Abstract:
A method of applying a layer of a flowable material to a substrate. The substrate is received with a rotatable chuck, and an amount of the flowable material is dispensed on to the substrate. The substrate is spun on the rotatable chuck, thereby spreading the flowable material across the substrate and conveying a surplus amount of the flowable material away from the substrate. An exhaust stream is created with a vacuum source. At least a portion of the surplus amount of the flowable material conveyed away from the substrate is entrained into the exhaust stream, which exhaust stream is conveyed into an exhaust system. A pressure drop is created in the exhaust stream across a vane anemometer within the exhaust system. The blow back of the entrained portion of the surplus amount of the flowable material from a downstream position in the exhaust system to the substrate is thereby reduced. Thus, positioning the vane anemometer in the exhaust system tends to create a sudden and distinct pressure drop across the vane anemometer, which tends to reduce the occurrence of blow back of the flowable material from the portion of the exhaust system that is downstream from the vane anemometer, and which is at a relatively lower pressure, to the portion of the exhaust system that is upstream from the vane anemometer, and which is at a relatively higher pressure, and which is where the substrate is processed.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to the field of application techniques for flowable material. More particularly, this invention relates to an improved exhaust flow control in a photoresist application system.  
         BACKGROUND OF THE INVENTION  
         [0002]    One of the central and recurring processes by which integrated circuits are formed is that of applying a coating of photosensitive material (typically called photoresist) to the substrate on which the integrated circuits are formed, patterning and developing the photosensitive material, and then selectively processing the underlying portions of the integrated circuits exposed in the patterned portions of the layer of photosensitive material. This series of process steps is repeated over and over again, with many different variations, during the process of fabricating the integrated circuits. Therefore, any problem with the photolithography processing of the substrates tends to have dramatic and far reaching effect on the cost of the manufacturing process and the integrity of the integrated circuits produced.  
           [0003]    Traditionally, substrates are coated with the layer of photoresist using a photoresist application system. A photoresist application system optionally includes a number of components to apply and set the photoresist layer to the substrate. A photoresist dispenser dispenses the photoresist onto the top surface of the substrate. After the desired amount of photoresist is dispensed onto the substrate, a chuck holding the substrate is ramped up according to a predetermined acceleration profile to one or more predetermined rotational rates. The acceleration and rotation of the substrate causes the photoresist to spread across the surface of the substrate. Excess photoresist is shed from the substrate at it spreads out to the edges of the substrate, and then off of the edges of the substrate.  
           [0004]    The excess photoresist that is spun off of the substrate enters a cup that is disposed around the substrate, and which is a part of an exhaust system. The exhaust system uses an exhaust stream of gas, typically ambient air, to help drawn the excess photoresist down through the exhaust system and away from the substrate to a collection point, such as a sump. The flow of the exhaust stream helps to reduce a blow back of the excess photoresist back onto the substrate. When blow back occurs, the effected areas of the substrate do not expose or develop properly and the substrate must be reworked or the quality of the integrated circuits underlying the effected areas tends to be compromised. Thus, the exhaust stream plays a relatively important role in ensuring that the photoresist coating process proceeds in a proper manner.  
           [0005]    As generally indicated above, the excess photoresist has a tendency to blow back onto the substrate if the pressure differential of the exhaust stream through the exhaust system becomes positive. Therefore, it is desirable for the exhaust stream pressure to decrease along the length of the exhaust system. However, even when there is a gradual decrease in pressure along the length of the exhaust system, various temporary and spurious conditions can cause droplets of photoresist to blow back onto the substrate.  
           [0006]    Thus, there is a need for a system and a method for applying photoresist to a substrate that reduces blow back of the entrained portion of the excess amount of the flowable material from a downstream position in the exhaust system to the substrate.  
         SUMMARY OF THE INVENTION  
         [0007]    The above and other needs are provided by a method of applying a layer of a flowable material to a substrate. The substrate is received with a rotatable chuck, and an amount of the flowable material is dispensed on to the substrate. The substrate is spun on the rotatable chuck, thereby spreading the flowable material across the substrate and conveying a surplus amount of the flowable material away from the substrate. An exhaust stream is created with a vacuum source. At least a portion of the surplus amount of the flowable material conveyed away from the substrate is entrained into the exhaust stream, which exhaust stream is conveyed into an exhaust system. A pressure drop is created in the exhaust stream across a vane anemometer within the exhaust system. The blow back of the entrained portion of the surplus amount of the flowable material from a downstream position in the exhaust system to the substrate is thereby reduced.  
           [0008]    Thus, positioning the vane anemometer in the exhaust system tends to create a sudden and distinct pressure drop across the vane anemometer, which tends to reduce the occurrence of blow back of the flowable material from the portion of the exhaust system that is downstream from the vane anemometer, and which is at a relatively lower pressure, to the portion of the exhaust system that is upstream from the vane anemometer, and which is at a relatively higher pressure, and which is where the substrate is processed.  
           [0009]    In various preferred embodiments, the flowable material is photoresist, the substrate is a semiconductor wafer, and the rotatable chuck and the exhaust system are components of a photoresist application system. The external surface of the vanes of the vane anemometer are preferably made of a material that is resistant to the adherence of the flowable material. Most preferably, the vane anemometer has magnets in the tips of the vanes, and the rotational rate of the vane anemometer is detected with sensors located outside of the exhaust system, which sensors detect the magnets located within the vanes of the vane anemometer.  
           [0010]    In an especially preferred embodiment, a signal corresponding to the velocity of the exhaust stream in the exhaust system is produced and reported. An alarm condition is signaled when the velocity of the exhaust stream in the exhaust system is less than a set point velocity. In other embodiments the application of the flowable material to additional substrates is prohibited when the velocity of the exhaust stream in the exhaust system is less than the set point velocity.  
           [0011]    In an apparatus for applying a layer of the flowable material to the substrate, a rotatable chuck receives the substrate, and a dispensing unit dispenses an amount of the flowable material onto the substrate, The rotatable chuck rotates the substrate and thereby spreads the flowable material across the substrate, and conveys a surplus amount of the flowable material away from the substrate. An exhaust system receives the surplus amount of the flowable material. A vacuum source creates an exhaust stream within the exhaust system, and the exhaust stream entrains at least a portion of the surplus amount of the flowable material conveyed away from the substrate and received by the exhaust system. A vane anemometer disposed within the exhaust system creates a pressure drop in the exhaust stream across the vane anemometer within the exhaust system, and thereby reduces blow back of the entrained portion of the surplus amount of the flowable material from a downstream position in the exhaust system to the substrate.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Further advantages of the invention will become apparent by reference to the detailed description of preferred embodiments when considered in conjunction with the following drawings, which are not to scale so as to more clearly depict the details, wherein like reference characters designate like or similar elements throughout the several views, and wherein:  
         [0013]    [0013]FIG. 1 is a cross sectional view of a photoresist application system according to a first embodiment of the invention, and  
         [0014]    FIGS.  2  is a cross sectional view of a photoresist application system according to a second embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Referring now to FIG. 1, there is depicted a first embodiment of a photoresist application system  10  according to the present invention. In its basic configuration the photoresist application system  10  consists of a relatively few different elements. However, in more elaborate embodiments a great number of elements are selectively added to the basic elements described herein. Thus, the embodiment described herein is considered to be exemplary of the basic elements and not an exhaustive recitation of all the different combinations of elements that may be added to the photoresist application system  10 .  
         [0016]    A photoresist dispensing tube  12  dispenses an amount of photoresist on to the substrate  14 . Preferably, the amount of photoresist dispensed from the photoresist dispensing tube  12  is a metered amount, which is dependent upon one or more of several different variables, such as the size of the substrate  14 , the desired thickness of the resultant layer of photoresist on the substrate  14 , and the type of photoresist dispensed from the photoresist dispensing tube  12 . Preferably, the photoresist is dispensed from the photoresist dispensing tube  12  while the substrate  14  is stationary, but in alternate embodiments the photoresist is dispensed from the photoresist dispensing tube  12  while the substrate  14  is rotating at some relatively low rotation rate. The photoresist is also preferably dispensed in a position on the top surface of the substrate  14  that is substantially near the center of the substrate  14 , which is also substantially the center for rotation of the substrate  14 , as described in more detail below. However, in alternate embodiments the photoresist is dispensed in various locations across the top surface of the substrate  14 .  
