Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2000-10-26
2002-09-17
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S075000
Reexamination Certificate
active
06452213
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor thin film substrate, a semiconductor device, a semiconductor device manufacturing method and an electronic apparatus, and more particularly, to the technology for manufacturing transistors (for example, a thin film transistor (TFT)) using polycrystalline films (polycrystalline semiconductor thin films), a semiconductor thin film substrate for manufacturing the thin film transistor, and the technology which is effectively applicable to manufacturing processes for electronic apparatus such as a liquid crystal display device, an information processing apparatus and so on that incorporate the thin film transistors.
Thin film transistors so far used in conventional image display devices and so on are formed using polycrystalline silicon which is fabricated on an insulating base made of glass, quartz or the like by a recrystallization method such as excimer laser anneal or the like, using amorphous silicon or micro-crystalline silicon formed by a plasma CVD method or the like as a precursor.
A conventional method of manufacturing a polycrystalline semiconductor thin film (polycrystalline silicon thin film) and a thin film transistor will be described below with reference to
FIGS. 1
a
-
1
d,
2
,
3
,
4
a-
4
c,
and
5
a-
5
c.
As illustrated in
FIG. 1
a,
a silicon oxide film (SiO
2
film)
102
and an amorphous silicon thin film
103
are sequentially formed on an insulating base
101
, for example, a glass base
101
. Next, as illustrated in
FIG. 1
b,
the surface of the amorphous silicon thin film
103
is irradiated with excimer laser light
105
which has light flux of rectangular or elongated cross-section. As indicated by an arrow
106
, the laser light
105
is moved (scanned) to anneal the overall surface of the amorphous silicon thin film
103
with the excimer laser light
105
. The amorphous silicon thin film
103
changes from an amorphous structure to a polycrystalline silicon thin film
104
by this annealing through a melt/solidification process, as illustrated in
FIG. 1
c.
The foregoing process is referred to as an excimer laser annealing process (excimer laser crystallization), and is used for fabricating a high quality polycrystalline thin film on a base made of a low melting point material such as glass. The excimer laser annealing process is described in detail, for example, in “1996 Society for Information Display International Symposium Digest of Technical Papers,” pp. 17-20, and “IEEE Transactions on Electron Devices,” vol. 43, no. 9, 1996, pp. 1454-1457, and so on.
FIG. 1
d
is a schematic diagram illustrating a TFT which has been formed using the aforementioned polycrystalline silicon thin film
104
. In the polycrystalline silicon thin film
104
, semiconductor regions
110
,
111
are formed by diffusing a predetermined impurity element. These semiconductor regions
110
,
111
constitute a source region and a drain region of a field effect transistor. Also, a gate insulating film
112
made of SiO
2
is provided on the surface of the polycrystalline silicon thin film
104
between the semiconductor regions
110
,
111
, and a gate electrode
113
is provided on the gate insulating film
112
. In this structure, a source-to-drain current can be controlled by a voltage applied at the gate electrode
113
. For example, the gate has a length of 4 &mgr;m and a width of 4 &mgr;m.
FIG. 2
is a graph related to the dependency of the silicon crystal grain diameter on an irradiated laser energy density in the conventional excimer laser crystallization. In this example, an amorphous silicon thin film
103
formed on an insulating base
101
, for example, a glass base
101
, has a thickness of 100 nm, and is crystallized by XeCl excimer laser-based annealing (at wavelength of 308 nm). As can be seen from the graph, the amorphous silicon thin film is not crystallized at a laser energy density below 100 mJ/cm
2
since the thin film is not melted with such energy. However, the thin film is melted from its surface as the energy density exceeds 100 mJ/cm
2
, resulting in crystal nuclei produced on a solid-liquid interface of the amorphous silicon thin film
103
and resulting formation of crystal grains (for example, crystal grains
104
a
).
As the laser energy density is increased, the amorphous thin film is melted deeper. As a result, larger crystal grains are produced (for example, crystal grains
4
b
). The production of crystal nucleus from a solid-liquid interface in this way is referred to as “inhomogeneous nucleation.” In
FIG. 2
, Ec indicates a laser energy density at which the solid-liquid interface reaches the insulating base
101
. As the laser energy density exceeds Ec, the overall amorphous thin film is melted, and enters into a supercooling state. As a result, crystal nuclei are produced within the thin film at random to form micro-crystals
104
c
of diameters equal to or less than 0.05 &mgr;m. Such production of crystal nuclei is referred to as “homogeneous nucleation.”
For fabricating a polycrystalline silicon thin film transistor (TFT) having satisfactory characteristics, for example, a TFT exhibiting a mobility &mgr; of 100 cm
2
/V.s, the grain diameter of silicon crystals must be 0.2 &mgr;m or more. Therefore, the amorphous thin film should be crystallized with the laser energy density set at Ec. In this example, Ec is set at 230 mJ/cm
2
. It should be noted however that the value of the laser energy density in the prior art may vary since it depends on the nature of the amorphous silicon film (for example, an employed growth method, its film thickness, and so on), the temperature of the base, and the wavelength and pulse width of the excimer laser. Details in this respect are found, for example, in “Applied Physics Letters,” vol. 63, no. 14, 1993, pp. 1969-1971, and so on.
FIG. 3
is a schematic plan view showing a positional relationship between semiconductor regions
110
,
111
and a gate electrode
113
of a TFT. A channel is formed between the semiconductor regions
110
,
111
, and the length of the channel is equal to the gate length. The channel length may be, for example, 4 &mgr;m. Also, an average crystal grain diameter (crystal diameter
104
b
) of crystals comprising the polycrystalline silicon thin film is 0.25 &mgr;m.
Therefore, it is readily estimated that larger crystal grains are desirably formed in the channel for improving the characteristics of the TFT (increasing the carrier mobility to achieve a faster operation).
Thus, as a technique for forming position-controlled large crystal grains, a method of controlling a laser intensity distribution has been proposed.
FIGS. 4
a
to
4
c
illustrate the formation of a gate insulating film (SiO
2
film)
112
, which is patterned to define a region in which large crystal grains should be formed, on a similar structure illustrated in
FIG. 1
a.
As an excimer laser is irradiated, the temperature at a region of an amorphous silicon thin film beneath the gate insulating film
112
becomes higher than the remaining region, so that a resulting temperature distribution is as indicated by a temperature curve
114
in
FIG. 4
b.
In this event, the crystallization initiates from ends of the gate insulating film
112
to form large crystal grains
121
a
due to a strong (large) temperature slope. Also, in this event, the crystal grains produced from both ends of the gate insulating film
112
grow to collide with one another in a region beneath the gate insulating film
112
, resulting in formation of a crystal grain boundary
122
, as illustrated in
FIG. 4
c.
In regions other than the region beneath the gate insulating film
112
, since the temperature slope is weak (small), a polycrystalline silicon thin film
4
is formed with crystal grains having smaller grain diameters than the crystal grains in the region beneath the gate insulating film
112
.
As a technique for forming position-controlled large crystal grains, a method of irradiating excimer laser using a mask
123
is known, as illustrated in
FIGS. 5
Kamo Takahiro
Kaneko Yoshiyuki
Kimura Yoshinobu
Ohkura Makoto
Shiba Takeo
Hitachi , Ltd.
Jackson Jerome
Mattingly Stanger & Malur, P.C.
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