Patent Publication Number: US-6214737-B1

Title: Simplified sidewall formation for sidewall patterning of sub 100 nm structures

Description:
TECHNICAL FIELD 
     The present invention generally relates to making sub-lithographic structures using sidewall patterning techniques. In particular, the present invention relates to sub 100 nm conductive structures using sidewall patterning techniques. 
     BACKGROUND ART 
     In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This includes the width and spacing of interconnecting lines and the surface geometry such as corners and edges of various features. Since numerous interconnecting lines are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern. 
     The requirement of small features (and close spacing between adjacent features) requires high resolution photolithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the resist, and an exposing source (such as optical light, X-rays, or an electron beam) illuminates selected areas of the surface through an intervening master template, the photomask, for a particular pattern. The lithographic coating is generally a radiation-sensitized coating suitable for receiving a projected image of the subject pattern. Once the image is projected, it is indelibly formed in the coating. The projected image may be either a negative or a positive of the subject pattern. Exposure of the coating through the photomask causes a chemical transformation in the exposed areas of the coating thereby making the image area either more or less soluble (depending on the coating) in a particular solvent developer. The more soluble areas are removed in the developing process to leave the pattern image in the coating as less soluble polymer. 
     Projection lithography is a powerful and essential tool for microelectronics processing. However, lithography is not without limitations. Patterning features having dimensions of about 0.25 μm or less with acceptable resolution is difficult at best, and impossible in some circumstances. Patterning conductive features including metal lines and silicon substances (such as amorphous silicon and polysilicon) with small dimensions is required in order to participate in the continuing trend toward higher device densities. Procedures that increase resolution, improve critical dimension control, and provide small conductive features are therefore desired. 
     SUMMARY OF THE INVENTION 
     The present invention provides methods of forming sub-lithographic features. The present invention also provides sub-lithographic conductive features that are particularly useful for forming metal lines, gate conductors and interconnects. As a result, the present invention effectively addresses the concerns raised by the trend towards the miniaturization of semiconductor devices. 
     In one embodiment, the present invention relates to a method of forming a conductive structure having a width of about 100 nm or less, involving the steps of providing a substrate having a conductive film; patterning a mask over a first portion of the conductive film wherein a second portion of the conductive film is exposed; partially etching the second portion of the conductive film to reduce the thickness of the second portion without completely removing the second portion thereby forming a sidewall in the conductive film; removing the mask; depositing a sidewall film over the conductive film, the sidewall film having a vertical portion adjacent the sidewall of the conductive film and a horizontal portion in areas not adjacent the sidewall of the conductive film; removing the horizontal portion of the sidewall film exposing a third portion of the conductive film; and etching the third portion of the conductive film thereby providing the conductive structure having a width of about 100 nm or less underlying the vertical portion of the sidewall film. 
     In another embodiment, the present invention relates to a method of forming a polysilicon structure, involving the steps of providing a substrate having a polysilicon film thereon; patterning a photoresist over a first portion of the polysilicon film exposing a second portion of the polysilicon film; partially etching the second portion of the polysilicon film to reduce the thickness of the second portion without completely removing the second portion thereby forming a sidewall in the polysilicon film; removing the photoresist; depositing uniformly a sidewall film over the polysilicon film, the sidewall film having a thickness from about 100 Å to about 2,000 Å and a vertical portion adjacent the sidewall of the polysilicon film; directionally etching the sidewall film exposing a third portion of the polysilicon film while substantially retaining the vertical portion of the sidewall film adjacent the sidewall; and etching the third portion of the polysilicon film thereby providing the polysilicon structure underlying the vertical portion of the sidewall film. 
     In yet another embodiment, the present invention relates to a method of forming a thin metal line, involving the steps of providing a substrate having a metal layer thereon; patterning a photoresist over a first portion of the metal layer exposing a second portion of the metal layer; partially etching the second portion of the metal layer to reduce the thickness of the second portion without completely removing the second portion thereby forming a sidewall in the metal layer; removing the photoresist; depositing uniformly a silicon containing film over the metal layer, the silicon containing film having a thickness from about 100 Å to about 2,000 Å and a vertical portion adjacent the sidewall of the metal layer; directionally etching the silicon containing film exposing a third portion of the metal layer while substantially retaining the vertical portion of the silicon containing film adjacent the sidewall; and etching the third portion of the metal layer thereby providing the thin metal line underlying the vertical portion of the silicon containing film. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates in a cross-sectional view the methods of forming a sub-lithographic structure according to several aspects of the present invention. 
     FIG. 2 illustrates in a cross-sectional view the methods of forming a sub-lithographic structure according to several aspects of the present invention. 
     FIG. 3 illustrates in a cross-sectional view the methods of forming a sub-lithographic structure according to several aspects of the present invention. 
     FIG. 4 illustrates in a cross-sectional view the methods of forming a sub-lithographic structure according to several aspects of the present invention. 
     FIG. 5 illustrates in a cross-sectional view the methods of forming a sub-lithographic structure according to several aspects of the present invention. 
    
    
     DISCLOSURE OF INVENTION 
     The present invention involves making sublithographic structures by sidewall patterning of conductive layers using a mask. The present invention more specifically involves methods of forming sub-lithographic features in a simple and efficient manner. 
     A conductive film is provided over a substrate. The substrate is typically a silicon substrate optionally with various elements and/or layers thereover; including metal layers, barrier layers, dielectric layers, device structures, active elements and passive elements including polysilicon gates, wordlines, source regions, drain regions, bit lines, bases, emitters, collectors, conductive lines, conductive plugs, etc. The conductive film is provided over a portion of the substrate or over the entire substrate. 
     The conductive film contains a material that conducts or semiconducts electrical current. Conductive films include metal layers and silicon layers, such as polysilicon and amorphous silicon. Metal layers include layers of a single metal as well as metal alloy layers containing two or more metals. Specific examples of metal layers include aluminum, copper, gold, nickel, palladium, platinum, silver, tantalum, titanium, tungsten, zinc, aluminum-copper alloys, aluminum alloys, copper alloys, titanium alloys, tungsten alloys, titanium-tungsten alloys, gold alloys, nickel alloys, palladium alloys, platinum alloys, silver alloys, tantalum alloys, zinc alloys, metal silicides, and any other alloys thereof. 
     The conductive film may be formed in any suitable manner. The thickness of the conductive film is typically from about 500 Å to about 3,000 Å. In another embodiment, the thickness of the conductive film is from about 750 Å to about 2,500 Å. In another embodiment, the thickness of the conductive film is from about 1,000 Å to about 2,000 Å. 
     A mask is provided over a portion of the conductive film wherein portions of the conductive film are exposed. In this connection, the mask acts as a sidewall template mask. The mask is provided over a portion of the conductive film or over the entire conductive film and etched back so that the mask has at least one sidewall substantially perpendicular to the substrate surface. In most embodiments, the mask has two or more sidewalls substantially perpendicular to the substrate surface. The mask contains a material that is easily formed into a pattern, such as a photoresist. 
     The exposed portions of the conductive film are partially etched wherein at least one step is formed in the conductive film. Dry or wet etching techniques may be employed. In a preferred embodiment, dry etch techniques are used. In one embodiment, the conductive film is partially etched using an anisotropic etching process. In another preferred embodiment, reactive ion etching (RIE) is employed. The specific etchant depends upon the specific identity of the conductive film material. Dry etch techniques typically involve using a plasma containing fluorine or chlorine compounds, such as one or more of BCl 3 , CCl 4 , SiCl 4 , Cl 2 , NF 3 , SF 6 , CH 3 F, CF 4  and CHF 3 . 
     The amount of the conductive film etched or removed varies depending upon the desired height of the resultant sub 100 nm conductive structure. Partial etching removes about 10% to about 75% of the thickness of the exposed portions of the conductive film. In another embodiment, partial etching removes about 20% to about 60% of the thickness of the exposed portions of the conductive film. In yet another embodiment, partial etching removes about 30% to about 50% of the thickness of the exposed portions of the conductive film. Thus, the thickness of the exposed portions of the conductive film after etching is from about 125 Å to about 2,700 Å. The removed portions of the conductive film form one or more steps or sidewalls therein. After the conductive film is partially etched, the conductive film may contain a series of plateau like regions. 
     The mask is stripped or removed from the substrate using any suitable technique. A sidewall film is deposited over all or a portion of the partially etched conductive film. In a preferred embodiment, the sidewall film is deposited over the entire partially etched conductive film. The sidewall film conforms to the profile of the partially etched conductive film; that is, the sidewall film is formed in a substantially uniform thickness over the partially etched conductive film. 
