Patent Grant
11956885
The invention generally relates to the field of microwave remote plasma sources and microwave power delivery systems. More specifically, the invention relates to impedance matching mechanisms in power delivery systems for improved power coupling into plasma sources and reduced reflected power from the plasma sources.
In addition, the prior art system 100 can include multiple transmission line elements for interconnecting the various components. Typically, for low power systems, e.g., about 1 kilowatt (kW) and below, one or more coaxial cables are used instead of waveguides for interconnecting these components, such as the coaxial cable segments 112a, 112b shown in
In the prior art system 100, if microwave power from the power generator 104 is not coupled efficiently to the plasma generated inside of the plasma applicator 102, a portion of that power is likely to be reflected back toward the power generator 104.
Therefore, in the instances where the coupling of microwave energy into the plasma is not optimal, some power may be reflected from the applicator 102 back toward the generator 104 as explained above. The automatic impedance matching network 108 is configured to only minimize power reflection upstream (i.e., between the power generator 104 and the impedance matching network 108). The automatic impedance matching network 108 thus considers as a load everything on the downstream side of the network 108, including the applicator 102 with the plasma therein, the downstream coaxial cable 112b, and the waveguide 114 to coaxial cable transition. Therefore, because the automatic impedance matching network 108 prevents reflected power from passing upstream, a significant portion of the power reflected from the plasma applicator 102 is forced to be dissipated downstream, such as being absorbed in the downstream coaxial cable 112b between the impedance matching network 108 and the plasma applicator 102. The reflected microwave power can be absorbed in the downstream coaxial cable 112b via conduction losses and dielectric losses. Conduction losses are resistive losses in the inner and outer conductors of the coaxial cable 112b. Dielectric losses are losses in the dielectric material used to construct the coaxial cable 112b for maintaining proper spacing between inner and outer conductors. In general, excessive reflected power in a coaxial cable, such as in the downstream coaxial cable 112b, can cause a buildup of high electric field, which can in turn cause excessive heat dissipation and subsequent cable overheating. As explained above, in the case of poor microwave coupling into plasma, reflected power can be rather high, on the order of several hundred watts. The downstream ⅞ coaxial cable 112b is not typically configured to dissipate such high reflected power and can easily overheat.
Another disadvantage of the prior art system 100 is that even though the solid-state microwave generator 104 can operate within a frequency band, such as between about 2.4 and about 2.5 GHz, due to the frequency limitation of the impedance matching network 108, the operating frequency for the low-powered remote plasma generator system 100 (e.g., a 1 kW system) is effectively fixed, e.g., at 2.45 GHz.
Therefore, there is a need for an impedance matching mechanism in a remote microwave power delivery system capable of improving power coupling and reducing reflected power between a variable frequency solid-state microwave generator and a plasma applicator. Specifically, it is desirable to design an impedance matching mechanism to prevent overheating of the downstream coaxial cable in the power delivery system, such as the coaxial cable 112b of the power delivery system 100 described above with reference to
In one aspect, a plasma-generating system is provided. The system includes a variable-frequency microwave generator configured to generate microwave power and a plasma applicator configured to use the microwave power from the microwave generator to (i) ignite a process gas therein for initiating a plasma in a plasma ignition process and (ii) maintain the plasma in a steady state process. The system also includes a coarse tuner connected between the microwave generator and the plasma applicator. At least one physical parameter of the coarse tuner is adapted to be set to achieve coarse impedance matching between the microwave generator and the plasma generated during both the plasma ignition process and the steady state process. A load impedance of the plasma generated during the plasma ignition process and the steady state process is adapted to vary over an impedance range. The microwave generator is configured to tune an operating frequency at the set physical parameter of the coarse tuner to achieve at least one of (i) ignition of the process gas during the plasma ignition process or (ii) maximization of the microwave power delivered to the plasma in the steady state process.
In another aspect, a method is provided for generating plasma in a system that includes a variable-frequency microwave generator connected to a plasma applicator. The method includes disposing a coarse tuner between the microwave generator and the plasma applicator such that the coarse tuner is positioned adjacent to the plasma applicator and configuring one or more physical parameters of the coarse tuner to achieve coarse impedance matching between the microwave generator and plasma generated by the plasma applicator during both plasma ignition and steady state plasma generation. A load impedance of the plasma generated during plasma ignition and steady state plasma generation is adapted to vary over an impedance range. The method further includes flowing a process gas into a plasma tube of the plasma applicator, setting a frequency of the microwave generator to an initial frequency value to initiate microwave power, coupling the microwave power to the plasma applicator to ionize the process gas therein, and iteratively fine tuning the frequency of the microwave generator relative to the initial frequency without altering the one or more physical parameters of the coarse tuner. Each iteration comprises determining if the process gas in the plasma tube is ignited for initiating a plasma at the microwave power corresponding to the tuned frequency, and discontinuing fine tuning the frequency of the microwave generator if ignition is detected.
Any of the above aspects can include one or more of the following features. In some embodiments, the coarse tuner is immediately adjacent to the plasma applicator without a coaxial cable connection therebetween. In some embodiments, the coarse tuner includes an integrated coupling element for coupling microwave power from the microwave generator to a microwave cavity of the plasma applicator.
In some embodiments, the coarse tuner is a fixed stub tuner that includes at least a stub and a coupling antenna. The fixed stub tuner is disposed proximate to a dielectric plasma tube. The at least one physical parameter of the fixed stub tuner comprises one of (i) a distance between the stub and a longitudinal axis of the dielectric plasma tube and (ii) a length of the stub. In some embodiments, the stub length is 1.21 inches and the distance is 2.96 inches. In some embodiments, at least one of the stub length or the distance is adjustable to achieve the coarse impedance matching. In some embodiments, the fixed stub tuner is a quarter wavelength fixed stub tuner. In some embodiments, the fixed stub is electrically shorted to prevent microwave radiation to the environment.
In some embodiments, the coarse impedance matching comprises modifying the load impedance of the plasma over the impedance range such that a maximum of power absorbed by the plasma is within an operating bandwidth of the variable-frequency microwave generator. In some embodiments, an automatic impedance matching network is absent between the microwave generator and the plasma applicator. In some embodiments, an isolator is located between the microwave generator and the coarse tuner to minimize reflected power from the plasma applicator to the microwave generator.
In some embodiments, a process pressure of the plasma applicator is set after the process gas flow is stabilized.
In some embodiments, the iterative fine tuning of the frequency of the microwave generator comprises iteratively increasing the frequency from the initial frequency by a predetermined step until an upper bound is reached. In some embodiments, the iterative fine tuning of the frequency of the microwave generator comprises iteratively decreasing the frequency from the initial frequency by a predetermined step until a lower bound is reached.
In some embodiments, the microwave power delivered to the plasma is maximized after ignition is detected. Maximizing the microwave power includes setting the frequency of the microwave generator to a second initial frequency value to generate microwave power, coupling the microwave power to the plasma applicator to maintain the plasma therein, and iteratively tuning the frequency of the microwave generator relative to the second initial frequency without altering the one or more physical parameters of the coarse tuner until a threshold frequency is reached. Each iteration includes calculating a value of the microwave power delivered to the plasma and recording the calculated microwave power value and the corresponding tuned frequency. Maximizing the microwave power also includes determining a maximum of the calculated microwave power values recorded, and setting the microwave generator to the tuned frequency corresponding to the maximum calculated microwave power value for maintaining the plasma in the plasma applicator in a steady state. In some embodiments, calculating a value of the microwave power delivered to the plasma comprises determining a forward power value and a reflected power value, and determining a difference between the forward power value and the reflected power value to calculate the value of the microwave power delivered to the plasma. In some embodiments, the iterative fine tuning of the frequency of the microwave generator comprises iteratively increasing the frequency from the second initial frequency by a predetermined step until the threshold frequency is reached. In some embodiments, the iterative fine tuning of the frequency of the microwave generator comprises iteratively decreasing the frequency from the second initial frequency by a predetermined step until the threshold frequency is reached. In some embodiments, initiating the plasma in the plasma tube and maximizing the microwave power delivered to the plasma after ignition are achieved without adjusting impedance matching between the microwave generator and a load of the plasma.
The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the technology.
In some embodiments, the plasma applicator 302 includes a plasma discharge tube 308 in which one or more process gases are excited by the microwave energy coupled thereto to generate the plasma 320 in the discharge tube 308. The plasma applicator 306 also includes a coupling element 310 attached to the outer housing of the plasma applicator 302 for coupling the microwave energy from the microwave generator 304 into a microwave cavity 312 of the plasma applicator 302. In some embodiments, the plasma applicator 302 includes a plasma detector (not shown) for detecting when plasma 320 is ignited. In some embodiments, the plasma applicator 302 is substantially similar to the microwave plasma applicator described in U.S. Pat. No. 9,831,066, which is owned by the assignee of the instant application and is hereby incorporated by reference in its entirety. In operation, the plasma applicator 302 is configured to couple the microwave power from the microwave generator 304 to (i) ignite the one or more process gases in the plasma discharge tube 308 for initiating the plasma 320 in a plasma ignition process and (ii) maintain the plasma 320 in a steady state process after the initial ignition process.
In some embodiments, the variable frequency solid-state microwave generator 304 is configured to operate within one or several Industrial, Scientific and Medical (ISM) frequency bands, including but not limited to about 2.4 GHz to about 2.5 GHz, about 902 MHz to about 928 MHz or about 5.725 GHZ to about 5.875 GHz. The microwave generator 304 can include a directional coupler (not shown) for measuring forward and reflected power at the output of the microwave generator 304. The microwave generator 304 can also include one or more built-in isolators (not shown), such as installed at the output of each of the power amplifier stages of the generator 304 to protect the power amplifiers from high reflected power. In some embodiments, the microwave generator 304 is substantially similar to the microwave generator described in U.S. Pat. No. 9,595,930, which is owned by the assignee of the instant application and is hereby incorporated by reference in its entirety.
In some embodiments, a coarse tuner 316 is disposed between the microwave generator 304 and the plasma applicator 302 in the power delivery system 300. The coarse tuner 316 can be placed upstream and adjacent to the plasma applicator 302 along the coaxial cable 306, such as integrated with the coupling element 310 connected immediately upstream of the microwave applicator 302 and attached to the outer housing of the microwave applicator 302. In alterative embodiments, the coarse tuner 316 is a standalone component upstream and adjacent (e.g., immediately adjacent/attached) to the plasma applicator 302. The coarse tuner 316 can be connected to the plasma applicator 302 without a coaxial cable (or any other connection element) therebetween. The type and location of the coarser tuner 316 can be chosen to prevent overheating of one or more connection elements in the system 300 by reducing power reflection from the microwave cavity 312 of the plasma applicator 302. In some embodiments, the coarse tuner 316 is configured to replace the automatic impedance matching network 108 of
In some embodiments, the coarse tuner 316 of the system 300 is a quarter wavelength fixed stub coarse tuner for achieving impedance matching in microwave power transmission and communication systems. Advantages of using a fixed stub tuner includes improved microwave power coupling into the plasma. However, unlike the load impedance in a typical power delivery system in which a fixed stub tuner is used (e.g., where the load impedance is fixed), the load impedance of the plasma load in the remote microwave plasma system 300 of
In some embodiments, the quarter wavelength fixed stub coarse tuner 316 is positioned immediately upstream of the microwave cavity 312 so as to minimize any reflected power and standing wave in the transmission line elements upstream of the plasma applicator 302. For example, as explained above, the fixed stub tuner 316 can be integrated with the coupling element 310 attached to the outer housing of the plasma application 302, as shown in
As shown in
In some embodiment, the third end 410 of the fixed stub tuner 400 is electrically shorted to prevent microwave leakage to the environment. Alternatively, the third end 410 of the fixed stub tuner 400 is electrically open. In some embodiments, the center conductor 402 is mechanically supported by the stub segment 418 of the fixed stub tuner 400, having the same inner and outer conductor diameters, and the center conductor 402 is attached to the stub segment 418 via the short segment 417 by, for example, a flat head screw. More specifically, the short segment 417 can be attached with screws to the outer conductor housing 404 and the center conductor 402. In some embodiments, the length 412 of the stub segment 418 is fixed prior to operating the power delivery system 300, such as to about a quarter wavelength at the operating frequency (e.g., 2.45 GHZ).
In some embodiments, the power delivery system 300 further includes a plasma detector 504, shown in
In some embodiments, one or more parameters of the fixed stub tuner 400 are adjusted and set prior to operating the power delivery system 300 in order to achieve coarse impedance matching between the microwave generator 304 and the plasma load in the plasma applicator 302. This coarse matching is able to accomplish a reasonably good impedance match for a range of load impedances generated by the plasma 320 during both the plasma ignition process and the steady state plasma maintenance process. For example, for the course turner configuration 400 of
For the coarse tuner 600 of
Such an adjustable stub is beneficial during laboratory experiments, as it allows the adjustment of the stub length 620 to optimize microwave power coupling and minimize reflected power in a real laboratory setup. In some embodiments, the maximum effective stub length 620 (e.g., the maximum length the stub can be retracted from the junction 618) can be set to approximately between one quarter wavelength and full wavelength at the center frequency of the operating frequency band (e.g., between about 1.2 inches and about 4.82 inches at about 2.45 GHz). Once the position corresponding to the optimal power coupling is found experimentally, the stub position can be locked to be used for other experiments and/or actual plasma generation within the power delivery system 300. Other configurations of adjustable short segment for a coarse tuner implemented on coaxial lines are possible and are within the scope of the present invention.
In some embodiments, the coarse tuner 600 is similarly connected to the power delivery system as the coarse tuner 400 as explained above with reference to
Referring back to
In some embodiments, several different coarse tuners (e.g., different fixed stub tuners 400, 600 with different dimensions for the stub-to-applicator distance 502 and/or stub length 412 or 620) can be designed for the power delivery system 300, where each coarse tuner's performance (e.g., power coupling and reduced reflected power capabilities) is optimized for a narrower range of plasma impedances, so as to cover a useful subset of process recipes in one semiconductor application, for example. In addition, different plasma applicators can be used in conjunction with different coarse tuners to cover different applications.
In some embodiments, because plasma impedance can vary significantly, especially before and after plasma ignition, the operating frequency of the solid-state microwave generator 304 can be adjusted within a predefined frequency range to fine tune impedance matching at the set physical parameter(s) of the coarse tuner 316. The fine tuning of the microwave generator 304 after the coarse impedance matching by the coarse tuner 316 can achieve at least one of (i) ignition of the process gas during the plasma ignition process or (ii) maximization of the microwave power delivered to the plasma in the steady state process, which is described in detail below in relation to
With the tuned coarse tuner 316 in place, the process 900 starts by providing a process gas to the plasma discharge tube 308 of the plasma applicator 304 (step 904). Once the process gas flow is stabilized, process pressure can be set and stabilized. The process 900 can then proceed to tune the frequency of the microwave generator 304 for plasma ignition in the applicator 302. First, the frequency of the microwave generator 304 is set to an initial frequency value to initiate microwave power at that set frequency (step 906). The resulting microwave power is coupled to the plasma applicator 302 via the integrated tuner 316 and coupling element 310 to ionize the process gas in the plasma applicator 302 (step 908). From the initial frequency, the frequency of the microwave generator 304 is iteratively adjusted without altering the physical parameters of the coarse tuner 316 (step 910). Each iteration can include determining if the process gas in the plasma discharge tube 308 is ignited at the microwave power corresponding to the tuned frequency (step 914) and discontinuing the frequency adjustment of the microwave generator 304 if plasma ignition is detected (step 916). Detection of plasma ignition can be accomplished using the plasma detector 504 described above with reference to
The process 1000 starts by setting the frequency of the microwave generator to a second initial frequency value to generate microwave power (step 1002). The resulting microwave power is coupled to the plasma generator 302 to maintain the plasma 320 in the applicator 302. From the second initial frequency, the frequency of the microwave generator 304 can be iteratively adjusted without altering the physical parameters of the coarse tuner 316 until a threshold frequency value is reached (step 1004). Each iteration can involve calculating a value of the microwave power delivered to the plasma 320 (step 1006) and recording the calculated microwave power value and the corresponding frequency value at that iteration (step 1008). After the threshold frequency is reached (step 1010), a maximum of the recorded microwave power values is determined (step 1012) and the microwave generator 304 is set to the frequency value at which the maximum recorded microwave power is generated (step 1014).
In some embodiments, the iterative fine tuning of the frequency of the microwave generator 304 involves iteratively increasing the frequency from the second initial frequency (e.g., about 2400 MHZ) by a predetermined step (e.g., about 2 MHZ) until an upper bound (e.g., about 2500 MHZ) is reached. Alternatively, the iterative fine tuning of the frequency of the microwave generator 304 can involve iteratively decreasing the frequency from the second initial frequency (e.g., about 2500 MHZ) by a predetermined step (e.g., about 2 MHZ) until a lower bound (e.g., about 2400 MHZ) is reached. In some embodiments, at each frequency iteration, the value of the microwave power calculated is a difference between the forward power measured and reflected power measured. For the power delivery system 300 of
There are several advantages associated with incorporating a coarse tuner (e.g., the fixed stub tuner 400 of
While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 5223457 | Mintz | Jun 1993 | A |
| 5621331 | Smith et al. | Apr 1997 | A |
| 5869817 | Zietlow | Feb 1999 | A |
| 5965034 | Vinogradov | Oct 1999 | A |
| 6855906 | Brailove | Feb 2005 | B2 |
| 8647585 | Hancock | Feb 2014 | B2 |
| 8900521 | Hancock | Dec 2014 | B2 |
| 9595930 | Garuti et al. | Mar 2017 | B2 |
| 9675716 | Hancock | Jun 2017 | B2 |
| 9831066 | Kamarehi | Nov 2017 | B1 |
| 10790118 | Kamarehi | Sep 2020 | B2 |
| 11049694 | AuBuchon | Jun 2021 | B2 |
| 11222770 | Kamarehi | Jan 2022 | B2 |
| 11404248 | Kraus | Aug 2022 | B2 |
| 11670489 | AuBuchon | Jun 2023 | B2 |
| 20100296977 | Hancock | Nov 2010 | A1 |
| 20130146225 | Chen | Jun 2013 | A1 |
| 20140066838 | Hancock | Mar 2014 | A1 |
| 20150279626 | Chen | Oct 2015 | A1 |
| 20170173846 | Chen | Jun 2017 | A1 |
| 20170232122 | Hancock | Aug 2017 | A1 |
| 20180130638 | Ahmad | May 2018 | A1 |
| 20180151332 | Kaneko et al. | May 2018 | A1 |
| 20180269037 | Kamarehi | Sep 2018 | A1 |
| 20180345569 | Chen | Dec 2018 | A1 |
| 20200022246 | Chen | Jan 2020 | A1 |
| 20200171180 | Hancock | Jun 2020 | A1 |
| 20200381217 | Kraus | Dec 2020 | A1 |
| 20210005430 | Kamarehi | Jan 2021 | A1 |
| 20210098236 | AuBuchon | Apr 2021 | A1 |
| 20210313153 | AuBuchon | Oct 2021 | A1 |
| 20220243333 | Kamarehi | Aug 2022 | A1 |
| 20230057795 | Pokidov | Feb 2023 | A1 |
| Number | Date | Country |
|---|---|---|
| 10-2020-0001970 | Jan 2020 | KR |
| 2012021325 | Feb 2012 | WO |
| 2021045991 | Mar 2021 | WO |
| Entry |
|---|
| “SmartmatchTM Intelligent Microwave Matching Unit Model AX3060,” Installation and Operations Manual, Part No. OM32000, Rev. E, Jul. 2001, 90 pgs. |
| “Three Stub Tuner Model AX3041,” Installation and Operations Manual, Part No. OM30001, Rev. C, May 2013, 17 pgs. |
| “Notification of Transmittal of the International Search Report and the Written Opinion of the International Searching Authority, or the Declaration,” International Filing Date: Aug. 2, 2022, International Application No. PCT/US2022/039149, Applicant: MKS Instruments, Inc., dated Nov. 17, 20202, pp. 1-11. |
| Number | Date | Country | |
|---|---|---|---|
| 20230057795 A1 | Feb 2023 | US |
