Patent Description:
This disclosure generally relates to devices and methods used in the manufacture of light emitting devices (LEDs) for attaching phosphors to LED dies, and LEDs formed using the devices and methods.

Semiconductor light emitting diodes and laser diodes (collectively referred to herein as "LEDs") are among the most efficient light sources currently available. The emission spectrum of an LED typically exhibits a single narrow peak at a wavelength determined by the structure of the device and by the composition of the semiconductor materials from which it is constructed. By suitable choice of device structure and material system, LEDs may be designed to operate at ultraviolet, visible, or infrared wavelengths.

LEDs may be combined with one or more wavelength converting materials (generally referred to herein as "phosphors") that absorb light emitted by the LED and in response emit light of a longer wavelength. For such phosphor-converted LEDs ("pcLEDs"), the fraction of the light emitted by the LED that is absorbed by the phosphors depends on the amount of phosphor material in the optical path of the light emitted by the LED, for example on the concentration of phosphor material in a phosphor layer disposed on or around the LED and the thickness of the layer. Phosphors may be embedded in a silicone matrix that is disposed in the path of light emitted by the LED.

Phosphor-converted LEDs may be designed so that all of the light emitted by the LED is absorbed by one or more phosphors, in which case the emission from the pcLED is entirely from the phosphors. In such cases the phosphor may be selected, for example, to emit light in a narrow spectral region that is not efficiently generated directly by an LED.

Alternatively, pcLEDs may be designed so that only a portion of the light emitted by the LED is absorbed by the phosphors, in which case the emission from the pcLED is a mixture of light emitted by the LED and light emitted by the phosphors. By suitable choice of LED, phosphors, and phosphor composition, such a pcLED may be designed to emit, for example, white light having a desired color temperature and desired color-rendering properties.

In one aspect, a converter layer bonding device includes a release liner and an adhesive layer coating the release liner, the adhesive layer is solid and non-adhesive at a first temperature, and is adhesive at an elevated temperature above the first temperature. The adhesive layer may have a thickness of less than <NUM>. The adhesive layer may have a thickness between <NUM> and <NUM>. The adhesive layer may be configured to bond to a substrate at the elevated temperature, and the release liner may be configured to be removable after the adhesive layer is bonded to the substrate. The adhesive layer may be configured to form a bond layer between the substrate and a phosphor in contact with the adhesive layer opposite the substrate.

In another aspect, a method for forming a converter layer bonding device includes mixing an adhesive material and a solvent to form an adhesive mixture, coating a sheet of release liner with the adhesive mixture, and drying the solvent from the adhesive mixture coated onto the release liner to form an adhesive layer, the adhesive layer being solid and non-adhesive at a first temperature, and adhesive at an elevated temperature above the first temperature.

In yet another aspect, a light emitting device includes a light emitting diode, a phosphor, and a bond layer between and in contact with the light emitting diode and phosphor, the bond layer configured to bond the light emitting diode to the phosphor, the bond layer having a thickness that is uniform, the thickness varying less than <NUM> %. The thickness of the bond layer may be less than <NUM>. The thickness of the bond layer may be between <NUM> and <NUM>. The bond layer may be transparent. A surface of the phosphor or the light emitting diode in contact with the bond layer may have a surface roughness, and the bond layer may conform to the surface roughness while maintaining the uniform thickness. An edge of the phosphor and an edge of the bond layer may align on a same plane. The bond layer may include multiple channels, which may have openings on an edge of the bond layer.

In yet another aspect, a light emitting device may include a plurality of individually addressable light emitting diodes mounted on a single chip, a plurality of phosphor tiles, and a bond layer between each of the individually addressable light emitting diodes and phosphor tiles, the bond layer having a thickness that is uniform, the thickness varying less than <NUM>% between each individually addressable light emitting diode and phosphor tile across the single chip. The thickness of the bond layer may be less than <NUM>. The thickness of the bond layer may be between <NUM> and <NUM>. The bond layer may be transparent. The plurality of LED die may be mounted on a tile, a portion of the plurality of LED die may have a height from the tile that varies from another portion of the plurality of LED die, the bond layer maintaining uniform thickness on the plurality of LED die. Surfaces of the plurality of phosphor tiles in contact with the bond layer may have a surface roughness, and the bond layer may conform to the surface roughness while maintaining the uniform thickness. Surfaces of the plurality of light emitting diodes in contact with the bond layer may have a surface roughness, and the bond layer may conform to the surface roughness while maintaining the uniform thickness. The bond layer may include multiple channels. The multiple channels may have openings on an edge of the bond layer. The bond layer may include a first bond layer in contact with the plurality of phosphor tiles and a second bond layer in contact with the plurality of light emitting diodes and the first bond layer. The first bond layer may have a different physical characteristic from the second bond layer. The first bond layer may include multiple channels. The first bond layer may have a different refractive index from the second bond layer.

In yet another aspect, a method of forming a light emitting device includes aligning a converter layer bonding device over a phosphor, the converter layer bonding device including an adhesive layer adhered to a release liner, a first surface of the adhesive layer opposite the release liner facing a surface of the phosphor, bringing the first surface of the adhesive layer and the surface of the phosphor into contact at an elevated temperature, the elevated temperature being a temperature at which the adhesive layer adheres to the phosphor, cooling the adhesive layer adhered to the phosphor, removing the release liner from the adhesive layer, bringing one or more LED die into contact with a second surface of the adhesive layer opposite the first surface, and heating the adhesive layer, LED die, and phosphor to cure the adhesive layer and form a bond layer between the LED die and the phosphor. The adhesive layer may be solid and non-adhesive at a first temperature below the elevated temperature. The G* (at <NUM>) of the adhesive layer at the first temperature may be greater than <NUM> KPa, and the G* (at <NUM>) of the adhesive layer at the elevated temperature may be between <NUM> KPa and <NUM> KPa. Bringing the first surface of the adhesive layer and the surface of the phosphor into contact at an elevated temperature may include applying a vacuum to the converter layer bonding device and the phosphor. The method may further include dicing the phosphor and the bonding layer between the LED die. The method may further include, after removing the release liner, cutting the phosphor and adhesive layer into n x m arrays, and wherein bringing one or more LED die into contact with the adhesive layer opposite the phosphor comprises bringing each LED die into contact with an n x m array. The method may further include cutting channels into the adhesive layer on the converter layer bonding device. The method may further include, before bringing one or more LED die into contact with adhesive layer, aligning a second converter layer bonding device over the adhesive layer, the second converter layer bonding device comprising a second adhesive layer adhered to a second release liner, a first surface of the second adhesive layer opposite the second release liner facing the second surface of the adhesive layer, bringing the second adhesive layer and a surface of the adhesive layer opposite the phosphor into contact at an elevated temperature, the elevated temperature being a temperature at which the second adhesive layer adheres to the adhesive layer, cooling the second adhesive layer adhered to the adhesive layer, removing the second release liner from the second adhesive layer, and bringing the LED die into contact with a surface of the second adhesive layer opposite the adhesive layer.

