FIELD OF THE INVENTION

This invention relates to the field of light emitting devices, and in particular to the transfer of optical thin films and barrier films to a light emitting structure during the manufacture of the light emitting devices.

BACKGROUND OF THE INVENTION

Light emitting devices are often composed of multiple functional elements, such as a light emitting element, a wavelength conversion element, an optical element, a protective element, and so on. These elements may be formed by applying a coating upon a formed structure, laminating a pre-formed film upon the structure, adhering a preformed cap upon the structure, encapsulating the structure, and so on.

Optical elements, such as distributed Bragg reflectors (DBRs), are often used to control the emission pattern or directionality of the light from the light emitting element, to achieve a desired color angular distribution, to enhance certain color distributions or to remove certain spectral content, as in a UV filter. A DBR is a film formed from multiple layers of alternating materials with varying refractive indices, or by periodic variation of some characteristic (such as height) of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection of an optical wave and the many reflections combine with constructive interference to act as a high-quality reflector. Other optical elements, such as anti- reflection (AR) films, may also be formed using multiple layers of material.

The layers of the DBR or AR film may be formed by vacuum deposition and by a layer-by-layer (LBL) spraying technique. It is difficult and/or expensive to coat uniform, conformal, optical films directly on planar and non-planar surfaces, and/or over low modulus materials and/or materials with large coefficient of thermal expansion (CTE), such as silicones. For example, vacuum deposition technologies are "directional" and do not conform well to non-planar substrates, especially with steep or undercutting features. In addition, vacuum deposited coatings may exhibit undesirable stresses and low adhesion, resulting in brittle and unreliable coatings.

SUMMARY OF THE INVENTION

It would be advantageous to provide a less costly or less complex technique for creating a device with a layer-by-layer film. It would also be advantageous to increase the yield associated with devices having a layer-by-layer film.

To better address one or more of these concerns, in an embodiment of this invention, a layer-by-layer film, such as a distributed Bragg reflector, an anti-reflection coating, a color filter, a barrier coat, etc. is formed on a substrate, then transferred to the surface of an underlying structure and released from the substrate. Because the layer-by- layer process cannot be used on a hydrophobic surface, such as a siliconized substrate commonly used as a release film, or on perfluorinated release polymers, such as ETFE, the substrate may be modified by casting a releasable film containing charged functional groups, or non-charged ones, but capable of generating charged groups by gas-phase reaction (CVD, plasmas, corona discharge, ozone) or liquid-phase reactions (such as hydrolysis) after coating. Alternatively, the substrate may be provided with a sacrificial layer, such as a UV and thermal release tape. The creation of a pre-formed layer-by-layer film on a releasable substrate may be controlled to provide flexible optical films of uniform thickness and high quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: FIGs. lA-lF illustrate an example process flow for forming a light emitting device with a pre-formed layer-by-layer film on a releasable substrate.

FIGs. 2A-2D illustrate a second example process flow for forming a light emitting device with a pre-formed layer-by-layer film on a releasable substrate.

FIGs. 3A-3C illustrate a third example process flow for forming a light emitting device with a pre-formed layer-by-layer film on a releasable substrate.

Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention.

However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

Also for simplicity and clarity, the invention is presented herein using the paradigm of a light emitting device, even though one of skill in the art will recognize that the principles of this invention may be applied to the creation of other devices having an LBL film.

Layer-by-layer (LBL) techniques are often used to provide thin film coatings. The deposition of each layer of an LBL film is a process in which poly electrolytes and nanoparticles are electrostatically adsorbed onto oppositely charged substrate surfaces. The initial deposition of the polymers or nanoparticles reverses the substrate surface charge, effectively quenching further adsorption, thereby automatically controlling the thickness of the formed layer. In order to achieve a desired thickness, the process rapidly alternates between oppositely charged solutions or suspensions to create each subsequent layer. Other materials may be used to form the layers, including metals, ceramics, and biological molecules

The LBL approach may also be used to form barrier coats that protect the underlying structure, generally to prevent or minimize the diffusion of compounds that could interfere with LED longevity, such as color stability, substrate reflectivity, and device efficiency. In some embodiments, the LBL film may provide a vapor barrier layer, an oxygen barrier layer, or a barrier to volatile organic compounds (VOCs).

