Mass flow control method and apparatus

Abstract

A self-verifiable mass flow control (MFC) device and method for providing self-verifying mass flow control are provided. The MFC includes a chamber configured to receive a fluid, an upstream valve disposed upstream of the chamber, and a downstream control valve disposed downstream of the chamber. The MFC further includes a pressure drop element disposed downstream of the downstream control valve and first and second pressure sensors. The MFC's controller is configured to control operation of the downstream control valve by toggling between flow-based feedback control and pressure-based feedback control. In the flow-based feedback control, flow is monitored based on the rate of pressure decay in the chamber detected by the first pressure sensor when the upstream control valve is closed. In the pressure-based feedback control, pressure upstream of the pressure drop element detected by the second pressure sensor is monitored.

Description

[Technical Field]

[0001] Related Applications This application claims the benefit of U.S. Provisional Patent Application No. 63 / 433,310, filed December 16, 2022, the entire teachings of which are incorporated herein by reference. [Background technology]

[0002] Semiconductor manufacturing processes often require the delivery of multiple different gases and gas mixtures in varying amounts throughout multiple processing steps. Gases are typically stored in tanks at the processing facility, and gas metering systems are used to deliver metered amounts of gas from the tanks to processing tools such as chemical vapor deposition reactors, vacuum sputtering systems, and plasma etch systems. Other components, such as valves, pressure regulators, mass flow controllers (MFCs), mass flow ratio controllers (FRCs), mass flow meters (MFMs), and mass flow verifiers (MFVs), can be included within the gas metering system or in the flow path from the gas metering system to the processing tool. Components such as MFCs, FRCs, MFMs, and MFVs are provided to ensure accurate delivery of process gases.

[0003] Generally, a mass flow controller (MFC) controls and monitors the flow rate of a fluid (e.g., gas or vapor) in real time, allowing the mass flow rate of gas through a device to be measured and controlled. Traditional pressure-based MFCs include a flow control valve and a pressure-dropping element, such as a flow nozzle. MFCs can measure the flow rate using one or more pressure sensors. MFCs control the flow rate based on a predetermined setpoint, typically preset by the user or an external device, such as a semiconductor tool. The setpoint can be changed at each step of the process. Summary of the Invention

[0004] A self-verifying mass flow control device and method for providing self-verifying mass flow control are provided.

[0005] The mass flow controller (MFC) includes a chamber configured to receive a fluid, an upstream valve disposed upstream of the chamber, and a downstream control valve disposed downstream of the chamber. The MFC further includes a pressure drop element disposed downstream of the downstream control valve and first and second pressure sensors. The first pressure sensor detects fluid pressure in the chamber, and the second pressure sensor detects fluid pressure downstream of the downstream control valve and upstream of the pressure drop element. The MFC further includes a controller configured to control operation of the downstream control valve. The controller is configured to toggle operation of the control valve between flow-based feedback control and pressure-based feedback control. In flow-based feedback control, flow is monitored based on a pressure decay rate in the chamber detected by the first pressure sensor when the upstream control valve is closed. In pressure-based feedback control, pressure upstream of the pressure drop element detected by the second pressure sensor is monitored.

[0006] The controller of the MFC may be further configured to record the pressure detected by the second pressure sensor during flow-based feedback control for use during pressure-based feedback control. A pressure set point for the pressure-based feedback control may be established based on the recorded pressure. Alternatively, or additionally, a coefficient for use in calculating the flow rate during pressure-based feedback control may be determined. The coefficient may be determined based on the recorded pressure.

[0007] During flow-based feedback control, the MFC's controller measures the fluid flow rate Q and sets the measured flow rate to a flow rate setpoint Q. sp For example, the measured flow rate Q can be determined according to the following formula:

number

[0008] During pressure-based feedback control, the MFC controller controls the flow rate of the fluid, Q cf is calculated and the calculated flow rate is the flow rate set point Q sp For example, the calculated flow rate Q cf can be calculated according to the following formula:

number

number

[0009] The controller of the MFC can include a flow controller and a pressure controller. The flow controller can be configured to monitor the flow rate based on the damping rate and compare the monitored flow rate with a flow setpoint. The pressure controller can be configured to monitor the pressure detected by the second pressure sensor and compare the monitored pressure with a pressure setpoint. Optionally, the upstream valve can be a control valve.

[0010] A method for controlling a flow rate includes toggling operation of a downstream control valve of a mass flow controller between flow-based feedback control and pressure-based feedback control. The flow-based feedback control includes monitoring a flow rate based on a pressure decay rate of a fluid in a chamber of the mass flow controller. The pressure decay rate is based on a pressure detected by a first pressure sensor upon closure of a valve disposed upstream of the chamber, the first pressure sensor detecting the fluid pressure in the chamber. The pressure-based feedback control includes monitoring a pressure upstream of a pressure reduction element based on a pressure detected by a second pressure sensor. The pressure reduction element is disposed downstream of the downstream control valve, and the second pressure sensor detects the fluid pressure downstream of the downstream control valve and upstream of the pressure reduction element.

[0011] The method may further include recording the pressure detected by the second pressure sensor during flow-based feedback control for use during pressure-based feedback control. A pressure setpoint for the pressure-based feedback control may be established based on the recorded pressure. Alternatively, or additionally, a coefficient for use in calculating the flow rate during pressure-based feedback control may be determined. The coefficient may be determined based on the recorded pressure.

[0012] During flow-based feedback control, the method measures the flow rate Q of a fluid and calculates the flow rate setpoint Q. sp The measured flow rate can be calculated according to Equation 1 above.

[0013] During pressure-based feedback control, this method cf is calculated and the calculated flow rate is the flow rate set point Q sp The calculated flow rate Q can be compared with cf can be calculated according to the above formula 2, and the coefficients used in the above formula 2 can be calculated according to the above formula 3.

[0014] The mass flow controller (MFC) includes a chamber configured to receive a fluid, an upstream valve disposed upstream of the chamber, and a downstream control valve disposed downstream of the chamber. The MFC further includes a pressure drop element disposed downstream of the downstream control valve and first and second pressure sensors. The first pressure sensor detects fluid pressure in the chamber, and the second pressure sensor detects fluid pressure downstream of the downstream control valve and upstream of the pressure drop element. The MFC further includes a controller configured to control operation of the downstream control valve. The controller is configured to monitor the pressure upstream of the pressure drop element detected by the second pressure sensor, determine a flow rate based on the monitored pressure and a coefficient, control operation of the downstream control valve based on adjusting the determined flow rate to a flow setpoint, determine a verification flow rate based on a pressure decay rate in the chamber detected by the first pressure sensor when the upstream control valve is closed, and update the coefficient based on the verification flow rate.

[0015] The mass flow controller (MFC) includes a chamber configured to receive a fluid, an upstream valve disposed upstream of the chamber, and a downstream control valve disposed downstream of the chamber. The MFC further includes a pressure drop element disposed downstream of the downstream control valve and first and second pressure sensors. The first pressure sensor detects fluid pressure in the chamber, and the second pressure sensor detects fluid pressure downstream of the downstream control valve and upstream of the pressure drop element. The MFC further includes a controller configured to control operation of the downstream control valve, the controller maintaining a controlled flow rate based on pressure detected by at least one of the first pressure sensor and the second pressure sensor, and recalibrating pressure-based flow control based on the pressure detected by the second pressure sensor using a pressure decay rate detected by the first pressure sensor during controlled flow.

[0016] The method for controlling a flow rate includes maintaining a controlled flow rate by controlling operation of a downstream control valve based on pressure detected by at least one of a first pressure sensor and a second pressure sensor. The first pressure sensor detects a fluid pressure in a chamber configured to receive a fluid. The second pressure sensor detects a fluid pressure downstream of the downstream control valve and upstream of a pressure reduction element, the pressure reduction element being disposed downstream of the downstream control valve. The method further includes recalibrating the pressure-based flow control based on the pressure detected by the second pressure sensor using a pressure decay rate detected by the first pressure sensor during the controlled flow rate.

[0017] The mass flow controller (MFC) includes a chamber configured to receive a fluid, an upstream valve disposed upstream of the chamber, and a downstream control valve disposed downstream of the chamber. The MFC further includes a pressure drop element disposed downstream of the downstream control valve and first and second pressure sensors. The first pressure sensor detects fluid pressure in the chamber, and the second pressure sensor detects fluid pressure downstream of the downstream control valve and upstream of the pressure drop element. The MFC further includes a controller configured to control operation of the downstream control valve. The controller is configured to perform pressure-based flow control based on monitoring the pressure upstream of the pressure drop element detected by the second pressure sensor. The controller is further configured to determine a verification flow rate based on the pressure decay rate in the chamber detected by the first pressure sensor when the upstream control valve is closed, while maintaining the pressure-based flow control or the pressure decay rate-based flow control in the chamber, and recalibrate the pressure-based flow control based on the verification flow rate.

[0018] The MFC may be further configured to toggle between providing pressure-based flow control and providing flow control based on monitoring a flow rate based on a pressure decay rate in the chamber detected by the first pressure sensor, or the MFC may be further configured to maintain a controlled flow rate based on pressure-based flow control when determining the verification flow rate.

[0019] The method for controlling a flow rate includes performing pressure-based flow control of actuation of a downstream control valve of a mass flow controller, and determining a verification flow rate based on a pressure decay rate in a chamber of the mass flow controller while maintaining the pressure-based flow control or the pressure decay rate-based flow control in the chamber. The method further includes recalibrating the pressure-based flow control based on the verification flow rate. The pressure decay rate is based on a pressure sensed by a first pressure sensor when the upstream control valve is closed, the first pressure sensor sensing the fluid pressure in the chamber. The pressure-based flow control is based on monitoring a pressure upstream of a pressure drop element sensed by a second pressure sensor, the pressure drop element being located downstream of the downstream control valve, and the second pressure sensor sensing the fluid pressure downstream of the downstream control valve and upstream of the pressure drop element.

[0020] The method may further include toggling between providing pressure-based flow control and providing flow control based on monitoring a flow rate based on a pressure decay rate in the chamber detected by the first pressure sensor. Alternatively, the method may further include maintaining a controlled flow rate based on the pressure-based flow control when determining the verification flow rate.

[0021] The pressure drop element can be a critical flow nozzle.

[0022] The foregoing will become apparent from the following more detailed description of exemplary embodiments, as illustrated in the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the embodiments. [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a schematic diagram of an example mass flow controller. [Figure 2] FIG. 1 is a diagram showing an example of feedback control provided in a mass flow controller. [Figure 3]3 is a flowchart illustrating an exemplary operation of the mass flow controller with feedback control shown in FIG. 2. [Figure 4] FIG. 10 is a diagram showing another example of feedback control provided in the mass flow controller. [Figure 5] 5 is a flowchart illustrating an exemplary operation of the mass flow controller with feedback control shown in FIG. 4. [Figure 6] FIG. 10 is a diagram showing another example of feedback control provided in the mass flow controller. [Figure 7] 7 is a flowchart illustrating an exemplary operation of the mass flow controller with feedback control shown in FIG. 6. DETAILED DESCRIPTION OF THE INVENTION

[0024] A description of exemplary embodiments follows.

[0025] MFCs typically include a system controller as part of a feedback control system, which provides a control signal to a control valve based on a comparison of the set flow rate and the measured flow rate. The feedback control system thus operates the flow control valve to maintain the measured flow rate at the set flow rate. Pressure-based mass flow control involves attaching a pressure sensor and a temperature sensor to the critical flow nozzle to measure mass flow rate under critical flow conditions, or attaching two pressure and temperature sensors to measure mass flow rate under non-critical flow conditions. Either method typically obtains the pressure upstream of the flow nozzle for use in flow calculations.

[0026] In traditional MFCs, the feedback control system assumes that the MFC is calibrated within a certain tolerance. To test whether the MFC is within the calibration tolerance, the MFC is typically tested offline using a mass flow verifier (MFV). While offline testing is highly accurate, it does not provide a solution to the problem of the MFC going out of calibration during process execution. In many cases, an out-of-calibration MFC is not detected until the process is complete, which can result in reduced product yield or even the loss of the entire product. Yield loss can be costly and is clearly undesirable.

[0027] A prior art method for providing self-verification is described in U.S. Patent No. 10,801,867, the entire contents of which are incorporated herein by reference. As described therein, a flow verification check of an MFC can be performed during a transition from a non-zero setpoint to a zero setpoint. The flow verification is based on the rate of pressure decay as fluid continues to flow from a reservoir upstream of the MFC.

[0028] Other prior art methods and devices for providing self-verification in an MFC require complex hardware configurations, such as devices that require sensitive valve position feedback components to provide independent flow measurements, or devices that include multiple additional components to provide a separate mass flow verifier (MFV) essentially built into the MFC. There is a need for improved mass flow controllers and methods that allow for in-field verification and / or calibration. Additionally, there is a need to provide self-verifying MFCs in smaller footprint configurations.

[0029] A self-verifiable mass flow control device and method for self-verifying mass flow control are provided. The provided device includes an improved control system and hardware configuration.

[0030] 1, mass flow controller 100 receives a fluid flow at inlet 122, which flows through a body 126 of the device. MFC 100 includes an upstream valve 102 and a downstream valve 104. As shown, both valves 102, 104 are adjustable control valves, although upstream valve 102 may alternatively be an on / off type valve.

[0031] As used herein, the term "control valve" refers to a valve capable of providing a controllable range of opening states, such as between an on state and an off state, and excludes on / off type valves. The opening of an adjustable control valve is controllable in response to a control signal, thereby controlling the flow rate through the valve. Adjustable control valves include proportional control valves. Examples of control valves suitable for use as adjustable control valves in the provided devices include solenoid valves, piezoelectric valves, and stepper motor valves.

[0032] The MFC further comprises a chamber 110, a temperature sensor 106 for detecting the temperature of the fluid in the chamber 110, and a pressure sensor 112 for detecting the fluid pressure in the chamber 110. The chamber 110 is disposed between the upstream valve 102 and the downstream control valve 104 and is provided for pressure decay rate control measurements. The MFC further comprises a critical flow nozzle 116. A second pressure sensor 114, and optionally a second temperature sensor 108, are disposed downstream of the control valve 104 and upstream of the critical flow nozzle 116. The second pressure sensor 114 is provided for pressure control measurements. The fluid exits the MFC at an outlet 124.

[0033] The MFC 100 further includes a controller 120 that can receive sensed temperature and pressure information from the sensors 106, 108, 112, and 114 and provide control signals to operate the valves 102 and 104. The MFC 100 combines pressure decay rate flow control and pressure control to regulate the flow rate of fluid through the device to a setpoint. For example, flow rate monitoring can be performed by alternating between obtaining upstream pressure decay rate flow measurements (e.g., for comparison to a flow setpoint) and downstream pressure measurements (e.g., for comparison to a pressure setpoint). The pressure setpoint for the downstream pressure control period can be determined on the fly when the flow rate through the device is regulated to the flow setpoint and stabilized during the upstream pressure decay rate flow control period.

[0034] In an example operation of the MFC 100, an upstream pressure decay rate is obtained based on pressure measurements obtained from the pressure sensor 112 when the upstream shutoff valve 102 is closed, and this pressure decay rate is used to calculate a flow rate for initial flow control. During this period, the controller 120 can compare the measured flow rate to a flow setpoint and generate a control signal for operation of the adjustable control valve 104. When the upstream shutoff valve 102 is open to fill a volume (e.g., the volume of the chamber 110 and fluid conduit), mass flow control is provided by controlling the downstream pressure of the device. During this period, the controller 120 can generate a flow control signal for operation of the adjustable control valve 104 based on pressure measurements obtained from the pressure sensor 114. While this operation is described with initial flow control provided based on pressure decay rate measurements for use in calculating a flow rate for comparison to the flow setpoint, the device may instead operate at an initial pressure setpoint for initial pressure control, and a subsequent upstream pressure decay period can be invoked to verify the pressure setpoint.

[0035] Traditional pressure-based MFCs typically require that the gas properties be known to determine the flow rate measurement of the gas passing through the device. Measuring the pressure decay rate provides a gas-independent method of determining flow rate. The pressure decay rate method may be undesirable for use with mass flow controllers because re-establishing the inlet gas flow rate when refilling the internal volume of the device can disrupt the flow control provided by the device.

[0036] The MFC devices and methods provided herein can overcome the problems of conventional pressure-based MFCs by performing initial and / or intermittent periods of flow control based on pressure decay rate measurements, during which the pressure downstream of chamber 110 and upstream of critical flow nozzle 116 is also monitored. The downstream pressure (P2) detected by downstream sensor 114 during such periods can be used to establish a pressure setpoint for use during the pressure control period. Thus, the MFC can self-verify its accuracy at any point in the process or periodically throughout the process while providing continuous mass flow control.

[0037] The flow-based feedback control period can be relatively short so that the volume of chamber 110 is not discharged so much that the critical flow condition at nozzle 116 is disturbed. For example, the upstream pressure (P1) can be monitored to remain above a threshold, as described further below. Optionally, valve 102 can be a proportional control valve, which gradually reintroduces gas flow into the volume to prevent potential disruptions.

[0038] In one configuration, the MFC's controller 120 can include two controllers: a flow controller 210 and a pressure controller 220, as shown in Figure 2. The MFC can maintain a continuous controlled flow rate by toggling between controllers 210 and 220.

[0039] The flow controller 210 provides flow-based control (Kq) and generates a valve input (VI) to the control valve (valve 104) to perform mass flow control (MFC) for the system. The flow-based control is based on the measured flow rate (Q) and the flow set point (Q sp ) can be compared. The measured flow rate can be based on the essentially gas-independent pressure decay rate principle. In particular, the flow rate (Q) can be measured according to the following equation, where V is the volume of the chamber (e.g., chamber 110) and T stp is the standard temperature, and P stp is the standard pressure, P1 is the pressure of the fluid in the chamber sensed by a first pressure sensor (eg, pressure sensor 112), and T is the sensed temperature of the fluid (eg, temperature sensor 106).

number

[0040] The pressure controller 220 provides pressure-based control (Kp) and generates a valve input (VI) to the control valve (valve 104) to perform mass flow control (MFC) of the device. Under critical flow conditions, the flow rate (Q) of the gas is controlled by the downstream outlet pressure (e.g., P in FIG. 1). d ) and is proportional to the pressure upstream of the nozzle (e.g., nozzle 116) (e.g., P2 in FIG. 1). The flow rate through a device based on this principle can be expressed as follows, where k is a function of the molecular weight of the gas (mw), the specific heat ratio of the gas (γ), and the temperature of the gas (T):

number

[0041] By controlling the nozzle upstream pressure (P2), the flow rate (Q) passing through the nozzle can be effectively controlled. The critical flow rate condition generally occurs when the nozzle upstream pressure (P2) is greater than the nozzle downstream pressure (P d ) is at least twice as large as the

number

[0042] Pressure-based control is based on the measured pressure (P2) and the pressure set point (P sp ) can be compared to this pressure set point (P sp ) can be determined on the fly once the flow rate has stabilized through the upstream pressure decay rate flow control.

[0043] A flow chart illustrating an example process 300A for the operation of an MFC is shown in FIG. sp ) is provided to the device (302) and the upstream shutoff valve (e.g., valve 102) is closed (304). A flow rate (Q) based on the pressure decay rate determined from the measured upstream pressure and temperature is calculated (306). The calculated flow rate (Q) is provided to a flow feedback controller (Kq), which generates a control command (VI) to the downstream control valve of the device (308). If the flow rate is not stable (310), measurements based on the pressure decay rate can continue. If the flow rate is stable (310), the downstream pressure (P2) is recorded and the pressure setpoint (P) of the pressure feedback controller is set. sp ) is set (312). At this stage, the operation of the device switches from flow control to pressure control (316). Alternatively, the upstream pressure (P 1) Flow-based control can also continue until the upstream pressure falls below a threshold (314). Switching to pressure-based control (316), the upstream shutoff valve can be opened as needed if the upstream pressure is below a threshold, or the upstream shutoff valve can remain closed until the downstream pressure (P2) stabilizes, as shown in the flowchart. During pressure control, the downstream pressure (P2) is provided as an input to a pressure feedback controller (Kp), which generates a control command (VI) to the device's downstream control valve 104 (318). The control valve adjusts the monitored pressure (P2) to a pressure setpoint (P sp) (320). If the upstream shutoff valve 102 is not already open, once the monitored pressure stabilizes (322), the shutoff valve 102 is opened and the volume is recharged (324). Once the upstream pressure (P1) stabilizes, the MFC can revert to flow-based control (326). However, pressure-based control can continue for a period of time until the upstream pressure stabilizes.

[0044] In another implementation, the MFC's controller 120 can include a feedback controller 410 that can switch between flow-based and pressure-based control, as shown in Figure 4. Flow-based control can be performed using Equation 1, as described above with respect to Figure 2. Pressure-based control further processes downstream pressure measurements to determine a flow setpoint (Q sp )

[0045] During pressure-based feedback control, the MFC controller 120 controls the flow rate of the fluid Q cf is calculated and the calculated flow rate is the flow rate set point Q sp For example, the determined flow rate Q cf can be calculated according to the following formula:

number

number

[0046] A flowchart illustrating another example process 300B for operation of an MFC is shown in FIG. 5. Process 300B is similar to process 300A shown in FIG. 3, except that the device includes a single feedback controller that can provide both upstream pressure decay rate feedback control and pressure-based feedback control. Specifically, the feedback controller calculates (308B) a control command for controlling the downstream control valve based on the pressure decay rate measurement, and during this flow-based feedback period, adjusts the coefficient (K) used during the pressure-based feedback period. cf ) is calculated (312B). During the pressure-based feedback control period, the pressure-based flow rate (Q cf ) is determined (318B), and the feedback controller calculates a control command for controlling the downstream control valve based on a comparison of this measurement to the flow setpoint (320B).

[0047] In this way, the provided MFC 100 can alternate between periods when the upstream shutoff valve is open and periods when it is closed. The device can use the upstream pressure measurement (P1) for mass flow control during periods when the upstream shutoff valve is closed and the downstream pressure measurement (P2) for mass flow control during periods when the upstream shutoff valve is open. The upstream pressure measurement can be used to calculate the flow rate through the device based on the principle of pressure decay rate, which is essentially gas-independent. Such gas-independent measurements can provide information about the device's pressure setpoint, which is determined when the flow rate determined by the decay rate stabilizes; otherwise, gas characteristics must be known and more complex calculations must be performed to control the flow rate. The downstream pressure measurement can be used for pressure-based control directly (e.g., by comparison to a pressure setpoint) or indirectly (e.g., by converting it to a calculated flow rate based on coefficients determined during flow-based feedback control and comparing it to a flow setpoint).

[0048] By alternating between flow-based and pressure-based feedback control, the MFC can provide uninterrupted flow control while validating the pressure-based setpoint. In a configuration such as that shown in Figure 1, flow control with downstream pressure-based control is very stable even as the upstream pressure changes during volume filling. Such an MFC configuration is particularly advantageous for low-flow applications (e.g., flow setpoints below 200 sccm).

[0049] In yet another example configuration, the MFC's controller 120 may include a feedback controller 510 configured to provide pressure-based feedback control, as shown in Figure 6. As shown, flow control is achieved by monitoring downstream pressure as per Equation 2, with verification and / or recalibration performed using the upstream pressure decay flow measurement rate. If the flow error between Qcf and Q exceeds a predetermined threshold, K cf Coefficients can be determined or updated on the fly (e.g., K cf =Q / P2).

[0050] For example, the controller 120 may use the measured pressure P2 and the coefficient K cf Based on the initial or provided value of flow rate Q cf The determined flow rate Q cf is the flow rate set point Q sp , the operation of the control valve 104 can be controlled.

[0051] Measured flow rate Q cf is the flow rate set point Q sp Once the downstream pressure (P2) is controlled and stabilized, cf The controller can be further configured to perform verification based on pressure decay rate measurements. In particular, the upstream valve 102 can be closed and the upstream pressure P1 monitored to obtain a flow measurement Q, such as via Equation 1. The upstream valve 102 may be closed for a short enough time (e.g., less than about 5 seconds) to obtain a flow measurement. The flow measurement Q is calculated as Q cf It can be used to verify.

[0052] For example, Q and Qcf If the error (ErrQcf) between them exceeds a predetermined threshold (e.g., ErrQcf > 0.01), the controller can update the coefficients according to Equation 3. The error (ErrQcf) is given as follows:

number

[0053] Alternatively, the controller may determine Q based on the measured and verified flow rate Q. cf can be replaced periodically with an updated value. Verification can be repeated intermittently or as needed by repeating the above steps.

[0054] A flow chart illustrating an example process 600 for the operation of an MFC is shown in FIG. sp ) is provided to the device (602), and the downstream pressure (P2) is measured and provided as an input to a flow feedback controller to determine a control valve command (604). The control valve controls the flow rate (Q cf ) to the flow rate set point (Q sp ) (606). This process continues until the downstream pressure (P2) stabilizes (608). The upstream shutoff valve is then closed (610) and a flow measurement of the pressure decay rate is obtained based on the upstream pressure (P1) (612). The upstream shutoff valve is then opened and the gas supply is resumed (614). The flow rate (Q) based on the downstream pressure is then calculated. cf ) can be verified by determining the error (ErrQcf) (616). If the error exceeds a threshold (618), the coefficients can be updated and the device recalibrated (620).

[0055] In the downstream pressure-based flow measurement described above and shown in Figures 2, 4, and 6, Qcf is represented by Equation 2 and / or Equation 4, and the downstream pressure-based flow measurement may also be obtained by a lookup table (e.g., a table showing the relationship between pressure and flow rate). When a lookup table is used instead of or in addition to Equation 2 and / or Equation 4, the MFC calculates the pressure-based flow measurement (Qcf). cf The lookup table can be updated when the flow error between the pressure drop rate (Q) and the upstream pressure decay rate flow measurement (Q) exceeds a predetermined threshold.

[0056] Although the MFC is shown in FIG. 1 as including a critical flow nozzle, the MFC may alternatively include other types of pressure drop elements or flow restrictors, such as laminar flow elements, porous media flow restrictors, orifices, or tubes.

[0057] The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

[0058] While exemplary embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and detail can be made therein without departing from the scope of the embodiments encompassed by the appended claims.

Claims

1. 1. A mass flow controller, comprising: a chamber configured to receive a fluid; an upstream valve disposed upstream of the chamber; a downstream control valve disposed downstream of the chamber; a pressure reducing element disposed downstream of the downstream control valve; a first pressure sensor for detecting a fluid pressure within the chamber; a second pressure sensor detecting a fluid pressure downstream of the downstream control valve and upstream of the pressure reduction element; a controller configured to control operation of the downstream control valve, the controller comprising: a flow-based feedback control including monitoring the flow rate based on a pressure decay rate in the chamber detected by the first pressure sensor when the upstream control valve is closed; and pressure-based feedback control including monitoring a pressure upstream of the pressure drop element detected by the second pressure sensor.

2. The controller 10. The mass flow controller of claim 1, further configured to record the pressure sensed by the second pressure sensor for use during pressure-based feedback control during flow-based feedback control.

3. The mass flow controller of claim 2 , wherein the controller is further configured to establish a pressure set point for pressure-based feedback control based on the recorded pressure.

4. The mass flow controller of claim 2 , further configured to determine a coefficient used in calculating flow rate during pressure-based feedback control based on the recorded pressure.

5. During flow-based feedback control, the controller measures the flow rate Q of the fluid and sets the measured flow rate to a flow setpoint Q. ｓｐ The mass flow controller of claim 1 , configured to compare:

6. The controller is further configured to calculate the measured flow rate Q according to the following formula: [Equation 1] where V is the volume of the chamber and T ｓｔｐ is the standard temperature, and P ｓｔｐ is the standard pressure, and P １ 6. The mass flow controller of claim 5, wherein T is the pressure of the fluid in the chamber sensed by the first pressure sensor and T is the sensed temperature of the fluid.

7. During pressure-based feedback control, the controller ｃｆ and set the flow rate to the flow rate set point Q ｓｐ The mass flow controller of claim 1 , configured to compare:

8. The controller determines the flow rate Q ｃｆ and further configured to calculate [Equation 2] In the ceremony, K ｃｆ is a coefficient, and P ２ 8. The mass flow controller of claim 7, wherein: is the pressure sensed by the second pressure sensor.

9. The controller calculates a coefficient K ｃｆ is further configured to determine according to the following formula: [Equation 3] where Q is the stabilized flow rate desired during flow-based feedback control, and P ｃｆ 9. The mass flow controller of claim 8, wherein Q is the pressure sensed by the second pressure sensor when flow is stabilized at the stabilized flow rate Q.

10. The controller determines the flow rate Q of the fluid based on a look-up table. ｃｆ 8. The mass flow controller of claim 7 configured to determine:

11. The mass flow controller of claim 1 , wherein the controller comprises a flow controller and a pressure controller.

12. The mass flow controller of claim 11 , wherein the flow controller is configured to monitor the flow rate based on the decay rate and compare the monitored flow rate to a flow setpoint.

13. 12. The mass flow controller of claim 11, wherein the pressure controller is configured to monitor the pressure sensed by the second pressure sensor and compare the monitored pressure to a pressure setpoint.

14. 10. The mass flow controller of claim 1, wherein the pressure reduction element is a critical flow nozzle.

15. The mass flow controller of claim 1 , wherein the upstream valve is a control valve.

16. The mass flow controller of claim 1 , further comprising a temperature sensor for detecting a temperature of the fluid.

17. 1. A method for controlling a flow rate, comprising: toggling operation of a downstream control valve of the mass flow controller between flow-based feedback control and pressure-based feedback control; The flow-based feedback control includes monitoring the flow rate based on a pressure decay rate of a fluid in the chamber of the mass flow controller, the pressure decay rate being based on a pressure sensed by a first pressure sensor upon closure of a valve disposed upstream of the chamber, the first pressure sensor sensing the fluid pressure in the chamber; the pressure-based feedback control includes monitoring a pressure upstream of a pressure reduction element based on a pressure sensed by the second pressure sensor, the pressure reduction element being located downstream of the downstream control valve, and the second pressure sensor sensing a fluid pressure downstream of the downstream control valve and upstream of the pressure reduction element.

18. 20. The method of claim 17, further comprising recording the pressure sensed by the second pressure sensor for use during pressure-based feedback control during flow-based feedback control.

19. 20. The method of claim 18, further comprising establishing a pressure set point for pressure-based feedback control based on the recorded pressure.

20. 20. The method of claim 18, further comprising determining a coefficient for use in calculating flow rate during pressure-based feedback control, the coefficient being determined based on the recorded pressure.

21. 20. The method of claim 17, further comprising, during flow-based feedback control, measuring the flow rate Q of the fluid and comparing the measured flow rate to a flow set point Qsp.

22. and calculating the measured flow rate Q according to the following formula: [Equation 4] where V is the volume of the chamber and T ｓｔｐ is the standard temperature, and P ｓｔｐ is the standard pressure, and P １ 22. The method of claim 21, wherein T is the pressure of the fluid in the chamber sensed by the first pressure sensor and T is the sensed temperature of the fluid.

