A New Method for Silicon Crystallization: 3D Laser Interferences for ThinFilm Transistors Applications
J. Brochet, F. Templier, B. Aventurier
Laboratoire des Technologies et Composants pour la Visualisation, CEA-Leti, MinatecCampus, 17 rue des Martyrs, 38054 Grenoble CEDEX 9, France
M. Audier.
Laboratoire des Matériaux et du Génie Physique, INP Grenoble, Minatec, 3 Parvis Louis
Néel BP 257, 38016 Grenoble CEDEX 1, France
Contact author: F. Templier ftemplier@cea.fr ; tel : 04.38.78.43.87
Abstract
In this paper, we crystallized thin film of amorphous silicon
deposited on glass substrate using a pulsed laser and an
interferometer system at low temperature. We propose a process
to realize poly-Si circular TFTs. Using laser interferences,
periodic crystallization in FCC Bravais pattern with a period of
652nm is expected. Analyses of layers treated by laser
interferences were done by optical microscopy, Transmission
Electron Microscopy (TEM) and Scannig Electron Microscopy
(SEM). Layers microstructure was observed and presence of Si
crystals was established.
1. Introduction
So far, polycrystalline silicon is not used for large displays.
Despite its very good mobility and its great stability under
electrical stress [1] [2], its use is limited to small and medium
displays because of its spatial inhomogeneity and grain
size/grain boundaries dispersion leading to threshold voltage
spatial dispersion. Low temperature poly-Si TFTs for active
matrix display is usually obtained by use of excimer laser which
leads to inhomogeneity in grain size and grain boundaries.
In order to integrate poly-Si TFTs in large area electronics,
the key point is homogeneity on large area.
Nebel and al. [3] previously proposed a method to
periodically crystallize amorphous silicon with three-beam laser
interferences. Periods px and py depend on wavelength and
angles between the three beams. A typical application is a
laterally structured p-electrode for p-i-n solar cells.
In this paper, we propose a method for periodical
amorphous silicon crystallization in FCC Bravais lattice with
periods of 652 nm. This method was previously used for
metallic oxide growth by photolysis in a 3D interferences
network [4]. We also propose a fabrication process using this
method for low-temperature poly-Si circular TFTs on glass
substrate where homogenous characteristics on large area are
expected.
2. Experimental
2.1. Laser interferences
We have used a pulsed Nd-YAG laser with 10 Hz
frequency emitting in IR at λ = 1065 nm. The beam is amplified
in a pump chamber. This leads to an increase of the coherence
length of laser pulses up to a distance of 3 m for 10 ns pulses.
Then, we obtain a UV beam at λ = 355 nm by generation and
separation of the 2nd and 3rd harmonic, which is achieved by
non-linear optical process. Finally, a beam expander expands the
diameter of the beam from 3 to 8 mm. This beam is split by an
interferometer into 4 beams with different intensities. These
beams converge in a 8 mm diameter beam on the silicon surface.
This geometry was previously used for holographic lithography
[5]. However, different intensities and different polarizations
were established in this theoretical approach to obtain an
interference pattern with the highest possible symmetry and a
maximum contrast of intensity.
Figure 1: Interferometer system
Interferences created by this mean between the four beams
create a Face Centred Cubic pattern of interference maxima and
minima. At the interference minima, temperature is below the
threshold of crystallization while at interference maxima, the
intensity is high enough to induce crystallization. The period
between two interference maxima is 652 nm, as predicted by the
theory [6]. This method was used on 80 nm hydrogenated
amorphous silicon film deposited on glass substrate.
Dehydrogenation was done by thermal treatment at 450°C for 1h
in nitrogen-rich atmosphere prior to the laser crystallization
experiments. Figure 1 shows the optical principle of the
interferometer.
Figure 2 shows a picture and corresponding Fourier
transform of the (111) section of the 3D interference array. The
picture was captured with a CCD camera.
Transmission Line Method (TLM) measurements to asses that
dopants were activated.
Then, the 2nd step of photo-lithography was realised and
drain and source contact were obtained by deposition of
Molybdenum (Mo) and a lift-off operation. After spreading,
insulation and development of the resin, a 100 nm thick drain
metal layer was deposited by sputtering and then lift-off was
done.
Finally, a last step of contact annealing was done at 400°C
during 30 min to improve carriers’ injection.
Figure 2: Picture and corresponding Fourier transform of
the (111) section of the 3D interference array.
We have used the 4-beam interferences laser under
different conditions of power and number of pulses applied on
the samples. Experiments were done under 3 different power
conditions: P = 60 mJ/cm², P = 90 mJ/cm² and P = 120 mJ/cm².
For each power condition, we applied several numbers of pulses:
n = 1 to 50. In order to be sure that amorphous silicon films were
crystallized and to verify the periodicity of the crystallization,
we performed optical microscopy, transmission electron
microscopy and scanning electron microscopy on our samples.
Pictures of these observations are given in part 3.
2.2.Process for circular TFTs
Figure 3 represents the 2-masks process steps for circular
TFTs fabrication.
After the crystallization step, the surface was cleaned with a
mixture of H2SO4/ H2O2 (2/3 ;1/3). Then, gate oxide of 100 nm
thick and gate metal (Al) of 200 nm thick were deposited by
Plasma Enhanced Chemical Vapor Deposition (PECVD) and
sputtering, respectively. Hydrogenation annealing was done at T
= 450°C, in a H2 atmosphere, during 30 min.
