Radio frequency power source for generating an inductively coupled plasma

A system for converting DC power into a RF electromagnetic field for maintaining a plasma in a processing chamber, the system comprising:

• a coil constructed to surround the processing chamber and configured to couple RF power into the plasma, said coil having two ends;
• and
• a RF power generator including a free-running oscillator having a DC power supply and a RF power output, said power output being connected to a load impedance which includes said coil, said RF power generator being operative for supplying constructed and arranged to supply RF current to said coil in order to generate the RF power that is coupled into the plasma, wherein:
• said free-running oscillator comprises:
• a vacuum tube having a cathode, a plate and a grid;
• a grid-leak circuit connected to said grid;
• a feedback circuit coupled to said vacuum tube;
• and a DC supply circuit constructed and arranged to heat said cathode;
• and
• at least part of said coil is connected to form a part of said feedback circuit wherein:
• said part of said coil that forms a part of said feedback circuit is spaced from at least one of said ends of said coil, and
• said DC power supply produces a time varying DC voltage that varies the RF power coupled into the plasma, and
• the DC voltage has the form of a train of pulses having rise times no more than 30 microseconds and fall times no more than 50 microseconds.

A method for converting DC power into a RF electromagnetic field for maintaining a plasma in a processing chamber, the method comprising:

• placing a helical coil around the processing chamber for coupling RF power into the plasma;
• providing a RF power generator including a free-running oscillator having a vacuum tube constituting an active component of the RF power generator, a feedback circuit coupled to said vacuum tube, a DC power input and an RF power output;
• connecting the RF power output to a load impedance which includes the helical coil for supplying RF current to the helical coil and connecting at least part of the helical coil to form a part of the feedback circuit;
• and
• introducing an ionizable gas into the chamber and delivering DC power to the DC power input in order to activate the oscillator to generate the RF power that is coupled into the plasma, wherein a modified Hartley oscillator is employed to enable automatic transition between start and run conditions without retuning.