U.S. patent application number 12/099021 was filed with the patent office on 2008-08-07 for array of light emitting devices to produce a white light source.
This patent application is currently assigned to Lumileds Lighting U.S., LLC. Invention is credited to Robertus G. Alferink, Michael D. Camras, William R. Imler, Michael R. Krames, Frank M. Steranka, Helena Ticha, Ladislav Tichy, Franklin J. Wall.
Application Number | 20080186702 12/099021 |
Document ID | / |
Family ID | 35709278 |
Filed Date | 2008-08-07 |
United States Patent
Application |
20080186702 |
Kind Code |
A1 |
Camras; Michael D. ; et
al. |
August 7, 2008 |
Array of Light Emitting Devices to Produce a White Light Source
Abstract
A device is provided with an array of a plurality of phosphor
converted light emitting devices (LEDs) that produce broad spectrum
light. The phosphor converted LEDs may produce light with different
correlated color temperature (CCT) and are covered with an optical
element that assists in mixing the light from the LEDs to produce a
desired correlated color temperature. The phosphor converted LEDs
may also be combined in an array with color LEDs. The color LEDs
may be controlled to vary their brightness such that light with an
approximately continuous broad spectrum is produced. By controlling
the brightness of the color LEDs, light can be produced with a
fixed brightness over a large range of white points with a high
color rendering quality.
Inventors: |
Camras; Michael D.;
(Sunnyvale, CA) ; Imler; William R.; (Oakland,
CA) ; Wall; Franklin J.; (Vacaville, CA) ;
Steranka; Frank M.; (San Jose, CA) ; Krames; Michael
R.; (Mountain View, CA) ; Ticha; Helena;
(Racanska, CZ) ; Tichy; Ladislav; (Racanska,
CZ) ; Alferink; Robertus G.; (Son, NL) |
Correspondence
Address: |
PATENT LAW GROUP LLP
2635 NORTH FIRST STREET, SUITE 223
SAN JOSE
CA
95134
US
|
Assignee: |
Lumileds Lighting U.S., LLC
San Jose
CA
|
Family ID: |
35709278 |
Appl. No.: |
12/099021 |
Filed: |
April 7, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10987241 |
Nov 12, 2004 |
|
|
|
12099021 |
|
|
|
|
Current U.S.
Class: |
362/231 ;
257/E33.073; 362/235 |
Current CPC
Class: |
H05B 45/20 20200101;
H01L 2924/0002 20130101; H01L 25/0753 20130101; H01L 33/58
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
362/231 ;
362/235 |
International
Class: |
F21V 9/10 20060101
F21V009/10 |
Claims
1. An apparatus comprising: at least one phosphor converted light
emitting device that produces light with a broad spectrum; a
plurality of color light emitting devices that produce light with a
narrow spectrum; and a controller for controlling the brightness of
the light produced by the color light emitting devices, wherein the
combination of the broad spectrum light produced by the at least
one phosphor converted light emitting device and the narrow spectra
light from the brightness controlled plurality of color light
emitting devices produces a resulting spectrum that is more
continuous than the broad spectrum produced by the at least one
phosphor converted light emitting devices.
2. The apparatus of claim 1, further comprising a board, the at
least one phosphor converted light emitting device and the
plurality of color light emitting devices are mounted on the
board.
3. The apparatus of claim 1, wherein there is a greater number of
phosphor converted light emitting devices than color light emitting
devices.
4. The apparatus of claim 3, wherein there is over twice as many
phosphor converted light emitting devices as color light emitting
devices.
5. The apparatus of claim 1, wherein the color light emitting
devices produce blue, cyan, amber and red light.
6. The apparatus of claim 1, wherein the controller individually
controls the brightness of the color light emitting devices to vary
the correlated color temperature of the resulting spectrum.
7. A method comprising: providing at least one phosphor converted
light emitting device that produces light with a broad spectrum;
providing a plurality of color light emitting devices, each of
which produces light with a narrow spectrum; mixing the light
produced by the at least one phosphor converted light emitting
device and the plurality of color light emitting devices to produce
light with a resulting spectrum that is more continuous than the
broad spectrum produced by the at least one phosphor converted
light emitting devices; controlling the brightness of the light
produced by the plurality of color light emitting devices to vary
the correlated color temperature of the resulting spectrum.
8. The method of claim 7, further providing a board and mounting
the at least one phosphor converted light emitting device and the
plurality of color light emitting devices on the board.
9. The method of claim 7, wherein a greater number of phosphor
converted light emitting devices are provided than color light
emitting devices.
10. The method of claim 9, wherein over twice as many phosphor
converted light emitting devices as color light emitting devices
are provided.
11. The method of claim 7, wherein the color light emitting devices
produce blue, cyan, amber and red light.
12. The method of claim 7, wherein the brightness of the light
produced by each color light emitting device is controlled
separately to vary the correlated color temperature.
13. The method of claim 7, wherein the brightness of each different
colored light produced by the color light emitting devices is
controlled separately to vary the correlated color temperature.
14. A method comprising: providing a plurality of phosphor
converted light emitting devices that produce light with a broad
spectrum, the phosphor converted light emitting devices producing
light with different correlated color temperature; arranging the
plurality of phosphor converted light emitting devices in an array;
and covering the array of phosphor converted light emitting devices
with an optical element that assists mixing of the light with
different correlated color temperatures to produce light with a
desired correlated color temperature.
15. The method of claim 14, wherein the optical element is bonded
to the phosphor converted light emitting devices.
16. The method of claim 14, wherein the optical element is a dome
mounted over the phosphor converted light emitting devices and
filled with an encapsulant.
17. The method of claim 14, wherein arranging the plurality of
phosphor converted light emitting devices comprises mounting the
plurality of phosphor converted light emitting devices to a
board.
18. The method of claim 14, the method further comprising:
providing a plurality of color light emitting devices, each of
which produces light with a narrow spectrum; arranging the
plurality of color light emitting devices in the array with the
plurality of phosphor converted light emitting devices.
19. The method of claim 18, further comprising controlling the
brightness of the light produced by the plurality of color light
emitting devices to vary the correlated color temperature of the
resulting spectrum.
20. An apparatus that produces broadband light with a desired
correlated color temperature, the apparatus comprising: an array of
a plurality of phosphor converted light emitting devices that
produce light with a broad spectrum with different correlated color
temperatures; and an optical element disposed over the array of the
plurality of phosphor converted light emitting devices, the optical
element mixing the light with different correlated color
temperatures to produce light with the desired correlated color
temperature.
21. The apparatus of claim 20, wherein the optical element is
bonded to the phosphor converted light emitting devices.
22. The apparatus of claim 20, wherein the optical element is a
dome mounted over the phosphor converted light emitting devices and
filled with an encapsulant.
23. The apparatus of claim 20, further comprising a board to which
the array of the plurality of phosphor converted light emitting
devices is mounted.
24. The apparatus of claim 20, wherein the array further comprises
a plurality of color light emitting devices.
25. The apparatus of claim 24, the apparatus further comprising a
controller for controlling the brightness of the light produced by
the plurality of color light emitting devices to vary the
correlated color temperature to the desired correlated color
temperature.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is a divisional application of and
claims priority to U.S. patent application Ser. No. 10/987,241,
filed Nov. 12, 2004, entitled "Bonding an Optical Element to a
Light Emitting Device", by Michael D. Camras et al, which is
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates generally to light emitting
devices and, more particularly, to an array of light emitting
devices to produce a white light source.
BACKGROUND
[0003] Adding or mixing a number of different color light emitting
devices (LEDs) can be used to produce light with a broad spectrum.
The spectrum produced, however, consists of the peaks of the narrow
band spectra produced by the individual LEDs. Consequently, the
color rendering of such a light source is poor. White light sources
with high color rendering, such as that produced by a halogen lamp,
have a continuous or near continuous spectrum over the full visible
light spectrum (400-700 nm).
[0004] Thus, a white light source with high color rendering that is
produced using an array of LEDs is desired
SUMMARY
[0005] In accordance with one embodiment of the present invention,
a plurality of phosphor converted light emitting devices may be
combined in an array to obtain light with a desired correlated
color temperature (CCT). In one embodiment, the phosphor converted
light emitting devices produce light with different CCTs. An array
of the plurality of phosphor converted light emitting devices may
be covered with an optical element that optionally can be filled
with a material that assists in light extraction and mixing the
light to produce light with the desired CCT. In another embodiment,
a plurality of color light emitting devices are combined with the
plurality of phosphor converted light emitting devices and the
brightness of the color light emitting devices are controlled to
produce light with the desired characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1A illustrates a side view of an LED die mounted on a
submount and an optical element that is to be bonded to the LED
die.
[0007] FIG. 1B illustrates the optical element bonded to the LED
die.
[0008] FIG. 2 illustrates an embodiment in which multiple LED dice
are mounted to a submount and a separate optical element is bonded
to each LED die.
[0009] FIG. 3 illustrates an embodiment in which multiple LED dice
are mounted to a submount and a single optical element is bonded to
the LED dice.
[0010] FIG. 4 is a flow chart of one implementation of producing
such an LED device with wavelength converting material covering the
optical element.
[0011] FIG. 5 illustrates an embodiment in which a layer of
wavelength converting material is disposed between the bonding
layer and the optical element.
[0012] FIG. 6 illustrates an embodiment in which a layer of
wavelength converting material is deposited on the LED die.
[0013] FIG. 7 illustrates an array of LEDs, which are mounted on a
board.
[0014] FIG. 8 is a graph of the broad spectrum produced by a
phosphor converted blue LED.
[0015] FIG. 9 is a CIE chromaticity diagram for the spectrum shown
in FIG. 8.
[0016] FIG. 10 is a graph of the spectra produced by phosphor
converted LEDs and colored LEDs, which are combined to produce an
approximately continuous spectrum.
[0017] FIG. 11 is a portion of a CIE chromaticity diagram that
shows the variation in the CCT that may be produced by varying the
brightness of the colored LEDs.
[0018] FIG. 12 is a portion of another CIE chromaticity diagram
that illustrates variable CCT values for an array of 29 phosphor
converted LEDs and 12 color LEDs.
DETAILED DESCRIPTION
[0019] FIG. 1A illustrates a side view of a transparent optical
element 102 and a light emitting diode (LED) die 104 that is
mounted on a submount 106. The optical element 102 is to be bonded
to the LED die 104 in accordance with an embodiment of the present
invention. FIG. 1B illustrates the optical element 102 bonded to
the LED die 104.
[0020] The term "transparent" is used herein to indicate that the
element so described, such as a "transparent optical element,"
transmits light at the emission wavelengths of the LED with less
than about 50%, preferably less than about 10%, single pass loss
due to absorption or scattering. The emission wavelengths of the
LED may lie in the infrared, visible, or ultraviolet regions of the
electromagnetic spectrum. One of ordinary skill in the art will
recognize that the conditions "less than 50% single pass loss" and
"less than 10% single pass loss" may be met by various combinations
of transmission path length and absorption constant.
[0021] LED die 104 illustrated in FIGS. 1A and 1B includes a first
semiconductor layer 108 of n-type conductivity (n-layer) and a
second semiconductor layer 110 of p-type conductivity (p-layer).
Semiconductor layers 108 and 110 are electrically coupled to an
active region 112. Active region 112 is, for example, a p-n diode
junction associated with the interface of layers 108 and 110.
Alternatively, active region 112 includes one or more semiconductor
layers that are doped n-type or p-type or are undoped. LED die 104
includes an n-contact 114 and a p-contact 116 that are electrically
coupled to semiconductor layers 108 and 110, respectively. Contact
114 and contact 116 are disposed on the same side of LED die 104 in
a "flip chip" arrangement. A transparent superstrate 118 coupled to
the n layer 108 is formed from a material such as, for example,
sapphire, SiC, GaN, GaP, diamond, cubic zirconia (ZrO2), aluminum
oxynitride (AlON), AlN, spinel, ZnS, oxide of tellurium, oxide of
lead, oxide of tungsten, polycrystalline alumina oxide (transparent
alumina), and ZnO.
[0022] Active region 112 emits light upon application of a suitable
voltage across contacts 114 and 116. In alternative
implementations, the conductivity types of layers 108 and 110,
together with respective contacts 114 and 116, are reversed. That
is, layer 108 is a p-type layer, contact 114 is a p-contact, layer
110 is an n-type layer, and contact 116 is an n-contact.
[0023] Semiconductor layers 108 and 110 and active region 112 may
be formed from III-V semiconductors including but not limited to
AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb,
II-VI semiconductors including but not limited to ZnS, ZnSe, CdSe,
ZnO, CdTe, group IV semiconductors including but not limited to Ge,
Si, SiC, and mixtures or alloys thereof.
[0024] Contacts 114 and 116 are, in one implementation, metal
contacts formed from metals including but not limited to gold,
silver, nickel, aluminum, titanium, chromium, platinum, palladium,
rhodium, rhenium, ruthenium, tungsten, and mixtures or alloys
thereof.
[0025] Although FIGS. 1A and 1B illustrate a particular structure
of LED die 104, the present invention is independent of the
structure of the LED die. Accordingly, other types of LED
configurations may be used instead of the specific configuration
shown. Further, the number of semiconductor layers in LED die 104
and the detailed structure of active region 112 may differ. It
should be noted that dimensions of the various elements of LED die
104 illustrated in the various figures are not to scale.
[0026] The LED die 104 is mounted to submount 106 via contacts
elements 120, such as solder bumps, pads, or other appropriate
elements, such as a layer of solder. Contact elements 120 will be
sometimes referred to herein as bumps 120 for the sake of
simplicity. Bumps 120 are manufactured from Au, Sn, Ag, Sb, Cu, Pb,
Bi, Cd, In, Zn or alloys thereof including AuSn, SnSb, SnCu, SnAg,
SnAgBi, InSn, BiPbSn, BiPbCd, BiPbIn, InCd, BiPb, BiSn, InAg, BiCd,
InBi, InGa, or other appropriate material with a melting
temperature that is greater than the temperature that will be used
to bond the optical element 102 to the LED die 104, but is
preferably Au or AuSn. In one implementation, the melting
temperature of bumps 120 is greater than 250.degree. C. and
preferably greater than 300.degree. C. The submount 106 may be,
e.g., silicon, alumina or AlN and may include vias for backside
connections.
[0027] The LED die 104 is mounted to the submount 106, e.g., using
thermosonic bonding. For example, during the thermosonic bonding
process, the LED die 104 with bumps 120 are aligned with the
submount 106 in the desired position while the submount 106 is
heated to approximately 150-160.degree. C. A bond force of, e.g.,
approximately 50-100 gm/bump, is applied to the LED die 104 by a
bonding tool, while ultrasonic vibration is applied. If desired
other processes may be used, such as thermo-compression, to bond
the LED die 104 to the submount 106. As is well known in the art,
with thermo-compression higher temperatures and greater bonding
forces are typically required.
[0028] In some embodiments, an underfill may be used with the LED
die 104 and submount 106. The underfill material may have good
thermal conductivity and have a coefficient of thermal expansion
that approximately matches the LED die 104 and the submount 106. In
another embodiment, a protective side coat, e.g., of silicone or
other appropriate material, may be applied to the sides of the LED
die 104 and the submount 106. The protective side coating acts as a
sealant and limits exposure of the LED 104 and the bumps 120 to
contamination and the environment.
[0029] For more information regarding producing bumps 120 from Au
or Au/Sn and for submounts with backside vias and bonding LED dice
with Au or Au/Sn bumps to a submount, see U.S. Ser. No. 10/840,459,
by Ashim S. Haque, filed May 5, 2004, which has the same assignee
as the present disclosure and is incorporated herein by reference.
It should be understood, however, that the present invention is not
limited to any specific type of submount and that any desired
submount configuration may be used if desired.
[0030] After the LED die 104 is mounted to the submount 106, the
optical element 102 is thermally bonded to the LED die 104. In one
embodiment, a layer of bonding material is applied to the bottom
surface of the optical element 102 to form transparent bonding
layer 122 that is used to bond optical element 102 to LED die 104.
In some embodiments, the transparent bonding layer 122 may be
applied to the top surface of the LED die 104, e.g., to superstrate
118, (as indicated by the dotted lines 122 in FIG. 1A). The bonding
layer 122 can be applied to the LED die 104 prior to or after
mounting the LED die 104 to the submount 106. Alternatively, no
bonding layer may be used and the optical element 102 may be bonded
directly to the LED die 104, e.g., the superstrate 118. The
transparent bonding layer 122 is, for example, about 10 Angstroms
(.ANG.) to about 100 microns (.mu.m) thick, and is preferably about
1000 .ANG. to about 10 .mu.m thick, and more specifically, about
0.5 .mu.m to about 5 .mu.m thick. The bonding material is applied,
for example, by conventional deposition techniques including but
not limited to spinning, spraying, sputtering, evaporation,
chemical vapor deposition (CVD), or material growth by, for
example, metal-organic chemical vapor deposition (MOCVD), vapor
phase epitaxy (VPE), liquid phase epitaxy (LPE), or molecular beam
epitaxy (MBE). In one embodiment, the optical element 102 may be
covered with a wavelength converting material 124, which will be
discussed in more detail below.
[0031] In one implementation, the bonding material from which
transparent bonding layer 122 is formed from glass such as SF59,
LaSF 3, LaSF N18, SLAH51, LAF10, NZK7, NLAF21, LASFN35, SLAM60, or
mixtures thereof, which are available from manufactures such as
Schott Glass Technologies Incorporated, of Duryea, Pa. and Ohara
Corporation in Somerville, N.J. Bonding layer 122 may also be
formed from a high index glass, such as (Ge, As, Sb, Ga)(S, Se, Te,
Cl, Br) chalcogenide or chalcogen-halogenide glasses, for
example.
[0032] In other implementations, bonding layer 122 may be formed
from III-V semiconductors including but not limited to GaP, InGaP,
GaAs, and GaN; II-VI semiconductors including but not limited to
ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe; group IV semiconductors and
compounds including but not limited to Si, and Ge; organic
semiconductors, metal oxides including but not limited to oxides of
antimony, bismuth, boron, copper, niobium, tungsten, titanium,
nickel, lead, tellurium, phosphor, potassium, sodium, lithium,
zinc, zirconium, indium tin, or chromium; metal fluorides including
but not limited to magnesium fluoride, calcium fluoride, potassium
fluoride, sodium fluoride, and zinc fluoride; metals including but
not limited to Zn, In, Mg, and Sn; yttrium aluminum garnet (YAG),
phosphide compounds, arsenide compounds, antimonide compounds,
nitride compounds, high index organic compounds; and mixtures or
alloys thereof.
[0033] In implementations where the LED die 104 is configured with
the n-contact and p-contact on opposite sides of the die 104, the
transparent bonding layer 122 or 122' may be patterned with, for
example, conventional photolithographic and etching techniques to
leave the top contact uncovered by bonding material and thus to
permit contact to make electrical contact with a metallization
layer on the optical element 102, which may serve as a lead, as is
described in U.S. Ser. No. 09/880,204, filed Jun. 12, 2001, by
Michael D. Camras et al., entitled "Light Emitting Diodes with
Improved Light Extraction Efficiency" having Pub. No. 2002/0030194,
which is incorporated herein by reference.
[0034] In one implementation, the optical element 102 is formed
from optical glass, high index glass, GaP, CZ, ZnS, SiC, sapphire,
diamond, cubic zirconia (ZrO2), AlON, by Sienna Technologies, Inc.,
polycrystalline aluminum oxide (transparent alumina), spinel,
Schott glass LaFN21, Schott glass LaSFN35, LaF2, LaF3, and LaF10
available from Optimax Systems Inc. of Ontario, N.Y., an oxide of
Pb, Te, Zn, Ga, Sb, Cu, Ca, P, La, Nb, or W, or any of the
materials listed above for use as bonding materials in transparent
bonding layer 122, excluding thick layers of the metals.
[0035] The transparent optical element 102 may have a shape and a
size such that light entering optical element 102 from LED die 104
will intersect surface 102a of optical element 102 at angles of
incidence near normal incidence. Total internal reflection at the
interface of surface 102a and the ambient medium, typically air, is
thereby reduced. In addition, since the range of angles of
incidence is narrow, Fresnel reflection losses at surface 102a can
be reduced by applying a conventional antireflection coating to the
surface 102a. The shape of optical element 102 is, for example, a
portion of a sphere such as a hemisphere, a Weierstrass sphere
(truncated sphere), or a portion of a sphere less than a
hemisphere. Alternatively, the shape of optical element 102 is a
portion of an ellipsoid such as a truncated ellipsoid. The angles
of incidence at surface 102a for light entering optical element 102
from LED die 4 more closely approach normal incidence as the size
of optical element 102 is increased. Accordingly, the smallest
ratio of a length of the base of transparent optical element 102 to
a length of the surface of LED die 104 is preferably greater than
about 1, more preferably greater than about 2.
[0036] After the LED die 104 is mounted on the submount 106, the
optical element 102 is thermally bonded to the LED die 104. For
example, to bond the optical element 102 to the LED die 104, the
temperature of bonding layer 122 is raised to a temperature between
about room temperature and the melting temperature of the contact
bumps 120, e.g., between approximately 150.degree. C. to
450.degree. C., and more particularly between about 200.degree. C.
and 400.degree. C., and optical element 102 and LED die 104 are
pressed together at the bonding temperature for a period of time of
about one second to about 6 hours, preferably for about 30 seconds
to about 30 minutes, at a pressure of about 1 pound per square inch
(psi) to about 6000 psi. By way of example, a pressure of about 700
psi to about 3000 psi may be applied for between about 3 to 15
minutes.
[0037] The thermal bonding of the optical element 102 to the LED
die 104 requires the application of elevated temperatures. With the
use of bumps 120 that have a high melting point, i.e., higher than
the elevated temperature used in the thermal bonding process, the
LED die 104 may be mounted to the submount 106 before the optical
element 102 is bonded to the LED die 104 without damaging the LED
die/submount connection. Mounting the LED die 104 to the submount
106 prior to bonding the optical element 102 simplifies the pick
and place process.
[0038] Bonding an optical element 102 to an LED die 104 is
described in US Pub. No. 2002/0030194; Ser. No. 10/633,054, filed
Jul. 31, 2003, by Michael D. Camras et al., entitled "Light
Emitting Devices with Improved Light Extraction Efficiency"; Ser.
No. 09/660,317, filed Sep. 12, 2000, by Michael D. Camras et al.,
entitled "Light Emitting Diodes with Improved Light Extraction
Efficiency; Ser. No. 09/823,841, filed Mar. 30, 2001, by Douglas
Pocius, entitled "Forming an Optical Element on the Surface of a
Light Emitting Device for Improved Light Extraction" having Pub.
No. 2002/0141006, which have the same assignee as the present
application and which are incorporated herein by reference.
Further, the process of bonding optical element 102 to LED die 104
described above may be performed with devices disclosed in U.S.
Pat. Nos. 5,502,316 and 5,376,580, incorporated herein by
reference, previously used to bond semiconductor wafers to each
other at elevated temperatures and pressures. The disclosed devices
may be modified to accommodate LED dice and optical elements, as
necessary. Alternatively, the bonding process described above may
be performed with a conventional vertical press.
[0039] It should be noted that due to the thermal bonding process,
a mismatch between the coefficient of thermal expansion (CTE) of
optical element 102 and LED die 104 can cause optical element 102
to detach from LED die 104 upon heating or cooling. Accordingly,
optical element 102 should be formed from a material having a CTE
that approximately matches the CTE of LED die 104. Approximately
matching the CTEs additionally reduces the stress induced in the
LED die 104 by bonding layer 122 and optical element 102. With
suitable CTE matching, thermal expansion does not limit the size of
the LED die that may be bonded to the optical element and, thus,
the optical element 102 may be bonded to a large LED die 104, e.g.,
up to 16 mm.sup.2 or larger.
[0040] FIG. 2 illustrates an embodiment in which multiple LED dice
204a, 204b, and 204c (sometimes collectively referred to as LED
dice 204) are mounted on a submount 206. The LED dice 204 are
schematically illustrated in FIG. 2 without showing the specific
semiconductor layers. Nevertheless, it should be understood that
the LED dice 204 may be similar to LED die 104 discussed above.
[0041] The LED dice 204 are each mounted to submount 206 as
described above. Once the LED dice 204 are mounted on submount 206,
individual optical elements 202a, 202b, and 202c are bonded to LED
dice 204a, 204b, and 204c, respectively, in a manner such as that
described above.
[0042] If desired, the LED dice 204 may be the same type of LED and
may produce the same wavelengths of light. In another
implementation, one or more of the LED dice 204 may produce
different wavelengths of light, which when combined may be used to
produce light with a desired correlated color temperature (CCT),
e.g., white light. Another optical element (not shown in FIG. 2)
may be used to cover optical elements 202a, 202b, and 202c and aid
in mixing the light.
[0043] FIG. 3 illustrates an embodiment of an LED device 300 that
includes multiple LED dice 304a, 304b, and 304c (sometimes
collectively referred to as LED dice 304) mounted on a submount 306
and a single optical element 302 bonded to the LED dice 304. The
LED dice 304 may be similar to LED die 104 discussed above.
[0044] The use of a single optical element 302 with multiple LED
dice 304, as shown in FIG. 3, is advantageous as the LED dice 304
can be mounted close together on submount 306. Optical components
typically have a larger footprint than an LED die to which it is
bonded, and thus, the placement of LED dice with separate optical
elements is constrained by the size of the optical elements.
[0045] After the LED dice 304 are mounted to the submount, there
may be slight height variations in the top surfaces of the LED dice
304, e.g., due to the differences in the height of the bumps 320
and thickness of the dice. When the single optical element 302 is
thermally bonded to the LED dice 304, any differences in the height
of the LED dice 304 may be accommodated by the compliance of the
bumps 320.
[0046] During the thermal bonding process of the optical element
302 to the LED dice 304, the LED dice 304 may shift laterally due
to the heating and cooling of the submount 306. With the use of
some bumps 320, such as Au, the compliance of the bumps 320 can be
inadequate to accommodate lateral shift of the LED dice 304.
Accordingly, the coefficient of thermal expansion of the optical
element 302 (CTE.sub.302) should approximately match the
coefficient of thermal expansion of the submount 306 (CTE.sub.306).
With an approximate match between CTE.sub.302 and CTE.sub.306 any
movement of the LED dice 304 caused by the expansion and
contraction of the submount 306 will be approximately matched by
the expansion and contraction of the optical element 302. A
mismatch between CTE.sub.302 and CTE.sub.306, on the other hand,
can result in the detachment of the LED dice 304 from the optical
element 302 or other damage to the LED device 300, during the
heating and cooling of the thermal bonding process.
[0047] With the use of sufficiently small LED dice 304, the thermal
expansion of the LED dice 304 themselves during the thermal bonding
process may be minimized. With the use of large LED dice 304,
however, the amount of thermal expansion of the LED dice 304 during
the thermal bonding process may be large and thus, the CTE for the
LED dice 304 also should be appropriately matched to the CTE of the
submount 306.
[0048] The LED dice 304 may be, e.g., InGaN, AlInGaP, or a
combination of InGaN and AlInGaP devices. In one implementation,
the submount 302 may be manufactured from AlN, while the optical
element 302 may be manufactured from, e.g., SLAM60 by Ohara
Corporation, or NZK7 available from Schott Glass Technologies
Incorporated. In another implementation, an Alumina submount 306
may be used along with an optical element 302 manufactured from
sapphire, Ohara Glass SLAH51 or Schott glass NLAF21. In some
implementations, a bulk filler 305 between the LED dice 304 and the
submount 306 may be used. The bulk filler 305 may be, e.g.,
silicone or glass. The bulk filler 305 may have good thermal
conductivity and may approximately match the CTE of the submount
306 and the dice 304. If desired, a protective side coating may be
applied alternatively or in addition to the bulk filler 305.
