US20070020939A1 - Controlled geometry hardmask including subresolution elements - Google Patents

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

CROSS-REFERENCE TO RELATED APPLICATION
• This application is a divisional of application Ser. No. 10/883,215, filed Jul. 1, 2004, pending.
BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• 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.
• 2. State of the Art
• 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.
• 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.
• 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 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.
• A method of forming features using conventional loose photo patterning will be better understood with reference toFIGS. 1A-1D.FIG. 1A showsportion100 ofsubstrate2, such as p or n type silicon or other semiconductor substrate material, including afirst layer4 deposited onsubstrate2.First layer4 is typically a silicon nitride layer approximately 900 Å in thickness.First layer4 is formed into a selected geometry, as shown inFIG. 1A, using conventional photolithography and anisotropic etch processing. Referring toFIG. 1B, ahardmask layer6 of, for example, 300 Å thick tetraethyloxysilicate (TEOS) silicon dioxide is deposited onfirst layer4. As shown inFIG. 1C,hardmask layer6 is anisotropically etched to leave only the portion ofhardmask layer6 covering the sidewalls offirst layer4.First layer4 is then completely removed fromsubstrate2 by a dry or wet etch to form the sidewall spacer hardmask shown inFIG. 1D usable for further etching ofsubstrate2 to define selected structural feature patterns therein.
• While conventional loose photo patterning allows for forming fine geometry hardmask features, it also results in a phenomenon known as “sputtering.” As shown inFIG. 1D, sputtering occurs whenhardmask layer6 exhibits an asymmetric profile, which results in a poorly defined profile in the etched features ofunderlying substrate2. These asymmetries ofhardmask layer6 produce different etch rates adjacent the inner and outer edges ofhardmask layer6 when theunderlying substrate2 is etched. As the aspect (height or depth to width) ratios of etched features insubstrate2 increase, the phenomenon of sputtering is aggravated and it becomes more important for the profile ofhardmask layer6 to be symmetric and, preferably, rectangular.
• 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
• 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.
• 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.
• 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.
• 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.
• 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
• In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:
• FIGS. 1A-1D are schematic sectional views illustrating a conventional loose photo patterning process.
• FIGS. 2A-2I are schematic sectional views illustrating an exemplary embodiment of a method of the present invention.
• FIGS. 3A-3D are schematic sectional views illustrating another exemplary embodiment of a method of the present invention.
• FIGS. 4A-4K are schematic sectional views illustrating yet another exemplary embodiment of a method of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
• 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.
• 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 toFIG. 2A,substrate2 is provided made from 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.Substrate2 may also be a glass material useful for forming reticles, such as soda-lime glass, borosilicate glass, or quartz. Then, as shown inFIG. 2B, afirst layer4 havingupper surface16 may be formed on and adhered toportion200 ofsubstrate2 using techniques such as, for example, chemical vapor deposition (CVD), physical vapor deposition (PVD), or atomic layer deposition (ALD).First layer4 may be formed from materials such as silicon nitride. In this exemplary embodiment,first layer4 may be formed from 500 Å thick silicon nitride.
• Referring toFIG. 2C, following deposition offirst layer4,second layer8 may be deposited on top of and adhered to theupper surface16 offirst layer4.Second layer8 may be formed from TEOS-type silicon dioxide.Second layer8 may be deposited using techniques such as, for example, CVD, PVD, or ALD. In this exemplary embodiment,second layer8 may be formed from 500 Å thick TEOS-type silicon dioxide.
• Referring toFIG. 2D, following deposition ofsecond layer8,portion200 may be covered with a patterned and developedphotoresist12, etched, and discrete structures in the form ofregions210 formed from etched first andsecond layers4,8 using conventional photolithography techniques. Referring toFIG. 2E,regions210 are shown after etching and subsequent removal ofphotoresist12. As shown inFIG. 2E,regions210 formed of the remaining segments offirst layer4 andsecond layer8 may exhibit a geometry defined by substantiallyvertical sidewalls24 extending fromsubstrate2 to exposedupper surface26 ofsecond layer8.
• Referring toFIG. 2F, following forming ofregions210,hardmask layer6 may be formed overregions210 and exposed portions ofsubstrate2 betweenregions210.Hardmask layer6 adheres toregions210 and intervening portions ofsubstrate2.Hardmask layer6 may be formed from materials such as TEOS-type silicon dioxide, silicon nitride, polysilicon, titanium nitride, aluminum oxide (Al₂O₃), amorphous carbon, or other suitable material, depending on the material ofsubstrate2,first layer4,second layer8, and the intended etch chemistry to be used withhardmask layer6. In an exemplary embodiment,hardmask layer6 may be formed from 300 Å thick TEOS-type silicon dioxide.Hardmask layer6 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).
