Abstract:
A system and method of improving a photolithography process is disclosed. A phase shift filter is placed between two lenses located between a reticle mask and a wafer. The two lenses combined with the phase-shift filter performs a adjustment of the mask image in the spatial frequency domain, projecting an image that is equivalent to the differentiation of the light intensity of the mask image, thereby reproducing a sharper defined mask pattern on the wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to photolithography of semiconductor devices, and more particularly, to a photolithography system having a frequency domain filter mask. 
     2. Background Information 
     Photolithography is a process that is commonly used in the manufacture of integrated circuits. The well-known process involves the deposition of a photoresist layer onto an underlying substrate layer. The photoresist is then selectively exposed to light, which chemically alters the photoresist. The photoresist is then developed and those portions of the photoresist that are exposed to light are either hardened or softened, depending upon whether or not the photoresist is “negative” or “positive” photoresist, respectively. 
     The pattern that is projected onto the photoresist layer is contained on a mask that is placed within the photolithography exposure tool. The mask is also referred to as a reticle mask. The most common type of photolithography exposure tool is the stepper machine. A mask is placed between the illuminating light and the photoresist. The reticle is typically formed from patterned chromium coated on glass or quartz. The pattern is transferred onto the photoresist by projecting an image of the mask onto the photoresist. 
     As features on the mask become closer and closer together, diffraction effects begin to appear when the width of the openings on the mask is comparable to the wavelength of the light source. The diffraction effect blurs the light image projected onto the photoresist, resulting in poor resolution. The pattern formed on the photoresist layer will not be an exact replica of the pattern on the reticle mask, producing errors in the manufacturing process. One prior art method of preventing diffraction patterns from interfering with the desired patterning of the photo-resist is to cover selected openings in the mask with a transparent layer that shifts one of the sets of exposing rays out of phase, which will null the interference pattern. This approach is referred to as a phase shift mask. 
     FIG. 1 shows a prior art phase shift mask. The phase shift mask has parts of the openings in the photoresist layer covered by a phase-shifting layer. This generally requires the deposition of a layer of silicon dioxide onto the mask or reticle and a photomasking process to remove the oxide layer from alternate patterns. Covering every other opening works well for repeated array patterns such as logic and memory products. 
     Nevertheless, use of the phase shift mask has several disadvantages. First, the design of a phase shift mask is a relatively complicated procedure that requires significant resources. Secondly, because of the nature of a phase shift mask, it is difficult to check whether or not defects are present in the phase shift mask. 
     Therefore, what is needed is a new method of providing the high resolution capabilities of a phase shift mask using a simpler approach. 
     SUMMARY OF THE INVENTION 
     A system and method of improving a photolithography process is disclosed. A phase-shift filter is placed between two focusing lenses located between a reticle mask and a wafer. The two focusing lenses combined with the phase-shift filter performs an amplitude and phase adjustment of the mask image in the spatial frequency domain, projecting an image that is equivalent to the differentiation of the light intensity of the mask image onto the wafer, thereby more accurately reproducing the mask pattern on the wafer. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The present invention will be described in conjunction with the following drawings, wherein: 
     FIG. 1 is a schematic drawing of a prior art phase-shift mask. 
     FIG. 2 is a schematic diagram of a lens pair for transforming an image in the spatial domain to the spatial frequency domain and then back to the spatial domain. 
     FIG. 3 is a schematic diagram of an optical system formed in accordance with the present invention. 
     FIG. 4 is a schematic diagram of another optical system formed in accordance with the present invention. 
     FIG. 5 is a diagram of an embodiment of a phase-shift filter. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention uses a first focusing lens, a phase-shift filter, and a second focusing lens to produce a replicate image of a mask pattern with sharper defined edges on a semiconductor wafer. The mask pattern is formed on a reticle mask, or photolithographic mask. The term “lens” will generally refer to a “focusing lens” hereinafter. The light emitting from a light source passes though the reticle mask, the first lens, the phase-shift filter, the second lens, and then projects an image of the mask pattern onto the wafer. The first lens produces a Fourier-transformed image of the mask pattern. The phase-shift filter adjusts the phase and amplitude of the Fourier-transformed image to produce an “adjusted Fourier-transformed” image. The second lens produces an inverse-Fourier transformed image of the adjusted Fourier-transformed image, which is then projected onto the wafer. As will be described below in more detail, the inverse-Fourier transform of the adjusted Fourier-transformed image is an accurate replica of the original mask pattern with sharply defined edges. 
     The openings of a mask that defines the mask pattern can be characterized as slits. When the slit widths on the mask are comparable to the wavelength of the light source, diffraction will occur when the light passes through the slits on the mask and onto the wafer. Due to diffraction, the image of the slit (slit image) formed on the wafer is blurred at the edges. The light intensity will be higher near the center of the slit image, decreasing gradually at the edges. Thus the boundaries of the slit image will not be clearly defined. By performing a differentiation operation on the light intensity pattern formed after the light passes through the mask, the edges of the image projected onto the wafer can be sharpened, resulting in a more clearly defined image. The differentiation operation of the light intensity pattern is achieved by utilizing a phase-shift filter to adjust the amplitude and phase of the image in the frequency domain. 
     Fourier Analysis 
     Referring FIG. 2, a light source  202 , a first lens  206 , and a second lens  210  are aligned along the optical axis  214  of the first and second lenses  206  and  210 . The focal length of the first lens  206  is equal to f′, and the focal length of the second lens  210  is equal to f. The first lens  206  has a front focal plane  204  and a back focal plane. The front direction refers to the direction towards the light source  202 . The second lens  210  has a front focal plane and a back focal plane  212 . The back focal plane of the first lens and the front focal plane of the second lens coincide at a spatial frequency plane  208 . 
     For purpose of illustration, assume that the x-axis is the horizontal axis, the y-axis is the vertical axis, and the z-axis is the optical axis  214 . A two-dimensional pattern u(x, y) is placed at the front focal plane  204 . According to Fourier optics theory, the image formed at the spatial frequency plane  208  is the two-dimensional Fourier transform of u(x, y), which is represent by the formula U(fx, fy). The symbols fx, fy represent the coordinates on the spatial frequency plane. The relationship between the u(x, y) and U(fx, fy) can be written as: 
     
