Abstract:
Systems and methods of solvent temperature control for wafer coating processes are provided. In an embodiment, a method for spin coating a wafer includes attaching the wafer to a rotatable chuck. The chuck is then rotated, and solvent is dispensed onto the wafer. The solvent dispensing temperature is controlled while the solvent is dispensed onto the wafer.

Description:
TECHNICAL FIELD 
       [0001]    The technical field relates to temperature control for solvents used in wafer coating processes, and more particularly relates to temperature control for solvents used in spin coating processes for wafers. 
       BACKGROUND 
       [0002]    The semiconductor industry is continuously moving toward the fabrication of smaller and more complex microelectronic components. Market pressures are expected to force manufactures to continue to reduce the size of microelectronic in the foreseeable future. The reduced size of microelectronic components has resulted in modified production processes handling smaller tolerances, with closer spacing of the various components. One result of the smaller, more closely spaced components is a reduced tolerance for mis-alignment in the manufacturing process. 
         [0003]    Microelectronics are often produced on a silicon wafer. Some wafers will undergo hundreds or thousands of different manufacturing steps to produce a product. Spin coating is a common manufacturing technique that is utilized in the production of many microelectronic components. For example, lithographic methods spin coat photoresist compounds onto wafers to position particular features or components on the wafer. Anti-reflective coatings are also spin coated onto wafers in some production processes. 
         [0004]    Spin coating is used to provide a layer of material with a relatively uniform thickness on the wafer, but the thickness of the spin coated layer is not completely uniform. As the size and spacing of microelectronic components shrink, variations in thickness of a spin coated layer become more significant. Smaller, more closely packed components require stricter tolerances, so a coating layer with a non-uniform thickness can result in unwanted residues or unwanted over-removal in thicker or thinner regions, respectively. Non-uniform thickness can have other undesired effects as well, such as shading of neighboring regions for angled manufacturing processes. Many manufacturing processes downstream from a spin coating process are calibrated to a specific coating layer thickness, so variations in thickness from one wafer to the next are also problematic. 
         [0005]    Therefore, systems and methods of producing more uniform and more consistent coating layer thicknesses are desired. Many coating layers are applied at specified temperatures and spin rates, and the thickness of the coating varies as the manufacturing conditions change. A variety of solvents are used in the spin coating process, and variations in the solvent temperature introduce variations in the manufacturing conditions. These variations can result in inconsistent or non-uniform coating layer thicknesses. 
         [0006]    Accordingly, it is desirable to provide a system for controlling the temperature of solvents during the spin coating process to improve the uniformity and consistency of the spin coated layer. In addition, it is desirable to develop a method for controlling the temperature of the solvents during the spin coating process. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
       BRIEF SUMMARY 
       [0007]    A spin coating system is provided for coating a wafer. The system includes a rotatable chuck with an attachment point for attaching the wafer to the chuck. A solvent dispensing system is configured to dispense solvent on the wafer, and a solvent temperature sensing element is coupled to the solvent dispensing system. The spin coating system also has a heat transfer system coupled to the solvent temperature sensing element and to the solvent dispensing system, and the heat transfer system is configured to adjust the temperature of solvent in the solvent dispensing system. 
         [0008]    In another embodiment, the spin coating system includes a rotatable chuck with an attachment point for attaching the wafer to the chuck. A solvent dispensing system has a nozzle positioned to dispense on the wafer. A heat transfer system includes a heat transfer device coupled to the solvent dispensing system, and a heat transfer temperature sensing element. The heat transfer system is configured to adjust the temperature of a heat transfer fluid in response to readings from the heat transfer temperature sensing element. 
         [0009]    A method for spin coating the wafer includes attaching the wafer to the chuck. Solvent is dispensed onto the wafer, and the chuck is rotated. The solvent dispensing temperature is controlled while the solvent is dispensed onto the wafer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The various embodiments will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
           [0011]      FIG. 1  is schematic drawing of a spin coating system in accordance with an exemplary embodiment with a wafer in position for receiving a coating material. 
