Abstract:
A plasma chamber having a switchable bias frequency superimposed onto plasma source frequency and applied to the cathode. A power supplier capable of generating multiple RF bias frequencies is coupled into a match network through a switch. The match network couples one of the bias frequencies to the cathode. Another match network applied a source RF power to the cathode. One parallel connection of variable shunt capacitor and fixed capacitor are provided between ground and input of the switch and another is connected between ground and the input of the source RF match network.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The subject invention relates to RF power suppliers and matching networks used in plasma chambers and, more specifically, RF power suppliers and matching networks enabling application of multiple-frequency RF power. 
         [0003]    2. Related Art 
         [0004]    Plasma chambers utilizing dual or multiple RF frequencies are known in the art. Generally, a plasma chamber of dual frequencies receives RF bias power having frequency below about 15 MHz, and an RF source power at higher frequency, normally 27-200 MHz. In this context, RF bias refers to the RF power which is used to control the ion energy and ion energy distribution. On the other hand, RF source power refers to RF power which is used to control the plasma ion dissociation or plasma density. For some specific examples, it has been known to operate etch plasma chambers at, e.g., bias of 100 KHz, 2 MHz, 2.2 MHz or 13.56 MHz, and source at 13.56 MHz, 27 MHz, 60 MHz, 100 MHz, and higher. 
         [0005]    Recently it has been proposed to operate a plasma chamber at one bias frequency and two source frequencies. For example, it has been proposed to operate a plasma etch chamber at bias frequency of 2 MHz and two source frequencies of 27 MHz and 60 MHz. In this manner, the dissociation of various ion species can be controlled using the two source RF frequencies. Regardless of the configurations, in the prior art each frequency is provided by an individual RF power supplier and each individual power supplier is coupled to an individual matching network. 
         [0006]      FIG. 1  is a schematic illustration of a prior art multiple frequency plasma chamber arrangement, having one bias RF power and two source RF power generators. More specifically, in  FIG. 1  the plasma chamber  100  is schematically shown as having an upper electrode  105 , lower electrode  110 , and plasma  120  generated in between the two electrodes. As is known, electrode  105  is generally embedded in the chamber&#39;s ceiling, while electrode  110  is generally embedded in the lower cathode assembly upon which the work piece, such as a semiconductor wafer, is placed. As also shown in  FIG. 1 , a bias RF power supplier  125  provides RF power to the chamber  100  via match circuit  140 . The RF bias is at frequency f 1 , generally 2 MHz or 13 MHz (more precisely, 13.56 MHz), and is generally applied to the lower electrode  110 .  FIG. 1  also shows two RF source power suppliers  130  and  135 , operating at frequencies f 2  and f 3 , respectively. For example, f 2  may be set at 27 MHz, while f 3  at 60 MHz. The source power suppliers  130  and  135  deliver power to chamber  100  via match networks  145  and  150 , respectively. The source power may be applied to the lower electrode  110  or the top electrode  105 . Notably, in all of the Figures the output of the match networks is illustrated as combined into a single arrow leading to the chamber. This is used as a symbolic representation intended to encompass any coupling of the matching networks to the plasma, whether via the lower cathode, via an electrode in the ceiling, an inductive coupling coil, etc. For example, the bias power may be coupled via the lower cathode, while the source power via an electrode in the showerhead or an inductive coil. Conversely, the bias and source power may be coupled via the lower cathode. 
       SUMMARY 
       [0007]    The following summary of the invention is intended to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
         [0008]    Various aspects of the subject invention provide plasma chamber having a switchable bias frequency superimposed onto plasma source frequency and applied to the cathode. According to an embodiment of the invention, a power supplier capable of generating multiple RF bias frequencies is coupled into a match network through a switch. The match network couples one of the bias frequencies to the cathode. Another match network applies a source RF power to the cathode. One parallel connection of variable shunt capacitor and fixed capacitor are provided between ground and input of the switch and another is connected between ground and the input of the source RF match network. The fixed capacitor provides protection for its parallel-connected variable shunt capacitor. 