[0049] In one implementation, all of the LED dice 304 may be the
same type and produce different or approximately the same
wavelengths of light. Alternatively, with an appropriate choice of
LED dice 304 and/or wavelength conversion materials, different
wavelengths of light may be produced, e.g., blue, green and red.
When LED dice 304 are the same type, the CTE for the LED dice 304
will be approximately the same. It may be desirable for the CTE of
the LED dice 304 to closely match the coefficient of thermal
expansion of the optical element 302 and the submount 306 to
minimize the risk of detachment or damage to the LED dice 304
during the thermal bonding process.
[0050] In another implementation, the LED dice 304 may be different
types and produce different wavelengths of light. With the use of
different types of LED dice, the CTE of the dice can vary making it
difficult to match the CTE for all the LED dice 304 with that of
the optical element 302 and the submount 306. Nevertheless, with a
judicious choice of the optical element 302 and submount 306 with
CTEs that are as close as possible to that of the LED dice 304,
problems associated with detachment of the LED dice 304 or other
damage to the device 300 during the thermal bonding process may be
minimized. Additionally, with the use of relatively small LED dice
304, e.g., the area smaller than approximately 1 mm.sup.2, problems
associated with thermal bonding a single optical element 302 to
multiple dice 304 may also be reduced. The use of a bulk filler 305
may also prevent damage to the device during thermal processing or
operation.
[0051] As shown in FIG. 3, in one implementation, the optical
element 302 may be coated with a wavelength converting material
310, such as a phosphor coating. In one embodiment, the wavelength
converting material 310 is YAG. FIG. 4 is a flow chart of one
implementation of producing such a device. As illustrated in FIG.
4, the LED dice 304 are mounted to the submount 306 (step 402) and
the optical element 302 is bonded to the LED dice 304 (step 404).
After the optical element 302 is bonded to the LED dice 304, a
layer of the wavelength converting material 310 is deposited over
the optical element 302 (step 406). The device can then be tested,
e.g., by applying a voltage across the active regions of the LED
dice 304 and detecting the wavelengths of light produced by the
device (step 408). If the device does not produce the desired
wavelengths (step 410), the thickness of the wavelength converting
material is altered (step 411), e.g., by depositing additional
wavelength converting material 310 over the optical element 302 or
by removing some of the wavelength converting material by etching
or dissolution and the device is again tested (step 408). The
process stops once the desired wavelengths of light are produced
(step 412).
[0052] Thus, the thickness of the wavelength converting material
310 coating is controlled in response to the light produced by the
LED dice 304 resulting in a highly reproducible correlated color
temperature. Moreover, because the deposition of the wavelength
converting material 310 is in response to the specific wavelengths
produced by the LED dice 304, a variation in the wavelengths of
light produced by LED dice 304 can be accommodated. Accordingly,
fewer LED dice 304 will be rejected for producing light with
wavelengths outside a useful range of wavelengths.
[0053] It should be understood that the process of coating the
optical element with a wavelength converting material may be
applied to the embodiments shown in FIGS. 1B and 2 as well.
[0054] In another implementation, the coating of wavelength
converting material may be placed between the LED die and the
optical element, e.g., within, over, or under the bonding layer
322. FIG. 5, by way of example, illustrates an LED die 502 mounted
to a submount 504 and bonded to an optical element 506 via bonding
layer 508, where a layer of wavelength converting material 510 is
disposed between the bonding layer 508 and the optical element 506.
The wavelength converting material 510 may be bonded to the bottom
surface of the optical element 506 by bonding layer 509 prior to or
during the bonding the optical element 506 to the LED die 502. The
wavelength converting material 510 may be, e.g., a phosphor
impregnated glass or wavelength converting ceramic that is formed
independently and then bonded to the LED die 502 and optical
element 506. In some embodiments, the wavelength converting
material 510 may be bonded directly to one or both of the LED die
502 and optical element 506. In one embodiment, the optical element
506, LED die 502 and wavelength converting material 510 may be
bonded together simultaneously. In another embodiment, the
wavelength converting material 510 may be bonded first to the
optical element 506 and subsequently bonded to the LED die 502,
e.g., where the bonding layer 509 has a higher bonding temperature
than the bonding layer 508. A suitable wavelength converting
material, such as a phosphor impregnated glass, is discussed in
more detail in U.S. Ser. No. 10/863,980, filed on Jun. 9, 2004, by
Paul S. Martin et al., entitled "Semiconductor Light Emitting
Device with Pre-Fabricated Wavelength Converting Element", which
has the same assignee as the present application and is
incorporated herein by reference.
