U.S. patent application number 09/756123 was filed with the patent office on 2001-11-22 for method and apparatus for controlling the thickness of a gate oxide in a semiconductor manufacturing process.
This patent application is currently assigned to Advanced Micro Devices, Inc.. Invention is credited to Wollesen, Donald L..
Application Number | 20010042509 09/756123 |
Document ID | / |
Family ID | 25386232 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010042509 |
Kind Code |
A1 |
Wollesen, Donald L. |
November 22, 2001 |
Method and apparatus for controlling the thickness of a gate oxide
in a semiconductor manufacturing process
Abstract
A method and apparatus for controlling the growth of an oxide,
such as a gate oxide, in a semiconductor device manufacturing
process takes into consideration the ambient atmospheric pressure
in order to reduce the variance in gate oxide thicknesses between
wafer lots. The pressure in the oxide diffusion tube is maintained
at a constant pressure near the ambient atmospheric pressure during
the oxide diffusion process. Alternatively, the furnace time is
changed from lot to lot as a function of changes in the ambient
atmospheric pressure in order to maintain the gate oxide thickness
at a constant value between wafer lots.
Inventors: |
Wollesen, Donald L.;
(Saratoga, CA) |
Correspondence
Address: |
McDERMOTT, WILL & EMERY
600 13th Street, N.W.
Washington
DC
20005-3096
US
|
Assignee: |
Advanced Micro Devices,
Inc.
Sunnyvale
CA
|
Family ID: |
25386232 |
Appl. No.: |
09/756123 |
Filed: |
January 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09756123 |
Jan 9, 2001 |
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09033642 |
Mar 3, 1998 |
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6187092 |
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09033642 |
Mar 3, 1998 |
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08885140 |
Jun 30, 1997 |
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6271151 |
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Current U.S.
Class: |
118/715 |
Current CPC
Class: |
H01L 21/28211 20130101;
C30B 33/005 20130101; Y10T 117/1008 20150115; H01L 21/28202
20130101 |
Class at
Publication: |
118/715 |
International
Class: |
C23C 016/00 |
Claims
What is claimed is:
13. An arrangement for controllably growing an oxide layer on an
object in a semiconductor device manufacturing process, comprising:
an oxide diffusion tube; and a gas supply arrangement that
maintains a constant gas pressure within the oxide a diffusion tube
during the growing of an oxide layer on an object, the constant gas
pressure being approximately ambient atmospheric pressure.
14. The arrangement of claim 13, wherein the gas supply arrangement
includes a low pressure source that creates a negative pressure
within the oxide diffusion tube.
15. The arrangement of claim 13, wherein the gas supply arrangement
includes a pressure pump that creates a positive pressure within
the oxide diffusion tube.
16. The arrangement of claim 13, wherein the constant gas pressure
is within a range between approximately 1/2 atmospheres below the
ambient atmospheric pressure and approximately 1/2 atmospheres
above the ambient atmospheric pressure.
17. An arrangement for controllably growing an oxide layer on an
object in a semiconductor device manufacturing process, comprising:
an oxide diffusion tube; an ambient pressure monitor that
determines the ambient atmospheric pressure; a heater in thermal
contact with the oxide diffusion tube to controllably heat the
oxide diffusion tube; a control arrangement coupled to the ambient
pressure monitor and to the heater, the control arrangement
controlling the heater to heat the oxide diffusion tube for an
amount of time that is a function of the determined ambient
atmospheric pressure.
18. The arrangement of claim 17, wherein the control arrangement
includes means for dynamically controlling the heater in response
to changes in the ambient atmospheric pressure during the growing
of an oxide layer.
19. The arrangement of claim 18, wherein the means for dynamically
controlling the heater includes means for changing the amount of
time the heater will heat the oxide diffusion tube.
20. The arrangement of claim 17, wherein the control arrangement
includes means for setting the amount of time the heater will heat
the oxide diffusion tube, the amount of time remaining fixed
throughout the growing of an oxide layer.
