JP3551906B2 - Mass flow controller - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a mass flow controller for controlling a mass flow of a fluid having a relatively small flow such as a gas.
[0002]
[Prior art]
Generally, in order to manufacture a semiconductor product or the like, for example, a CVD film formation or an etching operation is repeatedly performed on a semiconductor wafer or the like. A mass flow controller capable of controlling the flow is used.
This type of mass flow controller mainly includes a sensor unit for detecting a mass flow rate of a trace gas, a flow control valve, and a control circuit unit for controlling the same. The sensor section has a sensor formed by winding an electric heating coil around a sensor tube through which a small percentage of the total gas amount passes, and most of the gas flows through the bypass. The control circuit controls the valve opening of the flow control valve based on the value detected by the sensor, so that the gas flow of the set value flows. Further, in order to control the valve opening, since the entire gas flow rate itself is very small, the valve opening must be accurately controlled within a stroke range of, for example, about several tens of μm. Since a large change in thrust can be generated within the stroke range, a laminated piezoelectric element or an electromagnet type actuator is generally used, and the valve opening of the valve is operated based on a set value. It has become.
[0003]
As a method of controlling the gas flow rate by controlling the valve opening degree, that is, the operation amount of the valve, a position PID control method, a speed-type PID control method, and the like are known. It is necessary to control the generation of particles that cause product defects in the semiconductor processing chamber due to the inflow of an unusual gas flow. And control without time delay is required.
[0004]
[Problems to be solved by the invention]
By the way, the control circuit system of the mass flow rate control device described above has been conventionally configured with an analog circuit because of good tracking performance when a setting signal or an actual flow rate changes, but an additional communication function is added. In response to demands for higher accuracy in flow rates and flow rates, microcomputers and the like have been mounted to perform processing while calculating various data.
A mass flow controller equipped with this microcomputer or the like usually converts a sensor signal output from a sensor unit and a flow rate setting signal input from outside to set a desired flow rate into a digital signal, and then converts the digital signal into a microcomputer. The opening degree of the valve (valve opening degree) is determined by comparing these two signals, and the opening degree is output to the flow control valve as an opening degree command signal. That is, the comparison processing and the opening degree determination processing are calculated in digital form by the microcomputer.
The control of the valve opening is cyclically and repeatedly performed at predetermined intervals, for example, every 10 msec, thereby performing feedback loop control. Such processing based on a digital form is very effective when resetting the PID control constant so as to conform to the characteristics of the flow control valve, or when performing processing involving various storage and judgment functions.
[0005]
However, as described above, in the case of this digital processing, a delay for digitally converting the sensor signal occurs inevitably, and unlike analog processing capable of complete continuous processing, discontinuous processing is performed. Therefore, for example, a step-like command voltage is applied to the valve voltage, and controllability or responsiveness may be inferior to analog control in some cases.
The present invention has been devised in view of the above problems and effectively solving the problems. An object of the present invention is to provide a mass flow controller capable of exhibiting ideal responsiveness having both advantages of analog processing and digital processing.
[0006]
[Means for Solving the Problems]
The invention according to claim 1 controls the flow control valve provided in the fluid passage through which the fluid flows based on a flow rate setting signal input from the outside and a sensor signal from the flow rate sensor means, thereby controlling the flow rate of the fluid. In the mass flow control device for controlling the mass flow rate, the control circuit has a digital operation circuit system and an analog circuit system for controlling the responsiveness of the flow control valve, and the analog circuit system includes the flow rate setting signal and the sensor signal. A valve driving unit that outputs the valve voltage having a magnitude based on the output of the comparing unit; and a control that switches and selects a control constant that is connected to the comparing unit and used for the comparing unit. A constant switching unit, performs feedback control to determine the valve voltage of the flow control valve by comparing the sensor signal and the flow rate setting signal, Calculation circuit system, which is constituted so as to output a switching unit control signal for controlling the control constant switching unit based on the flow rate setting signal.
As described above, at least the sensor signal is compared with the flow rate setting signal (hereinafter, also simply referred to as “setting signal”), and the feedback loop for determining the valve voltage based on the result is performed by the analog circuit system, so that the complete Control can be performed continuously and with excellent responsiveness, and since other complicated control relating to responsiveness is performed by a digital circuit system, it is possible to achieve high precision of response control, Therefore, the advantages of digital processing and the advantages of analog processing can be achieved at the same time.
[0007]
In this case, before Symbol analog circuitry includes a comparator for comparing the flow rate setting signal and said sensor signals, and a valve driving unit which outputs the valve voltage of a magnitude based on the output of the comparator portion of .
