system and method for projection lithography with immersed image-aligned diffractive element

Abstract

A novel system and method and computer program product for exposing a photoresist film with patterns of finer resolution than can physically be projected onto the film in an ordinary image formed at the same wavelength. A hologram structure containing a set of resolvable spatial frequencies is first formed above the photoresist film. If necessary the photoresist is then sensitized. An illuminating wavefront containing a second set of resolvable spatial frequencies is projected through the hologram, forming a new set of transmitted spatial frequencies that expose the photoresist. The transmitted spatial frequencies include sum frequencies of higher frequency than is present in the hologram or illuminating wavefront, increasing the resolution of the exposing pattern. These high spatial frequency transmitted waves can be evanescent, or they can propagate at a steeper obliquity in a higher index medium than is possible in a projected image. A further method is described for designing lithographic masks to fabricate the hologram and to project the illuminating wavefront. In other embodiments, a simple personalization based on Talbot fringes and plasmonic interference is performed.

1 . A lithographic processing method of exposing a photosensitive medium formed in a wafer of semiconductor material to form an image of specified spatial frequency modulation, said method comprising:
 forming, atop a semiconductor wafer stack and adjacent to a photosensitive medium layer, a modulated diffractive hologram structure having a first spatial frequency modulation in its profile and refractive indices; and,   illuminating the formed diffractive hologram structure with a wave field having a second spatial frequency modulation, which combines with the first spatial frequency modulation to produce a specified spatial frequency modulation specified for the image.   
   
   
       2 . The method according to  claim 1 , wherein a diffractive hologram structure includes a grating, wherein a formed image includes alternating bright and dark regions of a specified spatial frequency modulation formed in the photosensitive medium layer. 
   
   
       3 . The lithographic method as claimed in  claim 1 , further comprising: inhibiting said photosensitive medium layer from exposure during forming of said diffractive hologram structure; and, switching on sensitivity of said photosensitive medium layer after forming said diffractive hologram structure. 
   
   
       4 . The lithographic method as claimed in  claim 1 , wherein said spatial frequency modulation produced by said hologram is a sum of frequencies of said first and second spatial frequency modulations. 
   
   
       5 . The lithographic method as claimed in  claim 1 , wherein a second spatial frequency modulation includes wave fields of evanescent orders. 
   
   
       6 . The lithographic method as claimed in  claim 1 , further comprising:
 printing, using partially coherent projection lithography, said hologram structure having spatial frequency content of up to 2*P*NA/λ diffraction orders, wherein P is the period of the first spatial frequency modulation, NA is the numerical architecture of the projection lens and λ is the wavelength of the first spatial frequency modulation used to print the hologram.   
   
   
       7 . The lithographic method as claimed in  claim 6 , wherein amplitudes of said diffraction orders is an adjustable variable when forming said hologram structure. 
   
   
       8 . The lithographic method as claimed in  claim 6 , wherein said forming of said hologram structure results in a patterned image exploiting frequency doubling such that an amplitude bandlimit of said printed image is 4*P*NA/λ. 
   
   
       9 . The lithographic method as claimed in  claim 1 , further including: implementing a patterned reduction mask for modulating said wave field to form said second spatial frequency modulation. 
   
   
       10 . The lithographic method as claimed in  claim 1 , wherein said modulated diffractive hologram structure is formed in a photosensitive medium layer having an increased or decreased transmission characteristic under irradiation. 
   
   
       11 . The lithographic method as claimed in  claim 1 , wherein said modulated diffractive hologram structure is formed in a semiconductor film using imprint lithography. 
   
   
       12 . The lithographic method as claimed in  claim 1 , wherein said modulated diffractive hologram structure is formed in an opaque semiconductor film by etching apertures into said opaque film. 
   
   
       13 . The lithographic method as claimed in  claim 1 , wherein said modulated diffractive hologram structure is formed in a photosensitive medium layer by exposing and developing a conventional photoresist layer. 
   
   
       14 . The lithographic method as claimed in  claim 1 , wherein a resulting lithography includes image patterns wherein the spatial frequency content of an image pattern achieves a non-evanescent spatial frequency of about 1.911/λ, where λ is the exposing wavefront wavelength. 
   
   
       15 . The method according to  claim 2 , wherein a pitch spacing between an alternating bright and dark region ranges between 50 nm and the size of the image. 
   
   
       16 . A method for designing a mask implemented for fabricating a hologram structure in a photosensitive layer of a semiconductor stack and for generating an optimized wavefield for illuminating said hologram structure to achieve a specified image target comprising alternating bright and dark pattern region in a photosensitive material layer of said stack, said method comprising:
 selecting a preliminary wave field and hologram diffractive properties using a scalar model at an artificially reduced spatial frequency wavelength;   implementing wavefront engineering method to generate a physical structure for said hologram in a preliminary solution and a 1 st  stage wave field for modulating said hologram;   implementing local optimization to provide an optimized design for said hologram and a wavefront at the artificially decreased wavelength;   incrementing the wavelength in a small upward increment;   refining, at said incremented wavelength, said local optimization to tune in a solution for the current incremented wavelength by adjusting variables of a hologram profile, an index of the hologram features and a film stack beneath the hologram; and,   iterating between steps d)-e) to arrive at a solution which is valid at an operating wavelength, said solution including a 2 nd  stage wave field for modulating said hologram, wherein a spatial frequency modulation in a resulting image includes the same bright and dark regions as the image target; and,   generating a pair of lithographic masks suitable to generate 1 st  stage and 2 nd  stage wave fields at respective first and second sets of spatial frequencies using said wavefront engineering method.   
   
   
       17 . The mask design method as claimed in  claim 16 , further including: employing an electromagnetic optimizer when iterating between steps d)-e), said optimizer maximizing lithographic process window utilization and, maximizing a range of light dose and focus fluctuations within which the bright and dark polarity patterns in the developed image match the target patterns to within an acceptable tolerance. 
   
   
       18 . The mask design method as claimed in  claim 16 , wherein said iterating between steps d)-e) results in a solution valid at a specified operating wavelength. 
   
   
       19 . The mask design method as claimed in  claim 16 , wherein said hologram structure is a parent grating structure, said iterating between steps d)-e) resulting in a solution having a minimized zero order diffraction by the parent grating by an interlayer designed to produce reflections capable of canceling a propagating 0 th  order wavefront. 
   
   
       20 . A method of exposing a photosensitive resist layer with an image including a pattern of specified spatial frequency modulation, said method comprising:
 forming a first parent grating structure over a film stack, said film stack including a photosensitive layer of material;   exposing said first parent grating structure with a wave field energy at a first modulation frequency to form a diffractive hologram structure above said photosensitive material layer;   illuminating said formed diffractive hologram structure with wave field energy at a second modulation frequency to form an image pattern at said photosensitive material layer,   wherein said formed image pattern exhibits enhanced spatial frequencies including a sum of first and second modulations frequencies when the wave field energy at a second modulation frequency is diffracted by said diffractive hologram structure.   
   
   
       21 . The method of  claim 20 , wherein said hologram and illuminating wave field are formed by reduction mask projection lithography, and information in a combined image is bounded by an amplitude spatial frequency limit at about 1.91/λ, where λ is the exposing wavefront wavelength. 
   
   
       22 . The method of  claim 21 , further including, personalizing said image pattern formed in said resist layer by said image pattern by illuminating said diffractive hologram structure via a reduction mask at said second modulation frequency. 
   
   
       23 . The method of  claim 21 , further including:
 removing said diffractive hologram structure; and   personalizing said image pattern formed in said resist layer by said image pattern by illuminating said diffractive hologram structure via a reduction mask at said second modulation frequency after said removing.   
   
   
       24 . The method of  claim 21 , further including: using said diffractive hologram structure to print many different patterns in said photosensitive material layer by changing the exposing pattern via a reduction mask at a second modulation frequency. 
   
   
       25 . A program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine to perform method steps for designing a mask implemented for fabricating a hologram structure in a photosensitive layer of a semiconductor stack and for generating an optimized wavefield for illuminating said hologram structure to achieve a specified image target comprising alternating bright and dark pattern region in a photosensitive material layer of said stack, said method steps comprising:
 a) selecting a preliminary wave field and hologram diffractive properties using a scalar model at an artificially reduced spatial frequency wavelength;   implementing wavefront engineering method to generate a physical structure for said hologram in a preliminary solution and a 1 st  stage wave field for modulating said hologram;   implementing local optimization to provide an optimized design for said hologram and a wavefront at the artificially decreased wavelength;   incrementing the wavelength in a small upward increment;   refining, at said incremented wavelength, said local optimization to tune in a solution for the current incremented wavelength by adjusting variables of a hologram profile, an index of the hologram features and a film stack beneath the hologram; and,   iterating between steps d)-e) to arrive at a solution which is valid at an operating wavelength, said solution including a 2 nd  stage wave field for modulating said hologram, wherein a spatial frequency modulation in a resulting image includes the same bright and dark regions as the image target; and,   generating a pair of lithographic masks suitable to generate 1 st  stage and 2 nd  stage wave fields at respective first and second sets of spatial frequencies using said wavefront engineering method.