Abstract:
Innovative techniques are disclosed for fabricating microelectronic devices using an alternating phase shift mask. Some embodiments of the invention encompass a double exposure technique that utilize high resolution line patterning such that two opaque lines intersect at an angle. After development, substantially circular images may be formed. In certain embodiments, high resolution disk imaging as small as 60 nm is possible.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Patent Application No. 61/808,283 entitled “Double Exposure Technique For High Resolution Disk Imaging” filed on Apr. 4, 2013 for Ling Wang et al. which is incorporated herein by reference. 
    
    
     BACKGROUND 
     High resolution fine patterning is an important process for device fabrication.  FIG. 1  illustrates the conventional method for producing an isolated image on a photoresist which involves using a chrome on glass mask (COG)  50  with a chrome dot  22 . A single exposure is performed through COG mask  50  to transfer a dot image to the substrate. For a positive photoresist on a wafer, the chrome dot will block a region from being exposed. After development, a dot  75  is formed at the unexposed region of the wafer  100  as shown in  FIG. 1 . 
     Future generations of microelectronic devices necessitate smaller critical dimensions. The dots produced by the method of  FIG. 1  gradually become deformed as they are scaled to smaller sizes.  FIGS. 2A-2D  illustrate images obtained using the conventional process of  FIG. 1 .  FIG. 2A  depicts a relatively smooth dot of 250 nm diameter. Similarly, a smooth dot having a size of 200 nm ( FIG. 2B ), and as low as 160 nm ( FIG. 2C ) can also be formed using the prior method. At just below 150 nm, dots produced by the process of  FIG. 1  begin to show distortion as indicated by  FIG. 2D . Thus,  FIGS. 2A-2D  demonstrate that the images developed with the prior method become deformed as the resolution is decreased below 150 nm. Therefore, a new method is needed for achieving an isolated image on a photoresist for microelectronic fabrication. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a conventional method for producing a pattern on a wafer. 
         FIGS. 2A-2D  illustrate a sequence of images produced by a prior art method. 
         FIG. 3  is a diagram of an imaging system using an alternating phase shift mask. 
         FIG. 4A  illustrates a method for producing an image in accordance with one embodiment of the invention. 
         FIG. 4B  illustrates a pattern formed by a process of the invention. 
         FIGS. 5A-5D  illustrate a sequence of scalable images produced in accordance with one embodiment of the invention. 
         FIGS. 6A and 6B  illustrate a diagram of an embodiment of the invention that uses a single mask to pattern the photoresist. 
         FIG. 7  is a flowchart illustrating an embodiment for producing a pattern. 
     
    
    
     DETAILED DESCRIPTION 
     Representative embodiments of the invention will now be described in association with  FIGS. 3-7 .  FIGS. 3-7  are not drawn to the scale of an actual device or system, and are merely illustrative of the embodiments described herein. 
       FIG. 3  illustrates an imaging system suitable for implementing the invention. The imaging system comprises a light source, a mask  318  and a wafer  330 . Photoresist  350  is coated onto wafer  330  for further processing. In certain embodiments, mask  318  is an alternating phase shift mask (alt-PSM) comprising a quartz material and an opaque area  310 . The quartz material has a thick region  305  and a thin region  315  bounded on one side by opaque area  310 . Opaque area  310  marks a transition  312  between the phase shifts of thick region  305  and thin region  315  on alt-PSM  318 . The electric field transmitted by thin region  315  is out of phase by 180° by that transmitted by thick region  305 . In several embodiments, opaque area  310  comprises chromium. However, in other embodiments, opaque area  310  may comprise any other metallic or opaque material. 
     During exposure, the pattern of alt-PSM  318  is projected onto photoresist  350 . As indicated by  FIG. 3 , light energy of a suitable image radiation is transmitted through transparent regions  305 ,  315  to contact photoresist  350 . Light transmitted through alt-PSM  318  enters photoresist  350  at regions  352 A and  352 B. As photoresist  350  is a positive resist, portion  352 A and portion  352 B will be stripped away some time after development. Opaque area  310  on mask  318  prevents light from transmitting through portion  322 . Unexposed portion  322  remains after development. and contains an image of the pattern transferred by alt-PSM  318 . 
     An embodiment of the invention will now be discussed in association with  FIGS. 4A and 4B .  FIG. 4A  illustrates an embodiment of the disclosure wherein a high resolution image  410  is formed on a substrate  400  using mask  415  and mask  425 . Each mask can be an alt-PSM. In certain embodiments, mask  415  has an opaque line  320  and mask  425  has an opaque line  340 . Although only one opaque line  320  is shown on phase shift mask  415  and one opaque line  340  is shown on phase shift mask  425 , it is understood that several embodiments of the disclosure are directed to phase shift masks that include more than one opaque line. Thus, any reference to a single opaque area is not intended to limit the disclosure to embodiments comprising a mask with solely a single opaque line. 
     A photoresist (not shown) is provided on a substrate, such as a wafer (not shown). A first phase shift mask  415  with multiple line-shaped opaque areas is placed over the photoresist. Then a suitable image radiation is provided by a light source to expose the photoresist with the pattern on mask  415 . A portion of the substrate may be exposed multiple times in a stepper to obtain the desired images. During exposure, opaque areas of the first mask  415  cause first regions of the photoresist to remain unexposed. The first mask  415  is then replaced with a second phase shift mask  425 . Similar to the first mask, the second mask  425  has multiple line-shaped opaque areas. In one embodiment, opaque lines on mask  415  and mask  425  have line widths of approximately 50-150 nm. In some embodiments, the line widths of both masks are equal. 
     In certain embodiments, the opaque line  340  of second mask  425  is placed over the resist in a direction substantially perpendicular to the major axis of first opaque line  320  to cause second regions of the photoresist to remain unexposed in the second exposure. Chromium (chrome) is a suitable material for the opaque lines, although other metals can also serve as the opaque area on the glass mask. Light is blocked from entering the resist regions in both exposures where the chrome lines of mask  415  and  425  intersect. By employing transparent regions  305 ,  315  with respective phase shifts of 0 and 180 degrees, the light diffracted into chrome lines  320  and  340  between these adjacent transparent areas  305 ,  315  interfere destructively (to cancel out each other), resulting in the chrome areas blocking the underlying photoresist portions from the light source during exposure. The photoresist exposed through the second mask  425  forms multiple latent images at the intersection of the unexposed areas. For simplicity, only a single latent image  410  is shown as being produced in  FIG. 4A . 
     After the double exposure, latent images appear in regions that are unexposed due to light being blocked by opaque lines  320 ,  340 . The photoresist is then developed and a plurality of substantially circular disks is formed. In summary, the double exposure creates one or more island images in resist and the optical proximity effects naturally round the four corners to make the island a perfect or near-perfect circular disk. 
     Although  FIG. 4A  illustrates an embodiment using two phase shift masks, the process of  FIG. 4A  can be implemented with a single phase shift mask (PSM) instead. In such an embodiment, chrome lines  320  and  340  appear in different regions of the same PSM. 
     A step-and-repeat apparatus (stepper) exposes the full pattern of the mask by sequentially stepping each field of wafer  450  during each exposure. After the first and second exposure, a pattern of disks is formed as illustrated in  FIG. 4B . Each disk in  FIG. 4B  has a pitch  430  of approximately 0.5 microns, where the pitch  430  is measured from the center of one disk to the center of an adjacent disk. Moreover, in certain embodiments a disk having a pitch of 1-20 microns is also possible. Yet in other embodiments the disk can have a pitch of 25-100 microns. Still, it is possible to form disks having a pitch of approximately 100-300 microns. In certain embodiments, the disks produced are relatively isolated, in that patterned features are absent between a majority of the disks. 
     Embodiments of the present disclosure can result in images that are scalable to a smaller degree than the images shown in  FIGS. 2A-2D . One advantage that can be achieved with certain embodiments is an improved contrast image. Although light intensity increases when feature sizes become smaller in these embodiments, the fine pattern retains its resolution because it is formed within the sensitivity parameters of the photoresist. 
     On the other hand, an enhanced image is produced by implementing several embodiments of the present invention. For example,  FIGS. 5A-5D  illustrate the scalability of the images obtained by following certain embodiments of the invention. Unlike in the prior art, the image obtained at 140 nm is smooth and remains smooth as it is scaled to 100 nm ( FIG. 5A ), 80 nm ( FIG. 5C ) and even down to 60 nm ( FIG. 5D ). 
     After development, the pattern of images obtained in several embodiments of the invention has a pitch of at least 0.5 microns. The pitch in these embodiments can be as large as 300 microns or greater, and will generally have a constant pitch with a high contrast image. Moreover, in certain embodiments associated with  FIGS. 5A-5D , a disk having a pitch of 1-20 microns is also possible. Yet in other embodiments the disk can have a pitch of 25-100 microns. Still, it is possible to form disks having a pitch of approximately 100-300 microns. In the aforementioned embodiments, the disks produced are relatively isolated, in that intervening structures are not present between at least a majority of disks. 
     Turning to  FIGS. 6A and 6B , an example of how PSM line imaging is used to form patterns on a photoresist will now be explained. The substrate to be patterned is shown as a wafer  600  in  FIG. 6A . Wafer  600  is divided into fields  610  that will be patterned by mask  630  of  FIG. 6B . Mask  630  is an alt-PSM with multiple opaque lines that block light from penetrating the photoresist during exposure. One possible way of laying out the opaque lines on alt-PSM  630  is shown in  FIG. 6B . The upper half of alt-PSM  630  includes opaque lines  620  oriented vertically, while the lower half of alt-PSM  630  has opaque lines  640  oriented horizontally. In certain embodiments, the opaque lines are chrome lines. 
     Numerous other configurations for locating the chrome lines in the mask(s) of the present invention are possible. For example, a column of horizontal lines  620  can alternate with a column of vertical lines  640 . Alternatively, a group of vertical chrome lines can be interspersed with a group of horizontal chrome lines. The line width of each line will vary based on the feature to be patterned. However, in one embodiment suitable line widths can range from approximately 50 nm to approximately 150 nm. 
     In the embodiment of  FIGS. 6A-6B , a photoresist is coated on a wafer, and the coated wafer is placed in a stepper, and then alt-PSM  630  is provided above the photoresist. During the first exposure, a light source illuminates the mask through the photoresist to expose a portion of the wafer. Then alt-PSM  630  is shifted to a new column (one of either A-2, A-3, A-4, . . . A-n−1 or A-n) and the exposure process is repeated. Due to the chrome lines of alt-PSM  630 , portions of the photoresist are unexposed during the first exposure. Then the photoresist is exposed again by shifting the mask to a region where the chrome lines are oriented horizontally. Alt-PSM  630  is positioned so that the horizontal chrome lines overlap the vertical chrome lines of the first exposure. 
     In one embodiment, the chrome lines are oriented at a right angle to each other. And then a second exposure is performed. The resulting unexposed areas form a latent image of substantially circular disks. After development, substantially circular disks are formed. 
     In other embodiments, a fine pattern of alternate shapes is also feasible. One process for these other embodiments is summarized in  FIG. 7 , where either a single alt-PSM or multiple alt-PSMs may be used. First, a photoresist is placed on a substrate to form a coating via block  750 . Then a first alt-PSM having a chrome line is placed in a first position on the substrate in block  752 . In several embodiments, opaque area in blocks  752  and  756  comprise chrome lines. However, in other embodiments, the opaque line may comprise any other metallic or opaque material lines having a line width ranging from approximately 50 nm to approximately 150 nm. 
     Afterwards, the coating is exposed through the mask in block  754 . During the first exposure, the chrome line on the first alt-PSM prevents portions of the photoresist from being exposed. Thereafter, the first mask on the photoresist is replaced with a second alt-PSM, also having an opaque area, such as a chrome line. In block  756 , the chrome line of the second alt-PSM is oriented at an angle α, relative to the chrome line of the first mask, wherein α is ≧30 degrees and ≦90 degrees. During the second exposure, in block  758 , additional portions of the photoresist are exposed. At the intersection of unexposed portions maintained by the first and second exposure, latent images are formed. The process proceeds to block  760 , wherein the latent images are developed to form a pattern of high resolution images. 
     The pattern formed by process  700  will depend on the angle of block  756 . When opaque lines of the alt-PSMs are oriented at 45 degrees relative to each other, substantially elliptical images are formed on the substrate. In the case where two alt-PSMs have their opaque lines oriented at a right angle to each other, substantially circular images are produced. The opaque lines of  FIG. 7  can have a line width ranging from approximately 50 nm to approximately 150 nm. 
     Although  FIG. 7  describes two alt-PSMs, the process of  FIG. 7  can be implemented with only a single mask. In such an embodiment, one group of chrome lines is disposed horizontally on the mask, and a second group of chrome lines is disposed vertically in a different region of the same mask. 
     The innovative techniques described above can be applied to fabricate future generations of near-field optical transducers, MEMS, semiconductors, and any other high resolution disk imaging applications. 
     The above detailed description is provided to enable any person skilled in the art to practice the various embodiments described herein. While several embodiments have been described, it should be understood that these are for illustration purposes only and should not be taken as limiting the scope of the invention. 
     Various modifications to these embodiments will be readily apparent to those skilled in the art, and generic principles defined herein may be applied to other embodiments. Thus, many changes and modifications may be made to the above embodiments, by one having ordinary skill in the art, without departing from the spirit and scope of the invention.