Abstract:
A method for efficient plasma etching of surfaces inside three-dimensional structures can include positioning an inner electrode within the chamber cavity; evacuating the chamber cavity; adding a first inert gas to the chamber cavity; regulating the pressure in the chamber; generating a plasma sheath along the inner wall of the chamber cavity; adjusting a positive D.C. bias on the inner electrode to establish an effective plasma sheath voltage; adding a first electronegative gas to the chamber cavity; optionally readjusting the positive D.C. bias on the inner electrode reestablish the effective plasma sheath voltage at the chamber cavity; etching the inner wall of the chamber cavity; and polishing the inner wall to a desired surface roughness.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 61/982,017, filed on Apr. 21, 2014, the entirety of which is incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    The present invention was made in the course of federally sponsored research or development pursuant to U.S. Department of Energy Project RF #325111. The United States Government may have certain Walk-In-Rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The invention relates generally to a method for plasma etching of surfaces and more specifically to a method for efficient plasma etching of surfaces inside three-dimensional structures. 
       BACKGROUND 
       [0004]    Generally, in reactive ion etching processes, an electric field plays a fundamental role. The chemistry in many discharges is affected strongly by the ion flux and energy distribution in the sheath and at the surface of walls and electrodes, which are in turn determined by the sheath electric field. Using measurements of electric fields in plasmas to provide a direct insight into the physics of discharges can be experimentally demanding. Moreover, the control of the electric field based on measurements is almost impossible to establish by feedback mechanisms. Therefore, the electric field in the sheath has to be controlled by external parameters, such as radiofrequency power and frequency combined with d.c. bias, radiofrequency circuit parameters, electrode dimensions and shape, pressure and composition of gas mixture. In order to control the electric field in the sheath with an external parameter, a link between one or more external parameters and a particular component of the electric field must be established. 
         [0005]    Corrugated electrodes have been used in a number of applications in a variety of research areas, including such disparate fields as the development of biomedical and environmental chemistry devices or the development of capacitors, but were rarely applied to asymmetric discharges. In those rare cases where corrugated electrodes were applied to asymmetric discharges, the corrugated electrodes were applied in planar geometry with the aim to increase the average sheath thickness, reduce the electron and ion flux to the surface, and decrease the density of power dissipated into the electrode material. The driven electrode expansion concept is aimed to reduce the asymmetry of the dissipated power and the asymmetry in sheath voltage which is illustrated by the sheath voltage ratio relationship with the surface area ratio, as expressed in and decrease the density of power dissipated into the electrode material. The driven electrode expansion concept is aimed to reduce the asymmetry of the dissipated power and the asymmetry in sheath voltage which is illustrated by the sheath voltage ratio relationship with the surface area ratio, as expressed in 
       Equation (1) 
       [0006]    
       
         
           
             
               
                 
                   
                     
                       V 
                       1 
                     
                     
                       V 
                       2 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           A 
                           2 
                         
                         
                           A 
                           1 
                         
                       
                       ) 
                     
