Abstract:
Methods of using a plasma generator to ash a work piece is provided. In an exemplary embodiment, the method includes flowing gas that has a gaseous component able to form plasma under conditions of radio-frequency energy excitation into the container. A proportion of the gas is directed to a first region of the container to form a higher gas density in the first region of the container and a corresponding lower gas density in a second region of the container. Sufficient energy is applied to the gas in at least the first region to excite a proportion of the gaseous component able to form plasma.

Description:
TECHNICAL FIELD 
       [0001]    The present technology relates generally to methods used in the fabrication of semiconductor devices, and more particularly, the present technology relates to methods of generating plasma used in ashing and surface treatment procedures. 
       BACKGROUND 
       [0002]    In semiconductor manufacturing, plasma ashing is the process of removing the photoresist from an etched wafer. Plasma in this context is an ionized form of a gas. A gas ionizing apparatus, also referred to as a plasma generator, produces a monatomic reactive species of oxygen or another gas required for the ashing process. Oxygen in its monatomic or single atom form, as O rather than O 2 , is the most common reactive species. The reactive species combines with the photoresist to form ash which is removed from the work piece with a vacuum pump. 
         [0003]    Typically, monatomic oxygen plasma is created by exposing oxygen gas (O 2 ) to a source of energy, such as a RF discharge. At the same time, many charged species, i.e. ions and electrons, are formed which could potentially damage the wafer. Newer, smaller circuitry is increasingly susceptible to damage by charged particles. Originally, plasma was generated in the process chamber, but as the need to avoid charged particles has increased, some machines now use a downstream plasma configuration, where plasma is formed remotely and channeled to the wafer. This reduces damage to the wafer surface. 
         [0004]    Monatomic oxygen is electrically neutral and although it does recombine during the channeling, it does so at a slower rate than the positively or negatively charged particles, which attract one another. Effectively, this means that when substantially all of the charged particles have been neutralized, the reactive neutral species remains and is available for the ashing process. 
         [0005]    Current plasma generating apparatus present a variety of challenges during ashing procedures. Generally, plasma is generated using a coil, often copper, wrapped around a dielectric tube, such as quartz or aluminum/sapphire tube. The coil is energized with a radio frequency (RF) voltage from an appropriate RF generator. Plasma formation is initiated by capacitively coupling the electric field through the quartz to the rarefied gas inside the quartz tube. As the power level and current through the coil are increased, the plasma switches from a capacitively coupled mode to an inductively coupled mode. Significant voltages exist on the coil. Difficulties arise in trying to isolate the high voltage components to prevent these components from breaking down and arcing to cause damage to other components. In addition, the high voltages generate a high electric field across the quartz and can cause significant ion bombardment and sputtering on the inside of the quartz tube thus reducing its lifespan and increasing its maintenance needs. A reduction in the ion bombardment energy may be helpful. 
         [0006]    In addition, as illustrated in schematic cross section in  FIG. 1 , prior art plasma sources  10  for ashing have smaller diameter plasma generation regions  12 , in quartz cylindrical containers  15 , than the work pieces  20  that are to be treated. Accordingly, plasma flows from a smaller diameter plasma generation region  12  of the quartz cylinder  15  of about 76 mm diameter that is surrounded by a RF induction coil  14 , to a larger diameter distribution region  16  of a diameter approximating the work piece diameter, often about 300 mm diameter. In the distribution region  16 , the oxygen atoms (O), which are the desired product in the plasma generator effluent, are spread out or dispersed over a larger cross sectional area than that of the generation region  12  in an attempt to control the flux of O atoms to the surface of a work piece  20 . In addition, the distribution region  16  includes a diffuser  18  of some kind to further facilitate a desired plasma distribution over the surface of the work piece  20 . Significant numbers of O atoms are lost in this process. 
         [0007]    Ion bombardment of the quartz cylinder  15  poses another significant challenge. When a small diameter plasma source  10  is used, the plasma density should be very high in order to generate enough O atoms to perform ashing at an acceptable rate. This high plasma density coupled with the high energy fields (E-fields) present in the coil  14  causes significant ion bombardment of the quartz container  15  and hence a reduced container lifespan. One method to ameliorate this effect is to place a Faraday shield  22  between the quartz container  15  and the coil  14 , as illustrated in the schematic cross section of  FIG. 2 . This effectively prevents the E-fields from penetrating the quartz container  15  and consequently reduces the sputtering of the quartz container  15 . The addition of the Faraday shield  22  reduces one problem at the expense of creating additional problems. The Faraday shield  22  is complex, increases cost, requires water cooling and consumes power that would otherwise be delivered to the plasma. 
         [0008]    In addition, present day plasma generator apparatus suffer from non-uniform plasma production. Generally, when an oxygen-containing gas flows through the container, plasma generation is initiated in the tube adjacent the coil. But since the E-field has limited penetration into the container, the peak area for energy dissipation is near the inner wall of the container. Due to this limited penetration of the E-field, the plasma forms a ring  25  inside the quartz container  15 , as seen from above, and as schematically shown in  FIG. 3 , with the area of peak power dissipation being near the inner wall of quartz container  15 . There is a hole  26  corresponding to a nearly field-free region where there is little or no energy dissipation from the excitation fields. For example, in the 76.2 mm diameter tube on the Gamma 2130™ of Novellus Systems, Inc. [San Jose Calif.], the size of the central hole  26  in the ring  25  is small, although quite visible under certain conditions. While gas flows through the entire cross section of the quartz container  15 , oxygen in the gas flow is mainly dissociated in the ring  25  to produce O atoms. Very little of the oxygen in the remainder of the gas flow is dissociated to O atoms. Accordingly, a large portion of the incoming gas flow, namely gas in the vicinity of the center of the cylindrical gas flow in container  15 , is not subjected to sufficient energy for ionization. 
         [0009]    In addition, present day plasma generators are difficult to adapt to ashing larger wafers. If the quartz container  15  is increased in diameter, the peak plasma region remains approximately the same size and is still located near the wall. The hole  26  in the ring  25  increases in size dramatically as the diameter of the quartz container  15  is increased. The majority of the gas flows down the center of the quartz container  15  and is never directly ionized. Thus, few O atoms are produced in the central region of the quartz container  15 . The efficiency of producing O atoms in larger diameter quartz containers is therefore expected to be low. 
         [0010]    Accordingly, it is desirable to provide an improved method of plasma generation that is suitable for use in ashing procedures in semiconductor fabrication. It is also desirable that the method provide a more uniform distribution of O atoms over a large diameter work piece, such as a 300 mm or larger wafer. It is further desirable to provide a method that does not require Faraday shields, but that also provides an acceptable quartz container lifespan. In addition, it is desirable to provide a method that converts oxygen more efficiently to O atoms. Further, it is desirable to provide an improved method of ashing a semiconductor work piece using a plasma generator. Other desirable features and characteristics of the technology will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A more complete understanding of the present technology may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers denote like elements throughout the figures and wherein: 
           [0012]      FIG. 1  is a simplified cross sectional view of a prior art plasma generator for use in ashing in semiconductor fabrication; 
           [0013]      FIG. 2  is a simplified cross sectional view of a prior art plasma generator with a Faraday shield for use in ashing in semiconductor fabrication; 
           [0014]      FIG. 3  is a cross sectional view along line  3 - 3  of  FIG. 2 ; 
           [0015]      FIG. 4  is a simplified schematic of an induction coil typical of prior art plasma generators; 
           [0016]      FIG. 5A  is a simplified side view in cross section of a large diameter plasma generator in accordance with an exemplary embodiment that utilizes an embodiment of a gas dispersion nozzle, and a quartz container with a conical upper portion; 
           [0017]      FIG. 5  B is a simplified side view in cross section of a large diameter plasma generator in accordance with an exemplary embodiment that utilizes an embodiment of a gas dispersion nozzle, and a domed quartz container; 
           [0018]      FIG. 6  is a simplified side view in cross section of a large diameter plasma generator in accordance with an exemplary embodiment that utilizes an embodiment of a gas dispersion nozzle, and a domed quartz container, with a Faraday shield interposed between the RF coil and the quartz container; 
           [0019]      FIG. 7A  is a simplified top view of a surface of an exemplary nozzle used in connection with plasma generators in accordance with exemplary embodiments; 
           [0020]      FIG. 7B  is a simplified side view in cross section of a nozzle with hemispherical outlet end used in connection with plasma generators in accordance with exemplary embodiments; 
           [0021]      FIG. 8  is a simplified side view in cross section of a large diameter plasma generator in accordance with an exemplary embodiment that utilizes an alternative embodiment of a gas dispersion nozzle, and a quartz container with a conical upper portion; 
           [0022]      FIG. 9  is a simplified side view in cross section of a plasma generator in accordance with another exemplary embodiment utilizing a flow directing baffle; and 
           [0023]      FIGS. 10  A-B illustrate simplified schematic representations of exemplary embodiments of symmetrical multi-segmented induction coils for use in connection with plasma generators in accordance with exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The following detailed description is merely exemplary in nature and is not intended to limit the described embodiments or the application and uses of the described embodiments. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. 
         [0025]    Exemplary embodiments provide methods for diverting a portion of an incoming gas flow into a region of higher plasma density than another region of the apparatus. The region of higher gas density is located in a container of suitable dielectric material, such as a quartz container, and specifically within or proximate the strongest region of a plasma-generating energy field to which the container is subjected. Accordingly, a higher proportion of the incoming ionizable components in the gas flow is ionized (or “converted to plasma”) when sufficient appropriate excitation energy is applied. 
         [0026]    An example embodiment of a plasma generator apparatus  100  with a conical upper portion is illustrated in  FIG. 5A , and another embodiment with a domed upper portion is illustrated in  FIG. 5B . The following description applies to both figures, except with respect to the differences relating to the shape of the upper portion of the container. The plasma generator is not limited to use in semiconductor fabrication to ash work pieces, but may also be used in other applications. The apparatus described herein can also be used for general surface treatment, such as cleaning organic material from any surface and not just in the semiconductor industry. Coupled with different chemistry that contains nitrogen, oxygen, hydrogen and compounds that might contain carbon and fluorine, this apparatus may be used for cleaning and surface treatment of a variety of work pieces, for example: cleaning organic material from parts, removing biological contamination, enhancing adhesion prior to deposition of another layer, reduction of metal oxides, or for etching a range of materials. 
         [0027]    The plasma generator  100  includes an upper portion  110  that is conical ( FIG. 5A ) or domed ( FIG. 5B ) and that caps a gas flow tube  125 . The apex  112  of the cone  110 , or highest point  112  of the dome  110 , is uppermost for receiving gas entering in gas stream  300  (depicted by arrows in the drawings) at a plasma generator inlet  101 . The cone or dome  110  and tube  125  of the plasma generator  100  may be fabricated of quartz, as is conventional, or another suitable material. The ionizable gas component in the gas stream  300  may be, for example oxygen, argon, helium, hydrogen, nitrogen, and fluorine-containing compounds. The diameter  120  of the tube  125  may be from about 200 mm up to 500 mm and/or typically about 300 mm for a work piece that approximates that size. The gas flow tube  125  has a larger diameter  120  with features to permit dispersion of plasma generated across the entire cross section of the larger diameter container  100  in which the plasma is generated. The term “larger” diameter in the specification and claims with reference to a container, within which plasma is generated, means a container diameter that approximates, but may not precisely equal, the diameter of a work piece to be subjected to ashing. In certain embodiments the larger diameter therefore may encompass a cylinder  125  having a diameter  120  in excess of about 200 mm, or in the range from about 300 mm to about 500 mm or more. A larger diameter may be typically at least about 300 mm for a work piece of that size, or more for larger work pieces. In the embodiment shown, the apex  112  of the cone  110 , or highest point  112  of the dome  110 , has a gas distribution nozzle  114  with a plurality of through holes  116  disposed in the nozzle  114 , as shown more clearly in the top view of  FIG. 7A . The through holes  116  direct incoming gas along the sloping inner sidewalls  118  of the cone or dome  110  toward the region of higher plasma density  130 . 
         [0028]    The nozzle  114  shown in top view in  FIG. 7A  may be spherical, hemispherical or pyramidal, or of any other suitable shape. Another exemplary embodiment of a nozzle  170  with a hemispherical-tip  172  is illustrated in  FIG. 7B . The nozzle tip  172  has a plurality of through holes  116  therein to direct gas along inner surfaces of walls of the cone  110  (or dome, if it is a domed container) to a region of higher gas density  130  for plasma generation in that region. 
         [0029]    Referring to  FIGS. 5A-B  and  FIG. 7A , the nozzle  114  directs gas flow via through holes  116  (shown in  FIG. 7A ). The gas exiting the through holes  116  (gas shown by downstream arrows  300  in the Figures) is directed to preferentially flow along the inner sidewalls  118  of cone or dome  110 . The gas flow then impacts the inner walls  126  of the gas flow tube  125 . The impact area is in the higher plasma region  130  which encompasses the intersection of inner sidewalls  118  and inner tube walls  126 . The directed gas flow creates a region  130  of high gas pressure (high gas density) as the gas flow changes direction from flowing parallel to the inner sidewalls  118  of the dome  110  to flowing downward in tube  125  parallel to its inner walls  126 . An energizing coil  140  surrounds the outer surface  113  of the cone or dome  110  and the outer surface  124  of the tube  125  to supply energy at the appropriate frequency into this region  130  to ionize gas components. 
         [0030]    Because a large proportion, or even a major portion, of the gas flow is directed by the nozzle  114  and the container inner sidewalls  118  into region  130 , region  130  is a zone of highest plasma density  130 . Excitation energy is applied from the outside of the tubular container  125  directly into this region  130 . This permits more efficient gas component ionization because it ameliorates the effect of the energy level diminishing (and ionization decreasing) as the energy penetrates farther into the container. Of course, the flowing of more gas through the region of highest power dissipation, region  130 , increases the production of radicals and atoms as well, in this case O atoms. 
         [0031]    A gas distributor plate  150  is disposed at the exit end  102  of the generator  100 . This gas distributor plate  150  has a plurality of through holes, or is of a porous construction. It provides means to control the O atom flux that impinges upon the work piece being treated. As the gas impinges upon and travels through the gas distributor plate  150 , some charged species are neutralized thereby reducing the potential for charged particle damage to the work piece  200 . 
         [0032]    In accordance with another exemplary embodiment, a diameter  120  of the tube  125  and a diameter  210  of work piece  200  are approximately the same. In accordance with another exemplary embodiment, a diameter of the tube  125  (shown by double-headed arrow  120 ) and the diameter of work piece  200  (shown by double-headed arrow  210 ) are approximately the same. While equality of diameter is not necessary, embodiments may have equal diameters of tube  125  and work piece  200 , or diameters that approximate equal size. This feature significantly or completely reduces the need to expand the tube  125  near its exit end  102  to approximate the work piece diameter to facilitate distribution of the gas flow. In general, it is preferable that a characteristic dimension of the apparatus, such as tube diameter in the example of a quartz cylinder, approximates a characteristic dimension of a work piece, such as the diameter of a circular work piece surface that is presented transverse to the direction of gas flow. In this regard, the plasma generation region is increased in size thereby allowing a reduction in overall plasma density while still increasing the O atom production generated in the flowing gas. Increasing the volume of the plasma reduces the plasma density in the region near the container wall. This in turn results in less ion bombardment and less container wall heating. 
         [0033]    The plasma generator  100  may be used in conjunction with a Faraday shield  144 , shown in  FIG. 6 , or may be used in conjunction with an induction coil circuit  160  that has a symmetrical coil  140  that has reduced peak voltage, as discussed below. Preferably, but not necessarily, to reduce damage to the quartz components (dome  110  and tube  125 ) the peak voltage V p  should be reduced by a factor of about 2 or even by a factor of about 4, if necessary to protect the container from premature aging. 
         [0034]    As a preliminary matter, the prior art driving the induction circuit  160  is shown in  FIG. 4 . One end of the coil  140  is grounded and the other end is powered by a high frequency alternating current generator  162  through a matching network and capacitor. The peak voltage V p  is seen at one end of the coil, and the other end is grounded. According to an exemplary embodiment of the present technology, the induction circuit  160  is configured, as shown in  FIG. 10A , so that there are two capacitors  164 , one outside each end of the coil  140 . This configuration, and configurations like this example, will be referred to as a “symmetrical coil” configuration. The capacitors  164  are chosen such that the total impedance of each capacitor is one half the impedance of the original capacitor shown in  FIG. 4 . This maintains the total impedance of the capacitor-coil induction circuit  160  unchanged. Accordingly, the voltage drop V p  across the entire coil  140  will be identical for the same current flowing through the coil  140  and, therefore, the resulting plasma generation capability will be the same. Thus, instead of a real ground (zero voltage, for example) located at only the end of the coil  140  as in  FIG. 4 , there is a now a pseudo ground (also zero voltage like the real ground, in this example) located at the center  166  of the coil  140  as well. This means that the highest voltage seen on the coil relative to ground at points  168  is V p /2. This reduces the peak voltage by a factor of 2, and thereby reduces all electric fields by a factor of 2. Furthermore, this also reduces the electrical field across the quartz walls of the plasma generator (which reduces ion bombardment energy) by a factor of 2. It also reduces all of the other electrical standoff voltage requirements by a factor of 2. 
         [0035]    The effect may be further enhanced by dividing the coil into a plurality of symmetrical segments, as shown in  FIG. 10B . As shown in  FIG. 10B , subdividing the coil  140  into two symmetrical segments reduces peak voltage V p  to one-quarter of the peak for an asymmetrical coil, V p /4. Accordingly, dividing the coil into N segments, reduces peak voltage to 1/(2N) of the peak voltage of an asymmetrical coil, which is shown in  FIG. 4  for comparison. When peak voltage V p  has been reduced so that any voltage-induced effects to the quartz components of the plasma generator  100  are at an acceptable level, there is no longer any need for a Faraday shield. 
         [0036]    In accordance with an exemplary embodiment of the present invention, illustrated schematically in  FIG. 8 , the gas inlet  101  is of a different design than the nozzle  114  of  FIG. 5 . The inlet  101  includes a tube  180  that has a closed end  181 , and a series of outlet holes  182  in the vicinity of the closed end  181  that direct gas to the region  130 , proximate an intersection between the dome or cone  110  and the tube  125 . Region  130  is adjacent induction coil  140  that is wrapped around the outer surface  113  of the dome  110  and the outer surface  124  of the tube  125  to provide excitation energy to gas in region  130 . In region  130 , the gas  300  is ionized and flows downward in the cylindrical container  125  to outlet  102 . At the outlet  102 , the gas encounters a gas distributor plate  150  which neutralizes some charged gas species that were formed during gas ionization. The gas stream  300  passes through holes or pores  184  in gas distributor  150 , and exits as gas stream  302  to impinge on an upper surface of the work piece  200  to perform a desired function, such as surface ashing. 
         [0037]      FIG. 9  illustrates another exemplary embodiment of a plasma generator apparatus  100 . This apparatus lacks a dome or cone upper section. Rather, incoming gas flows into an apparatus  100  that includes a tube  125  and is diverted within the tube  125  to flow around an axially-centered round baffle plate  190 . This diversion causes gas stream  300  to flow towards sides of the inner wall  126 . An induction coil  140  surrounds the outer surface  124  of container  125  in an area adjacent region  130  where the diverted gas flow  300  impacts the inner wall  126  of the container  125 . The induction coil  140  applies energy in that region  130  to ionize gas components. By diverting gas out of the central region  135  of the tube and forcing the gas towards the inner walls  126  of the tube, gas flow is forced into region  130  and gas is concentrated in that region  130 . The application of energy to the concentrated gas results in a greater ionization of the gas. The diameter of the baffle plate  190 , or other characteristic baffle dimension if not a circular baffle, may be selected taking into account the inside diameter  120  of tube  125  and the gas flow rate, to determine an optimum gas flow rate and pressure in the region  130  in which the gas is closest to the coil  140 . After ionization, the gas  300  then flows downward in the cylindrical container  125  to outlet  102 . At the outlet  102 , the gas encounters a gas distributor plate  150  which neutralizes some of the charged gas species that were formed during gas ionization. The gas stream  300  passes through holes or pores  184  in gas distributor  150 , and exits as gas stream  302  to impinge on an upper surface of the work piece  200  to perform a desired function, such as surface ashing. 
         [0038]    While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.