Abstract:
A thermal processing apparatus and method with predictive temperature correction. Distances are measured from a backside of the wafer relative to a reference plane. Heat is transferred to the backside of the substrate in relation to the measured distances. This allows a baking unit to uniformly heat the substrate to compensate for irregularities or warpage.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an apparatus and methods for thermally processing substrates, such as semiconductor wafers. 
       BACKGROUND OF THE INVENTION 
       [0002]    Coating/developing units, using photolithography processes for manufacturing semiconductor devices and liquid crystal displays (LCD&#39;s), generally coat a resist on a substrate, expose the resist coating to light to impart a latent image pattern, and develop the exposed resist coating to transform the latent image pattern into a final image pattern having masked and unmasked areas. This permits deposition or treatment of selected portions of the surface of the semiconductor wafer. Such a series of processing stages is typically carried out in a coating/developing system having discrete heating sections, such as a post apply baking unit and a post exposure baking unit. Each heating section of the coating/developing system incorporates a hotplate with a built-in heater. 
         [0003]    Feature sizes of semiconductor device circuits have been scaled to less than 0.1 microns. Typically, the pattern wiring that interconnects individual device circuits is formed with sub-micron line widths. Consequently, the heat treatment temperature of the resist coating should be accurately controlled to provide reproducible and accurate feature sizes and line widths. The substrates or wafers (i.e., objects to be treated) are usually treated or processed under the same process (i.e., individual treatment program) in units (i.e., lots) each consisting of, for example, twenty-five wafers. Individual processes define heat treatment conditions under which baking is performed. Wafers belonging to the same lot are heated under the same conditions. 
         [0004]    The post exposure bake (PEB) process serves multiple purposes in photoresist processing. First, the elevated temperature of the bake drives the diffusion of the photoproducts in the resist. A small amount of diffusion may be useful in minimizing the effects of standing waves, which are the periodic variations in exposure dose throughout the depth of the resist coating that result from interference of incident and reflected radiation. Another main purpose of the PEB may be to drive an acid-catalyzed reaction that alters the solubility of the polymer layer used in many chemically amplified resists. PEB may also play a role in removing solvent from the wafer surface. 
         [0005]    Hotplates having uniformities within a range of a few tenths of a degree centigrade are currently available and are generally adequate for current process methods. Hotplates are calibrated using a flat bare silicon wafer with imbedded thermal sensors. However, actual production wafers carrying deposited films on the surface of the silicon may exhibit small amounts of warpage because of the stresses induced by the deposited films. This warpage may cause the normal gap between the wafer and the hotplate (referred to as the proximity gap), to vary across the wafer from a normal value of approximately 100 μm by as much as a 100 μm deviation from the mean proximity gap (normal value). 
         [0006]    This variability in the proximity gap changes the heat transfer into the wafer causing temperature variations on the wafer surface. These temperature differences in a PEB may result in a change in critical dimension (CD) in that area of several nanometers, which can approach the entire CD variation budget for current leading edge devices, and will exceed the projected CD budget for smaller devices planned for production in the next few years. 
         [0007]    What is needed, therefore, is a method for heating a substrate during the pre- and post-exposure bake processes in a thermal processing system that is tolerant of warpage. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is premised on the realization that in a post exposure bake the topography of the bottom surface of a semiconductor wafer may be measured prior to the post exposure bake process utilizing an inline metrology unit. The topographical data measured by an inline metrology unit may then be conveyed to a control system for a hotplate as the wafer is transferred to the hotplate in a baking unit. Different heating elements in the hotplate are controlled to compensate for the differences in distances from the hotplate surface to the surface of the wafer, as measured in the inline metrology unit. 
         [0009]    By imbedding proximity sensors into a surface, above which the wafer is positioned prior to the baking step, one can measure the gap at a plurality of points between each individual wafer and a reference plane on the surface containing the sensors prior to baking. This may provide individual profiles of the warpage for each wafer. From the profile data, individual temperature offsets to compensate for the proximity gap variation may be calculated from a reference look-up table, and the appropriate adjustments may be made to the individual heating element zones beneath the areas of proximity variation. In other embodiments, the topography measurements may be taken after the baking step and stored. The stored measurements are used to adjust the heat applied to the wafer in subsequent baking steps. 
         [0010]    Because the ramp up of the temperature of a cold wafer is a dynamic event, small adjustments in control set points may stabilize during the ramp event. By this method, each wafer will see a customized heating event matching the physical shape of that individual wafer. The two primary advantages of this approach are individual wafer physical measurement for custom compensation, and high speed on the fly correction with no loss in production. This same method may be used in other similar wafer heating processes such as the post apply bake. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
           [0012]      FIG. 1  is a flow diagram depicting an exemplary processing sequence for a semiconducting wafer. 
           [0013]      FIG. 2  is a diagrammatic view of a thermal processing apparatus including an inline metrology unit and a baking unit. 
           [0014]      FIG. 3A  is an isometric view showing a wafer with an alignment notch. 
           [0015]      FIG. 3B  is a top view of the wafer in  FIG. 3A . 
           [0016]      FIG. 4  is a top plan view of an inline metrology unit of the thermal processing apparatus of  FIG. 2 . 
           [0017]      FIG. 5  is a cross-sectional view taken generally along line  5 - 5  in  FIG. 4 . 
           [0018]      FIG. 6  is an isometric view showing detail of the base of the inline metrology unit of  FIGS. 4 and 5 . 
           [0019]      FIG. 7  is a diagrammatic top view of the base of  FIG. 6 . 
           [0020]      FIG. 8  is a top plan view of a baking unit of the thermal processing apparatus of  FIG. 2 . 
           [0021]      FIG. 9  is a cross-sectional view taken generally along line  9 - 9  in  FIG. 8 . 
           [0022]      FIG. 10  is a detailed view of a portion of  FIG. 8 . 
           [0023]      FIG. 11  is an isometric view showing detail of the base of the baking unit of  FIGS. 8 and 9 . 
           [0024]      FIG. 12  is a diagrammatic top view of the base of the baking unit showing a representative arrangement for the heating elements. 
           [0025]      FIG. 13  is a top view of a base in an inline metrology unit having an alternative arrangement for the proximity sensors. 
           [0026]      FIG. 14  is a top view of a base of a baking unit with heating elements arranged in correspondence with the proximity sensors in  FIG. 13 . 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Photolithography processes for manufacturing semiconductor devices are divided into a series of lithography sequences. Each sequence may add a layer on to what may eventually become a multi-layer device. There are many different options that may be contained in any particular sequence.  FIG. 1  shows an exemplary lithography sequence that may be used in conjunction with the present invention. 
         [0028]    Referring now to  FIG. 1 , a wafer is loaded on to a coating/developing unit, also known as a Track tool, for processing. In block  10 , the wafer may first be transferred to a vapor prime module on the Track tool where the wafer receives a pre-treatment to a surface to improve adhesion of a photoresist coating that will be applied to the wafer. In block  11 , the wafer may then be transferred to a cool plate to stabilize the wafer temperature prior to coating. In block  12 , once stabilized, the wafer may be transferred to a spin coating device where the wafer is coated by spinning the wafer while dispensing a liquid photoresist or other material to an extremely uniform thin film. In block  13 , the wafer may again be transferred to a cool plate to stabilize the temperature of the wafer prior to baking. 
         [0029]    In block  14 , the wafer may then be baked in a post apply bake step where the wafer is baked to drive out any residual solvents, leaving a photosensitive polymer film. In block  15 , after the bake, the wafer may be transfer to a cool plate to cool and stabilize the temperature of the wafer after the bake. In block  16 , in some lithography sequences, a second film coating for a top coat or an anti-reflective film coat may be applied. If the second layer is applied, blocks  11 - 15  are repeated in the sequence shown in  FIG. 1 . 
         [0030]    In block  17 , at the completion of the coating process, in some lithography sequences and in accordance with an embodiment of the present invention, the wafer may be evaluated for conformance to parameters. Conventional metrology tools perform their measurements “offline” that is, they are separate units and the production of the wafer must be interrupted for the measurements to be taken, which may introduce delays into the process. For example, a bare wafer thickness measurement may be taken using a metrology unit, preferably in-line with the rest of the process to minimize the wafer being loaded and unloaded from the Track tool. The in-line metrology unit (i.e. IM unit) may provide the thickness measurement to confirm the photoresist film quality, or the IM unit may perform an analysis for patterned wafer defect (macro inspection). In block  18 , after the evaluation of the wafer, the wafer may then leave the Track tool and be transferred to a scanner for pattern exposure. After exposure, the wafer may then be transferred back to the Track tool. 
         [0031]    In block  19 , the wafer may then be transferred to a baking unit for a post exposure bake. This is one of the more critical bake activities, which is sensitive to temperature non-uniformities. The bake activates the chemistry in the exposed regions of the photoresist. In block  20 , the wafer may then be transferred after the bake, to a cool plate to cool and stabilize the temperature of the wafer. In block  21 , once stabilized, the wafer may then be transferred to a develop unit where the exposed pattern regions are rinsed away, typically with an alkaline fluid, followed by a water rinse. In block  22 , the wafer may then be transferred to a bake unit where a post develop bake, or “hard” bake is performed to stabilize the patterned film for resistance to subsequent etching or implant processing. 
         [0032]    In block  23 , the wafer may be transferred after the hard bake to a cool plate to cool and stabilize the temperature of the wafer. In block  24 , after cooling and in some lithography sequences, a metrology unit, in-line or off line, may be used for an After Develop Inspection (ADI) or an Optical Digital Profilometry (ODP) critical dimension measurement. After these measurements, the wafer leaves the Track tool. 
         [0033]    Because of the critical nature of the Post Exposure Bake (PEB) step in a lithography sequence, an embodiment of a thermal processing apparatus  30  may be included in the lithography sequence at the first metrology measurement as indicated above. The thermal processing apparatus  30  may include an inline metrology unit  40  (i.e., IM unit) in combination with a baking unit  80  used for the PEB as shown in  FIG. 2 . The topography measurements from the inline metrology unit  40  transfer to a control unit  122 , which may operate baking unit  80 . 
         [0034]    With reference to  FIG. 2  and as discussed above, the wafer  70  is processed in the thermal processing apparatus  30 . The wafer  70  is initially transferred to the IM unit  40 , as part of the sequence for processing the wafer  70  as discussed above. A series of proximity sensors obtain a plurality of distance measurements in the IM unit  40 . These measured distances may then be stored in the control unit  122 , or in other embodiments, may be transferred and stored offline. The measured distances may then be used by the control unit  122  to activate and control the heating elements  120  of a hotplate  90  contained in a baking unit  80  as the wafer  70  is being transferred to the baking unit  80 . The control unit  122  controls the power supplied to the heating elements  120  to make adjustments to temperatures of the elements based on the measured distances to account for non-uniformities in the wafer  70  and to provide for a uniform heating of the wafer  70 . 
         [0035]    With reference now to  FIGS. 4-7 , the inline metrology unit  40  of the thermal processing apparatus  30  may comprise a series of outer walls  62  which house a cylinder  52 , common base and support arm  56 , a base  60 , and a horizontal support wall  63 . The base  60  is positioned in a circular cut out in the horizontal support wall  63  and is further supported by a horizontal supporting member  61 . An opening  68  in the outer walls  62  allow for the wafer  70  to be transferred to and from the inline metrology unit  40 . 
         [0036]    The base  60  includes through holes  50  that align with lift pins  48 . The lift pins  48  extend from the common base and support arm  56 . The common base and support arm  56  are connected to, and supported by, a rod  54  of a vertical cylinder  52 . When the rod  54  is actuated to protrude from the cylinder  52 , the lift pins  48  protrude from the base  60 , thereby lifting the wafer  70 . Likewise, when the rod  54  is retracted into the cylinder  52  the lift pins  48  recede into the through holes  50  lowering the wafer toward a top surface  60   a  of the base  60 . Projections  64  on the top surface  60   a  of the base  60  accurately position the wafer  70 . In addition to the projections  64  and as best show in  FIGS. 3A ,  3 B, the wafer  70  may contain notch  70   n  that may be used to position the wafer in the inline metrology unit providing an orientation reference for the distance measurements. The top surface  60   a  also includes a plurality of smaller projections (“proximity pins”)  58  adapted to support the semi-conductor wafer  70  from its bottom surface  70   b  so that the bottom surface  70   b  of a wafer  70  does not contact the top surface  60   a  of the base  60  of the inline metrology unit  40 . 
         [0037]    The top surface  60   a  of the base  60  includes a plurality of proximity sensors  42 . The number and location of the proximity sensors  42  may be determined by the configuration of the hotplate  90  in the baking unit  80 . A sufficient number of proximity sensors  42  are utilized to provide sufficient data to control heating elements  120  in hotplate  90 . Accordingly, the number of sensors  42  scales with the number of heating elements  120 . In an embodiment in which the hotplate  90  has a series of concentric heating elements  120 , each of the heating elements  120  may be monitored by at least 3 sensors  42  ( FIG. 7 ) and these sensors  42  may be located at the same distance from a center point corresponding to one of the concentric heating elements  120  ( FIG. 11 ). A variety of different types of proximity sensors  42  may be used including but not limited to infrared, acoustic, inductive, eddy current, and capacitive type proximity sensors, as well as laser interferometers. 
