Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Non-single crystal – or recrystallized – material with...
Reexamination Certificate
2002-08-15
2004-10-19
Elms, Richard (Department: 2824)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Non-single crystal, or recrystallized, material with...
C257S065000, C257S066000, C257S067000, C257S255000
Reexamination Certificate
active
06806498
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a semiconductor film for use in thin film transistors (TFTs) used in liquid crystal displays, photosensors such as linear sensors, photovoltaic devices such as solar cells, memory LSIs for SRAMs (static random access memories) and the like. The invention also relates to a method and an apparatus for producing the semiconductor film. More particularly, the semiconductor film is a crystalline semiconductor thin film formed on, for example, a glass substrate or the like, by laser annealing an amorphous material or the like. The invention further relates to semiconductor device using the semiconductor thin film and a method of producing the device.
BACKGROUND ART
Conventionally, high-quality silicon semiconductor thin films used in thin film transistors (TFTs) or the like have been fabricated on an amorphous insulating substrates by using plasma CVD methods and plasma CVD apparatuses utilizing glow discharge. The hydrogenated amorphous silicon (a-Si) films produced by these manufacturing methods and apparatuses have been improved over many years of research and development and reached to a standard applicable as high quality semiconductor thin films. The hydrogenated amorphous thin films have found use in electric optical devices such as switching transistors for pixels in active matrix liquid crystal displays for lap-top or note-type personal computers, engineering workstations, and automobile navigation systems, photosensors for image sensors in facsimile machines, and solar batteries for electronic calculators, and in various integrated circuits. One of the most significant advantages of the hydrogenated amorphous silicon is that the formation with high reproducibility and stability on a large-area substrate is achieved at a process temperature as low as 300° C.
Meanwhile, the recent advancement of increased device sizes and greater pixel densities (higher definitions) in displays and image sensors has led to the demand for silicon semiconductor thin films that can achieve further high speed driving. In addition, in order to reduce device weight and manufacturing cost, such thin films should be applicable to driver elements formed in the peripheral circuit area of a liquid crystal display, which also requires the thin films to be capable of operating at high speed. However, the foregoing amorphous silicon shows a field effect mobility of 1.0 cm
2
/V·sec at best and thus cannot attain electric characteristics that can meet the requirements as mentioned above.
In view of the problems, research has been conducted on techniques for improving the field effect mobility and the like by forming a semiconductor thin film having crystallinity, and the developed manufacturing processes include:
(1) a manufacturing method in which by mixing a silane gas with hydrogen or SiF
4
, and employing a plasma CVD method, the deposited thin film is crystallized; and
(2) a manufacturing method in which by using amorphous silicon as a precursor, the crystallization of the film is effected.
In the method (1), the crystallization proceeds along with the formation of the semiconductor thin film, and the substrate must be heated to a relatively high temperature (600° C. or higher). This necessitates the use of a heat-resistant substrate such as costly quartz substrates, makes it difficult to use low-priced glass substrates, and therefore has a drawback of high manufacturing cost. Specifically, for example, Corning 7059 glass widely used in active matrix type liquid crystal displays has a glass transition temperature of 593° C., and therefore if subjected to a heating treatment at 600° C. or higher, the glass substrate will undergo considerable mechanical deformations such as shrinkage and strain, which makes it difficult to appropriately perform the forming processes of semiconductor circuits and the producing processes of liquid crystal panels. Furthermore, when a multi-dimensional integration is desired, there is a possibility of thermally damaging the previously-formed circuit area.
In the method (2) above, an amorphous silicon thin film is formed on a substrate, and the formed thin film is heated to obtain a polycrystalline silicon (polycrystal silicon: p-Si) thin film. This method generally utilizes a solid phase epitaxy method in which the heat treatment is performed at approximately 600° C. for a long time, and a laser annealing method (especially an excimer laser annealing method).
In the solid phase epitaxy method, the substrate on which an amorphous thin film is formed needs to be heated and maintained at a temperature of 600° C. or higher for 20 hours or longer, and thus this method also has drawbacks of high manufacturing cost and so forth.
In the excimer laser annealing method, an amorphous silicon thin film is irradiated with an excimer laser light, which is a UV light having a large light energy, to cause crystallization, as disclosed in, for example, IEEE Electron Device Letters, 7 (1986) pp. 276-8, IEEE Transactions on Electron Devices, 42 (1995) pp. 251-7. This method thereby achieves, without directly heating the glass substrate, a polycrystalline silicon thin film having relatively good electrical characteristics such as a high field effect mobility (higher than 100 cm
2
/V·sec). More specifically, amorphous silicon has a transmissivity characteristic as shown in
FIG. 1 and
, for example, shows an absorption coefficient of about 10
6
cm
−1
for a laser beam having a wavelength of 308 nm by a XeCl excimer laser. Therefore, most of the laser beam is absorbed in the layer from the surface to a depth of about 100 Å, the substrate temperature is not raised significantly (to approximately not higher than 600° C.), and amorphous silicon alone is brought to a high temperature to cause crystallization (polycrystallization or single crystallization). This allows the use of low-cost glass substrates. In addition, it is possible to irradiate a limited area of the substrate with the light beam to crystallize, and this allows a multi-dimensional integration in which a pixel region not requiring high speed characteristics so much is left to be an amorphous thin film, while a peripheral region of the pixel region is crystallized so as to form a driver circuit thereon. Further, it is also possible to form high quality crystalline thin films in a specific region on a substrate one after another, without thermally damaging the circuits already formed on the substrate. Furthermore, this technique permits the integration of CPUs (central processing units) and the like on the same substrate.
As an example of a semiconductor device using p-Si as described above, explained below is a typical construction of a TFT and a manufacturing method thereof.
FIGS.
2
(
a
) and
2
(
b
) schematically show a TFT
110
having a coplanar structure. FIG.
2
(
a
) is a plan view of the TFT
110
, and FIG.
2
(
b
) is a cross sectional view thereof taken along the line P-P′ in FIG.
2
(
a
). As shown in FIGS.
2
(
a
) and
2
(
b
), the TFT
110
has an insulating substrate
111
on which there are provided a undercoat layer
112
, a p-Si film
113
, a first insulating film (gate insulting film)
114
, a second insulating film
116
, and three electrodes, namely, a gate electrode
115
, a source electrode
117
s
, and a drain electrode
117
d
. The p-Si film
113
is a crystalline semiconductor layer composed of Si (silicon). The p-Si film
113
is formed on the undercoat layer
112
, patterned in a predetermined shape. The p-Si film
113
comprises a channel region
113
a
, a source region
113
b
, and a drain region
113
c
, and the source region
113
b
and the drain region
113
c
are disposed on both sides of the channel region
113
a
. The source region
113
b
and drain region
113
c
are formed by doping impurity ions such as phosphorus ions and boron ions.
The first insulating film
114
is made of, for example, silicon dioxide (SiO
2
), and is formed over the p-Si film
113
and the undercoat layer
112
. The gate electrode
115
is a metal thin fi
Goto Masashi
Izuchi Masumi
Kuramasu Keizaburo
Mino Yoshiko
Nishitani Hikaru
Elms Richard
Matsushita Electric - Industrial Co., Ltd.
Menz Doug
Parkhurst & Wendel L.L.P.
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