Pressure-type flow rate control apparatus

A pressure-type flow rate control apparatus for use especially in the gas supply system in semiconductor manufacturing facilities. The flow control apparatus is provided with a bore-variable orifice, which permits easy switching of the fluid flow rate control range as well as size reduction of the pressure-type flow control apparatus, and offers other advantages including improved gas replaceability, prevention of dust formation, and reduced manufacturing costs of the flow control system. The apparatus comprises an orifice, a control valve provided on the upstream side of the orifice, a pressure detector provided between the control valve and the orifice, and a control unit to calculate a fluid flow rate Q on the basis of a pressure P1 detected by the pressure detector with the equation Q=KP1 (K=constant) and to output in a drive for the control valve the difference between the set flow rate signal Qs and the calculated flow rate signal Q as control signal Qy, wherein the pressure P1 on the upstream side of the orifice is regulated by actuating the control valve for controlling the flow rate of the fluid downstream of the orifice with the ratio P2/P1 between the pressure P1 on the upstream side of the orifice and the downstream pressure P2 maintained at not higher than the ratio of the critical pressure of the controlled fluid, characterized in that a direct touch type metal diaphragm valve unit functions as the orifice and that the ring-shaped gap between the valve seat and the diaphragm serves a variable orifice wherein the gap is adjusted by the orifice drive.

FIELD OF THE INVENTION 
The present invention relates to an improvement of a pressure type flow 
rate control apparatus for fluids such as gases. More particularly, the 
present invention relates to the flow control of a gas supply system 
chiefly for use in semiconductor manufacturing facilities. 
BACKGROUND OF THE INVENTION 
Mass flow controllers have been widely used as flow rate control 
apparatuses for the gas supply system in semiconductor manufacturing 
plants. Recent years have seen the development of pressure-type flow rate 
control apparatuses that are replacing the mass flow controllers. Among 
the newly developed pressure-type control apparatuses are those disclosed 
in unexamined Japanese patent publications Nos. 8-335117 and 8-338546. 
FIG. 11 shows the pressure-type flow rate control apparatus which the 
inventors disclosed earlier in the above-mentioned unexamined Japanese 
patent publication No. 8-338546. The operating principle of that 
pressure-type flow rate control apparatus is this: The flow rate Qc of 
fluid on the downstream side of the orifice is calculated with an equation 
Qc=KP1 (K: constant), with the ratio P2/P1 of pressure P2 at the 
downstream side of an orifice 5 to pressure P1 of the upstream side held 
below the gas critical pressure ratio. The difference between the 
calculated flow rate Qc and the set flow rate Qs is input in a valve drive 
3 for a control value 2 as control signal Qy to regulate the degree of 
opening of the control valve 2 for adjusting the pressure P1 upstream of 
an orifice 5, such that the calculated flow rate Qc=the set flow rate Qs 
(that is, the control signal Qy=0) is achieved. Thus, the flow rate on the 
downstream side of the orifice is regulated to the aforesaid set flow rate 
Qs. 
Referring to FIG. 11, the reference number 1 indicates a pressure-type flow 
rate control apparatus; 2, a control valve; 3, a control valve drive; 4, a 
pressure detector; 5, an orifice; 7, a control unit; 7a, a temperature 
correction circuit; 7b, a flow rate calculation circuit; 7c, a comparison 
circuit; 7d, an amplification circuit; 21a and 21b, amplification 
circuits; 22a and 22b, A-D conversion circuits; 24, an inverted amplifier; 
25, a valve; Qc, signal for calculated flow rate; Qs, signal for set flow 
rate; and Qy, control signal (Qc-Qs). 
The aforesaid pressure-type flow control system permits setting the flow 
rate Q on the downstream side of the orifice at a desired level with high 
precision through adjustment of the pressure P1 on the upstream side of 
the orifice by actuating the control valve 2. Thus, the apparatus is a 
highly effective tool in practice. 
However, the problem with that pressure-type flow rate control apparatus is 
that because the orifice 5 is fixed in diameter, the application is 
limited to a specific range of flow rates and no switch-over in the flow 
rate ranges is possible. 
To make the switch-over possible, it is necessary to so design the orifice 
5 as to be readily replaceable, and to prepare a plurality of orifices 5 
with different bores or calibers ready for use. But a problem is that 
non-uniformity in precision of processing of those orifices 5 leads 
directly to errors in flow rate control. 
