Abstract:
Described herein are techniques for supplying radio frequency (RF) power to a large area plasma source so as to produce a plasma that is substantially uniform in two spatial dimensions. The RF power may be supplied by a power supply system, which may comprise a RF source and a distribution network. The distribution network may comprise a matching network, and a branching circuit that divides the RF power into several branches. Each of the branches of the distribution network may include a phase shifter that shifts the RF signal (which carries the RF power) by an odd multiple of 90°, and a blocking filter which blocks any harmonics and other unwanted frequencies which are reflected from a plasma source. The output of the branches may be coupled to feed points that are spatially distributed over the one or more electrodes of the plasma source.

Description:
FIELD OF THE INVENTION 
     The present invention relates to systems and methods for distributing radio frequency (RF) power to electrodes of a large area (or linear) plasma source, and more particularly relates to simultaneously distributing the RF power to a plurality of the electrodes and distributing the RF power from one or more RF sources. 
     BACKGROUND 
     Plasma processing of large area substrates (e.g., more than two meters by two meters in size) is needed for a cost-effective manufacturing of display screens, thin film photovoltaic panels, light filters for windows and other large area mass-market products. However, to produce a substrate with properties that are the same over nearly the entire area of the substrate (which is necessary for good product yields), the plasma involved in the processing of the substrate should have approximately constant properties over its full extent in the processing chamber. 
     When high frequency RF power (at frequencies of 13.56 MHz and above) is employed to excite the plasma, the electrical potentials on the electrode (i.e., powered electrode) for forming this plasma may have a quarter-wavelength on the same order of magnitude as the size of the electrode. Consequently, a substantial variation in the amplitude of the RF voltage and current may be present across the electrode, causing a non-constant power injection into the plasma along the length of the electrode. This results from the fact that RF currents propagate as electromagnetic waves on the surface of the metal electrode, in much the same way as a loaded transmission line. The electrical power may be absorbed into the plasma both capacitively and inductively due to shunt or induced currents in the plasma as EM waves propagate along the electrode surface. This variation in the amplitude of the RF voltage across an electrode has been shown to be substantial for parallel plate electrodes when the power is provided at a single contact point on an electrode that extends over the full area of the plasma. For linear electrodes, larger variations occur when power is provided at a single contact point on a linear electrode that is larger than a small fraction of a wavelength. Such non-uniformity in the RF voltage amplitude (and similarly in the RF current) often results in a significant non-uniformity of the process over the surface of the substrate, which is not desirable. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention, RF power (at one or more frequencies) and AC electric current is evenly divided and then distributed (with a high degree of stability) in equal amounts to multiple contact points on parallel plate or linear electrodes. Such even distribution of current enables the formation of plasma with a high degree of uniformity, which in turn enables a high yield in the mass production on large substrates of electrical components and devices (e.g., display screens, thin film photovoltaic panels, etc.). By evenly and stably dividing and distributing the current from power sources, a smaller number of power sources and matching networks are needed, which improves system reliability and lowers system cost. For example, reliability improves because each RF source or matching network has roughly the same mean time between failures (MTBF) regardless of size, so that a power system with fewer RF sources and matching networks will have fewer points to fail and thus a proportionately larger MTBF (i.e., a larger MTBF being more desirable). 
     In accordance with one embodiment of the invention, the power supply system is scalable to a large number (e.g., greater than 30) of feeds (i.e., a feed being an electrical connection) to a substantial number of electrodes (e.g., greater than 8) for one or more RF sources and at a wide range of drive frequencies (e.g., from about 100 kHz to over 100 MHz). The power supply system must maintain an even and stable distribution of the RF current even under a substantial range of power delivered to each of the feeds and under a substantial range of plasma conditions. This stability means that the current delivered to each feed is constant, even if there is a substantial variation in the impedance of an electrode situated in a plasma source or processing chamber. Such variations may be due to the scanning of a substrate or variation in local properties (e.g., surface variations in the substrate or structure around the substrate) due to using a photo mask. The even current distribution results in a substantially constant power density of the plasma along the length and width of any plasma source and in all plasma sources, which in turn causes the rate of a deposition process to remain highly uniform and stable over time when a large area substrate is scanned within the one or more plasma sources. 
