Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2001-03-29
2003-11-25
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S149000
Reexamination Certificate
active
06653179
ABSTRACT:
DESCRIPTION
Method for manufacturing a thin film semiconductor device, method for manufacturing a display device, method for manufacturing thin film transistors, and method for forming a semiconductor thin film.
TECHNICAL FIELD
This invention relates to a method for manufacturing thin film semiconductor devices involving thin film transistors formed in an active matrix type display device in an integrated form. More particularly, the invention relates to a method for forming polycrystalline semiconductor thin film transistors.
BACKGROUND ART
Crystallization annealing by laser light has been developed as a part of technologies to proceed with the manufacturing process of thin film semiconductor devices under low temperatures. This technique first irradiates laser light onto a semiconductor thin film of a non-single-crystalline material such as amorphous silicon or polycrystalline silicon with a relatively small grain size, which is formed on an insulating substrate, to locally heat it, and then makes the semiconductor film change into a polycrystal with a relatively large grain size during its cooling process (crystallization). The crystallized semiconductor thin film is used as an active layer (channel region) to integrally build thin film transistors. By employing this crystallization annealing, thin film semiconductor devices can be made under low process temperatures, and this enables the use of inexpensive glass substrates instead of expensive quartz substrates with a higher heat resistivity.
In crystallization annealing, line-shaped laser light normally elongated in the scanning direction is intermittently irradiated while making its shots partly overlapped. By overlapping shots of the laser light, the semiconductor thin film can be crystallized more uniformly. Crystallization annealing which uses line-shaped laser light (line beam) is schematically shown in FIG.
1
. Laser light
50
shaped into a line extending in the Y direction of an insulating substrate
1
of glass, for example, is irradiated onto the surface of the insulating substrate
1
having formed a semiconductor thin film. In this process, the insulating film substrate
1
is moved in the X direction relative to the irradiated region. In this example, a line beam
50
released from an excimer laser source is irradiated intermittently in a partly overlapped fashion. That is, the insulating substrate
0
is scanned in the X direction relative to the line beam
50
through a stage. Crystallization annealing is conducted by moving the stage after each shot by a pitch smaller than the width of the line beam
50
to ensure that the line beam
50
can irradiate the entire surface of the insulating substrate
1
. Excimer laser sources used in conventional crystallization annealing release pulses of 100 Hz or higher frequency, and the pulse width of each line beam is smaller than 50 ns.
Thin film semiconductor devices integrating thin film transistors are used in many active matrix type display devices, or the like. In order to realize display devices excellent in image quality, it is important to integrate thin film transistors having good operation properties all over the substrate. For this purpose, it is necessary to uniformly stack a semiconductor thin film of a polycrystal with a relatively large grain size. Additionally, needless to say, when taking the production yield into consideration, crystal grains having a large grain size must be uniformly built in all over the substrate. There are some methods for obtaining such a polycrystal, such as increasing the laser light irradiation energy, increasing the number of shots of overlapping irradiation, and making a crystal core in an amorphous semiconductor thin film before irradiation of laser light, for example. Even with these methods, however, no techniques have been successful in making sufficiently large, uniform crystal grains. Therefore, no system-on-panels remarked as the final target of low-temperature-process have been realized heretofore. A system-on-panel pertains to a device including built-in peripheral devices such as a video driver and a timing generator on a common substrate in addition to a switching element for driving pixels and thin film transistors used as horizontal scanners and vertical scanners. For realizing a system-on-panel, mobility u of individual thin film transistors has to be increased to 80 cm
2
/V·s through 300 cm
2
/V·s. For this purpose, it is necessary to further decrease the grain size of polycrystalline semiconductor thin film.
As shown in
FIG. 1
, when the line beam is irradiated to partly overlap between one shot and another, streaks appear to extend in the direction (Y direction) normal to the forward direction (X direction). In a microscopic view, these streaks are unevenness in crystal grain size. When the thin film transistors are integrated on the insulating substrate, unevenness in crystal grain size is observed as unevenness of the operation property, and it is therefore difficult to fabricate a display device ensuring a high quality throughout the entire surface of the insulating substrate. The first object of the invention is directed to solution of these problems of conventional techniques and provides a method for obtaining a polycrystal having a uniform, large grain size all over the surface of the insulating substrate by irradiating laser light onto a semiconductor thin film.
Next explained is another problem the invention intends to overcome. Thin film transistors are widely used as switching elements of active matrix type display devices. Especially as the semiconductor thin film to form the active layer of thin film transistors, polycrystalline silicon has been used for years. Polycrystalline silicon thin film transistors are used not only as switching elements, but they can be used also as circuit elements, and peripheral driving circuits can be built in together with pixel-driving switching elements on a common substrate. However, to form these peripheral driving circuits, high-performance thin film transistors are required. Particularly, their mobility is desired to be high.
Solid phase growth has been known for years as a technique for making high-quality polycrystalline silicon on an insulating substrate. This is a method which makes a silicon film as a precursor film by LP-CVD and then anneals it. Regarding the relation between conditions of deposition by LP-CVD and subsequent heating and the crystal grain size, it is known desirable to form amorphous silicon at a temperature not higher than 580° C. and anneal it at about 600° C., for example, in order to obtain polycrystalline silicon with a large grain size. In case of solid phase growth by heating, if an amorphous silicon film is annealed at 600° C. for 12 hours, for example, the crystal grain size reaches 100 through 2000 nm. In general, the larger the grain size, the higher the mobility. However, in solid phase growth by annealing, crystal patterns are not constant, and a lot of twin crystal defects and dislocation defects are observed in crystal grains through a crystal image. Because of these defects, although polycrystalline. silicon obtained by solid phase growth has a large grain size, its mobility is only around 70 cm
2
/V·s.
Laser annealing has also been used as a technique for making high-quality polycrystalline silicon. With this method, silicon thin films can be crystallized at relatively low temperatures without heating the entire substrate so high. When laser light is irradiated onto a silicon thin film to be crystallized, the energy is absorbed only by the very surface of the silicon thin film. Thereafter, the inner portion of the thin film melts due to heat conduction, and re-crystallizes during its cooling process. In polycrystalline silicon films made in this manner, crystal grains are distributed about uniformly. Also when reviewing its lattice image, crystal defects are less than those by solid phase growth. However, in the case of laser annealing, the crystal grain size is relatively as small as 200 through 300 nm, a lot of c
Hayashi Hisao
Minegishi Masahiro
Shimogaichi Yasushi
Takatoku Makoto
David Vu
Nelms David
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