BONDING AN OPTICAL ELEMENT TO A
LIGHT EMITTING DEVICE
FIELD OF THE INVENTION
The present invention relates generally to light emitting devices and, more particularly, to bonding an optical element to one or more light emitting device dice.
Semiconductor light emitting devices (LEDs) are typically covered with an optical element to protect the semiconductor structures, increase light extraction efficiency, and assist in focusing the emitted light. One type of material used to encapsulate LEDs is epoxy. Epoxy, however, is a low refractive index material and thus is not as effective as a higher index material at reducing losses due to total internal reflection at the semiconductor/low index encapsulant interface. Further, epoxy and other organic encapsulants typically suffer from yellowing when used with LEDs that operate with high temperature and/or short wavelengths. Moreover, epoxy encapsulants typically have coefficients of thermal expansion that poorly match those of the semiconductor materials in the LED. Consequently, the epoxy encapsulant subjects the LED to mechanical stress upon heating or cooling and may damage the LED.
Thus, an improved optical element with increased light extraction efficiency and is resistant to yellowing or other degradation and a method of bonding such an optical element to an LED is desirable.
In accordance with one embodiment of the present invention, a device is produced by mounting at least one light emitting device (LED) die on a submount and subsequently bonding an optical element to the LED die. The LED die is electrically coupled to the submount through contact elements, such as solder bumps or pads, that have a higher temperature melting point than is used to bond the optical element to the LED die. In one implementation, a single optical element is bonded to a plurality of LED dice that are mounted to the submount and the submount and the optical element have approximately the same coefficients of thermal expansion. Alternatively, a number of optical elements may be used. The LED or the optical element may be covered with a coating of wavelength converting material. In one implementation, the device is tested to determine the wavelengths produced and the thickness of the wavelength converting material is altered, i.e., increased or possibly decreased, until the desired wavelengths are produced.
BRIEF DESCRIPTION OF THE DRAWINGS
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.
FIG. IB illustrates the optical element bonded to the LED die.
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.
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.
FIG. 4 is a flow chart of one implementation of producing such an LED device with wavelength converting material covering the optical element.
FIG. 5 illustrates an embodiment in which a layer of wave5 length converting material is disposed between the bonding layer and the optical element.
FIG. 6 illustrates an embodiment in which a layer of wavelength converting material is deposited on the LED die.
FIG. 7 illustrates an array of LEDs, which are mounted on 10 a board.
FIG. 8 is a graph of the broad spectrum produced by a phosphor converted blue LED.
FIG. 9 is a CIE chromaticity diagram for the spectrum shown in FIG. 8. 15 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.
FIG. 11 is a portion of a CIE chromaticity diagram that shows the variation in the CCT that may be produced by 20 varying the brightness of the colored LEDs.
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.
25 DETAILED DESCRIPTION
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
30 bonded to the LED die 104 in accordance with an embodiment of the present invention. FIG. IB illustrates the optical element 102 bonded to the LED die 104.
The term "transparent" is used herein to indicate that the element so described, such as a "transparent optical element,"
35 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 ordi
40 nary 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.
LED die 104 illustrated in FIGS. 1A and IB includes a first
45 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
50 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 ann-contact 114 and a p-contact 116 that are electrically coupled to semiconductor layers 108 and 110, respectively. Contact 114 and
55 contact 116 are disposed on the same side of LED die 104 in a "flip chip" arrangement. A transparent superstate 118 coupled to the n layer 108 is formed from a material such as, for example, sapphire, SiC, GaN, GaP, diamond, cubic zirconia (Zr02), aluminum oxynitride (AlON), A1N, spinel, ZnS,
60 oxide of tellurium, oxide of lead, oxide of tungsten, polycrystalline alumina oxide (transparent alumina), and ZnO.
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
65 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.