Stafford, Texas

CEM: Liquid Delivery System with Vapor Control

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 have been used. These devices, however, can often not handle sufficient quantities of liquid with a low vapor pressure, or perform in an imperfect way. Moreover, they cannot instantaneously provide vapor of a mixture of liquids with different vapor pressures.

Bronkhorst has therefore developed a unique patented system to realize Mass Flow Control of Vapors with an innovation: the CEM-Liquid Delivery System (LDS). It can be applied for atmospheric, pressurized and vacuum processes, vaporizing liquid flows from 0,25 to 1200 g/h of water; for most other fluids the maximum capacity will even be higher.

Controlled Evaporative Mixer

At room temperature the liquid, for instance TEOS, HMDSO, Cupraselect or water, is drawn from a container with an inert gas blanket, or membrane, and measured by a liquid mass flow meter type μ-FLOW, LIQUI-FLOW  or mini CORI-FLOW. The required flow rate is controlled to the set point value by a control valve (C) forming an integral part of the patented liquid flow and carrier gas mixing valve (M). The then formed mixture is subsequently led into the evaporator to achieve total evaporation (E). This explains the abbreviation of CEM viz.: Control – Evaporation – Mixing, the 3 basic functions of the Liquid Delivery System. A complete system also incorporates a readout/control unit, including power supply, for operation of the CEM-system devices.

Advanced Bubbler Alternative

  • Accurately Controlled gas / liquid mixture
  • Fast response
  • high reproducibility
  • very stable vapor flow
  • flexible selection of gas / liquid ratio
  • lower working temperature than conventional systems
  • optional control by PC / PLC (RS232 / Fieldbus)

Vapor Flow Control Requires Gas and Liquid Flow Control

The set-up of a CEM-system usually consists of:

Mass Flow Controller for Gases

For measurement and control of the carrier gas flow. The amount of carrier gas required for the evaporation depends on the application (flow range, liquid, pressure, temperature). We advise a certain minimum of carrier gas flow for the transportation of the liquid source from the mixing valve into the heat-exchanger (see the diagrams below). To avoid a large pressure drop across the system, the gas flow should be restricted to approx. 100 ln/min for the 1000 W unit and approx. 10 ln/min and 4 ln/min respectively for the smaller units. For more information about Bronkhorst MFCs see EL-FLOW and Metal Sealed Digital Mass Flow / Pressure Meters and Controllers.

Mass Flow Meter for Liquids

For 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 our leaflets describing the LIQUI-FLOW, μ-FLOW, and mini CORI-FLOW.

CEM 3-way Mixing Valve and Evaporator

CEM, or Controlled Evaporator Mixer, for control of the liquid source flow and mixing the liquid with the carrier gas flow resulting in total evaporation; complete with Temperature Controlled Heat-Exchanger to add heat to the mixture to realize complete vaporization. (Tmax 200°C / Pmax 100 bar).

Power Supply and Readout System

Flow Control System for the control of the gas flow, liquid flow and temperature of the heat-exchanger.

Interconnecting Cables
  • 1 x cable MFC – electronics
  • 1 x cable LFM – electronics
  • 1 x cable heat-exchanger – electronics,
  • 1 x power cord heat- exchanger – electronics

Dimensions CEM 3-way Mixing Valve and Evaporator

Advantages of Exchanging Bubbler Systems with a CEM

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 required 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 the defined quantity of liquid is mixed with the carrier gas and vaporized. The method is straightforward, and virtually any concentration can be made in a matter of seconds with high accuracy and repeatability.

Classic Bubbler System
Classic Bubbler System

CEM Controlled Evaporation Mixing
CEM Controlled Evaporation Mixing


The CEM-system has been successfully applied in a large number of different applications in wide variety of markets. For example, tools (drills, screwdrivers, saw blades, etc.) and machine parts are coated to improve their wear resistance, electric and dielectric (insulating) layers are deposited in semiconductor and solar cell manufacturing processes, the heat insulation of glazing is improved by a coating applied to its surface and the humidity in reactor/process chambers is controlled to ensure optimum performance.

CVD (Chemical Vapor Deposition)

Chemical Vapor Deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials. The process is not only used in the semiconductor industry to produce thin films for LEDs, transistors and DRAMs, but also in surface treatment, hardening processes and 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:

  • ALD (Atomic Layer Deposition) or ALCVD (Atomic Layer CVD): deposits successive layers of different substances to produce layered, crystalline films.
  • APCVD (Atmospheric pressure CVD): CVD processes at atmospheric pressure.
  • MOCVD (Metal Organic CVD) – CVD processes based on metalorganic precursors.
  • PECVD (Plasma-Enhanced CVD): CVD processes that utilize a plasma to enhance chemical reaction rates of the precursors.
Defined Humidification of Gases

The CEM-system is ideally suited to the accurate adjustment of dew or moisture. The intrinsic characteristics of large dynamic range and high accuracy ensure that the moisture level can be controlled with great flexibility from only a few ppm up to virtually 100%, whilst also maintaining a very high stability in dew point. Full functionality is retained with operating pressures as high as 100 bar.

Calibration of Gas Chromatographs, Mass Spectrometers and Gas Sensors

Through the combination of Mass Flow Controllers for liquids and the CEM-system, gas phase concentrations can be produced as desired. Thus Mass Spectrometers or Gas Chromatographs can be calibrated as the reference stream from the CEM is both highly reproducible and highly accurate due to the direct action of the Mass Flow Controllers.

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