U.S. patent application number 10/364214 was filed with the patent office on 2003-07-31 for silicon thin film, group of silicon single crystal grains and formation process thereof, and semiconductor device, flash memory cell and fabrication process thereof.
Invention is credited to Ikeda, Yuji, Kanaya, Yasuhiro, Kunii, Masafumi, Noguchi, Takashi, Usui, Setsuo.
Application Number | 20030143375 10/364214 |
Document ID | / |
Family ID | 15494131 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030143375 |
Kind Code |
A1 |
Noguchi, Takashi ; et
al. |
July 31, 2003 |
Silicon thin film, group of silicon single crystal grains and
formation process thereof, and semiconductor device, flash memory
cell and fabrication process thereof
Abstract
A process of forming a silicon thin film includes the steps of:
irradiating a pulsed rectangular ultraviolet beam on an amorphous
or polycrystalline silicon layer formed on a base body, to thereby
form a silicon thin film composed of a group of silicon single
crystal grains which are each approximately rectangular-shaped and
which are arranged in a grid pattern on the base body. In this
process, the moved amount of a ultraviolet beam irradiating
position in a period from completion of an irradiation of the
rectangular ultraviolet beam to starting of the next irradiation of
the rectangular-ultraviolet beam is specified at 40 .mu.m or less,
and a ratio of the moved amount to a width of the rectangular
ultraviolet beam measured in the movement direction thereof is in a
range of 0.1 to 5%. Further, a selected orientation of the silicon
single crystal grains to the surface of the base body is
approximately the <100> direction.
Inventors: |
Noguchi, Takashi; (Kanagawa,
JP) ; Kanaya, Yasuhiro; (Kanagawa, JP) ;
Kunii, Masafumi; (Kanagawa, JP) ; Ikeda, Yuji;
(Kanagawa, JP) ; Usui, Setsuo; (Kanagawa,
JP) |
Correspondence
Address: |
Jeffrey F. Craft
Sonnenschein Nath & Rosenthal
Wacker Drive Station , Sears Tower
P.O. Box 061080
Chicago
IL
60606-1080
US
|
Family ID: |
15494131 |
Appl. No.: |
10/364214 |
Filed: |
February 11, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10364214 |
Feb 11, 2003 |
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09616685 |
Jul 14, 2000 |
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6548830 |
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Current U.S.
Class: |
428/143 ;
257/E21.134; 257/E21.209; 257/E21.414; 257/E29.294 |
Current CPC
Class: |
Y10T 428/24372 20150115;
H01L 21/2026 20130101; H01L 21/0242 20130101; H01L 21/02691
20130101; H01L 29/66765 20130101; H01L 21/02422 20130101; H01L
21/02686 20130101; H01L 21/02595 20130101; H01L 21/02609 20130101;
H01L 29/78678 20130101; H01L 21/02502 20130101; H01L 29/40114
20190801; H01L 21/02381 20130101; B82Y 10/00 20130101; H01L
21/02598 20130101; H01L 21/02491 20130101; H01L 21/02532 20130101;
H01L 21/02488 20130101 |
Class at
Publication: |
428/143 |
International
Class: |
B32B 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 22, 1996 |
JP |
P08-150306 |
Mar 24, 1997 |
JP |
P09-088728 |
Claims
What is claimed is:
1. A process of forming a silicon thin film, comprising the step
of: irradiating a pulsed rectangular ultraviolet beam on an
amorphous or polycrystalline silicon layer formed on a base body,
to thereby form a silicon thin film composed of a group of silicon
single crystal grains on said base body; wherein the moved amount
of a ultraviolet beam irradiating position in a period from
completion of an irradiation of said rectangular ultraviolet beam
to starting of the next irradiation of said rectangular ultraviolet
beam is specified at 40 .mu.m or less, and a ratio of said moved
amount to a width of said rectangular ultraviolet beam measured in
the movement direction thereof is in a range of 0.1 to 5%, whereby
forming a silicon thin film composed of a group of silicon single
crystal grains which are each approximately rectangular-shaped and
which are arranged in a grid pattern on said base body, a selected
orientation of said silicon single crystal grains to the surface of
said base body being approximately the <100> direction.
2. A process of forming a silicon thin film according to claim 1,
wherein a length of one side of said silicon single crystal grain
approximately rectangular-shaped is 0.05 .mu.m or more.
3. A process of forming a silicon thin film according to claim 1,
wherein an average thickness of said silicon thin film is in a
range of 1.times.10.sup.-8 m to 1.times.10.sup.-7 m.
4. A process of forming a silicon thin film according to claim 1,
wherein said base body is made of silicon oxide or silicon
nitride.
5. A process of forming a silicon thin film according to claim 1,
wherein opposed sides of said silicon single crystal grain
approximately rectangular-shaped are approximately in parallel to
the movement direction of the ultraviolet beam irradiating position
or intersect the movement direction of the ultraviolet beam
irradiating position at approximately 45.degree..
6. A process of forming a group of silicon single crystal grains
comprising: a step (a) of irradiating a pulsed rectangular
ultraviolet beam on an amorphous or polycrystalline silicon layer
formed on a base body, to thereby form a silicon thin film composed
of a group of silicon single crystal grains which are each
approximately rectangular-shaped and which are arranged in a grid
pattern on said base body, a selected orientation of said silicon
single crystal grains to the surface of said base body being
approximately the <100> direction; and a step (b) of
separating adjacent ones of said silicon single crystal grains to
each other; wherein the moved amount of a ultraviolet beam
irradiating position in a period from completion of an irradiation
of said rectangular ultraviolet beam to starting of the next
irradiation of said rectangular ultraviolet beam is specified at 40
.mu.m or less; and a ratio of said moved amount to a width of said
rectangular ultraviolet beam measured in the movement direction
thereof is in a range of 0.1 to 5%.
7. A process of forming a group of silicon single crystal grains
according to claim 6, wherein said step (b) of separating adjacent
ones of said silicon single crystal grains to each other comprises
a step of oxidizing said silicon thin film formed in said step (a)
to form each region made of silicon oxide between the adjacent ones
of said silicon single crystal grains.
8. A process of forming a group of silicon single crystal grains
according to claim 6, wherein a length of one side of each of said
approximately rectangular-shaped silicon single crystal grains in
said silicon thin film formed in said step (a) is 0.05 .mu.m or
more.
9. A process of forming a group of silicon single crystal grains
according to claim 6, wherein an average thickness of said silicon
thin film formed in said step (a) is in a range of
1.times.10.sup.-8 m to 1.times.10.sup.-7 m.
10. A process of forming a group of silicon single crystal grains
according to claim 6, wherein said base body is made of silicon
oxide or silicon nitride.
11. A process of forming a group of silicon single crystal grains
according to claim 6, wherein opposed sides of each of said
approximately rectangular-shaped silicon single crystal grains in
said silicon thin film formed in said step (a) are approximately in
parallel to the movement direction of the ultraviolet beam
irradiating position or intersect the movement direction of the
ultraviolet beam irradiating position at approximately
45.degree..
12. A silicon thin film comprising a group of silicon single
crystal grains which are each approximately rectangular-shaped and
which are arranged in a grid pattern on a base body, wherein a
selected orientation of said silicon single crystal grains to the
surface of said base body is approximately the <100>
direction.
13. A silicon thin film according to claim 12, wherein a length of
one side of said silicon single crystal grain approximately
rectangular-shaped is 0.05 .mu.m or more.
14. A silicon thin film according to claim 12, wherein an average
thickness of said silicon thin film is in a range of
1.times.10.sup.-8 m to 1.times.10.sup.-7 m.
15. A silicon thin film according to claim 12, wherein said base
body is made of silicon oxide or silicon nitride.
