Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – By reaction with substrate
Reexamination Certificate
1997-06-30
2001-08-07
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
By reaction with substrate
C438S585000, C438S773000
Reexamination Certificate
active
06271151
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the field of semiconductor manufacturing processes, and more particularly, to the formation of an oxide layer during the manufacturing process.
BACKGROUND OF THE INVENTION
The formation of oxide layers are important steps in the manufacturing of semiconductor devices. In thermal oxidation, an oxide film is grown on a slice of silicon by maintaining the silicon in an elevated temperature in an oxidizing ambient, such as dry oxygen or water vapor. Thermally grown silicon dioxide is used to form a stable gate oxide for field effect devices, for example.
Controlling the gate oxide thickness is an important manufacturing process control issue. As the gate oxide thickness is reduced to below 150 Å, the growth kinetics changes from parabolic to linear with time. This is explained in Grove, The Physics and Technology of Semiconductor Devices, pages 22-33. In other words, for gate oxides, once the thickness is above 150 Å, it is a self-limiting process and therefore makes it easier to control the gate oxide thickness and reduce the variance between devices. However, for gate oxides of less than 150 Å, the linearity of the growth kinetics with time makes control over the gate oxide thickness much more difficult. The oxide thickness as a function of time may be expressed as:
X
o
A
/
2
=
1
+
t
+
τ
A
2
/
4
B
-
1
where
:
A
≡
2
D
(
1
k
s
+
1
h
)
B
≡
(
2
D
C
*
N
1
)
and
,
τ
=
x
i
2
+
Ax
i
B
and
C
*
=
Hp
G
with
H=Henry's Law constant
P
G
=bulk gas pressure
D=Diffusivity of O
2
is Si
N
1
=2.2×10
22
SiO
2
molecules/cm
3
in the oxide
k
s
=chemical surface-reaction rate constant for oxidation
h=gas phase mass transfer coefficient
In a typical oxide/diffusion arrangement, a wafer carrier is positioned within an oxide diffusion tube, this wafer carrier holding a number of wafers on which a gate oxide layer is to be grown. The processing of the wafers in the oxide diffusion tube, usually made of quartz, involves providing a supply of gas containing the oxidizing medium, such as oxygen or water vapor, so that it flows through the oxide diffusion tube. An oxidation furnace concentrically surrounding the oxide diffusion tube is used to heat the tube. The process is normally performed at ambient atmospheric pressure.
The thickness of the oxide layer is normally controlled through varying either the temperature and/or the furnace time, i.e. the amount of time the wafers are subjected to the gas containing the oxidizing medium and the elevated temperature. Although strict control is made of the temperature and the flow of gas through the oxide diffision tube the variance in the gate oxide thickness tends to be approximately ten percent.
SUMMARY OF THE INVENTION
There is a need for a method and apparatus for growing gate oxide in a manner that will provide a more accurate control of the gate oxide thickness so that there will be less variance in the thickness of the gate oxide in the final product.
This and other needs are met by embodiments of the present invention which provide a method for controlling the growth of an oxide in a semiconductor device manufacturing process. The present invention recognizes that the thickness of the oxide is proportional to the bulk gas pressure. Normally, the oxidation diffusion process is performed at ambient atmospheric pressure. However, the standard atmospheric pressure varies on a regular basis according to weather patterns, for example from 28 mm Hg to 32 mm Hg. Hence, the pressure may easily vary by approximately 6 or 7 percent. Accordingly, in certain embodiments of the present invention, the pressure of the gas within the oxide diffusion tube is maintained at a constant pressure. Unlike pressure diffision tubes that have been used in the past to provide diffision at greatly elevated pressures of several atmospheres in order to speed up the diffusion process, in the present invention the pressure is maintained at approximately ambient atmospheric pressure. With the pressure maintained at a constant value, and assuming that the temperature and gas flows are regulated as normal, the variance in the gate oxide thickness is reduced.
The earlier stated needs are also met by other embodiments of the invention which provide a method of controlling the growth of an oxide in a semiconductor device manufacturing process, in which an ambient atmospheric pressure is determined. Rather than controlling the pressure in the oxide diffusion tube, the amount of time the wafer is subjected to the elevated temperature is controlled as a function of the determined ambient atmospheric pressure and the temperature to which the wafer will be subjected. In other words, although the pressure is not maintained constant, the furnace time will be set to account for the actual ambient atmospheric pressure.
The earlier stated needs are also met by an arrangement for controllably growing an oxide layer on a wafer in a semiconductor device manufacturing process. This arrangement includes an oxide diffusion tube and a gas supply arrangement that maintains the constant gas pressure within the oxide diffusion tube during the growing of the oxide layer on the wafer. This constant gas pressure is approximately ambient atmospheric pressure. The use of a gas pressure that is approximately ambient atmospheric pressure, rather than a high pressure system, avoids the added danger and expense involved with such systems. The present invention can therefore be used with conventional oxide diffusion tube systems if provided with a gas supply arrangement that maintains a constant gas pressure as provided in the present invention.
In other embodiments of the invention, an ambient pressure monitor is used to determine the ambient atmospheric pressure. A control arrangement is coupled to the ambient pressure monitor and to the heater that heats the oxide diffusion tube. The control arrangement controls the heater to heat the oxide diffusion tube for an amount of time that is a function of the determined ambient atmospheric pressure. This setting of the furnace time may be done manually, automatically, or even dynamically, in response to a changing ambient atmospheric pressure, which tends not to change very rapidly.
By reducing the variance in the thickness of the gate oxide, speed variances in microprocessors forming the final product will be reduced. Furthermore, a better control will be achieved for tunnel oxides on flash memories and electrically erasable memory cells.
The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4268538 (1981-05-01), Toole et al.
patent: 4599247 (1986-07-01), Bean et al.
patent: 5118286 (1992-06-01), Sarin
patent: 5211729 (1993-05-01), Sherman
patent: 5445975 (1995-08-01), Gardner et al.
patent: 5510146 (1996-04-01), Miyasaka
patent: 5851293 (1998-12-01), Lane et al.
patent: 5862057 (1999-01-01), Xia et al.
Wolf et al. (Silicon Processing for VLSI Era) vol. 1, pp. 210, 217, 170, 230, 1989.*
Wolf, Stanley, “silicon processing for the VLSI ERA” vol. 1, pp. 161-195, 1986.*
Kinetics of Oxide Growth, “Thermal Oxidation”, The Physics and Technology of Semiconductor Devices, pp. 22-33.
Advanced Micro Devices , Inc.
Bowers Charles
Nguyen Thanh
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