Abstract:
A system and method for maintaining a plasma in a plasma region by supplying RF power at a fundamental frequency to the plasma region together with a gas in order to create an RF electromagnetic field which interacts with the gas to create a plasma that contains electromagnetic energy components at frequencies that are harmonics of the fundamental frequency. The components at frequencies that are harmonics of the fundamental frequency are reduced by placing RF energy absorbing resistive loads in energy receiving communication with the plasma, the resistive loads having a frequency dependent attenuation characteristic such that the resistive loads attenuate electrical energy at frequencies higher than the fundamental frequency more strongly than energy at the fundamental frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application No. PCT/US02/37326, filed on Nov. 21, 2002, which, in turn, relies upon and claims benefit of the filing date of U.S. Provisional Patent Application No. 60/337,171, filed Dec. 10, 2001, the contents of both which are incorporated herein by reference in their entireties. 
     The present application is related to co-pending applications entitled “Device and Method For Coupling Two Circuit Components Which Have Different Impedances”, PCT Application US01/40073, filed Feb. 9, 2001, now U.S. Pat. No. 6,700,458, and “Method and Device for Attenuating Harmonics in Semiconductor Plasma Processing Systems”, PCT Application US01/04135, filed Feb. 9, 2001, now U.S. Patent Publication No. US-2003-0057844. Each of these applications is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     In the past, various techniques have been proposed for selective attenuation of harmonic frequencies created in plasma processing systems. These techniques utilize either a low-pass filter or a trap circuit. For example, U.S. Pat. No. 5,302,882 entitled “LOW-PASS FILTER FOR PLASMA DISCHARGE” discloses such techniques. 
     In the plasma processing industry, capacitively coupled plasma sources are widely used for dry etching and plasma enhanced chemical deposition. Dry etching is a process for removing a layer of material from a wafer surface. This removal is a result of combined mechanical and chemical effects of high-energy plasma ions striking the substrate surface In plasma enhanced chemical deposition, a layer of a material is deposited on the substrate surface. This material is introduced into the plasma either by sputtering a target made of the material or by supplying a gas which contains the material or from which the material is produced by a chemical reaction. The material may be ionized by the plasma and can then be attracted to the substrate by an electric field. 
     Plasma processing is commonly used in the semiconductor fabrication industry. The trend in the semiconductor fabrication industry has been toward integrated circuits having smaller elemental features. As a result, etch and deposition rate uniformity over the wafer surface has become more important, particularly when a layer is being etched or deposited according to a pattern. At the same time, recent developments in plasma source technology have led to the increased use of very high frequency RF excitation, e.g., from 60 to 300 MHz, and possibly even higher, to initiate and sustain the plasma. 
     The use of these very high excitation frequencies provides a benefit in the form of increased power coupling to the plasma, and thus excitation efficiency, that is likely caused by an increase of plasma electron temperature. This increase of RF power coupling affects the plasma density and the harmonic generation in the plasma. However, maintaining high etching and deposition rate uniformity levels at these very high excitation frequencies and with strong harmonics present has proven to be a difficult feat, for a number of reasons. 
     For example, as the plasma RF excitation frequency is increased, the wavelength of the RF wave decreases. Thus, RF electromagnetic field spatial variations are more pronounced at these higher frequencies and this adversely affects process uniformity. In addition, another trend in the industry is to process larger wafers, 300 mm diameter wafer technology presently being implemented. Of course, as wafer diameter increases, the wavelength-to-wafer-diameter ratio decreases. 
     Plasma acts as a nonlinear RF circuit element and thus acts as a source of harmonics of the fundamental excitation frequency. These harmonics, due to their higher frequencies, have an even higher power coupling efficiency to the plasma than the fundamental. Therefore, harmonics, even if present at very low power levels, can significantly affect process uniformity due to their very unfavorable wavelength-to-wafer-diameter ratio. 
     Since harmonics of the RF fundamental excitation frequency have comparatively short wavelengths, they are far more likely to set up resonances in various places in the process chamber, RF transmission lines, cavities, etc., since their half-wavelengths are comparable to the dimensions of these places. 
