JPH11351922A - Flow rate measuring method of gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To conserve a gas and time while guaranteeing a demand accuracy with an inexpensive measuring device when measuring the flow rate of the gas constantly flowing into a closed container. SOLUTION: A capacity C of a container, a set flow rate F of a gas, an allowable measuring error R of a measured flow rate of the gas and a changing speed L of a pressure attributed to a slight leakage of the gas from the container are defined. With a resolving power P of a pressure measuring means and a time interval T for observation, points on a lattice give the time and the pressure for the observation and a rise in pressure to be measured is indicated by an inclination between the maximum measured rise in pressure 27 and the minimum measured rise in pressure 26. With a due consideration given to this condition and other conditions of a measurable minimum pressure Pmin and a measurable maximum pressure Pmax of the pressure measuring means and a waiting time S for starting measurement, a pressure 22 of starting the observation of the pressure rise and a pressure 24 of ending it are derived by an expression allowed for a measuring accuracy. Thus, a rising rate of the pressure in the closed container can be measured in a shorter time considering a measuring accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for measuring a gas flow rate.

[0002]

2. Description of the Related Art Some apparatuses for controlling the flow rate of a gas have a mechanism for measuring the flow rate in order to feedback-control the flow rate. However, in order to check whether or not the control of the gas flow control device is normal, the measurement must be performed by means different from the measurement mechanism. FIG. 3 is a block diagram showing a configuration example of an apparatus provided with a gas flow control mechanism. When the gas whose flow rate is controlled by the flow control unit 11 flows into the closed container 12, the rate of increase in the pressure of the closed container is measured by the pressure measuring unit 13, and the measured value is taken into the central processing unit 14. Since the flow rate of the gas is nothing but the rate of change of the pressure, it can be obtained by calculating the flow rate of the gas in the central processing unit 14 from the time required for measurement and the increased pressure. Specifically, the time required to supply gas at a constant flow rate to the closed container 12 having the capacity C stored in the parameter storage unit 15 and to reach from the pressure p ₁ at which measurement was started to the pressure p ₂ at which measurement was completed. As d

[0003]

The measured flow rate f is determined by f = C (p ₂ −p ₁ ) / d. Conventionally, the pressure p ₁ for starting the measurement and the pressure p ₂ for ending the measurement have been set to appropriate values empirically.

[0004]

However, there are limitations on the measurement accuracy of the device for reading the actual pressure and the range of the pressure that can be measured, and also on the accuracy of the device for measuring time. In particular, in order to construct a measuring device at low cost, it is necessary to use a device having a measurement accuracy that is not very good. For this reason, if the pressure at which the measurement of the pressure rise is started and the pressure at which the measurement is completed are appropriately determined, the measurement accuracy is poor, and the flow rate may not be measured correctly. On the other hand, in order to improve the measurement accuracy, it is necessary to take a sufficiently long measurement time.
This not only wastes the measurement time but also wastes gas.

The object of the present invention is to guarantee the required accuracy without wasting gas or time with an inexpensive flow meter.

[0006]

According to the present invention, in order to solve the above-mentioned problems and achieve the object, a pressure at which measurement is started and a pressure at which measurement is completed are calculated by a calculation formula taking into account measurement accuracy. .

According to the present invention, the gas flow rate can be measured with the required measurement accuracy while reducing the amount of gas used in a short time by using an inexpensive measuring device.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS.

In the present embodiment, the capacity C of the closed container, the minimum pressure P _min measurable by the pressure measuring means, the maximum pressure P _max measurable by the pressure measuring means, and the minimum pressure measurable by the pressure measuring means are determined. Width P, minimum time interval T for observing the pressure measuring means, set flow rate F of gas, allowable measurement error R between the measured flow rate of gas and the actual flow rate (R> 0), and minute amount of gas from the closed container. The rate of change of pressure L due to leakage,
It is assumed that the time S must wait at least from the start of gas flow to the start of observing a rise in pressure.

When a constant flow rate of gas is introduced into a closed container, the time is plotted on the horizontal axis and the pressure is plotted on the vertical axis, and this is represented by a graph such as the actual measured pressure rise 25 in FIG. Where 21
Indicates a measurement start time, 22 indicates a measurement start pressure, 23 indicates a measurement end time, 24 indicates a measurement end pressure, 26 indicates a minimum measured pressure rise, and 27 indicates a maximum measured pressure rise.

However, as shown in the graph of FIG. 1, the actual pressure can be recognized only up to the minimum value P, and the pressure can be monitored only every time T. Therefore, actually, the time and pressure at which a point on the grid having a width T in the horizontal axis direction and a width P in the vertical axis direction as shown in FIG. 1 are observed. At this time, the number of grids in the horizontal axis direction from the start of measurement to the end thereof is m, the number of grids in the vertical axis direction is n, and the measurement error R of the measured flow rate allowed for the actual flow rate is Considering the case where the graph looks like the maximum measured pressure rise 27,

[0012]

(1 + R) F ≧ C {(n + 1) P / (m-1) TL} must be satisfied. Also, for the case where the graph of the minimum measured pressure rise 26 becomes,

[0013]

(1−R) F ≦ C {(n−1) P / (m + 1) T−L} must be satisfied. By transforming (Equation 10),

[0014]

## EQU12 ## m ≧ {(n + 1) P / T} / ｛(1 + R) F
/ C + L｝ +1 (Equation 12)

[0015]

[Mathematical formula-see original document] m≤ {(n-1) P / T} / {(1-R) F
/ C + L｝ -1 When m is deleted from (Equation 12) and (Equation 13), and the left side is transformed into an inequality having only n,

[0016]

[Equation 14] n ≧ ｛(CL / F + 1) ² −R ² ｝ FT / RC
P + (CL / F + 1) / R Now, the pressure p ₁ to start the measurement, when the pressure p ₂ to stop measuring,

[0017]

Since p ₂ −p ₁ = nP, from (Equation 14), regarding p ₁ and p ₂ ,

[0018]

## EQU16 ## p ₂ −p ₁ ≧ ｛(CL / F + 1) ² −R ² ｝ FT
The following conditional expression is derived: / RC + (CL / F + 1) P / R

On the other hand, the time S portion in which the initial pressure rise may not be linear due to, for example, an unstable flow rate at the beginning of gas introduction should not be an object of observation. for that reason,

[0020]

Equation 17 _{p 1 ≧ (P / C +} L) / S made conditional expression is derived. Furthermore, from the range of pressure that can be measured by the pressure observation means,

[0021]

[Equation 18] p ₁ ≧ P _min

[0022]

## EQU19 ## The conditional expression p ₂ ≤P _max holds. From these (Equation 16), (Equation 1)
From the conditional expression of 9),

[0023]

[Equation 20] When P _min ≧ (F / C + L) S

[0024]

[Equation 21] p ₁ = P _min

[0025]

When P _min <(F / C + L) S

[0026]

The pressure p ₁ where p ₁ = (F / C + L) S is defined as the measurement start pressure,

[0027]

P _max ≦ {(CL / F + 1) ² −R ² } FT / RC
+ (CL / F + 1) P / R + p ₁

[0028]

P ₂ = P _max P _max > {(CL / F + 1) ² −R ² } FT / RC + (CL / F
+1) For P / R + p ₁

[0029]

P ₂ = {(CL / F + 1) ² −R ² } FT / RC +
And (CL / F + 1) P / R + p 1 becomes the pressure p ₂ measured pressure at the end.

By deriving the pressure at which the observation of the pressure rise is started and the pressure at which the pressure rise is observed using a mathematical expression taking into account the above measurement accuracy, the pressure rise speed in the closed vessel can be measured in a short time while considering the measurement accuracy. Can be.

FIG. 2 shows a configuration diagram of a reaction chamber of a semiconductor manufacturing apparatus in which the gas flow rate measuring method according to the present embodiment is performed. A gas line 30 for introducing a reaction gas into the reaction chamber 32 shown in FIG. 2 is provided, and a mass flow controller 31 is connected to the gas line 30 to control the flow rate to be constant. In this embodiment, in order to confirm whether the mass flow controller is operating normally,
In practice, it can be used to measure how much flow is being controlled.

