Abstract:
A method and system for adjusting a height-to-diameter ratio of a plasma processing chamber, either dynamically or before substrate processing, to control a uniformity of a plasma and/or match a uniformity of a plasma to at least one of a process type and a wafer configuration and/or type. By adjusting the height of the chamber, the position of electrons near a chamber wall can be moved toward a center of the chamber and vice versa.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a division of application Ser. No. 10/195,553, filed Jul. 16, 2002, and is related to and claims priority under 35 U.S.C. 119(e) to U.S. provisional application serial No. 60/307,183, filed Jul. 24, 2001, the entire contents of which are herein incorporated by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention is related to plasma processing systems, particularly to a plasma processing system, which uses variable aspect ratio plasma source.  
         BACKGROUND OF THE INVENTION  
         [0003]    Manufacturers of semiconductor integrated circuits (IC) are faced with severe competitive pressure to improve their products. This pressure in turn is driving the manufacturers of the equipment used by IC manufacturers to improve the performance of their equipment. One particular type of tool that is widely used, and that is therefore particularly susceptible to these competitive pressures, is the plasma reactor. These reactors can be used to remove material, or (with modifications) they can be used to deposit material.  
           [0004]    The mechanisms for either deposition or removal are complex, but in either case, it is essential to control the physical processes at the surface of the wafer. Control of these processes is the focus of significant technological development. Etch uniformity is a particular concern, and the manufacturer of plasma reactors that can improve overall uniformity enhances its potential for increasing market share. Current plasma reactors do not provide adequate etch uniformity over a wide range of processes.  
           [0005]    In fact, in known plasma sources, plasma density uniformity is often related to a plasma source&#39;s height-to-diameter aspect ratio. As the height-to-diameter aspect ratio of the plasma source increases, the electron density at the center of the source increases. Conversely, as the height-to-diameter aspect ratio decreases, the electron density at the edges of the source increases.  
           [0006]    In high height-to-diameter aspect ratio cylindrical plasma sources, gaseous species can easily diffuse to the center of the source. In general, a shortcoming of high aspect ratio plasma sources is a spatially non-uniform plasma density except for a narrow range of process conditions (e.g. narrow range of pressure). In particular, for low pressure (usually less than 50 mTorr), the plasma density in an inductively coupled plasma (ICP) source tends to be greatest in the center of the chamber and lowest at the edge of the chamber. On the contrary, the inverse can be true for higher pressures (i.e. greater than 50 mTorr). In fact, the narrow range of pressure wherein the plasma density is spatially homogeneous is sensitive to the process chemistry, gas species, etc., and so the optimal pressure may vary from one process to another.  
           [0007]    Various patents and articles describe plasma systems, including: U.S. Pat. Nos. 6,042,687, 5,716,485, and 6,020,570.  
           [0008]    U.S. Pat. No. 6,042,687 entitled “Method and apparatus for improving etch and deposition uniformity in plasma semiconductor processing,” assigned to Lam Research Corp. (Fremont, Calif.), describes a plasma processing system and method for processing substrates such as by chemical vapor deposition or etching. The system utilizes a secondary gas concentrated near the periphery of the substrate, improving etching/deposition uniformity across the substrate surface.  
           [0009]    U.S. Pat. No. 5,716,485 entitled “Electrode designs for controlling uniformity profiles in plasma processing reactors,” assigned to Varian Associates Inc. (Palo Alto, Calif.), describes an electrode design for reducing the problem of non-uniform etch in large diameter substrates. The electrode opposite the substrate being etched in a plasma reactor can be tailored as to its shape so as to control the uniformity of the etching across the substrate. This is achieved with a number of generally dome-shaped electrode structures including generally cone-shaped electrodes, generally pyramid-shaped electrodes and generally hemispherically-shaped electrodes. The dome-shaped electrodes serve to disperse the high concentration of ions from the center of the reactor out toward the periphery of the substrate and thereby even out the ion density distribution across the substrate being etched. The electrodes are useable in diode plasma reactors, triode plasma reactors and ICP plasma reactors.  
           [0010]    U.S. Pat. No. 6,020,570 entitled “Plasma Processing Apparatus,” assigned to Mitsubishi Denki Kabushiki Kaisha (Tokyo, Japan), describes an electrode design for reducing the problem of non-uniform etch in large diameter substrates. The electrode opposite the substrate being etched in a plasma reactor can be tailored as to its shape so as to control the uniformity of the etching across the substrate. This is achieved with a number of generally ring-shaped electrode structures.  