         [0009]    According to an aspect of the invention, an RF matching circuitry is provided having capability for switchably coupling one of two bias frequencies and one source frequency to a cathode. The circuitry comprises: a switch having one input and a first and a second outputs; a first match network having input coupled to the switch&#39;s first output and output coupled to the cathode, the first match network is tuned to operate at a first bias frequency below 10 MHz; a second match network having input coupled to the switch&#39;s second output and output coupled to the cathode, the second match network is tuned to operate at a second bias frequency higher than the first bias frequency but below 15 MHz; and, a third match network having output coupled to the cathode, the third match network is tuned to operate at a source frequency higher than the second bias frequency. The circuitry may also comprise: a parallel resonance circuit coupled between the output of the third match network and the cathode, the parallel resonance circuit can be tuned to be centered at the same frequency as the second bias frequency; a variable shunt capacitor coupled between ground and the input of the switch; a second variable shunt capacitor coupled between ground and the input of the third match network; a fixed capacitor coupled between ground and the input of the switch; and/or a second fixed capacitor coupled between ground and the input of the third match network. 
         [0010]    According to an aspect of the invention, an RF matching circuitry is provided, having the capability for switchably coupling one of two bias frequencies to a cathode. The circuitry comprises: a switch having one input and a first and a second outputs; a first match network having input coupled to the switch&#39;s first output and output coupled to the cathode, the first match network tuned to operate at a first bias frequency below 10 MHz; a second match network having input coupled to the switch&#39;s second output and output coupled to the cathode, the second match network tuned to operate at a second bias frequency higher than the first bias frequency but below 15 MHz; a variable shunt capacitor coupled between ground and the input of the switch; and a fixed capacitor coupled between ground and the input of the switch. 
         [0011]    According to another aspect of the invention, a plasma processing chamber operable with two switchable RF bias power is provided, comprising: a chamber body configured to maintain plasma within an evacuated interior; a cathode configured for coupling RF energy to the plasma; a first RF power generator operable to selectively generate either a first bias frequency below 10 MHz or a second bias frequency higher than the first bias frequency but below 15 MHz; a switch having an input coupled to the first RF power generator and having a first and second outputs; a first match network having input coupled to the switch&#39;s first output and output coupled to the cathode, the first match network tuned to operate at the first bias frequency; a second match network having input coupled to the switch&#39;s second output and output coupled to the cathode, the second match network tuned to operate at the second bias frequency; a second RF power generator operable to generate RF source power at frequency higher than 15 MHz; and, a third match network having input coupled to the second RF power generator and output coupled to the cathode. In one example the first bias frequency is about 2 MHZ, the second bias frequency is about 13 MHz, and the source power frequency is one of 27 MHz, 60 MHz, and 100 MHz. The circuitry may further comprise a parallel resonance circuit coupled between the output of the third match network and the cathode, the parallel resonance circuit tuned to be centered at about 13 MHz with a 2 MHz band. The circuitry may further comprise parallel connection of a variable shunt capacitor and a fixed capacitor coupled between ground and the input of the switch. The circuitry may further comprise parallel connection of a variable shunt capacitor and a fixed capacitor coupled between ground and the input of the third match network. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
           [0013]      FIG. 1  is a schematic illustration of a prior art multiple frequency plasma chamber arrangement, having one bias RF power and two source RF power generators. 
           [0014]      FIG. 2  is a schematic illustration of a first embodiment of the invention of a multiple frequency plasma chamber arrangement, having two bias RF power and one source RF power generators. 
           [0015]      FIG. 3  depicts an embodiment of an RF power matching circuitry. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 2  is a schematic illustration of an embodiment of the invention of a multiple frequency plasma chamber arrangement, having two switchable RF bias power coupled to a match network. In  FIG. 2 , two RF bias power suppliers  225  and  255  provide switchable f 1  and f 2  RF bias power to the chamber  200  via switch  232  that is coupled to match circuits  240  and  245 , respectively. The RF bias is at frequency f 1 , generally 2 MHz or 2.2 MHz, while the RF bias frequency f 2  is generally 13 MHz (more precisely, 13.56 MHz). Both RF bias are generally applied to the lower electrode  210 . In this manner, an improved ion energy control is enabled. For example, for higher bombardment energy, such as for front-end etch applications, the 2 MHz source is used, while for softer bombardment, such as for back-end etch application, the 13 MHz bias is utilized.  FIG. 2  also shows a source RF power supplier  235 , operating at frequency f 3 , for example, 27 MHz, 60 MHz, 100 MHz, etc. The source power  235  is delivered to chamber  200  via match network  250  and is applied to the lower electrode  210 . The source power is used to control the plasma density, i.e., plasma ion dissociation. 
