Abstract:
A method for making a diffractive optical element (DOE) includes forming a first mask that exposes a portion of a substrate, depositing a first film over the substrate, removing the first mask to form a first optical element on the substrate, forming a second mask that exposes a portion of the first optical element, depositing a second film over the substrate, and removing the second mask to form a second optical element. A method for making a DOE includes patterning a first material to expose a portion of a substrate, depositing a first film over the substrate, planarizing the first film and the first material to form a first optical element, patterning a second material to expose a portion of the first optical element, depositing a second film over the substrate, and planarizing the second film and the second material to form a second optical element.

Description:
DESCRIPTION OF RELATED ART  
       [0001]     U.S. Pat. No. 5,218,471 (“Swanson et al.”) describes a method for fabricating a diffractive optical element (DOE). Specifically, Swanson et al. describes successive etching after applying masks. The etch depth of each mask is binary weighted. With such a method, 2ˆN of phase levels can be achieved using only N masks.  
       SUMMARY  
       [0002]     In one embodiment of the invention, a method for making a diffractive optical element (DOE) includes forming a first mask that exposes a portion of a substrate, depositing a first film over the substrate, removing the first mask to form a first optical element on the substrate, forming a second mask that exposes a portion of the first optical element, depositing a second film over the substrate, and removing the second mask to form a second optical element.  
         [0003]     In another embodiment of the invention, a method for making a DOE includes patterning a first material to expose a portion of a substrate, depositing a first film over the substrate, planarizing the first film and the first material to form a first optical element, patterning a second material to expose a portion of the first optical element, depositing a second film over the substrate, and planarizing the second film and the second material to form a second optical element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIGS. 1A, 1B ,  1 C,  1 D,  1 E, and  1 F illustrate cross-sections of structures formed in a method for fabricating a diffractive optical element in one embodiment of the invention.  
         [0005]      FIGS. 1G, 1H ,  1 I, and  1 J illustrate cross-sections of structures formed in a continuation of the method in  FIGS. 1A  to  1 F in one embodiment of the invention.  
         [0006]      FIGS. 2A, 2B ,  2 C,  2 D,  2 E, and  2 F illustrate cross-sections of structures formed in a method for fabricating a diffractive optical element in one embodiment of the invention.  
         [0007]      FIGS. 2G, 2H ,  21 , and  2 J illustrate cross-sections of structures formed in a continuation of the method in  FIGS. 2A  to  2 F in one embodiment of the invention.  
         [0008]      FIGS. 3A, 3B ,  3 C,  3 D,  3 E,  3 F, and  3 G illustrate cross-sections of structures formed in a method for fabricating a diffractive optical element in one embodiment of the invention.  
         [0009]      FIGS. 3H, 31 ,  3 J, and  3 K illustrate cross-sections of structures formed in a continuation of the method in  FIGS. 3A  to  3 E in one embodiment of the invention. 
     
    
       [0010]     Use of the same reference numbers in different figures indicates similar or identical elements. The figures are not drawn to scale and are for illustrative purposes only.  
       DETAILED DESCRIPTION  
       [0011]      FIGS. 1A  to  1 E illustrate a lift-off method for fabricating a diffractive optical element (DOE) in one embodiment of the invention.  
         [0012]     In  FIG. 1A , a lift-off mask  102  is formed over a substrate  104 . Mask  102  may have sidewalls  106  with a re-entry profile. Sidewalls  106  define a window  108  that exposes a portion of substrate  104 . Substrate  104  can be a silicon substrate and mask  102  can be a photoresist that is spun on, exposed, and developed.  
         [0013]     In  FIG. 1B , a thin film  110  is deposited over substrate  104 . As a result, thin film  110  collects on mask  102  and the exposed portion of substrate  104 . Thin film  110  can be a dielectric (e.g., Si, SiO 2 , or TiO 2 ) deposited by electron beam (e-beam) evaporation or sputtering. When a thin film is deposited by evaporation, the thickness can be controlled with great accuracy (e.g., within 10% of the target thickness) using in-situ thickness monitors in the evaporating equipment.  
         [0014]     In  FIG. 1C , mask  102  is removed to lift off the thin film collected thereon and to leave behind the thin film collected on substrate  104 . The remaining thin film forms an optical element  110 A. Mask  102  can be chemically removed by a resist stripper.  
         [0015]     In  FIG. 1D , a lift-off mask  112  is formed over substrate  104 . Mask  112  may have sidewalls  116  with a re-entry profile. Sidewalls  116  define a window  118  that exposes a portion of optical element  110 A. Mask  112  also covers sidewalls  115  of optical element  110 A to provide the proper offset for another optical element to be formed on top of optical element  110 A.  
         [0016]     A thin film  120  is deposited over substrate  104 . As a result, thin film  120  collects on mask  112  and the exposed portion of optical element  110 A. Mask  112  is removed to lift off the thin film collected thereon and to leave behind the thin film collected on optical element  110 A. The remaining thin film  120  forms an optical element  120 A ( FIG. 1E ).  
         [0017]     In  FIG. 1E , a lift-off mask  122  is formed over substrate  104 . Mask  122  may have sidewalls  126  with a re-entry profile. Sidewalls  126  define a window  128  that exposes a portion of optical element  120 A. Mask  122  also covers sidewalls  125  of optical element  120 A to provide the proper offset for another optical element to be formed on top of optical element  120 A.  
         [0018]     A thin film  130  is deposited over substrate  104 . As a result, thin film  130  collects on mask  122  and the exposed portion of optical element  120 A. Mask  122  is removed to lift off the thin film collected thereon and to leave behind the thin film collected on optical element  120 A. The remaining thin film  130  forms an optical element  130 A ( FIG. 1F ).  
         [0019]     As described above, the same process can be repeated a number of times to create a stack of optical elements having the desired thicknesses and shapes.  
         [0020]      FIG. 1F  illustrates a structure  100  having optical elements  110 A,  120 A,  130 A, and  140 A formed from the process described above. In one embodiment, structure  100  is a DOE such as a transmissive grating. Of course a reflective grating can be made if reflective thin films are used.  
         [0021]     In another embodiment, structure  100  is a mold for fabricating a DOE using a conventional ultraviolet (UV) replication process.  
         [0022]     In another embodiment, one or more structures  100  form a mold  142  for fabricating a DOE using a conventional injection molding process. In this embodiment, substrate  104  is a metal substrate and optical elements  110 A,  120 A,  130 A, and  140 A are made from metal thin films (e.g., Ni).  
         [0023]     In another embodiment, structure  100  forms an imprint mask for fabricating a DOE using conventional step and flash imprint lithography. In this embodiment, substrate  104  is a metal substrate and optical elements  110 A,  120 A,  130 A, and  140 A are made from metal thin films (e.g., Ni).  
         [0024]     In another embodiment illustrated in  FIGS. 1G  to  1 J, structure  100  is a model for a mold used to fabricate a DOE. In this embodiment, substrate  104  is a metal substrate and optical elements  110 A,  120 A,  130 A, and  140 A are made from metal thin films (e.g., Cu, Au, or W). In  FIG. 1G , a layer of metal  150  is formed over model  100  by plating model  100  with metal  150  (e.g., Ni).  
         [0025]     In  FIG. 1H , model  100  and substrate  104  are removed to form a mold  150 A. Mold  150 A defines a cavity  152  having the form of a DOE. Model  100  and substrate  104  can be removed by chemical wet etches. In  FIG. 1I , a material is deposited in mold  150 A to form a DOE  160 . In  FIG. 1J , DOE  160  is separated from mold  150 A and ready to be used. Depending on its material, DOE  160  can be a transmissive or reflective grating.  
         [0026]      FIGS. 2A  to  2 F illustrate a lift-off method for fabricating a DOE using binary weighted masks in one embodiment of the invention.  
         [0027]     In  FIG. 2A , a lift-off mask  202  is formed over a substrate  204 . Mask  202  defines a window  208  that exposes a portion of substrate  204 . Substrate  204  can be a silicon substrate and mask  202  can be a photoresist that is spun on, exposed, and developed. Although not illustrated, it is understood that mask  202  may have sidewalls with a re-entry profile.  
         [0028]     A thin film  210  is deposited over substrate  204 . As a result, thin film  210  collects on mask  202  and the exposed portion of substrate  204 . Thin film  210  can be a dielectric (e.g., Si, SiO 2 , or TiO 2 ) deposited by e-beam evaporation or sputtering.  
         [0029]     In  FIG. 2B , mask  202  is removed to lift off the thin film collected thereon and to leave behind the thin film collected on substrate  204 . The remaining thin film forms an optical element  210 A. Mask  202  can be chemically removed by a resist stripper.  
         [0030]     In  FIG. 2C , a lift-off mask  212  is formed over substrate  204 . To implement the binary weighted scheme, mask  212  defines a window  218 A that exposes a portion of optical element  210 A and a portion of substrate  204 . Mask  212  also defines a window  218 B that exposes another portion of substrate  204 . Although not illustrated, it is understood that mask  212  may have sidewalls with a re-entry profile.  
         [0031]     A thin film  220  is deposited over substrate  204  and collects on mask  212  and the exposed portions of optical element  210 A and substrate  204 . To implement the binary weighted scheme, thin film  220  has half the thickness of thin film  210 .  
         [0032]     In  FIG. 2D , mask  212  is removed to lift off the thin film collected thereon and to leave behind the thin film collected on optical element  210 A and substrate  204 . The remaining thin film forms optical elements  220 A,  220 B, and  220 C. After the use of two masks, a four level structure is formed.  
         [0033]     In  FIG. 2E , a lift-off mask  222  is formed over substrate  204 . To implement the binary weighted scheme, mask  222  defines windows  228 A,  228 B,  228 C, and  228 D. Window  228 A exposes a portion of optical element  220 A. Window  228 B exposes a portion of optical element  210 A. Window  228 C exposes a portion of optical element  220 B. Window  228 D exposes a portion of optical element  220 C and a portion of substrate  204 . Although not illustrated, it is understood that mask  222  has sidewalls with a re-entry profile.  
         [0034]     A thin film  230  is deposited over substrate  204  and collects on mask  222  and the exposed portions of substrate  204  and optical elements  210 A,  220 A,  200 B, and  220 C. To implement the binary weighted scheme, thin film  230  has half the thickness of thin film  220 .  
         [0035]     In  FIG. 2F , mask  222  is removed to lift off the thin film collected thereon and to leave behind the thin film collected on substrate  204  and optical elements  210 A,  220 A,  220 B, and  220 C. The remaining thin film forms optical elements  230 A,  230 B,  230 C,  230 D, and  230 E. After the use of three masks, an eight level structure  200  is formed. In one embodiment, structure  200  is a DOE such as a transmissive grating. Of course a reflective grating can be made if reflective thin films are used.  
         [0036]     As described above, the same process can be repeated a number of times to create a stack of optical elements having the desired thicknesses and shapes. Furthermore, the thin film layers may be deposited in the order of increasing thickness instead of decreasing thickness. By depositing the thin films in the order of increasing thickness, the photoresist lift-off masks can be spun on more evenly.  
         [0037]     In another embodiment, structure  200  is a mold for fabricating a DOE using a conventional UV replication process.  
         [0038]     In another embodiment, structure  200  forms a mold for fabricating a DOE using a conventional injection molding process. In this embodiment, substrate  204  is a metal substrate and thin films  210 ,  220 , and  230  are metal thin films (e.g., Ni).  
         [0039]     In another embodiment, structure  200  forms an imprint mask for fabricating a DOE using conventional step and lift imprint lithography. In this embodiment, substrate  204  is a metal substrate and thin films  210 ,  220 , and  230  are metal thin films (e.g., Ni).  
         [0040]     In another embodiment illustrated in  FIGS. 2G  to  2 J, structure  200  is a model for a mold used to fabricate a DOE. In this embodiment, substrate  204  is a metal substrate and thin films  210 ,  220 , and  230  are metal thin films (e.g., Cu, Au, or W). In  FIG. 2G , a layer of metal  250  is formed over model  200  by plating model  200  with metal  250  (e.g., Ni).  
         [0041]     In  FIG. 2H , model  200  and substrate  204  are removed to form a mold  250 A. Mold  250 A defines a cavity  252  having the form of a DOE. Model  200  and substrate  204  can be removed by chemical wet etches. In  FIG. 2I , a material is deposited in mold  250 A to form a DOE  260 . In  FIG. 2J , DOE  260  is separated from mold  250 A and ready to be used. Depending on its material, DOE  260  can be a transmissive or reflective grating.  
