Methods and apparatus for RF power delivery

RF power is delivered to a load in an RF power processor for carrying out process operations. The RF power delivery is performed using a variable frequency RF power amplifier and a control system. The control system maintains RF power delivery using control signals derived from reference RF signals used by the RF power amplifier and measurements of a characteristic of the RF power. The magnitude of the delivered RF power is controlled using measurements of the power input to the RF power amplifier.

BACKGROUND
 The present invention relates to improved methods and apparatus for
 delivering radio frequency (RF) power for RF power process operations.
 RF power is used in a wide variety of applications for carrying out process
 operations. Exemplary of such process operations is the use of RF
 induction power for heating. RF induction heating involves coupling RF
 power to a material, such as a workpiece, that absorbs the RF power and
 converts the RF power into heat. In other words, the currents induced in
 material by the RF power are converted into heat because of the electrical
 resistance of the material that absorbs the RF power. In this manner, the
 RF power can be used to heat the workpiece without having physical contact
 between the power source and the object. This type of heating can be used
 when the workpiece is the material that absorbs the RF power, and the
 workpiece is heated directly by the RF power. Alternatively, the workpiece
 may be in contact with or near a second material that absorbs RF power.
 The second material absorbs the RF power and creates heat. The heat is
 then transferred to the workpiece by conduction, convection, or radiation.
 In another example of RF heating, the RF power can be coupled to a gas to
 produce a thermal plasma. Free electrons in the thermal plasma absorb the
 RF power and they are raised to high energy levels. These energetic free
 electrons interact with other gas phase species to produce a high
 temperature mixture capable of transferring thermal energy to other gases,
 liquids, or solids.
 The thermal plasmas mentioned above can be used to promote chemical
 reactions. Chemical reactions can be promoted because of the high
 temperatures of the thermal plasma. Alternatively, thermal plasmas are
 able to promote chemical reactions because of the ability of the energetic
 electrons to break chemical bonds and allow chemical reactions to occur
 that would proceed with difficulty under non-plasma conditions.
 The manufacture of optical fiber pre-forms is an example of the use of
 thermal plasmas generated using RF power. The RF thermal plasma provides
 the energy for driving the chemical reactions in gas mixtures of silicon
 compounds, oxygen, and dopants. The chemical reactions cause deposition of
 doped silica layers.
 Another example involving RF power thermal plasmas is the operation of
 high-pressure gas lasers. In gas laser operation, the important
 characteristic of the RF plasma is the light emission that occurs because
 of the plasma. The thermal energy that is produced is generally not
 considered important to the operation of the laser.
 In other applications, RF power is used to produce non-thermal plasmas,
 also referred to as non-equilibrium plasmas. The fabrication of
 semiconductor devices is one area in which non-thermal plasmas are
 extensively used. The non-thermal plasmas are used for etch processes
 wherein the non-thermal plasmas are used to generate reactive species in a
 gas to accelerate reactions between the species and a solid surface. The
 etch process can be a general removal of components on the surface as in a
 cleaning process or the selective removal of material from certain areas
 on the surface through use of a masking material that has been previously
 patterned. Non-thermal plasmas are used to promote deposition reactions
 wherein gas phase species are caused to react to form a solid product that
 deposits on surfaces. During the manufacture of semiconductor devices,
 etch processes involving RF plasmas and deposition processes involving RF
 plasmas are used repeatedly during the fabrication process. One of the
 main benefits of using the non-thermal plasma is the ability of the
 non-thermal plasma to stimulate chemical reactions that would otherwise
 require temperatures that are too high for use in the fabrication of
 semiconductor devices.
 RF power non-thermal plasmas are also used as cleaning processes in the
 fabrication of semiconductor devices. The non-thermal plasmas are commonly
 used to strip photoresist materials from semiconductor wafers as part of
 post etch wafer clean procedures. The photoresist material serves as a
 mask material during etch processes used in patterning the surface of the
 devices. Resist material is stripped from the surface of the wafers by
 creating a non-thermal plasma in a gas containing oxidizing species such
 as oxygen and possibly halogen species that are capable of reacting with
 and volatilizing the resist material. In some applications, the
 non-thermal plasma is maintained at a position upstream of the wafer.
 Reactive species generated in the non-thermal plasma flow downstream and
 react with the wafer surface for the stripping process.
 Another cleaning process that uses non-thermal plasmas is the cleaning of
 reaction chambers used in semiconductor manufacturing. Sometimes, the
 reaction chambers used in plasma etch processes experience a buildup of
 deposits from the etch process. These deposits need to be removed as part
 of the reactor maintenance process. In addition, the reactors that are
 used in deposition processes for semiconductor device fabrication undergo
 a buildup of deposits on the reactor walls; the wall deposit must be
 removed as part of reactor maintenance. Non-thermal plasmas generated
 using RF power and gases containing species that are reactive with the
 deposits have been used to volatilize and removed the deposits built up on
 the walls of etch reactors and deposition reactors.
 RF power plasmas have also been used for decomposition of chemical
 compounds that are hazardous or otherwise undesirable. Some of the
 compounds are highly refractory in nature and are difficult to decompose.
 Examples of compounds that have been decomposed or abated with plasmas
 include chlorofluorocarbons (CFC) and perfluorocompounds (PFC).
 The applications given above where RF power is used as part of a process
 makeup only a small fraction of the applications for RF power. There are
 numerous additional processing applications for RF power. However, the
 methods and apparatus typically used to deliver RF power have deficiencies
 and may be inefficient for use in some RF power process operations. Some
 of the deficiencies are common for multiple applications. The existing
 deficiencies in the prior methods and apparatus for RF power delivery may
 limit the use of RF power for possible new applications.
