Abstract:
A method and apparatus is provided for obtaining a low average electron energy flux onto a substrate in a processing chamber. A processing chamber includes a substrate support therein for chemical processing. An energy source induced plasma, and ion propelling means, directs energetic plasma electrons toward the substrate support. A dipole ring magnet field is applied perpendicular to the direction of ion travel, to effectively prevent electrons above an acceptable maximum energy level from reaching the substrate holder. Rotation of the dipole magnetic field reduces electron non-uniformities.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    Pursuant to 37 C.F.R. § 1.78(a)(4), the present application claims the benefit of and priority to co-pending Provisional Application No. 62/142,868 (Attorney Docket No. TEA-057) filed on Apr. 3, 2015, and entitled DIPOLE RING MAGNET ASSISTED MICROWAVE RADIAL LINE SLOT ANTENNA PLASMA PROCESSING METHOD AND APPARATUS, which is expressly incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to plasma processing with microwave (MW) plasmas, particularly surface wave plasmas (SWP), for example as produced with a Radial Line Slot Antenna, and more particularly, to a method and apparatus providing relatively low electron energies and plasma uniformity near a treated substrate. 
       BACKGROUND OF THE INVENTION 
       [0003]    Typically, during semiconductor processing, a plasma etch process is utilized to remove or etch material along fine lines or within vias or contacts patterned on a semiconductor substrate. The plasma etch process generally involves positioning a semiconductor substrate with an overlying patterned, protective layer, for example a photoresist layer, into a processing chamber and etching exposed areas of the substrate through the pattern. 
         [0004]    Once the substrate is positioned within the chamber, it is etched by introducing an ionizable, dissociative gas mixture into the chamber at a pre-specified flow rate, while adjusting a vacuum pump to achieve a processing pressure. Then, plasma is formed when a portion of the gas species is ionized by collisions with energetic electrons. The gas may be ionized by direct current, radio frequency, microwave energy, or other energy sources known to the art. The energetic electrons dissociate some of the gas species in the gas mixture to create reactant species suitable for the exposed-surface etch chemistry. Once the plasma is formed, exposed surfaces of the substrate are etched by the chemistry at a rate that varies as a function of plasma density, average electron energy, and other factors. The process is adjusted to achieve optimal conditions, including an appropriate concentration of desirable reactant and ion populations to more selectively act upon various desired features (e.g., trenches, vias, contacts, etc.) in the exposed regions of substrate. The exposed regions of the substrate where etching is required are typically formed of materials such as silicon dioxide (SiO 2 ), poly-silicon and silicon nitride, for example. 
         [0005]    Dissociative attachment, in forming the ions during the exposed substrate etch process, prefers electron energy to be as low as possible. An example is where the chemistry is a Cl2+e- process yielding Cl+Cl-, the negative ions being extracted for etching. Moreover, it is highly desirable to accomplish this reaction in a spatial afterglow region of a plasma, to avoid periodic plasma density deficiencies and the resultant increased risk of damage from unmitigated electromagnetic waves reaching the substrate, for example in microwave plasma sources. 
         [0006]    While plasma etching has proven to be generally effective, process efficiency may be negatively impacted by a variety of factors. For example, undesirably high average electron energies (T e ) tend to impede ion formation, and thus results in reduced dissociative attachment at the substrate. Attempts to attenuate the negative effects noted above have included the introduction of Multi Pole Magnet (MPM) assemblies, for example as described in U.S. Pat. No. 5,595,627, entitled “Plasma Etching Method” and hereby expressly incorporated herein by reference. These configurations are not conducive to correcting magnetic irregularities by rotating the magnet assembly. Additionally, attempts to improve ion formation at the target substrate, have often necessitated more complex processing chemistries. 
         [0007]    Therefore, an apparatus and method for uniformly applying low average electron energy plasma etch, to a substrate, is needed. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with principles of the present invention, a low average electron energy flux onto a substrate in a processing chamber is achieved by providing a cross-B magnetic field in the processing chamber between a plasma source and a substrate. The cross-B magnetic field directs high energy electrons moving through the plasma process space, leaving electrons arriving in the region near the substrate at an acceptably low ion energy. 
         [0009]    In accordance with certain embodiments of the invention, the processing chamber is surrounded with a magnet array that forms a magnetic field through the chamber that is generally perpendicular to an axial path through the chamber from the plasma source, at one end of the chamber, to a substrate support at the other end of the chamber. In accordance with more particular embodiments of the invention, the field is rotated about the central axis of the chamber to enhance azimuthal uniformity of the flux onto the wafer. 
         [0010]    In certain methods of the present invention, energy from a plasma source is imparted into a processing space to generate a plasma at one end of a processing chamber. Energetic electrons propagate toward a substrate that is supported at an opposite end of the processing chamber. A dipole magnetic field is imposed across a process space between the plasma source and the supported substrate, perpendicular to the direction of electron motion, with sufficient strength, height, and position to cause electrons having energies below an acceptable maximum level to divert from the substrate, but to allow electrons having energies at or below the acceptable maximum energy to be concentrated onto the substrate. The process may also include the step of reducing electron density non-uniformity by rotating the magnetic field coaxially about the processing chamber. 
         [0011]    In certain embodiments, the dipole magnetic field is applied by utilizing an array of magnets spaced around the exterior of the processing chamber. In another embodiment, the array of magnets is comprised of a collection of magnetic columns. In yet another embodiment, the magnetic columns are comprised of a stacked plurality of discrete magnetic elements. In any of the above mentioned embodiments, the field may be applied utilizing permanent magnets. 
         [0012]    According to principles of the present invention, a plasma processing apparatus is provided which includes a substrate support at a first end, and an energy source coupled to a second end of the processing chamber. The apparatus is further configured with a Dipole Ring Magnet (DRM) assembly, which envelops the exterior of the processing chamber. The DRM field is configured to pass perpendicular to, and act upon, the plasma region between the energy source and the substrate support, which results in filtering the higher energy electrons and allowing lower energy electrons to propagate from the plasma source to a quiescent region near the target substrate. In one embodiment he DRM assembly may configured to rotate coaxially around the processing chamber, serving to smooth out electron density irregularities. 
         [0013]    These and other objectives and advantages of the present invention are set forth in the following detailed description of the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a cross sectional view of one embodiment of the disclosed processing system. 
           [0015]      FIG. 2  is a top view of the processing chamber and DRM configuration. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0016]    A plasma processing method and apparatus are disclosed in various embodiments. However, one skilled in the relevant art will recognize that the various embodiments may be practiced without one or more of the specific details, or with alternative methods, materials, or components. Well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. 
         [0017]    Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth in order to provide a thorough understanding of the invention. Nevertheless, the invention may be practiced with alternative specific details. Furthermore, the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale. 
         [0018]    References throughout this specification to “one embodiment” or “an embodiment” or “certain embodiments” or variations thereof means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention, but do not denote that they are present in every embodiment. Thus, the appearances of the phrases such as “in one embodiment” or “in an embodiment” or “in certain embodiments” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0019]    Nonetheless, it should be appreciated that, contained within the description are features which, notwithstanding the inventive nature of the general concepts being explained, are also of an inventive nature. 
         [0020]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several embodiments or several views,  FIG. 1  illustrates a plasma processing system  10  according to certain embodiments of the invention. The plasma processing system  10  may be, for example, a dry plasma etching system or a plasma enhanced deposition system. 
         [0021]    One embodiment of the plasma processing system  10  includes a processing chamber  12  having a chamber wall  14  configured to enclose a process space  16 . The chamber wall  14  is typically a cylinder having a central axis  15 . The processing chamber  12  has therein a substrate support  18  configured to support a substrate  20  in the process space  16 . Furthermore, the plasma processing chamber  12  has a plasma source  22  coupled to the processing chamber  12  and configured to energize a plasma in the process space  16 . During operation of the system  10 , the substrate  20  is exposed to plasma electrons, or process chemistry, or both, in process space  16 . The plasma source  22  of the system  10  may be a surface-wave plasma (SWP) source that includes a radial line slot antenna  24 . The plasma source  22  may be energized by direct current, radio frequency, microwave energy, or other energy sources known to the art. A quartz window  26  may be included to form a sealed interface between the plasma source  22  and the process pace  16 . The plasma source  22  and the substrate support  18  are coaxially aligned on the chamber axis  15 . 
         [0022]    As seen in  FIG. 1 , the plasma processing system  10  is provided with a plurality of individual magnetic columns  30 , which, when combined together, form a dipole ring magnet (DRM) as illustrated further in  FIG. 2 . The magnetic columns  30  are situated coaxially outward from the chamber wall  14 , and run substantially parallel with the chamber wall  14  and parallel to adjacent magnetic columns  30 . To cooperate with the other magnetic columns  30  to form a DRM, each magnetic column  30  is rotated incrementally about its axis, which is parallel to the axis  15  of the chamber  12 , such that the magnetic forces of each individual magnetic column  30  combine to result in an overall dipole magnetic field  44 . In one embodiment, the field lines pass generally from the northernmost point of the dipole ring, to the southernmost portion of the dipole ring. The rotations of the magnets are related such that the lines of the B-field of the DRM through the processing space  16  in the chamber are parallel and perpendicular to the chamber axis  15 . 
         [0023]    In use, the magnetic columns  30  are positioned and dimensioned to allow the resulting magnetic field lines to act perpendicularly upon some portion of the process space  16 . The columns originate at a first distance  32  below the quartz window  26 , and extend to a second distance  34  below the quartz window  26 . In one embodiment, the first distance originates where the plasma is no longer exothermic. The length of each magnetic column  30  should be dimensioned so that it is long enough to achieve electron cooling and low energy selection, and short enough to minimize plasma density loss. If the wafer is radio frequency (RF) biased, the magnetic columns  30  could be well above the substrate  20  to minimize DRM RF heating. Alternatively, terminal ends of the magnetic columns  30  could be placed in close proximity to the substrate  20  to take advantage of the DRM RF plasma. In one embodiment, the magnetic columns  30  originate at a first distance  32  of approximately 10 cm below the quartz and extend to a second distance  34  of approximately 45 cm below the quartz window  26 . In one exemplary configuration, the substrate  20  could be placed coincident with the plane formed by the terminal ends of the magnetic columns  30  to eliminate DRM RF plasma effects. 
         [0024]    In use, a plasma source  22  is energized to generate a plasma within the process space  16 , which forms at the end of the chamber adjacent the window  26  and propagates toward the substrate support  18  into the plasma space  16 . The Radial Line Slot Antenna  24 , coupled to a microwave source, may be used to form a surface wave plasma. Details of the Radial Line Slot Antenna  24  in an SWP source are described in U.S. Pat. No. 8,114,245, hereby expressly incorporated herein by reference. In the plasma, if left unchecked, a majority of the produced electrons are excessively energetic, which results in unacceptably low negative ion concentrations in the region near the substrate  20 . This can result in poor performance of the plasma etch upon the substrate. However, by introducing the magnetic columns  30 , the resulting DRM field advantageously influences the distribution of electron energy levels near the substrate  20 . 
         [0025]    As high energy electrons travel from the top of the process space  16  to the substrate  20 , the most energetic electrons are highly influenced by the DRM field and are deflected away from the substrate  20  with a force perpendicular to their direction of travel. Conversely, lower energy electrons are less significantly impacted by the DRM magnetic field forces, and are permitted to propagate further downward into the chamber toward the substrate  20 . The presence of the DRM field results in an electron energy gradient  36 , wherein less desirable high energy electron concentrations reside at the top of the process space  16 , or are directed into the chamber wall  14  where they are sent to ground potential. Conversely, lower energy electrons propagate into a quiescent region  38  near the substrate  20 , where their reduced energy facilitates negative ion production and effective etch performance. 
         [0026]      FIG. 2  is a top view of the plasma processing system  10 . The plurality of magnetic columns  30  cooperate to form a dipole ring magnet (DRM)  40 , which surrounds the chamber wall  14 . Each of the magnetic columns  30  are immovably mounted with a degree of incremental rotation so that the column field lines  42  cooperate to produce the desired collection of DRM field lines  44 . For clarity of discussion, the resultant magnetic field of the DRM  40  can be described as having a north, east, south and west pole. The resulting DRM field lines  44  can be arbitrarily directed by orienting two opposite magnetic columns  30 N and  30 S so that their column field lines  42  are coincident with the center of the process space  16 . 
         [0027]    By way of example in  FIG. 2 , the northernmost magnet column  30 N and southernmost column  30 S, are positioned so that their respective column field lines  42  are oriented due south and pass through the center of the substrate support  18 . To further cooperate with those two selected magnetic columns  30 N,  30 S, the easternmost magnetic columns  30 E and westernmost column  30 W are positioned so that their respective column field lines  42  are oriented due north. Magnetic columns  30   i  subtending the arc between the aforementioned four magnetic columns  30 N,  30 S,  30 E,  30 W, are progressively rotated by a fixed angular measurement. This angular positioning is determined by 180/(n+1), where n equals the number of magnetic columns  30  positioned between adjacent “compass” poles. For example, in  FIG. 2 , there are five magnetic columns positioned between the northernmost and easternmost magnetic columns  30 . Therefore, each magnetic column  30  is rotated thirty degrees as they progress around the perimeter of the chamber wall  14 , although these rotations may be modified to maintain parallel field lines in the chamber or to obtain some other field shape, if desired. Necessarily, diametrically opposed magnetic columns  30  have column field lines  42  oriented in the same direction. For example, in a proper configuration, the northeast and southwest magnetic columns  30  have column field lines  42  oriented in the same direction. 
         [0028]    Overall, what results, are DRM field lines  44  which are oriented horizontally with respect to the target substrate, and pass perpendicular to the direction of electron travel. While other ring magnetic assemblies are known to the art of plasma etch processing, they are often implemented in a multi-pole configuration. For example, adjacent columns may have alternating poles facing the center of the substrate  20 , configured to direct electron densities away from the chamber wall  14 . However, a multi-pole magnet disadvantageously possesses a strong radial gradient and azimuthal field symmetry. The DRM  40  of the disclosed invention, however, aims to beneficially concentrate low energy electrons near the substrate  20  being processed while maintaining a high degree of uniformity. Because of the DRM  40  improved radial uniformity, the entire DRM  40  may be coaxially rotated with respect to the substrate  20  and chamber wall  14 , to diminish any electron concentration irregularities that may exist. 
         [0029]    Although only certain 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 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.