Abstract:
A chromeless photomask includes a main pattern portion and a complementary pattern portion formed in the surface of the transparent mask substrate adjacent to an outer peripheral edge of the main pattern portion. The main pattern and complementary portions are each formed by recessing a surface of a transparent mask substrate to produce respective protrusions and recesses that induce a phase difference of 180 degrees in light rays passing therethrough. The complementary pattern portion is designed to produce interference that prevents distortion in the photoresist pattern formed at a region by and corresponding to the edge of the main pattern portion of the photomask. Accordingly, the present invention provides for a relatively large secondary mask alignment margin.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to photolithographic equipment and, in particular, an exposure apparatus, for use in forming a pattern on a semiconductor substrate. More specifically, the present invention relates to a chromeless photomask and to a method of manufacturing the same.  
           [0003]    2. Description of the Related Art  
           [0004]    In general, photomasks for use in photolithography are constructed of a transparent mask substrate formed of quartz, for example, and an opaque, metallic light-blocker layer formed of a desired pattern of chromium on the transparent mask substrate. The chromium light-blocker layer shields the exposure light, and causes light passing through the transparent substrate to undergo diffraction and interference, such that a corresponding micro-pattern can be transferred to a photoresist deposited on a semiconductor substrate. Nowadays, though, the density of semiconductor devices is at a very high level as exemplified by 256 or greater MDRAMs. Also, the line widths of the devices are as small as 0.25 μm or less. The photomasks comprising chromium light-blocker layers and that rely on the phenomena of diffraction or interference to transfer a micro-pattern to a semiconductor substrate are limited in their ability to form narrow and accurate micro-patterns. Phase shift masks and chromelsss photomasks have been developed as viable alternatives to address these limitations. The phase shift masks or chromelsss photomasks use a transparent pattern formed on or in a transparent mask substrate to create a phase difference that produces a photoresist pattern.  
           [0005]    A chromeless photomask transmits 100% of light through both a recessed area and a non-recessed area. However, the light passing through the recessed area is out of phase by 180 degrees from the light passing through the non-recessed area. As a result, the light rays passing through the recessed and non-recessed areas produce interference by which a predetermined pattern can be formed on a photoresist deposited on a semiconductor substrate. As long as the photomask pattern is well-designed, a chromeless photomask can be used to create a much finer pattern on a substrate than can be created using a conventional photomask having a chromium light-blocker layer.  
           [0006]    An example of a conventional chromeless photomask is shown in FIG. 12. Referring to FIG. 12, a conventional chromeless photomask  1 ′ includes a core area C including a main pattern  110  of closely spaced areas and a peripheral area P surrounding the core area C as separated a predetermined distance therefrom. The chromeless photomask consists of raised and recessed portions used to create an interference pattern by which a corresponding photoresist pattern can be formed on a semiconductor substrate.  
           [0007]    More specifically, the main pattern  110  in the core area C includes a plurality of identical areas which are closely spaced to produce interference when light passes through the core area C, whereby an image corresponding to the main pattern  110  can be effectively transferred to a photoresist. However, neighboring peripheral area P 1  surrounding the main pattern  110  may cause problems in the forming of the photoresist pattern. In particular, the outer areas of the main pattern  110  are affected by the neighboring peripheral areas P 1  such that an insufficient interference of light occurs at these areas leading, in turn, to a distorted photoresist pattern. In addition, when the photomask  1 ′ is misaligned along an X-axis, for example, during the exposure process, an undesirable “bar” appears along an area of the photoresist corresponding to one edge of the main pattern  110 , and a great amount of pattern distortion occurs along an area corresponding to the opposite edge of the main pattern  110 . Moreover, the forming a photoresist pattern using a conventional chromeless photomask is vulnerable to the problem of lens flare.  
         SUMMARY OF THE INVENTION  
         [0008]    Accordingly, an object of the present invention is to provide a photomask capable of forming a photoresist pattern without edge distortion and wherein an increased alignment margin is attained with respect to a secondary aspect of the mask alignment procedure.  
           [0009]    According to one aspect of the present invention, the photomask comprises a light-transmitting mask substrate, and a light-transmitting main pattern portion and a light-transmitting complementary pattern portion each formed by raised and recessed portion portions of material that will induce a phase difference of 180 degrees in light rays passing therethrough. The complementary pattern portion is formed adjacent the outer peripheral edge of the main pattern portion. The raised and recessed portions of the main pattern portion constitute a first pattern dedicated to produce the interference that forms the desired pattern on a photoresist layer. The raised and recessed portions of the complementary pattern portion constituting a second pattern that will produce interference that will enhance the portion of the desired pattern formed on the photoresist layer by light passing through the outer peripheral edge of the main pattern portion.  
           [0010]    Preferably, the mask substrate is formed of a 100% light-transmitting (exposure light) material, for example, quartz. The form of the second pattern defined by the complementary pattern portion may be simpler than that defined by the main pattern portion. Preferably, this second pattern is a pattern of identical stripes spaced a predetermined distance from one another and aligned along the outer peripheral edge of the main pattern portion. Thus, the interference between light rays passing through the complementary raised and recessed portions is such that a corresponding portion of a photoresist layer below the complementary pattern portion is maximally shielded from or is minimally exposed to the exposure light. Either of these effects will enhance that portion of the photoresist pattern formed at a region by and corresponding to the outer edge of the main pattern portion of the photomask.  
           [0011]    In an exposure apparatus, such as a stepper, of photolithographic equipment, the photomask is supported between a light source and a projection lens of the apparatus.  
           [0012]    Preferably, the pitch of the complementary raised portion and the complementary recessed portion of the photomask is constant such that a regular interference pattern is produced by light rays passing therethrough. The pitch of the stripes formed by complementary raised portion and the complementary recessed portion is designed based on the wavelength λ of the light emitted by the light source of the exposure apparatus, and the diffraction angle θ of the light passing through the transparent mask substrate. Preferably, the pitch is equal to or smaller than 0.61*(λ/sin θ).  
           [0013]    According to yet another aspect of the present invention, a method of manufacturing a chromeless photomask having the structure described above includes providing a mask substrate having a light-blocker mask layer on a surface thereof, removing select portions of the mask layer to form a main mask pattern and a complementary mask pattern, subsequently etching the mask substrate to a predetermined depth using the light-blocker mask layer as an etch mask to form a main pattern and a complementary pattern in the mask substrate, wherein each of the main pattern and the complementary pattern in the mask substrate will induce a phase difference of 180 degrees in light rays passing therethrough, and subsequently removing the light-blocker mask layer from the mask substrate.  
           [0014]    The light-blocker mask layer is preferably a chromium layer. In this case, the selected portions of the light-blocker layer may be removed using an e-beam lithographer so that fine and accurate main and complementary mask patterns can be formed.  
           [0015]    On the other hand, the mask substrate may be etched by plasma dry etching, and preferably, using a CF-based reactant gas. The use of the CF-based reactant gas in plasma dry etching facilitates the etching of the mask substrate especially when it is made of quartz, and enables the entire surface of the mask substrate to be etched uniformly.  
           [0016]    After the substrate is etched, the main and complementary mask patterns of the light-blocker mask layer may be etched away, and the surface of the substrate may then be subjected to a wet cleaning process. Preferably, the light-blocker mask layer is removed from the mask substrate by wet etching using a metal etchant. Suitable metal etchants for this wet etching include solutions of sulfuric acid (H 2 SO 4 ), hydrogen chloride (HCl), nitric acid (HNO 3 ), acetic acid (CH 3 COOH), and mixtures of these acid solutions. The use of the metal etchant is preferred because it can readily etch the metallic light-block mask layer without damaging the mask substrate. Alternatively, the light-blocker mask layer may be removed using dry etching.  
