Abstract:
A plasma system for processing large area substrates. In one embodiment the system includes a plurality of radiofrequency (rf) plasma sources removably attached to the rf transparent windows of a processing chamber. The number and distribution of sources is varied to provide the size and uniformity of the plasma field required to treat the substrate. A plurality of plasma probes, such as Langmuir probes, Faraday cups and optical sensor are positioned within the chamber and in electrical communication with the plasma sources adjust the rf field produced by the individual sources to maintain the desired degree of field uniformity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of copending U.S. application Ser. No. 08/503,973, filed Jul. 19, 1995. 
     
    
     
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    Plasma treatment of large area substrates such as glass or semiconductor substrates used in the fabrication of flat panel displays or 300 mm silicon wafers respectively pose a series of problems that do not occur in the processing of small area substrates. One problem is simply the generation of plasmas of sufficient area to treat the large area substrate. A second problem is the maintenance of the uniformity of the plasma density and chemistry over such a large area.  
           [0004]    The use of inductively or transformer coupled plasma sources (ICP and TCP, respectively) are affected both by the difficulty of maintaining plasma uniformity using inductive coil antenna designs and the cost of fabricating and maintaining such systems which require large and thick quartz windows for coupling the antenna radiation into the processing chamber. The use of such thick quartz windows results in an increase in rf power (and reduction in efficiency) due to heat dissipation within the window.  
           [0005]    The use of Electron Cyclotron Resonance (ECR) and Helicon type sources are limited by the difficulty in scaling the resonant magnetic field to large areas when a single antenna or waveguide is used. Furthermore, most ECR sources utilize microwave power which is more expensive and difficult to tune electrically. The use of hot cathode plasma sources results in the contamination of the plasma environment due to the evaporation of cathode material, while cold cathode sources result in contamination due to exposure of the cold cathode to the plasma generated.  
           [0006]    The present invention avoids these problems encountered by previous large area plasma processing systems.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention relates to an easily scalable and maintainable system for producing a uniform plasma of any size. In one embodiment a plurality of rf plasma sources are removably attached to the dielectric windows such as glass or quartz of a vacuum processing chamber, while in another embodiment each of the plurality of sources includes its own window and is attached to the chamber. Plasma measuring probes within the chamber provide information on the plasma uniformity and this information is used to control the rf energy applied to each of the rf plasma sources so as to maintain the desired uniformity. In one embodiment, the plasma measuring probes are Langmuir probes. In another embodiment, the probes are Faraday cups. In yet another embodiment, the probes are optical probes.  
           [0008]    In another embodiment, a plasma source includes a quartz window with an integral tube for gas introduction. Several of such plasma sources using different gases may be combined in a linear array for the sequential treatment of substrates in an inline processing system.  
           [0009]    In a further embodiment multiple antennae rf sources are used to provide a uniform plasma during the pulsing phase and steady state in order to provide a uniform ion implantation dose to a large area substrate. The plasma source is stationary and the wafers are transported through the plasma or the wafers are stationary and the plasma is transported past the wafers.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]    This invention is pointed out with particularity in the appended claims. The above and further advantages of this invention may be better understood by referring to the following description taken in conjunction with the accompanying drawing, in which:  
         [0011]    [0011]FIG. 1 is a block diagram of an embodiment of the plasma treating system of the invention;  
         [0012]    [0012]FIG. 2 is a top view of a configuration of sources to provide a large area plasma coverage using small area plasma sources;  
         [0013]    [0013]FIG. 3 is a plan view of an embodiment of a wafer holder with built-in faraday cups;  
         [0014]    [0014]FIG. 3 a  is a plan view of an embodiment of a Si test wafer with imbedded faraday cups;  
         [0015]    [0015]FIG. 4 is a perspective view of an embodiment of the invention configured as a volume source;  
         [0016]    [0016]FIG. 5 is a perspective view of an embodiment of plasma source of the invention with integral gas feed;  
         [0017]    [0017]FIG. 6 is a cross-sectional view of an embodiment of a continuous plasma treatment device using a plurality of plasma sources shown in FIG. 5;  
         [0018]    [0018]FIG. 7 is a schematic diagram of a system utilizing two plasma sources;  
