Mass Flow Controller Pressure Sensing for Low-Setpoint Stability
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Solution Overview
Problem
Pressure-based mass flow controllers face challenges in maintaining accuracy and repeatability at low setpoints due to large downstream pressure variations, which reduce the flow sensor signal-to-noise ratio and stability, especially in semiconductor processes transitioning from near-zero outlet pressures to atmospheric pressures.
Innovation Solution
A mass flow controller design featuring a combination absolute and differential pressure transducer assembly with a laminar flow element, where the absolute pressure transducer is upstream and the differential pressure transducer is downstream of the control valve, allowing for more accurate flow rate calculations and improved stability across varying pressures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a downstream flow sensor is used to measure mass flow rate, then the flow rate can be calculated using pressure drop across the flow restrictor, but large downstream pressure variations reduce the flow sensor signal-to-noise ratio and stability
Solution Approach 1:
The patent introduces an upstream flow sensor as an intermediary measurement point, located before the control valve where pressure conditions are more stable. This upstream sensor acts as a mediator that avoids the harmful downstream pressure variations while still enabling accurate flow rate calculation through the established pressure-drop relationship.
Solution Approach 2:
The patent extracts the flow sensing function from the downstream location and places it upstream of the control valve. By separating the measurement function from the valve assembly, the system eliminates the negative impact of downstream pressure excursions on measurement stability while maintaining the ability to calculate actual flow rate.
2Adaptability or versatility
If absolute pressure sensors with downstream laminar flow elements are used to handle atmospheric outlet pressures, then the controller can operate at atmospheric pressure, but the low-flow pressure drop of the flow restrictor is reduced by a factor of 50 or more
Solution Approach 1:
The upstream flow sensor serves as an intermediary that measures pressure conditions before they are affected by the control valve and downstream pressure variations. This allows the system to maintain accurate flow measurement across the full outlet pressure range from hard vacuum to atmospheric pressure without the signal degradation that occurs with downstream sensing.
3Ease of operation
If the flow control valve is positioned downstream of the laminar flow element, then the valve can control flow to the process chamber, but downstream pressure variations directly affect the flow sensor reading
Solution Approach 1:
The patent segments the flow control system into distinct functional zones: an upstream measurement zone with the flow sensor before the control valve, and a downstream control zone with the valve and process chamber. This segmentation isolates the measurement function from pressure variations caused by valve operation, allowing independent optimization of both measurement accuracy and control capability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration enhances the accuracy and repeatability of mass flow rate measurements by minimizing the impact of downstream pressure variations, providing more precise control over fluid flow, especially at low setpoints and high outlet pressures.
Implementation Method 1
Pressure-based mass flow meters use a defined flow restriction to create a pressure drop responsive to the flow to be measured
Implementation Method 2
an absolute pressure transducer having an absolute pressure membrane and exposed to absolute pressure in the third cavity
Implementation Method 3
a differential pressure transducer having a first differential pressure membrane and a second differential pressure membrane and exposed to differential pressure between the third cavity and the second cavity
Data Source
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AI summary
An example mass flow controller comprises a flow pathway comprising a first cavity and a second cavity, a laminar flow element and a pressure transducer assembly comprising an absolute pressure transducer exposed to absolute pressure in the third cavity in fluid communication with the first cavity, and a differential pressure transducer exposed to differential pressure between the third cavity and the second cavity. The mass flow controller also comprises a flow control valve downstream of the laminar flow element and actuated according to pressures sensed by pressure transducer assembly.