           [0012]      FIG. 2  is a side view of a wafer with a coating, where the coating is shown removed from one side of the wafer and present on the other side of the wafer; 
           [0013]      FIG. 3  is a schematic drawing of a heat transfer system in accordance with an exemplary embodiment; 
           [0014]      FIG. 4  is a side view of a heat exchanger in accordance with an exemplary embodiment; 
           [0015]      FIG. 5  is a cutaway perspective view of a solvent line with a tracing line in accordance with an exemplary embodiment; 
           [0016]      FIG. 6  is a schematic view of a solvent dispensing system in accordance with an exemplary embodiment; 
           [0017]      FIG. 7  is a schematic view of a coating fluid dispensing system in accordance with an exemplary embodiment, and a wafer on a chuck; 
           [0018]      FIG. 8  is a schematic view of a spin coating system with a heat transfer system coupled to a solvent dispensing system for dispensing reduced resist consumption solvent, in accordance with an exemplary embodiment; 
           [0019]      FIG. 9  is a schematic view of a spin coating system with a heat transfer system coupled to a solvent dispensing system for dispensing back side rinse solvent, in accordance with an exemplary embodiment; and 
           [0020]      FIG. 10  is a schematic view of a spin coating system with a heat transfer system coupled to a solvent dispensing system for dispensing edge bead removal solvent, in accordance with an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following detailed description is merely exemplary in nature and is not intended to limit the various embodiments or the application and uses thereof. Furthermore, there is no intention to be bound by any theory presented in the preceding background or the following detailed description. 
         [0022]    Coatings are routinely applied to wafers by spin coating during the microelectronics manufacturing process. The spin coating process often includes the application of various solvents to a wafer. The temperature of the wafer is equilibrated within relatively narrow tolerances before beginning the spin coating process, and relatively small changes in the wafer temperature can change the coating thickness and uniformity. It has been discovered that variations in the temperature of some solvents will change the thickness of the coating. Therefore, a temperature control system is added to the spin coating system to control the solvent temperature during application. 
         [0023]    Referring now to  FIG. 1 , in accordance with an exemplary embodiment, a spin coating system  10  is used to apply various coatings  12  to a wafer  14 . The wafer  14  is typically made of silicon, and may be essentially pure silicon, doped silicon, or another semiconductor, such as selenium (Se) or other semiconductor materials. The spin coating system  10  includes a chuck  16  that rotates about its axis to spin the wafer  14 . A motor  18  is typically connected to the chuck  16  to provide the motive force, but other sources of motive force could also be used as long as the chuck  16  can be rotated. The chuck  16  includes an attachment point  20  for attaching the wafer  14  to the chuck  16 . The chuck  16  and wafer  14  may be positioned within an enclosure  22  for the spin coating process. In many embodiments, operation of the spin coating system  10  is highly automated, so a robot or other mechanical device controls the various steps in the process. 
         [0024]    The spin coating process will vary in different embodiments, and many of the steps described below are repeated or omitted in various embodiments. The process begins by connecting the wafer  14  to the chuck  16 . In many embodiments, the attachment point  20  is a vacuum system, similar to a suction cup, that holds the wafer  14  in place. The attachment point  20  is generally designed such that there is no damage or permanent change to the wafer  14  from attachment to the chuck  16 . The wafer  14  is thin in many embodiments, such as about 500 to about 1,000 microns, so care is needed to prevent breaking or damaging the wafer  14  during placement and attachment to the chuck  16 . 
         [0025]    After attachment to the chuck  16 , the wafer  14  is brought to a controlled hold temperature  24 . The hold temperature  24  varies for different coating materials, solvent loadings, spin rates, etc., but in one embodiment the hold temperature  24  is about room temperature. For example, a hold temperature  24  of about 20 degrees centigrade (° C.) is used in some embodiments, but hold temperatures of about 22° C., 25° C., 28° C., or other temperatures can also be used. In other embodiments, the hold temperature  24  is noticeably above or below room temperature. The wafer  14  is held on the chuck  16  for a hold time to allow the wafer  14  to equilibrate to the hold temperature  24 , where the space inside the enclosure  22  is maintained at the hold temperature  24 . The hold time may be about 1 second to about 2 minutes, but other hold times are also possible. 
         [0026]    Once the wafer  14  has equilibrated to the hold temperature  24 , various materials are applied to the wafer. The wafer  14  has a coating side  30  and a back side  32  opposite the coating side  30 . In one embodiment, a solvent for reduced resist consumption solvent, also referred to as a reduced resist consumption solvent (RRC),  34  is applied near a center point of the wafer  14  on the coating side  30 , such as within 1 centimeter (cm) of the center point. However, in other embodiments, the RRC  34  is applied to the coating side  30  at locations further removed from the center point. In this description, materials designated as “solvents” are materials that are not intended to remain on the wafer  14  to aid in further processing or manufacturing steps. Most of the applied solvents do not remain on the wafer  14  when the wafer  14  is removed from the spin coating system  10 , but there may be a residue in some embodiments. The solvents may either spin or drip off the wafer  14  or evaporate, and no appreciable solvent residue is left behind for manufacturing processes downstream from the spin coating process. 
         [0027]    In some embodiments, the RRC solvent  34  is applied prior to the coating fluid  36 , where the coating fluid  36  contains a material that will remain on the wafer  14  after the wafer  14  leaves the spin coating system  10 . In other embodiments, the RRC solvent  34  is applied while the coating fluid  36  is applied. The RRC solvent  34  aids the coating fluid  36  in spreading over the wafer  14 , and results in a lower volume of coating fluid  36  to achieve a target thickness. Many coating fluids  36  are expensive, so the RRC solvent  34  saves on coating fluid costs. The RRC solvent  34  is applied either before the wafer  14  begins to spin, after the wafer  14  begins to spin, or the RRC solvent  34  application begins before the wafer  14  starts to spin and continues as the wafer  14  starts spinning. The total amount of RRC solvent  34  applied can be about 0.2 to about 2 milliliters (ml), but other quantities are also possible. Several different RRC solvents  34  are available, such as gamma butyrolactone, 1-methoxy-2-propanol aka propylene glycol monomethyl ether (PGME), 1-methoxy-2-propanol acetate (PGMEA), ethyl lactate, n-butyl acetate, and 4-methyl-2-pentanol. 
         [0028]    Either after or during the RRC solvent  34  application, the coating fluid  36  is applied to the coating side  30  of the wafer  14 . The coating fluid  36  is applied either before or after the wafer  14  begins spinning in various embodiments. In many embodiments, the RRC solvent  34  and the coating fluid  36  are dispensed onto the wafer  14  at close to the same point, and often within about 5 millimeters (mm) of each other. Centrifugal force from the spinning wafer  14  spreads the coating fluid  36  over the surface of the coating side  30  of the wafer  14 . The coating fluid  34  typically includes a residual material that will remain on the wafer  14  as a coating  12 , and a solvent that keeps the residual material in a liquid state for the spin coating process. The solvent in the coating fluid  34  is evaporated to leave a solid coating  12  on the wafer  14 . In some photoresist embodiments, the residual material includes a resin, which serves to bind the coating  12 , and an inhibitor or sensitizer that is the photoactive ingredient. Many other embodiments are also possible for the coating fluid  34 . The wafer  14  may be moved to a different location to evaporate the solvent from the coating fluid  34 , but the evaporation of the solvent is still considered part of the spin coating system  10 . Moving the wafer  14  to another location to evaporate the solvent from the coating fluid  36  frees the chuck  16  for another wafer  14 , which lowers the spin coating cycle time. There may be one or more applications of coating fluid  36 , and the RRC solvent  34  can be applied or skipped between different coating fluid applications. In many embodiments, the coating  12  is between about 35 nanometers (nm) to several microns, but other thicknesses are also possible. 
         [0029]    The coating fluid  36  tends to flow to the wafer outer edge  26 , and sometimes flows onto the wafer back side  32 . This leaves a residue on the wafer back side  32 , and the residue can interfere with downstream manufacturing processes. A back side rinse (BSR) solvent  38  can be applied to the wafer back side  32  to remove the residue. The BSR solvent  38  is applied during or after the coating fluid  36  application, and if applied concurrently with the coating fluid  36  application the BSR solvent  38  application continues for a period of time after the coating fluid  36  application stops. The BSR solvent  38  dissolves and removes the residue from the wafer back side  32 . 