       
           U ( fx, fy )= F[u ( x, y )]  (Equ. 1) 
       
     
     The notation F[ ] represents the Fourier transform operator. 
     When the image U(fx, fy) passes through the second lens  210  and is projected on the back focal plane  212 , the image at the back focal plane  212  will be the inverse-Fourier transform of the image formed at the spatial frequency plane  208 . If nothing is placed at the spatial frequency plane  208  to alter the amplitude and phase of the image at spatial frequency plane  208 , then the image projected on the back focal plane  212  is just the original pattern u(x, y). This is because the inverse-Fourier transform of a Fourier-transformed image is the same image itself. This can be written as: 
     
       
           F   −1   [F[u ( x, y )]]= u ( x, y ) 
       
     
     The notation F −1 [ ] represents the inverse-Fourier transform operator. 
     According to Fourier transformation theory, the Fourier transform of the derivative of u(x, y) is proportional to (fx+fy)·U(fx, fy), and can be expressed as: 
     
       
           F[u′ ( x, y )]=2π·( fx+fy )·exp( jπ/ 2)· U ( fx, fy )  (Equ. 2) 
       
     
     Thus if a phase-shift filter is placed at the spatial frequency plane  208 , such that the phase and amplitude of the image at the spatial frequency plane  208  is modified by an amount of “2π·(fx+fy)·exp(jπ/2)”, then the image formed at the back focal plane  212  will be the derivative of the light intensity pattern at the front focal plane  204 . 
     In the above formulas, the derivative of the pattern u(x, y) is taken along both the x-direction and the y-direction. If the only the diffraction effects along the x-direction needs to be considered, then the equations above can be simplified by taking out the fy components. Such situation occurs when the pattern u(x, y) has features in the x-direction that are comparable to the wavelength of the light source  202 , and the features along the y-direction are larger than one wavelength. Since the diffraction effect is significant only in the x-direction, differentiation of the image is required only in the x-direction, and the equations above can be simplified: 
       F[u′ ( x, y )]= F[d ( u ( x, y ))/ dx]= 2 π·fx ·exp( jπ/ 2) U ( fx, fy ).  (Equ. 3) 
     The derivative of a pattern will enhance the parts of the pattern that changes rapidly. Typically, the edges of the pattern are the places where there are significant changes. Thus the derivative of an image with blurred edges will result in an image having a similar pattern as the original but with more sharply defined edges. One embodiment of this invention uses a pair of lenses and a phase-shift filter to generate the derivative of a photolithography mask pattern, thereby reducing the distortion caused by the diffraction effect. 
     Photolithography Using Phase-Shift Filter in the Spatial Frequency Plane 
     Turning to FIG. 3, a schematic illustration of an embodiment of the present invention is shown. A photolithography system  300  includes a light source  302 , a reticle mask  304 , a first lens  306 , a phase-shift filter  308 , a second lens  310 , and a wafer  312  that are all aligned along the optical axis  314 . The reticle mask  304 , first lens  306 , phase-shift filter  308 , second lens  310 , and the wafer  312  are placed perpendicularly to the optical axis  314 . The light source  302  is typically an ultraviolet (UV) or deep ultraviolet (DUV) light source, although it may be any type of radiation source normally used in photolithography. An example of the light source  302  is a KrF laser emitting DUV radiation with a wavelength of 2480 angstrom. 
