Abstract:
A method of forming a structure having sub-lithographic dimensions is provided. The method includes: forming a chamfered mandrel on a substrate, the mandrel having an angled surface; and performing an angled ion implantation to obtain an implanted shadow region in the substrate, the implanted shadow mask having at least one sub-lithographic dimension.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of semiconductor manufacture; more specifically, it relates to a method of fabricating structures having sub-lithographic dimensions.  
         BACKGROUND OF THE INVENTION  
         [0002]    Optical lithography is one of the key processes driving the semiconductor industry. For example, in dynamic random access memories (DRAM), decreasing the minimum lithographic dimension (the smallest image that can be printed) has accounted for about 60% of the reduction in silicon area used by DRAM chips. For logic chips, channel length is one of the key components of performance (devices having shorter channels lengths are faster) and is driven to a significant extent by optical lithography and minimum lithographic dimensions.  
           [0003]    However, as image sizes have shrunk, it has been increasing difficult to print very small dimensions even with advanced lithographic tool and processes. To counter this difficulty, trim technology has been developed. In normal trim technology, the original image which is at the lithographic limit is “trimmed” to a sub-lithographic dimension but the dimensional tolerance on the trimmed image is not “trimmed” but is the same as on the original image, so tolerance does not scale with image size. Shadow trim allows sub-lithographic images to be formed and is illustrated in FIGS. 1 and 2 and described below. However, shadow trim technology suffers the problem that the edges of trimmed images are not “sharp” but “fuzzy.” These two problems with normal and shadow trim technology lead to formation of devices that have a wide range of performance parameters that often offset the gains obtained by making the individual devices smaller.  
           [0004]    [0004]FIG. 1 is partial cross-sectional view of a related art shadow lithographic process. FIGS. 1 and 2 illustrate shadow trim technology as applied to ion implantation. In FIG. 1, formed on a substrate  100  are shadow masks  105 . Each shadow mask  105  has a shadow sidewall  110  and an exposed sidewall  115 . Each shadow mask  105  is “H1” high and are spaced “W1” apart. An angled ion implantation (in the present example, BF 2 + ions at an angle φ is performed. Shadow sidewall  110  projects a shadow region  120  extending from shadow sidewall  110  into substrate  100  between shadow masks  105 . Shadow region  120  is “S1” wide. “S1” is equal to “H1”×tangent φ. Shadow region  120  includes an un-implanted region  125  and a transition region  130 . Extending from exposed sidewall  115  to shadow region  120  is a fully implanted region  135 . Fully implanted region  135  is “D1” wide. “D1” is equal to “W1”−“S1.” Transition region  130  is located between un-implanted region  125  and fully implanted region  135 . The interface between un-implanted region  125  and transition region  130  is designated by the letter “B.” The interface between transition region  130  and fully implanted region  135  is designated by the letter “A.” 
           [0005]    [0005]FIG. 2 is a plot of the relative amount of BF2+ implanted as a function of horizontal distance for the related art process illustrated in FIG. 1. In FIG. 2, it is seen that the width of transition region, which is the tolerance on “D1” (defined between points “A” and “B” in FIG. 1) is about 60 Å. Addition of 60 Å to the tolerance on “D1” (see FIG. 1) is unacceptable in technologies where the total tolerance allowed for all causes is 300 Å or less.  
           [0006]    Clearly, what is needed is a method of forming sub-lithographic images having small transition regions and dimensional tolerances that scale with the image dimensions.  
         SUMMARY OF THE INVENTION  
         [0007]    A first aspect of the present invention is a method of forming a structure having sub-lithographic dimensions, comprising: forming a chamfered mandrel on a substrate, the mandrel having an angled surface; and performing an angled ion implantation to obtain an implanted shadow region in the substrate, the implanted shadow region having at least one sub-lithographic dimension.  
           [0008]    A second aspect of the present invention is a method of forming a structure having sub-lithographic dimensions, comprising: forming a chamfered mandrel on a substrate, the mandrel having a vertical sidewall, a top surface and an angled surface, the angled surface connecting the vertical sidewall to the top surface; and performing an angled ion implantation to obtain an implanted shadow region in the substrate, the implanted shadow region having at least one sub-lithographic dimension.  
           [0009]    A third aspect of the present invention is a method of forming a structure having sub-lithographic dimensions, comprising the steps of: (a) forming a first hard mask layer on a substrate; (b) forming a chamfered mandrel on the substrate, the mandrel having a vertical sidewall, a top surface and an angled surface, the angled surface connecting the vertical sidewall to the top surface; (c) forming a second hard mask layer over the chamfered mandrel and portions of the first hard mask not protected by the chamfered mandrel; (d) performing an angled ion implantation into the second hard mask layer to form an implanted region and an un-implanted region in the second hard mask layer; (e) removing either the un-implanted region or the implanted region; and (f) removing portions of the first hard mask layer exposed by step (e) to form an image in the first hard mask layer, said image having at least one sub-lithographic dimension. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0011]    [0011]FIG. 1 is partial cross-sectional view of a related art shadow lithographic process;  
         [0012]    [0012]FIG. 2 is a plot of the relative amount of BF 2 + implanted as a function of horizontal distance for the related art process illustrated in FIG. 1;  
         [0013]    [0013]FIG. 3 is a plot of angle of incidence versus relative sputtering yield for a sputtering process according to the present invention;  
         [0014]    [0014]FIGS. 4A and 4B are partial cross-sectional views illustrating edge oblation during a sputtering process according to the present invention;  
         [0015]    [0015]FIGS. 5A through 5C are partial cross-sectional views illustrating a first embodiment of the present invention;  
         [0016]    [0016]FIG. 6 is a plot of the relative amount of BF 2 + implanted as a function of horizontal distance for the first embodiment of the present invention illustrated in FIGS. 5A through 5C;  
         [0017]    [0017]FIGS. 7A through 7D are partial cross-sectional views illustrating common process steps for second and third embodiments of the present invention;  
         [0018]    [0018]FIGS. 8A through 8E are partial cross-sectional views illustrating process steps of the second embodiment of the present invention;  
         [0019]    [0019]FIGS. 9A through 9C are partial cross-sectional views illustrating process steps of the third embodiment of the present invention;  
         [0020]    [0020]FIG. 10 is a partial cross-sectional view illustrating an alternative shape for mandrels of the present invention;  
         [0021]    [0021]FIGS. 11A through 11D are partial cross-sectional views illustrating a fourth embodiment of the present invention; and  
         [0022]    [0022]FIGS. 12A through 12D are partial cross-sectional views illustrating a fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    [0023]FIG. 3 is a plot of angle of incidence versus relative sputtering yield for a sputtering process according to the present invention. Incident angle is defined as the angle between a line normal to the surface of the target and the direction of the sputtering species, a 0° degree incident angle being perpendicular to the surface of the target. The peak-sputtering rate is found to occur at the incident angle θ, where θ is about 53° and is generally independent of the material of being sputtered and the sputtering species.  
         [0024]    [0024]FIGS. 4A and 4B are partial cross-sectional views illustrating edge oblation during a sputtering process according to the present invention. In FIG. 4A, a projecting structure  140  having a top surface  145  and sidewalls  150  is formed on a substrate  155 . Sidewalls  150  meet top surface  145  in edges  160 .  
         [0025]    In FIG. 4B, chamfered structure  140 A is formed by subjecting projecting structure  140  (see FIG. 4A) to about a 0° sputtering process that preferentially attacks edges  160  (see FIG. 4A) to produce angled surfaces  165 . Angled surfaces  165  are formed at about the same angle θ illustrated in FIG. 3 and described above. In FIG. 4B, θ is measured from top surface  145  to angled surface  165 . The angle θ is about 53° (the peak of the sputtering yield curve of FIG. 3).  
         [0026]    [0026]FIGS. 5A through 5C are partial cross-sectional views illustrating a first embodiment of the present invention. In FIG. 5A, mandrels  170  having top surfaces  175  and sidewalls  180  are formed on a substrate  185 . Sidewalls  180  meet top surfaces  175  in edges  190 . An optional, thin conformal layer  192  may be formed over mandrels  170  and substrate  185  where the substrate is not covered by mandrel. Conformal layer  192  is used as a sputtering endpoint detection layer allowing tight control of the sputtering process.  
         [0027]    In FIG. 5B, mandrels  170  (see FIG. 5A) are subjected to a sputtering process in order to produce chamfered mandrels  195  having angled surfaces  200 . Conformal layer  192  (see FIG. 5A) is removed by the sputtering process and an isotropic etch to expose sidewalls  180 . Angled surfaces  200  are formed at an angle α measured from top surfaces  205  to angled surfaces  200 . In one example, the sputtering species is Ar, substrate  185  is silicon, mandrels  195  are SiO 2  and the angle α is about 53°. Other examples of sputtering species include N 2  and Ne. Suitable tools for e sputtering SiO 2  in Ar ions include the AME 5000 and AME 8300 (hexode system manufactured by Applied Materials Corp, Santa Clara, Calif. In general, lower the pressure and/or higher power enhance the sputtering rate. Chamfered mandrel  195  may be formed of thermal oxide, tetraethoxysilane (TEOS) oxide, high-pressure deposition (HDP) oxide, silicon nitride, polysilicon, photoresist and combinations thereof.  
         [0028]    In FIG. 5C, chamfered mandrels  195  each have a shadow sidewall  210  and an exposed sidewall  215 . Shadow and exposed sidewalls  210  and  215  are “H2” high and are spaced “W2” apart. An angled ion implantation at an incident angle of β is performed. In one example, β is equal to about α to α±δ, where δ is about 0° to 1° (see FIG. 5B) The lower δ, the sharper the edge of implanted  235 . In the present example, BF 2 + ions are implanted at an incident angle of about 52° to 54°. Stated generally, the angle of incidence (β) of the implanted species is equal to or slightly lesser or sightly greater than the angle (α) of angled surface  200  (see FIG. 5B). Shadow sidewall  210  projects a shadow region  220  extending from shadow sidewall  210  onto substrate  185  between mandrels  195 . Shadow region  220  is substantially un-implanted, is “S2” wide and is self aligned from shadow sidewall  210 . “S2” is equal to about “H2”×tangent (β−δ) to “H2”×tangent (β+δ). Extending from exposed sidewall  215  to shadow region  220  is an implanted region  235 . Implanted region  235  is “D2” wide. “D2” is equal to “W2”−“S2.” If “W2” is a minimum lithographic dimension, then “D2” is a sub-lithographic dimension and implanted region  235  is a sub-lithographic feature. The interface between shadow region  220  and implanted region  235  is designated by the letters “A ” and “B.” 
         [0029]    One skilled in the art would be aware that chamfered mandrels  195  may be formed on top of a protective layer formed on the top surface of substrate  185 , the purpose of the protective layer being to protect the substrate from the sputtering step, the ion implant step or both and that all or a portion of the protective layer may be removed by the sputtering step or by a purposeful removal step prior to or after the ion implantation step.  
         [0030]    [0030]FIG. 6 is a plot of the relative amount of BF 2 + implanted as a function of horizontal distance for the first embodiment of the present invention illustrated in FIGS. 5A through 5C. In FIG. 6, it is seen that the width of transition region, which is the tolerance on “D2” (defined between points “A” and “B” in FIG. 5C) is less than 10 Å. The tolerance on the sub-lithographic image, in this case, implanted region  235  (see FIG. 5C) is small because of the sharp transition between points “A” and “B.” Other ion implant species include other boron containing ions, and ions containing arsenic, phosphorus, hydrogen, helium, oxygen, nitrogen and germanium.  
         [0031]    [0031]FIGS. 7A through 7D are partial cross-sectional views illustrating common process steps for second and third embodiments of the present invention. In FIG. 7A, a first hard mask layer  240  is formed on a top surface  245  of a substrate  250 . Mandrels  255  having a top surfaces  260  and sidewalls  265  are formed on a top surface  270  of first hard mask layer  240 . Mandrels  255  are spaced “W3” apart. Sidewalls  265  meet top surfaces  260  in edges  272 . An optional, thin conformal layer  274  may be formed over mandrels  255  and mask layer  240  where the mask layer is not covered by mandrel. Conformal layer  274  is used as a sputtering endpoint detection layer.  
