Active solid-state devices (e.g. – transistors – solid-state diode – Field effect device – Having insulated electrode
Reexamination Certificate
1999-04-20
2001-07-31
Smith, Matthew (Department: 2825)
Active solid-state devices (e.g., transistors, solid-state diode
Field effect device
Having insulated electrode
C257S093000, C257S633000, C438S149000, C438S151000, C349S042000, C349S122000, C349S138000
Reexamination Certificate
active
06268631
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a glass substrate on which a semiconductor device is formed and, more particularly, to a bottom layer formed on the surface of a glass substrate and a method of fabricating the bottom layer. Furthermore, the invention relates to a method of heat-treating a glass substrate.
BACKGROUND OF THE INVENTION
A thin-film transistor (TFT) is known as a thin-film semiconductor device fabricated on a glass substrate. TFTs formed on such a glass substrate are disposed in a pixel driver portion and also in a peripheral circuit for a liquid crystal display and are used to display images with high information content. Furthermore, these TFTs are employed in image sensors and in other integrated circuits.
Where a glass substrate is used, the following advantages can be derived:
(1) Since it is optically transparent to visible light, the glass substrate can be easily utilized in a device such as a liquid crystal display through which light is transmitted.
(2) It is inexpensive. However, the upper limit of the thermal treatment temperature is restricted by the heatproofness, i.e., the maximum usable temperature, of the glass substrate.
Corning 7059 glass is generally used as a glass substrate taking account of the problem of impurity release from the glass substrate, price problem, and other problems. The transition point of this glass is 628° C. and the strain point is 593° C. Other known practical industrial glass materials having strain points of 550-650° C. are listed in Table 1 below.
TABLE 1
7059D(CGW)
7059F(CGW)
1733(CGW)
LE30(HOYA)
TRC5(OHARA)
E-8(OHARA)
N-0(NEG)
OA2(NEG)
strain
593
593
640
625
643
625
point (° C.)
thermal
50.1
50.1
36.5
38.0
52.0
37.0
−7.0
38.0
expansion
coefficient (×10
−7
)
transmission
89.5
89.5
91.9
90.0
N.A.
91.0
N.A.
90.0
(%)
(400 nm)
(400 nm)
(400 nm)
(450 nm)
(450 nm)
(450 nm)
composition
SiO
2
49
49
57
60
59
60
Al
2
O
3
10
10
16
15
15
15
B
2
O
3
15
15
11
6
7
6
R
2
O
0.1
2
1
2
AN1(AGC)
AN2(AGC)
NA35(HOYA)
NA45(HOYA)
strain
625
616
650
610
point (° C.)
thermal
44.0
47.0
39.0
48.0
expansion
coefficient (×10
−7
)
transmission
90.0
89.8
N.A.
N.A.
(%)
(500 nm)
(500 nm)
composition
SiO
2
56
53
51
Al
2
O
3
15
11
11
B
2
O
3
2
12
13
R
2
O
0.1
0.1
0.1
Where an amorphous silicon film formed on a glass substrate by CVD is crystallized by heating, a high temperature, e.g., above 600° C., is generally needed. Therefore, where a Corning 7059 glass substrate is used, the substrate is shrunk by the heating.
An active-matrix liquid crystal display is known as a device utilizing TFTs formed on a glass substrate. To fabricate this liquid crystal display, it is necessary to form tens of thousands to several millions of TFTs on the glass substrate in rows and columns. To manufacture the TFTs, processes using numerous masks are necessitated. Consequently, shrinkage of the substrate is a great impediment to the manufacturing process.
Especially, where it is necessary to make a mask alignment before thermal treatment, substrate shrinkage caused by the thermal treatment is a problem.
In a process for heat-treating substrates, it is common practice to place these plural substrates in vertical posture within a heating furnace, taking account of the processing speed. Where the substrates are heated above their strain point, warpage of the substrates is conspicuous.
