Abstract:
Manufacturing of semiconductor devices often involves performed photolithography to pattern and etch the various features of those devices. Such photolithography involves masking and focusing light onto a surface of the semiconductor device for exposing and etching the features of the semiconductor devices. However, due to design specifications and other causes, the semiconductor devices may not have a perfectly flat light-incident surface. Rather, some areas of the semiconductor device may be raised or lowered relative to other areas of the semiconductor device. Therefore, focusing the light on one area causes another to become unfocused. By carefully designing a photomask to cause phase shifts of the light transmitted therethrough, focus across all areas of the semiconductor device can be achieved during photolithography, which results in sharp and accurate patterns formed on the semiconductor device.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The disclosure relates generally to a photolithography process for manufacturing semiconductor devices, and specifically to a photomask capable of adjusting focal planes among different regions of the semiconductor devices, and a method of manufacturing the semiconductor devices using the photomask. 
         [0003]    2. Related Art 
         [0004]    In conventional semiconductor manufacturing methods, photomasks are used during photolithography in order to expose a semiconductor wafer to a pattern of intense light. This exposure causes a pattern to be formed on the semiconductor wafer, which alone or in combination with additional exposures (using the same or different photomask), can form the basis of the integrated circuit to formed on the semiconductor wafer. After the exposure, etching and/or deposition can be performed in order to form the circuit elements in the semiconductor wafer. 
         [0005]    Conventional photomasks included a transparent baseplate having opaque elements corresponding to areas that are not to be exposed. The advent of smaller semiconductor features have necessitated the use of phase-shifting photomasks. Such phase-shifting photomasks are specifically designed only to allow for passing light to exit the photomask with either a zero degree phase shift (in exposure areas) or a 180 degree phase shift (in non-exposure areas). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0006]    Embodiments are described herein with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical or functionally similar elements. Additionally, generally, the left most digit(s) of a reference number identifies the drawing in which the reference number first appears. 
           [0007]      FIG. 1  illustrates an exemplary photolithography apparatus using a photomask according to an exemplary embodiment; 
           [0008]      FIG. 2  illustrates an exemplary semiconductor device to be manufactured using the photomask according to an exemplary embodiment; 
           [0009]      FIG. 3  illustrates a cross-sectional view of an exemplary multi-shifting photomask according to an exemplary embodiment; 
           [0010]      FIG. 4  illustrates a plan view of the exemplary photomask according to an exemplary embodiment; 
           [0011]      FIG. 5  illustrates an exemplary method for manufacturing a semiconductor device using the exemplary multi-shifting photomask according to an exemplary embodiment; 
           [0012]      FIGS. 6A and 6B  illustrate a comparison between a semiconductor pattern formed with a conventional photomask versus a semiconductor pattern formed with the exemplary photomask; and 
           [0013]      FIG. 7  illustrates a graphical representation of the phase-shift caused by the exemplary photomask versus an adjusted focal plane corresponding thereto. 
       
    
    
     DETAILED DESCRIPTION 
     Lithography Apparatus and System Properties 
       [0014]      FIG. 1  illustrates a photolithography apparatus  100  according to an exemplary embodiment. 
         [0015]    The apparatus  100  includes a photomask  110  that receives incident light  102  from a light source (not shown). The photomask  110  includes a substrate  112  with an absorber  114  disposed on a light-exit surface. In an embodiment, the substrate  112  is a quartz blank and the absorber  114  is represented by a MoSI. Conventionally, light that passes through the substrate  112  without passing through the absorber  114  passes through with 100% or near 100% transmission and a 0 degree phase shift. On the other hand, light that passes through both the substrate  112  and the absorber  114  of conventional photomasks passes through with approximately 6-8% transmission and a 180 degree phase shift. The exemplary photomask  110  differs from the conventional photomask, as will be described in detail below. 
         [0016]    Once the light passes through the photomask  102 , the light proceeds to a projection lens  120 , which focuses the incoming light onto a wafer  150  surface. The focusing of the light on the wafer  150  surface causes the surface of the wafer  150  to be etched, thereby forming a desired structural pattern thereon. 
         [0017]    In many respects, performing photolithography is akin to taking a photograph. The point or plane at which the light is focused produces a clear and sharp picture, whereas other points/planes are blurred. The corresponding consequence in photolithography is that only a focal plane of the projection lens  120  produces a sharp pattern that closely corresponds to a desired pattern. However, due to differences in layer structures among different regions of the wafer  150 , the entire surface of the wafer  150  may not lie in the same focal plane. 
         [0018]    For example,  FIG. 2  illustrates a cross-sectional view of a portion of an exemplary semiconductor wafer  150 . As shown, the wafer  150  includes a first area  210 A and a second area  210 B. In the first area  210 A, a first layer stack  225   a  has a height h A . Meanwhile, in the second area  210 B, a second layer stack  225   b  has a height h B , where h A ≠h B . The difference in height between the first area  210 A and the second area  210 B generally is attributed to differences in the number of, or general layout of, the layers among the first layer stack  225   a  and the second layer stack  225   b . However, other factors (by design or circumstance) are conceivable, and can be corrected for provided that they are known or can be measured prior to photolithography. Because of the differences in height among the areas of the wafer  150 , some areas of the wafer  150  may fall outside the focal plane of the projection lens, thereby resulting in a distorted etching pattern. 
         [0019]    An example of this can be seen in  FIG. 6A , which illustrates a wafer surface resulting from performing photolithography using a conventional photomask. The vertical line  601   a  represents a transmission between a first area  610 A and a second area  620 A. The first area  610 A may be raised (nearer to the projection lens) or recessed (further from the projection lens) relative to the second area  620 A. As a result, a focal plane of the first area  610 A differs from a focal plane of the second area  620 A. Therefore, as shown in  FIG. 6A , when the projection lens is configured to focus light for the focal plane of the first area  610 A, the second area  620 A becomes “out of focus,” and an inaccurate (“blurry”) pattern results. 
         [0020]    The exemplary photomask  110  described herein has been devised to address these issues, and to improve overall photolithography patterning of semiconductor wafers, particularly of semiconductor wafers having a surface with multiple areas of differing heights (e.g., non-uniform focal planes). 
         [0021]    Photomask 
         [0022]      FIG. 3  illustrates the exemplary photomask  110  according to an exemplary embodiment. The photomask  110  includes the substrate  112  and the absorber  114  disposed on a light-exit surface of the substrate  112 . The substrate  112  has a substantially planar incident surface (illustrated as the upper surface in  FIG. 3 ). Opposite the incident surface, the substrate has a light-exit surface that includes a plurality of protrusions  320  arranged in a tooth-like manner. In an embodiment, the protrusions  320  are integrally formed with the substrate  112 . These protrusions  320  can be designed to compensate for differences in focal areas of the semiconductor device, as will be discussed in further detail below. 
         [0023]    The absorber  114  includes a plurality of absorber portions (e.g.,  114 A,  114 B, and  114 C) disposed on the surfaces of the substrate protrusions  320 . These portions of the absorber  114  each have substantially the same thickness T Ab , and are disposed on a substantially even horizontal plane relative to their longitudinal axes. In other words, in an embodiment, each of the substrate protrusions  120  extend along a common plane, on which the absorber portions reside. By adjusting the depths of the protrusions relative to this common plane, the photomask  110  can account for differing focal planes of the semiconductor device. 
         [0024]    As discussed above with respect to  FIG. 2 , a first area  210 A of a semiconductor device  150  may have a different height (and therefore a different focal plane) relative to a second area  210 B of the semiconductor device  150 . By adjusting the depths of the protrusions  320  of the photomask  110 , these differences in the focal planes can be accounted for to provide a focused exposure in all areas. For example, in  FIG. 3 , an imaginary dividing line  150  is illustrated as separating the photomask  110  into a first area (Area A) that corresponds to the first area  210 A of the semiconductor device  150 , and a second area (Area B) that corresponds to the second area  210 B of the semiconductor device  150 . 
         [0025]    In Area A of the photomask  110 , the protrusions  320  have a depth H A , whereas in Area B of the photomask  110 , the protrusions  320  have a depth H B , where H A &gt;H B . By forming the light-exit surface of the photomask  110  in this manner, the focal plane of the Area A will be raised (e.g., closer to the photomask  110 ) relative to the focal plane of the Area B. 
         [0026]    A magnified view  390  is provided in  FIG. 3  to more clearly illustrate these features. As shown in the magnified view  390 , a first protrusion  320 A in the Area A has a depth H A  relative to the bottom horizontal plane. Likewise, a second protrusion  320 B in the Area B has a depth H B  relative to the bottom of the horizontal plane. By manufacturing the protrusions  320  in the Area A to have the depth H A , the focal plane of the Area A can be adjusted corresponding to the height of the first area  210 A of the semiconductor device  150 . Similarly, by manufacturing the protrusions  320  in the Area B to have the depth H B , the focal plane of the Area B can be adjusted corresponding to the height of the second area  210 B of the semiconductor device  150 . 
         [0027]    Photomask Design 
         [0028]    As discussed above, different areas of the photomask  110  can be designed to have different focal planes corresponding to different areas of the semiconductor device  150 . The process of calculating and designing the photomask according to the height differences of the semiconductor device  150  is described in detail herein. 
         [0029]    As shown in the magnified view  190  of  FIG. 3 , the absorber  114  has a thickness T Ab , the first protrusions  320 A in a first Area A have a depth H A , and the second protrusions  320 B in a second Area B have a depth H B . A phase angle of the transmitted light can be calculated as: 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    where ηs is the refractive index of the substrate  112 , ηab is the refractive index of the absorber  114 , and Hs is the depth of the protrusions in a particular area. For example, in Area A of the photomask  110 , Hs would be set to be equal to H A . The calculated phase θ is the phase shift from 180 degrees. In other words, a calculated phase angle θ of 2.5 degrees corresponds to a total phase shift of 182.5 degrees, whereas a calculated phase angle θ of −2.5 degrees corresponds to a total phase shift of 177.5 degrees. 
         [0030]    Adjusting the phase angle of the light passing through the photomask  110  causes a corresponding adjustment to the focal plane in that area.  FIG. 7  illustrates a graphical representation of the relationship between the phase angle θ and the focal plane. As shown in  FIG. 7 , the focal plane shifts inversely proportional to the phase shift. Thus, as the phase shift increases, the focal plane decreases (is set further away from the photomask  110 ). For example, a phase shift of 2.5 degrees results in the focal plane being shifted by −0.025 um (0.025 um further from the photomask  110 ). The values depicted in  FIG. 7  are for illustrative purposes only, and are not meant to be exact. 
         [0031]    Using the properties of the photomask  110  described above, the photomask can be designed to compensate for areas of the semiconductor device having different heights. For example, for areas of the semiconductor device  150  that are raised (e.g., first area  210 A), corresponding areas of the photomask  110  (e.g., Area A) can be set to have a phase angle θ that is less than 0 degrees. Using equation (1), above, the depth of the protrusions in the Area A can be calculated accordingly. Similarly, for areas of the semiconductor device  150  that are recessed (e.g., second area  210 B), corresponding areas of the photomask  110  (e.g., Area B) can be set to have a phase angle θ that is greater than 0 degrees. 
         [0032]    It should be noted that, in practice, a substantial portion of the semiconductor device may be at the same surface height, with smaller localized areas having other different heights. In this scenario, the photomask  110  may have a primary focal plane even with the surface height of that substantial portion. Other areas of the photomask  110  corresponding to the differing localized areas of the semiconductor device  150  can then be designed to have deeper or shallower depths of protrusions so as to adjust the focal planes accordingly in those areas. 
         [0033]      FIG. 4  provides an example of such a photomask  110 . As shown in  FIG. 4 , the photomask  110  has a primary area  410 E (Area E) that is constructed to be at a zero degree phase shift (from 180 degrees). Other localized areas  410 A- 410 D are then constructed with varying protrusion depths according to whether corresponding areas of the semiconductor device are recessed or raised from a primary plane of the semiconductor device. As shown, more than one separate localized area may have the same design needs. For example, there are two distinct Area A&#39;s  410 A that will be constructed to the same specifications in the example of  FIG. 4 . 
         [0034]    By constructing the photomask  110  to compensate for the different focal planes of the semiconductor wafer, an accurate pattern can be etched over all areas. An example of this can be seen in  FIG. 6B , which illustrates a wafer surface resulting from performing photolithography using an exemplary photomask according to an embodiment. The vertical line  601   b  represents a transition between a first area  610 B and a second area  620 B. The first area  610 B may be raised (nearer to the projection lens) or recessed (further from the projection lens) relative to the second area  620 B. As a result, a focal plane of the first area  610 B differs from a focal plane of the second area  620 B. However, as shown in  FIG. 6B , when the projection lens is configured to perform the exposure using an exemplary photomask, both the first area  610 A and the second area  620 B are kept “in focus” and are accurately patterned. 
         [0035]    Method for Preparing a Multi-Phase Shift Photomask 
         [0036]      FIG. 5  illustrates an exemplary method  500  for preparing the multiple phase-shift photomask described herein. The method  500  will be described with respect to  FIGS. 2-4  for illustrative purposes, although method  500  is not limited to this example. 
         [0037]    In step  510 , an electron beam writer is used to write a pattern onto a photomask plate. In step  520 , the imaged pattern is then developed to form a template, following which a base material (e.g., Chrome and/or MoSI) is etched away based on the printed pattern. 
         [0038]    Following the etch, according to an embodiment, in step  530  an area phase shift write is performed. This area phase shift write defines one or more focus adjustment areas (e.g.,  410 A in  FIG. 4 ). After the area phase shift write, an area substrate etch is performed in step  540 . The area substrate etch etches away the substrate in the current area to a desired depth in order to achieve the desired phase shift (according to equation (1), above). The area phase shift write and the area substrate etch (steps  530  and  540 ) are steps performed specifically to achieve the area phase shifts described above with respect to the exemplary embodiments, and are repeated as necessary for each focal area. In an embodiment, these steps are repeated for each individual area regardless of whether any of those areas are to be etched to the same depths, and in another embodiment, these steps are performed for multiple areas that are to have the same depths at the same time. 
         [0039]    In step  550 , after the phase shift area write and the area substrate etch have been performed for all the areas, a general phase shift area write is performed. In step  560 , a base layer is removed in the phase shift areas, after which the method concludes. 
         [0040]    Method and Apparatus for Exposing a Semiconductor Wafer Using the Exemplary Photomask 
         [0041]    The setup and process for exposing a semiconductor device using the exemplary photomask described herein is detailed below with reference to  FIGS. 1 ,  2  and  4 . 
         [0042]    As discussed above, the exemplary multi-phase photomask is designed with areas of different phase-shift properties in correspondence with areas of a semiconductor device that lie in different focal planes (see  FIGS. 2 and 4 ). In other words, based on design specifications, the semiconductor device will have some surface areas that are raised/lowered with respect to other surface areas, as shown in  FIG. 2 . As shown in  FIG. 4 , for example, the exemplary photomask can be designed to include corresponding areas that adjust the phase of transmitted light so as to focus the light at each of the focal planes of the semiconductor areas. 
         [0043]    Therefore, when seeking to perform photolithography of a semiconductor device having multiple areas of differing focal planes, the exemplary photomask can be disposed in an exposure apparatus, such as the photolithography apparatus  100  illustrated in  FIG. 1 . With reference to  FIG. 1 , the photolithography apparatus  100  includes a light source (not shown) that emits incident light  102 . The photomask is disposed in the path of this light. Light that is transmitted from the photomask will be focused by a projection lens  120  onto the surface of the semiconductor device  150 . 
         [0044]    The photomask should be positioned in the apparatus such that its phase shift areas are optically aligned with the corresponding focal plane areas of the semiconductor device. In other words, the photomask should be positioned such that substantially all light transmitted by a first phase shift area of the photomask will be focused by the projection lens  120  onto the corresponding focal plane area of the semiconductor device. Additionally, in an embodiment, the photomask is also positioned at such a distance from the projection lens  120  that the light focused by the projection lens will be substantially “in-focus” at each of the focal plane areas of the semiconductor device. In an embodiment, the photomask can be focally positioned by determining that light transmitted by a primary phase-shift area of the photomask is in-focus on the primary focal plane of the semiconductor device. 
         [0045]    Once properly positioned, the photolithography apparatus  100  can expose the semiconductor device  150  by emitting high intensity light from its light source. This light will become incident upon the exemplary photomask, and redirected by the projection lens to a surface of the semiconductor device  150 . Due to the unique properties of the exemplary photomask, the exposure produces sharp and accurate patterns over all areas of the semiconductor device  150 , regardless of whether they are recessed/raised relative to other areas. 
         [0046]    Several advantages are achieved through the use of the exemplary photomask. For example, because the photomask allows for the transmitted light to be focused across all areas of the exposure medium, sharp and accurate features can be etched. This substantially improves manufacturing yield, and reduces manufacturing defects. In addition, because the exposure can be performed simultaneously for all areas of the exposure medium, the photomask can be used in existing photolithography apparatuses without significant added cost or difficulty. Several other advantages will be apparent to those of ordinary skill in the art. 
       CONCLUSION 
       [0047]    It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections (if any), is intended to be used to interpret the claims. The Summary and Abstract sections (if any) may set forth one or more but not all exemplary embodiments of the invention as contemplated by the inventor(s), and thus, are not intended to limit the invention or the appended claims in any way. 
         [0048]    While the invention has been described herein with reference to exemplary embodiments for exemplary fields and applications, it should be understood that the invention is not limited thereto. Other embodiments and modifications thereto are possible, and are within the scope and spirit of the invention. For example, and without limiting the generality of this paragraph, embodiments are not limited to the software, hardware, firmware, and/or entities illustrated in the figures and/or described herein. Further, embodiments (whether or not explicitly described herein) have significant utility to fields and applications beyond the examples described herein. 
         [0049]    Embodiments have been described herein with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined as long as the specified functions and relationships (or equivalents thereof) are appropriately performed. Also, alternative embodiments may perform functional blocks, steps, operations, methods, etc. using orderings different than those described herein. 
         [0050]    References herein to “one embodiment,” “an embodiment,” “an example embodiment,” or similar phrases, indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it would be within the knowledge of persons skilled in the relevant art(s) to incorporate such feature, structure, or characteristic into other embodiments whether or not explicitly mentioned or described herein. 
         [0051]    The breadth and scope of the invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.