Patent ID: 12188800

DESCRIPTION OF THE REFERENCE CHARACTERS

200. . . Flow Rate Control Device (Mass Flow Controller)IN . . . Inflow Port1. . . Block BodyV . . . Control ValveC . . . Control Device2. . . Main Flow Path3. . . Diverting Element100. . . Thermal Flow MeterSP . . . Flow Rate Detector CircuitRu . . . Upstream-Side Electrical Resistance ElementRd . . . Downstream-Side Electrical Resistance Element4. . . Sensor Flow Path5. . . Sensor Output Generator6. . . Slope Effect Estimator7. . . Flow Rate Calculator

BEST EMBODIMENTS FOR IMPLEMENTING THE INVENTION

A thermal flow meter100and a flow rate control device200that is provided with this thermal flow meter100of the present embodiment are used to supply a plurality of different types of gases that contain component gases such as, for example, SF6and the like at set flow rates to an interior of a vacuum chamber in, for example, a semiconductor manufacturing process.

As is shown inFIG.1, the flow rate control device200is formed having a thin rectangular parallelepiped shaped outer configuration, and is used by being connected to a line through which a component gas is flowing. As is shown inFIG.2, the flow rate control device200is provided with a block body1that is connected to the line through which a gas is flowing, and that has a main flow path2, which forms a portion of this line, formed as an internal flow path, the thermal flow meter100that is mounted on a part-mounting surface of the block body1, a control valve V that is mounted on a downstream side from the thermal flow meter100, and a control device C that governs the control of at least the control valve V. In other words, the flow rate control device200is what is known as a mass flow controller in which those instruments that are necessary for performing flow rate control, namely, the block body1, the thermal flow meter100, the control valve V, and the control device C are packaged into a unit.

Here, as is shown inFIG.2, in the flow rate control device200, the block body1is formed having an elongated rectangular parallelepiped shaped outer configuration, and the main flow path2is formed extending in the direction of this elongation. The flow rate control device200is designed such that the standard orientation thereof is an orientation in which the control device200is mounted in such a way that a longitudinal direction of the block body1coincides with a horizontal direction. In other words, in a case in which, as is shown inFIG.10at (b), the flow rate control device200is vertically disposed such that the gas inside the main flow path2flows in a vertical direction, errors are generated in a zero-point output of the flow rate output from the thermal flow meter100due to a thermosiphon effect. The thermal flow meter100of the present embodiment is provided with a structure that corrects a slope effect, which is an error in the zero-point output that is caused by a thermosiphon effect.

The control device C is what is known as a computer that is provided with a CPU, memory, an A/D converter, a D/A converter, and various input/output tools, and that is formed such that, upon a program stored in the memory thereof being executed so as to cause the various devices to operate in mutual collaboration, the control device C performs the functions of a calculator CAL of the thermal flow meter100, and the functions of a valve controller9that controls an aperture of a control valve. The valve controller9performs feedback control on the aperture of the control valve V so that any deviation between a measured flow rate output from the calculator CAL of the thermal flow meter and a set flow rate that has been set by a user is reduced.

Next, the thermal flow meter100will be described in detail.

As is shown inFIG.2, the thermal flow meter100is provided with a sensor flow path4having a U-shaped configuration that branches off from the main flow path2along which gas is flowing, and merges once again with the main flow path2at a merging point located on the downstream side of the branch point, and with a diverting element3that is provided on the main flow path2between the branch point and the merge point and serves as a resistance element.

The diverting element3splits the flow at a predetermined diversion ratio between the main flow path2and the sensor flow path4, and is formed by a resistance component such as a bypass element having constant flow characteristics. A resistance component formed by inserting a plurality of narrow tubes into an outer pipe, or by stacking a plurality of thin circular plates in each of which are formed a plurality of through holes, or the like may be used as the diverting element3.

As is shown inFIG.2, the sensor flow path4is formed from metal (for example, stainless steel) as a capillary having a U-shaped configuration. A portion of a flow rate detector circuit SP that is used to detect the flow rate of a gas is provided on a portion of the sensor flow path4that is disposed in parallel with the flow direction of the main flow path2, in other words, is parallel with the direction of elongation of the block body1. The flow rate detector circuit SP detects the flow rate of a gas using the movement of heat that is generated by the flow of the gas that has been diverted onto the sensor flow path4.

As is shown inFIG.3, the flow rate detector circuit SP is a heating resistance wire whose electrical resistance value increases or decreases in conjunction with changes in temperature. The flow rate detector circuit SP is formed by an upstream-side electrical resistance element Ru in the form of a coil that is wound around an outer circumferential surface of the narrow tube forming the sensor flow path4, and by a downstream-side electrical resistance element Rd in the form of a coil that is wound around the sensor flow path4on the downstream side from the upstream-side electrical resistance element Ru. Here, the upstream-side electrical resistance element and the downstream-side electrical resistance element each operate as both a heater and a temperature sensor.

Furthermore, the flow rate detector circuit SP is a constant temperature drive type of circuit and, as is shown inFIG.3, includes an upstream-side constant temperature control circuit CTu that is formed by a bridge circuit of which a part is formed by the upstream-side electrical resistance element Ru, and a downstream-side constant temperature control circuit CTd that is formed by a bridge circuit of which a part is formed by the downstream-side electrical resistance element Rd.

The upstream-side constant temperature control circuit CTu is formed by an upstream-side bridge circuit in which a series resistor group that is made up of the upstream-side electrical resistance element Ru and a temperature-setting resistor R1that is connected in series to this upstream-side electrical resistance element Ru is connected in parallel with a series resistor group in which two fixed resistors R2and R3are connected in series, and by a feedback control circuit in the form of an operational amplifier that feeds a difference (Vu) between a potential at a connection point between the upstream-side electrical resistance element Ru and the temperature-setting resistor R1and a potential at a connection point between the two fixed resistors back to the upstream-side bridge circuit so as to maintain the balance of the upstream-side bridge circuit.

In the same way as the upstream-side constant temperature control circuit CTu, the downstream-side constant temperature control circuit CTd is formed by a downstream-side bridge circuit in which a series resistor group that is made up of the downstream-side electrical resistance element Rd and the temperature-setting resistor R1that is connected in series to this downstream-side electrical resistance element Rd is connected in parallel with a series resistor group in which the two fixed resistors R2and R3are connected in series, and by a feedback control circuit in the form of an operational amplifier that feeds a difference (Vd) between a potential at a connection point between the downstream-side electrical resistance element Rd and the temperature-setting resistor R1and a potential at a connection point between the two fixed resistors back to the downstream-side bridge circuit so as to maintain the balance of the downstream-side bridge circuit.

Here, the material used for the upstream-side electrical resistance element Ru has the same temperature coefficient of resistance as the material used for the downstream-side electrical resistance element Rd. In addition, the upstream-side electrical resistance element Ru and the downstream-side electrical resistance element Rd are both feedback-controlled by their respective feedback control circuits so that they have the same resistance value as the temperature-setting resistor R1. In other words, because their resistance value is kept constant, the respective voltages Vu and Vd are controlled so that the temperatures of the upstream-side electrical resistance element Ru and the downstream-side electrical resistance element Rd are also kept constant. In the present embodiment, Vu and Vd are used as an upstream-side voltage Vu and a downstream-side voltage Vd which are voltages that are applied in order to cause the upstream-side electrical resistance element Ru and the downstream-side electrical resistance element Rd to generate heat.

As is shown inFIG.4, the thermal flow meter100is additionally provided with the aforementioned calculator CAL that calculates the flow rate of a gas from the upstream-side voltage Vu and the downstream-side voltage Vd that are output from the flow rate detector circuit SP. The calculator CAL is formed so as to take (Vu−Vd)/(Vu+Vd) as the sensor output, and so as to correct any error in the zero-point output that is caused by a thermosiphon effect generated in a case in which the flow rate control device200is, for example, disposed vertically.

Here, an outline of the zero-point output correction function performed by the calculator CAL is shown inFIG.5. In a case in which the flow rate control device200has been disposed vertically so that a gas inflow port IN faces downwards, then even in a case in which there is no inflow or outflow of gas to the flow rate control device200, due to convection of the gas that is generated within the sensor flow path4, a gas flow is generated from the upstream-side electrical resistance element Ru towards the downstream-side electrical resistance element Rd. Because of this, in a case in which the gas inflow port IN faces downwards, a positive value error is generated in the pre-correction sensor output. Moreover, in the pre-correction sensor output, the error in the zero-point output increases proportionally as the pressure of the gas sealed inside the flow rate control device200increases. In the same way, in a case in which the inflow port IN of the flow rate control device200faces upwards, because a flow from the downstream-side electrical resistance element Rd towards the upstream-side electrical resistance element Ru due to convection is detected, the pre-correction sensor output becomes a negative value and an error is generated. In this case as well, the error in the zero-point output increases proportionally as the pressure of the gas sealed inside the flow rate control device200increases.

The calculator CAL is formed so as to estimate a slope effect, which is an error in the zero-point output in each state, and to cause the slope effect to approximate the actual flow rate by correcting the slope effect from the pre-correction sensor output.

