Patent Publication Number: US-7716993-B2

Title: Gas flow rate verification unit

Description:
TECHNICAL FIELD 
   The present invention relates to a gas flow rate verification unit that verifies a flow rate of a flow rate control device used in a gas system in a semiconductor manufacturing process. 
   BACKGROUND ART 
   In a film deposition device or a dry etching device in a semiconductor manufacturing process, special gas such as silane or phosphine, corrosive gas such as chlorinated gas, combustible gas such as hydrogen gas, or the like are used. 
   Flow rates of these gases should strictly be controlled. 
   The reason of this is because the gas flow rate directly affects a quality of the process. Specifically, the gas flow rate greatly affects a film quality in a film deposition process or a quality of a circuit processing in an etching process, whereby a yield of a semiconductor product is determined according to precision of the gas flow rate. 
   Another reason is that most of these gases are harmful to a human body and environment or have explosiveness. These gases are not allowed to be directly disposed in the atmosphere after they are used, so that a device used in a semiconductor manufacturing process should be provided with detoxifying device in accordance with a type of gas. However, the detoxifying device described above has limited processing capacity in general. Therefore, when the flow rate more than the allowable value flows, it cannot perfectly process the gas, so that the deleterious gas might be flown out in the atmosphere or the detoxifying device might be broken. 
   Moreover, since these gases, especially high-purity dust-free gas that can be used in a semiconductor manufacturing process, are expensive, and limitation is imposed on some gases for their use due to natural deterioration, they cannot be preserved in a large quantity. 
   In view of this, a known mass flow controller serving as a flow rate control device has conventionally been mounted in a semiconductor manufacturing process circuit so that a gas flows in an optimum flow rate for every type of gas. The mass flow controller described above changes the set flow rate by changing the applied voltage thus responding to changes in a process recipe. 
   However, these gases used in the semiconductor manufacturing process, especially the material gas for the film deposition among the so-called process gases, might cause precipitation of solid substances in a gas line due to its characteristics, so that the flow volume might be changed. The mass flow controller is formed with a capillary tube inside in order to supply a fixed flow rate with high precision. Even a small amount of precipitation of the solid substance on this portion could deteriorate the flow precision of the gas to be supplied. Further, since a gas with high corrosivity for an etching process or the like is flown, the corrosion of the mass flow controller cannot be avoided even if a material having a high corrosion resistance such as a stainless material or the like is used. As a result, a secular deterioration could occur, deteriorating the flow precision. 
   As described above, in the mass flow controller, the relationship between the applied voltage and the actual flow rate changes, so that the actual flow rate might possibly change. Therefore, the mass flow controller needs to be periodically subject to flow rate verification calibration. 
   The flow rate verification of the mass flow controller is basically performed by using a film flowmeter. However, this measurement is performed with a part of a pipe removed. After the measurement, the pipe should be assembled in the original state, and a leakage check should be executed. Therefore, the work is very time-consuming. Accordingly, it is ideal that the flow rate verification can be executed without removing the pipe. 
   As a method for performing the flow rate verification with the pipe assembled, there has been a method, as disclosed in Patent Document 1, in which a gas flow rate verification unit U is mounted downstream of a mass flow controller so as to constitute a gas mass flow measurement system.  FIG. 19  shows a block diagram of a gas mass flow verification system. 
   As shown in  FIG. 19 , the gas mass flow verification system includes the gas flow rate verification unit U that includes a valve component  151 , a chamber  153 , a transducer assembly  154 , and a valve component  152  and is mounted downstream of a mass flow controller  10 . The chamber  153  has a known volume. The transducer assembly  154  is connected to a gas flow line  150  downstream of the chamber  153 , and the valve components  151  and  152  are connected to the gas flow line  150  positioned downstream and upstream of the transducer assembly  154 , thereby making the volume constant. The transducer assembly  154  outputs a signal directly indicating a PV/RT on the basis of pressure and temperature between the valve components  151  and  152 . Here, P denotes a pressure, V denotes a volume, R denotes a gas constant, and T denotes an absolute temperature. 
   The gas mass flow verification system described above measures the actual flow rate of the mass flow controller  10  on the basis of the signal indicating the PV/RT outputted from the transducer assembly  154  without individually measuring pressure and temperature of the chamber  153 . The gas mass flow verification system compares the actual flow rate with the preset flow rate of the mass flow controller  10 , thereby verifying the flow rate of the mass flow controller  10 . 
   Patent Document 1: Japan Patent No. 3022931 
   DISCLOSURE OF INVENTION 
   Problems to be Solved by the Invention 
   However, when the inventors conducted an evaluation test with the conventional gas flow rate verification unit U, it is confirmed that the conventional unit U has wide variations in the verification precision regarding flow rate control of the mass flow controller  10 , having less reliability as shown in  FIG. 10  ( iii ). 
   Specifically, the inventors conducted the evaluation test by measuring the flow rate of the mass flow controller  10  with a high flow precision flowmeter and comparing the measured value with the flow rate measured by the conventional flow rate verification unit U. As shown in  FIG. 10(   iii ), in a case that the flow rate of N 2  gas to be supplied to the mass flow controller  10  is 100 sccm in the conventional unit U, an error between the flow rate calculated by the conventional unit U and the flow rate controlled by the mass flow controller  10  is 0.012%. On the other hand, in a case where 500 sccm of N 2  gas is supplied to the mass flow controller  10 , the error between the flow rate calculated by the conventional unit U and the actual flow rate supplied to the mass flow controller is 1.150%. Consequently, as the flow rate increases in the conventional unit U, the error of the flow rate measured by the conventional unit U becomes larger, resulting in less reliability. 
   In this way, if the verification precision of the gas flow rate verification unit U varies depending upon the flow rate controlled by the mass flow controller  10 , there is a possibility that the gas flow rate verification unit U measures 500 sccm and recognize in error that the flow rate of the mass flow controller  10  is accurate even though the mass flow controller  10  does not control the gas flow rate as the preset flow rate (500 sccm) when the mass flow controller  10  is applied a voltage for controlling a large flow rate (500 sccm). Moreover, there is a possibility that the flow rate verification unit U fails to measure the flow rate of 500 sccm and performs an unnecessary calibration to the mass flow controller  10  even though the mass flow controller  10  precisely controls the gas flow to the preset flow rate (500 sccm). Such a defect directly affects the yield in the semiconductor manufacturing process or the like and is very problematic. 
   The present invention has been made in view of the above circumstances and has an object to overcome the above problems and provide a gas flow rate verification unit capable of enhancing reliability of the flow rate verification. 
   Means for Solving the Problems 
   A gas flow rate verification unit according to the present invention is provided with the following configuration. 
   (1) A gas flow rate verification unit mounted downstream of a flow control device comprises a first cutoff valve that is connected to the flow rate control device for inputting a gas, a second cutoff valve for outputting the gas, a communication member communicating the first and second cutoff valves with each other, a pressure detector detecting a pressure of the gas supplied between the first and second cutoff valves, a temperature detector detecting a temperature of the gas supplied between the first and second cutoff valves, and a control device verifying a flow rate of the gas flowing through the flow rate control device by using a result of the pressure detected by the pressure detector and a result of the temperature detected by the temperature detector, and a volume from the first cutoff valve to the second cutoff valve is not more than a volume from an outlet of the flow rate control device to the first cutoff valve. 
   (2) In the present invention according to (1), preferably, the communication member is a channel block in which a first port communicating with an output of the first cutoff valve, a second port communicating with an input port of the second cutoff valve, and a third port communicating with the pressure detector are open at a same side face, and an internal channel that communicates the first port, the second port, and the third port with one another is formed. 
   (3) In the present invention according to (2), preferably, the temperature detector is a bar-like temperature sensor, and the channel block has a mounting portion in which the temperature sensor is mounted. 
   (4) In the present invention according to any one of (1) to (3), preferably, the gas flow rate verification unit is housed in a gas box including a gas unit in which the flow rate control device is mounted. 
   (5) In the present invention according to any one of (1) to (4), preferably, the control device includes a volume measuring device for measuring a volume from the flow rate control device to the first cutoff valve by calculating an increasing pressure value per a unit time from the time when the pressure detector detects a fixed initial pressure to the time when the pressure detector detects a target pressure when the gas is filled between the flow rate control device and the second cutoff valve by the target pressure, detecting the temperature of the gas upon the pressure detection by the temperature detector, measuring a volume of the gas in a tank (tank volume) from the flow rate control device to the second cutoff valve by use of the pressure increase value and the gas temperature, and subtracting the volume from the first cutoff valve to the second cutoff valve from the tank volume. 
   (6) In the present invention according to any one of (1) to (4), preferably, the second cutoff valve is connected to a vacuum pump, and the control device includes a volume measuring device for measuring the volume from the flow rate control device to the first cutoff valve by measuring the tank volume from the flow rate control device to the second cutoff valve by use of a pressure change and a temperature change between the first and second cutoff valves when a portion between the first and second cutoff valves is evacuated by the vacuum pump and then the gas filled between the flow rate control device and the first cutoff valve is discharged between the first and second cutoff valves and by subtracting the volume from the first cutoff valve to the second cutoff valve from the tank volume. 
   (7) In the present invention according to any one of (1) to (4), preferably, the control device samples pressure values detected by the pressure detector at a predetermined interval to calculate a gradient between a newly sampled pressure value and a sampled pressure value prior to the newly sampled pressure value and verifies a flow rate of the gas when the calculated gradient becomes within a measurable range. 
   (8) In the present invention according to any one of (1) to (4), preferably, the control device samples the pressure values detected by the pressure detector at a predetermined interval to calculate a correlation coefficient with respect to the gradient of the newly sampled pressure value and verifies the flow rate of the gas when the calculated correlation coefficient becomes within a measurable range. 
   Advantages of the Invention 
   In the gas flow rate verification unit of the present invention which has the above-mentioned configuration, the volume from the first cutoff valve to the second cutoff valve is not more than the volume from the outlet of the flow rate control device to the first cutoff valve, so that the pressure between the first and second cutoff valves is likely to be uniform even if the gas flow rate supplied from the flow rate control device to the portion between the first and second cutoff valves is changed. Therefore, the flow rate verification unit of the present invention can correctly detect the pressure and the temperature between the first and second cutoff valves by the pressure detector and the temperature detector and verify the gas flow rate by use of the results of the pressure detection and the temperature detection. Consequently, according to the gas flow rate verification unit of the present invention, an error of the measured flow rate with respect to changes in the controlled flow rate is reduced, so that reliability of the flow rate verification can be enhanced. 
   In the gas flow rate verification unit of the present invention which has the above-mentioned configuration, the first cutoff valve, the second cutoff valve, and the pressure detector are mounted in the channel block to be integrated in a manner that an output port of the first cutoff valve is communicated with a first port of the channel block, an input port of the second cutoff valve is communicated with a second port of the channel block, and the pressure detector is communicated with a third port of the channel block. Thereby, the volume between the first and second cutoff valves can be reduced to downsize the gas flow rate verification unit. Further, by reducing the volume between the first and second cutoff valves is reduced, time taken for the pressure between the first and second cutoff valves to reach the target pressure can be shortened, thereby shortening the verification time of the gas flow rate. 
   In the gas flow rate verification unit of the present invention, a bar-like temperature sensor is mounted in a mounting portion of the channel block to measure the temperature of the channel block, thereby detecting the temperature of the gas supplied between the first and second cutoff valves. Therefore, the temperature sensor can be mounted in the gas flow rate verification unit with the volume between the first second cutoff valves being reduced. 
   In the flow rate verification unit of the present invention with the above-mentioned configuration, the gas flow rate verification unit is housed in a gas box including a gas unit in which the flow rate control device is mounted, so that there is no need to change a structure or an external pipe of the gas box for arranging an installation space for the gas flow rate verification unit. Accordingly, the gas flow rate verification unit of the present invention can provide an excellent installation property. 
   In the flow rate verification unit of the present invention with the above-mentioned configuration, the control device includes a volume measuring device. The volume measuring device calculates an increasing pressure value per a unit time from the time when the pressure detector detects a fixed initial pressure to the time when the pressure detector detects a target pressure when the gas is filled between the flow rate control device and the second cutoff valve by the target pressure. Concurrently, in the volume measuring device, a gas temperature during the pressure detection is detected by the temperature detector. Then, after measuring a tank volume from the flow rate control device to the second cutoff valve by use of the pressure increase value and the gas temperature, the volume from the first cutoff valve to the second cutoff valve is subtracted from the tank volume to measure the volume from the flow rate control device to the first cutoff valve. Consequently, in the gas flow rate verification unit of the present invention, even if the volume from the outlet of the flow rate control device to the first cutoff valve varies depending upon a system structure in which the unit is mounted, the influence caused by the variation is eliminated, thereby precision in the gas flow rate verification unit can be kept satisfactory. 
   In the flow rate verification unit of the present invention with the above-mentioned configuration, the second cutoff valve is connected to a vacuum pump, and the control device is connected to the pressure sensor that detects the pressure between the outlet of the flow rate control device and the first cutoff valve. The control device includes the volume measuring device. In the volume measuring device, when the portion between the first cutoff valve and the second cutoff valve is evacuated by the vacuum pump, and then the gas filled between the flow rate control device and the first cutoff vale is discharged between the first and second cutoff valves, the tank volume from the flow rate control device to the second cutoff valve is measured by use of the pressure change and the temperature change between the first and second cutoff valves to subtract the volume from the first cutoff valve to the second cutoff valve from the tank volume, thus measuring the volume from the flow rate control device to the first cutoff valve. Consequently, according to the gas flow rate verification unit of the present invention, even if the volume from the flow rate control device to the first cutoff valve varies depending upon the system structure in which the unit is mounted, the influence caused by the variation is eliminated, so that the precision in the gas flow rate verification can be kept satisfactory. 
   In the flow rate verification unit of the present invention with the above-mentioned configuration, a gradient of the pressure value detected by the pressure detector or correlation coefficient with respect to the gradient of the pressure value are calculated, and the flow rate of the gas is verified when the calculated gradient or the correlation coefficient become within a measurable range. Therefore, the flow rate verification can be performed without waiting a dead time for stabilizing the pressure detector to detect a measurement start pressure, and accordingly the flow rate verification time can be shortened. 

   
       FIG. 1  is a schematic structural view of a gas box having a gas flow rate verification unit incorporated therein according to a first embodiment of the present invention; 
       FIG. 2  is a side view of a gas unit shown in  FIG. 1 ; 
       FIG. 3  is a side view of the gas flow rate verification unit shown in  FIG. 1 ; 
       FIG. 4  is a top view of the gas flow rate verification unit shown in  FIG. 1 ; 
       FIG. 5  is a sectional view of the gas flow rate verification unit taken along a line A-A in  FIG. 4 ; 
       FIG. 6  is an electric block diagram of a controller shown in  FIG. 1 ; 
       FIG. 7  is a flowchart showing a flow rate verification method executed by the gas flow rate verification unit according to the first embodiment; 
       FIG. 8  is a block diagram of an evaluation device; 
       FIG. 9  is a graph showing a relation between pressure and time wherein a longitudinal axis indicates the pressure and a lateral axis indicates the time; 
       FIG. 10  is a graph showing errors between a flow rate calculated by a gas flow rate verification system and a flow rate measured by a high precision flowmeter in each of three evaluation devices wherein black dots indicate errors in a case of the flow rate 100 sccm while black triangles indicate errors in a case of the flow rate 500 sccm, and each evaluation test is conducted under the same condition (5-13 kPa); 
       FIG. 11  is a block diagram showing one example of a gas supply integration unit provided with a gas flow rate verification unit according to a second embodiment of the present invention; 
       FIG. 12  is an electric block diagram of a controller shown in  FIG. 11 ; 
       FIG. 13  is a flowchart showing a gas flow rate verification method executed by a gas flow rate verification unit according to the third embodiment; 
       FIG. 14  is a graph showing a data obtained by sampling pressure values detected by a pressure sensor at an interval of a predetermined time in the gas flow rate verification unit according to the third embodiment of the present invention; 
       FIG. 15  is a graph showing a data obtained by sampling the pressure values detected by the pressure sensor at an interval of a predetermined pressure in the gas flow rate verification unit according to the third embodiment of the present invention; 
       FIG. 16  is a view showing a relation between a gradient of the data shown in  FIG. 14  or  FIG. 15  and a measurable range; 
       FIG. 17  is a view showing a relation between correlation coefficient of the data shown in  FIG. 14  or  FIG. 15  and a measurable range; 
       FIG. 18  is a view showing a result of an experiment for flow rate verification precision of the gas flow rate verification units in the first and third embodiments; and 
       FIG. 19  is a block diagram of a conventional flow rate control device absolute flow rate check system. 
   

   EXPLANATION FOR REFERENCE CODES 
   
       
       
         
             1  Gas box 
             2  Gas unit 
             10  Mass flow controller (flow rate control device) 
             11  Gas flow rate verification unit 
             12  First cutoff valve 
             13  Second cutoff valve 
             14  Pressure sensor (pressure detector) 
             15  Temperature sensor (temperature detector) 
             16  Controller (control device) 
             18  Channel block (communication member) 
             21  Second port (output port) 
             26  First port (input port) 
             47  Volume measurement program (volume measuring device) 
             58  Vacuum pump 
             62  Volume measurement program (volume measuring device) 
         
       
     
  
   BEST MODES FOR CARRYING OUT THE INVENTION 
   Embodiments of a gas flow rate verification unit according to the present invention will be explained below with reference to the accompanying drawings. 
   First Embodiment 
     FIG. 1  is a schematic structural view of a gas box  1  having a gas flow rate verification unit  11  incorporated therein.  FIG. 2  is a side view of a gas unit  2  shown in  FIG. 1 . 
   As shown in  FIG. 1 , the gas flow rate verification unit  11  is mounted in the gas box  1 , for example. The gas box  1  has a box-like shape and a gas supply integration unit having plural (twelve in  FIG. 1 ) gas units  2  integrated therein. As shown in  FIGS. 1 and 2 , each of the gas units  2  includes a fluid control device such as a regulator  3 , a pressure gauge  4 , an input cutoff valve  5 , a mass flow controller  10  which is one example of a “flow rate control device”, an output cutoff valve  6 , and the like each being fixed on an upper surface of a channel block  7  and integrally coupled in series. 
   As shown in  FIG. 1 , an installation space for installing a pipe  8  for charging a process gas from each gas unit  2  is provided between the gas units  2  and the gas box  1 . In the installation space, the surrounding of the pipe  8  is a dead space. The gas box  1  is configured such that the gas flow rate verification unit  11  is fixed to the dead space with a volt or the like. The gas flow rate verification unit  11  communicates with the mass flow controller  10  of each gas unit  2  to verify the flow rate of the mass flow controller  10 . The components of the gas flow rate verification unit  11  are made into a unit, thereby the gas flow rate verification unit  11  can be integrally attached to or detached from the gas box  1 . 
   &lt;Structure of Gas Flow Rate Verification Unit&gt; 
     FIG. 3  is a side view of the gas flow rate verification unit  11  shown in  FIG. 1 .  FIG. 4  is a top view of the gas flow rate verification unit  11  shown in  FIG. 1 . 
   As shown in  FIGS. 3 and 4 , the gas flow rate verification unit  11  includes a first cutoff valve  12 , a second cutoff valve  13 , a pressure sensor  14  as a “pressure detector”, a temperature sensor  15  that is a “temperature detector”, a controller  16  as “a control device”, and others. In the gas flow rate verification unit  11 , a sensor cover  17  is screwed to cover the pressure sensor  14  so as to prevent changes in the setting by the user&#39;s touch to the pressure sensor  14  at a time of installation of the unit. 
     FIG. 5  is a sectional view of the gas flow rate verification unit  11  taken along a line A-A in  FIG. 4 . It is to be noted that  FIG. 5  is a sectional view only illustrating the main components. A controller  16  should be illustrated in  FIG. 5 , but the controller  16  is omitted since  FIG. 5  is used for explaining the structure of the channel. 
   The first cutoff valve  12 , the pressure sensor  14  and the second cutoff valve  13  are fixed by bolts  40  on a top surface of the channel block  18 , serving as a “communicating member.” The temperature sensor  15  is mounted in the channel block  18 . 
   The first cutoff valve  12  and the second cutoff valve  13  are electromagnetic valves having the same structure. The outer configuration of the first and second cutoff valves  12  and  13  are made such that driving portions  24  and  30  are coupled to metallic bodies  19  and  25 . First ports  20  and  26  as “input ports” and second ports  21  and  27  as “output ports” are provided respectively with the bodies  19  and  25 , in which valve seats  22  and  28  are further provided to communicate the first ports  20  and  26  with the second ports  21  and  27  respectively. Diaphragms  23  and  29  are displaceably mounted between the bodies  19  and  25  and the driving portions  24  and  30 . The Cv values of the first and second cutoff valves  12  and  13  are desirably not less than 0.09 in order to reduce the influence to the gas flow. In the first embodiment, the Cv values of the first and second cutoff valves  12  and  13  are set to 0.10. 
   The pressure sensor  14  is a capacitance manometer. The pressure sensor  14  holds a metallic diaphragm  31 , which is formed thin to have a thickness of about 0.1 mm to be displaced according to the gas pressure inputted to a detection port  39 . A metal substrate  32  is fixed to a back-pressure side of the diaphragm  31 . A conductive electrode is wired on the metal substrate  32 . The metal substrate  32  is arranged with a predetermined space between the metal substrate  32  and the diaphragm  31 . In the pressure sensor  14  described above, when a pressure receiving surface of the diaphragm  31  receives a gas pressure and displaces, the space between the metal substrate  32  and the diaphragm  31  is changed to change capacitance of the metal substrate  32 . Consequently, the change in the capacitance is detected as a change in the gas pressure. 
   The temperature sensor  15  is a bar-like thermoelement. 
   The channel block  18  is formed of a metal such as a stainless shaping into a rectangular parallelpiped. The upper surface of the channel block  18  in the figure is formed with a first port  33 , a second port  34 , and a third port  35 . A main channel  36  is formed in the channel block  18  passing through from the right side in the figure. In the channel block  18 , an “internal channel” is formed by communicating the first port  33 , the second port  34 , and the third port  35  with the main channel  36 . A stopcock  37  is welded to the main channel  36  to assure air tightness of the channel. The internal channel of the channel block  18  is formed to make its sectional area substantially same as those of a channel communicating with the second port  21  of the first cutoff valve  12  and with the first port  26  of the second cutoff valve  13 . This is for allowing the pressure of the gas supplied to the gas flow rate verification unit  11  to be easily uniform in the channel block  18 . In the first embodiment, the section of the internal channel (the main channel  36 , or the like) is set to have a diameter of 4 mm. Further, an insertion hole  38 , which is one example of a “mounting portion”, is drilled at the outside of the main channel  36  in the direction orthogonal to the main channel  36  from the side face. 
   The second port  21  of the body  19  in the first cutoff valve  12  is connected to the first port  33  of the channel block  18  through an unillustrated gasket, and the first cutoff valve  12  is screwed from above in the figure with a volt  40 , thereby the first cutoff valve  12  is fixed to the top surface of the channel block  18  in the figure as the unillustrated gasket being crushed. 
   The first port  26  of the body  25  in the second cutoff valve  13  is connected to the second port  34  of the channel block  18  through an unillustrated gasket, and the second cutoff valve  13  is screwed from above in the figure with the volt  40 , thereby the second cutoff valve  13  is fixed to the top surface of the channel block  18  in the figure as the unillustrated gasket being crushed. 
   The detection port  39  of the pressure sensor  14  is connected to the third port  35  of the channel block  18  through an unillustrated gasket, and the pressure sensor  14  is screwed from above in the figure with the volt  40 , thereby the pressure sensor  14  is fixed to the top surface of the channel block  18  in the figure as the unillustrated gasket being crushed. 
   The temperature sensor  15  is inserted into the insertion hole  38  to be mounted in the channel block  18 . 
   Thus, in the gas flow rate verification unit  11 , the first cutoff valve  12 , the second cutoff valve  13 , the pressure sensor  14 , and the temperature sensor  15  are integrally mounted in one channel block  18 , as shown in  FIG. 5 . The controller  16  of the gas flow rate verification unit  11  described above is fixed to the side face of the channel block  18  as shown in  FIGS. 3 and 4 . 
   &lt;Electric Structure of Control Device&gt; 
     FIG. 6  is an electric block diagram of the controller  16 . 
   The controller  16  has a computer function including a CPU  41 , an input/output interface  42 , a ROM  43 , a RAM  44 , and a hard disk drive (hereinafter referred to as “HDD”)  45 . 
   The input/output interface  42  is connected to the first cutoff valve  12 , the second cutoff valve  13 , the pressure sensor  14 , and the temperature sensor  15  to receive and send signals. 
   Volume storing device  46  is provided in the HDD  45 . In the volume storing device  46 , a known volume Vk, a system channel volume Ve, and a (volume of the gas in a tank) tank volume V are stored. The “known volume Vk” means the volume between the first cutoff valve  12  and the second cutoff valve  13 , more specifically, the volume in the sealed space formed between the valve seat  22  of the first cutoff valve  12  and the valve seat  28  of the second cutoff valve  13  when the first and second cutoff valves  12  and  13  are closed. The “system channel volume Ve” means the volume from the outlet of the mass flow controller  10  to the first cutoff valve  12 , more specifically, the volume from the outlet of the mass flow controller  10  to the valve seat  22  of the first cutoff valve  12  when the first cutoff valve  12  is closed. The “tank volume V” means the volume from the outlet of the mass flow controller  10  to the valve seat  28  of the second cutoff valve  13  when the first cutoff valve  12  is opened and the second cutoff valve  13  is closed. The known volume Vk is measurable upon the manufacture of the gas flow rate verification unit  11 , so it is stored beforehand in the volume storing device  46  after the gas flow rate verification unit  11  is manufactured and before the gas flow rate verification unit  11  is mounted in an external system. On the other hand, the system channel volume Ve and the tank volume V cannot be measured before the gas flow rate verification unit  11  is mounted in the external system. Therefore, they are measured ex-post after the gas flow rate verification unit  11  is manufactured and mounted in the external system, and accordingly stored in the volume storing device  46 . 
   The ROM  43  stores a flow rate verification program  48  and a volume measurement program  47  that is “a volume measuring device”. The flow rate verification program  48  appropriately controls the opening and closing operation of the first and second cutoff valves  12  and  13  to detect the pressure and the temperature between the first and second cutoff valves  12  and  13  by the pressure sensor  14  and the temperature sensor  15 , and performs the flow rate verification of the mass flow controller  10  on the basis of the result of the detection. The volume measurement program  47  measures the system channel volume Ve and the tank volume V. 
   &lt;Relationship Between Known Volume and System Channel Volume&gt; 
   As shown in  FIGS. 3 ,  4  and  5 , the gas flow rate verification unit  11  in the first embodiment does not have a chamber which is provided in the conventional technique. In the gas flow rate verification unit  11 , the known volume Vk is set to be equal to or less than the system channel volume Ve. The reason of the known volume Vk set to be equal to or less than the system channel volume Ve is to prevent the pressure of the gas outputted from the mass flow controller  10  from being varied (dispersed) in the gas flow rate verification unit  11  due to the reduced channel in the gas flow rate verification unit  11 . Accordingly, it is desirable that the known volume Vk in the gas flow rate verification unit  11  is reduced as much as possible upon mounting the first cutoff valve  12 , the pressure sensor  14 , the temperature sensor  15 , and the second cutoff valve  13 . In the first embodiment, the system channel volume Ve from the outlet of the mass flow controller  10  of each gas unit  2  to the valve seat  22  of the first cutoff valve  12  constituting the gas flow rate verification unit  11  is set to 100 cc in the gas box  1  shown in  FIG. 1 , while the known volume Vk from the valve seat  22  (see  FIG. 5 ) of the first cutoff valve  12  constituting the gas flow rate verification unit  11  to the valve seat  28  (see  FIG. 5 ) of the second cutoff valve  13  is set to 10 cc. 
   &lt;Verification Method&gt; 
   Next, the outline of the verification method by the gas flow rate verification unit  11  according to the first embodiment will be explained. The outline is explained here since the verification method will specifically be described later in the explanation of the evaluation test.  FIG. 7  is a flowchart showing a flow rate verification method executed by the gas flow rate verification unit  11  according to the first embodiment. 
   The gas flow rate verification unit  11  performs the flow rate verification for every one line of the gas unit  2 . Specifically, at step  101  (hereinafter referred to as “S 101 ”), the system is initialized to delete the data acquired in the previous flow rate verification. At S 102 , the gas supply integration unit is purged to remove unnecessary gases in the channel. 
   It is determined at S 103  whether or not the tank volume has already been measured. When it is determined that the tank volume has not yet been measured (S 103 : NO), the tank volume V is measured at S 104 , and then, the program proceeds to S 105 . On the other hand, when it is determined that the tank volume has been measured (S 103 : YES), the program directly proceeds to S 105 . 
   It is determined at S 105  whether or not the pressure value detected by the pressure sensor  14  is equal to or more than a fixed measurement start pressure P 1 . When the detected pressure value is less than the fixed measurement start pressure P 1  (S 105 : NO), the controller  16  waits until the pressure sensor  14  measures the fixed measurement start pressure P 1 . On the other hand, when the pressure sensor  14  measures the fixed measurement start pressure P 1  (S 105 : YES), it is determined at step S 106  whether or not the pressure value detected by the pressure sensor  14  is a target pressure P 2 . The controller  16  waits until the pressure sensor  14  detects the target pressure P 2  (S 106 : NO). Specifically, the gas flow rate verification system  11  waits until the pressure sensor  14  detects the target pressure P 2 . After the pressure sensor  14  detects the target pressure P 2  (S 106 : YES), the controller  16  calculates the flow rate Q at S 107 . 
   Accordingly, the flow rate Q is measured by the processes at S 105  to S 107 . The method of measuring the tank volume V and the flow rate Q will be explained in detail in the evaluation test. 
   Then, at S 108 , 1 is added to the number of times of the verification ek, and at S 109 , it is determined whether or not the number of times of the verification ek is a fixed number of times of the verification e. When the number of times of the verification ek is not the fixed number of times of the verification e (S 109 : NO), the controller  16  returns to S 102  to repeat the purge and measurement of the flow rate Q. After the tank volume V and the flow rate Q are measured until the number of times of the verification ek becomes the fixed number of times of the verification e (S 109 : YES), the measured values of the flow rate Q are averaged at S 110 , and the average value is compared to the set flow rate of the mass flow controller  10  for executing the verification. Upon the verification, the corrected value of the flow rate Q is set as needed. Thus, the flow rate verification for one gas unit  2  is completed. 
   At S 111 , 1 is added to a number of the gas units that have been verified uk, and at S 112 , it is determined whether or not the number of the gas units that have been verified uk reaches the total number u of the gas units  2  mounted in the gas supply integration unit. When the number of the gas units that have been verified uk does not reach the total number u (S 112 : NO), which means there are the gas units  2  that have not yet been verified, so the controller  16  returns to S 102  to execute the flow rate verification of the mass flow controller  10  mounted in the next gas unit  2 . On the other hand, when the number of the gas units that have been verified uk reaches the total number u (S 112 : YES), which means that the flow rate verification has been completed for all of the gas units  2  mounted in the gas supply integration unit, the flow rate verification process is ended. 
   &lt;Evaluation Test&gt; 
   The present inventors have conducted the evaluation test for the gas flow rate verification unit  11  according to the first embodiment.  FIG. 8  is a block diagram of an evaluation device  50 . 
   The evaluation device  50  is configured by connecting four gas units  2 A,  2 B,  2 C and  2 D to the gas flow rate verification unit  11  in parallel. In the following explanation, four gas units are collectively referred to as “gas unit  2 ” if there is no need to distinguish each of the gas units. The fluid control devices constituting the gas units  2  are designated with only the numeral with the appended alphabets “A”, “B”, “C”, and “D” omitted. 
   The gas unit  2  is formed by integrally coupling a filter  51 , a manual valve  52 , a regulator  53 , a pressure thermometer  54 , the mass flow controller  10 , and an output cutoff valve  55  in series from the upstream side. In the gas unit  2 A, a high-precision flowmeter  56  is provided between the pressure thermometer  54 A and the mass flow controller  10 A in order to precisely measure the controlled flow rate of the mass flow controller  10 A. The gas units  2 A,  2 B,  2 C and  2 D are connected to a gas supply valve  57  in parallel, and coupled to a vacuum pump  58  through the gas supply valve  57 . A pressure gauge  59  is provided on the system channel that communicates the gas unit  2  with the gas supply valve  57  for detecting the pressure in the system channel. The gas flow rate verification unit  11  is provided on a branch channel that is branched from the system channel and coupled between the gas supply valve  57  and the vacuum pump  58 . 
   The evaluation test was carried out by appropriately changing the structure of the evaluation device  50 . Specifically, the devices described below were used in the evaluation test: (i) the evaluation device  50 A that is configured to use the gas flow rate verification unit  11  as unchanged as shown in  FIG. 8 ; (ii) the evaluation device  50 B that is configured such that a chamber  60  of 500 cc was mounted in the gas flow rate verification unit  11  to communicate with the main channel  36  of the channel block  18  as indicated by a dotted line in  FIG. 8 ; and (iii) the evaluation device  50 C that is configured by replacing the gas flow rate verification unit  11  shown in  FIG. 8  with the conventional gas flow rate verification unit U shown in  FIG. 19 . 
   &lt;Evaluation Testing Method&gt; 
   The evaluation test was carried out for the evaluation devices  50 A,  50 B and  50 C used in (i), (ii) and (iii). In the evaluation test, the tank volume V (Ve+Vk) and the system channel volume Ve were firstly measured, and subsequently, an error measurement was repeated five times to calculate the average value of the errors. The errors measurements was made by calculating the errors caused between the flow rate calculated by the gas flow rate verification unit and the flow rate measured by the high precision flowmeter  56 . The errors were measured in case where the control flow rate of the mass flow controller  10  was set to a large flow rate (500 sccm) and in case where the control flow rate of the mass flow controller  10  was set to a small flow rate (100 sccm), respectively. The results of the measurement of the errors in each of the cases (i), (ii), and (iii) were compared as shown in  FIG. 10 . The method of the evaluation test is specifically described below. 
   &lt;Measurement of Volume&gt; 
   When the gas flow rate verification unit  11  is connected to an external system, the system channel volume Ve from the outlet of the mass flow controller  10  to the valve seat  22  of the first cutoff valve  12  constituting the gas flow rate verification unit  11  varies depending upon the structure of the channel of the external system. In other words, the tank volume V varies depending upon the external system. Therefore, prior to the gas flow rate verification, the gas flow rate verification unit  11  measures the tank volume V and the system channel volume Ve. The controller  16  executes the volume measurement program  47 , thereby the tank volume V and the system channel volume Ve are measured. 
   In the measurement of the tank volume V, the output cutoff valves  55 B,  55 C and  55 D of the gas units  2 B,  2 C and  2 D and the gas supply valve  57  are closed, while the manual valve  52 A and the output cutoff valve  55 A of the gas unit  2 A, and the first cutoff valve  12  and the second cutoff valve  13  of the gas flow rate verification unit  11  are opened, and then, N 2  gas is supplied to the mass flow controller  10 A in increments of 50 sccm, while drawing the vacuum by use of the vacuum pump  58 . After the flow rate is stabilized, the second cutoff valve  13  of the gas flow rate verification unit  11  is closed. Accordingly, the pressure in the system channel and the pressure in the channel of the gas flow rate verification unit  11  increase, so that the pressure value detected by the pressure sensor  14  increases. In this case, after the second cutoff valve  13  is closed, the time from when the pressure sensor  14  measures the measurement start pressure P 1  (5 kPa in the first embodiment) to when the pressure sensor  14  measures the target pressure P 2  (13 kPa in the first embodiment) is counted, and the temperature is measured by the temperature sensor  15 . 
   As shown in  FIG. 9 , the pressure increase amount ΔP that is the increase amount from the fixed measurement start pressure P 1  to the target pressure P 2  is obtained by subtracting the fixed measurement start pressure P 1  from the target pressure P 2 . The pressure sensor  14  detects the pressure at a constant interval (e.g., at an interval of 0.1 second). Therefore, the measurement time Δt during when the pressure between the first cutoff valve  12  and the second cutoff valve  13  increases from P 1  to P 2  is obtained by counting the number of times of the pressure detection from the detection of the measurement start pressure P 1  by the pressure sensor  14  to the detection of the target pressure P 2  by the pressure sensor  14 . The pressure increase amount ΔP is divided by the measurement time Δt, thereby the increasing pressure amount ΔP/Δt per unit time is obtained. The gas constant of the used gas (N 2  gas in the first embodiment) is used as the gas constant R. The gas temperature T is obtained from the detection by the temperature sensor  15 . The flow rate Q is obtained by inputting the set flow rate (flow rate measured by the high precision flowmeter  56  (50 sccm in the first embodiment)) of the mass flow controller  10 . Therefore, the obtained numerical values are applied into an equation 2 that is a modification of the equation 1, which is the basis of the calculation of the flow rate, thereby the tank volume V is calculated. 
   
     
       
         
           
             
               
                 
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   In the equation, ΔP indicates the pressure (Pa), Δt indicates the measurement time (s), V indicates the tank volume (m 3 ), R indicates the gas constant (J/mol·K), and T indicates the gas temperature (K). 
   
     
       
         
           
             
               
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   The tank volume V was repeatedly measured ten times as described above, and the average value of the tank volumes V is calculated. This average value is defined as the tank volume V, and stored in the volume storing device  46  of the controller  16 . 
   The tank volume V corresponds to the volume obtained by adding the known volume Vk and the system channel volume Ve. The known volume Vk has been stored beforehand in the volume storing device  46 . Therefore, the known volume Vk is subtracted from the tank volume V to measure the system channel volume Ve. The measured system channel volume Ve is stored in the volume storing device  46 . 
   &lt;Measurement of Error&gt; 
   The errors are measured in such a manner that the controller  16  executes the flow rate verification program to carry out the flow rate verification, and the flow rate calculated by the gas flow rate verification unit and the flow rate measured by the high precision flowmeter  56  are compared. 
   When the flow rate verification of the gas unit  2 A is carried out, the output cutoff valves  55 B,  55 C, and  55 D of the gas units  2 B,  2 C and  2 D, and the gas supply valve  57  are closed, while the manual valve  52 A and the output cutoff valve  55 A of the gas unit  2 A and the first cutoff valve  12  and the second cutoff valve  13  of the gas flow rate verification unit  11  are opened. In this state, N 2  gas is supplied to the mass flow controller  10 A. After the N 2  gas is supplied for 30 seconds, the second cutoff valve  13  of the gas flow rate verification unit  11  is closed in order to stabilize the control flow rate of the mass flow controller  10 A. 
   Accordingly, the pressure in the gas flow rate verification unit  11  increases. Then, the time from when the pressure sensor  14  measures the measurement start pressure P 1  (5 kPa) to when the pressure sensor  14  measures the target pressure P 2  (13 kPa) is counted. The reason of counting the time is because the pressure increase time varies depending upon the flow rate. Specifically, the time for the pressure to increase from 5 kPa to 13 kPa is 7.5 seconds in the case of the flow rate of 100 sccm, and 1.5 seconds in the case of the flow rate of 500 sccm. After the pressure sensor  14  detects 13 kPa, the second cutoff valve  13  is opened to proceed to the next flow rate verification. 
   The gas flow rate verification unit  11  calculates the flow rate as described below. The pressure increase amount ΔP between the first cutoff valve  12  and the second cutoff valve  13  is obtained by subtracting the fixed measurement start pressure P 1  from the target pressure P 2 . The pressure sensor  14  detects the pressure at a constant interval (e.g., at an interval of 0.1 second). Therefore, the measurement time Δt during when the pressure between the first and second cutoff valves  12  and  13  increases from P 1  to P 2  is obtained by counting the number of times of the pressure detection from the detection of the measurement start pressure P 1  by the pressure sensor  14  to the detection of the target pressure P 2  by the pressure sensor  14 . The pressure increase amount ΔP is divided by the measurement time Δt, thereby the increasing pressure value ΔP/Δt per unit time is obtained. The gas constant of the used gas (N 2  gas in the first embodiment) is used as the gas constant R. The gas temperature T is obtained from the detection of the temperature sensor  15 . The tank volume V is grasped since it is stored in the volume storing device  46  by the above-mentioned volume measurement. Therefore, the obtained numerical values (the increasing pressure amount ΔP/Δt per unit time, the gas constant R, the temperature T, and the tank volume V) are applied into the equation 1, thereby the flow rate Q is calculated. 
   The gas flow rate verification unit  11  compares the calculated flow rate Q with the set flow rate of the mass flow controller  10 . When they agree with each other, it is determined that the mass flow controller  10  appropriately controls the flow rate. If they do not agree with each other, it is determined that the mass flow controller  10  does not appropriately control the flow rate, and the calibration of the mass flow controller  10  is performed as needed. 
   The flow rate Q calculated by the gas flow rate verification unit  11  is compared to the flow rate measured by the high precision flowmeter  56  to determine an error. The reason of this is as follows. The high precision flowmeter  56  has very high detection precision, and hence, the flow rate measured by the high precision flowmeter  56  is extremely close to the true value of the flow rate controlled by the mass flow controller  10 A. Therefore, when the flow rate Q calculated by the gas flow rate verification unit  11  and the flow rate measured by the high precision flowmeter  56  are compared to obtain an error, the precision in the flow rate verification of the gas flow rate verification unit  11  can be determined. 
   &lt;Result of Evaluation&gt; 
   The result of the evaluation through the measurement of the errors as described above will be explained with reference to  FIG. 10 . When the inventors evaluated the conventional gas flow rate verification unit U by using the evaluation device  50 C, the error between the flow rate Q 13  calculated by the conventional gas flow rate verification unit U and the true value of the mass flow controller  10 A measured by the high precision flowmeter  56  is small such as 0.012% (see black dot in  FIG. 10 ) as shown in  FIG. 10(   iii ), in the case of the flow rate of the mass flow controller  10 A is 100 sccm. The present inventors have considered that the reason why the precision in the flow rate verification is excellent in the conventional unit U is because the chamber  153  is provided as shown in  FIG. 19 . 
   Specifically, in general, the volume between the valve components  151  and  152  decreases, so that the pressure rises in a short period, when the valve components  151  and  152  are close to each other. When the increasing pressure amount (the gradient of the graph in  FIG. 9 ) per unit time becomes too great, the valve component  152  is opened before the transducer assembly  154  outputs the signal directly indicating the PV/RT, resulting in the flow rate verification being impossible. Consequently, the present inventors have considered that, in order to surely execute the flow rate verification, the chamber is required between the valve component  151  and the valve component  152  to increase the known volume Vk. 
   It is further desirable that the increasing pressure amount (the gradient of the graph in  FIG. 9 ) per unit time is decreased in order to execute the flow rate verification with high precision. However, when the increasing pressure amount is too decreased, there arises a problem that the time for the flow rate verification is longer. Therefore, the present inventors have considered that, in order to secure the pressure measuring time, the volume of the chamber  153  has to be determined considering the flow rate verification time allowed in the semiconductor manufacturing process. 
   On the other hand, the present inventors have considered that the gas flow rate verification unit  11  is configured by integrating the first cutoff valve  12 , the second cutoff valve  13 , the pressure sensor  14 , and the temperature sensor  15  into the channel block  18 , by which the size of the gas flow rate verification unit  11  is made compact more than the conventional gas flow rate verification unit U in which the valve components  151  and  152  are connected by use of a pipe. The present inventors have made the evaluation device  50 B by mounting a chamber  60  in the gas flow rate verification unit  11  in order to enhance the precision in the flow rate verification. In this case, the present inventors have mounted the chamber  60  having a volume of 500 cc, which is greater than the chamber  153  used in the conventional gas flow rate verification unit U, in the gas flow rate verification unit  11 , in order to remarkably enhance the precision in the flow rate verification. 
   When the present inventors further conducted the evaluation test by using the evaluation device SOB the error between the flow rate Q 12  calculated by the gas flow rate verification unit provided with the chamber  60  and the true value was 0.099% (see black dot in the figure), in the case of the flow rate of the mass flow controller  10 A of 100 sccm, as shown in  FIG. 10(   ii ). Considering the meaning of the presence of the chamber, the precision in the flow rate verification should be enhanced by the increased volume of the chamber  60  than the chamber  153 . However, the result of the evaluation of the gas flow rate verification unit provided with the chamber  60  was poorer than the result of the evaluation of the conventional gas flow rate verification unit U. 
   When the present inventors conducted the evaluation test by using the evaluation device  50 A, the error between the flow rate Q 11  calculated by the gas flow rate verification unit  11  and the true value was 0.014% (see black dot in the figure), in the case of the flow rate of the mass flow controller  10 A of 100 sccm, as shown in  FIG. 10(   i ). Considering the meaning of the presence of the chamber, the result of the evaluation of the gas flow rate verification unit  11  must be poorer than the result of the evaluation of the conventional gas flow rate verification unit U, and further, must be poorer than the result of the evaluation of gas flow rate verification unit provided with the chamber  60 . However, the result of the evaluation was more satisfactory than the result of the evaluation of the gas flow rate verification unit provided with the chamber  60 , and further, the result of the evaluation became poorer than the result of the evaluation of the conventional gas flow rate verification unit U only by 0.002%. 
   From the above-mentioned results of the evaluation, the present inventors have found that the precision in the flow rate verification does not depend upon the presence or absence of a chamber. 
   The present inventors evaluated the precision in the flow rate verification by using the evaluation devices  50 A,  50 B and  50 C with the flow rate increased to 500 sccm, in order to examine the precision in the flow rate verification within a flow-rate verifiable range. 
   In case where 500 sccm of N 2  gas was supplied to the evaluation device  50 A, the error between the flow rate Q 21  calculated by the gas flow rate verification unit  11  and the true value was 0.515% (see black solid triangle in the figure), as shown in  FIG. 10(   i ). Comparing the error (0.014%) in the case of flowing a small amount of 100 sccm of N 2  gas and the error (0.515%) in the case of flowing a large amount of 500 sccm of N 2  gas, the difference between them was 0.501%. 
   In case where 500 sccm of N 2  gas was supplied to the evaluation device  50 B, the error between the flow rate Q 22  calculated by the gas flow rate verification unit U provided with the chamber  60  and the true value was 0.982% (see black solid triangle in the figure), as shown in  FIG. 10(   ii ). Comparing the error (0.099%) in the case of flowing a small amount of 100 sccm of N 2  gas and the error (0.982%) in the case of flowing a large amount of 500 sccm of N 2  gas, the difference between them was 0.883%. 
   In case where 500 sccm of N 2  gas was supplied to the evaluation device  50 C, the error between the flow rate Q 23  calculated by the conventional gas flow rate verification unit U and the true value was 1.150% (see black solid triangle in the figure), as shown in  FIG. 10(   iii ). Comparing the error (0.012%) in the case of flowing a small amount of 100 sccm of N 2  gas and the error (1.150%) in the case of flowing a large amount of 500 sccm of N 2  gas, the difference between them was 1.138%. 
