Coating processes – Coating by vapor – gas – or smoke – Mixture of vapors or gases utilized
Reexamination Certificate
1997-11-26
2004-09-28
Chen, Bret (Department: 1762)
Coating processes
Coating by vapor, gas, or smoke
Mixture of vapors or gases utilized
C427S255110, C427S376100
Reexamination Certificate
active
06797323
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART
The present invention relates to a method of forming a silicon oxide layer in the production of, for example, a semiconductor device.
In the production of, for example, a metal oxide semiconductor (MOS) device, it is required to form a gate oxide of silicon oxide on a surface of a silicon semiconductor substrate. Further, in the production of a thin film transistor (TFT), it is required to form a gate oxide of silicon oxide on a surface of a silicon layer provided on an insulating substrate as well. It can be said that a silicon oxide layer fully takes part in the reliability of a semiconductor device. A silicon oxide layer is therefore constantly required to have high dielectric breakdown resistance and long-term reliability.
For example, when a MOS semiconductor device is produced, conventionally, a surface of a silicon semiconductor substrate is cleaned by RCA cleaning prior to the formation of a gate oxide. In the RCA cleaning, the surface of the silicon semiconductor substrate is cleaned with an NH
4
OH/H
2
O
2
aqueous solution and then further cleaned with an HCl/H
2
O
2
aqueous solution to remove fine particles and metal impurities from the surface. Meanwhile, when the RCA cleaning is carried out, the surface of the silicon semiconductor substrate reacts with the cleaning liquid to form a silicon oxide layer having a thickness of approximately 0.5 nm to 1 nm. This silicon oxide layer will be simply referred to as “oxide layer” hereinafter. The oxide layer is non-uniform in thickness and contains a residual component of the cleaning liquid. The oxide layer is therefore removed by immersing the silicon semiconductor substrate in a hydrofluoric acid aqueous solution, and further, a chemical component is removed with pure water. As a result, there can be obtained a silicon semiconductor substrate surface which is mostly terminated with hydrogen and only partly terminated with fluorine. In the present specification, obtaining a silicon semiconductor substrate surface which is mostly terminated with hydrogen and only partly terminated with fluorine will be represented as “exposing the surface of a silicon semiconductor substrate”. Thereafter, the above-obtained silicon semiconductor substrate is introduced into a process chamber (oxidation chamber) to form a silicon oxide layer on its surface.
With a decrease in thickness of a gate oxide and an increase in diameter of a substrate, an apparatus for the formation of a silicon oxide layer has been being converted from a horizontal-type apparatus in which a process chamber extends in the horizontal direction to a vertical-type apparatus in which a process chamber extends in the vertical direction. The horizontal-type apparatus for the formation of a silicon oxide layer is referred to as “horizontal-type processing apparatus” hereinafter, and the vertical-type apparatus for the formation of a silicon oxide layer is referred to as “vertical-type processing apparatus” hereinafter. The reason therefor is as follows. Not only the vertical-type processing apparatus can easily cope with an increase in the diameter of a substrate as compared with the horizontal-type processing apparatus, but also the vertical-type processing apparatus can serve to decrease the formation of a layer of silicon oxide caused by atmosphere taken into a process chamber of the vertical-type processing apparatus during the transfer of a silicon semiconductor substrate into the process chamber. The above layer of silicon oxide will be referred to as “natural oxide” hereinafter. However, even the use of the vertical-type processing apparatus results in the formation of a natural oxide having a thickness of approximately 2 nm on the surface of the silicon semiconductor substrate. The natural oxide contains a large amount of impurities derived from atmosphere, and the presence of the natural oxide is not at all negligible when a gate oxide is decreased in thickness. There have been therefore proposed methods for preventing the formation of the natural oxide to the lowest level possible, such as (1) a method in which a nitrogen gas atmosphere is formed in a substrate transfer portion provided in a vertical-type processing apparatus by flowing a large volume of nitrogen gas (nitrogen gas purge method), and (2) a method in which a substrate transfer portion is vacuumed and then inert gas such as nitrogen gas is introduced into the substrate transfer portion (vacuum loadlock method).
Thereafter, in a state where an inert gas atmosphere is formed in the process chamber (oxidation chamber), a silicon semiconductor substrate is brought into the process chamber. Then, an atmosphere of the process chamber is replaced with an oxidative atmosphere to form a gate oxide. For the formation of the gate oxide, there is generally employed a method in which a surface of a silicon semiconductor substrate is thermally oxidized by introducing high-purity water vapor into the process chamber maintained at a high temperature (wet oxidation method). In this method, a gate oxide having high electric reliability can be obtained as compared with a method in which the surface of a silicon semiconductor substrate is oxidized with dry oxygen gas (dry oxidation method). Included in the above wet oxidation method is an oxidation method using pyrogenic gas (hydrogen gas combustion oxidation method) in which hydrogen gas is mixed with oxygen gas at a high temperature and is combusted and the so-generated water vapor is used for oxidizing silicon in the silicon semiconductor substrate. The oxidation method using pyrogenic gas is widely used. In the oxidation method using pyrogenic gas, generally, oxygen gas is supplied into a combustion chamber disposed outside the process chamber and being maintained at 700 to 900° C., and then hydrogen gas is supplied into the combustion chamber to combust the hydrogen gas with the oxygen gas at a high temperature. The so-obtained water vapor is used as oxidizing species.
FIG. 13
shows a schematic view of a vertical-type processing apparatus based on an oxidation method using pyrogenic gas. The vertical-type processing apparatus comprises a double-tubular structured process chamber
10
which is made of fused quartz and perpendicularly held, a gas inlet port
12
for introducing water vapor and/or gas into the process chamber
10
, a gas exhaust port
13
for exhausting the water vapor and/or the gas from the process chamber
10
, a heater
14
for maintaining the interior of the process chamber
10
at a predetermined ambient temperature through a cylindrical liner tube
16
made of SiC, a substrate transfer portion
20
, a gas introducing portion
21
for introducing inert gas such as nitrogen gas into the substrate transfer portion
20
, a gas exhaust portion
22
for exhausting the gas from the substrate transfer portion
20
, a shutter
15
for partitioning the process chamber
10
and the substrate transfer portion
20
, and an elevator unit
23
for bringing silicon semiconductor substrates into and out of the process chamber
10
. Attached to the elevator unit
23
is a fused quartz boat
24
on which the silicon semiconductor substrates are to be placed. Further, hydrogen gas supplied to a combustion chamber
30
is mixed with oxygen gas at a high temperature and combusted in the combustion chamber
30
, to generate water vapor. The water vapor is introduced into the process chamber
10
through a piping
31
, a gas passage
11
and the gas inlet port
12
. The gas passage
11
corresponds to a space between an inner wall and an outer wall of the double-tubular structured process chamber
10
.
The outline of a conventional method of forming a silicon oxide layer with the vertical-type processing apparatus based on an oxidation method using pyrogenic gas will be explained below with reference to FIG.
13
and
FIGS. 82
,
83
and
84
.
[Step-10]
Nitrogen gas is introduced into the process chamber
10
through a piping
32
, the combustion chamber
30
, the piping
31
, the gas passage
11
and the ga
Kashiwagi Akihide
Kataoka Toyotaka
Kimura Hideki
Suzuki Atsushi
Suzuki Toshihiko
Chen Bret
Sonnenschein Nath & Rosenthal LLP
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