WO2003034471A1 - Self-aligned hybrid deposition - Google Patents

Abstract

A method of fabricating an organic device is provided. A first layer is deposited over a substrate through a mask (220) by a first process that results in the first layer having a first area of coverage. A second layer is then deposited over the substrate through the mask by a second process that results in the second layer havine a second area of coverage that is different from the first area of coverage.

Description

SELF-ALIGNED HYBRID DEPOSITION

Cross-Reference to Related Applications

[0001] This patent application claims priority benefits to the following United States

patent applications: 60,317,215 (filed September A, 2001), 60/316,264 (filed on September A,

2001), 60/316,968 (filed on September 5, 2001), and 60/332,090 (filed November 21, 2001).

These patent applications are incorporated by reference in their entireties. This patent

application is related to simultaneously filed patent application no. , attorney docket

no. 10020 / 21702, which is incorporated by reference in its entirety.

Statement Regarding Government Rights

[0002] This mvention 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 mvention.

Field of the Invention

[0003] The present invention is directed to a method of fabricating organic

semiconductor devices such as organic light emitting devices. More particularly, the

invention is directed to the fabrication of such devices where organic materials are deposited

through a mask. Summary of the Invention

[0004] A method of fabricating a device is provided. The method may be referred to

as a "hybrid" method, because different deposition processes are used to deposit organic

layers and metal layers, and the differences in these processes are favorably used to facilitate

the method. A shadow mask is positioned at a particular distance from a substrate. An

organic layer is then deposited through the mask using organic vapor phase deposition

(OVPD). A metal (or conductive metal oxide) layer is subsequently deposited through the

same mask using vacuum thermal evaporation (NTE). The organic layer may be made

reliably and controUably larger than the metal layer because of the differences between OVPD and NTE, even though both the organic and metal layers are deposited through the same

mask. A method of fabricating an organic device is provided. A first layer is deposited over

a substrate through a mask by a first process that results in the first layer having a first area of

coverage. A second layer is then deposited over the substrate through the mask by a second

process that results in the second layer having a second area of coverage that is different from

the first area of coverage.

[0005] Another method of fabricating a device is also provided. The hybrid method

described above is used to deposit a first organic layer, and then a first metal layer, through a

mask, where the mask is disposed at a first position. The mask is then be moved to a second

position that may be determined relative to the first position. The hybrid method is then used

to deposit a second organic layer, and then a second metal layer, through the mask. The mask

may then be moved to a third position that may be determined relative to the second position.

The hybrid method is then used to deposit a third organic layer, and then a third metal layer,

through the mask. The substrate may be moved in addition to or instead of the mask.

[0006] i each method provided, the substrate may be cooled during the deposition of the organic layers.

[0007] It is an object of the invention to provide an improved method of fabricating

organic semiconductor devices that use less steps than prior methods.

[0008] It is a further object of the invention to provide an improved method of

fabricating organic semiconductor devices that takes advantage of the ONPD process.

Brief Description of the Drawings

[0009] Figure 1 shows a vacuum thermal evaporation system.

[0010] Figure 2 shows a vacuum thermal evaporation system.

[0011] Figure 3 shows an organic vapor phase deposition system.

[0012] Figure 4 shows an organic vapor phase deposition system.

[0013] Figure 5 shows simulated results for deposition through a shadow mask,

showing the effect of varying deposition pressure.

[0014] Figure 6 shows simulated results for deposition through a shadow mask,

showing the effect of varying the separation between mask and substrate.

[0015] Figure 7 shows simulated results for deposition through a shadow mask,

showing the effect of varying mask thickness.

[0016] Figure 8 shows simulated results for deposition through a shadow mask,

showing the effect of varying the effective boundary layer thickness.

[0017] Figures 9 and 10 show simulated results for deposition through a shadow mask

with a carrier gas bulk flow velocity.

[0018] Figure 11 shows a general schematic of the substrate-shadow mask assembly

used in an experiment. The insets show scanning electron micrographs that detail the top

mask and the nickel mesh which was placed directly onto the substrate.

[0019] Figure 12 shows scanning electron micrographs of some representative Alctø patterns formed on silver-coated glass substrates after deposition through shadow masks.

[0020] Figure 13 shows simulation results for Alq₃ / Ag micropattems formed by

hybrid ONPD-NTE deposition, where the same shadow mask was used for both.

[0021] Figure 14 shows a scanning electron micrograph of experimental results for

Alq₃ / Ag micropattems formed by hybrid ONPD-NTE deposition, where the same shadow

mask was used for both.

[0022] Figure 15 illustrates a bell-shaped profile of a pixel deposited by ONPD

through a shadow-mask.

[0023] Figure 16 shows simulated deposition results obtained from a Monte-Carlo

simulation using 10⁵ particles, mfp — lOOμm, s = 7μm, t = 3μm, and a= 135°.

[0024] Figure 17 shows a simulated deposition results obtained from a Monte-Carlo

simulation using 10⁵ particles, mfp = lOμm, s = 7μm, t = 3μm, and a= 135°.

[0025] Figure 18 shows pixel shape factor, η, as a function of mfp.

[0026] Figure 19 shows simulated deposited results obtained from a Monte-Carlo

simulation using 10⁶ Al ^ molecules in a background of Ν₂ with mfp = 20μm, illustrating the

effect of varying mask thickness. The inset shows shape factor as a function of mask

thickness.

[0027] Figure 20 shows simulated deposition results, illustrating the effect of varying

mask - substrate separation.

[0028] Figure 21 shows simulated deposition results, illustrating the effect of different

mask side- wall geometries at various mask thicknesses.

[0029] Figure 22 shows shape factor as a function of mask thickness for the different

side- wall geometries of Figure 21. [0030] Figure 23 shows simulated normalized height as a function of position under

different assumptions about bulk flow velocity.

[0031] Figure 24 shows simulated ONPD deposition results where the carrier gas has

a bulk flow velocity.

[0032] Figure 25 shows a schematic diagram of an experimental OVPD system.

[0033] Figure 26 shows a micrograph of Alα^ layers deposited by ONPD on Si, and

interference microscopy results showing fringes near the edge of the layer.

[0034] Figure 27 shows a plot of the light intensity from interference microscopy

along the radius of the layer of Figure 26, and the corresponding pixel height profile

calculated from the interference pattern.

[0035] Figure 28 shows experimentally measured profiles for the layers of Figure 26.

[0036] Figure 29 shows experimental and simulated results for ONPD through a

shadow mask.

[0037] Figure 30 shows simulated results for ONPD through a shadow mask using a

variety of parameters.

[0038] Figure 31 shows a scanning electron micrographs of an Alq₃ film on Si

pattemed by VTE through a mesh with w = 7.5μm and s ~ Oμm at 10^"6 Torr, and a scanning

electron micrograph of Alα^ patterns deposited on Si using OVPD at 0.1 Torr through a

nickel mesh with t = 3.5μm, w = 7.5μm, and s < lμm.

[0039] Figure 32 shows a schematic representation of hybrid deposition.

Detailed Description

[0040] The same mask, with the same apertures, may be used to pattern both metal and organic layers of an organic device. Organic layers deposited by organic vapor phase deposition (OVPD) are subject to a natural and controllable spreading, depending upon

process parameters such as the vapor pressure and the geometry of the mask and substrate.

Metal layers deposited by vacuum thermal evaporation (VTE) generally have much less

spreading. As a result, it is possible to deposit different patterns though the same mask

apertures, such that the patterned organic layers are larger than the patterned metal layers.

