Abstract:
A method for forming a thin film on a semiconductor wafer. The method includes loading a semiconductor wafer into a process chamber while the process chamber is under vacuum pressure, or alternatively, while the partial pressure of the reactive gas is substantially zero. The process gas is introduced under pressure into the process chamber. The semiconductor wafer is unloaded from the process chamber while the process chamber is under a vacuum pressure, or alternatively while the partial pressure of the reactive gas is substantially zero.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to surface treatment of a semiconductor device and more particularly to a method for forming an ultra thin film on the surface of a semiconductor wafer.  
           [0003]    2. Description of the Related Art  
           [0004]    It is known that a layer of thin native silicon dioxide (SiO 2 ) tends to form naturally on bare silicon surfaces. Typically, the presence of the native oxide is undesirable, since the quality and consistency of the native oxide layer is unknown and unpredictable. For this reason, the thin layer of native oxide is generally removed from the surface of the silicon substrate before processing.  
           [0005]    In the manufacture of integrated circuits, however, SiO 2  has long been used as a dielectric for integrated circuits because of its excellent thermal stability and relatively good dielectric properties (k˜4.0). Commonly the operational voltage requirement for most integrated circuits is ˜5 volts. Thus, it is frequently desirable to form an SiO 2  insulating layer directly on the surface of the silicon semiconductor substrate or wafer, which will not break or overheat when subjected to the operational voltage.  
           [0006]    Unfortunately, most conventional manufacturing processes used for growing thin films are inefficient and wasteful. Typically, most conventional manufacturing processes are batch type processing methods, which may process from between 100 to 150 wafers per processing cycle. Because of the non-uniform nature of the processes and because of an inability to control growth, batch type processes yield many unusable wafers. These conventional processes also require relatively high cycle times. For example, some process can require from 8 to 10 hours for ramping up (heating) and ramping down (cooling) between processing cycles.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method for forming an ultra thin layer of dielectric material on a silicon surface. Preferably, the ultra thin layer can be made of SiO 2  or similar materials, such as SiN and Ta 2 O 5 . In the present invention, silicon substrates or wafers are loaded onto an appropriate wafer carrier and then introduced into a semiconductor wafer processing system. A wafer transport mechanism can be used to remove a single silicon wafer from the carrier and transport the wafer to a processing chamber. The processing chamber may be, for example, a furnace, an annealer, or other chamber for conducting thermal processing.  
           [0008]    In accordance with the present invention, the silicon wafer is loaded into the processing chamber while the processing chamber is under a vacuum pressure. The semiconductor wafer and chamber are heated. Once the chamber reaches a steady-state processing temperature, a process gas, such as oxygen, is introduced into the chamber under pressure. The chemical reaction which takes place in the processing chamber causes the oxygen to react with the surface of the silicon wafer to form an ultra thin SiO 2  layer thereon. The growth rate of the layer is dependent on the pressure of the reactive gas, which can be controlled to produce the desired thickness of the thin film layer. The thickness of the ultra thin SiO 2  layer may be on the order of between about 10 Å to 50 Å. Advantageously, the thin layer of SiO 2  may be formed within about 10-20 minutes in a process temperature of about 800° C. to about 850° C., whereafter the wafer is removed from the chamber and cooled.  
           [0009]    In some embodiments, the oxygen may react with Ta (Source TaETO) to form an ultra thin layer of Ta 2 O 5 . The Ta 2 O 5  layer may range in thickness from between about 50 Å to 250 Å. Advantageously, the thin layer of Ta 2 O 5  may be formed within about 10-20 minutes in a deposition process temperature of about 300° C. to about 500° C., or in an annealing process of between about 400° C. to about 800° C.  
           [0010]    In one aspect of the invention, a method is provided for forming a thin film on a semiconductor wafer. The method includes loading a semiconductor wafer into a process chamber while the process chamber is under vacuum pressure, or alternatively, while the partial pressure of the reactive gas is substantially zero. The process gas is introduced under pressure into the process chamber. The semiconductor wafer is unloaded from the process chamber while the process chamber is under a vacuum pressure, or alternatively while the partial pressure of the reactive gas is substantially zero.  
           [0011]    Because the method of the present invention provides a controllable thin layer growth rate, a higher percentage yield of wafers can be achieved in a shorter cycle time. In addition, since higher yields are produced from smaller wafer batch sizes, the overall footprint of the processing system for a required productivity level can be reduced, which saves valuable manufacturing space. Beneficially, the increase in throughput saves energy and reduces waste. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a simplified diagram of the processing system of the present invention;  
         [0013]    [0013]FIG. 2 is a flow diagram of the process method in accordance with the present invention;  
         [0014]    [0014]FIG. 3A is a simplified illustration of an embodiment of a furnace in accordance with the present invention; FIG. 3B is a simplified illustration of a heating element for use in the furnace embodiment of FIG. 3A;  
         [0015]    [0015]FIG. 4A is a simplified illustration of an embodiment of a furnace in accordance with the present invention; FIG. 4B is a simplified illustration of a heating element for use in the furnace embodiment of FIG. 4A;  
         [0016]    [0016]FIG. 5A is a simplified diagram of an alternative embodiment of a processing system in accordance with the present invention;  
         [0017]    [0017]FIG. 5B is a simplified illustration of a furnace for use with the processing system of FIG. 5A;  
         [0018]    [0018]FIG. 6 is a schematic illustration of a side view of one embodiment of a semiconductor wafer processing system in accordance with the present invention;  
         [0019]    FIGS.  7 A- 7 C are simplified illustrations of an embodiment of FIG. 6; and  
         [0020]    [0020]FIG. 8 is a graph representation of the pressure/temperature variation within the processing chamber as a function of time in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]    [0021]FIG. 1 is a simplified diagram of a processing system  10  that establishes a representative environment for the present invention. Processing system  10  may include a loading station  12 , which has multiple platforms  17  for supporting and moving a wafer cassette  14  up and into a loadlock  16 . Wafer cassette  14  may be a removable cassette, which is loaded onto platform  17 , either manually or with automated guided vehicles (AGV). Wafer cassette  14  may also be a fixed cassette, in which case wafers are loaded onto cassette  14  using conventional atmospheric robots or loaders (not shown). Once wafer cassette  14  is inside loadlock  16 , processing system  10  can be pumped down to vacuum. A wafer transport system  18  housed within transfer chamber  20 , described in greater detail below, rotates toward loadlock  16  and picks up at least one wafer  22  from cassette  14 . A processing chamber  24 , also under vacuum, receives wafer  22  from wafer transport system  18  through a gate valve  29 .  
         [0022]    Wafer transport system  18  is capable of lifting wafer  22  from wafer cassette  14  and, through a combination of linear and rotational translations, transporting the wafer through vacuum chamber valves  28  and  29 , and depositing the wafer at the appropriate position within furnace  24 . Similarly, wafer transport system  18  is capable of transporting wafer  22  from one processing chamber  24  to another (not shown) and from a processing chamber back to wafer loading station  12 .  
         [0023]    In one embodiment, wafer transport system  18  includes a robot arm  30  and a controller  32 . Robot arm  30  may be any conventional wafer processing robotic arm, which provides R (translation) and Θ (rotation) movements. A gripper or end effector (not shown) may be attached to the end of robot arm  30 . The end effector may be made of a heat resistant material, such as quartz, for picking-up and placing wafer  22 . An example of a commercially available type of robot arm is the SHR3000 robot (“SHR3000 robot”) from the JEL Corporation of Hiroshima, Japan. The SHR3000 robot can rotate 340°, has 200 mm of vertical motion, and can extend its arms 390 mm in the horizontal plane. Another example of a type of wafer processing robot is disclosed in U.S. patent application Ser. No. 09/451,677, filed Nov. 30, 1999, which is herein incorporated by reference for all purposes.  
         [0024]    Once wafer  22  is positioned in chamber  24 , transport system  18  retracts and gate valve  29  closes to begin processing. After wafer  22  is processed, gate valve  29  opens to allow transport system  18  to pick-up and remove wafer  22  from the processing chamber.  
         [0025]    [0025]FIG. 2 is a flow diagram of an embodiment of the method of the present invention, which can be performed using processing system  10  of FIG. 1. In this embodiment, processing chamber  24  is a furnace. Furnace  24  may be any conventional type wafer processing furnace, such as any lamp-based or resistively heated furnace. In accordance with the present invention, furnace  24  can be pumped down ( 40 ) to a vacuum pressure using a conventional pumping system  32 . Pumping down ( 40 ) furnace  24  ensures that substantially all moisture and oxygen are removed from the furnace to prohibit the formation of a native SiO 2 . Optionally, furnace  24  may be filled with an inert gas ( 42 ), such as Argon or Helium, to ensure that residual oxygen and moisture are substantially removed from furnace  24 . As further described below, furnace  24  may also be filled with N 2  for diluting the reactive gas.  
