Abstract:
One embodiment of the invention provides a system that facilitates exposing a wafer through at least two masks during an integrated circuit manufacturing process. The system includes a radiation source and two or more illuminators. Each of these illuminators receives radiation from the radiation source, and uses the radiation to illuminate a reticle holder. The radiation that passes through each reticle holder is then combined in an optical combiner, before passing through an imaging optics, which projects the combined radiation onto a semiconductor wafer.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention relates to the process of manufacturing an integrated circuit. More specifically, the invention relates to a method and an apparatus for exposing a wafer using multiple masks during an integrated circuit manufacturing process. 
     2. Related Art 
     Integrated circuits can be produced through an optical lithography process that involves creating a mask with a pattern specifying where the various features of the integrated circuit are to be placed and then passing radiation through the mask to expose the pattern on a semiconductor wafer. This pattern defines where the surface of the semiconductor wafer is to be etched or where new material is to be added to create the integrated circuit. 
     FIG. 1 illustrates the process of exposing a target wafer through a mask. As shown in FIG. 1, radiation emanates from source  102 , which can be any suitable source of radiation such as a laser. This radiation passes through illumination optics  104 , and then through mask  106 , to imaging optics  108 . Mask  106  includes the circuit patterns to be projected onto target  110 . Imaging optics  108  directs the radiation, including the circuit patterns onto target  110 . Target  110  is typically a semiconductor wafer treated to be sensitive to the radiation being used. The above-described process transfers the circuitry delineated on mask  106  to target  110 . 
     As features on integrated circuits become progressively smaller, designers have resorted to splitting the mask into multiple parts so that features can be placed on the separate masks to improve resolution and reduce interference between various features of the circuit and simplify mask manufacturing. 
     Using multiple masks requires that each of the masks is used to expose the semiconductor wafer. There are several ways that this might be accomplished. One possibility is to use two steppers and to expose the semiconductor wafer through a first mask in the first stepper, and then to expose the semiconductor wafer through a second mask in the second stepper. Since steppers are very expensive—several million dollars each—using two steppers is a very expensive option. 
     Another possibility is to use two reticle holders within a single stepper, where each reticle holder can be moved into a position where the beam of radiation is passed through the first mask for a first exposure and then passed through the second mask for the second exposure. While this eliminates the expense of having two steppers, changing reticle holders slows the manufacturing process and reduces throughput. Note, the terms “mask” and “reticle” are used interchangeably throughout this application. 
     What is needed is a method and an apparatus that allow multiple masks to be used for exposing a semiconductor wafer without the problems discussed above. 
     SUMMARY 
     One embodiment of the invention provides a system that facilitates exposing a wafer through at least two masks during an integrated circuit manufacturing process. The system includes a radiation source and two or more illuminators. Each of these illuminators receives radiation from the radiation source, and uses the radiation to illuminate a reticle holder. The radiation that passes through each reticle holder is then combined in an optical combiner, before passing through an imaging optics, which projects the combined radiation onto a semiconductor wafer. 
     In one embodiment of the invention, the radiation source includes two or more sources so that each illuminator receives radiation from a different source. 
     In one embodiment of the invention, the radiation source includes a beam splitter that splits the radiation from the radiation source into a separate beam for each illuminator. 
     In one embodiment of the invention, although there are more complex illumination pathways, only a single imaging optics is used for an overall saving. The invention provides improved throughput of wafers in fabrication and better processing control since the time between both exposures and processing is shorter. 
     In one embodiment of the invention, the system includes a phase shifting device interposed in the path of the radiation in one or more of the illuminators so that the phase of the radiation in one illuminator can be adjusted relative to the phase of the radiation in the other illuminators. Adjusting the relative phase of the radiators may reduce mask fabrication costs as described below. 
     In one embodiment of the invention, adjusting the phase of the radiation in one illuminator relative to the phase of the radiation in the other illuminators allows splitting a dark field mask that includes 0-degree and 180-degree phase shifters into two dark field binary masks. 
     In one embodiment of the invention, the system includes adaptive optics at the optical combiner to locally adjust for differences in phase between the illuminators and the objects (masks) at each point within the field. 
     In one embodiment of the invention, the system includes an optical path-lengthening device interposed in a path of radiation within at least one illuminator. This optical path-lengthening device causes temporal incoherence between the associated illuminator and the other illuminators. Temporal incoherence is useful in cases where only a double exposure is wanted without any phase shift effects. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates the process of exposing a target wafer through a mask. 
     FIG. 2 illustrates projecting an image defined by a pair of masks onto a target in accordance with an embodiment of the invention. 
     FIG. 3 illustrates projecting an image defined by a pair of masks onto a target using multiple light sources in accordance with an embodiment of the invention. 
     FIG. 4 is a flowchart illustrating the process of projecting an image defined by a pair of masks onto a target in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Projecting a Pair of Masks 
     FIG. 2 illustrates projecting an image defined by a pair of masks onto a target in accordance with an embodiment of the invention. Source  202 , which can be any suitable source of radiation such as a laser, provides radiation for the system. This radiation passes through illumination optics  204  to beam splitter  206  where the radiation is split into two beams; a first beam directed toward mask  208  and a second beam directed toward optical device  212 . Note that optical device  212  could be placed in the first beam instead of the second beam. Optionally, optical device  212  could be placed after mirror  213  or mirror  210 . Note that these optical structures are inside a single stepper/scanner system. 
     Optical device  212  can perform many functions. Optical device  212  can control the phase delay of the beam to compensate for path differences between illumination optics  204  and imaging optics  218 . Optical device  212  can also control the phase of the beam to provide a phase shifted coherent beam at imaging optics  218 . Using phase shifted coherent beams allows using two 0-degree dark field masks, with one beam shifted by 180 degrees, in place of a single dark field mask with both 0-degree and 180-degree phase shifts. 
     Another example where phase shifting in the optical device  212  can be useful is in a four-phase phase-shifting mask. More specifically, a single mask having four-phases can be decomposed into two masks, each having only two phases to manufacture. Optical device  212  can shift the phase in one path by 90 degrees relative to the other path. Optical device  212  can also modify the optical path such that the illuminations of mask  208  and mask  214  are incoherent with respect to each other. This provides the ability to expose target  220  through the separate masks without unwanted interference between the two beams. For example, incoherent illumination is useful for dual exposure AAPSM (one binary mask, one phase shifting mask) and contact arrays (two separate masks so that contacts are partially isolated and can be exposed using attenuated phase shifting masks and low-σ illumination). In an alternate embodiment of the invention, optical device  212  can include a set of optical elements (e.g. adaptive optics) that change the coherence of the light. 
     After passing through optical device  212 , the second beam is reflected from mirror  213  through mask  214  to optical combiner  216 . The first beam from beam splitter  206  passes through mask  208  and is reflected by mirror  210  to optical combiner  216 . Optical combiner  216  combines the two beams and directs the combined beam to imaging optics  218 . In one implementation of the invention, one or more of the mirror  210 , mirror  213  and the optical combiner  216  include adaptive optics for adjusting the phase front differences between the masks to compensate for small phase differences between the illumination paths. Note, placement of the adaptive optics on the mirrors is preferred over placement on the optical combiner  216  in some embodiments. Adaptive optics are useful so the overall effects of the illumination are seen identically through two very different paths. One embodiment of adaptive optics includes actuated micromirrors on mirror  210  or mirror  213 . An alternative embodiment of adaptive optics includes a deformable mirror on mirror  210  or mirror  213 . 
     Imaging optics  218  then projects the combined beam from optical combiner  216  onto target  220 . Target  220  is typically a semiconductor wafer treated to be sensitive to the radiation being used. This process transfers the circuitry delineated on masks  208  and  214  to target  220 . 
     Using Multiple Radiation Sources 
     FIG. 3 illustrates projecting an image defined by a pair of masks onto a target using multiple light sources in accordance with an embodiment of the invention. In this embodiment, radiation is provided by sources  302  and  312 . Sources  302  and  312  can be any suitable sources of radiation, such as lasers. Radiation from source  302  passes through illumination optics  304  and mask  306  and is directed by mirror  310  to optical combiner  316 , while radiation from source  312  passes through illumination optics  314  and mask  308  and is similarly directed to optical combiner  316 . Note that since these radiation sources are separate, the radiation from each source may actually be coherent, but incoherent with respect to the other source. In embodiments with multiple sources, it is possible to use mixtures of incoherent and coherent sources in some embodiments to achieve desired lithographic effects. 
     The illumination from radiation sources  302  and  312  can include arbitrary spatial frequency components at different strength depending on the source and the spatial filter used for the optics. The exposure process can be a time-sequence, instead of simultaneous exposures, at different exposure doses so as to mimic a variable spatial frequency sweep at different illumination strengths. This is analogous to channel equalization where the signal source is encoded to counter any suppression in the passband 
     Optical combiner  316  combines the beams, which have passed through masks  306  and  308  and directs this combined beam toward imaging optics  318 . Imaging optics  318  projects the combined beam onto target  320 . Target  320  can include a semiconductor wafer treated to be sensitive to the radiation being used. This process transfers the circuitry delineated on masks  306  and  308  to target  320 . 
     Optical combiner  316  can include adaptive optics to correct for variations in the optical path length and within the optics. Adaptive optics can include movable micromirrors and deformable mirror surfaces. 
     Projecting the Masks 
     FIG. 4 is a flowchart illustrating the process of projecting an image defined by a pair of masks onto a target in accordance with an embodiment of the invention. The system starts when radiation is received from radiation source  202  (step  402 ). The radiation from radiation source  202  then passes through illumination optics  204  (step  404 ). Beam splitter  206  splits the radiation from illumination optics  204  into two beams (step  406 ). Note that this example uses two beams, however, the system can be designed to use more than two beams and two masks, with appropriate modifications to the illuminators. 
     Next, the beams are applied to the illuminators so that masks  208  and  214  can be illuminated (step  408 ). One of these beams is applied through optical device  212 . Note that optical device  212  can be interposed in either beam, or an optical device can be interposed in both beams. Optical device  212  then adjusts the beam parameters to achieve a desired effect (step  410 ). For example, optical device  212  can control the phase delay of the beam to compensate for path differences between illumination optics  204  and imaging optics  218 . Optical device  212  can also control the phase of the beam to provide a phase shifted coherent beam at imaging optics  218 . Using phase shifted coherent beams allows using two 0-degree dark field masks with one beam shifted by 180 degrees in place of a single dark field mask with both 0-degree and 180-degree phase shifts. Another example where phase shifting can be useful is in a four-color phase shift mask, where only two colors need to be manufactured. Optical device  212  can shift the phase in one path by 90 degrees relative to the other path. Optical device  212  can also lengthen the optical path such that the illuminations of mask  208  and mask  214  are incoherent with respect to each other. This provides the ability to expose target  220  through the separate masks without unwanted interference between the two beams. 
     Next, one beam from beam splitter  206  illuminates mask  208  while the other beam from beam splitter  206  illuminates mask  214  (step  412 ). Note that the beam, which illuminates mask  214 , is processed by optical device  212  and is then reflected by mirror  213  onto mask  214 . The beams pass through masks  208  and  214  and are then directed to optical combiner  216 . The beam that passes through mask  208  is reflected by mirror  210  to reach optical combiner  216 . Optical combiner  216  combines the beams from masks  208  and  214  into a single beam directed toward imaging optics  218  (step  414 ). 
     Finally, imaging optics  218  projects the combined beam onto target  220  (step  416 ). Target  220  is typically a semiconductor wafer treated to be sensitive to the radiation being used. This process transfers the circuitry delineated on masks  208  and  214  to target  220 . 
     The preceding description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, the invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The method of this invention can be performed using the apparatus described in relation to FIGS. 2 and 3. The apparatus can be placed inside a stepper/scanner with appropriate modifications to contain the two or more illuminators. 
     The foregoing descriptions of embodiments of the invention have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the forms disclosed. Accordingly, many modifications and variations will be apparent. Additionally, the above disclosure is not intended to limit the invention. The scope of the invention is defined by the appended claims.