Abstract:
A multi-hole orifice plate for flow control includes an orifice plate for controlling the flow rate of a fluid, wherein the opening area of one orifice necessary for the passage of a predetermined flow rate of fluid is divided to provide a plurality of orifices having a total opening area equal to said opening area.

Description:
TECHNICAL FIELD 
     The present invention relates to the improvement of an orifice plate for flow control and a pressure-controlled flow controller using the same. The invention particularly relates to a multi-hole orifice plate for flow control configured such that, in a pressure-controlled flow controller for use in a gas supply apparatus for a semiconductor manufacturing system, etc., the range of the pressure ratio P 2 /P 1  between the pressure P 1  upstream of the orifice and the pressure P 2  downstream of the orifice, at which the critical expansion conditions of an orifice-passing fluid are established, can be kept wide and stable, thereby making it possible to achieve high-precision flow control over a wide flow range; and also relates to a pressure-controlled flow controller using the same. 
     BACKGROUND ART 
     When the pressure ratio P 2 /P 1  between the pressure P 1  upstream of the orifice and the pressure P 2  downstream of the orifice is equal to or lower than the pressure ratio at which the critical expansion conditions of a gas are established, the orifice-passing gas flows at the speed of sound, and a variation in the pressure P 2  downstream of the orifice is not transmitted to the upstream side. As a result, when the orifice has a fixed diameter, regardless of the kind of gas, the flow rate of the gas passing through the orifice plate changes in direct proportion to the gas pressure P 1  upstream of the orifice. 
     Meanwhile, utilizing such characteristics of an orifice, a large number of fluid flow controller using an orifice have been developed. 
       FIG. 13  shows an example of the configuration of the pressure-controlled flow controller using an orifice previously disclosed by the present inventors. The flow controller  21  includes a control valve  22 , a pressure detector  23 , a temperature detector  24 , an orifice  25 , an arithmetic and control unit  26 , amplifiers  27   a  and  27   b , A/D converters  28   a  and  28   b , and the like (JP-A-8-338546). 
     The fluid pressure P 1  upstream of the orifice  25  is detected by the pressure detector  23  and input to the arithmetic and control unit  26 . In the arithmetic and control unit  26 , the flow rate Qc is calculated using the arithmetic expression Qc=KP 1 , while Qc is compared with the flow command value Qs, and the control signal Qy corresponding to the difference between the two, Qc−Qs, is input to an actuator  30  of the control valve  22 . 
     In addition, the control valve  22  is opened/closed by the control signal Qy in the direction that the difference Qc−Qs approaches zero, whereby the flow rate downstream of the orifice  25  is constantly maintained at the set flow rate (flow command value) Qs. 
     Further, the orifice  25  is formed by making a small hole having an inner diameter of 0.01 to 0.20 mm in a metal plate having a thickness of 0.02 to 0.20 mm by pressing, electric discharge machining, or etching. The diameter of the orifice is appropriately selected according to the required gas flow rate to be controlled. 
     Furthermore, although electric discharge machining or etching is generally used to form an orifice  25 , in some cases, the orifice is formed by so-called cutting using a drill in order to reduce the processing cost (JP-A-11-117915). 
       FIG. 14  shows the flow control characteristics of the pressure-controlled flow controller of  FIG. 13  in the case where the gas is nitrogen gas, and shows the case where the pressure downstream of the orifice  25  is atmospheric pressure. 
     As clearly shown in  FIG. 14 , at a range where the pressure P 1  upstream side exceeds about twice the pressure P 2  on the downstream side, the relation between the flow rate Qc and P 1  is kept linear, and Qc is in direct proportion to the pressure P 1  upstream of the orifice. Thus, by automatically controlling the pressure P 1  upstream of the orifice, the feedback control of the flow rate through the orifice is possible. Furthermore, in  FIG. 14 , A shows the flow control characteristics in the case where the diameter of the orifice is φ0.37 mm, while B is in the case of 0.20φ. 
     As clearly shown in  FIG. 14 , when the lines A and B are both within a range of P 2 &lt;0.5P 1  (i.e., P 2 /P 1 &lt;0.5), the linearity is well maintained, and the flow rate can be precisely controlled by regulating P 1 . 
     However, it is known that the actual lower limit of P 1  at which the critical expansion conditions of a gas are established (P 2 /P 1 &lt;0.5 or P 1 /P 2 &gt;2) (i.e., the lower limit of P 1  at which the linearity is maintained) slightly changes with the inner diameter of the orifice, and the greater the diameter of an orifice is, the smaller the range of P 2 /P 1  at which critical expansion conditions are established tends to be. That is, when P 2  is constant, the lower limit of the control range of P 1  increases. 
     Specifically, with an increase in the diameter of an orifice, the critical pressure ratio P 2 /P 1 &lt;0.5 decreases to about P 2 /P 1 &lt;0.45, and when P 2  is constant, the lower limit of the control range of P 1  increases, resulting in a smaller control range of P 1 . 
     In other words, when the diameter of an orifice increases with an increase in the controlled flow rate of a flow controller, the control range of the critical pressure ratio P 2 /P 1  becomes smaller. This results in various inconveniences in the case where, for example, the gas is supplied to a vacuum chamber of a semiconductor manufacturing system. 
     As mentioned above, the conventional pressure-type flow controller using an orifice plate provided with one orifice has the drawback that the pressure ratio P 2 /P 1  at which critical expansion conditions are established varies with an increase in the diameter of the orifice, resulting in the variation of the flow (pressure) control range. Therefore, in the technical field of pressure-controlled flow controllers applied to semiconductor manufacturing systems, there has been a strong demand for the advent an orifice plate for flow control, in which even when the diameter of an orifice changes, no variation occurs in the actual pressure ratio P 2 /P 1  at which critical expansion conditions are established; and also a pressure-controlled flow controller using the same. 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP-A-8-338546 
     Patent Document 2: JP-A-11-117915 
     SUMMARY OF INVENTION 
     Technical Problem 
     A main object of the present invention is to solve the problems of conventional orifice plates for flow control and pressure-controlled flow controllers using the same mentioned above, that is, the problems that the actual pressure ratio P 2 /P 1  at which critical expansion conditions are established varies (decreases) with an increase in the inner diameter of an orifice, whereby the control range of the pressure ratio P 2 /P 1  becomes smaller, and also the flow control accuracy of the pressure-controlled flow controller decreases, etc., and provide an orifice plate for flow control, which is configured such that even when the inner diameter of an orifice is increased with an increase in the fluid flow rate, the pressure ratio P 2 /P 1  in the actual flow control can always be kept constant, and also the production cost for the orifice plate can be reduced; and also provide a pressure-controlled flow controller using the same. 
     Solution to the Problems 
     First, with respect to an orifice plate having one orifice (hereinafter referred to as single-hole orifice plate), the present inventors examined the degree of actual variation of the ratio P 2 /P 1  between the pressure P 1  upstream of the orifice and the pressure P 2  downstream of the orifice (hereinafter referred to as pressure ratio P 2 /P 1 ), at which the critical expansion conditions of a fluid are established, in accordance with a change in the diameter φ of the orifice. 
       FIG. 1  is a system diagram of a testing device (flow measuring device) used for testing the pressure ratio P 2 /P 1 , flow characteristics and the like of a conventional single-hole orifice plate for flow control and the multi-hole orifice plate for flow control according to the present invention;  1  is a gas inlet,  2  is a pressure regulator,  3  is a pressure meter,  4  is a molblock flow meter,  5  is a pressure-controlled flow controller,  6  is a control valve,  7  is an orifice plate,  8  is a pressure detector upstream of the orifice,  9  is a pressure detector downstream of the orifice,  10  is a regulation valve for the pressure P 2  downstream of the orifice,  11  is an evacuation pump, P 1  is the pressure upstream of the orifice, and P 2  is the pressure downstream of the orifice. Furthermore, the maximum flow range F.S. is based on N 2  gas. 
     The orifice plates used for the test were the following three kinds: 50-μm-thick steel plates having orifices of φ=67 μm, φ=179 μm, and φ=250 μm, respectively. The orifice of φ=67 μm is used in a pressure-controlled flow controller  5  with a maximum flow range (full scale) F.S.=130 sccm, the orifice of φ=250 μm is used in a pressure-controlled flow controller  5  with a maximum flow range F.S. of 850 sccm, and the orifice of φ=250 μm is used in a pressure-type flow controller  5  with a maximum flow range F.S.=1600 sccm. 
     In the test, first, the pressure Po of the pressure meter  3  is regulated to 300 kPa abs by the pressure regulator  2 . Next, the set flow rate of the pressure-controlled flow controller  5  is set at 100% F.S. (a rated flow rate), and the evacuation pump  11  is operated. Subsequently, while regulating the regulation valve  10  upstream of the evacuation pump  11  to regulate the pressure P 2  downstream of the orifice, the gas flow rates in the molblock flow meter  4  and the pressure-type flow controller  5  are each measured. Furthermore, N 2  gas was used as the test gas. 
