Plasma diagnostics

 

 

 

 

 

 

 

 

 

  

MWI 2650 and 2650/A - Microwave interferometer


 

Features

·      efficient and reliable electron density measurement

·      no limitation for the characterization of chemically reactive plasmas (etching, thin film deposition etc.)

·      non-invasive method

·      no assumptions about plasma sheath or ion composition needed

·      high temporal resolution (5 µs)

·      easy implementation into existing setups due to flexible reference path using a BNC-type coaxial cable

Applications

·      characterization of technical plasmas

·      plasma- and process control

·      process development

·      fundamental plasma research

 

JE PlasmaConsult GmbH has developed an advanced and very cost effective Microwave Interferometer (MWI 2650) operating at 26.5 GHz with an output power of 40 mW. It uses a dielectric resonator oscillator for high frequency stability and a super-heterodyne frequency conversion scheme for the registration of the phase shift by the plasma, which is proportional to the plasma density. This technique allows the use of a flexible coaxial cable instead of a rigid Ka-band microwave waveguide for the reference path. 

 

The PlasmaConsult MWI 2650 microwave interferometer consists of three main components:

  • Control and display unit

  • sender (horn antenna)

  • receiver (horn antenna)

All cables and connectors are included

 

Basics

The electron density is a key plasma parameter for the understanding and modelling of plasma sources. A classic method to determine the electron density in low-pressure glow discharges is the invasive Langmuir probe technique. However, in thin film deposition and etching plasmas Langmuir probes are considered to be less accurate or even non-applicable, because of probe surface coating by dielectric layers or probe surface etching, thus reducing the probe surface during the measurement.

Microwave (MW) interferometry is an interesting complement to the classical Langmuir probe in such cases, since it is a non-intrusive and less contamination sensitive method. Furthermore, it is a much faster method ( 5 μs temporal resolution) and does not need assumptions about the plasma sheath or the ion composition.

 

Working Principle

In general, microwave interferometers measure changes in the phase length of one path with respect to another. This phase length change corresponds directly to the line-of-sight integrated electron density of the plasma.

The MWI 2650 consists of two horn antenna, one is emitting a microwave beam at 26.5 GHz, and the other horn acts as a receiver. The plasma reactor should be equipped with two opposite, MW transparent dielectric windows. When the plasma is turned on, the presence of free electrons lowers the dielectric permittivity and decreases the phase length of the plasma path.

Within the electronic unit, this phase shifted signal is compared to the unaffected signal of the reference path. The measured phase shift ΔΦ is used to derive the line-of-sight integrated electron density between the two horn antenna. Its absolute value (in units of cm-3) is shown in the electronic unit display.

Diagnostics of atmospharic pressure plasma

The version MWI 2650-A is suited for atmospheric pressure plasmas. However, the electron density is not given directly in the electron unit display. Instead the phase shift and damping is shown. Using certain tables and graphs, the user can now calculate the electron density. Certain assumptions about the plasma collision frequency between electrons and heavy particles have to be made. 

 

Technical data  MWI 2650:

Operation frequency:            26.5 GHz

MW output power:                 40 mW

Min. electron density:

        pulsed plasma:                (10 9 - 1010)/ l  [cm-3]

                                              with l plasma path length in [cm].

         cw plasma:                     (1010-1011) / l  [cm-3]

Max. electron density:          4 x 1012 cm-3

temporal resolution:             5 µs

Max. gas pressure:               2000 Pa

plasma path length:             6,5  - 100 cm (smaller dimensions optionally)

Min. size reactor window:    50 mm

 

 

Technical data  MWI 2650-A:

Operation Frequency:            26.5 GHz

MW output power:                 40 mW

Min. electron density:

depending on plasma collision frequency:

                                             10 9 cm-3 for a plasma path length of 10 cm and in case the plasma collision frequency is of the order of  26.5 GHz, e.g. cold plasma with low electron temperature (e.g. a flame) 

                                             

                                             1010 cm-3  for a plasma path length of 10 cm and in case the plasma collision frequency is one order of magnitude higher than 26.5 GHz, e.g. a thermal plasma with electron temperature of several eV (e.g. plasma torch)

 

Max. electron density:          3 x 1013 cm-3

Phase shift:                           0 - 270°, resolution 0.1°     

Damping:                              0 - 10 dB, resolution 0.02 dB

 

Application note: