Abstract:
In connection with wafer planarization, an apparatus for forming a layer of material having a substantially uniform thickness and substantially parallel first and second major surfaces includes a pair of pressing elements and a stop. Each of the pair of pressing elements has a flat pressing surface. The pressing surfaces are opposed to one another and operable to compress a quantity of the material therebetween. The stop is positioned at least partially between the pressing surfaces and has a thickness substantially equal to the desired uniform thickness of the layer. The stop is positioned to establish a spacing between the flat pressing surfaces that is substantially equal to the thickness of the stop and thereby to the desired uniform thickness of the layer when the pressing elements engage the stop. As a result, engagement of the stop by the pressing surfaces during pressing of the material forms a layer of the material of substantially uniform thickness with substantially parallel major surfaces formed by the flat pressing surfaces. The layer is then used in semiconductor processing to provide a flat surface on a layer of a substrate assembly, thereby enhancing the planarization of the substrate assembly.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to methods and apparatus for forming a uniform layer of material for use in connection with manufacturing a substrate assembly during semiconductor processing, and also the layer itself. The invention also relates to a method of planarizing a semiconductor wafer.  
           [0002]    As used herein, “substrate” refers to the lowest layer of semiconductor material in a semiconductor wafer, and “substrate assembly” refers to a substrate having at least one additional layer with structures formed thereon.  
           [0003]    “Semiconductor flat” refers to a surface of the substrate assembly having a precision flat surface within desired tolerances. A significant aspect of semiconductor processing is planarization, i.e., ensuring that the semiconductor flat and other layers are planar within a predetermined specification.  
           [0004]    Production methods for semiconductors are known. A particular class of methods involves: etching or otherwise forming desired channels or trenches in a substrate assembly surface, applying a dielectric epoxy layer which fills the trenches over the substrate assembly surface, using an apparatus to press the substrate assembly having the epoxy layer to achieve desired surface characteristics (e.g., flatness) on the epoxy layer, and then removing the pressed substrate assembly from the apparatus for further processing. The epoxy may be of a type which is cured with ultraviolet radiation.  
           [0005]    Removing the pressed substrate assembly from the apparatus is difficult, however, because the epoxy begins bonding with the pressing surface. Therefore, according to some methods, the epoxy layer is first covered with a layer of a cover material before the pressing takes place. The cover material is selected to allow easy removal/release of the pressed substrate from the apparatus.  
           [0006]    In addition, the cover or release member must be transparent to the ultraviolet radiation if an epoxy of the type cured by ultraviolet radiation is used to cure the epoxy layer beneath the cover material. It has been previously determined that fluorinated ethylene-propylene (FEP) can be used as the cover material. Some types of FEP are transparent to ultraviolet radiation, and thus do not affect the epoxy curing by ultraviolet radiation passing through the cover.  
           [0007]    The cover material is placed over the epoxy layer before the substrate assembly is pressed, and thus the cover material surface characteristics are transferred to the substrate assembly surface. If the cover material is a uniform layer, which is defined as a layer having parallel major (top and bottom) surfaces that are planar, within predetermined tolerances, the pressing action applied through the cover material will be uniformly transferred to the epoxy layer as desired. As one result, if the cover material is a uniform layer, the substrate assembly surface can be formed to the same flatness as the pressing surface.  
           [0008]    In practice, achieving a sufficiently uniform layer of a cover material such as of FEP has not been achieved utilizing known techniques. Because of the nature of FEP material and the desired thickness of a typical cover (about 0.020 in. thick), the dimensions of a FEP cover are difficult to control.  
           [0009]    For example, in one approach where ultraviolet transmissive FEP has been heated to a temperature below its melting point and pressed between two optical flats during pressing, the major surfaces of the resulting FEP layer end up significantly skewed or out of parallel from one another. As used herein, optical flats are defined as precision pressing surfaces, e.g., surfaces that are flat to within one quarter of a wavelength of light.  
