RF Generators. Requirements:

Transcription

1 Requirements: RF Generators to deliver a requested forward power (adjustable) level into an RF system power level is adjusted manually, or power level is controlled by a digital or analog input signal from the system controller to deliver a specific frequency (usually not adjustable) into an RF system frequency is pre-set in the construction of the generator usually MHz waveform is usually desired to be a perfect sinusoid some generators deliver different waveforms (pulse, clipped, etc.) that can result in undesirable harmonics output impedance must be very close to that which is specified (usually 50 )

2 Specifications: RF Generators 1250 W, 1500 W are common configurations larger wafer sizes are requiring higher power levels: 2500W for 200 mm wafers 5000W for 300 mm wafers mounted vertically on a rack, usually in the subfab require water cooling VERY heavy! in excess of 75 lbs. Very expensive: $10,000 - $20,000

3 Block Diagram: RF Generators

4 AC Input: typically 220VAC must be well-grounded must be capable of delivering in excess of 10 amps of current Transformer: step up AC input voltage to > 1000 volts Rectifier and Filter: convert AC input to DC to drive RF generator subsystems (modern systems have replaced the transformer/rectifier combination with a switching power supply)

5 Oscillator: produces an output waveform (AC) with only a DC input frequency selection (triggering) is determined by a very precisely sliced quartz crystal signal is very low power, ~1mW output signal is split into multiple (up to 4) signals to drive 4 separate amplifier stages Buffer Amplifier: isolates the output of the oscillator from the input of the power amp prevents electrical cross-talk between the two provides filtering to eliminate harmonic distortion

6 Power Amplifier: amplifies the sine wave from the oscillator to the desired power level amplification is dependent on the DC voltage level from the control circuitry designed to deliver high power to low-impedance loads (50 includes additional filtering of undesired frequencies and harmonics

7 Power Generation and Dissipation: The output impedance of the generator is in series with the load. If the impedance is matched, then equal power is dissipated in each. This means: The RF generator must be capable of delivering twice the amount of power required by the load, and The RF generator must be capable of internally dissipating half of that power. This dissipation takes place in the transistors of the power amplifier. They must be properly heat sinked and, usually, water cooled.

8 Power Generation and Dissipation: Example (matched impedance): 2000 Watts consumed by generator (V x I into power amp stages) 1000 W is dissipated in the generator 1000 W is sent to the load» 300 W is dissipated in the match» 700 W actually makes it to the load (35% efficient) Example (non-matched impedance): 2000 Watts consumed by generator (V x I into power amp stages) 1000 W is dissipated in the generator 1000 W is sent to the load, 500 W is reflected back» an additional 500 W is dissipated in the generator (total 1500 W)» 200 W is dissipated in the match» 300 W actually makes it to the load (only 15%!)

9 Signal Recombination and Filtering: The four separated and amplified signals are recombined into one larger signal and delivered to the output. Measurement: built-in power meter that determines the output power level and generates a proportional signal signal is used to operate the output meter of the power supply signal is also used to drive the control circuitry Control: controls the DC voltage level of the rectified AC input into the power amplifier control signal responds to the power measurement signal this is a closed loop control system

10 Power Regulation: RF Generator Evaluation Defines the variation in output power from the setpoint. Regulation % = (power deviation / power commanded) x 100%

11 Harmonic Distortion: RF Generator Evaluation It is nearly impossible to generate an AC waveform at one frequency only. Even the best crystal oscillator circuits generate a range of frequencies that are centered around the target (or principal ) frequency. Often, the frequency spectrum will contain smaller frequency peaks at multiples of the principal frequency. These smaller peaks are called harmonics. Harmonics of a MHz signal will appear at 27.12, 40.68, and MHz, gradually diminishing.