Laser irradiation process

Details Laser irradiation process Laser irradiation process

1999-07-15

2001-02-13

Chaudhuri, Olik (Department: 2814)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth from solid or gel state

Using heat

C117S010000, C438S487000, C438S488000

Reexamination Certificate

active

06187088

ABSTRACT:

REFERENCE TO RELATED APPLICATION AND INCORPORATION BY REFERENCE
This application is based on application NO.HEI10-219200 filed in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a laser irradiation process comprising irradiating a pulse laser beam on a non-single crystal semiconductor film for annealing; in particular it relates to a laser irradiation process for forming a channel layer in a polycrystalline silicon thin film transistor which may be, for example, used for a liquid crystal display or a contact type image sensor.
2. Description of the Related Art
Recently, a thin film transistor comprising a polysilicon film as a channel layer on a glass substrate has been intensely developed for application in, for example, a liquid crystal display or a contact type image sensor. A laser annealing has been a common technique for preparing a polysilicon film in the light of reduction in a process temperature and improvement of a throughput; the process comprises forming a silicon film as a precursor on which an ultraviolet pulse laser beam is then irradiated to cause crystallization via fusion for forming a polycrystalline structure.
A common laser irradiation process is irradiation of a pulse laser beam having a rectangular or linear irradiation region.
JP-A 9-246183 has disclosed a process for irradiating a laser beam having a trapezoidal profile in a linewidth direction, which will be described with reference to FIG.
8
. The process comprises irradiating a laser having a trapezoidal profile as shown in the figure only once to provide a polycrystalline structure. The energy density in the center of the trapezoidal beam profile is equal to or higher than a microcrystallization threshold for an amorphous semiconductor (amorphous silicon). There exists a region with a slightly lower energy than the microcrystallization threshold in the slope of the trapezoidal beam profile. It is believed that a polycrystalline semiconductor region with a large grain size can be formed.
This process may form a polycrystalline semiconductor region with a large grain size in a region with a slightly lower energy than the microcrystallization threshold, but the large crystal grains are randomly arranged and generally their size is widely distributed. Thus, when such a polycrystalline semiconductor region is used as, for example, a channel layer for a TFT (Thin Film Transistor), TFT properties such as a mobility may significantly vary.
The above publication has disclosed an irradiation process where a linear laser is scanned in its linewidth direction, i.e., a direction perpendicular to the line direction, which may provide a large area of polycrystalline silicon film. Thus, a variety of processes for irradiating a laser beam with a trapezoidal profile by scanning have been proposed. These processes, however, have a common problem that a crystal structure in a polycrystalline silicon film formed is poorly uniform. For example, Nouda et al, Shingaku Giho Vol. SDM92-112, p. 53 (1992) has disclosed that a beam end of a pulse laser beam may significantly vary the size of a crystal formed by the next irradiation, due to dependency of the state of the melted film by laser irradiation on the film structure before irradiation. In particular, a fused state is considerably changed in an interface between an already-irradiated region (crystallized region) and a non-irradiated region (amorphous region) when an amorphous silicon film is used as a precursor.
FIG. 9
shows a grain size distribution for a polycrystalline silicon structure formed by a laser annealing using a laser beam having a common trapezoidal energy density profile. FIG.
9
(
b
) shows a grain size distribution in a polycrystalline region formed by irradiating a pulse laser having the profile shown in FIG.
9
(
a
) on an amorphous silicon film. Then, a pulse laser beam is irradiated by scanning with a pitch x, so that the grain size distribution may vary as shown in FIG.
9
(
c
). In the figure, a local minimum in a grain size can be observed in a region around the beam end in FIG.
9
(
b
). Finally, a polycrystalline silicon film having a grain size distribution shown in FIG.
9
(
d
) is formed, except the region where irradiation is initiated or stopped. In other words, there occurs unevenness in a grain size due to change in a crystal structure caused by the beam end. The grain size shown in FIG.
9
(
d
) is an average size, and when such a trapezoidal beam profile is used, large crystal grains are randomly arranged, leading to a wide distribution of grain size as described above.
SUMMARY OF THE INVENTION
In the light of the above problems, an objective of this invention is to provide a laser irradiation process for forming a large-size polycrystalline silicon film exhibiting an even grain-size distribution and a good grain arrangement.
This invention provides a laser irradiation process comprising irradiating a pulse laser beam on a non-single crystal semiconductor film to form a polycrystalline semiconductor film, where an energy profile along one direction in a pulse laser beam irradiation region meets the conditions (A) and (B) and comprising irradiating the pulse laser beam on the same position multiple times:
(A) there are the first region having an energy density of E
a
or higher and the second regions on both sides of the first region having an energy density of less than E
a
, where E
a
is a microcrystallization energy for an amorphous semiconductor film; and
(B) an energy density slope has an absolute value of 20 to 300 J/cm
3
in a boundary region in the second region extending to 1 &mgr;m from the boundary line between the first and the second regions.
In the laser irradiation process of this invention, a pulse laser exhibiting a profile having a given energy density slope in a boundary region is irradiated on the same position multiple times. Thus, a polycrystalline silicon film where large grains with an even size distribution are orderly arranged can be formed in the vicinity of the boundary region. Such a polycrystalline semiconductor film may be used to provide a high efficient device.
This invention also provides a laser irradiation process comprising irradiating a pulse laser beam on a non-single crystal semiconductor film to form a polycrystalline semiconductor film, where an energy profile along one direction in a pulse laser beam irradiation region meets the conditions (A) and (B) and comprising irradiating the pulse laser beam on the same position multiple times:
(A) there are alternately the first regions having an energy density of E
a
or higher and the second regions having an energy density of less than E
a
, where E
a
is a microcrystallization energy for an amorphous semiconductor film; and
(B) an energy density slope has an absolute value of 20 to 300 J/cm
3
in a boundary region in the second region extending to 1 &mgr;m from the boundary line between the first and the second regions.
This process may provide, in one step, a plurality of polycrystalline semiconductor structures where large grains with an even size are orderly arranged, for improving a process efficiency.
In the above laser irradiation process, an energy profile in a direction perpendicular to the above direction may meet the following conditions (A) and (B):
(A) there are the first region having an energy density of E
a
or higher and the second regions on both sides of the first region having an energy density of less than E
a
, where E
a
is a microcrystallization energy for an amorphous semiconductor film; and
(B) an energy density slope has an absolute value of 20 to 300 J/cm
3
in a boundary region in the second region extending to 1 &mgr;m from the boundary line between he first and the second regions.
In the above laser irradiation process, an energy profile in a direction perpendicular to the above direction may meet the following conditions (A) and (B):
(A) there are alternately the first regions having an energy density of E
a
or high

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