         [0026]    As understood with reference to the graph of FIG. 8, furnace  24  can be preheated to a steady state temperature T S , which can range from about 200° C. to about 1200° C. At least one silicon wafer  22  is loaded ( 44 ) into furnace  24  using transport system  18 . After the wafer is loaded ( 44 ) into furnace  24 , wafer  22  is heated from the initial temperature T S  to a processing temperature T P . The processing temperature T P  can range from between about 200° C. to about 1200° C.; preferably a range of between about 400° C. to about 1100° C.  
         [0027]    In one embodiment, as the wafer temperature in furnace  24  approaches processing temperature T S , a process gas, such as oxygen, is introduced ( 48 ) into chamber  24 . The rate of flow of the process gas through processing chamber  24  or the partial pressure of the reactive gas is controlled to control the desired rate of growth. It should be understood that when furnace  24  has been pulled to vacuum, the pressure line in FIG. 8 represents the actual pressure P A  of furnace  24 . Wafer  22  is held in furnace  24  exposed to the oxygen for a time long enough to accomplish the growth of the layer of SiO 2  The thickness of the SiO 2  layer can range from about 10 Å to about 50 Å; preferably between about 10 Å to about 30 Å. Generally, the processing time can range from about 1 to 20 minutes, depending on the process temperature and process ambient conditions.  
         [0028]    In an alternative embodiment, the growth rate of the thin film layer can be controlled by controlling the partial pressure P P  (FIG. 8) of the reactive gas relative to all gases introduced ( 48 ) into furnace  24 . For example, an inert gas, such as Helium or Argon, may be introduced into furnace  24 , creating a specific chamber pressure. The reactive gas can be introduced such that the partial pressure P P  of the reactive gas relative to the chamber pressure is at the desired pressure level for formation of the thin film layer. Optionally, an inert gas, such as N 2  or the like, can be introduced into furnace  24  prior to, with, or after the introduction of the reactive gas to dilute the reactive gas to the desired partial pressure P P . For example, with no intent to limit the invention thereby, under a partial pressure of 1 Torr the growth rate of O 2  can be maintained at 10-20 Å/hr. and at a partial pressure of 1 atm the growth rate of O 2  can be maintained at 1-2 Å/min.  
         [0029]    Referring again to FIG. 2, the growth of SiO 2  on the wafer surface can be stopped at processing temperatures by pulling furnace  24  to vacuum ( 49 ) before removing wafer  22  from the furnace. The wafer is then removed ( 50 ) from chamber  24 , using transport system  18 . The wafer is allowed to cool to between about 50° C. and 90° C. before being returned to cassette  14 . In an alternative embodiment, the growth of the SiO 2  layer can be slowed or almost stopped by removing the wafer from furnace  24 . Removal of the wafer causes the wafer to cool below processing temperatures.  
         [0030]    [0030]FIGS. 3A and 4A, are simplified illustrations of embodiments of furnace  24 . In each embodiment, furnace  24  may include a closed-end process inner chamber  52 , which defines an interior cavity  54 . In one embodiment, inner chamber  52  may be constructed with a substantially rectangular cross-section, having a minimal internal volume surrounding wafer  22 . For example, the volume of inner chamber  52  may be no greater than about 5000 mm 3 , preferably the volume is less than about 3000 mm 3 . One result of the small chamber volume is that uniformity in temperature is more easily maintained. Additionally, the small tube volume allows furnace  24  to be made smaller, and as a result, system  10  may be made smaller, requiring less clean room floor space. Inner chamber  52  may be made of quartz, silicon carbide, Al 2 O 3 , or other suitable material.  
         [0031]    In one embodiment, inner chamber  52  includes a wafer support structure  56 , which supports wafer  22  during processing. Wafer support structure  56  may be formed into the inner wall of inner chamber  52 . An open central portion of wafer support structure  56  allows wafer  22  to be supported on a peripheral edge  58  of wafer  22 .  
         [0032]    [0032]FIGS. 3A, 4A,  3 B and  4 B illustrate embodiments for use with heating elements of reactor  24 . The heating elements are configured to surround inner process chamber  52 . In the embodiment, shown in FIGS. 3A and 3B, the heating elements include heating device  60 . Heating device  60  includes a plurality of tubes  62 , preferably aluminum tubes, disposed in parallel across a top and bottom portion of inner chamber  52 . Each aluminum tube  62  includes a resistive heating element  64  disposed therein.  
