Abstract:
A manufacturing method and apparatus for IC fabrication controls the ion angular distribution at the surface of a wafer with electrodes in a wafer support that produce electric fields parallel to the wafer surface without disturbing plasma parameters beyond the wafer surface. The ion angular distribution function (IADF) at the wafer surface is controlled for better feature coverage or etching. Grid structure is built into the substrate holder within the coating at the top of the holder. The grid components are electrically biased to provide electric fields that combine with the sheath field to distribute the ion incidence angles from the plasma sheath onto the wafer. The grid can be dynamically biased or phased to control uniformity of the effects.

Description:
The present invention relates to integrated circuit (IC) fabrication and to the control of the ions onto the surface of a wafer during processing. More particularly, the invention relates the control of the angles of incidence of ions onto a wafer surface during plasma processing of a semiconductor wafer held on RF-biased wafer holders in processing tools for thin film material processing. 
     BACKGROUND OF THE INVENTION 
     In plasma processing systems, semiconductor wafers are placed on the RF biased substrate holders, such as electrostatic chucks (ESCs) or chucks provided with mechanical wafer clamps, where the wafers are exposed to ions from the plasma. The energy of the ions incident on the wafers from the plasma is typically independently controlled by varying the bias power level, while the ion density is usually controlled by control of an independent plasma source. The plasma source can be, for example, an inductively coupled plasma (ICP) source, a dual capacitively-coupled plasma source (CCP) having a secondary top electrode biased at another RF frequency, a helicon plasma source, a microwave plasma source, a magnetron, or some other independently operated source of plasma energy. 
     An RF biased ESC operates as CCP source in which a wafer having a surface to be etched or coated is placed on an electrode. The wafer support electrode is typically coupled to an RF generator through a blocking capacitor and an impedance matching unit, which does not allow real current to flow from the electrode to the RF generator.  FIG. 1A  is a diagram that represents a bulk plasma  10  in a plasma processing chamber in which a substrate  15  is supported on a substrate support  14  for processing. A plasma sheath  12  occupies a boundary between the plasma  10  and the surface of the wafer  15 . Between the bulk plasma  10  and the plasma sheath  12  is a region that can be referred to as the plasma presheath  13 . 
     At the plasma-to-surface boundary, electrons escape from the plasma and charge the wafer surface with a negative charge. This occurs because the plasma and the biased ESC electrode will find a condition at which electron current reaching the electrode from the plasma exactly balances ion current averaged over one RF cycle. Since electrons are more mobile than ions, the electrode acquires a negative potential that will limit electron current and encourage positive ion current from the plasma to the wafer surface. This negative potential is called the self-bias voltage and results in an energetic ion bombardment of the surface from the plasma, which can, for example, etch the wafer surface. Since the wafer is not formed of a conductive material, and since the substrate holder is usually coated with a dielectric or insulating material, the actual self-bias on the conductive portion of holder closer to the RF source is more negative than the self-bias on the top of the support. Changing applied RF bias power can control the ion bombardment energy and can partially affect the ion flux as well. 
     The initial kinetic energy of the ions in bulk plasma  10  is related to the gas temperature and, typically, is within a range of 0.05 to 0.1 electron volts (eV). Ion motion in the plasma  10  is random due to collisions with background gas atoms or molecules. Also, a directional component may be present in the plasma  10  due to ambipolar diffusion of the ions in an ambipolar electric field inside the plasma  10 , which is usually small, approximately 0.1 to 1 V/cm, but which can contribute to asymmetry in feature processing by bombarding ions. Before ions enter the sheath  12  from the plasma  10 , they gain some directional energy within the presheath  13 . This energy is approximately half the electron temperature T e  of the plasma  10 . Sheath theory tells us that ions enter plasma sheaths with a so-called Bohm velocity, which is proportional to kT e /m i , where m i  is the mass of the ion. In an actual plasma sheath  12 , ions are accelerated by a sheath voltage E that is the difference between the plasma voltage and the bias voltage on the wafer  15 . The sheath voltage E is much larger than the energy gain in the presheath  13 , and is much-much larger than the kinetic energy of the ions in the bulk plasma  10 . 
     Due to the combined random kinetic energy gained in the bulk plasma  10  and directional energy gained within the presheath  13  and sheath  12 , ions will hit the surface of the wafer  15  with a particular angle, or in a particular distribution of angles. This distribution of angles can be referred to as the Ion Angular Distribution Function (IADF)  16 , which can be defined for given process conditions. In typical systems of the prior art, most of the ions are within a small angle cone  17  of less than about 20 degrees when pressures are in the range of 1 milli-Torr (mTorr) to several 100 s of milli-Torr. 
     Experimental data from literature show the IADF at the wafer  15  in an ICP plasma process to be approximately as illustrated as curve  18  in  FIG. 1B , shown for pressures at about 2 mTorr, to that as illustrated as curve  19  at pressures at about 50 mTorr. The graphs are plots of the angular distribution of ions incident at a given point on the surface of a wafer, with the angle being measured relative to a vector normal to the wafer surface at the given point. The graph of  FIG. 1B  shows the distribution at angles measured in a vertical plane through the given point. This distribution is not necessarily the same in every direction or in every vertical plane through the point, and may therefore be regarded as a three-dimensional function of an angle φ to the normal, through a plane oriented at some angle θ to a reference line on the wafer&#39;s surface. 
     Furthermore, the IADF is not necessarily the same at every point on the surface of the wafer, and can vary across the surface of the wafer. Therefore, the IADF can also be considered as a function of φ and θ, that additionally varies as a function of the x,y coordinates on the wafer surface, or more conveniently, of the R,Θ coordinates of the points on the wafer surface. It is not uncommon to experience different ion incidence angles between the center and the edge of a wafer, for example. 
     The IADF plays an important role in feature coverage in ionized physical vapor deposition (IPVD) and other ion-controlled deposition and etching processes. Control of IADF to produce ideal feature coverage has been a challenge in the prior art. In the prior art, there is no independent process variable that allows direct control of the IADF at the wafer surface that is in contact with the plasma that leaves basic plasma process conditions unchanged. 
     Accordingly, there is a need for control of the ion angular distribution function (IADF) in IC plasma processes. 
     SUMMARY OF THE INVENTION 
     An objective of the present invention is to provide a method and apparatus for controlling the IADF at a wafer surface during plasma processing. A particular objective of the invention is to provide IADF-controlled plasma processing, and particularly IADF control that is independent of wafer holder RF bias power and frequency and of operating pressure. 
     One objective of the invention is to control the IADF at one or more given points on the surface of a wafer, for example, to deliver ions to sidewall surfaces of features on a wafer. A further objective of the invention is to control the variation of IADF across the surface of a wafer, either to vary the IADF function across the wafer surface or to make the IADF uniform across the wafer surface, for example, to compensate for the effects of conditions that would undesirably affect IADF. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a diagram illustrating a bulk plasma, a pre-sheath and a plasma sheath in a typical plasma processor of the prior art. 
         FIG. 1B  is a graph of typical ion angular distribution functions (IADF) in an ICP plasma represented by  FIG. 1A  at 2 and 50 mTorr. 
         FIG. 2A  is a diagram showing preferred ion angular incidence vectors for an enhanced sidewall coverage according to certain exemplary embodiments of the present invention. 
         FIG. 2B  is a diagram of one prior art technique for producing ion angular incidence of  FIG. 2A . 
         FIG. 2C  is a graph of an IADF with peaks at angles of 10 degrees and 20 degrees from perpendicular to the surface of a wafer. 
         FIG. 3  is a perspective view of biasing grid structure for a wafer support for IADF control according to one embodiment of the present invention. 
         FIGS. 4A and 4B  are diagrams of electric fields parallel to a wafer surface that can be produced in a sheath by the structure of  FIG. 3 . 
         FIG. 4C  is an enlarged portion of the diagram of  FIG. 4A  showing a simulation of the sheath electric fields. 
         FIG. 5A  is a graph showing certain versions of signals that can be used for energizing grid wires to produce the fields of  FIGS. 4A-4C . 
         FIG. 5B  is a graph, similar to  FIG. 5A , but showing the signals of x direction grid conductors phase shifted relative to the y direction grid conductors. 
         FIGS. 6A-D  are diagrams of ion incidence vectors at particular points on a wafer without and with parallel electric fields produced by the signals of  FIGS. 5A and 5B . 
         FIG. 7  is a simulation diagram showing a focusing effect of electric fields produced by certain embodiments of the IADF control structure of  FIG. 3 . 
         FIGS. 8A and 8B  are graphs, similar to  FIG. 5A , showing signals for energizing the structure of  FIG. 3  according to certain embodiments of the present invention. 
         FIGS. 9A-D  are diagrams, similar to  FIGS. 4A and 4B  showing electric fields produced in a sheath field according to some embodiments of the invention. 
         FIG. 10  is a simulation diagram showing electric fields of  FIG. 9A . 
         FIGS. 11A-D  are simulation diagrams showing electric fields relating to  FIGS. 9A-D . 
         FIG. 12  is a partial cross-sectional view showing the manufacturing detail of the substrate holder of  FIG. 3 . 
         FIG. 13  is a block diagram of one control system for operating a substrate support according to a biasing scheme of  FIGS. 4A-D  and  5 A. 
         FIG. 14  is a block diagram similar to  FIG. 13  of an alternative embodiment of a control system. 
         FIG. 15A  are is a diagram of biasing grid structure for an alternative embodiment of the wafer support of  FIG. 3 . 
         FIG. 15B  is a diagram, similar to  FIG. 15A , of biasing grid structure for another alternative embodiment of the wafer support. 
         FIG. 15C  is a perspective diagram of a section of the grid structure of  FIG. 15B . 
         FIG. 16  is a block diagram of one control system for operating a substrate support according to a biasing scheme of  FIGS. 9A-D  and  8 A. 
     
    
    
     DETAILED DESCRIPTION 
     An example of an ion angular distribution function (IADF)  20  produced by the present invention that creates increased sidewall coverage of features in ionized physical vapor distribution (IPVD) is shown in cross-section in  FIG. 2A  under conditions typical for IPVD. The IADF  20  is not typical of current prior art plasma processing. Mathematically, the IADF  20  can be described as a rotation of the natural IADF  16  ( FIG. 1A ) at certain angle  22  around an axis  21  that is perpendicular to the surface of the substrate  15 , as illustrated in  FIG. 2B . One such IADF  20  might, for example, be that produced in a vacuum process in which a well-collimated ion beam impinges along the axis  21  on the surface of a wafer  15  that is inclined at the angle  22  to the axis  21  and is rotating around the axis  21 , where the process occurs in high vacuum to yield a very low dispersion of the ion beam. The IADF  20 , so produced, is compared to the IADF  16  by curves  23  and  24  in  FIG. 2C , in which IADF  20  is represented by curve  23  for an angle  22  of 10 degrees and by curve  24  for an angle  22  of 20 degrees. 
       FIG. 2C  represents an ideal situation with the effects of reactive gas background, plasma chemistry, etc., excluded. But the performance of systems having such properties can be enhanced by employing techniques that would alter the IADF from that of IADF  16  to that of IADF  23  or  24 . In plasma processing, a parameter that has a noticeable impact on the IADF is background gas pressure. As curves  23  and  24  demonstrate, the lower the pressure, the smaller a cone angle  17 . IADF is also affected by the biasing RF frequency and power, but these also have an impact on the Ion Energy Distribution Function (IEDF) making bias frequency and power in many circumstances impractical for IADF control due to their adverse impact on the processing rates, selectivity, and surface chemistry, for example. 
     Embodiments of the invention may be applied to a generic plasma processing system having a vacuum chamber, one or more plasma sources (CCP, ICP, magnetron, etc.) with appropriate RF components (matching networks, generators, probes, etc.), a wafer holder, biased or unbiased by RF or DC voltage sources, a gas flow system, a vacuum pump system, and other components typical of plasma processing systems. Plasma in such a system is generated by the plasma sources. A self-bias potential is created at the wafer surface, which potential, being negative relative to the plasma potential, causes an energetic ion bombardment of the wafer surface. With such a system, IADF can be initially determined from the background gas pressure and the RF frequency and biasing power at the substrate holder. 
     According to an embodiment of the invention illustrated in  FIG. 3 , a substrate holder  14  is provided with IADF control structure  28  that includes a chuck body or electrode  30 , which may be an electrostatic chuck. The chuck is covered by a dielectric or insulating material  31 , for instance Al 2 O 3 , SiO 2 , or other ceramic material, having a total thickness d T , as illustrated in  FIG. 3 . Within the dielectric material, conductive grid structure  32  is incorporated, for example, as two-layers of parallel conductors  33 ,  34  intersecting in a rectangular manner. The parallel conductor of the layers  33  and  34  are equally spaced and oriented in respective x- and y-directions. The dielectric layer  31  may be formed of three layers  31   a ,  31   b ,  31   c , between which are clamped the two conductor layers  33  and  34 . 
     The individual conductors of each of the layers  33 ,  34  are biased at controllable electric potentials according to certain schemes. The biasing schemes may be such that two neighboring conductors of each layer  33  or  34  create electric fields that have components parallel to the surface of the wafer  15 , as illustrated in  FIGS. 4A and 4B . Alternating potential can be applied to all odd conductors (2k−1)  33   a  of layer  33  ( FIG. 4A ), for example, and same signal but with opposite phase shift (ΔΦ=π) or otherwise inverted can be simultaneously applied to all even conductors (2k)  33   b  of the layer  33  ( FIG. 4B ). That is, for x-axis orientation conductors  33 , voltages on conductors i=1, 3, 5, . . . , 2k−1, . . . equal V x   2k−1 =V x0  sin(ωt), and voltages on conductors i=2, 4, 5, . . . , 2k, . . . equal V x   2k =V x0  sin(ωt+π) =−V x   2k−1 . Similarly, the same alternating potential scheme can be applied to all odd conductors  34   a  of layer  34  with inverted signals applied to all even  34   b  conductors of layer  34 . 
     Writing the expressions in more generalized form including an x-to-y phase shift ΔΦ xy , the equations will appear (using y-axis equations as an example) as voltages on conductors j=1, 3, 5, . . . , 2k−1, . . . equal to V y   2k−1 =V y0  sin(ωt+ΔΦ xy ), and voltages on conductors j=2, 4, 5, . . . , 2k, . . . equal to V y   2k =V y0  sin(ωt+π+ΔΦ xy )=−V y   2k−1 . When ΔΦ xy =0 the electric field in the sheath  12  will focus spatially as shown in  FIG. 7 , that is, into individual spots in the manner of a “focus-defocus” action. The phase differences ΔΦ xy =π/2 will generate a sequence of focused sheath E-field vectors into lines in the x-direction with transition into focused sheath E-field vectors into lines in the y-direction at frequency 2πω. This will generate additional motion into the sheath E-field that is an azimuthal component of the E-field at a given point on the surface of the wafer in a local coordinate system centered at the point in question. 
       FIGS. 4A and 4B  demonstrate the electric fields in the direction of their components parallel to the wafer surface. For simplicity, the electric field components due to the plasma and wafer self-bias potentials are not shown in  FIGS. 4A and 4B . For illustration purposes, the results of 3D electrostatic simulation of the electric fields between two conductors of either of the conductor layers  33  or  34  are enlarged and shown in  FIG. 4C . The examples use the same amplitudes and frequencies for the signals on the two sets of grid wires  33  and  34 . While this is the most likely use, particularly where the wires of both wire sets are equally spaced, these parameters can be varied to accommodate asymmetrical wafer or chamber conditions. Variable distance between individual conductors could require also adjusting potential amplitude to produce desired values of the electric fields across the whole wafer area. Providing different potential amplitude for each conductor, while usually less practical, may be useful for specific geometries, such as to accommodate effects at the edge of the wafer. 
     Signal shapes and phasings for pairs of conductor of the set  33  in the x-direction and for pairs of conductors of the set  34  in the y-direction are shown in  FIGS. 5A and 5B , which illustrate principles that can be employed in various embodiments of the invention. Alternating harmonic or sinusoidal signals are often the most practical, though other forms of signal may be implemented, including pulsed signals, rectangular or triangular wave shape signals, etc. The signals should be selected to produce a substantial electric field parallel to the wafer surface, which is achieved by applying regular and inverted signals on alternating wires. The term “regular and inverted signals” is a more general term that includes harmonic signals for the respective (2k) and (2k−1) parallel conductors. Further, alternating wires need not be adjacent wires, as will be seen in examples described below. 
     Examples of the signals for pairs of conductors in each of the x- and y-directions are shown in  FIG. 5A . Phase shift is shown between two (2k) and (2k−2) parallel conductors. For simplicity, in  FIG. 5A , there is no phase difference between the signals in x- and y-direction (Δφ xy =0) and the signal amplitudes on all the grid wires is the same as are the wire spacings in  FIG. 5A . However, arbitrary phase difference between signals in the x- and the y-directions can be utilized. For example,  FIG. 5B  shows the signals on the y grid inverted but shifted relative to the phase on the x grid. 
     Some of the effects of phase shifting can be understood with reference to  FIGS. 6A-D  and  7 . When the phase difference between the x and y conductors is zero (ΔΦ xy =0), the electric field in the sheath will focus spatially into individual spots in a “focus-defocus” action as a result of a change of the radial component of E-field in a coordinate system local to each spot. At zero phase difference between the x- and y-directions, the E-field in the sheath will be pulsing between a focused and unfocused status. The transverse E-field component will create a “spot-like” pattern on the wafer. Only the local radial component of the transverse E-field will change according to 2πωt, with the local azimuthal component being zero, as illustrated in  FIGS. 6A and 6B , respectively, depending on the location on the wafer relative to the grid wires. This is shown in the top view by the solid arrows in  FIG. 6D , where in the electric field will be focused in solid spots that are labeled A and de-focused from A into the open spots that are labeled B. The identical situation will be produced also for Δφ xy =π, 2π, 3π, . . . . 
     The introduction of phase difference will generate a pattern of a focused E-field in the sheath with its lines in the x-direction, with a transition into a focused sheath E-field with lines in a y-direction directly above the grid wires, but with a non-zero azimuthal or rotational component of E-field at each spot midway between the grid wires in local coordinate system, as illustrated in  FIG. 6C . The motion of the vectors will range from circular (Δφ xy =π/2), to elliptical (when Δφ xy  is non-zero or different from a multiple of π/2), as illustrated by the broken lines in  FIG. 6D , to linear (Δφ xy =0), depending on phase angle and position relative to the grid. 
     In plasma the combination of the sheath electric field (E sheath ) and superimposed field from the grid (E grid ) will create specific (E sheath(grid) ) field as it is shown in  FIGS. 6A-C . In the 3D simulation of the static electric field for the portion of a rectangular grid shown in  FIG. 7 , the odd number (2k−1) conductors were biased by +50 V and the even number conductors (2k) by −50 V. The plasma potential was assumed to be +20 V, and the self-bias at the electrode was −50 V. One can see that ions experience a focusing effect into areas “A” between the positively biased conductors. 
     Frequency differences between x and y conductors will add additional rotations and variations, particularly at areas were the elliptical patterns are present. Such phase and frequency differences are not necessary to the invention, but can be used in connection with the invention, and may be helpful in some applications, and can be helpful, for example, to smooth or soften grid pattern effects across a wafer. 
     Application of the principles described above can be used to control or improve sidewall coverage at selected places on a wafer. Without more, however, such applications do so in a grid-like pattern can be over the wafer. For example, the fields shown in  FIGS. 4A-C  can exist along the lines  41  or  42  in  FIG. 6D , which are between and equidistant from the oppositely biased conductors. This is useful for demonstrating principles of various embodiments of the invention, and for certain special applications. 
     For example, with x- and y-conductors phased at π/2 relative to each other, a symmetrical IADF as in  FIG. 2A  or  6 C will be produced at the centers of each of the squares defined by the perpendicular grid wires of sets  33  and  34 . But off the centers of the squares and directly under the grid wires, the effects will not be so symmetrical. So, unless a process can advantageously utilize such patterned IADF control, such as where dies on the wafer correspond to the grid pattern, additional application of other principles of the invention might be preferable. 
     The principles described above can be used to control side wall coverage, or otherwise control IADF in plasma processes, in a uniform manner. By switching, shifting, or otherwise moving the fields, produced by biasing the conductors as described above, over the wafer in such a way that the integrated or average field parallel to the wafer is the same at all points and in all directions over the wafer, a uniform effect can be achieved. Some embodiments for doing so are described below. 
     Referring to  FIGS. 4A-C  and  6 D, in positions exactly above the grid conductors, the electric field (E sheath(grid) ) has a very small component parallel to the wafer (E ∥ ˜0). By replacing each of the grid wires in the examples described above with a pair of wires, and applying potentials alternately between the two, E ∥  can be made effectively uniform across the wafer on a time averaged basis. By doing so, another set of grid wires is provided midway between the wires of the original set, and the bias potential is switched between them. In this way, E ∥  is alternately switched between zero and maximum over the wires of each set, producing an uniform average E ∥  over the wafer. 
     That is, instead of inverting the bias on adjacent conductors of a set, a signal sequence is applied for groups of four adjacent conductors in a way that provides homogeneous and non-zero average E ∥  above the whole wafer or an area thereof. The signal shape, phasing and conductor group is illustrated in  FIGS. 8A and 8B . The signals in  FIG. 8A  are shown being switched after each single cycle, though multiple cycles per switching period can be used as well, as illustrated in  FIG. 8B . In  FIG. 9 , it can be seen that each position immediately above a grid conductor is exposed for some period of time to an electric field parallel to wafer, E ∥ . 
     For example, the individual conductors to which the signals of  FIG. 8A  are applied in each of the layers  33 ,  34  can be considered as arranged in groups of four conductors,  33   a ,  33   b ,  33   c  and  33   d , and  34   a ,  34   b ,  34   c  and  34   d , respectively. The E ∥  fields produced are those shown in  FIGS. 9A ,  9 B,  9 C and  9 D, respectively. The signal shape, as illustrated in  FIG. 8A , can be described as follows:
         for x-axis conductors:
           i=1, 5, 9, . . . , a, . . . V x   a  (Δφ=0)   i=2, 6, 10, . . . , b, . . . V x   b =V x   a  (Δφ=π)   i=3, 7, 11, . . . , c, . . . V x   c =V x   a  (Δφ=0)   i=4, 8, 12, . . . , d, . . . V x   d =V x   a  (Δφ=π)   
           for y-axis conductors:
           j=1, 5, 9, . . . , a, . . . V x   a  (Δφ=0+Δφ xy )   j=2, 6, 10, . . . , b, . . . V x   b =V x   a  (Δφ=π+Δφ xy )   j=3, 7, 11, . . . , c, . . . V x   c =V x   a  (Δφ=0+Δφ xy )   j=4, 8, 12, . . . , d, . . . V x   d =V x   a  (Δφ=π+Δφ xy )
 
The expressions for y-axis conductors are written more generally to allow for phase shift between x- and y-axis conductors. Identical amplitudes can be used, particularly grids conductors are equidistant in both directions. A single source can be used with a phase shifter and signal inventor to bias the entire grid.
   
               

       FIG. 10  is a simulation of the biasing scheme of  FIGS. 4A-C  and  5 A that shows results on potential distribution and electric field between the plasma  10  and the holder electrode  14 , through the wafer  15 , with the grid structure conductors  33   a  biased at +50 V and conductors  33   b  biased at −50 V. The self-bias potential at electrode  14  is taken at about −100 V, and the plasma  10  potential at about 20 V. The electric field lines are focused between two positively biased conductors  33   a  and  33   b  (area “A”). 
       FIG. 1A  shows the x-direction electric field and potential distribution with the bias scheme of  FIG. 8A  at t=π/2 in the cycle ( FIG. 9A ), where E ∥  is maximum over conductors  33   b  and  33   d  and zero over conductors  33   a  and  33   c .  FIG. 11B  illustrates the fields with the same scheme at t=3π/2 ( FIG. 9B ), where E ∥  is also maximum over conductors  33   b  and  33   d , but reversed in direction, and is also zero over conductors  33   a  and  33   c .  FIG. 11C  shows the x-direction electric field and potential distribution with the same bias scheme of  FIG. 8A  at t=5π/2 in the cycle ( FIG. 9C ), where E ∥  is maximum over conductors  33   a  and  33   c  and zero over conductors  33   b  and  33   d , while  FIG. 11D  illustrates the fields with the same scheme at t=7π/2 ( FIG. 9D ), where E ∥  is also maximum over conductors  33   a  and  33   c , but reversed in direction, and is zero over conductors  33   b  and  33   d . Groups of six or more conductors can be used and switched in pairs to achieve greater smoothing effects. 
     With the y-direction conductors similarly biased, over a cycle of t=0 to t=4π, the E ∥  field vector in the sheath will have generally had the same average intensity, direction and presence across the entire wafer. That is, the potential applied to each group of four conductors  33   a ,  33   b ,  33   c  and  33   d , generates, within the complete signal sequence or cycle, a parallel electric field above each conductor on the wafer surface. The bias is similarly applied to the y-direction conductors  34   a ,  34   b ,  34   c  and  34   d . A phase angle of, for example, π/2 can be imposed between the x and y conductors to create an angularly symmetrical distribution. 
     The range of frequencies of the grid potential used to alternate the horizontal electric field should be selected to insure that the ions pass from the plasma, through the sheath and onto the wafer, in much shorter time than the polarity switching time of energized conductors. Assuming sheath width to be about 10 mm, plasma potential to be about 20 V, and wafer self-bias to be about −50 V, the time for argon ions to pass through the sheath will be approximately 5.4×10 −7  seconds, which corresponds to a switching time of about 934 kHz. Thus, the frequency of the grid potential should be significantly less than 1 MHz, and practical values suggest around 100 kHz and less. The lower limit for applied frequency is given by the typical processing time in plasma and the type signal sequence. Practical values for frequency should be at least about 10-100 Hz. Different frequencies can be applied to the x and y conductors to achieve such effects as rotating IADFs. 
     The amplitude of the alternating potential on the grid conductors at the IADF control structure is based on typical voltage characteristics in plasma processing system. With plasma potential in a range of around +15 to 30 V and self-bias from −10 to −100 V, the electric field normal to wafer is about 250 to 1300 V/cm for sheath of about 1 mm in thickness, or 50 to 260 V/cm for a 5 mm sheath thickness. To produce an angle of 10 degrees declination to the normal of the ion incidence vector, a horizontal field of about 44 to 230 V/cm is needed at the wafer surface for a 1 mm sheath, and of about 9 to 46 V/cm for a 5 mm sheath. To produce declination angle 20 degrees, a horizontal field of about 91 to 470 V/cm is needed at the wafer surface for 1 mm sheath, and of about 18 to 94 V/cm for a 5 mm sheath. In that the wafer thickness is about 0.8 mm and the total insulator thickness is several hundreds of micrometers but less than 1 mm, and with the distance between two parallel conductors with opposite potential selected at about 1.4 to several millimeters, the alternating amplitude should be from several volts up to about 100 V, and in extreme cases several 100 s of volts. The most optimal potential value will vary depending on the particular process and can be obtained by experimentation for given plasma process conditions or by simulation. 
     The grid for the control of the IADF ( FIG. 3 ) can be formed on the substrate holder by coating the chuck or base electrode  30  with dielectric material using the typical manufacturing methods, such as plasma coating by spraying, thermal CVD, sputtering, etc., as illustrated in  FIG. 12 . The dielectric materials can be materials like Al 2 O 3 , Si 3 N 4 , SiLK, SiC, etc., with the choice depending on the particular application and the chemistry involved in process. Other suitable polymer materials like PMMA, Teflon may be used as well. The conductive lines are created using a deposition through a mask or reticule, with the pattern of conductors  34  in the y-direction formed of equidistant lines. PVD deposition techniques are typically used. Alternatively, a pattern can be produced by blanket deposition of the metal, applying a photosensitive or photoresist layer, exposing the layer through a mask, developing and then etching the layer to pattern the metal, and then removing the resist. 
     After the first layer of conductors is applied, the intermediate insulator layer  31   b  is deposited in a similar manner as the first insulator  31   a . Then the conductive pattern of x-direction conductors  33  is applied in a manner similar to that used for application of the conductors  34 . Then, a top insulating layer  31   c  is applied followed by planarization of the coating, and polishing, cleaning, etc. Details of creating buried electrodes in ceramics are known to persons skilled in that technology. 
     The IADF control is a structured electrode that can be incorporated into the a mono- or bi-polar electrostatic chuck (ESC). The methods of creating buried electrodes in such chucks are known to those skilled in ESC manufacturing. Current ESCs have been designed for wafer size up to 300 mm. The ESCs are coated by suitable insulating material in which RF electrodes and chucking electrodes are buried. Recent ESC designs utilize a single electrode configuration both for RF and ESC biasing. The ESC electrodes used for RF biasing can be used to provide both RF bias and chucking voltage. Either mono- or bi-polar chucks can be used in plasma processing tools. The IADF control or structured electrode can fulfill the function of controlling the ion directionality within a plasma sheath, and can also function as a chucking electrode and an RF biasing electrode. Other electrical biasing can be applied to the structure, for example for heating the wafer by buried heaters, etc. 
     An IADF control system  50  for a IADF control structure  28  as illustrated in the diagram of  FIG. 13 , which has the sets of conductors  33  and  34  respectively oriented in the x- and y-directions. In each set of the parallel conductors  33  and  34 , the odd and even conductors are configured to be biased by alternating potential from an IADF control unit  51 . The system  50  also includes an RF bias control unit  52  to bias the same electrode  33  and  34  to use the same electrodes  33  and  34  to apply bias voltage to the chuck  14 . The control unit  51  includes an alternating potential source  53 , having a direct output  53   a  and in inverted output  53   b , to produce the E ∥  potential. The RF bias control unit  52  has an RF source  55  connected through a matching network  56  to the electrodes  33  and  34 . To prevent the RF signal from the bias control unit  52  from flowing into the IADF control unit  51 , low-pass filters  54  connected between the source  53  and the conductors  33  and  34 . A high-pass filter  57  is also used to protect a matching network  56  and RF generator  55  from the potential source  53 . In lieu of low pass filters  54  and high pass filter  57 , notch filters or other isolation schemes can be used. 
     An ESC control unit  58  is also provided, that may be connected in a similar way to the electrodes  33  and  34  to supply chucking potential. Band-stop filters  59  may be used to avoid interference on the ESC control unit  58  from the lower RF frequency grid potential and at the RF bias frequency (typically 13.56 MHz), though allowing chucking and de-chucking the wafer. 
     Another version of an IADF control system  50  is the system  60  illustrated in  FIG. 14  for energizing the sets of conductors  33  and  34  of the IADF control structure  28  in a manner similar to that of system  50  of  FIG. 13 . The odd and even conductors of each of the sets are biased by alternating potential and supplied from the source  53  of the IADF control unit  51 . The RF bias control unit  52  biases the same electrodes  33  and  34 . To prevent RF signal to flow into IADF control unit  51 , the low-pass filters  54  are provided. Two high-pass filters  57   a  and  57   b  can be provided to protect matching network  56  and RF generator  55  in RF bias control unit  52 . The ESC control unit  58  is connected in similar way using the band-stop filter  59  to avoid interference of the potential at grid frequency and the RF frequency bias frequency. 
     Alternatives to the IADF structure  28  of  FIG. 3  are illustrated in  FIG. 15A  and in  FIGS. 15B and 15C . The embodiment  28   a  of  FIG. 15A  has sets of conductors  33  and  34  each arranged in a rectangular arrays  63  and  64 , each spaced by the intervening insulator  31   b . The circles show connections of two perpendicular wires of the rectangular arrays. The electrode structure  28   a  has a lower inductance than structure  28 , which has an advantage for RF biasing with the same electrodes. The electrical potential can be applied to the structure  28   a  in the same manner as applied the electrodes of structure  28  by systems  50  and  60  of  FIGS. 13 and 14 . The opposite polarity is imposed between electrodes  63  and  64 . Despite the difference in geometrical configuration, the performance of the structure  28  and  28   a  will be similar. 
     Another embodiment for the IADF control structure  28  is the structure  28   b  shown in  FIGS. 15B  and C, in which radial grids or arrays  65  and  66  replace the rectangular array  63  and  64 , of structure  28   a , each formed of concentric circular conductors connected to radial conductors, with conductors  65  and  66  separated by the intermediate dielectric layer  31   b . With the structure  28   b , the opposite polarity conductors is imposed between the two conductor arrays  65  and  66 . Other combinations of structures can be used according to principles of the present invention. 
     The structures  28   a  and  28   b  perform similar to the electrodes described in connection with  FIGS. 4A and 4B , which are described for simplicity. Performance similar to that of  FIGS. 9A-D  can result by doubling each of the arrays and separately controlling each in accordance with the signals of  FIG. 8A . This is achieved by use of a version of the control system  50 , such as control system  70  illustrated in  FIG. 16 . In the control system  70 , the doubled sets of the conductors  33  and  34  each contains multiple groups of four conductors  33   a - 33   d  and  34   a - 34   d , respectively, energized by IDAF control unit  71  according to the signals of  FIG. 8A . Conductors  33   a ,  34   a  and  33   c ,  34   c  of grid are biased by alternating potential supplied by the grid source  53 , with the conductors  33   a  and  34   a  being biased through a low pass filter  54  from the direct output  53   a  of the grid supply  53  while the conductors  33   c  and  34   c  are biased through a low pass filter  54  from the grid supply  53  through an inverter  53   b . Conductors  33   b ,  34   b  and  33   d ,  34   d  of each grid are biased by alternating potential and supplied through a 2π phase shifts or delays  73  and  74  from grid source  53 , with the conductors  33   b  and  34   b  being biased through a low pass filter  54  from the direct output  53   a  of the grid supply  53  while the conductors  33   d  and  34   d  are biased through a low pass filter  54  from the grid supply  53  through an inverter  53   b . As such, the biasing of pairs  33   a  and  33   b , and pairs  34   a  and  34   b , is alternated with the biasing of pairs  33   c  and  33   d , and pairs  34   c  and  34   d . That means the a-b pairs are biased when the c-d pairs are at zero potential, and the c-d pairs are biased when pairs a-b are at zero potential, as in  FIG. 8A , or similarly as in  FIG. 8B . 
     Also in  FIG. 16 , the RF bias control unit  52 , equipped with high pass filters  57 , can be used to bias the same electrode as IADF control unit  71  does. To prevent RF signal to flow into the IADF control unit  71 , the low-pass filters  54  are used, while the high-pass filters  57  are used to protect the matching network  56  and RF generator  55 . The ESC control unit  58  is connected in a similar way as in  FIGS. 13 and 14  through band-stop filter  59  to avoid interference of the potential at grid frequency, though allowing chucking and de-chucking the wafer. 
     Although only certain exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention.