Waller, William (6/43 College Street, Gladesville, NSW 2111, AU)
Sainty, Wayne (6/43 College Street, Gladesville, NSW 2111, AU)
Waller, William (6/43 College Street, Gladesville, NSW 2111, AU)
| 1. | A cathode system for an ion source, the cathode system comprising at least two filaments and a filament control system, wherein the filament control system provides a heating current to a first of the filaments to cause electron emission therefrom, and wherein the control system monitors the state of the first filament and switches the heating current from the first filament to a second of the filaments in response to a failure condition in the first filament. |
| 2. | A cathode system according to claim 1 wherein the filament control system monitors at least one of the filament voltage, the filament current and the filament resistance. |
| 3. | A cathode system according to claim 2 wherein the filament control system monitors both of the filament voltage and the filament current. |
| 4. | A cathode system according to claim 1 further comprising an audible signal generator that is activated upon failure of the first filament. |
| 5. | A cathode system according to claim 4 wherein the audible signal generator is activated upon failure of the second filament. |
| 6. | A cathode system according to claim 1 further comprising a visual indicator that is activated upon failure of the first filament. |
| 7. | A cathode system according to claim 1 wherein the filament control system monitors the temperature of the filament. |
| 8. | A cathode system according to claim 1 wherein the filament control system monitors an output parameter of the ion source. |
| 9. | A cathode system according to claim 8 wherein the output parameter of the ion source is the anode current. |
| 10. | A cathode system according to claim 1 wherein the first and second filaments share a common electrical connection. |
| 11. | A cathode system according to claim 10 wherein the common electrical connection is an earth connection. |
| 12. | An ion source comprising an anode, an electron emitting cathode, an ionization region disposed between the anode and the cathode, an electric potential generator, a gas supply, and a cathode control system, wherein the cathode is disposed at one end of the ionisation region; wherein the anode is disposed at an opposite longitudinal end of the ionisation region; wherein the gas supply supplies an ionisable gas to the ionisation region; wherein the cathode control system causes the cathode to emit electrons; wherein the electric potential generator generates an electric potential between the cathode and the anode such that electrons emitted by the cathode flow toward the anode through the ionisation region and cause ionisation of the gas; wherein the cathode comprises at least two filaments; wherein the cathode control system provides a heating current to a first of the filaments to cause thermal electron emission therefrom; and wherein the control system monitors the state of the first filament and switches the heating current from the first filament to a second of the filaments in response to a failure condition in the first filament. |
| 13. | An ion source according to claim 12 wherein the filament control system monitors at least one of the filament voltage, the filament current and the filament resistance. |
| 14. | An ion source according to claim 13 wherein the filament control system monitors both of the filament voltage and the filament current. |
| 15. | An ion source according to claim 12 further comprising an audible signal generator that is activated upon failure of the first filament. |
| 16. | An ion source according to claim 15 wherein the audible signal generator is activated upon failure of the second filament. |
| 17. | An ion source according to claim 12 further comprising a visual indicator that is activated upon failure of the first filament. |
| 18. | An ion source according to claim 12 further comprising a plurality of filament supports for providing an electrical connection to the filaments, wherein the first and second filaments share one of the plurality of filament supports. |
| 19. | An ion source according to claim 18 wherein the shared filament support provides an earth connection. |
| 20. | An ion source according to claim 18 wherein the first and second filaments are comprised of a single length of filament and wherein the approximate midpoint of the length is mounted at the shared filament support. |
| 21. | An ion source according to claim 12 wherein the cathode is automatically reset to operate the first filament upon replacement of the first filament. |
| 22. | A method of operating a cathode of an ion source comprising at least two filaments, the method comprising providing a heating current to a first filament of the cathode, monitoring the state of the first filament, detecting a failure condition of the first filament, and switching the heating current to a second filament in response to the detected failure condition. |
Background of the invention In the field of thin film fabrication, gridless ion sources have been known, for example from US 4862032, as providing a broad area ion beam capable of providing ion assistance for the thin film deposition process.