         [0038]    The proximity sensors  42  are configured to determine the distances from a reference plane to the bottom surface  70   b  of the semiconductor wafer  70 . The distance measurements obtained in the inline metrology unit  40  may be stored in control unit  122  for later use to control the hotplate  90  in the baking unit  80 . 
         [0039]    In addition to measuring the distances, in some embodiments, the IM unit  40  may also make other measurements to evaluate processing properties of the wafer  70 . The IM unit  40  may provide a thickness measurement to confirm the photoresist film quality or the IM unit  40  may perform an analysis for patterned wafer defect (macro inspection). These measurements may be made simultaneously with the distance measurements using measuring device(s)  69  and are conventionally performed by making measurements on a top side of the wafer  70 . 
         [0040]    Wafer  70  may then be transferred to the other intervening modules for processing as illustrated in  FIG. 1  and described above. Wafer  70  may then be transferred to a post exposure bake unit (“baking unit”)  80 . As discussed above, the post exposure bake activates the chemistry in the exposed regions of the photoresist. The topography data, which was measured in the inline metrology unit  40  and stored in the control unit  122 , may be retrieved prior to the wafer arriving at the baking unit  80 . In alternative embodiments, the topography data may be stored offline and delivered to the control unit  122  concurrently with the arrival of the wafer  70  at the baking unit  80 . The topography data from the inline metrology unit  40  may be used to control the temperatures of heating elements  120  of the hotplate  90  to compensate for differences in distances of various points between the hotplate  90  and wafer  70 . 
         [0041]    With reference to  FIGS. 8-11 , the baking unit  80  of the thermal processing apparatus  30  heats wafers  70  to temperatures above room temperature. Each baking unit  80  may include a processing chamber  82 , a hotplate  90 , and at least one resistance heater forming the heating elements  120  embedded in the hotplate  90 . In some embodiments, the heating elements may be arranged in a concentric ring fashion as best seen in  FIG. 11 . 
         [0042]    The hotplate  90  has a plurality of through-holes  108  and a plurality of lift pins  106  inserted into the through-holes  108 . The lift pins  106  are connected to and supported by an arm  104 , which is further connected to and supported by a rod  102  of a vertical cylinder  100 . When the rod  102  is actuated to protrude from the cylinder  100 , the lift pins  106  protrude from the hotplate  90 , thereby lifting the wafer  70 . 
         [0043]    The upper surface of the hotplate  90  includes projections  118 , which facilitate accurate positioning of the wafer  70 . In addition to the projections  118 , the notch  70   n  (FIGS.  3 A and  3 B) in the wafer may be used to position the wafer such that the distance measurements obtained in the inline metrology unit  40  correspond to the heating elements  120 ,  120 ′,  120 ″ of hotplate  90 . Proximity pins  116 , which are located on the upper surface of the hotplate  90 , support wafer  70  above hotplate  90 . When the wafer  70  is delivered to the hotplate  90 , the proximity pins  116  contact the bottom surface  70   b  of the wafer  70  and elevate the wafer  70  above the hotplate  90  forming a gap. The gap is sufficient to expose the bottom surface  70   b  of the wafer  70  to the elevated temperatures produced by the hotplate  90  and prevent the bottom surface  70   b  of the wafer  70  from contacting the hotplate  90  to prevent contamination and strain. 
         [0044]    As discussed above, the wafers  70  carry a layer of processable material, such as photoresist. The layer contains a substance that is volatized and released at the process temperature. This volatile substance evaporates off of the wafer  70  when the layer is exposed to the heat energy produced by the hotplate  90  at temperatures sufficient to heat the wafer  70  to process temperatures. An exhaust port  98   a  at the center of the lid  98  communicates with an exhaust pipe  99 . The waste product generated from the surface of the wafer during the heat treatment is exhausted through the exhaust port  99   a  and vented from the processing chamber  82  via exhaust pipe  99  to an evacuation unit (not shown). 
         [0045]    The temperature of each heating element  120 ,  120 ′,  120 ″ of hotplate  90  ( FIG. 11 ) is established by control unit  122 . The control unit  122  utilizes the measured distances determined by proximity sensors  42  in the inline metrology unit  40  to establish set temperatures by adjusting the power for the individual heating element  120 ,  120 ′,  120 ″ to uniformly heat a wafer  70  during processing. Heating elements  120 ,  120 ′,  120 ″ selectively adjust areas on the hotplate  90  to compensate for differences in the measured distances from various points of the semi-conductor wafer  70  to the hotplate. 
         [0046]    The temperature required for each heating element to uniformly heat the bottom surface  70   b  of semi-conductor wafer  70  can be determined empirically by testing the hot plate  90  using sensors located at various distances from the surface of the hot plate  90  and storing this data. Alternatively, this can be determined utilizing the following algorithm: 
         [0000]    
       
         
           
             
               ρ 
                
               
                   
               
                
               
                 C 
                 p 
               
                
               L 
                
               
                 
                    
                   T 
                 
                 
                    
                   t 
                 
               
             
             = 
             
               
                 
                   
                     k 
                     air 
                   
                   δ 
                 
                  
                 
                   ( 
                   
                     T 
                     - 
                     
                       T 
                       plate 
                     
                   
                   ) 
                 
               
               - 
               
                 h 
                  
                 
                   ( 
                   
                     T 
                     - 
                     
                       T 
                       ambient 
                     
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    where ρ is the density of silicon; C p  is the heat capacity of silicon; L is the thickness of the wafer; T is the temperature of the resist-coated wafer, K air  is the thermal conductivity of air, δ is the thickness of the gap between the hot plate  90  and the wafer  70 ; and h is a coefficient for heat lost from the top surface of the wafer to the surroundings. Thus, the control unit  122  can either utilize stored empirical data or the algorithm in order to determine the set point for each heating element  120  on the hotplate  90 . 
         [0047]    With reference to  FIGS. 13 and 14  and in an alternative embodiment, a hotplate  90 ′ may include a plurality of heating elements  126 ,  126 ′,  126 ″,  126 ′″ with each one of these elements having a plurality of individual segments. The number of individual segments in each of the heating elements  126 ,  126 ′,  126 ″,  126 ′″ may increase with increasing radius. Each of the heating element segments  126 ,  126 ′,  126 ″,  126 ′″ may be selectively activated by the control unit  122  responsive to distances measured from the proximity sensors  42  located in the top surface  60   a ′ of the base  60 ′ of the inline metrology unit  40 . To provide the measured distances needed to control these heating element segments  126 ,  126 ′,  126 ″,  126 ′″, the top surface  60   a ′ may include at least one proximity sensor  42  for each of the heating element segments  126 ,  126 ′,  126 ,  126 ′″. The particular arrangement of the heating element segments  126 ,  126 ′,  126 ″,  126 ′″ may vary depending upon the desired application. 
         [0048]    Adjusting the heating elements  120  of the hotplate  90  allows the baking unit  80  to uniformly heat the wafer  70  compensating for irregularities or warpage of the wafer  70 . Heating elements  120  may be adjusted for a wafer  70  prior to the wafer  70  arriving at the baking unit  80  so that the time required for the hotplate  90  to come to the proper temperature for each wafer  70  being processed may be minimized. Making topography measurements of the wafer  70  in the inline metrology unit  40  maintains efficiency, as the wafer  70  does not need to leave and be returned to the Track tool during processing. This invention addresses the uneven heating problem due to variability in the proximity gap with the prior art, while maintaining efficient processing of the wafer  70 . 
         [0049]    While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicants&#39; general inventive concept.