                     n 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where V is the sheath voltage, A is the electrode surface area, n is between 1.3 and 3 in the present set-up, and the indices “1” and “2” refer to the driven and the processed electrode, respectively. 
         [0007]    The driven electrode surface expansion concept has never been applied to an asymmetric discharge with a cylindrical coaxial geometry. A need, therefore, exists for a system and a method to apply the driven electrode surface expansion concept to an asymmetric discharge with a cylindrical geometry. 
         [0008]    Additionally, it is known that in wet etching processes the etching rate increases with the temperature. However, increasing the etching rate by increasing the temperature has never been attempted in dry plasma processes. A need, therefore, exists for a system and a method for increasing the etching rate by increasing the temperature of a dry plasma process. 
       SUMMARY 
       [0009]    According to various embodiments, the method can include positioning an inner electrode within the chamber cavity; evacuating the chamber cavity; adding a first inert gas to the chamber cavity; regulating the pressure in the chamber; generating a plasma sheath along the inner wall of the chamber cavity; adjusting a positive D.C. bias on the inner electrode to establish an effective plasma sheath voltage; adding a first electronegative gas to the chamber cavity; optionally readjusting the positive D.C. bias on the inner electrode reestablish the effective plasma sheath voltage at the chamber cavity; etching the inner wall of the chamber cavity; and polishing the inner wall to a desired surface roughness. 
         [0010]    Application of plasma etching to Nb cavities, according to various embodiments, can have at least two major benefits in the cost reduction of the next generation particle accelerators. The first involves the potential increase in consistency in performance in the quality factor Q, which is defined as a stored energy divided by the dissipated energy within one RF cycle and gradient achieved from cavity to cavity. This is possible since the final stage of plasma etching produces a pure niobium surface free from sub-oxides and residue from wet chemistry. A variety of superior high quality surfaces can be intentionally created through plasma processing, such as pure niobium pentoxide without suboxides, or superconducting niobium nitride, which has excellent stability as a diffusion barrier and a low secondary electron yield. Such surface modifications can be done in the same process cycle with the plasma etching process. The second cost benefit of plasma etching arises from its nearly insignificant process cost compared to wet chemistry, not only in terms of basic process costs but also in terms of impact on the environment and personnel safety. The proposed work will demonstrate that plasma etching provides (a) high etching rates; (b) low surface roughness; (c) better control of the final SRF surface; (d) improved RF performance at substantially lower operational costs; and (e) reduced environmental hazard due to the use of hydrofluoric acid in the chemical bath. 
         [0011]    Gas plasma etching method according to various embodiments can result in the improved surface features and lower surface roughness resulting in superior RF performance and higher-gradient SRF cavities, simplified, cleaner, and less expensive manufacturing process (compared to conventional wet chemical etching processes). 
         [0012]    The use of reactive gases containing chlorine (Cl) atoms was a natural choice for the plasma treatment of Nb, since Nb compounds with Cl have a high vapor pressure and low boiling temperature. All experiments on thin films were performed using low-frequency RF discharges at low or moderate pressures. Depending on experimental conditions, etching rates varied from a few nm/min in the case of physical sputtering to a few hundred nm/min during reactive ion etching. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other features, aspects, and advantages of the present invention will become better understood with reference to the following description and appended claims, and accompanying drawings where: 
           [0014]      FIG. 1 : is a photograph of an RF plasma etching system according to various embodiments; 
           [0015]      FIG. 2 : is a photograph of ring samples with the diameter of the outer electrode; 
           [0016]      FIG. 3 : is a photograph of a structure to be etched according to various embodiments; 
           [0017]      FIG. 4 : is a photograph of a structure to be etched according to various embodiments; 
           [0018]      FIG. 5 : is a photograph of a plurality of structures to be etched according to various embodiments 
           [0019]      FIG. 6 : is a schematic illustration of a tube; 
           [0020]      FIG. 7 : is a schematic illustration of a large pitch bellows tube; FIG. FIG. 
           [0021]      FIG. 8 : is a schematic of a small pitch bellows tube; 
           [0022]      FIG. 9 : is a schematic illustration of a disc loaded tube; 
           [0023]      FIG. 10 : is a photograph of a driven electrode positioned at the axis of a cylindrical vessel; 
           [0024]      FIG. 11 : is a photograph of a driven electrode positioned at the axis of a cylindrical vessel, during an RF discharge; 
           [0025]      FIGS. 12 a  and 12 b   : are schematic illustrations of a device for moving the inner (driven) electrode inside the structure which is supposed to be etched; 
           [0026]      FIG. 12 c   : is a schematic illustration of a device for moving the inner (driven) electrode inside the structure which is supposed to be etched; 
           [0027]      FIG. 13 : is a schematic illustration of block diagram of an RF plasma system according to various embodiments; 
           [0028]      FIGS. 14 a  and 14 b   : are schematic illustrations of a heating method applied to an RF plasmasystem according to various embodiments; 
           [0029]      FIGS. 15 a  and 15 b   : are schematic illustrations of ring samples placed inside the cavity of an RF plasma system according to various embodiments; 
           [0030]      FIG. 16 : is a schematic illustration showing a driven electrode movement in the three dimensional etching; 
           [0031]      FIGS. 17 a  and 17 b   : are schematic illustrations showing coaxial plasma segments according to various embodiment; 
           [0032]      FIG. 18 : is a schematic diagram of an RF coupling vacuum feed through according to various embodiments; 
           [0033]      FIGS. 19 a  and 19 b   : is a schematic diagram of a plurality of spoke cavities; 
           [0034]      FIG. 20 : is a schematic illustration showing gas diffuser; 
           [0035]      FIG. 20 a   : is a schematic illustration showing incorporation of the gas diffuser into the inner electrode. 
       
