Coating processes – Measuring – testing – or indicating
Reexamination Certificate
1998-09-25
2001-02-27
Padgett, Marianne (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S554000, C427S555000, C427S559000, C438S798000, C438S799000
Reexamination Certificate
active
06194023
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a method of making a poly-crystalline silicon semiconductor film from an amorphous silicon film and, more particularly, to a method of manufacturing a poly-crystalline silicon film in which laser pulses are applied to anneal an amorphous silicon film deposited on a glass substrate while the substrate or the laser beams are moved in a predetermined direction. The poly-crystalline silicon (p-Si) film is suitable for thin film transistors (TFTs), such as pixel switching and driver circuit TFTs used for a liquid crystal display device.
At the present, active-matrix type liquid crystal display devices are mass-produced. Such liquid crystal display devices, however, include amorphous silicon (a-Si) insulation-gate type TFTs. Since the mobility of electrons under electric fields (&mgr;FE) in a-Si TFTs is equal to or lower than 1 cm
2
/Vs, the a-Si TFTs are not sufficient in capability for high resolution, high speed and high performance display devices. Poly-crystalline silicon (p-Si) TFTs, on the other hand, have been experimentally made by means of a laser annealing process in which excimer laser pulses are applied to anneal a-Si films and make the same into p-Si films for TFTS. The mobility of electrons under electric fields in such p-Si TFTs ranges from 100 cm
2
/Vs to 200 cm
2
/Vs. The p-Si TFTs are expected to be essential components for high resolution, high speed and sophisticated function (e.g., driver-circuit-integrated) display devices.
Such an annealing process is carried out by laser annealing equipment
50
shown in FIG.
4
. The equipment
50
includes an excimer laser generator
51
, an optical system module
52
, an annealing chamber
54
, a control apparatus
55
, a substrate cassette station
56
and a manupulation robot
57
. The excimer laser generator
51
generates a XeCl gas excimer laser
53
with the wavelength of 308 nm, for instance. As shown in
FIG. 5
, the laser
53
is applied to an a-Si film substrate
62
through a reflection mirror
61
. The substrate
62
is moved in the direction
63
at a regular speed. The laser pulse beam size on the a-Si surface is 200 mm long and 0.4 mm wide, for instance. This pulse beam is oscillated at the frequency of 300 Hz. As a pulse applied region is gradually moved, the a-Si film in the region is successively poly-crystallized.
It is noted that a grain size (or diameter) of p-Si is a decisive factor of the mobility of electrons in p-Si TFTs under electric fields. The grain size mainly depends on energy density of the laser called energy fluence. The relationship between the grain size and the fluence is schematically shown in FIG.
6
. Generally, the grain size becomes larger as the energy fluence is increased. However, the grain size is not changed even if the energy fluence is increased in value from F1 through F2. The grain size again becomes larger in response to the fluence increase in value from F2 through F3. The p-Si, however, is transformed into micro-crystalline silicon in the case where the fluence value is greater than F3. In such micro-crystalline silicon, the mobility of electrons under electric fields (&mgr;FE) decreases so that the micro-crystalline silicon does not provide desired TFT characteristics.
A checking technique for the grain size is to etch the p-Si in Secco's solution and to observe the etched grain with a scanning electron microscope. This technique is used to properly set the fluence in the middle of the region between F1 and F2 where the grain size is not substantially changed. Laser oscillation intensities are always changed. Nevertheless, where the laser annealing is carried out in that region, a uniform grain size of p-Si can be obtained regardless of such laser oscillation intensity changes.
The inventors of the present invention have discovered that the fluence range between F1 and F2 is determined in accordance with a moving direction of a glass substrate relative to the shorter axis of the laser beams, and that a uniform grain size of p-Si cannot be obtained from a laser annealing process in which such a moving direction is not properly selected.
As described above, the laser beam is 200 mm long and 0.4 mm wide. Its profile is shown in
FIG. 7
in which the X- and Y-axes represent the width of the laser beam and the energy fluence of the laser beams, respectively. The fluence distribution is not constant but is declined slightly as shown. The laser beam scanning direction +X or −X is a moving direction with respect to the substrate. The “+X scan” is to move the substrate from the lower energy fluence to the higher energy fluence along the X-axis. The “−X scan” is in the direction reverse to the “+X scan”. The energy fluence is defined by an average value between the lower and higher values. The micro-crystal generation level energy fluence can be schematically defined as in FIG.
8
.
FIG. 9
shows experimental data carried out by the inventors under the following conditions:
Substrate Size: 300 mm×400 mm
Film Layer Structure: Glass Substrate/SiNx/a-Si
Fluence Range: 380 mJ/cm
2
through 430 mJ/cm
2
Atmosphere: Nitrogen Gas with the Normal Atmosphere Pressure
Laser Irradiation: 35 &mgr;m Pitch (10-Time Irradiation)
Laser Repetition Frequency: 300 Hz
Substrate Temperature: Room Temperature
Substrate Washing: No Washing before Laser Annealing
Quite importantly, the inventors have discovered the fact that the −X scan is wider in margin of the energy fluence to cause 0.3 &mgr;m or larger average grain size of poly-crystalline silicon than the +X scan as illustrated in FIG.
9
.
SUMMARY OF THE INVENTION
An object of this invention is to provide a method of manufacturing a poly-crystalline silicon film with a substantially uniform grain size.
An object of the present invention is to provide a method of manufacturing a high mobility poly-crystalline silicon film on a glass substrate.
An object of the present invention is to provide a method of manufacturing a poly-crystalline silicon film on a glass substrate in which an effective excimer laser annealing process is carried out in accordance with an optimum choice of a moving direction of the laser beams relative to the substrate.
An object of the invention is to provide a method of mass-producing a poly-crystalline silicon film used for a driver circuit integrated type liquid crystal display device or for a high performance liquid crystal display device.
The inventors have experimentally tested laser anneal processes to make amorphous silicon films on a glass substrate into poly-crystalline silicon films and have analyzed the poly-crystalline films in greater detail. They have extensively investigated laser annealing processes under burst irradiation states with the relative position between the laser beams and the substrate fixed. As a result, they have discovered that the grain distribution in the poly-crystalline silicon is correlated with a moving direction of the substrate relative to the laser beams.
A first aspect of this invention is to provide a method of manufacturing a poly-crystalline silicon film which includes the steps of forming an undercoat layer on a glass substrate, depositing an amorphous silicon film on the undercoat layer or on the glass substrate, and annealing the amorphous silicon film by irradiating a pulse laser beam to make the same into poly-crystalline silicon film while moving the glass substrate or the irradiated pulse laser beams in a moving direction.
In order to determine the moving direction of the substrate or the annealing laser beams prior to irradiating the laser beam, laser beam pulses with different energy fluence are applied to different places of the amorphous silicon film on the glass substrate and make the amorphous silicon film into a poly-crystalline silicon film.
An area of the poly-crystalline silicon film is then divided by a reference line into two sections where larger and smaller grain sizes are contained, respectively. Such grain sizes of the poly-crystalli
Fujimura Takashi
Goto Yasumasa
Imai Nobuo
Matsuura Yuki
Mitsuhashi Hiroshi
Kabushiki Kaisha Toshiba
Padgett Marianne
Pillsbury Madison & Sutro LLP
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