In a gridless ion source, a gas is ionized through collisions with energetic electrons. The source of electrons is often a heated filament disposed in the ion expulsion path. The filament is degraded by temperature stresses and by impinging ions until ultimately the filament breaks, and the heating current circuit is broken. The lifetime of the filament therefore places a limitation on the maximum length of the thin film deposition processes that utilize the ion source. To replace the filament, the vacuum chamber in which the ion source is disposed needs to be brought up to atmosphere, the filament replaced, and the chamber re-pumped to attain its operating pressure.
Modern thin film processes can now take of the order of hours, even days, for a single run and if the filament breaks during the process the entire run may be lost. With much automation of the process, it is often not practical to have a technician permanently monitor the state of the filament.
Summary of the invention In a first aspect, the invention resides in a cathode system for an ion source, the cathode system comprising at least two filaments and a filament control system, wherein the filament control system provides a heating current to a first of the filaments to cause electron emission therefrom, and wherein the control system monitors the state of the first filament and switches the heating current from the first filament to a second of the filaments in response to a failure condition in the first filament.
In a further aspect, the invention resides in an ion source comprising an anode, an electron emitting cathode, an ionization region disposed between the anode and the cathode, an electric potential generator, a gas supply, and a cathode control system, wherein the cathode is disposed at one end of the ionisation region; wherein the anode is disposed at an opposite longitudinal end of the ionisation region; wherein the gas supply supplies an ionisable gas to the ionisation region; wherein the cathode control system causes the cathode to emit electrons; wherein the electric potential generator generates an electric potential between the cathode and the anode such that electrons emitted by the cathode flow toward the anode through the ionisation region and cause ionisation of the gas; wherein the cathode comprises at least two filaments; wherein the cathode control system provides a heating current to a first of the filaments to cause thermal electron emission therefrom; and wherein the control system monitors the state of the first filament and switches the heating current from the first filament to a second of the filaments in response to a failure condition in the first filament.
In one embodiment, the control system monitors the current of the first filament.
In an alternative embodiment the control system monitors the voltage and/or the resistance of the first filament.
In a further embodiment the control system monitors the temperature of the first filament.
In a further embodiment the control system monitors the state of the first filament by monitoring another component of the ion source, for example the anode current.
Preferably the cathode control system monitors the state of the second filament current at least after the heating current has been switched to the second filament.
Preferably the ion source control system further comprises an audible signal generator that is activated in response to a detection of a failure condition in the first
filament. Preferably the audible signal generator is also activated upon detection of a failure condition in the second filament.
The filament is supported by a plurality of filament supports through which electrical connection to the filament is provided. In one embodiment, each filament is supported by an individual set of filament supports such that each of the filaments is electrically isolated from the other. In an alternative embodiment, two filaments share a common filament support, and thus a common electrical connection. Preferably, the common electrical connection is an earth connection.
In a further aspect, the invention resides in a method of operating a cathode of an ion source comprising at least two filaments, the method comprising providing a heating current to a first filament of the cathode, monitoring the state of the first filament, detecting a failure condition of the first filament, and switching the heating current to a second filament in response to the detected failure condition.
Brief description of the drawings The invention will now be described by way of example only with reference to preferred embodiments and to the accompanying drawings in which :- Figure 1 shows a schematic cross section of an ion source; Figure 2 shows a plan view of the ion source of Figure 1; Figure 3 shows a schematic circuit for a cathode control system; Figure 4 shows a plan view of an alternative embodiment of the invention; and Figure 5 shows an alternative circuit for a cathode control system.
Detailed description of preferred embodiments With reference to Figures 1 and 2, there is shown an ion source 100 in accordance with a preferred embodiment of the invention. The ion source 100 includes a base plate 101 that screws or otherwise engages with a cylindrical shroud 102. The shroud has an inner sloping surface 103 that defines an open end 116 of an ionisation region 113 to be described below. The base plate 101 has a collar 105, extending upward from which is a threaded section 106 for engagement with the shroud 102. The base 101 has an upper annular face 107. An inner circumferential flange 108 extends from the face 107 to
locate a ring magnet 114 thereon. The magnet is preferably a high flux rare earth magnet such as an NdFeB magnet.