    
    
       [0036]    It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention as well as to the examples included therein. All numeric values are herein assumed to be modified by the term “about,” whether or not explicitly indicated. The term “about” generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the term “about” may include numbers that are rounded to the nearest significant figure. 
         [0038]    Various embodiments of the invention describe methods and apparatus to etch, to purify, and to passivate a large-area Niobium (Nb) or other contaminated metallic surfaces using a cylindrical radiofrequency discharge in a gas mixture composed of specified amounts of argon (Ar), chlorine (Cl 2 ), helium (He), and oxygen (O 2 ). A radio frequency discharge can be generated between two coaxial full or segmented cylindrical electrodes in the capacitively-coupled regime, whereby the exposed inner surface of the outer electrode is being treated. U.S. Provisional Application Ser. No. 61/880,415, titled, “Efficient Plasma Etching of Surfaces Inside Three Dimensional Structures,” filed Sep. 20, 2013, which is hereby incorporated by reference in its entirety, describes the use of a smooth central driven electrode in an asymmetric RF discharge with the processed surface acting as the grounded electrode, which can be treated at external room temperature. 
         [0039]    Various embodiments of the present invention address the asymmetry of the discharge generated between two coaxial full or segmented electrodes in a capacitively-coupled regime, by using a corrugated, or otherwise area-enlarged driven electrode. Additionally or alternatively, various embodiments apply temperature variation on the sample surface, which can be the surface of the outer electrode. Additionally or alternatively, various embodiments introduce controlled motion of the driven (inner) electrode within the sample cavity to be etched. Hereinafter, the enlarged-area electrode will be referred to as the “corrugated electrode.” 
         [0040]    Through extensive experimentation with an externally heated grounded cylindrical electrode it has been determined that a substantial increase in etching rate can be achieved, under certain conditions, with increasing temperature. The temperature is an important point in ion-assisted etching. According to various embodiments, temperature enhances the chemical reactivity of the surface being etched. According to various embodiments it is desirable to etch an outer electrode. Therefore, the outer wall of an RF Plasma etching cylinder can be heated by various ways, such as with heating tape.  FIG. 1  is a photograph of an RF plasma etching system  1  according to various embodiments. Heating tape is shown wrapped around the outer electrode cylinder of the RF plasma etching system  1 . The outer cylinder forms the outer electrode of the system  1 . An inner electrode can be positioned inside the outer cylinder. Heating the outer cylinder of the system  1  can improve the chemical reactivity of the inner surface of the outer cylinder, which is the surface to be etched according to various embodiments of the invention. 
         [0041]    Various embodiments address the technology of processing superconducting radio frequency (SRF) cavities for particle accelerators. The cavities can be made of bulk niobium and the processing is aimed at producing a layer of pure superconducting material to avoid local heating due to impurities and oxide formation and subsequent loss of superconductivity. Plasma processing (plasma etching or plasma cleaning) of three-dimensional metal structures, such as cylindrical cavities, tubes or more complex components, as SRF cavities, would be substantially cheaper and more environmentally friendly compared to the commonly used wet (acid) processing. These cavities can be positioned as part of the RF Plasma etching system  1  to form the outer electrode. An inner electrode can be positioned within the cavity. The outer electrode can also be heated. The inner electrode can be a driven electrode and can be translated within the cavity. 
         [0042]    Static plasma generation, as described in U.S. Provisional Application Ser. No. 61/880,415, would not be sufficient for uniform mass removal in plasma processing of a complex structure. Static plasma generation involves a fixed, unmovable driven electrode inside the structure to help produce plasma for processing. In this case, the loading effect, or the amount of the substrate exposed, reduces the processing rate, which would make uniform processing more difficult to perform. The solution is to apply the motion on the driven electrode, where it does not fully cover the longitudinal dimension of the structure, but activates the plasma and performs processing on a given segment only. 
         [0043]    According to various embodiments, a large-area cylindrical SRF cavity or part of it, with surface impurities and covered with Niobium Pentoxide (Nb20s) can be positioned on the perimeter of a cylindrical chamber, acting as the outer electrode in an RF plasma etching system. This can be illustrated by the ring samples shown in  FIG. 2 , which shows ring samples with the diameter of the outer electrode. To verify the concept experimentally ring samples were used as substitutes for a part of outer treated surface. The ring samples were positioned over the inner surface of the cavity. Verification of the etching process is done by measuring mass difference of the rings before and after the procedure. These ring samples are just for experimental purpose in real application etch the whole inner surface of the cylinder is etched. 