     Preferably, the reticle mask  304  is formed by chromium on quartz in accordance with conventional techniques, and has a width of about 15 cm. The reticle mask  304  carries the mask pattern  330  that is desired to be imprinted onto the wafer  312 . The wafer  312  is typically coated with a photoresist layer, so that after the photolithography process, a replica of the mask pattern is formed on the photoresist layer on the wafer  312 . The reticle mask  304 , the first lens  306 , the phase-shift filter  308 , the second lens  310 , and the wafer  312  are mounted on a support frame of the photolithographic machine that is not shown in the figure. The support frame has adjustment mechanisms so that the distances between the reticle mask  304  and the first lens  306 , between the first lens  306  and the phase-shift filter  308 , between phase-shift filter  308  and the second lens  310 , and between the second lens  310  and the wafer  312 , can all be fine tuned to produce the sharpest image on the wafer  312 . 
     The focal length of the first lens  306  is f′, and the focal length of the second lens  310  is f. The reticle mask  304  is situated between the light source  302  and the first lens  306 . The first lens  306  has two focal planes. Define the front focal plane  320  of the first lens  306  as the one that is closer to the light source  302 , and the back focal plane as the one that is farther away from the light source  302 . Likewise, the second lens  310  has a front focal plane that is closer to the light source  302 , and a back focal plane  326  that is farther away from the light source  302 . In this embodiment, the back focal plane of the first lens  306  coincides with the front focal plane of the second lens  310 , and is called the spatial frequency plane  322 . This is because the image formed at the back focal plane of the first lens  306  is the Fourier transform of the image at the front focal plane  304 . 
     In operation, light emitting from the light source  302  passes through the reticle mask  304 , the first lens  306 , the phase-shift filter  308 , the second lens  310 , and then projects an image upon the wafer  312 . The first and second lenses  306  and  310  are conventional focusing optical lenses commonly used in many of the photolithography machines. Preferably, the lens has an effective exposure diameter of 30 cm. The phase-shift filter  308  is situated at the spatial frequency plane  322 . Typically, the phase-shift filter  308  has a certain thickness, and the center plane of the phase-shift filter  308  coincides with the spatial frequency plane  322 . The wafer  312  is situated at the back focal plane  326  of the second lens  310 . 
     The phase-shift filter  308  is formed from two parts: an attenuator  342  and a phase shifter  344 . The attenuator  342  is made of a glass or quartz substrate and some coating material such as Ag, CrO, CrON, or MoSiON. The thickness of the attenuator  342  and the coating of the attenuator  342  is designed such that an image is modified in the fx-direction according to the formula:                  S   1          (     fx   ,   fy     )       =     fx   ·       S   0          (     fx   ,   fy     )                     =     {               fx   ·       S   0          (     fx   ,   fy     )                       fx     &gt;   0                     fx   ·       S   0          (     fx   ,   fy     )       ·     exp        (   jπ   )                       fx     &lt;   0     ,                                          
     where S 0 (fx) is the image before passing through the attenuator  342 , and S 1 (fx) is the image after passing through the attenuator  342 . The term “exp(jπ)” is produced by adjusting the thickness of the substrate of attenuator  342  for the parts “fx&lt;0” such that light passing through it has a phase shift equal to π. The light rays passing through the attenuator  342  at a position closer to the fy-axis has a smaller amplitude (i.e., darker), and the light rays passing farther away from the fy-axis axis has a greater amplitude (i.e., brighter). Here, the notation fx and fy are used to denote the coordinates on the spatial frequency plane  322 . 
     The phase shifter  344  is typically made of glass, or quartz, and preferably has a refractive index of about 1.5. The phase shifter  344  shifts the phase of the image in the amount of 
     
       
         ΔΦ=(2π/λ)· a ·( n− 1)  (Equ. 4) 
       
     
     where ΔΦ is the amount of phase shift, “a” is the thickness of the phase shifter, “n” is the index of refraction of the phase shifter, and λ is the wavelength of the light source  302 . By adjusting the thickness of the phase shifter  344  (the thickness will depend on the wavelength of light source used), an amount of phase shift equal to π/2 can be achieved. 
     The combined effect of the attenuator  342  and the phase shifter  344  is to change the amplitude and phase of an incident image according to the formula: 
     
       
           S   2 ( fx, fy )=2 π·fx ·exp( jπ/ 2)· S   0 ( fx, fy )  (Equ. 5) 
       
     
     Where S 2 (fx, fy) is the image formed after passing through the phase-shift filter  308 . The term “2π” is just a constant and can be achieved by adjusting the overall opacity of the attenuator. Preferably, the combined thickness of the substrate of the attenuator  342  and the phase shifter  344  is adjusted such that light rays passing through the phase-shift filter  308  have a phase shift of π/2 in the region “fx&gt;0”, and have a phase shift of 3π/2 in the region “fx&lt;0”. 
     Referring to FIG. 5, an embodiment of phase-shift filter  500  comprises an attenuator  502  formed on a substrate  504 . The attenuator is preferably a coating made of Ag, CrO, CrON, or MoSiON. The substrate  504  is comprised of a first phase-shift portion  506  in the region “fx&lt;0”, and a second phase-shift portion  508  in the region “fx&gt;0”. The thickness of the first phase-shift portion  506  is designed such that the light rays passing through the first phase-shift portion  506  have a phase shift of 3π/2. The thickness of the second phase-shift portion  508  is designed such that the light rays passing through the second phase-shift portion  508  have a phase shift of π/2. The coating  502  is opaque on the fy-axis (thus allowing no light to pass through), and gradually becomes more transparent as |fx| becomes larger (thus allowing more light to pass through). An image passing through the coating  502  is modified in the fx-direction according to the formula: 
     