In addition, a control constant switching unit that switches and selects a control constant is connected to the comparison unit.
In a further, pre SL digital arithmetic circuit system, you output switching unit control signal for controlling the control constant switching unit based on the flow rate setting signal.
[0008]
Further, for example, as defined in claim 2 , the digital arithmetic circuit system performs correction of a zero point shift and correction of a linear shift of the flow rate sensor means based on the flow rate setting signal and the sensor signal. A corrected flow rate setting signal may be output.
Further, for example, as defined in claim 3 , when the set flow rate is changed from a zero flow state to a predetermined flow rate, the digital operation circuit system uses the valve voltage immediately before the fluid starts flowing when the flow control valve starts to open. A certain initial valve voltage may be obtained.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment of a mass flow controller according to the present invention will be described in detail with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram showing a mass flow rate control device according to the present invention, FIG. 2 is a configuration diagram showing a control constant switching section shown in FIG. 1, and FIG. FIG. 4 is a graph showing a relationship between a setting signal and a sensor signal for explanation, and FIG. 4 is a graph showing a relationship between a valve voltage and a flow rate.
[0010]
Here, a case will be described as an example where a gas fluid flows as the fluid.
As shown in the drawing, the mass flow controller 2 has a fluid passage 4 formed of, for example, stainless steel or the like, and most of the flow is caused to flow upstream of the fluid passage 4 in the gas flow direction. A bypass 6 is provided, and on the downstream side, a flow control valve 10 having, for example, a diaphragm 8 as a valve body for controlling the flow rate of the gas fluid is provided. The control device 2 includes an analog circuit AN that controls the valve opening degree of the flow control valve 10 in a feedback loop, among the controls related to the responsiveness of the flow control valve 10, and a responsiveness other than the feedback loop. And a digital operation circuit system DE for performing other control related to the above.
[0011]
First, the analog circuit system AN will be described. A sensor tube 14 which constitutes a part of the flow rate sensor means 12 is connected to both ends of the bypass 6. The fluid can flow. A pair of electric heating coils 16 for control are wound around the sensor tube 14. A mass flow rate of the gas fluid is detected by a sensor control circuit 18 connected to the coil, and the detected flow rate is output as a sensor signal. Has become. As described above, the flow sensor unit 12 is mainly configured by the sensor tube 14, the electric heating coil 16, and the sensor control circuit 18. The sensor control circuit 18 is configured to detect, for example, heat transfer generated due to a flow of a gas fluid as a change in electric resistance by a Wheatstone bridge, and is configured by an analog circuit.
[0012]
The detected flow rate is output as the sensor signal S1 as described above,
The data is input to the comparison unit 22 formed by an analog circuit. The comparison unit 22 corrects a setting signal ( flow setting signal) S0 input from the outside by applying a predetermined correction process to the digital operation circuit system DE to correct the setting signal S0− 1 is analogized and input. Further, a control constant switching unit 20 for selectively applying a predetermined control constant is connected to the comparison unit 22. The control constant switching unit 20 appropriately and selectively switches, for example, the control constant of PID control. For example, the control constant switching unit 20 divides the flow range up to the maximum flow rate into a plurality of areas, and sets the control constant of the area corresponding to the set flow rate. It is used selectively. Specifically, as shown in FIG. 2, a plurality of, for example, ten control constant resistances R1 to R10 having different resistance values, and circuits of changeover switches SW1 to SW10 respectively connected in series thereto are provided in parallel to form an analog circuit. The switching switches SW1 to SW10 are selectively switched by a switching unit control signal from a digital operation circuit system described later. In this case, for example, the range from zero flow rate to the maximum flow rate is equally divided into ten regions, and the resistances R1 to R10 are applied so as to provide optimal control constants for each region. For example, a resistor R1 is selected in a range of 0 to 10% of the maximum flow rate, and a resistor R2 is selected in a range of 11 to 20%. You have to choose. In FIG. 2, the capacitor 38 is a capacitor for determining a time constant in combination with the resistors R1 to R10.
[0013]
The comparing section 22 compares the sensor signal S1 with the setting signal S0-1 and outputs a comparison signal S2 by taking the difference into account with the control constant. The comparison signal S2 from the comparison unit 22 is input to a valve drive unit 26 also constituted by an analog circuit. The valve drive unit 26 controls the flow control according to the magnitude of the comparison signal. A valve voltage is output as a drive voltage toward the valve 10.
Thus, an analog circuit system AN forming a feedback loop is configured.
