Patent Abstract:
A photomask and a method of fabricating the photomask. The photomask including: a substrate transparent to a selected wavelength or wavelengths of radiation, the substrate having a top surface and an opposite bottom surface, the substrate having a printable region and a non-printable region; the printable region having first opaque regions raised above the top surface of the substrate adjacent to clear regions, each opaque region of the first opaque regions having sidewalls and a top surface; the non-printable region comprising a second opaque region raised above the top surface of the substrate, the second opaque region having sidewalls and a top surface; and a capping layer on the sidewalls of the first opaque regions and the sidewalls of the second opaque region.

Full Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to the field of photomasks for the manufacture of integrated circuits; more specifically, it relates to a photomask for the manufacture of integrated circuits and to a method of fabricating the photomask mask. 
       BACKGROUND OF THE INVENTION 
       [0002]    Integrated circuit fabrication utilizes photolithography masks having opaque and clear areas corresponding to features on an integrated circuit that the mask is used to fabricate. Generally several masks, each having a pattern of clear and opaque areas corresponding to a particular fabrication level are required to build a functional semiconductor device. In use, a photosensitive layer (hereinafter photoresist layer) on an integrated circuit substrate (hereinafter wafer) is exposed to optical radiation projected through the photomask to form latent images in the photoresist layer. After developing the photoresist layer, a positive or negative pattern (relative to the pattern of clear and opaque regions on the photomask) comprising islands of photoresist is reproduced on the wafer. 
         [0003]    One type of photolithographic mask is called a binary mask (as opposed to a phase shift mask) in which there are two levels of transmission and no phase change of the radiation passing through the mask, one level in the opaque regions that essentially blocks the optical radiation and one level in the clear regions that passes the optical radiation. 
         [0004]    A second type of mask is called an attenuated phase shift mask having three levels of transmission, one level in the opaque regions that essentially blocks the optical radiation, a second level in the clear regions that passes the optical radiation and a third level in semi-opaque regions that blocks about 94% of the optical radiation, but the optical radiation that is not blocked is phase shifted by 180 degrees compared to the optical radiation passing through the clear regions. 
         [0005]    A third type of mask is called an alternative phase shift mask having three levels of transmission, one level in the clear regions that essentially blocks the optical radiation, a second level in clear regions that passes the optical radiation and a third level in thin substrate clear regions that passes and phase-shifts the optical radiation by 180 degrees compared to the optical radiation passing through the thin substrate clear regions. 
         [0006]    In such masks, it is necessary to ensure that the relative transmission levels and/or optical radiation wavelength phase do not change if consistent image reproduction is to be consistent from wafer to wafer. 
       SUMMARY OF THE INVENTION 
       [0007]    A first aspect of the present invention is a photomask, comprising: a substrate transparent to a selected wavelength or wavelengths of radiation, the substrate having a top surface and an opposite bottom surface, the substrate having a printable region and a non-printable region; the printable region having first opaque regions raised above the top surface of the substrate adjacent to clear regions, each opaque region of the first opaque regions having sidewalls and a top surface; the non-printable region comprising a second opaque region raised above the top surface of the substrate, the second opaque region having sidewalls and a top surface; and a capping layer on the sidewalls of the first opaque regions and the sidewalls of the second opaque region. 
         [0008]    A second aspect of the present invention is a method of fabricating a photomask, comprising: on a substrate transparent to a selected wavelength or wavelengths of radiation, the substrate having a top surface and an opposite bottom surface, defining a printable region and a non-printable region; forming in the printable region, first opaque regions raised above the top surface of the substrate adjacent to clear regions, each opaque region of the first opaque regions having sidewalls and a top surface; forming in the non-printable region, a second opaque region raised above the top surface of the substrate, the second opaque region having sidewalls and a top surface; and forming a capping layer on the sidewalls of the first opaque regions and the sidewalls of the second opaque region. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0010]      FIGS. 1A through 1C  are cross-sectional views illustrating fabrication of a binary photomask according to the present invention; 
           [0011]      FIGS. 2A through 2C  are cross-sectional views illustrating fabrication of an attenuated phase shift photomask according to the present invention; and 
           [0012]      FIGS. 3A through 3C  are cross-sectional views illustrating fabrication of an alternating phase shift mask photomask according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    In a binary mask the opaque regions have, in one example, pass essentially none of a selected wavelength or group of wavelengths of optical radiation, i.e. the mask design wavelength(s) and the clear regions pass, in one example, about 99% or more of the optical radiation. 
