7.3 Light Extraction Efficiency Improvement Technology
133
In fact, due to the absorption and scattering loss in the film, the continuous stacking
more layers does not improve the reflectance after the film system reaches a certain
number of layers, Sometimes the reflectance decreases due to an increase in absorption and scattering loss. Therefore, the absorption and scattering losses in the film
system and the maximum number of layers of the dielectric film system are limited.
Omnidirectional full-band high-reflection structure design: High-reflection mirror
is optimized by high-reflection design to improve LED light extraction efficiency.
7.3.4 Flip-Chip Structure
Most of the LEDs used for illumination are high-current-driven power chips, which
require good current spreading and heat dissipation. There are several kinds of
deferent flip chip structures such as thin-film type (TF), flip chip (FC) and direct
attach (DA) flip chip. The TF structure can be divided into the silicon substrate
TF structure chip and the metal-based TF structure chip according to the substrate
material used in the chip.
Flip-chip is fabricated by flipping the traditional lateral structure LED on high
thermal conductivity substrate. The light emitted from the transparent sapphire
substrate can avoid absorption of electrodes and leads, and increase the interface
total reflection critical angle. The heat conductive path is changed from sapphire
with poor thermal conductivity to materials with higher thermal conductivity (such
as silicon, aluminum nitride, etc.). Thus, the thermal resistance is greatly reduced.
Flip-chip structure largely solves the problems faced by the lateral LED chip, such
as high operating voltage, current crowding, current saturation, low active region
utilization, and poor current spread. Flip-chip effectively improves the electrical,
optical, and thermal performance of power LEDs.
For thin-film (TF) chip structure, the LED epitaxial layer is transferred from the
original substrate to a new substrate by laser lift-off or electrical chemical etching,
where the new substrate has good electrical and thermal conductivity. After transferred to a new substrate, n-GaN side is up. Such an approach makes it easier to
integrate the chip with metal reflectors and the surface roughening substrate and
to enhance device efficiency of light. The main methods to achieve film transfer
are bonding and electroplating. The new substrate materials mainly include silicon
substrates and metal substrates represented by copper.
Direct attach (DA) flip-chip LED combines the advantages of FC—LED and
advanced TF—LED, where wire-bonding is not required during package process.
The current spreading and heat dissipation problems of the conventional TS chip
are effectively solved. Furthermore, the damage caused by the substrate stripping
process to the film can be avoided.
133
In fact, due to the absorption and scattering loss in the film, the continuous stacking
more layers does not improve the reflectance after the film system reaches a certain
number of layers, Sometimes the reflectance decreases due to an increase in absorption and scattering loss. Therefore, the absorption and scattering losses in the film
system and the maximum number of layers of the dielectric film system are limited.
Omnidirectional full-band high-reflection structure design: High-reflection mirror
is optimized by high-reflection design to improve LED light extraction efficiency.
7.3.4 Flip-Chip Structure
Most of the LEDs used for illumination are high-current-driven power chips, which
require good current spreading and heat dissipation. There are several kinds of
deferent flip chip structures such as thin-film type (TF), flip chip (FC) and direct
attach (DA) flip chip. The TF structure can be divided into the silicon substrate
TF structure chip and the metal-based TF structure chip according to the substrate
material used in the chip.
Flip-chip is fabricated by flipping the traditional lateral structure LED on high
thermal conductivity substrate. The light emitted from the transparent sapphire
substrate can avoid absorption of electrodes and leads, and increase the interface
total reflection critical angle. The heat conductive path is changed from sapphire
with poor thermal conductivity to materials with higher thermal conductivity (such
as silicon, aluminum nitride, etc.). Thus, the thermal resistance is greatly reduced.
Flip-chip structure largely solves the problems faced by the lateral LED chip, such
as high operating voltage, current crowding, current saturation, low active region
utilization, and poor current spread. Flip-chip effectively improves the electrical,
optical, and thermal performance of power LEDs.
For thin-film (TF) chip structure, the LED epitaxial layer is transferred from the
original substrate to a new substrate by laser lift-off or electrical chemical etching,
where the new substrate has good electrical and thermal conductivity. After transferred to a new substrate, n-GaN side is up. Such an approach makes it easier to
integrate the chip with metal reflectors and the surface roughening substrate and
to enhance device efficiency of light. The main methods to achieve film transfer
are bonding and electroplating. The new substrate materials mainly include silicon
substrates and metal substrates represented by copper.
Direct attach (DA) flip-chip LED combines the advantages of FC—LED and
advanced TF—LED, where wire-bonding is not required during package process.
The current spreading and heat dissipation problems of the conventional TS chip
are effectively solved. Furthermore, the damage caused by the substrate stripping
process to the film can be avoided.