Thermal Mass Flow Controllers and Meters
1. What is Thermal Mass Flow?
Thermal mass flow technology uses thermodynamic principles to derive actual mass flow. A thermal mass flow sensor can be combined with an integral valve and PID controller in one compact, efficient instrument that can accurately control the flow of gases or liquids over a wide range of flow rates.
TMF products exhibit high repeatability and accuracy, providing an effective way to ensure a reliable and stable process.
Thermal flow meters were developed in the late 1960s and early 1970s. Brooks Instrument and Tylan General were leaders in the development of the first thermal mass flow devices. In the early 1980s, the emerging semiconductor industry quickly became the largest consumer of mass flow devices. A new entrant in the mass flow market, Unit Instruments, developed a line of products targeted at that segment. Metal seal and ultra-high purity devices were the primary advances through the 1980s. Through the 1990s, mass flow product developments included digital I/O, higher flow rates and more industrialized packaging. In the first decade of the new century, digital bus protocols, MEMS sensors, pressure transient insensitive devices and multi-gas/multi-range (MultiFlo™) devices were the primary advances. In 2009, Brooks Instrument acquired certain assets from Celerity Instrumentation, which included all mass flow devices formerly branded as Unit, Tylan, Mykrolis, Millipore and Celerity. Brooks Instrument has a rich portfolio of mass flow technology and is a leading supplier of mass flow devices to a broad range of industries and applications.
3. Principle of Operation
There are five primary elements to a mass flow controller:
1) Body: The body provides a compact platform for all of the other mass flow controller elements, as well as the primary flow path for the fluid being measured.
2) Restrictor/bypass: The restrictor (bypass) causes a restriction in the flow path, causing a reduced flow through the sensor. It is important that the ratio of flow through the restrictor vs. the flow through the sensor be constant. A variety of bypass techniques can be employed depending on the flow rate and application.
3) Sensor: The sensor is the heart of the mass flow controller. It uses heat and a differential temperature measurement to provide a signal that is proportional to mass flow. There are two-wire, three-wire and MEMS sensors available. The type of sensor used depends on the supplier and the application.
4) Circuit board: The circuit board is the brain of the mass flow controller. It manages the external inputs and outputs (I/O) to and from the device, as well as the internal I/O with the sensor and control valve. If the flow signal does not match the set point signal (command input), the valve drive is adjusted to reposition the valve as required.
5) Valve + orifce/jet: The valve combined with the orifice (jet) is the flow control element. There are several types of valves used on mass flow controllers depending on the flow rate and application. The majority of flow controllers use a normally closed solenoid-type control valve. This valve has a coil winding around a valve stem. When the coil is energized, it creates a magnetic field that is modulated to adjust the height of the plunger and valve seat assembly inside the valve stem. When energy is applied, the plunger is attracted to the top or pole piece in the valve stem, raising the valve seat off the orifice and allowing fluid to flow through the device. From full closed to full open, the valve seat only moves a few thousandths of an inch. While these valves control flow, it is not recommended that they be used as a positive shut-off valve.
When you put all of these elements together, you have a fully functioning mass flow controller.
For a typical mass flow sensor, a precision power supply provides a power heat input (P) at the heater, which is located at the midpoint of the sensor tube. At zero (no flow) conditions, the heat reaching each temperature sensor is equal. The temperatures T1 and T2, therefore, are equal. When fluid flows through the tube, the upstream sensor is cooled, and the downstream sensor is heated, which produces a temperature difference. The temperature difference T2-T1 is directly proportional to the gas mass flow.
The equation is: DT = A * P * Cp * m, where:
- DT = Temperature difference T2 - T1 (°K)
- Cp = Specific heat of the gas at constant pressure (kJ/kg-°K)
- P = Heater power (kJ/s)
- m = Mass flow (kg/s)
- A = Constant of proportionality (S2-°K2/kJ2)
A bridge circuit interprets the temperature difference, and a differential amplifier generates a linear signal directly proportional to the fluid mass flow rate. The flow restrictor performs a ranging function similar to a shunt resistor in an electrical ammeter. The restrictor provides a pressure drop that is linear with flow rate. The sensor tube has the same linear pressure drop/flow relationship. The ratio of the restrictor flow to the sensor tube flow remains constant over the range of the meter. Different restrictors have different pressure drops and produce controllers with different full-scale flow rates. The span adjustment in the electronics affects the fine adjustment of the controller's full-scale flow. In addition to the mass flow sensor, the mass flow controller has an integral control valve and control circuit. The control circuit senses any difference between the flow sensor signal and adjusts the valve to increase or decrease the flow.
4. When are thermal mass flow devices used?
Thermal mass flow devices are used in a variety of industries and applications where accurate repeatable gas or liquid mass flow is required. Some of the typical industries and applications are listed below:
- Hydraulic systems test
- Ventilation R&D
- Hardening canopies for jet aircraft
- Silicon carbide for aircraft brakes
- Sample gas preparation and measurement
- Calibration gas control
- Engine test
- Emissions test
- Leak test
- Verification of SHED (Sealed Housing for Evaporative Determination) operations
- Reactor gas control for fermentation
- Bioreactor gas control of dissolved oxygen and pH
- Tablet coating
- Measurement of gases in chemical processes and manufacturing
- Catalyst research
- Pilot plant flow control
- Manufacture of computers
- Memory devices and other electronic equipment
Food & Beverage
- Process control in bottling, drying, mixing, cooling
- Protective gases for packaging
- Control of additives and pigments
- Flavor dosing
- Control fuel delivery
- Atmospheric simulations
- Production efficiency testing
- Flame control
- Gas mixing and blending
- Gas distribution
- Gas consumption measurement for internal accounting purposes
- Burner control
- Blanket gas control
- Metal organic chemical vapor deposition
- Vapor control
- Equipment performance validation
- Medical equipment manufacturing
- Specialty coatings
- Blank gas control
- Improved quality of manufactured metals
- Protective gases in packages
- Chemical deposition
- Thin film coatings
- Chemical vapor deposition
- Back-side wafer cooling and other critical processes in the production of semiconductor chips
High Flow Purge
Semiconductor wafers are manufactured using chemical and epitaxial processes, and are extremely sensitive to the presence of oxygen, humidity and other contaminants. In order to avoid damage during processing, the GF101, GF121 and GF126 high-flow MFCs are used in systems where the surrounding environment of the wafers and /or reticles are purged with a clean inert gas like nitrogen or clean dry air.
High-Flow Front Opening Universal Pods (FOUP) Applications
These specialized Front Opening Universal Pod devices hold 300mm wafers securely and safely and need high flow purge between etch and post-etch clean steps, which must be carefully controlled because exposure to moisture and oxygen immediately begins to destroy nanometer-scale critical dimensions of features on the wafer surface.
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