 One frequently encountered problem with prior RF power delivery systems is
 that the equipment tends to be large and heavy. There are instances in
 which the size of the RF power generator greatly exceeds the size of the
 processing chamber. Problems resulting from the large size of the
 equipment include taking up excess space on a factory floor. The excess
 space required by the equipment can be quite expensive if it is in a
 high-cost factory, such as a cleanroom used in semiconductor
 manufacturing. The large size also makes transporting the equipment
 difficult. Moving the apparatus frequently requires more than one person
 and the use of moving equipment.
 A second problem with existing RF power delivery systems is their
 complexity. The existing systems frequently have redundant systems and
 extra capabilities that are unnecessary. In addition, the effort to derive
 data for controlling the RF power delivery is unnecessarily complex.
 Here is one example of how a typical old-style RF power delivery system
 operates. Low frequency AC power is rectified and then switched to provide
 current to the RF amplifier. The RF amplifier drives current through an
 output match network and then through an RF power measurement circuit to
 the output of the power supply. The output match is usually designed to
 provide RF power that matches an impedance of 50 ohms. The 50 ohm
 impedance match is necessary in order to have the same characteristic
 impedance as the industry standard coaxial cables. Power flow through the
 50 ohm coaxial cable section is measured again by a load match controller.
 The instrument used for measuring the power is also designed to be
 compatible with the 50 ohm impedance of the coaxial cable. A load match,
 usually a variable RF match with a motorized automatic tuner, transforms
 the RF power again to enable the RF power to be coupled to a load. Motors
 in the tuner can change the values of variable capacitors and inductors
 present in the tuner. The variable load match is necessary in order to
 accommodate changes in the impedance of the load. Specifically, the
 variable load match makes it possible for the RF amplifier to provide RF
 power at a constant impedance to a load that may have a variable
 impedance. This arrangement allows constant RF power delivery if the load
 impedance changes.
 In the example just presented, the RF power is measured multiple times at
 different points between the RF amplifier and the load. In addition,
 special coaxial cable and measuring instruments are necessary in order to
 comply with the industry standards. Furthermore, the RF power measurements
 typically include forward RF power measurements and reflected RF power
 measurements.
 The coaxial cables described in the previous example present an additional
 problem for high frequency RF power. The length of the coaxial cable may
 be long enough, with respect to the wavelength of the RF power, to allow
 standing wave formation from the forward RF power and reflected RF power.
 Standing waves in the coaxial cable produce high peak currents and
 voltages that can damage the coaxial cable.
 For low frequency RF power delivery systems, the operation may be different
 from that of the previous example. The low frequency RF power delivery
 systems use a low frequency RF power amplifier. The low frequency RF
 amplifier is coupled through a fixed load match to a load. Because the RF
 frequency is low, the transmission line between the RF amplifier and the
 load generally is much shorter than a quarter wavelength, so that no
 standing wave pattern occurs in the coaxial cable. Consequently, there are
 no problems with high peak voltages and peak currents for low frequency RF
 power delivery systems.
 In the typical old-style RF power delivery systems, whether low frequency
 or high frequency, forward RF power and reflected RF power are both
 measured. Usually the forward and reflected RF power measurements are made
 with a device such as a dual directional coupler. The forward and
 reflected power measurements are used as part of the RF power control
 system to maintain a constant power delivery to the load. Measurements of
 the reflected RF power is used in high frequency RF power delivery systems
 to control the variable load match such that match conditions are obtained
 that give a substantially zero reflected power. In low frequency RF power
 systems, the reflected power measurements are used to adjust the forward
 RF power to obtain the desired net RF power delivery to the load.
 Clearly, there are numerous applications requiring RF power delivery
 systems. Unfortunately, typical methods and apparatus for old-style RF
 power delivery systems have characteristics that may be undesirable for
 some applications. There is a need for RF power delivery methods and
 apparatus that are simple in operation, use a minimum number of parts,
 minimize redundancy, and provide high reliability.
 SUMMARY
 This invention seeks to provide methods and apparatus that can overcome the
 deficiencies of known RF power delivery systems. Practicing this invention
 makes it possible to achieve RF power delivery to a load with lower
 complexity and fewer parts than are typically required in known RF power
 delivery systems.
 Aspects of the present invention are accomplished using a simpler system
 for controlling the RF power delivery to a load. One aspect of the
 invention is the control of RF power delivery by measuring a
 characteristic of the output RF power at an RF power coupling element and
 comparing the measured characteristic to a reference RF signal (a small RF
 signal) used by a variable frequency RF power amplifier and adjusting the
 frequency of the reference RF signal to maintain a constant phase angle
 between the reference RF signal and the measured signal for the RF power
 characteristic at the RF power coupling element.
 Another aspect of the invention is a method for controlling an apparatus
 for delivering RF power to the load. The method is performed with an RF
 power coupling element, and a variable frequency RF power amplifier
 capable of delivering RF power to the load via the RF power coupling
 element. The method includes deriving a control signal by comparing a
 characteristic of the RF power at the RF power coupling element to a
 characteristic of a reference RF signal used by the RF power amplifier.
 The method also includes using the control signal to adjust the frequency
 of the reference RF signal used by the RF power amplifier so as to effect
 efficient RF power delivery to the load.
 It is a further aspect of the present invention to achieve efficient RF
 power delivery based on a particular attribute of the RF power. For
 example, the delivery of RF power can be controlled to maintain a constant
 magnitude of the RF power. Alternatively, the delivery of RF power can be
 controlled to maintain a constant magnitude of the RF current.
 Alternatively, the delivery of RF power can be controlled to maintain a
 constant magnitude of the RF voltage. Preferably, efficient RF power
 delivery is based on the magnitude of the RF power.
 In another aspect of the present invention, power output from an RF
 amplifier is applied directly to an RF match without the use of coaxial
 cables. A variable RF match can be used in embodiments of the present
 invention. However, the preferred embodiment is to use a fixed RF match.
 The fixed RF match contains a match network having capacitance and
 inductance values that approximate tuned conditions expected for the load.
