Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1998-06-03
2001-07-17
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C428S446000, C428S448000, C428S450000
Reexamination Certificate
active
06261705
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a polycrystalline silicon thin film (hereafter simply called the poly-Si film) and a semiconductor device wherein the poly-Si film is applied.
A poly-Si film which is grown according to a conventional growth method, for example, grown by a low-pressure CVD (Chemical Vapor Deposition) system at a temperature of 700° C. with a silane gas of a flow rate of 800 sccm, has a layer structure wherein percentage of grains of a preferred orientation (
110
) increases monotonously towards the film surface, as can be seen in FIG.
8
.
FIG. 8
is a cross section schematically illustrating a poly-Si film
83
grown on a SiO
2
surface oxidation layer
12
of a Si substrate
11
according to the conventional method. In
FIG. 8
, a numeral
831
denotes a base layer where the preferred orientation rate (
110
) is about 30%, a numeral
833
denotes a surface layer where the preferred orientation rate (
110
) is about 70%, and a numeral
832
denotes an intermediate layer where the preferred orientation rate (
110
) varies from about 30% to 70%. In the drawing, the base layer
831
, the intermediate layer
832
and the surface layer
833
are bounded by dashed lines schematically. However, there is no distinct boundary between them actually, and the preferred orientation rate varies continuously traversing the layers, which is the same in other drawings when there is no other description.
A poly-Si film of another preferred orientation such as (
111
) or (
100
) obtained by the conventional growth method shows a similar layer structure, that is, a structure wherein the preferred orientation rate increases monotonously towards the film surface.
However, problem of excessive dopant diffusion into base material or insufficient activation of the poly-Si film itself becomes more serious with the poly-Si film having such layer structure, as it is made thinner and thinner along with miniaturization and high-performance of the semiconductor device whereto the poly-Si film is applied.
FIG. 9
is a cross section schematically illustrating a MOS (Metal Oxide Semiconductor) transistor, wherein a source electrode
95
, a drain electrode
96
, field oxidation films
93
, a gate oxidation film
94
, a gate electrode
97
, Al (aluminum) wirings
98
and a passivation film
99
are configured on a n-type Si substrate
92
.
When the poly-Si film having such layer structure is used for the gate electrode
97
without sufficient thickness, the dopant diffuses into the n-type Si substrate
92
penetrating the gate oxidation film
94
, which causes a considerable variation of on/off characteristic of the MOS transistor and an operational delay because of unnecessary gate capacitance, both disabling to improve operational speed of the MOS transistor.
FIG. 10
is a cross section schematically illustrating a thin film transistor to be applied for driving an LCD (Liquid Crystal Display), for example, wherein an active layer
101
comprising a source electrode
102
and a drain electrode
103
is configured on a glass substrate
100
, together with a gate electrode
104
, Al wirings
105
, a gate oxidation film
106
and a passivation film
107
.
When the above conventional poly-Si is applied to the active layer
101
, operational speed of the thin film transistor of
FIG. 10
is also degraded because of small carrier mobility of the poly-Si film. Therefore, it can be said that the thin film transistor to be used for high-speed driver of the high quality LCD cannot be realized with the poly-Si film obtained according to the conventional growth method.
As above described, in the film transistor using the poly-Si film as its active layer, or in the MOS transistor using the poly-Si film as its gate electrode, characteristics of the poly-Si film, such as the diffusion and precipitation of dopant, the interface and surface state or the carrier mobility, which define characteristics of the transistor, becomes difficult to be controlled, when thickness of the poly-Si film is to be reduced according to reduction of the element size of the MOS transistor or the thin film transistor. In other words, performance improvement of the LCD or the IC is now being limited by electronic characteristics of the poly-Si film applied to the film transistor or the MOS transistor used therein.
Therefore, a primary object of the present invention is to provide a poly-Si film which has excellence in its characteristics themselves concerning transistor characteristics, such as the diffusion and precipitation of dopant, the interface and surface state or the carrier mobility, and excellence in controllability of those characteristics as well.
Another object of the invention is to provide high-quality semiconductor devices by applying this poly-Si film.
CONCEPTION OF THE INVENTION
Electronic characteristics of the poly-Si film depend deeply on surface, interface and grain boundaries of the poly-Si film. In these boundaries, carrier trap density becomes high. Especially among them, the grain boundary acts as a barrier to the carriers, degrading the carrier mobility. These boundaries also act as high-speed diffusion channels or precipitation channels of dopant, resulting in degradation of various characteristics.
Among the above boundaries, the grain boundary is most important to the electronic characteristics of the poly-Si film.
The grain orientation relates deeply to characteristics of the above surface, interface and grain boundaries. However, even when preferred orientation rate is simply reinforced, improvement of the carrier mobility and prevention of the high-speed diffusion or precipitation of P or B dopant cannot be realized at the same time.
As previously described, in the layer structure of the poly-Si film according to the conventional growth method, the preferred orientation rate and the grain size increase towards the film surface. The reason is as follows.
Growth speed of each crystal plane of a crystal grain differs and depends on growth condition such as the growth temperature or the flow rate of the material gas. In other words, a specific high-speed growth plane is determined by the growth condition. At the beginning of film growth process, among island-like fine grains generated on the amorphous base surface such as the oxidation film, island-like grains whereof the high-speed growth planes are facing upwards begin to grow prior to other island-like grains, and become more and more dominant by suppressing growth of grains having other orientations and uniting with grains having the same orientation.
In the conventional growth method of the poly-Si film, the same growth condition is maintained for determining an orientation, and consequently, the grain boundary energy becomes high as the film grows. For reducing the grain boundary energy, it is necessary to generate some plane defects, such as a coherent boundary between twin crystals where defect energy itself is low, for example, so as to disturb the crystal orientation. With such plane defects, the orientation characteristics of the poly-Si film can be changed, that is, the preferred orientation rate can be decreased or the preferred orientation axis itself can be alternated.
Many slits are generated arround the high-energy grain boundaries because of their distortion, resulting in the low carrier mobility, or transparency, and also in the high-speed diffusion or the precipitation of the dopant. With the low-energy grain boundaries, the situation is contrary. However, the problem cannot be resolved with a poly-Si film uniformly consisting of low-energy grain boundaries. Even is such poly-Si film can be realized, the dopant may not be injected sufficiently in the poly-Si film, or the injected dopant may be swept out to the base film.
As to the gate electrode of the MOS transistor, the dopant should be sufficiently injected until the gate bottom interfacing with the gate oxidation film, and, at the same time, the dopant diffusion into the gate oxidation film should be restricted within a low limit. Furthermore, dopant pe
Okumura Hiroshi
Tanikawa Akio
Jones Deborah
NEC Corporation
Stein Stephen
Young & Thompson
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