     The situation is further worsened by the use of components made of high permittivity (ε) and/or permeability (μ) materials, or by the presence of RF transmission structural features that have significant series inductance (L) and/or shunt capacitance (C). Both of these effects reduce the wavelength of the propagating electromagnetic wave in a structure, the former by directly changing the wave propagation velocity, the latter by creating a “slow-wave” structure. This wavelength reduction allows harmonics to resonate in places where they normally would not. 
     It can thus be seen that reduction of the power content of the harmonics of the RF excitation frequency would improve etch or deposition uniformity. 
     BRIEF SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to control the power levels of harmonics of the fundamental frequency of the RF excitation power in plasma processing systems. 
     The above and other objects are achieved, according to the present invention, by a plasma processing system composed of a chamber enclosing a plasma region, a source of RF power having a fundamental frequency and means for transmitting the RF power from the source into the plasma region for establishing an RF electromagnetic field which interacts with a gas in the plasma region to create a plasma; and energy. Energy controlling members that include RF absorbing loads are disposed in energy-receiving communication with the plasma region. The RF absorbing loads have a frequency dependent attenuation characteristic such that the RF absorbing loads remove electrical energy appearing in the plasma at frequencies higher than the fundamental frequency more strongly than energy at the fundamental frequency. 
     Objects according to the invention are also achieved by method for maintaining a plasma in a plasma region, which method includes supplying RF power at a fundamental frequency to the plasma region together with a gas in order to create an RF electromagnetic field which interacts with the gas to create a plasma that contains electromagnetic energy components at frequencies that are harmonics of the fundamental frequency, and removing those components from the plasma, wherein the step of removing is carried out by placing RF absorbing loads in energy-receiving communication with the plasma, the loads having a frequency dependent attenuation characteristic such that the loads remove electrical energy appearing in the plasma at frequencies higher than the fundamental frequency more strongly than energy at the fundamental frequency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a simplified block diagram of a plasma processing system in accordance with a preferred embodiment of the present invention; 
         FIG. 2   a  shows a simplified block diagram of a trapping assembly in accordance with a preferred embodiment of the present invention; 
         FIG. 2   b  is a cross-sectional view of a trapping assembly in accordance with a preferred embodiment of the present invention; 
         FIG. 3  illustrates a simplified schematic representation of a frequency selective trap element in accordance with a preferred embodiment of the present invention; 
         FIG. 4  shows an alternate embodiment of the present invention in which a trapping assembly is coupled between a match network and a lower electrode; and 
         FIG. 5  shows an alternate embodiment of the present invention in which a trapping assembly comprising a plurality of transmission lines is coupled between a match network and an upper electrode. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A plasma processing system of the type to which this invention is applied includes a chamber which encloses a plasma region filled with an ionizable gas and into which RF electromagnetic energy is coupled. The energy interacts with the gas to initiate and sustain a plasma. According to the invention, one or more components are provided to control the energy contained in harmonics of the fundamental frequency of the RF energy coupled into and out of the plasma. This harmonic attenuation can take place wherever a suitable impedance-matched coupling structure is present, or can be provided to couple the harmonic power out of the plasma. 
     In one embodiment, frequency selective trap elements are provided, that selectively absorb power associated with certain harmonics while not affecting the others. Desirably, resistive loads are coupled to the transmission line, which delivers the RF electromagnetic energy to the plasma. All harmonics are generated at impedances different from the impedance of the fundamental frequency, and every harmonic has a different impedance at which it can be attenuated. In order to be effective at trapping different harmonics, the impedance of the trapping assembly must be variable. So not only is the trap frequency selective, but its input impedance is also variable and matched to the impedance of the harmonics that need to be controlled at that frequency. As the input of the matching network is changed, the frequency of the trapping assembly is also changed. As the plasma density or plasma species changes, the impedance of the harmonics will also change. Therefore, the trapping networks and the matching networks have to be tunable. 
     The design and implementation of a plurality of resistive loads and associated trapping networks that are under automatic control allows precise tailoring of the harmonic content in the plasma. The presence of matching networks is implicitly necessary because of the need to have some physical connection to the electrode so that electrical power can be applied. Thus, a plurality of resistive loads and associated networks are under automatic control so as to allow precise tailoring of the harmonic content of the plasma. 
