Abstract:
A method of forming a resist layer of uniform thickness across a surface patterned with a varying density of high aspect ratio features. A selected material layer having an affinity to a resist coat to be applied over the selected material layer is applied to a wafer having a plurality of recesses before applying a resist coat. After the resist coat is applied over the selected material layer, the selected material diffuses partially into the resist coat to condition a portion of the resist coat to be insoluble in the presence of a developer which is applied after the resist coat. Those portions of the resist coat into which the selected material layer has not diffused then are removed by a developer leaving a uniform resist coat thickness across the wafer.

Description:
TECHNICAL FIELD 
     The present invention relates, in general, to the fabrication of semiconductor devices and, in particular, to the formation of a resist layer of uniform thickness across a surface patterned with a varying density of high aspect ratio features, such as an array of “deep trenches” formed in the manufacture of DRAM devices. 
     BACKGROUND OF THE INVENTION 
     Dynamic Random Access Memory (DRAM) cells can retain information only temporarily, on the order of milliseconds, even with power continuously applied. Therefore, the cells must be read and refreshed at periodic intervals. Although the storage time may appear to be short, it is actually long enough to allow many memory operations to occur between refresh cycles. The advantages of cost per bit, device density, and flexibility of use (i.e., both read and write operations are possible) have made DRAM cells the most widely used form of semiconductor memory to date. The earliest DRAM cells were three-transistor cells. Today, DRAM cells consisting of only one transistor and one capacitor have been implemented. 
     When forming semiconductor devices such as a DRAM cell having high aspect ratio features (i.e., the depths of recesses in the device surface are many times greater than the widths of the recesses), it is difficult to form a uniform resist layer across the device surface. Conventional “spin coating” methods are unsuccessful because, after application, the coated material undergoes a volume change during solvent evaporation and material densification. The shrinkage, due to volume change, draws material down into the features causing the coated material to be pulled down, thereby thinning the coated material in areas of high pattern density. 
     These non-uniformities are problematic when the resist is subsequently etched to form a recessed barrier or stopping layer of controlled and uniform depth. Such a barrier or stopping layer might be formed, for example, in the fabrication of a DRAM device to define vertical features into each trench in an array, such as to form a buried dopant out-diffusion of controlled depth, or to define the length of junctions and gate electrodes in a vertical transistor. 
     During such processing, variations in the thickness of the starting, top layer result in undesirable cross-wafer variations in recess depth. The amount of process variation within a chip and across a wafer for the resist recess process depends mainly upon: (1) the etch rate profile within a chip and across a wafer, and (2) the resist profile within a chip and across a wafer. 
     Referring to FIGS. 1 and 2, during solvent evaporation and material densification, the shrinkage of a resist  10 , due to volume change, draws material down into a plurality of trenches  12  in a wafer  14 . The result is non-uniform depths with the degree of variation in depth indicated by arrows  16  in FIG.  2 . Without precise and uniform control over vertical depths, device dimensions cannot be controlled accurately across the wafer, or even within the chip. 
     A variety of suggestions have been offered for solving this problem. Viscosity, spin speeds, baking and curing conditions and thickness all have been varied in attempts to improve the uniformity of the coating from the center to the edge of the wafer. However, significant center-to-edge thickness variation and pattern factor variation are still present. 
     Attempts to planarize the resist after baking are known. For example, chemical-mechanical planarization processes such as Chemical-Mechanical Polishing (CMP) have been applied. The results of these attempts have not been fully qualified. Moreover, the addition of a chemical mechanical planarization step adds greatly to production costs. 
     Resist planarization, using exposure methods, also has been proposed. An exposure step requires additional equipment, however, such as a photolithography tool or a flood exposure system. “Photolithography” is a process in which a light source illuminates a pattern and projects the image through a lens assembly onto a semiconductor wafer or substrate. Ultimately, the pattern is etched into the wafer. Such additional equipment results in additional production costs. 
     There remains a need, therefore, for an improved method of forming a resist layer of uniform thickness across a surface patterned with a varying density of high aspect ratio features. It is an object of the present invention to provide a new and improved method of resist filling and planarization for the fabrication of semiconductor devices. It is another object of the present invention to provide a method of resist filling and planarization for the fabrication of semiconductor devices that eliminates, or at least greatly reduces, center-to-edge thickness variations in the resist coating that is applied during the fabrication of the semiconductor devices. It is a further object of the present invention to provide a method of resist filling and planarization for the fabrication of semiconductor devices that does not add greatly to the cost of fabrication of the semiconductor devices. It is still another object of the present invention to provide a method of resist filling and planarization for the fabrication of semiconductor devices that does not require the use of additional, expensive equipment in the fabrication of the semiconductor devices. 
