MASS FLOW TECHNOLOGY

1. Mass Flow and Mass Flow Measurement Basics

The terms mass flow or mass transfer refer to the movement of a liquid or gaseous material in a conducting element such as a tube or channel. Examples of liquid mass flow include natural phenomena such as the movement of water through a river channel and technological processes such as the passage of oil through a pipeline. Mass flow is formally defined as the “amount of substance” (determined as mass, moles, or volume) which passes through a cross section of the conducting channel during a set amount of time. Figure 1 shows how the basic parameters of mass flow are defined. The figure on the left shows material flowing through a pipe as it reaches the surface that is defined by a cross- section of the pipe at time, t=0. The figure on the right shows the pipe at time, t=Δt, when a mass of material, Δm, has passed through this surface. The flow rate is then defined as the mass per unit time that has passed through the cross-sectional surface of the pipe. It is reported as mass/time, moles/time, volume/time, etc.

Figure 1. Basic parameters of mass flow

a. Mass Flow Meters and Controllers

         Figure 2. The rotameter

There are different ways to measure mass flow rates that are based on different physical principles. The simplest and most costeffective instrument for gas flow rate measurement is the rotameter (Figure 2). This instrument consists of a tapered tube with a float that is raised by the fluid flowing through. The height of the float in the tube varies linearly with the flow rate of the gas or liquid moving through the tube. Rotameters are ubiquitous in industrial settings owing to their simplicity, repeatability, and robust properties. They provide a rapid, visual capability for setting and monitoring gas flow, are compatible with both gases and liquids, and require no external power for operation. A low pressure drop across the rotameter ensures that the measurement produces little impact on process characteristics. The primary constraints on their use lie in the fact that they must be mounted vertically and they cannot generate an electrical signal that can be used in electronic control systems

b. Thermal Mass Flow Meters

Figure 3. Thermal Mass Flow Meter sensor configurations

Unlike rotameters, thermal mass flow meters (MFMs) and controllers (MFCs) generate an electronic signal that can be used in process automation schemes. For this reason, they can be found in many industrial and most high technology environments that require precise process control. MFMs and MFCs employ thermodynamic principles to derive mass flow rates. Figure 3 illustrates the underlying measurement principle that is used by a thermal mass flow meter. The sensor is mounted on a side stream that takes a known ratio of the gas flow passing through the MFM. In a three-wire sensor configuration, the MFM uses high temperature coefficient of resistance wires as sensors to measure the temperature differential (ΔT=T₂-T₁), across a heater mounted on the side stream as shown in Figure 4. This temperature differential is directly proportional to the mass flow rate, obeying the relationship:

A two wire sensor MFM, which is a more common configuration, employs high temperature coefficient of resistance wires as both sensor and heater, as shown on the right in Figure 4. In this configuration, circuitry adjusts the power to the heater coils to maintain a constant temperature differential between the two coils. As with the three-wire MFM, Equation (4) and the splitting ratio are used to determine the mass flow through the instrument.

Figure 4. Basic principles of thermal MFM measurements

c. Thermal Mass Flow Controllers

Figure 5. Basic components of a thermal mass flow controller

A mass flow controller (MFC, Figure 5) is a single instrument that combines both mass flow sensing and control of gas flow. It consists of a mass flow meter (MFM), a feedback controller, and a control valve. Typically, MFCs are encountered more frequently than MFMs in process environments.

Thermal MFCs are used for industrial flow control over a very wide range, with controllers available for flows between 0.01 sccm and 1000 SLPM. Thermal MFCs are both accurate and repeatable, precisely controlling gas flows between 2 and 100% of their Full Scale reading with a resolution of 0.1%. They are factory-calibrated to provide accurate and repeatable control of a specified range (Figure 6).

Figure 6. MFC indication vs. actual flow.

It is important to understand the different characteristics and sensitivities of thermal MFCs in order to ensure their accuracy and repeatability during operation (Figure 7).

Figure 7. Thermal MFC characteristics and sensitivities.

Gas type is an obvious characteristic of a given MFC calibration since the measurement of gas flow depends on the thermo-physical properties of the gas being measured. Factory calibration of an MFC is normally performed using high purity nitrogen gas after which a gas correction factor (GCF) or a multi-gas correction function is applied that adjusts the calibration for different gas types

Figure 8. Temperature effects on thermal MFC flow measurements

Ambient temperature will impact both the zero offset and the accuracy of mass flow measurement over the measurement span when using thermal MFCs. Figure 8 shows the impact of temperature variations on the indicated vs. actual flow in a thermal MFC. Two correction factors are associated with measurement variations due to ambient temperature. The Zero Offset Coefficient, T_c, is associated with the indicated zero in the MFC. A change in ambient temperature will shift the entire calibration curve of the MFC as shown on the left of Figure 8. The magnitude of the shift is typically of the order of ppm of Full Scale per °C. Changes in ambient temperature also shift the slope of the calibration curve over the measurement span of a thermal MFC. The Span T_c associated with an MFC exhibits behavior as shown on the right of Figure 8. The entire slope of the calibration curve is shifted, with the effect having a magnitude on the order of ppm of the reading per °C.

While thermal MFCs are not normally affected by downstream pressure (unless the sensor is located downstream from the control valve), changes in upstream pressure can result in variations between actual and indicated flows, as shown in Figure 9.

Figure 9. The impact of upstream pressure changes on thermal MFC flow indication

By integrating a pressure sensor into a thermal MFC, the effect of upstream pressure pertubance on MFC’s accuracy can be greatly reduced. Hence, MKS provides a pressure insensitive MFC integrated with a pressure sensor such as the P9 MFC. Probably the most important, and sometimes neglected, effect on thermal MFCs is the impact that mounting attitude has on measurement and control of gas flow. Mounting attitude can impact the output of the sensors in an MFC through effects that occur either outside or inside the gas flow path (see Figure 10).

Figure 10. Mounting sensitivities of thermal MFC

2. Salient Features

Accuracy (non-linear, hysteresis, non-repeatable)	±1% of Full Scale
Control Range	2.0 to 100% of Full Scale
Controller Setting Time	<2 seconds (to within 2% of set point)
Full Scale Ranges (nitrogen equivalent)	Various, from 10 to 30,000 sccm
Maximum Inlet Pressure	150 psig
Operational Pressure Differential	<5,000 sccm: 10 to 40 psid 10,000 to 30,000 sccm: 15 to 40 psid
Pressure Coefficient	0.02% of reading per psi
Repeatability	±0.2% of Full Scale
Resolution	0.1% of Full Scale
Temperature Coefficients	Zero: <0.04% of Full Scale per °C (400 ppm) Span: <0.08% of Full Scale per °C (800 ppm)
Warm-up Time	5 minutes/5 phút

3. Applications

a. Vacuum/Pressure Control

Since process pressure is a dynamic equilibrium between gas flow into a process chamber and gas flow out of the chamber, the gas flow into a chamber can be used to control the pressure (Figure 14).

Figure 14. Vacuum control using an MFC

Figure 15. Closed-loop pressure control using a Baratron® manometer and an MFC

b. Physical Deposition

Gas pressure and composition are critical parameters that determine plasma ignition and deposition control in physical depostion process equipment such as the sputter deposition tool shown in Figure 16. In a typical sputter process, the flow rate of an inert gas such as Argon must be accurately and repeatably controlled throughout the process and process-to-process. Argon flow to a sputter deposition process chamber can be precisely and repeatably controlled using a closed-loop pressure control scheme as shown in the Figure 16.

Figure 16. Closed-loop pressure control in a sputter deposition process chamber.

c. Chemical Vapor Deposition

In chemical vapor deposition (CVD) processes, the flow rates of multiple precursor gases into the process chamber must be precisely and repeatably controlled. The concentration and ratio of these precursor gases in the process chamber determine both the process characteristics (e.g. deposition rate and throughput) and the material properties of the thin films produced (e.g. chemical composition, mechanical properties such as stress and finish, and film thickness). In a typical CVD tool, the flow rate of each precursor gases must be individually measured and controlled using MFCs. The cumulative flow of the different precursor gases is additive in determining the total molecular gas flow into the chamber and the chamber pressure is determined by the relationship shown in Figure 17.

Figure 17. Gas flow and pressure in a CVD tool

d. Broad Selection for Varied Industries

MKS Instruments is a global leader in MFC technology and supplies a variety of different MFC types to industries such as semiconductor fabricators, display manufacturers, and photovoltaic cell manufacturers. MKS’ line of thermal MFCs include the fast and repeatable G-Series thermal MFCs that provide a cost-effective flow control solution for most industrial and technological applications. G-Series MFCs are available as either elastomer or metal-sealed units with flow ranges up to 300 SLPM. They are available with a wide choice of digital (RS485, Profibus™, EtherCAT®, Profinet or DeviceNet™) or analog (0–5 VDC or 4–20 mA) I/O options. MKS I-Series thermal MFCs are designed specifically for applications that have harsh environments where protection against water and dust is critical. I-Series mass flow controllers have IP66-rated enclosures that are dust tight and protect against powerful water jets. They offer digital (Profibus®) or analog (0–5 VDC or 4–20 mA) I/O. I-Series MFCs are available with flow ranges up to 500 SLPM. MKS’ P-Series thermal MFCs are high performance multi-gas, multi-range mass flow controllers that are designed for critical applications where accuracy, repeatability, and pressure insensitivity are required. They have gas parameters stored in memory that allow user selected gas measurement and control with 1% set point accuracy. As with other MKS thermal mass flow controllers, P-Series MFCs have analog (0–5 VDC) or digital (DeviceNet, RS485) I/Os. The C-series is a compact MFC using a Micro-Electro-Mechanical Systems (MEMS) based flow sensor designed for non-corrosive gas applications. The C-series MFC is available with analog (0–5 VDC) or digital (RS485, Modbus TCP/IP) I/Os.

MKS Instruments also supplies pressure based flow controllers (such as the metal-sealed 1640A MFC) and a number of specialized mass flow control solutions such as flow ratio controllers (Delta II, III, and IV Mass Flow Ratio Controllers for up to 4-zone ratio control), Mass Flow Verifiers and single- and dual-zone pressure controllers (G-series and P-series integrated pressure controllers and DPC Dual-Zone Pressure Controllers).