         [0019]    [0019]FIG. 8 depicts an embodiment of an arrangement of surface magnets for ECR plasma generation in a two source system; FIG. 8 a  depicts another embodiment of an arrangement of surface magnets for ECR plasma generation;  
         [0020]    [0020]FIG. 9 is a diagram of a Separation by Plasma Ion Implantation Technology (SPIT) treatment device;  
         [0021]    [0021]FIG. 10 is a graph of the ion current (implantation dose) provided by the device of FIG. 9;  
         [0022]    [0022]FIG. 11 is a diagram of an In-line a plasma processing system;  
         [0023]    [0023]FIG. 12 is a graph of the ion current (implantation dose) provided by the system of FIG. 11;  
         [0024]    [0024]FIG. 13 is a diagram of another embodiment of a plasma treatment device;  
         [0025]    [0025]FIG. 14 is a graph of the ion current (implantation dose) provided by the device of FIG. 13;  
         [0026]    [0026]FIG. 15 is a diagram of the high voltage connections to the wafer and robot arm of the device of FIG. 11; and  
         [0027]    [0027]FIG. 16 is a diagram of the high voltage connections to the wafer and robot arm of the device of FIG. 13. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    In brief overview and referring to FIG. 1, an embodiment of the system of the invention  10  includes a vacuum chamber  14  having a vacuum port  18  connected to a vacuum pump (not shown). In the embodiment shown, the system  10  includes a series of dielectric windows  26  vacuum sealed by O-rings  30  and attached by removable clamps  34  to the upper surface  22  of the vacuum chamber  14 . Removably attached to some of these dielectric windows  26  are rf plasma sources  40 , in one embodiment having a helical or pancake antennae  46  located within an outer shield/ground  44 . Other embodiments of the antennae using capacitive or inductive coupling may be used. Cooling of each antenna is accomplished by passing a cooling fluid through the antenna. Cooling is typically required only at higher power. The windows  26  without attached rf plasma sources  40  are usable as viewing ports into the chamber  14 . The removability of each plasma source  40  permits the associated dielectric window  26  to be cleaned or the plasma source  40  replaced without the vacuum within the system  10  being removed. Although glass windows are used in this embodiment, other dielectric material such as quartz or polyethylene may be used for the window material.  
         [0029]    Each antenna  46  is connected to a rf generator  66  through a matching network  50 , through a coupling capacitor  54 . Each antenna  46  also includes a tuning capacitor  58  connected in parallel with its respective antenna  46 . Each of the tuning capacitors  58  is controlled by a signal D, D′, D″ from a controller  62 . By individually adjusting the tuning capacitors  85 , the output power from each rf antenna  46  can be adjusted to maintain the uniformity of the plasma generated. Other tuning means such as zero reflective power tuning may also be used to adjust the power to the antennae. In one embodiment, the rf generator  66  is controlled by a signal E from the controller  62 . In one embodiment, the controller  62  controls the power to the antennae  46  by a signal F to the matching network  50 .  
         [0030]    The controller  62  adjusts the tuning capacitors  58  and the rf generator  66  in response to a signal A from a sensor  70  (such as a Real Power Monitor by Comdel, Inc., Beverly, Mass.) monitoring the power delivered to the antennae  46 , a signal B from a fast scanning Langmuir probe  74  directly measuring the plasma density and a signal C from a plurality of Faraday cups  78  attached to a substrate wafer holder  82 . The Langmuir probe  74  is scanned by moving the probe (double arrow I) into and out of the plasma. With these sensors, the settings for the rf generator  66  and the tuning capacitors  58  may be determined by the controller prior to the actual use of the system  10  to plasma treat a substrate. Once the settings are determined, the probes are removed and the wafer to be treated is introduced. In another embodiment of the system, the probes are left in place during processing to permit real time control of the system. In such an embodiment using a Langmuir probe, care must be taken to not contaminate the plasma with particles evaporating from the probe and to not shadow the substrate being processed. In yet another embodiment of the system, the characteristics of the system are determined at manufacture and the system does not include plasma probes.  
         [0031]    Referring to FIG. 2, the configuration of plasma sources  40  may be such that a plurality of physically smaller plasma sources  40  produce a uniform plasma over an area greater than that of sum of the areas of the individual sources. In the embodiment of the configuration shown, four four-inch diameter plasma sources  40  spaced at the corners of a square at six inch centers produce a plasma substantially equivalent to that generated by a single twelve inch diameter source. Therefore, by providing a vacuum chamber  14  with a plurality of windows  26 , the various configurations of plasma sources  40  may be formed to produce a uniform plasma of the shape and uniformity desired. Antennae such as those depicted do not result in rf interference between sources when properly shielded as shown.  
         [0032]    Multiple rf plasma sources can excite electron cyclotron resonance in the presence of a multi-dipole surface magnetic field. Such a surface magnetic field would, for example, be approximately 1 KG at the pole face and would drop to a few Gauss at about 10 cm. from the pole face. In such a system, electron cyclotron resonance may be established, with the electron cyclotron resonance frequency (in Hz) being given by the expression v=2.8×10 6  (B) where B is the magnetic field strength in Gauss. Thus, if the fundamental electron cyclotron resonance frequency is 13.56 MHz (that is, the frequency supplied by the rf generator) the magnetic field required (as applied by the magnets) is 4.8 G, for resonance coupling to take place. Higher harmonics of the fundamental resonance frequency may be achieved by increasing the magnetic field proportionately. Thus for a second harmonic to be coupled, the magnetic field would have to be increased to 9.6 G. Such ECR coupling is most effective at lower pressures (P&lt;1 mTorr). The use of the small rf plasma sources permit such magnets to be positioned so as to make electron cyclotron resonance possible.  
         [0033]    The faraday cups  78  used to measure the uniformity of the field and the plasma dose, in one embodiment, are positioned near one edge in the surface of the wafer holder  82  (FIG. 3). The flat edge  86  of wafer  90  is positioned on the wafer holder  82  such that faraday cups  78  of the wafer holder  82  are exposed to the plasma. In this way the plasma dose experienced by the wafer  90  can be directly measured. Alternatively, a special wafer  90 ′, as shown in FIG. 3 a , is fabricated with a plurality of faraday cups  78  embedded in the wafer  90 ′. This special wafer  90 ′ is used to set the rf generator  66  and the tuning capacitors  58  to achieve the desired plasma density and uniformity. Once the operating parameters have been determined, the special wafer  90 ′ is removed and the wafers  90  to be processed placed on the wafer holder  82 .  
         [0034]    Referring to FIG. 4, although the system  10  has been described in terms of a planar array of plasma sources  40  located on the upper surface of the vacuum chamber  14 , the plasma sources  40  may be distributed over other surfaces of the vacuum chamber  14 ′ to generate a uniform volume of plasma. Such a system is particularly effective in batch processing.  
         [0035]    Referring to FIG. 5, in another embodiment, a quartz window  100  is not attached to the vacuum chamber  14 , but instead encloses one end of the shield  44  of the plasma source  40 ′. In this embodiment, a tube  104  attached to an opening  108  in the quartz window  100  provides a gas feed to form a plasma of a specific gas. In this case, the plasma source  40 ′ is not attached to a window  26  in the wall of the vacuum chamber  14 , but is instead attached to the vacuum chamber  14  itself. Such plasma sources  40 ′ can produce plasmas from specific gasses as are required by many processes. Several such plasma sources  40 ′ can be aligned to sequentially treat a wafer  90  with different plasmas as in the embodiment of the inline system shown in FIG. 6. In this embodiment, wafers  90  are moved by a conveyor  112  through sequential zones, in this embodiment zones I and II, of a continuous processing line  114 . Each zone is separated from the adjacent zones by a baffle  116 . In one embodiment, the gas in zone I is SiH 4  used in Si-CVD processing, while the gas in zone II is PH 3  used in doping. In another embodiment, a cluster tool having load-locks to isolate each processing chamber from the other chambers, and equipped with a robot includes the rf plasma sources  40  of the invention for plasma CVD and plasma etching.  
         [0036]    [0036]FIG. 7 depicts an embodiment of the system of the invention using two plasma sources. In this embodiment each source is an inductive pancake antenna 3-4 inches in diameter. Each antenna  46  is constructed of a ¼ inch copper tube and contains 5-6 turns. Each antenna  46  is connected to a matching network  50  through a respective  160  pf capacitor. The matching network  50  includes a 0.03 μH inductor  125  and two variable capacitors  130 ,  135 . One variable capacitor  130  is adjustable over the range of 10-250 pf and the second capacitor  135  is adjustable over the range of 5-120 pf. The matching network  50  is tuned by adjusting the variable capacitor  130 ,  135 . The matching network  50  is in turn connected to an rf source  66  operating at 13.56 mHz. A series of magnets  140 ,  145  are positioned around the circumference of the chamber in alternating polarity every 7 cm to form a magnetic bucket.  
         [0037]    With the chamber operating at 1 m Torr pressure, the power to the antenna  46  is 25 W per antenna or about 50 W total. With the pressure in the chamber reduced to 0.1 m Torr, the power is increased to 200 W per antenna or 400 W total. The resulting plasma at 50 W total power has a substantially uniform density of 10 11 /cm 3 . The uniformity and the density may be further improved using four of such sources.  
         [0038]    Referring to FIG. 8, one embodiment of the arrangement of magnets for ECR generation utilizes a plurality of magnets  150  adjacent the antenna  46 . In this embodiment, the plurality of the magnets  150  is reversed between the antennae. FIG. 8 a  depicts another embodiment in which each source has its own set of magnets. Other magnet configurations are possible.  
