Abstract:
Atmospheric inductively coupled plasma torch comprising a vessel within which the plasma is generated and a coil wound around the periphery of the vessel. The coil has at least two spaced-apart winding layers. The coil is constructed such that all winding layers of a given multi-turn is wound before an adjacent multi-turn is wound. A first end of the coil is coupled to ground, and a second end of the coil is coupled to receive a RF driver signal that is configured to ignite the plasma to facilitate processing.

Description:
BACKGROUND OF THE INVENTION 
     In the manufacture of semiconductor products, substrates such as wafers undergo deposition and etching processes to form features thereon. The processing of semiconductor substrates often leaves residues, such as polymer deposition, between processing steps. Atmospheric inductively coupled plasma torches have been employed to clean substrates in preparation for further processing. 
     To facilitate discussion,  FIG. 1  shows a typical prior art atmospheric inductively coupled plasma torch  100 , which includes a double-wall cylinder  102 . Cylinder  102  is typically formed out of quartz or a similarly suitable material. A cooling gas inlet  104  permits a cooling gas, such as nitrogen or air for example, to be injected in between the cylinder walls to thermally regulate double-wall cylinder  102  during use. By employing an appropriate cooling gas, thermal damage to atmospheric inductively coupled plasma torch  100  due to the high heat dissipation from the plasma therein is prevented. 
     A coil  106  is shown wrapped around the outer periphery of double-wall cylinder  102 . During use, a process gas (e.g., hydrogen or nitrogen) is introduced into the interior volume of cylinder  102  through process gas inlet  108 . When an appropriate driver RF signal (e.g., at 40 MHz) is supplied to coil  106 , coil  106  acts as part of a series LC resonance circuit to ignite a plasma from the process gas. To help cool coil  106  during use, liquid cooling is typically employed. 
     The inductively coupled plasma formed within atmospheric inductively coupled plasma torch  100  is ejected from opening  120 . The hot jet of plasma or activated neutral species ejected from opening  120  may then be employed to remove or clean materials, such as unwanted polymer deposition after an ion implantation process, from substrates. 
     As is known, the induced voltage on coil  106  is a function of the frequency of the driver RF signal. At 40 MHz, a typical atmospheric inductively coupled plasma torch may experience up to 20 KV (peak-to-peak) between the ends of coil  106 , for example. The high induced voltage is necessary for igniting plasma at typical atmospheric conditions. 
     However, the high RF driver frequency employed in the prior art (e.g., 40 MHz or higher) presents cost and engineering challenges. For example, many processing systems already employ lower-frequency RF sources (e.g., 10-30 MHz, such as 13.56 MHz or 27.12 MHz) for etching and deposition. Accordingly, components and expertise for designing, manufacturing, qualifying, and maintaining lower-frequency subsystems are readily available at lower cost. Further, tool-to-tool repeatability is improved when a lower driver RF frequency is employed. 
     The invention relates to methods and apparatus for igniting and sustaining plasma at a lower driver RF frequency in an atmospheric inductively coupled plasma torch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
       To facilitate discussion,  FIG. 1  shows a typical prior art atmospheric inductively coupled plasma torch. 
         FIG. 2A-2D  show cut-away drawings of an example improved 2-layer coil that employs the ULLU winding pattern. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail with reference to a few embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps and/or structures have not been described in detail in order to not unnecessarily obscure the present invention. 
     As discussed earlier, the induced voltage on the coil that functions as part of a series LC resonance circuit is a function of the driver RF frequency. Lowering the driver RF frequency has the effect of lowering the induced voltage on the coil. Unless compensation is made, the lower induced voltage may be insufficient to ignite the plasma in an atmospheric inductively coupled plasma torch. 
     As is known, the induced voltage on the coil is also a function of the coil inductance, or L. Generally speaking, increasing the inductance of such a coil has the effect of increasing the induced voltage on the coil. By increasing the coil&#39;s inductance, a high induced voltage may be maintained even if the driver RF frequency is lowered. Alternatively, a higher induced voltage may be achieved if the driver RF frequency remains the same. 
     One way to increase the coil&#39;s inductance is by increasing the number of turns. Generally speaking again, the inductance of a cylindrical coil is proportional to the square of the number of turns. For example, for a cylindrical coil geometry (solenoid) and without considering the ends of the coil, dropping the driver RF frequency by a factor of 3 (e.g., going from 40 MHz to 13 MHz) would require compensation in the form of a 1.7× increase on the number of turns with 1.7 being roughly the square root of 3) to maintain roughly the same induced voltage across the coil. 
     However, it is realized by the inventors herein that increasing the number of turns of the coil by simply adding more windings presents other problems. With reference to  FIG. 1 , for example, the plasma length and other plasma characteristics are directly or indirectly affected when the coil length (CL in  FIG. 1 ) is increased. For many applications, the changes in the plasma length and in other plasma characteristics are undesirable. Even if the change in the plasma length and other plasma characteristics can be accommodated, there is a physical limit to the number of turns that can be added since form factor requirements may prevent the atmospheric inductively coupled plasma torch from exceeding a certain length or bulk, for example. 
     In the transformer art, it is known that the number of turns of a coil can be effectively increased by utilizing a multi-layer coil. In a two-layer coil of the type typically employed in transformers, for example, an outer coil layer of 100 turns may overlay an inner coil layer of 100 turns, effectively providing a coil with an effective number of turns greater than 100 without increasing the coil length. 
     However, the inventors herein realize that the atmospheric inductively coupled plasma torch application involves arcing risks that are not typically experienced in transformer coil designs. As discussed earlier, the induced voltage between the ends of the coil may approach multiple tens of thousands of volts to effectively ignite plasma at atmospheric conditions. If two coil layers are wound such that the coil ends are spatially close to one another, arcing may occur. 
     Suppose, for example, the winding for the first coil layer end may start at an arbitrary point X on the quartz cylinder, proceeds toward point Y as it is wound around the quartz cylinder to form an inner coil layer, advances from the inner coil layer to the outer coil layer at point Y, and is wound back toward point X on the quartz cylinder to form the other end of the coil. At this stage, both ends of the coil are near point X. If the voltage difference between the two ends of the coil exceeds the breakdown voltage of the medium situated between the two coil ends (typically air), it is possible that arcing may occur between both coil ends due to the high voltage difference and the closeness of both coil ends. 
     Embodiments of the invention relate to methods and apparatus for maintaining a plasma-ignition capable voltage on the coil of an atmospheric inductively coupled plasma torch without increasing the physical length of the coil or unduly increasing its bulk while minimizing arcing damage. The coil discussed herein represents the coil that is wound around the cylinder or vessel (since it&#39;s possible to have a non-cylindrical vessel) that is used to generate and contain the plasma. In various embodiments, novel winding patterns (and methods therefor) are provided to minimize the voltage difference between adjacent multi-turns. As the term is employed herein, a multi-turn refers to multiple (at least two) turns that are wound on top of one another and are wound continuously from the bottom turn to the top turn or vice versa. 
     In one or more embodiments, novel winding patterns (and methods therefor) keep the voltage difference between adjacent multi-turns to a few thousand volts (as compared to tens of thousands) to minimize arcing. In one or more embodiments, novel winding patterns (and methods therefor) keep the ends of the coil (i.e., the parts of the coil that experience the highest voltage difference) more physically separated than possible in the prior art to minimize arcing. 
     Generally speaking, the coil is wound such that all winding layers of a single multi-turn are completed before proceeding to the next multi-turn. One variation relates to whether the winding starts at the lowest (inner-most or closest to the cylinder) winding layer for the first turn of a multi-turn or at the uppermost (outer-most or furthest from the cylinder) winding layer for the first turn of the multi-turn. Another variation occurs in whether the cross-over from a multi-turn to its adjacent multi-turn occurs at the same winding layer (e.g., the cross-over is made from upper-most layer of multi-turn X to upper-most layer of multi-turn X+1, or the cross-over is made from lowest layer of multi-turn X to lowest layer of multi-turn X+1) or at a different winding layer (e.g., the cross-over is made from upper-most layer of multi-turn X to lowest layer of multi-turn X+1, or the cross-over is made from lowest layer of multi-turn X to upper-most layer of multi-turn X+1). 
     In the following discussion, a two-layer coil is described. However, it should be understood that the apparatus and methods herein can be extended to 3-layer, 4-layer or more by simply performing the windings for all layers of a multi-turn before starting on an adjacent multi-turn, for example. 
     In one or more embodiments, a LULU (lower-upper-lower-upper) winding pattern is formed between the first two adjacent multi-turns in a two-layer coil. The pattern repeats for the next two multi-turns, and for the next two, and so forth. The LULU pattern involves first winding the coil close to the plasma cylinder (the lower layer, which is the “L” part of the LULU pattern). This represents the first “L” in the pattern “LULU”. Next, the winding is done for the upper layer radially further away from the plasma cylinder in the same multi-turn. This represents the first “U” in the pattern “LULU”. These two windings (lower layer, then upper layer) form the first multi-turn (which happens to be double-turn for a two-layer coil). It should be noted that both individual turns or wraps of an L-U double-turn are located at the same distance d 1  with respect to the end  160  of the quartz cylinder. 
     Next, the coil is led to the lower layer of the adjacent multi-turn, again close to the plasma cylinder. This represents the second “L” in the pattern “LULU”. Next, the winding is done for the upper layer radially further away from the plasma cylinder in the same adjacent multi-turn. This represents the second “U” in the pattern “LULU”. These two windings (lower layer, then upper layer) form the second double-turn. The second double-turn is located at a sufficiently different distance d 2  (measured with respect to the end  160  of the quartz cylinder) to minimize cross talk with L-U pair  1  at distance d 1 . The third double-turn and fourth double-turn proceed similarly. 
     It should be noted that if three layers are involved, the pattern simply becomes “L 1 U 11 U 12 L 2 U 21 U 22 ”, for example, where the designation L 1  represents the lowest layer that is closest to the cylinder for the first triple turn, the designation U 11  denotes the intermediate layer for the first triple turn and designation U 12  denotes the upper most layer that is radially the furthest away from the cylinder for the first triple turn. Analogously, the designation L 2  represents the lowest layer that is closest to the cylinder for the second triple turn, the designation U 21  denotes the intermediate layer for the second triple turn and designation U 22  denotes the upper most layer that is radially the furthest away from the cylinder for the second triple turn. In this case, the lower layer is wound first, and then the intermediate upper layer radially further out for the same triple turn is wound. Afterward, the upper-most layer that is radially even further out is wound for the same triple turn. The coil is then led to the lower layer of the adjacent triple turn to repeat. If four layers are involved, the pattern becomes “L 1 U 11 U 12 U 13 L 2 U 21 U 22 U 23 ”, for example. In this manner, any number of layers may be accommodated. 
     Also note that it is possible to wind a ULUL pattern for the first two adjacent multi-turns in a two-layer coil. In other words, it is possible to wind upper and lower for one multi-turn, cross-over, and wind upper and lower for the adjacent multi-turn. The pattern repeats for the next two multi-turns, and for the next two, and so forth. For three layers, it becomes “U 1 L 11 L 12 U 2 L 21 L 22 ”, for example, where the designation U 1  represents the upper most layer for the first triple turn, the designation L 12  denotes the intermediate layer for the first triple turn and designation L 12  denotes the lowest layer that is closest to the cylinder for the first triple turn. Analogously, the designation U 2  represents the upper most layer for the second triple turn, the designation L 21  denotes the intermediate layer for the second triple turn and designation L 22  denotes the lowest layer that is closest to the cylinder for the second triple turn. 
     In one or more embodiments, a LUUL (lower-upper-upper-lower) winding pattern is formed between the first two adjacent multi-turns in a two-layer coil. The pattern repeats for the next two multi-turns, and for the next two, and so forth. The LUUL pattern involves first winding the coil layer close to the plasma cylinder. This represents the first “L” in the pattern “LUUL”. Next, the winding is done for the upper layer radially further away from the plasma cylinder in the same multi-turn. This represents the first “U” in the pattern “LUUL”. These two windings (lower layer, then upper layer) form the first multi-turn. 
     Next, the coil is crossed over to the upper layer of the adjacent multi-turn. This represents the second “U” in the pattern “LUUL”. Next, the winding is done for the lower layer closer to the plasma cylinder in the same adjacent multi-turn. This represents the second “L” in the pattern “LUUL”. These two windings (lower layer, then upper layer) form the second multi-turn. The third and fourth multi-turns proceed similarly. 
     It should be noted that if three layers are involved, the pattern simply becomes “L 1 U 11 U 12 U 21 L 2 ” for the first pair of adjacent multi-turns, for example, where the designation L 1  represents the lowest layer that is closest to the cylinder for the first double turn, the designation U 11  denotes the intermediate layer for the first double turn and designation U 12  denotes the upper most layer that is radially the furthest away from the cylinder for the first double turn. Analogously, the designation L 2  represents the lowest layer that is closest to the cylinder for the second double turn, the designation U 21  denotes the intermediate layer for the second double turn and designation U 22  denotes the upper most layer that is radially the furthest away from the cylinder for the second double turn. In this case, the lower layer is wound first, and then the intermediate upper layer radially further out for the same multi-turn is wound. Afterward, upper-most layer that is radially even further out is wound for the same multi-turn. The coil is then led to the upper-most layer of the adjacent multi-turn. Next, the intermediate upper layer of that adjacent multi-turn is wound. Next, the lower layer of that adjacent multi-turn is wound. If four layers are involved, the pattern becomes “L 1 U 11 U 12 U 13 U 23 U 22 U 21 L 2 ”, for example. In this manner, any number of layers may be accommodated. 
     Also note that it is possible to wind a ULLU pattern for the first two adjacent double turns in a two-layer coil. In other words, it is possible to wind upper and lower for one multi-turn, cross-over, and wind lower and upper for the adjacent multi-turn. The pattern repeats for the next two multi-turns, and for the next two, and so forth. For three layers, it becomes “U 1 L 11 L 12 L 22 L 21 U 2 ”, for example, where the designation U 1  represents the upper most layer for the first triple turn, the designation L 11  denotes the intermediate layer for the first triple turn and designation L 12  denotes the lowest layer that is closest to the cylinder for the first triple turn. Analogously, the designation U 2  represents the upper most layer for the second triple turn, the designation L 21  denotes the intermediate layer for the second triple turn and designation L 22  denotes the lowest layer that is closest to the cylinder for the second triple turn 
     In one or more embodiments, the winding employs a continuous conductor and follows a single direction (i.e., either clock-wise or counter-clockwise) and proceeds until all winding layers of a single multi-turn is completed. Then the winding proceeds to the next multi-turn and completes all winding layers of the next multi-turn. Then the winding proceeds to the next multi-turn and so on. Multi-turns are added linearly in one direction along the linear length of the cylinder as adjacent multi-turns are added. In one or more embodiments, the grounded end of the coil is at an outer-most winding. Alternatively or additionally, in one or more embodiments, the high voltage end of the coil is at an outer-most winding. In one or more embodiments, 2-6 mm copper or copper alloy tubing is employed as coil material. The outer surface can be silver plated for better conductivity at high RF frequencies (skin effect of RF current). 
     In one or more embodiments of the invention, the coil is a tube-in-a-tube configuration in which a smaller tube is disposed inside a larger tube through the use of a double wall tube (with appropriate spacer structures in between). A cooling fluid (such as high-purity water or a similarly suitable cooling fluid) is injected into one tube (either the inner or outer tube) at one end of the coil, travels to the other end of the coil in the same tube, and is diverted into the other tube for returning to the original end of the coil. This configuration simplifies plumbing installation and maintenance. If the coil end that is employed to inject and extract the cooling fluid is also the grounded coil end, tap water or other ‘none high-purity’ cooling fluids may be used for cooling since the cooling fluid is not introduced into or extracted from the high voltage coil end. Introducing or extracting the cooling fluid at the grounded side avoids RF current leakage to undesirable locations in the plasma torch device due to a residual conductivity of the cooling fluid. 
     The features and advantages of the invention may be better understood with reference to the figures and discussions that follow.  FIG. 2A-2D  show cut-away drawings of an example improved 2-layer coil that employs the ULLU winding pattern. In  FIG. 2A , U A  and L A  form the first multi-turn, and then L B  and U B  form the adjacent multi-turn. U A  represents the upper (outer) layer of the first multi-turn. L A  represents the lower (inner) layer of the first multi-turn. L B  represents the lower (inner) layer of the second multi-turn. U B  represents the upper (outer) layer of the second multi-turn. In this example, the sequence is U A L A L B U B . Note that the coil is wound continuously clockwise (looking up from the bottom of the coil in the figure) in the direction of arrow  252  and proceeds in the direction of arrow  254  as additional multi-turns are added. 
       FIG. 2B  represents the continuation of the ULLU winding pattern of  FIG. 2A  with the second multi-turn pair shown in more details. Again, U A  and L A  form the first multi-turn, and L B  and U B  form the adjacent multi-turn. 
       FIG. 2C  represents the continuation of the ULLU winding pattern of  FIGS. 2B and 2A , with a third multi-turn pair added. Again, U A  and L A  form the first multi-turn, and L B  and U B  form the adjacent multi-turn. U C  represents the upper (outer) layer of the third multi-turn. L C  represents the lower (inner) layer of the third multi-turn. In this example, the sequence is U A L A L B U B U C L C . 
       FIG. 2D  represents the continuation of the ULLU winding pattern of  FIGS. 2C ,  2 B, and  2 A, with a fourth multi-turn added. Again, U A  and L A  form the first multi-turn, and L B  and U B  form the adjacent multi-turn. U C  represents the upper (outer) layer of the third multi-turn. L C  represents the lower (inner) layer of the third multi-turn. L D  represents the lower (inner) layer of the fourth multi-turn. U D  represents the upper (outer) layer of the fourth multi-turn. At this point, the pattern is U A L A L B U B U C L C L D U D . 
     As can be appreciated from  FIG. 2D  as well, the two ends of the coil are spatially separated such that they are at opposite ends of the coil linearly speaking (i.e., along the direction of arrow  254 ). This would not have been possible if the entire lower layer had been wound first, and then the winding had doubled back on top of the lower layer to form the upper layer (as was commonly done with transformer windings). Preferably (but not absolutely required), the two ends of the coil are at 180-degree with respect to one another (as shown in  FIG. 2D ) to maximize spatial separation. 
     As can be appreciated from the foregoing, embodiments of the invention effectively increase the number of turns without increasing the overall height of the coil (the height of the coil solenoid), which increases the inductance of the coil to effectively increase the induced voltage across the length of the coil while minimizing arcing. By increasing the induced voltage on the coil, plasma may be more easily ignited and/or sustained with the same or lower RF driver frequency. 
     Having disclosed exemplary embodiments and the best mode, modifications and variations may be made to the disclosed embodiments while remaining within the subject and spirit of the invention as defined by the following claims.