 An RF power coupling element is connected directly with the output of the
 fixed RF match; this connection is also made without the use of coaxial
 cables. The power coupling element couples RF power to a load. A sensor
 measures phase information for a characteristic of the RF power in the RF
 power coupling element. Two examples of the characteristics of the RF
 power in the RF power coupling element that can be used are RF current and
 RF voltage. A controller receives the measured phase information and
 determines the phase angle between the characteristic of the RF power in
 the RF power coupling element and a reference RF signal used by the RF
 power amplifier. The controller drives the phase angle between the
 reference RF signal and the characteristic of RF power in the coil to a
 predetermined constant phase angle by adjusting the frequency of the
 reference RF signal used by the RF power amplifier. The adjustments to the
 frequency of the RF power amplifier reference RF signal also appear as
 changes in the frequency of the RF power output from the amplifier.
 Maintaining the predetermined constant phase angle between the
 characteristic of RF power in the RF power coupling element and the
 reference RF signal used by the RF power amplifier causes the efficiency
 of RF power delivery to the load to be substantially unchanged by changes
 in the load impedance. The correlation between the phase of the reference
 RF signal used by the RF power amplifier and the phase of the RF current
 in the RF power coupling element replaces the need to measure the
 reflected RF power. Consequently, the circuitry required for measuring
 reflected RF power and for measuring forward RF power becomes unnecessary
 for achieving efficient RF power delivery for embodiments of the present
 invention.
 In another aspect of the present invention, RF power can be delivered at a
 controlled magnitude of the RF power. In one embodiment, a meter measures
 the power input to an RF amplifier. The power input to the RF amplifier
 can be DC power from a DC power source, or DC power derived from an AC
 power source, or AC power from a sub-radio frequency AC power source such
 as a slow AC power source. The RF amplifier uses the input power to
 produce an output of RF power. Power output from the RF amplifier is
 applied directly to a fixed RF match without the use of coaxial cables.
 The fixed RF match contains a match network having capacitance and
 inductance values that approximate tuned conditions expected for the load.
 An RF power coupling element is connected directly to the output of the
 fixed RF match. As one embodiment, this connection can be made without the
 use of coaxial cables. The RF power coupling element couples RF power to
 the load. A sensor measures the phase information for a characteristic of
 the RF power in the RF power coupling element. Examples of the
 characteristic of the RF power that can be used include RF current and RF
 voltage. A controller receives the measured phase information and
 determines the phase angle between the characteristic of the RF power in
 the RF power coupling element and a reference voltage used by the RF power
 amplifier. The controller controls the phase angle to a constant value by
 adjusting the frequency of the reference RF signal used by the RF power
 amplifier. The adjustments to the frequency of the RF power amplifier
 reference RF signal also appear as changes in the frequency of the RF
 power output from the amplifier. Maintaining the constant phase angle
 between the characteristic of the RF power in the RF power coupling
 element and the reference RF signal used by the RF power amplifier causes
 the efficiency of RF power delivery to the load to be substantially
 unchanged by changes in the load impedance. A power controller is used to
 control the magnitude of the RF power delivered to the load. The power
 meter provides to the power controller a signal of the measured power
 input to the RF amplifier. The power controller also receives set point
 information for the desired RF power. A control system in the power
 controller determines an output signal based on the set point information
 and the measured input power to the RF amplifier. The output signal from
 the power controlled is a gain signal for adjusting the gain of the RF
 amplifier. The control system in the power controller adjusts the gain of
 the RF amplifier until the power input to the RF amplifier substantially
 equals the set point power. Controlling the input power to the RF
 amplifier at the set point for the power controller provides control of
 the magnitude of the RF power delivered to the load. The magnitude of the
 RF power is determined from the measured input power to the RF amplifier
 and the known RF power conversion efficiency for the RF amplifier.
 Another aspect of the present invention is a method or RF power delivery in
 which the RF power delivery efficiency is directly maximized using a
 sensor and a controller to maintain a predetermined constant phase angle
 between a characteristic of the RF power in an RF power coupling element
 and a reference RF signal used by an RF power amplifier. In one
 embodiment, the controller receives an input signal from the sensor that
 provides information about the phase of the characteristic of the RF power
 in the RF power coupling element. The controller outputs a signal to the
 RF amplifier adjusting the frequency of the reference RF signal used by
 the RF amplifier so that the frequency of the output RF power varies. The
 adjustments to the reference RF signal used by the RF amplifier to
 maintain the constant phase angle between the characteristic of the RF
 power in the RF power coupling element and the reference RF signal used by
 the RF amplifier results in efficient RF power delivery to the load. The
 RF power delivery is controlled electronically without moving parts by
 varying the frequency of the reference RF signal supplied to the RF power
 amplifier.
 In accordance with a further aspect of the present invention, the RF power
 delivery is also controlled so that a set magnitude of power is delivered
 to the load. In one embodiment of the present invention, the magnitude of
 the RF power is determined from the measured input power to the RF
 amplifier and the known RF power conversion efficiency for the RF
 amplifier. A power controller is used to control the magnitude of the RF
 power delivered to the load. The power meter provides to the power
 controller a signal of the measured power input to the RF amplifier. The
 power controller also receives set point information for the desired RF
 power. A control system in the power controller determines an output
 signal based on the set point information and the measured input power to
 the RF amplifier. The output signal from the power controller is a gain
 signal for adjusting the gain of the RF amplifier. The control system in
 the power controller adjusts the gain of the RF amplifier until the power
 input to the RF amplifier substantially equals the set point power.
 Controlling the input power to the RF amplifier at the set point for the
 power controller provides control of the magnitude of the RF power
 delivered to the load to a predetermined magnitude. The RF power magnitude
 can be calculated using the measured input power to the RF power amplifier
 and the known power conversion efficiency for the RF power amplifier.
 In one embodiment of the present invention, control of the reference RF
 signal to achieve efficient RF power delivery is simultaneous with the
 control of the magnitude of the RF power output from the RF amplifier. In
 another embodiment, the RF power magnitude is controlled after efficient
 RF power delivery is achieved through control of the reference RF signal.
 In accordance with another aspect of the invention, a load is matched to an
 RF power source that supplies sufficient RF power to perform a process
 step. The method uses a variable frequency RF power amplifier that
 delivers RF power to the load via a fixed RF match and RF power coupling
 element. The method comprises adjusting the frequency of a reference RF
 signal used by the RF amplifier to maintain a predetermined constant phase
 angle between the characteristic of the RF power through the RF power
 coupling element and a reference RF signal used by the RF amplifier to
 achieve a resonant frequency for coupling RF power to the load. The method
 further comprises measuring power input to the RF amplifier and providing
 a gain signal to the RF amplifier as a function of the measured input
 power and a set point power. Using the gain signal, the output RF power
 for the amplifier is adjusted to deliver RF power to the load at a
 controlled magnitude. The magnitude of the RF power is derived from the
 measured power input to the RF amplifier and the known power conversion
 efficiency of the RF power amplifier.
 Another aspect of the invention relates to the combination including an RF
 power processor for receiving RF power and using the RF power to carry out
 a process; a power coupling element for coupling RF power to the RF power
 processor; a fixed RF match connected to the RF power coupling element for
 matching the RF power to the RF power processor; a variable frequency RF
 amplifier connected to the match for providing RF power for delivery to
 the RF power processor; a sensor for determining phase information for the
 characteristic of the RF power in the power coupling element, the sensor
 providing a first output signal related to the phase information for the
 characteristic of the RF power in the RF coupling element; and a first
 controller responsive to the first output signal from the sensor, the
 first controller controlling the frequency of a reference RF signal used
 by the RF power amplifier to maintain a predetermined constant phase angle
 between the characteristic of the RF power in the RF power coupling
 element and the reference RF signal used by the RF power amplifier. The
 combination further comprising a meter for measuring power input to the RF
 amplifier, the meter having a third output signal related to the measured
 power input to the RF amplifier; and a second controller responsive to the
 third output signal from the meter and a power set point for controlling
 the gain of the RF amplifier to allow a controlled magnitude of RF power
 delivery to the RF power processor.
 In one embodiment, the first controller includes a phase locked loop
 responsive to the output signal from the sensor. The first controller then
 generates the second output signal that adjusts the frequency of the
 reference RF signal used as an input to the RF power amplifier.
 Adjustments are made to the frequency of the reference RF signal used by
 the RF power amplifier so as to maintain a constant phase angle between
 the phase of the characteristic of the RF power in the RF power coupling
 element and the phase of the reference RF signal provided to the RF
 amplifier.
 In the aspects of the present invention, embodiments of the RF power
 coupling element can have any form suitable for RF power delivery.
 Exemplary forms of RF power coupling elements are antennas, coils,
 cylindrical coils, planar coils, electrodes, rings, parallel plates,
 screens, and waveguides. Various types of RF power coupling elements are
 well known in the art.
 In various separate embodiments of the present invention, the load that
 receives the RF power may use the RF power for different applications.
 Exemplary functions of the loads for various applications are as follows.
 The load may absorb the RF power to produce heat for a heating process as
 in RF induction heating. The load may absorb the RF power to produce a
 thermal plasma such as those used for chemical processing, materials
 processing, analytical chemistry, or driving optical devices. The load may
 absorb the RF power to produce a non-thermal plasma such as those used for
 chemical processing or materials processing. The load may absorb RF power
 to produce non-thermal plasmas such as plasmas used for semiconductor
 device fabrication processes like etching, deposition, cleaning, doping,
 oxidation, drying, photoresist stripping, parts cleaning, reaction chamber
 cleaning, and annealing. The load may absorb RF power to produce a plasma
 for stimulating chemical reactions that cannot proceed or proceed slowly
 under non-plasma conditions. The load may absorb RF power to produce a
 plasma for decomposing chemical compounds. The load may absorb RF power to
 produce a plasma for synthesizing chemical compounds.
 In another aspect of the present invention, the delivered RF power can be
 used for the abatement of gaseous halogenated organic compounds, other
 refractory organic compounds, perfluorocompounds, and refractory inorganic
 compounds. The apparatus includes a non-thermal plasma generated by RF
 power as the means for generating free radicals in a dielectric reaction
 vessel. In a further aspect of the present invention, the treatment of
 gases can be enhanced by the addition of suitable ancillary reaction gases
 including water, methane, hydrogen, ammonia, hydrogen peroxide, oxygen, or
 mixtures thereof.
 One object of embodiments of the present invention is to provide methods
 and apparatus for RF power delivery without the need for excess equipment
 and complexity. The object is to achieve effective RF power delivery
 without dependence on moving parts, with minimum redundancy of parts, and
 with minimum repetition of measurements.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for RF power delivery that allow the delivery of RF
 power using equipment that is small and lightweight with respect to
 traditional RF power delivery equipment. The object is to have equipment
 for some applications they can be handled by one-person and that takes up
 a minimum of space in a factory or semiconductor fab.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for RF power delivery for heating as in RF induction
 heating.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for RF power delivery for generation of plasmas.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for RF power delivery for generation of thermal
 plasmas.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for RF power delivery for generation of non-thermal
 plasmas.
 Another object of embodiments the present invention is to provide methods
 and apparatus for power delivery for promoting chemical reactions.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for RF power delivery for generation of plasmas for
 semiconductor device fabrication steps such as etching, deposition,
 cleaning, doping, oxidation, drying, photoresist stripping, parts
 cleaning, reaction chamber cleaning, and annealing.