         [0039]    Referring to FIG. 9, an embodiment of a large area substrate consists of a high voltage wafer holder  210  on which many wafers  201  can be placed. If the holder  210  is biased at a large negative voltage (DC) and transported along the linear length of the plasma source  220 , a desired ion implantation dose will be reached. This is a simple way to achieve the separation of thin-film silicon layer by ion implantation as used in the Silicon-On-Insulator (SOI) technology. Alternatively, the plasma source  220  could be pulsed and the wafer holder  210  remains stationary to achieve the same effects.  
         [0040]    The multiple antennae plasma source  200  is especially useful for processing of silicon wafers  201  when large ion implantation dosages are desired. The throughput of such a system is extremely large in comparison with any other system including conventional high current beam-line implantation. For example, a particular thin-film transfer technique using hydrogen ion implantation into Si-wafer to separate the top silicon layer with the bulk wafer via hydrogen ion embitterment. The dosage required is 10 16  to 10 17  ions-cm −2 . Another technique is the Separation by Ion IMplantation of Oxygen (SIMOX) having oxygen implantation dosages of 10 18  ions-cm −2 . Being the best conventional high current ion implantation available in the market will require long implantation time, up to several hours. Using the large area plasma source  200  and the described process and apparatus allows the implantation time to approach seconds for batch processing of a large number of wafers  201 .  
         [0041]    Two methods can be used for the batch processing of wafers. The first method includes pulsing the large area plasma source  220  with a negative high voltage dc bias from the HVDC supply  230 , which is connected to the wafers  201 . When the plasma forms and expands to the wafers, all of the ions are accelerated to the wafer by the electric field at the wafer surface. As a result, all of the accelerated ions have the same energy because of the large dc electric field at the wafer surface in comparison to either the plasma potential or the kinetic energy of the ions as the plasma expands. As the implant dose will be controlled by the exposure time of the wafer to the plasma, the plasma is turned off when the desired dosage is reached. The large plasma source allows essentially infinite number of wafer exposure (implantation) at a short time, limited by the available high voltage power rating of the wafer bias. The throughput of such a system is potentially 100 to 1,000 times that of conventional technology.  
         [0042]    The uniformity of the implant can be controlled precisely as the individual rf coils can be tuned separately for the optimum plasma uniformity. FIG. 10 shows the dosage relation to the exposure time.  
         [0043]    The second method and apparatus is known as In-line processing, shown in FIG. 11. The large area plasma source  260  is operated steady-state and the wafers  201  are moving from one end to the other end as the exposure time t=S/V where S is the length of the plasma (linear length of the plasma source) and V is the speed of the wafer transport. FIG. 12 shows the ion dose relation with the distance or time relationship for this embodiment. Again, the uniformity can be adjusted by controlling the tuning and power level of each rf coil. The throughput of such a system is also extremely high.  
         [0044]    In such a system, with the adjusting of the plasma density and the ion dosage per sec (dose rate), it&#39;s possible to control the wafer temperature precisely (with and without external wafer cooling means). A simplified version of the in-line system, shown in FIG. 13, can also be used which results in simple equipment design and which will not need wafer cooling for most SPIT applications. FIG. 14 depicts the ion dose and temperature profile of an individual wafer. In such a system, the large area plasma source allows a simple connection procedure of the negative high-voltage bias to the wafer and the wafer transport robot system as shown in FIGS. 15 and 16.  
         [0045]    After the wafers are loaded in the loading port (FIG.  13 ), the robot transport system with high voltage connection  270  will connect to the wafer holder  275  moving through the plasma for the desired exposure as shown in FIG. 13 and repeat the procedure and send the wafer back for a second batch of exposure.  
         [0046]    Multiple antennae rf sources are used to provide a uniform plasma during the pulsing phase and the steady state as to give uniform ion implantation dose to a large area substrate which is biased at a negative voltage. When the plasma is pulsed on, other types of excitation depend on the particular ionization mechanism (i.e. in the case of electron cyclotron resonant ECR source, a resonance ionization process), the location of plasma initiation is often localized. As such, the uniformity of such sources during the turn-on phase is poor. It is also takes a long time for such plasma sources to reach steady-state with the desired uniformity (preferably less than 5%) for normal plasma processes. The multiple antennae source has multiple ionization locations and are individually tuned by the electronic and tuning circuits. As such, a uniform plasma is provided from the pulsing phase all the way to steady state by control of the individual antenna. This is done precisely with the feedback scheme disclosed in co-pending U.S. patent application Ser. No. 08/503,973, the disclosure of which is incorporated herein by reference.  
         [0047]    Having shown the preferred embodiment, those skilled in the art will realize many variations are possible which will still be within the scope and spirit of the claimed invention. Therefore, it is the intention to limit the invention only as indicated by the scope of the claims.