Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – On insulating substrate or layer
Reexamination Certificate
2002-06-25
2004-03-16
Lebentritt, Michael S. (Department: 2824)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
On insulating substrate or layer
C438S162000, C438S169000, C438S166000, C438S487000
Reexamination Certificate
active
06706570
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the configuration of a laser apparatus used for a semiconductor device manufacturing process and other purposes. In particular, the invention relates to a laser apparatus used to improve or restore, by application of laser light, the crystallinity of a semiconductor material partially or fully made of an amorphous component, a substantially intrinsic, polycrystalline semiconductor material, or a semiconductor material whose crystallinity has been lowered being damaged by ion application, ion implantation, ion doping, or the like.
The invention also relates to a laser illumination system used for a low-temperature process for producing TFTs that are used in a liquid crystal display device and, more specifically, to a technique for forming, on the same substrate, thin-film transistors having a large mobility disposed in a peripheral circuit area and a number of thin-film transistors having uniform characteristics disposed in association with respective pixels.
2. Description of the Related Art
A panel used in a process for manufacturing a liquid crystal display generally has a peripheral circuit area and a pixel area. The peripheral circuit area has a role of controlling the value of a current flowing through each pixel. As semiconductor devices in the peripheral circuit area have a larger mobility, the circuit configuration of the display can be made simpler and the display is allowed to operate at higher speed. On the other hand, pixels have a role of holding information sent from drivers. If semiconductor devices in the pixel area do not have a sufficiently small off-current, they cannot hold such information. Further, if off-current values are much different from one pixel to another, the pixels display differently the same information sent from the drivers. For the above reasons, a technique is now required which allows semiconductor devices having different characteristics to be selectively formed on the same substrate.
In recent years, extensive studies have been made to decrease the temperature of semiconductor device manufacturing processes. This is largely due to the necessity of forming semiconductor devices on an insulative substrate made of glass or the like, which substrate is inexpensive and has high workability. In general, when a glass substrate is exposed to a high-temperature atmosphere of 600° C. or more, it expands or is deformed, for instance. Therefore, it is now desired that the temperature of semiconductor device manufacturing processes be reduced as much as possible. The decrease of the process temperature is also required from miniaturization and multi-layering of devices.
In semiconductor device manufacturing processes, it is sometimes necessary to crystallize an amorphous component of a semiconductor material or an amorphous semiconductor material, to return to the original crystalline level the crystallinity of a semiconductor material which has been lowered by ion application, or to improve the crystallinity of an already crystalline semiconductor material. This is because if such materials are crystallized, the mobility of resulting semiconductor devices can be made very large.
Conventionally, thermal annealing is used for the above purpose. Where silicon is used as a semiconductor material, the crystallization of an amorphous material, the restoration of an original crystallinity level, the improvement of crystallinity, etc. are attained by performing thermal annealing at 600 to 1,100° C. for at least several tens of hours.
In general, the processing time of such thermal annealing can be shortened as the temperature increases. On the other hand, almost no improvement is obtained at a temperature lower than 500° C. Therefore, to decrease the process temperature, it is necessary to replace a conventional thermal annealing step with some other proper step.
There is known, as one of the techniques for satisfying the above need are, a technique of performing various kinds. of annealing by laser light illumination. Since laser light can supply high energy equivalent to that of thermal annealing to a desired, limited portion, this technique has an advantage that it is not necessary to expose the entire substrate to a high-temperature atmosphere. In general, there have been. proposed two laser light illumination methods.
In a first method, a CW laser such as an argon ion laser is used and a spot-like beam is applied to a semiconductor material. A semiconductor material is crystallized such that it is melted and then solidified soon due to a sloped energy profile of a beam and its movement.
In a second method, a pulsed oscillation laser such as an excimer laser is used. A semiconductor material is crystallized such that it is instantaneously melted by application of a high-energy laser pulse and then solidified.
The first method of using a CW laser has a problem of long processing time, because the maximum energy of the CW laser is insufficient and therefore the beam spot size is at most several millimeters.
In contrast, the second method using a pulse oscillation laser can provide high mass-productivity, because the maximum energy of the laser is very high and therefore the beam spot size can be made as large as several square centimeters.
However, to process a single, large-area substrate with an ordinary square or rectangular beam, the beam needs to be moved in the four orthogonal directions, which still remains to be solved from the viewpoint of mass-productivity.
This aspect can be greatly improved by deforming a beam into a linear shape that is longer than the width of a subject substrate, and scanning the substrate with the beam.
The remaining problem is insufficient uniformity of laser light illumination effects. Pulsed oscillation lasers as represented by an excimer laser in which laser oscillation is obtained by gas discharge have a tendency that the energy somewhat varies from one pulse to another. Further, the degree of the energy variation also varies with the output energy. In particular, when illumination is performed in an energy range where stable laser oscillation cannot be obtained easily, it is difficult to perform laser processing with uniform energy over the entire substrate surface.
Another problem associated with the use of a pulsed oscillation laser is that the laser light energy decreases as the laser is used over a long time, which attributes to degradation of a gas necessary for laser oscillation. This does not appear to be a serious problem because the laser light energy can be increased by raising its operation level. However, in practice, raising the operation level is not preferable because once the operation level is changed, it takes some time for the laser light energy to be stabilized.
By the way, it is conventionally very difficult to produce, only with laser light illumination, a crystalline silicon film having such a large mobility as enables fast operation of a liquid crystal display. In view of this, a method of improving the crystallinity after laser light illumination has been proposed in which thermal annealing for crystallization is performed at about 550° C. for several hours before the laser light illumination. Although this method can attain a mobility (about 20 cm
2
/Vs) as required for the pixel area and the off-current is small (about 10
−12
A) and has a small pixel-to-pixel variation (on the same order), it cannot provide a mobility (more than 100 cm
2
/Vs) as required for the driver area.
We have already proposed the following method for solving this problem.
In the first step, a metal element such as Ni is added to a semiconductor material that is deposited on a glass substrate. Various substances other than Ni can be used as long as they serve as nuclei when the semiconductor material is crystallized. However, according to our experiments, in the case where the semiconductor material is amorphous silicon, the addition of Ni effectively produced silicon films having the best crystallinity. The following descr
Tanaka Koichiro
Yamazaki Shunpei
Lebentritt Michael S.
Robinson Eric J.
Robinson Intellectual Property Law Office P.C.
Semiconductor Energy Laboratory Co,. Ltd.
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