         [0017]    A rotatable chuck  16  holds the substrate while the photoresist is dispensed from the photoresist dispensing tube  12 . The rotatable chuck  16  is preferably a vacuum chuck, in that a slight vacuum is drawn through the rotatable chuck  16  and applied to the back surface of the substrate  12 , which back surface resides against an upper surface of the rotatable chuck  16 . In this manner the substrate  14  is held securely against the rotatable chuck  16 , to a degree sufficient that the substrate  14  does not separate from the rotatable chuck  16  during subsequent rotational processing of the substrate  14 , but not to the point that the substrate  14  is in any way damaged by the force of the vacuum applied between the top surface of the rotatable chuck  16  and back surface of the substrate  14 .  
         [0018]    At some point, either before, during or after dispensing the photoresist from the photoresist dispensing tube  12  onto the surface of the substrate  14 , the substrate  14  is rotated to assist in spreading the pool of photoresist dispensed onto the substrate  14  across the surface of the substrate  14 . Again, one of the purposes of the photoresist application system  10  is to produce a relatively uniform layer of photoresist across the entire top surface of the substrate  14 . In one embodiment, the substrate  14  is rotated by rotating the rotatable chuck  16 , such as by connecting the rotatable chuck to a motor  20  with a spindle  18 . Of course, in other embodiments, other means may be used to power the rotation of the rotatable chuck  16 .  
         [0019]    To ensure that a sufficient amount of photoresist is provided to form a uniform layer of photoresist with a desired thickness across the surface of the substrate  14 , an excess amount of photoresist is delivered onto the substrate  14  through the photoresist dispensing tube  12 . As the substrate  14  is rotated by the rotatable chuck  16 , the surplus photoresist, which is not required to form the uniform layer of photoresist across the surface of the substrate  14 , is flung off of the substrate  16  from the outside edges of the substrate  16 . The surplus photoresist typically impacts against the sidewalls  15  of the photoresist application system  10 .  
         [0020]    Preferably at all time during processing, an exhaust stream is drawn through the photoresist processing system  10  by a vacuum source  36 , such as a vacuum pump. The exhaust stream is preferably drawn from the ambient air around the photoresist processing system  10 , and through an exhaust system  34 , that includes a cup  22 , a passage  24 , and a sump  26 . The exhaust stream is preferably drawn at a rate, and the elements of the exhaust system  34  are preferably designed with a configuration, such that the exhaust stream flows in a substantially laminar manner, especially around the substrate  14 . Laminar flow of the exhaust stream is preferred because it is desired that none of the surplus photoresist that is flung off of the substrate  14  be carried back onto the substrate  14  by turbulence within the exhaust stream.  
         [0021]    In its most preferred operation, the exhaust system conveys the surplus photoresist away from the substrate  14  by delivering it down through the exhaust system to the sump  26 . Although the sump  26  is depicted in FIG. 1 as relatively near to the substrate  14 , it is appreciated that this for convenience in the drawings, and that in actual practice the sump  26  is preferably much farther away from the substrate  14 .  
         [0022]    The, exhaust stream created in the exhaust system  34  aids in the flow of the surplus photoresist through the exhaust system  34  toward the sump  26 . As surplus photoresist is flung off of the substrate  14 , some amount of the surplus photoresist is entrained in the exhaust stream and drawn down through the exhaust system  34 . The surplus photoresist that impacts against the side wall  15  of the photoresist application system  10  tends to build up on the side wall  15  until it also begins to move down through the exhaust system  34  under the influence of various forces, including gravity and the flow of the exhaust stream. As the photoresist on the sidewalls  15  of the photoresist application system  10  builds up, a certain amount of the surplus photoresist may come loose from the sidewalls  15  and also be entrained in the exhaust stream.  
         [0023]    It is preferred that once the surplus photoresist is flung off of the substrate  14 , the surplus photoresist is conveyed away from the substrate  14  through the exhaust system  34  to the sump  26 , and does not further contact the substrate  14 . However, various conditions within the exhaust system  34  may work against this preference. For example, because of the location of the vacuum source  36 , a negative pressure preferably exists through the exhaust system  34 . By this it is meant that when the pressures at any two points within the exhaust system  34  are compared, the point that is closer to the vacuum system  36 , the downstream position, preferably has a pressure that is at least slightly lower than the pressure of the point that is closer to the substrate  14 , the upstream position. In this manner, the flow of the exhaust stream is preferably always directed away from the substrate  14  at one end of the exhaust system  34  and toward the vacuum source  36  at the other end of the exhaust system  34 . This flow characteristic of the exhaust stream tends to reduce the amount of surplus photoresist entrained in the exhaust stream that blows back onto the substrate  14 .  
         [0024]    However, if the pressure through the exhaust system  34  becomes positive, then the amount of surplus photoresist entrained in the exhaust stream that blows back onto the substrate  14  tends to increase. By a positive pressure it is meant that when the pressures at any two points within the exhaust system are compared, the point that is closer to the vacuum system  36  has a pressure that is not less than the pressure at the point that is closer to the substrate  14 . When such a positive pressure exists, the flow of the exhaust stream tends to be directed toward the substrate  14  at what should be the upstream end of the exhaust system  34  and away from the vacuum source  36  at what should be the downstream end of the exhaust system  34 . This flow characteristic of the exhaust stream tends to increase the amount of surplus photoresist entrained in the exhaust stream that blows back onto the substrate  14 , and thus is an undesirable flow characteristic.  
         [0025]    According to a preferred embodiment of the invention, an element is added to the photoresist application system  10  to create a relatively sharp pressure drop at a given position within the exhaust system  34 . In this manner, a relatively larger positive pressure is required to produce a flow of the exhaust stream toward the substrate  14  and blow back of the surplus photoresist onto the substrate  14 . In a most preferred embodiment, the pressure drop is created by one or more vane anemometer  38 .  
         [0026]    The vane anemometer  38  preferably has one or more vanes  40 , which are disposed at an angle to the flow of the exhaust stream within the exhaust system  34 . As the exhaust stream flows past the vane anemometer  38 , the vanes  40  of the vane anemometer  38  rotate. The rotation of the vanes  40  of the vane anemometer  38  is preferably proportional to the velocity of the flow of the exhaust stream within the exhaust system  34 , as described in more detail below.  
         [0027]    In a most preferred embodiment, the vanes  40  of the vane anemometer  38  are covered with or constructed of a material that tends to not be wet by the photoresist within the exhaust system  34 , such as the photoresist that is entrained within the exhaust stream. In other words, the material that covers the vanes  40  of the vane anemometer  38  is resistant to the adherence of the photoresist, and the photoresist is preferably shed by and not absorbed by the material that covers the vanes  40  of the vane anemometer  38 . A material such as Teflon tends to resist the absorption of most photoresist material. However, in embodiments where the invention is applied to applications other than photoresist application systems  10 , then another material may preferably be used, as selected at least in part according to the specific characteristics of the flowable material that is within the exhaust system  34 .  
         [0028]    Because at least a small amount of energy is absorbed from the exhaust stream to produce the rotation in the vane anemometer  38 , and because the presence of the vane anemometer  38  in the exhaust system  34  tends to restrict the flow of the exhaust stream to at least some degree, the pressure in the cup  22  of the exhaust system  34  tends to be at least slightly higher than the pressure in the passage  24  or the sump  26  of the exhaust system  34 . Thus, a larger positive pressure is required to produce a blow back of the entrained photoresist onto the substrate  14 , and smaller variations within the pressure characteristics of the exhaust system  34  tend to not rise to a level that is sufficient to create a blow back condition. In this manner the vane anemometer  38  tends to reduce the amount of photoresist that blows back onto the substrate  14 .  
         [0029]    However, in additional preferred embodiments of the invention, additional use is made of the vane anemometer  38 . In these additional embodiments, the vane anemometer  38  is used to detect the velocity of the exhaust stream within the exhaust system  34 . This is accomplished by detecting the speed of the rotation of the vanes  40  of the vane anemometer  38 , which as described briefly above, is preferably proportional to the velocity of the flow of the exhaust stream within the exhaust system  34 . In this manner, the vane anemometer  38  is used to detect the velocity of the exhaust stream, which detected velocity is preferably used to help control the use of the photoresist application system  10 , as described below.  
         [0030]    The rotational speed of the vane anemometer  38  can be detected in a number of different ways. For example, a mechanical linkage can be made between the rotating vanes  40  and a sensor. However, in the preferred embodiment, no mechanical connections exist between the vanes  40  and the rotational speed sensing mechanism. Further, it is preferred that no additional elements of the velocity sensing mechanism exist within the exhaust system  34 .  
         [0031]    In a most preferred embodiment, elements such as magnets  42  are disposed within the vanes  40  of the anemometer  38 . Most preferably, the magnets  42  reside at the tips of the vanes  40  of the anemometer  38 . The magnets  42  may be attached to the outside of the vanes  40 , or the vanes  40  may be constructed of the magnets  42 , or the magnets  42  may be embedded within the vanes  40 . Preferably, the magnets  42  reside within the cladding material of the vanes  40 , which cladding material is preferably substantially non-wettable by the photoresist, as described above. In this manner, the magnets  42  tend to not collect photoresist over time, which would tend to alter the rotational characteristics of the vane anemometer  38  over time. Alternately, the magnets  42  are additionally clad with the same or another material that is substantially non-wettable by the photoresist, or are themselves constructed of a material that is non-wettable by the flowable material within the exhaust system  34 .  
         [0032]    Preferably a sensor  28 , such as a coil, is disposed outside of the exhaust system  34  in proximity to the vane anemometer  38 , and detects the rotation of the vane anemometer  38 , by detecting the motion of the magnets  42 . The sensor  28  may include the electronics to convert the information detected by the sensor  28  to a rotational speed value, such as rotations per minute, and display the rotational speed value, or alternately the sensor  28  may convey raw sensor information via communication means  30  to a larger controller  32  that converts the raw sensor information to a rotational speed value, reports the value, and performs other functions, as described in more detail below. Further, one of either the sensor  28  or the controller  32  preferably converts the information to a velocity or a flow rate of the exhaust stream. Any of this information as detected or calculated can be used in the manner as described below.  
         [0033]    For example, it may be empirically determined that when the velocity of the exhaust stream falls below a certain set point, that the degree of photoresist blow back onto the substrate increases. In this case, the controller  32  is programmed to compare the readings received from the sensor  28 , whether those readings be raw sensor data or calculated values, to the exhaust stream velocity set point. When the readings from the sensor  28  do not violate the set point, then the controller  32  preferably signals that the photoresist application system  10  is functioning properly in regard to that aspect of the operation of the photoresist application system  10 .  
         [0034]    However, when the readings from the sensor  28  do violate the set point, then the controller  32  preferably signals that the photoresist application system  10  is not functioning properly in regard to that aspect of the operation of the photoresist application system  10 . This signal may take any one or more of a number of different forms. For example, the signal may be an audible signal such as an alarm, or a visual signal such as a warning light. Further, the signal may be an electronic signal to appropriate interlocks on the photoresist application system  10 , which interlocks prohibit further application of photoresist to additional substrates  14  until the alarm condition of the exhaust stream is corrected. Additionally, the signal may be a digital data signal that is sent via a computerized network to a management station, which provides data logging for and control of the photoresist application system  10 .  
         [0035]    The placement of the vane anemometer  38  is preferably selected according to the individual flow attributes of the exhaust system  34 , according to at least some of the following considerations. For example, if there is a tendency for the blow back to originate within the cup  22  of the exhaust system  34 , then placement of the vane anemometer  38  at a position between the substrate  14  and substantial portions of the cup  22  may be preferred, such as depicted in FIG. 2. Also as depicted in FIG. 2, multiple vane anemometers  38  may be preferred, depending upon where they are disposed in the exhaust system  34 , so that a substantial portion of the cross sectional area of that part of the exhaust system  34  in which the vane anemometers  38  are disposed is occupied by the vane anemometers  38 , and a pressure drop across the vane anemometers  38  can be created.  
         [0036]    Although the preferred embodiments as described above have been exemplified with the specific recitation of a photoresist application system  10 , it is understood that this is by way of example only, and the invention has application to other embodiments where flowable materials are applied to substrates other than semiconductor wafers. Thus, the foregoing description of preferred embodiments for this invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as is suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.