www.industrial-lasers.com JANUARY/FEBRUARY 2017 Industrial Laser Solutions 15
substrates. This high-temperature process was employed in
production when flat-panel displays first became popular.
But as panel sizes increased, the display industry sought an
alternative approach that could be performed at lower temperatures, and would therefore eliminate the use of expensive
quartz or thermal glass substrates. The alternative method
that emerged was LTPS annealing (FIGURE 1).
In this technique, a pulsed excimer laser line beam is
scanned over the a-Si film. Silicon efficiently absorbs the
308nm excimer output. This high absorption, combined with
the high pulse energy of the excimer laser, makes it possible
to achieve near-complete melt of the thin silicon layer with just
a few pulses. This near-complete melt is essential to achieve
the optimum crystal formation (in terms of resultant electronic
characteristics). The high absorption of the silicon also pre-
vents the UV light from penetrating significantly into the glass
substrate below, thus avoiding any consequential heating of
the glass. This permits the use of economical glass materials as the substrate.
In production LTPS systems, the rectangular output of the excimer laser is significantly reshaped into a long, thin line beam
typically having a length equal to the width, or half the width, of
the panel. This enables the entire panel to be processed in one
or two passes under the laser beam. This is critical to achieving the process utilization and rapid throughput necessary for
economically viable LTPS.
The excimer laser beam profile is also homogenized to
construct a highly uniform (so-called “top hat”) profile along
its entire length and width. This is required to produce a consistent melt that, in turn, yields the desired electronic characteristics uniformly across the entire panel surface.
The entire LTPS annealing process takes place inside an
environmentally controlled chamber, where the panel is translated under the line beam. Panel motion is precisely synchronized with laser pulsing so that the each point on the panel
surface is irradiated with equivalent energy. Typically, this is
approximately 20 pulses per increment of travel. Onboard
alignment tools and process monitoring equipment, including a number of beam monitoring devices and pulse energy
meters, continuously verify that all process parameters
remain within the target range.
Display circuitry is produced on large panels that are subsequently singulated to yield individual devices. The display
industry has been increasing the size of these glass panels
steadily over the years to achieve greater economies of scale.
Since a fixed level of laser energy density is required to achieve
the desired melt characteristics for LTPS, this means that laser
power must also increase, assuming that the laser line length
is scaled up with the panel dimensions.
The Coherent VYPER lasers used for LTPS are powerful
excimer lasers, with pulse energies as high as 2J. This energy
level is sufficient for LTPS at beam lengths of up to 750mm.
However, as the display industry transitioned to larger panels,
a new approach was required.
Excimer lasers cannot simply be scaled up to higher and
higher powers without limit. Eventually, various practical issues,
such as cooling, discharge uniformity, and beam quality, come
into play. We chose a design concept in which the output from
multiple lasers is combined to reach these higher power levels. Specifically, a single V YPER laser, which itself contains two
separate oscillator beams, outputs 1.2k W (FIGURE 2).
A TwinV YPER laser system offers 2.4k W, which is sufficient
for LTPS production on 1300mm-wide glass panels. And a
FIGURE 2. In the newest TwinVYPER system, four separate
beams are combined within the system to produce a single