Patent Publication Number: US-6713396-B2

Title: Method of fabricating high density sub-lithographic features on a substrate

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method of fabricating high density sub-lithographic features on a substrate. More specifically, the present invention relates to a method of fabricating high density sub-lithographic features on a substrate using common microelectronic processing techniques to form a plurality of sub-lithographic spacers on a substrate, wherein within a minimum resolution of a lithographic system, a density of features can be increased by a factor of two or more. 
     BACKGROUND OF THE ART 
     A standard method in the microelectronics industry for patterning features on a substrate uses well understood photolithographic processes. Typically, a layer of photoresist is coated onto a substrate material followed by exposing the photoresist with a light source through a mask. The mask includes patterned features, such as lines and spaces, that are to be transferred to the photoresist. After the photoresist is exposed, the photoresist is immersed in a solvent to define the patterns that were transferred to the photoresist. The patterns produced by this process are typically limited to line widths greater than a minimum resolution λ of a photolithographic alignment tool, which is ultimately limited by a wavelength of light of a light source used to expose the photoresist. At present, a state of the art photolithographic alignment tool is capable of printing line widths as small as about 100.0 nm. 
     Features patterned into the photoresist are transferred into the substrate material using well known microelectronics processes such as reactive ion etching, ion milling, plasma etching, or chemical etching, for example. Using standard semiconductor processing methods, lines of width λ or gratings (i.e. a line-space sequence) of a period 2λ can be created. 
     However, in many applications it is advantageous to have the line width or the period be as small as possible. Smaller line widths or periods translate into higher performance and/or higher density circuits. Hence, the microelectronics industry is on a continual quest to reduce the minimum resolution in photolithography systems and thereby reduce the line widths or periods on patterned substrates. The increases in performance and/or density can be of considerable economic advantage because the electronics industry is driven by a demand for faster and smaller electronic devices. 
     In FIG. 1 a , a prior method of fabricating lines narrower than a minimum feature size λ comprises controlling the etch process used to pattern a substrate material. A substrate  101  includes lines  103  having a minimum feature size λ that is greater than or equal to a minimum resolution λ of a lithographic system used to pattern the lines  103 . Because of the minimum resolution λ of the lithographic system, the lines  103  will be spaced apart by a space  105  that is also greater than or equal to λ. In FIG. 1 a , the line  103  and space  105  pattern has a period of 2λ. Accordingly, within the period of 2λ a density of features is two, that is, there is one line feature  103  and one space feature  105 . Similarly, within the distance of λ, the density of features is one, that is, there is either a line  103  or a space  105  within the distance of λ. 
     In FIG. 1 b , the lines  103  have their respective widths reduced to a width that is less than λ by controlled lateral plasma etching such that a vertical sidewall s of the lines  103  prior to etching (see arrow e) recedes in a lateral direction to a reduced width (see dashed arrow r) that is less than λ (i.e. &lt;λ). However, the density of lines  103  has not been increased by the above method. In fact, due to the lateral etching, the lines  103  are made narrower than λ (i.e. &lt;λ) and the spaces  105  are made wider than λ (i.e. &gt;λ) due to the recession of the vertical sidewalls S. As a result, the density of features ( 103 ,  105 ) within the period of 2λ remains two and the density of features within the distance of λ remains one. 
     Similarly, in FIG. 2 a , if the features in a substrate  107  include a grating  109  having a line  111  and a space  113  that have a feature size that is greater than or equal to λ. Within a period of 2λ, the number of features ( 111 ,  113 ) is two and the density of features within the distance of λ remains one. 
     In FIG. 2 b , after controlled lateral plasma etching, vertical sidewalls S have receded with the end result being the spaces  113  are wider than λ (i.e. &gt;λ) and the lines  111  are narrower than λ (i.e. &lt;λ). As before, the density of features ( 111 ,  113 ) within the period of 2λ remains two and the density of features within the distance of λ remains one. 
     Therefore, there is a need for a method of fabricating sub-lithographic sized features that have a width that is narrower than a minimum resolution of a lithographic system. There also exists a need for a method of fabricating sub-lithographic sized features that increases a density of features within a minimum resolution of a lithographic system. 
