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Thermal Mass Flow Controllers
A mass flow controller (MFC) for each gas line in a semiconductor tool’s gas panel measures and regulates the mass flow of the gas in order to set the gas entering the process chamber to the values in the process recipe. While pressure regulation and temperature control are needed for sensitive chemical vapor deposition (CVD), plasma etching, or thin film processes, gas flow control can be just as important.
Semiconductor process recipes involve precise ratios of gas phase chemical to assure the correct stoichiomtery and reaction rates. Due to the accuracy and precision required of the gas flow rate, mass flow controllers are often the most sensitive and expensive components installed in a gas panel.
Mass Flow Controller Design
Between their inlet and outlet ports, mass flow controllers contain a mass flow sensor and a proportional mass flow control valve. The MFC circuitry compares the signal from the mass flow sensor to a setpoint. The control circuitry then adjusts the proportional valve accordingly to bring the output mass flow to the setpoint. The sensor, valve, and circuitry are shown below in the image of an opened prototype MFC, which Glew Engineering helped the manufacturer to develop.
The internal components of a Tylan™ Mass Flow Controller
Thermal Mass Flow Controllers
There are a few main types of mass flow controllers used in semiconductor equipment. The first type of MFC uses a thermal sensor to measure the mass flow, due to their simplicity, and ability to detect small flow rates. The thermal MFC’s internal flow path directs flow in towards one of two paths. First, a small amount of the flow, approximately 10 sccm, is diverted through the thermal flow sensor, a capillary tube. The remaining flow is diverted through the bypass. The split must remain constant between the two flows, which requires laminar flow within the MFC. You can see the main and bypass tubes in the diagram below.
A simplified internal diagram of a thermal mass flow controller
The thermal flow sensor has two small coils of wire wrapped around the capillary tube. The flow rate through the sensor tube results in a temperature difference between the two coils. This temperature resistance results in a resistance difference between the two coils, which is run through a bridge circuit and amplified. Since the MFC is calibrated, and the thermal properties of the gas are known in advance, the MFC flow sensor temperature differential changes at a known rate. By either comparing the current necessary to create a constant preset temperature difference or directly comparing the temperature difference from a constant current, the MFC can determine the mass flow. Since the sensor is measuring a difference between temperatures of the two coils, it does not matter what the incoming temperature of the gas is. Similarly, the thermal properties of a gas are essentially constant over application pressures, so variable input pressures also do not affect the MFC’s measurements.
Pressure Mass Flow Controller
Alternatively, some mass flow controllers use pressure to regulate flow by taking advantage of “choked flow” through a nozzle. As I wrote about in Part 8, if gas is flowing through a nozzle at sonic speeds then the upstream pressure remains constant regardless of any change in downstream pressure. When the flow within the nozzle reaches the speed of sound, then the device is known as a sonic nozzle or choked flow device. The MFC measures the pressure upstream of the nozzle, and thus can calculate the flow. Then, the mass flow controller adjusts the incoming gas pressure with a servo valve in order to vary the mass flowing through the nozzle. MKS™ is a large manufacturer of these types of MFCs, and also manufacture a number of tools that utilize sensitive pressure transducers.
Other Types of Mass Flow Sensors
There are a few other designs for mass flow sensors on the market, though aren’t as common yet in the semiconductor industry. Coriolis flow meters route the gas through a series of bent tubes that are either rotating or vibrating; the forces necessary to change the direction of the gas in the elbows measurably and predictably affects the rotation or vibration. By comparing the tube movements with and without gas flow, the sensor can determine the mass flow. Another type of mass flow sensor introduces a small amount of helium to the process gas, and then measures the speed of sound in the mixture; the resulting speed of sound is equivalent to the mass weighted average of the helium and the original mixture, and the sensor can use this equation to calculate the incoming mass flow.
Mass Flow Controller Calibration
While the hardware in a thermal mass flow sensor may seem relatively simple, the sensor’s are not easy to construct in a repeatable manner, and rely on much manual skill. Consequently, the sensors must be individually calibrated. Also, the sensors must be calibrated for different gasses, because simple linear conversion factors are often inadequate in todays exacting processing environments. The advent of digital MFCs that can store calibration tables was of great benefit.
Furthermore, the orientation of the MFC can change the sensor temperature profile as well; if the tube through the temperature sensors is not level, then the heated gas will rise towards the higher end, causing the MFC to read an incorrect mass flow. That isn’t to say that a mechanical engineer can’t mount MFCs vertically; they simply have to be correctly calibrated before installation, and not subsequently reoriented. Even when there is no gas flowing, gas can rise through free convection on being heated by the senors, resulting in a shift in the zero flow value known as thermo-siphoning.
The other types of mass flow meters mentioned above are not affected by orientation, since they are simply reliant on the pressure, temperature and density. However, that reliance on density means that they still require calibration to the material properties of whichever gas they will be servicing. With any other gas or gas mixture besides their calibration gas, their internal calculations will be invalid and will result in an incorrect and unpredictable mass flow.
Mass Flow Control in the Semiconductor Tool
Since many semiconductor process reactions are very sensitive to the amounts of certain reactants, it’s essential that the semiconductor tool’s gas panel accurately controls the amount of each gas flowing through it. Due to the importance of this feature, manufacturers have designed many different types of sensors and controllers for sensitively and precisely regulating the gas flow. Each of these varieties of mass flow controller is better suited to certain environments, operating conditions, gas mixtures, and process types, so it can be best to get advice from semiconductor equipment experts when designing a new semiconductor tool.
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