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Claims  |
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What is claimed is:
1. 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 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, said RF power generator further comprises a control circuit coupled to said vacuum tube
and constructed and arranged to vary the RF power that is coupled into the plasma, and said control circuit comprises a control signal source constructed and arranged to apply a control signal to said vacuum tube grid.
2. The system according to claim 1, wherein said coil comprises a helical coil.
3. The system according to claim 1, wherein one end of said coil is grounded and the other end of said coil is open-circuited.
4. The system according to claim 1, wherein said vacuum tube is a triode.
5. The system according to claim 1, wherein said DC power supply produces a time varying DC voltage that varies the RF power that is coupled into the plasma.
6. The system according to claim 5, wherein the DC voltage is a time varying voltage having a repetition rate lower than the frequency of the RF power.
7. The system according to claim 5, wherein the DC voltage is a time varying voltage having a repetition rate lower than the frequency of the RF power.
8. The system according to claim 5, wherein the DC voltage has the form of a train of pulses or a sinusoid.
9. The system according to claim 5, wherein the DC voltage is in the form of a train of pulses, each pulse having a multi-level waveform.
10. The system according to claim 1, further comprising a temperature control assembly operatively associated with said coil and said vacuum tube and constructed and arranged to maintain said coil and said vacuum tube at selected temperatures.
11. 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.
12. The system according to claim 11 wherein said RF power generator further comprises a control circuit coupled to said vacuum tube and constructed and arranged to vary the RF power that is coupled into the plasma, and wherein said control
circuit comprises a control signal source constructed and arranged to apply a control signal to said vacuum tube grid.
13. The system according to claim 12 wherein the control signal has the form of a train of pulses or a sinusoid.
14. The system according to claim 11 wherein the DC voltage is in the form of a train of pulses, each pulse having a multi-level waveform.
15. 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.
16. The method according to claim 15, further comprising varying the density of the plasma in the chamber.
17. The method according to claim 16, wherein varying the density of the plasma in the chamber comprises varying the magnitude of the RF power coupled into the plasma.
18. The method according to claim 17, wherein varying the magnitude of the RF power coupled into the plasma comprises varying the magnitude of the DC power to the DC power input.
19. The method according to claim 16, wherein varying the density of the plasma in the chamber comprises varying the pressure of gas in the chamber.
20. The method according to claim 16 wherein the vacuum tube has a control grid, wherein varying the density of the plasma in the chamber comprises applying a variable amplitude control signal to the grid, and wherein providing a RF power
generator comprises providing a RF power generator having a control circuit coupled to said vacuum tube and constructed and arranged to vary the RF power that is coupled into the plasma.
21. The method according to claim 20 wherein applying a variable amplitude control signal to the grid comprises applying a time varying signal having a repetition rate lower than the frequency of the RF power.
22. The method according to claim 21 wherein applying a time varying signal comprises applying a control signal that has the form of a train of pulses or a sinusoid.
23. The method according to claim 21 wherein applying a time signal comprises applying a control signal that has the form of a train of pulses, each pulse having a multi-level waveform. |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to the generation of inductively coupled plasmas in apparatus for performing etching and deposition processes.
A variety of semiconductor fabrication operations involve deposition and etching processes performed on a semiconductor substrate mounted within a process chamber. Such processes typically involve the use of a low pressure, high density
discharge wherein a plasma is generated by the interaction of an ionizable gas with a radio frequency (RF) electromagnetic field. The coupling of RF power to a plasma in semiconductor process chambers can be categorized as either predominantly
capacitive or predominantly inductive. Many examples of each can be found in the prior art.
In the case of capacitive coupling, RF power is coupled to the bottom plate and/or the top plate of a parallel plate process chamber. In general, the top plate also serves as the ionizable gas feed, the bottom plate serves as the wafer holding
chuck and the remainder of the chamber is grounded.
Inductive coupling generally employs a planar geometry, or a cylindrical geometry, or a combination of the two geometries. Furthermore, low RF power is usually applied to a bottom electrode, or chuck, to provide a RF bias. FIGS. 1A, 1B and 1C
present some examples of the inductive discharge geometries.
FIG. 1A illustrates an example of planar geometry in which a planar multi-turn coil is located at the top of a process tube, or process chamber. FIG. 1B shows an example of cylindrical geometry in which a multi-turn cylindrical coil is wound
around a process tube, while FIG. 1C shows a modified version of cylindrical geometry in which the cylindrical coil is surrounded by a conductive shield. The structure shown in FIG. 1C is an example of a helical resonator. In each of the illustrated
arrangements, the coil is connected to receive a RF current and, thence, to induce an electromagnetic (EM) field parallel to the longitudinal axis of the cylindrical geometry. This resultant RF EM field, that is a manifestation of the RF current in the
coil, consists primarily of radially propagating EM waves proximate to the plasma volume when polarized by an electrostatic shield (to remove the azimuthally propagating field). The radially propagating waves interact with a small thin surface layer of
the bulk plasma. The thickness of this thin layer is often referred to as the skin depth. This interaction ultimately leads to energized electrons and subsequent gas ionization, and the formation of a plasma. In general, a process tube acts as a
protective barrier and delineates the inner plasma volume from the external structure. At least in the structures of FIGS. 1B and 1C, the process tube is made of a dielectric material that is transparent to the electromagnetic energy emanating from the
coil. It will be understood that these figures are schematic. Actual equipment can take a variety of forms in practice.
The coupling of RF power to a plasma in semiconductor processing is conventionally at a drive frequency of 13.56 MHz, using a 50 .OMEGA. RF power generator. This frequency is conveniently located within a RF band designated for industrial use.
However, the frequency of operation is not limited to this value in the prior art and, in fact, multiple frequencies are employed typically when using multiple coupling electrodes.
RF power is typically supplied to the coil by an oscillator having at least one active component that may be a solid state, or semiconductor, component, or a vacuum tube.
As is known in the art, energy can be inductively coupled into a process chamber through a helical resonator as described in Lieberman & Lichtenberg, Chapter 12 (Principles of plasma discharges and materials processing, John Wiley & Sons, Inc.,
1994). With a helical resonator, the coil (or helix) has a length equal to an integral number of quarter waves of the RF input. The coil surrounds the plasma chamber and is encased within a cylindrical container that is grounded. FIG. 1C shows the
basic structure of such a helical resonator including the coil, an electrostatic shield enclosed by the coil to minimize capacitive coupling of the RF field with the plasma, a dielectric process tube that is enclosed by the electrostatic shield and
separates the helical coil from the plasma, an outer conductor, or shield, surrounding the coil and an RF input line connected to a tap of the coil. As shown in FIG. 1C, the coil tap to which the RF input is applied is spaced from one end of the coil
which is grounded. The portion of the coil between the coil tap and ground effectively serves as part of the matching circuit, thus the tap position can be selected to achieve a match condition. Under a given set of conditions, proper definition of the
tap point location can provide impedance matching for the circuit.
However, the load impedance on a RF power generator is a function of the intrinsic impedance of the coil and the impedance presented by the plasma, the latter impedance being a function of the properties of the plasma. Therefore, fluctuations in
the process conditions can lead to fluctuations in the impedance as seen by the RF power generator. Furthermore, the impedance of the process chamber, in which the plasma is established, varies significantly between the condition prior to plasma
ignition and the run condition. In order to maintain efficient energy transfer from the RF power generator to the plasma, proper matching of the power supply output impedance to the load impedance is required.
One technique used in the prior art is a variable frequency power supply. The frequency is determined by a phase mag detector that determines the match conditions at the input of a fixed match network coupling to the tap of the coil. However,
systems of this type can be very expensive, and hence a fixed frequency power supply is generally employed in conjunction with a match network.
An example of a fixed frequency RF oscillator coupled to the coil of a helical resonator via an impedance matching network is shown in FIG. 2. The matching network is a .pi.-filter composed of a series connected inductor, L, and two shunt
connected variable capacitors C.sub.1 and C.sub.2. The matching network compensates for differences between the variable load impedance represented by the coil and the plasma, and the output impedance of the RF power generator. For example, as shown in
FIG. 2, when the source impedance Z.sub.s is equal to the load impedance Z.sub.MNi, this impedance including the impedances of the match network, the helical coil and the plasma load, then the power transfer can be maximized. In this particular case,
the input impedance to the match network-load circuit Z.sub.MNi is the complex conjugate of the source impedance Z.sub.s, and the output impedance of the match network Z.sub.MNo, as seen by the load, is the complex conjugate of the load impedance
Z.sub.L. Under this special condition, the coupling between the RF source and the combination of the match network and the plasma loaded coil can be represented as equivalent to a purely resistive circuit. Hence, the matching network is designed to
maximize power transmission from the RF power generator to its load.
Given feedback of the power transfer state (reflected/transmitted power levels using special detector circuits whose outputs approximate the difference in phase between the forward and reflected signals and the magnitude of the reflected signal),
matching networks have been developed to respond to changes in the load impedance. In particular, during plasma ignition and run conditions, the variable capacitors are adjusted to tune the load circuit, which includes the impedance match network, the
coil and the plasma load, to a resonant condition for the fixed frequency power supply. When the circuit impedances are matched, power reflected to the source at the match network juncture is minimized, or even zero, depending upon the accuracy of the
match, thus reducing damage to the power supply, which must ultimately absorb this reflected power. It is known, however, that the use of a matching network with a fixed frequency power supply presents a number of problems for the manufacturers of
semiconductor equipment.
Specifically, existing impedance matching networks are inherently unreliable, due in part to the fact that the maintenance required to assure operating reliability is relatively complex and often beyond the capabilities of maintenance personnel.
Furthermore, know n matching networks have an inadequate response time, at least in certain operating situations. In particular, if the power supplied to the plasma source is to be varied according to a pulse pattern, then the fastest matching
networks cannot adjust to maintain an optimum match between the power supply and the plasma source. This is true because the time scale for the fastest match networks is several hundreds of milliseconds, i.e., the rise or fall time for a response is
approximately several hundred milliseconds. However, to achieve a RF square wave pulse to within one percent accuracy, the minimum pulse time scale for these match networks might be several tens of seconds or 25 to 50 seconds. Therefore, in order to
accurately achieve millisecond pulsing, one requires a match network with rise and fall times, or a time scale, of the order of microseconds. Therefore, it has been necessary to accept power coupling conditions that are inefficient and that are even
variable from pulse to pulse or from run to run.
If an impedance mismatch should occur during substrate processing, substrate damage will be the likely result.
The use of a variable frequency RF power generator alleviates many of the problems encountered when employing a fixed frequency RF power generator and a match network. U.S. Pat. No. 5,688,357 (Hanawa) discloses a method of using a variable
frequency RF power generator composed of a solid state oscillator in conjunction with a control system that includes a method of sensing the reflected and/or transmitted power. The control system adjusts the frequency of the RF power source until the
reflected power is minimized and/or the transmitted power is maximized. A disadvantage of present solid state technology is the fact that RF power supplies having a solid state component are suitable for handling relatively low power levels, of the
order of 5 kW. However, power supplies capable of generating higher power levels, for example up to 15 kW, are necessary to process wafers having diameters of 300 mm. An alternative to the use of a solid state oscillator is the use of a vacuum tube as
the active component within an oscillator circuit that includes the load coil and plasma load.
Vacuum tube oscillators have been employed for more than 50 years to convert direct-current (DC) power to alternating-current (AC) power. A complete discussion of the design of vacuum tube oscillators may be found in "Vacuum-Tube Oscillators"
(Chapter XI of Principles of Electrical Engineering Series Applied Electronics, A First Course in Electronics, Electron Tubes and Associated Circuits by Members of the Staff of the Department of Electrical Engineering, MIT, John Wiley & Sons, Inc., New
York, 1943). According to the literature, vacuum tube oscillators have been categorized in two classes, namely negative-resistance oscillators and feedback oscillators. For particular use in the processing of semiconductors using low pressure plasma
discharges, feedback oscillators can comprise a vacuum tube as an amplifier and a coupling circuit wherein the coupling circuit includes the load coil, which may be a helical coil or electrical components that couple the RF power with a plasma. Examples
of typical feedback oscillators are the: Hartley oscillator, Colpitts oscillator, tuned-grid oscillator, and tuned-grid tuned-plate oscillator. Basic circuits for known Hartley and Colpitts oscillators are shown in FIGS. 3A and 3B, respectively, which
are found in Vacuum-Tube Oscillators, supra.
