Semiconductor device manufacturing: process – Formation of semiconductive active region on any substrate – Amorphous semiconductor
Reexamination Certificate
2002-05-30
2004-10-12
Pham, Long (Department: 2814)
Semiconductor device manufacturing: process
Formation of semiconductive active region on any substrate
Amorphous semiconductor
C438S487000, C438S509000
Reexamination Certificate
active
06803296
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors (hereinafter, referred to as TFTs) and a method of manufacturing the semiconductor device. For example, the present invention relates to an electro-optical device typified by a liquid crystal display panel and an electronic equipment mounted with the electro-optical device as a component.
Note that the term semiconductor device in this specification indicates devices in general capable of functioning with the use of semiconductor characteristics, and electro-optical devices, light emitting devices provided with EL elements and the like, semiconductor circuits and electronic equipment are all included in the category of the semiconductor device.
2. Description of the Related Art
In recent years, a technique of constituting a thin film transistor (TFT) by using a semiconductor thin film (with a thickness of approximately several to several hundred of nm) formed on a substrate having an insulating surface has attracted attention. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device, and needs to be developed promptly as, in particular, a switching element of an image display device.
An active matrix liquid crystal module, an EL module, and a contact image sensor are known as typical examples of the thin film transistors. Particularly, a TFT having a silicon film having a crystalline structure (typically, polysilicon film) as an active layer (hereafter, referred to as polysilicon TFT) has high filed effect mobility, and thus a circuit with various functions can be formed by using the TFT.
For example, in a liquid crystal module mounted to a liquid crystal display device, a pixel portion for performing image display for each functional block and a driver circuit for controlling the pixel portion, such as a shift register circuit, a level shifter circuit, a buffer circuit or a sampling circuit, which is based on a CMOS circuit are formed on one substrate.
Further, TFTs (pixel TFTs) are respectively arranged in several tens to several million of pixels in the pixel portion of the active matrix liquid crystal module, and pixel electrodes are provided to the respective pixel TFTs. Opposing electrodes are provided in an opposing substrate sandwiching liquid crystal with a substrate, and a sort of capacitor with the liquid crystal as dielectric is formed. A voltage applied to the respective pixels is controlled with a switching function of the TFT to control charge to the capacitor to thereby drive the liquid crystal. Thus, a light transmission amount is controlled, thereby displaying an image.
The pixel TFT consists of an n-channel TFT, and applies a voltage to the liquid crystal to drive it as a switching element. Since the liquid crystal is driven by an alternating current, a system called frame inversion driving is often adopted. In this system, in order to suppress power consumption at a low level, it is important to sufficiently lower an off-current value (drain current that flows at the time of off-operation of the TFT) for a characteristic required for the pixel TFT.
Further, in order to manufacture a TFT having superior electrical characteristics at lower cost, a laser annealing technique that enables processing for a short period of time has been essential.
Laser annealing is generally used for a process of crystallizing an amorphous semiconductor film, a process of improving crystallinity, and the like. Note that a laser often used for laser annealing is an excimer laser. A method of conducting laser annealing in which: a laser beam emitted from a pulse oscillation laser with large output is processed by an optical system so as to have a shape of a square spot several by several centimeters or a linear shape with a length of, for example, 10 cm or more on an irradiation surface; and an irradiation position of the laser beam is scanned relative to the irradiation surface, is preferably used since the method provides high productivity and is superior in mass-production. Particularly, when a laser beam having a linear shape (hereinafter referred to as linear beam) is used on the irradiation surface, differently from the case where a spot laser beam, which needs scanning in back and forth directions and right and left directions, is used, the laser beam can be irradiated over the irradiation surface only with the scanning in a direction perpendicular to a line direction of the linear beam, which provides high productivity. The reason the scanning is performed in the direction perpendicular to the line direction is that the perpendicular direction is the most effective scanning direction. Due to the high productivity, the use of the linear beam from a large-output laser, which is processed by an appropriate optical system, is becoming the main stream in laser annealing. Further, the linear beam is irradiated in an overlapping manner while gradually shifting in a short direction, whereby laser annealing is conducted to the entire surface of an amorphous silicon film to crystallize the film or improve the crystallinity.
Further, in order to manufacture a TFT at lower cost, it has been essential to manufacture the TFT on a glass substrate which is cheaper than a semiconductor substrate or a quartz substrate and which can attain a large surface area thereof.
In case of using the glass substrate, in order to prevent alkaline metal contained in the glass substrate from diffusing, a base insulating film comprised of an insulating film containing silicon as its main constituent (silicon oxide film, silicon nitride film, silicon oxynitride film, or the like) is provided, an amorphous silicon film is formed on the film, and then, laser light irradiation is conducted.
The present inventors found a large number of minute holes in the surface of the silicon film that has undergone laser irradiation through many experiments and studies. The minute hole is very small, and a photograph of the hole in SEM (magnification of 35 thousand) observation is shown in FIG.
26
. The present inventors found that variation is caused among a large number of TFTs formed on a substrate with the cause of unevenness of the surface of a semiconductor film due to the minute hole. In the case where the active layer of the TFT is formed at the position of the minute hole, the TFT has the poor electrical characteristics in comparison with other TFTs manufactured on the same substrate.
Further, the minute hole often occurs in the case where laser light is irradiated with a relatively high energy density or a relatively high overlap ratio. In particular, there is a tendency that the minute hole appears remarkably in the case where laser light is irradiated in a nitrogen atmosphere or a vacuum.
Moreover, the minute hole occurs in the case where the amorphous silicon film is formed on the base insulating film, but does not occur in the case where the amorphous silicon film is formed contacting the substrate without forming the base insulating film.
Based on the above, the present inventors made many experiments and studies from various angles in order to pinpoint the cause of occurrence of the minute hole. As a result, they further found that minute convex portions were formed in the surface of the amorphous silicon film before laser light irradiation. This minute convex portion is also very small (typically, with a diameter of 1 &mgr;m or less and a height of 0.05 &mgr;m or less), and a photograph of the convex portion in SEM (magnification of 50 thousand) observation is shown in FIG.
25
. Note that when the minute convex portion and the vicinity thereof are measured by EDX analysis, it is confirmed that the convex portion is not impurities such as dust.
When the minute convex portion is irradiated with laser light, the minute hole is easy to occur. The present inventors found that the minute convex portion is the cause of occurrence of the minute hole.
The minute convex portion is formed at the step of fo
Fish & Richardson P.C.
Pham Hoai
Pham Long
Semiconductor Energy Laboratory Co,. Ltd.
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