The organic device may be an organic light emitting device (OLED), or a different type of

organic device, such as an organic transistor or an organic solar cell. A wide variety of devices may be fabricated, including organic passive matrix displays and organic active

matrix displays.

[0041] An organic layer and then a metal layer may be deposited, in sequence,

through the same shadow mask. Because of differences in the deposition method, the organic

layer may have greater spreading, and the metal layer may be disposed entirely on the organic

layer, such that there is no contact between the metal layer and any layers underneath the

organic layer. Shorts are therefore advantageously avoided. This process may be utilized to advantageously fabricate an organic device, such as an organic light emitting device (OLED).

[0042] 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 heterostracture, 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. Patent No. 5,707,745, which is incorporated herein by reference in its

entirety. [0043] Many modern organic electronic and nanostructure devices require lateral

pattern resolution on the order of microns, but are incompatible with conventional

photolithography. In-situ patterning of the organic films is often used instead

[0044] Embodiments of the invention may involve the movement of a shadow mask

in between the deposition of various layers of material. A description of such movement may

be found in a patent application having attorney docket number 10020 / 11501, which is

incorporated herein by reference in its entirety. An array of devices may be fabricated by

moving the mask (or substrate) as follows. After using the mask, disposed in a first position, to deposit a first organic layer and a first metal layer, the mask may be moved to a second

position determined relative to the first position. A second organic layer, and then a second

metal layer, may be deposited through the mask at the second position. Because the second

position is determined relative to the first position, costly and time consuming alignment

steps may advantageously be avoided. After depositing the second organic layer and the

second metal layer, the mask may be moved to a third position determined relative to the

second position. A third organic layer, and then a third metal layer, may be deposited through

the mask at the third position. This process may be advantageously used, for example, to

fabricate a three color OLED display, where the first, second, and third organic layers are

each adapted to emit a different color of light.

[0045] An embodiment of the invention may be used to fabricate a multi-color array

of OLEDs having well-defined pixels with a resolution of 5 microns or greater. The fact that

separate alignment steps are not necessary for each material (metal as opposed to organic),

nor for each color, enhances the resolution available.

[0046] Embodiments of the present invention may involve the use of organic vapor

phase deposition (OVPD). A description of organic vapor phase deposition may be found in patent applications having attorney docket numbers 10020 / 37, 10020 / 3702, and 10020 /

3703, which are incorporated herein by reference in their entirety.

Vacuum Thermal Evaporation (VTE)

[0047] Figure 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.

[0048] Figure 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^"6 to 10^{" 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.

[0049] Because of the low pressures typically used for VTE, the molecular mean free

path, λ, (also referred to as mfp) may be quite large. For example, at 10^"7 Torr, λ is about lm.

As a result, for example, a mask-substrate separation of less than 50 μm can yield pixels of up

to -lOOμm 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 λ, 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^"3 to 10^"13 Pa is a preferred

range of pressures for VTE. [0050] Because source 210 is not a single point, patterned layer 240 may be slightly

larger than aperture 222. With reference to Figure 2, the length of the base of patterned layer

240, l₃, is given by:

1 (_-+ Q. (/ι+ /0 h —₂ ^~ _h (1)

where s = mask-substrate separation, t = mask thickness, I, = source width, l₂ = aperture width, and h = source-mask distance. This formula gives very close d values to those observed

experimentally.

Organic Vapor Phase Deposition

[0051] Organic Vapor Phase Deposition (OVPD) is well suited for depositing

amorphous and crystalline organic thin films for display, transistor, and photovoltaic

applications. It is radically different from vacuum thermal evaporation 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. Preferably, the carrier gas is inert. Control

over dopant concentration, purity, and crystallinity of the deposited films can be significantly

improved over vacuum thermal evaporation (VTE). Here, we examine film deposition from the vapor phase in experimental and modeling detail, with emphasis on shadow mask

patterning, a critical step in the fabrication of full-color OLED-based displays. While it is

relatively easy to achieve sharply defined pixels using vacuum thermal evaporation at

pressures < 10^"6 because the molecular mean free path, λ, is typically > 30 cm, the situation is

more complicated in OVPD. Because OVPD typically proceeds at pressures > 10^"2 Torr, with

0.1 μm < λ < 1 cm, the increased frequency of intermolecular collisions in the vicinity of the mask plane leads to pixels with comparatively more diffuse edges. Deposition pressures in

the range of 1 to 10³ Pa are also preferred, with the molecular mean free path (mfp) ranging

from 10³ to 1 mm, respectively. Nevertheless, in this work we show that deposition with

pattern definition of about lμm can be achieved under the appropriate conditions of substrate

temperature, reactor pressure, and mask geometry. Our results reveal the dynamics of growth

of ordered organic films in the different material transport regimes encountered in OVPD ~

the free molecular, the diffusive, and the intermediate of the two — and help identify useful

processing conditions and guide the design of OVPD systems.

[0052] The technique of Organic Vapor Phase Deposition (OVPD) is becoming

increasingly widespread in depositing amorphous and crystalline organic thin films for

display, transistor, and photovoltaic applications. It is particularly attractive for fabrication of

molecular organic LEDs due to low operation cost and ease of scale-up as compared to

vacuum thermal evaporation (VTE). Low Pressure OVPD (LP-OVPD) has been used

previously in depositing optically non-linear salts, optically pumped organic laser, efficient

organic light emitting diodes (OLEDs), and pentacene channel thin film transistors (TFTs).

Since OVPD inherently differs from VTE in the use of a carrier gas, the mass transport

mechanism is also radically different. Film deposition at intermediate (0.1 - 100) Knudsen

numbers encountered in OVPD are examined, both experimentally and by Monte-Carlo

computer simulation, h particular, we examine the patterning of organic films via shadow-

masking, which has special relevance to the deposition of multiple color pixels on the same

substrate, for example for use in a full color OLED display fabrication.

[0053] Thus, simulations of organic film patterning by OVPD may also examine the

transition flow regime. [0054] The Knudsen number, Kn = mfp/d, characterizes the different transport

regimes encountered in gas flow processes in terms of the molecular mean free path, mfp,

divided by the critical apparatus dimension, d. VoτKn«l, molecule-molecule collisions are

far more frequent than molecule-wall collisions, and mass transport is described by PoiseuiUe

flow. The analogous method of metallo-organic chemical vapor deposition (MOCVD), used

to growth inorganic semiconductor thin films and heterostractures, frequently occurs in this

flow regime.

[0055] For deposition of unpattemed thin films, OVPD typically takes place at

Kn«l. However, for the purpose of high resolution film patterning, other flow regimes may

be preferred, depending on P_dep and whether d corresponds to the pattern size, resolution, or

aperture width.

[0056] Although molecular flow through small channels has been previously studied

for a range of Kn using several different techniques (molecular beam epitaxy, gas dosing, gas permeation of membranes, aerosol filtration, etc.), with the variably collimated fluxes having

the same general characteristics as shown in Fig. 2, the results are not readily nor rigorously applicable to micropatterning of organic thin films by OVPD. Molecular beam studies

typically involve highly rarifϊed fluxes of identical molecules or atoms, leading to line-of-

sight deposition, h contrast, gas dosing experiments use dense gases and large channels

(diameter » mfp), with the hydrodynamics of the flow described by Navier-Stokes equations.

The gas flow pattern is also typically unaffected by the presence of a condensing surface. The

studies of gas and gas-driven particle transport in porous media are generally concerned with bulk variables.²²'²³ Technical literature contains several diffusion- and Monte-Carlo-based

studies of reactive ion etching and deposition in via holes and trenches for similar Kn regimes as in OVPD, albeit for sub-micron aperture dimensions. This study uses a combination of patterned deposition experiments and Monte-Carlo simulations to study micropatterning by

OVPD. The treatment is simplified by neglecting gas-phase and surface reactions and

molecule or particle re-emission which must be considered for reactive deposition or etch.

[0057] In OVPD, the carrier gas flow creates a hydrodynamic boundary layer at the

substrate, ranging from 1 mm to ~5 cm in depth. In previous studies the film deposition rate

has been shown to be limited by diffusion across this boundary layer. Generally, the organics

are a minority species (< 1% by mole) and diffuse through a background of the carrier gas,

suffering collisions en route to the substrate.

[0058] Monte-Carlo simulations of patterned ONPD indicate that when the mfp is on the

order of the pattern size, deposit edges often become less defined. The control of pattern

resolution is achieved by varying the mfp, the bulk gas flow velocity, and aperture geometry. The

Monte-Carlo simulations and ONPD experiments demonstrate that patterns as small as 1 μm can

be achieved, with the deposit profile depending strongly on the pressure, aperture shape and

distance from the substrate. A weaker dependence on the mfp and bulk flow velocity is also

found. Operating conditions and aperture geometries are identified for achieving highest

resolution.

[0059] Process parameters that may be significant for OVPD include evaporation and

• reactor wall temperatures (T_evap, T_waIj), carrier gas flow rate ( V), and deposition pressure

(P_dep), with preferred film deposition conditions characterized by 1 Pa < P_dep < 10³ Pa, 500 K

< T_emp,T_wa,ι < 700 K, and 10 seem < V < 1000 seem, hi this process window, the vapor

pressure of the organic molecular species is lO^' O¹ Pa, keeping the organic materials volatile yet chemically stable^5"7 and the gas phase molecular diffusion rate is on the order of the bulk

carrier gas flow, promoting film deposition rate and uniformity. Layer thickness control can

be achieved either by means of a mechanical shutter, or by regulating V, or a combination of

both. A preferred processing window for OVPD is as follows. The lower bound on the

pressure at the source is ~1 Pa. Below this value the pressure drop from the source inlet to

the substrate is insufficient to drive the gas flow, and that transport of organics can become

diffusion-dominated and poorly controlled. The upper bound to the pressure in the deposition

region, P_dep, is ~1 kPa, limited by the decreasing diffusivity of organic vapor with pressure. In the pressure range from 1 to 10³ Pa, the molecular transport changes from the Knudsen

regime encountered in VTE, to the diffusive regime, and the character of deposition may be

changed significantly.

[0060] Figure 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 δ from a substrate 330. The

carrier gas then impinges on substrate 330, where the organic material physisorbs onto the

substrate surface. Substrate 330 maybe cooled. Walls 340 of system 300 maybe 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.

[0061] Spatially and temporally separating evaporation and deposition in OVPD

allows independent and precise control over the deposition rate, dopant concentration, and

coating uniformity of molecular organic thin films. Preferably, the deposition pressures ranges between about 0.01 and 10 Torr. Amorphous thin films (e.g. α-NPD, Ak ), as well as

crystalline thin films (e.g. perylene, pentacene), may be grown.

[0062] In OVPD, the adsorbing organic molecules are uniformly distributed above the

entire substrate area, which can be regarded as a quasi-infinite collection of point-evaporation

sources situated near the mask. In the preferred range of pressures of 0.01 to 10 Torr used in

OVPD, λ for the carrier gas molecules (e.g. nitrogen, argon) ranges from 1 cm to 0.1 μm,

respectively. Hence, with shadow mask features on the order of a micrometer, the variability in the frequency of molecular collisions above and below the mask at the different pressures

can lead to a significant variation in the sharpness of the pixel profile, as illustrated in Figure

4. Figure 4 shows an OVPD system 400. A carrier gas is used to transport organic molecules from a source (not shown in Figure 4, see, for example, Figure 3). The molecules have an

average mean free path λ. 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.

[0063] With qualifications with respect to the highly molecular nature of OVPD, we

can state that, preferably, the ratio of carrier gas velocity to the mean molecular velocity, v 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, transport to the substrate, diffusion to the

substrate surface, and surface diffusion and immobilization are most relevant. Since the

efficient deposition of amoφhous thin films preferably involves minimal surface diffusion

and desorption, the lowest practicable substrate temperatures are preferably used. Two things

happen in this case: k_ads » k_des, while the crystallization rate, k_c, is very high, meaning that the

surface diffusing organic molecules become immobilized much faster than they diffuse to the

substrate. Thus, surface diffusion and immobilization is very fast and may not be considered

rate limiting for deposition of amorphous films. The rate-limiting steps are thus transport to

the surface and diffusion to the substrate surface.

[0064] As shown in previous work, the overall deposition rate, r_dep can be expressed

as:

Vdep = (2)

where P_org/RT is the concentration of the organic species, N dot is the carrier gas flow rate, δis

the BL thickness, and D_org is the diffusivity of organic molecules in the carrier gas. The

kinematic viscosity itself depends on pressure via: v = μ/p, where p = P/RT. Increasing the

background gas pressure, P_dep, may result in a sublinear decrease in the deposition rate, r_dep, due

to two opposing factors: a decrease in the diffusivity, D_org, which lowers r_dep, and a decrease in

δ, 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. [0065] h the vicinity of the substrate the system maybe engineered and modeled as a gas

j et 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, Stakes the form:

where is the kinematic viscosity of the gas, and a is a quantity that decreases linearly with N

dot and/or the rate of rotation in such a way that the formula may be used directly to estimate δ

in units of cm when v is in cmVs and bulk flow axial velocity in cm/s is used for a. For the

preferred conditions used in ONPD and in this work, such as T= 275°C, P_dep= 0.2 Ton and N

dot = 15 seem of nitrogen, c> is approximately equal to 1-10 cm. However, since the £is not

significantly smaller than the axial dimension of the typical deposition chamber (1-30 cm), the

term boundary layer must be applied in ONPD with caution.

OVPD through a shadow mask

[0066] The preceding discussion relies on the validity of the continuum assumption due

to the use of the uniform bulk diffusivity, D_org, and the boundary layer thickness, δ. This section

examines the validity of the continuum assumption when applied to shadow masking in OVPD.

[0067] The extent to which the organic molecules retain their initial bulk flow velocity

when they arrive at the mask plane is a factor that may affect OVPD. 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 thennalized. In this case, it can be seen qualitatively that the decrease in D_org due to higher P_dep will not make the patterns less sharp. Since D_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_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.

[0068] 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 max * (4)

where d_max is the pixel edge dispersion, as shown in Figure 4, and u is the carrier gas velocity in

the deposition chamber. Here, we assumed that λ 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_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 Ton) 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. Here, it should be noted that the continuum and hence the diffusion

assumptions are inconect for most of OVPD conditions. As remarked earlier, the Knudsen number (λ/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 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 δ is expected to affect the sharpness of the pattern.

Furthennore, the latter will be limited by the following factors: molecular mean free path, λ,

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_org and λ are intimately related, we

next examine how λ varies with P_dep and its effect on pattern sharpness.