   Examining the result of the evaluation, it was found that the unit providing the most stable precision in the flow rate verification within the flow-rate verifiable range was the gas flow rate verification unit  11 , and the unit providing the most unstable precision was the conventional gas flow rate verification unit U. If the variation in the precision in the flow rate verification within the flow-rate verifiable range is due to the chamber, the variation in the precision in the flow rate verification produced by the gas flow rate verification unit provided with the chamber  60  of 500 cc must be greater than the variation in the precision in the flow rate verification produced by the conventional gas flow rate verification unit U. However, in the result of the evaluation, the variation in the precision in the flow rate verification produced by the conventional gas flow rate verification unit U is greater than the variation in the precision in the flow rate verification produced by the gas flow rate verification unit provided with the chamber  60  of 500 cc. From the result of the evaluation, the present inventors have confirmed that the unit structure of the gas flow rate verification unit  11  is more excellent than the conventional gas flow rate verification unit U from the viewpoint of a compact size, and further, is more excellent than the conventional gas flow rate verification unit U in reducing the variation in the precision in the flow rate verification. 
   The reason why the precision in the flow rate verification of the gas flow rate verification unit  11  is more excellent than that of the conventional gas flow rate verification unit U will be examined below. 
   The first reason is that the known volume Vk is not more than the system channel volume Ve. While the system channel volume Ve is 100 cc, the known volume Vk of the gas flow rate verification unit  11  is 10 cc. The known volume Vk of the gas flow rate verification unit provided with the chamber  60  is more than 500 cc, and the known volume Vk of the conventional gas flow rate verification unit U is more than 250 cc. In other words, only the known volume Vk of the gas flow rate verification unit  11  is smaller than the system channel volume Ve. When the chamber is provided to increase the known volume Vk, the gas is slowly flown into the chamber as long as the flow rate is small, so that the pressure balance in the gas flow rate verification unit is easy to be uniform. However, when the flow rate becomes large, the pressure between the first cutoff valve  12  (valve component  151 ) and the second cutoff valve  13  (valve component  152 ) sharply rises before the gas enters the chamber to increase the pressure, whereby the pressure balance in the gas flow rate verification unit is made non-uniform. Therefore, when the pressure and the temperature of the chamber are detected to perform the flow rate verification, the pressure and the temperature between the first cutoff valve  12  (valve component  151 ) and the second cutoff valve  13  (valve component  152 ) cannot correctly be detected as the flow rate increases, with the result that it is considered that the error is likely to be generated between the flow rate verified by the gas flow rate verification unit and the true value. Accordingly, it is considered that the precision in the flow rate verification is enhanced by setting the known volume Vk to be equal to or less than the system channel volume Ve. 
   The second reason is that the channel structure is simple. The gas flow rate verification unit provided with the chamber  60  is the same as the conventional gas flow rate verification unit U in that both of them have chambers, but they are different in that the channel is composed of the channel block  18  in the gas flow rate verification unit provided with the chamber  60  while the channel is composed of the pipe in the conventional gas flow rate verification unit U. The gas flow rate verification unit provided with the chamber  60  has poor flow rate verification precision during the control of the small flow rate, compared to the conventional gas flow rate verification unit U. However, the precision in the flow rate verification of the gas flow rate verification unit provided with the chamber  60  is more excellent than that of the conventional gas flow rate verification unit U during the control of a large flow rate. From this result, it is considered that the configuration in which the channel of the gas flow rate verification unit is formed with the channel block  18  simplifies the channel structure, compared to the case in which the channel is formed with a pipe, thus the precision in the gas flow rate verification during the control of the large flow rate can be enhanced. 
   The third reason is the reduced change in the sectional area of the channel. The gas flow rate verification unit  11  is different from the gas flow rate verification unit provided with the chamber  60  in the presence of the chamber  60 . Comparing the error upon the small flow rate, the error in the gas flow rate verification unit  11  is smaller than the error in the gas flow rate verification unit provided with the chamber  60  by 0.085%, and comparing the error upon the large flow rate, the error in the gas flow rate verification unit  11  is smaller than the error in the gas flow rate verification unit provided with the chamber  60  by 0.467%. Specifically, the errors in the gas flow rate verification unit  11  upon the small flow rate and upon the large flow rate are reduced only by providing no chamber  60 , and further, the error is suppressed as the flow rate increases. From this result, it is considered that, since there is no change in the sectional area of the channel between the channel block  18  and the internal channel in the case of providing no chamber  60 , the flow of the gas is stabilized so as to enhance the precision in the flow rate verification, and further, even if the flow rate is increased, the error in the precision in the flow rate verification can be suppressed. 
   &lt;Operations and Effects of Gas Flow Rate Verification Unit according to First Embodiment&gt; 
   Accordingly, since the known volume Vk is not more than the system channel volume Ve in the gas flow rate verification unit  11  according to the first embodiment, the pressure between the first cutoff valve  12  and the second cutoff valve  13  is likely to be uniform even if the gas flow rate supplied between the first cutoff valve  12  and the second cutoff valve  13  is changed from the small flow rate of 100 sccm to the large flow rate of 500 sccm. Therefore, the gas flow rate verification unit  11  of the first embodiment can correctly detect the pressure and the temperature by the pressure sensor  14  and the temperature sensor  15 , even if the gas flow rate supplied between the first and second cutoff valves  12  and  13  increases, thereby the gas flow rate can precisely be calculated and verified with the use of the result of the pressure detected by the pressure sensor  14  and the result of the temperature detected by the temperature sensor  15 . Consequently, according to the gas flow rate verification unit  11  of the first embodiment, the error between the measured flow rate and the flow rate controlled by the mass flow controller  10  is reduced (see  FIG. 10(   i )), thus the reliability with respect to the flow rate verification can be enhanced. 
   According to the gas flow rate verification unit  11  of the first embodiment, since the first cutoff valve  12 , the second cutoff valve  13 , and the pressure sensor  14  are screwed to the upper surface of the channel block  18  from above with the volt  40  to be integrated (see  FIG. 5 ), the known volume Vk between the first cutoff valve  12  and the second cutoff valve  13  can be reduced to downsize the gas flow rate verification unit  11 . Particularly, the chamber is eliminated and the devices  12 ,  13  and  14  are integrated on the channel block  18 , so that the gas flow rate verification unit  11  of the first embodiment can provide a foot space about ⅔ times smaller than that of the conventional gas flow rate verification unit U. Since the known volume Vk between the first and second cutoff valves  12  and  13  is reduced, the time taken for the pressure between the first and second cutoff valves  12  and  13  to reach the target pressure can be shortened, thus shortening the verification time of the gas flow rate. 
   In the gas flow rate verification unit  11  of the first embodiment, the bar-like temperature sensor  15  is inserted into the insertion hole  38  of the channel block  18  to measure the temperature of the channel block  18 , thus the change in the temperature of the gas supplied between the first and second cutoff valves  12  and  13  (see  FIG. 5 ) is detected. Therefore, the temperature sensor  15  can be mounted in the gas flow rate verification unit  11  with the known volume Vk between the first and second cutoff valves  12  and  13  being reduced. 
   According to the gas flow rate verification unit  11  of the first embodiment, the gas flow rate verification unit  11  is housed in the gas box  1  by utilizing the dead space formed between the gas unit  2  having the mass flow controller  10  mounted therein and the gas box  1 . Therefore, there is no need to change the structure of the external pipe of the gas box  1  for arranging the installation space for the gas flow rate verification unit  11 . Accordingly, the gas flow rate verification unit  11  of the first embodiment can provide an excellent installation property. 
   In the gas flow rate verification unit  11  of the first embodiment, the system channel volume Ve is measured when the volume measurement program  47  is executed by the controller  16 . Specifically, when the gas is filled between the mass flow controller  10  and the second cutoff valve  13  only by the target pressure P 2 , the increasing pressure value ΔP/Δt per a unit time while the time from when the pressure sensor  14  detects the fixed measurement start pressure P 1  to the time when the pressure sensor  14  detects the target pressure P 2  is calculated, and the temperature sensor  15  detects the gas temperature T when the pressure reaches the target pressure P 2 . Then, the pressure increase value ΔP/Δt and the gas temperature T are applied into the equation 1 together with the control flow rate Q of the mass flow controller  10  and the gas constant R to measure the tank volume V from the mass flow controller  10  to the second cutoff valve  13 . Thereafter, the known volume Vk is read out from the volume storing device  46 , and the known volume Vk is subtracted from the tank volume V, thereby the system channel volume Ve is measured. Consequently, according to the gas flow rate verification unit  11  of the first embodiment, even if the system channel volume Ve varies depending upon the system structure in which the unit is mounted, the influence made by the variation is eliminated, so that the precision in the gas flow rate verification can be kept satisfactory. 
   Second Embodiment 
   The second embodiment of the gas flow rate verification unit according to the present invention will be explained with reference to the drawings.  FIG. 11  is a block diagram showing one example of a gas supply integration unit  63  provided with a gas flow rate verification unit  11 A. 
   The gas flow rate verification unit  11 A according to the second embodiment is used for performing the flow rate verification of the gas supply integration unit  63  shown in  FIG. 11 . The circuit structure of the gas supply integration unit  63  is the same as that of the evaluation device  50  (see  FIG. 8 ) described in the first embodiment, so that the numerals same as those in the evaluation device  50  are given to the fluid control devices. The structure of the controller  61  in the gas flow rate verification unit  11 A according to the second embodiment is different from the controller  16  in the first embodiment. Therefore, the points different from the first embodiment will mainly be explained, and the same referential codes in the first embodiment are given to the same components in the figures for suitably omitting the explanations thereof. 
   &lt;Electric Structure of Controller&gt; 
     FIG. 12  is an electric block diagram of a controller  61  used in the gas flow rate verification unit  11 A according to the second embodiment. 
   The controller  61  is different from the controller  16  (see  FIG. 6 ) in the first embodiment in that a pressure gauge  59 , a vacuum pump  58 , and an output cutoff valve  55  of the gas supply integration unit  63  are connected to an input/output interface  42 . The pressure gauge  59  detects the pressure of the system channel (see  FIG. 11 ) that communicates the gas units  2  with the gas supply valve  57 , and outputs a pressure detection signal to the controller  61 . The vacuum pump  58  evacuates the gas supply integration unit  63  in receipt of the instruction from the controller  61 . The output cutoff valve  55  is closed or opened in receipt of the instruction from the controller  61  so as to control the output of the process gas in the gas units  2 . 
   The controller  61  stores a volume measurement program  62 , which is “a volume measuring device”, into the ROM  43 . The volume measurement program  62  is different from the volume measurement program  47  in the first embodiment in a manner that the volume measurement program  62  calculates the system channel volume Ve and the tank volume V by utilizing the Combined gas law (Boyle-Charles law) while the measurement program  47  calculates the tank volume V by using the equation 2. 
   &lt;Measurement of Volume&gt; 
   The tank volume V and the system channel volume Ve are measured by executing the volume measurement program  62  by the controller  61 . Here, the case in which the volume is measured by using the gas supply integration unit  63  (see  FIG. 11 ) which includes the circuit structure same as that in the evaluation device  50  (see  FIG. 8 ) will be explained as an example. 
   Firstly, the output cutoff valves  55 A,  55 B,  55 C and  55 D and the gas supply valve  57  shown in  FIG. 11  are closed, and the first cutoff valve  12  and the second cutoff valve  13  of the gas flow rate verification unit  11 A are opened. Then, the vacuum pump  58  is driven to evacuate the downstream side of the output cutoff valve  55 A. After the pressure sensor  14  detects a predetermined pressure (5 kPa) and the completion of the evacuation is confirmed, the manual valve  52 A and the output cutoff valve  55 A of the gas unit  2 A is changed to the opened state from the closed state, and N 2  gas is supplied in the gas unit  2 A. When the second cutoff valve  13  is closed, the pressure in the channel from the mass flow controller  10 A to the second cutoff vale  13  increases. The output cutoff valve  55 A is changed from the opened state to the closed state to stop the supply of the N 2  gas at the point when the pressure sensor  14  detects the predetermined pressure (13 kPa). Subsequently, the first cutoff valve  12  is closed, the second cutoff valve  13  is opened, and then, the vacuum pump  58  is driven to form a vacuum region between the valve seat  22  of the first cutoff valve  12  and the valve seat  28  of the second cutoff valve  13 . Thereafter, the first cutoff valve  12  is changed from the closed state to the opened state so as to discharge the N 2  gas into the vacuum region. At this time, the pressure sensor  14  detects the variation of the pressure, and the temperature sensor  15  detects the temperature of the channel block  18 , i.e., the gas temperature. 
   The pressure P 11  detected by the pressure sensor  14  immediately before the N 2  gas is discharged to the vacuum region, the temperature T 11  detected by the temperature sensor  15  immediately before the N 2  gas is discharged to the vacuum region, a volume measurement completion pressure P 12  for completing the volume measurement after the N 2  gas is discharged to the vacuum region, and the temperature T 12  detected by the temperature sensor  15  when the pressure reaches the volume measurement completion pressure P 12  are applied to the Combined gas law (P 11 ·V 11 /T 11 =P 12 ·V 12 /T 12 ). Accordingly, the volume V 12  is obtained. Since the volume V 12  is the volume after the first cutoff valve  12  is opened, it corresponds to the tank volume V. Therefore, the known volume Vk is subtracted from the tank volume V to measure the system channel volume Ve. The tank volume V and the system channel volume Ve measured as described above are stored in the volume storing device  46 . 
   &lt;Operations and Effects of Gas Flow Rate Verification Unit According to Second Embodiment&gt; 
   As described above, in the gas flow rate verification unit  11 A according to the second embodiment, the second port  27  of the second cutoff valve  13  is connected to the vacuum pump  58 , and the controller  61  is connected to the pressure gauge  59  that detects the pressure between the outlet of the mass flow controller  10  and the valve seat  22  of the first cutoff valve  12  and the output cutoff valve  55  of the gas unit  2  (see  FIGS. 11 and 12 ). The controller  61  executes the volume measurement program  62  wherein the system channel volume Ve is measured. Specifically, when the portion between the first cutoff valve  12  and the second cutoff valve  13  is evacuated by the vacuum pump  58 , and the gas filled between the mass flow controller  10  and the first cutoff valve  12  is discharged between the first and second cutoff valves  12  and  13 , the pressure change and the temperature change between the first and second cutoff valves  12  and  13  are detected by the pressure sensor  14  and the temperature sensor  15  respectively, and the result of the pressure detection and the result of the temperature detection are applied to the Combined gas law to measure the tank volume V. The known volume Vk is read out from the volume storing device  46 , and the known volume Vk is subtracted from the tank volume V, thereby the system channel volume Ve is measured. Consequently, according to the gas flow rate verification unit  11 A of the second embodiment, even if the system channel volume Ve varies depending upon the system structure in which the unit is mounted, the influence caused by the variation is eliminated to keep the precision in the gas flow rate verification satisfactory. 
   Third Embodiment 
   The third embodiment of the gas flow rate verification unit according to the present invention will be explained with reference to the drawings. 
   A gas flow rate verification unit  11 B according to the third embodiment is obtained by improving the flow rate verification process of the gas flow rate verification unit  11  in the first embodiment in order to shorten the flow rate verification time. Therefore, the points different from the first embodiment will mainly be explained, and the same referential codes in the first embodiment are given to the same components in the figures for suitably omitting the explanations thereof. 
     FIG. 13  is a flowchart showing a gas flow rate verification method executed by the gas flow rate verification unit  11 B according to the third embodiment.  FIG. 14  is a graph showing data obtained by sampling the pressure values detected by the pressure sensor at an interval of a predetermined time in the gas flow rate verification unit according to the third embodiment of the present invention.  FIG. 15  is a graph showing a data obtained by sampling the pressure values detected by the pressure sensor at an interval of a predetermined pressure in the gas flow rate verification unit according to the third embodiment.  FIG. 16  is a view showing a relation between the gradient of the data shown in  FIG. 14  or  FIG. 15  and a measurable range X 1 .  FIG. 17  is a view showing a relation between the correlation coefficient of the data shown in  FIG. 14  or  FIG. 15  and a measurable range X 2 . 
   As shown in  FIG. 13 , the gas flow rate verification unit  11 B of the third embodiment is different from the first embodiment in that the gas flow rate verification unit  11 B monitors the gradient of the pressure value detected by the pressure sensor  14  and the correlation coefficient with respect to the gradient of the pressure value, and if the gradient or the correlation coefficient are within the measurable ranges X 1  and X 2  (see  FIGS. 16 and 17 ), it measures the flow rate Q to perform the verification even before the pressure sensor  14  detects the fixed measurement start pressure P 1 . 
   Specifically, after the tank volume V is measured at S 104 , the output cutoff valves  55 B,  55 C and  55 D of the gas units  2 B,  2 C and  2 D and the gas supply valve  57  are closed, while the manual valve  52 A and the output cutoff valve  55 A of the gas unit  2 A, and the first cutoff valve  12  and the second cutoff valve  13  of the gas flow rate verification unit  11  are opened at S 301 . The verification gas (e.g., N 2  gas) is supplied to the mass flow controller  10  in this state. After the flow rate is stabilized, the second cutoff valve  13  is closed. In this case, the pressure P 0  detected by the pressure sensor  14  is stored. At S 302 , it is determined whether or not a predetermined time Δt has elapsed by a clock pulse or the like. The unit waits until the predetermined time Δt has elapsed (S 302 : NO). 
   On the other hand, when the predetermined time Δt has elapsed (S 302 : YES), the pressure value P 1  is inputted from the pressure sensor  14  and stored as shown in  FIG. 14  at S 303 . Then, at S 304 , the gradient of the pressure variation is calculated. Specifically, the pressure value P 0  obtained prior to the pressure value P 1  is subtracted from the pressure value P 1  that is most lately obtained to calculate the increasing pressure value P 1 −P 0 , and the increasing pressure value P 1 −P 0  is divided by the time (predetermined time) Δt that is from the time when the pressure value P 0  is obtained to the time when the latest pressure value P 1  is obtained, thereby calculating the pressure increase ratio (gradient) P 1 /Δt per a unit time. 
   At S 305 , it is determined whether or not the calculated gradient P 1 /Δt is within the measurable range X 1  that is registered beforehand in the gas flow rate verification unit  11 B. The pressure value P detected by the pressure sensor  14  sharply rises before a certain time has elapsed as shown by Y 1  in  FIG. 14 , and then, increases with generally a constant gradient to reach the fixed measurement start pressure P 1 . The gas flow rate verification unit  11 B stores the relation between the time and the gradient into the HDD  45  as map data as shown in  FIG. 16 , and stores the range, in which a margin is provided in the range of the gradient before the pressure reaches the fixed measurement start pressure P 1  so as not to give adverse affect to the flow rate verification precision, as the measurable range X 1  on the map data. 
   The gradient P 1 /Δt between P 0 −P 1  shown in  FIG. 14  is sharp, and it is determined not to be within the measurable range X 1  registered beforehand to the gas flow rate verification unit  11 B (S 305 : NO). In this case, the gradient of the pressure value P might be varied, by which the flow rate Q might not be precisely measured. Therefore, the program returns to S 302  to obtain the next pressure value P 2  after the lapse of the predetermined time and executes the process described above. 
   When the pressure increase ratio (gradient) P n /Δt between the latest pressure value P n  and the pressure value P n-1  prior to the P n  is calculated, and the calculated gradient P n /Δt is determined to be within the measurable range X 1  registered beforehand to the gas flow rate verification unit  11 B (S 305 : YES), the pressure variation afterwards is generally stabilized and gives no adverse affect to the flow rate verification precision. Therefore, the program proceeds to S 306 . 
   At S 306 , the pressure value P n  when the gradient P n /Δt is determined to be within the measurable range X 1  is stored as a measurement start pressure P 21 . In other words, the point when the gradient P n /Δt is determined to be within the measurable range X 1  is a flow rate verification start timing. 