Disposed on the magnet 114 is a spacer 117, for example of aluminium, that provides a radiation shield to prevent the magnet 114 from overheating due to radiation from the anode 112.
The anode 112 has an end wall 120 and an outwardly sloping side wall 121. The side wall and end wall together define the ionisation region 113. Figure 1 shows one filament 111 supported at the open end 116 of the ionisation region 113 by filament support legs 130. The filament legs 130 are connected to the shroud 102 through insulating mountings 131 to electrically isolate the filament legs 130 from the shroud 102. The filament legs 130 are each electrically conducting and have an electrical connection point 132 for connecting into a filament supply circuit (not shown). One filament is shown in Figure 1 for clarity, however as observed in Figure 2, the ion source is provided with two filaments, individually mounted on their own set of filament legs and electrically isolated from each other.
A projection 123 extends from the anode end wall 120 into the ionisation region 113. The projection 123 shown in Figure 1 is curved having an apex located on an axis of the anode. In alternative embodiments, the projection may have angled faces or the like. The projection provides a focal point for the electrons emitted by the cathode.
The anode 112 is located within the shroud by upper and lower insulating rings 118,119. A gas chamber 140 is defined by the anode 112, the insulating rings 118, 119 and the inside surface of the shroud 102. The upper insulator 118 is a rigid insulator for holding and locating the anode 112 properly in place. The insulator 118 is also required to have a high temperature resistance and low thermal expansion in order that the insulator provides a seal for the gas chamber under operating conditions. Preferred materials for the upper insulator include glass, ceramic or polymers such as PEEK (polyethylethylketone). The lower insulator is preferably a high temperature elastomer ring that provides a resilient seal for the gas chamber 140 when the base 101 is screwed into the shroud.
An inlet 141 through the shroud is connectable to a gas line (not shown) that supplies gas to the gas chamber 140. Control of the gas flow is governed by a mass flow controller or similar control mechanism disposed upstream of the ion source, as is well known in the art.
Extending through the anode side walls 121 are a plurality of channels 125, each terminating in the ionization region 113 at an aperture 126 disposed adjacent the end wall 120. The channels 125 provide a conduit from the gas chamber 140 to the ionization region 113. The channels 125 extend downwardly (as depicted in Figure 1) from the outer anode wall to the ionization region such that the channels are pointed at the projection 123. This ensures that the incoming gas molecules are on average directed at the projection 123. Gas ionisation efficiency is thereby increased because the gas molecules are introduced in proximity to and in the direction of the region of highest electron concentration and electron energy.
As shown in Figure 1, the projection 123 is integrally formed with the end wall 120. Also shown within the anode 112 is a cavity 127 that receives a cooling fluid from an inlet conduit 150. The cavity 127 extends to an underside surface 128 of the end wall and the projection 123. The thickness of the end wall is preferably less than 10mm in order that the cooling fluid can sufficiently cool the projection. The minimum thickness of the end wall and projection is determined only by the limits of the manufacturing processes used to fabricate the anode. In practice, the thickness of the end wall is approximately 2mm.
The fluid conduit 150 is a coaxial conduit, having an inner conduit 151 for supplying fluid, eg water, to the cavity 127 and an outer conduit 152 for removing fluid from the cavity. The inner conduit 151 extends into the cavity so that the outlet end 153 of the conduit is disposed adjacent the underside surface 128 of the end wall. This ensures that the coolest water is directed at the end wall and projection, which receives the majority of the anode heat load. The outlet 153 of the inner conduit has a notch 154 so that in the event that the inner conduit is inserted into the cavity until the conduit abuts the underside surface of the end wall, the flow of water is not restricted.
The fluid conduit 150 extends through the central aperture of the ring magnet 114 and the base plate 101 and can be used to provide an electrical connection to the anode with electrical breaks provided upstream of the connection.