         [0017]    The arrangement of  FIG. 2  enables superimposed application of either f 1 /f 3  or f 2 /f 3  frequencies to the chamber. For example, f 1  can be 400 KHz to 5 MHz; f 2  can be 10 MHz to 20 MHz, but normally less than 15 MHz; and f 3  can be 27 MHz to 100 MHz or over. In one particular example, f 1  is 2 MHz, f 2  is 13.56 MHz, and f 3  is 60 MHz. Such an arrangement makes it very easy to run recipes that require switching between low and high frequency bias power in mid processing. 
         [0018]      FIG. 3  illustrated an embodiment of the match circuitry wherein two out of three available frequencies are switchably applied to the cathode of a plasma chamber. A high frequency, f 3  is coupled to the cathode via a match circuit  334  and a parallel resonance circuit  330 , while two lower frequencies, f 1  and f 2 , are coupled to switch  332  that switchably couples only one of them to the cathode via either of match circuits  320  or  322 . In this embodiment, the two RF frequencies f 1 /f 2  are provided by a single RF power generator that can switchably operate at either frequency f 1  or frequency f 2 . Each of the match circuits is made of a series connection of a capacitor and an inductor. In one example, match circuit  320  has a capacitor having values of 200-500 pF and an inductor at about 20-50 mH; match circuit  322  has a capacitor having values of 50-200 pF and an inductor at about 0.5-5 mH; match circuit  320  has a capacitor having values of about 25 pF and an inductor at about 0.2-0.3 mH. 
         [0019]    The parallel resonance circuit  330  is provided in order to prevent energy from the 13.56 MHz power source to flow into the 60 MHz source. That is, when the switch  332  couples the 2 MHz bias source, the bias frequency is thirty times smaller than the plasma source frequency of 60 MHz, so it cannot jump the match network  334 . However, when the switch  332  couples the 13.56 MHz bias power, the bias frequency is much closer to the plasma source frequency f 3  and it may jump the match network  334 . Therefore, a parallel resonance circuit is provided and is made of a parallel connection of a capacitor and inductor. In this example, where f 1 =2 MHZ, f 2 =13.56 MHz, and f 3 =60 MHZ, the parallel resonance circuit  330  is centered at 13 MHz, with variance or band of Δf=2 MHZ. This prevents bias frequency 13.56 from leaking into source power supplier F 3 . The resonance circuit acts as a short circuit to the 60 MHz frequency. 
         [0020]    In the example of  FIG. 3 , a variable shunt capacitor  305  is coupled before the switch  332 , such that it is common to both match networks  320  and  322 , depending on which is engaged by switch  332 . Another variable shunt capacitor  315  serves the match network  334  for f 3 . In this embodiment, both shunt capacitors are implemented using variable vacuum capacitors. Also, in this embodiment special protection measures are implemented to protect the variable shunt capacitors. A fixed capacitor  300  is coupled in parallel to shunt capacitor  305 . Fixed capacitor  300  protects shunt capacitor  305  from high RF currents when variable capacitor  305  is set for low capacitance values. Conversely, fixed capacitor  310  is coupled in parallel with variable capacitor  315 . Fixed capacitor  310  protects shunt capacitor  315  from high RF currents when variable capacitor  315  is set for low capacitance values. In this example variable shunt capacitor  305  can be varied from about 30 pF to 1500 pF and capacitor  300  is selected as about 100 pF. Similarly, in this example variable shunt capacitor  315  can be varied from about 10 pF to 150 pF and capacitor  310  is selected as about 120 pF. 
         [0021]    Any of the above embodiments can be used to operate a plasma chamber to provide a processing having a first period operating at a first bias frequency and a second period operating at a second bias frequency. For example, the chamber may be operated using a low bias frequency, e.g., about 2 MHz for the main etch step; however, in order to create a “soft landing” during the over etch the system may be switched to operate using a higher frequency bias, such as, e.g., about 13 MHz. 
         [0022]    Finally, it should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein. The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. For example, the described software may be implemented in a wide variety of programming or scripting languages, such as Assembler, C/C++, perl, shell, PHP, Java, etc. 
         [0023]    The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in the plasma chamber arts. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.