           [0011]    In current systems, once a wafer is loaded into a plasma reactor for a given process step, the reactor may require parameter changes to achieve uniform plasma density for the current wafer process. Current etch processes rely on one or two adjustment parameters to reduce wafer edge effects. As wafers with different film stacks are processed, these parameters must be adjusted also from one cassette of wafers (i.e., 25 wafers) to the next. Adjustments are required to sustain a desired wafer etch profile as the chamber changes due to accumulated depositions, temperature, or electrode erosion. These types of adjustment processes are time-intensive and costly.  
           [0012]    What is needed is a more time-efficient and cost-effective system for increasing the uniformity in a plasma processing reactor.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an object of the present invention to increase uniformity in a plasma processing reactor by utilizing a variable aspect ratio (VAR) plasma source. In one embodiment, the uniformity (of the plasma or electron density) is controlled (either generally or in the radial direction) using feedback, which enables the aspect ratio of the plasma reactor to be dynamically controlled.  
           [0014]    It is another object of the present invention to enable plasma source parameters to be varied over a wide range of wafer compositions, configurations and/or processes while maintaining radial plasma density uniformity.  
           [0015]    It is a further object of the present invention to dynamically adjust a height-to-diameter ratio of a VAR plasma source for different wafer processes.  
           [0016]    It is another object of the present invention to provide a plasma source useable over a wide range of wafer compositions, configurations and/or processes without varying other more dominant process parameters (e.g., the pressure).  
           [0017]    It is another object of the present invention to provide a plasma source that can change processes dynamically, that is, to etch or deposit stacks of material and tune the process optimally for each layer. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    A more complete appreciation of the invention and many of the attendant advantages thereof will become readily apparent with reference to the following detailed description, particularly when considered in conjunction with the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1 illustrates a simplified block diagram of a plasma processing system according to the present invention;  
         [0020]    [0020]FIG. 2 illustrates a simplified cross-sectional view of a variable aspect ratio (VAR) plasma source according to the present invention;  
         [0021]    [0021]FIG. 3 illustrates an expanded view of a vertically translatable gas injection electrode for a VAR plasma source according to the present invention; and  
         [0022]    [0022]FIG. 4 illustrates a flowchart illustrating a method of using the variable aspect ratio plasma source according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    The present invention is directed to a method and apparatus for controlling plasma formed in a plasma reactor (e.g., an inductively coupled plasma reactor) having a (grounded) anode, a bias electrode which serves as the substrate holder and plasma coupling device (e.g., an inductive coil that surrounds the cylindrical geometry). In particular, a Variable Aspect Ratio (VAR) Plasma Source is designed with a variable height and is moved vertically within the plasma reactor to correct for process changes over a wide range of etching processes and deposition processes.  
         [0024]    [0024]FIG. 1 illustrates a simplified block diagram of a plasma processing system according to the present invention. FIG. 1 shows a plasma processing system from a high-level perspective. Plasma processing system  100  comprises plasma reactor  110 , wafer handling and robotics system  120 , cooling system  130 , pumping system  140 , gas supply system  150 , controller  160 , first RF generator  170 , first matching network  172 , second RF generator  180 , and second matching network  182 .  
         [0025]    Plasma processing system  100  further includes communication line  165 , gas supply line  155 , cooling lines  135 , vacuum line  145 , first RF transmission line  175 , and second RF transmission line  185 .  
         [0026]    Controller  160  is operatively coupled via communication line  165  to gas supply system  150 , wafer handling and robotics system  120 , cooling system  130 , pumping system  140 , first RF generator  170 , first matching network  172 , second RF generator  180 , second matching network  182 , and plasma reactor  110 .  
         [0027]    In a preferred embodiment, plasma reactor  110  is pneumatically coupled to pumping system  140  via vacuum line  145 . For example, a control valve is used, and the controller monitors the valve position. Plasma reactor  110  is electrically coupled to first RF generator  170  via first matching network  172  and first RF transmission line  175 . Plasma reactor  110  is electrically coupled to second RF generator  180  via second matching network  182  and second RF transmission line  185 . Controller  160  monitors and controls matching networks using tunable elements in the matching networks. For example, tuning parameters associated with the tunable elements can be used to determine plasma impedances and operating points.  
         [0028]    Plasma reactor  110  is hydraulically coupled to cooling system  130  via cooling lines  135 . Plasma reactor  110  is fluidly coupled to gas supply system  150  via gas supply line  155 . Plasma reactor  110  is operatively coupled to wafer handling and robotics system  120  via a robotic arm (not shown).  