         [0030]    The coating fluid  36  also tends to leave a bead, or a slight rise, on the coating side  30  near the wafer outer edge  26 , so the coating  12  tends to be slightly thicker there, as shown in  FIG. 2 , with continuing reference to  FIG. 1 . An edge bead removal (EBR) solvent  40  is applied to the coating side  30  of the wafer  14  approximately at the wafer outer edge  26  to remove the slight rise in the coating  12 . In some embodiments, the EBR solvent  40  is applied within about 5 millimeters (mm) to about 0 mm of the wafer outer edge  26  on the coating side  30 , but other distances are also possible. 
         [0031]    During processing, the use of the BSR solvent  38  can change the thickness of the coating  12  on the coating side  30  of the wafer. Several tests were conducted measuring the coating thickness  42 , indicated by arrows, at several different locations on the wafer coating side  30 , so as to determine a coating thickness variation  44 . Some of the tests were conducted when no BSR solvent  38  was used, and others were conducted when a BSR solvent  38  was used. The standard deviation in coating thickness  42  for coatings  12  applied without a BSR solvent  38  was about 0.12 nm, and the standard deviation in coating thickness  42  for coatings  12  applied with a BSR solvent  38  was about 0.25 nm, or about twice that of no BSR solvent  38 . Coatings  12  when BSR solvent  38  was used also had an average thickness of almost 0.3 nm more than coatings  12  when no BSR solvent  38  was used. The BSR solvent  38  and the coating fluid  36  were dispensed on opposite sides of the wafer  14 , so there was no direct contact of the BSR solvent  38  and the coating fluid  36  over the vast majority of the wafer  14 . There may have been some minor interaction at the wafer outer edge  26 , but the larger coating thickness variations  44  were present over the entire coating side  30  of the wafer  14 . 
         [0032]    In some tests, the BSR solvent  38  was added while the coating fluid  36  was applied, and in other tests the BSR solvent  38  was added only after the coating fluid  36  was applied. The test results were essentially the same when the BSR solvent  38  was added during the coating fluid  36  application and when the BSR solvent  38  was added after the coating fluid  36  application. Bulk BSR solvent  38  was not kept in a temperature controlled area, and the difference in the hold temperature  24  (which is the wafer temperature when the coating fluid  36  is applied) and the bulk BSR solvent  38  was only about 1° C. to about 2° C. Because the BSR solvent  38  does not directly interact with the coating fluid  36 , it was determined the temperature change induced by the BSR solvent  38  impacted the coating thickness  42 . The BSR solvent  38  negatively impacted the coating thickness variation  44  even when applied after the coating fluid  36 , so it was theorized that the change in temperature impacted the coating fluid viscosity, thermal expansion, or some other property of the coating fluid  36  enough to change the coating thickness  42 . 
         [0033]    The coating  12  has a less consistent coating thickness  42  when the BSR solvent  38  is used, and there may be similar results for other solvents, such as the RRC solvent  34  and the EBR solvent  40 . Therefore, to reduce the coating thickness variation  44 , and to improve coating thickness  42  consistencies between different wafers  14 , a heat transfer system  50  is used to control the temperature of the solvents applied during the spin coating process, where an exemplary embodiment is shown in  FIG. 3  with continuing reference to  FIGS. 1 and 2 . The heat transfer system  50  is configured to adjust the temperature of the solvent in response to a reading from a temperature sensing element. Many solvents are relatively volatile, so they will evaporate when dispersed on a wafer  14 . The evaporation of the solvent will result in evaporative cooling, so the proper solvent dispensing temperature may be different than the hold temperature  24  for the wafer  14 . The best solvent dispensing temperature can be determined experimentally, and the best solvent dispensing temperature might vary from one solvent to another and from one coating fluid  36  to another. Therefore, different experimental testing for each solvent and coating fluid  36  can be used. 
         [0034]    Several tests can be run to experimentally determine the solvent dispensing temperature for use in the spin coating system  10 . Different experimental testing processes can be used, but one embodiment is described herein. The first step is to establish standard conditions for the coating process, including a hold temperature  24  and a rotation speed for the wafer  14 . Next, several wafers  14  are coated at the standard conditions, except the solvent dispensing temperature of a selected solvent is varied and recorded. The coating  12  is cured, and the coating thickness  42  is measured at several points on each wafer  14 . The coating thicknesses  42  are measured at the same general locations on each wafer  14 . The coating thickness variation  44  and total coating thickness  42  is determined based on the measurements, and an acceptable range is determined for the solvent dispensing temperature. The desired solvent dispensing temperature can be based on a variety of factors, such as (a) most consistent coating thickness on a single wafer  14 , (b) most consisting coating thickness for different wafers  14 , (c) a measured coating thickness closest to a desired coating thickness, or (d) other factors. The testing provides a desired solvent dispensing temperature, or a set point for the temperature, as well as an acceptable range for the solvent dispensing temperature. For example, the most consistent coating thickness  42  may be a BSR solvent dispensing temperature of about 24° C., but solvent dispensing temperatures ranging from about 23° C. to about 25° C. may give acceptably consistent coating thicknesses  42 . Once a desired solvent dispensing temperature is determined for one solvent, that solvent dispensing temperature is set as a standard condition and the process is repeated for the next solvent. 
         [0035]    Once the solvent dispensing temperature is determined, a heat transfer system  50  is used to control the solvent dispensing temperature to within an acceptable range. The heat transfer system  50  has several components. A heat transfer device  54  is a device that transfers heat from a heat transfer fluid  52  to the solvent. The heat transfer device  54  may be a heat exchanger  56 , a tracing line  58 , a radiant heat transfer system, or other devices, as shown in  FIGS. 4 and 5 , with continuing reference to  FIGS. 1-3 . In some embodiments, the heat transfer device  54  is coupled to a solvent line  70  for heat transfer. In many embodiments, insulation  71  is positioned about the solvent line  70  to minimize unwanted changes in solvent temperature. In other embodiments, the heat transfer device  54  is coupled to other components of the solvent dispensing system, as long as the solvent dispensing temperature can be controlled within an acceptable solvent dispensing temperature range. The heat transfer fluid  52  can be a gas, but in many embodiments it is a liquid such as water or a glycol and water mixture. In many embodiments the heat transfer system  50  also includes a heat transfer fluid tank  60  for storing bulk heat transfer fluid  52 , and a heat transfer pump  61  moves heat transfer fluid  52  from the heat transfer fluid tank  60  through the heat transfer device  54  and back to the heat transfer fluid tank  60 . A cooling element  62  and/or a heating element  64  are used to increase or decrease the temperature of the heat transfer fluid  52 , and the cooling and/or heating elements  62 ,  64  can be positioned in the heat transfer tank  60  or in other locations. In other embodiments, two streams of different temperature are mixed together to produce a heat transfer fluid  52  with the desired temperature, and the heat transfer fluid  52  is discarded after use. Many other embodiments are also possible. 
         [0036]    In the heat transfer system embodiment shown in  FIG. 3 , a heat transfer control unit  66  is coupled to a heat transfer temperature sensing element  68  and a solvent temperature sensing element  72 . The heat transfer temperature sensing element  68  is positioned to measure the temperature of the heat transfer fluid  52 , and the solvent temperature sensing element  72  is positioned to measure the temperature of the solvent. The temperature sensing elements  68 ,  72  can be thermocouples, thermometers, radiant heat sensing devices, or other devices capable of measuring temperature. In some embodiments, the solvent temperature sensing element  72  is positioned to measure the solvent temperature close to the point of application onto the wafer  14 , such as within about 1 to about 4 centimeters (cm) of a dispensing nozzle. The heat transfer control unit  66  is coupled to the cooling element  62  and/or the heating element  64 , and adjusts the amount of cooling/heating based on the temperature of one or both of the heat transfer temperature sensing element  68  or the solvent temperature sensing element  72 . 
         [0037]    The heat transfer system  50  can be oversized for the amount of solvent dispensed, so much so that controlling the temperature of the heat transfer fluid  52  effectively controls the solvent dispensing temperature. For example, in some embodiments less than 3 ml of solvent are dispensed on a wafer  14 . Therefore, a heat transfer system  50  that transfers heat from 1,000 ml of heat transfer fluid  52  to 3 ml of solvent would bring the solvent to the temperature of the heat transfer fluid  52 . If the heat transfer system  50  is sized and configured to flow very large quantities of heat transfer fluid  52  past the solvent line  70 , the heat transfer system  50  can control the temperature of the solvent by reading the heat transfer temperature sensing element  68 . In this embodiment, there is no need for a solvent temperature sensing element  72  because the solvent is brought to the temperature of the heat transfer fluid  52 . 
         [0038]    In an exemplary embodiment, solvent is dispensed from a solvent dispensing system  74 , as illustrated in  FIG. 6  with continuing reference to  FIGS. 1-5 . In some embodiments, the solvent dispensing system  74  includes a solvent line  70 , a solvent storage tank  76 , a solvent pump  78 , and a nozzle  80 . In other embodiments, the solvent tank  76  can be pressurized so no solvent pump  78  is needed, and other embodiments are also possible. The solvent storage tank  76  can be as simple as a drum, bucket, or tote bin, but in other embodiments the solvent storage tank  76  is a dedicated tank. A wide variety of pumps can be used for the solvent pump  78 , but the solvent pump  78  should be able to dispense small quantities of solvent, such as 0.1 to 20 ml at a time. In many embodiments, the total solvent discharged per wafer is less than 30 ml. A wide variety of effective nozzles  80  are available, and essentially any hole that allows solvent to escape the solvent line  70  can be used, but some nozzle designs are more effective than others. 
         [0039]    The coating fluid  36  is dispensed from a coating fluid dispensing system  82 , in an exemplary embodiment, as shown in  FIG. 7  with continuing reference to  FIGS. 1-6 . The coating fluid dispensing system  82  includes a coating fluid tank  84 , a coating fluid pump  86 , a coating fluid line  88 , and a nozzle  80 . In an exemplary embodiment, the nozzle  80  is positioned to dispense near the center point of the coating side  30  of a wafer  14  positioned on the chuck  16 , and the coating fluid nozzle  80  is positioned within 5 mm of the reduced resist consumption solvent line nozzle  80 . However, other embodiments are also possible. As with the solvent dispensing system  74 , several variations and different embodiments can be used. 
         [0040]      FIG. 8  illustrates one embodiment of a spin coating system  10  with a heat transfer system  50  coupled to a RRC solvent dispensing system  74 . The solvent storage tank  76  is positioned in a temperature controlled storage area  90 , and the temperature in the temperature controlled storage area  90  is set at the desired solvent dispensing temperature. The temperature controlled storage area  90  maintains a desired temperature with a storage area heating and/or cooling element  92  coupled to a storage area temperature sensing element  94 . The solvent line  70  enters a heat exchanger  56 , which is part of the heat transfer system  50 , to adjust the temperature. The heat transfer control unit  66  is coupled to two different heat transfer temperature sensing elements  68  and a solvent temperature sensing element  72 . A heat transfer pump  61  moves heat transfer fluid  52  through the heat exchanger  56 , and a coating fluid dispensing system  82  dispenses coating fluid  36  on the wafer. 
         [0041]      FIG. 9  illustrates another embodiment of a spin coating system  10  with a tracing line  58  and a BSR solvent dispensing system  74 . The solvent storage tank  76  is positioned within the temperature controlled storage area  90 , and the heat transfer control unit  66  is coupled to one heat transfer temperature sensing element  68 .  FIG. 10  illustrates yet another embodiment of a spin coating system  10  with a heat exchanger  56  and a solvent dispensing system  74  for EBR solvent  40 . The duty on the heat exchanger  56  can be reduced or eliminated by using a temperature controlled storage area  90 . In yet other embodiments, the temperature of the solvent is controlled with just the temperature controlled storage area  90  and insulation  71  on the solvent line  70 . 
         [0042]    While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the application in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing one or more embodiments, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope, as set forth in the appended claims.