Abstract:
A method for creating a roughened surface on a material exposed to light during a photolithographic process is provided. The roughened surface is created on a surface of the material via a plasma etch process. The roughened surface diffuses light incident to the material such that the diffused light causes insubstantial damage to a photoresist subsequently formed on the material.

Description:
TECHNICAL FIELD 
     The present invention generally relates to reducing reflectivity of surfaces in order to facilitate lithography. 
     BACKGROUND OF THE INVENTION 
     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 device dimensions at submicron levels on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as comers and edges of various features. 
     The requirement of small features with 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 mask, for a particular pattern. The lithographic coating is generally a radiation-sensitive 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 a photomask causes the image area to become 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. 
     Present techniques in optical projection printing can resolve images of sub-micron when photoresists with good linewidth control are used. However, reflection of light from substrate/resist interfaces produce variations in light intensity and scattering of light in the resist during exposure, resulting in non-uniform photoresist linewidth upon development. 
     Constructive and destructive interference resulting from reflected light is particularly significant when monochromatic or quasi-monochromatic light is used for photoresist exposure. In such cases, the reflected light interferes with the incident light to form standing waves within the resist. In the case of highly reflective substrate regions, the problem is exacerbated since large amplitude standing waves create thin layers of underexposed resist at the wave minima. The underexposed layers can prevent complete resist development causing edge acuity problems in the resist profile. 
     Antireflective coatings are known and used to mitigate the aforementioned problems, however, the use thereof presents additional problems such as, for example, introduction of particulate contamination, requirement of tight temperature tolerances during production, etc. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of reducing reflectivity of an underlayer surface to be covered by a resist. The reflectivity of the surface is reduced by roughening (e.g., dulling) the surface so that it is less reflective. Reducing reflectivity of the surface mitigates destructive and constructive interference and standing waves resulting from light reflected therefrom. The surface roughening is accomplished via a plasma etch process to create a plurality of divots (or roughness profile) in the surface. The roughening of the surface makes the surface substantially less reflective. Light incident to the roughened surface is diffused to such a degree that the aforementioned problems associated with resist damage resulting from reflected light is mitigated. Furthermore, the present invention does not require the use of anti-reflective coatings, and thus avoids many of the problems associated with using anti-reflective coatings. 
     In accordance with one specific aspect of the invention, a method for creating a roughened surface on an underlayer material is provided. The method includes the steps of: roughening the surface of the underlayer via a plasma etch; subsequently forming a photoresist on the underlayer; and exposing the photoresist to light to form a pattern. 
     Another aspect of the invention relates to a method of mitigating damage to a photoresist formed on an underlayer. The method includes the step of: using an underlayer having a roughened surface which diffuses light incident to the underlayer, the incident light being diffused to such a degree that the diffused beams of light result in insubstantial damage to the photoresist. 
     Yet another aspect of the present invention relates to an underlayer which mitigates damage to a photoresist layer formed on the underlayer. The underlayer includes a reflective material; and a roughened surface formed on a surface of the underlayer, the roughened surface diffusing light incident to the underlayer so that the diffused light is of an intensity which results in minimal damage to the photoresist, the roughened surface formed via a plasma etch. 
     In accordance with another aspect of the present invention, a method for creating a roughened surface on an oxide layer is provided. The method includes forming an oxide layer over an underlayer material; creating a roughened surface on a surface of the oxide layer via a plasma etch process; subsequently forming a photoresist on the oxide layer; and exposing the photoresist to light to form a pattern. 
