Abstract:
The inductively coupled plasma source and antenna geometry are significant factors in determining plasma and process uniformity inside the chamber. Growing demands for processing larger and larger wafers or LCD substrates and providing higher and higher degrees of plasma uniformity challenge the current ICP type antenna designs and push development of sources. Branching RF antenna, featuring a plurality of major and minor branches, provides improved coverage of processing area, reduced standing wave effect, improved uniformity of inductively coupled electromagnetic field, more uniform plasma production, and more homogeneous processing conditions throughout the whole processing area.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is related to and claims priority to U.S. provisional Ser. No. 60/325,199 filed on Sep. 28, 2001, which is herein incorporated by reference in its entirety. The present application is also related to International application serial no. PCT/US02/28140, filed on Sep. 25, 2002, which claims priority to U.S. provisional application Ser. No. 60/325,188 filed on Sep. 28, 2001. Those applications are herein incorporated by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to a plasma material processing apparatus and more particularly to apparatus for and method of plasma generation of the induction coupling type in which induction plasma is excited by applying electromagnetic energy to a branching RF antenna. 
   BACKGROUND OF THE INVENTION 
   Many types of plasma material processing methods are widely accepted for semiconductor fabrication and plasma generation including: sputter etching, plasma-enhanced chemical etching, reactive ion etching, plasma-enhanced vapor deposition, ionized sputter deposition and magnetically enhanced plasma etching. Different types of well-known plasma sources are used in these processes such as the popular inductively coupled plasma (ICP) sources as well as others including: capacitively coupled plasma (CCP) sources, microwave plasma sources (including those that utilize the electron-cyclotron resonance for improved efficiency of power deposition into the plasma), surface wave plasma sources, and helicon plasma sources. In many sources, radio frequency (RF) power can be applied to a RF antenna such that process gas supplied to the plasma generating space is excited, disassociated and ionized. This excitation occurs due to a radio frequency electromagnetic field formed by RF currents in the antenna generating the plasma. 
   The inductively coupled plasma source and antenna geometry are significant factors in determining plasma and processing uniformity inside the chamber. The growing demands for processing larger and larger wafers or LCD (liquid crystal display) substrates and providing higher and higher degrees of plasma uniformity challenge the current ICP type antenna designs and push development of sources. 
   Traditional spiral RF antennas are becoming too long for larger wafers or LCD substrates and cannot generate uniform plasmas. Furthermore, such RF antennas are unable to provide the required plasma homogeneity in both, flat and dome-shaped, geometries. Problems with RF antennas occur due to the lengthening of antenna elements relative to the electromagnetic wave and because of the standing wave effects. The standing wave effects become stronger during increased frequency operation of increased wafer or substrate size, limiting the RF antenna&#39;s area of application and reducing uniformity. 
   In addition, radially extended RF antennas are becoming non-efficient because they are not able to uniformly cover the entire plasma processing area over the substrate. Area coverage reduces outwardly from the endpoint such that there is satisfactory coverage closer to the center of the antenna, but unsatisfactory coverage between any two radially extending “arms” of the antenna. 
   The dome-type antennas are similarly unable to perform adequately given the increasing size of the plasma area and substrates. 
   Finally, the antennas wired around the sides of the vacuum chamber are becoming inefficient because they cannot provide adequate RF fields in the inner half of the plasma volume. 
   What is required is a redesigned RF antenna apparatus and method for generating more uniform plasma coverage over larger areas. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of the above-described problems whose appearance in conventional plasma material processing of the RF induction coupling type where induction plasma excitation is created by applying power to a RF antenna. Accordingly, it is an object of the present invention to provide a novel branching RF antenna for use in plasma material processing operations. 
   It is another object of the present invention to improve the uniformity of plasma generation. 
   It is still another object of the present invention to reduce standing wave effects for improved plasma material processing. 
   According to the present invention, there is provided a plasma reactor for generating uniform plasma using an inductively coupled plasma (ICP) source, comprising: a branching radio frequency (RF) antenna coupled to a RF power source for creating an electromagnetic field wherein the electromagnetic field excites process gas in a processing chamber and converts the process gas into plasma; and a window through which the electromagnetic field can penetrate into the plasma reactor. 
   In addition, there is also provided a method for generating uniform plasma using an inductively coupled plasma (ICP) source, the method comprising the steps of: placing a sample on a work surface; continuously exhausting plasma reactor to pressure-reduced conditions; inputting gas into the plasma reactor; and applying RF power to a branching RF antenna, the branching RF antenna providing a uniform field to the gas in a processing chamber wherein uniform plasma is generated. 
   In a first aspect of the present invention, the branching RF antenna comprises a plurality of major and minor branches with embedded cooling channels extending from a central feed element. This preferred embodiment provides more homogenous plasma generation and thus more uniform coverage of the plasma area. 
   The plasma processing system for subjecting a target object to a plasma process comprises a process chamber formed in a process vessel; a gas supply system for supplying a process gas to the process vessel; an exhaust system for exhausting and controlling pressure in the process chamber; a susceptor arranged in the process chamber, the susceptor having a work surface for supporting the target object in the process chamber; and an ICP RF source having a branching antenna for sustaining a large uniform plasma during the plasma process. 
   Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention where: 
       FIG. 1   a  is a schematic diagram showing a plasma etching system according to a preferred embodiment of the present invention; 
       FIG. 1   b  is a schematic diagram showing a plasma etching system in accordance with an alternate embodiment of the present invention; 
       FIG. 1   c  is a schematic diagram showing a plasma etching system according to another alternate embodiment of the present invention; 
       FIG. 2   a  illustrates a schematic view of a branching RF antenna in accordance with a preferred embodiment of the present invention; 
       FIG. 2   b  illustrates a schematic view of a branching RF antenna in accordance with an alternate embodiment of the present invention; 
       FIG. 3  illustrates a simplified view of a second type of branching RF antenna in accordance with an alternate embodiment of the present invention; 
       FIG. 4  illustrates a simplified view of a third type of branching RF antenna in accordance with an alternate embodiment of the present invention; 
       FIG. 5  illustrates a simplified view of a fourth type of branching RF antenna in accordance with an alternate embodiment of the present invention; 
       FIG. 6  illustrates a simplified view of an alternate configuration for the fourth type of branching RF antenna in accordance with an alternate embodiment of the present invention; 
       FIG. 7  illustrates a simplified view of a first type of branching in accordance with a preferred embodiment of the present invention; and 
       FIG. 8  illustrates a simplified view of a fifth type of branching RF antenna in accordance with an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. In the following description, the constituent elements having substantially the same function and arrangement are denoted by the same reference numerals, and repetitive descriptions will be made only when necessary. 
     FIG. 1   a  is a schematic diagram showing a plasma etching system  100  according to a preferred embodiment of the present invention. Plasma etching system  100  comprises an ICP source having a branching antenna. 
   An airtight process chamber  102  of plasma etching system  100  is constituted by a substantially cylindrical process vessel  104  and top plate  106 . Process vessel  104  and top plate  106  are made of a conductive material, such as stainless steel and are grounded through ground line  108 . 
   Faraday shield  140  is electrically coupled to the sidewalls of process vessel  104 . 
   In the illustrated embodiment, Faraday shield  140  comprises a number of conductive elements  145  and spaces  147  arranged in a first pattern. Conductive elements  145  are connected to process vessel  104  via grounding means  148 . In this manner, conductive elements  145  are electrically grounded. Alternately, a dual Faraday shield  140  can be used that consists of at least two layers of conductive elements. 
   In other embodiments of the present invention, the branching RF antenna can be used without a Faraday shield, and it can be located above the dielectric window, or below the dielectric window, or there can be no dielectric window at all, so the branching RF antenna is located within the processing chamber. Desirably, conductive elements  145  are the same size. Alternately, conductive elements  145  can have different sizes. Also, Faraday shield  140  can be DC biased, or a RF power can be applied to it. 
   Dielectric window  155  is coupled to process vessel  104  and provides a ceiling for process chamber  102 . Desirably, dielectric window  155  comprises a dielectric material and provides a dielectric window for antenna  110 . 
