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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
(.mu.FE) in a-Si TFTs is equal to or lower than 1 cm.sup.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.sup.2 /Vs to 200 cm.sup.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 (.mu.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.times.400 mm
Film Layer Structure: Glass Substrate/SiNx/a-Si
Fluence Range: 380 mJ/cm.sup.2 through 430 mJ/cm.sup.2
Atmosphere: Nitrogen Gas with the Normal Atmosphere Pressure
Laser Irradiation: 35 .mu.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 .mu.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-crystalline film in the two sections are compared.
The moving direction is determined in accordance with results obtained from the comparing step. The glass substrate, for instance, is moved in the moving direction taken from the smaller grain size section to the other larger grain size one.
The moving direction may be determined after performing the above acts on more than one amorphous silicon film.
A second aspect of the present invention is characterized in the step of applying one pulse laser beam at a time to the amorphous silicon film on the glass substrate. The applied pulse laser beam is varied from lower energy fluence through
higher energy one. The pulse laser beam applied area is divided into two sections by the reference line perpendicular to the moving direction.
Further, the comparing step starts comparing the average grain sizes in the two sections caused by the pulse laser beam with the lower energy fluence. The step continues comparing the same until the smaller average grain size section is reversed
to the larger one and vice versa, by the pulse laser beam with higher energy fluence.
The direction determining step determines the moving direction in which the substrate is moved from the larger grain size section to the smaller grain size section or in which the irradiated laser beams are moved from the smaller grain size
section to the larger grain size section. The higher energy fluence in this step, however, is limited to such an extent that it does not cause the reversed grain size sections. The irradiating step irradiates the annealing pulse laser beams while
moving the substrate or the annealing beams in the moving direction.
A third aspect of the invention is characterized in the pulse laser beam applying step in which the pulse laser beam is rectangular in shape on the substrate and the shorter axis of the rectangular pulse laser beam is substantially in parallel
with the moving direction of the glass substrate.
The pulse laser beam applying step also applies one pulse laser beam at a time to the amorphous silicon film on the glass substrate. The pulse laser beam is varied from lower energy fluence through higher energy one. The pulse laser beam
applied area is divided into two sections by the reference line perpendicular to the moving direction.
Further, the comparing step starts comparing the average grain sizes in the two sections caused by the pulse laser beam with the lower energy fluence. The step continues comparing the same until the smaller average grain size section is reversed
to the larger one, and vice versa, by the pulse laser beam with higher energy fluence.
The direction determining step determines the moving direction in which the substrate is moved from the larger grain size section to the smaller grain size section with higher energy fluence, or in which the annealing laser beams are moved from
the smaller grain size section to the larger grain size section. The higher energy fluence, however, is limited to such an extent that it does not cause the smaller grain size section to be reversed to the larger grain size one, and vice versa. The
irradiating step irradiates the annealing laser beams while moving the substrate or the annealing laser beams in the moving direction.
A fourth aspect of the invention is characterized in the pulse laser beam applying step in which the pulse laser beam is rectangular in shape on the glass substrate and the shorter axis of the rectangular pulse laser beam is substantially in
parallel with the moving direction of the glass substrate.
Further, the pulse laser beam applying step applies one pulse laser beam at a time to the amorphous silicon film on the glass substrate. The pulse laser beam is varied from lower energy fluence through higher energy fluence. The pulse laser
beam applied area is divided into two sections by the reference line perpendicular to the moving direction.
The comparing step starts comparing the average grain sizes in the two sections caused by the pulse laser beam with the higher energy fluence and continues doing the same until the larger average grain size section is reversed to the smaller
average grain size, and vice versa, by the pulse laser beam with lower energy fluence.
The direction determining step determines the moving direction for the substrate to move from the smaller grain size section to the larger grain size section. The lower energy fluence in this step, however, is limited to such an extent that it
does not cause the reversed grain size sections. The annealing laser pulse may be moved instead of the substrate but its moving direction is reverse to that of the substrate. The irradiating step irradiates the annealing laser beams while the substrate
or the annealing laser beams are moved in its moving direction.
A fifth aspect of the invention is characterized in the pulse laser beam applying step in which the pulse laser beam is rectangular in shape on the glass substrate and the shorter axis of the rectangular pulse laser beam is substantially in
parallel with the moving direction.
Further, the pulse laser beam applying step applies one pulse laser beam at a time to the amorphous silicon film on the glass substrate. The pulse laser beam is varied from lower energy fluence through higher energy fluence. The pulse laser
beam applied area is divided into two sections by the reference line perpendicular to the moving direction.