• Referring toFIG. 2G, following deposition ofhardmask layer6,hardmask layer6 andsecond layer8 may be anisotropically etched to leave only portions ofhardmask layer6 covering the sides ofregions210 comprised of the remaining portions offirst layer4 andsecond layer8 to form spacers28. In other words, the portion ofhardmask layer6 adhered to theupper surface26 ofsecond layer8 may be removed by etching, which also removes the portions ofhardmask layer6 onsubstrate2 betweenregions210.Second layer8 may not be completely etched through as is shown inFIG. 2G, leaving a sufficient thickness ofsecond layer8 remaining to allow spacer28 to be of stable (constant) thickness at a height at least coincident with the top offirst layer4.
• Referring toFIG. 2H, following etching ofhardmask layer6, upper portions of spacers28 formed fromhardmask layer6 and all ofsecond layer8 may be removed by an abrasive planarization process such as CMP. A CMP compound may be selected for the polishing slurry that will not removefirst layer4, instead stopping onupper surface16 thereof. In an exemplary embodiment, spacers28 may be about 2000 Å in height. The remaining spacers28 formed ofhardmask layer6 exhibit a symmetric, rectangular cross-section.
• Referring toFIG. 21,first layer4 may then be removed using a selective dry or wet etching process, leaving only the spacers28 formed ofhardmask layer6 onsubstrate2 as hardmask elements. Subsequently,substrate2 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 spacers28 formed ofhardmask layer6 as an etch mask. Due to the symmetrical cross-section of spacers28 and the rectangular, squared-off upper portion of the cross-section, the etch rate on each side of spacers28 is substantially the same, eliminating asymmetry in the pattern etched intosubstrate2. Etched features insubstrate2 using hardmasks of the present invention may have aspect ratios of up to 5:1 or more.
• 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 insubstrate2. Referring toFIG. 3A, a plurality of discrete structures in the form ofregions210 may be formed onsubstrate2.Regions210 shown inFIG. 3A includefirst layer4 as previously described, bounded by spacers28 made fromhardmask layer6.Regions210 shown inFIG. 3A may be formed as in the previous exemplary embodiment as shown and described with respect toFIGS. 2A-2H. Referring toFIG. 3B, aphotoresist30 may be applied toupper surface32 offirst layer4 and selectively patterned and developed using conventional photolithography techniques to coverupper surfaces32 of at least oneregion210.
• Referring toFIG. 3C, following application of resist30,first layer4 may be removed fromregions210 that are not protected withphotoresist30 using an appropriate selective dry or wet etching process. Next,photoresist30 may then be removed using conventional techniques. Thus, subresolution hardmask elements are formed of spacers28 wherefirst layer4 has been removed, while larger, conventionally dimensioned hardmask elements are formed from theregions210 wherefirst layer4 remains flanked by spacers28, the larger hardmask elements being of only slightly larger dimension than that of eachregion210 prior to deposition ofhardmask layer6. By intentionally undersizing a givenregion210 intended to form a larger hardmask element to allow for the added width provided by spacers28, the dimensions of larger hardmask elements may be precisely controlled. Subsequently,substrate2 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 spacers28 formed ofhardmask layer6. 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 oflayer4 as well assubstrate2, wells or apertures W₁and W₂of different depths may be formed, as depicted inFIG. 3D. As in the previous exemplary embodiment, due to the precise, squared-off cross-section of spacers28, the etch rate on each side of spacers28 as well as ofregions210 having flanking spacers28 is substantially the same. Etched features insubstrate2 using hardmasks of the present invention may have aspect ratios of up to 5:1 or greater.
• 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 toFIG. 4A,substrate2 is provided of 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.Substrate2 may also be a glass useful for forming reticles such as soda-lime glass, borosilicate glass, or quartz. Then, as shown inFIG. 4B, afirst layer4 havingupper surface16 may be formed on and adhered toportion400 ofsubstrate2 using techniques such as CVD, PVD, or ALD.First layer4 may be formed from materials such as silicon nitride or other suitable material as noted above. In an exemplary embodiment,first layer4 may be formed from 500 Å thick silicon nitride.
• Referring toFIG. 4C, following formation offirst layer4,second layer8 may be deposited on top of and adhered to theupper surface16 offirst layer4.Second layer8 may be formed from TEOS-type silicon dioxide or other suitable material as noted above.Second layer8 may be deposited using techniques such as CVD, PVD, or ALD. In an exemplary embodiment,second layer8 may be formed from 500 Å thick TEOS-type silicon dioxide. Next, referring toFIG. 4D,third layer34 havingupper surface37 may be formed from aluminum oxide (Al₂O₃). In an exemplary embodiment,third layer34 may be formed from 100 Å thick Al₂O₃.
• Referring toFIG. 4E, following deposition ofthird layer34,portion400 may be covered by a patterned and developedphotoresist36 and anisotropically etched using conventional techniques known to those of ordinary skill in the art to form discrete structures in the form ofregions410. Referring toFIG. 4F,regions410 are shown after etching and removal ofphotoresist36. As shown inFIG. 4F,regions410 may exhibit a geometry defined by substantiallyvertical sidewalls42 extending fromsubstrate2 toupper surfaces40.
