WO2013038304A1

WO2013038304A1 - Reflective coating for a light emitting device mount

Info

Publication number: WO2013038304A1
Application number: PCT/IB2012/054598
Authority: WO
Inventors: Frederic Stephane Diana; Brendan Jude Moran; Jeffrey Dellert Kmetec; Stefan Eberle; Mira Misra; Kuochou Tai; Werner Karl Goetz; Mikhail FOUKSMAN
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2011-09-14
Filing date: 2012-09-06
Publication date: 2013-03-21
Anticipated expiration: 2014-03-14
Also published as: TW201323766A

Abstract

A method according to embodiments of the invention includes disposing on a secondary mount (14) a plurality of light emitting devices (15). Each light emitting device includes a light emitting diode (LED 10) disposed on a primary mount (12). A reflective material (18) is disposed on a surface (16) of the secondary mount between the light emitting devices. After disposing the plurality of light emitting devices on the secondary mount, a lens (26) is disposed over at least one of the light emitting devices (15).

Description

REFLECTIVE COATING FOR A LIGHT EMITTING DEVICE MOUNT BACKGROUND FIELD OF INVENTION

[0001] The present invention relates to a reflective coating for a mount on which a light emitting device is mounted.

DESCRIPTION OF RELATED ART

[0002] Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes such as surface- emitting lasers (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as Ill-nitride materials. Typically, Ill-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, Ill-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p- type regions.

[0003] Fig. 1 illustrates three LEDs mounted on a submount, described in more detail in US 2011/0012149. To form the device illustrated in Fig. 6, conventional LEDs 10A, 10B, and IOC are formed on a growth substrate, then singulated and mounted on a submount wafer 22. A reflective underfill material is prepared. For example, particles of Ti0₂ (appearing white under white light), or other reflective particles such as Zr0₂, are added to a silicone molding compound that is suitable for underfilling. An underfill and reflective layer 54 for each LED is then formed, for example by injection molding. The mold is cooled to solidify the underfill material. The mold is then removed from the wafer 22, leaving hardened underfill material 54 encapsulating each LED and on the wafer 22 surface between each LED. Excess underfill material 54 over the growth substrate of each LED is removed, for example by blasting the entire surface of the wafer 22 with high-velocity microbeads, then the growth substrate for each LED is removed. Phosphor layers 62A, 62B, and 62C may be molded over each LED, resulting in the structures illustrated in Fig. 1. The submount wafer 22 is then singulated to form individual LEDs/submounts.

SUMMARY

[0004] It is an object of the invention to provide a reflective coating on a light emitting device mount.

[0005] A method according to embodiments of the invention includes disposing on a secondary mount a plurality of light emitting devices. Each light emitting device includes a light emitting diode disposed on a primary mount. A reflective material is disposed on a surface of the secondary mount between the light emitting devices. After disposing the plurality of light emitting devices on a secondary mount, a lens is disposed over at least one of the light emitting devices.

[0006] A structure according to embodiments of the invention includes a light emitting device attached to a secondary mount. The light emitting device includes a Ill-nitride light emitting diode disposed on a primary mount. A reflective material is disposed on the secondary mount in an area adjacent to the light emitting device.

[0007] Embodiments of the invention may improve the light output from a light emitting device disposed on a primary and secondary mount, by reducing the amount of light lost to low reflectivity surfaces on the secondary mount.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Fig. 1 illustrates three LEDs mounted on a prior art submount wafer with a reflective layer disposed between the LEDs. [0009] Fig. 2 illustrates a structure including a semiconductor LED disposed on a primary mount.

[0010] Fig. 3 illustrates three devices, each including a semiconductor LED disposed on a primary mount, disposed on a secondary mount.

[0011] Fig. 4 illustrates the structure of Fig. 3 after forming a reflective coating.

[0012] Fig. 5 illustrates the structure of Fig. 4 after removing excess reflective material from the tops of the light emitting devices and the sides of the secondary mount.

[0013] Fig. 6 illustrates the structure of Fig. 5 after forming lenses over each light emitting device.

[0014] Fig. 7 illustrates a secondary mount with three light emitting devices and a single lens. [0015] Fig. 8 illustrates a light emitting device disposed to a secondary mount with a frame. DETAILED DESCRIPTION

[0016] In embodiments of the invention, areas between light emitting devices on a secondary mount are made reflective to reduce light loss. Though in the examples below the semiconductor light emitting devices are Ill-nitride LEDs that emit blue or UV light, semiconductor light emitting devices besides LEDs such as laser diodes and semiconductor light emitting devices made from other materials systems such as other III-V materials, Ill-phosphide, Ill-arsenide, II- VI materials, ZnO, or Si-based materials may be used.

[0017] Fig. 2 illustrates a device 15 including semiconductor light emitting device 10 such as an LED disposed on a primary mount 12. To form the structure illustrated in Fig. 2, a

semiconductor structure is grown on a growth substrate. The growth substrate may be any suitable substrate such as, for example, sapphire, SiC, Si, GaN, or composite substrates. The semiconductor structure includes a light emitting or active region sandwiched between n- and p- type regions. An n-type region may be grown first and may include multiple layers of different compositions and dopant concentration including, for example, preparation layers such as buffer layers or nucleation layers, and/or layers designed to facilitate removal of the growth substrate, which may be n-type or not intentionally doped, and n- or even p-type device layers designed for particular optical, material, or electrical properties desirable for the light emitting region to efficiently emit light. A light emitting or active region is grown over the n-type region.

Examples of suitable light emitting regions include a single thick or thin light emitting layer, or a multiple quantum well light emitting region including multiple thin or thick light emitting layers separated by barrier layers. A p-type region may then be grown over the light emitting region. Like the n-type region, the p-type region may include multiple layers of different composition, thickness, and dopant concentration, including layers that are not intentionally doped, or n-type layers. The total thickness of all the semiconductor material in the device is less than 10 μηι in some embodiments and less than 6 μηι in some embodiments. In some embodiments, the semiconductor material may optionally be annealed at between 200 °C and 800 °C after growth.

[0018] A metal p-contact is formed on the p-type region. If a majority of light is directed out of the semiconductor structure through a surface opposite the p-contact, such as in a flip chip device, the p-contact may be reflective. A vertical device may be formed by attaching the semiconductor structure to primary mount 12 by, for example, thermosonic bonding, removing the growth substrate, and forming a metal n-contact on the surface of the semiconductor structure revealed by removing the growth substrate. A flip chip device may be formed by patterning the semiconductor structure by standard photolithographic operations and etching to remove a portion of the entire thickness of the p-type region and the entire thickness of the light emitting region, to reveal a surface of the n-type region on which a metal n-contact is formed. The p- and n-contacts may be redistributed by a stack of insulating layers and metals. Metal bonding layers may be formed on the n- and p-contacts. The LED 10 is then attached to primary mount 12, for example by soldering, thermosonic bonding with, for example, gold interconnects, or any other suitable bonding technique. The bonding layers or an additional underfill layer may support the semiconductor structure during removal of all or part of the growth substrate, or the growth substrate may remain part of the final device.

[0019] Additional layers such as wavelength converting layers, filter layers, dichroic layers, or optics may be formed over LED 10, after attaching LED 10 to primary mount 12, or over multiple LEDs after multiple devices are disposed on a secondary mount, as described below. LED 10 may be attached to primary mount 12 when primary mount 12 is still attached to a wafer of primary mounts. The surface 19 of primary mount 12 between neighboring LEDs may be made reflective, for example as described above in reference to Fig. 1.

[0020] Primary mount 12 may be a material with high thermal conductivity, such as ceramic or copper. Such materials may be expensive, compared to other submount materials such as, for example, silicon. With expensive materials, primary mount 12 may be made as small as possible.

[0021] Multiple devices 15 may be packaged on a single secondary mount, for example in a linear or two-dimensional array. As used herein, "device 15" or "light emitting device 15" refers to the structure illustrated in Fig. 2. Fig. 3 illustrates three of the devices 15 illustrated in Fig. 2 attached to a secondary mount 14. Devices 15 may be attached to secondary mount 14 by, for example, soldering. Secondary mount 14 may be, for example, a printed circuit board or a silicon mount. Secondary mount 14 provides mechanical support, heat dissipation, and electrical connection to devices 15. Particularly in cases where the area of primary mount 12 of Fig. 2 is limited, as described above, the regions 16 on secondary mount 14 between devices 15 may be a significant source of light loss.

[0022] Figs. 4-6 illustrate forming a reflective surface on secondary mount 14, according to embodiments of the invention. In Fig. 4, a material 18 that is highly reflective, over a range of angles of incidence and/or over a range of wavelengths, is disposed over devices 15 and secondary mount 14 such that material 18 fills the spaces 16 between devices 15. Reflective material 18 may be, for example, a Ti0₂- or V0₂-based white coating, or white pigments disposed in silicone. Other materials may be added to the mixture to optimize the thermo- mechanical properties of the material. In some embodiments, the coating of reflective material 18 is an electrical insulator. In some embodiments, the coating of reflective material 18 protects secondary mount 14 from environmental corrosion, heat from devices 15, and blue or UV light from devices 15. In particular, the coating may be formed as a barrier to corrosion of metal layers on secondary mount 14. A coating of reflective material 18 may be formed by any suitable technique including jetting, screen-printing, spray-coating, or a photolithography process with evaporation steps. The coating of reflective material 18 is formed thick enough to cover the low reflectivity surfaces of secondary mount 14 between devices 15.

[0023] In an optional step illustrated in Fig. 5, excess reflective material is removed, exposing the tops 22 of devices 15, to allow light to escape from devices 15, leaving reflective material 18 between devices 15. Excess reflective material may also be removed from the edges 20 of secondary mount 14, for example to allow electrical connectivity to this surface. In some embodiments, the coating of reflective material is a preformed sheet of appropriate size with openings as necessary, or is deposited such that removal of excess reflective material from the tops of devices 15 is not necessary.

[0024] Fig. 6 illustrates the structure of Fig. 5 after dome lenses 24 are formed over each light emitting device 15. Lenses 24 may be formed and disposed over devices 15 by any suitable technique. In some embodiments, lenses 24 are pre-formed lenses that are glued or adhered to devices 15 and/or secondary mount 14, or otherwise disposed over devices 15. Alternatively, lenses 24 may be formed in a low pressure overmolding process, as follows: A mold with indentations corresponding to the positions of the devices 15 on the secondary mount 14 is provided. The indentations are filled with a liquid, optically transparent material, such as silicone, which when cured forms a hardened lens material. The shape of the indentations will be the shape of the lens. The mold and the LED dice/support structure are brought together so that each LED die resides within the liquid lens material in an associated indentation. The mold is then heated to cure (harden) the lens material. The mold and the support structure are then separated, leaving a lens over each LED die. Alternatively, lenses 24 may be formed by high pressure injection molding, where the liquid material is injected at high pressure after an empty mold is encased around the object to be encapsulated. In Fig. 6, a single lens is formed over each light emitting device 15 but multiple lenses over a single device are contemplated and are within the scope of the invention.

[0025] Fig. 7 illustrates a single lens 26 formed over multiple light emitting devices 15. Lens 26 may be formed with the same techniques described above for lenses 24 of Fig. 6. Devices 15 may be more closely spaced in Fig. 7 as compared to Fig. 6, though they need not be. [0026] Both lenses 24 of Fig. 6 and lens 26 of Fig. 7 may be an optical component with a high index of refraction, for example greater than 1.5 in some embodiments and greater than 2 in some embodiments. In some embodiments, lenses 24 of Fig. 6 or lens 26 of Fig. 7 may be any suitable shape, such as dome lenses as illustrated, half-dome lenses, or Fresnel lenses. In some embodiments, wavelength converting material such as phosphor, scattering structures or materials, or other materials may be disposed within lenses 24 of Fig. 6 or lens 26 of Fig. 7. Wavelength converting materials, optical coatings such as filters, or other materials may be disposed over devices before the application of lenses, or over lenses 24 of Fig. 6 or lens 26 of Fig. 7. In some embodiments, no lenses are formed over light emitting devices 15.

[0027] Though Figs. 3, 4, 5, 6, and 7 illustrate multiple devices 15 on a secondary mount 14, embodiments of the invention may be applicable to a single device 15, as illustrated in Fig. 8. A single device 15 is disposed on a secondary mount 14. An optional frame 30 may be disposed on secondary mount 14 around device 15, or alternatively an optional reflector cup may be formed in secondary mount 14. The side walls of frame 30 or a reflector cup may be reflective, in some embodiments. A coating of reflective material 18 is disposed around device 15, as described above. A lens 28 may be disposed over device 15, by any of the techniques described above in reference to Fig. 6.

[0028] Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. For example, different elements of different embodiments may be combined to form new embodiments. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.

Claims

CLAIMS What is being claimed is:

1. A method comprising:

disposing on a secondary mount a plurality of light emitting devices, each light emitting device comprising light emitting diode disposed on a primary mount;

disposing a reflective material on a surface of the secondary mount between the light emitting devices; and

after disposing the plurality of light emitting devices on a secondary mount, disposing a lens over at least one of the light emitting devices.

2. The method of claim 1 wherein each primary mount comprises a ceramic mount.

3. The method of claim 1 wherein each light emitting diode comprises a Ill-nitride light emitting layer.

4. The method of claim 1 wherein disposing a reflective material comprises forming a coating comprising a reflective material.

5. The method of claim 4 wherein the coating comprises a white material disposed in silicone.

6. The method of claim 4 wherein the coating is formed by one of jetting, screen- printing, spray-coating, or evaporation.

7. The method of claim 1 wherein a lens is disposed over each light emitting device.

8. The method of claim 1 wherein a single lens is disposed over multiple light emitting devices.

9. The method of claim 1 wherein the secondary mount is a PC board.

10. The method of claim 1 wherein disposing a lens comprises:

aligning a mold with the at least one light emitting device;

filling the mold with liquid lens material;

treating the liquid lens material to form a solid lens; and

removing the mold.

1 1. A structure comprising:

a light emitting device comprising a Ill-nitride light emitting diode disposed on a primary mount; a secondary mount, wherein the light emitting device is attached to the secondary mount; and

a reflective material disposed on the secondary mount in an area adjacent to the light emitting device.

12. The structure of claim 1 1 wherein the light emitting device is a first light emitting device, the structure further comprising a second emitting device comprising a Ill-nitride light emitting diode disposed on a primary mount, wherein:

the second light emitting device is attached to the secondary mount; and

the reflective material is disposed between the first and second light emitting devices.

13. The structure of claim 12 further comprising a lens disposed over the first and second light emitting devices.

14. The structure of claim 12 further comprising a first lens disposed over the first light emitting device and a second lens disposed over the second light emitting device.

15. The structure of claim 1 1 wherein the reflective material is set back from an edge of the secondary mount.

16. The structure of claim 1 1 further comprising a lens disposed over the light emitting device.