In a number of processes, two or more components must be mixed together. In instances where it comprises a vapor of a liquid in a carrier gas, traditional Bubbler Systems and the more recent Vapor Source Controllers can be useful for vapor generation. However, these devices often cannot handle sufficient quantities of liquid with a low vapor pressure. Or they perform in an imperfect way. Moreover, they cannot instantaneously provide vapor of a mixture of liquids with different vapor pressures.
Process Solutions Corp. offers a unique system to realize Mass Flow Control of Vapors with an innovation: the CEM-Vapor Generation System. This vapor generation system is useful for atmospheric, pressurized, and even vacuum processes. It’s capable of vaporizing liquid flows from 0.25 to 1,200 g/h of water. And for most other fluids, the maximum capacity will even be higher.
At room temperature the liquid, for instance TEOS, HMDSO, Cupraselect, or water, draws from a container with an inert gas blanket, or membrane. Then, a liquid mass flow meter type μ-FLOW, LIQUI-FLOW or mini CORI-FLOW measures the liquid. The flow rate is controlled to the set point value by a control valve (C), which forms an integral part of the liquid flow and carrier gas mixing valve (M).
Then, the mixture enters the evaporator to achieve total evaporation (E). So, this explains the abbreviation of CEM viz.: Control – Evaporation – Mixing. These are the 3 basic functions of the Vapor Delivery System. Additionally, a complete vapor generation system incorporates a readout / control unit for easy operation of the CEM-system devices.
The set-up of a CEM-system usually consists of a gas mass flow meter, liquid mass flow meter and controller, a vaporizer, a digital readout and controller, along with all necessary cables.
Gas mass flow controllers allow for the measurement and control of the carrier gas flow. The amount of carrier gas for evaporation depends on the application (flow range, liquid, pressure, temperature). And we advise a certain minimum of carrier gas flow for the transportation of the liquid source from the mixing valve into the heat-exchanger.
To avoid a large pressure drop across the system, gas flow should not exceed approximately 100 ln/min for the 1,000 W unit or 10 ln/min and 4 ln/min respectively, for smaller units. For more information about Bronkhorst MFCs see EL-FLOW and Metal Sealed Digital Mass Flow / Pressure Meters and Controllers.
Liquid mass flow meters and controllers allow for the measurement of the liquid source flow. Bronkhorst offers liquid flow meters for 1.5-30 mg/h up to 0.4-20 kg/h of water equivalent. For more information, see the LIQUI-FLOW, μ-FLOW, and mini CORI-FLOW.
CEM 3-way Mixing Valve and EvaporatorCEM, or Controlled Evaporator Mixers allow for control of the liquid source flow and mixing of the liquid with the carrier gas flow, resulting in total evaporation. And it’s complete with Temperature Controlled Heat-Exchanger to add heat to the mixture to realize complete vaporization. (Tmax 200°C / Pmax 100 bar).
Our flow Control System allows for the control of the gas flow, liquid flow, and temperature of the heat-exchanger.
In many cases, small concentrations of vapors are made by bubbling a carrier gas through the fluid. This method requires optimum pressure and temperature control of the bubbler system, which is expensive. Moreover, the response time is very slow. The absolute accuracy is low and the bubbler system has a poor long term stability.
We propose a much more direct approach, viz., control the necessary quantity of liquid to reach the desired concentration, under room temperature, with a LIQUI-FLOW, μ-FLOW or mini CORI-FLOW Mass Flow Meter. This instrument forms part of a CEM-system in which a specific quantity of liquid mixes with the carrier gas before it’s vaporized. So with this straightforward technique, virtually any concentration can be made in a matter of seconds with high accuracy and repeatability.
The CEM-system is useful in a large number of applications in wide variety of markets. For example, tools (drills, screwdrivers, saw blades, etc.) and machine parts receive a coating to improve their wear resistance. And electric and dielectric (insulating) layers are necessary in semiconductor and solar cell manufacturing processes. Additionally, the heat insulation of glazing sees improvement by coating its surface. The humidity in reactor / process chambers is controlled to ensure optimum performance.
Chemical Vapor Deposition (CVD) is a chemical process that produces high-purity, high-performance solid materials. The process is useful in the semiconductor industry to produce thin films for LEDs, transistors, and DRAMs. Also, it’s useful for surface treatment, hardening processes, and in the manufacturing of high temperature superconductors.
In a typical CVD process, the surface (wafer or substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. The CEM-system is used in various types of CVD processes. For example:
The CEM-system is ideal for the accurate adjustment of dew or moisture. The intrinsic characteristics of large dynamic range and high accuracy ensure the control of moisture level with great flexibility from only a few ppm up to virtually 100% while also maintaining a very high stability in dew point. Full functionality remains with operating pressures as high as 100 bar.
Calibration of Gas Chromatographs, Mass Spectrometers and Gas SensorsThrough the combination of Mass Flow Controllers for liquids and the CEM-system, gas phase concentrations generate as necessary. Since the reference stream from the CEM is both highly reproducible and highly accurate due to the direct action of the Mass Flow Controllers, it is able to calibrate Mass Spectrometers or Gas Chromatographs.
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