16. A silicon thin film according to claim 12, wherein said group
of silicon single crystal grains are formed by irradiating a pulsed
rectangular ultraviolet beam on an amorphous or polycrystalline
silicon layer formed on said base body, and wherein the moved
amount of a ultraviolet beam irradiating position in a period from
completion of an irradiation of said rectangular ultraviolet beam
to starting of the next irradiation of said rectangular ultraviolet
beam is specified at 40 .mu.m or less, and a ratio of said moved
amount to a width of said rectangular ultraviolet beam measured in
the movement direction thereof is in a range of 0.1 to 5%.
17. A silicon thin film according to claim 16, wherein opposed
sides of said silicon single crystal grain approximately
rectangular-shaped are approximately in parallel to the movement
direction of the ultraviolet beam irradiating position or intersect
the movement direction of the ultraviolet beam irradiating position
at approximately 45.degree..
18. A group of silicon single crystal grains, comprising a
plurality of silicon single crystal grains which are each
approximately rectangular-shaped and which are arranged in a grid
pattern on a base body, wherein a selected orientation of said
silicon single crystal grains to the surface of said base body is
approximately the <100> direction, and adjacent ones of said
silicon single crystal grains are separated from each other.
19. A group of silicon single crystal grains according to claim 18,
which are formed by a process comprising: a step (a) of irradiating
a pulsed rectangular ultraviolet beam on an amorphous or
polycrystalline silicon layer formed on a base body, to thereby
form a silicon thin film composed of a group of silicon single
crystal grains which are each approximately rectangular-shaped and
which are arranged in a grid pattern on said base body, a selected
orientation of said silicon single crystal grains to the surface of
said base body being approximately the <100> direction; and a
step (b) of separating adjacent ones of said silicon single crystal
grains to each other; wherein the moved amount of a ultraviolet
beam irradiating position in a period from completion of an
irradiation of said rectangular ultraviolet beam to starting of the
next irradiation of said rectangular ultraviolet beam is specified
at 40 .mu.m or less, and a ratio of said moved amount to a width of
said rectangular ultraviolet beam measured in the movement
direction thereof is in a range of 0.1 to 5%.
20. A group of silicon single crystal grains according to claim 19,
wherein said step (b) of separating adjacent ones of said silicon
single crystal grains to each other comprises a step of oxidizing
said silicon thin film formed in said step (a) to form each region
made of silicon oxide between the adjacent ones of said silicon
single crystal grains.
21. A group of silicon single crystal grains according to claim 19,
wherein a length of one side of each of said approximately
rectangular-shaped silicon single crystal grains in said silicon
thin film formed in said step (a) is 0.05 .mu.m or more.
22. A group of silicon single crystal grains according to claim 19,
wherein an average thickness of said silicon thin film formed in
said step (a) is in a range of 1.times.10.sup.-8 m to
1.times.10.sup.-7 m.
23. A group of silicon single crystal grains according to claim 19,
wherein opposed sides of each of said approximately
rectangular-shaped silicon single crystal grains in said silicon
thin film formed in said step (a) are approximately in parallel to
the movement direction of the ultraviolet beam irradiating position
or intersect the movement direction of the ultraviolet beam
irradiating position at approximately 45.degree..
24. A group of silicon single crystal grains according to claim 18,
wherein said base body is made of silicon oxide or silicon
nitride.
25. A process of fabricating a semiconductor device, comprising the
steps of: irradiating a pulsed rectangular ultraviolet beam on an
amorphous or polycrystalline silicon layer formed on a base body,
to form a silicon thin film composed of a group of silicon single
crystal grains on said base body; and forming a source/drain region
and a channel region in said silicon thin film or said silicon
single crystal grains; wherein the moved amount of a ultraviolet
beam irradiating position in a period from completion of an device
according to claim 25, wherein opposed sides of said silicon single
crystal grain approximately rectangular-shaped are approximately in
parallel to the movement direction of the ultraviolet beam
irradiating position or intersect the movement direction of the
ultraviolet beam irradiating position at approximately
45.degree..
30. A process of fabricating a flash memory cell, comprising: a
step (a) of irradiating a pulsed rectangular ultraviolet beam on an
amorphous or polycrystalline silicon layer formed on a tunnel oxide
film, to form a silicon thin film composed of a group of silicon
single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on said
tunnel oxide film, a selected orientation of said silicon single
crystal grains to the surface of said tunnel oxide film is
approximately the <100> direction; and a step (b) of
separating adjacent ones of said silicon single crystal grains to
each other, whereby forming a floating gate composed of said group
of silicon single crystal grains; wherein the moved amount of a
ultraviolet beam irradiating position in a period from completion
of an irradiation of said rectangular ultraviolet beam to starting
of the next irradiation of said rectangular ultraviolet beam is
specified at 40 .mu.m or less, and a ratio of said moved amount to
a width of said rectangular ultraviolet beam measured in the
movement direction thereof is in a range of 0.1 to 5%.
31. A process of fabricating a flash memory cell according to claim
30, wherein said step (b) of separating adjacent ones of said
silicon single crystal grains to each other comprises a step of
oxidizing said silicon thin film formed in said step (a) to form
each region made of silicon oxide between the adjacent ones of said
silicon single crystal grains.
32. A process of fabricating a flash memory cell according to claim
30, wherein a length of one side of each of said approximately
rectangular-shaped silicon single crystal grains in said silicon
thin film formed in said step (a) is 0.05 .mu.m or more.
33. A process of fabricating a flash memory cell according to claim
30, wherein an average thickness of said silicon thin film formed
in said step (a) is in a range of 1.times.10.sup.-8 m to
1.times.10.sup.-7 m.
34. A process of fabricating a flash memory cell according to claim
30, wherein opposed sides of each of said approximately
rectangular-shaped silicon single crystal grains in said silicon
thin film formed in said step (a) are approximately in parallel to
the movement direction of the ultraviolet beam irradiating position
or intersect the movement direction of the ultraviolet beam
irradiating position at approximately 45.degree..
35. A semiconductor device comprising a source/drain region and a
channel region formed in a silicon thin film composed of a group of
silicon single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on a
base body or formed in said silicon single crystal grains, wherein
a selected orientation of said silicon single crystal grains to the
surface of said base body is approximately the <100>
direction.
36. A semiconductor device according to claim 35 wherein a length
of one side of said silicon single crystal grain approximately
rectangular-shaped is 0.05 .mu.m or more.
37. A semiconductor device according to claim 35, wherein an
average thickness of said silicon thin film is in a range of
1.times.10.sup.-8 m to 1.times.10.sup.-7 m.
38. A semiconductor device according to claim 35, wherein said base
body is made of silicon oxide or silicon nitride.
39. A semiconductor according to claim 35, wherein said group of
silicon single crystal grains are formed by irradiating a pulsed
rectangular ultraviolet beam on an amorphous or polycrystalline
silicon layer formed on said base body, and wherein the moved
amount of a ultraviolet beam irradiating position in a period from
completion of an irradiation of said rectangular ultraviolet beam
to starting of the next irradiation of said rectangular ultraviolet
beam is specified at 40 .mu.m or less, and a ratio of said moved
amount to a width of said rectangular ultraviolet beam measured in
the movement direction thereof is in a range of 0.1 to 5%.
40. A semiconductor device according to claim 39, wherein opposed
sides of said silicon single crystal grain approximately
rectangular-shaped are approximately in parallel to the movement
direction of the ultraviolet beam irradiating position or intersect
the movement direction of the ultraviolet beam irradiating position
at approximately 45.degree..
41. A semiconductor device according to claim 35, which is a thin
film transistor of a bottom gate type.
42. A flash memory cell comprising a floating gate composed of a
plurality of silicon single crystal grains which are each
approximately rectangular-shaped and which are formed on a tunnel
oxide film, a selected orientation of said silicon single crystal
grains to the surface of said tunnel oxide film being approximately
the <100> direction; wherein said silicon single crystal
grains are arranged in a grid pattern on said tunnel oxide film and
adjacent ones of said silicon single crystal grains are separated
from each other.