The basic premise behind the operation of a feedback-oscillator is that the device acts as an amplifier wherein a portion of the output power is fed back as input to the amplifier such that oscillations may be maintained. Hence, any device
capable of a periodic output with an output power greater than the input power required to drive the oscillations may be referred to as self-excited. More precisely, if a component of the output power is fed back to, for example, the cathode of the
vacuum tube with the proper magnitude and phase, then oscillations can be sustained. Sometimes, it is useful to view the feedback connected vacuum tube oscillator as a negative-resistance element.
FIGS. 4A and 4B present a simplified schematic diagram and an equivalent circuit diagram, respectively, of a feedback oscillator corresponding to the Hartley oscillator shown in FIG. 3A. FIGS. 4A and 4B are also found in Vacuum-Tube Oscillators,
supra. In FIG. 4A, the circuit is composed of a vacuum tube amplifier and a coupling network. As shown, the vacuum tube amplifier has an output voltage E.sub.p (plate to cathode), an input voltage E.sub.g (grid to cathode), and a voltage gain K=E.sub.p
/E.sub.g. The coupling network sees an input voltage E.sub.p and has an output voltage of E.sub.fb where .beta.=E.sub.fb /E.sub.p is the voltage ratio of the coupling network. In order to generate self-excited oscillations, the voltage gain of the
amplifier K must be at least equal to the inverse of the feedback voltage ratio .beta., or K.gtoreq.1/.beta..
FIG. 4B presents an equivalent circuit diagram of the same circuit as FIG. 4A. However, it assumes the circuit to be a linear Class A circuit. Substituting for the value of K in the circuit of FIG. 4B, it is possible to show the following
condition for sustained oscillations, commonly referred to as the Barkhausen criterion, ##EQU1##
where Z is the impedance of the load circuit, and .mu. and g.sub.m the gain and the mutual conductance, respectively, of the vacuum tube. Clearly, .beta. is a complex voltage ratio, since the impedance more than likely includes reactive
components, wherein the real and imaginary parts must be independently equal to satisfy equation (1). These two criteria place constraints on the magnitude and phase, and hence define a necessary condition for operation. In fact, sometimes the real
part of equation (1) sets the condition for the mutual conductance of the tube g.sub.m, and the complex part of equation (1) generally sets the frequency of operation.
As shown in FIG. 3A, the load circuit of the Hartley oscillator whose impedance is Z comprises two inductors L.sub.1 and L.sub.2 in parallel with a capacitor C, wherein the common node between the two inductors is directly connected to the vacuum
tube cathode.
In connection with plasma generation in spectrometers, European patent EP 568920A1 (Gagne) discloses the use of a triode vacuum tube within a Colpitts oscillator circuit for coupling RF power to an atmospheric plasma. However, the oscillation
circuit is disclosed as having a poor efficiency, approximately 40 to 60%, for coupling power to the plasma. Additionally, when designed as a feedback oscillator for coupling RF power to a low pressure, high density plasma, the Colpitts oscillator was
unable to transition from a plasma ignition condition to a run condition without manual circuit tuning. In order to overcome these problems and improve the robustness of the oscillator circuit, a Hartley oscillator has been employed to enable automatic
transition between start and run conditions. Furthermore, the Hartley oscillator circuit was found to be more efficient; approximately 78%.
BRIEF SUMMARY OF THE INVENTION
It is an object of the present invention to provide a plasma generating system with a RF power generator which alleviates the drawbacks and shortcomings noted above.
Another object of the invention is to provide such a system with a RF power generator that can generate high power levels, in the range of 15 kW and higher, but is less costly than existing power supplies capable of operating at such power levels
A further object of the invention is to provide a RF high power generator that is capable of power transfer to the plasma source, while adjusting rapidly to changes in the RF power level, e.g. with match network time scales of the order of one or
several microseconds, and continuously maintaining a matched impedance coupling circuit during variations in the plasma source impedance.
It is a further object of the invention to provide a RF generator that operates stably during start and run conditions and is capable of automatic transition between start a | | |
