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RF in Science and IndustryJonathan 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. Bandwidth Availability

6.780 MHz 30 kHz Local acceptance*13.560 MHz 14 kHz Worldwide*27.120 MHz 326 kHz Worldwide*40.680 MHz 40 kHz Worldwide915.00 MHz 26 MHz Reg. 2 (Americas)2.45 Ghz 100 MHz Worldwide

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Examples of ISM RF• Heating of lossy materials

– 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 HeatingPolar 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

SAGARF-VacuumTimber dryingsystem

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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:

• 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. PressureMFP (cm) = 5x10-3/p (torr)

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Plasma Sputtering

Energetic ions impact target and dislodge atoms ormolecules. 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 oxygenin 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 + e-

Si + 4F Si F4

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Plasma Etching with Mask

Plasma

Mask

RF

DC field superimposed on RF helps accelerate F- ions

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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

• Food

• Construction

• Metallurgy

• Semiconductors (Discrete, IC, Photovoltaic)

• Optics

• Health and Medicine

• Laboratory science