Preparing a plurality of orifices with different bores present problems 
such as lack of economy and poor control precision. 
Also, in a flow rate controller of a fixed flow rate type using the 
so-called sonic velocity nozzle (or orifice), sectional area-variable 
nozzles or orifices have been developed for permitting a change in the 
flow rate range, and disclosed in unexamined Japanese utility model 
publication No. 56-41210 and examined Japanese utility model publication 
No. 60-42332. 
However, these sectional area-variable orifices are all those with 
mechanisms similar to needle-type valves and are inevitably accompanied by 
many dead spaces in the fluid flow path. That makes complete gas switching 
or replacement difficult and causes much dust. For this reason, those 
sectional area-variable orifices are not very suitable for use in the gas 
supply system in semiconductor manufacturing facilities. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to solve the problems encountered 
with the prior art pressure-type flow rate control apparatuses. The 
problems include: (a) because the orifice has a fixed diameter, the flow 
rate range cannot be switched over from one range to another, (b) 
processing an orifice with high precision is a difficult job and 
non-uniformity in processing precision directly leads to errors in 
controlled flow rate, resulting in failure to achieve a highly reliable 
and stable flow rate control, and (c) the prior art sectional 
area-variable orifices cannot effect complete gas switching or replacement 
and produce dust, and therefore cannot be used properly in the gas supply 
system in semiconductor manufacturing facilities. The foregoing object may 
be achieved by providing a pressure-type flow rate control apparatus which 
permits an easy adjustment of the orifice sectional area and a 
high-precision flow rate control at a wide range of flow rates without 
difficulty and which is superior in gas switching or replacement and 
prevention of dust, such that it is usable in the gas supply system in 
semiconductor manufacturing facilities. 
The inventors have conceived an idea of using as a sectional area-variable 
orifice a direct touch type metal diaphragm valve unit which is high in 
cleanliness and in replaceability of gases. These are indispensable 
features for equipment used in the gas supply system in semiconductor 
manufacturing facilities. Then, the inventors conducted research to 
determine if the fluid passage of the direct touch type metal diaphragm 
valve unit is capable of controlling the flow rate as efficiently as the 
so-called supersonic velocity orifice or nozzle. 
FIG. 1 shows a flow rate control testing apparatus using as a sectional 
area-variable orifice the aforementioned direct touch type metal diaphragm 
valve unit. In FIG. 1, the reference number 2 indicates a pressure control 
valve; 3, a control valve drive; 4, a pressure detector; 5, a variable 
orifice (a direct touch type metal diaphragm valve unit); 6, an orifice 
drive; 7, a control circuit; 8a, a gas inlet; 8b, a gas outlet; 9, a mass 
flow meter; 10, a vacuum chamber; 10a, a vacuum gauge; and 11, a vacuum 
pump. 
The aforesaid control valve 2 is the same direct touch type metal diaphragm 
valve unit as disclosed in unexamined Japanese patent publication 
8-338546. The drive 3 is provided with a piezoelectric element type drive 
unit. The drive 3 for the control valve 2 may alternatively be other drive 
units, such as a magnetostrictive element type, a solenoid type, a motor 
type, an air pressure type, and a thermal expansion type. 
The aforesaid pressure detector 4 is a semiconductor strain gauge. To be 
concrete, the pressure detector 4 is incorporated in the valve main body 
of the pressure-control valve 2, as in unexamined Japanese patent 
publication No. 8-338546. 
A direct touch type metal diaphragm valve, which will be described later, 
is used as the variable orifice 5, and the drive 6 is provided with a 
pulse motor drive, that is, a linear actuator working on a pulse motor and 
a ball screw mechanism. 
The control circuit 7' compares a detected pressure signal Qp1 from the 
pressure detector 4 on the upstream side of the orifice with the set 
pressure Qps, and inputs a control signal Qy in the control valve drive 3 
to bring the pressure difference to zero, thereby controlling the control 
valve 2. 
The direct touch type metal diaphragm valve unit, which constitutes the 
variable orifice 5, includes a stainless steel valve main body 12, a 
diaphragm 13 made of stainless steel or a nickel-cobalt alloy, and a pulse 
motor type drive 6 to press down the diaphragm 13 as shown in FIG. 2. The 
valve main body 12 is provided with a fluid inlet 12a, a valve seat 12b, a 
valve chamber 12c, and a fluid outlet 12e. 