         [0044]    To illustrate the variety of structures that can be etched according to various embodiments,  FIGS. 3-5  show three images which are samples of possible structures to be etched.  FIG. 3  is a photograph of a first exemplary structure  3  to be etched according to various embodiments. More specifically,  FIG. 3  is a photograph of a 1.5 GHz 9-cell SRF cavity.  FIG. 4  is a photograph of a second exemplary structure  4  to be etched according to various embodiments. More specifically,  FIG. 4  is a photograph of a 1.5 GHz single-cell SRF cavity. Comparing  FIG. 4  with  FIG. 5 , to etch 6 GHz SRF cavity, you do not need to move the electrode as its length is small, but it is not the case in case of 9-cell as it is a meter long cavity (approximately).  FIG. 5  is a photograph of a plurality of exemplary structures  5  to be etched according to various embodiments. 
         [0045]    More specifically,  FIG. 5  shows a plurality of 6 GHz cavity. Due to their complex geometries, it can be difficult to etch the inner surface of such cavities. As  FIG. 5  is a small structure, approximately 10-12 cm in height and approximately 200-300 square cm area, it might be prudent to etch it without the movement of the inner electrode, but in case of  FIG. 3  and  FIG. 4  the movement of the inner electrode is beneficial and almost required. 
         [0046]    According to various embodiments an electrode, including a driven (inner) electrode can be employed. The electrode  6  can have a cylindrical symmetry, as illustrated in  FIG. 6 . The electrode  6  can be designed and constructed so that it has an equal or larger surface area than the treated outer electrode.  FIG. 7  is a schematic illustration of an electrode  71  in the form of a large pitch bellows tube, having cylindrical end segments  72  and a corrugated central section  73 .  FIG. 8  is a schematic illustration of an electrode  81  in the form of a small pitch bellows tube, having cylindrical end segments  82  and a corrugated central section  83 .  FIG. 9  is a schematic illustration an electrode  91  in the form of a disc loaded tube, having cylindrical end segments  92  and a corrugated central section  93 . Comparing the electrodes  71 ,  81 , and  91  of  FIGS. 7, 8 , and  9  shows the corrugated section of an electrode according to various embodiments of the invention can have differently sized corrugations. The size and type of corrugations can be selected to provide varying surface areas to the electrode. 
         [0047]      FIGS. 7-9  illustrate the shape of various electrodes. For example, if a vessel having a 5 cm inner diameter is to be etched, the inner electrode must have an outer diameter of less than 5 cm. Therefore, if a cylindrical inner electrode is employed, the inner electrode surface area will always be less than the surface area of the outer electrode in the case the inner electrode is a straight tube. This difference in surface areas can create more voltage drop at the inner electrode than at the outer electrode. To reverse voltage drop and to place more surface area on the inner electrode, various embodiments of the present invention utilize a corrugated or wiggling pattern on the surface of the inner electrode. By controlling the distance between two pitches and the depth between two pitches it is possible to optimize the electrode area ratio beneficial to the specific etching process. Generally, the surface area of the inner electrode should be approximately equal to the surface area of the outer electrode, i.e. the inner surface area of the cavity to be etched. In general for the reactive ion etching the surface to be etched should be placed on the smaller area electrode, or should be smaller area electrode. However, according to various embodiments, the surface to be etched is the larger area electrode, or is the outer electrode. Therefore, according to various embodiments, the corrugated-type structures can be employed. In typical RF plasma etching process are limited to planar surfaces, where the surface to be etched is placed on the smaller-area electrode. On the other hand, according to various embodiments of the present invention, the surface to be etched can be the larger area electrode, or the outer electrode. To reduce heat load and sheath voltage drop at the driven electrode various embodiments opt for the corrugated type structure. 
         [0048]    As illustrated in  FIGS. 10 and 11 , the driven electrode  102 ,  112  can be positioned at the axis of the cylindrical vessel  101 ,  111 , containing the hollow cylindrical electrode made of Niobium. The inner electrode material, shape and connection were chosen based on the outer electrode to be etched. The bright light shown in the image is plasma and whitish looking element is the inner electrode. 
         [0049]    Etching gases are introduced into the initially evacuated etching chamber in a stepwise manner. The etching gases can include but are not limited to Cl2, SF6, BF3, CF4, CCl2, and combinations thereof with a mixture of Argon, Helium or nitrogen or any inert kind of gas. Any corrosive gas or any corrosive gas in mixture with inert type gas, which makes the volatile product with the surface material to be etched, can be used. 
         [0050]    The etching vessel wall, i.e. the outer wall of the cavity to be etched, can be uniformly heated by means of an external tape heater so that the surface temperature of the processed electrode is elevated to a spatially and temporary constant value in the range from 100 to 1500 K. The processing procedure described in U.S. Provisional Application Ser. No. 61/880,415 is initiated. 
         [0051]    Dry plasma etching systems and methods according to various embodiments include external heating of the etched cylindrical surface. Measured in the temperature range between room temperature and about 1,500 Kelvin, it has been determined that the layer thickness removal rate, k, approximately obeys the Arrhenius law, 
         [0000]    
       
