Dual-stage wet oxidation process utilizing varying H2/O2 ratios

Details Dual-stage wet oxidation process utilizing varying H2/O2 ratios Dual-stage wet oxidation process utilizing varying H2/O2 ratios

2000-01-13

2002-04-16

Chaudhuri, Olik (Department: 2814)

Semiconductor device manufacturing: process

Coating of substrate containing semiconductor region or of...

By reaction with substrate

C427S255270, C427S255370

Reexamination Certificate

active

06372663

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to a wet oxidation process for forming silicon oxide layers and more particularly, relates to a dual-stage wet oxidation process for forming silicon oxide layers wherein different H
2/
O
2
are utilized in two separate stages.
BACKGROUND OF THE INVENTION
The formation of silicon oxide on a silicon substrate is a frequently conducted process in the fabrication of semiconductor devices. One of the methods for forming silicon oxide is thermal oxidization which is carried out by subjecting a silicon wafer to an oxidizing ambient at elevated temperatures. A common objective of an oxidizing system is to obtain a high quality silicon oxide film of uniform thickness while maintaining a low thermal budget (the product of temperature and time). Methods have been developed to increase the oxidation rate and to reduce the oxidation time and temperature. Two of such methods are the dry oxidation method and the wet oxidation method by using an external torch.
The substances used to grow thermal oxides on a silicon surface are dry oxygen and water vapor. In a dry oxygen reaction, silicon oxide is formed by Si+O
2
→SiO
2
, while for water vapor, the reaction is Si+2H
2
O→SiO
2
+2H
2
. In both cases, silicon is consumed and converted into silicon dioxide.
In a dry oxidation process, silicon dioxide layers can be formed in a temperature range of 400° C.~1150° C. The process is typically performed in a resistance-heated furnace or in a rapid thermal processing chamber with heat provided by tungsten halogen lamps. In a typical dry oxidation process, a horizontal furnace tube may be used in which a batch of wafers is introduced into the furnace tube positioned in a slow moving wafer boat and then heated to an oxidation temperature in a ramp-up process. The wafers are held at the elevated temperature for a specific length of time and then brought back to a low temperature in a ramp-down process. In the dry oxidation process, oxygen mixed with an inert carrier gas such as nitrogen is passed over the wafers that are held at an elevated temperature.
A wet oxidation process can be performed by either bubbling oxygen through a high purity water bath maintained at between 85° C. and 95° C., or by a direct reaction of hydrogen with oxygen producing water vapor in a pyrogenic steam oxidation process.
The thermal budget required to grow a silicon oxide layer to a certain thickness is considerably smaller in a wet oxidation process than that in a dry oxidation process. The wet oxidation process for producing a silicon oxide film can therefore be carried out more efficiently and at a lower cost. However, because of a residual water content, silicon oxide films formed by the wet oxidation process exhibit a lower dielectric strength and has higher porosity to impurity penetration than silicon oxide films formed in a dry oxidation process. As a compromise, wet oxidation process is frequently used in conjunction with dry oxidation process such that a high quality oxide film can be grown with minimized oxidation time required. This is performed by beginning and ending an oxidation process in dry oxygen while using the wet oxidation process for the intermediate stage which reduces the thermal budget while increasing the overall oxide growth rate. By using this dry oxidation-wet oxidation-dry oxidation process sequence, high quality silicon oxide films can be grown on both sides of the oxide layer in order to provide properties of the three-layered film comparable to those of a single layer grown by a dry oxidation process alone.
Another benefit of the wet oxidation process is that the apparatus used for carrying out the wet oxidation may also be used to carry out a dry oxidation process. For instance, as shown in
FIG. 1
, a wet oxidation apparatus
10
consists of an oxidation chamber
12
, an external torch
14
, and a conduit
16
that connects the external torch
14