           [0017]    A photoresist pattern free of edge distortion can be formed by photolithography using the chromeless photomask according to the present invention because the chromeless complementary pattern portion is formed adjacent to and along the edge of the chromeless main pattern portion of the photomask. Such pattern distortion in the edge of the photoresist pattern can be prevented even when a slight misalignment occurs in a secondary mask alignment process. In addition, the possibility of flare due to a defect in the lens of the stepper can be reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments thereof made with reference to the attached drawings, in which:  
         [0019]    [0019]FIG. 1 is a plan view of a chromeless photomask according to the present invention;  
         [0020]    [0020]FIG. 2 is an enlarged plan view of a pattern unit of the chromeless photomask according to the present invention  
         [0021]    [0021]FIG. 3 is an enlarged plan view of portion A in FIG. 2;  
         [0022]    [0022]FIG. 4 is a sectional view taken along line IV-IV in FIG. 3;  
         [0023]    [0023]FIG. 5 is a flowchart of an embodiment of a method of manufacturing a chromeless photomask according to the present invention;  
         [0024]    [0024]FIGS. 6 through 10 are sectional views each illustrating a respective step of the method a manufacturing the chromeless photomask according to the present invention;  
         [0025]    [0025]FIG. 11A is a simplified schematic diagram of an alignment and exposure apparatus used in photolithography;  
         [0026]    [0026]FIG. 11B is a plan view of a contact pattern transferred to a photoresist layer on a semiconductor substrate using the chromeless photomask of FIG. 3 according to the present invention; and  
         [0027]    [0027]FIG. 12 is a plan view of a conventional chromeless photomask. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.  
         [0029]    Referring first to FIGS. 1 and 2, a chromeless photomask according to the present invention includes a transparent mask substrate  1  having a core area C and a peripheral area P. The mask substrate  1  is formed of a light-transmitting material, and preferably of a transparent material such as quartz. The core area C of the mask substrate  1  defines a main pattern portion  10 , which is used to pattern a photoresist on a semiconductor substrate. A portion P 1  of the peripheral area P of the mask substrate directly adjacent the core area C defines a complementary pattern portion  20 .  
         [0030]    Referring to FIGS. 3 and 4, the main pattern portion  10  is made by forming recesses to a predetermined depth in the surface of the mask substrate  1 , resulting in a main raised portion  101  and a main recessed portion  102 . Both the main raised portion  101  and the main recessed portion  102  transmit light because the main raised and recessed portions  101  and  102  are of transparent material, namely quartz. The difference in height between the main raised and recessed portions  101  and  102  is designed to induce a 180-degree phase difference between light rays passing through the main raised and recessed portions  101  and  102  and, in turn, to cause the wavefronts of the light rays propagating from the main raised and recessed portions  101  and  102  to undergo interference at the photoresist on the semiconductor substrate. Thus, the interference pattern will result in the image of a micro-pattern being formed on the photoresist, which micro-pattern has a CD or line width that is narrower than that of the main pattern portion  10 .  
         [0031]    The complementary pattern portion  20  has a second pattern that is different from the first pattern defined by the main pattern portion  10 , e.g., is simpler than the first pattern. In particular, the complementary pattern portion  20  is a pattern of stripes made by forming grooves to a predetermined depth in the surface of the mask substrate  1 , resulting in a plurality of alternating complementary raised and recessed portions  201  and  202 . The pitch of the complementary raised and recessed portions  201  and  202  is constant so that light rays passing through the complementary raised and recessed portions  201  and  202  of the mask substrate  1  undergo interference on the photoresist that enhances the interference pattern otherwise produced by the edges of the main pattern portion  10  adjacent the peripheral area P. Accordingly, a sharp photoresist pattern free of distortion and having a uniform critical dimension can be produced.  