         
           
             
               
                 
                   k 
                   = 
                   
                     A 
                      
                     
                         
                     
                      
                     
                        
                       
                         
                           - 
                           
                             
                               E 
                               a 
                             
                             
                               k 
                               B 
                             
                           
                         
                          
                         
                           1 
                           T 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0052]    where A is the pre-exponential factor, Ea that we labeled “the activation energy,” expressed in form of the energy per molecule in Joules, kB is the Boltzmann constant, and temperature T is expressed in Kelvins. 
         [0053]    By way of non-limiting example, the following values for the constants in the simple Arrhenius plot of Eq. (2) can be obtained for a chlorine/argon mixture with given parameters: 
         [0000]    
       
         
           
             A 
             ≅ 
             
               1.65 
               × 
               
                 10 
                 5 
               
                
               
                   
               
                
               nm 
                
               
                 / 
               
                
               min 
             
           
         
       
       
         
           
             
               
                 E 
                 a 
               
               
                 k 
                 B 
               
             
             ≅ 
             
               3.58 
               × 
               
                 10 
                 3 
               
                
               
                   
               
                
               K 
             
           
         
       
     
         [0054]    Therefore, the common activation energy per molecule is 
         [0000]        E   a ≅(8.62×10 −5  eV/K)×(3.58×10 3  K)≅0.309 eV
 