         [0014]    In a attenuated phase-shift mask, a small amount of radiation passes through the opaque regions (in one example, passing about 6% or less of the optical radiation) and undergoes a phase shift relative to the phase of the radiation passing through the clear regions (passing about 99% or more of the optical radiation). 
         [0015]    In an alternating phase-shift mask, the radiation passing through the thinned clear regions (passing about 99% or more of the optical radiation) of the substrate undergoes a phase shift relative to the phase of the radiation passing through the non-thinned clear regions. The opaque regions have, in one example, an essentially zero radiation transmission level. 
         [0016]      FIGS. 1A through 1C  are cross-sectional views illustrating fabrication of a binary photomask according to the present invention. In  FIG. 1A , a photomask  50  comprises a quartz or glass substrate  100  having a top surface  105  and a bottom surface  110 . Photomask  50  includes a non-printable region  115  and a printable region  120 . In one example, non-printable-region  115  surrounds the entire periphery of printable region  120 . Non-printable region  115  comprises an opaque layer  125  on top surface  105  of substrate  100  and an anti-reflective layer  130  on a top surface of opaque layer  125 . Formed in printable region  120  is a pattern of opaque regions  135  and clear regions  140  corresponding to a pattern of shapes to be transferred to a wafer by a photolithographic process using photomask  50 . Each of opaque region  135  comprises opaque layer  125  and anti-reflective layer  130 . 
         [0017]    In one example, substrate  100  comprises quartz or glass. In one example, first opaque layer  125  is chrome formed by evaporation or sputter deposition. Chrome is particularly reactive under semiconductor device fabrication conditions and application of the embodiments of the present invention to chrome containing masks is particularly advantageous. In one example, opaque layer  125  is between about 300 Å and about 1000 Å thick. In one example, anti-reflective layer  130  is chrome oxide (CrO 2 ) or chromium oxynitride (Cr 2 O x N y ). In one example, anti-reflective layer  130  is between about 30 Å and about 500 Å thick. 
         [0018]    The pattern of opaque regions  135  and clear regions  140  may be formed by (1) forming a chrome layer on the substrate, a chrome oxide layer on top of the chrome layer and a photoresist layer on top of the chrome oxide layer, (2) exposing selected regions of the photoresist layer to optical or e-beam radiation, (3) developing the photoresist layer, (4) etching away the chrome oxide and chrome where they not protected by photoresist, and (5) removing any remaining photoresist. 
         [0019]    In  FIG. 1B , a conformal capping layer  145  is formed on all exposed surfaces of opaque layer  125 , anti-reflective layer  130  and exposed top surface  105  of substrate  100  in clear regions  140 . Capping layer  145  is transparent to mask design wavelength(s) and thin to prevent attenuation of optical radiation. In one example, capping layer  145  comprises silicon dioxide (SiO 2 ). When capping layer  145  comprises SiO 2 , it may be formed by plasma enhanced CVD (PECVD). In one example, capping layer  145  is between about 10 Å and about 50 Å thick. Capping layer  145  prevents the material (e.g. Cr) in opaque regions  135  from chemical attack and prevents the top surface of clear regions  140  from contamination. Capping layer  145  should be thick enough to prevent diffusion of underlying layers but thin enough not to block light. 
         [0020]    In  FIG. 1C , spacers (i. e. sidewall capping layers)  150  are formed from capping layer  145  using a reactive ion etch (RIE) to remove the capping layer from the top surface of antireflective layer  130  and from top surface  105  of substrate  100  in clear regions  140  (except where spacers  150  contact top surface  105  along the periphery of clear regions  140 ). When capping layer  145  is SIO 2 , the RIE etch may be fluorine based. Spacers  150  completely cover all exposed edges of opaque layer  125 . 
         [0021]      FIGS. 2A through 2C  are cross-sectional views illustrating fabrication of an attenuated phase shift photomask according to the present invention. In  FIG. 2A , a photomask  55  comprises substrate  100  having non-printable region  115  and printable region  120 . Non-printable region  115  comprises an attenuating layer  155  on top surface  105  of substrate  100 , an opaque layer  125  on a top surface of attenuating layer  155  and an anti-reflective layer  130  on the top surface of opaque layer  125 . Formed in printable region  120  is a pattern of optical radiation attenuating regions  160  and clear regions  140  corresponding to a pattern of shapes to be transferred to a wafer by a photolithographic process using photomask  55 . Each of attenuating regions  160  comprises attenuating layer  155 . 
         [0022]    In one example, attenuating layer  155  is between about 300 Å and about 1000 Å thick. In one example, attenuating layer  155  is molybdenum silicide (MoSi). In one example, anti-reflective layer  130  is between about 30 Å and about 500 Å thick. 
         [0023]    The pattern of opaque regions  135  and clear regions  140  may be formed by (1) forming a molybdenum silicide layer on the substrate, forming a chrome layer on the molybdenum silicide, a chrome oxide layer on top of the chrome layer and a photoresist layer on top of the chrome oxide layer, (2) exposing selected regions of the photoresist layer to optical or e-beam radiation, (3) developing the photoresist layer, (4) etching away the chrome oxide, chrome and molybdenum silicide where they not protected by photoresist, (5) forming a second photoresist layer to protect the non-printable regions and (6) etching the exposed chrome oxide and chrome and (6) removing the second photoresist layer. 
         [0024]    In  FIG. 2B , conformal capping layer  145  is formed on all exposed surfaces of opaque layer  125 , anti-reflective layer  130 , attenuating layer  155  and on exposed top surface  105  of substrate  100  in clear regions  140 . 
         [0025]    In  FIG. 2C , optionally, spacers  150  are formed by etching layer  145  with RIE. Spacers  150  completely cover all exposed edges of opaque layer  125 . 
         [0026]      FIGS. 3A through 3C  are cross-sectional views illustrating fabrication of an alternating phase shift mask photomask according to the present invention. In  FIG. 3A , a photomask  60  comprises substrate  100  having non-printable region  115  and printable region  120 . Non-printable region  115  comprises an opaque layer  125  and an anti-reflective layer  130  on the top surface of opaque layer  125 . Formed in printable region  120  is a pattern of opaque regions  135 , thinned clear regions  140 A and clear regions  140 B corresponding to a pattern of shapes to be transferred to a wafer by a photolithographic process using photomask  60 . 
         [0027]    Photomask  60  may be formed from photomask  50  illustrated in  FIG. 1A  and described supra by protecting some clear regions  140  (see  FIG. 1A ) with photoresist and etching into substrate  100  to form trenches  160  where an opening has been lithographically formed in the photoresist layer and then removing the photoresist. 
         [0028]    In  FIG. 3B , conformal capping layer  145  is formed on all exposed surfaces of opaque layer  125 , opaque region  130 , exposed top surface  105  of substrate  100  in clear regions  140  and the sidewalls and bottom of trenches  160 . 
         [0029]    In  FIG. 3C , optionally, spacers  150  are formed. Spacers  150  completely cover all exposed edges of opaque layer  125 . 
         [0030]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.

Technology Classification (CPC): 6