The structure of the calculator CAL will now be described in detail with reference to the function block diagram shown inFIG.4.

The calculator CAL performs at least the functions of a sensor output generator5, a slope effect estimator6, a flow rate calculator7, and a receiving portion8.

The sensor output generator5is formed so as to receive inputs of the upstream-side voltage Vu and the downstream-side voltage Vd, which are voltages that are applied to the upstream-side electrical resistance element Ru and the downstream-side electrical resistance element Rd, and to then calculate and output (Vu−Vd)/(Vu+Vd) as the sensor output. Here, the voltage differential (Vu−Vd) is a value that changes in accordance with the flow rate of the gas flowing through the sensor flow path4, and a voltage sum (Vu+Vd) corresponds to a temperature index of the gas flowing through the sensor flow path4. By dividing the voltage differential by the voltage sum, the temperature effect on the flow rate is corrected in this sensor output.

The slope effect estimator6estimates a slope effect that is generated in the sensor output in accordance with the attitude of the sensor flow path4based on at least the Prandtl number of the gas being measured. In the present embodiment, the slope effect estimator6estimates the slope effect based not only on the Prandtl number, but also on the Nusselt number and the Grashof number of the gas. Moreover, the pressure and temperature of the gas, which are obtained from various sensors provided within the flow rate control device200or from various sensors provided separately for the semiconductor manufacturing process, are also input into the slope effect estimator6, and the slope effect estimator6outputs a slope effect based on these values.

The slope effect estimator6calculates the Prandtl number from the temperature, pressure, and physical property values of the gas, and calculates the Nusselt number from the Grashof number. In addition, based on a relational expression between the value of the Nusselt number and the voltage differential (Vu−Vd) forming a portion of the sensor output, the slope effect estimator6estimates a voltage differential (Vu0−Vd0) in a state in which there is no flow as the slope effect. In order to perform these functions, the slope estimator6is provided with at least a temperature acquisition portion61, a pressure acquisition portion62, a physical property values storage portion63, a Nusselt number calculating portion64, and a zero-point output calculating portion65.

The temperature acquisition portion61acquires an output signal from a temperature sensor (not shown in the drawings) that is provided, for example, in the block body1of the flow rate control device200as the temperature of the gas, and outputs this temperature to the Nusselt number calculating portion64. Note that it is also possible for the temperature acquisition portion61to acquire information from another temperature sensor that is provided on the line to which the flow rate control device200is connected.

The pressure acquisition portion62acquires an output signal from a pressure sensor (not shown in the drawings) that measures the pressure of the gas present within the main flow path of the flow rate control device200. The pressure sensor may be provided, for example, in the actual flow rate control device200itself so as to measure the pressure of the gas flowing through the main flow path, or may be provided on a flow path that connects on-off valves (not shown in the drawings) that are provided upstream and downstream respectively from the flow rate control device200to the flow rate control device200. The pressure of the gas sealed inside the flow rate control device200that the pressure acquisition portion62outputs to the Nusselt number calculating portion64is a pressure that is acquired in a state in which, for example, the respective on-off valves are both closed, and there is no inflow or outflow of gas in relation to the flow rate control device200.

In a case in which the Nusselt number is denoted by Nu, the Grashof number is denoted by Gr, the Prandtl number is denoted by Pr, the constant of proportionality is denoted as A, and the index is denoted by n, then the Nusselt number calculating portion64calculates the value of the Nusselt number Nu based on Nu=A(Gr×Pr)n. Here, in the present embodiment, the constant of proportionality A is taken as 1, and the index n is taken as 2. This is because it was discovered by the inventors of the present application from the experiment results shown inFIG.6that, in a case in which values such as these are employed, the Nusselt number Nu can be calculated from the product of the Grashof number Gr and the Prandtl number Pr.

Here, the accuracy when estimating the Nusselt number Nu in a case in which the Prandtl number Pr is not used will be described based on the measurement results shown inFIG.7. Note that in order to enable an easier comparison to be made between the graphs shown inFIG.6andFIG.7, intervals between the auxiliary lines of the respective axes thereof have been adjusted so the unit quantities thereof are substantially the same. As is shown inFIG.7, although a correlation exists between the Nusselt number Nu and the Grashof number Gr, in those regions where the value of the Grashof number is small, there is increased scattering of the Nusselt number Nu relative to the Grashof number Gr. In other words, even if an approximate straight line is calculated between the Grashof number Gr and the Nusselt number Nu, and the Nusselt number Nu is estimated from the Grashof number Gr based on this approximate straight line, compared with a case in which the Nusselt number Nu is estimated from (Gr×Pr)2, as is the case in the present embodiment, the estimation accuracy is markedly inferior. From the results obtained by comparingFIG.6andFIG.7, it can be confirmed that a method of calculating the Nusselt number Nu using (Gr×Pr)2is suitable for the structure of the thermal flow meter100of the present embodiment.

The method of calculating the Nusselt number Nu of the present embodiment will now be described in further detail. In a case in which an internal diameter of the sensor flow path4is denoted by L, a resistance value of the upstream-side electrical resistance element Ru or the downstream-side electrical resistance element Rd is denoted by R, a thermal conductivity of the fluid being measured is denoted by λ, a molar specific heat at constant pressure of the fluid being measured is denoted by Cp, a viscosity of the fluid being measured is denoted by η, a density of the fluid being measured is denoted by ρ, a gravitational acceleration is denoted by g, a volumetric expansion of the fluid being measured is denoted by β, and a temperature differential between the upstream-side electrical resistance element Ru or the downstream-side electrical resistance element Rd and the fluid being measured is denoted by ΔT, then the Grashof number Gr and the Prandtl number Pr are expressed respectively by the following.
Pr=Cpη/λ, and
Gr=ρgL3βΔT/η2.

Based on information such as the type of gas which the receiving portion8has received from a user, and on the pressure and temperature thereof acquired by the pressure acquisition portion62and the temperature acquisition portion61, the Nusselt number Nu calculating portion64reads the molar specific heat at constant pressure Cp, the volumetric expansion β, and the density ρ and the like stored in the physical property values storage portion63. Next, the Nusselt number Nu calculating portion64substitutes each of the read physical property values, as well as the acquired pressure and temperature, into the above-described calculation formulae for the Grashof number Gr and the Prandtl number Pr, and calculates the respective values. Finally, the Nusselt number Nu calculating portion64calculates a square of the product of the Grashof number Gr and the Prandtl number Pr as the Nusselt number Nu. The Nusselt number Nu thus calculated is then output to the zero-point output calculating portion65.

The zero-point output calculating portion65calculates the slope effect based on a relational expression between the voltage differential (Vu0−Vd0) in a state in which there is no slope effect in the form of a flow and the Nusselt number Nu. More specifically, in a case in which the internal diameter of the sensor flow path4is denoted by L, the resistance value of the upstream-side electrical resistance element Ru or the downstream-side electrical resistance element Rd is denoted by R, the thermal conductivity of the fluid being measured is denoted by λ, and the temperature differential between the upstream-side electrical resistance element Ru or the downstream-side electrical resistance element Rd and the fluid being measured is denoted by ΔT, then based on Nu=L×((Vu0−Vd0)2/R)/(L2×ΔT)/λ, the zero-point output calculating portion65calculates the voltage differential (Vu0−Vd0). Here, the positivity or negativity of (Vu0−Vd0) is determined in such a way that (Vu0−Vd0) is positive in a case in which the gas intake port of the flow rate control device200is on the lower side, and is negative in a case in which the gas intake port is on the upper side.

The flow rate calculator7corrects the slope effect estimated by the slope effect estimator6for the pre-correction sensor output (Vu−Vd)/(Vu+Vd) output from the sensor output generator5, and then calculates the flow rate of the gas based on the corrected sensor output. In other words, the flow rate calculator7calculates the flow rate by correcting any shift in the zero-point output by subtracting the slope effect (Vu0−Vd0) from the pre-correction voltage differential (Vu−Vd), and substituting the corrected sensor output {(Vu−Vd)−(Vu0−Vd0)}/(Vu+Vd) into a predetermined flow rate calculation function. More specifically, if the flow rate is denoted by F and the flow rate calculation function is denoted by Sens(X), then the flow rate is converted into F=Sens({(Vu−Vd)−(Vu0−Vd0)}/(Vu+Vd)).

According to the thermal flow meter100and the flow rate control device200that are formed in this manner, as is shown in the graph inFIG.8, a slope effect that appears for a sensor output can be accurately corrected so that an accurate flow rate is obtained. Here, the actual measurement results shown inFIG.8contain two sets of measurement results for a plurality of types of gas. One set of measurement results is for the inflow port IN facing upwards, while the other set is for when the inflow port IN faces downwards. As is shown inFIG.8at (a), depending on the gas type, there is a marked increase in the amount of shift in the zero-point output proportionally as the sealing pressure within the flow rate control device200becomes higher. In contrast, as is shown inFIG.8at (b), by employing the correction method of the present embodiment, a sizable decrease can be achieved in the amount of shift in the zero-point output irrespective of the type of gas and sealing pressure. It is thought that this is due to the fact that, because the slope effect estimator6estimates the slope effect based on a Prandtl number, which is a value that is affected by the pressure and thermal conductivity of a fluid, not only can the actual size itself of the convection generated by a thermosiphon effect be corrected, but the differing effects on the zero-point output caused by differences between the ease of heat propagation depending on the gas type can also be corrected.

Here, as a comparative example, the results obtained when the zero-point output was corrected by calculating the Nusselt number Nu using only the Grashof number Gr and without using the Prandtl number Pr, as was shown inFIG.7, are shown inFIG.9. Note that, in order to enable an easier comparison to be made between the graphs shown inFIG.8andFIG.9, intervals between the auxiliary lines of the respective axes thereof have been adjusted so the unit quantities thereof are substantially the same. Compared with the zero-point output in the case shown inFIG.9at (a) in which nothing has been corrected, as is shown inFIG.9at (b), it is possible to correct the zero-point output to a certain extent even in a case in which only the Grashof number Gr is used. However, as can be understood by comparingFIG.8at (b) andFIG.9at (b), it can be confirmed that using the Prandtl number Pr as well, as is the case in the present embodiment, enables the correction accuracy, particularly in areas where the sealed-in pressure, is high to be improved. In this way, by employing the thermal flow meter100of the present embodiment, it is possible to correct a zero-point output with a high degree of accuracy regardless of the type of gas and regardless of the pressure of the sealed-in gas.

Moreover, because the slope effect estimator6is able, based on the pressure and temperature of a gas and on the respective physical property values of a gas, to calculate the size of a slope effect and, in a case in which information about the mounting orientation of the flow rate control device200has been input, to also determine the positivity or negativity that appears as a zero-point output, it is no longer necessary to employ an additional sensor such as a gyro sensor that is not normally employed in the flow rate control device200.

In other words, without making any hardware modifications from a normal thermal flow meter, simply by making software modifications it is possible to accurately correct measurement errors that are caused by a thermosiphon effect.

Additional embodiments will now be described.

The structure of the slope effect estimator is not limited to the structure described in the foregoing embodiment. Namely, it is acceptable for the slope effect estimator to estimate a slope effect based at least on the Prandtl number of a fluid. For example, it is also possible for the slope effect estimator to estimate a slope effect based on a relational expression between the Prandtl number and a voltage differential (Vu0−Vd0) that shows an error in the zero-point output. Alternatively, it is also possible for the slope effect estimator to be formed in such a way as to estimate a slope effect based on a relational expression between the Nusselt number and the Prandtl number without using the Grashof number.

The way in which a slope effect is expressed is not limited to being a voltage differential (Vu0−Vd0). For example, it is also possible to individually calculate each of the zero-point output Vu0and the zero-point output Vd0so that the upstream-side voltage Vu and the downstream-side voltage Vd can both be individually corrected. If this type of structure is employed, then the temperature index Vd+Vu can also be corrected.

The mounting orientation of the flow rate control device and the type of fluid (gas) that is being measured are set in advance by a user via the receiving portion, however, it is also possible for this information to be acquired automatically by the flow rate control device. For example, it is also possible to employ a structure in which the flow rate control device is provided with a gyro sensor, and for the orientation of the intake port of a fluid and the attitude of the sensor flow path to be acquired so that the positivity or negativity of the slope effect can be set automatically. In addition, it is also possible for the amount of correction of the slope effect to be changed in accordance with the slope angle. Moreover, because it is possible to estimate the thermal conductivity of the fluid flowing through the flow path from the temperature index Vu+Vd, it is also possible to identify the type of fluid from this type of value and to consequently obtain other physical property values that are required.

The thermal flow meter correction method of the present invention is not limited to being used in a constant temperature drive system, and may also be applied, for example, in a constant current drive system as well as in other types of systems. For example, in a thermal flow meter in a constant current drive system, it is sufficient if the current detection circuit is provided with a bridge circuit that includes the above-described upstream-side electrical resistance element and the above-described downstream-side electrical resistance element, and with a constant current circuit that supplies a constant current to this bridge circuit.

In addition to these, various other modifications and combinations of embodiments can be made insofar as they do not depart from the spirit or scope of the present invention.

Industrial Applicability

According to the present invention, it is possible to provide a thermal flow meter that takes into account the effects on a slope effect that are due to pressure and to differences between the thermal conductivities of fluids and is thereby able to correct errors occurring when measuring a flow rate that are caused by a thermosiphon effect at a greater level of accuracy than is possible conventionally.