Abstract:
A system for fabricating patterns on a semiconductor, the system includes a first aperture having two openings aligned in a first axis, a first mask, a second aperture having two openings aligned in a second axis, and a second mask. The system may be implemented with the second axis substantially perpendicular to the first axis.

Description:
BACKGROUND 
     This description relates to fabricating semiconductors. 
     Patterns may be fabricated on a semiconductor (e.g., a silicon wafer) by transmitting beams of light through a mask and onto a surface of the semiconductor. In order to produce patterns with extremely small pitches (i.e., the distances between lines) on a semiconductor a phase shifting mask (PSM) may be used. PSMs cause the shifting of the phase of a light source so that the peaks of one wave of light lines up with the valleys of an adjacent wave, effectively canceling each other out and producing a dual-beam image (a “shadow” image) between the waves that is smaller than the two waves themselves. The dual-beam image may be used to fabricate patterns having pitches as low as one-half the theoretical minimum pitch of the light source. In the PSM fabrication technique, light source beams are transmitted through zero degrees and 180 degrees and, when passing through the PSM mask, result in cancellation of the zero degree order of the light. 
     Fabrication of semiconductor patterns may be achieved by performing a double-light exposure which refers to a first light exposure in a lateral axis (e.g., an “x-axis exposure”) followed by a second light exposure in a second orthogonal axis (e.g., a “y-axis exposure”). 
     “Negative” photoresistive materials (“resists”) may be used as part of a semiconductor patterning process. Negative resists refers to the property of a resist that becomes insoluble when exposed to a light beam, therefore the exposed area of the negative resists remains on the substrate after processing of the semiconductor substrate. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a dipole illumination apparatus corresponding to a first exposure of a substrate. 
         FIG. 2  is a diagram of a dipole illumination apparatus corresponding to a second exposure for a substrate. 
         FIG. 3  is a diagram of a substrate that may be produced using the apparatus of  FIGS. 1 and 2 . 
         FIG. 4  is a flowchart of a process that may be performed using the apparatus of  FIGS. 1 and 2 . 
     
    
    
     DESCRIPTION 
       FIGS. 1 and 2  depict a dipole illumination system  10  that may be used to form patterns using a double-light exposure on a semiconductor substrate  12 . System  10  includes a first dipole aperture  20  and a first mask  25  that are used for forming x-axis features on substrate  12  (during a first exposure  14  of substrate  12 , see  FIG. 1 ), and a second dipole aperture  30  and a second mask  35  that are used for forming y-axis features (during a second exposure  16  of substrate  12 , see  FIG. 2 ). In more detail, first mask  25  includes openings  26  that allow the passing of spatial frequencies in a lateral (x-axis) direction, and first aperture  20  includes a first set of dipole openings  20   a  and  20   b  that diffract light source beams  9  in the x-axis. As will be explained, dipole opening  20   b  causes the 0 th  and +1 order of light in the x-axis to be “collected” (e.g., combined) on the pupil of lens  18 , and dipole opening  20   a  causes the 0 th  and −1 order of light in the x-axis to be collected on the pupil of lens  18 . Similarly, second mask  35  includes openings  36  to allow the passing of spatial frequencies in a longitudinal (y-axis) direction, and second aperture  30  includes a second set of dipole openings  30   a – 30   b  that diffract light source beams  9  in the y-axis onto lens  18 . Dipole opening  30   b  causes the 0 th  and +1 order of light in the y-axis to be collected on the pupil of lens  18 , and dipole opening  30   a  causes the 0 th  and −1 order of light in the y-axis to be collected on the pupil of lens  18 . The smaller the pitch between features (e.g., openings) in masks  25  and/or  35 , the smaller the intensity of the diffracted ±1 orders of light passed through those features from apertures  20  and/or  30 , respectively. Therefore, at relatively small pitches the decreased intensity of the diffracted ±1 orders of light at the center of the lens  18  is similar to that created using a PSM where the 0 th  order light at the center of the lens is effectively cancelled. 
       FIG. 1  includes top-down views  40  and  42  that depict the distribution of diffracted light onto the top  19  of lens  18  after passing through dipole openings  20   b  and  20   a , respectively. In both top-down views  40  and  42 , the diffracted beam patterns include a 0 th  order light  40   b,  and ±1 light  40   a  and  40   c . As shown in top-down view  40 , the diffraction of light beam  9  through first dipole opening  20   b  causes the x-axis −1 light  40   a  to be shifted off lens  18 , the 0 th  order light  40   b  to be shifted to an edge of the entrance pupil of lens  18 , and x-axis +1 light  40   c  to be shifted onto the top  19  of lens  18  (and passed through lens  18  to substrate  12 ). Referring to top-down view  42 , the diffraction of light beam  9  through mask  25  by second dipole opening  20   a  causes the x-axis +1 light  40   c  to be shifted off lens  18 , 0 th  order light  40   b  to be shifted to an edge of lens  18 , and the x-axis −1 light  40   a  to be shifted onto the top  19  of lens  18  (and passed through lens  18  to substrate  12 ). 
     Similarly,  FIG. 2  includes top-down views  50  and  52  that depict the distribution of diffracted light onto the top  19  of lens  18  after passing through dipole openings  30   b  and  30   a , respectively. In both top-down views  50  and  52 , the diffracted beam patterns include a 0 th  order light  50   b , and ±1 light  50   a  and  50   c . Referring to top-down view  50 , the diffraction of light beam  9  through first dipole opening  30   b  causes the y-axis −1 light  50   a  to be shifted off the lens  18 , the 0 th  order light  50   b  to be shifted to an edge of the entrance pupil of lens  18 , and x-axis +1 light  50   c  to be shifted onto the top  19  of lens  18  (and passed through lens  18 ). Similarly, as shown in top-down view  52 , the diffraction of light beam  9  through second dipole opening  30   a  causes the y-axis −1 light  50   c  to be shifted off the lens  18 , the 0 th  order light  50   b  to be shifted to an edge of lens  18 , and the y-axis +1 light  50   c  to be passed onto the top  19  of lens  18 . 
     Masks  25  and  35  are examples of so-called binary masks, e.g., masks arranged to have spatial frequencies in a single direction, or axis. Binary masks are relatively simple to fabricate and inspect for defects as compared to phase-shift masks (PSMs), and in particular, as compared to chromeless PSMs. Therefore, the use of system  10  to fabricate patterns on a semiconductor may be performed using relatively simple binary masks. 
     In an embodiment, the resolution (e.g., the “pitch”) of line patterns formed on a semiconductor depends, in part, on the location and/or the diameter of the dipole opening on an aperture. For example, the further apart dipole openings  20   a – 20   b  are located from the center  22  of aperture  20  the smaller the pitch of system  10 . The diameter and locations from the center line of the dipole openings may be expressed in terms of σ c , which refers to units of partial coherence (i.e., Numerical Aperture Condenser/Numerical Aperture Imaging). In an embodiment, σ c  is within a range 0 to 1, inclusive. 
     In an embodiment, pitch is defined by the equation:
 