   In a preferred embodiment, gas supply line  133  is coupled to the processing chamber  102  to supply processing gas. Gas supply line  133  is coupled to gas source unit  134  through at least one opening/closing valve (not shown) and at least one flow control valve (not shown). The gas source unit  134  has gas sources for a plurality of different gases to be supplied to process chamber  102 , e.g., CF4, C4 F8, CO, O2, Ar, and N2. 
   In an alternate embodiment, gas supply line  133  is coupled to Faraday shield  140 , which is used to supply processing gas to chamber  102 . 
   Branching RF antenna  110  is arranged above Faraday shield  140 . In a preferred embodiment, branching RF antenna comprises a plurality of major and minor branches. Preferably, the branching RF antenna branches are equally loaded, an advantage for the RF power source and for design simplicity. Maintaining equal loads, especially in the case of n major branches when each major branch covers an n-th part of the plasma area is desirable. However, the principle of equal loads is not required for the invention. In alternate cases, different loads can be used, and other antenna configurations can be used. 
   Branching RF antenna  110  is connected through a matching unit  112  and transmission line  113  to RF power supply  114 . Desirably, RF power supply  114  outputs a plasma generating RF power to branching RF antenna  110  and operates in a RF frequency range of 10-1000 MHz. Alternately, a different number of matching units and/or transmission lines can be used. 
   In alternate embodiments, branching RF antenna  110  can be connected to at least one other RF power supply (not shown), so different branches can be fed by different RF power supplies, possibly with different frequencies. 
   Susceptor  116  comprises a conductive material and is arranged in the lower portion of the process chamber  102 . Susceptor can be grounded or electrically isolated in the case when RF power is not applied to the susceptor  116  (as in  FIG. 1   b ). However, in preferred embodiment ( FIG. 1   a ), the susceptor is under RF potential supplied by a second RF power supply  126  through the corresponding matching network  124 . The RF frequency of the second RF power supply  126  is in the range of 500 kHz-10 MHz. This provides additional biasing potential on the wafer W and improves directionality of charged particle fluxes going from the plasma (not shown) to the wafer W. 
   The upper surface of the susceptor  116  serves as a wafer-holding surface and an insulating layer  115  made of, e.g., polyimide is adhered to the upper surface. On the insulating layer  115 , a conductive electrostatic chuck electrode  117  and a resistive layer  121  are arranged. The chuck electrode can be prepared by forming a silver or palladium film on the lower surface of the resistive layer. A conductive line (not shown) covered by an insulating cable is provided in the susceptor and connected to the chuck electrode  117 . On the other end, this conductive line is connected to a DC power supply through a switch (not shown). 
   A gas supplying passage (not shown) is provided through the resistive layer  121  to the rear surface of the wafer W for supplying a heat conductive gas, such as He. Further, there is a plurality of pusher pins, e.g. three, for moving a wafer up and down with respect to the upper surface of the resistive layer. 
   The susceptor is mounted on the cooling block  119  made of a thermo-conductive material such as aluminum, which carries tubes with circulating coolant such as liquid nitrogen. The cooling block  119  is coupled to the bottom part of the process chamber through an insulating member  118 . 
   An elevating shaft  120  is movable in the vertical direction by the elevating mechanism (not shown). It is designed to move the entirety of the work table  123  including susceptor  116 , electrostatic chuck  117 , cooling block  119 , as well as the wafer W, in vertical direction, so the distance between the wafer W and the branching RF antenna  110  can be properly adjusted. 
   Bellows  122  comprises an airtight member. Bellows  122  is coupled to insulating member  118 , surrounds elevating shaft  120 , and is coupled to the bottom surface of process chamber  102 . Hence, even if the susceptor  116  is moved vertically, the airtightness in process chamber  102  is not impaired. 
   Process chamber  102  is connected to exhaust line  136  of an exhaust system. Exhaust line  136  is connected to exhaust pump  138  through an opening/closing valve and a flow control valve (not shown). Exhaust pump  138  can exhaust process chamber  102  and set process chamber  102  at a vacuum of, e.g., from 10 mTorr to 100 mTorr. 
