WO2008017795A1

WO2008017795A1 - Method of forming an electronic component

Info

Publication number: WO2008017795A1
Application number: PCT/GB2006/002949
Authority: WO
Inventors: Paul Raymond Drury; Robert Harvey
Original assignee: Xaar Technology Limited
Priority date: 2005-08-08
Filing date: 2006-08-08
Publication date: 2008-02-14
Also published as: GB0516278D0; TW200711519A

Abstract

A method of forming a component for use in printed electronics. The method comprises the steps of preparing a plurality of discrete deposition locations (20a, 20b, 20c) on a substrate (2), and depositing first and second components, such as light emitting polymers (24a, 24b), by fluid-processing on the discrete deposition locations. The fluid- processing comprises the steps of : supplying the first component (24a) in a fluid; contacting the fluid containing the first component with the deposition locations, attracting at least a proportion of the first component to a selected deposition location; removing fluid and any unfixed first component; supplying the second component (24b) in a fluid,- contacting the fluid containing the second component with the deposition locations,- attracting at least a proportion of the second component to a selected deposition location, for example by dielectrophoresis; and removing fluid and any unfixed second component.

Description

METHOD OF FORMING AN ELECTRONIC COMPONENT

The present invention relates to the manufacture of electronics components and in an important example to displays and other components manufactured and used in the printed electronics industry.

Displays comprising organic polymers are well known in the art. The displays typically comprise light emitting polymers (LEPs) and are known as Organic Light Emitting Diodes (OLEDs).

LEP based display technology is widely accepted as a possible replacement to the cathode ray tube and liquid crystal displays. The displays are self-luminous without requiring a backlight, thin and light weight and require a low diving voltage. Wide viewing angles are also possible.

A thin film of the polymer is deposited on a glass or plastic substrate and sandwiched between two electrodes. Holes and electrons are injected from the electrodes and light is emitted from the polymeric material.

One particular benefit of the polymer systems is that they can be solution processed into large area thin films using relatively inexpensive technology. In a known technique the solution is deposited by inkjet printing into "wells" or depressions formed on a substrate, each well forming a pixel. The polymers are dissolved or dispersed in an excess of carrier fluid which is removed through a process of carefully controlled evaporation. The wells prevent the mixing of the light emitting polymeric material, which may otherwise result in inoperative pixels. The evaporation must be controlled to ensure a constant thickness of polymeric material within the pixel, else it is possible to observe a "coffee-ring" phenomena, where a thicker ring and a very thin centre of polymer may be observed. Both these problems reduce the image quality and lifetime of a display.

At particularly small display sizes, and especially where a high resolution is required, the wells are extremely small, often of the order 25μm by 100μm. In order to deposit fluid within the wells the inkjet printers must be capable of extreme accuracy and must be aligned to eject a droplet of fluid into the exact centre of each well, a target typically specified as 5μm by 5μm. This is possible, but time consuming. A further concern is that the polymeric material must extend right to the edge of the well, completely coating the electrode in the base of the well. Failure for this to occur will most likely lead to the 'roof electrode contacting the electrode beneath the polymeric material, the resultant short-circuit making the component element inoperable.

Conventional methods of inkjet formed electronic components, such as OLED display screens, have generally relied upon forming an initial array of wells on the substrate through photolithographic processes. A layer of polyimide or photoresist may be deposited over a substrate and subsequently a grid of barrier structures formed by laser-ablation, photo-etching or the like. However, at the nanometre scale the substrate may be volatile due to thermal or mechanical vibration and interactions with the fluids deposited. Deformations of the substrate will be transferred to the barrier structure, thus producing an irregular array of wells. To deposit the electronically active components in the wells an inkjet addresses the substrate by moving in parallel lines, depositing droplets with constant pitch. This leads to misplacement of the droplets within the wells.

