Abstract:
An extreme ultraviolet lithography system may have a spatial filtering system in projection optics that reduce flare. A flare filter may be provided at the pupil plane to pass the required diffraction orders (at minimum 0 th  and +1 or 0 th  and −1 orders) of the light from the mask, while blocking the effects of scattering from various mirrors used in the projection optics. By reducing flare, process window and critical dimension variation can be improved.

Description:
BACKGROUND  
       [0001]     This relates generally to projection optics used in lithography for fabricating integrated circuits by transferring patterns from a mask to the integrated circuit wafer.  
         [0002]     In integrated fabrication, extreme ultraviolet radiation (EUV) may be utilized to expose a mask and to transfer a pattern on the mask to an integrated circuit wafer. The mask or grating may be exposed to the extreme ultraviolet radiation. The light from the mask or grating is focused by a projection optical system onto the wafer.  
         [0003]     Flare may arise due to the surface scattering from the mirrors comprising the compound projection optic. The EUV lithographic projection optics include multilayer mirrors, and the extreme ultraviolet flare is due to mid-spatial frequency roughness of the mirror surfaces. The scattering from the mirror surfaces will be imaged as background DC light at the wafer plane. Flare can reduce process window and increase critical dimension variation across the field. However, due to the short scattering range of extreme ultraviolet wavelengths, the flare is essentially constant over the field, making its effect on critical dimensions and process window relative easy to predict and possibly correctable through mask design. Thus, flare is less likely a concern for extreme ultraviolet lithography as long as the amount of open frame flare is below ten percent. However, being proportional to one over the wavelength squared, flare in extreme ultraviolet systems can be difficult to control. Angular dependence of short range flare can lead to local critical dimension variations. It is also challenging to reduce the intrinsic flare or flare in open field below ten percent.  
         [0004]     For any lithographic system, the diffraction pattern seen at the optic&#39;s exit pupil plane can be predicted if the mask patterns are known. For the perfect system or flare-free system, the exit pupil has diffraction patterns in certain areas of the pupil. Extreme ultraviolet mirrors have some amount of roughness and contribute to scattering at the pupil plane, which will eventually become background light at the wafer plane. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a depiction of an extreme ultraviolet lithography system in accordance with one embodiment of the present invention;  
         [0006]      FIG. 2  is a side elevation of a projection optics for an extreme ultraviolet lithography system in accordance with one embodiment of the present invention;  
         [0007]      FIG. 3  is a depiction of the pupil view for diffraction patterns of dense lines and spaces (or grating mask patterns) with scattering from rough mirror surfaces;  
         [0008]      FIG. 4  is a depiction of a flare filter to block the scattering portion or unused portion of pupil in accordance with one embodiment of the present invention;  
         [0009]      FIG. 5  is a depiction of the effect of the flare filter in accordance with one embodiment of the present invention;  
         [0010]      FIG. 6  is a depiction of a frequency doubling flare filter which blocks 0 th  order diffraction pattern in accordance with one embodiment of the present invention; and  
         [0011]      FIG. 7  is a depiction of the effect of the flare filter shown in  FIG. 6  in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows an engineering test system (ETS) embodiment of an extreme ultraviolet (EUV) lithography system as an example EUV system. However, the present invention is not limited to the use of one type of system and is applicable to any EUV system.  
         [0013]     The ETS system may include a drive laser beam which illuminates a C 1  multilayer coated collector  110  in one embodiment. The collector  110  may direct the laser towards the C 2 , C 3  pupil optics grazing incidence mirror assembly  105  in one embodiment. A laser-produced plasma generates extreme ultraviolet radiation in a vacuum in one embodiment. As another example, a discharge source may be used to produce the EUV radiation.  
         [0014]     The radiation from the grazing incidence mirror assembly  105  and condenser C 3  may pass through a spectral purity filter  115  on the way to a C 5  mirror  120 . The C 5  mirror  120  is a grazing incidence mirror that reflects the illuminated light to the mask. From the C 5  mirror  120  the radiation may pass to a reticle stage  107 . The reticle stage  107  includes the mask whose pattern is to be transferred to a wafer. The reticle is also a reflective multilayer coated mask.  
         [0015]     From the reticle, the radiation passes through projection optics  113 . Then, the radiation enters the wafer stage  107  which actually includes the wafer to receive the pattern.  
         [0016]     The projection optic  113  is shown in more detail in  FIG. 2  according to one embodiment. The projection optic  113  is the optical system between the mask  111  and the wafer. The mask  111  defines a mask plane. The mask  111  may be part of the reticle stage  107  in  FIG. 1 .  
         [0017]     Radiation from the mask  111  may pass to a first mirror  120   d , reflect to a second mirror  120   a , pass through a third mirror  120   c , and be reflected from a pupil plane mirror  120   b  in one embodiment. The pupil plane mirror  120   b  may it include a flare filter  14 , to be described in more detail hereinafter. The radiation reflected from the mirror  120   b  may then be reflected from the mirror  120   c , through the mirror  120   d , to impact the wafer as indicated. The center line of the optics is indicated at A.  
         [0018]     The mirrors  120   a ,  120   b ,  120   c , and  120   d , may be part of a single, multilayer mirror in some embodiments. In other embodiments, other reflective arrangements may be used. For example, in one embodiment, six mirrors may be used in the projection optic  113 . While the filter  14  is shown located in the pupil plane, it can be located in the projection optic  113 , anywhere between the mask and the wafer, depending on the optical design.  
         [0019]     Radiation conveying the information recorded in the gratings in the mask  111  may have diffracted orders including the zero th  ±1 th  and ±2 th , etc. orders. The mask  111  may be a binary mask with 1:1 lines and space in a single pitch, as one example.  
         [0020]     Referring to  FIG. 3 , an image of the unfiltered pupil view shows scattering as indicated by cross-hatching. The zero th  order image is indicated, as are the ±1 th  orders as indicated in this simple example for 1:1 lines and space mask patterns.  
         [0021]     The flare filter  14 , shown in  FIG. 4 , is placed at the pupil plane. The filter  14  has openings  20  designed to transmit the zero th  and ±1 th  orders and blocking the rest of pupil area. The pupil is indicated.  
         [0022]     In an extreme ultraviolet (EUV) lithography system, the filter  14  may be placed in front of the mirrors, such as the mirror  120   b , at the pupil plane as indicated in  FIG. 2 .  
         [0023]     Referring to  FIG. 5 , the result of the application of the flare filter  14  to the image shown in  FIG. 3  is that the amount of flare may be dramatically reduced in some cases. The flare only results from the greater size of the openings  20  and  22 , relative to the actual size of the zero th  and ±1 th  orders.  
         [0024]     Through the use of a spatial filter  14  at the projection optics exit pupil plane, the effect of extreme ultraviolet flare may be reduced. This technique does not need the use of special mask features to reduce extreme ultraviolet flare. By putting the spatial filter in the exit pupil plane, the amount of flare can be reduced or even minimized. This relaxes the requirements for multilayer mirror polishing in mid spatial frequency ranges.  
         [0025]     The information needed for defining the flare reduction filter is the diffraction patterns from the mask. The diffraction patterns from the mask can be calculated if the mask contents are well known. With the information of the mask diffraction patterns, one can create a filter to block the background scattering at the pupil plane. Lithography friendly mask designs for critical layers are becoming more popular for 65 nanometer node and beyond. The mask design usually has unidirectional features with a lower number of pitches. This makes it easy to predict the diffraction patterns at the pupil plane. If the diffraction patterns are known, designing a filter to reduce the extreme ultraviolet flare is feasible. Since extreme ultraviolet lithography systems require from six to eight mirrors, installing the filter in the pupil plane may be advantageously implemented when the system is defined.  
         [0026]     The flare filter may be a simple and inexpensive device where the pupil plane can be accessible like a simple mask through automated filter exchanger. The pupil filter blocks the appropriate areas in the pupil plane that are not used for imaging. For very complex mask structures and orientations, the flare filter may be less useful.  
         [0027]     If the flare of a given system and mask is F, then the amount of flare with a flare blocking filter is F times the transmitting area of the filter, divided by the area of the pupil. This can be on the order of 0.2 or even smaller depending on the illumination condition.  
         [0028]     As another application of the flare filter, referring to  FIG. 6 , a filter  14   a  may block the zero th  order of light to create a frequency doubling effect for extreme ultraviolet light as indicated in  FIG. 7 . This increases the patterning capability of the extreme ultraviolet lithography system without using phase shifting masks or other illumination tricks or double patterning tricks. Thus, in  FIG. 6 , the mask blocks the 0 th  order and passes the ±1 th  order. This provides a frequency doubling effect.  
         [0029]     References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application.  
         [0030]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.