RF delivery configuration in a plasma processing system

1. Field of the Invention

This invention relates generally to plasma processing systems, and more particularly to radio frequency (RF) power generation and distribution in such systems.

2. Description of the Background Art

Plasma processing systems are employed in the fabrication of semiconductor devices. In a plasma processing system, such as a plasma enhanced chemical vapor deposition (PECVD) system, radio frequency (RF) power is supplied to the system's chamber to maintain a plasma therein. A semiconductor substrate in the chamber is exposed to the plasma and process gases, thereby performing a fabrication process on the substrate. In a PECVD system, for example, the fabrication process may be deposition of a thin film of dielectric material on the substrate.

FIG. 1 shows a schematic diagram of a typical RF delivery configuration. In the configuration of FIG. 1 , an RF generator 101 is coupled to one or more electrodes in a plasma processing chamber 105 . RF generator 101 may be a so-called “high-frequency” RF generator operating at a frequency of about 13.56 MHz and having an output impedance of about 50 Ω (Ohms). RF generator 101 is coupled to plasma processing chamber 105 via a 50 Ω coaxial cable 102 . Coaxial cable 102 typically has a fixed length specifically selected for the characteristics required by plasma processing chamber 105 .

To prevent power loss due to signal reflection, impedance matching network 103 performs an impedance transformation to match the load impedance presented by plasma processing chamber 105 to the characteristic impedance of cable 102 . Impedance matching network 103 may be an automatic impedance matching network or a fixed impedance matching network. An automatic impedance matching network, which is also referred to as an “auto-match,” is so named because it employs tuning components that are automatically adjusted in response to the impedance presented to it. A fixed impedance matching network, which is also referred to as a “fixed-match,” is so named because it employs components that are not adjusted during normal operation. That is, the components of a fixed-match have preset values. A matching network 103 may be connected to an electrode in plasma processing chamber 105 using a connection scheme 104 , which may include combiners, splitters, direct connections using electrically conductive elements, or combinations thereof.

The load impedance presented by a plasma processing chamber can have relatively high phase angles, which can lead to high circulating currents and RF delivery stability issues. Matching network 103 thus requires components that can handle high circulating currents, thereby increasing the cost of matching network 103 . The high load currents also raise reliability issues by degrading the components of matching network 103 , lower the efficiency of matching network 103 because of I 2 R power losses, and prevent accurate voltage-current probe readings between matching network 103 and the chamber.

In the example of FIG. 1 , matching network 103 is customized to the load impedance presented by chamber 105 . Thus, a manufacturer offering different models or types of plasma processing chambers may have to design, manufacture, and stock different matching networks 103 . This not only increases the overall cost of the plasma processing system, but also raises logistical issues.

In one embodiment, a system for delivering radio frequency (RF) power to a plasma processing system includes an automatic impedance matching network configured to receive RF power from an RF generator, and a fixed impedance matching network coupled between the automatic impedance matching network and the plasma processing chamber. The fixed impedance matching network may be configured to transform a first impedance presented by the chamber to a second impedance that allows the automatic impedance matching network to operate within a tuning range.

These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.

FIG. 1 shows a schematic diagram of a typical RF delivery configuration.

FIG. 2 shows a schematic diagram of an RF delivery configuration in accordance with an embodiment of the present invention.

FIG. 3A shows a schematic diagram of an automatic impedance matching network in accordance with an embodiment of the present invention.

FIG. 3B shows a schematic diagram of an automatic impedance matching network in accordance with another embodiment of the present invention.

FIG. 4A shows a minimum tuning range in a Smith chart for an automatic impedance matching network in accordance with an embodiment of the present invention.

FIG. 4B shows a tuning range in a Smith chart for an automatic impedance matching network in accordance with another embodiment of the present invention.

FIG. 4C shows a tuning range in a Smith chart for an automatic impedance matching network in accordance with another embodiment of the present invention.

FIG. 5A shows a fixed impedance matching network in accordance with an embodiment of the present invention.

FIG. 5B shows a fixed impedance matching network in accordance with another embodiment of the present invention.

FIG. 6 shows a schematic diagram of an RF delivery configuration in accordance with another embodiment of the present invention.

FIG. 7 shows a schematic diagram of an RF delivery configuration in accordance with another embodiment of the present invention.

FIG. 8 shows a schematic diagram of an RF delivery configuration in accordance with another embodiment of the present invention.

FIG. 9 shows a schematic diagram of an integrated match circuit in accordance with an embodiment of the present invention.

The use of the same reference label in different drawings indicates the same or like components.

In the present disclosure, numerous specific details are provided such as examples of apparatus, circuits, components, and methods to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.

Referring now to FIG. 2 , there is shown a schematic diagram of an RF delivery configuration in accordance with an embodiment of the present invention. In the example of FIG. 2 , an RF generator 201 is coupled to a plasma processing chamber 225 via a coaxial cable 202 , an automatic impedance matching network 203 (“auto-match 203 ”), a connection scheme 206 (e.g., a coaxial cable), a fixed matching network 207 (“fixed-match 207 ”), and a connection scheme 208 . In one embodiment, RF generator 201 is a commercially available high-frequency RF generator operating at a frequency of about 13.56 MHz and having an output impedance of about 50 Ω (Ohms). A coaxial cable 202 couples RF generator 201 to the rest of the system. To minimize reflections, coaxial cable 202 has a characteristic impedance that matches the output impedance of RF generator 201 . In one embodiment, the characteristic impedance of coaxial cable 202 is about 50 Ω. As can be appreciated, the coaxial cables disclosed herein may also be replaced with other connection schemes.

In one embodiment, plasma processing chamber 225 is a chamber that employs a plasma in the fabrication of integrated circuits. For example, plasma processing chamber 225 may be a plasma-enhanced chemical vapor deposition (PECVD) or a high-density plasma chemical vapor deposition (HDP-CVD) chamber. Plasma processing chamber 225 may be a commercially available plasma processing chamber such those available from Novellus Systems, Inc. of San Jose, Calif.

Plasma processing chamber 225 typically presents a load impedance that is not matched to the output impedance of RF generator 201 and the characteristic impedance of intervening coaxial cables. To minimize reflections and to alleviate the problems discussed in connection with the configuration of FIG. 1 , fixed-match 207 is employed between plasma processing chamber 225 and an auto-match 203 . Fixed-match 207 sees the load impedance presented by plasma processing chamber 225 at a node 212 and transforms that load impedance to another impedance at a node 211 . In one embodiment, the impedance presented by fixed-match 207 at node 211 allows auto-match 203 to operate within a tuning range. Fixed-match 207 beneficially isolates the rest of the system from the relative low load impedance presented by plasma processing chamber 225 , thereby allowing auto-match 203 to be constructed using lower cost components, allowing for the use of a voltage-current (VI) probe between fixed-match 207 and auto-match 203 , and alleviating issues relating to high phase angles of load currents including RF delivery stability issues, component degradation and I 2 R power losses.

Connection scheme 208 connects fixed-match 207 to plasma processing chamber 225 . Connection scheme 208 may comprise electrically conducting elements directly connecting fixed-match 207 to an electrode in plasma processing chamber 225 . Depending on the application, connection scheme 208 may also include combiners, splitters, and other means for connecting fixed-match 207 to an electrode in plasma processing chamber 225 . Various connection schemes 208 may be employed without detracting from the merits of the present invention.

Connection scheme 206 couples fixed-match 207 to other components downstream from node 211 . The characteristic impedance of connection scheme 206 is selected to minimize reflections. In one embodiment where the fixed-match 207 presents an impedance between 10 Ω to 90 Ω at node 211 , connection scheme 206 may be a coaxial cable having a characteristic impedance of about 30 Ω. Connection scheme 206 may have a relatively short (e.g., less than 8 feet), predetermined fixed length, for example. A connection scheme, such as connection scheme 206 , having a characteristic impedance lower than the standard 50 Ω advantageously allows for higher power handling. The relatively short length of connection scheme 206 helps minimize power loss.

A voltage-current probe (“VI probe”) measures voltage, current, and phase, and may thus be employed as a troubleshooting or process control tool. Because a typical VI probe provides inaccurate readings in high phase angle environments, VI probes are not effective when employed “in front” of a plasma processing chamber. In the example of FIG. 2 , fixed-match 207 protects a VI probe from the high phase angles of plasma processing chamber 225 and the high load currents associated with such load impedance. This allows VI probe 205 to be coupled between auto-match 203 and fixed-match 207 . Of course, the use of VI probe 205 is optional. Preferably, VI probe 205 comprises a non-invasive, commercially available VI probe that is generally employed in the 50 Ω side of plasma processing applications. A non-invasive VI probe 205 allows auto-match 203 to see the impedance presented by fixed-match 207 as if VI probe 205 is not coupled between them. VI probe 205 may be connected to fixed match 207 via connection scheme 206 , and to auto-match 203 via a connection scheme 204 . In one embodiment, connection scheme 204 comprises a removable stub. The stub may be a connection scheme consisting of adapters placed between two terminals. The stub may be removed to allow VI probe 205 to be installed in its place. Other means for connecting VI probe 205 to auto-match 203 may also be employed without detracting from the merits of the present invention.

Auto-match 203 transforms the impedance present at its node 213 to an impedance that matches the characteristic impedance of coaxial cable 202 and the output impedance of RF generator 201 . Auto-match 203 may comprise an auto-match 203 A shown in FIG. 3A or an auto-match 203 B shown in FIG. 3B . In one embodiment where the characteristic impedance of coaxial cable 202 and the output impedance of RF generator 201 is about 50 Ω, the impedance presented by auto-match 203 at node 214 is also about 50 Ω. Auto-match 203 employs components that are automatically tuned to perform this impedance transformation. Fixed-match 207 protects auto-match 203 from the high phase angle and low load impedance of plasma processing chamber 225 and the high load currents associated with such load impedance. This allows auto-match 203 to be implemented using lower cost component and allows it to reliably operate.

Fixed-match 207 presents an impedance at node 211 that is within the tuning range of auto-match 203 . In other words, fixed-match 207 transforms the impedance seen at node 212 to within the tuning range of auto-match 203 . In one embodiment where auto-match 203 is configured to operate within a minimum tuning range of around a 30 Ω3:1 VSWR circle, fixed-match 207 is configured to present an impedance of about 10 Ω to 90 Ω at node 211 . A fixed-match may be customized to present a target impedance or range for a particular plasma processing chamber. Thus, by providing a particular fixed-match for a particular plasma processing chamber, a single auto-match may be employed for different plasma processing chambers. That is, a single auto-match design may be employed to work with different plasma processing chambers by providing a custom fixed-match for each type of chamber. As will be more apparent below, this is advantageous because a fixed-match has cheaper and less components compared to an auto-match. Also, unlike an auto-match, a fixed-match has few or no moving parts.

FIG. 3A shows a schematic diagram of an auto-match 203 A in accordance with an embodiment of the present invention. Auto-match 203 A is a specific embodiment of auto-match 203 shown in FIG. 2 . Auto-match 203 A may include an automatic impedance matching circuit 310 (auto-matching circuit 310 ) and an absorptive filter circuit 312 . In the example of FIG. 3A , resistor R 1 represents the output impedance of RF generator 201 , which is about 50 Ω in one embodiment.

Auto-matching circuit 310 comprises components that are automatically adjusted to transform the impedance seen at a node 215 to another impedance presented at node 214 . The range within which the components of auto-matching circuit 310 may be adjusted to present a target impedance at node 214 is also referred to as a “tuning range.” Auto-matching circuit 310 may be automatically tuned to match to the characteristic impedance of cable 202 and the output impedance of RF generator 201 . In one embodiment, auto-matching circuit 310 comprises variable capacitors C 1 and C 2 , and an inductor L 1 . Capacitors C 1 and C 2 may include moveable plates that may be adjusted to change their capacitance. A monitoring circuit (not shown) comprising a detection circuit commands motors (not shown) to move tuning elements, thereby changing their value to keep auto-matching circuit 310 operating within its tuning range.

In one embodiment, auto-match 203 A includes an absorptive filter 312 . Absorptive filter 312 allows energies at a fundamental frequency to pass while absorbing energies at frequencies away from the fundamental frequency. Absorptive filter 312 may be configured to pass energies at a fundamental frequency of the output signal of RF generator 201 , which is 13.56 MHz in one embodiment. Energies away from the fundamental frequency are absorbed by absorptive filter 312 by dissipating them in a resistive element, such as resistor R 2 or R 4 .

In the example of FIG. 3A , capacitor C 4 , inductor L 3 , and resistor R 3 form a parallel-series resonant circuit that allows energies at the fundamental frequency to pass but rejects residual energies at frequencies that are away from the fundamental frequency. A parallel resonant circuit with a resistive element for dissipating energies at frequencies away from the fundamental frequency are shown as comprising capacitor C 3 , inductor L 2 , and resistor R 2 on one end, and comprising capacitor C 5 , inductor L 4 , and resistor R 4 on another end. At the fundamental frequency, parallel resonant circuit comprising capacitor C 3 and inductor L 2 and parallel resonant circuit comprising r capacitor C 5 and inductor L 4 appear as high impedance paths to ground and do not affect the RF power. At frequencies away from the fundamental frequency, the parallel resonant circuit shunts energies to resistor R 2 (or resistor R 4 ), which dissipates the energies. Absorptive filter circuit 312 helps minimize variations in the supplied RF power, and helps prevent fabrication processes from shifting. Absorptive filter circuits are also disclosed in commonly-assigned U.S. application Ser. No. 09/774,446, filed by Stephen E. Hilliker on Jan. 30, 2001, which is incorporated herein by reference in its entirety.

In one embodiment where absorptive filter 312 is configured to allow energies at a fundamental frequency of 13.56 MHz to pass and reject energies at harmonic frequencies, and auto-matching circuit 310 has a tuning range around 30 Ω 3:1 VSWR circle, the components of auto-match 203 A have the approximate values shown in Table 1.

FIG. 3B shows a schematic diagram of an auto-match 203 B in accordance with another embodiment of the present invention. Auto-match 203 B is a specific embodiment of auto-match 203 shown in FIG. 2 . Similar to auto-match 203 A, auto-match 203 B includes components that are automatically adjusted based on readings of a phase/magnitude detector 250 to transform the impedance seen at node 213 to another impedance presented at node 214 . Auto-match 203 B includes a “single leg” absorptive filter comprising capacitors C 13 – 15 , inductor L 20 , and resistors R 20 – 22 . The values of the components of auto-match 203 B may be selected based on a tuning range and target absorptive filter characteristics.

FIG. 4A shows a minimum tuning range in a Smith chart for an auto-match 203 in accordance with an embodiment of the present invention. In the example of FIG. 4A , the auto-match 203 is configured to present an impedance of about 50 Ω at node 214 (see FIG. 2 ), and has a tuning range centered at about 30 Ω at node 213 . The VSWR circle 401 represents the minimum tuning range of an auto-matching circuit (e.g., see auto-matching circuit 310 in FIG. 3A ) within the auto-match 203 , which is a 3:1 VSWR circle around 30 Ω in this example. Because an absorptive filter (e.g., see absorptive filter 312 in FIG. 3A ) has no appreciable effect on the tuning range of an auto-matching circuit, the tuning range of the auto-matching circuit is the tuning range of the auto-match 203 .

As can be appreciated, the tuning range of an auto-match 203 may be varied to meet the needs of specific applications. In the example Smith chart of FIG. 4B , the minimum tuning range of an auto-match 203 has been extended to cover lower resistance values. FIG. 4C shows another example tuning range in a Smith chart for an auto-match 203 .

FIG. 5A shows a fixed-match 207 A in accordance with an embodiment of the present invention. Fixed-match 207 A is a specific embodiment of fixed match 207 shown in FIG. 2 . Fixed-match 207 A is configured to transform the impedance presented by plasma processing chamber 225 at node 212 to an impedance that is within the tuning range of an auto-match 203 at node 211 . That is, fixed-match 207 A may be configured to present an impedance that is within the tuning range of auto-match 203 . In one embodiment where auto-match 203 has a tuning range of 3:1 VSWR circle around 30 Ω, the components of fixed-match 207 A have the values shown in Table 2.

FIG. 5B shows a fixed-match 207 B in accordance with another embodiment of the present invention. Fixed-match 207 B is a specific embodiment of fixed match 207 shown in FIG. 2 . Fixed-match 207 B is configured to transform the impedance presented by plasma processing chamber 225 at node 212 to an impedance that is within the tuning range of an auto-match 203 at node 211 . That is, fixed-match 207 B may be configured to present an impedance that is within the tuning range of auto-match 203 . In one embodiment where auto-match 203 has a tuning range of 3:1 VSWR circle around 30 Ω, the components of fixed-match 207 B have the values shown in Table 3.

In light of the present disclosure, those of ordinary skill in the art will appreciate that the use of a fixed-match in conjunction with an auto-match as described herein may be adapted to work with other RF delivery configurations. For example, FIG. 6 shows a schematic diagram of an RF delivery configuration in accordance with another embodiment of the present invention. In the example of FIG. 6 , RF generator 201 supplies RF power to either a dome 612 or an electrostatic chuck 611 of a plasma processing chamber 610 . Plasma processing chamber 610 may be an HDP-CVD chamber such as those of the type available from Novellus Systems, Inc., for example. Other types of plasma processing chambers may also be employed without detracting from the merits of the present invention. For example, dome 612 or chuck 611 may be other types of electrodes in a plasma processing chamber.

Still referring to FIG. 6 , an RF switch box 606 allows RF power from RF generator 201 to be switched between chuck 611 and dome 612 , each of which has a corresponding fixed match 207 (i.e., 207 - 1 , 207 - 2 ). A coaxial cable 607 (i.e., 607 - 1 , 607 - 2 ), which may be a relatively short 300 cable, connects RF switch box 606 to the selected fixed-match 207 . Each fixed-match 207 may be connected to its corresponding electrode (i.e., dome 612 or chuck 611 ) via a connection scheme 609 (i.e., 609 - 1 , 609 - 2 ). Connection scheme 609 may be an RF conductor, for example. Depending on the setting of RF switch box 606 , auto-match 203 will work with either fixed-match 207 - 1 or fixed-match 207 - 2 . RF generator 201 , coaxial cable 202 , auto-match 203 , connection scheme 204 , and VI probe 205 function as in the configuration of FIG. 2 .

FIG. 7 shows a schematic diagram of an RF delivery configuration in accordance with another embodiment of the present invention. In the example of FIG. 7 , an integrated match circuit 704 allows RF power to be delivered from RF generator 201 , RF generator 701 , or both to selected or all of chamber electrodes 703 (i.e., 703 - 1 , 703 - 2 , . . . 703 -n). Integrated match circuit 704 may include a combiner for combining the output signals of RF generator 201 and RF generator 701 . RF generator 701 may be a so-called low-frequency RF generator operating at a frequency of about 400 KHz and having an output impedance of about 50 Ω, for example. The output signal of RF generator 701 is coupled to an electrode 703 via a coaxial cable 702 , which is a 50 Ω cable in one embodiment. Integrated match circuit 704 may include a fixed-match (e.g., fixed-match 207 ) for each chamber electrode 703 . A splitter 705 allows the output of integrated match 704 to be fed to chamber electrodes 703 . In the example of FIG. 7 , RF generator 201 , coaxial cable 202 , auto-match 203 , connection scheme 204 , and VI probe 205 function as in the configuration of FIG. 2 .

FIG. 9 shows a schematic diagram of an integrated match circuit 704 A in accordance with an embodiment of the present invention. Integrated match circuit 704 A is a specific embodiment of the integrated match circuit 704 shown in FIG. 7 . Referring to FIG. 9 , integrated match circuit 704 A comprises a low frequency fixed-match 903 and a high-frequency fixed match 905 . Fixed-match 903 accepts a low-frequency RF signal at a node 901 , while fixed-match 905 accepts a high frequency RF signal at a node 902 . Fixed-match 903 transforms an impedance seen at its output to match to the characteristic impedance of cable 702 and output impedance of RF generator 701 (see FIG. 7 ) at node 901 . Fixed-match 905 transforms an impedance seen at node 906 to an impedance that is within the tuning range of an auto-match 203 coupled to node 902 . A combiner circuit 904 allows the output of fixed-match 903 to be combined with that of fixed match 905 . The output of integrated match circuit 704 A at node 906 is provided to a splitter for delivery to the electrodes of the plasma processing chamber. In one embodiment, the components of integrated match circuit 704 A have the approximate values shown in Table 4.

FIG. 8 shows a schematic diagram of an RF delivery configuration in accordance with another embodiment of the present invention. In the example of FIG. 8 , RF power from an RF generator 801 is coupled to dome 612 of plasma processing chamber 610 via a coaxial cable 802 , fixed-match 207 - 1 , and connection scheme 609 - 1 (see also FIG. 6 ). RF generator 801 may be a low-frequency RF generator operating at a frequency of about 400 KHz and having an output impedance of about 50 Ω, for example. In one embodiment, coaxial cable 802 has a characteristic impedance of about 50 Ω. RF switch box 606 allows RF power from RF generator 201 to be coupled to either dome 612 or chuck 611 depending on the process to be performed in plasma processing chamber 610 . Fixed-match 207 - 2 is paired with auto-match 203 when RF switch box 606 is configured to switch RF generator 201 signal to chuck 611 . Otherwise, auto-match 203 is paired with fixed-match 207 - 1 . The other components of the configuration of FIG. 8 have already been discussed in connection with FIGS. 2 and 6 .

Improved RF delivery configurations in a plasma processing system have been disclosed. While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.