In a preferred embodiment, the ion source is operated by providing a mains rectified voltage signal 0-300 V to the anode, as described in Applicant's co-filed application titled"Ion source control system", the entire contents of which are herein incorporated by reference. An AC heating current of approximately 16A is passed through either of the cathode filaments to stimulate electron emission. Electrons generated at the cathode are influenced by the anode potential and are accelerated toward it. The magnetic field imparts a spiral motion on the electrons increasing the time which the electrons spend in the ionisation region and thus their potential to ionise gas molecules, and focussing the electrons toward the longitudinal axis. When the anode voltage is sufficiently high, the electrons gain enough energy that collisions with gas molecules cause ionisation. Positive ions created in the plasma experience the opposite effect to the electrons and are accelerated away from the anode out of the open end 116 of the ionisation region.
The general theory of operation of the ion source is known from Applicant's previous applications PCT/AU99/00591and PCT/AU01/01548 the entire contents of which are herein incorporated by reference.
A cathode control circuit will now be described with reference to Figure 3. The circuit 300 receives at its input end 301 a variable 0-5 V input signal. The input is passed through a high gain amplifier 302 and thereafter to a power device 303. In a preferred embodiment, the power device is based on a phase controlled triac 321 controlling a transformer 322, the secondary side of which 323 outputs to the filament. The power device 303 scales the variable input signal to a high current signal required for electron emission from the filament.
In the circuit 300, two filaments 311,312 are provided. The selection of the filament is controlled by a relay 305 which in turn is governed by a logic circuit 340 to be described below. The filaments are connected to ground 314 with a sense resistor 315 connecting through to the secondary winding 323 of the power device transformer,
completing the power gain circuit. The voltage across the sense resistor 315 is provided to a feedback block 316 that connects to the input circuit at a summing junction 317. The feedback block 316 contains scaling components that match the typical voltages across the sense resistor with the desired filament currents for feeding back to the drive components such as the amplifier 302 and power device 303.
In operation, the input end 301 receives a 0-5 V signal as selected by the operator.
The summing junction 317 adds the feedback signal from the feedback block 316.
Initially, the feedback signal from the feedback block is low and thus the output of the amplifier is relatively high. The high input drives the power device 303, increasing the current output of the secondary winding 323 of the transformer 322 and thus increasing the current in the first filament 311. The filament current is detected in the sense resistor 315 by the feedback block 316. As the feedback signal increases, the output of the summing junction 317 decreases and the circuit achieves a steady state. In general, the operation of a single cathode filament circuit is known in the art.
The logic circuit 340 contains a comparator that receives the output 345 from the amplifier 302 and compares it to a reference voltage 344, typically 9 V. Under steady state operation, the amplifier output signal 345 is low, typically 5 V. When the first filament 311 fails, the feedback circuit 316 provides zero return signal to the summing junction 317. The input signal 301 therefore provides the sole driver to the amplifier and the amplifier is therefore driven to its maximum output, typically l OV. This causes the switching of the comparator of the logic circuit 340, in turn switching the relay 305 to the second filament circuit 312.
The logic circuit 340 has a delay built in to it that prevents the circuit from switching due to transient signals that are not indicative of a failure condition in the first filament.
The relay circuit 305 has in it an LED 341 and a buzzer 342 that are activated when the relay is switched to the second filament circuit. The LED is permanently on, whilst the buzzer is operated for a brief period, e. g. 10 seconds, to provide an audible indication that the first filament has failed.
When the power to the logic circuit 340 is dropped, e. g. when the ion source is turned off to allow the filament to be replaced, the state of the relay 305 is switched back to the first filament circuit. Thus the filament control system always attempts to power the first filament when the ion source is first switched on.
Referring now to Figure 5 there will be described an alternative filament change over circuit. The change over circuit 500 includes an operational amplifier 501 that receives as inputs 502,503 voltages representative of the filament current and filament voltage respectively. The input voltages are derived from the filament circuit 514 and are filtered and scaled by scaling components 517 eg voltage dividers, resistors etc, so that under normal operating conditions, the voltage inputs 502,503 are matched and the output of the op amp 501 is low. The output of the op amp 501 feeds the base 508 of a transistor 504 via a resistor 505. The transistor activates a relay 506 comprising a double pole switch. A first pole of the switch 507 connects a voltage rail 511, eg 12 volts, to the base 508 of the transistor 504, thereby providing a latch circuit. The second pole 509 operates a second relay 510 in the filament circuit 514 that switches the filament circuit 514 from the first filament 515 to the second filament 516.
The operation of the circuit will now be described. As mentioned above, under normal operating conditions the output of the op amp 501 is low, providing a low base voltage to the transistor. In this state, the relay 506 is unenergised and the two switches 507,509 are both open. The relay 510 is also unenergised ensuring that the filament circuit 514 is set to the first filament 515.
Upon failure of the first filament, the filament voltage input 503 to the op amp 501 remains unchanged, but the filament current input 502 is reduced because no current is able to flow. Because the voltage inputs 502,503 are now unmatched, the output of the op amp becomes high enough to activate the transistor 504 which in turn energises the relay 506. The first switch 507 closes and latches the voltage rail 511 to the base 508 of the transistor 504, thereby maintaining the relay 506 in the energized state regardless of the input coming from the op amp 501. The second pole 509 is also closed, thus connecting the voltage rail 511 to the second relay 510 thereby switching in the second
filament 516. This state of the circuit is maintained due to the latching of the circuit through the first switch 507.
In order to replace the first filament, the entire circuit 500 must be switched off so that no voltages are present in the vacuum chamber when the operator replaces the filaments. During the filament replacement, the transistor 504 is reset because no voltage is provided to the base 508. Further down the circuit, the relay 506 is de-energised as is the latching circuit and the second relay 510. Thus the circuit 500 is reset to operate the first filament when the ion source is next operated.
As described for the previous embodiments, the activation of the second filament circuit can coincide with the activation of a visible and/or audible alarm to indicate the failure of the first filament.
Whilst the switching of the filament in the embodiments described above rely on monitoring the filament supply circuit, other embodiments are possible. For example, the logic circuit 340 may receive an input from a sense resistor placed in the anode supply line that measures the anode current. If a filament fails, the anode current will quickly go to zero. The low signal can be sensed across the sense resistor and provided to the logic circuit to switch the filament.
An alternative embodiment is illustrated in Figure 4. In this embodiment, three filament legs 430,431, 432 are provided to support two filaments 440,441. Filament leg 431 is thus shared by the two filaments and therefore provides a common electrical connection. It is preferred that the shared connection is an earth connection, for example as may be connected to the ground connection 314 of the circuit 300 described above and shown in Figure 3.
For ease of mounting, the dual filaments can be provided by a single length of filament. The single strand can be fixed at the first leg 430, extended to the common filament leg 431 where it is fixed in the middle of the strand, and then bent to extend to the third filament leg 432. Because the central leg 431 is connected to ground, a voltage applied to only one of the other filament legs will cause a current to flow in only one half of the strand between the active filament leg and the grounded leg.
Whilst the invention has been described with particular reference to gridless open ended ion sources, it will be appreciated by the skilled addressee that the invention can be equally applied to other types of ion sources that utilize a thermal emission cathode, in particular where the cathode is disposed in the ion expulsion path of the ion source.
Further, whilst only straight filaments have been shown, it will be understood by the skilled addressee coiled, ie helical, and other more complex filament shapes can be used.
It will be appreciated by the skilled addressee that whilst a dual filament system has been described, any greater number of filaments may be provided, the number required being dependent on the average lifetime of each filament, and the length of the process for which the ion source is required to operate continuously. A greater number of filaments can be provided for by replicating the logic components of the cathode control circuit and by replacing the two position relay 305 with a multi-position switch, one position for each filament line.
The present invention as herein described in the preferred embodiments assists in providing continuity of operation for the length of the deposition process because as soon as the first filament is blown, the circuit automatically switches to the back-up filament.
In addition, the audible warning and first filament status indicator light allow an operator to quickly know the filament status. In between deposition runs, the operator can check the indicator lights, and if necessary, replace the blown filaments, and the back-up filament if deemed necessary to ensure the following deposition process will complete satisfactorily.
Throughout the specification, the word"comprising"and variations such as "comprise","comprises"etc, are used inclusively, that is, the features specified may be combined with further features in various embodiments of the invention.
While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all changes which come within the meaning and range of equivalency are therefore intended to be embraced herein.