[0055] FIG. 6 illustrates another embodiment, similar to the
embodiment shown in FIG. 5, except a wavelength converting material
520 is bonded directly to the LED die 502 (and optionally over the
edges of the LED die 502) prior to or during bonding of the optical
element 506. Thus, as shown in FIG. 6, the wavelength converting
material 520 is placed between the LED die 502 and the bonding
layer 509. If desired, an additional layer of wavelength converting
material may be deposited over the optical element 506 in FIGS. 5
and 6, as discussed above.
[0056] In another implementation, the coating of wavelength
converting material may be located over the LED die or dice
remotely, e.g., on an envelope of glass, plastic, epoxy, or
silicone with a hollow space between the envelope and the LED die
or dice. If desired, the hollow space may be filled with a material
such as silicone or epoxy.
[0057] Related U.S. patent application having Serial No.
application Ser. No. 10/987,241, filed Nov. 12, 2004, entitled
"Bonding an Optical Element to a Light Emitting Device", by Michael
D. Camras et al, which has the same assignee as the present
disclosure, and is incorporated herein by reference.
[0058] FIG. 7 illustrates an array 600 of LEDs 602, which are
mounted on a board 604. The board 604 includes electrical traces
606 that are used to provide electrical contact to the LEDs 602.
The LEDs 602 may be phosphor converted devices manufactured, e.g.,
as described above. The LEDs 602 may each produce white light with
different CCTs. By mixing the white light with different CCTs in
array 600, a light with a desired CCT may be produced. If desired,
the LEDs 602 may be covered with a transparent element 608 of e.g.,
glass, plastic, epoxy, or silicone. The transparent element 608 may
be filled, e.g., with epoxy or silicone, which assists the
extracting and mixing of the light and to protect the LEDs 602. It
should be understood that array 600 may include any number of LEDs
602 and that if desired, one or more of the LEDs may produce
non-white light. Moreover, if desired, a plurality of the LEDs 602
may be bonded to a single optical element 603, or one or more of
the LEDs 602 may not include optical element 603.
[0059] As illustrated in FIG. 7, individual or groups of LEDs 602
may be independently controlled, e.g., by controller 610, which is
electrically connected to the traces 606 on the board 604. By
independently controlling LEDs 602 or groups of LEDs 602, a high
color rendering, e.g., over 85, with a constant brightness may be
achieved. Further, the white points produced by the array 600 may
be tuneable over a large range of CCT, e.g., between 3000 K and
6000 K. By way of example, a number of phosphor-converted (PC) blue
LEDs that produce white light may be used in combination with LEDs
with different colors, such as blue, cyan, amber and red to produce
a light with a desired CCT. As shown in the graph of FIG. 8, the
phosphor converted blue LEDs generates light with a broad spectrum
702 in the green area in combination with a peak in the blue
region. The thickness of the phosphor may be tuned to produce
approximately equal peak values for both the green and blue parts
of the spectrum. FIG. 9 shows a CIE chromaticity diagram for the
spectrum shown in FIG. 8, which illustrates the x and y color
coordinates 752 above the black bodyline 754. Of course, PC LEDs
that produce spectra having peaks in other area may be used if
desired. Alternatively, if desired, PC LEDs that produce different
spectra, i.e., white light having different CCTs may be used
together.
[0060] A majority of the LEDs 602 in the array 600 of FIG. 7 may be
PC LEDs that generate the spectrum shown in FIG. 8. The remaining
LEDs 602 shown in FIG. 7 may be color LEDs, e.g., LEDs that produce
blue, cyan, amber and red. The brightness of the color LEDs may be
adjusted by controller 610. The combination of fully powered PC
LEDs with colored LEDs generates an approximately continuous
spectrum, as illustrated in FIG. 10. FIG. 10 shows a graph with the
spectrum 702 from the PC LEDs along with spectra 704, 706, 708 and
710 from the blue, cyan, amber and red colored LEDs combined to
form spectrum 720. As illustrated in the portion of the CIE
chromaticity diagram shown in FIG. 11, by varying the brightness of
the colored LEDs, an area that covers part of the black body line
764 can be obtained. By way of example, one embodiment that
included 29 PC LEDs and 12 color LEDs, e.g., 3 blue, 3 cyan, 3
amber, and 3 red, is capable of producing a brightness of 800 lumen
with a color rendering between 85 and 95 and a CCT between 3200 K
and 5800 K. FIG. 12 illustrates a portion of the CIE chromaticity
diagram that illustrates variable CCT values for an array of 29 PC
LEDs and 12 color LEDs. Of course, any number of PC LEDs and color
LEDs may be used.
[0061] Although the present invention is illustrated in connection
with specific embodiments for instructional purposes, the present
invention is not limited thereto. Various adaptations and
modifications may be made without departing from the scope of the
invention. Therefore, the spirit and scope of the appended claims
should not be limited to the foregoing description.
* * * * *