Then the first photolithography step was realized: spreading
of the resin by spin-coating, insulation with the gate mask and
development. The gate contact was etched by Alu Etch at T =
30°C during 2 min. The SiO2 layer was etched by Reacting Ion
Etching (RIE) with a gaz mixture of CHF3 + O2.
The following step consists in Boron implantation at E = 10
keV and 1015 at/cm² dose. This step causes degradation at the
surface of the poly-Si layer. Thermal annealing is not suitable
because of the glass substrate. Therefore laser annealing was
used with the same laser used to crystallize the amorphous layer,
but at a lower energy in order to not recrystallize the layer and
affect the study. This is a critical step so it is necessary to realize
Figure 3: 2-Masks process steps for circular TFTs
fabrication
3. Structural analysis
Different analyses were performed on our samples to
observe laser interferences effects on amorphous layers.
Figure 4: optical micrograph of laser-treated a-Si:H. One
can see a period of 652nm between dots. On the inset,
Fourier transform of the micrographs
Figure 4 shows a picture obtained by optical microscopy
observation. The inset is the Fourier Transform of the picture.
This was helpful to see the good periodicity after laser treatment
but this is not sufficient to say that the crystallization occurs on
the films. Nevertheless we can say that there is a periodic microstructure of the amorphous layer with periods of 652 nm
confirmed by the Fourier Transform of the picture, where we
can see that the structure occurs in FCC lattice as expected.
Then we performed TEM observations, as shown on figure
5 and figure 6. Samples were prepared using the so-called
scratching method. The surface is scratched using a diamond tip,
forming small squares in order to remove a thin layer of the
material. Small fragments obtained from the sample are
collected by adhesion to a carbon film deposited on a copper
grid support. This keeps the nanostructures in their initial
orientations. This technique presents advantages of being
simple, quick and easy to implement. It is also inexpensive since
it does not require complicated equipment. However, due to the
pulverizing of the initial material, this technique will lose the
microstructure organization at high scale.
Figure 6 shows HR-TEM pictures of a microcrystal. On the
right, (111) planes are visible. The inset shows Fourier
transform. Once indexation of the FT is done, one can see the
FCC Bravais lattice, or diamond structure of the crystalline
silicon. Based on this picture, we extract crystallites size of 80
nm.
These observations allowed us to highlight a crystallization
of the amorphous silicon layer when we apply laser interference
treatment. However, we can not assess the crystalline fraction of
the material.
Figure 7 shows SEM pictures of crystallized silicon after
etching with a solution of K2Cr2O7 + HF (Secco etching [7])
during 5 s. Considering the first picture with low magnification,
it seems that crystallization occurs from seed of µc-Si. These
seeds are periodic and probably originate from the maxima
interference of the FCC array with a period of 652 nm. On the
picture taken with a higher magnification, it seems that the
whole amorphous silicon film was crystallised with average Si
grain size around 80 nm.
Figure 5: TEM pictures on bright-field mode. Inset:
Fourier transform of the TEM picture.
Figure 5 and 6 show observations made in bright-field
mode with different magnifications. First, in figure 5 we see a
wide shot of the sample. In the bright-field mode, crystallites
appear darker than the amorphous material. The inset is a
Fourier transform of the TEM picture. One can see diffraction
rings corresponding to (111), (220) and (311) crystalline silicon
planes.
Figure 7: SEM pictures after SECCO etching
(K2Cr2O7 + HF).
Figure 6: HR-TEM pictures. One can see a microcrystal and
the distance between (111) plane. The inset shows the Fourier
transform of the picture and the FCC bravais lattice.
These observations are not sufficient to say if the
amorphous layer is fully crystallised with grains and grain
boundaries.
alkaline glass produced using diode pumped solid state
continuous wave laser lateral crystallization”, Jpn J. of
Appl Phys, Part 1, vol. 43, pp. 1269-76, 2004
4. Conclusion
This is, to our knowledge, the first paper on laserinterferences crystallisation of amorphous silicon for low
temperature polysilicon thin-film transistors on glass substrates.
Structural analyses show that the amorphous layer is partially
crystallized after laser interference treatment. Micro-structure of
the amorphous layer observed by optical microscopy is FCC
Bravais lattice. SEM pictures show that crystallization seems to
occur in the whole layer. This technique is promising but would
require significant additional work on crystallization conditions
and structural characterization to demonstrate its real interest in
the production of active matrix OLED displays. Fabrication of
TFTs with this material would give an opportunity to assess the
homogeneity on large area.
[3]
C. E. Nebel and al., “Laser-Interference Crystallization
of Amorphous Silicon: Applications and Properties”,
Phys. Stat. Sol. (a), vol. 166, pp. 667-74, 1998
[5]
M. Campbell et al., “Fabrication of photonic crystals for
the visible spectrum by holographic lithography”, Nature
vol. 404, pp.53-56, 2000.
[4]
M. Salaün, “croissance d’oxydes métalliques par
photolyse dans un réseau d’interférences 3D”, ph.D
thesis, INP Grenoble, 2008
5. References
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G.K. Giust and al, “High-performance laser processed
polysilicon thin-film transistors”, IEEE Electron Device
Letters, vol.20, pp. 77-9, 1999
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M. Duneau, F. Delyon, and M. Audier, “Holographic
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A. Hara and al, “High performance low temperature
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