma..sup.2P.sub.dep
where T.sub.gas=gas temperature, .sigma.=average molecular
diameter, P.sub.dep=deposition pressure. For Alq.sub.3/N.sub.2
system, for example, .lamda.>>1500, 150, 15, 1.5 .mu.m for
P.sub.dep=0.01, 0.1, 1, and 10 Torr, respectively, at T=275.degree.
C. Under these conditions, in the time it takes the vapor molecules
to traverse the distance from nozzle to substrate, the time for
lateral dispersion of the jet is minimized. However, the smaller
the working distance, the more difficult the construction of the
apparatus will be, and the more difficult it will be to keep the
substrate cold, while keeping the nozzles hot to prevent
condensation. Therefore, it is undesirable to decrease the working
distance much below the threshold necessary to achieve the required
pixel resolution.
[0078] Preferably, the cross-sectional area of an individual
nozzle, A.sub.cs, and its shape are dictated by the shape of the
pattern that is to be obtained. Since the operating pressure is
likely to be such that the mean free path of the molecules is
short, the working distance is also small, on the order of microns.
Given the flow velocity, working distance, and pressure drop across
the nozzle, its shape is designed to suit the desired lateral
dispersion; ideally, the nozzle width corresponds to the width of
the pixel deposited.
[0079] In OVPD with shadow masking, the step preceding diffusion is
delivery of vapors by the carrier gas to the vicinity of the
substrate. Bulk flow velocity here is about 1-10 m/s. The final
step in deposition is the diffusion of molecules across the
boundary layer, where the molecular velocities are isotropic,
giving rise to the pixel edge dispersion. Preferably, for a given
mask-substrate geometry and material system, this dispersion is
dictated by .lamda. only. Changing the carrier gas from N.sub.2 to
Ar for example, may have a minor effect, since
.sigma..sub.AVE=.sigma..sub.N2+.sigma..sub.Alq3, and
.sigma..sub.Alq3>.sigma..sub.N2+Ar. The difference in the mass
of the carrier gas molecule makes no difference; heavier molecules
merely travel slower, so that the momentum transferred in an
Alq.sub.3-N.sub.2 or Ar collision with a thermalized, isotropic
velocity distribution is the same.
[0080] In OVJD, however, we gain another control knob for the pixel
shape. Since the carrier molecules are pushed through a conduit by
a large pressure drop at velocities on the order of the thermal
velocity, .about.100-1,000 m/s, the momentum transfer in a
collision is no longer isotropic, nor is it governed by the thermal
velocity distribution. Rather, it is unidirectional,
substrate-directed and is proportional to the mass of the carrier
gas molecule (its velocity is controlled by the pressure drop now,
rather than gas temperature). By using a heavier carrier gas,
therefore, we can achieve better directionality of deposition and
therefore sharper edge profiles and in the case of vapor deposited
OLED displays, sharper pixels.
[0081] OVJD differs from and improves upon shadow-masking in many
ways, including (but not limited to): the elimination of thin and
flimsy shadow masks; elimination of dust contamination from
organics condensed on the mask; eliminates the problem of
mask-substrate separation control for large area applications;
improves deposition (material) efficiency; offers the advantage of
control over pixel shape; offers spatially specific deposition; and
finally, an OVJD apparatus has the potential for portability and
private use.
[0082] The deposition of organic thin films of Aluminum (Alq.sub.3)
was carried out using a multi-barrel glass reactor system with in
situ temperature and thickness measurement capability, described in
detail elsewhere. (M. A. Baldo, et al. Phys. Lett., 71: 3033
(1997), which is incorporated by reference in its entirety).
Alq.sub.3 is an example of the small molecule organic materials
that are preferred for many OLEDs. Briefly, the reactor vessel is
an 11 cm diameter by 150 cm long Pyrex.RTM. cylinder. It is heated
by means of a three-zone furnace enabling source temperature
control via positioning of each cell along the temperature gradient
within the tube. Each source is separately encased in a 2.5 cm
diameter by 75 cm long glass barrel. Carrier gas flow was regulated
by mass flow controllers, while the deposition pressure is kept
between 0:1 and 10 Torr by adjusting the pump throttle valve and
the total carrier flow rate from 10 to 50 sccm. A 40 lpm vacuum
pump with a liquid nitrogen cold trap is used to exhaust
uncondensed carrier and organics. Organic vapors condense onto a
rotating water-cooled substrate positioned behind a mechanically
operated shutter. Film thickness and growth rate are monitored by a
quartz crystal microbalance calibrated using the ellipsometrically
measured organic film thickness.
