Patent Publication Number: US-7224030-B2

Title: Method and apparatus for producing rectangular contact holes utilizing side lobe formation

Description:
This application is a divisional of patent application Ser. No. 09/559,090, entitled “Method for Producing Quadratic Contact Holes Utilizing Side Lobe Formation,” filed on Apr. 27, 2000, which application is incorporated herein by reference. 

   FIELD OF THE INVENTION 
   This invention relates generally to photolithography for semiconductor fabrication and more particularly to forming quadratic contact holes and similar features using constructive interference of side lobe formations. 
   BACKGROUND OF THE INVENTION 
   As semiconductor feature sizes continue to shrink into the sub-micron range, the effects of light diffraction during photolithographic processes become more pronounced. Deep ultraviolet (UV) exposure tools use light sources having a 248 nm wavelength. When such tools are used to form semiconductor devices having feature sizes of 200 to 300 nm, the effects of light diffraction become quite pronounced. 
   One area where diffraction effects are a particular problem is in the formation of small rectangular features, such as contact holes and vias. Square features on a photomask pattern become rounded due to diffraction when the substrate wafer is exposed. Rounded holes have a greater resistance than rectangular contact holes of similar size, resulting in greater contact resistance. In order to prevent the increased resistance, a larger diameter round contact hole would be required, resulting in lower packing density (i.e. a greater spacing requirement). In either event, the rounded contact hole is less desirable than a rectangular contact hole of similar size. 
   The use of attenuated phase shift masks is well-known to offset the problems associated with light diffraction in small feature size products. The prior art offers many solutions which are geared toward eliminating side lobe patterns resulting from diffraction effects. For instance, Tzu teaches the use of an opaque ring around the phase shifting features to reduce the effects of side lobe formation in U.S. Pat. No. 5,935,736. Choi et al. teach the formation of “dummy” open regions in the phase shift mask, which dummy regions allow light to pass through unimpeded and 180 degrees out of phase with diffracted light that would otherwise form side lobes below the dummy open region in U.S. Pat. No. 5,591,550. Garza, in U.S. Pat. No. 5,795,682, also teaches the use of guard rings to destructively interfere with, and hence eliminate, side lobe patterns. Sugawara teaches the use of alternating patters of phase shifting and non-phase shifting patterns to cause destructive interference of side lobe patterns in U.S. Pat. No. 5,487,963. 
   The shortcoming of the prior art is that the various techniques to reduce or eliminate side lobe formation do not address the need for forming good rectangular patterns in dense patterns of small features. The present invention overcomes the shortcoming of the prior art, as will become apparent from the following description of preferred embodiments of the invention. 
   SUMMARY OF THE INVENTION 
   In one aspect, the present invention provides a method of forming a quadratic hole on a surface comprising coating the surface with a photoresist layer and exposing the photoresist layer to a light source, the light source having passed through a photomask. The photomask comprises a first window corresponding to the desired quadratic feature, whereby light from the light source and passing through the first window exposes a first portion of the desired quadratic feature on the surface, and a plurality of adjacent windows, whereby light passing through the adjacent windows form diffraction patterns. The diffraction patterns constructively interfere to expose a second portion of the desired quadratic feature on the surface. Finally, the photoresist layer is selectively dissolved to remove exposed portions of the photoresist layer and the portions of the surface underlying the portions of the photoresist layer that were removed are etched to form quadratic holes. 
   In another aspect, the present invention provides for an integrated circuit. The circuit includes a substrate, a conductive layer overlying the substrate, and an insulating layer overlying the conductive layer. The integrated circuit further includes a rectangular contact hole passing through the insulating layer to the conductive layer, the rectangular contact hole having been formed by exposing a photoresist layer on the surface of the insulating layer through a photomask with a plurality of rectangular windows, the rectangular windows having a pitch such that, for a given partial coherence of the exposing light source, the side lobe features formed from the rectangular windows constructively interfere to expose a rectangular portion of the photoresist layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features of the present invention will be more clearly understood from consideration of the following descriptions in connection with accompanying drawings in which: 
       FIG. 1  illustrates a system for photolithographically forming patterned features on the surface of a substrate, 
       FIGS. 2   a  and  2   b  illustrate a prior art attenuated phase shift photomask and the spectral intensity profile of light passing through a small feature on the photomask; 
       FIGS. 2   c  and  2   d  illustrate the resulting exposure pattern including side lobe formation for a single small feature on a prior art attenuated phase shift photomask and for an array of small features on the prior art attenuated phase shift photomask; 
       FIGS. 3   a  and  3   b  illustrate in plan view a preferred embodiment phase shift mask and resulting exposure pattern, respectively; and 
       FIGS. 4   a  through  4   g  illustrate process steps for forming a square contact hole. 
   

   DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
   The making and use of the various embodiments are discussed below in detail. However, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     FIG. 1  illustrates in schematic form, a photolithographic processing stepper  10 . The stepper comprises a light source  2  which is used to illuminate a photosensitive layer (a photoresist layer)  4  formed on a semiconductor wafer  6  or similar substrate. Patterns are formed on photoresist layer  4  by passing light from light source  2  through a photomask  8  upon which is formed the pattern desired to be transferred. The light also passes through focusing lens  9  prior to impinging upon layer  4 . 
   Further details will now be provided for a preferred embodiment photomask  8 .  FIGS. 2   a  and  2   b  illustrate a photomask  8  having a single pattern formed thereon, the pattern being a square opening. The pattern consists of a window region  10  surrounded by attenuating regions  12 . Attenuating region  12  is formed by coating the photomask with an opaque material such as molybdenum silicide in the case of a 248 nm wavelength exposure light (for other wavelength exposure lights, different opaque material would be employed as is well known in the art). Preferably, this coating provides for about a 6% transmittance of light, although transmittance in the range of 4% to 20% will give acceptable results. Note from  FIG. 2   b  that light passing through region  12  travels one half a wavelength further through the photomask than light passing through open window  10 , resulting in a 180° phase shift between light passing through open window  10  and region  12 . This phase shift will tend to lessen, but not eliminate, the intensity of the diffraction patterns  16  formed as light passes through open window  10 , as shown in  FIG. 2   b . Curve  14  of  FIG. 2   b  represents the intensity of light passing through photomask  8  and impinging upon photoresist layer  4 . As shown, the light intensity is highest beneath open window  10 , where the light from light source  2  passes through photomask  8  essentially unimpeded. Second order diffraction of the light results in peaks  16 . Approximately 6% of the light from light source  2  passes through attenuating region  12 , phase shifted by 180° as discussed above, which reduces but does not eliminate side lobe formation. 
     FIG. 2   c  illustrates the resulting pattern on the surface of photoresist layer  4 . Note that two features are exposed on the layer. The first feature  18  corresponds to the region beneath open window  10 . Note that the feature is essentially round due to corner rounding and other known diffraction phenomena. Side lobe  20  is also formed, corresponding to the region of light intensity  16  arising from the diffraction of light passing through open window  10 . 
     FIG. 2   d  illustrates the resulting exposure pattern when a plurality of open windows  10  are formed on photomask  8 . For each such open window  10 , a feature  18  is exposed on photoresist layer  4 . Again, features  18  show the corner rounding effects of diffraction resulting from the exposure process. Also shown are features  22 , which correspond to those regions where side lobes  20  of adjacent open windows  10  constructively interfere with one another such that the combined intensity of the side lobes  20  in the regions  22  is sufficient to fully expose the photoresist layer  4 . This results in unintended and undesirable features being formed on the semiconductor wafer  6 , possibly resulting in short-circuits, unwanted current paths, and other performance degrading results. 
   In the preferred embodiments of the present invention, undesirable side lobe features  22  are eliminated and corner rounding of contact hole  18  is minimized by positioning open windows  10  relative to each other such that for an open window  10 , the side lobes  20  associated the surrounding open windows  10  constructively interfere with each other in the region that corner rounding of contact  18  would otherwise occur. By positioning the side lobes to occur where it is desirable to expose the photoresist, undesirable features  22  are eliminated, and the shape of desired feature  18  is improved. The following paragraphs further illustrate the invention with preferred embodiment examples. 
   EXAMPLE 1 
     FIG. 3   a  illustrates a first exemplary photomask  8  used to create a pattern of small quadratic contact holes  18 . Photomask  8  comprises attenuating region  12  and open windows  10 . The resulting pattern on photoresist layer  4  is shown in  FIG. 3   b . The desired pattern to be exposed on photoresist layer  4  is an array of square shaped contact holes of size 225 nm by 225 nm. Photoresist layer  4  was comprised of a 5000 Angstrom thick layer of JSR M22G photoresist available from JSR, Corp., Tokyo, Japan, with a bottom antireflective coating of approximately 900 Angstroms thickness. The wafer was exposed using a Nikon DUV Scanner S203B with a numerical aperture of 0.68. 
   The photomask itself was formulated using attenuated phase shift photomask technology wherein attenuating region  12  provided for 6% transmittance. The pattern on the photomask was 4× the desired pattern on layer  4 , with appropriate demagnification being accomplished via focusing lens  9 . Deep UV light source  2  provides a partially coherent light beam with a 248 nm wavelength. Simulations dictated that side lobes  20  would constructively interfere in the desired corner regions of contact holes  18  when the 225 nm contact holes were spaced apart with a pitch of 350 nm (using a 248 nm wavelength light source). Hence open windows  10  were patterned on photomask  8  with a pitch of 350 nm. 
