FIG. 1. The structural characteristic of polysilicon films listed in Table I.
The specimen number of the film is denoted in upper right-hand corner of
the photographs. The plane view micrographs ͑a͒, ͑b͒, ͑c͒, ͑g͒, and ͑h͒ were
taken, for comparison, at the same magnification. ͑a͒ Specimen ͑0C͒ sub-
jected only to furnace annealing at 600 °C/12 h denoted hereafter ͑FA͒. Due
to the high density of microtwins a mottlelike contrast is evident. ͑b͒ Speci-
men ͑4C͒ which was subjected to FA followed by a laser annealing ͑LA͒ at
350 mJ cmϪ2, the microtwins persist. ͑c͒ Specimen ͑5C͒, which was sub-
jected to FA plus LA at 380 mJ cmϪ2. At this stage a dramatic reorganization
inside the grains occurs resulting in elimination of the microtwins. No sub-
stantial movement of the grain boundaries was observed in this case. ͑d͒
XTEM micrograph from specimen ͑5A͒, which was subjected only to LA at
380 mJ cmϪ2. Small grains and higher roughness at the surface are the main
characteristics of this film. ͑e͒ XTEM dark field micrograph from specimen
͑4C͒, which was subjected to FA plus LA at 350 mJ cmϪ2. Only a single
grain of the polysilicon film is diffracting. In the lower part of the grain a
high density of defects is evident denoted by arrows. ͑f͒ XTEM micrograph
from specimen ͑5C͒, which was subjected to FA and LA at 380 mJ cmϪ2. A
very large grain, free of defects with smooth-free surface is evident. ͑g͒
Specimen 1A, which was subjected only to LA at 200 mJ cmϪ2. The struc-
ture presents a complete crystallization with a grain size around 50 nm. ͑h͒
Specimen 4A, which was subjected only to LA at 350 mJ cmϪ2. The struc-
ture presents small grains with a grain size around 250 nm. ͑i͒ Specimen 5A,
which was subjected only to LA at 380 mJ cmϪ2. The structure is the same
as ͑h͒ with a slightly larger grain size.
FIG. 2. Calculated molten thickness xm , vs laser energy density E relative
to 100 nm thick polysilicon film on glass.
of the grain boundaries occurs. The small increase of the
grain size is attributed to the smoothening of the grain
boundaries which for films annealed only at 600 °C are very
obscured and consequently they have higher surface energy.
Amorphous silicon films annealed only by laser at the same
energy density give grains five times smaller as it is shown in
Figs. 1͑d͒ and 1͑i͒. The films exhibit significant roughness
which appears as protrusions at the grain boundaries reveal-
ing significant mass transfer to the boundaries as it shown in
Fig. 1͑d͒. This behavior can be understood by considering
that the melting point of a-Si is roughly 20% lower than that
of crystalline Si,7 and the amorphous to crystalline reaction
is exothermic with an energy release of 0.12 eV/atom.8
From Table I it is evident that in polysilicon films, which
were annealed only by laser, the grain size increases propor-
tionally with the laser energy. In contrast the grain size of the
already crystallized films at 600 °C is rather insensitive to
the laser energy. The in-grain existing microtwins are acti-
vated and start to move above a threshold energy located
around 250 mJ cmϪ2. In order to simulate the thermal pro-
cess during the laser annealing we used a computer program
based on heat flow calculation ͑HFC͒.9 In the simulations the
thermal and optical parameters of the polysilicon film were
approximated with those of crystalline silicon. In Fig. 2 the
molten thickness xm, is shown versus the laser energy den-
sity E. As can be seen, the polysilicon surface starts to melt
around 220 mJ cmϪ2 and xm linearly increases for increasing
energy density. This behavior can explain the TEM observa-
tion: indeed the onset for the in-grain reordering is close to
the energy required for initiating the surface melting. As the
energy density increases the microstructure reordering occurs
in the region, where the melt–regrowth process occurs. In
fact, a laser energy density of 350 mJ cmϪ2 can melt only
nearly 60% of the film, leaving most of the microtwins in the
lower part as it is shown in Fig. 1͑e͒. Some of them can
propagate in the melted part of the films during the recrys-
tallization process counteracting the beneficial effect of the
laser annealing. For the film shown in Fig. 1͑f͒ a laser energy
density of 380 mJ cmϪ2 was sufficient to melt 80% of the
film thickness, leaving only the lower 20 nm of the film in
the solid phase. However even in this part the temperature
was sufficiently high to eliminate the microtwins leaving a
that the mean grain size of the furnace annealed films is
about 1 m as it is shown in Fig. 1͑a͒. Amorphous silicon
films deposited by silane and annealed under the same con-
ditions give a mean grain size 0.25 m only.6 Low laser
energy densities up to 280 mJ cmϪ2 do not affect the struc-
ture of the specimens already annealed at 600 °C. The size of
the grains as well as the microtwin density do not change,
revealing that the laser energy was not sufficient to activate
movement of the twins. In contrast the a-Si films are easily
crystallized, when irradiated with the same energy densities
as it is shown in Table I. A laser energy of the order of 350
mJ cmϪ2 improves the quality of the films previously an-
nealed at 600 °C/12 h, Fig. 1͑b͒. However improvement oc-
curs only in the upper half of the films as it is revealed by the
XTEM micrograph in Fig. 1͑e͒. Amorphous Si films which
were irradiated only by laser at the same energy are shown in
Fig. 1͑h͒. Irradiation by laser at 380 mJ cmϪ2 of the films
which were already subjected to a furnace annealing at
600 °C/12 h, results in elimination of the microtwins as it is
shown in Figs. 1͑c͒ and 1͑f͒, respectively. Only very few
large twins remain, denoted by an arrow in the plane view
micrograph in Fig. 1͑c͒. The mean size of the grains is
slightly larger than the size of the furnace annealed speci-
mens, Fig. 1͑a͒, revealing that in this case a small movement
Appl. Phys. Lett., Vol. 66, No. 11, 13 March 1995
Caluccio et al.
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Carluccio