         [0029]    Controller  160  (e.g., a computer controller) includes memory to store process instructions. In operation, upon command from controller  160  and in accordance with the process instructions stored in the memory of controller  160 , wafer handling and robotics system  120  places a silicon wafer to be processed into plasma reactor  110 . The aspect ratio of plasma reactor  110  is adjusted. Pumping system  140  pumps down plasma reactor  110 . Gas from gas supply system  150  is introduced to plasma reactor  110  according to a pre-determined gas mixture recipe. Then, first RF generator  170  couples power to plasma reactor  110 , which, in the presence of an ionizable gas at a pre-determined pressure within plasma reactor  110 , creates a plasma that provides a population of ions and chemical environment suitable for etching the wafer. Second RF generator  180  couples power to the substrate holder to provide a bias suitable for attracting positively charged ions to the substrate surface to energize the surface etch chemistry. Cooling system  130  provides cooling for the plasma reactor  110  as the wafer is etched.  
         [0030]    Controller  160  monitors and controls operational parameters for plasma reactor  110 . For example, controller  160  can provide instructions to plasma reactor  110  to adjust the aspect ratio; to cooling system  130  to stabilize the temperature of the reactor wall and/or chuck; to gas supply system  150  to change the process gas; to first RF generator  170  to change the power being supplied to the plasma; and/or to second RF generator  180  to change the power being supplied to the plasma.  
         [0031]    [0031]FIG. 2 illustrates a simplified cross-sectional view of a variable aspect ratio (VAR) plasma source according to the present invention. In a preferred embodiment, plasma reactor  110  (FIG. 1) comprises VAR plasma source  200 .  
         [0032]    VAR plasma source  200  includes chuck assembly  210 , plasma source assembly  240 , and VAR assembly  230 . Plasma source assembly  240  is coupled to chuck assembly  210  and VAR assembly  230 .  
         [0033]    Plasma source assembly  240  includes process chamber  205 , housing  245 , and plasma source  250 . Desirably, housing  245  is cylindrically shaped as shown in cross-section in FIG. 2 and comprises at least one outlet  218 . For example, chuck assembly  210 , housing  245 , and VAR assembly  230  can be formed as cylinders and share a common axis  204 . VAR assembly  230  comprises housing  232  and a vertically translatable gas inject electrode (to be discussed in greater detail in FIG. 3). VAR assembly  230  comprises temperature control (not shown) so that the temperature of the VAR assembly  230  can be monitored and controlled.  
         [0034]    In a preferred embodiment, plasma source  250  comprises an inductively coupled plasma (ICP) source. In another embodiment, plasma source  250  can comprise an electrostatically shielded radio frequency (ESRF) plasma source. Plasma source  250  is coupled to housing  245 .  
         [0035]    As shown in FIG. 2, ESRF plasma source includes inductive coil  252 , chamber  254 , process tube  256 , and electrostatic shield  258 . Inductive coil  252  is generally fabricated from copper tubing and is desirably designed to be a quarter-wave resonator. Furthermore, inductive coil  252  is immersed within a bath of (dielectric) coolant such as Fluorinert and disposed about the perimeter of a dielectric process tube, which interfaces with the plasma processing region. The bath of coolant is recirculated in chamber  254  via an inlet flow of coolant and a corresponding outlet flow of coolant through coolant supply lines in order to provide plasma source cooling.  
         [0036]    Electrostatic shield  258  is slotted and reduces capacitive coupling between the inductive coil  252  and the plasma processing region. Electrostatic shield  258  is generally fabricated from aluminum, and it is electrically grounded. RF power is coupled to the inductive coil  252  from first RF generator  290  through first impedance match network  292 , and first transmission line  294 . Desirably, the ICP source is utilized to generate a plasma from an ionizable gas.  
         [0037]    Process tube  256  is generally fabricated from a dielectric material such as quartz or alumina. In addition, process tube  256  acts as a window for coupling RF power to the plasma, and it preserves the vacuum integrity of the chamber.  
         [0038]    The electrical and mechanical design of an inductively coupled plasma source including the inductive coil, electrostatic shield, process tube, coil enclosure, impedance match network, tap location, etc. is well known to those of skill in the art. For further details, refer to U.S. Pat. 5,234,529, which is herein incorporated by reference in its entirety.  
         [0039]    Chuck assembly  210  includes grounded chuck susceptor  212 , insulator  214 , and electrode  216 . In a preferred embodiment, insulator  214  is used to electrically isolate grounded chuck susceptor  212  and electrode  216 . In addition, electrode  216  is a biasable electrode.  