The flow control valve 10 has, for example, a laminated piezoelectric element 29 that generates a large change in thrust within a small stroke range as an actuator for vertically driving the diaphragm 8. Is controlled by As the actuator, for example, a solenoid using an electromagnet may be used instead of the piezoelectric element 29.
[0014]
On the other hand, the digital operation circuit system DE includes, for example, a microcomputer that operates digitally. A setting signal output from an external device such as a semiconductor manufacturing device is input to the digital operation circuit system DE. This setting signal is usually an analog signal of 0 to 5 V, and after being converted into a digital signal by the A / D converter 28, is input to the digital operation circuit system DE. The digital operation circuit system DE performs a correction according to the characteristics of the flow rate sensor means 12, for example, a zero point shift correction and a linear shift correction, on the setting signal, and outputs the corrected setting signal. Has become.
Since the corrected setting signal is output as a digital signal, it is converted into an analog signal by the D / A converter 30 and input to the comparing unit 22.
Further, the sensor signal S1 from the sensor control circuit 18 is converted into a digital signal by the A / D converter 40 and then input to the digital operation circuit system DE, where the correction applied to the setting signal S0 is reversed. After that, the signal is converted into an analog signal by the D / A converter 42 and output to the outside as the flow rate output signal S5.
The digital operation circuit system DE is connected to an interface 32 for performing digital communication with the outside as necessary. Further, in addition to the above-described correction, the digital arithmetic circuit DE outputs a switching unit control signal to the control constant switching unit 20 or outputs a switching zero signal to the valve driving unit 26 as described later. When a setting signal to a predetermined flow rate is input from the state, a signal is output to instruct the valve voltage to be applied at a stretch to a voltage immediately before the fluid starts flowing out, or a sensor signal is constantly monitored and related. The data is updated as needed.
[0015]
Next, an example of a control method performed by using the mass flow controller configured as described above will be specifically described.
First, the gas fluid flows out flow control valve 10 opens in response thereto the setting signal S 0 output from an external gas used system is input into the device 2. When the gas fluid flows through the fluid passage 4, a part of the gas flows through the sensor pipe 14 of the flow rate sensor unit 12, and most of the gas flows through the bypass 6. To systems, for example, semiconductor manufacturing equipment.
The flow rate of the gas fluid flowing through the sensor tube 14 is detected by a constant current type sensor control circuit 18 using a bridge circuit, and the mass flow rate flowing through the entire fluid passage 4 is obtained. This flow rate is sent to the comparison unit 22 as a sensor signal S1. Output to
[0016]
On the other hand, for example, a setting signal S0 input from the outside as an analog signal of 0 to 5 V is converted into a digital signal by the A / D converter 28 and then input to the digital operation circuit system DE, where various corrections are performed. Added. Then, the digital operation circuit system DE outputs a corrected setting signal S0-1 to which a predetermined correction has been applied. After the signal S0-1 is converted into an analog signal by the D / A converter 30, Are input to the comparison unit 22.
The comparison unit 22 compares the sensor signal S1 with the corrected setting signal S0-1 by analog processing in consideration of the optimal control constant for the PID control switched and selected by the control constant switching unit 20, This result is output as a comparison signal S2 to the valve drive unit 26. The valve drive unit 26 applies a valve voltage S3 based on the comparison signal S2 to the piezoelectric element 29 of the flow control valve 10 to adjust the valve opening of the diaphragm 8.
As described above, the flow rate of the gas fluid is fed back in a closed system that reaches the sensor control circuit 18 that outputs the sensor signal S1, the comparison unit 22 (including the control constant switching unit 20), the valve driving unit 26, and the flow control valve 10. Will be controlled. In this case, since the closed feedback loop is entirely constituted by the analog circuit AN as described above, the closed feedback loop is controlled intermittently, for example, every 10 msec using a digital control system as in the conventional device. Unlike the case, since the feedback control can be continuously performed, the control accuracy can be improved.
[0017]
Also, in order to perform feedback control by digital processing as in a conventional device, it is necessary to convert each signal in an analog state into a digital signal, or conversely, to convert a digital signal into an analog signal finally. However, it is inevitable that a delay occurs in the control due to the conversion operation every time the conversion operation is performed. However, in the case of the present embodiment, since the control of the feedback loop is all performed by analog processing as described above, There is no delay, and the flow rate can be controlled quickly and with high accuracy.
If it is not necessary to apply various corrections to the setting signal S0, the setting signal S0 may be directly input to the comparing unit 22. Similarly, when the control constant of the PID control is fixed, the control constant switching unit 20 becomes unnecessary.
[0018]
As described above, of the control relating to the responsiveness of the flow control valve 10, the control relating to the feedback loop from the detection of the sensor signal S1 to the output of the valve voltage S3 is performed by analog processing. The operation of issuing a command to correct the zero point deviation, the correction of the linear deviation, the switching of the control constant, and the operation of setting the valve voltage immediately before the start of the flow of the sensor means 12 are performed by the digital operation circuit system DE. The operation of issuing the control constant switching command is a process that requires storage and comparison judgment, and it is not practical in terms of mounting size and cost if it is performed by an analog circuit. Be sure to do it.
[0019]
Next, each of the above corrections or operations will be described.
First, for the operation of issuing a control constant switching command, the digital operation circuit system DE determines how much the setting signal S0 indicates with respect to the full scale, which is the maximum flow rate of the applied flow control valve 10. Then, the switching unit control signal S4 is output so as to select a resistance value (corresponding to a control constant) corresponding to the flow rate in FIG. 2, and the switch SW of the corresponding resistance is turned on. In FIG. 2, since the setting signal S0 (0 to 5V) indicates a flow rate within the range of 10 to 20% of the full scale flow rate, the switch SW2 is closed and the resistance value of the resistor R2 is selected as a control constant. Is shown. It should be noted that the number of divided regions for the full-scale flow rate is not limited to the ten regions in the illustrated example.
[0020]
Next, the correction of the zero point deviation and the correction of the linear deviation of the flow rate sensor unit 12 will be described.
Generally, as shown in FIG. 3, the actual characteristic (solid line) of the flow rate sensor means 12 is such that the relationship between the set signal (set flow rate) and the sensor signal (detected flow rate) is an ideal straight line (broken line). , And has a characteristic that the zero point is shifted from the origin, and this characteristic differs depending on the individual flow control valve 10 and the flow sensor means 12. Therefore, the characteristics of the flow rate sensor means 12 and the flow rate control valve 10 are measured in advance at the time of factory shipment and stored in advance in a storage unit (not shown) of the digital operation circuit system DE. In order to suppress the control error, the above-described zero point shift correction and straight line shift correction are performed on the setting signal S0.
In the zero point shift correction, an actual characteristic (solid line) is shifted downward as indicated by an arrow A to obtain a characteristic curve (dash-dot line) passing through the origin. Then, the correction of the straight line deviation is obtained by calculating the deviation between the characteristic curve indicated by the one-dot chain line and the ideal characteristic indicated by the broken line as a displacement amount indicated by an arrow B, and correcting the setting signal S0 in consideration of the displacement amount.
This makes it possible to perform flow control with higher control accuracy.
[0021]
Next, an operation of setting the valve voltage immediately before the start of the flow will be described.
First, as shown in FIG. 4, the relationship between the valve voltage and the flow rate is not linear, and when the voltage is gradually increased from the fully closed state (0 V), even if the voltage is increased, the flow rate does not change and becomes zero. There is an area indicating the status. In FIG. 4, almost no gas flows up to 50V.
The reason is that when the drive voltage is increased, a force is generated in the direction of opening the valve body (diaphragm 8), but the force does not immediately displace, and the force (surface pressure) by which the valve body presses the valve seat is reduced. Used to do. Then, a voltage rise after the surface pressure becomes zero appears as a displacement. The flow control valve has a characteristic that gas flows at a maximum flow rate (full scale) of 2000 cc / min when the valve voltage is 150 V.
For a valve actuator of a mass flow controller such as a mass flow controller, in addition to the above-described laminated piezoelectric element, a solenoid, thermal expansion, and the like are used. Actuators are also designed to apply some surface pressure. In addition, such design and adjustment must be made in order to absorb mechanical displacement of the valve and misalignment due to the difference in thermal expansion of parts due to changes in the ambient temperature. is necessary.
[0022]
Thus, in the case shown in FIG. 4, approximately 50 V is the outflow valve voltage.
Under such characteristics, when the set flow rate is changed from the zero flow state to the predetermined flow rate, if the PID control is always performed from the zero valve voltage state, much time is required until the gas starts to flow. Therefore, a delay occurs in the control. Therefore, in order to prevent this control delay, when a setting signal S0 that rapidly changes the flow rate from a zero flow state to a predetermined flow rate value is input, the flow-out valve voltage, for example, about 50 V The digital operation circuit system DE issues a command to the valve drive unit 26 to output a small voltage value, for example, a voltage smaller by 5 V, for example, 45 V, to the piezoelectric element 29 as an initial valve voltage. Then, the process immediately shifts to the PID control. Therefore, in this case, even when the flow rate is rapidly changed from zero flow rate to the maximum flow rate (100% flow rate), a drive voltage of 45 V is output as the initial valve voltage. By performing such control, the occurrence of overshoot can be reliably suppressed, and the flow rate can be quickly and stably set to a desired flow rate.