43. A flash memory cell according to claim 42, wherein said
plurality of silicon single crystal grains are formed by a process
comprising: a step (a) of irradiating a pulsed rectangular
ultraviolet beam on an amorphous or polycrystalline silicon layer
formed on said tunnel oxide film, to thereby form a silicon thin
film composed of a group of silicon single crystal grains which are
each approximately rectangular-shaped and which are arranged in a
grid pattern on said tunnel oxide film, a selected orientation of
said silicon single crystal grains to the surface of said tunnel
oxide film being approximately the <100> direction; and a
step (b) of separating adjacent ones of said silicon single crystal
grains to each other; wherein the moved amount of a ultraviolet
beam irradiating position in a period from completion of an
irradiation of said rectangular ultraviolet beam to starting of the
next irradiation of said rectangular ultraviolet beam is specified
at 40 .mu.m or less, and a ratio of said moved amount to a width of
said rectangular ultraviolet beam measured in the movement
direction thereof is in a range of 0.1 to 5%.
44. A flash memory cell according to claim 43, wherein said step
(b) of separating adjacent ones of said silicon single crystal
grains to each other comprises a step of oxidizing said silicon
thin film formed in said step (a) to form each region made of
silicon oxide between the adjacent ones of said silicon single
crystal grains.
45. A flash memory cell according to claim 43, wherein a length of
one side of each of said approximately rectangular-shaped silicon
single crystal grains in said silicon thin film formed in said step
(a) is 0.05 .mu.m or more.
46. A flash memory cell according to claim 43, wherein an average
thickness of said silicon thin film formed in said step (a) is in a
range of 1.times.10.sup.-8 m to 1.times.10.sup.-7 m.
47. A flash memory cell according to claim 43, wherein opposed
sides of each of said approximately rectangular-shaped silicon
single crystal grains in said silicon thin film formed in said step
(a) are approximately in parallel to the movement direction of the
ultraviolet beam irradiating position or intersect the movement
direction of the ultraviolet beam irradiating position at
approximately 45.degree..
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a new silicon thin film, a
group of silicon single crystal grains, and formation processes
thereof, and a semiconductor device, a flash memory cell, and
fabrication processes thereof.
[0002] A silicon thin film composed of a group of silicon single
crystal grains formed on a base body has been used for various
kinds of semiconductor devices such as a thin film transistor
(hereinafter, referred to as a "TFT") and a semiconductor device
based on a SOI (Silicon On Insulator) technique, and for solar
cells; and it is being examined to be applied to production of
micromachines.
[0003] In the field of semiconductor devices, for example, a
stacked SRAM using TFTs as load elements has been proposed. TFTs
have been also used for LCD (Liquid Crystal Display) panels. And,
in general, a silicon thin film composed of a group of silicon
single crystal grains is used for a TFT required to be enhanced in
electric characteristics such as a carrier mobility (.mu.),
conductivity (.sigma.), on-current characteristic, subthreshold
characteristic, and on/off current ratio. Concretely, efforts are
being made to improve characteristics of SRAMs and TFTs by
increasing sizes of silicon single crystal grains in combination
with reduction in density of twin crystals for lowering a trap
density in the silicon single crystal grains.
[0004] The enlargement in size (up to 1 .mu.m) of silicon single
crystal grains for improving the electric characteristics of such a
silicon thin film has been examined by a SPC (Solid Phase
Crystallization, solid phase crystallization from amorphous
silicon) technique or an ELA (Excimer Laser Anneal, liquid phase
crystallization using excimer laser). One process of forming a
silicon thin film using the ELA technique has been known, for
example, from a document ["Dependence of Crystallization Behaviors
of Excimer Laser Annealed Amorphous Silicon Film on the Number of
Laser Shot", B. Jung, et al., AM-LCD 95, PP. 177-120]. This
document describes that a silicon thin film composed of silicon
single crystal grains whose selected orientation is approximately
the <111> direction can be formed by repeatedly irradiating
excimer laser beams on an amorphous silicon layer. Another process
of forming a silicon thin film using the ELA technique has been
also known, for example, from a document ["Crystal forms by
solid-state recrystallization of amorphous Si film on SiO.sub.2",
T. Noma, Appl. Phys. Lett. 59 (6), Aug. 5, 1991,pp. 653-655]. This
document describes that silicon single crystal grains are oriented
in the <110> direction, and they include fine {111} twin
crystals.
[0005] A process of forming a silicon thin film by graphoepitaxial
growth using a strip-heater has been known, for example, from a
document [Silicon graphoepitaxy using a strip-heater oven", M. W.
Geis, et al., Appl. Phys. Lett. 37(5), Sep. 1, 1980, pp. 454-456].
This document describes that a silicon thin film formed on
SiO.sub.2 is composed of a (100) aggregation structure.
[0006] A silicon thin film composed of a group of silicon single
crystal grains has been also formed by a chemical-vapor deposition
(CVD) process or a random solid-phase growth process. For example,
the formation of polysilicon crystal grains by CVD has been known
from Japanese Patent Laid-open Nos. Sho 63-307431 and Sho
63-307776. In the techniques disclosed in these documents, the
selected orientation of silicon single crystal grains is the
<111> direction. Incidentally, in the case where a silicon
thin film composed of a group of silicon single crystal grains
having large sizes is formed by a normal chemical vapor deposition
process, it cannot satisfy a uniform quality, a reduced leak, and a
high mobility. In the random solid-phase growth process, it is
possible to form a silicon thin film composed of a group of silicon
single crystal grains having an average grain size of 1 .mu.m or
more; however, it is difficult for silicon single crystal grains to
selectively grow. Further, in the TFT using the silicon thin film
formed by such a process, since grain boundaries tend to be present
in a TFT active region, there occurs a problem that TFT
characteristics are varied depending on the presence of the grain
boundaries, to thereby shorten the life time of the TFT.
[0007] In all of the techniques disclosed in the above-described
references, any attempt has been not made to regularly arrange a
group of silicon single crystal grains on an insulating film. If a
group of silicon single crystal grains can be regularly arranged on
an insulating film, the TFT characteristics can be highly
controlled and equalized, and also one TFT can be formed in each of
the silicon single crystal grains. This is expected to further
develop the SOI technique.
[0008] A process of arranging silicon nuclei or crystal nuclei at
desired positions and forming silicon single crystal grains having
large sizes on the basis of the silicon nuclei or crystal nuclei
has been known, for example, from Japanese Patent Laid-open Nos.
Hei 3-125422, Hei 5-226246, Hei 6-97074, and Hei 6-302512. In the
technique disclosed in Japanese Patent Laid-open No. Hei 3-125422,
micro-sized silicon nuclei or crystal nuclei must be formed by
patterning using a lithography process; however, there is a
limitation to accurately form these micro-sized silicon nuclei or
crystal nuclei by the present lithography technique. In the case
where the sizes of silicon nuclei or crystal nuclei are large,
polycrystals tend to be formed with twin crystals and dislocations
easily produced, resulting in the reduced throughput. In the
techniques disclosed in Japanese Patent Lid-open Nos. Hei 5-226246,
Hei 6-97074, and Hei 6-302512, it is necessary to irradiate an
energy beam enabling fine convergence and direct scanning onto an
amorphous silicon layer or to carry out ion implantation.
Accordingly, these techniques have problems that not only the step
of forming silicon single crystal grains is complicated, but also
it takes a lot of time to form silicon single crystal grains
because of the necessity of a solid phase growth step, resulting in
the reduced throughput.
[0009] On the other hand, non-volatile memories are being
extensively developed at present. In particular, a flash memory
having a floating gate structure is being examined from the
viewpoint of the reduced size of the memory cell and the lowered
voltage. In a flash memory, data is written or erased by injecting
or discharging an electric charge into or from the floating gate.
Of various electric charge injecting methods, a channel hot
electron injection method or a method of allowing a
Fowler-Nordheim's tunnel current to flow by applying a high
electric field (for example, 8 MV/cm or more) on a tunnel oxide
film are generally used.
[0010] In such a flash memory cell, it has been known that a
threshold voltage after erasion of data is varied depending on
variations in sizes of polycrystalline silicon grains forming a
floating gate, for example, from a document ["Non-volatile Memory
and Its Scaling", Journal of Japan Society of Electron Information
Communication, Vol. 9, No. 5, pp. 469-484 (May, 1996)]. Further, as
one means for realizing a future fine flash memory cell operated at
a low voltage, a flash memory including a floating gate composed of
silicon nanocrystals has been proposed in a document ["A silicon
nanocrystal based memory", S. Tiwari, et al., Appl. Phys. Lett. 68
(10), 4, pp. 1377-1379, Mar. 4, 1996]. Additionally, as one form of
a non-volatile memory to lead the next generation over the present
semiconductor device, a single electron memory operated at a low
voltage using a small storage electric charge (electron) has been
proposed in a document ["A Room-temperature Single-Electron Memory
Device Using Fine-Grain Polycrystalline Silicon", K. Yano, et. al.,
IEDM93, PP. 541-544].
[0011] To realize a flash memory cell hard to be varied in a
threshold voltage after erasion of data, it is necessary to make as
small as possible variations in sizes of silicon crystal grains
forming a floating gate. Also, to realize a fine flash memory cell
operated at a low voltage, it is necessary to regularly form fine
silicon crystal grains on a thin insulating film (tunnel oxide
film) with an excellent controllability.
SUMMARY OF THE INVENTION
[0012] An object of the present invention is to provide a process
of easily, effectively forming a group of silicon single crystal
grains in a grid pattern on a base body with the reduced variations
in grain size, and a group of silicon single crystal grains formed
by the process.
[0013] Another object of the present invention is to provide a
process of easily, effectively forming a silicon thin film composed
of a group of silicon single crystal grains which are arranged in a
grid pattern on a base body with the reduced variations in grain
size, and a silicon thin film formed by the process.
[0014] A further object of the present invention is to provide a
process of fabricating a semiconductor device using the above
silicon thin film or the above group of silicon single crystal
grains, and a semiconductor device fabricated by the process.
[0015] An additional object of the present invention is to provide
a process of fabricating a flash memory cell using the above
silicon thin film or the above group of silicon single crystal
grains, and a flash memory cell fabricated by the process.
[0016] To achieve the above object, according to the present
invention, there is provided a process of forming a silicon thin
film, including the step of: irradiating a pulsed rectangular
ultraviolet beam on an amorphous or polycrystalline silicon layer
formed on a base body, to thereby form a silicon thin film composed
of a group of silicon single crystal grains on the base body;
wherein the moved amount (L) of a ultraviolet beam irradiating
position in a period from completion of an irradiation of the
rectangular ultraviolet beam to starting of the next irradiation of
the rectangular ultraviolet beam is specified at 40 .mu.m or less,
and a ratio (R=L/W) of the moved amount to a width (W) of the
rectangular ultraviolet beam measured in the movement direction
thereof is in a range of 0.1 to 5%, preferably, in a range of 0.5
to 2.5%, whereby forming a silicon thin film composed of a group of
silicon single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on the
base body, a selected orientation of the silicon single crystal
grains to the surface of the base body being approximately the
<100> direction.
[0017] According to the present invention, there is provided a
silicon thin film including a group of silicon single crystal
grains which are each approximately rectangular-shaped and which
are arranged in a grid pattern on a base body, wherein a selected
orientation of the silicon single crystal grains to the surface of
the base body is approximately the <100> direction.
[0018] To achieve the above object, according to the present
invention, there is provided a process of fabricating a
semiconductor device, including the steps of: irradiating a pulsed
rectangular ultraviolet beam on an amorphous or polycrystalline
silicon layer formed on a base body, to form a silicon thin film
composed of a group of silicon single crystal grains on the base
body; and forming a source/drain region and a channel region in the
silicon thin film or the silicon single crystal grains; wherein the
moved amount (L) of a ultraviolet beam irradiating position in a
period from completion of an irradiation of the rectangular
ultraviolet beam to starting of the next irradiation of the
rectangular ultraviolet beam is specified at 40 .mu.m or less, and
a ratio (R=L/W) of the moved amount to a width (W) of the
rectangular ultraviolet beam measured in the movement direction
thereof is in a range of 0.1 to 5%, preferably, in a range of 0.5
to 2.5%, whereby forming a silicon thin film composed of a group of
silicon single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on the
base body, a selected orientation of the silicon single crystal
grains to the surface of the base body being approximately the
<100> direction.
[0019] According to the present invention, there is provided a
semiconductor device including a source/drain region and a channel
region formed in a silicon thin film composed of a group of silicon
single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on a
base body or formed in the silicon single crystal grains, wherein a
selected orientation of the silicon single crystal grains to the
surface of the base body is approximately the <100>
direction.
[0020] As the semiconductor device of the present invention or the
semiconductor device fabricated by the process of fabricating a
semiconductor device of the present invention, there may be
exemplified a top gate type or a bottom gate type thin film
transistor used for LCD panels, a semiconductor device based on the
SOI technique (for example, a thin film transistor as a load
element of a stacked SRAM), and a MOS type semiconductor device.
The silicon thin film and the formation process thereof according
to the present invention can be applied not only to fabrication of
these semiconductor devices but also to production of solar cells
and to fabrication of micromachines.
[0021] In the silicon thin film and the formation process thereof,
and the semiconductor device and the fabrication process thereof
according to the present invention, a length of one side of a
silicon single crystal grain approximately rectangular-shaped may
be 0.05 .mu.m or more, preferably, 0.1 .mu.m or more. Here, the
wording "a silicon single crystal grain approximately
rectangular-shaped" means not only a perfectly rectangular silicon
single crystal grain but also an imperfectly rectangular silicon
single crystal grain having a chipped corner. The length of one
side of an imperfectly rectangular silicon single crystal grains
having a chipped corner means the length of one side of a virtual
rectangular single crystal grain obtained by filling up the chipped
corner. The same shall apply hereinafter. The average thickness of
a silicon thin film may be in a range of 1.times.10.sup.-8 m to
1.times.10.sup.-7 m, preferably, in a range of 1.times.10.sup.-8 m
to 6.times.10.sup.-8 m, more preferably, in a range of 1.times.10-8
m to 4.times.10.sup.-8 m. When the average thickness of a silicon
thin film is less than 1.times.10.sup.-8 m, there is a difficulty
in fabrication of a semiconductor device using the silicon thin
film. On the other hand, when it is more than 1.times.10.sup.-7 m,
the thickness of an amorphous or polycrystalline silicon layer
required to ensure the thickness of the silicon thin film is
excessively thick, and consequently, the selected orientation of
the silicon single crystal grains is possibly out of approximately
the <100> direction. The average thickness of a silicon thin
film may be measured by an ellipsometer, interference spectrometer,
or the like.
[0022] In the silicon thin film and the semiconductor device using
the silicon thin film according to the present invention, a group
of silicon single crystal grains constituting the silicon thin film
may be formed by irradiating a pulsed rectangular ultraviolet beam
on an amorphous or polycrystalline silicon layer formed on a base
body, and the moved amount (L) of a ultraviolet beam irradiating
position in a period from completion of an irradiation of the
rectangular ultraviolet beam to starting of the next irradiation of
the rectangular ultraviolet beam may be specified at 40 .mu.m or
less, and a ratio (R=L/W) of the moved amount to a width (W) of the
rectangular ultraviolet beam measured in the movement direction
thereof may be in a range of 0.1 to 5%, preferably, in a range of
0.5 to 2.5%. In the silicon thin film and the formation process
thereof, and the semiconductor device and the fabrication process
thereof according to the present invention, opposed sides of the
silicon single crystal grain approximately rectangular-shaped may
be approximately in parallel to the movement direction of the
ultraviolet beam irradiating position or intersect the movement
direction of the ultraviolet beam irradiating position at
approximately 45.degree.. Crystal planes constituting these two
sides are the {220} planes. That is, a crystal plane constituting
one side of a silicon single crystal grain approximately
rectangular-shaped is the {220} plane.
[0023] To achieve the above object, according to the present
invention, there is provided a process of forming a group of
silicon single crystal grains including: a step (a) of irradiating
a pulsed rectangular ultraviolet beam on an amorphous or
polycrystalline silicon layer formed on a base body, to thereby
form a silicon thin film composed of a group of silicon single
crystal grains which are each approximately rectangular-shaped and
which are arranged in a grid pattern on the base body, a selected
orientation of the silicon single crystal grains to the surface of
the base body being approximately the <100> direction; and a
step (b) of separating adjacent ones of the silicon single crystal
grains to each other; wherein the moved amount (L) of a ultraviolet
beam irradiating position in a period from completion of an
irradiation of the rectangular ultraviolet beam to starting of the
next irradiation of the rectangular ultraviolet beam is specified
at 40 .mu.m or less, and a ratio (R=L/W) of the moved amount to a
width (W) of the rectangular ultraviolet beam measured in the
movement direction thereof is in a range of 0.1 to 5%, preferably,
0.5 to 2.5%.
[0024] According to the present invention, there is provided a
group of silicon single crystal grains, including a plurality of
silicon single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on a
base body, wherein a selected orientation of the silicon single
crystal grains to the surface of the base body is approximately the
<100> direction, and adjacent ones of the silicon single
crystal grains are separated from each other.
[0025] To achieve the above object, according to the present
invention, there is provided a process of fabricating a flash
memory cell, including: a step (a) of irradiating a pulsed
rectangular ultraviolet beam on an amorphous or polycrystalline
silicon layer formed on a tunnel oxide film, to form a silicon thin
film composed of a group of silicon single crystal grains which are
each approximately rectangular-shaped and which are arranged in a
grid pattern on the tunnel oxide film, a selected orientation of
the silicon single crystal grains to the surface of the tunnel
oxide film is approximately the <100> direction; and a step
(b) of separating adjacent ones of the silicon single crystal
grains to each other, whereby forming a floating gate composed of
the group of silicon single crystal grains; wherein the moved
amount (L) of a ultraviolet beam irradiating position in a period
from completion of an irradiation of the rectangular ultraviolet
beam to starting of the next irradiation of the rectangular
ultraviolet beam is specified at 40 .mu.m or less, and a ratio
(R=L/W) of the moved amount to a width (w) of the rectangular
ultraviolet beam measured in the movement direction thereof is in a
range of 0.1 to 5%, preferably, 0.5 to 2.5%.
[0026] According to the present invention, there is provided a
flash memory cell including a floating gate composed of a plurality
of silicon single crystal grains which are each approximately
rectangular-shaped and which are formed on a tunnel oxide film, a
selected orientation of the silicon single crystal grains to the
surface of the tunnel oxide film being approximately the
<100> direction; wherein the silicon single crystal grains
are arranged in a grid pattern on the tunnel oxide film and
adjacent ones of the silicon single crystal grains are separated
from each other. In addition, the thickness of each of the silicon
single crystal grains separated from each other may be in a range
of 1.times.10.sup.-8 m to 1.times.10.sup.-7 m, preferably,
1.times.10.sup.-8 m to 8.times.10.sup.-m, more preferably, in a
range of 2.times.10.sup.-8 m to 5.times.10.sup.-8 m.
[0027] The flash memory cell of the present invention or the flash
memory cell fabricated by the process of fabricating a flash memory
cell of the present invention basically includes a source/drain
region and a channel region formed in a semiconducting substrate or
a silicon layer, a tunnel oxide film formed thereon, a floating
gate formed on the tunnel oxide film, an insulating film covering
the floating gate, and a control gate.
[0028] In the group of silicon single crystal grains or the flash
memory cell using the same according to the present invention, the
group of silicon single crystal grains may be formed by a process
including: a step (a) of irradiating a pulsed rectangular
ultraviolet beam on an amorphous or polycrystalline silicon layer
formed on the base body (or tunnel oxide film), to thereby form a
silicon thin film composed of a group of silicon single crystal
grains which are each approximately rectangular-shaped and which
are arranged in a grid pattern on the base body (tunnel oxide
film), a selected orientation of the silicon single crystal grains
to the surface of the base body (tunnel oxide film) being
approximately the <100> direction; and a step (b) of
separating adjacent ones of the silicon single crystal grains to
each other; wherein the moved amount (L) of a ultraviolet beam
irradiating position in a period from completion of an irradiation
of the rectangular ultraviolet beam to starting of the next
irradiation of the rectangular ultraviolet beam is specified at 40
.mu.m or less, and a ratio (R=L/W) of the moved amount to a width
(W) of the rectangular ultraviolet beam measured in the movement
direction thereof is in a range of 0.1 to 5%, preferably, in a
range of 0.5 to 2.5%.
[0029] In the group of silicon single crystal grains and the
formation process thereof, and the flash memory cell and the
fabrication process thereof according to the present invention, the
step (b) of separating adjacent ones of the silicon single crystal
grains to each other preferably includes a step of oxidizing the
silicon thin film formed in the step (a) to form each region made
of silicon oxide between the adjacent ones of the silicon single
crystal grains. Alternatively, the step (b) of separating adjacent
ones of the silicon single crystal grains to each other preferably
includes a step of etching the silicon thin film formed in the step
(a) to form each space between the adjacent ones of the silicon
single crystal grains. The length of one side of each of the
approximately rectangular-shaped silicon single crystal grains in
the silicon thin film formed in the step (a) is preferably as short
as possibly; however, it is preferably 0.05 .mu.m or more from a
practical standpoint. The average thickness of the silicon thin
film formed in the step (a) may be in a range of 1.times.10.sup.-8
m to 1.times.10.sup.-7 m, preferably, 1.times.10.sup.-8 m to
6.times.10.sup.-8 m, more preferably, 1.times.10.sup.-8 m to
4.times.10.sup.-8 m. The opposed sides of each of the approximately
rectangular-shaped silicon single crystal grains in the silicon
thin film formed in the step (a) may be approximately in parallel
to the movement direction of the ultraviolet beam irradiating
position or intersect the movement direction of the ultraviolet
beam irradiating position at approximately 45.degree.. Crystal
planes constituting these two sides are the {220} planes. That is,
a crystal plane constituting one side of a silicon single crystal
grain approximately rectangular-shaped is the {220} plane.
[0030] As the base body or the tunnel oxide film in the present
invention, there may be exemplified, while not exclusively, silicon
oxide (SiO.sub.2), silicon nitride (SiN), SiON, a stacked structure
of SiO.sub.2-SiN, and a stacked structure of
SiO.sub.2-SiN-SiO.sub.2. The base body or the tunnel oxide film may
be formed by oxidization or nitrization of the surface of a silicon
semiconducting substrate, or may be formed of a suitable film on a
semiconducting substrate, a layer, or an interconnection layer by
CVD or the like.
[0031] As a ultraviolet beam, there may be exemplified a XeCl
excimer laser having a wavelength of 308 nm and a full-solid
ultraviolet laser. The width (W) of a rectangular ultraviolet beam
measured in the movement direction is preferably in a range of 40
.mu.m to about 1 mm. The length of a rectangular ultraviolet beam
measured in the direction perpendicular to the movement direction
may be freely selected. It is desired to use a ultraviolet beam
having an extremely sharp rise in energy at an edge portion. As a
ultraviolet beam source for generating such a ultraviolet beam,
there may be exemplified, while not exclusively, a combination of a
XeCl excimer laser source, and attenuator, a beam homogenizer for
equalizing a rectangular beam, and a reflection mirror.
[0032] When the moved amount (L) of a ultraviolet beam irradiating
position in a period from completion of an irradiation of a
rectangular ultraviolet beam to stating of the next irradiation of
the rectangular ultraviolet beam is more than 40 .mu.m or a ratio
(R=L/W) of the moved amount to the width (W) of the ultraviolet
beam measured in the movement direction of the ultraviolet beam
irradiating position is more than 5%, there is a fear that a group
of silicon single crystal grains which are each approximately
rectangular shaped and which are arranged in a grid pattern on a
base body are not formed, or that the selected orientation of the
silicon single crystal grains to the surface of the base body is
out of approximately the <100> direction. Further, when the
movement ratio (R=L/W) is less than 0.1% of the width (W) of the
ultraviolet beam measured in the movement direction of the
ultraviolet beam irradiating position, the throughput becomes
excessively low. In addition, the base body may be moved with the
ultraviolet beam source kept fixed; the ultraviolet beam source may
be moved with the base body kept fixed; or both the ultraviolet
beam source and the base body may be moved.
[0033] In the case where 30% or more of silicon single crystal
grains constituting a group of silicon single crystal grains are
selectively oriented approximately in the <100> direction
with respect to the surface of a base body, the selected
orientation of the silicon single crystal grains constituting the
group of silicon single crystal grains to the surface of a base
body is specified as approximately the <100> direction. In
addition, the approximately <100> direction of silicon single
crystal grains contains the case that the <100> direction of
the silicon single crystal grains is not strictly in parallel to
the direction perpendicular to the surface of a base body. The
selected orientation is sometimes called "a preferred orientation".
A polycrystalline structure in the form of a film or the like in
which crystals are not random-oriented but a large number of the
crystals have a crystal axis, crystal plane and the like oriented
in a specified direction, is called "an aggregate structure" or "a
fiber structure". In this structure, the oriented crystal axis is
called the selected orientation.
[0034] In the present invention, as described above, by specifying
the moved amount (L) of a ultraviolet beam irradiating position in
a period from completion of an irradiation of a rectangular
ultraviolet beam to starting of the next irradiation of the
rectangular ultraviolet beam to be 40 .mu.m or less, and also
specifying a ratio (R=L/W) of the moved amount to the width (W) of
the ultraviolet beam measured in the movement direction of the
ultraviolet beam irradiating position to be in a range of 0.1 to
5%, there can be formed a silicon thin film composed of a group of
silicon single crystal grains which are each approximately
rectangular-shaped and which are arranged in a grid pattern on a
base body, a selected orientation of the silicon single crystal
grains being approximately the <100> direction. The reason
for this is unclear, but it may be considered as follows: namely,
by irradiating a pulsed rectangular ultraviolet beam (having
extremely sharp rise in energy at an edge portion) in a certain
region of an amorphous or polycrystalline silicon layer while
overlapping and slightly shifting the ultraviolet beams, there is
established a repetition of a thermal equilibrium state by heat
reservation and a cooling (solidifying) state, to thereby form a
group of these silicon single crystal grains. Also, the reason why
the selected orientation of silicon single crystal grains to the
surface of a base body is approximately the <100> direction
may be considered to be due to the free energy on the surface of Si
to the base body made of, for example, SiO.sub.2.
[0035] According to the present invention, there can be easily,
effectively formed a silicon thin film composed of a group of
silicon single crystal grains arranged in a grid pattern on a base
body (insulating film), a selected orientation of the silicon
single crystal grains to the surface of the base body being
approximately the <100> direction. Accordingly, it becomes
possible to highly control and equalize characteristics of a TFT
formed of the silicon thin film or to improve a semiconductor
device bases on the SOI technique by forming a TFT in a fine
silicon single crystal grain. Further, since the crystallinity of
the silicon thin film is improved in terms of macro-structure,
characteristics of a TFT used for LCD panels and the like can be
enhanced. Also, there can be realized a flash memory cell (nano dot
memory) capable of being operated at a low voltage by directly
applying a tunneling effect and electron accumulation.
Additionally, by forming a floating gate of a flash memory cell of
a silicon thin film of the present invention, variations in sizes
of silicon grains forming the floating gate can be reduced, so that
there can be realized a flash memory cell hard to be varied in
threshold voltage after erasion of data.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1A is a schematic view illustrating a process of
forming a silicon thin film in Example 1, and FIGS. 1B and 1C are
schematic sectional views of the silicon thin film formed on a base
body;
[0037] FIG. 2 is a transmission type electron microscopic
photograph of the silicon thin film obtained in Example 1;
[0038] FIG. 3 is a copy of an AFM photograph of the surface of the
silicon thin film obtained in Example 1;
[0039] FIG. 4 is a copy of another AFM photograph of the surface of
the silicon thin film obtained in Example 1;
[0040] FIG. 5 is a copy of an AFM photograph of the surface of a
silicon thin film obtained in Comparative Example 1;
[0041] FIG. 6 is a copy of an AFM photograph of the surface of a
silicon thin film obtained in Comparative Example 2;
[0042] FIGS. 7A to 7C are schematic sectional views of a silicon
thin film formed on a base body, illustrating a process of
fabricating a semiconductor device in Example 2;
[0043] FIG. 8 is a schematic sectional view of a bottom gate type
thin film transistor in Example 2;
[0044] FIG. 9 is a graph showing evaluation results of
characteristics of n-type thin film transistors of a bottom gate
type in Examples 2, 3 and Comparative Example 2;
[0045] FIGS. 10A to 10C are schematic sectional views of a silicon
thin film formed on a tunnel oxide film, illustrating a process of
fabricating a flash memory cell in Example 4; and
[0046] FIGS. 11A and 11B are schematic sectional views, continued
to FIGS. 10A to 10C, illustrating the process of fabricating a
flash memory cell in Example 4; and
[0047] FIG. 12 is a schematic sectional view showing one example in
which a floating gate of a flash memory cell is formed of the
silicon thin film of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] Hereinafter, the present invention will be described in
detail using the following examples with reference to the
accompanying drawings.
EXAMPLE 1
[0049] This example concerns a silicon thin film and a formation
process thereof according to the present invention. In this
example, a silicon thin film composed of a group of silicon single
crystal grains was formed on a base body made of SiO.sub.2 by
irradiating a pulsed ultraviolet beam on an amorphous silicon layer
formed on the base body. The irradiation conditions of the
ultraviolet beam and the like are shown in Table 1.
1 TABLE 1 XeCl excimer laser ultraviolet beam (wavelength: 308 nm)
irradiated amount 320 mJ/cm.sup.2 pulse width about 26 nano seconds
frequency about 200 Hz beam shape rectangular shape: 400 .mu.m
(width, W) .times. 150 mm (length) moved amount L 4 .mu.m movement
ratio R 1% (= 4 .mu.m/400 .mu.m .times. 100)
[0050] Concretely, a SiN film 11 having a thickness of 50 nm was
formed on a substrate 10 made of quartz and a base body 12 made of
SiO.sub.2 was formed on the SiN film 11 to a thickness of 100 nm.
Then, an amorphous silicon layer 13 having a thickness of 30 nm was
formed on the base body 12 by PECVD. This state is shown by a
schematic sectional view of FIG. 1A. Next, a pulsed ultraviolet
beam was irradiated on the amorphous silicon layer 13 formed on the
base body 12 in the conditions shown in Table 1. This state is
shown by a schematic sectional view of FIG. 1B. In FIG. 1E, a
region of the silicon layer 13 on which the preceding ultraviolet
beam was irradiated is shown by the dotted line, and a region of
the silicon layer 13 on which the present ultraviolet beam was
irradiated is shown by the chain line. In this example, since the
movement ratio (R=L/W) is 1%, a certain point of the amorphous
silicon layer 13 is 100 times exposed to the pulsed ultraviolet
beam. Although the substrate 10 was moved with a ultraviolet beam
source kept fixed in this example, the ultraviolet beam source may
be moved with the substrate 10 kept fixed, or both the substrate 10
and the ultraviolet beam source may be moved. A schematic sectional
view of a silicon thin film 14 thus obtained is shown in FIG. 1C.
In FIG. 1C, grain boundaries are shown by the dotted line. Each of
the silicon single crystal grains has a sectional shape in which
the central portion is recessed and the peripheral portion
projects.
[0051] The silicon thin film thus obtained was observed by a
transmission type electron microscope. The result is shown by an
electron microscopic photograph in FIG. 2. In addition, a sample
for microscopic observation is only a silicon thin-film obtained by
etching the substrate 10, SiN film 11 and base body 12 using a
mixed solution of HF/H.sub.2O={fraction (1/2.)} As can be seen from
the photograph of FIG. 2, the silicon thin film thus obtained is
composed of a group of silicon crystal grains which are
approximately rectangular-shaped. The selected orientation of the
silicon single crystal grains to the surface of the base body was
approximately the <100> direction. The length of one side of
the silicon single crystal grain approximately rectangular-shaped
was 0.1 .mu.m or more. The opposed two sides of the silicon single
crystal grain approximately rectangular-shaped were approximately
in parallel to the movement direction of the ultraviolet beam
irradiating position. Crystal planes constituting the two sides
were the {220} planes. Depending on the observation points, the
opposed two sides of the silicon single crystal grain approximately
rectangular-shaped intersected the movement direction of the
ultraviolet beam irradiating position at approximately
45.degree..
[0052] When a polycrystalline silicon is perfectly non-oriented, a
ratio of a refraction intensity I.sub.111 at the {111} plane to a
refraction intensity I.sub.220 at the {220} plane was
I.sub.111:I.sub.220=5:3. Besides, the ratio of I.sub.111:I.sub.220
in the silicon thin film obtained in this example was 1:4. From the
analysis of the refraction intensity ratio, it is revealed that the
selection orientation of the silicon single crystal grains to the
surface of the base body is approximately the <100>
direction. In addition, about 30% of the silicon single crystal
grains constituting the group of silicon single crystal grains in
the silicon thin film were selectively orientated in the
<100> direction to the surface of the base body, and the
remaining silicon single crystal grains were oriented at random to
the surface of the base body. Also, it was frequently observed that
the adjacent ones in a unit constituted of several silicon single
crystal grains correspond to each other in the crystal
orientation.
[0053] The surface of the silicon thin film obtained in this
example was observed and measured by an AFM (Atomic Force
Microscope). The measured results are shown in Table 3, and the
surface observation photographs are shown in FIGS. 3 and 4. In
addition, the observation field in FIG. 3 is 3 .mu.m.times.3 .mu.m,
and the observation field in FIG. 4 is 20 .mu.m.times.20 .mu.m. As
can be seen from FIGS. 3 and 4, the silicon thin film is composed
of a group of silicon single crystal grains arranged in a grid
pattern on the base body. That is, the silicon single crystal
grains are regularly arranged in a checkerboard pattern. Also, in
FIG. 4, there are observed some linear stripes which leftward,
downward extend from the right, upper portion. A gap between the
stripe is about 4 .mu.m, which substantially corresponds to the
moved amount L of the ultraviolet beam irradiating position. The
opposed two sides of the silicon single crystal grain approximately
rectangular-shaped were approximately in parallel to the movement
direction of the ultraviolet beam irradiating position or
intersected the movement direction of the ultraviolet beam
irradiating position at approximately 45.degree.
Comparative Example 1
[0054] A silicon thin film was formed on a base body in the same
manner as in Example 1, except that the moved amount L and the
movement ratio R were different from those in Example 1. The moved
amount L and the movement ratio R are shown in Table 2.
[0055] The surfaces of the silicon thin films obtained in
Comparative Example 1 were observed and measured. The measured
results are shown in Table 3, and the surface observation
photographs are shown in FIG. 5 (Comparative Example 1A) and FIG. 6
(Comparative Example 1B). In addition, the observation field in
each of FIGS. 5 and 6 is 3 .mu.m.times.3 .mu.m. As can be seen from
FIGS. 5 and 6, for each of the silicon thin films obtained with the
moved amount L set at 40 .mu.m or more, single crystal grains are
not arranged in a grid pattern on the base body. Further, it is
revealed that as the moved amount L is made larger, the number of
irregularities of the silicon thin film is made smaller.
[0056] Further, experiments were repeated with the irradiated
amount of the XeCl excimer laser beam changed into 280 mJ/cm.sup.2,
320 mJ/cm.sup.2, 340 mJ/cm2 and 360 mJ/cm.sup.2; however, any
silicon thin film composed of silicon single crystal grains
selectively oriented in the <100> direction to the surface of
the base body was not formed.
2 TABLE 2 moved amount L (.mu.m) movement ratio R (%) Comparative
40 10 Example 1A Comparative 200 50 Example 1B Comparative 400 100
Example 1C
[0057]
3 TABLE 3 movement ratio R Ra (.mu.m) RMS (.mu.m) Example 1 1%
11.71 14.50 Comparative Example 1A 10% 8.66 10.83 Comparative
Example 1B 50% 4.81 5.98 Comparative Example 1C 100% 5.21 6.30
EXAMPLE 2
[0058] This example concerns a semiconductor device and a
fabrication process thereof according to the present invention. In
this example, an n-type thin film transistor having a bottom gate
structure was fabricated using the process of forming a silicon
thin film described in Example 1. An insulating layer 21 made of
SiO.sub.2 was formed on the surface of a glass substrate 20, and a
polycrystalline silicon layer doped with an impurity was deposited
over the surface by CVD. The polycrystalline silicon layer was
patterned, to form a gate electrode 22. Next, a base body 23 made
of SiO.sub.2 was formed over the surface by CVD. The base body 23
also functions as a gate oxide film.
[0059] As in Example 1, an amorphous silicon layer 24 having a
thickness of 40 nm was formed on the base body 23 made of SiO.sub.2
by PECVD, as shown in FIG. 7A. Next, a pulsed ultraviolet beam was
irradiated on the amorphous silicon layer 24 thus formed (see FIG.
7B), to form a silicon thin film 25 composed of a group of silicon
single crystal grains on the base body (see FIG. 7C). The
irradiation conditions of the ultraviolet beam and the like are the
same as those shown in Table 1. In addition, a region of the
silicon layer 24 on which the preceding ultraviolet beam was
irradiated is shown by the dotted line, and a region of the silicon
layer 24 on which the present ultraviolet beam was irradiated is
shown by the chain line.
[0060] An impurity was doped in a source/drain region forming area
of the silicon thin film 25 by ion implantation, followed by
activation of the impurity thus doped, to form a source/drain
region 26 and a channel region 27. An insulating layer 28 made of
SiO.sub.2 was then deposited by CVD, and opening portions were
formed in the insulating layer 28 at positions over the
source/drain region 26 by lithography and RIE. An interconnection
material layer made of an aluminum alloy was deposited on the
insulating layer 28 including the opening portions by sputtering,
followed by patterning of the interconnection material layer, to
form an interconnection 29 on the insulating layer 28 (see FIG. 8).
The interconnection 29 is connected to the source/drain region 26
via the interconnection material layer buried in the opening
portions.
EXAMPLE 3
[0061] An n-type thin film transistor having a bottom gate
structure was fabricated in the same manner as in Example 2, except
that the moved amount L and the moved amount/beam width R were
different from those in Example 2. The moved amount L and the moved
amount/beam width R in each of Examples 2 and 3 are shown in Table
4.
Comparative Example 2
[0062] An n-type thin film transistor having a bottom gate
structure was fabricated in the same manner as in Example 2, except
that the moved amount L and the moved amount/beam width R were
different from those in Example 2. The moved amount L and the moved
amount/beam width R in Comparative Examples 2 are shown in Table
4.
[0063] The characteristics of the n-type thin film transistors of a
bottom gate type obtained in Examples 2, 3 and Comparative Examples
2A, 2B were evaluated by measuring a drain current (I.sub.ON) with
V.sub.d=10 V and V.sub.g=15 V. The results are shorn in FIG. 9. As
can be seen from FIG. 9, when the movement R is 5% or less, the
drain current (I.sub.ON) becomes higher.
4 TABLE 4 moved amount L (.mu.m) movement ratio R (%) Example 2 4 1
Example 3 20 5 Comparative Example 2A 40 10 Comparative Example 2B
360 90
EXAMPLE 4
[0064] This example concerns a group of silicon single crystal
grains and a formation process thereof, and a flash memory cell and
a fabrication process thereof according to the present invention.
Hereinafter, this example will be described with reference to FIGS.
10 and 11.
[0065] First, element isolation regions 31 having a LOCOS structure
was formed in a silicon semiconducting substrate 30, followed by
ion implantation for well formation, channel stop, and threshold
value adjustment. The element isolation region may be of a trench
structure. After that, fine particles and metal impurities on the
surface of the silicon semiconducting substrate 30 were removed by
RCA cleaning, and then the surface of the silicon semiconducting
substrate 30 was cleaned by a solution of 0.1% hydrofluoric acid.
Next, a tunnel oxide film (equivalent to a base body) 32 having a
thickness of 3 nm was formed on the exposed surface of the silicon
semiconducting substrate 30 by a known oxidation process.
[0066] After that, as shown in FIG. 10A, an amorphous silicon layer
33 having a thickness of about 40 nm was formed on the tunnel oxide
film 32 by PECVD, as in Example 1. A pulsed ultraviolet beam was
irradiated on the amorphous silicon layer 33 (see FIG. 10B), to
form a silicon thin film 34 composed of a group of silicon single
crystal grains on the tunnel oxide film 32 (see FIG. 10C). The
irradiation conditions of the ultraviolet beam and the like are the
same as those shown in Table 1. In FIG. 10B, a region of the
silicon layer 33 on which the preceding ultraviolet beam was
irradiated is shown by the dotted line, and a region of the silicon
layer on which the present ultraviolet beam was irradiated is shown
by the chain line.
[0067] The silicon thin film thus obtained was observed by a
transmission type electron microscope and an AFM. It was observed
that silicon single crystal grains 35 each having an approximately
rectangular shape (length of one side: about 0.3 .mu.m) were
arranged in a grid pattern on the base body. The selected
orientation of the silicon single crystal grains to the surface of
the base body was approximately the <100> direction. In
addition, about 30% of the silicon single crystal grains
constituting the group of the silicon single crystal grains were
oriented in the <100> direction to the surface of the base
body, the remaining silicon single crystal grains were
random-oriented to the surface of the base body. Also, in some
silicon single crystal grains, the <100> direction was not
strictly in parallel to the direction perpendicular to the surface
of the base body. Further, it was frequently observed that the
adjacent ones in a unit constituted of several silicon single
crystal grains correspond to each other in the crystal orientation.
Additionally, opposed two sides of the silicon single crystal grain
approximately rectangular-shaped were approximately in parallel to
the movement direction of the ultraviolet beam irradiating position
or intersected the movement direction of the ultraviolet beam
irradiating position at approximately 45.degree..
[0068] After that, the adjacent silicon single crystal grains 35
were separated from each other. Concretely, the silicon thin film
thus obtained was oxidized in an oxygen gas atmosphere under the
temperature condition of 1000.degree. C..times.20 min to form each
region 36 made of silicon oxide (SiO.sub.2) between the adjacent
silicon single crystal grains 35A (see FIG. 11A). The average
thickness of the silicon single crystal grain 35A thus oxidized was
about 10 nm, and the size thereof was 7-13 nm. These silicon single
crystal grains 35A, spaced at intervals (about 0.3 .mu.m), were
arranged in a arid pattern on the tunnel oxide film (base body 32.
That is, the silicon single crystal grains were regularly arrange
in a checkerboard pattern. Thus, a floating gate 37 composed of a
plurality of the silicon single crystal grains 35A was formed. In
general, the oxidation of silicon preferentially proceeds from
grain boundaries. The silicon single crystal grains 35 was
selectively orientated approximately in the <100> direction
to the surface of the tunnel oxide film (base body) 32, so that the
thicknesses and the sizes of the silicon single crystal grains can
be preferably adjustable.
[0069] After that, the regions 36 made of SiO.sub.2 were patterned,
to remove the unnecessary regions 36 made of SiO.sub.2 and the
silicon single crystal grains 35A. Then, an insulating film 38 was
formed over the entire surface by CVD, and a polycrystalline
silicon layer doped with an impurity was formed on the insulating
film 38 by CVD, followed by patterning of the polycrystalline
silicon layer and the insulating film 38. Thus, a control gate 39
formed of the polycrystalline silicon layer was formed.
[0070] After that, an impurity was doped in a source/drain region
forming area of the exposed silicon semiconducting substrate 30 by
ion implantation, followed by activation of the impurity thus
doped, to form a source/drain region 40 and a channel region 41
(see FIG. 11B). Then, an insulating film made of, for example
SiO.sub.2 was deposited over the entire surface by CVD, and opening
portions were formed in the insulating layer at positions over the
source/drain region 40 by photolithography and RIE. An
interconnection material layer made of an aluminum alloy was then
deposited on the insulating layer including the opening portions by
sputtering, followed by patterning of the interconnection material
layer, to accomplish an interconnection on the insulating layer.
The interconnection is connected to the source/drain region 40 via
the interconnection material layer buried in the opening portions.
Thus, a flash memory cell (nano dot memory) was fabricated.
[0071] While the present invention has been described with
reference to the examples, such description is for illustrative
purposes only, and it is understood that many changes may be made
without departing from the scope of the present invention. For
example, although the amorphous silicon layer was formed on the
base body in the examples, a polycrystalline silicon layer may be
formed on the base body. When a pulsed ultraviolet beam is
irradiated on an amorphous or polycrystallinbe silicon layer, the
base body may be heated. The surface of a silicon thin film may be
planarized by etching-back. Additionally, in the semiconductor
device and the fabrication process thereof according to the present
invention, one transistor element can be fabricated from one
silicon single crystal grain formed on a base body by forming a
source/drain region and a channel region in the silicon single
crystal grain. In this case, the adjacent silicon single crystal
grains may be separated from each other by patterning a silicon
thin film by lithography and etching for removing the unnecessary
silicon single crystal grains. Alternatively, the adjacent silicon
single crystal grains may be separated from each other by oxidizing
a silicon thin film formed on a base body made of a material having
an etching selection ratio to silicon oxide, for example, silicon
nitride, to form each region made of silicon oxide between the
adjacent silicon single crystal grains, and etching the silicon
oxide. Further, as shown by a schematic sectional view of FIG. 12,
a floating gate of a flash memory cell may be formed of a silicon
thin film according to the present invention or a floating gate of
a flash memory cell may be formed on the basis of the process of
forming a silicon thin film according to the present invention.
* * * * *