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
2002-04-11
2003-10-28
Jackson, Jerome (Department: 2811)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C257S066000, C257S075000
Reexamination Certificate
active
06639245
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor device in use for an active matrix display device, wherein a liquid crystal is driven by a thin film transistor, particularly, a driver monolithic type liquid crystal display device, wherein a peripheral drive circuit is on the same substrate, and the like and a method of manufacturing the same.
Among thin type liquid crystal display devices with low power consumption, those using a thin film transistor (hereinafter, referred to as TFT) for a drive element are mainly used for a display unit of a personal computer or the like, a portable TV (television) and the like due to its high performance such as high contrast, high response speed and so forth, and its market size has been greatly expanded in recent years.
The TFTs in use for a liquid crystal display device include the one wherein a CG silicon film is used as a semiconductor of its active portion. As described in Japanese Patent Laid-Open Publication No. 6-244103, this CG silicon film is a silicon film having excellent crystallinity obtained by depositing a trace of a kind of metal element such as nickel Ni or the like on the surface of an amorphous silicon film and thereafter heating the film.
Since the CG silicon film has lower power consumption and faster response than an amorphous silicon film and a polycrystalline silicon and has an advantage that a future sheet computer can be manufactured by utilizing its high mobility, it is considered promising as a material for manufacturing a next-generation driver monolithic type liquid crystal display device.
The CG silicon film is a crystalline silicon film formed by adding a metal element for promoting crystallinity into an amorphous silicon film and thereafter heating the film. A method of removing the metal element introduced into silicon Si at this time is disclosed in Japanese Patent Laid-Open Publication No. 10-223533. In this Japanese Patent Laid-Open Publication No. 10-223533, a part of the formed CG silicon film is doped with a 5th group element P (phosphorous) in high concentration and subsequently heat treated, thereafter the metal element is removed from a region to be used as an active portion of the TFT by gettering the metal from the region doped with P (phosphorous).
Furthermore, methods of forming a CG silicon film include a method called longitudinal growth and a method called lateral growth. The longitudinal growth is a method wherein a metal element is directly added to the whole surface of an amorphous silicon film and then heated to allow crystal growth. Meanwhile, the lateral growth is a method wherein, for example, a SiO
2
film formed on an amorphous silicon film is photopatterned so that a part of the amorphous silicon film is exposed, a metal element is added to the exposed portion and the film is heated so that crystals are grown in a direction horizontal with respect to a substrate into a portion where the amorphous silicon film is not exposed.
Here, the longitudinal growth, which is the most relevant to the present invention, will be explained in detail.
FIGS. 11A-11D
show how the longitudinal growth occurs in a CG silicon film.
FIGS. 11A-11D
are all plan views viewed from the film surface side. In
FIGS. 11A-11D
, reference numeral
71
denotes an amorphous silicon film. Reference numeral
72
denotes a Si crystal, which is to be a nucleus. Reference numeral
73
denotes a CG silicon crystal (also referred to as “domain”). Reference numeral
74
denotes a domain boundary.
First, as shown in
FIG. 11A
, a catalyst metal element is added to the surface of an amorphous silicon film
71
on a quartz substrate.
Subsequently, when solid-phase crystal growth is allowed in this amorphous silicon film
71
at temperature of about 600° C. for about 1 hour, a Si crystal
72
which is to be a nucleus is formed at several points on the quartz substrate as shown in FIG.
11
B. The generation density of the Si crystals
72
which are to be nuclei is affected by the quality of the amorphous silicon film
71
, the concentration of an added metal element and so forth.
When solid-phase crystal growth is further allowed for a long time, CG silicon crystals
73
grow radially from the crystal to be a nucleus as a center as shown in FIG.
11
C. The region of these CG silicon crystals grown from one nucleus as a center is referred to as a domain. While the inside of this domain
73
is polycrystalline, this can be considered as a so-to-speak quasi-single crystal since these crystals are known to have better continuity than those of p-Si.
When solid-phase crystal growth is further allowed for a long time, the grown domains
73
are finally bumped against each other as shown in FIG.
11
D. Then, the whole surface of the substrate becomes a CG silicon film, and the growth finishes. In
FIG. 11D
, a place where the domains
73
are bumped against each other is referred to as a domain boundary
74
. The size of these domains depends on the formation conditions, but a big domain may exceed 200 &mgr;m in diameter.
Meanwhile, when a TFT is formed by the CG silicon film, either the longitudinal growth or the lateral growth is employed. However, when a TFT is formed by lateral growth, the amount of a catalyst metal to be introduced must be about ten times more than the amount introduced for longitudinal growth. The reason for this is that, if the amount of the catalyst metal to be introduced is reduced, a distance of crystal growth in the horizontal direction becomes shorter, or a portion where few crystals grow is generated. Therefore, the CG silicon film by lateral growth inevitably contains a more amount of the catalyst metal than the silicon film by longitudinal growth. Thus, it is highly likely that the amount of the residual catalyst metal after gettering is naturally more in the laterally grown CG silicon film than in the longitudinally grown CG silicon film. Since metal element residues in such a silicon film adversely affect TFT characteristics (particularly OFF characteristics), changes with time such as deterioration or the like and so forth, a more excellent TFT can be obtained by reducing the metal element in the TFT to a minimum.
Consequently, the longitudinally grown CG silicon film is considered to be more suitable to formation of an active region of a TFT than the laterally grown CG silicon film.
Meanwhile, when a TFT is formed by the CG silicon film, either the longitudinal growth or the lateral growth is employed. However, when a TFT is formed by lateral growth, the amount of a catalyst metal to be introduced must be about ten times more than the amount introduced for longitudinal growth. The reason for this is that, if the amount of the catalyst metal to be introduced is reduced, a distance of crystal growth in the horizontal direction becomes shorter, or a portion where few crystals grow is generated. Therefore, the CG silicon film by lateral growth inevitably contains a greater amount of the catalyst metal than the silicon film by longitudinal growth. Thus, it is highly likely that the amount of the residual catalyst metal after gettering is naturally more in the laterally grown CG silicon film than in the longitudinally grown CG silicon film. Since metal element residues in such a silicon film adversely affect TFT characteristics (particularly OFF characteristics), changes with time such as deterioration or the like and so forth, a more efficient TFT can be obtained by reducing the metal element in the TFT to a minimum.
Since each of the TFTs for pixels of the display unit is responsible for the display of a different pixel, the differences in characteristics lead to differences in an electric potential applied to each pixel electrode, a charge holding time or the like, which are directly reflected on differences in transmittance of liquid crystal. That is, in display on a TFT panel using a longitudinally grown CG silicon film, light transmittance of each pixel varies depending on the presence or absence of a domain boundary in a TFT active region of the display unit. Therefo
Gotoh Masahito
Ueda Tohru
Conlin David G.
Edwards & Angell LLP
Hartnell, III George W.
Jackson Jerome
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