Abstract:
Methods for forming accurate, symmetric cross-section spacers of hardmask material on a substrate such as a silicon wafer or quartz substrate, for formation of precise subresolution features useful for forming integrated circuits. The resulting symmetrical hardmask spacers with their symmetric upper portions may be used to accurately etch well-defined, high aspect ratio features in the underlying substrate. Some disclosed methods also enable simultaneous formation of hardmask structures of various dimensions, of both conventional and subresolution size, to enable etching structural features of different sizes in the underlying substrate.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the fabrication of integrated circuits. More specifically, the present invention relates to a method of producing fine geometry hardmasks with a controlled profile and height to enable the fabrication of precise subresolution features for integrated circuits and other structures.  
         [0003]     2. State of the Art  
         [0004]     In fabrication of semiconductor devices including integrated circuitry, such as memory dice, conventional photolithography limits the ability to produce very fine structural features. Currently, photolithography is only capable of producing structural features of about 100 nm in minimum dimension. This inadequacy limits the ability of a manufacturer to produce extremely small structural features for integrated circuits through conventional photolithography processes. A capability to further reduce the dimensions of structural feature size is particularly important to the fabrication of semiconductor memory devices to enable increasing the number of memory cells on such semiconductor memory devices of a given size.  
         [0005]      U.S. Pat. No. 6,514,849 to Hui et al., U.S. Pat. No. 6,537,866 to Shields et al., U.S. Pat. No. 6,455,433 to Chang et al., U.S. Pat. No. 6,110,837 to Linliu et al., U.S. Pat. No. 5,916,821 to Kerber, U.S. Pat. No. 5,776,836 to Sandhu, and U.S. Pat. No. 5,296,410 to Yang attempt to overcome some of the problems associated with conventional photolithography.  
         [0006]     An alternative to using conventional photolithography is a technique called “loose photo patterning.” Generally described, loose photo patterning allows creating smaller mask features than would be possible with conventional photolithography. In loose photo patterning, mask features of conventional size are formed using conventional photolithography and dry etching, followed by coating such features with a layer of material. The layer of material is then removed from the top of the mask feature and the mask feature is subsequently etched away. These side coatings can be used as a hardmask to form so-called “subresolution” structural features, indicating that such structural features are of smaller dimensions than are achievable by using photolithography to form them directly. In other words, such features are smaller than the finest resolution photolithography processes can produce. Furthermore, all subresolution features will be the thickness of the coating used to coat the standard mask feature. Loose photo patterning allows creating mask features as small as 10 nm, which would not be possible with conventional photolithography.  
         [0007]     A method of forming features using conventional loose photo patterning will be better understood with reference to  FIGS. 1A-1D .  FIG. 1A  shows portion  100  of substrate  2 , such as p or n type silicon or other semiconductor substrate material, including a first layer  4  deposited on substrate  2 . First layer  4  is typically a silicon nitride layer approximately 900 Å in thickness. First layer  4  is formed into a selected geometry, as shown in  FIG. 1A , using conventional photolithography and anisotropic etch processing. Referring to  FIG. 1B , a hardmask layer  6  of, for example, 300 Å thick tetraethyloxysilicate (TEOS) silicon dioxide is deposited on first layer  4 . As shown in  FIG. 1C , hardmask layer  6  is anisotropically etched to leave only the portion of hardmask layer  6  covering the sidewalls of first layer  4 . First layer  4  is then completely removed from substrate  2  by a dry or wet etch to form the sidewall spacer hardmask shown in  FIG. 1D  usable for further etching of substrate  2  to define selected structural feature patterns therein.  
         [0008]     While conventional loose photo patterning allows for forming fine geometry hardmask features, it also results in a phenomenon known as “sputtering.” As shown in  FIG. 1D , sputtering occurs when hardmask layer  6  exhibits an asymmetric profile, which results in a poorly defined profile in the etched features of underlying substrate  2 . These asymmetries of hardmask layer  6  produce different etch rates adjacent the inner and outer edges of hardmask layer  6  when the underlying substrate  2  is etched. As the aspect (height or depth to width) ratios of etched features in substrate  2  increase, the phenomenon of sputtering is aggravated and it becomes more important for the profile of hardmask layer  6  to be symmetric and, preferably, rectangular.  
         [0009]     Therefore, due to the limits of conventional photolithography and loose photo patterning it is desirable to develop a method which results in hardmask elements with an accurately controlled profile and height, enabling the semiconductor device fabricator to achieve an accurately etched profile in a substrate underlying the hardmask.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The present invention, in a number of embodiments, includes methods for forming accurate, symmetric cross-section hardmask elements on an underlying substrate to enable the fabrication by etching of precise structural features in the substrate and resulting end products incorporating such features. The resulting hardmask elements may be used to accurately etch well-defined, high aspect ratio features in the substrate free of sputtering defects.  