   In order to sufficiently expose photoresist layer  4  in the side lobe regions, the layer was overexposed at a rate of 78 mJ/cm 2 . The partial coherence of light source  2  was selected at 0.44 sigma in order to generate sufficient side lobes to expose the corner regions of contact holes  18 . A lower partial coherence would result in more side lobe formation (the intensity ratio between the main peak and the side lobe would become smaller). A greater partial coherence would hence result in lesser side lobe formation. Depending on the size of the contact mask feature, then, partial coherence can be readily adjusted for optimum side lobe formation. Typically, a partial coherence in the range of 0.3–0.65 will result in desirable side lobe formation for most desired feature sizes and pitches. 
   The resulting exposed photoresist layer  4  is illustrated in  FIG. 3   b . Note that a pattern of square contact holes is formed from the combination of rounded feature  18  forming the main portion of the contact hole and side lobe features  22  forming the corners portions. Although shown separately for illustration purposes, features  18  and  22  form a single feature (contact hole) in actual practice. Note that a novel and improved result is obtained by using the side lobe features to advantageous effect, rather than trying to minimize or eliminate those features as is taught in the prior art. 
   In the following examples, only select ones of the resulting contact holes  18  are required for further processing of an integrated circuit. The other contact holes  18  are necessary only to ensure that the selected contact holes have the desired square shape. Further processing is therefore preferred in order to eliminate unnecessary contact holes. 
   EXAMPLE 2 
   In this example, photoresist layer  4  has been exposed using the methods of any of the preceding examples, resulting in the pattern illustrated in  FIG. 3   b . Only the center-most contact hole  18 , however, is needed for the circuit, the surrounding contact holes having been formed solely to ensure that the center-most contact hole has an acceptable square shape and to prevent undesirable side lobe features for forming around the desired contact hole. 
   In order to block out (remove) the unnecessary contact holes, a second exposure involving a second photoresist layer and a blocking photomask is employed. This blocking photomask will have an open window  10  only corresponding to the desired (center-most) contact hole, and the remaining contact holes will be placed under an attenuating region  12  of the blocking photomask. In this way, only the desired contact hole will be exposed during the second exposure step. In order to prevent the first photoresist layer  4  from being dissolved along with the exposed portions of the second photoresist layer, the first photoresist layer can be crosslinked prior to the second photolithographic step through radiation exposure, chemical cross-linking, e-beam exposure, or other well known cross-linking methods. 
   EXAMPLE 3 
   In this example. photoresist layer  4  has been exposed using the methods of the preceding examples, resulting in the pattern illustrated in  FIG. 3   b .  FIGS. 4   a  through  4   g  illustrate the process steps to eliminate the unnecessary contact holes.  FIG. 4   a provides a cross section view of semiconductor wafer  6 , including conducting region  24  formed atop the wafer and insulating layer  26  formed atop the conducting region. Photoresist layer  4  has been photolithographically processed as described above, resulting in the patterned layer  4  illustrated in  FIG. 4   a . As will be apparent to one skilled in the art, desired contact bole  10 ′ is intended to provide a conductive pathway through insulating layer  26  by which electrical contact with buried conductive region  24  can be made. The conductive pathway through insulating layer  26  is formed by filling contact hole  10 ′ with conductive material. 
   In  FIG. 4   b , insulating layer  26 , which is preferably a silicon dioxide or polysilicon layer is etched using known etching techniques. Three contact holes are etched into the layer because of the pattern of photoresist layer  4 . In  FIG. 4   c , photoresist layer  4  is removed. In  FIG. 4   d  a second photoresist layer  28  is applied and a blocking pattern is formed in layer  28 . This blocking pattern is formed over desired contact hole  10 ′ only. Once second photoresist layer  28  has been exposed with the blocking pattern, the unexposed portions of the layer  28  is removed as shown in  FIG. 4   e . Next, unnecessary contact holes  10  are filled with an insulator material  30 , such as plasma deposited polymer, spin coated polymer, or the like, as shown in  FIG. 4   f . In this way, the unnecessary contact holes are removed. Finally, as shown in  FIG. 4   g , the remaining photoresist layer  28  is removed, leaving a single contact hole  10  in the desired location. 
   As will be apparent from the above described examples, the present invention can be embodied in numerous embodiments. Square contact holes can be obtained for a variety of photolithographic processes, provided that the pitch and size of the open windows are adjusted to cause side lobe formation in the region of the contact hole corners, and that the illumination conditions (particularly partial coherence) is adjusted such that the photoresist layer is sufficiently exposed. Other light sources, such as commonly used 193 nm and 365 nm wavelength light sources can be employed with the teachings of the present invention, provided the partial coherence, numerical aperture, and focus are properly adjusted, as taught herein, for the size and pitch of the desired rectangular features. 
   While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. It should be understood that the present invention also relates to machine readable media on which are stored reticle designs meeting the requirements of this invention, or program instructions for performing methods of this invention. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.