and the oxidation chamber
12
for providing fluid communication therein between. The wet oxidation apparatus
10
further includes conduit
20
for feeding an inert gas into conduit
16
for purging both the conduit
16
and the oxidation chamber
12
, conduit
22
for flowing oxygen into the external torch
14
by a carrier inert gas, and conduit
24
for flowing hydrogen into the external torch
14
with an inert carrier gas. An exhaust conduit
28
takes away unused or excess water vapor in the oxidation chamber
12
. The flow of gases in conduits
20
,
22
and
24
is controlled by valves
30
,
32
and
34
, respectively.
The convention wet oxidation apparatus
10
shown in
FIG. 1
has been used for many years. In a normal silicon oxide growth process, in order to achieve high growth rates while minimizing the thermal budget of the process, the maximum H
2
/O
2
gas mixture ratio of 1.8 is used for producing thick silicon oxide layers, i.e. layers thicker than 2000 Å. At the high H
2
/O
2
gas mixture ratio of 1.8, the partial pressure of water vapor in the reaction chamber is very high which causes a loading effect, i.e., the lesser number of wafers are loaded in the reaction chamber, the poorer is the wafer-to-wafer coating uniformity.
In the conventional thick silicon oxide growth process carried out by the water oxide method, the process is carried out by a single step pyrolysis technique at a high H
2
/O
2
ratio of about 1.8. The gas mixture ratio of 1.8 for H
2
/O
2
is the highest possible within a safety limit without the danger of causing an explosion in the furnace. After the gas mixture is burned in a torch, the high H
2
/O
2
gas mixture ratio produces high water pressure in the furnace tube and thus achieves a high growth rate of silicon oxide. However, the excess water vapor left in the furnace tube does not stop reacting on the plurality of wafers positioned in the furnace until the water vapor is purged out by an inert gas.
The reaction mechanism in the wet oxidation process can be shown as follows:
2H
2
O+Si→+SiO
2
+2H
2
. (The original reaction)
2H
2
+O
2
→2H
2
O(H
2
reacts with O
2
at high temperature in tube)
2H
2
O+Si→SiO
2
+2H
2
(Secondary reaction occurs before H
2
O is purged out)
The secondary reaction causes an effect known as the loading effect in which when the furnace tube is loaded only with a few wafer and that the wafers are charged from the top of the boat, the loading effect is very serious in the top than the bottom due to the different gas flow conditions leading to poor wafer-to-wafer uniformity.
The loading effect occurring in a conventional wet oxidation process for growing silicon oxide is shown in FIG.
2
. As described above, the excess wet vapor in the furnace tube does not stop reacting even after the main pyrolysis step is completed, until all the water vapor is purged out by an inert gas such as nitrogen. The loading effect becomes more serious in the upper chamber of the furnace than the lower chamber when more wafers are positioned in the upper chamber. As shown in
FIG. 2
, at the lower end of the total number of wafers processed, a deviation in the thicknesses of the silicon oxide layers formed on wafers positioned in the upper chamber can be as high as 100 Å. This deviation is greatly reduced, as shown by the data in
FIG. 2
to about 20 Å, when 144 wafers are loaded in the furnace tube. In a semiconductor fabrication facility, the furnace tube cannot always be loaded full of wafers, it is therefore inevitable that whenever there are only a few wafers loaded in the furnace tube, the uniformity in thickness of the silicon oxide layers formed on the wafers becomes unacceptable.
For instance, data plotted in
FIG. 2
are obtained by a conventional wet oxidation recipe in which three burn steps are first carried out; the first burn step for about 2 minutes at an O
2
flow rate of 10 liter/min, a second burn step of about 1 minute at a O
2
flow rate of 3.5 liter/min, followed by a third burn step for about 1 minute with a H
2
flow rate of 3

Affiliated with

Also associated with

Rating

Dual-stage wet oxidation process utilizing varying H2/O2 ratios does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Dual-stage wet oxidation process utilizing varying H2/O2 ratios, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dual-stage wet oxidation process utilizing varying H2/O2 ratios will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2907707