         [0011]     In one exemplary embodiment, a method for forming hardmasks on a substrate is provided. First, a substrate is provided. Next, a first layer of a material may be formed on the substrate. Following formation of the first layer, a second layer may be formed on the exposed surface of the first layer. Next, a plurality of discrete structures may be formed on the substrate by masking, patterning and etching the first and second layers to a geometry defined by an exposed surface of the second layer and substantially vertical sidewalls extending upward from the substrate to the exposed surface. Following formation of the plurality of structures, a hardmask layer made may be deposited over the substrate and the plurality of discrete structures. Portions of the hardmask layer adhered to the exposed surface of the second layer may then be removed by an etching process, which also removes the portions of the hardmask layer on the substrate between the discrete structures, while the portions of the hardmask layer flanking the discrete structures remain as spacers, exhibiting an asymmetric profile. The discrete structures may then be planarized by an abrasive process such as chemical mechanical planarization (CMP) to remove the entire second layer and the laterally adjacent, uppermost, asymmetric ends of the remaining hardmask portion spacers. Following planarization, the first layer may be removed by a selective etching process, leaving only the remaining spacers formed as portions of the sidewalls of the original hardmask layer, providing well-defined, symmetrical hardmask elements for etching of the underlying substrate. The method of the above exemplary embodiment produces hardmask features having a well-defined, symmetric cross-section.  
         [0012]     In another exemplary embodiment, a method for forming hardmask elements of various sizes on a substrate is disclosed. First, a substrate is provided. A first layer of a material may then be formed on the substrate. Following formation of the first layer, a second layer may be deposited on the exposed surface of the first layer. A plurality of discrete structures may then be formed by masking, patterning and etching the first and second layers to a geometry defined by an exposed surface of the second layer and substantially vertical sidewalls extending from the substrate to the exposed surface. Following formation of the plurality of structures, a hardmask layer may be deposited over the substrate and the plurality of discrete structures. The portion of the hardmask layer adhered to the exposed surface of the second layer may then be removed by an etching process, which also removes the portions of the hardmask layer between the discrete structures, while portions of the hardmask layer flanking the structures remain as spacers which exhibit an asymmetric profile. The discrete structures may then be planarized by an abrasive process such as CMP to remove the entire second layer and the laterally adjacent, uppermost ends of the remaining hardmask portion spacers. Following planarization, the exposed upper surface of the first layer of material of at least one of the structures may be protected and the unprotected portions of the first layer of the structures removed by an etching process. The method of the above exemplary embodiment produces hardmask features of various sizes having a well-defined, symmetric cross-section.  
         [0013]     In yet another exemplary embodiment, a method for forming hardmasks of various sizes on a substrate is disclosed. A substrate is provided and a first layer of a material may be formed on the substrate. Following formation of the first layer, a second layer may be deposited on the exposed surface of the first layer. After formation of the second layer, a third layer of a material may be formed on the exposed surface of the second layer. A plurality of discrete structures may then be formed by masking, patterning and etching the first layer, the second layer, and the third layer to a geometry defined by an exposed surface of the third layer and substantially vertical sidewalls extending from the substrate to the exposed surface. After formation of the plurality of discrete structures, the exposed upper surface of at least one of the plurality of discrete structures may be protected. A hardmask layer may be deposited over the substrate and the plurality of discrete structures. Following deposition of the hardmask layer, the portions of the hardmask layer on the substrate and on the upper surfaces of the discrete structures may be removed by an etching process, the etching also removing the third layer and a portion of the second layer on any unprotected discrete structure. The plurality of discrete structures may be planarized to remove the second layer, the planarization stopping on the first layer of the unprotected discrete structures and the third layer of the at least one protected discrete structure. Finally, the exposed portions of the first layer may be removed by an etching process. The method of the above exemplary embodiment produces hardmask features of various sizes having a well-defined, symmetric cross-section.  
         [0014]     These features, advantages, and alternative aspects of the present invention will be apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0015]     In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:  
         [0016]      FIGS. 1A-1D  are schematic sectional views illustrating a conventional loose photo patterning process.  
         [0017]      FIGS. 2A-2I  are schematic sectional views illustrating an exemplary embodiment of a method of the present invention.  
         [0018]      FIGS. 3A-3D  are schematic sectional views illustrating another exemplary embodiment of a method of the present invention.  