In yet another aspect, a method of forming a light emitting device includes attaching a plurality of LED die to a tile, aligning a converter layer bonding device over the plurality of LED die, the converter layer bonding device comprising an adhesive layer adhered to a release liner, a first surface of the adhesive layer opposite the release liner facing surfaces of the plurality of LED die that are opposite the tile, bringing the first surface of the adhesive layer and surfaces of the plurality of LED die into contact at an elevated temperature, the elevated temperature being a temperature at which the adhesive layer adheres to the LED die, cooling the adhesive layer adhered to the plurality of LED die, removing the release liner from the adhesive layer, leaving portions of the adhesive layer adhered to each LED die and remaining portions of the adhesive layer being removed with the release liner, bringing a plurality of phosphor tiles each into contact with a portion of the adhesive layer adhered to each of the plurality of LED die, heating the adhesive layer, LED die, and phosphor to cure the adhesive layer and form a bond layer between the LED die and the phosphor. The adhesive layer is solid and non-adhesive at a first temperature below the elevated temperature, and adhesive at the elevated temperature. Bringing the first surface of the adhesive layer and surfaces of the plurality of LED die into contact at an elevated temperature may include applying a vacuum to the converter layer bonding device and the plurality of LED die. A height of each of the plurality of LED die from the tile varies, and the bond layer has a uniform thickness, the thickness varying less than <NUM> % over the plurality of LED die.

The following detailed description should be read with reference to the drawings, in which identical reference numbers refer to like elements throughout the different figures. The drawings, which are not necessarily to scale, depict selective embodiments and are not intended to limit the scope of the invention.

As used herein, spatially relative terms, such as "beneath", "below", "lower", "above", "upper" and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Thus, for example, the term "below" can encompass both an orientation of above and below, depending on the orientation of the device.

Light emitting pixel arrays are light emitting devices in which a large number of small light emitting devices, such as, for example LEDs, are arrayed on a single chip. The individual LEDs, or pixels, in a light emitting pixel array may be individually addressable, may be addressable as part of a group or subset of the pixels in the array, or may not be addressable. Thus, light emitting pixel arrays are useful for any application requiring or benefiting from fine-grained intensity, spatial, and temporal control of light distribution. These applications may include, but are not limited to, precise special patterning of emitted light from pixel blocks or individual pixels. Depending on the application, emitted light may be spectrally distinct, adaptive over time, and/or environmentally responsive. The light emitting pixel arrays may provide pre-programmed light distribution in various intensity, spatial, or temporal patterns. The emitted light may be based at least in part on received sensor data and may be used for optical wireless communications. Associated electronics and optics may be distinct at a pixel, pixel block, or device level.

Light emitting pixel arrays have a wide range of applications. Light emitting pixel array luminaires can include light fixtures which can be programmed to project different lighting patterns based on selective pixel activation and intensity control. Such luminaires can deliver multiple controllable beam patterns from a single lighting device using no moving parts. Typically, this is done by adjusting the brightness of individual LEDs in a 1D or 2D array. Optics, whether shared or individual, can optionally direct the light onto specific target areas.

Light emitting pixel arrays may be used to selectively and adaptively illuminate buildings or areas for improved visual display or to reduce lighting costs. In addition, light emitting pixel arrays may be used to project media facades for decorative motion or video effects. In conjunction with tracking sensors and/or cameras, selective illumination of areas around pedestrians may be possible. Spectrally distinct pixels may be used to adjust the color temperature of lighting, as well as support wavelength specific horticultural illumination.

Street lighting is an important application that may greatly benefit from use of light emitting pixel arrays. A single type of light emitting array may be used to mimic various street light types, allowing, for example, switching between a Type I linear street light and a Type IV semicircular street light by appropriate activation or deactivation of selected pixels. In addition, street lighting costs may be lowered by adjusting light beam intensity or distribution according to environmental conditions or time of use. For example, light intensity and area of distribution may be reduced when pedestrians are not present. If pixels of the light emitting pixel array are spectrally distinct, the color temperature of the light may be adjusted according to respective daylight, twilight, or night conditions.

Light emitting arrays are also well suited for supporting applications requiring direct or projected displays. For example, warning, emergency, or informational signs may all be displayed or projected using light emitting arrays. This allows, for example, color changing or flashing exit signs to be projected. If a light emitting array is composed of a large number of pixels, textual or numerical information may be presented. Directional arrows or similar indicators may also be provided.

Vehicle headlamps are a light emitting array application that requires large pixel numbers and a high data refresh rate. Automotive headlights that actively illuminate only selected sections of a roadway can be used to reduce problems associated with glare or dazzling of oncoming drivers. Using infrared cameras as sensors, light emitting pixel arrays activate only those pixels needed to illuminate the roadway, while deactivating pixels that may dazzle pedestrians or drivers of oncoming vehicles. In addition, off-road pedestrians, animals, or signs may be selectively illuminated to improve driver environmental awareness. If pixels of the light emitting pixel array are spectrally distinct, the color temperature of the light may be adjusted according to respective daylight, twilight, or night conditions. Some pixels may be used for optical wireless vehicle to vehicle communication.

An example of a type of light emitting pixel array is a micro-LED, also referred to as a µLED. <FIG> and <FIG> illustrate an example of a micro-LED. <FIG> shows a plan view of a micro-LED array <NUM>, along with an exploded view of a 3x3 portion of LED array <NUM>. In <FIG>, each square <NUM> represents a single LED, or pixel. As shown in the 3x3 portion exploded view, LED array <NUM> may include pixels <NUM> with a width w1, which may be between <NUM> and <NUM>, for example approximately <NUM> or less, e.g. <NUM>. The gaps, or lanes, <NUM> between the pixels may be separated by a width, w2, which may be between <NUM> and <NUM>, for example, approximately <NUM> or less, e.g., <NUM>. The lanes <NUM> may provide an air gap between pixels or may contain other material, as shown in <FIG>. The distance d1 from the center of one pixel <NUM> to the center of an adjacent pixel <NUM> may be approximately <NUM> or less (e.g., <NUM>). Such a micro-LED array may have hundreds, thousands, or millions of LEDs positioned together on substrates that may have, for example, an area in the range of centimeters, although the size of the area may vary. It will be understood that the widths and distances provided herein are examples only, and that actual widths and/or dimensions may vary. For instance, the width, w2, may be in the order of at least a millimeter, to form a sparse micro-LED, but may be larger or smaller.

It will be understood that although rectangular pixels arranged in a symmetric matrix are shown in <FIG> and <FIG>, pixels of any shape and arrangement may be applied to the embodiments disclosed herein. For example, LED array <NUM> of <FIG> may include <NUM>,<NUM> pixels in any applicable arrangement such as a <NUM> x <NUM> matrix, a <NUM> x <NUM> matrix, a symmetric matrix, a non-symmetric matrix, or the like. It will also be understood that multiple sets of pixels, matrixes, and/or boards may be arranged in any applicable format to implement the embodiments disclosed herein.

Micro-LEDs can be formed from pcLEDs. One method for forming a micro-LED is to use epitaxial growth to form the multiple individual LEDs <NUM> on a die in a flip-chip construction as in known in the art. <FIG> illustrates a side view of a portion of one type of micro-LED device taken through line AA of <FIG>.

<FIG> shows the pixels <NUM> and lanes <NUM>. Each pixel <NUM> is formed of an LED die <NUM>, which is positioned on, for example, a chip <NUM>, which may provide the electrical signals to the LED die <NUM>. A phosphor <NUM> is over and on LED dies <NUM>. The phosphor <NUM> may be formed of phosphor particles contained in a matrix, for example, garnet particles contained in silicone. Alternatively, or in addition, the phosphor <NUM> contains a densely sintered phosphor ceramic, such as Lumiramic™. In one example, the individual LEDs produce a blue light and the phosphor converts the blue light to a white light to produce a micro-LED that is monochrome white at a CCT of about <NUM>. <FIG> shows phosphor <NUM> as a single plate covering both the LED dies <NUM> and the gaps <NUM> between the LED dies <NUM>. However, phosphor <NUM> may be singulated, and cover just the individual LED dies <NUM>, as shown in more detail in examples below.

One method of forming a pcLEDs, including micro-LEDs, is to separately form a phosphor converting material into a tile (which also may be referred to as a plate, platelet, wafer, film or other shape), such as, for example, Lumiramic™ , as phosphor <NUM>. The phosphor, which is also referred to as a phosphor tile, is then attached or bonded to the separately formed LED die or plurality of LED dies.

The phosphor tile is typically attached or bonded to an LED or substrate using a layer of an adhesive positioned between the LED dies and the phosphor. The dimensions and uniformity of this adhesive layer are critical to the optical and thermal performance of the device, as well as its mechanical robustness.

In current manufacturing, there are several different methods used to apply this adhesive layer. One approach is to dispense a small volume of an adhesive solution (typically a liquid containing silicones and solvents) onto the surface of the LED die. A phosphor plate is then placed on top of the die. The droplet of adhesive solution then flows out the edges of the plate, and once dried and cured, forms a thin bond-line between die and phosphor. Another approach is to coat a similar adhesive solution onto a continuous phosphor wafer or film by a method such as spin-coating, spray-coating, or aerosol-jet printing. After coating the phosphor wafer or film, the phosphor is singulated to form plates, which are then attached to the LED die and the adhesive film cured. Each of these methods has its own drawbacks.

In the adhesive solution dispense approach, the final gap between the ceramic phosphor and the die is typically determined by a combination of factors, such as: the surface tension and viscosity of the adhesive solution, force and time of the "pick and place" tool, volume of the droplet, position of the droplet relative to the center of the surface of the die, temperature of the die, time elapsed between droplet dispense and attach, and rate of solvent evaporation. Unfortunately, this is a complex process that is difficult to control, and thus results in large variability of the average thickness between devices as well as large variability within each device. Additionally, this is a serial process (each adhesive solution droplet is dispensed sequentially), which decreases throughput. This process also does not scale well to micro-LEDs due the impractically small volumes of adhesive required. In the approach where the phosphor wafer or film is coated with adhesive first and then singulated, surface roughness can lead to adhesive layer non-uniformity. Even if the surface is smooth, if the surface energy of the converter layer of film is too low or too variable, the adhesive solution could partially de-wet coating, leading to an incomplete layer.

<FIG> shows a converter layer bonding device <NUM> that can be used to form pcLEDs, and in particular, is useful for forming light emitting pixel arrays, such as micro-LEDs. Converter layer bonding device <NUM> includes a release liner <NUM> coated with an adhesive layer <NUM>.

Release liner <NUM> may be any material, generally in the form of a flexible sheet, capable of holding adhesive layer <NUM> and capable of releasing adhesive layer <NUM> in operation (as shown below in <FIG>). Thus, the release liner <NUM> may be optimized (e.g., for roughness, slippage, and surface energy) so that the adhesive layer <NUM> coats the release liner <NUM> evenly, and so that the release liner <NUM> can release cleanly from the adhesive layer <NUM> after adhesive layer <NUM> is transferred to a substrate. Release liner <NUM> may be a sheet of flexible material, such as polyethylene terephthalate ("PET"), such as, for example, PANAC Corporation SP-PET - <NUM>-<NUM> BU. Release liner <NUM> may be coated with a transfer coating (not shown) positioned between the release liner <NUM> and the adhesive layer <NUM> that enhances the release of the adhesive layer <NUM>. Such a transfer coating may be, for example, a siliconized release coating, examples of which include PANAC Corporation's PDMS release coating on PET liners and as further described in <NPL>. In particular, when the release liner <NUM> is removed (as shown in the examples below), to ensure a clean transfer of the adhesive layer <NUM> to the substrate, the peel strength between the adhesive layer <NUM> and release liner <NUM> may be below <NUM> N/m, for example, between <NUM> - <NUM> N/m.

Adhesive layer <NUM> is the adhesive that is transferred to a substrate and that forms a bond layer (as shown below in <FIG>). The material used to form the adhesive layer <NUM> may be chosen to have the following properties. The first is that the material can be coated evenly onto the release liner <NUM> to form the converter layer bonding device <NUM>. The second is that the material forms an adhesive layer <NUM> that is dry, not tacky, and relatively hard, i.e., it does not flow, at a first, lower temperature, e.g., room temperatures. That is, at a first temperature, such as room temperatures, e.g., <NUM> - <NUM>, the adhesive layer <NUM>, while adhering to the release liner <NUM> on which it was formed, is not adhesive enough to attach to a substrate, such as a phosphor tile or LED die. For example, the adhesive layer <NUM> at a first temperature, such as room temperature, may follow the Dahlquist Criterium of G*(at <NUM>) ><NUM> KPa (<NUM>. 1MPa), for example G* > 300KPa (<NUM> MPa). Third, the material forming the adhesive layer <NUM> becomes tacky and reflows at elevated temperatures. That is, when heat is applied to the adhesive layer <NUM>, it becomes adhesive, and is then capable of attaching to a substrate. For example, an elevated temperature is chosen such that the G* (at <NUM>) of adhesive layer <NUM> becomes between G* = <NUM> KPa and G* = <NUM> KPa, with tan delta typically between <NUM> and <NUM>, for example, at between <NUM> and <NUM>, e.g., <NUM>. Fourth, the material used to form the adhesive layer <NUM> is capable of forming a bond layer that provides a strong bond between a phosphor and a target substrate. The bond layer may be transparent, or may have additional optical characteristics, such as scattering, R1 gradient, and emissivity, as disclosed in more detail below.

In particular, the adhesive layer <NUM> may not be adhesive enough to attach to a substrate at a first, lower temperature, but becomes adhesive enough at elevated temperature to attach to a substrate, such as a phosphor or LED die, and, after cooling, has a stronger attachment to the substrate than to the release liner <NUM>, such that the release liner may be easily removed.

The thickness T of the adhesive layer <NUM> is chosen to match the desired target thickness of the bond layer in the final device, and may be in a range of <NUM> to <NUM> , for example, less than <NUM>, in the range of between <NUM> and <NUM>. The adhesive layer <NUM> is also formed so that the thickness T is uniform across the layer, for example, T will have a deviation (variation) of less than <NUM>%, for example, less than <NUM>%, across the adhesive layer <NUM>. The material used to form the adhesive layer <NUM> may be, for example, a siloxane adhesive.

<FIG> show a flow chart and illustration of a method of making a converter layer bonding device <NUM>. At S310, the release liner <NUM> to be used may be coated with a siliconized coating to enhance the release properties as described above. At S320 an adhesive mixture <NUM> may be prepared by mixing the adhesive material with a solvent, for example, a resin and solvent, such as a methylphenylsiloxane (for example, LPS-9501D from Shin-Etsu Chemical Co. ) and cyclohexanone in mass ratios between <NUM>:<NUM> and <NUM>:<NUM>. The concentrations of adhesive material and solvent may be chosen to achieve the desired viscosity of the adhesive mixture <NUM>. The viscosity of the adhesive mixture <NUM> may be chosen to optimize wetting of the release liner <NUM>, while still achieving the desired thickness T of the adhesive layer <NUM> in the converter layer bonding device <NUM>. For example, the viscosity of the adhesive mixture <NUM> may range between <NUM> and <NUM>,<NUM> mPa's (or cP).

As shown in <FIG>, at S330 the adhesive mixture <NUM> is coated onto the release liner <NUM>. Any method that can suitably coat the release liner <NUM> with a uniform layer of the adhesive mixture <NUM> at the desired thickness may be used, such as, for example, spin-coating, gravure coating, etc. <FIG> illustrates, as an example, a spin-coating process for coating release liner <NUM> with the adhesive mixture <NUM>. In <FIG>, the release liner <NUM> is positioned on a spin-coating support <NUM> and the adhesive mixture <NUM> is deposited from nozzle <NUM> as is known by persons having ordinary skill in the art.

As shown in <FIG>, at S340 the adhesive mixture <NUM> coated onto release liner <NUM> is dried to remove solvent. Depending on the adhesive used, at S350 the adhesive mixture may be additionally cured to stabilize the material and improve uniformity of the subsequent transfer from the converter layer bonding device <NUM>. The resulting converter layer bonding device <NUM> is shown in <FIG>. This process results in an adhesive layer <NUM> that may be thin (may be under <NUM>), uniform, defect-free, and can be made in a large area, such as such as from <NUM> to <NUM> wide, and even tens of meters to thousands of meters, such as in a roll. Note that roll-to-roll methods such as gravure coating technology may be used for large area coating.

<FIG> show a flow chart and illustration of an example of the method of using the converter layer bonding device <NUM>. A vacuum lamination process may be used to transfer the adhesive layer <NUM> onto a substrate <NUM>.

As shown in <FIG>, at S410, a converter layer bonding device <NUM> may be aligned over a substrate <NUM>. As shown in <FIG>, the adhesive layer <NUM> of the converter layer bonding device <NUM> is facing a surface <NUM> of the substrate <NUM> to which the adhesive layer <NUM> is to be applied.

As shown in <FIG>, at S420, a vacuum may be applied to the converter layer bonding device <NUM> and substrate <NUM>, and at S430, the converter layer bonding device may be brought into contact with the substrate at elevated temperatures. The temperature used depends on the particular adhesive material that forms the adhesive layer. In general, the temperature is high enough to make the adhesive layer <NUM> flow and become tacky, i.e., adhesive, enough to stick to the substrate <NUM>. The elevated temperature may be chosen such that the G* (at <NUM>) is between G* = <NUM> KPa, and G* = <NUM> KPa, with tan delta typically between <NUM> and <NUM>. At the same time, the temperature should be low enough to prevent excessive flow, so that the adhesive layer <NUM> maintains its shape and coverage of the substrate <NUM>. The substrate <NUM> with the converter layer bonding device <NUM> attached may then be cooled, for example, back to room temperature.

As shown in <FIG>, at S440, once the adhesive liner <NUM> has cooled, the release liner <NUM> may be removed, leaving a continuous layer of the adhesive behind on the substrate <NUM>. As the adhesive layer <NUM>, after heat treatment, is more strongly attached to the substrate than the release liner <NUM>, the release liner <NUM> may be removed, for instance, mechanically by peeling off the adhesive layer <NUM> that is attached to substrate <NUM>. As noted above, the ensure clean transfer, the release liner may be designed to have a peel strength between the adhesive layer <NUM> and release liner <NUM> of below <NUM> N/m, for example <NUM>-<NUM> N/m. The adhesive layer <NUM> remains on the substrate <NUM> after removal of release liner <NUM>.

As shown in <FIG>, at S450, the sample <NUM> to be bonded to substrate <NUM> may be brought into contact with the transferred adhesive layer <NUM>. Heat may be applied to cure the adhesive layer and form a bond layer <NUM> between the substrate <NUM> and sample <NUM>. The bonded substrate <NUM> and sample <NUM> may then be cooled.

The thickness of the resulting bond layer <NUM> may be very thin, and be between <NUM> and <NUM>, for example, between <NUM> and <NUM>. The bond layer <NUM> has a uniform thickness over the surface of the substrate to which it is applied, and the variation in thickness across the bond layer may be less than <NUM>%, and for example, may be less than <NUM>%. For example, for a bond layer <NUM> that has a thickness of <NUM>, the variation in thickness across the bond layer between substrate <NUM> and sample <NUM> would be less than +/- <NUM> at a <NUM>% variation, and +/-<NUM> at a <NUM>% variation. Additionally, if the substrate <NUM> has a relatively large area on which numerous minute LED dies are form, such as, for example, as is used for making a light emitting pixel array, such as a micro-LED, the adhesive layer <NUM> and subsequent bond layer <NUM> provides a thin and uniform bond layer over the entire area of the device and form the numerous minute light emitters. Thus, for a micro-LED having thousands to millions of individually addressable LEDs over and area with, for example, <NUM> - <NUM> per side (<NUM> - <NUM>,<NUM> <NUM>) or for somewhat larger arrays, for example, <NUM> - <NUM> per side (<NUM>,<NUM> - <NUM>,<NUM><NUM>), the bond layer <NUM> is uniform, which improves the performance of the device. Further advantages of the bond layer <NUM> formed from the converter layer bonding device <NUM> as disclosed herein are described in more detail below.

The bond layer <NUM> may be transparent. The bond layer <NUM> may also include dispersed particles and/or dyes which could provide additional optical, physiochemical or mechanical characteristics, such as higher refractive index, enhanced light scattering, light absorption or emission at different wavelengths. To form a bond layer <NUM> having dispersed particles and/or dyes, the particles and/or dyes may be included in the adhesive mixture <NUM> (at S320) used to form the adhesive layer <NUM> on the converter layer bonding device <NUM>.

<FIG> illustrate an example application of the converter layer bonding device in which the adhesive layer is transferred onto a phosphor, followed by die attach. A converter layer bonding device <NUM> may be prepared as described above with respect to FIGs.

In <FIG>, a phosphor film or wafer <NUM>, such as, for example, a Lumiramic™ tile, may be mounted onto a carrier tape <NUM>. The phosphor wafer <NUM> may be O<NUM>-plasma treated to improve adhesion of subsequently-transferred adhesion layer <NUM>. Vacuum lamination may then be used to transfer the adhesive layer <NUM> onto the phosphor wafer <NUM> as follows. As shown in <FIG>, the converter layer bonding device <NUM> may be aligned over the phosphor <NUM> with the adhesive layer <NUM> facing the phosphor <NUM>. As shown in <FIG>, vacuum may be applied, then the converter layer bonding device <NUM> brought into contact with phosphor <NUM> at elevated temperature (for example, between <NUM> and <NUM>, e.g., <NUM>) so that the adhesive layer <NUM> may be in contact with the surface of the phosphor <NUM> to be bonded. As shown in <FIG>, once the converter layer bonding device is cooled, the release liner <NUM> may be removed, leaving adhesive layer <NUM> behind on phosphor <NUM>. In <FIG>, LED die <NUM> may be attached to the adhesive layer <NUM>, using, for example, a pick-and-place tool as is known by persons having ordinary skill in the art. The LED die <NUM> are positioned onto the adhesive layer <NUM> with the light emitting side of the LED die facing and in contact with the adhesive layer <NUM>. The adhesive layer <NUM> may then be cured at an elevate temperature (for example, between <NUM> and <NUM>, e.g., <NUM>) to form bond layer <NUM> that is transparent. As shown in <FIG>, the phosphor <NUM> may then be singulated by forming slots <NUM> through the bond layer <NUM> and phosphor <NUM> between each LED die <NUM> if desired.

This method illustrated in <FIG> may be used to form light emitting pixel arrays, such as micro-LEDs. The representative LED die <NUM> and bonded phosphor portions <NUM> may be a portion of a large array of LED die. As shown in <FIG>, the light emitting device shown in <FIG> may be electrically connected to a signal source, such as with a substrate <NUM>, that provides each LED die <NUM> with a signal, such that each LED die <NUM> is individually addressable. The carrier tape <NUM> may be removed by methods known to persons having ordinary skill in art, resulting in a micro-LED having a thin and uniform bond layer <NUM> attaching the phosphor to the numerous LED die. In another example, each LED die <NUM> represents a die with multiple pixels and phosphor <NUM> a phosphor tile to form multiple light emitting pixel arrays, and advantageously each will have a bond layer <NUM> with a consistent, uniform thickness.

The resulting LED devices each have a phosphor portion <NUM> bonded to an LED die <NUM>. The converter layer bonding device <NUM> and method can form a bond layer <NUM> between each LED die <NUM> and phosphor portion <NUM> that is very thin, between <NUM> and <NUM>, for example, between <NUM> and <NUM>, as described above, and have a very uniform thickness, as described above. Additionally, the bond layer <NUM> substantially maintains the shape of the of the adhesive layer <NUM>, and thus does not flow out of or over the edges of the LED die <NUM> and phosphor <NUM>. Additionally, when singulated, the bond layer <NUM> can be cleanly diced, which also leaves the edge of the bond layer flush with the edge of the phosphor <NUM> and/or the LED die <NUM>.

<FIG> illustrate an example application of the converter layer bonding device in which the adhesive layer is transferred onto a phosphor, followed by phosphor array patterning and LED die attach. A converter layer bonding device <NUM> may be prepared as described above with respect to FIGs.

<FIG> shows adhesive layer <NUM> transferred onto phosphor <NUM> on carrier tape <NUM> by a vacuum lamination method as described above with respect to <FIG>. As shown in <FIG>, after removal of the release liner <NUM>, the phosphor <NUM> and adhesive layer <NUM> may be patterned, for example, into n x m arrays of phosphor tiles <NUM>. <FIG> is a plan view of the n x m arrays of phosphor tiles <NUM> of <FIG>. Patterning of the phosphor <NUM> and adhesive layer <NUM> may accomplished by any applicable manner of dicing, segmenting, dividing, apportioning, slicing or compartmentalizing as is known in the art, such as, for example, sawing, etching, applying a mask to dice, using one or more lasers, and/or chemical treatments. Patterning may include forming slits <NUM> through the phosphor <NUM> and adhesive layer <NUM>, but not the carrier tape <NUM>, to form individual phosphor elements <NUM> of the n x m arrays of phosphor tiles <NUM>. Patterning may also include slicing through the carrier tape <NUM> to form openings <NUM>, which separate n x m arrays <NUM> if multiple arrays are to be formed. As shown in <FIG>, the n x m arrays <NUM> may be placed onto to LED die <NUM> such that the adhesive layer <NUM> is in contact with the light emitting face <NUM> of the LED die <NUM>. LED die <NUM> may be on substrate <NUM> for this purpose. A pick-and-place tool may be used to place the n x m arrays of phosphor tiles <NUM> on the LED dies <NUM>, as in known in the relevant art. The n x m arrays <NUM> on the LED dies <NUM> may then be heated to an elevated temperature (for example, between <NUM> and <NUM>, e.g., <NUM>) to heat and fully cure the adhesive layer <NUM> and form bond layer <NUM> that is transparent.

The method shown in <FIG> may be used to form micro-LEDs. LED die <NUM> in <FIG> may include numerous minute LEDs that form the micro-LED, and each n x m phosphor array <NUM> may be aligned over each of the minute LEDs <NUM> in LED die <NUM>. That is, the dimension and count of the minute LED die pixels <NUM> in LED die <NUM> match up with those of the n x m arrays <NUM>. Thus, multiple micro-LEDs may be formed using this method, and advantageously, each will have a bond layer with a consistent, uniform thickness.

<FIG> show transferring the adhesive layer onto LED die on tile, which LED die may have significant surface topography. A converter layer bonding device <NUM> may be prepared as described above with respect to FIGs.

As shown in <FIG>, LED die <NUM> may be attached to a substrate, such as a tile. This may create a significant (potentially in the range of > <NUM>) height variations in the top of the LED die <NUM>. Before transferring the adhesive layer <NUM>, the LED die surface may be O2-plasma treated to improve adhesion of subsequently-transferred adhesive layer <NUM>. Vacuum lamination, as described above with respect to <FIG> may then be used to transfer the adhesive layer <NUM> onto the die surface, as shown in <FIG>.

Once the converter layer bonding device is cooled, release liner <NUM> may be removed as shown in <FIG>. Removal of the release liner <NUM> leaves a uniform adhesive layer <NUM> only on the surfaces <NUM> of the LED die <NUM>. Portions <NUM> of the adhesive layer <NUM> that are between LED die <NUM> are not adhered to the LED die <NUM> and thus are not transferred and remain on the release liner <NUM>. As shown in <FIG>, phosphor platelets <NUM> may then be placed on the adhesive layer <NUM> on the LED die <NUM> at elevated temperatures (for example, between <NUM> and <NUM>, e.g., <NUM>) to fully cure adhesive layer <NUM> and form bond layer <NUM> that is transparent. A pick-and-place tool may be used to place the phosphor platelets <NUM>.

<FIG> show a cross-sectional view and plan view, respectively, of a converter layer bonding device having a patterned adhesive layer that can be used, for example, to improve oxygen permeability, which reduces browning and thus improves transparency and device performance (as described, for example, in Cree® XLamp® LEDs Chemical Compatibility, CLD-AP63 REV 6A, August, <NUM>, Cree, Inc. A converter layer bonding device <NUM> may be prepared as described above with respect to FIGs. Cutting the channels <NUM> may be accomplished by methods including, for example, dicing, laser ablation or laser cutting, and stamping, as is known by persons having ordinary skill in the art, and is possible because the adhesive layer <NUM> is relatively solid at a first, lower temperature, such as room temperature. Channels <NUM> of a given kerf are cut into adhesive layer <NUM>, but release liner <NUM> is left intact. The channels <NUM> may be open on at least one of the edges of the adhesive layer <NUM>, so as to allow ambient gasses, such as air or pure oxygen, to pass into the adhesive layer <NUM>. The size and spacing of the channels depend on the application. The channels may only be large enough to allow gas to pass into the layer, and may be spaced close enough together so that the gas entering the channels can diffuse significantly into the bond layer, but not so many channels or so close together to weaken the bonding. For example, channels with a width of <NUM> may be diced into the layer with a pitch of <NUM>. The resulting patterned converter layer bonding device <NUM> may be used in a similar manner to the converter layer bonding device <NUM> as disclosed herein.

<FIG> illustrate use of a patterned converter layer bonding device <NUM>. In <FIG>, a phosphor film or wafer <NUM> may be mounted onto a carrier tape <NUM>. The phosphor wafer <NUM> may be O<NUM>-plasma treated to improve adhesion of subsequently-transferred adhesion layer <NUM> having channels <NUM>. Vacuum lamination is then used to transfer the adhesive layer <NUM> having channels <NUM> onto the phosphor <NUM> as disclosed above with respect to <FIG>. As shown in <FIG>, after aligning the converter layer bonding device <NUM> with the phosphor wafer <NUM>, vacuum may be applied, then the converter layer bonding device <NUM> may be brought into contact with phosphor <NUM> at elevated temperature (for example, between <NUM> and <NUM>, e.g. <NUM>) so that the adhesive layer <NUM> with channels <NUM> may be in contact with the surface of the phosphor <NUM> to be bonded. As shown in <FIG>, once the patterned converter layer bonding device <NUM> is cooled, the release liner <NUM> may be removed, leaving adhesive layer <NUM> having channels <NUM> adhered on phosphor <NUM>. As shown in <FIG>, LED die <NUM> may be attached to the adhesive layer <NUM> having channels <NUM>, using, for example, a pick-and-place tool as is known in the relevant art. The LED die <NUM> are positioned onto the adhesive layer <NUM> with the light emitting side of the LED die <NUM> facing and in contact with the adhesive layer <NUM>, and also positioned over the channels <NUM>. The adhesive layer <NUM> having channels <NUM> may then be cured at an elevate temperature (for example, between <NUM> and <NUM>, e.g., <NUM>) to form transparent bond layer <NUM> having channels <NUM>. As shown in <FIG>, the phosphor <NUM> may then be singulated by forming slots <NUM> through the bond layer <NUM> and phosphor <NUM> between each LED die <NUM> if desired.

The resulting transparent bond layer <NUM> between LED die <NUM> and phosphor <NUM> has open channels <NUM> of a certain aspect ratio, for example, having a <NUM> width and a depth (or height) that is the thickness T of the bond layer <NUM> or less than the thickness T of the bond layer, and, for example, may be <NUM>. Other examples of bond layers with channels, additives used to modify the optical characteristics, and use of a second material to back-fill the channels may be found in <CIT> titled "Fabrication For Precise Line-Bond Control and Gas Diffusion Between LED Components". These channels <NUM> may decrease the path length for oxygen to diffuse into the bond layer <NUM>, reducing the browning that can occur to high refractive index/high phenyl-content siloxanes. Although <FIG> illustrate a pattern with multiple straight channels <NUM>, other patterns may be used. Also, the patterned converter layer bonding device <NUM> may be used in place of the converter layer bonding device <NUM> without patterns in any of the applications disclosed herein.

<FIG> illustrate an application of stacked adhesive layers by a multiple lamination process onto a phosphor wafer for improved oxygen permeability. In <FIG>, a first converter layer bonding device <NUM> is prepared with a first adhesive layer <NUM> on first release liner <NUM> (as described with respect to <FIG>). First adhesive layer <NUM> may, for example, be patterned to include channels <NUM> as described above with respect to <FIG>. As shown in <FIG>, the first adhesive layer <NUM> with channels <NUM> may be transferred to phosphor <NUM> in a manner as described above with respect to <FIG>. As shown in <FIG>, a second converter layer bonding device <NUM>, having second adhesive layer <NUM> on second release liner <NUM> may then be positioned onto and transferred to first adhesive layer <NUM>. Second converter layer bonding device <NUM> may be prepared as described above with respect to <FIG>. Second adhesive layer <NUM> may have different physicochemical characteristics than first adhesive layer <NUM>, such as, for example, higher oxygen permeability. As shown in <FIG>, the second adhesive layer <NUM> of second bonding device <NUM> is positioned so as to be in contact with the first adhesive layer <NUM>. The vacuum lamination process is then repeated, and the second converter layer bonding device <NUM> is heated to an elevated temperature at which the second adhesive layer <NUM> becomes adhesive, to adhere second adhesive layer <NUM> to first adhesive layer <NUM>. The second converter layer bonding device is then cooled and the second release liner <NUM> is removed, leaving the second adhesive layer <NUM> adhered to the first adhesive layer <NUM>, which is adhered to phosphor <NUM>. This process may be repeated to add additional adhesive layers. The resulting stack of at least two adhesive layers <NUM> and <NUM>, may have different optical and/or physical properties and/or morphologies.

As shown in <FIG>, LED die <NUM> may be attached to the second adhesive layer <NUM> with the light emitting side of the LED die <NUM> facing and in contact with the side of the second adhesive layer <NUM> opposite the phosphor <NUM>. The LED die <NUM> may be positioned on the second adhesive layer <NUM> at an elevated temperature (e.g. <NUM>). A pick-and-place tool may be used for this purpose, as is known to persons having ordinary skill in the art. The multilayer adhesive stack of first adhesive layer <NUM> and second adhesive layer <NUM> is then fully cured to form multilayer bond layer <NUM> having a first bond layer <NUM> formed from the first adhesive layer <NUM> and a second bond layer <NUM> formed from second adhesive layer <NUM>. The multilayer bond layer <NUM> and phosphor <NUM> may be singulated. The resulting portion of the multilayer bond layer <NUM> may have differentiated properties in each of the different layers. <FIG> illustrate an example where one layer has channels <NUM> of a certain aspect ratio and the other layer does not have channels. The channels or/and the increased permeability of the stack towards oxygen will result in increased oxygen concentration in the bond layer, reducing the adhesive browning intrinsic to high refractive index/high phenyl-content siloxanes. Other embodiments of a multilayer bond layer may be made with layers that do not have channels but have other different physical properties, or that both have channels but the channels differ, for instance in direction of pattern or width of cut, or both layers may be the same. The different layer may have different physical characteristics, such as higher oxygen permeability, refractive index, or other optical characteristics, including, but not limited to scattering light, optical absorption and emission.