Further problems occur when a multi-layer component is to be formed on a non-rigid substrate. The substrate may flex or stretch in the time between a first polymer composition is deposited and a second polymer composition is deposited. Each inkjet station must be individually controlled and adjusted. For this reason, it is extremely difficult to form displays or other printed electronics components on a reel-to-reel basis.

Whilst some efforts have been made to improve the jetting accuracy of the printhead in order to more accurately place the droplets with increasingly fine pitch, the deformation of the substrate and well structure remains a fundamental limitation on the process. Without detailed knowledge of the deformation of the substrate, and the exact location of the wells, it is impossible to fill the wells with a satisfactory level of accuracy. Achieving such knowledge would require an extremely sophisticated tracking scheme and - if practicable at all - would be expensive and time consuming. It is an object of the present invention to seek to address the above problems and to seek to provide an improved method of forming on a substrate an electronics component.

Accordingly, the present invention consists in one aspect in a method of forming on a substrate an electronics component having at least one functional electrode and a body of polymeric material in a defined physical relationship with the functional electrode, the method comprising the steps of forming an electrode on a substrate; depositing on the electrode a drop of a fluid containing polymer; and applying an electric field through use of said electrode to cause polymer to move toward said defined physical relationship with the electrode.

An ink jet printhead can be used in the deposition of one or more drops of fluid containing polymer. This technique is efficient and economic in its consumption of polymer, but may not achieve the highest degree of accuracy or uniformity. The application of an electric field then causes the polymer to move to precisely the required physical relationship with the electrode. In this sense, the method of the invention is self aligning.

According to a further aspect of the present invention there is provided a method of forming a component for use in printed electronics said method comprising the steps: preparing a plurality of discrete deposition locations on a substrate, and depositing first and second components by fluid-processing on said discrete deposition locations wherein fluid-processing comprises the steps: a) supplying the first component in a fluid, b) contacting the fluid containing the first component with the deposition locations, c) attracting at least a proportion of the first component to a selected deposition location, d) removing fluid and any unfixed first component, e) supplying the second component in a fluid, f) contacting the fluid containing the second component with the deposition locations, g) attracting at least a proportion of the second component to a selected deposition location, h) removing fluid and any unfixed second component.

The first and second components may be dissolved or dispersed in the fluid and may be a polar component or more preferably a non-polar component. The first and second components may be attracted to the selected deposition locations by the application of a uniform or more preferably non-uniform electric field.

The field is preferably continually applied whilst the fluid and unfixed component is removed.

Fluid may be contacted with the deposition locations by placing the substrate comprising the deposition locations in a bath containing the fluid. Preferably the fluid is contacted with the selected deposition locations by depositing the fluid from a droplet deposition apparatus such as an inkjet print head or pippette. The deposition location may be an electrical track deposited by any known means, the deposition location may be used in the application of the non-uniform or uniform electric field. The tracks may be the part of the driver circuit that drives the LEPs.

According to a still further aspect of the present invention there is provided a method of forming a component for use in printed electronics, comprising the steps of providing a substrate and forming at least one functional element on the substrate by droplet deposition of polymer in a fluid and positioning polymer through dielectrophoresis or electrophoresis.

Dielectrophoresis (DEP) may be used as a method for depositing neutral particles. It is defined as the motion of neutral, polarisable matter produced by a non-uniform electric field. DEP is distinguished from electrophoresis, which is the motion of charged particles in a uniform electric field.

The motion induced by DEP lies in the fact that a net force can arise upon neutral particles lying in a non-uniform electric field. The force may be thought of as arising from an imaginary two-step process of (1) alignment of an electric dipole in a particle placed in the electric field and (2) unequal forces on the ends of that dipole. The force of an electric field on a charge is equal to the amount of the charge and to the local field strength at that charge. Since the two equal charges of the induced dipole of the particle lie in unequal field strengths of the diverging field, a net force arises. When the particle is suspended in a fluid, the ability to polarise the fluid enters into the equation. If, for example, the particle is more polarisable than the fluid, then the net force is such as to impel the particle to move towards the region of greater field strength. If, for example, the fluid is more polarisable than the particle, then the net force acting on the particle is such as to impel the particle to move away from the region of greater field strength.

The effect is independent of the direction of the field and hence rapidly alternating fields can induce unidirectional motion in a neutrally charged particle.

A spherical particle of radius r and complex permittivity ξ_p, suspended in a fluid of absolute complex dielectric permittivity ξ_m experiences a net force due to DEP that may be written as:

FDEP = 2πr³ξ_mα_rVE²

Where VE² = gradient of the square of the electric field (root mean square value) quantifying the non-uniformity of the electric field. V = del vector operator

OCr= real component of the Clausius-Mossoti factor; the effective polarisability of the particle with respect to the suspending fluid

In the above expression: Re = "the real part of "

c _P- c ^m = the complex dielectric properties of the particle and its medium,

with £" * = £- \ σ / ω, in which: J =VTJ £ = permittivity σ = conductivity ω = angular frequency of the applied field (ω = 2πf) For a vacuum, which has no mobile charges, £ = 1 , and, for a conductor S = infinity.

£*v- £ * ■» can be positive or negative, depending on the relative magnitudes of £*? and £ *m , controlled movement from and to areas of high electric field strengths is possible. In other words, a material with a higher dielectric constant will experience a force tending to move it to a stronger electric field, displacing a material with a lower dielectric constant in the process. The field E appears as VE² in the above equations and hence reversing the bias does not reverse the DEP force. It is possible to employ AC voltages in the range from 50Hz to 500 MHz. The factor α_r is frequency dependent. At low frequencies polarizability is mainly determined by the conductivity and at high frequencies by the permittivity. The present invention will now be described, by way of example only, with reference to the following drawings in which:

Figure 1 depicts a display formed from solution processing by an inkjet printer

Figures 2a to 2e depict a known method of forming a display using an inkjet printhead.

Figure 3 shows an isometric view of droplets of fluid containing polymer being deposited on electrodes formed on a substrate according to a first embodiment of the present invention.

Figure 4 shows an isometric view of the electrodes, having been coated by polymer, according to a first embodiment of the present invention.

Figure 5 is an isometric view showing an electrical component following laser ablation of the polymer coating according to a first embodiment of the present invention. Figures 6a to 6g depict a method of forming an electrical component according to the present invention.

Figures 7a to 7h depict a further method of forming a display according to the present invention.

Figures 8a and 8b depict a further method of forming a component.

Figure 1 depicts a display formed from solution processing by an inkjet printer. A clear substrate 2 is formed with a plurality of barrier elements 8a, 8b, 8c, 8d. The barriers are typically formed of a photoimageable polymer or may themselves be deposited from an inkjet printer. The barriers define wells into which the active components are deposited. The first active components consists of a lower, transparant, electrode 4, for example Indium Tin Oxide (ITO). This electrode acts as the active electrode.

A hole injecting layer 6 is deposited on to the ITO. This is formed of poly(3,4 - ethylene dioxythiophene) / poly (styrenesulphonate) (PEDOT: PSS). On to the layer of the hole injecting material is deposited differing light emitting polymers 12,14,16, each of which emits light of a different colour.

A ground electrode 10 is deposited over the light emitting polymers. This layer also provides a protective function. The electrodes are driven by a driver circuit (not shown) which may be passive or active.

The inkjet printer (not shown) preferably has mechanical actuators formed of piezoelectric material. Bubblejet printers, which use a high heat to eject fluid, can damage the fluid ejected. The barriers prevent bleeding of the light emitting polymers but can use up to 30% of the available substrate area. This affects the maximum amount of light that may be emitted.

Figures 2a to 2e depict a known method of forming a display using an inkjet printhead. Figure 2a depicts a substrate 2 onto which a plurality of fluid droplets are ejected. As in Figure 1 , barriers are formed that define wells to receive the droplets. At the base of each well is a transparent electrode. The droplets are ejected with pitch w; the distance between the path of the leading droplet and the barrier is depicted as x. The distance x must be carefully controlled in order that the well is overfilled with fluid as shown in Figure 2b. Following carefully controlled evaporation a layer of polymeric material remains, covering the base of the well as shown in Figure 2c. The roof electrodes form a good electrical contact with the polymer.

Figure 2d shows a well following droplet deposition where the distance x was too large. Following evaporation a portion of the base of the well remains uncovered and the roof electrodes contact the base electrode, resulting in a short-circuited pixel as shown in Figure 2e.

Figure 3 shows an isometric view of droplets of fluid containing polymer being deposited on electrodes formed on a substrate according to a first embodiment of the present invention. A plurality of elongate electrodes is formed side-by-side in a transverse array direction. The electrodes are grouped in closely-spaced pairs, with the distance between subsequent pairs substantially greater than the distance between the two electrodes in a pair. The printhead moves along the length of each electrode pair, depositing train of droplets that covers the electrode pair with polymer containing fluid.

Unlike the conventional method - as depicted in Figures 1 and 2 - placement errors in both the timing direction (as defined by successive droplets in a single scan of the printhead) and transverse array direction (perpendicular to the scan direction) may be tolerated. The electrodes are thus covered with a layer of fluid. An alternating voltage is then applied to the ends of the pairs of covered electrodes, resulting in the polymer being attracted to the surface of said electrodes by dielectrophoresis. The carrier fluid is carefully evaporated, leaving a covering layer of polymer over the electrodes as shown in Figure 4. Advantageously, this may occur at least partially whilst the alternating voltage is applied. The resulting structure then undergoes laser ablation to separate the polymeric material into islands, thus forming individual electronic elements. The laser moves in the transverse direction, and is of sufficiently low power to vaporise the polymer whilst leaving the electrodes substantially unaffected. The end connections of each pair of electrodes are joined together so that the same signal may be sent to both of the electrodes. Ground electrodes are then deposited over the top of the component to allow each element to be individually addressed as shown in Figure 5.

Figures 6a to 6g depict a method of forming an electrical component according to the present invention. Figure 6a depicts an end view of a substrate with a plurality of conductors formed thereupon. The conductors are grouped in pairs; in this embodiment, the pairs are further grouped into R, G and B groups.

Figures 6a to 6g depict a method of forming an electrical component according to the present invention.

Figure 6a depicts an end view of a substrate with a plurality of elongate conductors, similar to those shown in Figures 3 to 5, formed thereupon. The conductors are grouped in pairs; in this embodiment the pairs are further grouped into R, G and B groups corresponding to groups of Red, Green and Blue pixels in a display component manufactured according to the present invention.

Figure 6b depicts fluid carrying sacrificial coating material being applied to the G and B electrode pairs by droplet deposition using an inkjet printhead. The printhead scans along the length of the elongate electrodes in a similar manner to that shown in Figure 3.

An alternating voltage is applied to the ends of the electrode pairs, attracting the sacrificial coating material to said electrodes by DEP. The fluid carrying the sacrificial coating material is carefully evaporated. This may advantageously occur during at least a portion of the time when the alternating voltage is applied. A thin coating of sacrificial material remains on the R electrode pairs as shown in Figure 6c.

Figure 6d depicts fluid carrying red wavelength light emitting polymer being deposited on the R electrode pairs by inkjet printing. An alternating voltage is applied to the R electrodes as before and the red wavelength LEP is attracted by DEP to the electrode pairs. Figure 6e shows the component with the R electrodes thus covered by red wavelength LEP.

Optionally, a sacrificial coating may be also be deposited over the red wavelength LEP by droplet depostion in fluid and subsequent evaporation of the fluid carrier and attraction of the coating material by DEP. Such a process is illustrated in Figure 6f.

The sacrificial material coating the G and B electrodes is then removed by solvent washing as shown in Figure 6g. The resulting formation may then undergo further deposition and coating steps with sacrificial layers applied to electrode pairs not intended to be coated. Such a process may be repeated numerous times, thus ^""creating a complermultilayered structure.

A suitably engineered ionic fluid, which dissolves only the sacrificial coating material with little effect on the LEP, may be advantageously used for the washing step.

Figures 7a to 7e depict a further method of forming a display according to the present invention.

Figure 7a depicts a substrate 2 onto which a plurality of independently addressable electrical tracks or points are provided 20a, 20b, 20c. The electrical tracks or points are formed of ITO and will, for the purpose of clarity, be called deposition locations for the remainder of the description for this example. Hole injecting layers 22a, 22b, 22c are formed on the deposition locations by ejection from a drop-on-demand inkjet printer and subsequent coating utilising electrophorectic deposition or diθlectrophoretic deposition. The process of dielectrophoretic deposition will be described in greater detail with respect to Figure 8(c).

The base 2, deposition locations 20a to 20c and hole injecting layers 22a to 22c are covered with fluid containing light emitting polymer by droplet deposition. A voltage is applied to selected deposition location 20a to create a non-uniform field. Polymer is attracted through DEP to the selected deposition location. The light emitting polymer forms a layer 24 as in Figure 7c. Any unfixed polymer is washed from the surface and the attracted polymer is encapsulated by a protective, sacrificial layer 26 as in Figure 7d formed by droplet deposition and, optionally, coating by DEP. The surface of the substrate remote from the deposition location 20a may also be coated with a sacrificial protective layer (not shown) which is removed when the unfixed polymer or fluid is washed from the surface of the deposition location 20a.

A second light emitting polymer applied in fluid to the base by droplet deposition and a voltage is applied to a selected second deposition location 20b to create a nonuniform electric field. The deposition location attracts polymer by DEP and forms a layer 24b, as in Figure 7e. Unfixed or attracted polymer is washed from the surface and the attracted polymer is encapsulated by a protective sacrificial layer 26, formed as before. The surface of the substrate remote from the deposition location 20b may also be coated with a sacrificial protective layer (not shown, with the exception of layer 26) which is removed when the unfixed polymer or fluid is washed from the surface of the deposition location 20b

The steps are repeated for a third light emitting polymer 24c in Figure 7g.

Advantageously, the polymer material will, under application of a non-uniform electric field, form a self-limiting layer thickness.

Smaller amounts of liquid and polymer may be used and the application of the non- uniform electric field controls the polymer on the deposition location, which is the location provided with the field, and allows for a greater degree of freedom with regard to the drying of the solvent. The attraction of the polymer to the deposition location prevents the "coffee-ring" phenomena observed in the prior art.

Figure 8a depicts a further method of depositing material by DEP. Two parallel tracks 24a(1) and 24a(2) are provided between which an alternating field is applied. As discussed earlier the direction of the field does not affect the direction in which a particle moves - it moves continually towards the higher field.

The tracks or deposition locations are placed in contact with a fluid containing the particles to be deposited and the field applied. The particles are attracted to the deposition locations as the area of the highest field.

Once the particles have been fixed and any excess fluid removed the tracks are bridged together to form a single driving electrode 28 and driven simultaneously to apply a field between the driving electrode and a ground electrode 30 to luminesce the polymer 29. Appropriate materials for deposition by DEP are known as dendrimers, also known as "snowflakes". These are branched molecules having a core connected to a plurality of dendrons. The dendrons are selected to provide a functionality based on its ability to be deposited by DEP. Other dendrimers can be added that aid dispersing of the core in the fluid. Whilst the present invention has been described with regard to displays other printed electronics components are also envisaged as being recipients of the benefit of the invention e.g. the manufacture of transistors, TFTs and other electrical components. Those skilled in the art of fluid processable materials will realise that the inventive method described herein may be used with all such materials, enabling the formation of complex, multi-layer components. The inherent self-aligning of subsequent layers ensures that even with volatile substrate or polymer layers, adjacent layers are correctly aligned.

Polar material may be deposited on the deposition locations by a process of electrophoretic deposition.

Claims

1. A method of forming on a substrate an electronics component having at least one functional electrode and a body of polymeric material in a defined physical relationship with the functional electrode, the method comprising the steps of forming an electrode on a substrate; depositing on the electrode a drop of a fluid containing polymer; and applying an electric field through use of said electrode to cause polymer to move toward said defined physical relationship with the electrode.

2. A method according to Claim 1 , wherein said electrical field has a maximum at the electrode and wherein said polymer moves through dielectrophoresis.

3. A method according to Claim 1 , wherein the polymer is polar and moves through electrophoresis.

4. A method according to any preceding claim, wherein the fluid is applied to the electrode from an ink jet printhead.

5. A method according to any preceding claim, wherein two electrode parts are formed on the substrate, and wherein the electric field is applied by the application of a potential across the electrode parts.

6. A method according to Claim 5, further comprising the step of electrically connecting the electrode parts to form said functional electrode.

7. A method according to any preceding claim, wherein the polymer is a light emitting polymer.

8. A method according to any preceding claim, further comprising the step of removing excess fluid.

9. A method according to any preceding claim of forming on a common substrate an array of electronics components.

10. A method according to Claim 9, a first set of said components having a body of a first polymeric material and a second set of said components having a body of a second, different polymeric material.

11. A method according to Claim 9 or Claim 10, further comprising the step of dividing moved polymer to form a plurality of bodies of polymeric material in respective components.

12. A method according to Claim 11 , in which the polymer is divided by laser ablation.

13. A method of forming a component for use in printed electronics said method comprising the steps: preparing a plurality of discrete deposition locations on a substrate, and depositing first and second components by fluid-processing on said discrete deposition locations wherein fluid-processing comprises the steps: a) supplying the first component in a fluid, b) contacting the fluid containing the first component with the deposition locations, c) attracting at least a proportion of the first component to a selected deposition location, d) removing fluid and any unfixed first component, e) supplying the second component in a fluid, f) contacting the fluid containing the second component with the deposition locations, g) attracting at least a proportion of the second component to a selected deposition location, h) removing fluid and any unfixed second component.

14. A method according to Claim 13, wherein the first and or second components are attracted to the selected deposition location by applying an electric field extending to the selected deposition location.

15. A method according to Claim 14, wherein the first and or second components are polar.

16. A method according to any one of Claim 13 to Claim 15, wherein the first and or second components are dissolved or dispersed in the fluid.

17. A method according to Claim 13, wherein the first and or second components are attracted to the selected deposition location by applying a non-uniform electric field to the selected deposition location.

18. A method according to Claim 17, wherein the first and or second components are non-polar.

19. A method according to any one of Claim 17 or Claim 18, wherein the first and or second components are dissolved or dispersed in the fluid.

20. A method according to any one of Claim 13 to Claim 19, wherein the fluid is applied to the deposition locations from a droplet deposition apparatus.

21. A method according to any one of Claim 13 to Claim 20, wherein the first and second components are polymeric.

22. A method according to Claim 21, wherein the polymeric material is a light emitting polymer.

23. A method of forming a component for use in printed electronics, comprising the steps of providing a substrate and forming at least one functional element on the substrate by droplet deposition of polymer in a fluid and positioning polymer through dielectrophoresis or electrophoresis.

24. A printed electronics component manufactured by a method according to any preceding claim.

25. A component according to Claim 24, comprising a transistor or a capacitor.

26. A component according to Claim 24, comprising a display device.