         [0032]    The pitch of the complementary raised and recessed portions  201  and  202  is designed for based on the wavelength λ of the exposure light produced by the photo stepper of the photolithography equipment that employs the photomask, and the angle of diffraction θ of light passing through the complementary pattern portion  20 . Moreover, the shorter the wavelength, the easier it is to form a fine pattern. However, as the wavelength of the light and the design rule of the pattern become smaller, the angle of diffraction becomes greater. One might, therefore, think to reduce the distance between the photomask and a lens of the photo stepper. In general, though, the distance between the lens and the photomask is fixed in the photo stepper. Therefore, the inventors have conducted experiments to optimize the pitch of the complementary raised and recessed portions  201  and  202  with the aim of providing a photomask that can form the finest non-distorted pattern. Through these experiments, a pitch equal to or less than 0.61*(λ/sin θ) was determined to be most appropriate for forming a fine, accurate photoresist pattern using the phase shifting effect produced by a photomask according to the present invention.  
         [0033]    [0033]FIG. 5 is a flowchart of a method of manufacturing a chromeless photomask according to an embodiment of the present invention. FIGS. 6 through 10 illustrate the steps in the flowchart of FIG. 5.  
         [0034]    Referring to FIG. 6, a mask substrate  1  having a light-blocker mask layer  50  on a surface thereof is prepared (step S 1  in FIG. 5). The mask substrate  1  is a flat rectangular plate formed of transparent quartz. The light-blocker mask layer  50  is formed of chromium so as to be 100% light-shielding (opaque) with respect to the exposure light (the light-blocker mask layer  50  will be hereinafter referred to as the Cr layer  50 ). The Cr layer  50  may be formed using a physical vapor deposition process, for example, a metal evaporating or metal sputtering process. Alternatively, the Cr layer  50  may be formed using an electrochemical method, for example, an electrolysis plating method.  
         [0035]    As described above, the mask substrate  1  with the Cr layer  50  can be manufactured by forming the Cr layer  50  on a bare mask substrate. Alternatively, the mask substrate  1  with the Cr  50  may be purchased ahead of time.  
         [0036]    Referring to FIGS. 7 and 8, a main pattern portion  10   a  and a complementary pattern portion  20   a  are formed in the Cr layer  50  on the mask substrate  1  using an electron beam lithographer (step S 2 ). More specifically, the Cr layer  50  is irradiated with an e-beam having a predetermined spot diameter to form a main pattern portion  10   a  of the Cr layer  50 . The Cr layer  50  is also scanned with an e-beam to form a complementary pattern portion  20   a  around the main pattern portion  10   a  as spaced a predetermined distance therefrom. The complementary pattern portion  20   a  of the Cr layer  50  consists of a plurality of stripes spaced a predetermined distance from one another.  
         [0037]    Referring to FIG. 9, the mask substrate  1  is then etched using the main pattern portion  10   a  and the complementary pattern portion  20   a  as an etch mask (step S 3  in FIG. 5). As a result, the patterns of the main pattern portion  10   a  and the complementary pattern portion  20   a  are transferred into the mask substrate  1 , i.e., the main pattern group  10  and the complementary pattern portion  20  are formed.  
         [0038]    In step S 3 , the mask substrate  1  is etched using plasma dry etching with a CF-based reactant gas capable of etching silicon oxide layers, which are equivalent to the quartz constituting the mask substrate  1 . Plasma dry etching is preferred because, as an anisotropic etching method, it makes it easy to control the etch rate such that a fine, uniform pattern can be formed over the entire surface of the mask substrate  1 .  
         [0039]    As is clear from the description above, the portions of the mask substrate  1  that are etched away form the main and complementary recessed portions  102  and  202 . The surface of the mask substrate  1  is etched to a depth at which a phase difference of 180 degrees can be induced between light passing through the main and complementary recessed portions  102  and  202  and light passing through the main and complementary raised portions  101  and  201 . The depth of etching of the mask substrate  1  is controlled by controlling the etching time.  
         [0040]    Referring to FIG. 10, the remaining portions of the Cr layer  50 , namely the main pattern portion  10   a  and a complementary pattern portion  20   a  used as an etch mask, are removed (step S 4 ). Preferably, the Cr layer  50  is removed by wet etching with a metal etchant, and the resulting structure is cleaned to remove any particles produced by the etching process. The metal etchant includes at least one acid solution and thus, it can easily remove the (metal) Cr layer  50 . Suitable acid solutions for the metal etchant include solutions of sulfuric acid (H 2 SO 4 ), hydrogen chloride (HCl), nitric acid (HNO 3 ), acetic acid (CH 3 COOH), and mixtures of the forgoing acid solutions. Alternatively, the Cr layer  50  may be removed using a dry back-etching technique.  
         [0041]    [0041]FIG. 11A shows the basic structure of an alignment and exposure apparatus, namely, a stepper, used in photolithography. The stepper includes a light source  3  that emits light of a predetermined wavelength, a mask mount portion  7  equipped with the photomask  1  according to the present invention, and a focusing lens  5  disposed below the mask mount portion  7  and aligned with the light source  3  to focus light emitted from the light source  3  onto a semiconductor substrate (not shown) through the photomask  1 . The diffraction angle θ of light passing through the photomask  1  is established by the wavelength λ of light emitted from the light source  3  and the pitch of the complementary raised and recessed portions  201  and  202  of the complementary pattern portion  20  of the photomask  1 . For the reasons described earlier, the pitch of the complementary raised and recessed portions  201  and  202  is preferably equal to or smaller than 0.61*(λ/sin θ).  
         [0042]    [0042]FIG. 11B shows the contact pattern transferred to a photoresist layer on a semiconductor substrate using the chromeless photomask of FIG. 3 according to the present invention. Although the chromeless photomask of FIG. 3 has no direct pattern for forming contacts, a photoresist pattern for contacts C 1  and C 2  is formed by the interference of light rays passing through the raised and recessed pattern portions of the chromeless photomask. Also, the critical dimension of the photoresist patterns formed at locations corresponding to the edge of the main pattern portion  10  (FIG. 3) is maintained uniform due to the interference of the light rays passing through the complementary pattern portion  20  (FIG. 3). That is, the interference of the light rays passing through the complementary raised and recessed portions  201  and  202  of the complementary pattern portion  20  prevents a portion of the photoresist layer from being irradiated such that no pattern is formed in that portion of the photoresist layer. Also, a desired photoresist pattern free from edge distortion can be formed even when there is a slight misalignment in the exposure process. Thus, the chromeless photomask according to the present invention provides an increased alignment margin in the photolithography process. Still further, lens flare at a location corresponding to the edge of the photoresist pattern can be prevented. Therefore, the photolithography process can be performed with a high degree of reliability.  
         [0043]    Finally, although the present invention has been particularly shown and described with reference to the preferred embodiments thereof, the present invention is not so limited.  
         [0044]    For instance, the complementary pattern portion  20  may be a pattern of dotted lines or spots, instead of a pattern of stripes as described above.  
         [0045]    Also, the main and complementary raised portions  101  and  201  and the main and complementary recessed portions  102  and  202  have been described as being formed of the same material. Alternatively, the main and complementary raised portions  101  and  201  can be formed of a different material, but possessing the same light transmittance, as that of the main and complementary recessed portions  102  and  202 . As another alternative, the main and complementary raised portions  101  and  201  may be formed of a material, for example, a phase shifting material, possessing a light transmittance different than that of the material constituting the main and complementary recessed portions  102  and  202 . In these cases, after a light transmitting material is deposited on a surface of the mask substrate  1  to a predetermined thickness, the main and complementary pattern portions  10  and  20  are formed in the light transmitting material layer using an e-beam lithographer. A fewer number of steps are required for manufacturing such photomasks compared to the case of manufacturing a photomask using a Cr layer according to the present invention. Therefore, such photomasks are more easily manufactured at a lower cost.  
         [0046]    Therefore, although the present invention has been particularly shown and described with reference to the preferred embodiments thereof, various changes in form and details may be made thereto without departing from the spirit and scope of the present invention as defined by the following claims.