First, the central processing unit 34 calculates a measurement start pressure p ₁ and a measurement end pressure p ₂ from the flow rate set by the mass flow controller 31. However, the capacity C of the reaction chamber 32, the minimum pressure P _min that can be measured by the vacuum gauge 33 as the pressure measuring means, the maximum pressure P _max that can be measured by the vacuum gauge 33, the resolution P of the vacuum gauge 33, and the intake of the vacuum gauge 33 The measurement time interval T of the mass flow controller 31 for observing the value, the set flow rate F of the mass flow controller 31, the allowable measurement error R of the measured flow rate of the gas, the rate of change L of the pressure due to the slight leakage of the gas from the reaction chamber, and the gas The time S required to wait at least for the time required for the flow rate to stabilize from the start of flowing the gas to the start of observing the rise in pressure and for the time required for stopping the evacuation of the reaction chamber 32 is stored in the parameter storage unit 35 in advance. Have been.

Next, the mass flow controller 31 is controlled at the set flow rate F, and at the same time, the exhaust of the reaction chamber 32 is stopped by closing the exhaust valve 36. Subsequently, the vacuum gauge 33 is monitored, and a time d from when the measurement start pressure is reached to the measurement end pressure is measured. And

[0034]

The measured flow rate f is calculated by the central processing unit 34 according to f = C (p ₂ −p ₁ ) / d.

[0035]

As described above, according to the method of the present invention, the flow rate of gas can be measured with the required measurement accuracy while reducing the amount of gas used in a short time by using an inexpensive measuring device.

[Brief description of the drawings]

FIG. 1 is a graph showing a change in pressure in a container when a constant flow rate of gas is introduced into the container in consideration of measurement accuracy of an apparatus having a gas flow control mechanism.

FIG. 2 is a configuration diagram of a reaction chamber of a semiconductor manufacturing apparatus in which a gas flow rate measuring method according to an embodiment of the present invention is performed.

FIG. 3 is a block diagram showing a configuration example of an apparatus having a gas flow control mechanism;

[Explanation of symbols]

 Reference Signs List 11 flow rate control unit 12 sealed container 13 pressure measurement unit 14, 34 central processing unit 15, 35 parameter storage unit 21 measurement start time 22 measurement start pressure 23 measurement end time 24 measurement end pressure 25 actual measured pressure rise 26 minimum measured pressure Rise 27 Maximum measurement pressure rise 31 Mass flow controller 32 Reaction chamber 33 Vacuum gauge 36 Exhaust valve

Claims (1)

[Claims] 1. A gas to be measured is supplied to a closed vessel at a constant flow rate, the pressure in the closed vessel is measured at regular intervals, and the capacity C of the closed vessel and the minimum pressure P that can be measured by a pressure measuring means. _min , the maximum pressure P measurable by the pressure measuring means
_max , the minimum pressure width P that can be measured by the pressure measuring means, the minimum time interval T for observing the pressure measuring means, the set flow rate F of the gas, the allowable measurement error R of the measured flow rate of the gas, The value of the pressure L that changes per unit time due to a minute leak and the value of the time S that must wait at least from the start of flowing gas to the start of observing a rise in pressure are stored, and P _min ≧ (F / C + L) ) In the case of S ## EQU2 ## p ₁ = P _{min In the case of} P _min <(F / C + L) S ## EQU4 ## From the pressure p _{1 where} p ₁ = (F / C + L) S, P _max ≦ {(CL / F + 1) ² -R ² } FT / RC +
(CL / F + 1) P / R + [6] When _{p 1 p 2 = P max P} max> {(CL / F + 1) 2 -R 2} FT / RC + (CL / F
+1) P / R + p ₁ p ₂ = {(CL / F + 1) ² −R ² } FT / RC + (C
As L / F + 1) P / R + p time required to reach ₁ comprising a pressure p ₂ d, determine the actual flow rate f of the gas flow rate control means by Equation 8] _{f = C (p 2 -p 1} ) / d A method for measuring a gas flow rate, characterized in that: