Abstract:
A method and system of making a mask with a transparent substrate thereon is provided. A first resolution enhancement structure is formed on the first portion of the transparent substrate. A second resolution enhancement structure is formed on a second portion of the transparent substrate, with the second resolution enhancement structure different from the first resolution enhancement structure.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates generally to photolithography and more particularly to patterning different types of features. 
     2. Background Art 
     Integrated circuits are now used in almost every type of electronic product ranging from toys to massive computers. These integrated circuits are all generally made by a photolithographic process, which involves manufacturing a template containing patterns of the electrical circuit as transparent and opaque areas. The patterned template is referred to as a “reticle” or “mask”. 
     A radiation source, such as a light, is used to copy or “pattern” multiple images of the mask onto a photosensitive material or photoresist on the surface of a silicon wafer. Once features are patterned on the photoresist, further processing is performed to form various structures on the silicon wafer, which is subsequently cut up to form the integrated circuits. In addition to repeatedly “patterning” features onto the photoresist on the silicon wafer with a single mask, multiple masks are used to pattern different photoresist layers to form different structures at different levels on the silicon wafer. 
     In conventional industry practice, the masks are fabricated starting from an initial mask blank, which is transparent to the imaging light. Typically, the mask blank consists of fused silica or quartz. The mask blank is coated by an opaque film, typically a chromium based material. The opaque film is also processed using another mask and a photoresist to create openings in the opaque film to expose and permit light to pass through the openings and through the transparent quartz. 
     As the size and density of features start to be below the wavelength of the light used to pattern the images, a different class of masks is necessary because the light is subject to diffraction and interference effects. Diffraction effects are due to the wave nature of light, which cause peaks and valleys to occur in the intensity of light passing through an opening, such as an opening in the opaque film, and falling on the photoresist of the silicon wafer. Interference effects occur with side-by-side openings where the peaks and valleys of the light wave can interfere so as to cancel each other out or can reinforce and amplify each other depending on the location of the openings. 
     This is a problem when the openings are used to pattern features having sizes below the wavelength of the light used and near the resolution limit of the light. Optical distortion becomes extremely high and the correspondence between an image on the mask and the feature on the photoresist is no longer one-to-one because of information loss. This is particularly problematic where different types of features are in the same integrated circuit, such as closely spaced repeating features and spaced-apart non-repeating features. 
     Solutions to this problem has been long sought but has equally long eluded those skilled in the art. 
     DISCLOSURE OF THE INVENTION 
     The present invention provides for a method of making a mask with a transparent substrate. A first resolution enhancement structure is formed on a first portion of the transparent substrate. A second resolution enhancement structure is formed on a second portion of the transparent substrate, with the second resolution enhancement structure different from the first resolution enhancement structure. This method of making a mask allows the patterning of different types of features in the same processing step thus reducing manufacturing time and cost. 
     Certain embodiments of the invention have other advantages in addition to or in place of those mentioned above. The advantages will become apparent to those skilled in the art from a reading of the following detailed description when taken with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic of a photolithographic system using the present invention; and 
         FIG. 2  is a schematic of a mask with representations of the electric field of the light on the mask, the electric field on the photoresist, the intensity of the light on the photoresist, and an integrated circuit in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , therein is shown a simplified schematic of a photolithographic system  100  using the present invention. In the photolithographic system  100 , radiation is directed from an illumination source  102  through a patterned mask  104  and a lens  106  onto a semiconductor wafer  108 . 
     The semiconductor wafer  108  includes a photoresist layer  110  on a semiconductor substrate  112 , which will form a plurality of integrated circuits when completed. 
     The patterned mask  104  includes a light-transparent substrate  114 , of a material such as fused silica or quartz, with a patterned mask coating  116 . 
     The term “horizontal” as used in this application is defined as a plane parallel to the conventional plane or surface of the light-transparent substrate  114 , where the patterned mask coating  116  is deposited, regardless of the orientation of the substrate. The term “vertical” refers to a direction perpendicular to the horizontal as just defined. Prepositions, such as “on”, “side”, “higher”, “lower”, “over”, and “under” are defined with respect to the conventional plane or surface being on the top surface of the light-transparent substrate  114 , which is shown in an upside down position in the figures. 
     The term “process” as used herein is defined without being limiting to include one or more of the following: depositing or growing semiconductor materials, masking, patterning, lithography, etching, implanting, removal, and/or stripping. 
     The present system is applicable to any wavelength of radiation and the modifications for other wavelengths would be obvious to those skilled in the art based on the description of the present invention provided herein. 
     The illumination source  102  produces light  118 , which the patterned mask  104  selectively allows through as patterned light  120  to be focused by the lens  106  on to selected areas of the photoresist layer  110  to reproduce the mask pattern in the patterned mask coating  116 . 
     In photolithographic systems, there is a minimum distance beyond which even a geometrically perfect lens cannot separate two points; i.e., when the two points are less than a minimum distance from each other, the two points cannot be separated or “resolved”. This is due to diffraction and interference effects. Diffraction effects are due to the wave nature of light, which cause peaks and valleys to occur in the intensity of light passing through an opening, such as an opening in the opaque film, and falling on the photoresist of the silicon wafer. Interference effects occur with side-by-side openings where the peaks and valleys of the light wave can interfere so as to cancel each other out or can reinforce and amplify each other depending on the location of the openings. 
     Depending upon how close two points are, the diffraction effect spreads the light from these two points across the imaging lens. If the two points are sufficiently close, the light will be diffracted out of the path of the lens. In this case, the points will be too close to each other and they will be under the limit of resolution of the system. The resolution of a non-perfect lens depends upon the wavelength of the light source and the numerical aperture (NA) of the lens. Two images are considered as being resolvable when the intensity between them drops to 80 percent of the image intensity. Thus, two images are considered resolvable when the equation is fulfilled:
         2D=0.6 {hacek over ((S)}/NA   where: 2D is the separation of the two images;
           {hacek over ((S)} is the wavelength of the illumination source  102 ; and   NA is the numerical aperture of the lens  106 .   
               

     Referring now to  FIG. 2 , therein is shown a schematic  130  of the patterned mask  104  with representations of mask electric fields  132  of the light  120  ( FIG. 1 ) from the patterned mask  104 , wafer electric fields  134  of the light  122  ( FIG. 1 ) on the photoresist layer  110  due to diffraction effects, and wafer intensities  136  of the light  122  ( FIG. 1 ) on the photoresist layer  110  (the intensities are the square of the wafer electric fields). 
     There is a major problem in patterning two or more distinctly different types of features at the same time in integrated circuits. For example, integrated circuit memory devices have core polysilicon wordlines and periphery polysilicon gates, which heretofore required different pattern masks because the two different types of features can be different sizes, and/or density features. As the feature size of the structures becomes smaller and smaller, they are harder and harder to image. 
     In another example, the polysilicon gates of the core transistors and the periphery transistors should be printed at the same time to minimize costs. Unfortunately, the core transistors are a regular array that where the transistors are very close packed together and the periphery transistors are random transistors that are substantially far apart. 
     On occasion, attempts to pattern the different types of features have used different enhancement resolution techniques in different processing steps. Unfortunately, these attempts have not been completely successful because of interface problems, stringer problems, and gouging. 
     The inventors have discovered that using different resolution enhancement techniques on different portions of the light-transparent substrate  114  can provide solve this major problem. In one approach, two different types of films are placed on the light-transparent substrate  114  and, in a second approach, one type of film is placed over a different type of film that is on the light-transparent substrate  114 . The different resolution enhancement techniques use different resolution enhancement structures. The following are some of the different enhancement structures, which may be used singularly or in combination as different portions of the pattern mask  104  ( FIG. 1 ) in accordance with the present invention. 
     A binary mask portion  150  is generally for repeating parallel features. An opaque film  151  has repeated parallel openings  152  and  153 . The mask electric fields  132  are two spaced apart rectangular blocks  154  and  155 . The wafer electric fields  134  are two peaked curves  156  and  157 . The mask intensities are two peaked curves  158  and  159 . Well-defined peaked curves will produce resolution-enhanced features. 
     A phase shift mask (PSM) portion  160  is also generally for repeating parallel features. The opaque film  151  has repeated parallel openings  161  and  162 . Since phase shifting of light occurs as it passes through different thicknesses of material, an extra layer of transmissive material  163  can be added to one optical path to cause a phase shift. This extra layer of transmissive material  163 , such as silicon or quartz, is placed in the opening  162  to produce a phase shift of 180°, and this corresponds to an optical path length difference of {hacek over ((S)}/ 2 . The mask electric fields  132  are two spaced apart rectangular blocks  164  and  165  with the rectangular block  165  having a phase shift of 180° from the rectangular block  164 . The wafer electric fields  134  are a sinusoidal curve  166 . The mask intensities are two well-defined peaked curves  167  and  168 . 
     A proximity effect mask portion  170  is for single features. The exact size and shape of the features patterned on a semiconductor wafer depend on their closeness, or “proximity”, to other structures. The proximity effects appear in the form of distorted shapes and, in extreme cases, no shapes at all. By using computer simulation, a pattern feature opening  171  in the opaque film  151  can be changed by adding or subtracting sub-structures called “optical-proximity-correction-features” (OPC features), such as scattering bar openings  172  and  173 . The scattering bar openings  172  and  173  are sub-resolution with lines placed adjacent to isolated lines on the same pitch as dense features. The mask electric fields  132  are a rectangular block  174  with two thin rectangular blocks  175  and  176  to either side, which are at the limits of resolution. The wafer electric fields  134  are a peaked curve  177  with two smaller curves on either side of two higher order diffractions. The mask intensity is a well-defined broad peaked curve  178  with steep sides. 
     Other examples of OPC features are serifs, which are additional open areas on corners to reduce corner rounding and feature length shortening, line-jogs, which are width variations to adjust for adjacent features. 
     An alternating phase shift mask (Alt-PSM) portion  180  generally gives the greatest improvement in resolution when closely spaced clear lines are separated on an opaque background. The opaque film  151  has repeated parallel openings  181 ,  182  and  183 . Since phase shifting of light occurs as it passes through different thicknesses of material, the light-transparent substrate  114  can be etched out to cause a phase shift or by depositing transparent material over the entire mask and then etching away the desired areas. The opaque film  151  alternates between openings which are not phase-shifted and openings which are phase-shifted. For example, for the Alt-PSM portion  180 , the alternative parallel openings  181  and  183  are etched out to produce a phase shift of 180°, and this corresponds to an optical path length difference of {hacek over ((S)}/ 2 . The mask electric fields  132  are three spaced apart rectangular blocks  184  through  186  with the rectangular blocks  184  and  186  having a phase shift of 180° from the rectangular block  185 . The wafer electric fields  134  are a sinusoidal curve  187 . The mask intensities are three well-defined peaked curves  188  through  190 . 
     An attenuated phase shift mask (Att-PSM) portion  200  generally gives the greatest improvement in resolution for alternating rows of rectangular transparent areas offset from one another or non-repetitive patterns. The Att-PSM portion  200  is fabricated by replacing the opaque film  151  with a partially transparent film  201 , such as a molybdenum silicon-oxynitride film, or patterned to be a half-tone film, such as a half-tone dotted chrome film, to allow only 5% to 10% of the light to pass through the partially transparent film  201 . The thickness of the partially transparent film  201  also produces a phase shift of 180°. 
     The 5% to 10% of the light will be too weak to expose the photoresist to the degree necessary for it to be washed away during development, but the negative amplitude of the phase-shifted light will be sufficient to destructively interfere with the non-phase-shifted light. The partially transparent film  201  has two openings  202  and  203 . The mask electric fields  132  are two spaced apart rectangular blocks  204  and  206  connected by a phase shifted small rectangular block  205 . The wafer electric fields  134  are two peaked curves  207  and  209  connected by a flattened region  208 . The mask intensities are two well-defined peaked curves  210  and  212  connected by a small curve  211 , which is too weak to expose the photoresist. 
     As previously described, the above resolution enhancement techniques and structures can be used with different adjacent films. They can also be used with one film over the other, such as the light-transparent substrate  114  having a partially transparent film  220  with an opaque film  222  over it. 
     An optical-proximity-correction (OPC) binary mask portion  230  is generally for sharp, repeating parallel features. The partially transparent film  220  and the opaque film  222  have repeated parallel openings, of which one opening  231  and an OPC feature  232  are shown. The mask electric fields  132  are a rectangular block  233  and a thin rectangular block  234 . The wafer electric fields  134  are a large peaked curve  235  and a small peaked curve  236 . The mask intensities are form a single well-defined peaked curve  237 . 
     An attenuated phase shift mask (Att-PSM) portion  240  has the opaque film  222  removed (as indicated by the dotted lines) from the partially transparent film  220 . The Att-PSM portion  240  thus only has the partially transparent film  220  to transmit only 5% to 10% of the light and to produce a phase shift of 180°. The partially transparent film  220  has two openings  241  and  242 . The mask electric fields  132  are two spaced apart rectangular blocks  243  and  245  connected by a phase shifted small rectangular block  244 . The wafer electric fields  134  are two peaked curves  246  and  248  connected by a flattened region  247 . The mask intensities are two well-defined peaked curves  249  and  251  connected by a small curve  250 , which is too weak to expose the photoresist. 
     The optical-proximity-correction (OPC) binary mask portion  230  and the Att-PSM portion  240  are shown for exemplary purposes producing the respective tightly spaced polysilicon wordlines  260  and isolated polysilicon gates  262  on respective core region  264  and peripheral region  266  on the semiconductor substrate  112  using the single patterned mask  104  ( FIG. 1 ). The tightly spaced wordlines structures would be proximally spaced and the isolated gate structures would be distally spaced. 
     While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations which fall within the spirit and scope of the included claims. All matters set hithertofore forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.