Abstract:
A processing system and associated method for vacuum evaporation of material onto a substrate. The processing system includes a loading chamber, a transfer chamber, and a thermal processing chamber arranged together to form a cluster tool. The cluster tool arrangement provides the system a continuous processing capability. The system also includes an evacuation system arrangement for evacuating the processing system to adequate processing pressure levels. The evacuation system arrangement includes a series of pumps, which are capable of maintaining the selected processing pressure levels for continuous thermal evaporation processing without the need for lowering the pressure to deep vacuum pressure levels.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to vacuum deposition, and more particularly to a system and method for vacuum deposition in a cluster tool.  
           [0003]    2. Description of the Related Art  
           [0004]    Vacuum evaporation systems are well known in the art. FIG. 1A illustrates a typical vacuum evaporations system  10 , which has magnetron sputtering and/or electron beam gun deposition capabilities. Magnetron sputtering and electron beam gun deposition are well established technologies for the coating of objects, such as semiconductor substrates. For example, sputtering is accomplished by bombardment with high-energy ions, usually ions of the rare gas argon (sputter gas), which is initiated by gas discharge processes. The ions of the sputter gas are subjected to controlled acceleration in the direction of the target at high kinetic energies, such that individual atoms can be knocked out of the target.  
           [0005]    As illustrated in FIG. 1A, vacuum deposition system  10  includes a single closed chamber  12  operatively coupled to a roughing pump  14  and a diffusion type pump  16  through a series of valves  18   a - 18   d.  The sputter process is typically carried out in closed chamber  12  under conditions of deep vacuum pressures to ensure a long a mean free path length of the vapor particles (i.e., sputtered particles) as possible.  
           [0006]    [0006]FIG. 1B illustrates a representative Pressure-Time profile for system  10 . As shown, valves  18   a - 18   d  are closed to maintain the pressure in chamber  12  initially at atmospheric pressure. Valve  18   a  is opened to allow roughing pump  14  to lower the pressure in chamber  12  to a first pressure P 0 . Once the pressure is lowered to a level where the roughing pump is no longer effective, valve  18   a  is closed while valves  18   b  and  18   c  are opened to allow diffusion pump  16  to evacuate chamber  12  down to deep vacuum pressure P 1 . Since in most evaporation processes the process is not controlled, the deep vacuum pressure supplied by diffusion pump  16  is necessary to compensate for the increase in pressure that occurs at time T 3 , when the evaporation process begins. Also, since the mean free path of the sputtered particles is longest at pressures below pressure P 0 , the deep pressure is necessary to increases the length of time that the sputtered particles can be effectively deposited.  
           [0007]    Unfortunately, because of the rapid increase in pressure during the evaporation process, the mean free path of the sputtered particles is changing from long to short very quickly, which can create a relatively non-uniform deposition. Another drawback of vacuum deposition system  10  is the inefficient use of the vacuum pumps. For example, the roughing vacuum pump is effective to evacuate chamber  12  up to pressure levels of from 0.7 Torr-0.5 Torr. At pressure levels of 0.7 Torr-0.5 Torr the efficiency of the mechanical pump is reduced dramatically because the movement of the remaining atmosphere in chamber  12  begins to take on molecular flow characteristics. This results in a substantial reduction in pumping speed as chamber  12  continues to be evacuated to about 0.2 Torr. The diffusion pump can then be used to further evacuate chamber  12  to the desired deep vacuum pressure levels. Unfortunately, since diffusion pumps, such as oil diffusion pumps, are ineffective when operated at pressures over 0.2 Torr microns, chamber  12  has had to be mechanically evacuated to the effective operating range of the diffusion pumps. The time taken to reduce the pressure in chamber  12  from 0.5 Torr-0.2 Torr can be significant and can reduces production rates appreciably. In addition, once the evaporation process has begun, the pressure in Chamber  12  rises significantly, which is usually above the operating range of diffusion pump  16 . This prevents the ability to continuously process substitutes.  
           [0008]    In addition to the length of time required to evacuate chamber  12  to its effective operating levels, chamber  12  is opened to atmosphere between operations. The evacuation time is further extended since water and oxygen molecules, as well as other contaminants, are introduced into chamber  12 . The presence of these contaminants can adversely effect the quality of the thin film layers. Purging chamber  12  of such molecules is possible, however, this further extends the production time.  
           [0009]    What is needed is a vacuum deposition system that provides a continuous processing capability, reduces processing cycle times, and increases throughput per cycle.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a processing system and associated method for vacuum evaporation of material onto a substrate. In accordance with the present invention, the system includes an evacuation system arrangement for evacuating the processing system to adequate processing pressure levels. Advantageously, the evacuation system arrangement includes pumps, which are capable of substantially maintaining the processing pressure levels for continuous thermal evaporation processing without the need for lowering the pressure to deep vacuum pressure levels. The processing system further includes a loading chamber, a transfer chamber, and a thermal evaporation processing chamber arranged together to form a cluster tool. The cluster tool arrangement provides the system a continuous processing capability.  
           [0011]    In one aspect of the present invention, a system is provided for vacuum depositing a thin film on a semiconductor substrate. The system includes a wafer processing chamber for receiving a substrate. The wafer processing chamber defines a transfer section and a processing section. The transfer section provides the system with the capability to insert and remove substrates on a continuous cycle. The processing section provides the system with a thermal evaporation processing capability in which the thin film is deposited on the substrate. A first evacuation device is provided in the system for evacuating the wafer processing chamber to an operating pressure. A second evacuation device is also provided for evacuating the wafer processing chamber to sustain the operating pressure throughout the wafer processing cycle. Once the processing of the substrate has occurred, the substrate can be transported between the transfer section and the processing section while the chamber is maintained at or near the operating pressure.  
           [0012]    In another aspect of the present invention, a method is provided for vacuum depositing a thin film on a substrate. The method includes providing a wafer processing chamber for receiving a substrate. The chamber is substantially and continuously evacuated to sustain a selected operating pressure level during a thermal evaporation process, which may be conducted in the wafer processing chamber.  
           [0013]    The present invention has many advantages over typical vacuum deposition systems and methods. For example, sputtered particles that emanate from the evaporated material during the evaporation process have a more average and consistent mean free path. Thus, the sputtered particles can be more uniformly deposited on the wafer. Because the present invention does not require deep vacuum pressure levels, the process chamber structure can be made smaller and more economically. By avoiding the need for deep vacuum pressures, the system of the present invention can be arranged in conjunction with a loading station and a transfer station to provide continues cycling of substrates within a closed vacuum environment. Thus, exposure of the substrates to contamination is minimized and processing cycle times are reduced. The time needed to reduce the chamber pressure to deep vacuum levels and the need to backfill the chamber between processing cycles is removed, thus throughput of processed substrates can be increased.  
           [0014]    These and other features and advantages of the present invention will be more readily apparent from the detailed description of the embodiments set forth below taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.  
         [0016]    [0016]FIG. 1A shows a typical vacuum deposition system;  
         [0017]    [0017]FIG. 1B is a representative Pressure-Time profile for the system of FIG. 1A;  
         [0018]    [0018]FIG. 2 is a simplified schematic illustration of an embodiment of the wafer processing system in accordance with the present invention  
         [0019]    [0019]FIG. 3 is a representative Pressure-Time profile in accordance with the present invention; and  
         [0020]    [0020]FIG. 4 is a flow chart representing embodiment of the present invention. 
     
    
       [0021]    The use of the same reference symbols in different drawings indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0022]    Embodiments of the present invention will be described with reference to the aforementioned figures. These figures have been simplified for ease of describing and understanding the embodiments.  
         [0023]    [0023]FIG. 2 is a simplified schematic illustration of one embodiment of a semiconductor wafer processing system  20  that establishes a representative environment of the present invention. Processing system  20  can include a loading station, which can have multiple platforms (not shown) for supporting and moving a wafer cassette  22  up and into a loadlock or loading chamber  24 . Wafer cassette  22  may be a removable cassette which is loaded onto the platform, either manually or with automated guided vehicles (AGV). Wafer cassette  22  may also be a fixed cassette, in which case wafers are loaded onto cassette  22  using conventional atmospheric robots or loaders (not shown). Once wafer cassette  22  is inside loading chamber  24 , loading chamber  24  and transfer chamber  26  can be pumped down to a vacuum pressure. A robot  28  housed within transfer chamber  26  can rotate toward loading chamber  24  and pick up a wafer  30  from cassette  22 . A thermal processing chamber  32 , which can also be under vacuum pressure, accepts wafer  30  from robot  28  through a gate valve. Robot  28  then retracts and, subsequently, the gate valve closes to begin the processing of wafer  30 . After wafer  30  is processed, the gate valve opens to allow robot  28  to pick-up and place wafer  30  into a cooling station (not shown). The cooling station cools the newly processed wafers before they are placed back into wafer cassette  22 .  
         [0024]    In one embodiment, thermal processing chamber  32  has a thermal evaporation capability, such as magnetron sputtering and/or electron beam source deposition capabilities. As shown in FIG. 2, a conductive source material holder  34 , such as a conductive crucible, is positioned within chamber  32 . Crucible  34  carries a preselected material  36  for thermal evaporation onto substrates  30 , which are typically positioned within chamber  32  on a substrate holder (not shown). Preselected material  36  may include, but is not limited to, metals, such as Al, Au, Ni, Cu, Ag, Ti, Ta and the like; alloys, such as AlSi, TiSi, and the like; and insulators, such as indium tin oxide (ITO).  
         [0025]    When conducting an electron beam source deposition, for example, a high voltage electron beam source (not shown) can be positioned within chamber  32  proximate to crucible  34 . In one configuration, the electron beam source includes a high voltage electron gun and a deflection magnet system arranged for bending electrons from the gun into crucible  34  for evaporating preselected material  36 . The magnet system forms a magnetic field in the region above crucible  34  to guide the high voltage electron beam into crucible  34  to cause material  36  to evaporate.  
         [0026]    In some embodiments, a reactive process gas, such as NH 3 , O 2 , and H 2 , may be introduced into chamber  32  during processing at any convenient position relative to crucible  34  and the electron beam source. In the case in which the activation gas is reactive with the evaporant material  36 , the deposited thin film can have constituents of both the evaporant material  36  and the reactive process gas. In one example, Ta from a solid can be combined with O 2  to from a Ta 2 O 5  Layer. In another example CuO or CuO 2  can be combined with H 2  to yield a Cu layer. In another embodiment, the reactive gas may be introduced into chamber  32  to prevent oxidation of evaporate material  36 . For example, Cu is placed in crucible  34  and evaporated, H 2  is introduced into chamber  32  to react with the Cu to prevent the unwanted oxidation. Various types of electron beam source assemblies are disclosed in U.S. Pat. Nos. 4,882,198, and 6,012,413, which are herein incorporated by reference in their entirety for all purposes.  
         [0027]    As shown in FIG. 2, processing chamber  32  is coupled to an evacuation system  40 . Evacuation system  40  is used to reduce the internal pressure of chamber  32  to a level in which efficient deposition, such as magnetron sputtering or electron beam source deposition, can occur. In accordance with the present invention, pressure levels of below about 0.5 Torr are not necessarily required, thus a diffusion type pump is not needed. In one embodiment, evacuation system  40  can provide the Pressure-Time profile depicted in the representative graph of FIG. 3. Evacuation system  40  can initially lower the chamber pressure level to a level at or near the desired operating pressure P 0 . The operating pressure P 0  of processing system  20  may range from about 10 −3  to about 10 −6  Torr. Once at the operating pressure P 0 , evacuation system  40  can maintain and sustain P 0  during the material evaporation process. To sustain the operating pressure P 0  the pumping speed of evacuation system  40  is such that it is greater than the speed of the pressure change caused by the evaporation. As best illustrated in FIG. 3, the increase in pressure caused by the evaporation process only slightly rises above the operating pressure, such that the operating pressure is considered to be substantially maintained. For example, the pumping speed of the evacuation system during processing can range from between 1000 Torr l/min and 50000 Torr l/min.  
         [0028]    One result of substantially maintaining the operating pressure during the evaporation process is that a more average mean free path is provided for the particles to travel before being deposited on the substrate. The average mean free path can be obtained and sustained at higher pressures, as well. These results allow for more uniform and consistent layers being deposited on the substrates.  
         [0029]    Evacuation system  40  allows process chamber  32  to be used in conjunction with transfer chamber  26  and loading station  24  to form a “cluster tool.” In a cluster tool, the three chambers  26 ,  24 , and  32  are arranged such that they are in gaseous communication with each other chamber. Thus, a substrate or plurality of substrates can be cycled through system  20  without having to be exposed to the external environment or to atmospheric pressure. Accordingly, system  20  can be used in a continuos operational mode, since the evacuation system  40  can maintain the P 0  at all times during and in between processes and because the system does not require deep vacuum pressures levels.  
         [0030]    Evacuation system  40  can include pumps, for example, a roughing pump  42  and a mechanical booster pump  44 , and associated pumping components (not shown), such as a pumping manifold for communicating chambers  24 ,  26 , and  32  to an inlet of roughing pump  42 , isolation or gate valves, and vent valves for venting to atmospheric pressure as required.  
         [0031]    In one embodiment, roughing pump  42  can be used to evacuate system  20 , including loading chamber  24 , transfer chamber  26  and processing chamber  32  (collectively the “system chamber”). In this embodiment, roughing pump  42  can be an electric motor driven reciprocating piston, compressor type pump, which is particularly efficient for pumping a gas at above absolute pressures of about 0.5 Torr. Roughing pump  42  should be capable of providing a pumping flow rate of about 1000 Torr l/min to about 50000 Torr l/min. In operation, the actual pumping flow rate is diminished as the pump  42  evacuates chambers  24 ,  26 , and  32  and when the gas density is reduced to a level where its flow can be characterized as molecular flow, or molecular in nature (generally below 0.5 Torr). An example of a suitable roughing pump for use in the present invention is available from Kashiyama Industries Ltd. of Tokyo, Japan.  
         [0032]    Roughing pump  42  is coupled in series to mechanical booster pump  44 . Booster pump  44  may be any pump that can provide a high pumping speed with a low base pressure. The low base pressure may range from about 10 −7  to about 10 −10  Torr. The actual pumping rate can be determined by the application. For example, booster pump  44  may be a turbo pump or similar type pump, such as TMO1000L available from Leybold Vacuum.  
         [0033]    [0033]FIG. 4 is a flow chart which describes an operational method  50  in accordance with the present invention. In this method  50 , a wafer platform loads  52  a wafer cassette carrying multiple wafers into the loading chamber. Once the wafer cassette is inside the loading chamber, the loading chamber, transfer chamber, and processing chamber can be pumped down  54  to a selected vacuum pressure. The vacuum pressure can be at or near a desired operating pressure as determined by the application. A mechanical roughing pump can be used to achieve the desired pressure level. In one embodiment, the roughing pump lowers the pressure level of the entire system down to a pressure level ranging from about 10 −2  Torr to about 10 −4  Torr.  
         [0034]    A robot being housed within the transfer chamber rotates toward the loading chamber to pick up and translate a wafer from the loading chamber to the thermal processing chamber. The thermal processing chamber accepts the wafer from the robot through a gate valve. The robot then retracts and, subsequently, the gate valve closes to begin the processing of the wafer. In one embodiment, the wafer processing can include a thermal evaporation process, such as a sputtering deposition or an electron beam source deposition. It is known that thermal evaporation processes that occur in a closed system tend to increase the internal pressure of the system. The increased pressure may increase the pressure level above the selected operation pressure levels. Thus, a booster pump, can be used to maintain the operating pressure level  56   a  during the thermal evaporation processing  56   b  of the wafer.  
         [0035]    After the wafer is processed, the gate valve opens to allow the robot to pick-up and place the wafer into an alternate location in the system  58 , for example, into a cooling station. The cooling station cools the newly processed wafers before they are placed back into the wafer cassette. The robot then picks up and places a different wafer into the processing chamber  60  to begin the processing again.  
         [0036]    At all times after the chambers have been pumped down to operating pressure, the roughing pump and turbo pump continue to substantially maintain the operating pressure level.  
         [0037]    While the principles of the invention have been described in connection with certain embodiments, it is to be understood that this description is not a limitation on the scope of the invention.