     This invention further includes a method of using the trapping network as a plasma harmonic detector to feed back the variations of the harmonics to the controller controlling the trapping network. The plasma harmonic detector detects the spectral content and spatial variations of the RF field in the plasma. The feedback signals from the plasma harmonic detector will adjust the matching networks to minimize a particular function. That particular function can be the spectral and spatial variations of certain harmonics at certain frequency. There is a multiplicity of the small matching networks around the electrode. By using the plasma harmonics detector with a specific algorithm, each of the matching networks can be tuned to achieve the best plasma uniformity results. 
     This invention still further includes a method of using the apparatus as a process reliability detector by measuring the voltage across the resistive elements in the trap. By monitoring the amount of the power dissipated by the resistive element, a very precise evaluation of the plasma process conditions is made. Measuring the amount of power the plasma available in its harmonic range makes a very subtle and precise measurement of the condition of the plasma. 
       FIG. 1  shows a simplified block diagram of a plasma processing system in accordance with a preferred embodiment of the present invention. Plasma processing system  100  comprises plasma excitation RF source  102  that supplies RF power at a fundamental frequency and a match network  104 . Trapping assembly  106  is coupled between match network  104  and an upper electrode  108 , which is located at the top of a plasma chamber  110 . Plasma chamber  110  encloses a plasma region in which plasma  112  will be initiated and maintained. A wafer chuck  114  is located at the bottom of the plasma region and is connected to a second RF source  116  via a second match network  118 . Electrodes  108 ,  114  and sources  102 ,  116  form a capacitively coupled RF plasma source that is used for performing an etch or deposition operation on a wafer mounted on chuck  114 . Source  116  acts primarily to impose a DC self-bias on wafer chuck  114 , which self-bias acts to attract ions to the surface of the wafer mounted on chuck  114 . Trapping assembly  106  is located on the main RF feed line to electrode  108 . 
     Controller  130  is coupled to trapping assembly  106 . Controller  130  receives measurement data from trapping assembly  106  and sends control data to trapping assembly  106 . Controller  130  processes a portion of the measurement data to create control data. For example, the controller can perform a fast Fourier transform (FFT). In addition, controller  130  is used to control system operations and monitor the process. Controller  130  can comprise a computer or embedded processor, such as a digital signal processor (DSP). These types of processors are known to those skilled in the art. 
     Plasma  112  can be excited and maintained by RF electromagnetic wave energy at the fundamental RF frequency that is passed to upper electrode  108  and plasma  112  by match network  104  and trapping assembly  106 . Trapping assembly  106  comprises a transmission line that is essentially transparent to RF electromagnetic wave energy at that frequency. Plasma  112 , in turn, converts some of the energy that it receives at the fundamental frequency into harmonics, and these are coupled back into upper electrode  108  and trapping assembly  106 . Energy at harmonic frequencies is strongly attenuated in the resistive loads of trapping assembly  106 , and a significant amount of this energy is dissipated in the form of heat along the length of trapping assembly  106 . The reduction of power at harmonic frequencies results in better electric field uniformity at and below upper electrode  108 , and thus better etch and deposition uniformity. 
       FIGS. 2   a  and  2   b  show a simplified block diagram of a trapping assembly in accordance with a preferred embodiment of the present invention. Trapping assembly  106  comprises transmission line  170  and a plurality of frequency selective trap elements  172 . Transmission line  170  has a frustoconical coaxial geometry. This geometry primarily serves to reduce reflection points between match network  104  and upper electrode  108 . Preferably, transmission line  170  has a constant characteristic impedance, which also helps to reduce reflections. By making the ratio of the outer diameter to the inner diameter of transmission line  170  constant, a constant characteristic impedance is maintained. Alternately, the impedance of transmission line  170  can vary along its length. 
     Transmission line  170  comprises inner conductor  174  and outer conductor  176 . Transmission line  170  can comprise any suitable configuration including a coaxial line, microstrip, or strip-line. 
     Outer conductor  176  comprises a conically shaped sheet of low-loss conducting material such as copper, silver-plated copper, aluminum, or silver-plated aluminum. Outer conductor  176  is coupled to element  199 . Element  199  is part of the process chamber wall and supports trapping assembly  106 . Outer conductor  176  is coupled to ground via element  199 . 
     Inner conductor  174  comprises a conically shaped block of low-loss conducting material such as copper, silver-plated copper, aluminum, or silver-plated aluminum. Inner conductor  174  is coupled to cooling plate  120 , and cooling plate  120  is coupled to electrode  108 . Inner conductor  174  comprises at least one cooling channel, as described below. 
     Frequency selective trap elements  172  are electrically coupled to both inner conductor  174  and outer conductor  176 . Frequency selective trap elements  172  on the transmission line are tuned to harmonic frequencies to selectively monitor and control the harmonic content of the plasma. Frequency selective trap elements  172  are arranged in the space outside the outer conductor  176  and are in electrical contact with the inner conductor  174  through an opening in the outer conductor  176 . Alternately, frequency selective trap elements  172  can be positioned between the inner conductor  174  and the outer conductor  176 . 
     Conductors  174 ,  176  and the above-mentioned cooling channel are all axially symmetrical in this embodiment although they do not necessarily need to be. Outer conductor  176  constitutes a RF ground return terminal. The usual two match network output terminals are connected to inner conductor  174  and outer conductor  176 , respectively. This is achieved by mounting a match network output capacitor  128  directly on top of the inner conductor  174 . Outer conductor  176  is connected within the enclosure of match network  104 , which enclosure serves as a ground conductor. 
     Upper electrode  108  is of the shower head type, provided with a plurality of passages (not shown) for delivery of process gas to the plasma region from a plenum  129  enclosed between electrode  108  and cooling plate  120 . The plenum is supplied with process gas by a gas feed line  132 . Gas feed line  132  is connected to a process gas source and extends along the vertical axis of the frustoconic outline of transmission line  170 . 
     The lower surface of electrode  108 , the surface which faces the plasma region, is covered with a shower-head plate  136 , i.e., a plate provided with gas passages aligned with passages. Plate  136  may be made of material compatible with the chamber process, e.g., doped silicon. Plate  136  acts to prevent sputtering of material from electrode  108 . In addition, silicon plate  136  is made of a material compatible with the chamber process, to prevent contamination, and as such acts to separate the plasma from the lower surface of electrode  108 . This is particularly advantageous when electrode  108  contains a material that is not chemically compatible with the process. 
     An alumina dielectric ring insulator  198  serves to extend coax transmission line below trapping assembly  106  and around cooling plate  120  and electrode  108 . The part of the transmission line constituted by insulator  198  does not absorb any RF and acts as a connection between the plasma and the trapping assembly  106 . Insulator  198  constitutes the dielectric of a coax line whose walls are metallic parts provided by cooling plate  120 , electrode  108 , and the chamber structure, a portion of which is shown as element  199 . 
     A quartz shield ring  138  is attached around plate  136  and below electrode  108 . Quartz shield ring  138  is provided to cover the screws that are used to attach silicon plate  136  to electrode  108 , thereby isolating those screws from the plasma environment to prevent process contamination. Electrode  108 , plate  136  and ring  138  are all attached to, and supported by, cooling plate  120 , which is in turn supported by insulator ring  198 , the latter itself being supported by the chamber wall structure  199 . 
     Cooling of the inner conductor  174  is performed through a coolant fluid circulated through a cooling channel  140  formed in inner conductor  174 . Cooling channel  140  is annular in shape and communicates with a coolant fluid source and a heat exchange element via inlet and outlet cooling lines  142 . As noted earlier herein, cooling channel  140  is axially symmetrical. The coolant fluid in channel  140  also acts to cool upper electrode  108 . 
     Match network  104  (details of which are not shown) is mounted on top of trapping assembly  106 , and all cooling and gas feed connections are made within its RF enclosure. Match network  104  can be constructed according to principles well known in the art. 
       FIG. 3  illustrates a simplified schematic representation of a frequency selective trap element in accordance with a preferred embodiment of the present invention. In the illustrated embodiment, frequency selective trap element  172  comprises input port  310  connected to inner conductor  174 , output port  312  connected to outer conductor  176 , control port  314 , transmission line  316 , coupling capacitor  318 , match network  320 , resistive load  322 , and probe  330 . In the illustrated embodiment, control port  314  is coupled to match network  320  and probe  330 . Alternately, other configurations can be envisioned. 
     Control port  314  is coupled to controller  130  and comprises both control and sensor functions. Control port  314  is configured using at least one shielded cable. Resistive load  322  comprises at least one high power resistor that is mounted on a thermally conductive surface, such as the outer conductor. 
     Match network  320  comprises a plurality of narrow band components, and wideband components. For example, variable capacitors and variable inductors can be used, or at higher frequencies, stub tuners and hybrid networks can be used. Match network  320  allows each frequency selective trap element  172  to be tuned to a particular harmonic frequency. For example, a control voltage can be provided to at least one varactor diode or at least one variable capacitor. Matching network techniques are known to those skilled in the art. In addition, match network  320  can provide measurement data from load resistor  322  and/or from match network  320  to controller  130 . For example, measurement data can include voltage, current, and/or power data. 
     Desirably, probe  330  provides measurement data that includes voltage and current information from transmission line  316 . Alternately, measurement data can include magnitude and phase information. Controller  130  uses the measurement data to determine which frequency components are present and sends control data to match network  320 . Desirably, match network  320  is tuned to the proper frequency, and the desired signal level is achieved at load resistor  322 . Alternately, the desired signal level can be achieved at match network  320  or probe  330 . 
     One or more frequency selective trap element  172  is used for each harmonic signal being controlled. Controller  130  is coupled to each one of the frequency selective trap elements  172  and tunes the match networks in all of the frequency selective trap elements  172  in the trapping assembly to achieve the proper harmonic profile. Desirably, proper harmonic profiles can be determined using experimental data from processes providing uniform etch rates. For example, historical data correlating process results to harmonic profiles can be used to produce algorithms for controller  130 . Harmonic profiles include fundamental and harmonic signal information. 
     Also, controller  130  controls the operating levels of the RF sources used to generate the plasma. Controller  130  can adjust these operating levels to control the power delivered to the plasma at the fundamental frequency and to a lesser degree the harmonic levels. For example, controller  130  may have to increase the power delivered to the plasma at the fundamental frequency in order to maintain the desired plasma density. In addition, controller  130  controls the operating frequencies of the RF sources used to generate the plasma and can tune the operating frequencies to further control the harmonic profile. Those skilled in the art will also recognize that controller  130  controls match networks  104  and  108  ( FIG. 1 ) and can use these system level match networks to control the harmonic profile. By controlling the fundamental level and the harmonic levels, controller  130  generates a high density, uniform plasma. 
       FIG. 4  shows an alternate embodiment of the present invention in which a trapping assembly is coupled between a match network and a lower electrode. Lower electrode comprises a wafer chuck for supporting wafer  470  while a plasma process is performed. RF power is supplied to match network  418  by power source  416 . 
     Trapping assembly  406  comprises transmission line  480  and a plurality of frequency selective trap elements  472 . Transmission line  480  is a coaxial transmission line comprising inner conductor  474 , outer conductor  476 , and dielectric layer  478 . At least one frequency selective trap element  472  is coupled between and in electrical contact with conductors  474  and  476 . 
     Frequency selective trap elements  472  selectively controls the amount of energy which arises within the plasma at frequencies that are harmonics of the fundamental frequency produced by power source  416  and also all other frequencies in the chamber associated with upper electrode plasma excitation (e.g., fundamental and harmonics of upper electrode), and which is conducted to trapping assembly  406  via chuck  414 , after being coupled into chuck  414  from the plasma. 
       FIG. 5  shows an alternate embodiment of the present invention in which a trapping assembly comprising a plurality of transmission lines is coupled between a match network and an upper electrode. Trapping assembly  506  comprises a plurality of transmission lines  570  and a plurality of frequency selective trap elements  572 . Desirably, at least one frequency selective trap element  572  is coupled to each transmission line  570 . Transmission line  570  comprises first conductor  574 , second conductor  576 , and dielectric  578 . Transmission lines  570  can comprise any suitable configuration including coaxial line, microstrip, or strip-line. Transmission lines  570  can have different physical characteristics. 
     One or more frequency selective trap elements can be tuned to selectively control the amount of energy, which arises within the plasma chamber at frequencies that are harmonics of the fundamental frequency. In addition, when multiple transmission lines are used in a trapping assembly, the transmission lines can be designed to make the trapping assembly more efficient. 
     Alternately, the transmission lines can also comprise an absorber material, which can be used to further control the harmonic levels. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.