   In a preferred embodiment, controller  170  is coupled to first RF power supply  114 , matching network  112 , gas source  134 , exhaust pump  138 , second RF power supply  126 , and second matching network  124 . Controller  170  comprises hardware and software to control the operation of first RF power supply  114 , matching network  112 , second RF power supply  126 , second matching network  124 , gas source  134 , and exhaust pump  138 . For example, controller  170  can control the frequency, phase, amplitude, and bias of the signals provided by the RF power supply. In addition, controller  170  can control process gas and flow rate to the process chamber  102 . Also controller  170  can control the temperature of Faraday shield  140 , branching RF antenna  110 , and the wafer W by controlling the flow of fluids through cooling channels (not shown). 
   In plasma etching system  100  shown in  FIG. 1   a , a process is performed as follows. First, wafer W is placed on the worktable  123  arranged in process chamber  102 . Subsequently, process chamber  102  is exhausted by the exhaust system connected to process chamber  102 , thereby setting the entire interior of process chamber  102  to a predetermined pressure-reduced atmosphere. 
   Worktable  123  with wafer W is moved vertically to the working position, so the distance between the wafer W and the branching RF antenna is set to predetermined value defined by the process. 
   While process chamber  102  is continuously exhausted, a process gas is supplied from process gas supply system  134  to process chamber  102 . In a preferred embodiment, the process gas is provided from gas supply system to at least one gas supply pipe, and from the at least one gas supply pipe to process chamber  102 . In alternate embodiment, the processing gas is supplied to the Faraday shield; it then enters process chamber  102  from Faraday shield  140  through gas supply holes (not shown in FIG.  1 ). 
   In this state, plasma generating RF power is provided to branching RF antenna  110  so that the process gas supplied to process chamber  102  is excited, dissociated, and ionized, thereby generating a uniform wide-area plasma. 
   In order to prevent Faraday shield  140  from being inductively heated, and to effectively use the input energy from branching RF antenna  110  for generating the plasma, no electric current passageway should be formed in any direction, which is the same as that of an RF electric field. For example, a branching RF antenna  110  may be formed using an almost radial configuration of elements and arranged to have a geometric center aligning with that of wafer W. In such a case, the RF electric field generated by branching RF antenna  110  has an electric field direction which is also mostly in the radial direction. For this reason, a corresponding Faraday shield  140  would be provided with a number of slots, which are arranged concentrically, so the conductive elements in the Faraday shield have mostly azimuthal direction. In other words, slots  147  extend in directions that are substantially perpendicular to the direction of the RF electric field generated by branching RF antenna  110 . With this arrangement, the electromagnetic field generated by branching RF antenna  110  is transmitted into process chamber  102  without being cut off, so that the RF electric field is generated in process chamber  102 , while capacitive component is considerably reduced. As a result, the input energy from branching RF antenna  110  is effectively used for generating the plasma. 
   In alternate embodiments, top plate  106  is not required. For example, the process chamber can be formed using a dielectric window, isolation layer, branching RF antenna, and/or gas dispensing apparatus. 
     FIG. 1   b  is a schematic diagram showing a plasma etching system according to an alternate embodiment of the present invention.  FIG. 1   b  illustrates a plasma etching system in which a RF power supply system is not coupled to the susceptor. 
     FIG. 1   c  is a schematic diagram showing a plasma etching system according to another alternate embodiment of the present invention.  FIG. 1   c  includes all the features described for the apparatus of  FIG. 1   a .  FIG. 1   c ., however, illustrates a different geometry for the branching RF antenna, and in the illustrated embodiment, geometry of the dielectric window and the Faraday shield is not flat, but curved. The curvature and configuration of the RF antenna is designed such that a uniform plasma is provided at the surface of wafer W. 
     FIG. 2  illustrates a simplified view of a branching RF antenna in accordance with a preferred embodiment of the present invention.  FIG. 2   a  illustrates a branching RF antenna with cooling channels in all branches, in accordance with a preferred embodiment of the present invention.  FIG. 2   b  illustrates a branching RF antenna with cooling channels in some branches, in accordance with an alternate embodiment of the present invention. 
   Branching RF antenna  200  comprises a plurality of major branches  208  and a plurality of minor branches  210 . Each major branch comprises a first number of branches that extend radially from central feed elements  202  in substantially the same direction to branching point  214 . Desirably, each branch in a major branch lies in a different plane, but all of the branches in a major branch have the same projection on the plane parallel to the substrate. For example, the branches in a major branch can be electromagnetically coupled and/or electrically coupled to each other but not physically coupled to each other. 
   At branching point  214 , at least one minor branch  210  is coupled to each major branch. Each minor branch extends from branching point  214  in a different direction. Desirably, a substantial portion of each minor branch lies in the same plane. As shown in  FIG. 2 , three minor branches are coupled to each major branch at the branching point, but this is not required for the invention. In alternate embodiments, at least two minor branches are coupled at one or more branching points to each major branch. 
   In preferred embodiment, branching RF antenna of  FIG. 2   a  is powered by RF power supply  114  ( FIG. 1 ) through the matching network  12  at the center  202  and is grounded at the outer points  206  on antenna periphery. This provides RF current through major branches  208  and minor branches  210 . Desirably, each branch in a major branch is independently coupled to central feed elements  202 . 
   In alternate embodiment, the outer points  206  are connected to the ground through capacitors (not shown). 
   Major branches  208  have symmetrical geometry and loading for significantly simplifying antenna tuning and ensuring more homogeneous plasma generation. Each major branch  208  covers different azimuthal area. 
   In alternate embodiments, a different number of major branches  208  and/or a different number of minor branches can be used. 
   In a preferred embodiment, antenna  200  has cooling channels  212  extending through all its branches. For high RF power, cooling channels  212  can be provided in both major and minor branches  208  and  210 . However, an absence of cooling channels  212  in some minor branches  210  ( FIG. 2   b ) can be tolerated when using moderate RF power, relying on minor branches  210  cooling via the high thermo-conductivity of the metal parts of the antenna  200  and proximity of minor branches  210  to other cooled branches. 
   In an alternative embodiment, branching RF antenna  200  can be placed on a dome-shaped or other non-flat surface. For a dome-shaped, the common center of all the major branches is located at the center of the dome (center of symmetry). For a stepped surface, the common center of all the major branches can be located at the center of the topmost step. 
   In an alternative embodiment, the branches of any branching RF antenna can be powered at the antenna periphery and grounded at the common center. However, this embodiment can complicate construction due to the increased number of points requiring connection to the power source. 
   In another embodiment, the major branches can be powered by different RF sources. Also, the major branches can be powered by RF with different phases and/or different frequencies. 
   Major and minor branches are fabricated using a metal such as anodized aluminum. For example, antenna branches can have a single conductive surface that can be fabricated using a metal such as anodized aluminum. Antenna branches  208 ,  210  can be fabricated differently for the different antenna configurations. 
   As shown in  FIG. 2 , the coupling angle between the minor branches and the major branches is less than ninety degrees, but this is not required for the invention. In alternate embodiments, these coupling angles can be equal to and/or greater than ninety degrees. 
   As shown in  FIG. 2 , each major branch comprises one branching point, but this is not required for the invention. In alternate embodiments, at least one major branch can comprise more than one branching point. 
     FIG. 3  illustrates a simplified view of a second type of branching RF antenna in accordance with an alternate embodiment of the present invention.  FIG. 3  is a simplified view showing a branching RF antenna in accordance with a preferred embodiment of the present invention.  FIG. 3  illustrates a branching RF antenna with cooling channels in all branches. 
   Branching RF antenna  300  comprises a plurality of major  308  branches and a plurality of curved minor branches  310 . Each major branch comprises a first number of branches that extend radially from central feed elements  302  in substantially the same direction to branching point  314 . Desirably, each branch in a major branch lies in a different plane, but all of the branches in a major branch have the same projection on the plane parallel to the substrate. For example, the branches in a major branch can be electromagnetically coupled and/or electrically coupled to each other but not physically coupled to each other. 
   At branching point  314 , at least two curved minor branches  310  are coupled to each major branch. Each curved minor branch extends from branching point  314  in a different direction. Desirably, a substantial portion of each curved minor branch  310  lies in the same plane, and curved minor branches  310  have the same projection on the plane parallel to the substrate. As shown in  FIG. 3 , two curved minor branches  310  are coupled to each major branch  308  at branching point  314 , but this is not required for the invention. In alternate embodiments, at least two minor branches are coupled at one or more branching points to each branch of the major branch. 
   Desirably, branching RF antenna  300  is powered by RF power supply  114  ( FIG. 1 ) through the matching network  112  at central feed elements  302  and is grounded at the outer points  306 . This provides RF current through major branches  308  and minor branches  310 . Each branch in a major branch can be independently coupled to central feed elements  302 . In other embodiments, the outer points  306  are connected to the ground through capacitors (not shown). 
   Major branches  308  have symmetrical geometry and loading for significantly simplifying antenna tuning and ensuring more homogeneous plasma generation. Each major branch  308  covers different azimuthal area. 
   In alternate embodiments, a different number of major branches  308  and/or a different number of minor branches can be used. 
   In the illustrated embodiment, antenna  300  has cooling channels  312  extending through all its branches. For high RF power, cooling channels  312  can be provided in both major and minor branches  308  and  310 . However, an absence of cooling channels  312  in minor branches  310  ( FIG. 4 ) can be tolerated when using moderate RF power, relying on minor branches  310  cooling via the high thermo-conductivity of the metal parts of the antenna  300  and proximity of minor branches  310  to other cooled branches. 
   In an alternative embodiment, branching RF antenna  300  can be placed on a dome-shaped or other non-flat surface. For a dome-shaped, the common center of all the major branches is located at the center of the dome (center of symmetry). For a stepped surface, the common center of all the major branches can be located at the center of the topmost step. 
   In another embodiment, the major branches can be powered by different RF sources. Also, the major branches can be powered by different phases and/or different frequencies. 
   In another embodiment, branching RF antenna  300  comprises a plurality of coplanar major branches  308  extending radially from central feed element  302  and a plurality of coplanar curved minor branches  310 . In this example, branching occurs in the plane of branching RF antenna  300  or on the same surface. 
   The decreased number of end points  306  simplifies construction of the antenna  300 . Preferably, all curved branching elements  310  have symmetrical geometry and loads to significantly simplify antenna tuning and ensure more homogeneous plasma generation. However, alternative embodiments need not require symmetry. 
   In alternative embodiments, similar to description of branching antenna of  FIG. 2 , the RF power feed can be arranged going to the end points  306 , while the center point  302  is grounded. In yet other alternative embodiments the grounding connecting can be arranged through capacitors (not shown). 
   Branching RF antenna  300  shown in  FIG. 3  can be adjusted to a dome-shape or other non-flat surface. 
     FIG. 4  illustrates a simplified view of a third branching RF antenna in accordance with an alternate embodiment of the present invention. 
   Branching RF antenna  400  comprises a plurality of major branches  408  and a plurality of minor  410  branches. Major branches  408  are coplanar and extend radially from central feed element  402 . Each major branch  408  comprises branching point  414 . A number of minor branches  410  are coupled to the branching point  414  of each major branch  408 . In addition, central feed element  402  can also be coupled to transmission line  113  (FIG.  1 ). 
   In the illustrated embodiment, branching RF antenna  400  has eight major branches  408 , which provide a common geometrical center. Each major branch  408  begins from central feed element  402  and extends radially. Branching point  414  is located a first length  424  from central feed element  402 . Major branches  408  have symmetrical geometry and loading for significantly simplifying antenna tuning and ensuring more homogeneous plasma generation. Each major branch  408  covers different azimuthal area. 
   In the illustrated embodiment, three minor branches  410  extend from each branching point  414 . Minor branch length  426  is the distance from branching point  414  to grounded outer point  406  on antenna periphery  404 . Along antenna periphery  404 , each minor branch  410  has distinct endpoint  406 . Moderate differences in the geometry and loads of minor branches  410  can be tolerated in this preferred embodiment. 
   In other alternate embodiments, a different number of major branches  408  and/or a different number of minor branches can be used. 
   In addition, branching RF antenna  400  can comprise cooling channels  412  that can extend through one or more major branches  408 . For high RF power, cooling channels  412  can be provided in both major and minor branches  408  and  410 . 
   In the illustrated embodiment, the flat coplanar branching RF antenna  400  provides a more uniform covering of a larger area thus providing a more homogenous plasma generation as required for larger substrates. 
   In another alternative embodiment, however, branching RF antenna  400  can be placed on a dome-shaped or other non-flat surface, rather than the preferred flat surface. For a dome-shaped, the common center of all the major branches is located at the center of the dome (center of symmetry). For a stepped surface, the common center of all the major branches can be located at the center of the topmost step. 
     FIG. 5  illustrates a simplified view of a fourth type of branching RF antenna in accordance with an alternate embodiment of the present invention. 
   Branching RF antenna  500  comprises a plurality of major branches  508  and a plurality of minor  510  branches. Major branches  508  are coplanar and extend radially from central feed. Each major branch  508  comprises branching point  514 . A number of minor branches  510  are coupled to the branching point  514  of each major branch  508 . Central feed element  502  is coupled to transmission line  113  (FIG.  1 ), and the ends  506  on the periphery are grounded. 
   In the illustrated embodiment, branching RF antenna  500  has three major branches  508 , which provide a common geometrical center. Each major branch  508  begins from central feed element  502  and extends mainly radially. Major branches  508  have symmetrical loading for significantly simplifying antenna tuning and ensuring more homogeneous plasma generation. Each major branch  508  covers different azimuthal area. 
   In the illustrated embodiment, three minor branches  510  extend from each branching point  514 . Along antenna periphery  504 , each minor branch  510  has distinct endpoints  506 . Moderate differences in the geometry and loads of minor branches  510  can be tolerated in this preferred embodiment. 
   In other alternate embodiments, a different number of major branches  508  and/or a different number of minor branches can be used. 
   In addition, branching RF antenna  500  can comprise cooling channels that can extend through major branches  508  and some of minor branches  510 . For high RF power, cooling channels can be provided in both major and minor branches  508  and  510 . 
   In the illustrated embodiment, the flat coplanar branching RF antenna  500  provides a more uniform covering of a larger area thus providing a more homogenous plasma generation as required for larger substrates. 
   In another alternative embodiment, however, branching RF antenna  500  can be placed on a dome-shaped or other non-flat surface, rather than the preferred flat surface. For a dome-shaped, the common center of all the major branches is located at the center of the dome (center of symmetry). For a stepped surface, the common center of all the major branches can be located at the center of the topmost step. 
     FIG. 6  illustrates an alternate configuration for the fourth type of branching RF antenna in accordance with an alternate embodiment of the present invention. As illustrated, there are not visible break points shown between the major and minor branches, and the projections on the wafer plane are not combined into single major branches, but there is still significant grouping of branches. 
   Near the common center  602  the antenna branches are significantly grouped (in the case illustrated in  FIG. 6 , there are three groups  608  of branches, corresponding to major branches  508  of FIG.  5 ). In the outer area, the antenna branches  610  go further from each other, similar to the minor branches  510  in FIG.  5 . 
   All branches are fed at the center  602  and grounded at the end points  606  in the periphery  604 . Alternately, the antenna branches can be fed at the end points  606  and grounded at the common center  602 . Yet alternately, the electrical grounding can be arranged through capacitors (not shown). Cooling channels (not shown) can go through the antenna branches. 
   In the illustrated embodiment, the flat coplanar branching RF antenna  600  provides a more uniform covering of a larger area thus providing a more homogenous plasma generation as required for larger substrates. 
   In another alternative embodiment, however, branching RF antenna  600  can be placed on a dome-shaped or other non-flat surface, rather than the preferred flat surface. For a dome-shaped, the common center of all the major branches is located at the center of the dome (center of symmetry). For a stepped surface, the common center of all the major branches can be located at the center of the topmost step. 