         [0032]    In FIG. 7B, mandrel  255  (see FIG. 7A) is subjected to a sputtering process in order to produce a chamfered mandrel  275  having angled surfaces  280 . Conformal layer  274  (see FIG. 7A) is removed by the sputtering process and an isotropic etch to expose sidewalls  265 . Angled surfaces  280  are formed at an angle α measured from top surface  260  to angled surface  280 . In one example, the sputtering species is Ar and the angle α is about 52° to 54°. Other sputtering species include N 2  and Ne. During the sputtering process, none to some of first hard mask layer  240  exposed between chamfered mandrels  275  may be removed. FIG. 7B and subsequent figures, illustrate the case in which first hard mask layer  240  is substantially un-sputtered by the sputtering process.  
         [0033]    In FIG. 7C, a conformal second hard mask layer  285  is formed over chamfered mandrels  275  and first hard mask layer  240  exposed between the chamfered mandrels. Angled surface  280  of chamfered mandrel  275  is replicated in an angled surface  290  of second hard mask layer  285 .  
         [0034]    In FIG. 7D, an angled ion implantation at an incident angle of β is performed. In one example, β is equal to about α to α±δ, where δ is about 1° (see FIG. 7B) Stated generally, the angle of incidence (β) of the implanted species is equal to or slightly lesser or slightly greater than the angle (α) of angled surface  280 . The ion implant species is designated “X” in FIG. 7D. Ion implant species “X” strikes second protective layer  285  only where the second protective layer is not “shadowed” by chamfered mandrel  275  to form un-implanted regions  285 A and implanted regions  285 B of second protective layer  285 . Un-implanted regions  285 A and implanted regions  285 B of second protective layer  285  have different physical and/or chemical attributes either upon implantation or subsequent to an activation or conversion process such as a heat treatment. Because un-implanted regions  285 A and implanted regions  285 B each have different chemical properties one may be removed (etched) selectively with respect to the other.  
         [0035]    [0035]FIGS. 8A through 8D are partial cross-sectional views illustrating process steps of the second embodiment of the present invention. FIG. 8A, continues processing from FIG. 7D and in a first example, substrate  250  is silicon, first protective layer  240  comprises a layer of Si 3 N 4  over SiO 2 , chamfered mandrel  275  is HDP oxide or TEOS oxide, second protective layer  285  is polysilicon and the implant species “X” is boron or phosphorous (see FIG. 7D) will be used in describing FIGS. 8A through 8D.  
         [0036]    In FIG. 8A, first protective layer  240  comprises an upper layer  241  of Si 3 N 4  over a lower layer  242  of SiO 2 . In FIG. 8A, implanted regions  285 B of second hard mask layer  285  (see FIG. 7D) are removed using KOH, leaving behind un-implanted regions  285 A. KOH etches doped polysilicon faster than un-doped polysilicon. In FIG. 8B, a chemical-mechanical-polish (CMP) processes is performed to remove upper portions of chamfered mandrel  275  (see FIG. 8A) to produce mandrels  275 A. In FIG. 8C, mandrels  275 A (see FIG. 8B) are removed using dilute or buffered HF. In FIG. 8D, upper layer  241  of first hard mask layer  240  (see FIG. 8C) is removed where the upper layer is not protected by un-implanted regions  285 A using any one of well-known reactive ion etch (RIE) processes selective Si 3 N 4  to SiO 2  or wet etch chemistries. In FIG. 8E, un-implanted regions  285 A (see FIG. 8D) are removed using KOH and lower layer  242  is removed using dilute or buffered HF where the lower layer is not protected by upper layer  241  to form islands  295 . Islands  295  have a width “W4.” 
         [0037]    If, in FIG. 7A, “W3” is a minimum lithographic dimension, then “W4” is a sub-lithographic dimension and islands  295  are sub-lithographic features.  
         [0038]    In a second example, substrate  250  is silicon, first protective layer  240  comprises a layer of Si 3 N 4  over SiO 2 , chamfered mandrel  275  is SiO 2 , second protective layer  285  is polysilicon and the implant species “X” is oxygen (see FIG. 7D). The oxygen implantation converts polysilicon to SiO 2 , (a heat cycle may be performed to fully convert the silicon to SiO 2 ) which may be etched in dilute or buffered HF.  
         [0039]    [0039]FIGS. 9A through 9C are partial cross-sectional views illustrating process steps of the third embodiment of the present invention. FIG. 9A, continues processing from FIG. 7D and an example wherein, substrate  250  is silicon, first protective layer  240  is Si 3 N 4 , chamfered mandrel  275  is HDP oxide or TEOS oxide, second protective layer  285  is polysilicon or Si 3 N 4  and the implant species “X” is oxygen (see FIG. 7D) will be used in describing FIGS. 9A through 9C.  
         [0040]    In FIG. 9A, implanted regions  285 B are converted to SiO 2  if second protective layer  285  is polysilicon and un-implanted regions  285 A (see FIG. 7D) are removed using KOH. If second protective layer  285  is Si3N4, implanted regions  285 B are converted to Si x ON y  and un-implanted regions  285 A are removed using and RIE process or wet etch process. In FIG. 9B, first protective layer  240  is removed where the first protective layer is not protected by chamfered mandrels  275  or implanted regions  285 B using any one of well-known RIE processes. In FIG. 9C, chamfered mandrels  275  and implanted regions  285 B are removed using dilute or buffered HF leaving islands  300 . Islands  300  are spaced apart a distance “D3.” 
         [0041]    If, in FIG. 9A, “W3” is a minimum lithographic dimension, then “D3” is a sub-lithographic dimension and images  302  (between islands  300 ) are sub-lithographic features.  
         [0042]    [0042]FIG. 10 is a partial cross-sectional view illustrating an alternative shape for mandrels of the present invention. In FIG. 10, formed on substrate  305  are mandrels  310 . Mandrels  310  have a base  315  in contact with substrate  305 , at least two vertical sides  320  extending up from the base and at least two angled sides  325  each meeting one vertical side an along edge  330 . Angled sides  325  extend upward from vertical sides  320  to meet in an edge  335 . Mandrel  310  is formed by extending the sputtering process illustrated in FIGS. 3, 4A and  4 B and described above until edge  335  is formed.  
         [0043]    [0043]FIGS. 11A through 11D are partial cross-sectional views illustrating a fourth embodiment of the present invention. In FIG. 11A, formed on a substrate  350  is a mandrel precursor layer  355 . Layer  355  may formed from thermal oxide, TEOS oxide, HDP oxide, silicon nitride, polysilicon and combinations thereof. Formed on mandrel precursor layer  355  are photoresist islands  360 .  
         [0044]    In FIG. 11B, photoresist islands  360  (see FIG. 11A) are reflowed to produce reflowed resist islands  360 A.  
         [0045]    In FIG. 11C, an first RIE process with a high selectivity to reflowed resist islands  360 A is performed to form trenches  365  in mandrel precursor layer  355 . The exact RIE process is a function of the materials of layer  355  and the type of photoresist used and a person skilled in the art would be able to select an appropriate RIE process.  
         [0046]    In FIG. 11D, an second RIE process with a low selectivity to reflowed resist islands  360 A is performed to form mandrels  370 . Mandrels  370  have angled surface  375  caused by etch back of reflowed resist islands  360 A (see FIG. 11C) forming residual resist islands  360 B. Residual resist islands  360 B may now be removed by wet stripping or dry ashing.  
         [0047]    [0047]FIGS. 12A through 12D are partial cross-sectional views illustrating a fifth embodiment of the present invention. In FIG. 12A formed on a (100) single crystal silicon substrate  380  (or a (100) single crystal silicon layer) is hard mask layer  385 . Hard mask layer may be silicon oxide, silicon nitride, or other material not etched significantly by strong aqueous bases.  
         [0048]    In FIG. 12B, hard mask layer  385  (see FIG. 12A) has been formed using one of any number of well known photolithographic/etch processes, into islands  390  exposing a top surface of substrate  380  between the islands.  
         [0049]    In FIG. 12C, substrate  380  trenches  400  formed in substrate  380 . Trenches  400  have a bottom  410  and sloped sidewalls  405 . Trenches  400  are formed by anisotropic etching substrate  380  in an aqueous or alcoholic solution of a strong base such as KOH, NaOH, tetramethylammonium hydroxide (TMAH) or ethylene diamine pyrocatechol (EDP.) Formation of sloped sidewalls in (100) silicon is well known. The slope is formed because the etch rate in the &lt;111&gt; crystallographic plane is faster than in any of the other planes.  
         [0050]    In FIG. 12D, a directional RIE process is performed deepening trench  400  and forming straight sidewalls  415  between sloped sidewalls and bottom  420 . Islands  390  may now be removed.  
         [0051]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.