Laser Lift Off
Contents
Laser lift off is typically used to transfer and detach films. Laser lift off excels in detaching the
sapphire substrate from GaN or AlN based epi layers as used in the LED industry. Excimer lasers at
248 and 193 nm excel in this application as they provide large (several mm wide) spots on target with
an homogeneity better than +- 5%. This technique can be used for wafer based LEDs as well as flip
chip. Also a number of other films can be transferred from “transparent” substrates.
Typically, in order to produce GaN LEDs a thin GaN film of a few microns thick is grown on a sapphire
substrate. Major costs of LED fabrication are the sapphire substrate and the scribe-and-break
operation. For the traditional LED configuration the sapphire is not removed, so both cathode and
anode are installed on the same side of the GaN epi layer, which has several drawbacks for high
brightness LEDs that require high current input and high light extraction efficiency. These drawbacks
include high current density inside the material, causing low efficiency due to current crowding, and
lower reliability and shorter lifetime. Also there is significant light loss through the sapphire.
However, a vertical LED structure provides the possibility of pumping a LED with more current,
eliminates the undesired current crowding and bottleneck inside the device, and significantly
increases the maximum light output and efficiency of the LED.
This vertical LED structure requires removing the sapphire before attaching the electrical contacts on
the active epi layer. Excimer lasers have been proven to be valuable tools for separating the sapphire
and GaN thin film. LED laser lift off dramatically reduces the time and cost of the LED fabrication
process, enabling the manufacturer to grow GaN LED film devices on the sapphire wafer, for
example, and then transfer the thin film device to a heat sink electrical interconnect. The process
allows for the creation of free standing GaN films, and the integration of GaN LEDs onto virtually any
carrier substrate. Additionally, the sapphire may potentially be reused; this technique increases LED
light output, and has low operating costs due to low stress on the UV laser.
The basic concept behind UV laser lift off is the difference in absorption of UV light by the epi layer as
compared to the sapphire. A typical green/blue LED with GaN has a bandgap of about 3.3 eV, and
strongly absorbs 248 nm radiation (5 eV) from a KrF excimer laser. Sapphire, however, is a poor UV
absorber due to its high bandgap energy (9.9 eV). The laser light goes through the back of a sapphire
wafer causing photo-induced decomposition at the GaN/sapphire interface, and creates a localized
explosive shock wave that debonds the interface. The same principle is also used for liftoff from AlN
(6.3eV bandgap) with 193nm (6.4eV) ArF excimer lasers.
In order to achieve successful lift off both beam homogeneity and wafer preparation are important.
Typically beam homogenization techniques with excimer lasers create a flat top beam on the wafer
with uniform energy density distribution across a 5mm by 5mm area. Regarding wafer preparation, it
is important to minimize the residual stress from the high-temperature epi layer growth on sapphire;
and to ensure adequate bonding between the epi layer and the carrier substrate to avoid fractures
along the epi during lift off.
Laser lift off is typically used to transfer and detach films. Laser lift off excels in detaching the
sapphire substrate from GaN or AlN based epi layers as used in the LED industry. Excimer lasers at
248 and 193 nm excel in this application as they provide large (several mm wide) spots on target with
an homogeneity better than +- 5%. This technique can be used for wafer based LEDs as well as flip
chip. Also a number of other films can be transferred from “transparent” substrates.
Typically, in order to produce GaN LEDs a thin GaN film of a few microns thick is grown on a sapphire
substrate. Major costs of LED fabrication are the sapphire substrate and the scribe-and-break
operation. For the traditional LED configuration the sapphire is not removed, so both cathode and
anode are installed on the same side of the GaN epi layer, which has several drawbacks for high
brightness LEDs that require high current input and high light extraction efficiency. These drawbacks
include high current density inside the material, causing low efficiency due to current crowding, and
lower reliability and shorter lifetime. Also there is significant light loss through the sapphire.
However, a vertical LED structure provides the possibility of pumping a LED with more current,
eliminates the undesired current crowding and bottleneck inside the device, and significantly
increases the maximum light output and efficiency of the LED.
This vertical LED structure requires removing the sapphire before attaching the electrical contacts on
the active epi layer. Excimer lasers have been proven to be valuable tools for separating the sapphire
and GaN thin film. LED laser lift off dramatically reduces the time and cost of the LED fabrication
process, enabling the manufacturer to grow GaN LED film devices on the sapphire wafer, for
example, and then transfer the thin film device to a heat sink electrical interconnect. The process
allows for the creation of free standing GaN films, and the integration of GaN LEDs onto virtually any
carrier substrate. Additionally, the sapphire may potentially be reused; this technique increases LED
light output, and has low operating costs due to low stress on the UV laser.
The basic concept behind UV laser lift off is the difference in absorption of UV light by the epi layer as
compared to the sapphire. A typical green/blue LED with GaN has a bandgap of about 3.3 eV, and
strongly absorbs 248 nm radiation (5 eV) from a KrF excimer laser. Sapphire, however, is a poor UV
absorber due to its high bandgap energy (9.9 eV). The laser light goes through the back of a sapphire
wafer causing photo-induced decomposition at the GaN/sapphire interface, and creates a localized
explosive shock wave that debonds the interface. The same principle is also used for liftoff from AlN
(6.3eV bandgap) with 193nm (6.4eV) ArF excimer lasers.
In order to achieve successful lift off both beam homogeneity and wafer preparation are important.
Typically beam homogenization techniques with excimer lasers create a flat top beam on the wafer
with uniform energy density distribution across a 5mm by 5mm area. Regarding wafer preparation, it
is important to minimize the residual stress from the high-temperature epi layer growth on sapphire;
and to ensure adequate bonding between the epi layer and the carrier substrate to avoid fractures
along the epi during lift off.
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