         [0033]    Each resistive heating element  64  includes a resistive heating element core and a filament wire. The core is usually made of a ceramic material, but may be made of any high temperature rated, non-conductive material. The filament wire is wrapped around the core to allow for an optimal amount of radiated heat energy to emanate from the element. The filament wire may be any suitable resistively heatable wire, which is made from a high thermal conductivity material for increased thermal response and high temperature stability, such as SiC, SiC coated graphite, graphite, NiCr, AlNi and other alloys. Preferably, the resistive heating filament wire is made of a combination Al—Ni—Fe material, known commonly as Kantal A-1 or AF, available from Omega Corp. of Stamford, Conn.  
         [0034]    Each tube  62  is in relative close proximity to each other element, for example, each tube  62  may be spaced between about 0 mm and 50 mm, preferably between about 1 mm and 20 mm. Accordingly, the close spacing provides for an even heating temperature distribution across wafer  22  when positioned in inner chamber  52 . The plurality of tubes  62  are contained in a quartz container  66  to reduce the possibility of metal contamination.  
         [0035]    [0035]FIGS. 4A and 4B illustrate an alternative embodiment of the heating element of reactor  24 . In this embodiment, heating device  70  includes a ribbon shaped heating element  71  wrapped around a quartz plate  72 . Each heating device  70  can be disposed in parallel across a top and bottom portion of inner chamber  52 . Alternatively, heating element  71  can include a plurality of individual resistive heating elements combined to form the heating element.  
         [0036]    Advantageously, a direct line voltage of between about 100 volts and about 500 volts may be used to power the resistive elements described above. Thus, no complex power transformer is needed in the present invention for controlling the output of the resistive heating elements.  
         [0037]    [0037]FIG. 5A is a simplified diagram of an alternative embodiment of processing system  100  in accordance with the present invention. Processing system  100  includes components consistent with the description of the embodiments above, where like components are numbered similarly. The alternative embodiment of FIG. 5A includes a transport system  102  capable of simultaneously transporting a plurality of wafers  22  from loadlock  16  to process chamber  104 . Further, process chamber  104  is capable of simultaneously receiving and processing the plurality of wafers  22 . In this alternative embodiment, wafer transport system  102  includes a robot arm  106  coupled to a plurality of end-effectors  108 . End-effectors  108  are arranged in a stacked configuration and spaced apart with sufficient space to simultaneously access a plurality of wafers  22  in cassette  14 . Wafer transport system  102  is capable of lifting the multiple wafers  22  from wafer cassette  14  and, through a combination of linear and rotational translations, transporting wafers  22  through vacuum chamber valves  28  and  29 , and depositing the wafer at the appropriate position within processing chamber  104 . Similarly, wafer transport system  102  is capable of transporting wafers  22  from one processing chamber  104  to another (not shown) and from a processing chamber back to wafer loading station  12 .  
         [0038]    In one embodiment, robot arm  106  is moved up and down as indicated by arrow  110 . In this manner, robot arm  106  can move the plurality of end-effectors  108  into position to pick up the wafers. In this embodiment, robot arm  106  controls five end-effectors  108 . Thus, each end effector  108  is capable of servicing approximately 20% of wafer cassette  14 .  
         [0039]    In yet another embodiment, robot arm  106  is fixed for movement in the vertical direction. In this embodiment, wafer loading station  12  includes the capability of moving wafer cassette  14  in the direction indicated by arrow  112  once wafer cassette  14  is in loadlock  16 . Wafer cassette  14  is moved incrementally a distance sufficient to allow each end-effector  108  to access a portion of wafers  22 .  
         [0040]    [0040]FIG. 5B is a simplified illustration of a front view of furnace  104 . As shown in FIG. 5B, furnace  104  is a series of stacked furnaces including a plurality of inner chambers  52 . Each inner chamber  52  is capable of receiving one wafer  22  delivered by robot arm  106  and end effectors  108  (FIG. 5A). Advantageously, in the stacked arrangement, the bottom heating device  114 , for example, can serve as the beating device for a subsequent inner chamber  52 . This arrangement saves energy, materials, and floor space.  
         [0041]    [0041]FIG. 6 is an illustration of yet another alternative embodiment of processing system  80  in accordance with the present invention. Processing system  80  includes components consistent with the description of the embodiments above, where like components are numbered similarly. Processing system  80  includes a process chamber  82  capable of processing a plurality of wafers  22 . In this embodiment, wafer  22  is removed from cassette  14  and transported through process system  80  by wafer transport system  86  into process chamber  82 . Wafer transport system  86  lifts a wafer  22  from wafer cassette  14  and, through a combination of linear and rotational translations, transports the wafer through transport chamber  88 , and deposits the wafer at the appropriate position within furnace  82 . Similarly, wafer transport system  86  is capable of transporting wafer  22  from one processing chamber to another (not shown) and from a processing chamber back to wafer loading station  12 .  
         [0042]    [0042]FIGS. 7A and 7B show an embodiment of process chamber  82  (FIG. 6) which includes a heating assembly  120  includes heating member or plate  121 , at least one heat source  122 , and a coupling mechanism  124 . Heating assembly  120  may be positioned suspended within process chamber  82 , in a cantilevered relationship on a wall of process chamber  82 . Alternatively, heating assembly  120  may rest on mounts emanating up from a floor of process chamber  82 .  
         [0043]    Heating plate  121  may have a large mass relative to wafer  22 , and may be fabricated from a material, such as silicon carbide, quartz, inconel, aluminum, steel, or any other material that will not react at high processing temperatures with any ambient gases or with wafer  22 . Arranged on a top surface of heating plate  121  may be wafer supports  126 . In a preferred embodiment, wafer supports  126  extend outward from the surface of heating plate  121  to support the single wafer  22 . Wafer supports  126  are sized to ensure that wafer  22  is held in close proximity to heating plate  121 . For example, wafer supports  126  may each have a height of between about 50 μm and about 20 mm, preferably about 2 mm to about 8 mm. The present invention includes at least three wafer supports  126  to ensure stability. However, the total contact area between wafer supports  126  and wafer is less than about 350 mm 2 , preferably less than about 300 mm 2 .  
         [0044]    Heating plate  121  may be formed into any geometric shape, preferably a shape which resembles that of the wafer. In a preferred embodiment, heating plate  121  is a circular plate. The dimensions of heating plate  121  may be larger than the dimensions of wafer  22 , such that the surface area of the wafer is completely overlaid by the surface area of heating plate  121 . Preferably, the diameter of heating plate  121  may be no less than the diameter of wafer  22 , preferably, the diameter of heating plate  121  is greater than the diameter of wafer  22 . For example, the radius of heating plate  121  is greater than the radius of wafer  22  by about a length of between about 1 mm and 100 mm, preferably 25 mm.  
         [0045]    In one embodiment, on a periphery of heating plate  121  is coupled at least one heat source  122 . Heat source  122  may be a resistive heating element or other conductive/radiant heat source, which can be made to contact a peripheral portion of heating plate  121  or may be embedded within heating plate  121 . The resistive heating element may be made of any high temperature rated material, such as a suitable resistively heatable wire, which is made from a high mass material for increased thermal response and high temperature stability, such as SiC, SiC coated graphite, graphite, AlCr, AlNi and other alloys. Resistive heating elements of this type are available from Omega Corp. of Stamford, Conn.  
         [0046]    Coupling mechanism  124  includes a mounting bracket  128  and electrical leads  130  to provide an electrical power connection to heat source  122 . Mounting bracket  128  may be coupled to an internal wall of process chamber  82  using conventional mounting techniques. Once mounted, electrical leads  130  can extend outside of process chamber  82  to be connectable to an appropriate power source. The power source may be a direct line voltage of between about 100 volts and about 500 volts.  
         [0047]    [0047]FIG. 7C is an illustration of yet another embodiment of the present invention. As shown in the figure, a plurality of heating plates  121  may be stacked together within process chamber  82 . In a preferred embodiment, mounting holes  132  (FIG. 7B) are provided on a periphery of heating plates  121  and extend therethrough. Any appropriate number of mounting holes may be used to ensure that each heating plate  121  is supported. However, each mounting hole is positioned, such that the loading/unloading of wafer  22  is not hampered. Preferably, as illustrated in FIG. 7B, each mounting hole  132  is positioned on a half of heating plate  121  near coupling mechanism  124 . This arrangement ensures that the loading/unloading of wafer  22  onto heating member  120  is not impeded. In one embodiment, a rod  134  or similar member is threaded through mounting holes  132  and spacers  136 . Spacers  136  keep heating plate  121  an appropriate distance away from one another, which ensures that wafer supports  126  and wafer  22  can be fit in-between the stacked heating plate by, for example, robot arm  106  (FIG. 5A) or wafer transport system  86  (FIG. 6). The distance between the stacked heating plates may be between about 10 mm and 50 mm, for example, about 20 mm. The top most stacked heating plate  138  may be the same in structure and performance as the other heating plates  121 , except that the top most heating plate  138  may not be used to support wafer  22 .  
         [0048]    The description of the invention given above is provided for purposes of illustration and is not intended to be limiting. The invention is set forth in the following claims.