     FIG. 7  illustrates a simplified view of a first type of branching in accordance with a preferred embodiment of the present invention. Each major branch  708  consists of a few branches having the same (or close to be the same) projections on the wafer surface. At the branching point  714 , the branches split off each other, thus forming the plurality of minor branches  710 . The cooling channels  712  can go through the antenna branches to ensure acceptable temperature regime for the antenna. 
   The present invention allows several types of antenna branching to be considered. The preferred type of branching corresponds to the case ( FIG. 7 ) when the projection of the antenna&#39;s field on the plane of the wafer looks like a major branch is branching (i.e. when the minor branches split off the major branches). Actually, each major branch consists of a few branches (see, FIG.  7 ); it is just their projection on the wafer plane looks like it is almost a single major branch. This provides more uniform electromagnetic (EM) field above the wafer and, respectively, more uniform plasma generation throughout the whole area of the wafer both near the wafer center and closer to its periphery. 
   An alternate type of branching is coplanar branching, where the major and minor branches of the antenna lie substantially in the same plane. 
   In addition, modifications to the branching principle are allowed. For example, when the projections on the wafer plane are not exactly combined into single major branches, but still there is significant grouping of branches (see,  FIG. 6 , as compared with FIG.  5 ), are covered under the present invention as well. 
   Another type of branching is illustrated in FIG.  8 . In this case, the full length of antenna (such, for example, as the flat spiral antenna) is divided into a few branches, so the electrical length of each branch is short enough to avoid standing wave effects). The principle of equal (or close to equal) electric load of each branch is beneficial in most usual situations and is chosen as a preferred embodiment, as the antenna tuning and uniform plasma generation would be most easily supported. 
   Still, there might be cases when one would prefer a non-uniform plasma generation (e.g., higher plasma generation rate at the wafer periphery). In that case, the principle of equal load for the branches might not be applicable. 
     FIG. 8  illustrates a simplified view of a fifth type of branching RF antenna and second type of branching significantly different from those presented in  FIGS. 2-6 , in accordance with embodiments of the present invention. Branching RF antenna  800  comprises a plurality of branches  810 ,  820  and  830  each branch is a part of a planar spiral. Each branch  810 ,  820  and  830  has a different radial area Branches  810 ,  820  and  830  are fed at the first ends, i.e.  812 ,  822  and  832 , respectively, while the other ends,  814 ,  824  and  834  are electrically grounded. In addition, a single RF power generator can power all branches or different RF power generators (with the same or different RF frequencies, phases or amplitudes) can power different branches. The powered and grounded ends of each branch can be switched, which would exchange powered and grounded ends. 
   In the illustrated embodiment, outer branch  830  comprises a single turn; middle branch  820  comprises about one and a half turns; and inner branch  810  comprises about two turns. A turn is a single revolution of the negative or positive direction. Alternatively, branches  810 ,  820 , and  830  can have different numbers of turns. Outer branch  830  has a first end  832  and second end  834 . Middle branch  820  has a first end  822  and second end  824 . Inner branch  810  has first end  812  and second end  814 . To obtain equal loads, a preferred configuration for antenna  800  should have a different number of turns for each branch with inner branch  810  having the highest number of turns. 
   Furthermore, in cases when the load for the branches differ due to the radial dependence of certain processes as well as the rate of plasma and gas diffusion, different matching networks and/or RF sources can drive the different branches. The specific tuning of the antenna branch parameters are needed to compensate for the radial dependence of processes thus providing a radially independent plasma density profile. When the generation of radially non-uniform plasma is required in special circumstances, the embodiment shown in  FIG. 8  can be conveniently applied. 
   The surface of branching RF antenna  800  shown in  FIG. 8  can be flat, but such an antenna can also be adjusted to the dome-shape or other non-flat surface. 
   The present invention can be effectively applied to plasma processing apparatus such as an etching apparatus. The present invention can also be applied to a plasma processing apparatus other than an etching apparatus, e.g., a film-forming apparatus or an ashing apparatus. The present invention can also be applied to a plasma processing apparatus for a target object other than a semiconductor wafer, e.g., an LCD glass substrate. Additional advantages and modifications will readily occurs to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.