           [0010]    The temperature range for processing the FEP is very narrow. An acceptable temperature is slightly below the melting glass flow transition point, which allows the FEP material to acquire the surface smoothness characteristics of the optical flats. Since high pressures are required to make the FEP surface conform to the optical flats surfaces, at temperatures below the glass transition point (i.e., in the plastic state), maintaining the material at a consistent thickness is very difficult. This difficulty is due to the uncontrolled movement of FEP material from the higher pressure zones to the lower pressure zones at the perimeter of the pressing mechanism. Consequently, the thickness of the layer is no longer satisfactorily uniform.  
           [0011]    When used as a cover layer, this non-uniformity in thickness caused variations in the thickness of the epoxy layer. Consequently, during subsequent semiconductor wafer processing, involving etching through the epoxy layer, undesirable non-uniform etching would occur because thinner portions of the epoxy layer would be etched through first. For example, FEP sheets exhibiting these problems had major surfaces which were flat to within about 30-35 angstroms, but which were only parallel to one another within ±0.010 in., have been obtained using known processes.  
           [0012]    Accordingly, it would be desirable to provide a method and apparatus by which FEP and other materials used as cover layers on a substrate assembly could be produced within desired uniform layer specifications.  
         SUMMARY  
         [0013]    Wafer planarization is enhanced utilizing a layer of material having a substantially uniform thickness and substantially parallel first and second major surfaces. The layer is used in producing a flat on or planarizing a substrate assembly.  
           [0014]    In one embodiment, an apparatus having a substantially uniform thickness and substantially parallel first and second major surfaces includes a pair of pressing elements and a stop. The layer of material formed by the apparatus used in producing a flat on semiconductors. Each of the pair of pressing elements has a flat pressing surface. The pressing surfaces are opposed to one another and operable to compress a quantity of the material therebetween. The stop is positioned at least partially between the pressing surfaces and has a thickness substantially equal to the desired uniform thickness of the layer. The stop is positioned to establish a spacing between the flat pressing surfaces that is substantially equal to the thickness of the stop and thereby to the desired uniform thickness of the layer when the pressing elements engage the stop. As a result, engagement of the stop by the pressing surfaces during pressing of the material forms a layer of the material of substantially uniform thickness with substantially parallel major surfaces formed by the flat pressing surfaces.  
           [0015]    The apparatus can also include a heater that heats the material to a temperature where it flows without melting. Further, the apparatus can include a compression force applicator to move one or both of the pressing surfaces. The compression force applicator can include a plurality of biasing elements.  
           [0016]    The pressing surfaces can be optical flats. The shim can have a plurality of projections extending inwardly from the border portion with overflow material recesses positioned between the projections. The projections can be of a triangular shape.  
           [0017]    In a specific example, the first and second major surfaces of the layer are each within 100 angstroms of being flat. Preferably, in this example, the first and major second surfaces of the layer are at least within 0.000005 in. of being parallel to one another. In this example, a stop portion of the shim is about 0.020 in. thick. The cover layer may also be transparent to ultraviolet radiation.  
           [0018]    According to an exemplary method, a layer is formed by heating material and pressing the material between first and second flat pressing surfaces. A stop is disposed between the first and second pressing surfaces to limit the extent to which the first and second pressing surfaces approach one another during pressing to thereby form a layer of substantially uniform thickness having first and second major surfaces with the first and major second surfaces being formed by the flat pressing surfaces. Thereafter, one of the first and major second surfaces of the formed layer may be applied to a flat surface of a substrate assembly. In this approach, the heating step may include heating the material until the material transitions to a plastic state without melting.  
           [0019]    The formed layer may be applied, for example, over an epoxy layer of a substrate assembly. The assembly may then be pressed by precision optical flats with the flatness of the optical flats being transferred to the epoxy layer through the formed layer. The formed layer in this case prevents the epoxy layer from adhering to the pressing apparatus.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a side view of an apparatus for achieving a uniform thickness of a material to be applied to a substrate.  
         [0021]    [0021]FIG. 2 is a top view of an upper lid of the apparatus of FIG. 1.  
         [0022]    [0022]FIG. 3 is a bottom view of a lower lid of the apparatus of FIG. 1.  
         [0023]    [0023]FIG. 4 is a top view of the shim of the apparatus of FIG. 1.  
         [0024]    [0024]FIG. 5 is a side sectional view of the shim of FIG. 4 along the line V-V.  
         [0025]    [0025]FIG. 6 is a magnified view of an edge portion of the shim sectional view of FIG. 5 showing a tooth portion.  
         [0026]    [0026]FIG. 7 is a side sectional view of the shim of FIG. 4 along the line VII-VII and corresponding to FIG. 6, but showing an open region.  
         [0027]    [0027]FIG. 8 is a side sectional view of an edge portion of the apparatus showing the upper optical flat beginning to press against material applied on the lower optical flat with the shim between the upper and lower optical flats, while being heated in an oven.  
         [0028]    [0028]FIG. 9 is a side sectional view of a portion of the apparatus of FIG. 8 showing the apparatus after pressing is complete with the upper and lower optical flats in contact with the shim and the material within the shim pressed to a uniform thickness.  
         [0029]    [0029]FIG. 10 is a graph of time-temperature profiles showing the temperatures of a heater element, an oven air temperature and a representative FEP material being pressed during a heating process.  
         [0030]    [0030]FIG. 11 is a schematic side view of a substrate assembly with a cover layer applied over an epoxy layer. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0031]    [0031]FIG. 1 shows one form of a press assembly  100  for achieving a desired uniform layer of a material to be applied on a substrate assembly during manufacture. The uniform layer is used in producing a flat on a semiconductor. The assembly  100  includes an upper lid  102 , a lower lid  104 , an upper optical flat  112 , a lower optical flat  114  and a stop which limits the extent to which flats  112 ,  114  approach one another and which may take the form of a shim  118 . In the illustrated embodiment, these components each have a generally circular periphery, and are coaxially aligned with each other. For clarity, the upper lid  102  and upper optical flat  112  are shown spaced from the shim  118 , lower optical flat  114  and lower lid  104 .  
         [0032]    During operation of the assembly  100 , the upper optical flat  112  and the lower optical flat  114  serve as pressing elements that are pressed together under predetermined heating conditions against the shim  118 , thereby pressing material applied on the lower optical flat  114  within the shim  118  to a uniform thickness. As shown in FIG. 1, a lower side of the shim  118  contacts an upper side or pressing surface of the lower optical flat  114 . A lower side of the lower optical flat  114  contacts a supporting surface  108  of the lower lid  104 .  
         [0033]    The shim  118  may be annular or ring-shaped with projections that extend inwardly and space the flats apart to a desired uniform distance when engaged by the flats. The projections may comprise a plurality of spaced apart fingers. In the specific form shown, the fingers comprise tooth points  134  that project inwardly at regularly spaced intervals (FIG. 4) from a border  136 . Alternatively, the shim  118  may take other forms such as being shaped as an ellipse, triangle, square, rectangle or other closed geometrical shape. The tooth points  134  do not span the entire interior of the shim  118  and thus define an open center area or void  144 . Communicating with the open center area  144  are overflow material receiving recesses, pockets or open regions  138  that lie between adjacent tooth points  134 . Material in the open center area  144  is pressed to a desired thickness B, which is equal to the thickness of the tooth points, when the upper optical flat  112  and the lower optical flat  114  are pressed together in a press direction A against the tooth points  134 .  
         [0034]    As described below, excess material is pressed outward from between the upper optical flat  112  and the lower optical flat  114  through the open regions  138 . The excess material flows outward from the open center area  144  through the open regions  138  into areas adjacent the periphery of the first optical flat  112  and the second optical flat  114 .  
         [0035]    The pressing action in the press direction A is achieved through a compression force or pressure applicator. In an illustrated embodiment, the pressing action is achieved using elongated fasteners or bolts  120  that slidably extend through apertures  122  in the upper lid  102  and apertures  124  in the shim  118 , and are threaded into apertures  126  in the lower lid  104 . Threaded ends of the bolts  120  are received in helicoils  132  positioned within the apertures  126 . The bolts  120  are each inserted through one or more biasing elements. In the form shown, the bolts  120  are each inserted through a pair of Belleville washers  128 ,  130  oriented in a stacked back-to-back orientation to create a pressing action when the bolts  120  are tightened. The illustrated assembly  100  is preferably secured together by six such bolts  120  at equally spaced intervals, but for clarity, only two bolts  120  are shown in FIG. 1. Prior to pressing, the upper optical flat  112  may be separated from the shim  118  by, for example, approximately {fraction (3/16)} in.  
         [0036]    The upper optical flat  112  and the lower optical flat  114  are cylindrically shaped and each have at least one precision pressing surface. The pressing surfaces are preferably flat to at least to within 100 angstroms and more preferably flat to at least within 50 angstroms. In a specific example, these optical flats are half-wavelength flats having a flatness of 30-35 angstroms. The optical flats may be made of a quartz material. Although the size of the flats may vary in a specific example, they have a diameter of approximately 9 in. and a thickness of approximately {fraction (11/2)} in. Thus, the upper lid  102 , the lower lid  104 , the shim  118  and the bolts  120  are sized accordingly.  
         [0037]    To prevent damage to the quartz material, the upper lid  102  and the lower lid  104  may have an upper supporting surface  106  and a lower supporting surface  108 , respectively, with beveled ends  110 . The edges  116  of the upper optical flat  112  and the lower optical flat  114  are spaced outward of the beveled ends  110 . As a result, the edges  116  of the upper optical flat  112  and lower optical flat  114  are not directly loaded during pressing. The upper lid  102  and the lower lid  104  may be made of a heat conducting material such as aluminum. The shim  118  may be, for example, made of stainless steel. The Belleville washers  128 ,  130  may also be made of stainless steel and rated at, for example, 150 lbs.  
         [0038]    [0038]FIG. 2 is a top view of the upper lid  102  showing its upper surface.  
         [0039]    [0039]FIG. 2 shows the six equally spaced apertures  122  separated from each other by an angle E (i.e., 60° ). FIG. 2 also shows the relative positions of the upper optical surface  106  and the bevel  110  on the lower surface of the upper lid  102 .  
         [0040]    [0040]FIG. 3 is a bottom view of the lower lid  104  showing its lower surface. Similar to the upper lid  102 , FIG. 3 shows the six equally spaced apertures  126  separated from each other by the angle E, as well as the uniform lower support surface  108  and the bevel  110  on the upper surface of the lower lid  104 . The apertures  126  of the lower lid  104  are fitted with helicoils  132  (not shown), as described above, for receiving threaded ends of the bolts  120 .  
         [0041]    [0041]FIG. 4 is a top view of the illustrated shim  118  showing its upper surface with the border portion  136  from which the inwardly projecting tooth points  134  extend. The six equally spaced apertures  124  shown in this example extend through the border portion or reinforcing section  136 . Each tooth point  134  defines an acute included angle F. Although variable, in the form shown, the angle F is 30°. Apexes of adjacent tooth points  134  are separated from each other by an acute tooth point spacing angle G. In the illustrated embodiment, the angle G, although it may be varied, is 10°, and thus there are 36 tooth points  134  total. There are also 36 open regions  138  interspersed between adjacent pairs of the tooth points  134 . The major surfaces (i.e., the top and the bottom) of the teeth  134  are formed to be parallel with each other within a desired tolerance. In a specific example, this is +0/-0.000005 in. The open central area of the shim, between the apexes of a pair of diametrically opposed tooth points  134 , is sized large enough to result in a uniform sheet of the desired size. For example, a circular central area having a diameter of 8.12 inches, between the apex of a tooth and the apex of a diametrically opposed tooth, may be used to produce a circular sheet of material having the desired uniform thickness and flatness, which is at least eight inches in diameter. The use of pointed teeth for the projections facilitates the flow of material past the projections and minimizes the possibility of non-uniformities in the sheet extending inwardly into the central area from the teeth. Alternatively, the sheet may be made significantly oversized, in which case non-uniformities at the edge of the sheet may be trimmed while still having a sheet of the desired size with the desired uniformity.  
         [0042]    [0042]FIG. 5 is a side sectional view of the shim  118  along the line V-V of FIG. 4. FIG. 6 is a magnified view of a right side portion of the sectional view in region VI of the shim  118  of FIG. 5. FIG. 6 shows the extent by which the tooth points  134  extend inwardly from the border portion  136 . As also shown in FIG. 6, the border portion  136  has a thickness H that is substantially greater than the thickness B of the tooth points  134  extending inwardly from the border portion  136 .  
         [0043]    [0043]FIG. 7 is a sectional view of the shim  118  along the line VII-VII of FIG. 4 on a scale comparable to FIG. 6. FIG. 7 shows the extent of the open regions  138  between adjacent tooth points  134 , as well as the adjacent tooth point  134 ′ in the counterclockwise direction.  
         [0044]    [0044]FIG. 8 is a partial side view of a right end of the upper optical flat  112 , the lower optical flat  114  and the shim  118 . The portion of the shim  118  shown in FIG. 8 is the same as in FIG. 7, i.e., showing one of the open regions  138  and the adjacent tooth point  134 ′. In FIG. 8, a layer  142  of cover material has been deposited on the lower optical flat  114  and over the tooth points  134  of the shim  118 , and the upper optical flat  112  and the lower optical flat  114  are being pressed together in the direction A, while being heated in an oven  300 . As shown in FIG. 8, the layer  142  has an initial thickness C that is about two times thicker than the desired layer thickness B.  
         [0045]    [0045]FIG. 9 is a view similar to FIG. 8, but showing the configuration after the upper optical flat  112  and the lower optical flat  114  have been pressed together until stopped by the shim  118 . As shown in FIG. 9, the layer  142  has been pressed to the thickness B uniformly, and excess material has been forced out from between the upper optical flat  112  and the lower optical flat  114  along the path D through the open regions  138 .  
         [0046]    Assume the layer  142  is to be of FEP, and the desired thickness B of the layer  142  is 0.020 in. To manufacture such a layer, one specific approach is as follows:  
         [0047]    (1) the layer  142  is initially deposited on the lower optical flat  114  within the open center area  144  of the shim  118  to a level about twice the desired thickness B (i.e., the starting thickness of the FEP may be about 0.040 in.);  
         [0048]    (2) the assembly  100  is heated in an oven to cause the layer  142  to flow, but is maintained below the melting point of FEP;  
         [0049]    (3) a spring force in the case applied by the Belleville washers  28 ,  30 , press the upper optical flat  112  and the lower optical flat  114  together, in a controlled manner;  
         [0050]    (4) excess FEP passes outward from between the upper optical flat  112  and the lower optical flat  114  and into the open regions  138 ;  
         [0051]    (5) after the desired thickness B is reached, i.e., when the upper optical flat  112  bears against the shim  118 , the assembly  100  is allowed to cool;  
         [0052]    (6) the excess FEP is then removed;  
         [0053]    (7) the bolts  120  are loosened and the upper optical flat  112  and the shim  118  are raised; and  
         [0054]    (8) the layer  142 , which is a uniform layer having a thickness B, is removed from the lower optical flat  114 .  
         [0055]    Alternatively, only the pressing surfaces, the shim  118  and the layer  142  need to be heated to cause the layer  142  to flow.  
         [0056]    The raw FEP is typically provided in sheets which are normally 0.04 in. thick. These sheets are typically formed using rollers and have significant thickness variations. Also, defects may exist in these sheets, such as bubbles. Typically, the raw material sheets are visually screened, and portions having bubbles or other significant defects that are likely to show up in the finished layer are discarded. However, minor bubbles or defects in the raw material near the expected edges of the finished layer may be allowed to remain as they disappear during pressing and flowing process of making the finished layer.  
