Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Electron or ion source
Reexamination Certificate
2000-03-15
2001-07-10
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Electron or ion source
C315S111410, C315S291000, C315S224000, C315S106000, C250S42300F, C250S492210
Reexamination Certificate
active
06259210
ABSTRACT:
FIELD OF THE INVENTION
The present invention is concerned with power control apparatus for controlling the power supplies of an ion source having an indirectly heated cathode.
BACKGROUND OF THE INVENTION
Hot cathode ion sources are well known and include for example the so called Freeman ion source and the so called Bernas ion source. These ion sources include a filament cathode which is directly heated by passing a current through the filament from a filament power supply. The ion source comprises an arc chamber containing the filament and to which a supply of gas or vaporised material is supplied. Once the cathode filament has been heated sufficiently with the filament current, thermionic electrons are emitted by the cathode. If the cathode is held at a substantial negative potential relative to an anode electrode in the arc chamber, a plasma is formed in the arc chamber with an arc current flowing between the cathode and the anode electrode. Typically, the anode electrode is in fact formed by the walls of the arc chamber.
In the plasma so formed in the arc chamber, molecules of the feedstock gas or vapour are ionised and these positive ions are extracted from the arc chamber through an aperture by an extraction electrode held at a negative potential relative to the arc chamber. The extracted ions may then be used to form an ion beam which may have a number of applications. One important application is in ion beam implantation where beams of ions of desired dopant materials are directed at semiconductor substrates (wafers) in order to implant the dopant in the semiconductor to provide desired conductivity conditions.
A Freeman type ion source, especially for ion beam implantation apparatus, is disclosed in U.S. Pat. No. 4,578,589. The Freeman ion source includes a filament power supply to provide a heating current through the cathode filament, and an arc power supply. U.S. Pat. No. 4,754,200 discloses a method of controlling the power supplies to optimise performance of the ion source, especially in its application in an ion implantation apparatus.
U.S. Pat. No. 5,262,652 discloses a Bernas-type ion source in an ion implanter application. Again the Bernas-type source has a directly heated cathode filament with a filament power supply to provide a heating current through the filament, and a separate arc power supply to provide a desired arc potential between the filament and the anode or arc chamber body.
The above mentioned U.S. Pat. No. 4,754,200 discloses circuits for controlling the ion source power supplies. Thus, it is known to apply a constant arc voltage between the filament and the arc chamber body (or anode) and then to adjust the filament supply to achieve a desired arc current. It is also known to control the filament power supply by means of a power demand signal, i.e. to achieve an output power from the filament power supply in accordance with an input power demand signal, where the filament input power demand is derived from an error in the arc voltage, the arc current being held constant.
U.S. Pat. No. 5,497,006 discloses an ion source of the Bernas-type, but with an indirectly heated cathode. In this arrangement, the cathode in the arc chamber of the source is formed of an electrically conductive button-like member which is indirectly heated from a separate filament located behind the button on the opposite side relative to the main plasma chamber of the ion source. The source includes not only an arc power supply and a filament power supply, but also a cathode bias power supply which provides a required bias potential between the filament and the cathode button. In operation, the filament is biased negatively relative to the cathode button so that electrons thermionically emitted by the filament are accelerated to strike the rear face of the button, thereby heating the button cathode so that this cathode in turn emits electrons into the plasma chamber to initiate or maintain the plasma arc of the source.
This arrangement with an indirectly heated cathode in the ion source greatly extends the life of the ion source before it is necessary to change either the cathode itself or the cathode heater filament. The above form of ion source with an indirectly heated cathode will be referred to hereinafter as an indirectly heated cathode-type ion source as hereinbefore defined.
U.S. Pat. No. 5,497,006 discloses an ion source having a filament power supply unit, a cathode bias power supply unit and an arc power supply unit, and a power control apparatus for controlling these power supplies. In the U.S. specification, programmable power supply units are used for each of these supplies. The arc power supply unit is controlled to provide an output arc voltage in accordance with an input voltage demand level. The bias voltage applied between the cathode button and the filament by the cathode bias power supply unit is set in accordance with the difference between a desired arc current and the measured current delivered to the arc by the arc power supply, so as to minimise this difference. Thus, if the arc current is below the demand value, the cathode bias supply voltage is increased to increase the heating energy delivered to the cathode button, thereby to reduce the impedance of the plasma in the arc chamber and consequently to increase the arc current. The filament power supply in U.S. '006, is in turn controlled to keep the current delivered by the cathode bias power supply equal to a desired level of current. Thus, the current from the cathode bias power supply is maintained at a required demand level by increasing or decreasing the voltage applied by the filament power supply to the filament.
SUMMARY OF THE INVENTION
The present invention sets out to improve the way in which the various power supplies of an indirectly heated cathode-type ion source are controlled, to improve stability, to maximise source life, and to provide quick response especially in terms of control of the plasma arc of the source.
According to one aspect of the invention, power control apparatus for controlling power supplies of an indirectly heated cathode-type ion source as hereinbefore defined, having a filament power supply unit, a cathode bias power supply unit and an arc power supply unit, comprises a bias supply controller responsive to a difference between a parameter of the arc power supply and a demand value of said parameter to produce a bias power demand signal representing an output power of the bias power supply unit required to minimise said difference and is further responsive to said bias power demand signal to maintain said output power of said bias power supply unit at said required power.
Because the bias supply controller operates to maintain the output power of the cathode bias power supply unit at a required level, the power delivered to the cathode is not effected by changes in impedance of the cathode bias load, that is the impedance of the electron flow in the region between the filament and the indirectly heated cathode itself. Such impedance changes can arise due to changes in the chemistry of materials in this region, changes in gas pressure or physical changes, for example.
In prior art control arrangements, where for example, the output voltage of the cathode bias power supply is directly controlled, a change in the cathode bias load impedance produces a change in the cathode bias current, and therefore a change in the power delivered to the cathode from the cathode bias power supply unit. In the prior art arrangement disclosed in U.S. Pat. No. 5,497,006, this change in cathode bias power must be compensated for by adjusting the filament power supply in order to return the cathode bias current to a required value. With this arrangement of the invention the output power of the cathode bias power supply unit is directly controlled to match a demand value, so that a change in bias load impedance does not require external compensation to keep the power delivered to the bias load at the demand value.
The arc power supply parameter used to produce a bias power deman
Applied Materials Inc.
Philogene Haissa
Tennant Boult Wade
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