         [0055]    In the case of more complicated structures, there can be a need for two dimensional motion. As used herein, an “X-Y translator” is a system that can move in two directions, if needed. The X-Y translator can move the driven electrode in at least two perpendicular directions. One-dimensional or linear motion can be sufficient for etching the structures illustrated  FIG. 3 ,  FIG. 4 , and  FIG. 5 , but in the case of a more complex structure, as illustrated in  FIGS. 19 a  and 19 b   . Electron bunch dynamics in complex light sources can require cavities with minimum wakefield instabilities.  FIG. 19 a    illustrates a plurality of spoke cavities for acceleration.  FIG. 19 b    illustrates a plurality of crab cavities for deflection. In order to etch the interior of more complex cavities as illustrated in  FIGS. 19 a  and 19 b   , it can be necessary to move the driven electrode in the multiple directions to etch the surface uniformly. 
         [0056]      FIGS. 12 a - b    are schematic illustrations of a driven electrode assembly  121  comprising a driven electrode  123 . The driven electrode  123  is attached to an x-y translator  122 . The translator  122  can move the driven electrode  123  in a stepwise manner and the processing can be performed segment by segment. The translator  122  motion can be controlled by a stepper motor with controllable speed and resolution. The driven electrode  123  can be moved back and forth in an axial direction  126 . The electrode motion can be performed during plasma operation. Plasma configuration is not affected by the motion, which has been verified experimentally. The inner electrode can be moved during the plasma operation, which did not switch off or show any fluctuation. Therefore, the driven electrode can be moved toward the area to be etched without disturbing the plasma conditions. 
         [0057]      FIG. 12  is a schematic illustration of a driven electrode assembly  121  having an x-y translator  122 , showing both axial direction  126  and a transverse direction  127  in which the driven electrode can be moved. 
         [0058]    The driven electrode assembly  121  can be used to move the inner (driven) electrode  123  inside the structure which is supposed to be etched. The driven electrode assembly  121  works by using the translator  122  to force plate  124  to squeeze and releasing the bellows  124 , which house the driven electrode  123 . When the bellows  124  are compressed the driven electrode  123  translates in and out of the driven electrode assembly  121 . Only a small portion of the electrode  123 , which is shown in the figure, is active as the rest is under atmospheric pressure and shielded. The moving electrode  123  contributes to etching in uniform way a long structure. 
         [0059]      FIG. 13  is a schematic block diagram of an RF plasma etching system  130 . As shown the system  130  can include a gas flow system  139  supplying gas to the cavity  140 , such as a Niobium cavity. The cavity  140  can be encircled by heating tape  138 . A driven electrode  141  can be disposed within the cavity  140 . Power can be supplied to the driven electrode  141  via a power supply system  131 . The driven electrode  141  can be moved via a bellow  132 . A pressure gauge  133  can be provided to monitor the pressure in the system  130 . Gas provided by the gas flow system  139  can be removed from the system via a vacuum system  137 , which can be monitored and controlled by a second pressure gauge  136 , a turbomolecular pump  135  and a valve  134 . To illustrate the segment wise etching of the cavity, the electrode has been depicted broken inside the cavity. The electrode can travel in stepwise fashion inside the cavity to etch it segment by segment. 
         [0060]      FIGS. 14 a - b    are schematic illustrations of an RF plasma etching system  142  according to various embodiments.  FIGS. 14 a - b    illustrate the heating method applied. More specifically, heating tape  145  is wrapped around the cavity or structure  150  to be internally etched. The heating tape  145  can heat the surface of the structure  150  to be etched. The heating tape  145  can be used to maintain the temperature of the structure  150  to be etched. A driven electrode  146  can be disposed within the structure  150 . The driven electrode  145  can have any structure as described in other embodiments, including but not limited to a tube-shape, a corrugated-shape, or a wiggling shape. A plasma region  151  can be created within the structure  150 . Power can be supplied to the driven electrode  145  via an RF power coupling  143 . The power supply can be RF or MW or DC (pulsed) power supply  147 . The driven electrode can be held under atmospheric pressure and shielded as indicated by dotted line  144 . Until dotted line  144  connects driven electrode  146  within structure  150 , all electrical connections are under atmospheric pressure, so there is no plasma production. The plasma region  151  only occurs in the shade region as illustrated. Gas can be evacuated from the system  142  via a turbomolecular pump  148 , backed by a roughing pump  149 . The cavity temperature of the structure  150  can be varied using the external heating tape  145 . The tape  145  can be wrapped around the external cavity wall and set at the required wall etching temperature. The power coupling area is not heated. In  FIG. 14 b   , an x-y translator  1401  is shown that comprises bellows  1402  and a connection  1403  to an electrical supply (not shown). 
         [0061]    Referring to  FIG. 18 , an RF coupling vacuum feedthrough  180  is shown. RF coupling vacuum feedthrough  180  contains an HN type connector with its holder  181 , 200 to 300 mm long coaxial connector with the inner conductor  182  ending with a thread  185  to be attached to the inner electrode  182 , and the outer conductor equipped with two conflate flanges, one of which being a miniconflat flange  183  with electric feedthrough. The cylindrical gap surrounding the conductor  182  is filled with air at atmospheric pressure and sealed off. Connector ending  184  can be covered with a ceramic insulator up to the connecting thread  185 . For purposes of the present application, HN connectors are medium-sized weatherproof units designed for high voltage applications. 
         [0062]      FIGS. 15 a - b    are schematic illustrations of an RF plasma etching system  152  according to various embodiments. All structural elements of the RF plasma etching system  152  are the same as in  FIGS. 14 a - b   , with like reference numerals indicating the same structures. The distinction between  FIGS. 15 a - b    and  FIGS. 14 a - b    is the presence of ring samples  153  disposed within the plasma region  151 . The driven electrode  146  is disposed within the ring samples 
         [0000]      153 . The ring samples  153  are placed inside the cavity for etching diagnostic purposes, to measure etch rate and surface properties of the etched sample. Therefore, the ring samples  153  are used for experimental purpose, for optimization of parameters like pressure, power, gas concentrations, temperature, bias etc. Flat and ring samples are not used during the etching of a cylindrical or any three-dimensional structure. 
         [0063]      FIG. 16  is a schematic illustration of an RF plasma etching system  160  according to various embodiments. The system  160  includes a connection  143  to supply power to a driven electrode  146 . The driven electrode  146  is under atmospheric pressure and shielded in section  144  of the system  160 . The driven electrode  146  can include one or more corrugated segments  164 . Depending on the squeezing and releasing of the bellows  161  in the translation segment  162  of the system  160 , the driven electrode  146 , a plasma region  151  can be created in whichever region of a cavity of a structure  163  that is exposed to the driven electrode. The structure  163  can be a single cell or multi-cell complex structure. As in other embodiments, a turbomolecular pump  148  backed by a roughing pump  149  can be provided to evacuate gas from the system  160 . The dotted lines of the corrugated segments  164  of the driven electrode  146  are presented to show the linear motion of the driven electrode  146  through the cavity of the structure  163  to be etched. Therefore,  FIG. 16  shows the driven electrode  146  moving in an exemplary three dimensional etching application. Because of loading effect and plasma properties variation, it can be difficult to uniformly etch a given structure by etching the whole structure at once. To overcome this problem, various embodiments can employ a translation stage equipped with bellows, such as bellows  161  to move the driven electrode  146  inside the structure  163  and perform segment-wise etching. Such embodiments allow for the etching of one sub section of the structure  163  at a time, by placing the driven electrode  146  at the corresponding axial position. When etching of the first segment is completed the driven electrode  146  can be translated to the next subsection by squeezing and releasing the bellows  161 , which is holding the manifold of the driven electrode  146 . 
         [0064]      FIGS. 17 a  and 17 b    are schematic illustrations of a coaxial plasma segment  170 . The segment  170  includes a structure  176  to be etched. A driven electrode  171  can be disposed within the structure  176  from a first position  172  to a second position  174 . At each position  172 ,  174 , the driven electrode  171  can be used to generate a plasma region  173 ,  175 . The plasma regions  173 ,  175  can be coaxial plasma regions and can depend on where the electrode is positioned within the structure  176 . The driven electrode  171  can have any structure as described in other embodiments, including but not limited to a tube-shape, a corrugated-shape, or a wiggling shape.  FIGS. 17 a  and 17 b    show that the plasma  173 ,  175  exists at the volume corresponding to the location  172 ,  174  where the driven electrode  171  is moved. Referring the shaded portions as plasma, the figures illustrate the synchronized motion of the driven electrode  171  and the plasma. 
         [0065]      FIG. 20  is the schematic illustration of coaxial conical gas diffuser that serves as the conduit for gas mixture intake.  FIG. 20 a    indicates that the gas diffuser is incorporated into the inner electrode. 
         [0066]    Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein. 
         [0067]    The reader&#39;s attention is directed to all papers and documents which are filed concurrently with this specification and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 
         [0068]    All the features disclosed in this specification (including any accompanying claims, abstract, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0069]    Any element in a claim that does not explicitly state “means for” performing a specified function, or “step for” performing a specific function, is not to be interpreted as a “means” or “step” clause as specified in 35 U.S.C §112, sixth paragraph. In particular, the use of “step of” in the claims herein is not intended to invoke the provisions of 35 U.S.C §112, sixth paragraph.