Gas flow control apparatus

Gas flow control apparatus for controlling the flow of a process gas through a conduit system to a process site, for halting the flow of gas when the process is completed, and for then developing a partial vacuum in the conduit system sufficiently high to reduce the concentration of process gas and maintain it in a gaseous state. The partial vacuum is developed by carrier gas flowing through a venturi in communication with the conduit system supplying the process gas. The carrier gas also expels any remnants of the process gas from all points between the venturi and the process site. Further, the system can be used for vaporizing gases from liquids for use in an atmospheric process at the process site, the carrier gas flowing through the venturi and vaporizing a liquid in a chamber upstream of the venturi. The vaporized gases can then be regulated by a mass flow controller.

BACKGROUND OF THE INVENTION 
The present invention relates generally to apparatus for controlling the 
flow of a gas. 
Industrial processes such as those used in the fabrication of semiconductor 
wafers in furnaces require a continuous flow of a process gas mixture to 
the furnace process site. A typical semiconductor fabrication process 
requires initial use of a particular mixture of gases, both reactive and 
inert, followed by a sequence of different gas mixtures as the process 
proceeds The rate of flow of the gases, their initiation and their shut 
off are under the control of one or more mass flow controllers. 
The gas mixture proceeds from a mixing station or manifold through a 
conduit system which includes conventional tubing, hoses, control valves, 
the mass flow controllers and like components. 
On completion of a processing step, closure of the associated control valve 
will halt the flow of process gas to the process site. However, remnants 
of the gas will remain in the system. Since the next process step may 
require use of a different mixture of gases, remnants of the first process 
gas mixture must be purged or expelled from the conduit system to avoid 
contaminating the new process gas. In addition, it is desirable to expel 
process gas remnants promptly after the end of all processing to minimize 
the harmful effects of any corrosive or reactive component gases remaining 
in the conduit system. Preferably, such purging should be done under 
automatic control. Although various ways of expelling remnants of process 
gases have been proposed, none has been entirely satisfactory. 
Some components of process gas mixtures are obtained by vaporizing a 
liquid. Various means for vaporizing such a liquid and controlling flow of 
the resulting vapor have been proposed, but each suffers certain 
drawbacks, particularly complexity and difficulty of controlling the rate 
at which the vapor enters the associated control valve, especially if the 
process is being carried out at a pressure close to atmospheric. 
It will be apparent from the foregoing that there is a need for gas flow 
control apparatus which provides a simple, automatically controllable 
means to purge process gas from as much of the conduit means as possible, 
and to minimize the effects of any such gas remaining in the conduit 
system after process gas shut off, and which also provides a simple means 
for vaporizing a liquid and controlling the flow rate of the vapor into 
the process gas mixture. 
SUMMARY OF THE INVENTION 
The present invention provides gas flow control apparatus operative to 
establish a partial vacuum in the conduit means through which a process 
gas flows toward the process site. The vacuum source is actuable on 
process gas shutoff to purge process gas from portions of the conduit 
means, and to establish a partial vacuum in the remainder of the conduit 
means to reduce the concentration of the process gas and maintain it in a 
gaseous state. The apparatus can also be operated during the process to 
vaporize a liquid in a chamber upstream of the conduit means to provide a 
component of the process gas. 
More particularly, the apparatus according to the present invention 
includes a vacuum eductor means having a process gas inlet, a process gas 
outlet, a venturi nozzle, a venturi tube in communication with the process 
gas outlet and spaced from the venturi nozzle to define a venturi space in 
communication with the gas process inlet, a carrier gas inlet in 
communication with the nozzle, and conduit means for carrying a process 
gas to the process gas inlet. Admitting carrier gas to the carrier gas 
inlet creates the partial vacuum in the conduit means and expels or purges 
process gas from the system downstream from the venturi space. 
The vapor phase of the liquid in the chamber is routed from the chamber in 
a fluid flow path which includes a mass flow controller so that the rate 
of vapor flow to the process gas inlet can be conveniently and easily 
controlled. 
As will be apparent, the carrier gas can also be employed to create a 
partial vacuum while the process of fabricating semiconductor wafers, for 
example, is underway. This will aid in causing the process gas to flow to 
the process site. Normally, however, the process gas is under sufficient 
pressure that this is not necessary, and the carrier gas is not admitted 
until the process step has ended and the flow of associated process gas 
has been cut off. 
Other aspects and advantages of the present invention will become apparent 
from the following detailed description, taken in conjunction with the 
accompanying drawings, illustrating by way of example the principles of 
the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawings for purposes of illustration, the invention is 
embodied in gas flow control apparatus of the kind used to mix, measure, 
monitor and control the mass flow of a process gas to a process site such 
as a furnace containing semiconductor wafers. 
Such apparatus typically includes a manifold 26 for accepting and mixing 
the various components of the process gas, and also some form of conduit 
system 16 for supplying the process gas, including conduits and 
passageways defined by conventional tubing, valves, flow controllers and 
the like. 
The usual process for fabricating semiconductors includes a number of 
separate process steps or stages which each use a different mixture of 
process gas components. It is important on conclusion of one process step 
that the process gas for that step not be present in a concentration 
sufficient to contaminate the process gas for the next step, particularly 
where the process gas includes a reactive component. The present apparatus 
is effective to prevent such contamination by developing a partial vacuum 
in the conduit system. This capability is also useful in vaporizing a 
liquid for use as a component of the process gas. This is very convenient 
when the process site is at or near atmospheric pressure since it is 
relatively easy to control the mass flow of the vacuum induced vapor. 
The development of the partial vacuum is accomplished by a venturi or 
vacuum eductor assembly 10 which defines a venturi space 13 through which 
the process gas flows. On process gas shut off a carrier gas is introduced 
into the eductor assembly to create the partial vacuum in the conduit 
system 16 upstream of the venturi space. This reduces the gas 
concentration sufficiently to maintain the remnants of the process gas in 
a gaseous state. Also, as indicated above, the partial vacuum can be used 
to vaporize a liquid for use as a component of the process gas during 
normal flow of the process gas. On process gas shut off, the continuing 
flow of carrier gas also purges any remaining process gas present 
downstream of the venturi space 13. 
As shown in FIGS. 1 through 4, the eductor assembly 10 includes a process 
gas inlet 12, as best seen in FIGS. 2 and 3, the venturi space 13, and a 
process gas outlet 14. 
The various process gas components pass from supply tanks or the like (not 
shown) through suitable conduits (not shown) which are coupled to inlet 
ports 18, 20, 22 and 24 that form an integral part of the mixing means or 
manifold 26. Passageways in the manifold 26 extend from the inlet ports to 
an elongated central conduit 28. 
The manifold 26 is welded or otherwise attached to a control valve 30 so 
that a central conduit 32 of the valve 30 is in gas tight fluid 
communication with the manifold conduit 28. In similar fashion, the 
control valve 30 is attached to the eductor assembly 10 in gas tight 
relation so that the mixture of gases from the conduit 32 flows into the 
process gas inlet 12. 
Any control valve 30 can be employed that is operable to turn on and shut 
off the gas flow, but a solenoid actuated valve is preferred because it 
can be remotely operated by an automatic control system programmed to 
start and stop the flow of process gas. This enables different 
combinations of process gas components to pass from the manifold 26 to the 
process site at predetermined intervals. 
When the valve 30 is the shut off point the conduit system 16 to be purged 
of harmful concentrations of process gas comprises the gas passages or 
conduits located downstream from the valve 30 and extending to the venturi 
space 13. Where valve 30 is left open, and the gas shut off points are 
valves (not shown) associated with gas supply tanks or the like, the 
conduit system 16 in which a partial vacuum is developed includes all of 
the conduits and passageways up to such shut off valves, which would 
include any delicate mass flow controllers located downstream of such 
valves. 
The eductor assembly 10 comprises a housing 34 having a central, 
longitudinally oriented bore 35 which slidably receives a cylindrical 
venturi nozzle 36 and a cylindrical venturi tube 38. The tube and the 
nozzle are aligned and their adjacent extremities include reduced diameter 
portions upon which the ends of a cylindrical nozzle sleeve or coupling 40 
are seated. 
The housing 34 also includes a transverse conduit 42 intersecting the bore 
35 and having an upstream end defining the process gas inlet 12. The 
opposite end of the conduit 42 terminates in an auxiliary port normally 
closed by a threaded plug 44. The plug bears against a suitable O-ring 46 
to provide a gas tight seal with the housing. The auxiliary port is 
useful, for example, in extracting process gas samples for analysis while 
the process is underway. 
The nozzle coupling 40 has transversely aligned openings that communicate 
with the conduit 42 to admit process gas, and to communicate with the plug 
44, respectively. 
The coupling 40 spaces the nozzle and tube ends apart sufficiently to 
define the venturi space 13. 
The fit between the coupling 40 and the nozzle 36 and tube 38 is made gas 
tight by a pair of O-rings 50 and 52 located in annular grooves provided 
in the nozzle and tube, respectively. 
The outer extremities of the venturi nozzle and tube are engaged, 
respectively, by the inner ends of coupling nuts 54 and 56 which are 
threadably received within threaded counterbores provided in the opposite 
extremities of the bore 35. O-rings 58, 60, 62 and 64 located between the 
nozzle, tube, the adjacent nuts 54 and 56, and the associated portions of 
the housing 34 provide a gas tight fit, as will be apparent. 
The nut 54 is coupled to a flexible conduit 66, as best seen in FIG. 1, 
which is attached to a suitable control valve 68. The valve 68 is 
connected by a conduit 70 to a source of inert carrier gas such as 
nitrogen. The other nut 56 is coupled to a conduit (not shown) which 
carries process gas from the process gas outlet 14 to the furnace or other 
process site. 
The gas passage through the venturi nozzle 36 includes a substantially 
constant diameter upper passage 72 which narrows to a relatively small 
diameter lower passage 74 in communication with a passage increasing 
slightly in diameter and opening into the venturi space 13. 
