Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2001-10-09
2003-03-18
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S162000, C257S059000, C257S067000
Reexamination Certificate
active
06534353
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a thin-film transistor and, more particularly, to a thin-film transistor (element) for use in a driving circuit for a liquid-crystal display device and to a method of fabricating the same.
BACKGROUND ART
At present, liquid-crystal display devices driven by thin-film transistors (TFTs) as thin-film semiconductor elements are used widely in a notebook personal computer, a vehicle navigator, and the like and requested to be further reduced in size, weight, and cost in the future. To respond to the request, there has been developed a polycrystalline silicon thin-film transistor which allows integral formation of even driving circuits for a pixel portion with a substrate having a display portion and the pixel portion for the display portion formed therein and higher performance thereof has been pursued. Referring to the drawings, a method of fabricating a conventional polycrystalline silicon thin-film transistor will be described.
FIG. 1
is a structural cross section of a thin-film transistor of the type termed “top-gate” produced in accordance with a conventional method. In the drawing,
1
denotes a transparent insulating substrate made of quartz, glass, or the like, in which glass is used normally in terms of cost;
2
denotes a polycrystalline silicon thin film;
3
denotes a gate insulating film;
4
denotes a gate electrode;
5
denotes an interlayer insulating film;
6
denotes a source electrode film;
7
denotes a drain electrode film; and
13
denotes an underlying layer (so-called undercoat), which is formed with the view to preventing some components of a substrate materiel from being diffused in the polycrystalline silicon thin film but may not be formed in some cases depending on the substrate material or a method of processing the substrate.
In practice, such thin-film transistors used as switches for the pixel portion and in driving circuits therefor are arranged in rows and columns in vertical and lateral directions and at locations determined by the display surface of a liquid-crystal display device as a product and by the driving circuits formed in the peripheral portion thereof. However, since the foregoing is so-called well-known technology and is not relevant directly to the present invention, intentional depiction thereof is omitted.
A method of fabricating the thin-film transistors, which is relevant directly to the present invention, will be described briefly herein below, though it is so-called well-known technology.
First, a silicon dioxide thin film
13
is formed as the underlying layer on the transparent insulating substrate
1
made of glass or the like by plasma chemical vapor deposition (PCVD), sputtering, or the like.
Then, an amorphous silicon thin film is formed entirely over the substrate or at a specified location thereon by PCVD, chemical vapor deposition (CVD), or sputtering.
Next, an excimer laser is applied to an amorphous silicon thin film thus formed to temporarily melt the amorphous silicon thin film (so-called laser annealing), thereby forming the polycrystalline silicon thin film
2
composed of grains (particles) each having a relatively large diameter by utilizing the crystallization of silicon during the solidification thereof.
Next, the polycrystalline silicon thin film is processed into a specified configuration determined by the arrangement of transistors (elements) on the substrate. In short, so-called patterning is performed.
Next, the gate insulating film
3
is formed on the patterned polycrystalline silicon thin film by normal pressure CVD, PCVD, sputtering, or like method and the gate electrode
4
is formed at a specified location on the gate insulating film
3
.
Next, the interlayer insulating film
5
is formed and contact holes are formed by etching in the portions of the interlayer insulating film in which the source and drain electrodes of each of the transistors are to be formed.
Next, the source and drain electrodes
6
and
7
of each of the transistors are formed by using the contact holes, whereby the polycrystalline silicon thin-film transistor is produced.
It will be appreciated that, if necessary, the cleaning of the substrate, the implantation of impurity ions required by the element to perform its intrinsic function, i.e., phosphorus (P) or boron (B) ions into the source and drain regions, a heat treatment subsequently performed to join a dangling bond or expel excess hydrogen, wiring required by the element to perform its intrinsic function, and the like are performed in addition to the foregoing process steps. Since these process steps are also well-known technology and not relevant directly to the present invention, the description thereof is omitted here.
A description will be given next to irradiation conditions for laser annealing.
To improve the characteristics of the thin-film semiconductor element, the film should have a large and uniform crystal grain diameter. If the crystal grain diameter is to be increased by laser annealing, it is effective to perform irradiation with high energy or irradiate the same portion several times. As a result of such irradiation, however, the grain diameter loses uniformity, the characteristics of the thin-film semiconductor element vary greatly, or heat is transmitted to the glass substrate to cause the deformation of glass or the diffusion of a glass component into the thin-film semiconductor, so that the performance of the semiconductor element degrades against expectations. It is to be noted that the heat resistance temperature of the glass substrate used in the liquid crystal display device is 600° C.
Under the present circumstances, therefore, poly-crystallization is performed by applying a laser under conditions which are a trade-off between the size and uniformity of the crystal grain diameter and the adverse effects of heat on the glass substrate.
In addition to the foregoing, there has been adopted the approach of optimizing the energy density of a laser beam in consideration of the thickness of a silicon film or the like, though the description thereof is omitted here since it is not relevant directly to the present invention.
However, the method encounters the following problems during melting recrystallization.
(1)
FIG. 2
is a cross-sectional view of a polycrystalline silicon thin film formed by melting recrystallization involving excimer laser annealing. As shown in the drawing, numerous projections
11
are formed at a surface of the polycrystalline silicon thin film
2
, particularly at the grain boundaries. Moreover, tramp materials (“impurities” in another technical field)
12
, which are unnecessary by nature for the transistor element to perform its intrinsic function, e.g., oxygen in an atmosphere, hydrogen from moisture, boron (B) from glass pieces jumped from a HEPA filter, and the like are taken in by the surface portion.
In this case, these tramp materials are not only located in large quantities in the surface which is chemically and physically unstable during the polycrystallization of the amorphous silicon during which the amorphous silicon is temporarily melted at a high temperature achieved by laser irradiation and then solidified but also segregated at the upper surface of silicon from the lower portion thereof with solidification (aggregate in a large quantity from inside silicon). In particular, the tramp materials are assumed to be segregated in large quantities in the projections, which are chemically unstable because of the segregated tramp materials. If oxygen is a tramp material, it is bonded to silicon as a semiconductor in an extremely complicated and unstable state instead of reacting therewith to form a silicon dioxide. It is to be noted that oxygen forms compounds with silicon, carbon, or the like belonging to Group IV at a ratio of either 1:1 (e.g., a carbon monoxide or silicon monoxide) or 2:1 (e.g., carbon dioxide gas or a silicon dioxide) and therefore does not achieve a constant composition. Under special conditions such as in the surface of the amorphous silicon which
Kawakita Tetsuo
Kuramasu Keizaburo
Sasaki Atsushi
Le Dung A
Matsushita Electric - Industrial Co., Ltd.
Nelms David
Parkhurst & Wendel L.L.P.
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