Download RA19 - Laser Lift-Off Techniques

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Laser Lift-Off Techniques
The term ‘Lift-Off’ has been applied to a wide variety of processes where layers are removed or
transferred by interaction at the substrate/layer interface. In some cases that interaction involves a
laser; an early example was the transfer of thin metallic films from one substrate to another by back
irradiation of the first ; the most general and obvious requirement is that the first substrate be
transparent to the laser beam.
LLO today generally refers to the removal of the sapphire support on which GaN layers have been
epitaxially grown for the rapidly expanding LED sector. Even very brief heating of the interface to
>1000°C results in decomposition of the GaN into Ga,- a low melting point metal, and nitrogen
which remains trapped at the interface. Early studies suggested heating of the part to above the m.p.
of the Ga, but this is rarely necessary, though heating of the part during processing may be
beneficial(see below).
1
2
GaN layer >
sapphire >
3
GaN
Ga
N2
transfer substrate
contact heterostructure
4
>
>
Ga remelt
& debond
>
irradiate with short >
pulse KrF excimer
Beam patterning for
direct generation of
mesa-like structures >
>
Requirements of LLO
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The LLO process is relatively insensitive to wavelength; it requires only that the sapphire be
transparent and the GaN opaque,- i.e. absorbing. There are reports of successful LLO from
193nm to 1064nm.
Excessive heating has to be avoided of the back-bond (usually has to be limited to 200-300°C)
which is only a few µm from the lift-off interface. This in turn means that the pulse must be of
short duration and correspondingly high peak energy density.
However the above sharp transients imply a high temperature gradient, which can cause
damage in the GaN bulk, complicated by the fact that the back-bond interface is rarely planar.
Theoretical temperature simulations have been done; the problem is essentially one
dimensional and the temporal profile of the laser pulse can be handled using Duhamel’s
theorem, but the temperature dependent properties, and above all the abrupt singularity when
lift-off occurs make this an intractable problem, so that required energy density is largely
phenomenological.
LLO is essentially a one-shot process. The threshold is very sharp, but because excessive
heating has to be avoided, the process window, in terms of energy density, is rather narrow,
and for this reason requires a large area, uniform intensity flat-top beam
Lateral temperature gradients are to be avoided, and this also favours a large area flat top
beam.
For the above reasons, excimer laser with a short pulse, large area beam at low rep. rate, is a natural
choice, with 248nm being the most common wavelength in use:•
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Required e.d. is in the range 0.4-0.8J/cm², it depends mainly on pulse duration, a little on
sample composition. A 30ns duration needs >600mJ/cm²
Parts are often warped, requiring efficient holding, and in larger wafer sizes where warp may
exceed optical DOF, some means of part mapping or autofocus may be necessary.
To minimize damage due to transients and temperature gradients, some workers have used
background heating, though it is not universal
Single device area is typically in the range 0.3-2mm, thus requiring on-target energies on the
order of 1-30mJ
The lower end of the device size range can be accessible to small compact lasers, with the
advantage of shorter pulse duration. For devices of 1mm and above, a larger laser is required,
but which may also allow processing of groups of 3,16,25 etc. device-sized areas at one time.
LLO samples may be pre-or post etched; either way, accurate registration of the sharp spot
edges with the etch or intended etch area is required, indicating high spec; X,Y stages.
For efficient processing of wafers, firing on-the-fly with correct pulse synchronization is required.
Optec LLO systems use large or small lasers, and offer high homogeneity beam, precision positioning,
point to point, on-the-fly & raster modes, running under ProcessPower software
Typical homogenized beam profile.
ProcessPower for LLO
R&D LLO system, in end test Optec;
see also LAP2000 DataSheet.