23. During pressure-based feedback control, the flow rate Q of the fluid ｃｆ is calculated, and the calculated flow rate is set to the flow rate set point Q ｓｐ 20. The method of claim 17, further comprising: comparing the

24. The obtained flow rate Q ｃｆ and further comprising calculating: [Equation 5] In the ceremony, K ｃｆ is a coefficient, and P ２ 24. The method of claim 23, wherein: is the pressure sensed by the second pressure sensor.

25. Coefficient K ｃｆ and further comprising determining according to the following formula: [Equation 6] where Q is the stabilized flow rate desired during flow-based feedback control, and P ｃｆ 25. The method of claim 24, wherein Q is the pressure sensed by the second pressure sensor when flow is stabilized at the stabilized flow rate Q.

26. The flow rate Q of the fluid ｃｆ 24. The method of claim 23, wherein determining is determined based on a lookup table.

27. 1. A mass flow controller, comprising: a chamber configured to receive a fluid; an upstream valve disposed upstream of the chamber; a downstream control valve disposed downstream of the chamber; a pressure reducing element disposed downstream of the downstream control valve; a first pressure sensor for detecting a fluid pressure within the chamber; a second pressure sensor detecting a fluid pressure downstream of the downstream control valve and upstream of the pressure reduction element; a controller configured to control operation of the downstream control valve, the controller maintaining a controlled flow rate based on pressure sensed by at least one of the first pressure sensor and the second pressure sensor, and recalibrating pressure-based flow control based on the pressure sensed by the second pressure sensor using a pressure decay rate sensed by the first pressure sensor during the controlled flow rate.

28. 1. A method for controlling a flow rate, comprising: maintaining a controlled flow rate by controlling operation of a downstream control valve based on pressure sensed by at least one of a first pressure sensor and a second pressure sensor, wherein the first pressure sensor senses a fluid pressure in a chamber configured to receive a fluid, and the second pressure sensor senses a fluid pressure downstream of the downstream control valve and upstream of a pressure reduction element, the pressure reduction element being disposed downstream of the downstream control valve; and recalibrating the pressure-based flow control based on the pressure sensed by the second pressure sensor using a pressure decay rate sensed by the first pressure sensor during controlled flow.

29. 1. A mass flow controller, comprising: a chamber configured to receive a fluid; an upstream valve disposed upstream of the chamber; a downstream control valve disposed downstream of the chamber; a pressure reducing element disposed downstream of the downstream control valve; a first pressure sensor for detecting a fluid pressure within the chamber; a second pressure sensor detecting a fluid pressure downstream of the downstream control valve and upstream of the pressure reduction element; a controller configured to control operation of the downstream control valve, the controller comprising: performing pressure-based flow control based on monitoring the pressure upstream of the pressure drop element as sensed by the second pressure sensor; determining a verification flow rate based on a pressure decay rate in the chamber sensed by the first pressure sensor when the upstream control valve is closed, while maintaining pressure-based flow control or chamber-based flow control; recalibrating the pressure-based flow control based on the verification flow rate; and the controller configured to:

30. 30. The mass flow controller of claim 28, wherein the controller is further configured to toggle between pressure-based flow control and flow control based on monitoring a flow rate based on a pressure decay rate in the chamber detected by the first pressure sensor.

31. 30. The mass flow controller of claim 28, wherein the controller is further configured to maintain a controlled flow rate based on the pressure-based flow control when determining the verification flow rate.

32. 1. A method for controlling a flow rate, comprising: providing pressure-based flow control of actuation of a control valve downstream of the mass flow controller; determining a verification flow rate based on a pressure decay rate in the chamber of the mass flow controller while maintaining the pressure-based flow control or the pressure decay rate-based flow control in the chamber; and recalibrating the pressure-based flow control based on the verification flow rate, wherein a pressure decay rate is based on a pressure sensed by a first pressure sensor when an upstream control valve is closed, the first pressure sensor sensing a fluid pressure in the chamber, and the pressure-based flow control is based on monitoring a pressure upstream of a pressure drop element sensed by a second pressure sensor, the pressure drop element being located downstream of the downstream control valve, and the second pressure sensor sensing a fluid pressure downstream of the downstream control valve and upstream of the pressure drop element.

33. 33. The method of claim 32, further comprising toggling between providing flow control based on the pressure and providing flow control based on monitoring a flow rate based on a pressure decay rate in the chamber detected by the first pressure sensor.

34. 33. The method of claim 32, further comprising maintaining a controlled flow rate based on the pressure-based flow control when determining the verification flow rate.

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