   At S 307 , it is determined whether or not the measurement time Δtx has elapsed from the measurement of the measurement start pressure P 21 . Before the measurement time Δtx has elapsed (S 307 : NO), the unit waits while monitoring the pressure value P of the pressure sensor  14 . 
   On the other hand, when the measurement time Δtx has elapsed (S 307 : YES), the pressure value P when the measurement time Δtx has elapsed is inputted from the pressure sensor  14  and stored as a measurement end pressure P 22 . 
   The flow rate Q is calculated at S 309 . Specifically, the pressure difference P 22 −P 21  between the measurement end pressure P 22  and the measurement start pressure P 21  is calculated, and the calculated pressure difference P 22 −P 21  is divided by the measurement time Δtx, thereby calculating the pressure increase ratio P/Δt. The calculated pressure increase ratio P/Δt, the tank volume V calculated at step S 104 , the temperature T detected by the temperature sensor  15 , and the gas constant R of the used gas are applied to the equation 1 so as to calculate the flow rate Q. 
   Thereafter, the program proceeds to S 108 . The process after S 108  is described above, so that the explanation thereof is omitted. 
   As another example, the pressure value may be obtained with an interval of a predetermined pressure, and the correlation coefficient with respect to the gradient of the variation in the pressure value may be monitored, thereby the flow rate verification timing may be judged. 
   Specifically, as shown in S 302  to S 304  in  FIG. 13 , every time the pressure detected by the pressure sensor  14  increases by the predetermined pressure ΔP, the pressure value P n  is stored. When the pressure value P n  is acquired at an interval of the predetermined pressure, the interval Δt n  of the pressure acquiring time is short to a certain time as shown by Y 2  in  FIG. 15 , but a certain time has elapsed, the interval Δt n  of the pressure acquiring time becomes generally constant. The correlation coefficient of the gradient of the pressure value approaches 1 within the range where the time Δt n  of the pressure acquiring time becomes generally constant. Therefore, the correlation coefficient with respect to the gradient ΔP/Δt n  of the latest pressure value P n  is calculated. The gas flow rate verification unit  11 B sets, as the measurable range X 2 , the range in which a margin is provided in the range of the gradient to make the correlation coefficient generally close to 1 within the range of giving no adverse affect to the flow rate verification precision for the flow rate Q. 
   Accordingly, when the calculated correlation coefficient is out of the measurable range X 2  ( 5305 : NO), the pressure variation is not stabilized, and the pressure variation might give adverse affect to the verification of the flow rate Q. Therefore, the program returns to S 302 , where the pressure value P n  is stored when the pressure increases by the predetermined pressure, and the process same as described above is executed. 
   On the other hand, when the correlation coefficient is within the measurable range X 2  (S 305 : YES), the pressure variation is generally stabilized, and might not give adverse affect to the verification of the flow rate Q. Therefore, the program proceeds to S 306 . The process after S 306  is as described above, so that the explanation thereof is omitted. 
   In case where the pressure is monitored with the time interval Δt as shown in  FIG. 14 , the flow rate verification start timing may be judged on the basis of whether or not the correlation coefficient with respect to the gradient of the variation of the pressure value belongs to the measurable range X 2  (see  FIG. 17 ). Alternatively, in case where the pressure is monitored with the pressure interval ΔP as shown in  FIG. 15 , the flow rate verification start timing may be judged on the basis of whether or not the gradient belongs to the measurable range X 1  (see  FIG. 16 ). 
   &lt;Operations and Effects of Third Embodiment&gt; 
   As explained above, if the pressure increase ratio (gradient) P n /Δt or ΔP/t n  or the correlation coefficient of the gradient of the pressure value P of P n /Δt or ΔP/t n  belongs to the measurable ranges X 1  or X 2  even before the pressure sensor  14  measures the fixed measurement start pressure P 1 , the gas flow rate verification unit  11 B of the third embodiment measures the flow rate to perform the verification (see S 302  to S 309  in  FIG. 13 ). On the other hand, the gas flow rate verification unit  11  in the first embodiment waits until the pressure sensor  14  detects the fixed measurement start pressure P 1 , and then, measures the flow rate Q to perform the verification (see S 105  to S 107  in  FIG. 7 ). 
   In the flow rate verification, the purge and the flow rate measurement are repeatedly performed fixed verification number of times e in order to enhance the verification precision. Therefore, the gas flow rate verification unit in the first embodiment takes several minutes to complete the flow rate verification for one gas unit  2 . On the other hand, the gas flow rate verification unit  11 B performs the flow rate verification without waiting a dead time from when the pressure is generally stabilized to when the pressure reaches the fixed measurement start pressure P 1 . Accordingly, the time to complete the flow rate verification for one gas unit  2  can be made within one minute. 
   Accordingly, the gas flow rate verification unit  11 B of the third embodiment performs the flow rate verification with the condition that the gradient of the pressure value or the correlation coefficient is within the measurable range X 1  or X 2  even before the pressure sensor  14  detects the fixed measurement start pressure P 1  resulting in the verification time being shortened compared to the gas flow rate verification unit  11  of the first embodiment. In general, the gas supply integration unit has a great number of gas units  2  installed therein. Therefore, if the verification time for each gas unit  2  can be shortened, the verification time for the overall gas supply integration unit can remarkably shortened, so that a remarkable effect can be provided. 
   When the flow rate verification is performed before the pressure of the pressure sensor  14  reaches the fixed measurement start pressure P 1 , like the gas flow rate verification unit  11 B of the third embodiment, there is a possibility that the precision is deteriorated. In view of this, a high precision flowmeter is mounted downstream of the mass flow controller  10  to measure the flow rate outputted from the mass flow controller  10  with the high precision flowmeter, and the flow rates measured by the gas flow rate verification units  11  and  11 B in the first and third embodiments were compared to the measured value of the high precision flowmeter so as to examine the precision. The result of the examination is shown in  FIG. 18 . 
     FIG. 18  is a view showing a result of the experiment for the flow rate verification precision of the gas flow rate verification units  11  and  11 B in the first and the third embodiments. 
   The structures are the same in the gas flow rate verification units  11  and  11 B, but the flow rate verification process is only different. Therefore, the gas flow rate verification units  11  and  11 B have the same tank volume V. It is supposed that the gas flow rate verification unit  11 B monitors the pressure value P of the pressure sensor  14  at an interval of a predetermined time to judge the flow rate verification timing according to the gradient of the pressure value. 
   As shown in  FIG. 18 , the gas flow rate verification unit  11 B of the third embodiment performs the flow rate verification with precision more excellent than that in the gas flow rate verification unit  11  of the first embodiment by only about 0.05%, even if the flow rate verification is executed before the pressure sensor  14  detects the fixed measurement start pressure P 1 . The increase is very slight such as 0.05% in terms of the numerical value. However, considering that the target precision of the mass flow controller  10  is 1%, the enhancement in the precision by 0.05% greatly contributes to the enhancement of reliability of the product. Accordingly, the gas flow rate verification unit  11 B in the third embodiment can shorten the flow rate verification time, and further, can enhance the flow rate verification precision, compared to the gas flow rate verification unit  11  in the first embodiment. 
   The present invention is not limited to the above-mentioned embodiments, but various modifications are possible. 
   (1) For example, in the above-mentioned embodiments, the first cutoff valve  12 , the second cutoff valve  13 , and the pressure sensor  14  of the gas flow rate verification unit  11  are fixed to the single channel block  18 . Alternatively, they may be connected with a pipe, or may be connected via plural channel blocks. Specifically, so long as the known volume Vk is not more than the system channel volume Ve, the channel of the gas flow rate verification unit  11  can appropriately be assembled. 
   (2) Although the mass flow controller  10  is employed as a flow rate control device in the above-mentioned embodiments, a device having a flow rate setting function, such as a pressure variation correcting constant flow rate valve or a flow rate control valve may be employed as a flow rate control device. 
   (3) For example, although a thermoelement is used as the temperature sensor  15  in the above-mentioned embodiments, a thermistor vacuum gauge or a Pirani gauge may be applied to a temperature detector. The temperature detector may be mounted to the side face of the channel block  18 , may be mounted so as to thrust into the channel block  18  from above, or may be mounted in the internal channel of the channel block  18 . 
   (4) For example, although the capacitive pressure sensor is employed as a pressure detector in the above-mentioned embodiments, a piezoresistance pressure sensor, manometer, or Mcleod gauge may be employed as a pressure detector. 
   (5) For example, although an electromagnetic valve of an electromagnetic driving system is employed as the first cutoff valve  12  and the second cutoff valve  13  in the above embodiments, a valve using the other driving system such as an air operate valve may be employed. Instead of the diaphragm valve, a poppet valve may be used. 
   (6) For example, although the gas flow rate verification unit  11  is housed in the gas box in the above-mentioned embodiments, the gas flow rate verification unit  11  may be connected to the gas unit that is mounted in a rail or a mounting plate and not housed in the gas box. 
   (7) For example, the tank volume V and the system channel volume Ve are stored ex post in the volume storing device  46  in the above-mentioned embodiments. Alternatively, when the gas flow rate verification unit  11  is incorporated in the gas box  1 , and the tank volume V and the system channel volume Ve are found, the tank volume V and the system channel volume Ve may be stored in the volume storing device  46  as an initial value. In this case, when a user modifies the channel structure in the gas box, the defect in the flow rate verification involved with the change in the channel structure can be prevented by executing the volume measurement explained in the above-mentioned embodiments. 
   (8) In the third embodiment of the present invention, the measurement start pressure P 21  and the measurement end pressure P 22  are calculated on the basis of the measurement time tx to calculate a flow rate. On the other hand, a target pressure P 23  may be set by adding an increase pressure, which is determined beforehand, to the measurement start pressure P 21 , and the time Δt taken for the pressure to increase from the measurement start pressure P 21  to the target pressure P 23  is counted in order to obtain the pressure increasing ratio (gradient) ΔP/Δt, per a unit time, in which the pressure increases from the measurement start pressure P 21  to the target pressure P 23 . In this case, the flow rate Q can be calculated by applying the obtained pressure increasing ratio ΔP/Δt per a unit time to the equation 1.