Fluid handling – Processes – With control of flow by a condition or characteristic of a...
Reexamination Certificate
2002-06-24
2004-03-30
Rivell, John (Department: 3753)
Fluid handling
Processes
With control of flow by a condition or characteristic of a...
C137S486000, C137S487500, C137S599130
Reexamination Certificate
active
06712084
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to mass flow sensing and control systems.
BACKGROUND OF THE INVENTION
Capillary tube thermal mass flow sensors exploit the fact that heat transfer to a fluid flowing in a laminar tube from the tube walls is a function of mass flow rate of the fluid, the difference between the fluid temperature and the wall temperature, and the specific heat of the fluid. Mass flow controllers employ a variety of mass flow sensor configurations. For example, one type of construction involves a stainless steel flow tube with one, and more typically two or more, resistive elements in thermally conductive contact with the sensor tube. The resistive elements are typically composed of a material having a high temperature coefficient of resistance. Each of the elements can act as a heater, a detector, or both. One or more of the elements is energized with electrical current to supply heat to the fluid stream through the tube. If the heaters are supplied with constant current, the rate of fluid mass flow through the tube can be derived from temperature differences in the elements. Fluid mass flow rates can also be derived by varying the current through the heaters to maintain a constant temperature profile.
Such thermal mass flow sensors may be attached as a part of a mass flow controller, with fluid from the controller's main channel feeding the capillary tube (also referred to herein as the sensor tube). The portion of the main channel to which the inlet and outlet of the sensor tube are attached is often referred to as the “bypass” of the flow sensor. Many applications employ a plurality of mass flow controllers to regulate the supply of fluid through a supply line, and a plurality of the supply lines may be “tapped off” a main fluid supply line. A sudden change in flow to one of the controller's may create pressure fluctuations at the inlet to one or more of the other controllers tapped off the main supply line. Such pressure fluctuations create differences between the flow rate at the inlet and outlet of an affected mass flow controller. Because thermal mass flow sensors measure flow at the inlet of a mass flow controller, but outlet flow from the controller is the critical parameter for process control, such inlet/outlet flow discrepancies can lead to significant process control errors.
In a semiconductor processing application, a process tool may include a plurality of chambers with each chamber having one or more mass flow controllers controlling the flow of gas into the chamber. Each of the mass flow controllers is typically re-calibrated every two weeks. The re-calibration process is described, for example, in U.S. Pat. No. 6,332,348 B1, issued to Yelverton et al. Dec. 25, 2001, which is hereby incorporated by reference. In the course of such an “In Situ” calibration, conventional methods require a technician to connect a mass flow meter in line with each of the mass flow controllers, flow gas through the mass flow meter and mass flow controller, compare the mass flow controller reading to that of the mass flow meter and adjust calibration constants, as necessary. Such painstaking operations can require a great deal of time and, due to labor costs and the unavailability of process tools, with which the mass flow controllers operate, can be very costly.
A mass flow sensor that substantially eliminates sensitivity to pressure variations would therefore be highly desirable. A convenient calibration method and apparatus for mass flow controllers would also be highly desirable. More flexible access to a mass flow controller would also be highly desirable. Apparatus and method for increasing the control performance of a mass flow controller would also be highly desirable.
SUMMARY OF THE INVENTION
In an illustrative embodiment, a mass flow controller in accordance with the principles of the present invention includes the combination of a thermal mass flow sensor and a pressure sensor to provide a mass flow controller that is relatively insensitive to fluctuations in input pressure. The new controller is relatively inexpensive, that is, it does not require a pair of expensive, precision, pressure sensors nor an all-stainless steel wetted surface differential sensor. Nevertheless, the new controller is adapted to control fluid flow over a broad range of fluid pressures. The new mass flow controller includes a thermal mass flow sensor, a pressure sensor, and an electronic controller. The thermal mass flow sensor is configured to measure the inlet flow of the controller. The pressure sensor senses the pressure within the volume in the channel between the flow sensor's bypass and an outlet control valve, which volume will be referred to herein as the “dead volume.” The pressure sensor and thermal mass flow sensor respectively provide signals to the controller indicating the measured inlet flow rate and the pressure within the dead volume. A temperature sensor may be employed to sense the temperature of the fluid within the dead volume. In an illustrative embodiment, the temperature sensor senses the temperature of the controller's wall, as an approximation of the temperature of the fluid within the dead volume. The volume of the dead volume is determined, during manufacturing or a calibration process, for example, and may be stored or downloaded for use by the electronic controller.
The controller employs the measured pressure within the dead volume to compensate the measured inlet flow rate figure and to thereby produce a compensated measure of the outlet flow rate as a function of the measured pressure and measure inlet flow rate. This compensated measure of outlet flow rate may be used to operate a mass flow controller control valve. By reading the pressure sensor output over a period of time, the electronic controller determines the time rate of change of pressure within the dead volume. Given the dead volume, the temperature of the fluid within the dead volume, and the input flow rate sensed by the thermal mass flow sensor, the electronic controller computes the fluid flow rate at the output of the mass flow controller as a function of these variables. The electronic controller employs this computed output fluid flow rate in a closed loop control system to control the opening of the mass flow controller output control valve. In an illustrative embodiment the pressure sensed by the pressure sensor may also be displayed, locally (that is, at the pressure sensor) and/or remotely (at a control panel or through a network interface, for example).
In accordance with another aspect of the principles of the present invention, a variable-flow fluid source, a receptacle of known volume, and a pressure differentiator may be used to calibrate a mass flow controller. The variable-flow fluid source supplies gas at varying rates to the mass flow controller being calibrated and at proportional rates to a receptacle of known volume. A pressure differentiator computes the time derivative of gas flow into the receptacle of known volume and, from that, the actual flow into the receptacle. Given the actual flow, the proportionate flow into the mass flow controller may be determined and the flow signal from the mass flow controller correlated to the actual flow. In an illustrative embodiment, a mass flow controller closes the outlet valve to form a receptacle of known volume (the dead volume). A pressure sensor located within the dead volume produces a signal that is representative of the pressure within the dead volume. With the outlet valve closed, the flow into the dead volume decreases exponentially while the pressure increases, until the pressure within the dead volume is equal to that at the inlet to the mass flow controller. The mass flow controller's electronic controller takes the time derivative of the pressure at a plurality of times. Given the dead volume/receptacle volume, the time derivative of the pressure within the dead volume, and the temperature of the gas, the controller computes the flow rate at those sample times. The ele
Ambrosina Jesse
Kottenstette Nicholas
Shajii Ali
Krishnamurthy Ramesh
McDermott & Will & Emery
MKS Instruments Inc.
Rivell John
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