     Another aspect of the present invention relates to a method for creating a roughened surface on an oxide layer serving as an anti-reflective coating for an underlayer. The method includes the steps of: using a plasma etch process to form the roughened surface, the plasma etch process including using argon as a reactant gas, the roughened surface being tailored to diffuse light incident to the oxide layer such that the diffused light does insubstantial damage to a photoresist formed on the oxide layer. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-sectional illustration of a portion of an underlayer having its surface roughened to reduce reflectivity thereof in accordance with the present invention; 
     FIG. 2 is a schematic cross-sectional illustration of a reflective underlayer formed on a silicon substrate; 
     FIG. 3 is a schematic cross-sectional illustration of the underlayer layer of FIG. 2 undergoing a roughening process; 
     FIG. 4 is a schematic cross-sectional illustration of the underlayer of FIG. 3 after the roughening process is substantially complete; 
     FIG. 5 is a schematic cross-sectional illustration of a photoresist layer formed on the roughened underlayer of FIG. 4; 
     FIG. 6 is a schematic cross-sectional illustration of the photoresist layer of FIG. 5 undergoing a patterning process; 
     FIG. 7 is a schematic cross-sectional illustration of an etch step being performed on the underlayer to pattern the underlayer material; 
     FIG. 8 is a schematic cross-sectional illustration of the patterned underlayer material after the etching step in FIG. 7 is substantially complete; 
     FIG. 9 is a schematic cross-sectional illustration of a stripping process to remove residual photoresist from the patterned underlayer material of FIG. 8; 
     FIG. 10 is a schematic cross-sectional illustration of the patterned underlayer material in accordance with the present invention; 
     FIG. 11 is a schematic cross-sectional illustration of a portion of an oxide layer with a roughened surface to serve as an anti-reflective coating for an underlayer material in accordance with the present invention; 
     FIG. 12 is a schematic cross-sectional illustration of a reflective underlayer formed on a silicon substrate; 
     FIG. 13 is a schematic cross-sectional illustration of an oxide layer formed over the underlayer; 
     FIG. 14 is a schematic cross-sectional illustration of the oxide layer of FIG. 12 undergoing a roughening process; 
     FIG. 15 is a schematic cross-sectional illustration of the oxide layer of FIG. 14 after the roughening process is substantially complete; 
     FIG. 16 is a schematic cross-sectional illustration of a photoresist layer formed on the roughened oxide layer of FIG. 15; 
     FIG. 17 is a schematic cross-sectional illustration of the photoresist layer of FIG. 16 undergoing a patterning process; 
     FIG. 18 is a schematic cross-sectional illustration of the patterned photoresist; 
     FIG. 19 is a schematic cross-sectional illustration of an etch step being performed on the oxide layer and underlayer to pattern the underlayer material; 
     FIG. 20 is a schematic cross-sectional illustration of the patterned underlayer material after the etching step in FIG. 19 is substantially complete; 
     FIG. 21 is a schematic cross-sectional illustration of a stripping process to remove residual photoresist and oxide from the patterned underlayer material of FIG. 20; and 
     FIG. 22 is a schematic cross-sectional illustration of the patterned underlayer material in accordance with the present invention; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. The method of the present invention will be described with reference to roughening a material so as to reduce reflectivity of a surface thereof. The roughened surface is a non-uniform surface condition resulting from the present invention, wherein light reflected from the roughened surface tends to be scattered/diffused to a degree where the scattered/diffused beams cause insubstantial damage to a photoresist layer formed on the material. The following detailed description is of the best modes presently contemplated by the inventors for practicing the invention. It should be understood that the description of this preferred embodiment is merely illustrative and that it should not be taken in a lIinse. 
     FIG. 1 is a schemtic illustration of an underlayer  20  having a top surface  22  roughened (or dulled) so as to substantially reduce reflectivity thereof in accordance with the present invention. As can be seen, the top surface  22  includes a roughened surface  24  which results in light  30  incident to the underlayer  20  being diffused such that the intensity of the respective diffused (e.g., scattered) beams  32 , are of such low intensity that they do not damage a thin photoresist  36  (FIG. 5) formed over the underlayer  20 . As mentioned above, such reduction of reflectivity via the roughened surface  24  mitigates the above-noted problems associated with constructive interference, destructive interference, standing waves and intense scattered beams caused by reflected light. 
     FIGS. 2-10 illustrate one specific methodology for carrying out the present invention. In FIG. 2, the underlayer  20  is shown formed on a silicon substrate  37 . The underlayer  20  has a substantially smooth surface  38 . Such a smooth surface  38  will reflect light incident thereto in a manner likely to cause damage to the photoresist  36 . However, as will be described in greater detail below, the smooth surface  38  will be roughened or dulled so as to reduce the reflectivity thereof. The underlayer  20  may include any material such as oxide, polysilicon, silicon nitride and aluminum, for example, which are known to have highly reflective characteristics. In the construction of integrated circuits, one or more metallic layers, such as aluminum or titanium, are deposited and subsequently patterned to provide ohmic or Schottky contacts and electrical connections between various circuit elements. It is to be appreciated that the present invention is intended to include any such materials which may serve as an underlayer and where reflectivity of light incident thereto results in damage to an overlying photoresist. Accordingly, semiconductor materials, silicon (including polysilicon, amorphous silicon, compound semiconductors and polycrystalline silicon), other metals (e.g., titanium, titanium alloys, tungsten, tungsten alloys, aluminum alloys, copper, copper alloys) and other reflective materials employed as underlayers in photolithographic processes are intended to fall within the scope of the present invention as defined by the hereto appended claims. The underlayer  20  may be formed by any of a variety of suitable techniques (e.g., chemical vapor deposition (CVD) processes including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), melting, sputtering and the like). The thickness of the underlayer  20  may be suitably tailored in accordance with the chosen material(s) and the desired performance characteristics of the resulting structures patterned from the underlayer  20 . 
     Turning now, to FIG. 3, the underlayer  20  is shown undergoing a roughening process  50  (e.g., plasma etch). The roughening process includes using any of magnetic enhanced reactive ion etching (MERIE), electron cyclotron etching (ECR), or conventional reactive ion etching (RIE) methods, for example. According to the present example, a MERIE method is used with a non-reactant gas of at least one of argon (Ar) (5-100 sccm); xenon (5-100 sccm); krypton (5-100 sccm); and helium (5-100 sccm) at a source power level within the range of about 20-500 W, bias power level within the range of about 50-200 W, and pressure within the range of about 3-20 mT. It is to be appreciated that the reactant gas be inert so as to mitigate damage to the underlayer  20 . The present invention is intended to include any suitable inert reactant gas or combination of substantially inert reactant gases. 
     FIG. 4 illustrates the underlayer  20  with a roughened surface  56  complete in relevant part. Next, as illustrated in FIG. 5, the photoresist layer  36  is formed on the roughened underlayer  20 . FIG. 6 illustrates the photoresist layer undergoing a patterning step  60 . An exposure source (not shown) illuminates selected areas of the photoresist surface through an intervening mask template. Exposure of the photoresist  36  through the photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. As the light beam  30  (FIG. 1) cuts through the photoresist  36  to pattern a line for example, the light beam  30  strikes the surface of the underlayer  20 . Since the underlayer  20  has the roughened surface  24 , the incident light  30  is scattered/diffused such that the scattered/diffused beams  32  (FIG. 1) have an intensity which results in minimal or negligible damage to the photoresist  36 . As a result, maintaining integrity of the photoresist  36  is facilitated by the present invention which in turn promotes improved resolution in the photolithographic process. 
     FIG. 7 illustrates an etching step  70  being performed on the underlayer  20  so as to pattern the underlayer  20  by etching away portions thereof not protected by the patterned photoresist  36 . FIG. 8 illustrates a patterned underlayer  74  after the etching step  70  has been completed. A stripping/cleaning step  80  is performed in FIG. 9 to remove residual photoresist  36  remaining on the patterned underlayer  74 . Any suitable stripping cleaning methodology may be employed to carry out this step  80 . FIG. 10 illustrate the patterned underlayer  74  substantially complete in relevant part. 
     Turning now to FIGS. 11-21, another embodiment of the present invention is discussed wherein the surface of an oxide layer is roughened so that the oxide layer can serve as an anti-reflective coating for an underlayer. FIG. 11 illustrates an underlayer  120  formed on a silicon substrate  121 . An oxide layer  122  is shown formed over the underlayer  120 . The surface  124  of the oxide layer  122  is roughened (or dulled) so as to substantially reduce reflectivity thereof in accordance with the present invention. As can be seen, the surface  122  includes a roughened surface  126  which results in light  130  incident to the oxide layer  122  being diffused such that the intensity of the respective diffused (e.g. scattered) beams  132 , are of such low intensity that they do not damage a thin photoresist  136  (FIG. 16) formed over the oxide layer  122 . As mentioned above, such reduction of reflectivity via the roughened surface  126  mitigates the above-noted problems associated with constructive interference, destructive interference, standing waves and intense scattered beams caused by reflected light. 
     In FIG. 12, the underlayer  120  is shown formed on the silicon substrate  121 . The underlayer  120  may include any material such as aluminum and/or titanium, which are known to have highly reflective characteristics. In the construction of integrated circuits, one or more metallic layers, such as aluminum or titanium, are deposited and subsequently patterned to provide ohmic or Schottky contacts and electrical connections between various circuit elements. It is to be appreciated that the present invention is intended to include any such materials which may serve as an underlayer and where reflectivity of light incident thereto results in damage to an overlying photoresist. Accordingly, semiconductor materials, silicon (including polysilicon, amorphous silicon, compound semiconductors and polycrystalline silicon), other metals (e.g., titanium, titanium alloys, tungsten, tungsten alloys, aluminum, aluminum alloys, copper, copper alloys) and other reflective materials employed as underlayers in photolithographic processes are intended to fall within the scope of the present invention as defined by the hereto appended claims. The underlayer  120  may be formed by any of a variety of suitable techniques (e.g., chemical vapor deposition (CVD) processes including low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD), melting, sputtering and the like). The thickness of the underlayer  120  may be suitably tailored in accordance with the chosen material(s) and the desired performance characteristics of the resulting structures patterned from the underlayer  120 . 
     In FIG. 13, the oxide layer  122  is formed over the underlayer  120 . The oxide layer  122  has a substantially smooth surface  138 . Such a smooth surface  138  will reflect light incident thereto in a manner likely to cause damage to the photoresist  136 . However, as will be described in greater detail below, the roughened oxide layer  122  will serve as an anti-reflective coating to mitigate damage to the photoresist  136  from reflected light. In the preferred embodiment, the oxide layer  122  includes SiO 2 , however, any suitable material (e.g., silicon oxy-nitride, oxide-nitride-oxide) may be employed to carry out the present invention. Any suitable technique for forming the oxide layer  122  may be employed such as plasma etch chemical vapor deposition (PECVD), or high density plasma chemical vapor deposition (HDPCVD) techniques. 
     Turning now, to FIG. 14, the oxide layer  122  is shown undergoing a roughening process  150 , which includes the use of a plasma etch employing an inert reactant gas. According to one specific embodiment of the invention, the roughening process includes: using any of magnetic enhanced reactive ion etching (MERIE), electron cyclotron etching (ECR), or conventional reactive ion etching (RIE) methods, for example. According to the present example, a MERIE method is used with a non-reactant gas of at least one of argon (Ar) (5-100 sccm); xenon (5-100 sccm); krypton (5-100 sccm); and helium (5-100 sccm) at a source power level within the range of about 20-500 W, bias power level within the range of about 50-200 W, and pressure within the range of about 3-20 mT. It is to be appreciated that the reactant gas be inert so as to mitigate damage to the underlayer  20 . The present invention is intended to include any suitable inert reactant gas or combination of substantially inert reactant gases. The roughening process  150  may be suitably tailored with respect to granularity so as to result in a desired roughened surface. 
     FIG. 15 illustrates the oxide layer  122  with a roughened surface  156  complete in relevant part. Next, as illustrated in FIG. 16, the photoresist layer  136  is formed on the underlayer  120 . FIG. 17 illustrates the photoresist layer  136  undergoing a patterning step  160 . An exposure source (not shown) illuminates selected areas of the photoresist surface through an intervening mask template. Exposure of the photoresist  136  through the photomask causes the image area to become either more or less soluble (depending on the coating) in a particular solvent developer. As the light beam  130  (FIG. 11) cuts through the photoresist  136  to pattern a line for example, the light beam  130  strikes the surface of the oxide layer  122 . Since the oxide layer  122  has the roughened surface  156 , the incident light  130  is scattered/diffused such that the scattered/diffused beams  132  (FIG. 11) have an intensity which results in negligible damage to the photoresist  136 . As a result, maintaining integrity of the photoresist  136  is facilitated by the present invention which in turn promotes improved resolution in the photolithographic process. 
     FIG. 18 illustrates a patterned photoresist  136 . In FIG. 19, an etching step  170  is performed on the underlayer  120  so as to pattern the underlayer  20  by etching away portions of the underlayer  120  and the oxide layer  122  not protected by the patterned photoresist  136 . FIG. 20 illustrates a patterned underlayer after the etching step  170  has been completed. A stripping/cleaning step  180  is performed in FIG. 21 to remove residual photoresist  136  and any oxide  122  remaining on the patterned underlayer  136 . Any suitable stripping cleaning methodology may be employed to carry out this step  180 . FIG. 21 illustrate the patterned underlayer  174  substantially complete in relevant part. It is to be appreciated that this embodiment provides for a patterned underlayer  136  having a substantially smooth surface as compared to the patterned underlayer  20  of the first embodiment. Accordingly, the present embodiment may be desired in situations where a roughened underlayer is not desired. 
     What has been described above are preferred embodiments of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims.