RF in Science and Industry
Jonathan Allen, Ph.D.
RF Electronics Consulting
Philadelphia CONET
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What is RF?
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Maxwell
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James Clerk Maxwell 1831-79
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RF Vs. Pwr. or LF
• LF: System dimensions << so propagation
times are insignificant within system.
• RF: Phase diff. due to propagation times.
• RF: Skin effect = (2/ o)1/2 [mks]
• In RF plasmas, ion and electron migration
per 1/2 cycle generally << sys. Dimensions.
• Small L and C values much more important
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LF Capacitors & Inductors
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RF Capacitors & Inductors
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ISM Bands (U.S.)
Center Freq.
6.780 MHz
*13.560 MHz
*27.120 MHz
*40.680 MHz
915.00 MHz
2.45 Ghz
Bandwidth
30 kHz
14 kHz
326 kHz
40 kHz
26 MHz
100 MHz
Availability
Local acceptance
Worldwide
Worldwide
Worldwide
Reg. 2 (Americas)
Worldwide
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Examples of ISM RF
• Heating of lossy materials
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Plastic welding (PVC)
Cooking
Drying
Glue curing
Medical (diathermy, ablation, cautery)
• Materials testing
• NMR/MRI
• Plasma processes
– Sputtering & deposition
– Etching
– Spectroscopy
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RF Heating
Polar molecules flip orientation as e-field reverses. Some of the
energy is dissipated as heat (dielectric loss).
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RF Gluing (also plastic welding)
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RF Drying
• Use for wood, foods, ceramic greenware
• Fast
• Selective--Heats only wet zones, uncured
resins.
• Uniform in depth.
• Controllable
– Reduce RF power as product approaches goal
• Often uses 27 or 40 MHz
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RF Drying Wood
SAGA
RF-Vacuum
Timber drying
system
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Drying Potato Chips
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RF Induction Heating
• Usually uses LF ~100 KHz for metals
• Localized heating possible  Zone refining
• No combustion products or oxygen (can heat in vacuum)
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Induction Heating Metal Rod
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Lab Measurement
The same properties of polar molecules that enable
RF heating also measure moisture content of wood,
flour, etc. with an RF capacitance bridge.
RF induction coils measure the thickness of
metallic films and foils based on skin-depth.
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Plasmas Generate:
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Free electrons
Positive ions
Radicals
Energetic particles
Spallation of target (sputtering)
Chemical reactions
Excited atoms and molecules
Light (glow discharge)
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Paschen Curves--Breakdown vs. Pressure
MFP (cm) = 5x10-3/p (torr)
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Plasma Sputtering
Energetic ions impact target and dislodge atoms or
molecules. These migrate to the substrate where
they deposit to form a thin film.
Sputtering may be reactive, such as an aluminum
target whose sputtered atoms reacting with oxygen
in the process gas to form an Al2O3 film.
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Plasma Sputtering
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Plasma Etching
Plasma produces chemically active radicals
which react with unmasked areas of wafer.
This etches away material.
e.g. Fluorine radicals and ions etch silicon:
C F4 + e-  C F3 + F + eSi + 4F  Si F4
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Plasma Etching with Mask
DC field superimposed on RF helps accelerate F- ions
Plasma
Mask
RF
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Amorphous Si Plasma Deposition
SiH4  Si  + 2 H2
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ICP Spectroscopy
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How to Generate RF
• Spark generator (obsolete, dirty)
• Power oscillator (efficient, cheap, but
frequency not well defined)
• Oscillator (usu. xtal), driving Power
amplifier (present industry standard)
– Vacuum tubes
– Bipolar transistors
– Power FETs
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Power Oscillator
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Oscillator-Amplifier System
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Power Tubes 4-400C, 5CX1500A
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RF Power Transistor
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Impedance Matching
RF generators conventionally have 50
(resistive) output impedance.
Loads can have any complex (and varying)
impedance. Matching networks allow the
generator always to see a 50 resistive load
and therefore operate efficiently.
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L Network
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Pi Network
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Univ. Matching Network, Can set as L or Pi
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Allen Matching Network
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Allen MN Physical Construction
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RF Instrumentation
• Current measurement
• Voltage measurement
– Broadband
– Frequency selective
• Directional power
• Circuit analysis
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RF Ammeter
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RF Ammeter (Square-law scale)
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Induction Ammeter
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VTVM with HF Probe (hp 410C)
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Calibrated Receiver (EMC-25)
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Single Frequency Detector
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Directional Wattmeter (Bird 43)
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RF Impedance Meter (hp 4815A)
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Grid Dip Meter (Measurements 59)
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Important Uses of RF
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Food
Construction
Metallurgy
Semiconductors (Discrete, IC, Photovoltaic)
Optics
Health and Medicine
Laboratory science
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