U.S. patent application number 10/345724 was filed with the patent office on 2003-07-10 for laser annealing apparatus to control the amount of oxygen incorporated into polycrystalline silicon films.
This patent application is currently assigned to Sharp Laboratories of America, Inc., Sharp Laboratories of America, Inc.. Invention is credited to Voutsas, Apostolos.
Application Number | 20030127435 10/345724 |
Document ID | / |
Family ID | 24621070 |
Filed Date | 2003-07-10 |
United States Patent
Application |
20030127435 |
Kind Code |
A1 |
Voutsas, Apostolos |
July 10, 2003 |
Laser annealing apparatus to control the amount of oxygen
incorporated into polycrystalline silicon films
Abstract
The invention provides an apparatus for reducing, or
eliminating, ambient air in connection with an excimer laser
annealing process. Nozzles are provided to direct a flow of gas,
preferably helium, neon, argon or nitrogen, at a region overlying
the target area of an amorphous silicon layer deposited on an LCD
substrate. The nozzles direct a flow of gas at sufficient pressure
and flow rate to remove ambient air from the region overlying the
target area. With the ambient air, especially oxygen, removed, the
laser can anneal the amorphous silicon to produce polycrystalline
silicon with less oxygen contamination. In a preferred embodiment,
an exhaust system is also provided to remove the gas.
Inventors: |
Voutsas, Apostolos;
(Portland, OR) |
Correspondence
Address: |
Matthew D. Rabdau
Sharp Laboratories of America, Inc.
5750 NW Pacific Rim Boulevard
Camas
WA
98607
US
|
Assignee: |
Sharp Laboratories of America,
Inc.
|
Family ID: |
24621070 |
Appl. No.: |
10/345724 |
Filed: |
January 15, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10345724 |
Jan 15, 2003 |
|
|
|
09653484 |
Aug 31, 2000 |
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Current U.S.
Class: |
219/121.65 ;
219/121.84 |
Current CPC
Class: |
B23K 26/1437 20151001;
B23K 26/1435 20130101; B23K 2101/36 20180801; B23K 26/147
20130101 |
Class at
Publication: |
219/121.65 ;
219/121.84 |
International
Class: |
B23K 026/14 |
Claims
What is claimed is:
1. A laser annealing apparatus for processing a semiconductor
material having a surface comprising: a) a laser for directing a
beam to a location on the surface of the semiconductor material;
and b) a nozzle for delivering a flow of gas to the location on the
surface of the semiconductor material, whereby ambient air is
removed from the location where the beam is directed.
2. The apparatus of claim 1, wherein the laser is an excimer
laser.
3. The apparatus of claim 1, wherein the excimer laser is a krypton
fluoride (KrF) laser or a xenon chloride (XeCl) laser.
4. The apparatus of claim 1, wherein the gas is inert.
5. The apparatus of claim 1, wherein the gas is argon, neon, helium
or nitrogen.
6. The apparatus of claim 1, wherein the flow of gas is at a flow
rate of between approximately 5 and 50 standard liters per
minute.
7. The apparatus of claim 1, wherein the nozzle is mounted to the
laser.
8. The apparatus of claim 1, further comprising an exhaust
port.
9. The apparatus of claim 1, further comprising an exhaust pump
attached to the exhaust port.
10. The apparatus of claim 1, wherein the exhaust port is formed by
an opening in a base, which the substrate rests upon.
11. The apparatus of claim 1, wherein the exhaust port is attached
to the laser.
12. The apparatus of claim 1, wherein the exhaust port is adjacent
the nozzle.
13. The apparatus of claim 1, further comprising a pump attached to
the nozzle for increasing the pressure out of a gas reservoir.
14. The apparatus of claim 1, further comprising a shroud mounted
to the laser
15. The apparatus of claim 1, wherein in the shroud is flexible and
adapted to fit over the substrate and provide an inlet for the
gas.
16. The apparatus of claim 1, wherein the shroud is composed of
plastic, rubber, metal or fabric.
17. The apparatus of claim 1, wherein a portion of the shroud is
corrugated to provide flex.
18. A kit for modifying existing excimer laser annealing equipment
comprising: a) at least one nozzle for removing ambient air from a
target area where a laser beam is directed to anneal a target
material.
19. The kit of claim 1, wherein the at least one nozzle is adapted
to mount to a base where the target material is placed.
20. The kit of claim 1, wherein the at least one nozzle is adapted
to mount to the laser.
21. The kit of claim 1, further comprising at least one exhaust
port.
22. The kit of claim 1, wherein the at least one exhaust port is
adapted to mount to the laser.
23. The kit of claim 1, wherein the at least one exhaust port is
incorporated into a replacement base which can readily replace the
base.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates generally to flat panel display
manufacturing systems and more particularly to an apparatus for
preparing polycrystalline silicon films on flat panel display
substrates.
[0002] Thin film transistors (TFTs) used in liquid crystal displays
(LCDs) or flat panel displays of the active matrix display type are
fabricated on silicon films deposited on a transparent substrate.
The most widely used substrate is glass. Amorphous silicon is
readily deposited on glass. Amorphous silicon limits the quality of
TFT that can be formed. If driver circuits and other components are
to be formed on the display panel, as well as switches associated
with each pixel, crystalline silicon is preferred.
[0003] Amorphous silicon can be crystallized to form crystalline
silicon by solid-phase crystallization. Solid-phase crystallization
is carried out by high temperature annealing. But, glass substrates
cannot withstand the temperatures necessary to melt and crystallize
silicon. Quartz substrates can withstand high temperature
annealing, but quartz substrates are too expensive for most LCD
applications.
[0004] Because glass deforms when exposed to temperatures above
600.degree. C., low-temperature crystallization (preferably below
550.degree. C.) is used for solid-phase processing of silicon on
glass. The low-temperature process requires long anneal times (at
least several hours). Such processing is inefficient and yields
polycrystalline silicon TFTs that have relatively low field effect
mobility and poor transfer characteristics. Polycrystalline silicon
produced by solid-phase crystallization of as-deposited amorphous
silicon on glass suffers due to small crystal size and a high
density of intragrain defects in the crystalline structure.
[0005] Excimer laser annealing (ELA) has been actively investigated
as an alternative to low-temperature solid-phase crystallization of
amorphous silicon on glass. In excimer laser annealing, a
high-energy pulsed laser directs laser radiation at selected
regions of the target film, exposing the silicon to very high
temperatures for short durations. Typically, each laser pulse
covers only a small area (several millimeters in diameter) and the
substrate or laser is stepped through an exposure pattern of
overlapping exposures, as is known in the art. More powerful lasers
with larger beam profiles are now available or are under active
development, reducing the number of exposures required. Regardless
of the number and pattern of exposures, ELA allows areas of
amorphous-film to be crystallized without damaging the underlying
glass substrate.
[0006] The major advantages of ELA are the formation of
polycrystalline grains with excellent structural quality and the
ability to process selected areas of a display panel.
Polycrystalline silicon produced on transparent substrates by ELA
has electron mobility characteristics rivaling IC driver circuits
currently mounted along the edges of the screen. Thus, it becomes
possible to incorporate driver circuitry onto the substrate,
simplifying manufacturing.
[0007] The most common problem that plagues ELA is the narrow
process window associated with the development of large and uniform
grain sizes. Surface roughness inherent to the process is also
troublesome. Research has suggested that improvements in surface
conditions, a reduction in defects, and increased crystal size are
associated with low oxygen content ELA polycrystalline silicon
films. Oxygen content can be controlled in several ways. The
industry standard currently being used is to perform ELA in a high
vacuum (10.sup.-7 Torr), or somewhat less efficacious, in a rough
vacuum (10.sup.-3 Torr). Alternatively, ELA has been carried out in
chambers filled with non-oxygen ambient gases such as He, Ar, or N
with varying results. The association between oxygen content and
polycrystalline silicon film quality is still being
investigated.