         [0057]    [0057]FIG. 10 is one example of a time-temperature profile of various temperatures in a pressing process in which FEP is used as the layer  142 . The curve  150  shows the temperature of a heating element within the oven. The curve  152  shows the air temperature within the oven. The two curves  154  represent the temperature of the FEP as measured by thermocouples  156 ,  158  and  160  at the periphery, center, and halfway between the periphery and the center, respectively, of the lower optical flat  114  (FIG. 1).  
         [0058]    The melting point of the specific FEP of this example is 270 C. It is desirable to heat the FEP until it transitions to a plastic state and begins to flow, but does not melt. At point a, following a soak of approximately 12 hours, the temperature of the layer  142  is stabilized at about 223 C. An extended soak period is used to prevent the possibility of overheating the layer  142  beyond the melting point. It is also desirable to heat the upper optical flat  112  and the lower optical flat  114  evenly, i.e., until the temperatures of the peripheries and the centers of the optical flats are within ½ to 1 C of each other.  
         [0059]    After point a, the temperature of the oven is raised, as shown in the curves  150  and  152 , to increase the temperature of the layer  142  slightly. Thereafter, the layer  142  reaches the temperature at which the FEP flows, and the pressing takes place until stopped by the shim  118 .  
         [0060]    In another example using PTFE as the layer  142 , a time-temperature profile similar to FIG. 10 may be used. The melting point of one specific PTFE is approximately 317 C, and the soak temperature is approximately 270 C. Besides these differences, the process is generally similar to the process described above for the layer  142  made of FEP. Of course, other temperature heating profiles may also be used.  
         [0061]    With the pressing complete, excess material is trimmed from the assembly  100  near the peripheries of the upper optical flat  112  and the lower optical flat  114  such as with a dull knife.  
         [0062]    The pressed uniform layer  142  is then allowed to cool, for example, slowly to avoid thermal shock. In one process, the pressed layer  142  is allowed to cool for approximately 6 hours. Over the course of the cool down period, the layer  142  may shrink by 0.050 to 0.100 in diameter. After the cool down period is concluded, the pressure is released, and the layer  142  is complete. The cover layer may be removed and used in subsequent semiconductor processing.  
         [0063]    [0063]FIG. 11 is a schematic side view of a substrate assembly with a cover layer. As shown in FIG. 11, the uniform layer  142  that has been pressed to uniform thickness has been applied over an epoxy layer  200  of a substrate assembly  202  before the substrate assembly  202  is subsequently pressed and cured with ultraviolet radiation. A pressing apparatus is shown schematically, in a state separated from the substrate assembly  202 , at  206 . The epoxy layer  200  has been applied to fill trenches  204  in the substrate assembly  202 .  
         [0064]    With the layer  142  in place between the pressing apparatus  206  and the epoxy layer  200 , the completed substrate assembly  202  is easily removed from the pressing/curing assembly (if necessary, air can be directed between the layer  142  and the pressing surface of the pressing apparatus  206  to facilitate removal). Because the layer  142  is uniform (the major surfaces are substantially flat and parallel), the precision of the pressing surface of the pressing apparatus  206  is transferred to the epoxy layer  200  of the substrate  202 . One suitable epoxy is DEN 431  Novalak resin mixed with a solvent to achieve a desired consistency.  
         [0065]    Although FEP is a preferred cover material for use as the layer  142 , other plastic materials that can be heated to a plastic state without melting can also be used, with consideration of the other requirements discussed above. One specific FEP is available from McMaster-Carr of Los Angeles, Calif. under the catalog designation 85375K114.  
         [0066]    In the methods and apparatus described above, one of the pressing surfaces remains stationary, whereas the other of the pressing surfaces is moved. Optionally, both pressing surfaces may be moved toward each other, as would be known to those with ordinary skill in the art.  
         [0067]    Having illustrated and described the principles of our invention with reference to several preferred embodiments, it should be apparent to those of ordinary skill in the art that the invention may be modified in arrangement and detail without departing from such principles. We claim as our invention all such modifications that fall within the scope of the following claims.