     The sidewall film is selected so that there is etch selectivity between the conductive film and the sidewall film. For conductive films of metal layers and silicon layers, sidewall films of one or more of silicon oxide, silicon oxynitride and silicon nitride are employed. In these embodiments, the sidewall film is a silicon containing film. The sidewall film may be made in any suitable manner, including chemical vapor deposition (CVD) techniques. CVD techniques include low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD). In one embodiment, a silicon nitride film may be made by PECVD or LPCVD employing gases containing silicon and nitrogen, such as using SiH 4  or SiCl 2 H 2  and N 2  or NH 3 . In another embodiment, a silicon oxide film may be made by PECVD or LPCVD employing gases containing silicon and oxygen, such as using SiH 4  or SiCl 2 H 2  and N 2 O or O 2 . In yet another embodiment, a silicon oxynitride film may be made by PECVD or LPCVD employing gases containing silicon, oxygen and nitrogen, such as using SiH 4  and N 2 O and NH 3 . 
     In embodiments where the conductive film is a metal, such as those listed above including aluminum, copper, titanium and tungsten, the sidewall film is preferably silicon oxide or silicon nitride, although silicon oxynitride may be employed as the sidewall film. In embodiments where the conductive film is a silicon layer such as polysilicon or amorphous silicon, the sidewall film is preferably silicon oxynitride, although silicon oxide or silicon nitride may be employed as the sidewall film. 
     The thickness of the sidewall film is selected depending upon the desired width of the resultant sub-lithographic conductive structure. In one embodiment, the sidewall film has a thickness from about 100 Å to about 2,000 Å. In another embodiment, the sidewall film has a thickness from about 200 Å to about 1,500 Å. In yet another embodiment, the sidewall film has a thickness from about 300 Å to about 1,000 Å. In one embodiment, the thickness of the sidewall film is substantially uniform around the partially etched conductive film in that the thickness does not vary by more than about 50 Å comparing the thickest and thinnest areas of the sidewall film. 
     For purposes of this invention, there are two main portions of the sidewall film. The first is a portion of the sidewall film adjacent the sidewall(s) of the partially etched conductive film and is referred to herein as the vertical portion. The second is a portion of the sidewall film that covers the flat surfaces (parallel or substantially parallel to the plane of the flat surface of the substrate) of the partially etched conductive film and is referred to herein as the horizontal portion(s). 
     A directional etch is performed to remove portions of the sidewall film covering the flat surfaces of the partially etched conductive film (the horizontal portions as defined above) while not substantially removing the vertical portion of the partially etched conductive film. Dry or wet etching techniques may be employed. In a preferred embodiment, a dry anisotropic etch process is employed whereby the horizontal portions of the partially etched conductive film are removed. In one embodiment, a RIE technique is employed. In a preferred embodiment, dry etch techniques are used. The specific etchant depends upon the specific identity of the sidewall film material. Dry etch techniques typically involve using a plasma containing one or more fluorine compounds, such as one of NF 3 , SF 6 , CH 3 F, C 4 F 8 , CF 4  and CHF 3 . The etch technique is selected so that the partially etched conductive film is not etched significantly etched or degraded. In this connection, the partially etched conductive film acts as an etch stop layer when performing a directional etch of the sidewall film. 
     While the directional etch removes the horizontal portions of the sidewall film, a small amount of the vertical portion of the sidewall film is also removed. However, in most instances, the small amount of the vertical portion removed substantially corresponds with the thickness of the sidewall film thus leaving a vertical portion of the sidewall film (having a height) equal to or slightly less than the height of the step (amount of thickness of the conductive film etched using the mask). Typically, the height of the vertical portion of the sidewall film is slightly less (from about 1% to about 20% less based upon the height of the step) than the height of the conductive film step. All or substantially all (from about 80% to about 99% of the original height) of the vertical portion of the sidewall film remains positioned adjacent the step or sidewall of the partially etched conductive film. 
     After directional etch of the horizontal portions of the sidewall film, the partially etched conductive film is etched in a directional manner wherein the remaining vertical portion of the sidewall film acts as a mask to the portion of the partially etched conductive film located underneath it. In one embodiment, the partially etched conductive film is etched using an anisotropic etching process. In another preferred embodiment, a RIE is employed. Dry or wet etching techniques may be employed. In a preferred embodiment, dry etch techniques are used. As noted above in connection with the partial etch of the conductive film, the specific etchant depends upon the specific identity of the conductive film material. Dry etch techniques typically involve using a plasma containing fluorine or chlorine compounds, such as one or more of BCl 3 , CCl 4 , SiCl 4 , Cl 2 , NF 3 , SF 6 , CH 3 F, CF 4  and CHF 3 . The etch technique is selected so that the vertical portion of the sidewall film or the underlying substrate is not etched or significantly etched or degraded. 
     Generally speaking, the etch techniques for the directional etch of the horizontal portions of the sidewall film and the directional etch of the partially etched conductive film are selected so that at least about 2:1 etch selectivity is achieved between the sidewall film material and the conductive material. In a preferred embodiment, the etch techniques for the directional etch of the horizontal portions of the sidewall film and the directional etch of the partially etched conductive film are selected so that at least about 3:1 etch selectivity is achieved between the sidewall film material and the conductive material. In a more preferred embodiment, the etch techniques for the directional etch of the horizontal portions of the sidewall film and the directional etch of the partially etched conductive film are selected so that at least about 5:1 etch selectivity is achieved between the sidewall film material and the conductive material. 
     Remaining on the substrate is one or more lines (depending upon the shape of the patterning of the partially etched conductive film) of vertical portions of the sidewall film over unetched portions of the conductive film constituting a sub-lithographic structure or feature. Alternatively, the remaining vertical portion of the sidewall film may be stripped or removed leaving a sub-lithographic structure or feature made of the conductive film material. 
     The dimensions of the conductive film portion of the resulting sub-lithographic feature depend primarily upon the specific identity of the sidewall film, the thickness of the sidewall film and the extent of the directional etch performed on the conductive film. The dimensions of the conductive film portion of the resulting sub-lithographic feature may be from about 100 Å to about 2,000 Å in width and from about 125 Å to about 2,700 Å in height. In another embodiment, the dimensions of the conductive film portion of the resulting sub-lithographic feature may be from about 200 Å to about 1,500 Å in width and from about 200 Å to about 2,200 Å in height. In yet another embodiment, the dimensions of the conductive film portion of the resulting sub-lithographic feature may be from about 300 Å to about 1,000 Å in width and from about 250 Å to about 1,800 Å in height. 
     The resulting sub-lithographic feature made in accordance with the present invention may be useful for subsequent processing of the substrate on which it is positioned. For example, the sub-lithographic feature made in accordance with the present invention may be used for forming a gate conductor (in embodiments where the conductive film contains silicon), for forming thin metal lines, and for forming metal patterns. 
     One embodiment of the methods of the present invention is now described in connection with the Figures. Referring to FIG. 1, a substrate  10  is provided with a conductive film  12  over a portion of the substrate  10 . Although not shown, the substrate  10  may include any suitable semiconductor material (one or more layers of semiconductor materials), for example, a monocrystalline silicon substrate. Also, although not shown, the substrate  10  may include one or more layers including substrate layers, diffusion regions, dielectric layers such as oxides, devices, polysilicon layers, and the like. 
     The conductive film  12  may be deposited over the entire substrate  10  and patterned to cover a desired portion of the substrate  10  using, for instance, photolithography and etching techniques. In this embodiment, the conductive film  12  comprises polysilicon. The thickness of the polysilicon conductive film  12  is about 2,000 Å. A photoresist  14  is deposited and patterned over the polysilicon conductive film  12  using suitable photolithography techniques so that it partially covers the polysilicon conductive film  12 , wherein a portion of the conductive film  12  is exposed (not underneath the photoresist  14 ). 
     Referring to FIG. 2, the exposed portion of the polysilicon conductive film  12  is partially etched and the photoresist  14  is then removed. In this embodiment, about 50% of the thickness of the exposed portion of the polysilicon conductive film  12  is removed creating sidewall or step  16  in the conductive film  12 . The height of sidewall  16  is about 1,000 Å. 
     Referring to FIG. 3, a sidewall film  18  is deposited over the polysilicon conductive film  12  in a suitable manner. In this embodiment, a silicon oxynitride layer is deposited by CVD techniques and the thickness of the silicon oxynitride sidewall film  18  is about 250 Å. The thickness of the sidewall film  18  is substantially uniform on the partially etched polysilicon conductive film  12 . There are two portions of the silicon oxynitride sidewall film  18 ; namely, a portion  18   a  adjacent the sidewall  16  of the conductive film  12  and portions  18   b  covering the flat surfaces of the conductive film  12 . 
     Referring to FIG. 4, a directional etch is performed removing portions of the sidewall film covering the flat surfaces of the polysilicon conductive film  12 . In this embodiment, an anisotropic dry etch using C 4 F 8  is performed for a time sufficient to remove the portions  18   b  of the sidewall film  18  over the flat surfaces of the conductive film  12 , but not so long as to remove substantial portions of portion  18   a  of the sidewall film  18  adjacent the sidewall  16  of the conductive film  12 . 