[0008] A significant problem with prior art systems for reducing
oxygen incorporation into polycrystalline silicon during ELA is the
need for a process chamber to house the target substrate. When a
process chamber (alternatively called: "chamber", "processing
chamber", or "substrate isolation chamber") is used, the beam of
the excimer laser must pass into the chamber through a quartz
window. Vacuum chambers, in particular, are costly. Chambers for
processing in non-air ambient at atmospheric pressure are somewhat
simpler than vacuum chambers, but still have quartz windows. The
quartz windows cost several thousand dollars and have only a
limited life, lasting only days or weeks in volume production. The
cost associated with a processing chamber is one reason ELA
equipment without substrate isolation is still being manufactured,
sold and used. The despite evidence that ELA performed in air
ambient produces polycrystalline silicon with inferior mobility
characteristics (and a higher oxygen content) compared with films
annealed in non-air ambient.
[0009] It would be advantageous to be able to effectively control
the amount of oxygen incorporated in ELA polycrystalline silicon
films, keeping the oxygen content below a predetermined threshold,
while minimizing the cost of production.
[0010] It would be advantageous to have ELA equipment that would
reduce, or eliminate, oxygen from the target area without the need
for an isolation chamber.
[0011] It would also be advantageous to improve the quality of ELA
polycrystalline silicon films on flat panel display substrates by
reducing oxygen incorporation with relatively simple changes to ELA
equipment.
[0012] It would also be advantageous if existing equipment could be
modified to reduce, or eliminate, oxygen from the target area
without the need for an isolation chamber.
SUMMARY OF THE INVENTION
[0013] Accordingly, a laser annealing apparatus for forming
polycrystalline silicon film on substrates using ELA is provided.
The laser annealing apparatus of the present invention comprises a
laser for directing a beam to a location on the surface of a
semiconductor material. A nozzle, or plurality of nozzles, is
positioned to direct a flow of gas over the location on the surface
of the semiconductor material. The gas is preferably helium, neon,
argon or nitrogen. The gas removes ambient air, especially oxygen,
from the location on the surface of the semiconductor material.
This allows the laser to anneal the semiconductor material in an
atmosphere with reduced oxygen, or preferably no oxygen. The
absence of oxygen allows the laser to produce a higher quality
polycrystalline region within the semiconductor material.
[0014] The apparatus is adapted to be retrofit to existing ELA
systems. It can be mount to a laser head or to a moveable base upon
which the semiconductor material is placed.
[0015] The apparatus will preferably include an exhaust system to
aid in removing the gas and ambient air.
[0016] In one preferred embodiment of the apparatus, a shroud is
provided to surround a laser beam produced by the laser. The shroud
incorporates an air path for the gas flow to the nozzle as well as
an exhaust port. In a further embodiment, the shroud is partially
sealed to the base supporting the semiconductor material forming an
enclosure. The shroud is preferably flexible to allow for the
movement of the base. The flexibility is preferrably provided by a
flex region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic cross-sectional view showing an ELA
system with gas flow nozzles for removing ambient air from a target
region according to an embodiment of the present invention.
[0018] FIG. 2 is a schematic cross-sectional view showing an ELA
system with gas flow nozzles mounted to a laser along with an
exhaust system for removing ambient air from a target region
according to an embodiment of the present invention.
[0019] FIG. 3 is a schematic cross-sectional view showing an ELA
system with gas flow nozzles and an exhaust system mounted to a
laser for removing ambient air from a target region according to an
embodiment of the present invention.
[0020] FIG. 4 is a schematic cross-sectional view showing an ELA
system with gas flow nozzles further comprising a shroud.
[0021] FIG. 5 is a schematic cross-sectional view showing an ELA
system with gas flow nozzles further comprising a flexible shroud
that forms an enclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0022] FIG. 1 is an illustration of the components of an excimer
laser annealing (ELA) apparatus 10. Several manufactures supply
excimer lasers suitable for use with the present apparatus,
including Sopra, S.A. of France, and Lambda Physik of Germany.
Excimer lasers and related systems are well known to those skilled
in the ELA art.
[0023] Excimer laser annealing apparatus 10 includes a laser head
14, which emits high-energy coherent optical radiation at a
selected wavelength. The type of laser used is a matter of design
choice. For example, XeCl lasers emit UV radiation at a wavelength
of 308 nm; KrF lasers operate at a wavelength of 248 nm. Laser 14
emits a pulsed beam 20 that is adjustable to various power levels.
Pulse durations can be varied over a wide range, typically between
10 and 200 ns, and the pulse repetition rates are generally
selectable between 0.1 Hz and 300 Hz. Beam cross-sections can vary
widely depending on the power level of the laser and the type of
optics used in the apparatus.