         [0019]      FIGS. 4A-4K  are schematic sectional views illustrating yet another exemplary embodiment of a method of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     In the description which follows, like features and elements in the several embodiments are identified with the same or similar reference numerals for the convenience of the reader.  
         [0021]      FIGS. 2A-2I  illustrate an exemplary embodiment of a method of the present invention suitable for producing hardmask elements having an accurate, symmetric, rectangular cross-section. Referring to  FIG. 2A , substrate  2  is provided made from a suitable semiconductor substrate such as silicon, polysilicon, or a layered semiconductor structure such as a silicon on insulator (SOI) structure, as exemplified by silicon on glass (SOG) and silicon on sapphire (SOS) structures. Substrate  2  may also be a glass material useful for forming reticles, such as soda-lime glass, borosilicate glass, or quartz. Then, as shown in  FIG. 2B , a first layer  4  having upper surface  16  may be formed on and adhered to portion  200  of substrate  2  using techniques such as, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD). First layer  4  may be formed from materials such as silicon nitride. In this exemplary embodiment, first layer  4  may be formed from 500 Å thick silicon nitride.  
         [0022]     Referring to  FIG. 2C , following deposition of first layer  4 , second layer  8  may be deposited on top of and adhered to the upper surface  16  of first layer  4 . Second layer  8  may be formed from TEOS-type silicon dioxide. Second layer  8  may be deposited using techniques such as, for example, CVD, PVD, or ALD. In this exemplary embodiment, second layer  8  may be formed from 500 Å thick TEOS-type silicon dioxide.  
         [0023]     Referring to  FIG. 2D , following deposition of second layer  8 , portion  200  may be covered with a patterned and developed photoresist  12 , etched, and discrete structures in the form of regions  210  formed from etched first and second layers  4 ,  8  using conventional photolithography techniques. Referring to  FIG. 2E , regions  210  are shown after etching and subsequent removal of photoresist  12 . As shown in  FIG. 2E , regions  210  formed of the remaining segments of first layer  4  and second layer  8  may exhibit a geometry defined by substantially vertical sidewalls  24  extending from substrate  2  to exposed upper surface  26  of second layer  8 .  
         [0024]     Referring to  FIG. 2F , following forming of regions  210 , hardmask layer  6  may be formed over regions  210  and exposed portions of substrate  2  between regions  210 . Hardmask layer  6  adheres to regions  210  and intervening portions of substrate  2 . Hardmask layer  6  may be formed from materials such as TEOS-type silicon dioxide, silicon nitride, polysilicon, titanium nitride, aluminum oxide (Al 2 O 3 ), amorphous carbon, or other suitable material, depending on the material of substrate  2 , first layer  4 , second layer  8 , and the intended etch chemistry to be used with hardmask layer  6 . In an exemplary embodiment, hardmask layer  6  may be formed from 300 Å thick TEOS-type silicon dioxide. Hardmask layer  6  may be deposited by a process useful for precisely defining a thickness thereof such as, for example, low pressure chemical vapor deposition (LPCVD) or atomic layer deposition (ALD).  
         [0025]     Referring to  FIG. 2G , following deposition of hardmask layer  6 , hardmask layer  6  and second layer  8  may be anisotropically etched to leave only portions of hardmask layer  6  covering the sides of regions  210  comprised of the remaining portions of first layer  4  and second layer  8  to form spacers  28 . In other words, the portion of hardmask layer  6  adhered to the upper surface  26  of second layer  8  may be removed by etching, which also removes the portions of hardmask layer  6  on substrate  2  between regions  210 . Second layer  8  may not be completely etched through as is shown in  FIG. 2G , leaving a sufficient thickness of second layer  8  remaining to allow spacer  28  to be of stable (constant) thickness at a height at least coincident with the top of first layer  4 .  
         [0026]     Referring to  FIG. 2H , following etching of hardmask layer  6 , upper portions of spacers  28  formed from hardmask layer  6  and all of second layer  8  may be removed by an abrasive planarization process such as CMP. A CMP compound may be selected for the polishing slurry that will not remove first layer  4 , instead stopping on upper surface  16  thereof. In an exemplary embodiment, spacers  28  may be about 2000 Å in height. The remaining spacers  28  formed of hardmask layer  6  exhibit a symmetric, rectangular cross-section.  
