Abstract:
A plasma chamber for activating a process gas, including at least four legs forming a toroidal plasma channel, each leg having a cross-sectional area, and an outlet formed on one leg, the outlet having a greater cross-sectional area than the cross-sectional area of the other legs. The plasma chamber further includes an inlet for receiving the process gas and a plenum for introducing the process gas over a broad area of the leg opposing the outlet to reduce localized high plasma impedance and gas flow instability, wherein the leg opposing the outlet defines a plurality of holes for providing a helical gas rotation in the plasma channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of and priority to International patent application no. PCT/US2007/081875, filed on Oct. 19, 2007. The disclosure of the above-identified application is incorporated herein by reference in its entirety. 
     BACKGROUND 
     Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application. 
     For example, some applications require the use of ions with low kinetic energy (i.e. a few electron volts) because the material being processed is sensitive to damage. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy. Still other applications, such as reactive ion beam etching, require precise control of the ion energy. 
     Some applications require direct exposure of the material being processed to a high density plasma. One such application is generating ion-activated chemical reactions. Other such applications include etching of and depositing material into high aspect ratio structures. Other applications require neutral activated gases containing atoms and activated molecules while the material being processed is shielded from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements. 
     Various plasma sources can generate plasmas in numerous ways including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into a plasma. Induction coils are typically used for inducing current into the plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave passing window into a discharge chamber containing a gas. Microwave discharges can be used to support a wide range of discharge conditions, including highly ionized electron cyclotron resonant (ECR) plasmas. 
     Compared with microwave or other types of RF plasma sources, a toroidal plasma source has advantages in low electric field, low plasma chamber erosion, compactness, and cost effectiveness. The toroidal plasma source operates with a low electric field and inherently eliminates current-terminating electrodes and the associated cathode potential drop. The lower plasma chamber erosion allows toroidal plasma sources to operate at higher power densities than other types of plasma sources. In addition, the use of high permeability magnetic cores couples electromagnetic energy to plasma efficiently, allowing the toroidal plasma source to operate at relatively low RF frequencies while lowering power supply costs. Toroidal plasma sources have been used to produce chemically reactive atomic gases including fluorine, oxygen, hydrogen, nitrogen, etc. for processing semiconductor wafers, flat panel displays, and various materials. 
     SUMMARY 
     No existing toroidal plasma source can operate at NF3 flow rate of above 24 standard liters per minute (slm). There are increasing demands for high power, high gas-flow-rate plasma sources to increase throughput in plasma processing, particularly in manufacturing of flat panel displays and solar panels. The gas flow rates required by these applications can be tens to hundreds slm. At such high flow rates, flow dynamics and gas flow patterns strongly affect gas-plasma interaction or dissociation rate of the process gas as well as the stability of the plasma. 
     Techniques have been developed to control gas flow to improve plasma stability and to increase gas-plasma interaction. However, in existing plasma source designs process gases are introduced into a plasma channel either through a single gas injection hole or multiple holes located in a small area in the plasma channel creating high plasma impedance near the gas injecting point. The localized gas concentration and high flow speeds cause flow instabilities and limits the amount of gases that can be processed through a plasma source. 
     The embodiments described herein provide an apparatus and a method for reducing localized high plasma impedance and gas flow instability in a plasma channel. 
     The apparatus consists of a plasma chamber for use with a reactive gas source, including at least four legs forming a toroidal plasma channel, each leg having a cross-sectional area, and an outlet formed on one leg, the outlet having a greater cross-sectional area than the cross-sectional area of the other legs to accommodate increased gas flow due to dissociation of inlet gas by the plasma. The plasma chamber further includes an inlet for receiving the process gas and a plenum for introducing the process gas over a broad area along the toroidal plasma channel to reduce localized high plasma impedance and gas flow instability. In one embodiment, the plenum introduces the process gas along the plasma channel leg opposing the outlet, via a plurality of holes for providing a helical gas rotation in the plasma channel. 
     In one embodiment, the holes can be substantially tangential to the plasma channel inner surfaces and are angled or oriented to create a helical gas rotation in the plasma channel. The holes can be angled between 30 degrees and 90 degrees relative to an axial direction of the plasma channel leg, and between 45 degrees and 90 degrees relative to a direction perpendicular to the axis of the plasma channel leg. In one embodiment, two separate but coherent gas rotations are introduced during gas injection to improve gas-plasma interactions and to maintain flow stability. 
     In one embodiment, the plasma chamber can further include at least one ignition device to initiate plasma discharge. The ignition device can be located between the plenum and the leg opposing the outlet, recessed from the plasma channel through a tube section, and with a purge hole in the tube section for assisting with ignition of the plasma. 
     In one embodiment, a transition angle between the vertical legs of the plasma channel and the outlet can be greater than 95 degrees. The transition angle can range between 100 and 180 degrees for minimizing flow turbulence. 
     In one embodiment, the plasma channel can be smoothed to prevent flow turbulence, pressure build-up, or interaction of plasma with walls of the plasma channel. The NF3 flow capability of the plasma chamber can be at least 30 slm. 
     A buffer for introducing a process gas to a plasma chamber can include an inlet for receiving the process gas and a plenum for introducing the process gas over a broad area of the plasma channel to reduce localized high plasma impedance and gas flow instability in the plasma channel. The plenum can define a plurality of holes for providing a helical gas rotation in the plasma channel. The holes can be substantially tangential to the plasma channel inner surfaces and are angled or oriented to create a helical gas rotation in the plasma channel. The holes can be angled between 30 degrees and 90 degrees relative to the axial direction of the plasma channel leg, and between 45 degrees and 90 degrees relative to the direction perpendicular to the axis of the plasma channel leg. A method for introducing a process gas into a plasma chamber includes introducing the process gas over a broad area of a plasma channel and creating a helical gas rotation in the plasma channel to reduce localized high plasma impedance and gas flow instability in the plasma channel. The method further includes providing at least two separate but coherent gas rotations during gas introduction to improve gas-plasma interactions and to maintain flow stability. The method further includes outputting the gas at an outlet location having a cross-section area greater than the cross-sectional area of the plasma channel to prevent flow turbulence near the outlet location. 
     A plasma chamber for use with a reactive gas source, including means for forming at least four legs to form a toroidal plasma channel, each leg having a cross-sectional area and means for forming an outlet on one leg, the outlet having a greater cross-sectional area than the cross-sectional area of the other legs. The plasma chamber further includes means for receiving a process gas and means for introducing the process gas over a broad area of the leg opposing the outlet to reduce localized high plasma impedance and gas flow instability, wherein the leg opposing the outlet defines a plurality of holes for providing a helical gas rotation in the plasma channel. 
     The embodiments described herein provide the following advantages over the prior art. The plasma source can generate high flow rates of activated gases used for etching, thin film deposition and chamber clean. The plasma source can be used to abate harmful or undesirable gases. The plasma source expands the operational capability of toroidal plasma sources thereby enabling users to achieve higher process throughput and lower process cost. The plasma source can operate at high gas flow rates and achieve high gas excitation or dissociation rate. The plasma source can extend the NF3 flow capability of toroidal plasma source to 30 slm or higher. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following more particular description of the embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the embodiments. 
         FIG. 1  is a schematic representation of a toroidal low-field plasma source for producing activated gases; 
         FIG. 2  illustrates of an embodiment of a swirl gas mixing device; 
         FIG. 3  shows a cross-sectional view of an embodiment of a toroidal plasma chamber; 
         FIG. 4  shows operation data of the plasma source demonstrating its operation at NF3 flow rate of up to 45 slm and at pressure of 100 torr; 
         FIG. 5A  shows a top view of a gas plenum; 
         FIG. 5B  shows a top view of another embodiment of a gas plenum; 
         FIG. 5C  shows a cross-sectional view of the gas plenum of  FIG. 5A  or  FIG. 5B ; 
         FIG. 6A  shows one side of an internal gas volume of the plasma channel; 
         FIG. 6B  shows the helical gas rotation in the plasma channel with respect to gas flowing in a vertical direction of  FIG. 6A ; 
         FIG. 6C  shows the helical gas rotation in the plasma channel with respect to gas flowing in a horizontal direction of  FIG. 6A ; 
         FIG. 7A  shows a bottom view of an embodiment of a gas outlet; 
         FIG. 7B  shows a cross-sectional view of the gas outlet of  FIG. 7A ; 
         FIG. 8A  shows the calculated pressure drop in the plasma source based on a total flow rate of 120 slm at the gas outlet; 
         FIG. 8B  shows the gas flow speed profile in the plasma source based on a total flow rate of 120 slm at the gas outlet; 
         FIG. 9A  shows a cooling structure  200  for the toroidal plasma source; and 
         FIG. 9B  shows a cross-sectional view of the cooling structure of  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic representation of an embodiment of a toroidal low-field plasma source  10  for producing activated gases. The source  10  includes a power transformer  12  that couples electromagnetic energy into a plasma  14 . The power transformer  12  includes a high permeability magnetic core  16 , a primary coil  18 , and a plasma chamber  20 . The plasma chamber  20  allows the plasma  14  to form a secondary circuit of the transformer  12 . The power transformer  12  can include additional magnetic cores and conductor coils (not shown) that form additional primary or secondary circuits. 
     The plasma chamber  20  can be formed from a metallic material such as aluminum or a refractory metal, a coated metal such as anodized aluminum, or can be formed from a dielectric material such as quartz. One or more sides of the plasma chamber  20  can be exposed to a process chamber  22  to allow charged particles generated by the plasma  14  to be in direct contact with a material to be processed (not shown). Alternatively, the plasma chamber  20  can be located at a distance from the process chamber  22 , allowing activated neutral gases to flow to the process chamber  22  while charged particles recombine during the gas transport. A sample holder  23  can be positioned in the process chamber  22  to support the material to be processed. The material to be processed can be biased relative to the potential of the plasma. 
     The plasma source  10  also comprises a switching power supply  50 . In one embodiment, the switching power supply  50  includes a voltage supply  24  directly coupled to a switching circuit  26  containing a switching semiconductor device  27 . The voltage supply  24  can be a line voltage supply or a bus voltage supply. The switching semiconductor device  27  can be a set of switching transistors. The switching circuit  26  can be a solid state switching circuit. An output  28  of the circuit  26  can be directly coupled to the primary winding  18  of the transformer  12 . 
     The toroidal low field plasma source  10  can include a means for generating free charges that provides an initial ionization event that ignites a plasma in the plasma chamber  20 . The initial ionization event can be a short, high voltage pulse that is applied to the plasma chamber. The pulse can have a voltage of approximately 500-10,000 volts and can be approximately 0.1 to 10 microseconds long. A continuous high RF voltage of 500-10,000 volts can also be used to produce the initial ionization event, and the voltage is disconnected after gas breaks down. A noble gas such as argon may be inserted into the plasma chamber  20  to reduce the voltage required to ignite a plasma. Ultraviolet radiation can also be used to generate the free charges in the plasma chamber  20  that provide the initial ionization event that ignites the plasma in the plasma chamber  20 . 
     In one embodiment, the high voltage electric pulse is applied to an electrode  30  positioned in the plasma chamber  20 . In another embodiment, the short, high voltage electric pulse is applied directly to the primary coil  18  to provide the initial ionization event. In another embodiment, the short, high voltage electric pulse is applied to an electrode  32  that is capacitively coupled to the plasma chamber  20  by a dielectric. In another embodiment, the plasma chamber  20  is exposed to ultraviolet radiation emitting from an ultraviolet light source  34  that is optically coupled to the plasma chamber  20 . The ultraviolet radiation causes the initial ionization event that ignites the plasma. 
     The toroidal low field plasma source  10  can also include a circuit  36  for measuring electrical parameters of the primary winding  18 . Electrical parameters of the primary winding  18  include the current driving the primary winding  18 , the voltage across the primary winding  18 , the bus or line voltage supply generated by the voltage supply  24 , the average power in the primary winding  18 , and the peak power in the primary winding  18 . 
     In addition, the plasma source  10  can include a means for measuring relevant electrical parameters of the plasma  14 . Relevant electrical parameters of the plasma  14  include the plasma current and power. For example, the source  10  can include a current probe  38  positioned around the plasma chamber  20  to measure the plasma current flowing in secondary of the transformer  12 . The plasma source  10  can also include an optical detector  40  for measuring the optical emission from the plasma  14 . In addition, the plasma source  10  can include a power control circuit  42  that accepts data from one or more of the current probe  38 , the power detector  40 , and the circuit  26  and then adjusts the power in the plasma by adjusting the current in the primary winding  18 . 
     In operation, a gas is bled into the plasma chamber  20  until a pressure substantially between 1 millitorr and 100 torr is reached. The gas can comprise a noble gas, a reactive gas or a mixture of at least one noble gas and at least one reactive gas. The circuit  26  containing switching semiconductor devices supplies a current to the primary winding  18  that induces a potential inside the plasma chamber  20 . The magnitude of the induced potential depends on the magnetic field produced by the core and the frequency at which the switching semiconductor devices operate according to Faraday&#39;s law of induction. An ionization event that forms the plasma can be initiated in the chamber. The ionization event can be the application of a voltage pulse to the electrode  30  in the chamber  20  or to the electrode  32  that is capacitively coupled to the plasma chamber  20 . The ionization event can also be the application of a high voltage to the primary winding. Alternatively, the ionization event can be exposing the chamber to ultraviolet radiation. 
     Once the gas is ionized, a plasma is formed which completes a secondary circuit of the transformer. The electric field of the plasma can be substantially between 1-100 V/cm. If only noble gases are present in the plasma chamber  20 , the electric fields in the plasma  14  can be as low as 1 volt/cm. If, however, electronegative gases are present in the chamber, the electric fields in the plasma  14  are considerably higher. 
       FIG. 2  illustrates an embodiment of a swirl gas mixer plate  60  according to the prior art. The swirl gas mixer plate  60  contains a number of concentric holes  62 , which are aligned tangentially to the inner surface of the plasma channel (not shown). In operation, the swirl gas mixer plate  60  injects feed gas helically into the plasma chamber  20 , creating a spiral flow and forcing the feed gas to mix and react with the plasma  14 . However, the swirl gas mixer plate  60  introduces the gas at a specified location in the plasma channel, leading to erosion at the location due to high impedance created by the gas. 
       FIG. 3  shows a cross-sectional view of an embodiment of a toroidal plasma chamber  100  for minimizing flow turbulence and flow-induced plasma instabilities, and improving gas-plasma interactions. The toroidal plasma chamber  100  includes a gas inlet  110 , a toroidal plasma channel  120 , and a gas outlet  130 . The plasma chamber is formed with multiple sections and with multiple dielectric breaks  136  along the plasma channel. The dielectric breaks prevent induced electric current from flowing in the plasma chamber, and distributes induced electric voltage uniformly across the multiple dielectric breaks  136  thereby reducing peak electric field in the plasma channel. 
     The gas inlet  110  includes a buffer or gas plenum  140  for introducing gas into the plasma channel  120  over a broad area to reduce localized high plasma impedance and gas flow instability. The plasma channel  120  include an upper leg  122 , a lower leg  124 , and two side legs  126  that form a race-track-shaped toroidal plasma topology. A plurality of gas injection holes  142  (better illustrated in  FIGS. 5A-5C ) generate two separate but coherent gas rotations during gas injection to improve gas-plasma interactions and to maintain flow stability. It should be noted that the gas flow path in the plasma channel  120  is smoothed (e.g., having no sharp corners) to prevent flow turbulence, pressure build-up, or interaction of plasma with the channel walls. In one embodiment, the upper leg  122  includes at least one ignition device  144  for providing an ionization event that forms the plasma. The ignition device  144  may be recessed from the plasma channel to reduce heat from the plasma to the electrode or the dielectric window. There can optionally be a purge hole  146  injecting a fraction of inlet gas into tube section  148  connecting the ignition device  144  and the plasma channel  120  to assist with the ignition of the plasma. The purge hole  146  delivers fresh inlet gas to the ignition device  144  and helps to bring charged particles generated at the ignition device  144  to the plasma channel  120 . The gas outlet  130  is substantially larger than the cross-section area of the plasma channel  120  to accommodate a higher amount of gas at the gas outlet  130  due to dissociation of the process gas, and to enable a smooth transition from the toroidal plasma channel to the gas outlet  130 . 
       FIG. 4  shows operational data of the plasma source  100  ( FIG. 3 ) demonstrating its operation at NF3 flow rate of up to 45 slm and at pressure of 100 torr. As shown, the plasma source  100  can operate at high gas flow rates and can achieve a high gas excitation or a dissociation rate. In one embodiment, the NF3 flow capability of toroidal plasma source  100  can be at least 30 slm or higher. 
       FIG. 5A  and  FIG. 5B  show top view of two embodiments of the gas plenum  140  ( FIG. 3 ) and  FIG. 5C  shows a cross-sectional view of the gas plenum  140 . The gas plenum  140  includes a plurality of holes  142  for introducing process gas into the plasma channel  120  ( FIG. 3 ). The gas injection holes  142  generate a helical gas rotation in the plasma channel  120 . The embodiment of  FIG. 5A  creates a symmetric rotation pattern in the two halves of the top leg of the plasma channel  120 , while the embodiment of  FIG. 5B  creates an anti-symmetric rotation pattern.  FIG. 6A  shows one side of an internal gas volume of the plasma channel  120  ( FIG. 3 ). The holes  142  are substantially tangential to the plasma channel  120  inner surfaces and are angled or oriented to generate helical gas rotation in the plasma channel  120 .  FIG. 6B  shows gas trajectories viewed along the axis of a side leg  126  of the plasma channel  120 .  FIG. 6C  shows the gas trajectories viewed along the upper leg of plasma channel  120 . The helical gas rotation forces the plasma to the center of the plasma channel, improving plasma stability as well as reducing erosion within the plasma channel  120 . The helical gas rotation also improves interaction between the process gas and plasma. The holes  142  are angled between 30 degrees and 90 degrees relative to an axial direction of the plasma channel  120  (generally shown as A), and between 45 degrees and 90 degrees relative to a perpendicular direction (generally shown as B) to the axis of the plasma channel  120 . The injection holes  142  are spread over a broad area in the plasma channel  120  to prevent localized concentration of inlet gas and high local plasma impedance. Two separate but coherent gas rotations are introduced during gas injection to improve gas-plasma interactions and to maintain flow stability. The holes  142  are also oriented tangential to the plasma channel surface to avoid pushing the plasma towards the surfaces of the plasma channel  120  by the inlet gas. 
       FIG. 7A  shows a bottom view of the gas outlet  130  of the plasma channel  120  ( FIG. 3 ) and  FIG. 7B  shows a cross-sectional view of the gas outlet  130  of the plasma channel  120 . In one embodiment, the cross-section area of the gas outlet  130  is greater than twice the cross-section area of the plasma channel  120  to prevent flow turbulence near the gas outlet  130 . In some embodiments, a transition angle  128  between the vertical legs  126  of the plasma channel  120  and the gas outlet  130  is greater than 95 degrees. In some embodiments, the transition angle  128  can range between 100 and 180 degrees for minimizing flow turbulence. 
       FIG. 8A  shows the calculated pressure drop in the plasma source  100  ( FIG. 3 ) based on a total flow rate of 120 slm at the gas outlet  130 .  FIG. 8B  shows the gas flow speed profile in the plasma source  100  based on a total flow rate of 120 slm at the gas outlet  130 . It should be noted that the highest pressure drop and flow speed occur at the transition section between the plasma channel  120  and the gas outlet  130  thereby illustrating the importance of having a transition angle at between 100 and 180 degrees for minimizing flow turbulence. 
       FIG. 9A  shows a cooling structure  200  for the toroidal plasma source  100  ( FIG. 3 ).  FIG. 9B  shows a cross-sectional view of the cooling structure  200  of  FIG. 9A . The cooling structure is symmetric on the two sides of the plasma chamber, and only one side is shown in  FIG. 9A  and  FIG. 9B . The cooling structure  200  includes an inlet tube  202 , an outlet tube  204 , and a plurality of channels  206 . The cooling structure  200  is segmented, similar to the plasma chamber  100 , to multiple sections. Individual cooling sections are mounted onto each plasma chamber section along a plasma channel. Dielectric tubes connect the different cooling sections to allow a coolant such as water to flow between the cooling sections. A thermally conductive pad or grease is used for improving thermal conduction from the plasma channel to the cooling structure. In operation, a coolant is forced through the channels  206  to cool the toroidal plasma source  100 . The ability to cool the plasma source  100  is beneficial because it reduces the temperature of the plasma chamber, protecting the plasma chamber material and vacuum seals. The ability to cool also allows the plasma source to operate at high power level and high gas flow rate, improving process throughput and reducing process cost. 
     One skilled in the art will realize the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting of the invention described herein. Scope of the invention is thus indicated by the appended claims, rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.