     SUMMARY OF THE INVENTION 
     The method of fabricating high density sub-lithographic features of the present invention solves the aforementioned problems by using common microelectronic processes including sub-lithographic spacer formation and Damascene processes to form a plurality of sub-lithographic spacers on a substrate. The sub-lithographic spacers have a period that is less than a minimum resolution of a lithographic system. Spacers, in microelectronics processing parlance, are films that cover vertical side walls of features on a substrate. Damascene processing refers to a technique for creating inlaid patterns of a first material in a matrix of a second material by deposition of the first material into a depression defined in the second material, followed by removal of a portion of the first material by a planarization process. For example, a planarization process such as chemical mechanical planarization (CMP) can be used to remove and planarize the first material. 
     A density of features, including the sub-lithographic spacers, within a minimum resolution of the lithographic system is increased by the method of the present invention. Moreover, the density of features within the minimum resolution of the lithographic system can be further increased by subsequent depositions of material, followed by anisotropic etching to selectively remove horizontal surfaces of the deposited material. The depositions of the material can be conformal depositions wherein a horizontal thickness and a vertical thickness of the deposited material are substantially equal to each other. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a cross-sectional view of a prior substrate having line features thereon with a period of twice a minimum resolution of a lithographic system. 
     FIG. 1 b  is a cross-sectional view of FIG. 1 a  after a prior controlled lateral etching process is used to reduce a width of the lines. 
     FIG. 2 a  is a cross-sectional view of a prior substrate having a grating thereon with features having a period of twice a minimum resolution of a lithographic system. 
     FIG. 2 b  is a cross-sectional view of FIG. 2 a  after a prior controlled lateral etching process is used to reduce a width of the features. 
     FIG. 3 a  is a cross-sectional view of photolithographic patterning of a mask layer according to the present invention. 
     FIG. 3 b  is a cross-sectional view of the mask layer of FIG. 3 a  after an etch process according to the present invention. 
     FIGS. 3 c  and  3   d  are cross-sectional views of a substrate including features with a minimum feature size after an etch process according to the present invention. 
     FIG. 4 is a cross-sectional view of a deposited spacer material according to the present invention. 
     FIG. 5 is a cross-sectional view of sub-lithographic spacers formed by an anisotropic etch process according to the present invention. 
     FIG. 6 is a cross-sectional view of another deposited spacer material deposited over the sub-lithographic spacers of FIG. 5 according to the present invention. 
     FIG. 7 is a cross-sectional view of additional sub-lithographic spacers formed by an anisotropic etch process according to the present invention. 
     FIGS. 8 and 10 are cross-sectional views of an inlaid material after a deposition process according to the present invention. 
     FIGS. 9 and 11 are cross-sectional views of an inlaid spacer formed by a planarization process according to the present invention. 
     FIGS. 12 a  and  12   b  are a cross-sectional view depicting formation of a feature carried by a substrate according to the present invention. 
     FIG. 13 is a cross-sectional view of a deposited spacer material according to the present invention. 
     FIG. 14 is a cross-sectional view of sub-lithographic spacers formed by an anisotropic etch process according to the present invention. 
     FIG. 15 is a cross-sectional view of additional sub-lithographic spacers formed by an anisotropic etch process according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description and in the several figures of the drawings, like elements are identified with like reference numerals. 
     As shown in the drawings for purpose of illustration, the present invention is embodied in a method of fabricating high density sub-lithographic features. The method includes depositing a mask layer on a substrate and then patterning the mask layer to define an image that includes a minimum feature size that is greater than or equal to a minimum resolution of a lithographic system used for patterning the mask layer. The mask layer is then etched to transfer the image to the substrate thereby defining a feature on the substrate. The feature includes the minimum feature size and also includes horizontal surfaces and vertical side wall surfaces. 
     A spacer material is deposited on the feature such that the spacer material covers the horizontal surfaces and the vertical side wall surfaces. The deposition continues until the spacer material has a predetermined thickness that is less than the minimum feature size. 
