Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
1997-03-28
2001-02-27
Wilczewski, Mary (Department: 2822)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S486000, C438S487000, C438S162000, C438S982000, C438S154000
Reexamination Certificate
active
06194254
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device using a thin film transistor (TFT) mounted on an insulating substrate such as a glass plate, and more particularly to a semiconductor device which can be utilized in an active matrix type liquid crystal displaying unit, or the similar matrix circuit.
2. Description of the Related Art
An active matrix type liquid crystal display unit using a TFT to drive a pixel, an image sensor, a three dimensional integrated circuit, and the like are known as a semiconductor device having a TFT on an insulating substrate such as a glass plate.
A thin film silicon semiconductor is generally used as the TFT mounted on such a device. In particular, for a high speed operation it is strongly required to establish a method for manufacturing a TFT comprising a crystalline silicon semiconductor. A method of conducting crystallization by forming an amorphous semiconductor film and applying a heat energy thereto (heat annealing) is known as a method for obtaining such a crystalline thin film silicon semiconductor.
There are some problems in manufacturing a semiconductor circuit using the crystalline silicon film thus formed. For example, a circuit that not only a matrix circuit but also the peripheral circuit for driving the same are constituted of the TFT (monolithic type active matrix circuit) is taken into account as an active matrix type circuit used in a liquid crystal display unit (i.e., a circuit that a controlling transistor is arranged in each pixel).
In this complicated circuit, characteristics required in the TFT vary depending on the position of the circuit. For example, the TFT used for controlling the pixel of the active matrix circuit is required to have sufficiently small leak current in order to maintain an electric charge stored in a capacitor constituted of a pixel electrode and an opposite electrode. However, a current driving ability may not be so high.
On the other hand, a large current switching at a short time is necessary in the TFT used in a driver circuit which supplies signals to a matrix circuit, and the TFT having a high current driving ability is required. However, a leak current may not be so low.
A TFT having a high current driving ability and a low leak current is most desirable. However, the TFT presently manufactured is far from such an ideal TFT, and if the current driving ability is high, the leak current is also high, and if the leak current is low, the current driving ability is low.
Therefore, the monolithic type active matrix circuit constituted using the conventional TFT attempts to improve the current driving ability and reduce the leak current by changing a channel length or a channel width of the TFT. However, if the circuit becomes finer, the change by a scale as conventionally employed is limited.
For example, in order to obtain a high current driving ability it is necessary to increase the channel width. The monolithic circuit uses the TFT having a channel width of 500 to 1,000 &mgr;m. However, if a higher current driving ability is required due to the increase in the number of pixels and the degree of gradation, it is difficult to further expand the channel width to 5 mm, 10 mm or the like from that the formation region of the peripheral circuit is limited.
On the other hand, it is desirable for the TFT used to control the pixel to obtain a clear image quality by increasing a charge retention ability. However, considering that the pixel region has a size of several hundreds &mgr;m square, it is impossible to increase the channel length to 50 &mgr;m, 100 &mgr;m or the like in order to decrease the leak current. As s result, since a scale of a matrix, a pitch and the number of pixels are largely limited in the conventional TFT monolithic type active matrix circuit, a displaying unit having a finer screen capable of obtaining a high quality image cannot be manufactured.
The above problems occur in not only the monolithic type active matrix circuit but also in other semiconductor circuits.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome the problems and further improve the characteristics of a circuit as a whole.
The present inventor has confirmed that some metal elements are effective to promote crystallization of an amorphous silicon film. The elements which promote the crystallization are Group VIII elements such as Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and Pt, 3d elements such as Sc, Ti, V, Cr, Mn, Cu and Zn, noble metal such as Au and Ag, and the like. Among the above, Ni, Cu, Pd and Pt have a large crystallization promoting effect. By adding those metal elements to the amorphous silicon film, the crystallization temperature can be lowered, whereby a time required for the crystallization can be shortened.
A method for adding the metal elements includes a method for forming the above-described metal element film or a thin film containing the metal element in contact with the upper or lower side of the amorphous silicon film. Further, it is confirmed that if the metal element is introduced by an ion implantation, substantially same effect is obtained. For example, it is confirmed that it is possible to lower the crystallization temperature in addition of nickel in an amount of 1×10
15
atoms/cm
3
or more.
The amount of the metal element added varies depending on the type of the metal element. If nickel is used, it is desired that the amount thereof is in the concentration range of from 1×10
17
to 1×10
20
atoms/cm
3
. If the concentration of nickel is more than 5×10
20
atoms/cm
3
, nickel silicide is formed locally, resulting in deterioration of characteristics as the semiconductor. Further, if the concentration of nickel is less than 1×10
17
atoms/cm
3
, the effect of nickel as a catalyst is decreased. A reliability as the semiconductor becomes high as the nickel concentration decreases.
Thus, it becomes apparent that the crystallization can be promoted by adding specific metal elements to the silicon film. In addition, it is confirmed that by selectively adding those metal elements to the silicon film, a crystal growth selectively generates from a region to which the metal element has been added, and the crystal growth region expands into its periphery. Further, according to more detailed observations, needle crystals are growing in the direction along the substrate surface not in the direction in the thickness of the substrate, in the silicon film to which those metal elements have been added.
A crystal grows in a needle form in the silicon film to which those metal elements have been added. The width (length) thereof is about 0.5 to 3 times the thickness of the silicon film, and a growth in a transverse direction (a side direction of the crystal) is small. For this reason, a grain boundary is formed in parallel to the crystal growth direction. Where nickel is used as the metal element, the crystal grows in the (
111
) direction. An example of this crystal growth is shown in
FIGS. 1A
to
1
C.
FIG. 1A
is a top view, showing a state that a crystal growth generates from the region to which a metal element has selectively been added. Region
2
is a silicon film region to which the metal element has been added and the crystal growth expands from the region
2
to the periphery. Ellipse region
3
is region crystal-grown in a transverse direction. Arrows show the direction of the crystal growth. An outer region
1
outside the region
2
is a region which is not crystallized.
FIG. 1B
is an enlarged view of a part of the region
3
, for example, a square region
4
. As is apparent from
FIG. 1B
, grain boundaries
6
and
7
generate in parallel to the direction of crystal growth (B to C) in the silicon film
5
. Therefore, the grain boundary is less in a cross section (face BC) which is in parallel to the direction of the crystal growth, but many grain boundaries are observed in a cross section (face BA) which is vertical to the direction of the crystal growth.
In a case wherein such a film
Costellia Jeffrey L.
Ferguson, Jr. Gerald J.
Nixon & Peabody LLP
Semiconductor Energy Laboratories Co., Ltd.
Wilczewski Mary
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