The comparing step starts comparing the average grain sizes in the two sections caused by the pulse laser beam with the lower energy fluence and continues doing the same until the smaller average grain size section is reversed to the larger
average grain size, and vice versa, by the pulse laser beam with higher energy fluence.
The direction determining step determines the moving direction in which the substrate is moved from the larger grain size section to the smaller grain size section with higher energy fluence, or in which the annealing laser beams are moved from
the smaller grain size section to the larger grain size section. The higher energy fluence, however, is limited to such an extent that it does not cause the smaller grain size section to be reversed to the larger grain size one, and vice versa. The
irradiating step irradiates the annealing laser beams while the substrate or the annealing laser beams are moved in the moving direction.
A sixth aspect of the invention is characterized in the pulse laser beam applying step in which the pulse laser beam is rectangular in shape on the substrate and the shorter axis of the rectangular pulse laser beam is substantially in parallel
with the moving direction of the glass substrate or the annealing laser beams.
Further, the pulse laser beam applying step applies one pulse laser beam at a time to the amorphous silicon film deposited on the glass substrate. The pulse laser beam is varied from lower energy fluence through higher energy one. The pulse
laser beam applied area is divided into two sections by the reference line perpendicular to the moving direction.
The comparing step starts comparing the average grain sizes in the two sections caused by the pulse laser beam with the lower energy fluence and continues doing the same until the larger average grain size section is reversed to the smaller one,
and vice versa, by the pulse laser beam with higher energy fluence.
The direction determining step determines the moving direction for the substrate to move from the larger grain size section to the smaller grain size section with the higher energy fluence. However, the higher energy fluence in this step is
limited to such an extent that it does not cause the reversed grain size sections. The annealing laser beams may be moved instead of the substrate but its moving direction is reverse to that of the substrate. When a 0.1 .mu.m or smaller average grain
size crystal is detected in the smaller grain size section after the moving direction has been determined, the applied laser beam is again adjusted in shape and energy fluence to avoid it and the moving is also checked. The irradiating step then
irradiates the annealing laser beams while the substrate or the annealing laser beams are moved in its moving direction.
A seventh aspect of the invention is characterized in the pulse laser beam applying step in which the pulse laser beam is rectangular in shape on the glass substrate and the shorter axis of the rectangular pulse laser beam is substantially in
parallel with the moving direction of the glass substrate or the annealing laser beams.
The pulse laser beam applying step applies one pulse laser beam at a time to the amorphous silicon film deposited on the glass substrate. The pulse laser beam is varied from higher energy fluence through lower energy one. The pulse laser beam
applied area is divided into two sections by the reference line perpendicular to the moving direction.
Further, the comparing step starts comparing average grain sizes in the two sections caused by the pulse laser beam with the higher energy fluence and continues doing the same until the larger average grain size section is reversed to the smaller
average grain size, and vice versa, by the pulse laser beam with lower energy fluence.
The direction determining step determines the moving direction for the substrate to move from the larger grain size section to the smaller grain size section with the higher energy fluence. However, the higher energy fluence in this step is
limited to such an extent that it does not cause the reversed grain size sections. The annealing laser beams may be moved instead of the substrate but its moving direction is reverse to that of the substrate. When 0.1 .mu.m or smaller average grain
size crystals, however, are detected in the larger grain size section after the moving direction has been determined, the applied laser beam is again adjusted in shape and energy fluence to avoid it. The moving direction is also checked again. The
irradiating step irradiates the annealing laser beams while the substrate or the annealing laser beams are moved in its moving direction.
An eighth aspect of the invention is to provide a method of manufacturing a poly-crystalline silicon film which includes 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 by irradiating an excimer laser beam to make the same into poly-crystalline silicon film while the glass substrate or the annealing laser beams are moved in a moving direction.
The method further includes the steps of measuring an intensity profile of the laser beam in the moving direction by using a fluorescent plate or a charge-coupled device to locate a plateau of the intensity profile, dividing the plateau into two
portions, comparing heights of the plateaus in the two portions, and determining the moving direction for the substrate to move from the higher plateau portion to the lower plateau portion. The annealing laser beams may be moved instead of the substrate
but its moving direction is reverse to that of the substrate.
A ninth aspect of the invention is an improvement to the method of the eighth aspect of the invention. The improvement is characterized in the pulse laser beam applying step in which the pulse laser beam in this step is rectangular in shape on
the substrate and the shorter axis of the rectangular pulse laser beam is substantially in parallel with the moving direction of the glass substrate or the annealing laser beams.