[0083] In addition to deposition of organic thin films using OVPD,
a conventional vacuum thermal evaporator was used. The
source-to-substrate distance was approximately 30 cm; the
deposition pressure was maintained at 10.sup.-6 Torr.
[0084] For the shadow-mask, we used a 5 .mu.m thick nickel mesh
consisting of 10 .mu.m lines that interlace, forming 15 .mu.m
square openings. This mesh was placed directly on top of 1 mm thick
silver-coated glass slides and covered with a 50 .mu.m thick nickel
mask containing round holes 1 and 0.3 mm in diameter. This
arrangement allows for the simultaneous measurement of deposition
for two values of s. Due to the profile of the nickel mesh, the
smallest value of s is .about.2 .mu.m. Here, the dispersion values,
d, for s=2 .mu.m will refer to the fuzziness of the square pixels
7-10 .mu.m on the side. Values of d corresponding to s=5 .mu.m
refer to the fuzziness of the circular deposition edge formed with
the 1 mm and 0.3 mm holes of the 50 .mu.m thick mask resting on top
of the mesh.
[0085] Additional shadow masks were fabricated integral to the
substrate, using a photoresist/chromium/photoresist (PR1/Cr/PR2)
sandwich structure and photolithography, to provide the most
accurate mask-substrate separations. Following the deposition of
Alq.sub.3, the resulting pixel patterns were examined using
scanning electron microscopy.
[0086] One embodiment is an OLED display "vapor-jet printer." In
this example, we deposit a 1000.times.2000 pixel array for a high
resolution color display (.about.30.times.50 cm outer dimension).
The "print-head" will consist of 1000 nozzles for the red
luminophore, 1,000 nozzles for the green, and 1,000 for the blue.
The rate of substrate and/or nozzle translation is dictated by the
rate of deposition and the amount of material to be deposited.
[0087] Each pixel is 100.times.100 .mu.m and requires 500 Angstroms
of the dye-doped layer. A typical deposition rate in the current
OVPD system is 10 .ANG./s, while the system has an efficiency of
5-10%. With the vapor jet deposition, the materials use efficiency
is likely to be 100%, but with 50% efficiency (100 .ANG./s), to
deposit the entire screen, at 5 seconds per pixel set, would take
.about.3 hours plus translation time. However, if 2 linear arrays
of nozzles are used, this time is cut in half. With the
conventional shadow masking technique, the deposition of each of
the luminophores in sequence can take from 3 to 10 minutes, with
whatever additional costs and time required to clean the shadow
mask, which could be substantial. However, with the combination of
a properly sized nozzle array and deposition rate, which implies an
increase in the organic vapor concentration and/or carrier gas flow
rate, while keeping the source in the saturation regime, the OLED
display "vapor-jet printer" can be at least comparable to OVPD
using a shadow mask in production time and cost, especially since
frequent cleanings would not be necessary, and material loss would
be reduced.
Additional Simulation
[0088] Consider a jet of carrier gas delivered through a
small-diameter capillary onto a cooled substrate. In the
Monte-Carlo simulation, the z-directed carrier gas velocity,
U.sub.z, can be increased to simulate a jet which broadens only by
the isotropic random molecular velocities superimposed onto this
flow-field. FIG. 14 shows the spatial concentration profile for a
simulated jet of N.sub.2 carrying Alq.sub.3, with mfp=10 .mu.m,
t=50 .mu.m, and U.sub.z=100 m/s, while the mean thermal speed, =500
m/s. Since the flow-field was not known in this flow regime, the
simulation kept dU.sub.z/dz=0 for simplicity. The figure shows that
the collimated jet can result in a deposit with well-defined edges
even for s>>mfp. Careful selection of U, P.sub.dep, .alpha.
and s may thus enable a printing method for molecular organic thin
films analogous to ink-jet printing for polymers, except where the
liquid solvent is replaced by a jet of highly volatile inert
carrier gas. In FIG. 14, carrier gas with organic molecules is
ejected from apertures 1415 in mask 1410, to impinge upon substrate
1420. Plots 1430, 1440 and 1450 illustrate different simulated
deposition results where the jet nozzle is located at different
distances from the substrate, and show a widening of the vapor jet
as it moves further from the nozzle.
[0089] Although the present invention is described with respect to
particular examples and preferred embodiments, it is understood
that the present invention is not limited to these examples and
embodiments. The present invention as claimed therefore includes
variations from the particular examples and preferred embodiments
described herein, as will be apparent to one of skill in the
art.
* * * * *