21. A semiconductor device having a gate oxide layer formed by the
process comprising the steps of: positioning at least one wafer on
which the gate oxide layer is to be grown within an oxide diffusion
tube; maintaining a constant pressure of gas within the oxide
diffusion tube, the pressure being approximately ambient
atmospheric pressure; and growing the gate oxide layer on the wafer
until a desired thickness of the gate oxide layer is achieved.
Description
[0001] This application is a Divisional of Application Ser. No.
08/885,140 filed Jun. 30, 1997.
TECHNICAL FIELD OF THE INVENTION
[0002] 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
[0003] 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.
[0004] Controlling the gate oxide thickness is an important
manufacturing process control issue. As the gate oxide thickness is
reduced to below 150 .quadrature., 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
.quadrature., 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
.quadrature., 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: 1 X o A
/ 2 = 1 + t + A 2 / 4 B - 1 where : A 2 D ( 1 k s + 1 h ) B ( 2 DC
* N 1 ) and , = x i 2 + Ax i B and C * = Hp G
[0005] with H=Henry's Law constant
[0006] P.sub.G=bulk gas pressure
[0007] D=Diffusivity of O.sub.2 is Si
[0008] N.sub.1=2.2.times.10.sup.22 SiO.sub.2 molecules/cm.sup.3 in
the oxide
[0009] k.sub.s=chemical surface-reaction rate constant for
oxidation
[0010] h=gas phase mass transfer coefficient
[0011] 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.
[0012] 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 diffusion tube the variance in the gate oxide
thickness tends to be approximately ten percent.
SUMMARY OF THE INVENTION
[0013] 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.
[0014] 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 diffusion
tubes that have been used in the past to provide diffusion 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic cross-sectional view of a
semiconductor device.
[0021] FIG. 2 is a sectional schematic depiction of an arrangement
for growing oxide on a semiconductor wafer in accordance with an
embodiment of the present invention.
[0022] FIG. 3 is a sectional schematic depiction of an arrangement
for growing oxide on a semiconductor wafer for another embodiment
of the present invention.
[0023] FIG. 4 is a sectional schematic depiction of an arrangement
for growing oxide on a semiconductor wafer for still another
embodiment of the present invention.
[0024] FIG. 5 is a sectional schematic depiction of a rapid thermal
oxidation arrangement for another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
[0025] FIG. 1 is a cross-sectional view of an exemplary
semiconductor device 10 to depict gate oxide. The device 10
includes a silicon substrate 12 that has a source region 14 and a
drain region 16. Field oxide 18 is provided and a gate 20 is
located over gate oxide 22. The thickness of the gate oxide 22 is
represented as x. The present invention improves upon the variance
in the gate oxide thickness x that is produced between wafers and
wafer lots in the manufacturing process. Reducing the variance
allows an improvement in the gate oxide control, and in turn,
permits tighter distributions that depend on the gate oxide
control. This includes microprocessor speed and write/erase cycles
on programmable cells. It also improves the gate oxide reliability
control.
[0026] It is to be understood that the present invention will be
described with the example of thermal oxidation to create a gate
oxide. However, the present invention is also applicable to
formation of other types of oxide layers in addition to gate oxide
layers. It is also applicable to other thermal oxides such as
silicon oxynitride, thermal oxides, or thermal nitridation.
[0027] The present invention recognizes that a variance in the
atmospheric pressure from the standard assumed ambient atmospheric
pressure of 29.92 mmHg will vary the thickness x of the gate oxide
22. The present invention either maintains a constant gas pressure
in an oxide diffusion tube, or varies the furnace time as a
function of the actual measured ambient atmospheric pressure. By
using either of these embodiments to control the oxide diffusion
process, the gate oxide thickness x in various wafer lots will
exhibit a variance that is much reduced than if the ambient
atmospheric pressure was not taken into consideration.
[0028] FIG. 2 is a sectional schematic depiction of a oxide
diffusion arrangement in accordance with an embodiment of the
present invention. The arrangement 30 includes an oxide diffusion
tube 32 that is sealed at its ends by end caps 34. In typical
installations, the tube 32 is made of quartz and is vertical,
although tube 32 may also be horizontal. A gas supply 36 is coupled
to the inlet of one of the end caps 34. The gas supply 36 provides
the carrier gas and the reaction gas. Typically, the mix of gas
includes nitrogen, argon and oxygen.
[0029] An oxidation furnace 38 surrounds the oxide diffusion tube
32. A liner 42 separates the oxide diffusion tube 32 from a heater
core 40. A timer 44 controls the amount of time that the heater
core will be heated (e.g., the furnace time).
[0030] Wafers 48 are loaded into the oxide diffusion tube on a
wafer carrier 46. The furnace time for heating the heater core 40
in the oxidation furnace 38 is controlled by a timer 44. For a
given gate oxide thickness, the timer will be set to a specific
time, and this time will be slightly varied according to the
measured atmospheric pressure.
[0031] Although the setting of the timer to the same time for two
different lots of wafers should theoretically produce gate oxides
having the same thickness, variations in the ambient atmospheric
pressure will cause the thickness to vary from lot to lot. In order
to overcome this problem, the embodiment of the present invention
in FIG. 2 provides a vacuum pump 50 at the exhaust of the oxide
diffusion tube 32. The vacuum pump 50 creates in the oxide
diffusion tube 32 a slight negative pressure with respect to
ambient atmospheric pressure. For example, the pressure created by
the vacuum pump 50 may be set to a value in a range from
approximately one half atmospheres below ambient atmospheric
pressure to just slightly below atmospheric pressure. For example,
if ambient atmospheric pressure is 29.92 mm Hg, the pressure in the
oxide diffusion tube 32 may be regulated to be 26 mm Hg. The vacuum
pump 50 has a regulating element to maintain the pressure at a
constant pressure in certain embodiments. As will be apparent to
those of ordinary skill in the art, the regulator may instead be a
separate component from the vacuum pump 50.
[0032] The arrangement 30 of the present invention operates at near
ambient atmospheric pressure, and therefore avoids the dangers and
added expense inherent in pressure diffusion arrangements, in which
the oxide diffusion tube is pressurized to several atmospheres. The
disadvantage of the pressurized diffusion tubes has made oxide
diffusion at ambient atmospheric pressure the standard in the
semiconductor industry.
[0033] FIG. 3 depicts another embodiment of the present invention
in which the gas in the oxide diffusion tube is pressurized to
slightly above ambient atmospheric pressure, (such as 32 mm Hg) up
to, for example, approximately one half atmosphere above ambient
atmospheric pressure. For this purpose, a pressure pump 52 (with an
appropriate regulator) is employed to maintain the constant gas
pressure within the oxide diffusion tube 32 for the duration of the
oxide diffusion process. In the embodiments of FIGS. 2 and 3, the
vacuum pump 50 and the pressure pump 52 can be located at either
end of the diffusion tube as there is not a significant pressure
drop across the tube 32.
[0034] With both of the embodiments of FIGS. 2 and 3, the furnace
time between wafer lots as controlled by the timer 44 will remain
the same, and the pressure in the oxide diffusion tube 32 will also
be maintained at a constant value according to the present
invention. Since both the temperature and the pressure are
maintained constant from lot to lot, in both of the embodiments of
FIGS. 2 and 3, the variance in the gate oxide thickness will be
reduced in comparison to the prior art methods which operate at
ambient atmospheric pressure but do not take into account the
variations in the ambient atmospheric pressure.
[0035] FIG. 4 depicts an additional embodiment of the present
invention in which the pressure in the oxide diffusion tube is not
regulated. However, in this embodiment, the ambient atmospheric
pressure is taken into consideration in the oxide diffusion process
to adjust other oxide diffusion control parameters.
[0036] A controller 54 receives signals from an ambient pressure
sensor 56 that detects the actual ambient atmospheric pressure in
the area of the oxide diffusion tube 32. Based on the actual value
of the ambient atmospheric pressure, the controller 54 will set the
timer 44 to control the furnace time in order to control the gate
oxide thickness to be the same from lot to lot. For example, assume
that for lot 1 the ambient atmospheric pressure, as determined by
the ambient pressure sensor 56, is at 29.92 mm Hg. The timer will
be set at a specific value t.sub.1 to achieve a desired gate oxide
thickness x. Now assume for lot 2 that the ambient pressure has
increased to 31 mm Hg. Since the gate oxide thickness is
proportional to the bulk gas pressure, the gate oxide thickness x
will be achieved in a shorter time period than for lot 1.
Accordingly, timer 44 is set to a shorter time t.sub.2 for lot 2
than it was for lot 1 in order to achieve the same gate oxide
thickness x in the wafers of lot 2 as was achieved for the wafers
of lot 1. Conversely, if the ambient pressure is less than 29.92 mm
Hg when a third lot of wafers is to be processed in the oxide
diffusion tube 32, the controller 54 will set the timer 44 to
increase the amount of time t.sub.3 of the oxide diffusion process
(i.e., the furnace time) to maintain a gate oxide thickness x.
[0037] The embodiment of FIG. 4 depicts an automatic control of the
timer 44 through controller 54 and an ambient pressure sensor 56
that provides a signal to the controller 54. The timer 44 may be
set by the controller 54 at the beginning of the oxide diffusion
process so that the amount of time that the wafer is subjected to
the temperature will remain the same throughout the oxide diffusion
process. Alternatively, the time can be controlled dynamically in a
feedback control method. Thus, if the ambient pressure changes
during the oxide diffusion process, the controller 54 may lengthen
or shorten the amount of time (through the timer 44) of heating by
the oxidation furnace 38. Alternatively, the timer 44 may be
manually set by an operator who has measured the ambient
atmospheric pressure with a barometer and set the timer 44
accordingly to account for the actual value of the ambient
atmospheric pressure. This method therefore bypasses the controller
54 and the ambient pressure sensor 56, but does not provide dynamic
feedback for changing ambient pressure conditions.
[0038] Although the present invention has been described thus far
with the example of thermal oxidation/diffusion tubes, the
above-described techniques are also applicable to RTO (rapid
thermal oxidation) equipment and techniques. An exemplary
embodiment of an RTO process arrangement is depicted in FIG. 5. An
RTO chamber 60 receives a wafer 62 (or a plurality of wafers 62)
that is to be processed. Supply gas is provided to the chamber 60
from a gas supply 64 through a valve 66 that is under the control
of a controller 68. An ambient sensor 70 compares the atmospheric
pressure to the pressure of the chamber atmosphere and provides a
comparison signal to the controller 68. A timer 72 provides the
furnace time for the wafers 62 in the chamber 60 to the controller
68. The chamber 60 is heated by, for example, infrared lamps 74.
The temperature in the chamber 60 is determined from a value
provided by a thermocouple 76 to the controller 68.
[0039] The control of the RTO process equipment depicted in FIG. 5
is the same as in the embodiment of the invention depicted in FIGS.
2-4.
[0040] The present invention reduces the variance in the thickness
of oxide layers, such as gate oxide, by taking into consideration
the actual ambient atmospheric pressure. The present invention
allows existing oxide diffusion arrangements at ambient atmospheric
pressures to be retrofitted at low expense to control the pressure
in the oxide diffusion tube during the oxide diffusion process.
Alternatively, the amount of time of the oxide diffusion process is
changed in certain embodiments to account for differences in the
ambient atmospheric pressure from the standard, assumed atmospheric
pressure. Again, this reduces the variation in the gate oxide
thickness from lot to lot.
[0041] Although the present invention has been described and
illustrated in detail, it is to be clearly understood that the same
is by way of illustration and example only and is not to be taken
by way of limitation, the spirit and scope of the present invention
being limited only by the terms of the appended claims.
* * * * *