     In accordance with one embodiment of the invention, RF power output from one or more impedance matching networks is split among multiple feeds to one or multiple plasma sources. (A plasma source is sometimes called a plasma load, when the plasma source is viewed as a component in an electrical circuit.) This power distribution “splitter” provides parallel circuits for parallel current paths, each of which circuits may have a phase shifter and one or more blocking filters for blocking unwanted frequencies (e.g., frequencies provided by other supplies reflected from the plasma load and/or harmonics of the supply frequency). These parallel circuits may be constructed using inexpensive passive components (such as capacitors and inductors) that can be compact and whose temperature can be well controlled. 
     In accordance with one embodiment of the invention, the phase shifters typically are in a “pi” configuration, but also may be in a “T” configuration. With suitably chosen, passive reactive elements combined in a “pi” or “T” configuration, the output RF current delivered to the terminations (e.g., contact points on electrode(s)) will be equal, regardless of varying magnitudes of the terminating impedances. The inventors found surprisingly that this is the case even with the terminating impedances varying by more than an order of magnitude! Circuit properties may be tuned to provide the proper transformation properties for the current, preferably to shift its phase by odd multiples of 90°, where this multiple need not be the same for all feed points (i.e., feed points being contact points on the electrode(s)). The impedance of the phase shifter for each drive frequency may in some embodiments be within about +/−50% of the load impedance at that feed point at that drive frequency to produce the desired current transformation. “Pi” or “T” phase shifters are preferable to a phase shifter constructed using coaxial delay lines. In the latter case, very long coaxial delay lines would be needed for RF frequencies of 27.12 MHz and below, complicating the temperature control of the coaxial delay lines (which is needed to properly control the impedance of the coaxial delay lines). 
     In accordance with one embodiment of the invention, when multiple RF frequencies are used to drive the plasma source(s), one or more blocking filters are located in each of the branches after the phase shifter. The one or more blocking filters (which may be constructed from simple passive components) may prevent any unwanted frequencies from propagating from the plasma load through the filter to the RF source. The end of each branch from the distribution network for each RF frequency is then connected to the feed lines of an electrode. In one embodiment, multiple frequencies of power are provided to each feed line of each electrode. This results in the equal and highly stable distribution of power at multiple frequencies to multiple electrodes in multiple plasma sources. 
     In accordance with a preferred embodiment of the invention, the parallel current paths of the distribution network may be constructed using printed circuit (PC) boards (with simple and inexpensive passive components assembled thereon). These boards may be compact and stackable in a chassis of appropriate size where adequate space is provided between boards to allow airflow between same. The PC boards may be maintained at a stable operating temperature by air cooling fans so that substantial numbers (e.g., greater than or equal to 8) of such boards may be housed in a single splitter box to provide current at a large number of feed points to complex multi-plasma source configurations. Boards for up to hundreds of feed points could be housed and kept at a stable temperature in such a chassis. The phase shifters and blocking filters may have the same design for all feed points with common component values (e.g., capacitance and inductance values) for a given drive frequency and feed line impedance. 
     Regarding the application to multiple electrodes or plasma sources for which equal RF current is to be provided, several factors may determine the plasma density and rate of ionization and dissociation in a plasma. These factors include the gas density of the plasma, the gap size between the electrodes, and the RF current density across the powered electrode(s). Thus, if the RF current per electrode section (i.e., an electrode may be composed of several electrically isolated sections) is made the same for all sections, the plasma density and ionization/dissociation rates will be the same for all electrode sections, thereby providing a highly uniform process. It is noted that plasma is difficult to control by other passive means since both reactive and resistive parts of the impedance will generally drop as the power to the plasma is increased, which tends to provoke an instability in most approaches to current distribution where more current will then flow to electrodes or sources having lower impedances. 
     In summary, the advantages of one embodiment of the invention include: 
     (1) Simplified process control due to the use of a single RF source and matching network for each RF frequency, which may provide power to as many as ten or more plasma sources. 
     (2) The use of multiple plasma excitation frequencies for a single feed point of RF power, which helps avoids cross-talk, in which currents from one RF power supply are reflected into another RF power supply having a different frequency output, which would interfere with a stable operation of the plasma source. 
     (3) A highly stable distribution of RF power that does not drift during long processes. 
     (4) An improved reliability because fewer RF sources and matching networks are used, resulting in an increased system-level MTBF. 
     (5) The cost of the RF power supply system as a whole is also reduced. 
     These and other embodiments of the invention are more fully described in association with the drawings below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts an apparatus for depositing at least one layer on a substrate, the apparatus comprising at least one RF source, in accordance with one embodiment of the invention. 
         FIG. 2  depicts an apparatus for depositing at least one layer on a substrate, the apparatus comprising at least two RF sources, in accordance with one embodiment of the invention. 
         FIGS. 3 a  and 3 b    each depicts a phase shifting circuit, in accordance with one embodiment of the invention. 
         FIG. 4  depicts components of a computer system in which computer readable instructions instantiating the methods of the present invention may be stored and executed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It should be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. Description associated with any one of the figures may be applied to a different figure containing like or similar components/steps. While the sequence diagrams each present a series of steps in a certain order, the order of some of the steps may be changed. 
       FIG. 1  depicts apparatus  10  for depositing at least one layer on substrate  138 , in accordance with one embodiment of the invention. Apparatus  10  may include electrodes  112 ,  132  and  142  situated within chamber  146 , such electrodes configured to generate plasma or products of plasma decomposition within plasma region  136 . Apparatus  10  may also include power supply system  148 , which may be electrically coupled to electrodes  112  and  132 . Power supply system  148  may comprise at least one RF source  102  configured to generate an RF signal at a first frequency (e.g., ω 1  radians or equivalently, X MHz where ω 1 =2π*X MHz), and distribution network  150  configured to distribute RF power from the RF source to the electrodes. Although not shown in detail, it should be recognized that apparatus  10  may further include elements for the introduction of one or more gasses (e.g., used to form a plasma in plasma region  136 ), vacuum pump or other evacuation means for chamber  146 , and other features common to plasma deposition apparatus. 
     Distribution network  150  may comprise impedance matching network  104  configured to match the impedance of the plasma load, and branching circuit  106  configured to distribute the RF signal into the parallel branches of distribution network  150 . Each of the branches may be coupled to one of electrodes  112  and  132 . As shown, each branch may supply RF power to one feed point of one of the electrodes, and the feed points may be distributed over the surface of an electrode to provide better uniformity of the amplitude of the RF voltage and current across each of the electrodes. 
     Further, each of the branches may comprises a phase shifter (i.e., phase shifter  108   a  in the first branch, phase shifter  108   b  in the second branch, phase shifter  108   c  in the third branch and phase shifter  108   d  in the fourth branch). Each of the phase shifters may cause a phase shift of an odd multiple of 90° in the RF current at the first frequency, ω 1 . As a result of the phase shifters, the output RF current delivered to the terminations (e.g., contact points on electrode(s)) will be substantially equal, regardless of varying magnitudes of the terminating impedances. More details regarding the phase shifters are described with respect to  FIGS. 3 a  and 3 b    below. 
     While electrodes  112  and  132  may be powered, electrode  142  may be grounded. Plasma within plasma region  136  may be formed between electrodes  112  and  142  and between electrodes  132  and  142 . Electrodes  112  and  132  may be interpreted as segments of a larger electrode (such segments being electrically isolated from one another). An electrode that is segmented into smaller electrically isolated portions may provide better uniformity in the amplitude of the RF voltage and current amplitude across the electrodes (as compared to a non-segmented electrode of the same size). While plasma region  136  is depicted “above” substrate  138 , this is for ease of illustration. In practice, it is understood that substrate  138  may be located within plasma region  136  in order to perform a deposition and/or etching process on the substrate. While not depicted, one or more gas sources may supply gas into chamber  146 , and one or more gas exhausts may exhaust gas from chamber  146 . 
       FIG. 2  depicts apparatus  100  for depositing at least one layer on substrate  138 , in accordance with one embodiment of the invention. Apparatus  100  may include electrodes  112 ,  132  and  142  situated within chamber  146 , such electrodes configured to generate plasma or products of plasma decomposition within plasma region  136 . Apparatus  100  may also include power supply system  158 , which may be electrically coupled to electrodes  112  and  132 . Power supply system  158  may include first RF source  102  configured to generate an RF signal at a first frequency, ω 1 ; second RF source  122  configured to generate an RF signal at a second frequency (e.g., ω 2  radians or equivalently, Y MHz where ω 2 =2π*Y MHz); first distribution network  160  configured to distribute RF power from first RF source  102  to one or more of electrodes  112 ,  132 ; and second distribution network  162  configured to distribute RF power from second RF source  122  to one or more of electrodes  112 ,  132 . 
     First distribution network  160  may comprise impedance matching network  104  configured to match the impedance of the plasma load, and branching circuit  106  configured to distribute the RF signal of RF source  102  into the parallel branches of distribution network  160 . Each of the branches may supply RF power to a feed point of one of the electrodes, and the feed points may be distributed over the surface of an electrode to provide better uniformity in the amplitude of the RF voltage and current across each of the electrodes. 
     Each of the branches of first distribution network  160  may comprise a phase shifter and a blocking filter (i.e., phase shifter  108   a  and blocking filter  110   a  in the first branch, phase shifter  108   b  and blocking filter  110   b  in the second branch, phase shifter  108   c  and blocking filter  110   c  in the third branch, and phase shifter  108   d  and blocking filter  110   d  in the fourth branch). Each of the phase shifters  108   a ,  108   b ,  108   c  and  108   d  may cause a phase shift of an odd multiple of 90° in the RF signal at the first frequency, ω 1 . Each of the blocking filters  110   a ,  110   b ,  110   c  and  110   d  may block at least the first harmonic of the first frequency (i.e., 2ω 1 ), as well as frequencies generated by other RF sources (e.g., RF source  122 ) in power supply system  158 , thereby preventing power of other frequencies from interfering with RF source  102 . 
     Second distribution network  162  may comprise impedance matching network  124  configured to match the impedance of the plasma load, and branching circuit  126  configured to distribute the RF signal of RF source  122  into the parallel branches of distribution network  162 . Each of the branches may supply RF power to a feed point of one of the electrodes, and the feed points may be distributed over the surface of an electrode to provide better uniformity in the amplitude of the RF voltage and current across each of the electrodes. 
     Similarly, each of the branches of second distribution network  162  may comprise a phase shifter and a blocking filter (i.e., phase shifter  128   a  and blocking filter  130   a  in the first branch, phase shifter  128   b  and blocking filter  130   b  in the second branch, phase shifter  128   c  and blocking filter  130   c  in the third branch, and phase shifter  128   d  and blocking filter  130   d  in the fourth branch). Each of the phase shifters  128   a ,  128   b ,  128   c  and  128   d  may cause a phase shift of an odd multiple of 90° in the RF signal at the second frequency, ω 2 . Each of the blocking filters  130   a ,  130   b ,  130   c  and  130   d  may block at least the first harmonic of the second frequency (i.e., 2ω 2 ), as well as frequencies generated by other RF sources (e.g., RF source  102 ) in power supply system  158 , thereby preventing power of other frequencies from interfering with RF source  122 . 
     As a specific example, phase shifters  108   a ,  108   b ,  108   c  and  108   d  may be 90° phase shifters configured to operate at X MHz; phase shifters  128   a  and  128   b  may be 90° phase shifters configured to operate at Y MHz; and phase shifters  128   c  and  128   d  may be 270° phase shifters configured to operate at Y MHz. Further, each of filters  110   a ,  110   b ,  110   c  and  110   d  may comprise a filter configured to block an RF signal at 2X MHz and a filter configured to block an RF signal at Y MHz; and each of filters  130   a ,  130   b ,  130   c  and  130   d  may comprise a filter configured to block an RF signal at 2Y MHz and a filter configured to block an RF signal at X MHz. 
     The end of each branch from first distribution network  160  may be connected to one or more feed points of electrodes  112 ,  132 . Similarly, the end of each branch from second distribution network  162  may be connected to one or more feed points of electrodes  112 ,  132 . In one embodiment, two or more frequencies of power may be provided to each feed point of each electrode. 
     In one embodiment of the invention, phase shifters  108   a ,  108   b ,  108   c  and  108   d  and filters  110   a ,  110   b ,  110   c  and  110   d  may be located on a first PC board, and branching circuit  106  may provide RF signals to the first PC board via coaxial cables. Similarly phase shifters  128   a ,  128   b ,  128   c  and  128   d  and filters  130   a ,  130   b ,  130   c  and  130   d  may be located on a second PC board, and branching circuit  126  may provide RF signals to the second PC board via coaxial cables. Alternatively, phase shifters  108   a ,  108   b ,  108   c ,  108   d ,  128   a ,  128   b ,  128   c  and  128   d  and filters  110   a ,  110   b ,  110   c ,  110   d ,  130   a ,  130   b ,  130   c  and  130   d  may all be located on the same PC board. 
     In one embodiment of the invention, power supply system  158  comprises three major subsystems connected in series, each performing one of the three essential functions: 
     The first major subsystem may include branching circuits  106  and  126 , which may be a PC board or a multi-connector branching structure, or branching structure with coaxial cables of equal length on the outputs. The branching circuits may provide power of a frequency or range of frequencies to parallel outputs of the branching circuits  106  and  126 , each of the parallel outputs approximately having the same effective path length. 
     The second major subsystem may include phase shifters  108   a - d  and  128   a - d , each of which cause the phase of the current along each output branch from the branching circuits to advance or regress by an odd multiple of 90° (as measured from the corresponding branching structure). The phase shifter for each branch in some embodiments may be constructed as a “pi” or “T” circuit (as shown below in  FIG. 3 ) employing passive reactive elements, such as capacitors and inductors. Values for the passive reactive elements should be chosen to produce both the desired odd multiple of 90° phase shift, as well as having the desired impedance to match that of the plasma load associated with that part of the electrode (e.g.,  112 ,  132 ) to which it is connected. 
     The third major subsystem may comprise one or more blocking filters (arranged in series) for each branch following the phase shifter (e.g., filters  110   a - d  and filters  130   a - d ). Each blocking filter may have a low impedance at the frequency that is desired to be passed (e.g., the frequency of a RF source), but a very high impedance at each of the unwanted frequencies. This filter in some embodiments may be an LC tank circuit (i.e., an inductor and a capacitor arranged in parallel) that is resonant at the frequency that is to be blocked. If not already apparent, an LC tank circuit has a very high impedance at the resonant frequency, resulting in the blocking of the resonant frequency. 
     It is noted that controller  152  may control the operation of RF sources  102  and  122  (e.g., control frequency, phase and/or amplitude of RF signal generated by the respective RF source). 
       FIGS. 3 a  and 3 b    each depicts a phase shifting circuit, in accordance with one embodiment of the invention.  FIG. 3 a    depicts a “pi” circuit, whereas  FIG. 3 b    depicts a “T” circuit. To produce a 90° phase shift, one may set θ (in the mathematical expressions of  FIG. 3 ) to 90°. Upon simplifying the mathematical expressions, one may calculate an inductance of Z o /ω o  for the inductor(s) and a capacitance of 1/(Z o ω o ) for the capacitor(s), where Z o  is the impedance of the plasma load associated with that part of the electrode (e.g.,  112 ,  132 ) at ω o . 
     As noted above, aspects of the present invention involve the use of a controller which may be instantiated as a processor-based system with a processor-readable storage media having processor-readable instructions stored thereon.  FIG. 4  provides an example of a system  400  that is representative of such a processor-based system. Note, not all of the various processor-based systems which may be employed in accordance with embodiments of the present invention have all of the features of system  400 . For example, certain processor-based systems may not include a display inasmuch as the display function may be provided by a client computer communicatively coupled to the processor-based system or a display function may be unnecessary. Such details are not critical to the present invention. 
     System  400  includes a bus  402  or other communication mechanism for communicating information, and a processor  404  coupled with the bus  402  for processing information. System  400  also includes a main memory  406 , such as a random access memory (RAM) or other dynamic storage device, coupled to the bus  402  for storing information and instructions to be executed by processor  404 . Main memory  406  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  404 . System  400  further includes a read only memory (ROM)  408  or other static storage device coupled to the bus  402  for storing static information and instructions for the processor  404 . A storage device  410 , which may be one or more of a floppy disk, a flexible disk, a hard disk, flash memory-based storage medium, magnetic tape or other magnetic storage medium, a compact disk (CD)-ROM, a digital versatile disk (DVD)-ROM, or other optical storage medium, or any other storage medium from which processor  404  can read, is provided and coupled to the bus  402  for storing information and instructions (e.g., operating systems, applications programs and the like). 
     System  400  may be coupled via the bus  402  to a display  412 , such as a flat panel display, for displaying information to a user. An input device  414 , such as a keyboard including alphanumeric and other keys, may be coupled to the bus  402  for communicating information and command selections to the processor  404 . Another type of user input device is cursor control device  416 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  404  and for controlling cursor movement on the display  412 . Other user interface devices, such as microphones, speakers, etc. are not shown in detail but may be involved with the receipt of user input and/or presentation of output. 
     The processes referred to herein may be implemented by processor  404  executing appropriate sequences of processor-readable instructions stored in main memory  406 . Such instructions may be read into main memory  406  from another processor-readable medium, such as storage device  410 , and execution of the sequences of instructions contained in the main memory  406  causes the processor  404  to perform the associated actions. In alternative embodiments, hard-wired circuitry or firmware-controlled processing units (e.g., field programmable gate arrays) may be used in place of or in combination with processor  404  and its associated computer software instructions to implement the invention. The processor-readable instructions may be rendered in any computer language. 
     System  400  may also include a communication interface  418  coupled to the bus  402 . Communication interface  418  may provide a two-way data communication channel with a computer network, which provides connectivity to the plasma processing systems discussed above. For example, communication interface  418  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN, which itself is communicatively coupled to other computer systems. The precise details of such communication paths are not critical to the present invention. What is important is that system  400  can send and receive messages and data through the communication interface  418  and in that way communicate with other controllers, etc. 
     Thus, methods and systems for distributing RF power to a plasma source have been described. It is to be understood that the above-description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     A diagnostic system may be associated with the RF power distribution system described above. Said diagnostic system may include circuits or devices to make measurements of the voltage or current provided at any subset of the feed points to one or more electrodes. Said diagnostic system may additionally include a computer or control system and one or more software programs or firmware programs that may determine based on the measured voltage or current from one or more of the branches whether the system is operating properly. Said diagnostic software system may have as one of its functions to detect abnormal or undesirable conditions in the plasma. It may also determine what type of abnormal condition is occurring and either terminate the process or calculate what adjustment to the process conditions may rectify the abnormal condition. The control program for the diagnostic system may then make the adjustments and/or alert the operations staff as to the abnormal conditions.