Where TFTs are formed on a glass substrate, if the used TFTs permit flow of a large electrical current, then generation of heat accompanying the operation is a problem. This problem associated with the heat generation arises from the difference in coefficient of thermal expansion between silicon and the glass substrate. That is, the coefficient of thermal expansion of silicon, i.e., a single crystal of silicon, is 148 W m
−1
K
−1
(300 K), while the coefficient of thermal expansion of the glass substrate, i.e., quartz glass, is 1.38 W m
−1
K
−1
(300 K). Since the coefficient of thermal expansion of the glass substrate is much lower than that of silicon in this way, during operation of TFTs, heat generated by the TFTs cannot escape. Hence, malfunction and thermal destruction due to the heat generation present problems.
Especially, these problems become conspicuous where crystalline silicon is used. In particular, TFTs using an amorphous silicon film treat weak electrical current and so the problem of heat generation is not so serious. On the other hand, TFTs using a crystalline silicon film permit flow of large electrical current. Therefore, generation of heat poses a great problem.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of carrying out a heating step for fabrication of a semiconductor device using a glass substrate without suffering from problems of shrinkage and flexure of the glass substrate and without suffering from the problem of heat generation during operation of TFTs. A film comprising aluminum nitride is formed on one or both surfaces of a glass substrate having a strain point of 550-690° C. to provide a glass substrate assembly.
The present invention is based on a fact we have discovered empirically. Specifically a glass substrate was heat-treated above its strain point. Then, the substrate was slowly cooled at a rate of 0.01-0.5° C./min. A later heating step was carried out at a temperature lower than the strain point of the glass substrate. During this step, the substrate was rapidly cooled at a rate of 10° C./min to 300° C./sec. Shrinkage of the glass substrate could be suppressed within 50 ppm.
By performing the above-described processing, glass substrates which generally have strain points of 550-690° C. and which result in shrinkage of less than 50 ppm in a heat-treating step conducted below 600° C. can be obtained.
In one feature of the invention, glass substrates are heat-treated while held substantially horizontal to solve the problem of warpage of the glass substrates when they are thermally treated. One example of an apparatus for holding the substrates substantially horizontal and thermally processing them is shown in FIG.
1
.
FIG. 1
schematically shows a heating furnace comprising a reaction tube
11
made of quartz, substrate-holding means
12
, or substrate holders, and substrates
13
held horizontal. The apparatus is further equipped with a heater (not shown) for heating the reaction tube
11
from outside. In addition, the furnace is equipped with a means for supplying desired gases into the reaction tube and also with means for moving the substrate-holding means out of the reaction tube.
FIG. 1
shows the condition in which the glass substrates
13
are held horizontal on the substrate-holding portions
12
to prevent the substrates from being warped and deteriorated in planarity if they are heat-treated. This scheme is advantageous where glass substrates are required to be heated above their strain point.
Where it is not desired to permit glass substrates to shrink in a heating step, it is possible to previously heat-treat the glass substrates. As a result, in a later heating step, shrinkage of the substrates is suppressed.
We conducted experiments and have found the following facts. If such a preheating step is carried out above the strain point of the glass substrates, and if they are slowly cooled after the thermal treatment, then they shrink to a large extent. If the subsequent heating step is carried out below the transition point of the glass substrates, and if they are quickly cooled after this thermal processing, then they hardly shrink. For example, a glass substrate assembly is prepared which comprises:
a glass substrate which shows a shrinkage of less than 50 ppm when rapidly cooled after a heat treatment conducted at a temperature lower than 600° C.; and
a film provided on one or both surfaces of said glass substrate and comprising aluminum nitride. Then, this glass substrate assembly is subjected to the heat treatment conducted at a temperature lower than 600° C. This glass substrate assembly hardly shrink even by this heat treat
Fukada Takeshi
Sakama Mitsunori
Teramoto Satoshi
Costellia Jeffrey L.
Malsawma Lex H.
Nixon & Peabody LLP
Semiconductor Energy Laboratoty Co., Ltd.
Smith Matthew
LandOfFree
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