     Next, using the measured flow rate Qs of the molblock flow meter  4  as the reference value, the error (set point error (S.P. %)) of the measured flow rate Qc of each pressure-controlled flow controller  5  was calculated as (Qc−Qs)×100/Qs (S.P. %). Furthermore, the flow measurement was measured at 100%, 50%, 20%, and 10% of the set flow rate of the pressure-controlled flow controller  5 . 
     With respect to the three kinds of pressure-controlled flow controllers having different orifice diameters (F S. 130 sccm, F.S. 850 sccm, F.S. 1600 sccm), each of  FIG. 2 ,  FIG. 3 , and  FIG. 4  shows the relation between the pressure ratio (P 2 /P 1 ) and the set point error (S.P. %) by using the set input (set flow rate) to each pressure-controlled flow controller  5  as a parameter. From the comparison of  FIG. 2  to  FIG. 4 , it was confirmed that with an increase in the flow range (rated flow rate S.P.) and a resulting increase in the orifice diameter, the set point error (S.P. %) approaches zero, that is, the range of P 2 /P 1  at which critical expansion conditions are established becomes smaller. 
     In addition, from the flow control results obtained using the testing device of  FIG. 1 ,  FIG. 5 ,  FIG. 6 , and  FIG. 7  each show the relation between the set flow rate (%) of the pressure ratio P 2 /P 1  when the set point error (S.P. %) is within ±1% and the flow linear error (F.S. %) based on the controlled flow rate at the time of 100% setting, that is, the relation between the set flow rate (%) and the linearity error (F.S. %). It was shown that within the range of the pressure ratio P 2 /P 1  at which critical expansion conditions are established, the flow linear error F.S. % of the single-hole orifice plate is within ±1% F.S. 
     The present invention has been created based on the results of the flow characteristic tests as shown in  FIG. 2  to  FIG. 7 . Noting that the smaller the diameter of the orifice of a single-hole orifice plate, the greater the range of the pressure ratio P 2 /P 1  at which critical expansion conditions are established, the present inventors have conceived that in the case when the controlled flow rate increases, rather than increasing the diameter of the orifice of a single-hole orifice plate according to the flow rate range as conventional, when the number of orifices of a multi-hole orifice plate on which each orifice has a small diameter is changed, thereby corresponding to the increase of the controlled flow rate, the flow rate increase can be managed while the range of the pressure ratio P 2 /P 1  at which the critical expansion conditions of the orifice-passing fluid are established is being kept maximum and constant. 
     A first aspect of the present invention is basically configured such that in an orifice plate for flow control, the opening area of one orifice necessary for the passage of a predetermined flow rate of fluid is divided to provide a plurality of smaller orifices having a total opening area equal to said opening area. 
     According to a second aspect of the present invention, in the first aspect, the plurality of orifices are formed by pressing. 
     According to a third aspect of the present invention, in the above first aspect, the orifice plate has a thickness of 20 to 200 μm and serves as an orifice plate for a pressure-controlled flow controller. 
     According to a fourth aspect of the present invention, in the above first aspect, the orifice plate has a thickness of 20 to 200 μm, the plurality of orifices each have a hole diameter of 0.010 to 0.200 mm, and the number of the plurality of orifices is 2 to 100. 
     According to a fifth aspect of the present invention, in the above first aspect, the plurality of orifices each has a longitudinal plane shape including a rectangular part and a trapezoidal part. 
     According to a sixth aspect of the present invention, in the above fifth aspect, a portion on the backside of the orifice plate having the plurality of orifices bored therein is finished by polishing. 
     According to a seventh aspect of the present invention, in the first aspect, when the ratio P 2 /P 1  between the pressure (P 1 ) upstream of the orifice and the pressure (P 2 ) downstream of the orifice is equal to or lower than the pressure ratio at which the critical expansion conditions of a gas are established, the gas flow rate Q changes in direct proportion to the gas pressure (P 1 ) upstream of the orifice. 
     An eighth aspect of the present invention is basically configured such that in a pressure-controlled flow controller, the multi-hole orifice according to any one of the preceding aspects is used as an orifice plate for flow control. 
     Advantageous Effects of Invention 
     In the present invention, in an orifice plate for flow control configured such that a fluid flows under critical expansion conditions, and the flow rate Q of the orifice-passing fluid is in direct proportion to the pressure P 1  upstream of the orifice, the opening area of one orifice necessary for the passage of a desired flow rate of fluid is divided to provide a plurality of orifices having a total opening area equal to said opening area. 
     As a result, even when the orifice opening area increases with an increase in the controlled flow rate, the pressure ratio P 2 /P 1  between the pressure P 1  upstream of the orifice and the pressure P 2  on the downstream side at which critical expansion conditions are established does not actually vary and is maintained at a constant value, whereby the reduction of the control range of P 2 /P 1  (flow control range) can be effectively prevented. In addition, in a pressure-controlled flow controller using the orifice plate, it is made possible to increase the flow control range and improve the control precision. 
     In addition, since the plurality of orifices can be easily formed by pressing, the orifice plate can be produced at a lower cost, compared with the conventional production by laser beam machining, etc. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a system diagram of a testing device used for testing the orifice flow characteristics. 
         FIG. 2  is a line chart showing the relation between pressure ratio P 2 /P 1  and set point error (S.P. %) of a pressure-controlled flow controller with F.S. 130 sccm. 
         FIG. 3  is a line chart similar to  FIG. 2  with F.S. 850 sccm. 
         FIG. 4  is a line chart similar to  FIG. 2  with F.S. 1600 sccm. 
         FIG. 5  is a line chart showing the relation between the set flow rate (%) at a pressure ratio P 2 /P 1  at which the set point error S.P. % is within ±1% and the error relative to full scale (linearity error) (F.S. %), with respect to a pressure-controlled flow controller with F.S. 130 sccm. 
         FIG. 6  is a line chart similar to  FIG. 5  with F.S. 850 sccm. 
         FIG. 7  is a line chart similar to  FIG. 5  with F.S. 1600 sccm. 
         FIG. 8A  shows a plane view of an example of the multi-hole orifice plate according to the present invention,  FIG. 8B  shows a back view of  FIG. 8A , and  FIG. 8C  shows a c-c sectional view of  FIG. 8B . 
         FIG. 9A  is a plan view showing another example of the multi-hole orifice plate according to the present invention,  FIG. 9B  shows a back view of  FIG. 9A  and  FIG. 9C  shows a sectional view of  FIG. 9B . 
         FIG. 10  is a plan view showing another example of the multi-hole orifice plate according to the present invention. 
         FIG. 11  is a line chart showing relation between the pressure ratio P 2 /P 1  and the set point error (S.P. %) similar to  FIG. 2  to  FIG. 4  in the case of using the multi-hole orifice plate of  FIG. 10 . 
         FIG. 12  is a line chart showing relation between the set flow rate (%) and the linearity error (F.S. %) similar to  FIG. 5  to  FIG. 7  in the case of using the multi-hole orifice plate of  FIG. 10 . 
         FIG. 13  is a block diagram of a known pressure-controlled flow controller. 
         FIG. 14  is a line chart showing the flow control characteristics of the pressure-controlled flow controller of  FIG. 13 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, the present invention will be described based on embodiments and drawings. 
       FIG. 8A ,  FIG. 8B , and  FIG. 8C  show an example of the orifice plate for flow control according to the present invention;  FIG. 8A  is a plan view,  FIG. 8B  is a back view, and  FIG. 8C  is a c-c sectional view. 
     In  FIG. 8A , a total of fifteen orifices  12  with a diameter of 0.085 mm are provided in an orifice plate  7   a  with an outer diameter of 3.5 mm and a thickness of 0.05 mm. 
     In addition, the longitudinal plane shape of the orifice  12  is formed by pressing to have a rectangular part  12   a  and a trapezoidal part  12   b  as shown in  FIG. 8C , and the depth of the orifice  12  is 0.05 mm, which is the same as the thickness of the orifice plate  7   a.    
     Furthermore, the portion on the back side of the orifice plate  7   a , where the orifices  12  are provided, is polished in a narrow shape to form a polished surface  12   c , and the front and back of the orifice plate  7   a  are distinguished by the polished surface  12   c.    
       FIG. 9A ,  FIG. 9B , and  FIG. 9C  shows another example of the multi-hole orifice plate  7   a  for flow control according to the present invention, which is the same as the multi-hole orifice plate for flow control of  FIG. 8A ,  FIG. 8B , and  FIG. 8C , except that the number of orifices  12  is 5, and the orifices  12  each has a diameter of 0.135 mm. 
       FIG. 10  is an enlarged plan view showing another example of the multi-hole orifice plate  7   a  for flow control according to the present invention, in which thirty-seven orifices  12  each having a diameter φ of 79 μm (0.079 mm) are provided. 
