Flame-free wet oxidation

Details Flame-free wet oxidation Flame-free wet oxidation

1999-01-15

2002-01-01

Bowers, Charles (Department: 2813)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

By reaction with substrate

C438S758000, C438S769000, C438S770000, C438S775000, C438S786000, C438S787000

Reexamination Certificate

active

06335295

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to methods and apparatus for performing wet oxidation of semiconductor wafers. More particularly, the invention relates to safe methods and apparatus for forming high quality oxides via wet oxidation.
Wet and dry oxidation processes are currently employed by the semiconductor industry for the purpose of forming oxide layers such as gate oxides and isolation oxides on semiconductor surfaces. Dry oxidation processes typically employ molecular oxygen, nitrous oxide, nitric oxide, or some combination thereof to react with a semiconductor substrate surface and produce a layer of semiconductor oxide. The nitrogen containing a species may be employed when it is desirable to impart some nitrogen to the oxide, as is the case with hardened gate oxides, for example. Because it is relatively slow in comparison to wet oxidation processes, dry oxidation is typically limited to the formation of relatively thin oxide films.
Wet oxidation is the subject of the present invention. Wet oxidation involves reacting ultra pure water with the semiconductor surface to form oxide layers. Water vapor is typically reacted with a silicon substrate at a temperature in a neighborhood of 700 degrees Centigrade to form oxide layers between about 50 and 6000 angstroms in thickness. The ultra pure water is typically produced by a “torch” which is a reactor where ultra pure gaseous hydrogen is reacted with ultra pure gaseous oxygen to produce the water.
FIG. 1
presents a cross-sectional diagram of a wet oxidation system
10
. The system
10
includes a “torch” II and a “furnace”
13
(terms used in the industry). As mentioned, hydrogen and oxygen react in the torch to form water. The water from torch
11
is piped to furnace
13
where it reacts with the silicon on multiple wafers to form silicon oxide layers.
Torch
11
contains a torch chamber wall
15
, which is typically made from quartz. Chamber wall
15
may assume a generally “jug-shaped” configuration having a diameter of roughly 2″-12″ and a height of roughly 12″-20″. Generally, the torch is substantially smaller than the furnace. Note that torch
11
and furnace
13
are not drawn to scale in FIG.
1
.
Ultra pure hydrogen and oxygen are introduced to the interior of torch
11
through an annular arrangement of quartz pipes. A hydrogen inlet
17
is provided through the bottom of torch chamber wall
15
. A quartz inlet is defined by an annular pipe
19
which circumferentially surrounds hydrogen inlet pipe
17
. The flow of hydrogen through inlet
17
is controlled by the size of an orifice
21
which may be a constriction at the end of quartz inlet pipe
17
.
Torch
11
also includes a heater
23
which jackets a portion quartz vessel
15
. Heater
23
may in one embodiment be a simple coil heater. It provides the energy necessary to ignite a hydrogen-oxygen flame
25
within the interior of torch
11
. In many designs, the interior of torch
11
is maintained at a temperature of about 540 to 700 degrees Centigrade. At atmospheric pressure, a temperature of about 540 degrees Centigrade is required for ignition. The hydrogen-oxygen reaction is highly exothermic, producing a flame temperature in the neighborhood of 5000 degrees Centigrade. The water produced by the reaction of hydrogen and oxygen in flame
25
is directed to the bottom of furnace
13
through a pipe
27
. There, it passes up through an injection tube
29
which is typically a hollow quartz tube oriented vertically in the center of the furnace.
Furnace
13
also includes a container wall
31
which may be made from quartz. It is generally cylindrically shaped and has a diameter of roughly 10″ and a height of roughly 45″. Wall
31
is jacketed by a very precise heater
33
which supplies sufficient energy to drive the oxidation reaction. To careful control the temperature within furnace
13
, multiple thermocouples may provided in close proximity to wafers
35
. Other thermocouples are provided in heater
33
. Typically, the furnace interior is maintained at a temperature of about 700 or more degrees Centigrade. Water from water injection column
29
disperses throughout the interior of furnace
13
and contacts wafers
35
supported on a quartz boat
37
. Quartz boat
37
is a ladder arrangement of horizontal quartz wafer support structures cut in or held in place by three or more vertical rails. In a typical design, quartz boat
37
may hold between 100 and 200 wafers. The water reacts with exposed silicon on wafers
35
to produce silicon oxide layers. An exhaust port
39
draws excess water together with any gaseous carriers and reaction products out of furnace
13
.
This design has certain shortcomings. Most of these shortcomings derive from the fact that a hydrogen-oxygen flame is produced. It is critically important that the position of this flame be carefully controlled so that it does not contact the quartz torch vessel wall
15
or inlet pipes
17
or
19
. Thus, the torch reactor must generally assume the jug-shape or some other shape which is unlikely to contact flame
25
. In addition, the size of orifice
21
must be carefully controlled so that the hydrogen flow rate is sufficiently high to prevent the flame from contacting quartz inlet pipes
17
or
19
. Should the flame contact any of the quartz components described herein, the quartz my devitrify and vaporize. This introduces particulates and other contaminants to the ultra pure water source, thereby precluding generation of a high quality oxide.
Still further, many precautions must be taken to ensure that hydrogen explosions do not occur. Typical safety mechanisms include interlocks to ensure that hydrogen does not flow without oxygen also flowing into torch
11
. Further, additional interlocks are provided to ensure that the ratio of hydrogen to oxygen always remains below 2:1 by volume. Typically, the hydrogen to oxygen ratio is about 1.9 to 1 by volume. Still further, the torch often includes a flame detector such as a flame detector
41
shown in FIG.
1
. Such a flame detector ensures that the hydrogen and oxygen are actually reacting. If there is no flame, an explosive mixture may be forming within the torch, within the furnace, or elsewhere.
In view of the above shortcomings, an improved wet oxidation system is necessary.
SUMMARY OF THE INVENTION
The present invention provides methods and apparatus for performing wet oxidation. Wet oxidation performed in accordance with this invention does not employ a flame. Therefore, contamination due to the flame impinging on quartz components of a torch is not a problem. Flame-free generation of water is accomplished by reacting hydrogen and oxygen under conditions that do not result in ignition. In a preferred embodiment, this is accomplished by providing a diluted hydrogen stream in which molecular hydrogen is mixed with a diluent such as a noble gas or nitrogen. This use of diluted hydrogen also reduces or eliminates the danger of explosion. This can greatly simplify the apparatus design by eliminating the need for complicated interlocks, flame detectors, etc.
One aspect of the invention provides a method of forming an oxygen containing layer on a semiconductor surface. The method may be characterized as including the following sequence: (a) forming water vapor by reacting gaseous hydrogen and gaseous oxygen without generating a flame; and (b) contacting the water vapor with the semiconductor surface under conditions which form the oxygen containing layer on the semiconductor surface. The oxide may be a silicon oxide layer or a nitrogen containing silicon oxide layer, for example. The water vapor should be sufficiently pure to meet the requirements of the semiconductor device fabrication industry. Generally, this means that the gaseous hydrogen and gaseous oxygen each will have a purity of at least about 99.999% by volume.
One technique for ensuring that the water vapor is formed without generating a flame involves providing the gaseous hydrogen as a mixture of molecular hydrogen and

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