         [0027]     Referring to  FIG. 2I , first layer  4  may then be removed using a selective dry or wet etching process, leaving only the spacers  28  formed of hardmask layer  6  on substrate  2  as hardmask elements. Subsequently, substrate  2  may be etched to form wells (shown by the dashed lines) or apertures having a controlled and accurate profile free from sputtering defects using the hardmask elements comprising spacers  28  formed of hardmask layer  6  as an etch mask. Due to the symmetrical cross-section of spacers  28  and the rectangular, squared-off upper portion of the cross-section, the etch rate on each side of spacers  28  is substantially the same, eliminating asymmetry in the pattern etched into substrate  2 . Etched features in substrate  2  using hardmasks of the present invention may have aspect ratios of up to 5:1 or more.  
         [0028]     In another exemplary embodiment of a method of the present invention, hardmask spacers of a symmetric geometry may be formed to various widths according to the design of the desired structural features to be etched in substrate  2 . Referring to  FIG. 3A , a plurality of discrete structures in the form of regions  210  may be formed on substrate  2 . Regions  210  shown in  FIG. 3A  include first layer  4  as previously described, bounded by spacers  28  made from hardmask layer  6 . Regions  210  shown in  FIG. 3A  may be formed as in the previous exemplary embodiment as shown and described with respect to  FIGS. 2A-2H . Referring to  FIG. 3B , a photoresist  30  may be applied to upper surface  32  of first layer  4  and selectively patterned and developed using conventional photolithography techniques to cover upper surfaces  32  of at least one region  210 .  
         [0029]     Referring to  FIG. 3C , following application of resist  30 , first layer  4  may be removed from regions  210  that are not protected with photoresist  30  using an appropriate selective dry or wet etching process. Next, photoresist  30  may then be removed using conventional techniques. Thus, subresolution hardmask elements are formed of spacers  28  where first layer  4  has been removed, while larger, conventionally dimensioned hardmask elements are formed from the regions  210  where first layer  4  remains flanked by spacers  28 , the larger hardmask elements being of only slightly larger dimension than that of each region  210  prior to deposition of hardmask layer  6 . By intentionally undersizing a given region  210  intended to form a larger hardmask element to allow for the added width provided by spacers  28 , the dimensions of larger hardmask elements may be precisely controlled. Subsequently, substrate  2  may be etched to form wells or apertures W of various widths (shown by the broken lines) and having a controlled and accurate profile due to the symmetric profile of the spacers  28  formed of hardmask layer  6 . Thus, both conventionally dimensioned features as well as subresolution-sized features may be formed. Alternatively, by using an etchant suitable for removal of the material of layer  4  as well as substrate  2 , wells or apertures W 1  and W 2  of different depths may be formed, as depicted in  FIG. 3D . As in the previous exemplary embodiment, due to the precise, squared-off cross-section of spacers  28 , the etch rate on each side of spacers  28  as well as of regions  210  having flanking spacers  28  is substantially the same. Etched features in substrate  2  using hardmasks of the present invention may have aspect ratios of up to 5:1 or greater.  
         [0030]      FIGS. 4A-4K  illustrate yet another exemplary embodiment of a method of the present invention suitable for producing hardmask spacers of various widths yet having an accurate, symmetric, rectangular geometry. Referring to  FIG. 4A , substrate  2  is provided of a suitable semiconductor substrate such as silicon, polysilicon, or a layered semiconductor structure such as a silicon on insulator (SOD) structure, as exemplified by silicon on glass (SOG) and silicon on sapphire (SOS) structures. Substrate  2  may also be a glass useful for forming reticles such as soda-lime glass, borosilicate glass, or quartz. Then, as shown in  FIG. 4B , a first layer  4  having upper surface  16  may be formed on and adhered to portion  400  of substrate  2  using techniques such as CVD, PVD, or ALD. First layer  4  may be formed from materials such as silicon nitride or other suitable material as noted above. In an exemplary embodiment, first layer  4  may be formed from 500 Å thick silicon nitride.  
         [0031]     Referring to  FIG. 4C , following formation of first layer  4 , second layer  8  may be deposited on top of and adhered to the upper surface  16  of first layer  4 . Second layer  8  may be formed from TEOS-type silicon dioxide or other suitable material as noted above. Second layer  8  may be deposited using techniques such as CVD, PVD, or ALD. In an exemplary embodiment, second layer  8  may be formed from 500 Å thick TEOS-type silicon dioxide. Next, referring to  FIG. 4D , third layer  34  having upper surface  37  may be formed from aluminum oxide (Al 2 O 3 ). In an exemplary embodiment, third layer  34  may be formed from 100 Å thick Al 2 O 3 .  