       
           S   3 ( fx, fy )=| fx|·S   2 ( fx, fy ), 
       
     
     where S 2 (fx) is the image before passing through the attenuator  502 , and S 3 (fx) is the image after passing through the attenuator  502 . 
     According to Fourier Optics theory, the image produced at the back focal plane of the first lens  306  (which is the spatial frequency plane  322 ) is the Fourier transform of the image at the front focal plane  320 . Assuming that the thickness of the phase-shift filter  308  is small compared with the focal length f′, the image projected onto the front end of the phase-shift filter  308  is the Fourier-transformed image of the mask pattern of the reticle mask  304 . The phase-shift filter  308  changes the amplitude and phase of the Fourier-transformed image according to Equation 5, and produces an “adjusted Fourier-transformed” image of the mask pattern. The image formed on the back focal plane  326  of the second lens  312  is the inverse-Fourier transform of the image at the front focal plane of the second lens  310  (which is the spatial frequency plane  322 ). Thus, the image projected onto the wafer  312  is the inverse-Fourier transform of the adjusted Fourier-transformed image of the mask pattern. 
     Assume that the reticle mask  304  has a two-dimensional mask pattern  330  that is described as u(x, y). The image u(x, y) is situated at the front focal plane  320  of the first lens  306 . The Fourier-transformed image at the front end of the phase-shift filter  308  is U 0 (fx, fy), where fx, fy are the coordinates on the spatial frequency plane  322 . The image formed after passing through the phase-shift filter is U 1 (fx, fy), and according to Equation 5, U 1 (x, y)=2π·fx·exp(jπ/2)·U 0 (fx, fy). According to Equation 3, U 1 (fx, fy) is substantially the same as F[u′(x, y)], which is the Fourier transform of the derivative of u(x, y) along the x-direction (i.e., d u(x, y)/dx). 
     The back end of the phase-shift filter  308  is near the front focal plane of the second lens  310  (under the assumption that the thickness of the phase-shift filter  308  is small compared with the focal lengths f′ and f). According to Fourier Optics theory, the image projected on the back focal plane  326  is the inverse Fourier transform of the image at the front focal plane of the second lens  310 . The image projected at the back focal plane  326  is F −1 [F[u′(x, y)]], which is just u′(x, y). Therefore, the image projected upon the wafer  312  situated at the back focal plane  326  is simply the derivative of the image of the mask pattern  330 . Here, the derivative of the image means the derivative of the light intensity of the image. The derivative of an image will have sharper edge patterns. Therefore, the combination of the first lens  306 , phase-shift filter  308 , and second lens  310  has the effect of transferring the image of the mask pattern  330  onto the wafer  312  with the edges more sharply defined. The blurring due to diffraction is reduced accordingly. 
     Referring to FIG. 4, a schematic illustration of another embodiment of the present invention is shown. This embodiment eliminates one lens from the previous embodiment. The distances between various components are also adjusted. A photolithography system  400  includes the light source  302 , the reticle mask  304 , a lens  406 , the phase-shift filter  308 , and the wafer  312  that are all aligned along the optical axis  314 . The reticle mask  304 , lens  406 , phase-shift filter  308 , and the wafer  312  are placed perpendicularly to the optical axis  314 . The reticle mask  304 , the lens  406 , the phase-shift filter  308 , and the wafer  312  are mounted on a support frame of the photolithographic machine that is not shown in the figures. The support frame has adjustment mechanisms so that the distances between the reticle mask  304  and lens  406 , between lens  406  and phase-shift filter  308 , and between phase-shift filter  308  and wafer  312 , can be fine tuned to produce a sharp image on the wafer  312 . 
     The focal length of the lens  406  is f. The reticle mask  304  is situated between the light source  302  and the lens  406 . The distance between the reticle mask  304  and the lens  406  is d o , with d o  greater than the focal length f. In operation, light emitting from the light source  302  passes through the reticle mask  304 , the lens  406 , the phase-shift filter  308 , and then projects an image upon the wafer  312 . The distance between the wafer  312  and the lens  406  is d i , with d i  greater than the focal length f. The distances d o  and d i  satisfy the thin lens equation: 1/f=1/d o +1/d i . The distance between the wafer  312  and the phase-shift filter  308  is q. The distance q is typically designed to be greater than 1000 times the smallest line width of the mask pattern  330 . 
     The two-dimensional mask pattern  330  of the reticle mask  304  is described as u(x, y). According to Fourier Optics theory described previously, the image projected onto the wafer  312  is the derivative of the image of the mask pattern  330 , magnified at a ratio of d i /d o . The mask is designed such that the dimension ratio of the mask pattern  330  to the pattern desired to be imprinted on the wafer is d o /d i . The combination of the lens  406  and the phase-shift filter  308  has the effect of transferring the image of the mask pattern  330  onto the wafer  312  with the edges more sharply defined. The blurring due to diffraction is reduced accordingly. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.