<FIG> illustrate another method of forming a multilayer bond layer, in this case by forming a converter layer bonding device that includes a multilayer adhesive layer.

Similar to the method disclosed above with respect to <FIG>, the release liner <NUM> to be used may be coated with a siliconized coating to enhance the release properties as described above (not shown). A first adhesive mixture <NUM> may be prepared by mixing a first adhesive material with a solvent. As shown in <FIG>, the first adhesive mixture <NUM> is then coated onto the release liner <NUM>. Any method that can suitably coat the release liner <NUM> with a uniform layer of the first adhesive mixture <NUM> at the desired thickness may be used, such as, for example, spin-coating, gravure printing, etc. <FIG> illustrates, as an example, a spin-coating process for coating release liner <NUM> with the first adhesive mixture <NUM>. In <FIG>, the release liner <NUM> is positioned on a spin-coating support <NUM> and the first adhesive mixture <NUM> is deposited from nozzle <NUM> as is known by persons having ordinary skill in the art.

As shown in <FIG>, the first adhesive mixture <NUM> coated onto release liner <NUM> is dried to remove solvent. Depending on the adhesive used, the first adhesive mixture <NUM> may be additionally cured to stabilize the material and improve uniformity of the subsequent transfer from the converter layer bonding device.

As shown in <FIG>, the release liner <NUM> and first adhesive layer <NUM> formed from the dried first adhesive mixture <NUM> may then be coated with a second adhesive mixture <NUM>. Any suitable method may be used to coat second adhesive mixture <NUM> onto the first adhesive layer <NUM>. <FIG> illustrates a spin-coating method as disclosed above.

As shown in <FIG>, the second adhesive mixture <NUM> may then be dried to remove solvent. The resulting converter layer bonding device <NUM> is shown in <FIG> with second adhesive layer <NUM> adhered on top of and in contact with first adhesive layer <NUM>, which is on release liner <NUM>. Both adhesive layers <NUM> and <NUM> may be thin (may be under <NUM>), uniform, defect-free, and can be made in a large area. The first adhesive layer <NUM> and second adhesive layer <NUM> may be different. The second adhesive layer <NUM> may be patterned as described above with respect to <FIG>. The first adhesive layer <NUM> may have different physical characteristics, such as higher oxygen permeability, refractive index, or other optical characteristics, including, but not limited to scattering light, optical absorption and emission, than the first adhesive layer <NUM>. The converter layer bonding device <NUM> may be used in any applications in which a converter layer bonding device <NUM> with a single adhesive layer is used, and may be used to transfer adhesive layers <NUM> and <NUM> to a substrate (e.g., a phosphor or LED die) in a same manner as disclosed above with respect to <FIG> for single adhesive layer <NUM>.

The bond layer that is formed using the converter layer bonding device and methods disclosed herein, and the pcLED devices, including micro-LEDs, formed using the converter layer bonding device and method, have several advantages, in particular, as compared to the adhesive dispensing process conventionally used in the art. Because the adhesive layer is transferred onto the substrate, such as the phosphor or LED die, as a dry film, there will be no excess adhesive extruded out when phosphor and die are brought into contact, and therefore no "wings" or "fillets" along the edges of the devices. The dry adhesive layer does not spread out when transferred, but maintains its shape. The adhesive layer stays where it is positioned. Additionally, during the curing process, the temperature is controlled so that the resulting bond layer also does not flow and spread, but maintains its shape. Additionally, there will be no significant bond-line variation, as the thickness of the bond layer is determined before application by the thickness of the adhesive layer. Additionally, in the example disclosed above with respect to FIG. 8A-8E, the patterning of the adhesive layer can be performed to allow air channels in the pcLED device.

The bond layer formed using the converter layer bonding device and method disclosed herein also has advantages over other coating methods, such as spin-coating and spray-coating for applying adhesive. <FIG> shows a schematic of an expanded view of a substrate <NUM> and the surface of the substrate <NUM>, which is rough and not flat. Phosphor tiles and LED dies may have such surface roughness. Adhesive layer <NUM> transferred onto the substrate <NUM> using the converter layer bonding device and method disclosed here, conforms to the native surface roughness of the substrate <NUM>, while maintaining a uniform layer thickness T. <FIG> shows, for comparison, a surface coated with a conventional solution-state method for application of adhesive, for example, spin coating or spray coating. As seen in FIG. 13B, an adhesive solution <NUM> flows to fill in the surface structures, and may leave portions of the surface exposed. Also, of note, is that the adhesive coating of <FIG> may result in small air voids between the adhesive layer and a sample surface to be bonded.

Claim 1:
A method for forming a light emitting device comprising:
aligning a converter layer bonding device (<NUM>) over a phosphor (<NUM>), the converter layer bonding device comprising an adhesive layer (<NUM>) adhered to a release liner (<NUM>);
bringing the adhesive layer and the phosphor into contact at an elevated temperature, the elevated temperature being a temperature at which the adhesive layer adheres to the phosphor;
cooling the adhesive layer adhered to the phosphor;
removing the release liner from the adhesive layer;
bringing one or more LED die (<NUM>) into contact with the adhesive layer opposite the phosphor; and
heating the adhesive layer, one or more LED die, and phosphor to cure the adhesive layer and form a bond layer between the one or more LED die and the phosphor.