[0069] A Monte-Carlo type simulation may be used to model deposition through a

shadow mask. With reference to Figure 4, a larger λ 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, λ has the form:

^{λ = ~} VFA2 • π • σ ²— • P Tdep^{~ (5)}

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_dep, represents the free molecular transport regime, where λ is

large and the carrier gas flow rate, V dot, limits material transport. Increasing V dot results in greater P_dep and transport becomes diffusion limited as λ 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 Ton optimum pressure range used in OVPD.

[0070] While Eq.(5) maybe used accurately with dilute, non-polar gases like helium and

argon, OVPD deals with a mixture of complex molecules, e.g. Alq₃, along with the carrier gas,

such as nitrogen or argon. The effective nominal mean free path and collision cross-section, λ

and σ, can be determined via modified expressions for the diffusivity through Eq. 5 and the

relationship:

Doorrgg —~ uλ (6)

Here, the Chapman-Enskog expression for the diffusivity of molecules with dipoles or induced-

dipoles may be used:

where M_t is the mass of the diffusing species i, T is the gas temperature, and σ^ is the average

collision cross-section, σ_AB = ['/2(σ_A + σ^)²]'^A. The quantity Ω_D,_AB is a dimensionless function of

the Lennard- Jones intermolecular potential and temperature. Frequently, for the materials

commonly used in OLEDs, reliable Lennard- Jones parameters are not available, and the Fuller conelation may be substituted:

where ∑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. As evident from Table 1, the values oϊD_ΛB 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 λ and σshould suffice for

determining trends with pressure, which trends may be used to control desired deposition

properties.

Simulation

[0071] A Monte-Carlo simulation was performed 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^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 following function was

used: FNOΓUA t

Pcoll = (9)

Vc

where F_N is the number of real molecules represented by one simulated molecule, σ_τ is the total

cross-section of the colliding molecules, u_r is their relative speed, Δt is the time interval allowed

for the collision to take place, while V_c is the volume of the cell in which the collision occurs.

The value of σ_τ can be calculated from d_eff, an effective collision diameter which scales with the

relative particle velocity, v_r:

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.

Simulation Results and Discussion

[0072] Figure 5 shows simulated results for deposition through a shadow mask in the

diffusive regime. For a nominal s = 10 μm and mask thickness of 18 μm, the deposition patterns

for λ = 8.25, 82.5, and 825 μm (P_dep * 0.01, 0.1, 1.0 Ton) are shown in Figure 5. Molecules

were launched from 2000 μm 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 Figure 5, no difference in d is observed for different

values of λ. 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 λ, which

conespond to small D_org. The simulation was performed with 30 μm wide mask openings, a

mask thickness of 18 μm, and a mask separation, 5 = 10 μm. Plots 510, 520 and 520 show

deposition thickness profiles for the mask (higher) and the substrate (lower) for λ = 8.25, 82.5,

825 with P_dep = 1.0, 0.1, 0.01 Ton. 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 otλ.

[0073] Figure 6 shows simulated results for deposition through a shadow mask in the

diffusive regime. The mask openings remain 30 μm wide, with t = 18 μm and λ — 82.5 μm,

while s = 3, 10, 20 μm, respectively, for plots 610, 620 and 630. Smaller values of s result in

sharper pixels. As long as s ~ λ, trapezoidal pixel shapes may be obtained, similar to vacuum

deposition. Pixel overlap starts to occur when s ~ t. Keeping the purely diffusive framework for

the simulation, Figure 6 shows how variation in s affects pixel edge dispersion. Since λ does not

affect d in this regime, we use λ = 82.5 μm; for t = 18 μm pixel cross-talk starts to occur when

s = 20 μm, i.e. as s approaches t, there is overlap of the neighboring pixels.

[0074] Figure 7 shows simulated results for deposition through a shadow mask in the

diffusive regime. The mask openings remain 30 μm wide, with A = 82.5 μm and 5 = 10 μm.

Here, mask thickness, t, is varied to 18, 36, and 54 μm, 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 λ, 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 λ.

[0075] Figure 8 shows simulated results for deposition through a shadow mask in the

diffusive regime. The mask openings are 30 μm wide, with λ = 82.5 μm and 5 = 10 μm. Plots

810 and 820 show results for δ = 410 and δ = 2060 μm, respectively. Here, the effective

boundary layer thickness is decreased from 2060 to 410 to 80 μm by adjusting the launching

point to be closer to the substrate. As δ approaches λ, the deposition efficiency increases, in

agreement with the continuum model for diffusion-limited transport to the substrate. Here, the

effective c> was varied by launching the molecules closer to the substrate.

[0076] 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.

[0077] 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_bulk,

approaches the molecular thennal 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.

An example of this vapor-jet deposition mode is illustrated in Figure 9, which shows a

concentration map where the bulk velocity is superposed on the random thermal motion of the

molecules, resulting in a jet-like molecular flux through the mask aperture.

[0078] Figure 10 shows a material concentration map which shows the j et-like character

of deposition with ultra-fast carrier flow. The overall deposition efficiency of this process may approach 100%, since pixels are patterned by the directed gas jets and no material is wasted in

coating the shadow mask. It should be possible to engineer a deposition system with individual

nozzles for each color pixel for efficient, precise, and more portable deposition systems. The

deposition profile of Figure 10 shows a well-defined, trapezoid-shaped pixel, similar to the

vacuum deposition case.

Experimental

[0079] The deposition of organic thin films of Alq₃ was carried out using a multi-banel

glass reactor system with in situ temperature and thickness measurement capability. Briefly, the

reactor vessel is an 11 cm diameter by 150 cm long Pyrex^® 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 banel. Carrier gas flow is regulated by mass flow controllers, while the

deposition pressure is kept between 0.1 and 10 Ton by adjusting the pump throttle valve and the

total carrier flow rate from 10 to 50 seem. A 40 1pm 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 orgamc film thickness.

[0080] In addition to deposition of organic thin films using OVPD, a conventional

vacuum thermal evaporator was used. The source-to-substrate distance was approximately 30cm;

the deposition pressure was maintained at 10^"6 Ton.

[0081] The shadow-masking anangements are illustrated in Figure 11. A mask 1110

comprising a mesh 1120 and a plate 1130 having holes was used. A clamp 1140 was used to

hold mask 1110 a fixed distance above a substrate 1150. Mesh 1120 comprised a 5μm thick nickel mesh consisting of 10 μm lines that interlace, forming 15 μm square openings. This mesh

was placed directly on top of lmm thick silver-coated glass slides and covered with a 50μm thick

nickel plate 1130 containing round holes 1 and 0.3mm in diameter. This anangement allows for

the simultaneous measurement of deposition for two values of s. Due to the non-square profile

of the metal wires comprising the nickel mesh, as shown in Figure 11, the smallest value of s is

~ 2μm. Here, the dispersion values, d, for s=2μm will refer to the fuzziness of the square pixels

7-1 Oμm on the side. Values of d conesponding to s=5μm refer to the fuzziness of the circular

deposition edge formed with the lmm and 0.3mm holes of the 50μm thick mask resting on top

of the mesh. The next largest separation of 50μm was achieved with a two-layer nickel mask

illustrated in Figure 11.

[0082] Additional shadow masks were fabricated integral to the substrate to provide the

most accurate mask-substrate separations. The masks were formed using a photoresist /

chromium / photoresist (PR1 / Cr / PR2) sandwich structure and photolithography.