[0023]
In this case, when the initial valve voltage is obtained, the digital operation circuit system DE obtains the flow valve voltage determined in advance, for example, a predetermined ratio of 50 V, for example, 90% as 45 V (= 50 V × 0.9). Alternatively, a voltage value that is smaller by a predetermined amount, for example, 5 V, than the outflow valve voltage, for example, 50 V, may be obtained as the initial valve voltage.
In any case, since the characteristics of the flow control valve 10 are unique to each control valve, the flow-out valve voltage is obtained in advance at the factory shipment stage or the like.
Also, in this case, the flow-out valve voltage may change due to the aging of the flow control valve 10 or the like, so that the digital operation circuit system DE constantly monitors and updates this.
[0024]
Here, the control characteristics of the device of the present invention and the conventional device as described above were actually measured, and the evaluation results will be described.
FIG. 5 is a graph showing the control characteristics of the device of the present invention and the conventional device (only digital circuit). FIG. 5A shows the characteristic curve of the conventional device, and FIG. 5B shows the characteristic curve of the device of the present invention. As shown in FIG. 5 (A), in the case of the conventional device, a control delay of about 2 seconds has occurred after the setting signal was input, but in the case of the device of the present invention shown in FIG. 5 (B). It was found that the control delay was about 0.7 seconds, and the control delay was significantly improved.
In this embodiment, the case where a gaseous fluid flows is described as an example. However, the present invention is not limited to this, and the present invention can be applied to a case where a liquid fluid flows.
[0025]
【The invention's effect】
As described above, according to the mass flow controller of the present invention, the following excellent operational effects can be exhibited.
By then comparing the sensor signal and the flow rate setting signal to a feedback loop to determine the valve voltage by this result to perform an analog circuit system, fully continuous, and to perform excellent control in response In addition, since other complicated control relating to the response is performed by a digital circuit system, high precision control can be achieved, and therefore, the advantage of digital processing and the advantage of analog processing can be achieved at the same time. be able to.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing a mass flow controller according to the present invention.
FIG. 2 is a configuration diagram illustrating a control constant switching unit illustrated in FIG. 1;
FIG. 3 is a graph showing a relationship between a setting signal and a sensor signal for explaining a correction control of a zero point shift and a straight line shift of a flow sensor.
FIG. 4 is a graph showing a relationship between a valve voltage and a flow rate.
FIG. 5 is a graph showing control characteristics of the device of the present invention and a conventional device (only a digital circuit).
[Explanation of symbols]
2 Mass flow controller 4 Fluid passage 6 Bypass 10 Flow control valve 12 Flow sensor means 14 Sensor tube 18 Sensor control circuit 20 Control constant switching unit 22 Comparison unit 26 Valve driving unit AN Analog circuit system DE Digital operation circuit system

Claims (3)

Mass flow control for controlling a mass flow rate of the fluid by controlling a flow rate control valve provided in a fluid passage for flowing the fluid based on a flow rate setting signal input from outside and a sensor signal from a flow rate sensor means. in the device, Propelled by one and a digital arithmetic circuitry and the analog circuitry with respect to the control of the response of the flow control valve,
The analog circuit system is connected to a comparison unit that compares the flow rate setting signal and the sensor signal, a valve driving unit that outputs the valve voltage having a magnitude based on an output of the comparison unit, and the comparison unit. A control constant switching unit for switching and selecting a control constant used for the comparison unit, and performing feedback control for determining the valve voltage of the flow control valve by comparing the sensor signal and the flow setting signal,
The mass flow controller according to claim 1, wherein the digital operation circuit system is configured to output a switching unit control signal for controlling the control constant switching unit based on the flow rate setting signal . The digital operation circuit system corrects a zero point shift and a straight line shift of the flow rate sensor unit based on the flow rate setting signal and the sensor signal, and outputs a corrected flow rate setting signal. to claim 1 Symbol placement of the mass flow controller. The digital arithmetic circuit system obtains an initial valve voltage which is a valve voltage immediately before the fluid starts flowing when the flow control valve starts to open when the set flow rate is changed from a state of zero flow rate to a predetermined flow rate. Item 3. The mass flow controller according to Item 1 or 2 .

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