Semiconductor device manufacturing: process – Chemical etching – Vapor phase etching
Reexamination Certificate
2001-11-16
2004-10-19
Vinh, Lan (Department: 1765)
Semiconductor device manufacturing: process
Chemical etching
Vapor phase etching
C438S692000, C438S715000, C438S770000, C156S922000, C118S715000
Reexamination Certificate
active
06806199
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a manufacturing process for a mirror finished silicon wafer capable of manufacturing the mirror finished silicon wafer having an excellent quality in which grown-in crystal defects are annihilated by heat-treating the mirror finished silicon wafer in a gas atmosphere of high safety at a lower cost without selection of a heat treatment furnace for use in the heat treatment, a mirror finished silicon wafer having an excellent quality, and a heat treatment furnace preferably used in the manufacturing process.
BACKGROUND ART
It has been known that there exist defects named so-called grown-in defects such as COP (Crystal Originated Particle), oxide precipitates and so on in a CZ silicon wafer. A proposal has been made on a heat treatment performed in a hydrogen atmosphere (hereinafter may be referred to as “hydrogen annealing”) as a method for annihilating grown-in detects in the vicinity of a wafer surface. This heat treatment is required to use hydrogen at a temperature of 1000° C. or higher, so it is necessary to take a countermeasure from the viewpoint of safety. Since such a treatment cannot be carried out in an ordinary open type furnace (a furnace with an unsealed opening side such as a horizontal furnace), the furnace is required to be modified with a sealed structure for increasing airtightness and an explosion-proof apparatus as a measure against an explosion, which have lead to a very high cost.
In
FIG. 3
, there is shown a schematic structure of an ordinary horizontal furnace. In
FIG. 3
, reference numeral
10
indicates a horizontal furnace, which has a quartz tube body, that is, a reaction tube
12
. A gas supply port
14
for supplying a gas is provided at a front end of the reaction tube
12
. At the rear end of the reaction tube
12
is provided a furnace opening
16
which is capable of opening and shutting by a cap
18
. Where a hole
20
is formed in the cap
18
, a supply gas is released to the outside of the furnace mainly through the hole
20
. Where no hole
20
is formed, the supply gas is released to the outside of the furnace through a clearance between the cap
18
and the furnace opening
16
. A wafer support Si boat
22
supporting vertically many wafers W is placed inside of the reaction tube
12
. A heater
24
is provided outside of the outer periphery of the reaction tube
12
and the many wafers W placed inside of the reaction tube can be heat-treated.
Meanwhile, it has been recently found that even a heat treatment carried out in an argon atmosphere (hereinafter may be referred to as “Ar annealing”) can annihilate the grown-in defects in the level equal to hydrogen annealing. Ar annealing is not explosive and then safer compared with hydrogen. Although the Ar annealing ensures safe operation, it has also been known that the annealing displays a characteristic behavior to a silicon wafer.
An example of such a characteristic behavior is a phenomenon that tiny pits are easily formed on a surface of a wafer subjected to the Ar annealing. This is caused by the following mechanism. An oxide film is formed by very small amounts of oxygen and water as impurities included in a raw material gas, or oxygen and water in the outside air involved through the furnace opening of the reaction tube in a heat treatment process, and then the oxide film is allowed to react with silicon (Si) according to the following reaction:
SiO
2
+Si→2SiO
As a result of the reaction, Si is etched and the etched portion is observed as pits. The pits serve as a cause for degrading a local surface roughness (micro-roughness) and a long-period surface roughness (haze) on a wafer surface. Thus, an Ar gas is sensitive to a trace of impurities and small changes in the environment such as fluctuations in temperature, so the Ar gas has a demerit of difficulty in handling.
As measures to prevent this phenomena from occurring, two methods have been mainly proposed: One proposal is that an impurity content in a raw material gas is restricted to 5 ppm or less, and a purge box is also provided at an opening of a heat treatment furnace to prevent the outside air from being involved when wafers are inserted into the furnace (JP A 99-135511).
The other proposal is a method in which wafers are kept at 300° C. or lower and inserted into the furnace in order to prevent the outside air from being involved into the furnace when the wafers are inserted into the furnace (JP A 99-168106). However, it is supposed with ease that these methods lead to complexity in the apparatus and lower productivity.
As described above, the hydrogen annealing and the Ar annealing can advantageously annihilate the grown-in defects and give an excellent oxide film dielectric breakdown strength characteristic, so they have been widely used recently. However, for the above-mentioned reasons, it has been considered that high quality annealed wafers cannot be produced in a furnace with poor airtightness such as a horizontal furnace.
In the recent trend that device makers and wafer makers introduce many heat treatment furnaces, an increasing number of the makers have introduced vertical furnaces for the purpose to save a floor space. A vertical furnace has been developed later than a horizontal furnace and enables a highly airtight structure; therefore, the vertical furnace has been profitably used in a variety of applications. Accordingly, in view of such recent introduction of the vertical furnaces, effective use of long-standing horizontal furnaces has been sought. However, since the horizontal furnaces lack airtightness as described above, the range of its use has been still limited.
As stated previously, the presence of the grown-in defects such as COP (Crystal Originated Particle) is taken up as one of causes for decreasing a product yield in a device process. The grown-in defect is one of causes for degrading the oxide film dielectric breakdown strength and disconnecting wiring. Particularly, the defects are the greatest factor for deterioration in the oxide film dielectric breakdown strength. In order to annihilate the COP, it has been found that the hydrogen annealing and the Ar annealing are effective. The hydrogen annealing is, however, problematic in a safety aspect because of the use of the hydrogen gas at high temperature. In order to avoid such a problem, there is used a safety apparatus which leads to complex and expensive facilities for the hydrogen annealing as well as to a decrease in productivity and an increase in a production cost.
On the other hand, the Ar annealing has a problem that pits are easily generated if an Ar gas is of low purity. Furthermore, at the same time that pits are generated, micro-roughness and haze on a wafer surface are deteriorated. It has been known that micro-roughness and haze affect the oxide film dielectric breakdown strength and mobilities of electrons and holes just under an oxide film of a transistor having a MOS structure (see J. Appl. Phys. 79(2), Jan. 15, 1996, p. 911). Especially, mobilities of carriers (electrons and holes) are required to improve with an increase in degree of integration of MOS transistors. It is therefore necessary not only to decrease grown-in defects, but also to reduce micro-roughness and haze.
The inventors have conducted, as shown in Experimental Example 1 described later, an investigation into and a research on a haze level on a wafer surface after Ar annealing performed in a horizontal furnace, which is generally used, of low cost and widely spread, and as a result, have found for the first time that the outside air intrudes into a reaction tube for a heat treatment through a poorly sealed part of a connection portion between the reaction tube and a supply line of a raw material gas, which deteriorates a haze level of a heat-treated wafer.
That is, a connection portion
28
between a gas supply port
14
of a front end of a quartz tube body (reaction tube)
12
of a heat treatment furnace
10
which is generally used as shown in
FIG. 3
, and a supply line
26
of a non-oxidative raw m
Akiyama Shoji
Kobayashi Norihiro
Arent & Fox PLLC
Shin-Etsu Handotai & Co., Ltd.
Vinh Lan
LandOfFree
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