When the pulse motor 14 is set to take the initial position, the diaphragm 
13 is pressed down by a guide slider 18 and a diaphragm presser 16 through 
a ball screw mechanism 19 against the elastic force of springs 17 and 15 
until the diaphragm 13 sits on the valve seat 12b for closing the valve. 
In the next step, when an orifice control signal Qz is input in the pulse 
motor 14, the pulse motor 14 rotates in such direction that the guide 
slider 18 is pulled up through the ball screw mechanism 19 and the 
diaphragm presser 16 is pushed upward by the elastic force of the spring 
15. 
As a result, the diaphragm 13 returns in the upward direction to the 
original position, moving away from the valve seat 12b, thereby forming a 
ring-shaped fluid passage (orifice) between the valve seat 12b and the 
diaphragm 13. 
In this embodiment, the pulse motor 14 is a so-called stepping motor 
producing 50,000 pulses per revolution. The ball screw mechanism 19 has a 
thread pitch of 0.5 mm/revolution. 
In that arrangement, inputting one pulse in the pulse motor 14 moves or 
displaces the diaphragm by 10 nm, thereby permitting a very high precision 
control of the opening degree of the orifice. In FIG. 2, the reference 
number 20 indicates a coupling; 21, a bearing; and 22, the shaft of the 
ball screw mechanism. 
The mass flow meter 9 measures the gas flow rate Q on the downstream side 
of the variable orifice 5 and outputs the detected flow rate signal Qx. 
The vacuum chamber 10, vacuum gauge 10a, and vacuum pump 11 are included in 
the semiconductor manufacturing facilities. The pressure within the vacuum 
chamber 10 is generally maintained at a vacuum of several torr. 
When testing flow rate characteristics of the variable orifice 5, an 
orifice control signal Qz was first input to set the opening degree of the 
variable orifice 5 to a specific level. Then, nitrogen gas N.sub.2 with a 
pressure of 6.0 kg/cm.sup.2 was fed into the gas inlet 8a. After that, the 
set pressure signal Qps was set to a level between 0 and 3 kgf/cm.sup.2 
abs for controlling the pressure control valve 2 while the flow rate of 
N.sub.2 on the downstream side of the orifice 5 was measured by the mass 
flow meter 9. 
As mentioned, the vacuum chamber 10 has a cubic volume of 9.26 liters and 
is maintained at a vacuum of about 1 torr by the vacuum pump 11. 
FIG. 3 shows the relationship between the upstream pressure, that is, the 
set pressure Qps and the gas flow rate Q sccm on the downstream side of 
the orifice where an area of the ring-shaped gap (fluid passage) of the 
variable orifice 5 was made equal by the orifice control signal Qz to the 
sectional area of an orifice with a circular bore section 0.14 mm in 
diameter. 
It is understood that sccm is the flow rate in cubic centimeters 
(cc)/minutes in terms of the standard state. 
FIG. 4 shows the relationship between the pressure of the upstream side of 
the orifice 5, that is, the set pressure Qps, and the gas flow rate Q sccm 
on the downstream side of the orifice where the orifice control signal Qz 
was changed to make an area of the ring-shaped gap (fluid passage) of the 
variable orifice equal to the sectional area of an orifice with a circular 
bore section 0.25 mm in diameter. 
As is evident from FIG. 3 and FIG. 4, there exists a relationship between 
the flow rate Q and the upstream pressure P1 represented approximately by 
Q=KP1 in the region where the pressure P1 on the upstream side of the 
variable orifice is not lower than 0.5 kgf/cm.sup.2 abs with the pressure 
P2 on the downstream side of the orifice being 1 torr or about 133.3 Pa. 
In other words, it is shown that the ring-shaped fluid passage (or the gap) 
between the diaphragm and the valve seat of the direct touch type metal 
diaphragm valve unit having a construction as shown in FIG. 2 has almost 
the same pressure-flow rate control characteristics as the so-called fixed 
orifice. 
FIG. 5 indicates the flow rate characteristics of the variable orifice 5 
and shows the relationship between the working stroke L of the variable 
orifice 5 (or length of the gap of the diaphragm 13) and the gas flow rate 
Q on the downstream side of the orifice under the conditions that the 
pressure P1 at the upstream side of the variable orifice 5 in the testing 
apparatus shown in FIG. 1 is held at 0.5 kgf/cm.sup.2 abs and the 
downstream pressure P2 at a vacuum degree of 1 torr. 
The experiment showed that the relationship between the stroke L (mm) and 
the flow rate Q (sccm) is such that they are almost linearly proportional 
to each other. The relationship is always observed where the working 
stroke L is between 0 and about 0.12 mm. 
FIG. 6 is a diagram depicting the relationship between the working stroke L 
(mm) of the variable orifice 5 and the orifice bore or diameter .O 
slashed. (mm), where each orifice diameter is calculated on the basis of 
each flow rate in FIG. 5 assuming that the orifice was circular in bore 
shape. It was found that the relationship between the stroke L (mm) and 
the orifice bore (mm) is always observed, i.e., is reproducible. 
That is, as is clear from FIG. 5 and FIG. 6, there always exists a fixed 
correlation between the working stroke L (mm) of the variable orifice 5 
and the flow rate Q (sccm) or between the working stroke L and the orifice 
bore diameter .O slashed. (mm). Therefore, it is possible to accurately 
bring the diameter .O slashed. (mm) of the variable orifice or the flow 
rate Q (sccm) to a desired level by changing the stroke L (mm). The 
present invention serves the purpose of a reliable variable orifice. 
The present invention was developed on the basis of the results of the 
testing of the pressure-flow rate characteristics with the direct touch 
type metal diaphragm valve unit shown in FIG. 2 serving as a variable 
orifice. The present invention provides a pressure-type flow rate control 
apparatus comprising an orifice 5, a control valve 2 provided on the 
upstream side of the orifice 5, a pressure detector 4 provided between the 
control valve 2 and the orifice 5, and a control unit 7, to calculate a 
fluid flow rate Q on the basis of a pressure P1 detected by the pressure 
detector 4 with the equation Q=KP1 (K=constant) and to output in a drive 
13 for the control valve 2 the difference between the set flow rate signal 
Qs and the calculated flow rate signal Q as control signal Qy, wherein the 
pressure P1 on the upstream side of the orifice is regulated by actuating 
the control valve 2 for controlling the flow rate Q of the fluid 
downstream of the orifice with the ratio P2/P1, of the pressure P2 on the 
downstream side of the orifice to the upstream pressure P1, maintained at 
not higher than the ratio of the critical pressure of the controlled 
fluid, characterized in that a direct touch type metal diaphragm valve 
unit functions as the orifice 5 and that the ring-shaped gap between the 
valve seat 12b and the diaphragm 13 serves as variable orifice 5. 
In one embodiment the variable orifice 5 is one in which the ring-shaped 
gap is regulated by a pulse motor type drive. The pulse motor drive may 
comprise a stepping motor and a ball screw mechanism. 
The variable orifice 5 is one in which the ring-shaped gap is regulated by 
a piezoelectric element type drive. 
The working stroke L of the diaphragm 13 ranges from 0 to 0.12 mm. 
The area of the ring-shaped gap (fluid passage) between the valve seat 12b 
and the diaphragm 13 is equal to a sectional area of a circular bore of 
0.14 to 0.25 mm in diameter.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Embodiments of the present invention will now be described with reference 
to the drawings. 
FIG. 7 is a schematic diagram of a pressure type flow rate control 
apparatus according to the present invention. In FIG. 7, reference numeral 
1 indicates a pressure type flow rate control apparatus; 2, a pressure 
control valve; 3, a valve drive; 4, a pressure detector; 5, a variable 
orifice; 6, an orifice drive; 7, a control unit; 10a, a vacuum gauge; 11, 
a vacuum pump; Qy, a control signal for control valve 2; Qp1, a detected 
pressure signal; Qz, an orifice control signal; Qs, a set flow rate 
signal; and Qos, a signal for setting the opening degree of orifice 5. 
Pressure control valve 2 in FIG. 7 is a direct touch type metal diaphragm 
valve unit of the construction shown in FIG. 8, identical to that 
disclosed in unexamined Japanese patent publication No. 8-338546. 