     A density of features within the minimum feature size is increased by anisotropically etching the spacer material to selectively remove the spacer material from the horizontal surfaces. As a result, the spacer material remains on the vertical side wall surfaces and defines a plurality of sub-lithographic spacers that are in contact with the vertical side wall surfaces and extend laterally outward of the vertical side wall surfaces. The sub-lithographic spacers include a thickness that is less than the minimum feature size. Consequently, a density of features within the minimum feature size is greater than 2.0. That density includes the feature and the sub-lithographic spacers. 
     Optionally, the density of features within the minimum feature size can be further increased by repeating the above mentioned depositing and anisotropic etching steps to define additional sub-lithographic spacers on previously defined sub-lithographic spacers. The additional sub-lithographic spacers also include a thickness that is less than the minimum feature size. 
     In FIG. 3 a , a mask layer  17  is deposited on a surface  12  of a substrate  11 . The mask layer  17  can be a layer of a photoresist material, for example. The mask layer  17  is patterned to define in the mask layer  17  an image that includes a minimum feature size λ. The minimum feature size λ is greater than or equal to a minimum resolution of a lithographic system that is used to pattern the mask layer  17 . For instance, the lithographic system can be a conventional photolithography system and the minimum resolution can be determined by a wavelength of a light source carried by the photolithography system and used to project an image on the mask layer  17 . 
     Referring again to FIG. 3 a , a mask  21  carries features ( 23 ,  25 ) that also have a minimum feature size λ that is greater than or equal to a minimum resolution of the lithographic system. The mask  21  is illuminated by a light source (not shown) and a portion of that light  43  is blocked by the opaque features  23  and another portion of that light  41  passes through the transparent features  25  and exposes the mask layer  17 . 
     In FIG. 3 b , those portions of the mask layer  17  that are exposed to the light  41  remain after the mask layer  17  is etched and those that were not exposed to the light are removed after the mask layer  17  is etched. After etching, images ( 18 ,  19 ) are defined in the mask layer  17 . The images ( 18 ,  19 ) also include the minimum feature size λ. For example, the images ( 18 ,  19 ) can be defined by submerging the mask layer  17  in a solvent that dissolves those portions of the mask layer  17  that were not exposed to the light  41 . As a result, the dissolved portions form the image  18  and the undissolved portions form the image  19 . 
     In FIG. 3 c , the image  19  covers some portions of the surface  12  of the substrate  11 , whereas the image  18  is coincident with the surface  12 . The images ( 18 ,  19 ) are then transferred to the substrate  11  by etching the substrate to define features ( 10 ,  20 ). The features ( 10 ,  20 ) include the minimum feature size λ. The feature  20  is a trench having vertical side wall surfaces  16  and a horizontal surface  14 ; whereas, the feature  10  is a line also having vertical side wall surfaces  16  and a horizontal surface  12 . Because the features ( 10 ,  20 ) include the minimum feature size λ, a minimum period between a repetition of the features is 2λ. 
     Accordingly, in FIG. 3 d , within a distance of λ, there is density of features equal to one (i.e. 1.0), that is, a single feature  10  or a single feature  20 . On the other hand, within a period of 2λ, there is a density of features equal to two (i.e. 2.0), that is, there is a feature  10  and a feature  20 . 
     In FIG. 4, a spacer material  31  is deposited on the horizontal surfaces ( 12 ,  14 ) and vertical side wall surfaces  16 . The deposition of the spacer material  31  continues until the spacer material  31  has a predetermined thickness (t H , t V ) that is less than the minimum feature size λ. That is, a thickness t H  of the spacer material  31  on the horizontal surfaces ( 12 ,  14 ) is less than λ (t H &lt;λ) and a thickness t V  of the spacer material on the vertical side wall surfaces  16  is less than λ (t V &lt;λ). For example, in a photolithographic process with the minimum feature size λ, the horizontal and vertical side wall thicknesses (t H , t V ) are typically in a range from about 0.1 λ to about 0.5 λ. The thicknesses (t H , t V ) need not be equal to each other (i.e. t H ≠t V ). 
     For all of the embodiments described herein the spacer materials (including the spacer material  31 ) and inlaid spacers as will be described below, can be conformally deposited such that the horizontal and vertical side wall thicknesses (t H , t V ) are substantially equal to each other (see FIGS. 4 through 7 and FIGS.  13  through  15 ). That is, t H =t V . Moreover, subsequent depositions to increase a density of features within the minimum feature size λ can also be conformal depositions. Additionally, the depositions of the spacer materials can be a combination of non-conformal depositions wherein t H ≠t V  and conformal depositions wherein t H =t V . 
