Patent Description:
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" or "downconverters") 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.

Documents <CIT>, <CIT>, and <CIT> disclose a downconverter layer transfer device having a release liner and a downconverter layer comprising separate downconverter pixels.

According to the invention, a downconverter layer transfer device is provided as recited in claim <NUM>. The downconverter layer transfer device may include a siliconized layer disposed between the release liner and the downconverter layer. The downconverter material may include at least one of a phosphor, an organic dye, a quantum dot, and a scattering agent. The shear modulus G* (at <NUM>) of the downconverter layer at the first temperature may be greater than <NUM> KPa, and the shear modulus G* (at <NUM>) of the downconverter layer at the elevated temperature may be between <NUM> KPa and <NUM> KPa.

In another aspect, a lighting device is provided, the lighting device including a plurality of individually addressable light emitting diodes mounted on a substrate, each light emitting diode having a light emitting surface, and a plurality of downconverter layer pixels, each downconverter layer pixel in contact with and adhered to the light emitting surface of one of the light emitting diodes, the downconverter layer pixels comprising an adhesive material and a downconverter material interspersed throughout the adhesive material. The adhesive may be a heat-curable adhesive material that, before curing, is solid and non-adhesive at a first temperature, and is adhesive at an elevated temperature above the first temperature.

According to the invention, furthermore a method of forming a lighting device is provided as recited in claim <NUM>. Further embodiments of the invention are provided by the dependent claims.

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 substrate, which may be a semiconductor die or 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 one of an array of LED dies <NUM> positioned on, for example, a substrate <NUM>. Substrate <NUM> may be, for example, a single semiconductor die or chip, a board or interposer, such as for example a CMOS (complementary metal oxide semiconductor) chip or an MCPCB (metal core printed circuit board). Substrate <NUM> may provide the electrical signals to each of the LED die <NUM> in the array of LED dies <NUM>. A downconverter <NUM> is over and on array of LED dies <NUM>. The downconverter163 may be formed, for example, of phosphor particles contained in a matrix, for example, garnet particles contained in silicone.

<FIG> shows downconverter <NUM> as singulated, and covering just the individual LED dies <NUM>. However, downconverter <NUM> may be a single layer covering both the LED dies <NUM> and the gaps <NUM> between the LED dies <NUM>. The singulated downconverter <NUM> shown in <FIG> may contain individual downconverter pixels <NUM>, <NUM>, and <NUM> positioned over individual dies <NUM>. Downconverter <NUM> may contain a variety of different downconverter materials, each in a different individual downconverter pixel positioned over individual LED dies <NUM>. For example, to form a micro-LED with red, green, and blue emitting pixels (an "RGB micro-LED"), the LED dies <NUM> may be blue light emitting LED dies, and individual phosphor pixel <NUM> may be a downconverter that converts blue light to red light, individual phosphor pixel <NUM> may be a downconverter that converts blue light to green or green/yellow light, and individual phosphor pixel <NUM> may include only a scattering agent so that the blue light from the LED die is transmitted without conversion. The red, green, and blue light emitted may be combined to form a white light. In one example, the downconverter converts the blue light to a white light to produce a micro-LED that is monochrome white at a CCT of about <NUM>. The amount of light emitted by the individual pixels <NUM> may be individually controlled so as to form a tunable micro-LED in which a mixture of unsaturated green, red, and blue lights emitted can establish a highly efficient tunable white light source. Downconverter <NUM> may also include spaces <NUM> between each individual downconverter pixel <NUM>, <NUM>, <NUM> and aligned with gap <NUM>.

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™. The tile is then attached or bonded to the separately formed LED die or array of LED dies. To assemble an RGB micro-LED using this method, after a layer of adhesive is applied to the LED die, a "pick and place" tool is used to place each of the individual phosphor tiles onto each individual LED die to form the individual red, green and blue pixels. This serial pick-and-place approach introduces significant attachment accuracy issues leading to wide gaps between neighboring pixels. The serial pick-and-place method is also time-consuming, posing a bottleneck for throughput, and thus slowing down the manufacturing process. Other methods that may be used include patterning red and green subpixels onto an array of blue die, either via a stamp process, photolithography or ink-jet printing. The stamp process methods can require, for a multi-color array, assembly of the downconverter pixels on the stamp, which involves use of serial pick-and-place to assemble. The photolithography method can require patterning directly onto an array of LEDs, which can be difficult due to variations in topography as well as differences in material types (for instance, lithography on GaN is different from lithography on silicon). Use of inkjet printing can be limited due to difficulty printing high aspect ratio phosphor pixels.