A tenth aspect of the 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 by irradiating an annealing excimer laser beam to make the same into poly-crystalline silicon film while the glass substrate or the annealing laser beam are moved in a moving direction.
The method further includes the steps of measuring an intensity profile of the laser in the moving direction by using a fluorescent plate or a charge-coupled device to locate a plateau in the intensity profile, dividing the plateau into two
portions, comparing heights of the plateaus in the two portions, determining the moving direction taken from the higher plateau portion to the lower plateau portion, and using an excimer laser beam of substantially no higher intensity components to be
mixed in the lower plateau portion for the annealing excimer laser beam.
An eleventh aspect of the invention is an improvement to the method of the tenth invention. The improvement is characterized in the pulse laser beam applying step in which the pulse laser beam is rectangular in shape on the substrate and the
shorter axis of the rectangular pulse laser beam is substantially in parallel with the moving direction.
The above-stated and other objects and advantages of the invention will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration
and are not to be construed as defining the scope or limit of the invention, reference being had for the latter purpose to the claims appended hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1(a) is an energy fluence profile of a laser beam;
FIG. 1(b) is a schematic plan view of grain sizes of poly-crystalline silicon made by the laser beam shown in FIG. 1(a);
FIG. 2 is a schematic plan view of grain sizes of poly-crystalline silicon and micro-crystalline silicon made by a laser beam which has higher energy fluence than that shown in FIG. 1(a);
FIG. 3 is a sectional view of the structure of a liquid crystal display device;
FIG. 4 is a partial cut-out perspective view of a poly-crystalline silicon manufacturing apparatus;
FIG. 5 is a schematic perspective view of a laser annealing system used in the manufacturing apparatus shown in FIG. 4;
FIG. 6 shows the relationship between energy fluence of the laser beam and grain sizes;
FIG. 7 shows an energy fluence profile of the laser beam and moving directions of a glass substrate relative to the laser beam;
FIG. 8 shows a schematic energy fluence profile of a micro-crystal generation level; and
FIG. 9 shows the relationship between average grain sizes and energy fluence of the laser beam.
EXPLANATION OF THE PREFERRED EMBODIMENT
With the structures set forth above, taken along X- and Y-axes the moving direction of the substrate relative to the annealing laser beam and the energy fluence, respectively, the intensity distribution of the pulse laser beams applied to the
amorphous silicon (a-Si) films is plotted and trapezoidal or rectangular in shape on the glass substrate as shown in FIG. 1(a). The flat region and the rising and falling regions at the edges are called the plateau "P(P1)" and the steepness "S",
respectively, in the specification.
Generally, the optical system is adjusted to make the intensity in the plateau P substantially uniform regardless of locations. The intensity is, however, actually weaker at either the front or the rear end with respect to the substrate moving
direction. It is because the intensity resolution of a measurement apparatus, such as a charge coupled device (CCD) profiler to measure the intensity distribution of the laser beams, is 5% so that it cannot detect the intensity of less than 5%. Where
the laser beams are adjusted in this way and are used to anneal an amorphous silicon film, the laser beam energy observed at a point on the substrate is gradually varied from a weaker level to a stronger one, or vice versa.
In the event that a fluence F3 laser beam is used to generate micro-crystalline silicon as set forth above with reference to FIG. 6, and which occurs between the stronger and weaker laser energy beams on the plateau P, the micro-crystalline
silicon grows at the portions to which a stronger energy beam is applied while poly-crystalline silicon with larger grain sizes grows at the portions to which weaker energy beam is applied.
An ordinary laser annealing process includes an overlapped scanning method in which the substrate is moved up to a length equivalent to 5% of the laser beam width. In this case, the laser beams are applied 20 times to a point on the substrate.
Where the laser annealing is carried out with the overlapped scanning method in which the fluence F3 to generate micro-crystalline silicon is set in the middle of the range between the stronger and weaker beams and the substrate is moved in a direction
from the weaker beam to the stronger one, poly-crystalline silicon with larger grain sizes grows in the first half of 20 times and micro-crystalline silicon grows in the latter half. As a consequence, the micro-crystalline silicon remains so that high
performance TFTs are not realized.
As a laser annealing method for poly-crystalline silicon TFTs, its laser annealing fluence is adjusted to be less than F3 and be ordinarily of intensity between F1 and F2 shown in FIG. 6. In practice, however, F2 is very close to F3 as shown in
FIG. 9. Further, the oscillation intensity of pulse laser beams fluctuates in the range of .+-.10%. Therefore, even though the laser annealing fluence is set to be of the intensity between F1 and F2, some of the applied laser beams are stronger in
energy by +10% than the set value, i.e., stronger than F3.
In the case that the substrate moving direction is set to be consistent with a direction from the weaker beam to the stronger one, micro-crystalline and poly-crystalline silicon are formed by the laser beam with energy fluence larger than F3 in
the region where those +10% stronger beams are applied. Since the next laser beams are applied to a different region, the laser beams are not applied to the region where the overlapped scanning is not carried out. As a result, TFTs with poor
characteristics are partially formed and defects are found in the TFTs for driver circuits and pixels.
Where the substrate movement is in the direction reversed to the one set forth above, i.e., the direction taken from the stronger beams to the weaker ones and the annealing energy fluence is set as in the above-mentioned case, the +10% stronger
beams are applied to regions and micro-crystalline silicon is formed in the regions. The stronger beams, however, are applied there in the initial scanning period. The weaker beams are then followed so that the micro-crystalline silicon is melted,
reactivated and returned to poly-crystalline silicon with ordinary grain sizes. Therefore, the formed TFTs are not of partially poor characteristics but are of entirely high performances on the substrate.
As explained above, where the substrate moving direction in a laser annealing process is taken from the stronger beams to the weaker ones in the plateau region, TFTs with high performances are obtained. It is noted, however, that a beam profiler
cannot detect the plateau region P of rectangular beams in the case that the beam are less different in intensity than 5%. Even in this case, the intensity difference between the laser beams can be detected, if a laser annealing process is carried out
for an amorphous silicon deposited substrate without moving the substrate and the laser annealed region is divided into two sections along the shorter axis of the laser beams to compare average grain sizes in the two sections. More particularly, the
grain sizes of poly-crystalline silicon in the laser annealed region are in proportion to the magnitude of fluence. The average grain size in the lower fluence region is smaller while that in the higher fluence region is larger. The intensity
difference in the plateau P can be judged from such comparisons of the average grain sizes. The substrate moving direction is properly determined to be taken from the stronger beams to the weaker ones.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The first embodiment of the present invention will be explained hereinafter with reference to the attached drawings.
Embodiment 1
The laser annealing process without moving a glass substrate, not shown, is applied to a region 11 in FIG. 1(b) where a-Si films have been deposited on the substrate. That is to say, a pulse-like laser beam is applied only once to the region.
The surface situation of the p-Si film is visible as shown in FIG. 1(b) when the film is etched in Secco's solution. The excimer laser beam size is approximately 200 mm.times.0.4 mm (the longer and shorter axes of the laser applied region 11,
respectively), although the shorter axis along the substrate moving direction is enlarged for the sake of easy understanding.
The laser applied region 11 indicates all the area where the a-Si films have been made into the p-Si films. The region 12 represents the plateau. The Secco's etching of the region 11 provides visualization of the grain sizes of p-Si films in
the plateau region 12. The region 11 is divided into two sections at a reference line 13 perpendicular to the glass substrate moving direction. The upper half of the region 12 has larger grain sizes. An average grain size in each divided region by the
line 13 is obtained from the grain distributions. There are many methods of calculating the average grain size. A computer software used in this embodiment is Image-Pro.RTM. Plus, Version 1.2 for Windows.TM., Media Cybernetics, 8484 Georgia Avenue,
Silver Spring, Md. 20910, U.S.A., which analyzes distance from the center of gravity in each crystal in a designated region. This software can be applied to figure out the average grain size of the crystal and also its statistical average.
The region 12 is divided into two sections by the line 13 perpendicular to the axis along the substrate moving direction. The average grain size of the P-Si in each divided section is calculated with the software. As a result, the average grain
size in the upper half section is 0.2 .mu.m and that in the lower one is 0.15 .mu.m.
This operation is carried out for regions where laser beams are applied in accordance with sequentially increased fluence. In particular, the fluence is successively increased to shift the plateau P1 to an upper position shown in FIG. 1(a).
The grain size difference in the sections on both sides of the line 13 shown in FIG. 1(b) is not found in the section to which very low fluence laser beams are irradiated. The grain size difference becomes more distinctive in the sections as the
fluence of the applied laser beams becomes higher. Thus, the comparison between grain sizes can be done in the above-mentioned way.
Further, when much higher energy fluence beams are applied to the regions, it is o | | |
