Abstract:
Methods to at least partially compensate for photoresist-induced spherical aberration that occurs during mask imaging used for photolithographic processing of semiconductor devices, LCD elements, thin-film magnetic heads, reticles and other substrates including photo-defined structures thereon. A photoresist or other photosensitive material may be irradiated with a mask pattern image including a selected nonzero spherical aberration value to compensate for photoresist-induced spherical aberration.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the processing of substrates such as semiconductor wafers using photolithography. More specifically, the present invention relates to methods for correcting photoresist-induced spherical aberration that occurs during photolithography.  
         [0003]     2. State of the Art  
         [0004]     Semiconductor devices including integrated circuitry, such as memory dice, are mass produced by fabricating hundreds or even thousands of circuit patterns on a single semiconductor wafer or other bulk semiconductor substrate using photolithography in combination with various other processes. In order to increase the number of memory cells on semiconductor memory devices for a given surface area, it is important to accurately control the optical resolution of the images produced during photolithography. These images are used to define structural features on a semiconductor substrate to fabricate the integrated circuitry of such semiconductor memory devices.  
         [0005]     Photolithography is a process in which a pattern is delineated in a layer of material, such as a photoresist, sensitive to photons, electrons, or ions. In photolithography, an object containing a pattern (e.g., reticle or mask) is exposed to incident light. The image from the reticle or mask is projected onto a photoresist that covers a wafer or other substrate. The photolithographic process typically involves exposing and developing the photoresist multiple times. At a given step, the photoresist is selectively exposed to photons, electrons, or ions, then developed to remove one of either the exposed or unexposed portions of photoresist, depending on whether a positive or negative photoresist is employed. Complex patterns typically require multiple exposure and development steps.  
         [0006]     There are three predominant conventional photolithography methods of optically transferring a pattern on a mask to a photoresist that is coated on a substrate. These methods are contact printing, proximity printing, and projection printing. Currently, projection printing is the most frequently used type of exposing system. Referring to  FIG. 1 , a conventional exposing system used in projection printing is shown. Exposing system  100  includes illumination controller  101  coupled to illumination source  102  for producing light. Illumination source  102  typically includes a mirror, a lamp, a light filter, and/or a condenser lens system. In the exposing system shown in  FIG. 1 , illumination source  102  irradiates mask  108  having the desired pattern to be projected onto photoresist  110 . Projection lens  104 , which may include a complex set of lenses and/or mirrors, focuses the image from mask  108  onto photoresist  110 . Photoresist  110  is developed and substrate  112  is subsequently processed as by etching to form the desired structures and photoresist  110  is then removed.  
         [0007]     When projecting a mask pattern onto a photoresist using the above-described process, it is typically necessary to compensate for spherical aberration induced by the exposing system. Spherical aberration is caused by using spherically shaped lenses and mirrors because truly spherical surfaces do not form sharp images. An example of a spherical aberration is shown in  FIG. 2 . Paraxial light rays  202  and peripheral light rays  204  are incident on a lens  206 . Paraxial light rays  202  and peripheral light rays  204  do not unite accurately at a focus. Rather than the light being focused at a point, it is focused along a line transverse to the plane of the target photoresist, resulting in a blurred image. The spherical aberration is defined as the distance between focal point  210  of paraxial light rays  202  and focal point  208  of peripheral light rays  204 . By using a complex system of lenses and/or mirrors, the spherical aberration of an optical system used in conventional photolithography may be, and typically is, adjusted to zero. In other words, the light incident on photoresist  110  in  FIG. 1  has a spherical aberration of zero.  
         [0008]     Systems for controlling spherical aberration of the light incident on a photoresist are disclosed in U.S. Pat. No. 5,432,587 to Nozue and U.S. Pat. No. 5,973,863 to Hatasawa et al. U.S. Pat. No. 5,636,000 to Ushida et al. discloses a projection lens system that controls environmental effects on the resulting projected image. U.S. Pat. No. 5,935,738 to Yasuzato et al. and U.S. Pat. No. 6,310,684 to Matsuura disclose methods to measure spherical aberration in projection lens systems.  
         [0009]     While these prior art methods and apparatus have provided the capability to compensate for spherical aberration within photolithography exposing systems, they do not completely overcome the problem. This is due to the fact that the point for focusing a mask pattern is not selected to be at the surface of a photoresist layer, but is at some point within the photoresist layer near the middle of its cross-sectional thickness. By focusing the mask pattern in this fashion, a more uniform exposure over the entire cross-section of the photoresist is achieved. This in turn provides mask features with well defined profiles.  FIG. 3  shows a graph of mask feature profiles that result from focusing at different focal points within the photoresist. On the left,  FIG. 3  shows that when the focus depth is too shallow, such as by focusing the light at or near the surface of the photoresist, the upper portion of photoresist  110  is overexposed, resulting in a mask feature profile  114 A that is rounded-off and not thick enough to provide sufficient protection for the subsequent etching process. At a median focus depth, mask feature profile  114 B has uniform sidewalls and maintains the thickness of the photoresist. At lower focus depths, the bottom portion of the photoresist is overexposed, resulting in undercutting as shown by mask feature profile  114 C.  
         [0010]     Because the light must pass through a portion of the photoresist layer to reach the focus point, however, the refractive index of the photoresist itself may induce spherical aberration. Referring to  FIG. 4A , the origin of photoresist-induced spherical aberration is shown, where the photolithography process is adjusted in the conventional manner to compensate for spherical aberration from the optical system. Incident light  302 , having a spherical aberration set at zero, arrives at photoresist  110  at various angles due to diffraction of light off of mask  108  (not shown, see  FIG. 1 ). Incident light  302  is refracted due to the difference in the index of refraction between air  306  and photoresist  110 . However, since incident light arrives at various different angles, it is refracted by different amounts in accordance with Snell&#39;s law and comes into focus at different depths within the photoresist  110 . Thus, refraction by photoresist  110  causes the focal points for light arriving at different incident angles to be separated along a line, resulting in spherical aberration  308 . This separation of the incident light  302  within photoresist  110  is the photoresist-induced spherical aberration.  FIGS. 4A and 4B  also illustrate how incident light  302  at differing angles would ideally unite accurately at a single focal point  304  (shown by broken lines in  FIG. 4A ) in the absence of refraction by photoresist  110 .  
         [0011]     Because the density of integrated circuitry patterns is constantly increasing to meet semiconductor technology requirements, photoresist-induced spherical aberration is progressively becoming a significant problem. Photoresist-induced spherical aberration causes the best focus to be a function of mask pattern density and feature size and pitch (spacing). This is due to the diffraction of light at different angles by various mask patterns. Reduced pitch gives higher diffraction. Thus, features like isolated lines will produce different focal points within the photoresist layer than dense line patterns. For each type of feature, there is a process window wherein light may be focused above or below the feature focal point and be within a tolerable range for forming an acceptable feature profile as shown in  FIG. 3 . Because the focal points for each feature are at different locations, the process windows for the various features will also be different. Therefore, to provide mask feature profiles that are within a tolerable range for all mask features having different sizes and pitches, it is necessary to focus the mask pattern at a point within the photoresist layer where portions of the process windows for each feature overlap, which is the overall process window for the mask pattern.  
         [0012]     Because the photoresist-induced spherical aberration causes the focal points for the different mask features to be farther apart, there will be less overlap of their corresponding process windows. Therefore, if spherical aberration is not corrected or compensated for, the acceptable overall process window will be smaller, reducing process tolerances. Failure to correct for spherical aberration can cause misregistration in the resulting resist pattern and, if focus is poor, the resulting resist pattern is not as “clean” or well defined, as shown on the right and left sides of  FIG. 3 .  
         [0013]     Accordingly, in order to improve the quality of patterns transferred to photoresists using photolithography, a need exists to reduce photoresist-induced spherical aberration.  
       BRIEF SUMMARY OF THE INVENTION  
       [0014]     The present invention, in a number of embodiments, includes methods to reduce, compensate for, or eliminate photoresist-induced spherical aberration exhibited in conventional photolithography processing. The present invention may be used in photolithography processing for fabrication of semiconductor devices, liquid crystal display elements, thin-film magnetic heads, reticles, and for many other applications that require accurate photolithographic imaging.  
         [0015]     One exemplary embodiment comprises a method of adjusting the spherical aberration of light used to pattern a photoresist. A substrate having a photoresist disposed thereon is provided. An illumination source for producing light is provided in conjunction with a mask to project a mask pattern image on the photoresist. An optical element or system may then be used to set the spherical aberration of the mask pattern image to have a nonzero value selected to reduce, compensate for or eliminate photoresist-induced spherical aberration effects. Photoresist-induced spherical aberration may be estimated from the resist refractive index and the nominal focus position in the resist. The mask pattern image having the selected spherical aberration is then used to irradiate the photoresist.  
         [0016]     In another exemplary embodiment, the compensatory spherical aberration of the mask pattern image incident on the photoresist may be set approximately equal to the photoresist-induced spherical aberration of the photoresist when the photoresist is irradiated with light having a spherical aberration of zero.  
         [0017]     In another exemplary embodiment, the compensatory spherical aberration of the mask pattern image incident on the photoresist may be determined according to a disclosed formula.  
         [0018]     In another exemplary embodiment, the compensatory spherical aberration of the mask pattern image incident on the photoresist may be determined experimentally from process measurements.  
         [0019]     In yet another exemplary embodiment, the mask pattern image having a predetermined compensatory spherical aberration may be focused at approximately the middle of the cross-section of the photoresist.  
         [0020]     These features, advantages, and alternative aspects of the present invention will be apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0021]     In the drawings, which illustrate what is currently considered to be the best mode for carrying out the invention:  
         [0022]      FIG. 1  illustrates a conventional projection printing-type photolithography system.  
         [0023]      FIG. 2  schematically illustrates a spherical aberration phenomenon attributable to passage of light through a lens.  
         [0024]      FIG. 3  illustrates mask feature profiles resulting from different focusing depths within a photoresist layer.  
         [0025]      FIG. 4A  schematically illustrates the phenomenon of photoresist-induced spherical aberration.  
         [0026]      FIG. 4B  schematically illustrates light rays focused to a point in the absence of a photoresist.  
         [0027]      FIG. 5A  is a schematic illustration of an exemplary embodiment of an exposing system used in the present invention.  
         [0028]      FIG. 5B  is a schematic illustration of a pellicle secured to a mask suitable for use in the present invention.  
         [0029]      FIG. 6  is a schematic illustration of light incident on a cross-section of photoresist wherein photoresist-induced spherical aberration is eliminated. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention includes methods to reduce, compensate for, or eliminate photoresist-induced spherical aberration exhibited in conventional photolithography processing.  FIG. 5A  illustrates the practice of the present invention in conjunction with the most common type of photolithography process used in semiconductor processing, projection printing. The present invention may employ step-and-repeat, step-and-scan type projection printing, or other similar systems.  
         [0031]     Referring to  FIG. 5A , an exemplary projection system  400  used in the method of the present invention is illustrated. Illumination controller  405  may be controllably coupled to illuminating source  402  for projecting light rays  401 . Illuminating source  402  may include, for example, a mirror, a lamp, a light filter, and/or a condenser lens system. The term “light” as used herein is not restricted to visible light, but may also include other forms of radiation energy such as photons, laser beams, or x-rays. Mask  108 , defining a pattern to be projected onto photoresist  110 , receives light rays  401  from illuminating source  402  to produce mask pattern image  406  representative of the pattern of mask  108 . The terms “mask” and “reticle” are used synonymously throughout the following description of the present invention. Optionally, mask  108  may be secured to hard or soft pellicle  414  to protect mask  108  from contaminants, as shown in  FIG. 5B . Hard pellicles include, for example, glass or polymer fibers.  
         [0032]     Again with reference to  FIG. 5A , projection lens apparatus  409  receives mask pattern image  406  from mask  108 . Projection lens apparatus  409  may be, for example, a reduction lens or a combination of lenses and/or mirrors for focusing mask pattern image  406  onto the surface of photoresist  110 . Typical semiconductor fabrication photolithography involves a four to ten times reduction of the pattern size on mask  108 . Projection lens apparatus  409  projects mask pattern image  406  to produce a projected pattern image  407  comprising light rays  403  (not shown, see  FIG. 6 ).  
         [0033]     Projection lens apparatus  409  may be configured for altering the spherical aberration of mask pattern image  406  to reduce, compensate for, or eliminate the phenomenon of photoresist-induced spherical aberration caused by photoresist  110 . Current optical systems used in photolithography are capable of varying the spherical aberration. Techniques for altering the spherical aberration in projection lens apparatus  409  are well known to those of ordinary skill in the art. For example, U.S. Pat. No. 5,973,863 to Hatasawa et al. (“Hatasawa”), the disclosure of which is incorporated herein by reference, teaches using a glass plate that is disposed at the bottom of the projection lens apparatus. As disclosed in Hatasawa, the spherical aberration of the projection lens may be controlled by altering the thickness of the glass plate and the radius of curvature of the glass plate.  
         [0034]     The spherical aberration may also be altered external to projection lens apparatus  409 . Systems for controlling spherical aberration external to projection lens apparatus  409  are disclosed in U.S. Pat. No. 5,432,587 to Nozue (“Nozue”), the disclosure of which is incorporated herein by reference. As disclosed in Nozue, a typical system for controlling spherical aberration of the light in projection lens systems includes varying the optical path between mask  108  and projection lens apparatus  409  or using a transparent parallel plate between mask  108  and projection lens apparatus  409 . Other known methods for controlling spherical aberration include moving the position of the reticle and/or the photoresist with respect to the projection lens apparatus  409 , as well as by slightly changing the wavelength of the projected light. However, unlike the present invention, which comprises altering the spherical aberration of projected pattern image  407  to a nonzero value to reduce, compensate for, or eliminate photoresist-induced spherical aberration, conventional optical systems used in photolithography typically set the spherical aberration to zero.  
         [0035]     Projected pattern image  407 , in accordance with the present invention, thus exhibits a predetermined spherical aberration set to reduce, compensate for, or eliminate the photoresist-induced spherical aberration from photoresist  110 . Thus, the spherical aberration value of the projected pattern image  407  is opposite that of the photoresist-induced spherical aberration. Projected pattern image  407  may then be irradiated onto photoresist layer  110  on substrate  112 . Substrate  112  may be a semiconductor substrate such as single crystal silicon, single crystal gallium arsenide, polysilicon, indium phosphide, a layered bulk semiconductor substrate (such as a silicon on insulator (SOI) substrate as exemplified by a silicon on glass (SOG) or silicon on sapphire (SOS) substrate), a glass (for example, soda-lime glass, borosilicate glass, or quartz) useful for forming reticles, or any other suitable material such as those used in forming liquid crystal displays and thin film magnetic heads. Substrate  112  having photoresist  110  may be supported and held in position on a holding device such as a chuck (not shown) which may be part of, or controlled by, a stepper (not shown), as known in the art.  
         [0036]     Referring to  FIG. 5B , hard or soft pellicle  414  may be used to protect mask  108 . Hard or soft pellicle  414  may also introduce a spherical aberration into mask pattern image  406  that must be corrected for by projection lens apparatus  409  or an apparatus external to it which is capable of setting the spherical aberration of the projected pattern image  407 .  
         [0037]      FIG. 6  illustrates the interaction of light rays  403  of projected pattern image  407  with photoresist  110  after adjustment in accordance with the present invention. Light rays  403  having a predetermined spherical aberration  412  irradiate photoresist  110 . In an exemplary embodiment, the nominal focal point  404  of light rays  403  may be set approximately at the middle of photoresist  110  to provide more uniform exposure over the entire cross-section of photoresist  110 . In other words, the highest intensity of light rays  403  may be focused at the middle of the cross-section of photoresist  110 . Spherical aberration  412  may be approximately equal to the value of the photoresist spherical aberration when photoresist  110  is irradiated with light having a spherical aberration of zero as previously described with reference to  FIG. 4A . Spherical aberration  412  may also be calculated based on the material properties of photoresist  110 . In an exemplary embodiment, spherical aberration  412  may be calculated using the following formula:  
       a   =         (       n   2     -   1     )     ·   t       8   ·     n   3     ·     s   4               
 where a is the spherical aberration  412  of light rays  403 , n is the refractive index of photoresist  110 , t is the thickness of photoresist  110 , and s is the focal distance into photoresist  110 . The refractive index of photoresist  110 , n, may be typically within a range of about 1.4 to 2.0. For organic photoresists, n is typically about 1.75. Photoresists  110  containing metal fillers may have a refractive index of up to about 2.0. 
 
         [0039]     In another exemplary embodiment, spherical aberration  412  may be determined by taking process measurements. The best focus of different size and density features on mask  108  may be measured. Then differences in nominal focal points of the different size and density features of mask  108  provide a useful tool for estimating the necessary spherical aberration  412  to be introduced into mask pattern image  406  to best accommodate the range of exhibited focal points and provide a suitable operating window.  
         [0040]     Although the foregoing description contains many specifics, these are not to be construed as limiting the scope of the present invention, but merely as providing certain exemplary embodiments. Similarly, other embodiments of the invention may be devised which do not depart from the spirit or scope of the present invention. The scope of the invention is, therefore, indicated and limited only by the appended claims and their legal equivalents, rather than by the foregoing description. All additions, deletions, and modifications to the invention, as disclosed herein, which fall within the meaning and scope of the claims are encompassed by the present invention.