The first layer of the LBL film may be formed by charging the surface of the underlying structure, such as the surface of the light emitting element, then spraying an oppositely charged layer of the material forming the first layer. Thereafter, the subsequent layers are formed upon each prior applied layer, alternating the charge of each layer. Because the first layer is electrostatically adsorbed on the surface of the underlying structure, the bond between the LBL film and the underlying structure is generally secure and optically efficient. If the LBL films contain chemically functional groups, the bonds with the substrate and adjacent layers can be further strengthen by the formation of covalent bonds.

LBL deposition directly on LED devices, however, may introduce an inefficiency in the conventional fabrication flow for creating light emitting devices. In addition, coating at or near the end of the fabrication process may lead to undesirably low yields, particularly if the coating occurs before the testing of the light emitting structures, or if the coating process does not provide uniform, conformal optical films. In some situations, the upper surface of the LED device may not be compatible with the LBL deposition process, precluding its use for such devices. For example, a wavelength conversion element is commonly used within an LED device, and is often formed as a phosphor-embedded silicone layer atop the light emitting surface of the LED device. This silicone layer may be hydrophobic, whereas the LBL deposition process cannot be applied to a hydrophobic surface.

In an embodiment of this invention, the use of a pre-formed layer-by-layer (LBL) film on a releasable substrate can be expected to simplify the process of creating an LBL film on the surface of a light emitting device. It may also provide for a higher quality LBL film, in that the thickness, uniformity, and quality of the LBL film may be controlled in a process that is substantially independent of the creation of the light emitting device. Non-conformal films can be discarded, avoiding the creation of a light emitting device with a non-conformal LBL film, resulting in a higher yield. Additionally, because the LBL films are produced independent of the fabrication of the light emitting devices, the manufacture of these films may achieve an economy-of-scale and other optimizations that are not achievable with LBL films that are created directly upon the light emitting device.

One of the challenges in creating a pre-formed LBL film on a releasable substrate is the creation of a releasable substrate that is compatible with the LBL process. As noted above, the deposition of LBL films is a process in which polyelectrolytes and

nanoparticles are electrostatically adsorbed onto oppositely charged substrate surfaces. Because electrostatic self-assembly drives this process, the initial substrate requires charged moieties; accordingly, LBL films cannot be grown on hydrophobic surfaces, such as siliconized release films and perfluorinated release polymers (such as ETFE).

In an embodiment of this invention, a treated silicone sheet, such as a treated

Polydimethylsioxane (PDMS)-coated Polyester sheet, such as poly ethylene terephthalate or PET, may be used as a substrate. Because PDMS is hydrophobic, the surface of the sheet is further modified by coating a releasable film that may contain anionic or cationic groups, or a film that can be treated with a corona discharge, ozone or plasmas in order to generate anionic groups on its surface, to provide a hydrophilic (non -hydrophobic) surface layer above the hydrophobic surface. In another embodiment of this invention, a sacrificial surface layer is placed on the substrate, such as a Polyethylene terpthalate (PET) substrate. The sacrificial surface may be, for example, an adhesive tape that becomes releasable upon the application of UV or thermal energy, including Nitto Denko's Revalpha thermal release tape,

Furukawa's UV wafer dicing tape, and Terepac's Digital Release Adhesive (DRA) that releases upon application of light and heat.

FIGs. lA-lF illustrate an example process flow for forming a light emitting device with a pre-formed layer-by-layer film on a releasable substrate.

FIG. 1 A illustrates an example sheet 100 comprising a substrate 110 with a coating 120 that provides a hydrophilic surface. As noted above, the coating 120 may comprise anionic or cationic groups, or the coating 120 may be a sacrificial surface layer. For example, the coating 120 may be the aforementioned Revalpha thermal release tape placed upon a silicone or other flexible substrate 110.

In another embodiment, Shin Etsu PLF-100 or Dow Corning LF-1000 may be applied as a thin coats (-50 um) on top of a releasable liner (siliconized PET) situated on the substrate 110. Shin Etsu PLF-100 and Dow Corning LF-1000 are solvent-based silicone-backbone phenylated resins that may be cured or partially cured (crosslink) after coating to achieve mechanical stability and support. UV-curable resins, such as UV- silicones may also be used. The top surface of these films may be "activated" by plasma (02, CH3-OH, etc.) corona discharge or UV-ozone, and/or grafted (in liquid or gas phase) with functional reagents such as trimethoxy aminopropyl silane, creating a hydrophilic surface as a 'primer' to facilitate LBL deposition.