• Referring toFIG. 4G, following formation ofregions410,photoresist38 may be selectively patterned over theupper surface40 of one ormore regions410. Theregions410 covered and protected byphotoresist38 will be referred to asregions420.
• Referring toFIG. 4H,hardmask layer6 may be formed oversubstrate2,regions410 formed offirst layer4,second layer8,third layer34 and, wherephotoresist38 is present,regions420.Hardmask layer6 covers portions ofsubstrate2 betweenregions410 and420.Hardmask layer6 adheres tofirst layer4,second layer8,third layer34,photoresist38 and intervening portions ofsubstrate2.Hardmask layer6 may be formed from materials such as TEOS-type silicon dioxide, silicon nitride, polysilicon, titanium nitride, amorphous carbon, or aluminum oxide (Al₂O₃) depending on the material ofsubstrate2,first layer4,second layer8,third layer34, and the intended etch chemistry to be used withhardmask layer6. In an exemplary embodiment,hardmask layer6 may be formed from 300 Å thick TEOS-type silicon dioxide.Hardmask layer6 may be deposited by a process such as low pressure chemical vapor deposition (LPCVD) or atomic layer deposition (ALD).
• Referring toFIG. 4I, following deposition ofhardmask layer6,hardmask layer6,third layer34 and a portion ofsecond layer8 may be anisotropically etched inregions410 where resist38 is not present, leaving only portions ofhardmask layer6 forming spacers28 covering the sides offirst layer4 andsecond layer8. In theregions410 wheresecond layer8 is etched, it is not completely etched. Instead, a sufficient thickness ofsecond layer8 remains after this etching step to ensure spacers28 are at a stable (constant) thickness at least to a level coincident with the top offirst layer4. Furthermore, in theregions420 wherethird layer34 is protected byphotoresist38, it is not completely removed. Instead, the remaining portion ofthird layer34 may be typically about 80 Å thick following etching.
• Referring toFIG. 4J, following etching ofhardmask layer6,regions410 andregions420 may be planarized using an abrasive process such as CMP. Whileregions420 are slightly higher thanregions410, 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 onfirst layer4 inregions410 and stop on the remaining thickness ofthird layer34 inregions420. The remaining sidewall portions ofhardmask layer6 forming spacers28 exhibit a symmetric, rectangular profile.
• Referring toFIG. 4K, after CMP,first layer4 may be removed fromregions410 using a selective dry or wet etching process, leaving only the spacers28 formed ofhardmask layer6 onsubstrate2 as subresolution hardmask elements. The material of the remaining portion ofthird layer34 is resistant to the etchant used to removefirst layer4. Therefore,third layer34 protects underlyingfirst layer4 andsecond layer8 inregions420 from being removed, leaving a much larger hardmask element extending between spacers28 bridged by the material ofthird layer34. Subsequently,substrate2 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 spacers28 formed ofhardmask layer6. The etch rate of the material ofsubstrate2 on each side of spacers28 is substantially the same. Etched structural features insubstrate2 using hardmask elements of the present invention may have aspect ratios of up to 5:1 or greater. Inregions420 wherethird layer34 remains, bridging spacers28 formed on the sides ofthird layer34,second layer8 andfirst layer4, hardmask features have a slightly greater dimension than the original photopatterned and etchedregions410 prior to formation ofhardmask layer6 thereover, which dimension may be compensated for by slightly undersizingregions410 to be used asregions420. As a result, the present invention may be used to facilitate simultaneous formation of both conventionally dimensioned and subresolution-dimensioned features insubstrate2.
• 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.

Claims (8)

1. An article of manufacture comprising:

a substrate; and

at least one subresolution hardmask element including a first side and a second side configured to promote etching of the substrate adjacent each of the first and second sides at substantially the same etch rate.

2. The article of manufacture ofclaim 1, wherein the at least one subresolution hardmask element includes a top surface substantially perpendicular to the first and second sides thereof.

3. The article of manufacture ofclaim 1, wherein the at least one subresolution hardmask element exhibits a rectangular cross-section.

4. An article of manufacture comprising:

a substrate; and

a plurality of hardmask elements, at least some of the hardmask elements being of subresolution dimensions and at least one of the hardmask elements being of a dimension larger than subresolution.

5. The article of manufacture ofclaim 4, wherein the at least some hardmask elements include a first side and a second side configured to promote etching of the substrate adjacent each of the first and second sides at substantially the same etch rate.

6. The article of manufacture ofclaim 4, wherein the at least some hardmask elements exhibit a rectangular cross-section.

7. The article of manufacture ofclaim 4, wherein the at least one hardmask elements comprises at least one layer of material flanked by spacers of subresolution dimension.

8. The article of manufacture ofclaim 7, wherein the at least one layer of material comprises a plurality of layers.

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