         [0040]    As illustrated in FIG. 2, RF power is coupled to electrode  216  from second RF generator  280  through second electrode match network  282 , blocking capacitor  284 , and second electrode RF transmission line  286 . In addition, substrate (e.g., a semiconductor wafer or LCD panel)  270  is shown on electrode  216 . Desirably, the second electrode is utilized to attract the population of positively charged ions to the wafer surface. More specifically, the plasma source RF power controls the ion density while the chuck RF power controls the ion energy.  
         [0041]    For example, first RF generator  290  delivers RF power (e.g., in the range of 1 to 5 kW) to ICP source. At substantially the same time, second RF generator  280  delivers RF power (e.g., in the range of 100 W to 3 kW) to electrode  216 . The RF energy applied in the presence of process gases (e.g., at a pressure of 1 to 1000 mTorr) ignites plasma within reaction chamber in the region above wafer  270 .  
         [0042]    VAR assembly  230  comprises housing  232  and vertically translatable gas injection electrode  300 , which is shown in detail in FIG. 3. Double-headed arrow  235  shows directions of movement for the injection plate in the vertically translatable gas injection electrode  300 .  
         [0043]    [0043]FIG. 3 illustrates an expanded view of a vertically translatable gas injection electrode for a VAR plasma source according to the present invention. Vertically translatable gas inject electrode  300  includes mounting plate  305 , a plurality of translators  310 , a plurality of translation means  315 , coupling rod  320 , structural member  325 , enclosure  330 , bellows  335 , skirts  337 , and injection plate  340 . In alternate embodiments, mounting plate  305  and/or structural member  325  are not required. Desirably, controller  160  (FIG. 1) is operatively coupled to the plurality of translation means  315 .  
         [0044]    Double-headed arrow  350  shows directions of movement for injection plate  340 . Clearance gap  345  between injection plate  340  and the inside wall of enclosure  330  allows such movement. Skirts  337  protect bellows  335  from RF energy as injection plate  340  is moved within the chamber. Skirts  337  are designed to minimize their impact on the plasma uniformity. For example, slots and material properties are chosen to minimize energy loss. In addition, skirts  337  are temperature controlled to minimize particle release from surface depositions.  
         [0045]    In a preferred embodiment, a drive mechanism comprises a translator and a translation means responsively coupled to the translator. Desirably, a drive mechanism comprises a screw jack as a translator and motor drive as a translation means. For example, drive mechanisms can be lead screw driven linear stages capable of providing vertical movement of the gas inject electrode relative to the plasma source and the chuck assembly. Desirably, three drive mechanisms are used and spaced at equal distances azimuthally, i.e. every 120 degrees (only two drive mechanisms are shown in FIG. 3). Since linear drive mechanism components are well known in the art and are readily available for integration into the apparatus of the present invention the details of these components, including lead screws, linear bearings, electrical drive motors, controllers, limit switches, and the like will not be described. It will be appreciated by those of skill in the art that different methods of providing vertical translation of gas inject electrode relative to the plasma source and chuck assembly (e.g. linear motors, pneumatic devices) may be provided and such methods fall within the scope of the invention. These elements are interrelated as shown in FIG. 3.  
         [0046]    Injection plate  340  includes a plurality gas orifices  342  fed gas through gas supply channels  344  from gas supply system  150  (FIG. 1). In a preferred embodiment, injection plate  305  are fabricated from aluminum and anodized for contact with the plasma. It will be appreciated by those of skill in the art that different methods of introducing gas to the reaction chamber are possible and different means to fabricate the gas inject electrode (i.e. materials, methods of fabrication, etc.) are possible, and such designs fall within the scope of this invention.  
         [0047]    In other embodiments, injection plate  305  can include layers of inject plates stacked together wherein the bottom-most inject plate is fabricated from a material such as silicon. The material for the gas injection plate may be chosen specifically for a particular process. For instance, a silicon gas inject electrode may be desirable for oxide etch applications in that it is compatible with the etch process and etched silicon can act as a fluorine radical scavenger. In addition, the bottom-most inject plate can also include an edge comprising a material tuned to optimize the uniformity of a process. Also, the bottom-most inject plate can include materials having thickness profiles and/or doping profiles that are optimize for etch or deposition processes.  
         [0048]    The gas injection plate  305  can be vertically translated via drive mechanisms discussed above. A tight clearance (i.e. ˜2 mm.) is provided between the gas inject plate and the outer wall of enclosure  340 . Rod  320  is used to translate movement from the drive mechanism to the injection plate. In a preferred embodiment, rod  320  is also used to provide process gases to injector plate  340 . Bellows  325  is extendably connected between the upper surface of the gas injection plate and the bottom surface of enclosure  330 . The bellows  325  preserves the vacuum integrity while allowing movement of the gas injection plate  340 .  
         [0049]    In operation, upon command from controller  160  shown in FIG. 1 and in accordance with empirical data stored in controller (shown in FIG. 1) first translation means, second translation means, and third translation means (not shown) drive the vertically translatable gas inject electrode to an optimized setting for the selected wafer etch process step. In doing so, the translation of the gas inject electrode leads to a variation of the (cylindrical) plasma source aspect ratio (height-to-diameter). This step optimizes etch uniformity for the current wafer process and can be repeated in order to dynamically regulate the aspect ratio to control the uniformity during the process.  
         [0050]    [0050]FIG. 4 illustrates a flowchart illustrating a method of using the variable aspect ratio plasma source according to the present invention. Procedure  500  shows a method of operating the apparatus of the present invention to optimize etch uniformity. Procedure  500  begins with step  510 .  
         [0051]    In step  510 , a wafer is placed upon the chuck assembly  210  via conventional means (e.g., transfer system robotic arm and lift pins, etc.) in the reaction chamber  205 .  
         [0052]    In step  520 , the VAR plasma source receives commands from the controller to achieve an optimum height-to-diameter ratio for the current wafer etch process. By adjusting the height of the vertically translatable gas inject electrode relative to the wafer, the radial component of the plasma density and electron density are optimized. For example, the optimal position for the vertically translatable gas inject electrode can be determined from wafer blanket and patterned etch tests completed a priori.  
         [0053]    Alternatively, the optimal position of the vertically translatable gas inject electrode relative to the wafer may be determined and/or re-determined in-situ once a plasma has been generated via spatially resolved optical emissions. For example, U.S. Patent Application No. 60/193,250 describes a technique for monitoring and recording spatially resolved (in a transverse directions parallel with the wafer surface) plasma optical emissions via optical spectroscopy, entitled “Optical monitoring and control system and method for plasma reactors”. This application is herein incorporated by reference in its entirety.  
         [0054]    In addition, the optimal position of the vertically translatable gas inject electrode relative to the wafer may be determined and/or re-determined in-situ once a plasma has been generated via microwave measurements. For example, U.S. Patent Applications (60/144,880; 60/144,833; 60/144,878; and 60/166,418) describe techniques for using microwave devices to make plasma density measurements. These applications are herein incorporated by reference in their entirety.  
         [0055]    In step  530 , the chamber is evacuated by the vacuum pumping system to a base pressure (e.g. 0.1 to 1 mTorr), process gas is introduced to the vacuum chamber at a prescribed flow rate (e.g., equivalent to 100 to 1000 sccm argon), and the gate valve (or vacuum pump throttle valve) is partially closed to achieve the desired process pressure (e.g. 1 to 100 mTorr). Following the introduction of an ionizable gas to the process chamber, RF power is provided to the first electrode (inductive coil) and second electrode (chuck electrode), and the plasma is generated.  
         [0056]    The etch process is run with a first set of operational parameters. The first set of operational parameters comprise process type, process time, chamber pressure, temperature, process gases, flow rates, first RF generator power, and second RF generator power. In some processes, the aspect ratio of the plasma source is adjusted during the process to achieve optimum wafer etch uniformity.  
         [0057]    In another embodiment, a deposition process can be run with operation parameters optimized during the deposition process.  
         [0058]    In step  540 , the wafer can be unloaded or removed from the reaction chamber (e.g., again by conventional means).  
         [0059]    To further improve etch uniformity, the etch uniformity on the wafer can be analyzed. The analysis results can be stored and used to recalculate the optimal position used for the vertically translatable gas inject electrode for another wafer or another set of wafers.  
         [0060]    In addition, the controller can dynamically adjust a height-to-diameter ratio of a VAR plasma source for different wafer processes including trench etching and/or via etching processes. The controller can dynamically adjust a height-to-diameter ratio of the VAR plasma source to maintain radial plasma density uniformity while operational parameters vary over a wide range of wafer compositions, configurations and/or processes. The controller can dynamically adjust a height-to-diameter ratio of the VAR plasma source to provide a plasma source that can change processes dynamically, that is, to etch or deposit stacks of material and tune the process optimally for each layer. For example, the ratio can be changed for break-thru, main etch, and over-etch conditions. The ratio can also be dependent upon the material such as silicon compounds and/or gallium compounds.  
         [0061]    In an alternative embodiment, a vertically moveable lower electrode is utilized (instead of or in addition to a moveable upper electrode). A vertically moveable lower electrode allows the exhaust manifold effect to be tuned and allows the amount of sidewall, which is available to act as a ground electrode for a parallel plate plasma, to be tuned.  
         [0062]    Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.