<FIG> shows a side view of a portion of a downconverter layer transfer device <NUM> that can be used to form pcLEDs, and, in particular, is useful for forming light emitting pixel arrays, such as RGB micro-LEDs. Downconverter layer transfer device <NUM> includes a release liner <NUM> coated with a downconverter layer <NUM>. Downconverter layer <NUM> includes one or more downconverter materials mixed with an adhesive material, as will be described in more detail below. Downconverter layer <NUM> may include portions of different types of downconverter materials in separate downconverter layer pixels <NUM>, <NUM>, <NUM>. For example, downconverter layer pixel <NUM> may include a red downconverter material, downconverter layer pixel <NUM> may include a green downconverter material, and downconverter layer pixel <NUM> may contain only a scattering agent. The number of different downconverter materials used in downconverter layer <NUM>, and the arrangement and colors of the different downconverter layer pixels may vary depending on the desired light emitting device to be manufactured. For example, an RGB pattern of downconverter layer pixels may repeat over the entirety of the downconverter layer transfer device. There may be a space <NUM> between each of the downconverter layer pixels <NUM>, <NUM>, and <NUM>. The size of the downconverter layer pixels <NUM>, <NUM>, <NUM>, and space <NUM> is set so that when the downconverter layer <NUM> is transferred to the LED array <NUM>, as described below, the downconverter layer pixels <NUM>, <NUM>, <NUM> are positioned on top of the individual LEDs <NUM>, to produce a light emitting device as shown above in <FIG> and <FIG>.

<FIG> shows only a portion of a downconverter layer transfer device with just four downconverter layer pixels. In general, a full downconverter layer transfer device may have an area matching the area of, or large enough to cover the light emitting portion of, the LED array <NUM> on which the downconverter will be positioned to form micro-LED <NUM>. A full downconverter layer transfer device may have an area that covers more than one LED array <NUM>, such that several LED arrays <NUM> may be manufactured at once. The number of individual downconverter layer pixels <NUM>, <NUM>, <NUM> may match the number of individual LED dies <NUM> in the LED array <NUM>, although this is not required. Thus the number of individual downconverter layer pixels in a single downconverter layer transfer device may be in the <NUM>'s to the <NUM>,<NUM>,<NUM>.

Release liner <NUM> may be any material, generally in the form of a flexible sheet, capable of holding downconverter layer <NUM> and capable of releasing downconverter 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 downconverter layer <NUM> coats the release liner <NUM> evenly, and so that the release liner <NUM> can release cleanly from the downconverter layer <NUM> after downconverter 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 in <FIG>) 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 downconverter 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.

Downconverter layer <NUM> includes the downconverter material and an adhesive material, and is the portion of the downconverter layer transfer device that is transferred and adhered to the substrate, i.e., the LED array <NUM> to form a micro-LED as will be described in more detail below with respect to <FIG>.

The adhesive material used to form the downconverter layer <NUM> may be chosen to have the following properties. The first is that the adhesive material can be coated evenly onto the release liner <NUM> to form the downconverter layer transfer device <NUM>. The second is that the adhesive material forms a downconverter 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 material in downconverter 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 an LED die. For example, the adhesive material in downconverter layer <NUM> at a first temperature, such as room temperature, may follow the Dahlquist Criterium of shear modulus G*(at <NUM>) ><NUM> KPa (<NUM>. 1MPa), for example shear modulus G* > 300KPa (<NUM> MPa). Third, the adhesive material forming the downconverter layer <NUM> becomes tacky and reflows at elevated temperatures. That is, when heat is applied to the downconverter layer <NUM>, it becomes adhesive, and is then capable of attaching directly to a substrate. For example, an elevated temperature is chosen such that the shear modulus G* (at <NUM>) of the adhesive material becomes between shear modulus G* = <NUM> KPa and shear modulus G* = <NUM> KPa, with tan delta typically between <NUM> and <NUM>, for example, at between <NUM> and <NUM>, e.g., <NUM>. Fourth, the adhesive material used to form the downconverter layer <NUM> is capable of forming a downconverter that provides a strong bond to a target substrate.

