8.7 Vertical Structure LEDs
171
8.7 Vertical Structure LEDs
Since the sapphire substrate of the conventional lateral structure LED is a poor
conductor of heat, the LED has a large rise in junction temperature due to difficulty
in discharging heat at a large operating current density. This can lead to rapid degradation of its luminous efficiency. The LED lifetime can also be drastically shortened,
which makes the conventional lateral structure LEDs more challenging to meet the
requirements of the general lighting field.
Compared with the lateral structure LED with poor thermal conductivity, the
vertical structure LED adopts a substrate with excellent thermal conductivity. This
greatly improves the heat dissipation capability. The material degradation is slower,
and the LED lifetime is increased. The vertical structure LED has the upper and lower
electrode structures. The injection current can be uniformly distributed in the lightemitting area, avoiding the current crowding phenomenon of the mesa structure LED.
In addition, the vertical structure LED upper surface is easy to produce a surface light
extraction structure, which is beneficial to improve the light extraction efficiency. On
the one hand, the vertical structure LED is particularly suitable for application in the
field of general illumination. On the other hand, the vertical structure LED needs
to make a new substrate and remove the sapphire substrate, which complicates the
manufacturing process and will also produces many process compatibility issues.
The technical solutions for preparing the transfer of substrate mainly include
bonding, electroplating, sapphire grinding and so on. In order to carry out the subsequent laser lift-off, ICP etching and other processes smoothly and to maintain high
yield, the substrate must be firmly combined with the LED epitaxial layer material. It must be resistant to acid and alkali. The thickness of the substrate should be
uniform and the coefficient of thermal expansion should be matched. How to choose
the appropriate substrate material and optimize the preparation process conditions
to achieve low stress and low damage as well as firmly adhered substrate transfer
technology is the most difficult and critical point in vertical structure LED fabrication
technology.
Laser lift-off is a one-step process for removing sapphire substrates. The yield of
vertical structure chips is generally decided in this step. Adjusting the energy uniformity and energy density of the laser spot is the basic measure to ensure high yield. The
extent of the effect of laser lift-off on the GaN epitaxial layer, the degree of damage
to the epitaxial layer, and whether it will adversely affect the overall photoelectric
performance of the chip, need to be clarified. One advantage of the vertical structure
LED technology is that the sapphire substrate can be reused. Usually, the back surface
of the sapphire substrate is not polished. To peel off the epitaxial wafer with rough
substrate, the process parameters and lining of the laser lift-off are required. The
relationship between the roughness of the substrate is well studied. Electroplating
technology, bonding technology and laser stripping technology are introduced as
below.
171
8.7 Vertical Structure LEDs
Since the sapphire substrate of the conventional lateral structure LED is a poor
conductor of heat, the LED has a large rise in junction temperature due to difficulty
in discharging heat at a large operating current density. This can lead to rapid degradation of its luminous efficiency. The LED lifetime can also be drastically shortened,
which makes the conventional lateral structure LEDs more challenging to meet the
requirements of the general lighting field.
Compared with the lateral structure LED with poor thermal conductivity, the
vertical structure LED adopts a substrate with excellent thermal conductivity. This
greatly improves the heat dissipation capability. The material degradation is slower,
and the LED lifetime is increased. The vertical structure LED has the upper and lower
electrode structures. The injection current can be uniformly distributed in the lightemitting area, avoiding the current crowding phenomenon of the mesa structure LED.
In addition, the vertical structure LED upper surface is easy to produce a surface light
extraction structure, which is beneficial to improve the light extraction efficiency. On
the one hand, the vertical structure LED is particularly suitable for application in the
field of general illumination. On the other hand, the vertical structure LED needs
to make a new substrate and remove the sapphire substrate, which complicates the
manufacturing process and will also produces many process compatibility issues.
The technical solutions for preparing the transfer of substrate mainly include
bonding, electroplating, sapphire grinding and so on. In order to carry out the subsequent laser lift-off, ICP etching and other processes smoothly and to maintain high
yield, the substrate must be firmly combined with the LED epitaxial layer material. It must be resistant to acid and alkali. The thickness of the substrate should be
uniform and the coefficient of thermal expansion should be matched. How to choose
the appropriate substrate material and optimize the preparation process conditions
to achieve low stress and low damage as well as firmly adhered substrate transfer
technology is the most difficult and critical point in vertical structure LED fabrication
technology.
Laser lift-off is a one-step process for removing sapphire substrates. The yield of
vertical structure chips is generally decided in this step. Adjusting the energy uniformity and energy density of the laser spot is the basic measure to ensure high yield. The
extent of the effect of laser lift-off on the GaN epitaxial layer, the degree of damage
to the epitaxial layer, and whether it will adversely affect the overall photoelectric
performance of the chip, need to be clarified. One advantage of the vertical structure
LED technology is that the sapphire substrate can be reused. Usually, the back surface
of the sapphire substrate is not polished. To peel off the epitaxial wafer with rough
substrate, the process parameters and lining of the laser lift-off are required. The
relationship between the roughness of the substrate is well studied. Electroplating
technology, bonding technology and laser stripping technology are introduced as
below.