 Another object of embodiments of the present invention is to provide
 methods and apparatus for removal of refractory compounds from waste
 streams. Refractory compounds include compounds that show a high degree of
 stability with respect to temperature and reactivity and are difficult to
 decompose.
 Another object of embodiments of the present invention is to provide new
 and useful methods and apparatus for the destruction of refractory
 compounds such as perfluorocompounds, such as carbon fluorides, carbon
 tetrafluoride, nitrogen triflouride, and sulfur hexafluoride by reactions
 facilitated by a plasma.
 Yet, another object of embodiments of the present invention is to provide
 methods and apparatus for gas waste treatment using a non-thermal plasma
 generated by RF power.
 A further object of embodiments of the present invention is to provide
 methods and apparatus that are suitable for processing waste streams
 emanating from an individual semiconductor process tool and that can
 become an integral part of the semiconductor device fabrication process.
 An advantage of embodiments of the present invention is the ability to
 provide an economical apparatus and method for the destruction of
 refractory compounds contained in gaseous waste streams.
 Another advantage of embodiments of the present invention is the ability to
 provide waste treatment of undiluted off gases from individual
 semiconductor device fabrication tools. Embodiments of the present
 invention can be made compact enough to be integrated into and attached
 directly to one or more than one wafer processing tools.
 The above and still further objects, features and advantages of the present
 invention will become apparent upon consideration of the following
 detailed descriptions of specific embodiments thereof, especially when
 taken in conjunction with the accompanying drawings.

DESCRIPTION
 Reference is now made to FIG. 1 wherein there is illustrated an RF power
 processor 20 such as a plasma chamber, a vacuum processing plasma chamber,
 a plasma reactor, or an RF induction heater. The RF power processor 20
 typically operates with a load capable of absorbing RF power and using the
 RF power for a process operation. Typical RF power operations and the
 characteristics of appropriate loads are well known in the art. An RF
 power coupling element 30 is disposed so as to inductively couple RF power
 to the RF power processor 20. An RF match 40 is connected with the RF
 power coupling element 30. A variable frequency RF power amplifier 50 is
 connected with the RF match 40 so that RF power output from the variable
 frequency RF power amplifier 50 can be applied to the RF power coupling
 element 30 via the RF match 40. The variable frequency RF power amplifier
 50 is a standard RF power amplifier that uses a reference RF signal and
 input electric power so that it is capable of providing a relatively high
 output RF power. Examples of two types of reference RF signals are RF
 current and RF voltage. A sensor 60 is located near the RF power coupling
 element 30. The sensor 60 is arranged so as to be capable of deriving
 information for one or more of the oscillating characteristics of RF power
 in the RF power coupling element 30. An example of the information derived
 by the sensor 60 includes phase information; another example is frequency
 information. The sensor 60 is also capable of providing a first output
 signal 70 related to the information representative of the characteristics
 of RF power in the RF power coupling element 30. A first controller 80
 receives the first output signal 70 from the sensor 60. The first
 controller 80 maintains a constant phase angle between the first output
 signal 70 and the reference RF signal used by the RF power amplifier 50.
 The first controller 80 maintains the constant phase angle by adjusting
 the frequency of the reference RF signal used by the RF power amplifier
 50. The first controller 80 provides the reference RF signal information
 as a second output signal 90 to the RF power amplifier 50. The RF power
 amplifier 50 uses the second output signal 90 and electric power input to
 produce the RF power applied to the RF match 40.
 Preferably, the constant phase angle used by the first controller 80 is a
 predetermined constant incorporate in the first controller 80 as a control
 parameter. Various commercially available controllers can be used for the
 first controller 80. Typical controllers that can be selected for the
 first controller 80 use a phase locked loop. A phase locked loop is a
 control system that uses feedback to maintain an output signal in a
 specific phase relationship with a signal supplied to the phase locked
 loop. When a phase locked loop is selected to provide the control for the
 first controller 80, the phase locked loop maintains a constant phase
 angle between the reference RF signal used by the RF power amplifier 50
 and the first output signal 70.
 Techniques for incorporating the constant phase angle into the first
 controller 80 are well known to those skilled in the art. It is typical
 for controllers suitable for use as the first controller 80 to allow
 setting the control parameters such as phase angles. The constant phase
 angle is derived incorporated as part of pre-tuning the controller. An
 example procedure for pre-tuning the controller includes: using the
 controller to control RF power delivery to a representative load such as a
 load similar to the load that the controller will be expected to handle;
 monitoring a parameter of the RF power delivery indicative of the RF power
 delivery efficiency; varying the control parameters of the controller,
 such as the phase angle, so as to tune the RF power delivery to achieve
 desired, preferably optimum, power delivery conditions; fixing the control
 parameter settings for the controller so that in subsequent use the
 controller is capable of controlling using the predetermined control
 parameters such as the predetermined phase angle. In other words, the
 controller is set with the predetermined control parameters including the
 predetermined constant phase angle.
 Consequently, the embodiment of the present invention shown in FIG. 1 is
 capable of maintaining the predetermined constant phase angle between the
 reference RF signal used by the RF power amplifier 50 and the
 characteristic of the RF power in the RF power coupling element 30 so as
 to effect desired, preferably efficient, RF power delivery to the load.
 Reference is now made to FIG. 2 of the drawings wherein there is
 illustrated the RF power processor 20. The RF power processor 20 typically
 operates with a load capable of absorbing RF power and using the RF power
 for a process operation. An RF power coupling element 30 is disposed so as
 to inductively couple RF power to the RF power processor 20. An RF match
 40 is connected with the RF power coupling element 30. A variable
 frequency RF power amplifier 50 is connected with the RF match 40 so that
 RF power output from the variable frequency RF power amplifier 50 can be
 applied to the RF power coupling element 30 via the RF match 40. The
 variable frequency RF power amplifier 50 is a standard RF power amplifier
 that uses a reference RF signal and input electric power so that it is
 capable of providing a relatively high output RF power. Examples of two
 types of reference RF signals are RF current and RF voltage. A sensor 60
 is located near the RF power coupling element 30. The sensor 60 is
 arranged so as to be capable of deriving phase information representative
 of one or more of the oscillating characteristics of RF power in the RF
 power coupling element 30. An example of the information derived by the
 sensor 60 includes phase information; another example is frequency
 information. The sensor 60 is also capable of providing a first output
 signal 70 related to the information representative of the characteristics
 of RF power in the RF power coupling element 30. A first controller 80
 receives the first output signal 70 from the sensor 60. The first
 controller 80 maintains a constant phase angle between the first output
 signal 70 and the reference RF signal used by the RF power amplifier 50.
 The first controller 80 maintains the constant phase angle by adjusting
 the frequency of the reference RF signal used by the RF power amplifier
 50.
 The first controller 80 provides the reference RF signal information as a
 second output signal 90 to the RF power amplifier 50. The RF power
 amplifier 50 uses the second output signal 90 and electric power input to
 produce the RF power applied to the RF match 40.
 FIG. 2 also illustrates a meter 100. The meter 100 is coupled with the RF
 power amplifier 50 to enable the meter 100 to make measurements on the
 input electric power provided to the RF power amplifier 50. The meter is
 capable of measuring one or more characteristics of the electric power
 provided to the RF power amplifier 50. Typically, the electric power
 provided to the RF amplifier is either DC or slow AC power such as AC
 frequencies less than about 600 Hz. Examples of the characteristics of the
 electric power that the meter 100 may be capable of measuring include
 input power magnitude, input power voltage, and input power current. The
 meter 100 produces a third control signal 120 containing information from
 the power measurements by the meter 100. A second controller 110 is used
 to control the magnitude of a characteristic of the RF power delivered to
 the load. The power meter 100 provides to the second controller 110 the
 third control signal 120 containing information from the power
 measurements by the meter 100. The second controller 110 also receives set
 point information 140 for the desired magnitude of the characteristics for
 the RF power. The second controller 110 derives an output signal as a gain
 signal 130 for controlling magnitude of the RF power delivered to the
 load. The gain signal 130 is based on the set point information 140 and
 the third control signal 120. The gain signal 130 from the second
 controller 110 is input to the RF power amplifier 50 for adjusting the
 gain of the RF power amplifier 50. The second controller 110 adjusts the
 gain of the RF power amplifier 50 to maintain the power input to the RF
 power amplifier 50 at a value that substantially equals the power
 specified by the set point information 140. Controlling the input power to
 the RF power amplifier 50 to the set point information 140 for the second
 controller 110 provides control of the magnitude of the RF power delivered
 to the RF power processor 20 to a predetermined magnitude.
 The magnitude of the RF power can be derived from measurements of the input
 power to the RF power amplifier 50 by the meter 100 and the known
 conversion efficiency of the RF power amplifier 50. The conversion
 efficiency for the RF amplifier can be measured experimentally as in
 performing a calibration. Alternatively, the conversion efficiency for the
 RF amplifier may be obtained from the manufacturer of the RF amplifier.
 Deriving the RF power magnitude from measurements of the input DC/slow AC
 power and the RF amplifier conversion efficiency eliminates the need for
 measuring the RF power output from the amplifier 50. In other words, there
 is no need to measure the relatively high RF power. Consequently, there is
 no need for RF power measuring components or circuits for the embodiments
 of the present invention.
 Preferably, the constant phase angle used by the first controller 80 is a
 predetermined constant incorporate in the first controller 80 as a control
 parameter. Various commercially available controllers can be used for the
 first controller 80. Typical controllers that can be selected for the
 first controller 80 use a phase locked loop. A phase locked loop is a
 control system that uses feedback to maintain an output signal in a
 specific phase relationship with a signal supplied to the phase locked
 loop. When a phase locked loop is selected to provide the control for the
 first controller 80, the phase locked loop maintains a constant phase
 angle between the reference RF signal used by the RF power amplifier 50
 and the first output signal 70. Techniques for incorporating the constant
 phase angle into the first controller 80 are well known to those skilled
 in the art. The example procedure for obtaining the preferred constant
 phase angle presented for the embodiment shown in FIG. 1 also applies to
 the embodiment shown in FIG. 2.
 Consequently, the embodiment of the present invention shown in FIG. 2 is
 capable of maintaining the predetermined constant phase angle between the
 reference RF signal used by the RF power amplifier 50 and the
 characteristic of the RF power in the RF power coupling element 30 so as
 to effect desired, preferably efficient, RF power delivery to the load at
 the controlled magnitude of the characteristic of the RF power determined
 by the set point. Examples of characteristics of the RF power for which
 the magnitude can be controlled in performing the RF power delivery
 include magnitude of the RF power, magnitude of the RF current, and
 magnitude of the RF voltage.
 Reference is now made to FIG. 3 of the drawings wherein there is
 illustrated the RF power processor 20. The RF power processor 20 typically
 operates with a load capable of absorbing RF power and using the RF power
 for a process operation. An RF power coupling element 30 is disposed so as
 to inductively couple RF power to the RF power processor 20. An RF match
 40 is connected with the RF power coupling element 30. A variable
 frequency RF power amplifier 50 is connected with the RF match 40 so that
 RF power output from the variable frequency RF power amplifier 50 can be
 applied to the RF power coupling element 30 via the RF match 40. The
 variable frequency RF power amplifier 50 is a standard RF power amplifier
 that uses a reference RF signal and input electric power so that it is
 capable of providing a relatively high output RF power. Examples of two
 types of reference RF signals are RF current and RF voltage. A sensor 60
 is located near the RF power coupling element 30. The sensor 60 is
 arranged so as to be capable of deriving phase information representative
 of one or more of the oscillating characteristics of RF power in the RF
 power coupling element 30. An example of the information derived by the
 sensor 60 includes phase information; another example is frequency
 information. The sensor 60 is also capable of providing a first output
 signal 70 related to the information representative of the characteristics
 of RF power in the RF power coupling element 30. An advanced controller
 150 receives the first output signal 70 from the sensor 60. The advanced
 controller 150 is capable of controlling at least two parameters
 simultaneously, based on multiple data inputs. In addition, the advanced
 controller 150 is capable of providing at least two control signals.
 Control systems are commercially available that can be used for the
 advanced controller 150. An example of a suitable control system would be
 microprocessor based control systems such as computers. The advanced
 controller 150 maintains a constant phase angle between the first output
 signal 70 and the reference RF signal used by the RF power amplifier 50.
 The advanced controller 150 maintains the constant phase angle by
 adjusting the frequency of the reference RF signal used by the RF power
 amplifier 50. The advanced controller 150 provides the reference RF signal
 information as a second output signal 90 to the RF power amplifier 50. The
 RF power amplifier 50 uses the second output signal 90 and electric power
 input to produce the RF power applied to the RF match 40.
 FIG. 3 also illustrates a meter 100. The meter 100 is coupled with the RF
 power amplifier 50 to enable the meter 100 to derive measurements of the
 input electric power provided to the RF power amplifier 50. The meter is
 capable of measuring one or more characteristics of the electric power
 provided to the RF power amplifier 50. Typically, the electric power
 provided to the RF amplifier is either DC or slow AC power such as AC
 frequencies less than about 600 Hz. Examples of the characteristics of the
 electric power that the meter 100 may be capable of measuring include
 input power magnitude, input power voltage, and input power current. The
 meter 100 produces a third control signal 120 containing information from
 the power measurements by the meter 100. The advanced controller 150
 controls the magnitude of a characteristic of the RF power delivered to
 the load. The power meter 100 provides to the advanced controller 150 the
 third control signal 120 containing information from the power
 measurements by the meter 100. The advanced controller 150 also receives
 set point information 140 for the desired magnitude of the characteristics
 for the RF power. The advanced controller 150 derives an output signal as
 a gain signal 130 for controlling magnitude of the RF power delivered to
 the load. The gain signal 130 is based on the set point information 140
 and the third control signal 120. The gain signal 130 from the advanced
 controller 150 is input to the RF power amplifier 50 for adjusting the
 gain of the RF power amplifier 50. The advanced controller 150 adjusts the
 gain of the RF power amplifier 50 to maintain the power input to the RF
 power amplifier 50 at a value that substantially equals the power
 specified by the set point information 140. Controlling the input power to
 the RF power amplifier 50 to the set point information 140 for the
 advanced controller 150 provides control of the magnitude of the RF power
 delivered to the RF power processor 20 to a predetermined magnitude.
 The embodiment shown in FIG. 3 accomplishes the same results as the
 embodiment shown in FIG. 2. However, the embodiment shown in FIG. 3 uses
 only one controller for controlling the RF reference signal and the gain
 of the RF power.
 A preferred characteristic of the RF power in the RF power coupling element
 30 for maintaining the predetermined constant phase angle is the phase of
 the RF current. Specifically, it is preferred to maintain the
 predetermined constant phase angle between the phase of the reference RF
 signal used by the RF power amplifier 50 and the phase of the RF current
 in the RF power coupling element 30. Furthermore, the preferred reference
 RF signal is RF voltage.
 It is preferable in the embodiments of the present invention shown in FIG.
 1, FIG. 2, and FIG. 3 to minimize the use of RF cable such as coaxial
 cable for the connections between the RF power amplifier 50, the RF match
 40, and the RF power coupling element 30. It is still more preferable to
 have no RF cable connections between the RF power amplifier 50, the RF
 match 40, and the RF power coupling element 30 so that RF power from the
 RF power amplifier 50 can be applied directly to the RF match 40 and RF
 power from the RF match 40 can be directly applied to the RF power
 coupling element 30. Minimizing the use of RF cable also makes it possible
 for the apparatus to be smaller in size, lighter in weight, and lower in
 cost.
 It is possible for the embodiments of the present invention shown in FIG.
 1, FIG. 2, and FIG. 3 to have RF match 40 include variable components for
 capacitance and inductance. The preferred embodiment for RF match 40 has
 only fixed components for capacitance and inductance; in other words, it
 is preferred to use a fixed match for RF match 40. The capacitance and
 inductance components used in RF match 40, preferably, are selected to
 produce approximately tuned conditions for RF power delivery to an
 expected load. Tuning requirements beyond the capability of the fixed
 match are handled by other parts and systems of the present invention such
 as the first controller 80 and the sensor 60. The ability to use the fixed
 match offers the advantages of having no moving parts in the match, simple
 operation, and immediate response. Overall, the fixed match offers greater
 reliability.
 The embodiments of the present invention shown in FIG. 1, FIG. 2, and FIG.
 3 achieve efficient RF power delivery using measurements and control
 systems that have not been used before. In fact, the measurements and
 control systems used by the embodiments shown in FIG. 1 and FIG. 2
 eliminate the need for some of the components and measurements typically
 used in prior art RF power delivery systems. As can be seen in FIG. 1,
 FIG. 2, and FIG. 3, the embodiments of the present invention have neither
 components nor other circuitry for measuring the magnitude of the RF power
 delivered to the load. Forward and reflected RF power cannot be measured
 using the embodiments shown in FIG. 1, FIG. 2, and FIG. 3. Nor is there a
 need for measuring the RF power magnitude for efficient RF power delivery
 using the embodiments of the present invention. This is in contrast to the
 typical prior art wherein the RF power measurements are used as control
 parameters to achieve efficient RF power delivery by either maximizing the
 forward RF power, minimizing the reflected RF power, or other behavior of
 the measured RF power. The prior art typically uses components such as
 dual directional couplers for measuring forward and reflected RF powers,
 whereas the present invention has no need for RF power measurement
 components.
 Another alternative used by prior art RF power delivery systems to achieve
 efficient RF power delivery includes measuring the phase angle between RF
 current and RF voltage on a line carrying the relatively high power output
 from an RF power amplifier. The prior art either adjusts the impedance of
 a variable RF match network or adjust the frequency of the RF power output
 so that the phase angle between the RF current and the RF voltage is zero
 on the line connected to the RF power coupling element. The prior art
 method requires collecting information for two separate phase measurements
 on the high power RF output (in other words, making two measurements on
 the large RF signal). As is known to those skilled in the art, making
 measurements on high power RF signals is difficult and undesirable.
 In contrast to the prior art RF power delivery systems, the embodiments of
 the present invention shown in FIG. 1, FIG. 2, and FIG. 3 are capable of
 accomplishing efficient RF power delivery by measuring phase information
 for only one characteristic of the RF power in the RF power coupling
 element 30. In other words, phase information is measured for only one
 characteristic of the relatively high RF power (specifically, making one
 measurement on the large RF signal). The phase information for the RF
 power in the RF power coupling element is been compared to the reference
 RF signal (i.e. a small RF signal) used by the RF power amplifier to
 derive the signal for controlling the RF power delivery. Since the
 reference RF signal used by the RF amplifier is generated by the
 controller, there is no need for special equipment to measure the phase
 information for the reference RF signal.
 The sensor 60, which provides the phase information, can be of any type
 that is capable of deriving phase information representative of one or
 more of the oscillating characteristics of RF power in the RF power
 coupling element 30. Examples of the characteristics of RF power for which
 the sensor 60 can be used are RF current and RF voltage. For deriving the
 phase of the RF current, the sensor 60 can be a current pickup such as a
 loop with multiple turns for inductively detecting phase information. In a
 preferred embodiment, the sensor 60 is a current pickup with a 100 turn
 loop. Alternatively, the sensor 60 can be a voltage pickup for deriving
 phase information for the RF voltage.
 The RF power coupling element 30 can be of any type suitable for RF power
 delivery systems. Many types of RF power coupling elements are known in
 the art. Exemplary types of RF power coupling elements are antennas,
 coils, cylindrical coils, planar coils, electrodes, rings, parallel
 plates, screens, and waveguides.
 Practicing the present invention does not require the use of the RF power
 amplifier 50 and the first controller 80 as they are presented in the
 embodiments shown in FIG. 1, FIG. 2, and FIG. 3. Other types of equipment
 and configurations can be used for practicing the present invention. For
 example, the embodiments shown in FIG. 1, FIG. 2, and FIG. 3 indicate that
 the reference RF signal is provided to the RF power amplifier 50. In an
 alternative embodiment, the RF amplifier may include a frequency generator
 such as a voltage controlled oscillator wherein the voltage controlled
 oscillator generates the reference RF signal based on an input control
 signal from a controller. For this alternative embodiment, the output for
 the controller could be a control signal like a DC voltage that determines
 the frequency for the reference RF signal used by the RF amplifier.
 Controllers that are capable of providing control signals for voltage
 controlled oscillators are well known in the art. Persons skilled in the
 art can provide substitute equipment configurations for additional
 embodiments of the present invention.
 Embodiments of the present invention can be used in a wide variety of RF
 power delivery applications. The type of application for the embodiments
 of the present invention determines the required apparatus for the RF
 power processor 20 (FIG. 1 and FIG. 2). For example, using the present
 invention embodiments for low-pressure plasma processing such as
 non-thermal plasma processing requires delivery of RF power to the RF
 power processor 20 (FIG. 1 and FIG. 2) wherein the RF power processor 20
 includes a plasma chamber. The plasma chamber must be capable of
 containing a gas at suitable pressure for generating the plasma. Plasma
 chambers of this type are well known in the art. Low-pressure plasma
 processing chambers, such as vacuum plasma processing chambers, are
 extensively used in applications such as plasma processes for electronic
 device fabrication.
 Embodiments of the present invention are particularly suited for plasma
 processing wherein the RF power is coupled to an ionizable gas to produce
 a plasma for stimulating chemical reactions. Exemplary reactions include
 reactions for synthesizing chemical products, reactions for decomposing
 chemical compounds, and reactions for surface treatment. For this type of
 application, the RF power processor 20 (FIG. 1, FIG. 2, and FIG. 3)
 includes a plasma chamber with means for receiving and removing the gas,
 such as gas inlets and gas exits.
 Embodiments of the present invention can be used to carry out plasma
 processing for which the plasma processing includes plasma treatment of a
 workpiece. Example workpieces include substrates such as semiconductor
 wafers that are subjected to plasma processes used in the fabrication of
 electronic devices and substrates subjected to plasma processes for
 fabrication of optical elements and devices. For applications of this
 type, the RF power processor 20 (FIG. 1, FIG. 2, and FIG. 3) also includes
 methods and apparatus for positioning the workpieces during plasma
 processing. The RF power processor 20 further includes means for receiving
 and removing ionizable gas suitable for plasma processing.
 Alternatively, embodiments of the present invention can be applied to
 plasmas operating at pressures other than low pressure, like at
 atmospheric pressure. Plasmas operating at atmospheric pressure,
 optionally, may be open to the atmosphere. An embodiment of the present
 invention is to generate a plasma open to the atmosphere without
 significant confinement of the plasma except the confinement determined by
 the RF power coupling element. This type of plasma may also be referred to
 as a plasma torch.
 While there have been described and illustrated specific embodiments of the
 invention, it will be clear that variations in the details of the
 embodiments specifically illustrated and described may be made without
 departing from the true spirit and scope of the invention as defined in
 the appended claims and their legal equivalents.