[0024] Laser 14 emits beam 20 that passes through a beam
homogenizer 26. The beam homogenizer is an optical system that
produces a substantially uniform beam profile as it strikes a
target surface. The size of beam 20 as it emerges from homogenizer
26, and any other optics (not shown), determines the size and
configuration of the target area on the substrate, which will be
irradiated by laser beam 20. Beam profiles vary substantially from
typical beams of between 10 and 50 mm.sup.2 up to so-called single
shot ELAs capable of annealing entire flat panel displays in a
single exposure. Currently, the flat panel displays that can be
annealed in a single shot are smaller than standard display
sizes.
[0025] Laser beam 20 is directed at a flat panel display substrate
36 onto which a layer of amorphous silicon film 38 has previously
been deposited. Substrate 36 is supported in apparatus 10 on a
movable stage 40 of any suitable type capable of repositioning
substrate 36 in a programmable manner to ensure that target regions
43 (where beam 20 strikes film 38) on the substrate are
sequentially or repeatedly irradiated. It will, of course, be
readily understood that the function of movable stage 40 could
alternatively be accomplished by moving laser head 14 to aim at
different target regions 43 on the substrate 36. Whichever way the
function is accomplished, the purpose of the ELA apparatus 10 is to
expose selected areas of the film 38 to excimer laser energy by
irradiating one or more target regions 43.
[0026] The power level of laser 14, the duration of the laser pulse
emitted, the size of laser beam 20, and the degree of overlap
between successive exposures will determine the number of exposures
or "shots" to which each target region on substrate 36 is exposed.
ELA systems are readily programmed to perform multiple exposures,
sometimes 100 exposures, or more, on each sub-region or target area
of the substrate, in order to properly anneal and crystallize the
amorphous silicon film. The present invention is not limited to any
specific laser annealing parameters and can be readily used with
excimer lasers of any suitable power level.
[0027] A particular advantage of the present invention is that ELA
apparatus 10 does not require an environmental chamber, or similar
sealed enclosure, surrounding substrate 36. Prior art systems have
utilized chambers that were generally designed to be evacuated to
either a high or a rough vacuum. Alternatively, the chamber has
been used to provide a controlled atmosphere within the chamber. In
ELA systems that include such environmental chambers, beam 20 from
laser 14 enters the chamber through a suitable window. Due to the
wavelength of the lasers used, the window is typically a quartz
window. The present invention provides for a system that reduces,
or eliminates, the need for an environmental chamber. The
elimination of the chamber will also eliminate the need for the
window and other equipment associated with the chamber.
[0028] Instead of the chamber, the present apparatus can be used in
a conventional LCD-processing cleanroom environment. With the
chamber eliminated, the apparatus 10 should provide a means for
controlling the atmosphere over target region 43.
[0029] One embodiment of the apparatus is shown in FIG. 1. The ELA
apparatus 10 includes a gas supply system 50 for directing gas onto
the surface of the silicon film 38 on substrate 36. Gas supply
system 50 includes one, or more, reservoirs 56 of a suitable gas,
preferably argon, neon, helium or nitrogen. The reservoir 56 can
contain a liquefied gas if fitted with a suitable vaporizer, or it
can contain pressurized gas.
[0030] Gas supply reservoir 56 is operatively connected to one, or
more, nozzles 60 through suitable manifolds or conduits 64 that
deliver gas to target region 43 on substrate 36. One, or more,
valves 70 are preferably provided to control the flow of gas from
reservoir 56 to nozzles 60. Although in a preferred embodiment a
high pressure gas supply tank will be used, it is also possible to
use a pump and still be within the scope of the present invention.
According, a pump 74 is shown. The number, shape, size, and
configuration of nozzles 60 is a matter of design choice and
optimization. The nozzles can take a variety of forms such as
showerheads, multiple individual nozzles, or elongated laminar flow
apertures. The nozzles will preferably be adjustable in orientation
to direct the gas flow to the desired location. The adjustability
is preferably accomplished using ball-type nozzles, flexible tubing
and brackets, or bendable nozzle ends. The method of adjusting the
nozzle direction is not critical to the present invention.