In particular, the adhesive material used in downconverter layer <NUM> may not be adhesive enough to attach directly to a substrate at a first, lower temperature, but becomes adhesive enough at elevated temperature to attach directly to a substrate, such as an 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 adhesive material used to form the downconverter layer <NUM> may be, for example, a siloxane adhesive.

The downconverter material used in downconverter layer <NUM> may be any downconverter material to be used for the application of the micro-LED, and that is compatible with the adhesive material. Downconverter material may include, for example, phosphor particles, such as garnet particles, organic dyes, such as emissive small molecules such as, for example, Alq<NUM> (Al(C<NUM>H<NUM>NO)<NUM>) or polymers such as for example, PPV (Poly(p-phenylene vinylene) , and/or quantum dots, such as, for example, colloidal semiconductor nanocrystals. Downconverter material may also be a scattering agent, such as, for example, TiO<NUM>, that scatters the light emitted by the LED <NUM>, but does not change the color of the light emitted.

The thickness T of the downconverter layer <NUM> is chosen to match the desired target thickness of the downconverter <NUM> 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> may also be formed so that the thickness T is uniform across the layer, for example, T may have a deviation (variation) of less than <NUM>%, for example, less than <NUM>%, across the downconverter layer <NUM> and between downconverter layer pixels <NUM><NUM>, <NUM>, <NUM>. That is, each of the downconverter layer pixels <NUM>, <NUM>, <NUM>, <NUM> has a thickness deviation (variation) of less than <NUM>%, for example, less than <NUM>%, as compared to the thickness T of any of the other downconverter layer pixels <NUM>, <NUM>, <NUM>, <NUM>. Uniform thickness of the downconverter layer pixels <NUM>, <NUM>, <NUM>, <NUM> is advantageous because the more uniform thickness results in a more uniform appearance of the emitted light. Also advantageously, because the downconverter material is in the adhesive layer, there is no need for a separate bonding layer to attach a tile or other layer containing downconverter material to the LED die. This removes an additional processing step in the formation of the lighting device and results in a device in which the downconverter layer is directly in contact with a light emitting surface of the LED die. Light emitted by the LED die does not need to pass through a bonding layer before passing into a downconverter layer. A further advantage is that there is consistent alignment between the downconverter pixels in the resulting device, as compared to those made by the pick-and-place method, in which there can be random variations in alignment as a result of the pick-and-place process.

<FIG> show a flow chart and illustration of an example of the method of using the downconverter layer transfer device <NUM>. A vacuum lamination process may be used to transfer the downconverter layer <NUM> onto an LED array <NUM> similar to the process disclosed in <CIT>.

As shown in <FIG>, at S310, a downconverter layer transfer device <NUM> may be aligned over an LED array <NUM>. The LED array <NUM> includes a plurality of individual LED dies <NUM> on a substrate <NUM>. As shown in <FIG>, the downconverter layer <NUM> of the converter layer bonding device <NUM> is facing a surface <NUM> of the LED array <NUM> to which the downconverter layer <NUM> is to be applied. The surface <NUM> is a surface through which the LED die emits light. Each of the downconverter layer pixels <NUM>, <NUM>, <NUM> is facing surface <NUM> of one of the individual LED die <NUM>. Edges <NUM> of the downconverter layer pixels <NUM>, <NUM>, <NUM> may align with edges <NUM> of the individual LED die <NUM> such that spaces <NUM> between downconverter layer pixels <NUM>, <NUM>, <NUM> are positioned over lanes <NUM> of LED array <NUM>. Achieving good alignment between the individual downconverter layer pixels <NUM>, <NUM>, <NUM> and individual LED die <NUM> is important for improved performance of the micro-LED. A variety of methods may be used to achieve alignment. For example, fiducial markers as are known to persons of ordinary skill in the art may be placed (not shown) on downconverter layer transfer device <NUM> and the LED array <NUM> to allow precise alignment.

As shown in <FIG>, at S320, a vacuum may be applied to the downconverter layer transfer device <NUM> and LED array <NUM>, and at S330, the downconverter layer transfer device <NUM> may be brought into contact with the LED array <NUM> at elevated temperatures. The temperature used depends on the particular adhesive material used in the downconverter layer transfer device <NUM>. In general, the temperature is high enough to make the adhesive material used in the downconverter layer <NUM> flow and become tacky, i.e., adhesive, enough to stick to the surface <NUM> of the LED dies <NUM>. The elevated temperature may be chosen such that the shear modulus G* (at <NUM>) is between shear modulus G* = <NUM> KPa, and shear modulus 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 material used in downconverter layer <NUM> generally maintains its shape and coverage of the LED dies <NUM>, without dripping or leaking over the edges of the LED dies <NUM>. The LED array <NUM> with the downconverter layer transfer device <NUM> attached may then be cooled, for example, back to room temperature.

As shown in <FIG>, at S340, the release liner <NUM> may be removed, leaving the downconverter layer <NUM> behind on the LED array <NUM>. As the adhesive material in downconverter layer <NUM>, after heat treatment, is more strongly attached to the LED dies <NUM> than the release liner <NUM>. The release liner <NUM> may be removed, for instance, mechanically, by peeling the release liner <NUM> off of the downconverter layer <NUM> that is attached to LED array <NUM>. As noted above, the ensure clean transfer, the release liner <NUM> may be designed to have a peel strength between the downconverter layer <NUM> and release liner <NUM> of below <NUM> N/m, for example <NUM>-<NUM> N/m. The downconverter layer <NUM> remains on the LED array <NUM> after removal of release liner <NUM>. In particular, the individual LED dies <NUM> have an individual downconverter layer pixel <NUM>, <NUM>, <NUM> attached to the surface <NUM> of the LED dies <NUM>.

At S350, additional curing of the adhesive material in the downconverter layer <NUM> may be performed after removal of the release liner <NUM>. For example, heat may be applied to fully cure the adhesive material in the downconverter layer <NUM> to convert it to the downconverter <NUM> as shown in <FIG>.

As shown in <FIG>, a fully cured downconverter layer <NUM> is bonded to the LED array <NUM>. In particular, individual downconverter pixels <NUM>, <NUM>, and <NUM>, which may contain different downconverter materials, are bonded to the individual LED dies <NUM> of the LED array <NUM>.

<FIG> illustrate an expanded view of the process of using the downconverter layer transfer device <NUM> at the level of the individual LEDs <NUM> of an LED array <NUM>, and individual downconverter pixels <NUM>, <NUM>, <NUM>. The downconverter layer transfer device <NUM> may be made to be large enough to transfer downconverters to more than one LED array <NUM>, as illustrated in <FIG>. Each LED array <NUM> contains a plurality of LEDs <NUM> to form, for example, a micro-LED. Each LED array <NUM> is positioned on a substrate <NUM>, which may be, for example, a carrier tape as is known by persons of ordinary skill in the art. The full downconverter layer transfer device <NUM> is shown, with the individual downconverter pixels formed in multiple arrays <NUM> on the release liner <NUM>. The vacuum lamination, process disclosed with respect to <FIG> is applied, so as to form downconverters on multiple LED arrays <NUM> in a single manufacturing process.

<FIG> show a flow chart and illustration of a method of making downconverter layer transfer device <NUM>. <FIG> shows release liner <NUM>, which may be a flexible sheet having the properties as described above with respect to release liner <NUM>. At S510, as shown in <FIG>, the release liner <NUM> to be used may be coated with a siliconized coating <NUM> to enhance the release properties as described above. Siliconized coating <NUM> is optional.

As shown in <FIG>, at S520, a patterned photoresist <NUM> may be formed on the release liner <NUM>. To form the patterned photoresist <NUM>, a UV-curable resist is coated on to the release liner <NUM>, and then imprinted to form a pattern structure with the inverse of the desired pixel dimensions. The imprinted UV-curable resist is then cured to leave patterned photoresist <NUM> with "wells" <NUM> in the photoresist pattern. As shown in <FIG>, in this example, patterned photoresist <NUM> is patterned into rows, but other patterns may be used depending on the final pattern of downconverter pixels that is desired. <FIG> also illustrates the patterned photoresist <NUM> having approximately the same width as the "wells" <NUM>, but the sizes may vary and the photoresist <NUM> may be much narrower than the wells <NUM>. In one example, a well may have a width of <NUM> and a depth of <NUM>.

