02 rf energy

JozwiakSpring 2004 Semitec 215Plasma and RF Energy

Introduction to RF Energy• Electromagnetic Radiation:• Hans Christian Oerstead (1819)

– demonstrated a link between electricity and magnetism– he noticed that a magnetic compass needle could be

deflected by placing it near a wire with electric current running through it

– developed left-hand rule

• James Clerk Maxwell (1850)– showed that electrical fields and magnetic fields can

couple together to form electromagnetic waves– once created, an electromagnetic wave will continue

forever until it is absorbed by matter


Introduction to RF Energy• Electromagnetic Radiation:• Electromagnetic waves:

– Energy radiating from vibrating charged particles (i.e. electrons).– Requires no matter to propagate (compare to sound waves).– Changing electric field <==> Changing magnetic field (coupling)


Introduction to RF Energy• Electromagnetic Waves:• Wavelength/ Frequency and the velocity of the wave are related by:

f = v = wavelength (meters)f = frequency (hertz or cycles/second)v = velocity (meters/second)

• For an electromagnetic wave, v is replaced by c (the speed of light):c = 2.9979 x 108 meters/second (in a vacuum)c = 2.9391 x 108 meters/second (in air)c = 2.2490 x 108 meters/second (in water)etc.


Introduction to RF Energy• The Electromagnetic Spectrum:• Logarithmic scale of electromagnetic waves ordered by frequency or wavelength.• Infinitely long at both ends (in

theory).• In practice, ranges from:• Long Waves

– 104 m, 300 Hz to • Gamma Rays

– 10-14 m, 1021 Hz • This is 18 orders of magnitude!


Introduction to RF Energy• Radio Waves:• Heinrich Hertz (1880)

– proved the existence of radio waves– demonstrated that radio waves travel at the speed of light– also demonstrated that radio waves can travel freely

through the air

• Characteristics of Radio Waves:– lowest frequencies in the electromagnetic spectrum– produced by electrons oscillating in an antenna– also produced by stars, sparks, and lightning– used mainly for communications


Introduction to RF Energy• Radio Waves:• Bands:

– Long Wave (LF): ~ 1-2 km, specialized radio applications

– Medium Wave (HF): ~ 100m, AM radio

– VHF (Very High Frequency): ~ 2m, FM radio, commercial aircraft

– UHF (Ultra High Frequency): < 1m, police radio, military, television


Introduction to RF Energy• RF Power (or RF Energy):

– RF Power: AC electrical power at radio frequencies• RF Power is NOT electromagnetic radiation

– Radio Waves: Electromagnetic waves at radio frequencies

• Radio waves are created (radiated) when RF power travels through an unshielded conductor (antenna)


Introduction to RF Energy• Non-Semiconductor Applications of RF Power:• Heating and Curing via Induction Heating:

– electrically conducting materials (usually metals) are heated by a non-contact method in an alternating magnetic field

– varying magnetic field induces an electric current inside the component just like a transformer

– only the object is heated– more efficient and consistent than radiative or resistive heating

• Applications for Induction Heating:– heat treatment of metals– heating prior to deformation (i.e. forging)– brazing and soldering– shrink fitting (motor rotors to shafts)


Introduction to RF Energy• Non-Semiconductor Applications of RF Power:• Drying of Synthetic and Natural Fibers:

– water in the product conducts RF-induced current until the product is dry

– product is not damaged due to over-drying

• Lumber Treatment:– Radio Frequency Vacuum (RFV) Drying places wood between

two large RF electrodes.– Bipolar nature of water molecules means that they rotate

quickly in response to the changing electric field.– Friction from moving water molecules creates steam that

permeates out of the wood. Dries from the inside out.– Reduced drying time, more efficient, higher quality.


RF Plasma• DC Power is rarely used for industrial plasma

generation.• Why use RF Power?1. Allows the Usage of Insulating Electrodes:

– In deposition and etching systems, the electrodes are coated with an insulating material.

• This reduces particulates in the chamber, and makes them easier to clean.

– The one-way nature of DC plasmas causes a charge to build up on the electrodes and extinguish the plasma.

– In an AC (RF) system, there is no net current flow to either electrode, so there is no charge buildup.


RF Plasma• Why use RF Power?2. Improved Power Coupling:

– RF current creates electromagnetic waves inside the plasma chamber.– These electromagnetic waves couple energy into the plasma much

more efficiently than DC does.

3. RF Plasmas are Easier to Strike:– Gasses in an RF plasma ionize more efficiently than in DC plasmas =>

more ions for the same amount of power.– RF plasmas can be sustained at lower gas pressures than DC plasmas.

4. Reduced Ion Bombardment:– Ion bombardment in DC plasmas can damage the wafer.– A properly biased RF plasma can reduce or eliminate ion

bombardment.


RF Plasma• RF Plasma Frequency:• Federal Communications Commission assigns

frequency bands for various uses.– RF plasma generation falls under ISM band (Industrial,

Scientific, Medical).

• The most widely used frequency for industrial plasma generation is 13.56 MHz (13.56 x 106 cycles/second).– Originally selected to reduce interference of higher-

order harmonics with other bands.– Other frequencies used include: 55 kHz, 100 kHz, 455 kHz,

and 2.45 GHz (microwave plasma).


RF Plasma• RF Plasma:• Why RADIO frequency?

– The Breakdown Voltage (voltage at which a gas ionizes) is a function of:

• the ionization potential of the gas• the mean free path• the frequency of the applied RF power

– most gasses ionize more efficiently at radio frequencies

– At lower frequencies, the behavior is more like a DC plasma.

• Non-conducting electrodes quickly charge up.

– At higher frequencies, the (relatively) heavy ions are not mobile.


RF Plasma• Self-DC Bias:• A pure AC signal (with no DC bias) injected into a plasma can create a plasma

voltage with both AC and DC components.– This is called “Self-DC bias”.

• Self-DC Bias is created because:

– At RF frequencies, the lighter electrons respond much faster than do the heavier ions.

– Electrodes of unequal area (usually the case) cause a greater flow of electrons in one direction than the other.


RF Plasma• Self-DC Bias:

increases as the 4th power of the relative areas of the electrodes!

• Self-DC Bias will be shorted out:– If conducting electrodes and no blocking capacitor are used.– OR If the electrodes are of equal area.

• Self-DC Bias will exist if:– A blocking capacitor is inserted in series with the electrodes, or – Non-conducting electrodes are used.

21_ VVV BiasDC 4

1

2

2

1

AREA

AREA

V

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RF Plasma• Desirable Self-DC Bias:

– Sputtering and Reactive Ion Etching (directional processes).– There is a net flow of ions towards the smaller of the two electrodes:

• the target in the case of sputtering, or• the wafer in the case of RIE.

– A blocking capacitor is used, and the electrodes are designed for the desired amount of DC bias.

• Undesirable Self-DC Bias:– PECVD and Plasma Etching (chemical processes).– Self-DC bias may result in an undesired level of ion bombardment on

the wafer.– Self-DC bias must be reduced or eliminated.– The blocking capacitor is removed, and the electrodes are designed

as symmetrical (equal area) as possible.

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