[0031] The purpose of the gas supply 50 is to direct a gas flow 65
across, or onto, surface 42 above target region 43 of silicon film
38 during irradiation of target region 43 by laser beam 20. Gas
flow 80 displaces the ambient atmosphere from the environment of
target region 43 during one or more exposures to the laser beam
20.
[0032] In the preferred embodiment shown in FIG. 1, nozzles 60 are
located in the area of the movable stage 40. The nozzles can be
mounted to the movable stage 40 so that nozzles 60 move with the
stage, thereby remaining stationary relative to the substrate 36.
Alternatively, the nozzles can be mounted near the stage, but not
attached to it, so that when the stage moves the gas flow 80
remains fixed. By fixing the gas flow, while the stage moves, the
gas flow can be oriented more precisely over the target region
43.
[0033] Gas flow 80 should be of sufficient volume and flow rate to
remove a significant portion of the ambient air, and particularly
the oxygen, from the target region 43. The term "remove" is
intended to convey the reduction, or elimination, of ambient air,
especially oxygen from the target region. The amount of oxygen
present at the surface should be reduced to the point where it will
not affect the polycrystalline silicon being formed during any
laser annealing steps. To determine the appropriate range for the
gas flow 80, the volume of air that is to be displaced, and
replaced with argon, neon, helium or nitrogen, along with the time
allowed for operation, should be considered. The time period for
the displacement of the air volume should be determined by the
laser discharge frequency. The displacement process should take
place within successive laser shots. For example, if the laser
operates at 100-300 Hz, the time between successive shots is 3.3-10
milliseconds. The displacement volume can be simply estimated by
the product of the area covered by the laser beam times the
thickness of the region of desired gas on top of the irradiated
surface. This thickness is preferably minimized to minimize the
displaced volume of ambient air. However, it should be a
sufficiently thick region to effectively inhibit diffusion of
oxygen from the ambient air to the surface of the irradiated
area.
[0034] For example, the diffusion coefficieint of O.sub.2 through a
stagnant layer of N.sub.2 is estimated as: D.sub.O2-N2=0.18
cm.sup.2/s. The diffusion length of O.sub.2 atoms through a region
of nitrogen can be estimated by: 1={square root}{square root over
((D.sub.O2-N2.multidot.t))- }; where 1 is the diffusion length and
t is the time allowed for the diffusion process. Solving for the
diffusion length using the diffusion coefficient provided above,
and setting t equal to the time between successive laser shots
(0.01 seconds), an upper limit for the diffusion length can be
estimated as 0.3 mm. This means that oxygen at a concentration
comparable to ambient air will diffuse through a depth of 0.3 mm of
nitrogen gas flow over the target region. By providing a nitrogen
gas flow to a thickness over the target region of three times this
estimated diffusion length it should be possible to ensure that the
interface between the irradiated target region and the region of
nitrogen will be virtually oxygen free. Thus the preferred
thickness of nitrogen is estimated as approximately three times the
diffusion length of oxygen through nitrogen, or 1 mm.
[0035] As a minimum, the area flooded by the nozzles needs to be
equal to the area of the laser beam. However, the area flooded by
the nozzles is preferably three to five times the area of the laser
beam. A typical laser beam is 7.5 cm.times.1.2 cm. So, preferably
the volume of ambient air, and corresponding volume of nitrogen, is
2.7-4.5 cm.sup.3 (or 2.7-4.5 ml).
[0036] Although the above discussion related to nitrogen as the
preferred gas, the amount of gas flow needed to displace ambient
air can be readily calculated for other desired gases, including
helium, neon and argon.
[0037] Referring now to FIG. 2, an apparatus 110, which is an
alternative embodiment of this invention, is shown in
cross-section. A laser head 114, which is preferably cylindrical,
is provided incorporating a laser, a homogenizer, and other optics,
if any (not shown). Laser head 114 may also be referred to
generally as the laser. Laser head 114 directs a laser beam 120 at
a flat panel display substrate 136 onto which a layer of amorphous
silicon film 138 has previously been deposited. Substrate 136 is
supported on a moveable stage 140 of any suitable type capable of
repositioning substrate 136 in a programmable manner to ensure that
target regions 143 (where beam 120 strikes film 138) may be
selected as desired.
[0038] In a preferred embodiment of the present invention, at least
one nozzle 160, and preferably nozzles, mounts to laser head 114.
The nozzle is attached to the laser head through an attachment
block 116, which preferably seals the nozzles to the laser head.
Mounting to laser head 114 is preferable as it provides consistent
alignment of a gas flow 165 with laser beam 120. Nozzle 160 is
connected to a gas reservoir, and valve system as described above
in connection with FIG. 1. The optional pump may also be
included.
[0039] In one embodiment of the present invention, gas flow 165
eventually dissipates into the surrounding ambient environment.
[0040] In another embodiment of the present invention, an exhaust
system 170 is provided. As shown in FIG. 2 the exhaust system 170
is incorporated into stage 140. Exhaust system 170 is formed by
apertures 172 in stage 140 connected to an exhausting pump (not
shown). Preferably, a baffle 174 is provided to direct the gas into
the exhaust system 170.
[0041] Referring now to FIG. 3, in another preferred embodiment the
apparatus 110 is configured such that the laser beam 120 is
directed substantially along the axis of the laser head 114. A ring
of nozzles 160 is provided adjacent the laser head 114 for
introducing gas flow 165. Exhaust system 170 is formed by a second
ring of nozzles 180 mounted outside of ring of nozzles 160. A
nozzle that is used as part of the exhaust system is also referred
to as an exhaust port. In operation, nozzles 160 will provide gas
flow at target region 143 as necessary to reduce, or eliminate
oxygen from the target region. The gas flow then exits through
exhaust system 170, which is connected to the exhaust pump (not
shown).
[0042] Although the exhaust system was described above as being
mounted outside the ring of nozzles 160, it also possible to mount
the exhaust system adjacent the laser, and mount the ring of
nozzles 16 outside the exhaust system.
[0043] Referring now to FIG. 4, a shroud 190 mounts to laser head
114 and surrounds laser beam 120. In one preferred embodiment, the
shroud is rectangular. Nozzles 160 are provided at an end 192 of
the shroud and connected to a pump (not shown) by a duct 194. The
duct 194 is preferably incorporated into the shroud. In the
rectangular configuration, the nozzles 160 preferably provide a
flow of gas across the laser beam 120 at the surface 142 of the
substrate 136. In a preferred embodiment, a second set of nozzles
180 is connected to an exhaust pump (not shown) to remove the gas
from the work area. The shroud allows the gas to be delivered to
the area immediately over the surface 142 of the substrate 136
while at least partially enclosing the laser beam 120. This is
useful if the distance of the laser head 114 is preferably further
from the surface than the preferred gas flow origin.
[0044] Referring now to FIG. 5, another preferred embodiment
incorporates a shroud. The shroud 190 attaches to the laser head
114 through an attachment block, or seal, 116. In this embodiment,
the shroud 190 rests on the movable base 140. An o-ring 195 is
provided to at least partially seal the shroud 190 to the base 140.
The combination of the shroud 190 and the base 140 forms a rough
enclosure 198. At least one nozzle 160 introduces the gas flow 165
into the enclosure 198. The gas flow 165 can be allowed to exit the
chamber through leakage at the edges of the shroud 190, or
alternatively through an exhaust port 200.
[0045] Since the shroud 190 is in contact with the moveable stage
150, it should to continue to allow the stage to move with
impairing its movement. In a preferred embodiment, flex zones 220
are provided to allow the shroud to flex in response to the
movement of the stage. The shroud is preferably made of a flexible
material, including thin flexible metal, plastic, rubber, or
fabric. In another embodiment, only the flex zones 220 should be
flexible and the remaining structures can be rigid.
[0046] Some of the preferred embodiments of the present invention
are adapted to be retrofit onto existing ELA systems without
chambers. Accordingly, the apparatus may be provided as components
to be mounted to the laser head.
[0047] Additional alternative embodiments are possible within the
scope of the present invention. Other variations of the apparatus,
or materials, within the scope of the present invention will occur
to those of ordinary skill in the art. Accordingly, the foregoing
disclosure and description thereof are for illustrative purposes
only and are not intended to limit the invention. This invention is
defined by the claims.
* * * * *