At S530, one or more adhesive downconverter mixtures <NUM>, <NUM> may be prepared by mixing the adhesive material, a downconverter material, and a solvent. For example, a resin and solvent, such as a methylphenylsiloxane resin and cyclohexanone solvent, or, in another example, a silicone resin (such as Dow Coming® LF-<NUM>) and a propylene glycol methyl ether acetate solvent, may be mixed with one or more downconverter materials, such as a phosphor, organic dye, quantum dot, or scattering agent. The downconverter material is dispersed throughout the adhesive. A separate adhesive downconverter mixture <NUM>, <NUM> is formed for each downconverter pixel type to be formed. For instance, if an RGB device is to be formed, an adhesive downconverter mixture with a red downconverter material, an adhesive downconverter mixture with a green downconverter material, and an adhesive downconverter mixture with a scattering agent, may each be formed. Mass ratios of adhesive material to solvent depend on the particular downconverter material used and the desired properties of light emitted from the device, and may be between, for example, <NUM>:<NUM> and <NUM>:<NUM>, or even greater. The concentrations of adhesive material and solvent may be chosen to achieve the desired viscosity of the adhesive downconverter mixtures <NUM>, <NUM>. The viscosity of the adhesive downconverter mixtures <NUM>, <NUM> may be chosen to optimize wetting of the release liner <NUM>, while still achieving the desired thickness T of the resulting downconverter layer <NUM>. For example, the viscosity of the adhesive downconverter mixture <NUM>, <NUM> may in a range between <NUM> and <NUM>,<NUM> mPa.

At S540, as shown in <FIG>, each adhesive downconverter mixture <NUM>, <NUM> is coated onto the patterned regions of the photoresist <NUM>. In particular, each adhesive downconverter mixture <NUM>, <NUM> is coated onto the photoresist <NUM> so as to fill certain of the "well" area <NUM> of the photoresist <NUM> with adhesive downconverter mixture <NUM>, <NUM>. Any method that can suitably coat the release liner <NUM> with a uniform layer of the adhesive downconverter mixture <NUM>, <NUM> at the desired thickness may be used, such as, for example, flexographoic printing, slot-die coating, or rotary screen printing. As shown in <FIG>, different adhesive downconverter mixtures <NUM>, <NUM> are filled into different wells <NUM>. For example, adhesive downconverter mixture <NUM> may be filled into every other well <NUM> and adhesive downconverter mixture <NUM> may fill the wells in between. Multiple, sequential coating passes may be needed for the separate adhesive downconverter mixtures <NUM>, <NUM>. That is, as shown in <FIG>, a first adhesive downconverter mixture <NUM> may be coated into a first sets of the wells <NUM>, and then a second adhesive downconverter mixture <NUM> may be subsequently coated into wells <NUM> not filled with first adhesive downconverter mixture <NUM>. If more than two adhesive downconverter mixtures are to be used, several coating passes, one for each of the different adhesive downconverter mixtures may be used.

At S550, S560 once the adhesive downconverter mixtures <NUM>,<NUM> are coated into the wells <NUM> of photoresist <NUM>, the adhesive downconverter mixtures <NUM>, <NUM> may be dried to form downconverter layer <NUM> having downconverter layer pixels <NUM>, <NUM>. The photoresist <NUM> may be developed to remove the photoresist <NUM>, as shown in <FIG>, leaving downconverter layer transfer device <NUM> with downconverter layer pixels <NUM>, <NUM> on the release liner <NUM>. Removal of the photoresist at S560 may also occur before or during drying and/or partially curing of the adhesive downconverter mixture <NUM>, <NUM> at S550.

Depending on the adhesive used, at S570 the adhesive downconverter mixture may be additionally cured to stabilize the material and improve uniformity of the downconverter layer <NUM>.

An advantage of using a photoresist <NUM> such as shown in <FIG> above is that the edges (or sidewalls) for example, edges <NUM> of downconverter layer pixel <NUM>, are well-defined, and there is no need for mechanical sawing of the downconverter layer to separate the pixels.

<FIG> shows a side view of a portion of an embodiment according to the invention of a downconverter layer transfer device <NUM> that can be used to form pcLEDs, and, in particular, is useful for forming light emitting pixel arrays, such as RGB micro-LEDs. In the downconverter layer transfer device <NUM> shown in FIG. 6A, downconverter layer <NUM> does not contain distinct and defined gaps or spaces between downconverter pixels, such as spaces <NUM> shown in <FIG>. Downconverter layer transfer device <NUM> includes a release liner <NUM> coated with a downconverter layer <NUM>. Downconverter layer <NUM> includes one or more downconverter materials mixed with an adhesive material, as described above. Downconverter layer <NUM> may include portions of different types of downconverter materials in separate downconverter layer pixels <NUM>, <NUM>, <NUM>. For example, downconverter layer pixel <NUM> may include a red downconverter material, downconverter layer pixel <NUM> may include a green downconverter material, and downconverter layer pixel <NUM> may contain only a scattering agent. The number of different downconverter materials used in downconverter layer <NUM>, and the arrangement and colors of the different downconverter layer pixels may vary depending on the desired light emitting device to be manufactured. For example, an RGB pattern of downconverter layer pixels may repeat over the entirety of the downconverter layer transfer device. As shown in FIG. 6A, the different adhesive downconverter pixels <NUM>, <NUM>, <NUM> are adjacent to each other, and may be in contact with each other at junctions <NUM>. At junctions <NUM> there may be some mixing between the downconverter materials of the two adjacent downconverter pixels. The size of the downconverter layer pixels <NUM>, <NUM>, <NUM> is set so that when the downconverter layer <NUM> is transferred to the LED array <NUM>, as described below, the downconverter layer pixels <NUM>, <NUM>, <NUM> are positioned on top of the individual LEDs <NUM>, to produce a light emitting device as shown above in <FIG> and <FIG>.

<FIG> shows a method for using the downconverter layer transfer device <NUM>. The method generally follows the method set forth in the flow chart of <FIG> for downconverter layer transfer device <NUM>, except that the downconverter layer transfer device <NUM> does not contain edges, such as edges <NUM> shown in <FIG>. As shown in <FIG>, a downconverter layer transfer device <NUM> may be aligned over an LED array <NUM>. The LED array <NUM> includes a plurality of individual LED dies <NUM> on a substrate <NUM>. As shown in <FIG>, the downconverter layer <NUM> of the converter layer bonding device <NUM> is facing a surface <NUM> of the LED array <NUM> to which the downconverter layer <NUM> is to be applied. The surface <NUM> is a surface through which LED die <NUM> emits light. Each of the downconverter layer pixels <NUM>, <NUM>, <NUM> is facing surface <NUM> of one of the individual LED die <NUM>. Junctions <NUM> between the downconverter layer pixels <NUM>, <NUM>, <NUM> may align so as to be between edges <NUM> of the individual LED die <NUM> such that junctions <NUM> between downconverter layer pixels <NUM>, <NUM>, <NUM> are positioned over lanes <NUM> of LED array <NUM>. Achieving good alignment between the individual downconverter layer pixels <NUM>, <NUM>, <NUM> and individual LED die <NUM> is important for improved performance of the micro-LED. A variety of methods may be used to achieve alignment, such as fiducial markers, as described above.

As shown in <FIG>, a vacuum may be applied to the downconverter layer transfer device <NUM> and LED array <NUM>, and the downconverter layer transfer device <NUM> may be brought into contact with the LED array <NUM> at elevated temperatures. The temperature used depends on the particular adhesive material used in the downconverter layer transfer device <NUM>, as described above with respect to <FIG>. The LED array <NUM> with the downconverter layer transfer device <NUM> attached may then be cooled, for example, back to room temperature.

As shown in <FIG>, the release liner <NUM> may be removed, leaving the downconverter layer <NUM> behind on the LED array <NUM>. After heat treatment, the portions of the adhesive material in downconverter layer <NUM> positioned on the LED die <NUM> are more strongly attached to the LED dies <NUM> than the release liner <NUM>. The release liner <NUM> may be removed, for instance, mechanically, by peeling the release liner <NUM> off of the downconverter layer <NUM> that is attached to LED array <NUM>. The portions <NUM> of the adhesive material in downconverter layer <NUM> at the junctions <NUM>, which are not in contact with an LED die <NUM>, remain attached to the release liner after heat treatment, and therefore, as shown in <FIG>, when the release liner is removed, the portions <NUM> of the downconverter layer at junction <NUM> are removed with the release liner <NUM>, leaving only the portions of the downconverter layer <NUM> that are positioned on the LED die <NUM> behind. Release liner <NUM> has a peel strength as noted above with respect to <FIG>. After removal of the release liner <NUM> and portions <NUM> of downconverter layer <NUM> at the junctions <NUM>, the individual LED dies <NUM> have an individual downconverter layer pixel <NUM>, <NUM>, <NUM> attached to the surface <NUM> of the LED dies <NUM>.

Additional curing of the adhesive material in the downconverter layer <NUM> may be performed after removal of the release liner <NUM>, resulting in a fully cured downconverter layer <NUM> bonded to the LED array <NUM>, as shown in <FIG>. In particular, individual downconverter pixels <NUM>, <NUM>, and <NUM>, which may contain different downconverter materials, are bonded to the individual LED dies <NUM> of the LED array <NUM>. The lighting device structure in <FIG> is essentially the same as the lighting device structure shown in <FIG> with the exception that without use of the phorotresist to form the downconverter layer <NUM>, the edges of the downconverter pixels <NUM>, <NUM>, <NUM> may not be as well-defined as those of downconverter pixel.

In addition, or as an alternative, to removal of portions <NUM> with removal of the release liner <NUM>, mechanical sawing may be used to separate downconverter layer pixels <NUM>, <NUM>, <NUM> positioned on LED dies <NUM>, and remove any remaining portion <NUM> of the downconverter layer <NUM>.

To prepare downconverter layer transfer device <NUM>, a method similar that shown in <FIG> may be used, except that a photoresist is not used. Thus, similar to S510 of <FIG>, a release liner <NUM> may be optionally coated with a siliconized coating <NUM>. A patterned photoresist is not formed on the release liner <NUM>, that is, S520 is skipped. Similar to S530, one or more adhesive downconverter mixtures may be prepared by mixing the adhesive material, a downconverter material, and a solvent. The adhesive downconverter mixture is then coated onto the release liner in a predetermined pattern to form the downconverter layer <NUM> (<FIG>), the predetermined pattern set so as to form the different downconverter layer pixels <NUM>, <NUM>, <NUM>. This may be achieved using, for example, contact printing techniques such as gravure or flexographic printing.

As shown in <FIG>, the different adhesive downconverter mixtures, when disposed on the release liner <NUM> to form the downconverter layer pixels <NUM>, <NUM>, <NUM> may be in contact with each other at junctions <NUM>, and there may be some mixing of the downconverter materials at the junctions <NUM> within the adhesive forming the downconverter layer <NUM>. However, when used to form the downconverter pixels, as shown in <FIG>, the junctions <NUM> where downconverter mixture may have mixed may be positioned to correspond to the gaps <NUM> between the LED dies <NUM>. The portions <NUM> may then be removed with the release liner when it is removed, as shown in <FIG>, and/or by mechanically removing downconverter layer <NUM> that may remain over gaps <NUM> between dies.

Claim 1:
A downconverter layer transfer device (<NUM>) comprising:
a release liner (<NUM>); and
a downconverter layer (<NUM>) disposed on the release liner, the downconverter layer including a downconverter material dispersed throughout an adhesive, the downconverter layer being solid and non-adhesive at a first temperature, and adhesive at an elevated temperature above the first temperature,
the downconverter layer comprising a first downconverter layer pixel (<NUM>) and a second downconverter layer pixel (<NUM>),
characterised in that
the first and second downconverter layer pixels have different downconverter materials, are disposed on different areas of the release liner, are adjacent to each other, and meet at a junction (<NUM>).