Abstract:
A wafer ( 18 ) is made using a mask ( 14 ) that has a quartz substrate ( 15 ) and a patterned stack ( 32 ) for providing a mask pattern. The patterned stack comprises an opaque layer ( 36 ) between two ARC layers ( 34, 38 ). The patterned stack reduces flare, which in turn improves critical dimension (CD) control. The stack reduces the reflections that come from the interface between the opaque layer ( 36 ) and quartz substrate ( 15 ). This stack also absorbs the reflections that come back from the direction of the wafer. The opaque layer ( 36 ) is silicon, which is opaque at wavelengths below 300 nanometers, and the ARC layers are non-stoichiometric silicon nitride.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates generally to semiconductor circuits, and more specifically, to the manufacture of semiconductor circuits.  
         BACKGROUND OF THE INVENTION  
         [0002]    Physical limitations associated with lithography that is used to fabricate semiconductor devices are an issue for small dimensions. In optical lithography, a photosensitive film, a photoresist, is patterned by a photomask. A photoresist is formed onto a wafer having integrated circuits. The photomask has areas composed of a light absorber in a feature that corresponds to the desired circuitry for the integrated circuits. Portions of the photoresist as determined by the photomask are exposed to light from a light source. After exposure, a desired device pattern remains in the photoresist on the wafer. Further processing may be performed to transfer the resulting photoresist pattern onto the wafer.  
           [0003]    However, in exposing the photoresist to the light source, openings in the photomask do not exactly transfer to the wafer as a result of light diffraction and reflection. A condition known as flare occurs when light is radiated onto areas of the wafer that should not be exposed to light. Flare caused by light being reflected from optical components degrades optical microlithography perfomance. The photomask is considered one of the optical components. A conventional photomask uses a quartz plate with an overlying absorber material, such as chromium. The chromium is used to form the desired features for the integrated circuit. The reflectivity from quartz and chromium is about forty percent that adds about two percent extra flare through the system. Elimination or reduction of such reflection is needed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]    The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements.  
         [0005]    [0005]FIG. 1 illustrates a photolithographic system having a photomask in accordance with the present invention;  
         [0006]    [0006]FIG. 2 illustrates in further detail the photomask for use in the system of FIG. 1; and  
         [0007]    [0007]FIG. 3 illustrates in flow chart form a lithographic process of patterning the photomask of FIG. 2 in a process of making an integrated circuit. 
     
    
       [0008]    Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of the embodiments of the present invention.  
       DETAILED DESCRIPTION  
       [0009]    [0009]FIG. 1 illustrates a lithographic system  10  generally having a light source  12 , a photomask  14  including a quartz plate  15 . The light source  12  selectively transmits light to optics  16  through the photomask  14 . The optics  16  focuses the light onto a photoresist on the surface of a wafer  18 . The open area of photomask  14  exposes areas  20 ,  22  and  24  of the photoresist on the wafer  18  with light. Areas  26  and  28  of the photoresist are not exposed to light because the light was blocked by the absorbing stacks  30  and  32  of photomask  14 . Each of absorbing stacks  30  and  32  has three layers of material and functions as a patterned opaque stack to form a mask pattern. For example, absorbing stack  32  has a first antireflective coating (ARC)  38 , an opaque layer  36  and a second antireflective coating (ARC)  34 . Opaque layer  36  functions to block all or nearly all light (e.g. ninety-nine percent), but in any event blocks at least eighty percent of the light. In one form, opaque layer  36  may be implemented with substantially pure silicon (including polysilicon). The term “substantially pure silicon” includes doped silicon, but not silicon-containing compounds. Alternative materials may include metal, silicides and other non-transparent materials. The first ARC  38  and the second ARC  34  may be implemented respectively by non-stoichiometric silicon nitride. Alternative ARC materials may include any transparent dielectric materials, such as SiO x N y , CaF 2 , MgF 2 , although not limited to these materials, to form a dielectric layer.  
         [0010]    Illustrated in FIG. 2 is a detail of the absorbing stacks  30  and  32  of photomask  14 . For convenience of illustration, elements that are common in FIG. 1 and FIG. 2 are provided with the same figure reference number. Assume initially that the mask  14  does not have an ARC layer such as ARC  38  in the absorbing stack  32  in any of the absorbing stacks. Further assume that incoming light  42  is not collimated and therefore not exactly perpendicular to the interface of quartz plate  15  and the absorbing stack  32 . As a result, the light  42  is reflected from the interface as represented by reflected light  44 . The reflected light  44  is then reflected from an upper surface  40  of quartz plate  15 . A reflected light  46  is then radiated through the pattern opening between absorbing stacks  30  and  32 . This light is passed through optics  16 . Because the reflected light  46  is directed to optics  16  at an angle other than ninety degrees, the light  46  will undesirably expose a portion of either area  26  or area  28 , or both. This undesired exposure is characterized as flare.  
         [0011]    Assume now that the structure of absorbing stacks  30  and  32  as illustrated in FIG. 2 exists. When light  42  strikes the interface of quartz plate  15  and the absorbing stack  32 , the light is not reflected as reflected light  44 . Instead, ARC  38  passes substantially all the light through instead of reflecting the light. When the light reaches the opaque layer  36 , the light will be absorbed. Therefore, the absorbing stacks  30  and  32  efficiently function to avoid the previously noted flare.  
         [0012]    Illustrated in FIG. 3 is a flowchart of a process  50  for patterning a photomask to make an integrated circuit on a wafer. In a step  52 , a mask substrate, such as quartz plate  15 , is provided. A first ARC, such as ARC  38 , is formed on the mask substrate in a step  54 . An absorber, such as absorber  36 , is formed on the first ARC in a step  56 . A second ARC, such as ARC  34 , is formed on the absorber in a step  58 . The first ARC, absorber and second ARC is patterned pursuant to a desired integrated circuit pattern to form a mask in a step  60 . A semiconductor wafer, such as silicon wafer  18 , is provided in a step  62 . Photoresist is applied to the silicon wafer  18  in a step  64  to form a photoresist layer. In a step  66 , the photoresist is patterned using the mask from the step  60 . In a step  68 , semiconductor processes are performed on the wafer  18  to complete manufacture of an integrated circuit.  
         [0013]    It should be noted that when absorbing stacks  30  and  32  are implemented using SiN x , silicon and SiN y , the absorbing stacks are easily etched using a dry or plasma etching process. Such processes are very selective, have excellent resolution at small dimensions and have low cost. The use of silicon for the absorbing stacks also has the advantage of being a low stress structure due to silicon having a low coefficient of thermal expansion that minimizes stress fractures and peeling of small features from the mask. The implementation of silicon nitride ARCs reduces flares that permits improved control of the critical dimensions of the features.  
         [0014]    The determination of the composition of silicon nitride, SiN x  and SiN y , may be optimized to minimize reflection. Depending on the wavelength of the light source  12 , the amount of reflection can be calculated as a function of both the extinction coefficient, k, of the silicon nitride and the thickness of the silicon nitride. Thus, a contour is generated that details an optimal silicon nitride thickness and extinction coefficient that results in minimal reflectivity characteristic. From this contour information, plots can be readily generated that illustrate a plot of how the reflectivity percentage of silicon nitride varies with respect to the extinction coefficient for various light wavelengths. Similarly, plots can be readily generated that illustrate how the reflectivity of silicon nitride varies with respect to the thickness of the silicon nitride film. The optimum value is then used to determine values of x and y for the silicon nitride composition.  
         [0015]    It should be noted that x and y may be the same value, but the first ARC layer may have a first non-stoichiometric composition and the second ARC layer may have a second non-stoichiometric composition that is different from the first non-stoichiometric composition.  
         [0016]    By now it should be appreciated that there has been provided a photolithographic system for use in making an integrated circuit wherein flare caused by light reflection from a photomask is minimized. In the illustrated form, first and second distinct ARC layers are used on a same side of a quartz plate. Between the first and second ARC layers is an absorbing layer. Although various absorbing layer materials may be used, Applicants have discovered that the use of silicon for the absorbing layer has numerous advantages over prior metals that have been used, such as chromium. Preferably, the absorbing layer has an extinction coefficient of at least one. For example, silicon has higher absorption than chromium in short wavelengths, such as below 300 nm. The use of silicon results in improved critical dimension control, better resolution and more stability in the photomask structure.  
         [0017]    In the foregoing specification, the invention has been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, various compositions of silicon nitride may be used for each of the ARC layers in the absorbing stacks  30  and  32 . Semiconductor wafers other than silicon-based wafers may be manufactured using the disclosed photolithography system. Various types of optics may be used to implement optics  16 . Various optical materials in addition to quartz plate  15  may be used for mask  14 . Differing types of light may be used as the light source  12 . Examples of the light source  12  include argon fluoride, krypton fluoride and fluoride lasers. Various wavelength light may be used; however the wavelength should not be greater than about 300 nanometers. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention.  
         [0018]    Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. As used herein, the terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The terms a or an, as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e., open language). The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.