     Referring to FIG. 5, a directional etch is performed removing exposed portions of the conductive film  12  (not underneath the sidewall film portion  18   a ) by suitable means. For example, a selective etch technique is used to remove the polysilicon conductive film  12  employing an anisotropic dry etch leaving patterned remaining portion  18   a  of the silicon oxynitride sidewall film  18 . The remaining portion  18   a  serves as a mask to prevent etching of the remaining portion of the polysilicon conductive film  12  therebeneath during the directional etch. In this embodiment, a dry etch technique including a fluorocarbon and a chlorocompound, such as using Cl 2  and SF 6 , is employed. The etch technique has selectivity in that the size of the remaining portion  18   a  of the silicon oxynitride sidewall film  18  is not substantially reduced. 
     In this embodiment, the height and width of patterned remaining polysilicon conductive film  12  beneath the remaining portion  18   a  is about 1,000 Å and about 250 Å. The patterned remaining polysilicon conductive film  12  may be used in making gate conductors. The patterned remaining polysilicon conductive film  12  is particulary useful for forming narrow gate conductors which enables the production of semiconductor devices having high densities. 
     Another embodiment of the methods of the present invention is now described in connection with the Figures. Referring again to FIG. 1, a substrate  10  is provided with a conductive film  12  over a portion of the substrate  10 . Although not shown, the substrate  10  may include any suitable semiconductor material (one or more layers of semiconductor materials), for example, a monocrystalline silicon substrate. Also, although not shown, the substrate  10  may include one or more layers including substrate layers, diffusion regions, dielectric layers such as oxides, devices, polysilicon layers, and the like. 
     The conductive film  12  may be deposited over the entire substrate  10  and patterned to cover a desired portion of the substrate  10  using, for instance, photolithography and etching techniques. In this embodiment, the conductive film  12  comprises an aluminum-copper alloy. The thickness of the aluminum-copper alloy conductive film  12  is about 1,500 Å. A photoresist  14  is deposited and patterned over the aluminum-copper alloy conductive film  12  using suitable photolithography techniques so that it partially covers conductive film  12 , wherein a portion of the conductive film  12  is exposed (not underneath the photoresist  14 ). 
     Referring to FIG. 2, the exposed portion of the aluminum-copper alloy conductive film  12  is partially etched and the photoresist  14  is then removed. In this embodiment, about 40% of the thickness of the exposed portion of the conductive film  12  is removed creating sidewall or step  16  in the conductive film  12 . The height of sidewall  16  is about 600 Å. 
     Referring to FIG. 3, a sidewall film  18  is deposited over the aluminum-copper alloy conductive film  12  in a suitable manner. In this embodiment, a silicon nitride layer is deposited by CVD techniques and the thickness of the silicon nitride sidewall film  18  is about 400 Å. The thickness of the silicon nitride sidewall film  18  is substantially uniform on the partially etched aluminum-copper alloy conductive film  12 . There are two portions of the silicon nitride sidewall film  18 ; namely, a portion  18   a  adjacent the sidewall  16  of the conductive film  12  and portions  18   b  covering the flat surfaces of the conductive film  12 . 
     Referring to FIG. 4, a directional etch is performed removing portions of the sidewall film covering the flat surfaces of the aluminum-copper alloy conductive film  12 . In this embodiment, an anisotropic dry etch using CHF 3  optionally with O 2  is performed for a time sufficient to remove the portions  18   b  of the silicon nitride sidewall film  18  over the flat surfaces of the aluminum-copper alloy conductive film  12 , but not so long as to remove substantial portions of portion  18   a  of the silicon nitride sidewall film  18  adjacent the sidewall  16  of the conductive film  12 . 
     Referring to FIG. 5, a directional etch is performed removing exposed portions of the aluminum-copper alloy conductive film  12  (not underneath the sidewall film portion  18   a ) by suitable means. For example, a selective etch technique is used to remove the aluminum-copper alloy conductive film  12  employing an anisotropic dry etch leaving patterned remaining portion  18   a  of the silicon nitride sidewall film  18 . The remaining portion  18   a  serves as a mask to prevent etching of the remaining portion of the aluminum-copper alloy conductive film  12  therebeneath during the directional etch. In this embodiment, a dry etch technique including at least one chlorocompound, such as using Cl 2  and BCl 3 , is employed. The etch technique has selectivity in that the size of the remaining portion  18   a  of the silicon nitride sidewall film  18  is not substantially reduced. 
     In this embodiment, the height and width of patterned remaining aluminum-copper alloy conductive film  12  beneath the remaining portion  18   a  is about 900 Å and about 400 Å. The patterned remaining aluminum-copper alloy conductive film  12  may be used in making interconnect lines, particulary narrow interconnect lines which enables the production of semiconductor devices having high densities. 
     Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including any reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiments of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several embodiments, such feature may be combined with one or more other features of the other embodiments as may be desired and advantageous for any given or particular application.