         [0042]      FIGS. 3A  to  3 G illustrate a Damascene like method for fabricating a DOE in one embodiment of the invention.  
         [0043]     In  FIG. 3A , an oxide layer  302  is formed on a substrate  304 . Oxide layer  302  can be SiO 2  deposited by plasma enhanced chemical vapor deposition (PECVD), and substrate  304  can be a silicon substrate. An etch mask  305  is then formed on oxide layer  302 . Etch mask  305  can be a photoresist that is spun on, exposed, and developed. Etch mask  305  defines a window  306  that exposes a portion of oxide layer  302 .  
         [0044]     In  FIG. 3B , the exposed portion of oxide layer  302  is removed. The exposed portion of oxide layer  302  can be removed by dry or wet etching.  
         [0045]     In  FIG. 3C , etch mask  305  is removed. Etch mask  305  can be chemically removed by a resist stripper. The remaining oxide layer  302  defines a window  308  that exposes a portion of substrate  304   
         [0046]     In  FIG. 3D , a thin film  310  is deposited over substrate  304 . As a result, thin film  310  collects on oxide layer  302  and the exposed portion of substrate  304 . Thin film  310  can be Si deposited by PECVD.  
         [0047]     In  FIG. 3E , oxide layer  302  and thin film  310  are planarized to a desired thickness. Oxide  302  and thin film  310  can be planarized by chemical mechanical polishing (CMP). The remaining thin film forms an optical element  310 A.  
         [0048]     As described above, the same process can be repeated a number of times to create a stack of optical elements having the desired thicknesses and shapes. The planarized surface provides a smooth surface for spinning on the photoresist etch mask used to form the next optical element. As the process is similar to the Damascene process currently used to form copper conductors in complementary metal oxide semiconductor (CMOS) processing, the thickness (i.e., layer to layer registration) and shape (i.e., feature size) of the optical elements can be controlled with great accuracy (e.g., 0.04 micron and 0.4 micron, respectively).  
         [0049]      FIGS. 3F and 3G  illustrate a five level structure  300  having optical elements  310 A,  320 A,  330 A,  340 A, and  350 A formed from the process described above. The remaining oxides  302 ,  312 ,  322 , and  332  can be optionally removed by dry or wet etching. However, in some circumstances it may be desired to retain the remaining oxides. In one embodiment, structure  300  is a DOE such as transmissive grating. Of course a reflective grating can be made if reflective thin films are used.  
         [0050]     In another embodiment, structure  300  is a mold for fabricating a DOE using a conventional UV replication process.  
         [0051]     In another embodiment, structure  300  forms a mold for fabricating a DOE using a conventional injection molding process. In this embodiment, substrate  304  is a metal substrate and optical elements  310 A,  320 A,  330 A,  340 A, and  350 A are made from metal thin films (e.g., Ni).  
         [0052]     In another embodiment, structure  300  forms an imprint mask for fabricating a DOE using conventional step and lift lithography. In this embodiment, substrate  304  is a metal substrate and optical elements  310 A,  320 A,  330 A,  340 A, and  350 A are made from metal thin films (e.g., Ni).  
         [0053]     In another embodiment illustrated in  FIGS. 3H  to  3 K, structure  300  is a model for a mold used to fabricate a DOE. In this embodiment, optical elements  310 A,  320 A,  330 A,  340 A, and  350 A are thin metal films (e.g., Cu, Au, or W). In  FIG. 3H , a layer of metal  350  is formed over model  300  by plating model  300  with metal  350  (e.g., Ni).  
         [0054]     In  FIG. 31 , model  300  and substrate  304  are removed to form a mold  350 A. Mold  350 A defines a cavity  352  having the form of a DOE. Model  300  and substrate  304  can be removed by chemical wet etches. In  FIG. 3J , a material is deposited in mold  350 A to form a DOE  360 . In  FIG. 3K , DOE  360  is separated from mold  350 A and ready to be used. Depending on its material, DOE  360  can be a transmissive or reflective grating.  
         [0055]     The above described processes can be performed in both a CMOS fab and an optoelectronic device fab. The advantage of the CMOS fab is that the DOEs can be made at high volume and with great precision. The advantage of the optoelectronic device fab is that the DOEs can be formed with optoelectronic devices on the same substrate.  
         [0056]     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Numerous embodiments are encompassed by the following claims.