Pressure detector 4 is a semiconductor strain gauge, which is fixed in a 
pressure detector receiving hole 12d of pressure control valve 2. 
The direct touch type metal diaphragm valve unit and pulse motor type drive 
6 shown in FIG. 2 serve as the variable orifice 5 and its drive 6. The 
variable orifice 5 and the drive 6 have the same construction as those 
shown in FIG. 2; accordingly the description thereof is omitted. 
The drive 6 is not limited to a pulse motor drive, and may be of the 
piezoelectric element type or the solenoid type. 
The operation of the pressure flow control system will now be explained. 
First, the set flow rate signal Qs and the orifice opening degree setting 
signal Qos are input into the control unit 7. 
Then, when the gas with a specific pressure P1 is fed into the gas inlet 
8a, the detected pressure signal Qp1, which corresponds to the upstream 
pressure P1 detected by the pressure detector 4, is input into the control 
unit 7, in which the flow rate Q=KP1 is calculated. 
Control unit 7 outputs a control signal Qy for the control valve 
corresponding to the difference between the set flow rate signal Qs and 
the calculated flow rate Q, whereby the pressure control valve 2 is opened 
or closed in such a direction that the difference between Qs and Q is 
reduced. 
Furthermore, if the bore of the variable orifice 5 is to be changed to 
switch over to some other control flow range, the setting of the orifice 
opening degree setting signal Qos is changed. Thus, the orifice control 
signal Qz is changed. As a result, the orifice drive 6 changes the working 
stroke L of the diaphragm 13, which in turn changes the orifice bore .O 
slashed. (mm) accordingly. 
In the embodiment shown in FIG. 7, the working stroke L is not subjected to 
what is called feedback control. Needless to say, the working stroke L of 
the orifice 6 may be detected and the detected value may be fed back to 
control unit 7 to effect feedback control of the working stroke L. 
Also, the embodiment illustrated in FIG. 7 is not provided with a 
correction circuit based on gas temperature, or an alarm circuit or a gas 
supply cut-off circuit which is activated when the pressure P2 on the 
downstream side of orifice rises, with the value of P2/P1 approaching (or 
exceeding) a critical value, unlike the prior art control apparatus shown 
in FIG. 11. Needless to say, those circuits may be provided. 
The control unit 7 shown in FIG. 7 is provided with a circuit which makes a 
correction so that the calculated value Q=KP1 of the flow rate Q may be 
equal to the pressure-flow rate curve as shown in FIG. 3 or FIG. 4 and 
with a storage unit to store data necessary to make such a correction. 
EXAMPLE 
FIG. 9 shows a main part of the valve main body 12 of the direct touch type 
metal diaphragm valve unit forming the variable orifice 5 used in the 
present invention. FIG. 10 is an enlarged view of the part indicated by 
the letter B in FIG. 9. 
The inside diameter .O slashed..sub.1 of the valve chamber 12c provided in 
the valve main body 12 is 15 mm. The inside diameter .O slashed..sub.2 of 
the fluid inlet passage is 0.4 mm. The valve seat 12b is 3 mm .O slashed. 
in outside diameter and the fluid outlet passage is 2.5 mm .O slashed. in 
inside diameter. 
Effects of the Invention 
The present invention uses a direct touch type metal diaphragm valve unit 
as a variable orifice in a pressure type flow control, and switches the 
control flow rate range by changing the working stroke of the diaphragm. 
This permits simplification of the construction of the orifice and 
eliminates all mechanically sliding parts in the fluid path, minimizing 
the generation of dust and particles to a nearly negligible level, unlike 
the prior art needle-type variable orifice. 
Using the direct touch type metal diaphragm valve unit as a variable 
orifice substantially reduces so-called dead spaces within the fluid path, 
and eliminates spaces in the fluid path that trap gas. That substantially 
improves gas replaceability. 
Furthermore, the orifice bore (that is, the flow rate range) can be 
accurately changed without difficulty through adjustment of the working 
stroke of the diaphragm. This significantly improves controlling 
efficiency as compared to the prior art, in which the orifice has to be 
replaced with another orifice having a different bore. As set forth above, 
the pressure type of flow rate control apparatus according to the present 
invention is useful especially in the gas supply system of semiconductor 
manufacturing facilities where super high-purity gases are handled.