[0083] Following the deposition of Aktø, the resulting pixel patterns were examined using

scanning electron microscopy.

Experimental Results and Discussion

[0084] Figure 12 shows scanning electron micrographs of the patterns resulting from

OVPD through a shadow-mask at P_dep ranging from 2x 10^'6 to 2 Ton. As the deposition pressure

increases, both simulations and experimental data indicate the loss of edge sharpness. Images

1210, 1220 and 1230 show results for P_dep = 2-10^"6, P_dep = 0.2 Ton, and P_dep = 2 Ton,

respectively. The separation 5 = 5 and 2.5 μm 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 Ton and separation of up to 15 μm it

is possible to achieve a pixel resolution on the order of several microns, which is sufficient for full color display applications.

[0085] To quantify the extent of pixel fuzziness, we define a dispersion parameter, d, to

be the distance along the x-direction where the pixel thickness is between 90 and 10% of its maximum at the center. This number is scaled by the mask-substrate separation, s, to yield a dimensionless pixel dispersion which is plotted versus the deposition pressure for both experiment and simulation.

Hybrid deposition

[0086] Figure 13 shows the results of a simulation for an Alctø / Ag micropattems formed by hybrid OVPD-VTE deposition, where the same shadow mask was used for both. Excellent alignment of metal layers 1310 with underlying organic layers 1320 is guaranteed automatically, while the spread of the organic prevents potential shorts around the pixel edge. The simulation results show an increase in dispersion with pressure similar to that of the experiment at the pressure range accessible in OVPD. While pressures between 10^"6 and 10^"1 Ton are not readily accessible with the cunent deposition system, the simulation results can fill in the gap. In the limit of very low deposition pressure, pixel edge dispersion may asymptote to a non-zero constant, characteristic of the vacuum chamber and mask geometry.

[0087] When fabricating a full-color OLED display, the various layers of patterned material are preferably aligned to one another. One practical scheme involves a combination of ONPD through a shadow-mask at moderate pressure, as shown in Figure 4, followed by the deposition of a metal contact at low pressure, as indicated in Figure 2, without moving the mask between the steps. The controlled dispersion of the organic coating will prevent formation of electrical shorts around the edge of the organic film. Tins concept is illustrated by the simulation

illustrated in Figure 13.

[0088] Figure 14 shows a scanning electron micrograph of a stracture experimentally

fabricated in accordance with the simulation of Figure 13. An Alq₃ organic layer 1410 was

deposited by ONPD, followed by an Ag metal layer 1420 deposited by VTE. Because of the

different parameters used when the layers were deposited, the organic layer was consistently and

reliably larger than the metal layer, even though they were deposited through the same shadow

mask, which remained immobile in the same position through both deposition of the organic and the metal. A second set of pixels may be deposited by simply translating the mask laterally

(perhaps in-situ), without the need for alignment of a mask to the previously deposited pattern,

and then depositing a second organic layer (perhaps that emits a different color), followed by a

metal layer. The process may be repeated once again for a three color display.

Table 1: Gas phase diffusivity coefficients for Alq₃, Ν₂, and Alq₃ in N₂ mixture.

T D_org (Kinetic Theory) D_org (Fuller et al.) D_org (Chapman -Enskog)

(K) (cm²/s) (cm²/s) (cm²/s)

273 0.0355 (N₂) 0.68 (N₂) 0.105 (Al b) 0.0629 (N₂) 548 _{Q 101 m} 2.30 (N₂) 0.356 (Alq,) 0.179 (N₂)

Additional Simulations

[0089] Although a continuum-based analysis may not be rigorously accurate for the

transport regime encountered in OVPD, some insight into the patterning by OVPD can be

gained by considering simple diffusion in confined geometries. [0090] Figure 15 illustrates a bell-shaped profile of a pixel deposited by OVPD

through a shadow-mask. A substrate 1510 is disposed underneath a mask 1520. The aperture

wall profile is characterized by an angle a (or α"). Mask 1520 has a thickness t and an

aperture 1525 of width w is separated from the substrate by s. A layer 1530 of material is

deposited tlirough mask 1520 onto substrate 1510. The molecular flux incident on the

substrate originates from an infinite number of uniformly distributed point sources in the

space bound by -w/2 < x < w/2, resulting in a the deposit having the bell-shaped profile

shown. For isotropic diffusion in the continuum regime, the base of the deposit will widen in

direct proportion to s. Thus, smaller s should result in greater pattern resolution. If 5 is kept constant, but the deposition pressure increases, the diffusivity of the gas molecules drops.

However, since diffusion 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. Moreover, in the continuum

assumption, the pattern formation does not depend on the shape of the aperture above the

edge closest to the substrate. In contrast, a simulation of the deposition, which looks at the

stochastic scattering of molecules in the vicinity of the substrate, is able to predict that the

deposited pattern shape depends not only on s, but also on mfp and the aperture shape.

[0091] Due to the discrete nature of transport at low number densities, several

collimating effects on the organic molecule flux through the mask aperture must also be

considered. Firstly, molecular flux collimation may be proportional to the aspect ratio, t/w, of

the aperture (at the expense of deposition efficiency). Secondly, some collimation may occur

if the heavy organic molecules retain a significant fraction of their initial bulk flow velocity,

U, when they arrive at the mask plane, provided v.<U (we assume fully developed flow in the deposition chamber, so that at the edge of the boundary layer, Hfor organic molecules is

assumed to be equal to £7 for the carrier gas molecules). In the OVPD regimes considered

here, bulk flow may not significantly contribute to flux collimation, since ϋ ~ 100-400 m/s,

while ZJ~ 1-10 m/s, and hence pure diffusion dominates near the substrate. However, the

effects of the bulk flow momentum of the carrier gas can still be investigated using Monte-

Carlo simulations. For example, it maybe that, if U/ . > 0.1, changes in t/can affect the deposit profile. Finally, since natural convection rate is insignificant compared to diffusion at

low number densities, the thermal driving force in the vicinity of the substrate is estimated to

be relatively small and is not further considered.

[0092] The discussion of pattern shape is aided by defining a shape factor, η, equal to

an area 1535 of layer 1530, which is bound by -w/2 < x < w/2 (cross-hatched region) divided

by the total deposit cross-sectional area. Then η is equal to unity for a "perfect", rectangular

deposit and decreases with increasing P, t/w, s.

[0093] One objective of the simulation is to understand how organic molecules are

transported in confined geometries, where the apparatus dimensions are on the order of the

mean free path of the molecules, and continuum-based descriptions do not apply. The model

addresses the patterning of the organic deposit on the substrate with the added constraint of

condensation on the surfaces of aperture with a finite thickness. The simulation represents

the stochastic nature of the diffusion process at the intermediate Kn. Therefore, the model

traces the paths of organic molecules in the vicinity of the substrate as they collide with the

lighter carrier gas molecules and deposit on cooled surfaces - the substrate and the aperture

edges. For simplicity the sticking probability is assumed to be unity. Using a prefened vapor

pressure of the organics of aboutlO^"2 Pa, and a prefened background carrier gas pressure on

the order of 1 to 10³ Pa, organic-organic collisions are rare. [0094] Bulk diffusive transport of gases is a mean free path phenomenon, whose rate is characterized by the diffusivity, D. Kinetic theory of simple gases conelates the bulk parameter D with the molecular based quantity, the mean free path, mfp:

D——u -mfp

³ dl)

for a single-component, low-pressure non-polar gas at low pressure mfp. Since the organic vapors used in OVPD consist of large molecules, geometrically and energetically more complex than the hard sphere atoms of kinetic theory, there may be some differences between the simulation and experimental results.