pitch=λ/(2 NA×σ c ), where λ=wavelength.
 
Therefore, using an exemplary light source beam wavelength of 193 nm, 0.6 NA and with dipole openings of 0.05 σ c , located at at σ c =0.95, the minimum pitch would equal approximately 170 nm. Or, as another example, using a light source beam wavelength of 193 nm stepper with 0.75 NA, and with openings of 0.05 σ c , located at σ c =0.95, the minimum pitch would equal approximately 135 nm.
 
       FIG. 3  shows a pattern that may be formed on a negative resist coating  55  on a surface  56  of substrate  12  using system  10 . As described herein, negative resists may be used to create patterns on a substrate, e.g., where the light exposure on the negative resist causes the exposed area to become insoluble and remain after processing of the substrate. In this example, first exposure  14  (using aperture  20  and mask  25 ) causes line/space pattern  50  to be formed on substrate  12 , and second exposure  16  (using aperture  30  and mask  35 ) causes line/space pattern  60  to be formed on substrate  12 . Pattern  70  represents a combined pattern of patterns  50  and  60 , which leaves voids  75   a – 75   n  between the exposed patterns  50  and  60 . 
     In an embodiment, voids  75   a – 75   n  expose areas in a lower layer of substrate  12 , i.e., a lower layer beneath negative resist coating  55 . In an embodiment, voids  75   a – 75   n  expose areas usable as electrical contacts in the lower layer, e.g., the contacts may be useful in the fabrication of a semiconductor device, such as a semiconductor memory device. 
       FIG. 4  depicts a flowchart of process  100  that may be used to form a pattern on a semiconductor. Process  100  includes transmitting ( 110 ) a light source beam through a first dipole aperture and a first mask in a first axis onto a semiconductor, and transmitting ( 120 ) a light source beam through a second dipole aperture and a second mask in a second axis onto the semiconductor to form a pattern on the semiconductor. Process  100  may optionally include forming a pattern on a negative resist layer on the semiconductor (not shown). 
     The invention is not limited to the specific embodiments described above. For example, some embodiments described the formation of a pattern that included contact “holes” that are essentially round in shape. However, other pattern geometries may be formed using the dipole illumination apparatus using different shapes and patterns formed on the mask(s). 
     Other embodiments not described herein are also within the scope of the following claims.