     SUMMARY OF THE INVENTION 
     To meet these and other objects, and in view of its purposes, the present invention provides a method of resist filling and planarization for the fabrication of semiconductor devices. In accordance with the present invention, the method includes the steps of providing a wafer having a plurality of recesses in a surface of the wafer and applying to the surface of the wafer and the surfaces of the recesses a layer of a selected material having an affinity to a resist coat to be applied subsequently over the selected material layer. A method of resist filling and planarization for the fabrication of semiconductor devices, in accordance with the present invention, also includes the steps of applying a resist coat over the selected material layer and allowing the selected material layer to diffuse partially into the resist coat to condition a portion of the resist coat closest to the wafer to be insoluble in the presence of a developer applied subsequently to the resist coat. A method of resist filling and planarization for the fabrication of semiconductor devices, in accordance with the present invention, further includes the step of applying a developer to the resist coat to remove portions of the resist coat spaced from the wafer by the portion of the resist coat into which the selected material layer has diffused and which has been conditioned to be insoluble in the presence of the developer and into which the selected material layer has not diffused and which has not been conditioned to be insoluble in the presence of the developer. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
     FIG. 1 shows the variations in thickness of a resist layer caused by a volume change during solvent evaporation and material densification according to prior art techniques for the fabrication of semiconductor devices; 
     FIG. 2 shows the non-uniform depths of trenches resulting from the variations in thickness of a resist layer caused by a volume change during solvent evaporation and material densification according to prior art techniques for the fabrication of semiconductor devices; 
     FIG. 3 shows the first step of a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention; 
     FIG. 4 shows the second step of a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention; 
     FIG. 5 shows the third step of a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention; 
     FIG. 5A is an enlargement of a portion of FIG. 5; and 
     FIG. 6 shows the fourth step of a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing the method of the present invention in detail, it will be useful to define two basic terms. First, “pH” is a measure of the acidity of a material. The material is considered acidic if the pH is below 7 and is considered basic if the pH is above 7. A material having a pH of 7 is considered neutral. If an acidic chemical species is added to a material, the pH of the material is lowered. In contrast, if a basic chemical species is added to a material, the pH of the material is increased. 
     Second, “diffusion” is a process during which chemical species move from a region of high concentration to a region of low concentration. If a material A contains a higher concentration of chemical species X than does a material B and material A and material B are placed in contact with one another and species X is a species that diffuses, species X can be made to move or diffuse from material A into material B. The amount of species X which diffuses into material B increases with the temperature of materials A and B. Consequently, it is often by application of heat treatment that diffusion can be made to occur in a controlled manner. 
     Referring to FIG. 3, a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention includes the step of providing a wafer  20  having a plurality of recesses  22  in a surface  24  of wafer  20 . Wafer  20  can be the substrate in which deep trenches, namely recesses  22 , are formed. For example, deep trenches are formed in the fabrication of a DRAM device to define the length of junctions and gate electrodes in a vertical transistor. Wafer  20  can be characterized as having high aspect ratio features because the depths of recesses  22  are many times greater than the widths of recesses  22 . 
     Next, a layer  26  of selected catalyzing material, shown by dotted lines, is applied to surface  24  of wafer  20  and the surfaces of recesses  22 . Selected material layer  26  has an affinity to a resist coat  28  to be applied subsequently over selected material layer  26  as shown in FIG.  4 . More specifically, selected material layer  26  contains one or more components that can diffuse into resist coat  28 . 
     Selected material layer  26  can be applied to surface  24  of wafer  20  and the surfaces of recesses  22  in either one step or two or more steps. Selected material layer  26  can be applied to surface  24  of wafer  20  and the surfaces of recesses  22  already containing the one or more components that can diffuse into resist coat  28  or the one or more components that can diffuse into resist coat  28  can be applied to a base layer after the base layer has been applied to surface  24  of wafer  20  and the surfaces of recesses  22 . 
     In a first embodiment of the present invention, selected material layer  26  contains a basic chemical species (i.e., pH greater than 7) which can be, for example, ammonia or an amine-containing film such as silicon nitride or silicon oxynitride. As such, selected material layer  26  contains a chemical species that can diffuse into resist coat  28  to be applied subsequently over selected material layer  26  causing a decrease in the acidity of that region of resist coat  28 . Selected material layer  26  can be applied by depositing a thin film, applying a wet chemical, plasma treatment, or the like. 
     In accordance with the first embodiment of the present invention, resist coat  28  is composed of a polymer that contains an acid-sensitive protecting group functioning such that when the pH is above a certain threshold value, it prevents dissolution of the polymer by a developer, such as tetra-methyl-ammonium-hydroxide (IMAH), but, when the pH is below the certain threshold, it does not protect the polymer from dissolution (i.e., the polymer can dissolve). Contained in resist coat  28  is an acid species, or an acid-generating species that creates acid within the resist coat in response to some treatment such as exposure to light (i.e., a photo-acid generator) or to heat (i.e., a thermal-acid generator such as para-nitro-benzyltosylate). 
     In a second embodiment of the present invention, selected material layer  26  contains an acid chemical species (i.e., pH less than 7) which can be, for example, canphore sulfonic acid, perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid. As such, selected material layer  26  contains a chemical species that can diffuse into resist coat  28  applied subsequently over selected material layer  26  causing an increase in the acidity of that region of resist coat  28 . As with the first embodiment of the present invention, the selected material layer  26  of the second embodiment can be applied by depositing a thin film, applying a wet chemical, plasma treatment, or the like. 
     In accordance with the second embodiment of the present invention, resist coat  28  is composed of a polymer, for example tetramethoxymethylglycoluril, that will cross-link in an acidic environment but will not cross-link in a non-acidic environment. Cross-linking of the polymer causes it to become insoluble in a developer, such as TMAH. 
     Referring to FIG. 4, a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention also includes the steps of applying a resist coat  28  over selected material layer  26  and allowing selected material layer  26  to diffuse partially into resist coat  28  a given distance creating a layer of reacted material as shown by FIG.  5 . In both embodiments of the present invention, selected material layer  26  diffuses partially into resist coat  28  a given distance creating a layer of reacted material in that portion of resist coat  28  closest to wafer  20 . In the first embodiment of the present invention, diffusion of the basic chemical species in selected material layer  26  into resist coat  28  causes an increase in the pH of that portion of resist coat  28  into which selected material layer  26  diffuses (i.e., this portion of resist coat  28  becomes less acidic or even basic) and this reduction in the acid concentration in this region is sufficient to allow the protecting group to render this region insoluble in developer. In the second embodiment of the present invention, diffusion of the acid chemical species in selected material layer  26  into resist coat  28  causes a decrease in the pH of that portion of resist coat  28  into which selected material layer  26  diffuses (i.e., this portion of resist coat  28  becomes more acidic) and this increase in the acid concentration in this region is sufficient to allow cross-linking of the polymers in resist coat  28  to render this region insoluble in developer. 
     By taking advantage of chemically amplified reactions, such as those that occur in Deep Ultraviolet (DUV) photoresists, selected material layer  26  can be very thin and of low concentration. For example, DUV photoresists react with very low concentrations of the selected material layers identified above. In effect, selected material layer  26  “poisons” by diffusion the bottom of resist coat  28 , lowering the development rate of the resist in the layer contacting surface  24  of wafer  20 , thereby developing a resist “footing” shown symbolically on an enlarged scale in FIG.  5 A. The arrows in FIG. 5A illustrate catalyst diffusion from selected material layer  26  into resist coat  28 . 
     Referring to FIG. 6, a method of resist filling and planarization for the fabrication of semiconductor devices according to the present invention further includes the step of removing unreacted portions of resist coat  28 . The portions of resist coat  28  that are removed are those portions of resist coat  28  spaced from wafer  20  and into which selected material layer  26  has not diffused. Such removal leaves a layer of reacted resist coat material  30  having a uniform resist height as represented by arrows  32  in FIG.  6 . Conventional etching techniques, using a liquid developer such as TMAH, can be applied to remove non-catalyzed portions of resist coat  28  into which selected material layer  26  has not diffused. 
     Those portions of resist coat  28  into which selected material layer  26  has diffused, namely the layer of reacted resist coat material  30 , are not etched away. The relatively uniform height  32  of the remaining reacted resist coat material  30  is determined by the diffusion length from surface  24  of wafer  20 . Therefore, reacted resist coat material  30  is substantially planar with respect to surface  24 , independent of the pattern and characteristics of recesses  22  and the liquid flow characteristics and material shrinkage occurring during and after the spin coating application of resist coat  28 . The phrase “substantially planar” means planar with allowance for minor deviations from planarity. 
     Once the uniform reacted resist coat material  30  is formed, processing using normal chemical downstream etching, or other such techniques, can be used to recess the resist to the desired depth. Because all of the resist is later removed from the wafer, there should be little, if any, concern given to contamination of the device from catalyzing selected material layer  26 . 
     Although illustrated and described above with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and ranger of equivalents of the claims and without departing from the spirit of the invention.