     Techniques for depositing the spacer material  31  include but are not limited to chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering, plating, and atomic layer deposition (ALD). 
     In FIG. 5, a density of features within the minimum feature size λ is increased by anisotropically etching the spacer material  31  to selectively remove the spacer material  31  from the horizontal surfaces ( 12 ,  14 ); however, the spacer material  31  remains on the vertical side wall surfaces  16  and defines a plurality of sub-lithographic spacers  33  that are in contact with the vertical side wall surfaces  16  and extend outward therefrom. The sub-lithographic spacers  33  are sub-lithographic because their thickness t V  is less than the minimum feature size λ (i.e. t V &lt;λ). 
     After the anisotropic etching, the density of features within the minimum feature size λ is three, that is, there are two sub-lithographic spacers  33  and a feature  24  (i.e. the feature  24  is the space between the spacers  33 , see reference numeral S in FIG.  5 ). Accordingly, the density of features within the minimum feature size λ is greater than 2.0. Similarly, the density of features within a period of 2λ is four, that is, there are two sub-lithographic spacers  33 , one feature  24 , and one feature  10  (see reference numeral D in FIG.  5 ). Consequently, the density of features within a period of 2λ is greater than 3.0. 
     The sub-lithographic spacers  33  have vertical side wall surfaces  22 . A distance λ S  between the vertical side wall surfaces  22  in the feature  24  is less than the minimum feature size λ (λ S &lt;λ). Additionally, a distance λ P  between the vertical side wall surfaces  22  within the feature  24  is less than the minimum feature size λ (λ P &lt;λ). As a result, an inlaid spacer, as will be described below, can optionally fill in the distance λ S  and will also have a sub-lithographic size that is less than the minimum feature size λ. 
     In FIG. 6, it may be optionally desired to further increase the density of features within the minimum feature size λ by repeating the deposition and the anisotropic etching steps as described above. A spacer material  51  is deposited on the horizontal ( 12 ,  14 ) and vertical side wall surfaces  22  of the previously formed sub-lithographic spacers  33  and completely fills in the feature  24 . The deposition continues until the spacer material  51  has a predetermined thickness (t H , t V ) that is less than the minimum feature size λ. 
     In FIG. 7, a density of features within the minimum feature size λ is further increased by anisotropically etching the spacer material  51  to selectively remove the spacer material  51  from the horizontal surfaces ( 12 ,  14 ); however, the spacer material  51  remains on the vertical side wall surfaces  22  and defines a plurality of sub-lithographic spacers  53  that are in contact with the vertical side wall surfaces  22  and extend outward therefrom. The sub-lithographic spacers  53  are sub-lithographic because their thickness t V  is less than the minimum feature size λ (t V &lt;λ). 
     After the anisotropic etching, the density of features within the minimum feature size λ is five, that is, there are four sub-lithographic spacers ( 33 ,  53 ) and a feature  26  (see reference numeral S in FIG.  7 ). Therefore, the density of features within the minimum feature size λ is greater than 4.0. Similarly, the density of features within a period of 2λ is six, that is, there are four sub-lithographic spacers ( 33 ,  53 ), one feature  26 , and one feature  10  (see reference numeral D in FIG.  7 ). Consequently, the density of features within a period of 2λ is greater than 5.0. 
     The sub-lithographic spacers  53  have vertical side wall surfaces  44 . A distance λ S  between vertical side wall surfaces  44  in the feature  26  is less than the minimum feature size λ (λ S &lt;λ). Additionally, a distance λ P  between the vertical side wall surfaces  44  within the feature  26  is less than the minimum feature size λ (λ P &lt;λ). As a result, an inlaid spacer, as will be described below, can optionally fill in the distance λ S  and the inlaid spacer will also have a sub-lithographic size that is less than the minimum feature size λ. 
     After the anisotropic etching step, the substrate  11  can be planarized along a plane (see dashed line p in FIGS. 4 and 6) to form a substantially planar surface. For example, a process such as chemical mechanical planarization (CMP) can be used to planarize the substrate  11 . 
     In FIGS. 8 and 10, after completion of the anisotropic etching steps, an inlaid material ( 37 ,  67 ) can be deposited on the substrate  11 . The inlaid material ( 37 ,  67 ) completely covers the horizontal surfaces ( 12 ,  14 ), the features ( 10 ,  20 ), and fills in any depressed regions in the substrate, such as those defined by a space between the vertical side wall surfaces ( 22 ,  44 ) of the sub-lithographic spacers ( 33 ,  53 ). 
     In FIGS. 9 and 11, the substrate  11  is planarized (see dashed line p) to form a substantially planar surface and to define an inlaid spacer ( 39 ,  69 ). For example, a process such as CMP can be used to planarize the substrate  11 . 
     Moreover, in FIGS. 9 and 11, the inlaid spacers ( 39 ,  69 ) are formed between the vertical side wall surfaces ( 22 ,  44 ) of the sub-lithographic spacers ( 33 ,  53 ). The distance λ S  between those vertical side wall surfaces ( 22 ,  44 ) is less than the minimum feature size λ. Accordingly, the inlaid spacers ( 39 ,  69 ) are also sub-lithographic because they have a thickness equal to the distance λ S  that is also less than then minimum feature size λ. 
     In FIG. 9, the density of features within the minimum feature size λ is greater than 2.0 as there are two sub-lithographic spacers  33 , and the inlaid spacer  39 . 
     In FIG. 11, the density of features within the minimum feature size λ is greater than 4.0 as there are two sub-lithographic spacers  33 , two sub-lithographic spacers  53 , the inlaid spacer  69 . 
     In FIG. 9, the density of features within the period 2λ is greater than 3.0 as there are two sub-lithographic spacers  33 , the inlaid spacer  39 , and the feature  10 . 
     In FIG. 11, the density of features within the period 2λ is greater than 5.0 as there are two sub-lithographic spacers  33 , two sub-lithographic spacers  53 , the inlaid spacer  69 , and the feature  10 . 
     In another embodiment of the present invention, as illustrated in FIG. 12 a , a feature layer  80  is deposited on a surface  82  of a substrate  71 . Using a photoresist and photolithography process as was described above, a layer of photoresist can be deposited on the feature layer  80 , exposed with an image, and the image resolved by etching to form a pattern  91  on the feature layer  80 . 
     In FIG. 12 b , the feature layer  80  is etched to define features ( 81 ,  85 ) that include horizontal surfaces ( 82 ,  84 ) and vertical side wall surfaces  86 . The features ( 81 ,  85 ) include the minimum feature size λ that is greater than or equal to a minimum resolution of a lithographic system used for patterning the feature layer  80 . 
     In FIG. 13, a spacer material  87  is deposited on the horizontal surfaces ( 82 ,  84 ), the vertical side wall surfaces  86  of the features ( 81 ,  85 ). The deposition continues until the spacer material  87  has a predetermined thickness (t H , t V ) that is less than the minimum feature size λ. 
     In FIG. 14, a density of features within the minimum feature size λ is increased by anisotropically etching the spacer material  87  to selectively remove the spacer material  87  from the horizontal surfaces ( 82 ,  84 ); however, the spacer material  87  remains on the vertical side wall surfaces  86  and defines a plurality of sub-lithographic spacers  83  that are in contact with the vertical side wall surfaces  86 . The spacers  83  are sub-lithographic because their thickness t V  is less than the minimum feature size λ (t V &lt;λ). 
     After the anisotropic etching, the density of features within the minimum feature size λ is three, that is, there are two sub-lithographic spacers  83  and one feature  85 . Accordingly, the density of features within the minimum feature size λ is greater than 2.0. Similarly, the density of features within a period of 2λ is four, that is, there are two sub-lithographic spacers  83 , one feature  85 , and one feature  81 . Consequently, the density of features within a period of 2λ is greater than 3.0. 
     The sub-lithographic spacers  83  have vertical side wall surfaces  94 . A distance λ S  between the vertical side wall surfaces  94  in the feature  85  is less than the minimum feature size λ (λ S &lt;λ). Additionally, a distance λ P  between the vertical side wall surfaces  94  within the feature  85  is less than the minimum feature size λ (λ P &lt;λ). As a result, an inlaid spacer (not shown), as was be described above, can optionally fill in the distance λ S  and will also have a sub-lithographic size that is less than the minimum feature size λ. 
     Optionally, the density of features within the minimum feature size λ can be further increased, as was described above in reference to FIGS. 6 and 7, by repeating the deposition and the anisotropic etching steps. For example, another layer of a spacer material (not shown) is deposited on the horizontal ( 82 ,  84 ) and vertical side wall surfaces  94  of the previously formed sub-lithographic spacers  83 . The deposition continues until the spacer material has a predetermined thickness (t H , t V ) that is less than the minimum feature size λ. 
     In FIG. 15, after the anisotropic etching step, a plurality of sub-lithographic spacers  93  are defined on the vertical side wall surfaces  94  of the previously formed sub-lithographic spacers  83 . The sub-lithographic spacers  93  are sub-lithographic because their thickness t V  is less than the minimum feature size λ (t V &lt;λ). 
     Furthermore, the density of features within the minimum feature size λ is five, that is, there are four sub-lithographic spacers ( 83 ,  93 ), a feature  92  (see FIG. 15 where  92  is a space between spacers  93 ). Therefore, the density of features within the minimum feature size λ is greater than 4.0. Similarly, the density of features within a period of 2λ is six, that is, there are four sub-lithographic spacers ( 83 ,  93 ), one feature  92 , and one feature  81 . Consequently, the density of features within a period of 2λ is greater than 5.0. 
     An inlaid material (not shown), as was described above, may be deposited and planarized to form an inlaid spacer (not shown) that fills the feature  92 . The inlaid spacer increases the density within the minimum feature size λ and within the period of 2λ as was described above. 
     Materials for the inlaid spacers ( 39 ,  69 ) and the sub-lithographic spacers ( 33 ,  53 ,  83 ,  93 ) include but are not limited to a metal, an electrically conductive material, a semiconductor material, silicon (Si), a dielectric material, and an optical material. The silicon can be polysilicon (α-Si). The metal can be a material including but not limited to aluminum (Al), tungsten (W), tantalum (Ta), and copper (Cu). 
     Materials for the substrates ( 11 ,  71 ) and the feature layer  80  include but are not limited to a metal, an electrically conductive material, a semiconductor material, silicon (Si), a dielectric material, a glass, and an optical material. The silicon can be single crystal silicon (Si) or polysilicon (α-Si). The metal can be a material including but not limited to aluminum (Al), tungsten (W), tantalum (Ta), and copper (Cu). 
     One use for the high density sub-lithographic features of the present invention includes a nano-imprint stamp in which one or more of the features, including the sub-lithographic spacers, are selectively etched to remove material along a horizontal surface thereof such that there is a variation in height among the features and the sub-lithographic spacers. Those variations in height can be transferred into a substrate that carries an imprint layer by pressing the nano-imprint stamp into the imprint layer. 
     Another use for the high density sub-lithographic features of the present invention include an optical component. For example, the optical component can be an optical grating, a polarizing filter, or a neutral density filter. The substrates ( 11 ,  71 ), the spacer, the inlaid spacers, and the feature layer  80 , can be optical materials with a high enough band gap to be optically transparent. 
     For instance, the substrate can be made from a material including but not limited to an optically transparent glass and the spacers or the inlaid spacers can be made from materials including but not limited to magnesium oxide (MgO), silicon oxide (SiO 2 ), tantalum oxide (Ta 2 O 5 ), calcium fluoride (CaF 2 ), and magnesium fluoride (MgF 2 ). 
     The depositions of the materials for the spacers and the inlaid spacers can be accomplished using processes including but not limited to chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), sputtering, plating, and atomic layer deposition (ALD). 
     The anisotropic etching step can be accomplished using techniques including but not limited to reactive ion etching, ion milling, chemical etching, and plasma etching. 
     Although several embodiments of the present invention have been disclosed and illustrated, the invention is not limited to the specific forms or arrangements of parts so described and illustrated. The invention is only limited by the claims.