Semiconductor device manufacturing: process – Gettering of substrate – By vapor phase surface reaction
Reexamination Certificate
1999-10-28
2003-02-25
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Gettering of substrate
By vapor phase surface reaction
C134S026000, C134S030000, C134S031000, C210S088000, C210S181000
Reexamination Certificate
active
06524934
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention is related to generation of water vapor. More specifically, the present invention is a method and apparatus for generating ultra-pure water vapor without utilizing an inert transporting gas.
2. Background Art
Ultra-pure water vapor is utilized in many applications and industries. One such use is for affixing a silicon oxide film by the water oxidation method in semiconductor and integrated circuit manufacturing processes. Another such use can be as an auxiliary reactant used during plasma photo resist stripping to aid in the removal of inorganic residues.
A first method to produce ultra-pure water vapor is a system to boil high purity water. Such a system is typically referred to as a boiler.
FIG. 1
illustrates a prior art boiler system. The boiler system includes a chamber
100
for boiling the water, a water inlet
105
, an upstream water purification and filtering system
110
, a heat source
115
, a water vapor outlet
120
, an outlet pressure and flow controlling device
125
, and a control and instrumentation system
130
. Water is purified and filtered in the upstream water purification and filtering system
110
prior to introduction to the chamber
100
. Heat is applied to the chamber
100
to produce steam (water vapor). The water vapor is then piped to the point of use through the outlet pressure and flow controlling device
125
.
The boiler method of producing water vapor has several significant short falls that renders a boiler ineffective for use in semiconductor manufacturing operations.
First, a boiler system will concentrate the impurities contained in the water it is boiling. If the water flowing into the boiler has been purified and filtered to low parts per million (PPM) or better impurity levels, the majority of the trace impurities will remain in the chamber
100
when the water vapor is released. This leads to a concentration of impurities in the chamber
100
. As time passes, the amount of impurities leaving the boiler can increase due to aerosols leaving with the saturated water vapor. These aerosols can be formed at the gas/liquid interface as vapor bubbles that rise to the surface, break and “splatter” liquid droplets into the vapor phase. Since these aerosols are formed from liquid in the boiler, the aerosols contain the same level of continuously increasing impurities. These high impurity levels can contaminate the product water vapor and the downstream delivery system.
Second, a boiler method is typically controlled by a feedback control process monitoring the pressure through the outlet pressure and flow controlling device
125
and adjusting the heat source
115
, to maintain a constant pressure. This often results in oscillation and instability in the output flow, particularly when transient or non-steady state flow rates are required. This oscillation effect can further increase the formation of aerosols-described above.
A second method of producing high purity water vapor is referred to as a bubbler.
FIG. 2
illustrates a prior art bubbler system. A bubbler consists of a sealed chamber
200
which is isolated from the out side air, a water inlet
205
, an upstream water purification and filtering system
210
, a heat source
215
, a water vapor outlet
220
, an inert gas inlet
230
and an inert gas flow control device
225
.
The chamber
200
contains a quantity of water
235
therein, maintained at a freely selected constant temperature. An inert gas enters through the inert gas inlet
230
and passes through the water. The result is an inert gas which contains a water component corresponding to the vapor pressure of water at the freely selected temperature. The control of the water concentration is accomplished by means of the temperature and vapor pressure relationship within the chamber
200
.
The bubbler method has several short falls. Accurate control of the water vapor concentration leaving the bubbler is dependant on the assumption that the carrier vapor achieves vapor liquid equilibrium with the bulk water. This requires accurate control of the liquid temperature, bubble sizes and distribution, bubble residence time in the water, and total operating pressure. In practice, simultaneous control of all these variables is difficult, and oscillations are likely to occur when transient or non-steady state flow rates are required. Obtaining pure water vapor is not possible with this method, due to the use of an inert gas to create the carrier bubbles. Impurity build-up similar to that experienced in a boiler system would also occur in this method.
A third method for generating high purity water vapor is one in which a standard gas contained in a cylinder is diluted.
FIG. 3
illustrates a gas dilution type system. A dilution system includes a cylinder
300
, containing a quantity of inert gas
305
, with the inert gas having a known concentration of water vapor. A dilution system also includes an inert gas flow control
310
, a diluent inert gas inlet
315
, a diluent inert gas flow control
320
, and an outlet
325
.
In the dilution method, the inert gas
305
is diluted to a selected dilution ratio using a quantity of diluent inert gas from the diluent inert gas inlet
315
. The water vapor concentration of the resulting gas mixture is determined by the inert gas flow control
310
and the diluent inert gas flow control
320
.
The dilution method also has several shortfalls. First, the reliability of the water concentration is low since there are no standard gases having highly accurate water concentrations. Second, it i difficult to generate high concentrations and large quantities of water vapor, and by definition, generation of pure water vapor is not possible. The mixture being diluted cannot have a water concentration higher than the dew point at ambient conditions, or liquid condensation inside the storage container will result. Heating the container will increase the available concentration, but doing so is not practical in modem semiconductor fabs, and will also exhibit similar problems to that described in the boiler system above.
A fourth method of producing high purity water vapor is commonly referred to as combustion in a quartz diffusion furnace or, more simply, combustion.
FIG. 4
a
illustrates a gas combustion system. A gas combustion system includes a combustion chamber
400
, an ultra-pure oxygen inlet
405
, an ultra-pure hydrogen inlet
410
, an oxygen flow control
415
, a hydrogen flow control
420
, an outlet
425
, a hydrogen gas nozzle
430
, a Si chip
435
for ignition held in a vicinity of a top side of the hydrogen gas nozzle
430
, and a heating lamp
440
for heating the Si chip
435
.
A vicinity at the tip end of the hydrogen gas nozzle
430
inside the chamber
400
attains a high temperature from about 1800° C. to 2000° C. due to flames of combustion. In addition, the amount of oxygen gas supplied to the chamber
400
is set to a level exceeding one half that of the hydrogen gas in order to completely combust the hydrogen gas H
2
and have excess oxygen remaining. This maintains safer operation of the system.
The combustion method achieves excellent practical effects in that high purity water is generated and can be instantaneously generated at a rate of several liters per minute. However, in this method, there is a problem in that if the flow rate of hydrogen gas or oxygen gas is reduced to decrease the water amount, combustion can easily be stopped. It is therefore, extremely difficult to provide controls for decreasing the amount of water vapor which is generated. The control range of a ratio of water vapor to oxygen is narrow. As a result, production of pure water vapor over wide pressure and flow rate ranges is very difficult, and may not be possible for systems requiring on/off flow rate demands.
The combustion method has an additional difficulty in that when combustion stops, raw gas is fed directly into the outlet
425
. An interlock mechanism becomes indispensable to prevent a hydrogen gas explosion when combustion stops. This adds additio
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