     The outer diameter, the thickness, and the like of the orifice plate  7   a  are the same as those of the orifice plates  7   a  of  FIG. 8A  and  FIG. 9A . 
     The diameter φ of 79 μm of the orifice  12  is equal to the diameter of the orifice of a single-hole orifice plate  7  of Pressure-Controlled Flow Controller F180 manufactured by Fujikin Incorporated, which is a control orifice plate with a rated flow rate of 180 sccm (F.S.). 
     Accordingly, the F.S. flow rate of the multi-hole orifice plate  7   a  for flow control of  FIG. 10  is equivalent to 180 sccm×37=6.660 sccm. 
       FIG. 11  shows a curve similar to  FIG. 2 , showing the relation with the pressure ratio P 2 /P 1 , wherein the single-hole orifice plate  7  of the tester of  FIG. 1  is replaced by the multi-hole orifice plate  7   a  of  FIG. 10 . 
     As is clear from the comparison between  FIG. 11  and  FIG. 2 , in the case of the multi-hole orifice plate  7   a  of the present invention, the pressure ratio P 2 /P 1  value at which the set point error (S.P. %) is within a range of ±1% does not fall below 0.45 even at the time of 10% input (10% of the set flow rate). At the time of 100% input (100% of the set flow rate), the P 2 /P 1  value of about 0.52 can be obtained. 
     In contrast, in  FIG. 3  in which the maximum flow rate is 850 sccm, the pressure ratio P 2 /P 1  at the time of 100% input is about 0.42. Even in  FIG. 4  in which the maximum flow rate is 1600 sccm, the pressure ratio P 2 /P 1  at the time of 100% input is about 0.40. This shows that in the case where the multi-hole orifice plate  7   a  of the invention of the present application is used, the range of P 2 /P 1  whereat critical expansion conditions are established can be expanded. In addition, the reason why the theoretical pressure ratio P 2 /P 1  whereat critical expansion conditions are established is slightly different from the pressure ratio P 2 /P 1  at which critical expansion conditions are established in actual measurement as above has not been theoretically analyzed yet and is currently under examination. However, it is assumed that it is affected by differences in the flowing condition of the fluid on the orifice outlet side. 
       FIG. 12  is a line chart similar to  FIG. 5  in the case of using the multi-hole orifice plate  7   a  of the present invention. The line chart shows the relation between the set flow rate (%) at a pressure ratio P 2 /P 1  whereat the set point error (S.P. %) is within ±1% and the error relative to the control flow at the time of 100% setting (flow linear error) (F.S. %). 
     As clearly shown in  FIG. 12 , it has been confirmed that the multi-hole orifice plate  7   a  of the present invention also results in a flow linear error (F.S. %) within ±1% F.S. 
     In addition, the pressure-controlled flow controller according to the present invention is given by replacing the orifice plate of the Pressure-Controlled Flow Controller F180 manufactured by Fujikin Incorporated, the pressure-controlled flow controller shown in  FIG. 13 , or the like with the orifice plate of the present invention. Thus, the detailed description will be omitted. 
     As described above, in the multi-hole orifice plate for flow control and the pressure-controlled flow controller using the same according to the present invention, by adjusting the number of orifices  12  based on the controlled flow rate even when the controlled flow rate increases, the range of the pressure ratio P 2 /P 1  at which critical expansion conditions are established can be maintained wide and constant, thereby making it possible to perform high-precision flow control stably over a wide range. 
     INDUSTRIAL APPLICABILITY 
     The multi-hole orifice plate according to the present invention can be applied not only to a pressure-controlled flow controller but also to any orifices in ordinary orifice devices inserted into a pipeline to control the fluid flow, fluid diverters, etc. 
     REFERENCE SIGNS LIST 
     
         
           1 : Gas inlet 
           2 : Pressure regulator 
           3 : Pressure meter 
           4 : Molblock flow meter 
           5 : Pressure-type flow controller 
           6 : Control valve 
           7 : Orifice plate (single-hole) 
           7   a : Multi-hole orifice plate 
           8 : Pressure detector on the orifice upstream side 
           9 : Pressure detector on the orifice downstream side 
           10 : Regulation valve for the pressure P 2  on the orifice downstream side 
           11 : Evacuation pump 
         P 1 : Pressure on the orifice upstream side 
         P 2 : Pressure on the orifice downstream side 
         P O : Pressure on the gas supply source side 
           12 : Orifice