         [0032]     Referring to  FIG. 4E , following deposition of third layer  34 , portion  400  may be covered by a patterned and developed photoresist  36  and anisotropically etched using conventional techniques known to those of ordinary skill in the art to form discrete structures in the form of regions  410 . Referring to  FIG. 4F , regions  410  are shown after etching and removal of photoresist  36 . As shown in  FIG. 4F , regions  410  may exhibit a geometry defined by substantially vertical sidewalls  42  extending from substrate  2  to upper surfaces  40 .  
         [0033]     Referring to  FIG. 4G , following formation of regions  410 , photoresist  38  may be selectively patterned over the upper surface  40  of one or more regions  410 . The regions  410  covered and protected by photoresist  38  will be referred to as regions  420 .  
         [0034]     Referring to  FIG. 4H , hardmask layer  6  may be formed over substrate  2 , regions  410  formed of first layer  4 , second layer  8 , third layer  34  and, where photoresist  38  is present, regions  420 . Hardmask layer  6  covers portions of substrate  2  between regions  410  and  420 . Hardmask layer  6  adheres to first layer  4 , second layer  8 , third layer  34 , photoresist  38  and intervening portions of substrate  2 . Hardmask layer  6  may be formed from materials such as TEOS-type silicon dioxide, silicon nitride, polysilicon, titanium nitride, amorphous carbon, or aluminum oxide (Al 2 O 3 ) depending on the material of substrate  2 , first layer  4 , second layer  8 , third layer  34 , and the intended etch chemistry to be used with hardmask layer  6 . In an exemplary embodiment, hardmask layer  6  may be formed from 300 Å thick TEOS-type silicon dioxide. Hardmask layer  6  may be deposited by a process such as low pressure chemical vapor deposition (LPCVD) or atomic layer deposition (ALD).  
         [0035]     Referring to  FIG. 4I , following deposition of hardmask layer  6 , hardmask layer  6 , third layer  34  and a portion of second layer  8  may be anisotropically etched in regions  410  where resist  38  is not present, leaving only portions of hardmask layer  6  forming spacers  28  covering the sides of first layer  4  and second layer  8 . In the regions  410  where second layer  8  is etched, it is not completely etched. Instead, a sufficient thickness of second layer  8  remains after this etching step to ensure spacers  28  are at a stable (constant) thickness at least to a level coincident with the top of first layer  4 . Furthermore, in the regions  420  where third layer  34  is protected by photoresist  38 , it is not completely removed. Instead, the remaining portion of third layer  34  may be typically about 80 Å thick following etching.  
         [0036]     Referring to  FIG. 4J , following etching of hardmask layer  6 , regions  410  and regions  420  may be planarized using an abrasive process such as CMP. While regions  420  are slightly higher than regions  410 , the CMP process is capable of simultaneously planarizing both regions due to the CMP pad being formed of a yieldable or deformable material. A CMP compound for a slurry may be selected that will stop on first layer  4  in regions  410  and stop on the remaining thickness of third layer  34  in regions  420 . The remaining sidewall portions of hardmask layer  6  forming spacers  28  exhibit a symmetric, rectangular profile.  
         [0037]     Referring to  FIG. 4K , after CMP, first layer  4  may be removed from regions  410  using a selective dry or wet etching process, leaving only the spacers  28  formed of hardmask layer  6  on substrate  2  as subresolution hardmask elements. The material of the remaining portion of third layer  34  is resistant to the etchant used to remove first layer  4 . Therefore, third layer  34  protects underlying first layer  4  and second layer  8  in regions  420  from being removed, leaving a much larger hardmask element extending between spacers  28  bridged by the material of third layer  34 . Subsequently, substrate  2  may be etched to form wells or apertures (shown by the dashed lines) having a controlled and accurate profile due to the symmetric profile of spacers  28  formed of hardmask layer  6 . The etch rate of the material of substrate  2  on each side of spacers  28  is substantially the same. Etched structural features in substrate  2  using hardmask elements of the present invention may have aspect ratios of up to 5:1 or greater. In regions  420  where third layer  34  remains, bridging spacers  28  formed on the sides of third layer  34 , second layer  8  and first layer  4 , hardmask features have a slightly greater dimension than the original photopatterned and etched regions  410  prior to formation of hardmask layer  6  thereover, which dimension may be compensated for by slightly undersizing regions  410  to be used as regions  420 . As a result, the present invention may be used to facilitate simultaneous formation of both conventionally dimensioned and subresolution-dimensioned features in substrate  2 .  
         [0038]     Although the foregoing description contains many specifics, these are not to be construed as limiting the scope of the present invention, but merely as providing certain exemplary embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the invention, as disclosed herein, which fall within the meaning and scope of the claims are encompassed by the present invention.