The lower tip of the nozzle 36 is conical and extends into a complementally 
configured conical depression formed in the upper end of the venturi tube 
38. This conical depression is in communication with a longitudinal 
passageway 76 extending through the tube 38 and terminating in an end 
which defines the process gas outlet 14. The passageway 76 is of 
relatively small diameter, increasing gradually in diameter toward the 
cavity defining the venturi space 13, and also increasing in diameter in 
the opposite direction toward the process gas outlet 14. 
In operation, the process gas components are admitted into the manifold 26 
in the proper proportion by mass flow controllers or the like (not shown) 
which are associated with the gas supply for each such component. The 
mixture then passes through the manifold conduit 28 into the control valve 
conduit 32. 
Energization of control valve 30 by the associated automatic control system 
allows process gas to pass from the control valve to the process gas inlet 
12. From this point the process gas passes through the transverse eductor 
assembly conduit 42, through the opening in the coupling 40, and into the 
venturi space 13. The process gas next flows through the venturi tube 
passageway 76, out of the process gas outlet 14, and on to the furnace or 
other process site. 
Assuming the overall process requires a modified gas mixture for the next 
process step, the automatic control system next closes the control valve 
30 and halts further flow of the first gas mixture. The automatic control 
system then opens the control valve 68 to allow carrier gas to flow 
through the conduit 66 and into the eductor assembly 10. 
The carrier gas is under pressure and its flow is therefore sufficiently 
rapid that a relatively low pressure is developed in the venturi space 13. 
This develops a partial vacuum in the conduit means 16 which, in this 
instance, constitutes the conduit 42 and the control valve conduit located 
downstream of the closure element of the valve. The partial vacuum is 
sufficient to maintain the process gas in vapor form, and to reduce the 
concentrations of reactive gas components to a level insufficient to 
attack the walls and other components which define the conduit means. 
The carrier gas flowing downstream from the venturi space 13 purges all 
process gas in the downstream conduits and passageways, extending all the 
way to the process site itself, so that the site is rendered neutral and 
ready for the next gas mixture. 
After a suitable interval, the central control system closes the carrier 
gas valve 68 and admits the next combination of process gas components to 
the manifold 26. The control valve 30 is then opened and the mixed gases 
pass into the eductor assembly 10 as before. 
After processing of the semiconductor wafers is finished, control valve 30 
is left open and the valves (not shown) which control the flow of gas 
components to ports 18, 20, 22 and 24 are closed. The carrier gas valve 68 
is then opened and carrier gas flowing past the venturi space 13 develops 
a partial vacuum in the conduit means, which now includes all of the 
conduits- and passageways up to the gas supply valves located upstream of 
the manifold 26. This reduces the concentration of process gas back up to 
the supply valves at a level that components located downstream of such 
valves are protected from corrosive attack. 
In addition to the ability of the present apparatus to prevent harmful 
levels of concentration of process gas, the apparatus is also useful to 
develop and control the flow of controlled quantities of a process gas 
component normally liquid at atmospheric pressures. 
With reference to FIG. 5, a vessel 78 defines a chamber 80 which is filled 
with the process gas liquid 82. A space defined between the liquid and the 
upper extremity 84 of the vessel is in communication with a flexible 
conduit 86 coupled to a suitable mass flow controller 88. The controller 
outlet conduit is coupled to inlet port 24 of manifold 26. 
When carrier gas is admitted through conduit 66 to the eductor assembly 10, 
the resulting partial vacuum vaporizes some of the liquid in the chamber 
80, and draws that vapor through the conduit 86 and the controller 88 to 
the manifold 26. From there the vapor flows through the eductor assembly 
10 to the process site. 
In its vapor form the amount of vapor flow into the manifold 26 is easily 
controlled by the controller 88. Thus, the present apparatus readily lends 
itself to use in processes in which a liquid must be vaporized for 
employment as a component of the process gas. 
Wherever elements of the present apparatus may be exposed to reactant 
components of the process gas, such elements are preferably fabricated of 
a nonmetallic, corrosion resistant material such as KEL-F, which 
manufactured by Minnesota Mining and Manufacturing Co. 
From the foregoing it will be appreciated that gas flow control apparatus 
according to the present invention provides an effective means for 
reducing the corrosive effect of reactant components of process gas so 
that process gas used in one stage of the overall process does not remain 
after gas shut off in sufficient concentrations to contaminate the process 
gas used in the next step. Further, on shut down of the entire system, the 
present apparatus provides an effective way to establish a partial vacuum, 
and consequently reduce process gas concentration, upstream of the process 
site all the way to the component gas supply valves. The carrier gas 
utilized to establish the partial vacuum can also purge out of the system 
all process gas downstream from the venturi which establishes the vacuum. 
In addition, the apparatus optionally employs the partial vacuum for 
vaporizing a liquid to provide a component of the process gas whereby the 
amount of such liquid utilized in an atmospheric process, for example, is 
easily controlled by regulating the flow of its vapor. 
Various modifications and changes may be made with regard to the foregoing 
detailed description without departing from the spirit of the invention.