[0095] The Monte-Carlo simulation proceeds as follows. The computational space is divided into an x-z grid extending infinitely in the -direction, whose purpose is to locate the substrate and mask surfaces, and track changes in the thickness of deposits. A particle representing an organic molecule is assigned a random initial location (x₀,y₀,z₀) inside the boundary layer and above the mask. A random initial direction is chosen and the particle

travels a distance r = [(x -x₀)⁺ ( ryo)⁺(^zr^zo)_^ll2 _> where (x;_;,y_;,z_;) is the particle's new location. The distance r is a minimum of the grid size or the mfpllO. The probability of collision, P_cgll, with a carrier gas molecule is given by P_coU = r/mfp, which is checked against a random number, rand, between 0 and 1. If P_coll<rand, the molecule is again allowed to proceed in the same direction for a distance r. If P_coll>rand, the particle collides with a locally generated carrier gas molecule having a velocity chosen randomly from a Maxwell-Boltzmann distribution. The collision causes the molecule to be deflected with a velocity and an angle consistent with momentum and energy conservation in an elastic collision of two hard spheres. If the path of the particle crosses the substrate plane or the aperture wall, the particle is assumed to stick to the surface with unity efficiency, while the thickness of the deposit is

updated. A single aperture is simulated, with periodic boundary conditions imposed in the x-

direction. The simulation run-time increases with the number of molecules used for the

deposition run and decreases for larger mfp. Effects of aperture geometry on deposited

pattern formation were modeled by assigning different aperture side-wall angle, α = 45, 90,

135°, and α' = 270°, as illustrated in Figure 15.

[0096] We consider the growth of films of the archetypal molecule, tris(8-

hydroxyquinoline) (Alq₃), having d~10 A. The collision cross-section for a mixture of Alq₃ in

a N₂ carrier gas is taken as an average of the respective cross-section of the two species in

self-diffusion, d = V2(d_AlgAd_N2), while P - P_dep in Eq. (1).

[0097] Figure 16 shows a simulated profile of the organic species concentration at the

end of a simulation run of 10⁵ particles, with s = 7μm, t = 3μm, and a= 135°, and mfp =

lOOμm, conesponding to a total deposition pressure of -37 Pa at T = 500K. Organic layer

1630 (simulated) was deposited on substrate 1620 through mask 1610, which is disposed a

distance s above mask 1610. The initial particle velocities were assigned from a random

thermal distribution and superimposed onto a z-directed velocity vector with a magnitude of

U= lOm/s. The size of the individual particle was enlarged to show the deposited film

thickness profile.

[0098] Figure 17 shows a simulated profile similar to that of Figure 16, where an

organic layer 1730 (simulated) was deposited on substrate 1720 through mask 1710, which is

disposed a distance s above mask 1710. The mfp used to generate the results of Figure 17 is a

factor of 10 less than that used for Figure 16. By decreasing the mfp by a factor of 10, a more

diffuse pattern is obtained. The films shown in Figure 17 may be unrealistically thick with respect to most device applications, but figure nevertheless illustrates the increased parasitic

deposition on the inner side of the mask and aperture walls when mfp is reduced. A film

thickness on the order of 1000 A is more practical, and subsequent simulation results will

provide only the thickness profile, without the shadow mask superimposed for clarity.

[0099] The following OVPD simulations used 10⁶ Alctø molecules in a N₂ carrier gas,

unless otherwise stated. The effects of deposition pressure on pattern resolution were

investigated by varying the mfp, proportional to the reactor pressure. Growth through an

aperture with w = 80μm, t = 70μm and with the substrate separated from the lower mask edge

by = 20μm was considered. The choices for 5 and t are consistent with deposition of high

resolution display picture elements (pixels) by VTE, where the substrate facing downward

evaporation geometry causes the mask to bow away from the substrate under gravity, leading

to s ~ 20μm. To stiffen the mask and minimize s, usually t > 70μm is used. Figure 18 shows

a plot of shape factor v. mean free path. The shape factor, η, increases only weakly as mfp

increases over four orders of magnitude (lμm < mfp ≤ lOOOμm, conesponding approximately

to 10³Pa < P_d ≤ lPa), as would be expected for a system approaching the Knudsen transport

regime. However, η does not reach unity due to parasitic deposition on the sidewalls of the

almost square aperture. As expected, the highest pattern edge resolution is achieved for the

largest mfp, i.e. the lowest P_dep. However, the pattern profile exhibits a dome-like shape due

to relatively large t/w ratio.

[0100] Figure 19 shows profiles of simulated organic layers similar to layer 1630 of

Figure 16, but generated with different parameters. Increasing the aperture thickness can

improve collimation of the molecular flux toward the substrate due to the condensation on the

walls of the aperture. Plots 1910, 1920, 1930 and 1940 show profiles for an aperture thickness of 50, 20, 10 and 5 μm, respectively, for s = mfp = 20μm. w = 300μm should have

no effect on spreading at the edges of the profiles. However, as tlw increases, the deposited pattern becomes significantly more domed due to the scavenging of the organic molecules by

the upper edges of the aperture. The shape factor decreases less than 5% over lμm < t <

lOOμm range. The pixel deposition efficiency decreases with increasing t due to parasitic

deposition on the aperture sidewall.

[0101] Figure 20 shows profiles of simulated organic layers similar to layer 1630 of

Figure 16, but generated with different parameters. The mask separation, s, can vary due to

bowing under gravity (in VTE), or possibly heating during the deposition process. Plots

2010, 2020, 2030 and 2040 show profiles for depositions where s = 2, 10, 22, and 50μm,

respectively. For each of the plots, t = 20μm, w = 300μm, and mfp = 20 μm. Appreciable

edge broadening may arise due to collisions in the mask-substrate gap for s > mfp. In

addition, the doming of the middle of the deposit is also pronounced and, in contrast to the

case of large tlw, is accompanied by the broadening of the pixel edges. The inset of 2050 of

Figure 20 plots η vs. 5 for this series of depositions, showing a rapid decrease in η with s.

Thus, optimal pattern resolution leading to a rectangular deposit is achieved for the smallest s

and t. Since OVPD can, in principle, be carried out with the mask positioned above the

substrate, thin masks can be used without compromising small s values due to mask bowing,

wliich would otherwise occur in VTE. One drawback of thin masks is that they are more

susceptible to thermally and mechanically induced stresses.

[0102] Figure 21 shows profiles of simulated orgamc layers similar to layer 1630 of

Figure 16, but generated with different parameters. The profiles of Figure 21 illustrate the

effect of different side- wall angles. Graph 2110 shows profiles generated with a side- wall angle α of 45 degrees, s was 10 μm, mfp was 20 μm, and the mask thickness t was varied between 5 and 80 μm to generate the different profiles of graph 2110. Graphs 2120, 2130 and 2140 were generated using the same parameters as graph 2110, except that α was 135, 270, and 45 degrees, respectively. Figure 21 shows that is possible to minimize t at the aperture edge, while keeping the mask thick elsewhere.

[0103] Figure 22 shows plots of shape factor v. mask thickness, based on the profiles of Figure 21. Plots 2210, 2220, 2230 and 2240 are based on the graphs 2110, 2120, 2130 and

2140, respectively. The variation in η with the aperture shape was investigated by varying

the aperture side- wall angle, a = 45, 90, 135, and α' = 270°. The aperture with a= 135°

results in the most diffuse pixels due to the lack of collimation of the approach angle of

physisorbing molecules. This occurs to a lesser extent with the biconical aperture ( = 270°),

but the sharpest patterns are achieved with α = 90° and α = 45° geometries. In addition to

allowing deposition of sharp patterns, these aperture angles result in the greatest deposition efficiency for a given deposit shape, while being more structurally robust than the other mask shapes for a given t.

[0104] As the mfp increases (conesponding to decreasing P_dep) the molecules keep more closely to their original bulk flow velocity as they enter and propagate through the boundary layer. In this case, the trajectories become, on average, more collimated, and the deposition profiles become sharper. Figure 23 shows plots of normalized height v. position for three cases. Plot 2310 shows a profile generated under purely diffusive deposition. Plot 2320 shows a profile generated where the bulk transport velocity (set to one tenth of the average thermal velocity) was added to the z-component of the thermalized velocity vector of the carrier gas molecules. Plot 2330 is an intermediate case, where the z-directed velocity of the carrier molecule decreases inversely with its proximity to the substrate. The purely

diffusive case results in the least sharp deposit edges, while plot 2320 yields the sharpest

patterns. The actual deposition mechanism is likely to be bound by these two extremes,

accounting for the complex hydrodynamics of the carrier gas flow, and where the heavy

organic molecules retain a fraction of the z-directed component of the bulk carrier gas flow.

[0105] Consider a j et of carrier gas delivered through a small-diameter capillary onto

a cooled substrate, h the Monte-Carlo simulation, the z-directed carrier gas velocity, U_z, can be increased to simulate a jet which broadens only by the isotropic random molecular

velocities superimposed onto this flow-field. Figure 24 shows the spatial concentration

profile for a simulated jet of N₂ carrying Alq₃, with mfp = lOμm, t = 50μm, and U_z = 100 m/s,

while the mean thermal speed, ύ = 500 m/s. Since the flow-field was not known in this flow

regime, the simulation kept dUJdz = 0 for simplicity. The figure shows that the coUimated jet

can result in a deposit with well-defined edges even for s » mfp. Careful selection of U, P_dep,

a 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 Figure 24, carrier gas with organic molecules is ejected from apertures

2415 in mask 2410, to impinge upon substrate 2420. Plots 2430, 2440 and 2450 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.

Additional Experimental

[0106] The deposition of organic thin films of Ak was carried out using a multi-

banel Quartz deposition system with in situ temperature and thickness measurement

capability. Figure 25 shows an illustration of the deposition system. An 11 cm diameter by 150 cm long Quartz cylinder 2510 serves as the chamber walls. The cylinder is fitted at the

upstream end with 4 evaporation source banels 2520 (only 2 are visible in Figure 25), which

consist of 2.5cm diameter by 100 cm long Quartz banels individually encasing quartz

evaporation cells. The main tube is heated by means of a three-zone furnace 2530 enabling

source temperature control via positioning of each cell along the temperature gradient within

the tube. Carrier gas is flown on the inside of cylinder 2510 as well as each of the source

banels 2520, regulated by mass flow controllers, while the deposition pressure is kept between 0.1 and 10 Ton by adjusting the pump throttle valve and the total carrier flow rate

from 10 to 100 seem. A 401pm vacuum pump with a liquid nitrogen cold trap is used to

exhaust uncondensed carrier and organics. Organic molecules from the vapor phase physisorb onto a rotating water-cooled substrate 2540, positioned behind a mechanically

operated shutter 2550. Fihn thickness and growth rate are monitored by a quartz crystal

microbalance calibrated using the ellipsometry to measure organic film thickness at the end of

the growth cycle. The system of Figure 25 was also used to generate experimental results

discussed earlier.

[0107] The deposition profiles of organic thin films obtained using OVPD were

compared with those from a conventional vacuum thermal evaporator. The source-to-

substrate distance was approximately 30cm; the deposition pressure was maintained at 10^"6

Ton.

[0108] Three types of shadow-masks were used. One was a 60μm thick, 1cm x 1cm

molybdenum square with circular openings having diameters of 1000, 500, and 100 μm. The

aperture profile in this mask was cylindrical in shape (α = 90°). Another mask employed was

a 75mm thick molybdenum sheet with circular apertures having nominal diameters of 1000,

300, and 100 μm. The openings in this mask had a double-beveled edge, forming a biconical aperture (α' = 270°). The third mask type was a Ni mesh, 3.5 ± 0.5 μm thick, with square

openings of 7.5 and 12.5 μm nominally, separated by equally wide lines. The masks were

fixed to silicon substrates using a retainer ring. The mask-substrate separation was controlled

by using shims of multiple layers of the Ni mesh placed between the Si substrate and the Mo

mask bottom surface, hi depositions through the Ni meshes, lcm x 1cm sheets of the meshes

were fixed to the substrate by sandwiching them between the substrate and the first or second

types of mask, and then clamped to the holder by the retainer. Due to the profile of the Ni

mesh, the smallest effective separation was 1.0 ± 0.5 μm, but could be greater in some places

due to the flexibility of the mesh itself.

[0109] Analysis of the deposited pattern profiles was performed using scanning

electron microscopy (SEM) and atomic force microscopy (AFM) for the smallest pixel sizes,

and interference microscopy for the larger pixels. The latter method entailed illuminating the

substrates with monochromatic light (λ = 540 ± lOrrm) and observing the fringe patterns

formed at the sloping edge of the bell-shaped pixel. Figure 26 shows an example of the

images obtained. Image 2610 shows SEM microscopy results for circular pixels (α = 90°)

deposited by OVPD. The cylindrical aperture mask used for this deposition included pixel

diameters of w = 100, 300, and 1000 μm. Image 2630 shows interference microscopy results

for region 2620 of image 2610.

[0110] Figure 27 shows a thickness profile 2710 of the image from Figure 26,

extracted from the digitized pixel image by counting the number of fringes from the edge

(plot 2720) and using:

„ λ l H-m —

2 n (i2) where H is the pixel thickness, m = 0, 1, 2, 3, etc. is the fringe order, λ = 540nm, and n = 1.74

is the refractive index of Alq₃.

[0111] Figure 28 shows measured pattern profiles for the deposited layers of Figure

26. Plots 2810, 2820, and 2830 show profiles for the 100, 300 and 1000 μm diameter layers,

respectively, each with s = Oμm, and t = 50 μm. Plots 2850, 2860, and 2870 show profiles for

the 100, 300 and 1000 μm diameter layers, respectively, each with s = 40μm, and t = μm.

Plot 2820 conesponds to a film nominally 2μm thick, while plot 2860 conesponds to a film

nominally 1.6 μm thick. Plot 2840 shows plot 2860, normalized to plot 2820 by multiplying by 2 / 1.6, and superimposed on plot 2820. These plots indicate that the pixel deposition

efficiency (infened from the area under each profile) decreases with the aperture aspect ratio,

t/w. The doming of the middle portion of each pixel for greater tlw is similar to that observed

in the simulation (see Figure 19). By comparing the normalized 5 = Oμm and s = 40μm

curves it is evident that increasing s decreases the efficiency of pattern deposition (due to

condensation on the back of the mask) and edge sharpness, as predicted by the simulation

(see Figure 20 and related discussion).

[0112] The highest resolution patterns by OVPD thus far have been achieved using a

masking set-up where the Ni mesh is sandwiched between the substrate surface and one of the

thicker masks. A sample patterned Alq₃ film deposited by OVPD at P_dep = 0.1 Ton was

imaged using atomic force microscopy (AFM), illustrated in Figure 29. Here, a Ni mesh with

t ~ 3μm and w ~ 6.0μm nominally was used as a mask (cross-section SEM shown in the inset

of Figure 29), with a likely s < lμm. The edge sharpness is about 3μm, obtained from the

thickness profile 2910 (circles) extracted from the image in the inset of Figure 29. Plot 2920 shows a simulation result (solid line). The experimental pattern profile is well-fitted by the simulation, using w = 6. Oμm, t = 3.5μm, 5 = 0.5μm, mfp = 20μm and a' = 270^°. The

simulated and experimental dimensions are nearly identical, suggesting that the stochastic

simulation may accurately describe the patterned deposition mechanism.

[0113] Figure 30 shows additional simulation results. Plots 3010, 3020, 3030, 3040

and 3050 show simulated results with various parameters. All plots used 5 = 0.5. All plots

used t = 3.5μm and w = 2 μm, except plot 3050, which used t = 7 μm and w - 6 μm. Plots

3010, 3020 and 3040 used mfp = 40 μm, while plots 3030 and 3150 used mfp = 20 μm. Plot 3010 used α = 90 degrees, plot 3020 used α = 60 degrees, and plots 3030, 3040 and 3050 used

α = 45 degrees.

[0114] Similar patterns have been obtained for depositions at pressures from 0.1 to 2

Ton, with the lower pressures favoring sharper pixels. Two extreme cases are illustrated by

SEMs of deposited films shown in Figure 31 : Image 3110 shows Alq₃ patterns deposited on

Si by VTE through Ni meshes at P_dep ~ 10^"6 Ton and s ~ 0.5μm, and image 3120 shows

analogous patterns deposited in OVPD at P_dep = 2 Ton. The vacuum-deposited patterns show

the trapezoidal profile discussed above, with l₂ < lμm, while OVPD patterns have edge

dispersion on the order of 1-3 μm. Simulations indicate that if apertures with = A5° with

minimal s are used, sub-micron resolution may be achievable.

[0115] Figure 32 shows hybrid deposition through a single mask. A mask 3220 is

disposed under a substrate 3210. First, an organic layer 3230 is deposited by OVPD through

mask 3220 onto substrate 3210. Then, a metal or metal oxide layer 3240 is deposited by VTE

through mask 3220 onto organic layer 3230. Because of differences in the processes, such as

base pressure, the organic layer is wider than the metal or metal oxide layer even though it

was deposited through the same mask in the same location, without any movement of the mask relative to the substrate during or in between the two deposition processes. This

phenomena may be advantageously used to deposit a second layer over a wider first layer,

such that the second layer has no contact with any layers underlying the first layer ~

desirable, for example, when the first layer is a light emitting layer, the second layer is an

electrode, and another electrode underlies the organic layer. The order may also be reversed,

such that the wider layer is deposited second.

[0116] Although the various embodiments describe OVPD and VTE as specific

processes for which base pressure and other parameters may be used to control the area of

coverage of a deposited layer, the concept may be extended to other deposition techniques, such as sputtering, e-beam, or generally physical vapor deposition. The area of coverage may

also be controlled within a particular process. For example, a wide organic layer may be

deposited over a nanower organic layer (or vice versa), both by OVPD, but with different

OVPD parameters.

[0117] It has been shown that OVPD may be used for patterned deposition of organic

thin films with micrometer scale resolution. We have found that, due to the low density of

carrier gas and organic molecules used in OVPD, the molecular mean free paths are

frequently on the order of the mask dimensions, suggesting that Monte-Carlo direct

simulations should be used for modeling this process. While it is likely that the deposition

takes place largely in the diffusive mode, experimental evidence suggests that organic

molecules retain a significant fraction of their velocity due to bulk transport by the carrier gas.

As the deposition pressure decreases, the mean free path increases, resulting in sharper pixel edges. Using a heavier carrier gas has no first-order effect on the pixel shape in the diffusive

regime, while it can improve edge sharpness in vapor-jet deposition mode. Mask-substrate

separations that are smaller than the mean free path of the system will frequently yield trapezoidal pixel profiles found in the line-of-sight vacuum deposition. A thicker mask helps

increase pixel edge sharpness, albeit at the detriment of mask-to-substrate deposition

efficiency. The simulated vapor-jet deposition process indicates that it may be possible to

pattern organics directly by ultra-fast gas jets, where the flow velocity matches or exceeds the

thermal velocity of the molecules. This process may result in close to 100% efficient

patterned organic vapor deposition. By depositing at the lower end of the pressure range

(<10Pa) and using appropriate aperture shapes (e.g. a< 90°) it is possible to achieve sub-

micron pattern resolution.

[0118] While the present invention is described with respect to particular examples

and prefened 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 prefened embodiments described herein, as will be apparent

to one of skill in the art. While some of the specific embodiments maybe described and

claimed separately, it is understood that the various features of embodiments described and

claimed herein may be used in combination.

Claims

WHAT IS CLAIMED IS:

1. A method of fabricated an organic device, comprising:

a) depositing a first layer onto a substrate through a mask at a first base pressure;

b) depositing a second layer through the same mask at a second base pressure;

wherein the base pressure is used to control the sizes of the first and second layers.

2. A method of fabricating an organic device, comprising:

a) depositing a first layer over a substrate through a mask using organic vapor

phase deposition, wherein the first layer is organic;

b) depositing a second layer over the first layer through the same mask by a

process that results in the second layer having a smaller area of coverage than

the first layer.

3. The method of claim 2, wherein the process of step b) is vapor thermal evaporation

(VTE).

4. A method of fabricating an organic device, comprising:

a) depositing a first layer over a substrate through a mask by a first process that

results in the first layer having a first area of coverage.

b) depositing a second layer over a substrate through the mask by a second

process that results in the second layer having a second area of coverage that is different from the first area of coverage.

5. The method of claim 4, wherein the base pressures at which the first and second

processes are carried out are different, and this difference is used to control the relative areas of coverage of the first and second layers.

6. The method of claim 4, wherein the first layer comprises a metal or a conductive

metal oxide, the first process is vapor thermal evaporation, the second layer comprises

an organic material, the second process is organic vapor phase deposition, and the second area of coverage is larger than the first area of coverage.

7. The method of claim 4, wherein the first layer comprises an organic material, the first

process is organic vapor phase deposition, the second layer comprises a metal or

conductive metal oxide, the second process is vapor thermal evaporation, and the first

area of coverage is larger than the second area of coverage.

8. The method of claim A, wherein the organic device is an organic light emitting device.

9. The method of claim 4, wherein the organic device is an organic transistor.

10. The method of claim 4, wherein the organic device is an organic solar cell.

11. The method of claim 7, wherein the metal or metal oxide layer is disposed entirely on

the organic layer, such that there is no contact between the metal or metal oxide layer

and any material underlying the organic layer. , ,

12. The method of claim 7, wherein the base pressure during the deposition of the first

layer is about 0.1 - 10 ton.

13. The method of claim 4, wherein the method is used to fabricate a passive matrix

display.

14. The method of claim 4, wherein the method is used to fabricate an active matrix

display.