Abstract:
A method and apparatus for forming a partition layer on a substrate by imaging a thermal or laser transfer medium onto the substrate using a imagewise controlled radiation source. A matrix of partition cells are created on the substrate that can, in a further step, be selectively filled with fluid or ink using an inkjet printing technique. The partition layer, the substrate, and fluids to be deposited are selected so that the fluids wet the substrate but not the partitions, preventing bending between adjacent cells.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
         [0001]    Not applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED R&amp;D  
         [0002]    Not applicable  
         REFERENCE TO MICROFICHE APPENDIX  
         [0003]    Not applicable  
         FIELD OF THE INVENTION  
         [0004]    The present invention pertains to processes involving the deposition of one or more fluid materials onto a substrate, especially in those cases where the accurate positioning of the fluids on the substrate is important, or where blending or coalescence between discrete fluid droplets or pools is undesirable. Examples of items with manufacturing processes that may benefit from this invention include color filters for liquid crystal displays (LCD), organic light emitting diode displays (OLED), and liquid semi-conducting polymer devices.  
         BACKGROUND OF THE INVENTION  
         [0005]    There are many industrial processes where accurate or controlled deposition of fluid substances is desirable. While this is true for the printing industry, especially with respect to inkjet printing, it also applies to other manufacturing processes, particularly in the electronics and microelectronics industry. Examples include processes involved in the manufacture of display screens and other electronic components.  
           [0006]    Recent advances in inkjet printing technologies have vastly improved inkjet printing as a means for the precise deposition of liquid droplets. Because inkjet printing makes it possible to deposit very small measures of fluid very accurately and cheaply, and because inkjet deposition typically requires fewer steps and less equipment than conventional methods such as photolithography, it is a very attractive candidate for many precision-manufacturing applications outside of conventional printing. Outside of the graphic arts, examples of proposed applications of inkjet printing include manufacture of color filters for flat panel displays, manufacture of OLED devices, deposition of microlenses on the tips of optical fibers, deposition of conductive ink in the manufacture of circuit boards, and the manufacture of 3-dimensional objects.  
           [0007]    In many of the proposed applications, the precise placement of the fluid substance, whether ink or polymer or other material, is critical. In the case of color filter manufacture for conventional flat panel displays or OLED devices, many such precisely placed fluid droplets must be placed very close to other such droplets without coalescing. This is particularly important in situations in which different kinds of fluids are being deposited on the same substrate, such as differently colored inks on a color filter substrate. In the case of color filters, it is vital to the quality of the filter to prevent blending between discrete ink droplets, as differently colored filter elements are typically situated adjacent to one another on a clear substrate, separated only by a minute barrier on the order of some microns across. In the case of organic light emitting diodes (OLED) and semiconducting polymers, the actual active device materials also need to be deposited with great accuracy and circumspection in order to ensure that the various sections of the devices remain well delineated.  
           [0008]    The use of fluid droplet deposition techniques, such as inkjet, for precision manufacturing applications, however, does present a number of difficulties. Firstly, inkjet technology typically has a spatial accuracy of approximately 40 microns while the display screen and semiconductor industries require accuracies at least an order of magnitude better. Secondly, there are particular problems with respect to controlling the position of the fluids once they have been deposited upon a substrate.  
           [0009]    For example, in the case where the fluid material is intended to adhere to the substrate, problems arise because the ability of the fluid to adhere to a surface is related to its ability to wet a surface, and the fact that a fluid capable of wetting a surface will also tend to spread upon that same surface. This can cause undesirable effects such as coalescence of adjacent droplets as well as blending of the fluids in adjacent droplets. Droplet spreading can also lead a non-uniform thickness of the layer on the substrate, as the fluid droplet layer may tend to be thinner at its boundaries when it spreads.  
           [0010]    In applications where it is necessary to deposit a pattern of one or more fluid substances that need to be contained within specific areas, the wetting of the substrate will prevent accurate and uniform placement of droplets, as well as non-uniform thicknesses in layers created with the fluid droplets. At the same time, the droplets are generally required to adhere well to the substrate upon which they are deposited.  
           [0011]    One area in which the specific aforementioned problem can be an issue is the manufacture of color filters for flat panel displays. Manufacturers are faced with a stringent requirement for the accurate placement of fluid substances without being able to tolerate any coalescence or blending between fluid droplets from adjacent patterned cells. Filter layer thickness and the distribution of colorant in the layer are also important to ensure that each colored region transmits light uniformly.  
           [0012]    The color filter of a liquid crystal display is typically fabricated with alternating cells of red, green, and blue material deposited on a transparent substrate at the same matrix spacing as the panel. The colored materials act as filters and therefore transmit light. To enhance contrast, a light-shielding region, known as a black matrix, usually demarcates the color cells. This black matrix typically has an optical density of greater than 3.0 in order to adequately block stray light from adjacent cells. It is also used to mask non-emitting areas such as LCD cell electrodes.  
           [0013]    Many different methods have been used or proposed for forming the partition walls. Typically, photolithography is used. However, other methods such as electrodeposition, screen offset printing, gravure printing, flexographic printing, and inkjet printing have also been suggested in the art.  
           [0014]    Most of these methods suffer from a number of deficiencies. In general the very precise methods are expensive and complicated, and often require additional processing steps or expensive equipment. The printing based methods tend to be cheaper, but tend to suffer the fact that the partitions are being made by applying a liquid to a surface, with the same attendant problems of liquid spreading and coalescence. Conventional manufacturing techniques like photolithography are costly due to the need for complex processes like vacuum chamber deposition as well as the multiple steps which are required to produce a finished color filter. On the other hand, currently available alternative methods are either just as complex, or lack the required precision. The resulting high fabrication costs have thus prevented flat panel displays from replacing conventional CRT displays in applications where cost is a primary concern. This has happened despite their many desirable characteristics, such as compact size and low radiation emissions.  
           [0015]    Other types of display devices, based on OLEDs, have requirements slightly different from conventional color filters. For example, because OLEDs are light emitting, the optical density required for the partition walls may not be as severe and hence controlling the thickness of the fluid layers may be less important. However, accurate placement is still needed. For example, an OLED display may be made up of OLED cells of different colours, which again would require that closely placed fluid droplets be prevented from mixing or coalescing.  
           [0016]    A number of methods of addressing this problem have been proposed, and the concept of using the surface energy characteristics of the fluids, substrates, and the partition barriers to control the placement of inkjetted droplets has specifically been suggested. Again, as with previously mentioned approaches to fabricating partition walls in general, the various approaches proposed for producing a desirable pattern of surface energy variations on a substrate of a colour filter have tended to be either expensive or complex. Additionally, they frequently require many processing steps, including post-processing steps, or lack the precision required for the manufacture of devices such as color filters for flat panel displays or OLED displays.  
           [0017]    The incorporation of conventional printing techniques has been proposed in various approaches, as the conventional printing industry has extensive experience with methods to produce large numbers of precisely patterned objects at low cost. However, the use conventional printing techniques requiring the use of liquid ink to produce precisely patterned surface energy variations on a substrate suffers the same problems mentioned earlier with regard to adhesion, wettability, and liquid spreading.  
           [0018]    One well-known printing technique that does not involve the deposition of a fluid in order to create a precise pattern on a surface is thermal transfer. This technology is employed extensively in the printing and imaging field for the purposes of proofing. In applying this technology to the filter, OLED and polymeric semiconductor devices addressed here, it has the benefit of not employing liquids that could spread or coalesce.  
           [0019]    In thermal transfer, a donor material layer is disposed on a carrier sheet. This medium is then either placed in close proximity with a substrate or in contact with that substrate, such that the donor layer faces the substrate. The donor layer is then transferred from the carrier sheet to the required areas of the substrate by a variety of means. These include illumination with a laser or another thermal source. When a laser is employed in this fashion, the process is referred to as laser induced film transfer.  
           [0020]    The mechanism of transfer varies from essentially explosive heating of the interlayer between the donor and carrier to detach a minute amount of donor material, to phase change processes. When a phase change is employed, the process is known as phase change transfer. All the mechanisms have the net result of transferring donor material in a precisely defined fashion from the carrier sheet to the substrate on which the image or pattern is required. Provided a well-defined laser-beam is used, images or patterns may be imagewise written onto the carrier to transfer the donor material imagewise to the substrate. This technology provides a method to obtain very well-defined edge structures and is inherently capable of the precision required for the devices discussed here.  
           [0021]    Unfortunately, there has to date been no simple method proposed for incorporating thermal transfer into an inkjet-based method for manufacturing colour filters, OLED devices or semiconducting polymer devices.  
         OBJECTS AND ADVANTAGES OF THE INVENTION  
         [0022]    It is a principle object of the present invention to provide a method for fabricating a surface comprising a pattern of varying surface energy that is useful in any device fabrication application where a fluid substance is to be accurately deposited onto a substrate.  
           [0023]    A more particular object of the present invention is to provide a method for fabricating a partition layer that is useful in any the manufacture of color filters for liquid crystal displays (LCD) and in the manufacture of organic light emitting diode displays (OLED) and liquid semi-conducting polymer devices.  
           [0024]    Yet another object of the present invention is to provide a method for fabricating a partition layer using materials which are commonly available from graphic arts media vendors.  
           [0025]    Yet a further object of the present invention is to provide a method for forming a partition pattern on a substrate using imaging techniques well established in the area of graphic arts industry.  
           [0026]    A further object of the present invention is to provide a simple method for fabricating on a surface a pattern to partition liquids such that the pattern requires no post-processing to impart the desired surface energy characteristics.  
           [0027]    It is another object of the present invention to allow fabrication of color filters, OLEDs, and other devices whose manufacture may require the precise deposition of fluids using transfer media that are substantially similar to the media used extensively in the graphic arts industry.  
           [0028]    A further object of the present invention is to allow the use of commonly available imaging systems in order to implement the methods of the invention, which once again has the advantage of reducing costs.  
           [0029]    Yet a further object of the invention is to allow for a simple two-step process with no required wait time between steps, requiring no post processing of the deposited partition material.  
           [0030]    Another object of the present invention is to provide a two-step process that may advantageously be performed on the same imaging platform.  
           [0031]    The method disclosed in this application for letters patent has several advantages over conventional methods of forming partition layers  
           [0032]    Beyond the obvious advantages provided by the attainment of the above objects, a further significant advantage of this approach is that it offers the advantage of economy of scale and results in a lower cost than would be achievable with a fully customized process. It also allows the process to benefit from developmental efforts for a larger market.  
         SUMMARY OF THE INVENTION  
         [0033]    A method for forming a partition layer on a substrate by imaging a thermal or laser transfer medium onto the substrate using a imagewise controlled radiation source is described. A matrix of partition cells is created on the substrate that can, in a further step, be selectively filled with fluid. The selective filling of partition cells can be accomplished using an inkjet printing technique. The partition layer, the substrate, and fluids to be deposited are selected so that the fluids wet the substrate but not the partitions, preventing unwanted interactions between fluids in adjacent cells. An apparatus for implementing this method is also described. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]    [0034]FIGS. 1 a - 1   d  show the process steps of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0035]    FIGS.  1 - a  through  1 - d  show cross sectional views of the process for forming partition barriers on a substrate according to the preferred embodiment of the present invention. It is to be understood that the present invention is executed in at least two dimensions to create a two-dimensional pattern to be used to control the placement of fluids. In general the material being deposited may have any desired surface energy to affect the subsequent placement of liquid droplets on the surface on which the material is deposited. In the preferred embodiment of the present invention, the invention is demonstrated at the hand of a low surface energy material that is transferred to a surface.  
         [0036]    In FIG. 1- a  a media donor sheet  10 , comprising carrier layer  11  and a low surface energy partition material layer  12  initially adhering to carrier layer  11 , is positioned above a substrate  13 . In the preferred embodiment of the present invention the transfer media fixture (not shown) serves to maintain donor sheet  10  in position above substrate  13 .  
         [0037]    In an alternative embodiment of the present invention, the use of phase change media requires that donor sheet  10  be maintained in contact with substrate  13 .  
         [0038]    Substrate  13  can be glass, chromium, or any other material to which the low surface energy partition material of layer  12  will adhere. Substrate  13  is mounted on a stage (not shown) that may be fixed or movable in one or more dimensions. Imagewise controllable radiation source  20  is attached to a translation stage (not shown) that can be translated over the area of the substrate. By way of example, the translation stage may be similar to those employed in flatbed printers.  
         [0039]    In an alternative embodiment of the present invention the imagewise controllable radiation source is affixed to a non-translating fixture.  
         [0040]    In yet a further embodiment of the present invention, transfer of the low surface energy partition material of layer  12  is performed not via laser or on a point-by-point basis, but rather by using area illumination through a mask that has been provided with the pattern desired.  
         [0041]    Returning to the preferred embodiment of the present invention, the radiation from imagewise controllable radiation source  20  is modulated in response to a data source (not shown) that determines the partition pattern to be formed. In the preferred embodiment of the present invention the lasers are switched on and off in response to the data source. The depth of modulation and functional form of the modulation may be adapted to suit the particular low surface energy partition material being used. On exposure to the radiation from imagewise controllable radiation source  20 , the low surface energy partition material of layer  12  is transferred to substrate  13  in the areas where the radiation impinges on donor sheet  10 .  
         [0042]    In selecting materials for the substrate, partition material and the fluid to be deposited as part of the preferred embodiment of the present invention, careful attention is paid to the relative surface energy of the materials as this determines the degree to which the fluid will wet the substrate and the partition material. The selection requirement is that the surface energy of substrate  13  is chosen to be greater than the surface energy of the fluid to be deposited on it, which should in turn be greater than the surface energy of the material of low surface energy partition material layer  12 . Under these conditions, the fluid adheres to the substrate but does not spread past the partition layers.  
         [0043]    An example of a thermal transfer medium that demonstrates properties for application in the preferred embodiment of the present invention, is available from Imation Corp of Oakdale Minn., and is sold under the trade name Imation Matchprint Digital Halftone Proofing System. The Imation Matchprint medium works through a laser induced thermal film transfer. The laser transfers the pigment as a film from the donor sheet  10  to substrate  13  resulting in clean edges.  
         [0044]    Another example of a similar medium is Dupont WaterProof®, available from Dupont Company of Wilmington, Del. This medium works slightly differently in that the donor sheet  10  and substrate  13  are in closer contact and the laser effects a phase change transfer, melting the low surface energy partition material layer  12 , which then sticks to the receiver layer.  
         [0045]    Other transfer media are available which operate through a laser ablation transfer where the donor undergoes an explosive decomposition and propulsion from donor to receiver. These commercially available media are primarily targeted at the color proofing market for the printing industry. Typically proofing media are available in a multitude of colors and a number of different formulations to suit various imaging devices. These media, with little customisation, can be used in forming partition layers on a substrate for applications that require accurate placement of fluids. In order to form good partition layers, the medium is not wettable by the fluid to be contained in the partition. At present, inks used in color filter production typically have a surface tension of the order of 29 mN/meter and typically include butyl carbitol acetate as solvent. The medium chosen should also be chemically stable in the presence of the solvent. Furthermore, the medium has sufficient exposure sensitivity to allow it be imaged using conventional imaging methods.  
         [0046]    A suitable imagewise controllable radiation source for exposing the transfer media and creating the partitions is the SQUAREspot™ thermal imaging head, produced by CreoScitex of Burnaby, British Columbia, Canada. The SQUAREspot™ thermal imaging head is used in many areas of graphic arts imaging including digital proofing and is capable of imaging at a resolution up to 5080 dots per inch, with 20W of available imaging power divided into more than 200 independent channels. The radiation source is a laser diode bar with a wavelength of 830 nm. This imaging head is particularly suited to imaging features with very clean edges due to the substantially square profile of the laser beams it produces. Advantageously the SQUAREspot™ thermal imaging head is available with a beam size appropriate for writing the partitions, The CreoScitex imaging system is available with a 5 um spot size that is well suited to form these thin partitions.  
         [0047]    In the present application for letters patent the term “imagewise controllable radiation source” is used to describe any radiation source that is imagewise controllable and which controllably emits radiation capable of transferring a medium imagewise from a carrier sheet to a substrate.  
         [0048]    Referring now to FIG. 1- b,  the imaging is partially completed and partition walls  2  have been deposited forming a cell  3  on the substrate  13 . FIG. 1- c  shows the imaging completed. At this stage a 2-dimensional grid partition pattern will have been formed.  
         [0049]    Referring now to FIG. 1- d,  on completion of imaging, the donor sheet is removed and the substrate  13  is ready to have fluid droplets  5  deposited into the cells forming areas of fluid  21 . Fluid  21  may comprise a colorant such as a pigment or dye, or may be another type of fluid such as the active materials employed in making an organic light emitting diode (OLED) or a liquid semiconducting polymer. In the present application for letters patent, the term active materials is used to describe materials that play an active role in the functioning of an optoelectronic or electronic device. The roles include, but are not limited to, the formation of the p-n junction of a diode, the emission of light upon stimulation and the creation of free electrons in response to the absorption of photons. It is also clear that a plurality of different fluids may be deposited in droplet form in the same cell to create more complex structures or mixtures for electronic devices.  
         [0050]    In the present application for letters patent the term electronic device is used to describe all devices of which the operation is based on the manipulation or behaviour of electrons, including specifically, but not limited to, those that absorb or emit light. This in particular includes all diodes, including specifically light emitting devices, and any optical filters, such as those employed by liquid crystal displays or organic light emitting devices (OLED).  
         [0051]    In the preferred embodiment of the present invention a translation stage typical of commercial flatbed plotters is used to allow the imaging head to move in one axis whereas the substrate moves on an orthogonal axis, thus allowing the entire surface to be traversed. In addition, in the preferred embodiment of the present invention, an integrated imaging unit is used, such that the imagewise controllable radiation source  20  and fluid deposition unit  4  share the same translation stage. This combination allows the step of imagewise transferring the partition medium and the step of depositing fluid to be performed in succession on the same platform. In the preferred embodiment of the present invention, fluid deposition unit  4  is an inkjet head.  
         [0052]    In an alternative embodiment, the imaging arrangement may consist of separate translation stages. It is clear to those skilled in the art that different arrangements may be employed to achieve the relative translation of the substrate and imagewise controllable radiation source  20 . Similarly, a variety of arrangements to obtain the relative translation of substrate  13  and fluid deposition unit  4  are known in the art.  
         [0053]    A particular alternative embodiment employs a flexible substrate mounted on a cylinder or drum. The cylinder rotates while the imagewise controllable radiation source traverses across the length of the cylinder. The motion of the imagewise controllable radiation source may be continuous, such that it writes a spiral swath around the cylinder, or it may be stepped, such that it writes circular swaths. These mechanical arrangements are well-established in the printing industry and will not be discussed in further detail here. The same arrangement may be applied to fluid deposition unit  4 . Fluid deposition unit  4  and imagewise controllable radiation source  20  may also be mounted on the same stage.  
         [0054]    In an alternative embodiment of the present invention, the fluid is deposited by a process other than ink-jetting. By way of example, the fluid may be deposited by a syringing method. In yet another example the fluid may be deposited by passing the substrate containing the partition cells through a bath of the fluid.  
         [0055]    It should be understood that the above descriptions of the simple and preferred embodiments are intended for illustrative purposes only, and are not intended to limit the scope of the present invention in any way. Those skilled in the art will appreciate that various modifications can be made to the embodiments discussed above without departing from the spirit of the present invention.