In FIG. IB, an LBL layer 130 may be formed by spraying the material forming the layers onto the hydrophilic coating 120, creating the sheet 101. The LBL layer 130 may be, for example, a Bragg reflector, a filter, an anti -reflection layer, a barrier layer, and so on. In FIG. 1C, an optional wavelength conversion layer 140 may be formed above the LBL layer. For example, a slurry of silicone containing one or more phosphors may be formed atop the LBL layer 130, then dried and partially cured (B-stage) to form a sheet 102. The thickness of the layer 140 may be controlled by passing the sheet 100 through a pair of rollers, applying a press plate, and so on. Optionally (not illustrated), a releasable protective sheet may be used to cover the partially cured silicone.

In another embodiment, the wavelength conversion layer 140 may also be preformed and adhered to the pre-formed LBL layer 130. In such an embodiment, the preformed wavelength conversion layer 140 may be partially cured, such that its surface is tacky, providing the adhesion to the LBL layer 130.

It is significant to note that although the silicone surface of the wavelength conversion layer 140 is incompatible with the LBL process, because silicone is hydrophobic, the fact that the LBL layer 130 is already formed, and no longer requires a hydrophilic surface, allows for this combination of an LBL layer 130 and a silicone-based wavelength conversion layer 140.

At this point, the sheet 102 comprises a wavelength conversion material 140 with an LBL film 130, formed above the coated substrate 110, 120. Assuming that the coated substrate 110, 120 is transparent, this sheet 102 may be tested for quality and uniformity, with defective sheets being discarded. The sheet 102 may also be tested for its wavelength conversion characteristics, and 'binned' based on the wavelength of the emitted color, for matching with similarly 'binned' light emitting devices to provide a desired combined color output, as detailed in U.S. Patent 7,344,952, "Laminating Encapsulant Film Containing Phosphor Over LEDs", issued 3 July 2008 to Haryanto Chandra, and incorporated by reference herein.

In the example of the LBL 130 forming a Bragg reflector, or other reflector, the coated substrate 110, 120 need not be transparent, and the sheet 102 may be tested by directing light toward the surface that is opposite the coated substrate 110, 120, and measuring the reflectance. Depending upon the particular process used, the sheet 102 may be sliced/diced along lines 180 to provide 'singulated' elements 103 that may be attached to a light emitting structure 160, as illustrated in FIG. ID. Alternatively (not illustrated), the sheet 102 may be applied to a plurality of light emitting structures on a tile, or other mount, then singulated when the tile is sliced/diced. If a protective sheet had been used to cover the semi-cured layer 140, it may be removed, exposing a surface of the layer 140.

Typically light emitting structure 160 is illustrated as containing a light emitting element 150 enclosed by a protective outer shell 155 in one plane, exposing a light emitting surface and an electrical connection surface. One of skill in the art will recognize that the light emitting structure 160 may take other forms, including a self- supporting light emitting element 150 without an outer shell 155. The light emitting element 150 may comprise (not illustrated) an active layer sandwiched between an n-type and p-type semiconductor.

Because the layer 140 is semi-cured, it adheres to the light emitting surface of light emitting structure 160. Subsequent curing then bonds the layer 140 to the light emitting structure 160, forming a light emitting structure 104, as illustrated in FIG. IE. If the layer 140 had been provided as a pre-formed sheet that is attached to the pre-formed LBL 130, this subsequent curing also bonds the layer 140 to the LBL 130.

At FIG. IF, the coated substrate 110, 120 is removed, leaving the LBL layer 130 of the light emitting device 105 exposed. If the coating 120 is a UV, heat, or light activated material, the appropriate UV, heat, and/or light energy is applied to release the coated substrate 110, 120 from the LBL layer 130 exposing a surface 135 of the LBL layer 130.

Depending upon the material used to form the coating layer 120, some residual amounts of the coating 120 may remain on the surface 135 of LBL layer 130. If light is expected to exit the surface 135 of the LBL layer 130, the material selected to form the coating layer 120 should be transparent to the light emitted by the light emitting device 150 and/or the light emitted by wavelength conversion layer 140. In the alternative, the coating layer 120 may be removable from LBL layer 130 without causing damage to any part of device 104. In the alternative, if the LBL layer 130 forms a reflector, as may be used in a side- emitting device, the residual coating 120 need not be transparent, and need not be removable. One of skill in the art will recognize that the residual coating 120 may be removed even in a reflector configuration, to provide optical and/or mechanical properties that are compatible with the particular application.

As noted above, it is significant to note that the light emitting device 105 includes an LBL layer 130 atop a silicone-based wavelength conversion layer 140.

Conventionally, an in-situ creation of an LBL layer 130 upon a surface would preclude the use of a silicone-based wavelength conversion layer 140, and require the use of a different material to form the wavelength conversion layer, which generally increases the cost and complexity of the manufacturing process.

It is also significant to note that the tested singulated element 103, may be applied only to tested light emitting structures 160, thereby increasing the efficiency and yield of the process.

It is also significant to note that the process for creating the LBL layer 130 is independent of the process for creating the structure 160, allowing each of these processes to be optimized independently, which typically will reduce the cost of each of these elements 130, 160. Additionally, the LBL that is independently formed may be designed to provide improved reliability, such that it will function even if cracks or other defects are created during processing.

FIGs. 2A-2D illustrate a second example process flow for forming a light emitting device with a pre-formed layer-by-layer film on a releasable substrate.

FIG. 2A illustrates a sheet 101 comprising an LBL film 130 upon a substrate 110 with a hydrophilic coating 120, as detailed above with regard to FIGs. 1 A-1B.

FIG. 2B illustrates a plurality of light emitting structures 160 with a wavelength conversion layer 140 being placed upon the sheet 101. The wavelength conversion layer may include a semi-cured silicone material that facilitates adherence to the LBL film 130 when the silicone material is cured. Alternatively, an adhesive layer may connect the layer 140 to the film 130. FIG. 2C illustrates a resultant light emitting structure 204 after singulation, and FIG. 2D illustrates the light emitting device 205 after removal of the coated substrate 110, 120. If the coating 120 is a UV, heat, or light activated material, the appropriate UV, heat, and/or light energy is applied to release the coated substrate 110, 120 from the LBL layer 130.

FIGs. 3A-3C illustrate a third example process flow for forming a light emitting device with a pre-formed layer-by-layer film on a releasable substrate. In this

embodiment, the LBL film comprises a reflector that will surround each light emitting element.

In this embodiment, as illustrated in FIG. 3 A, the LBL film 330 may be patterned on the coated substrate 110, 120 so as to produce gaps 335 in the film 330. The pattern may be produced in a UV or thermal activated tape 120 before it is applied to the substrate 110. Alternatively, an unpatterned LBL film 330 may be formed on the tape 120, and the pattern formed by selectively removing portions of the LBL-covered tape 120, or portions of the LBL film 330 on the tape 120, after it is on the substrate 110, for example, by photosensitive etching or similar process.

Light emitting elements 150 are placed in each gap, such that the reflective LBL film 330 surrounds each light emitting element 150, as illustrated in FIG. 3B. Thereafter, an optical element 310 may be formed over each light emitting element 150 and a portion of surrounding reflective LBL film 330. In this example, the optical element 310 is illustrated as a hemispherical dome, although one of skill in the art will recognize that other shapes may also be formed. The optical element 310 may include wavelength conversion material, or a separate wavelength conversion element (not illustrated) may be situated upon the light emitting element 150.

FIG. 3C illustrates a singulated light emitting device 305 after removal of the coated substrate 110, 120, using the removal techniques detailed above. The removal of the coated substrate 110, 120 may be performed before or after singulation. In embodiments of this invention, the singulation of the devices 105, 205, 305 occurs while the devices 105, 205, 305 are on the coated substrate 110, 120, but the singulation method (e.g. laser slicing/dicing) does not extend into the substrate 110, allowing the substrate 110 to be reused.

While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, it is possible to operate the invention in embodiments wherein the underlying structure is not a light emitting structure. For example, the underlying structure may include a photosensitive receiver. In like manner, the LBL layer need not be an optical element, as in the case of an LBL barrier layer that protects the underlying structure from external elements, such as vapor, oxygen, and VOCs. In this case, the underlying device may not be a light emitting or light receiving device.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.