RF in Science and Industry Jonathan Allen, Ph.D. RF Electronics Consulting Philadelphia CONET 1 What is RF? 2 Maxwell 3 James Clerk Maxwell 1831-79 4 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 5 LF Capacitors & Inductors 6 RF Capacitors & Inductors 7 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 8 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 9 RF Heating Polar molecules flip orientation as e-field reverses. Some of the energy is dissipated as heat (dielectric loss). 10 RF Gluing (also plastic welding) 11 12 13 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 14 RF Drying Wood SAGA RF-Vacuum Timber drying system 15 Drying Potato Chips 16 RF Induction Heating • Usually uses LF ~100 KHz for metals • Localized heating possible  Zone refining • No combustion products or oxygen (can heat in vacuum) 17 Induction Heating Metal Rod 18 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. 19 Plasmas Generate: • • • • • • • • Free electrons Positive ions Radicals Energetic particles Spallation of target (sputtering) Chemical reactions Excited atoms and molecules Light (glow discharge) 20 Paschen Curves--Breakdown vs. Pressure MFP (cm) = 5x10-3/p (torr) 21 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. 22 Plasma Sputtering 23 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 24 Plasma Etching with Mask DC field superimposed on RF helps accelerate F- ions Plasma Mask RF 25 Amorphous Si Plasma Deposition SiH4  Si  + 2 H2 26 ICP Spectroscopy 27 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 28 Power Oscillator 29 Oscillator-Amplifier System 30 Power Tubes 4-400C, 5CX1500A 31 RF Power Transistor 32 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. 33 L Network 34 Pi Network 35 Univ. Matching Network, Can set as L or Pi 36 Allen Matching Network 37 Allen MN Physical Construction 38 RF Instrumentation • Current measurement • Voltage measurement – Broadband – Frequency selective • Directional power • Circuit analysis 39 RF Ammeter 40 RF Ammeter (Square-law scale) 41 Induction Ammeter 42 VTVM with HF Probe (hp 410C) 43 Calibrated Receiver (EMC-25) 44 Single Frequency Detector 45 Directional Wattmeter (Bird 43) 46 RF Impedance Meter (hp 4815A) 47 Grid Dip Meter (Measurements 59) 48 Important Uses of RF • • • • • • • Food Construction Metallurgy Semiconductors (Discrete, IC, Photovoltaic) Optics Health and Medicine Laboratory science 49