Abstract:
The present invention is a system that selectively directs radiation of multiple wavelengths at a substrate to facilitate pattern formation. The system may include a wavelength discriminator to filter the radiation and an absorption layer to develop a localized heat source. The localized heat source may be employed to raise a temperature of an imprinting layer. This improves the flow rate and the fill factor of the material disposed within the imprinting layer, thus reducing the time required to fill the features defined on a mold.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The field of the invention relates generally to imprint lithography. More particularly, the present invention is directed to a patterning system that produces and selectively directs infrared radiation at a substrate to develop a localized heat source.  
         [0002]     Micro-fabrication involves the fabrication of very small structures, e.g., having features on the order of micro-meters or smaller. One area in which micro-fabrication has had a sizeable impact is in the processing of integrated circuits. As the semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate, micro-fabrication becomes increasingly important. Micro-fabrication provides greater process control while allowing increased reduction of the minimum feature dimension of the structures formed. Other areas of development in which micro-fabrication has been employed include biotechnology, optical technology, mechanical systems and the like.  
         [0003]     An imprint lithography technique is disclosed by Chou et al. in  Ultrafast and Direct Imprint of Nanostructures in Silicon , Nature, Col. 417, pp. 835-837, June 2002, which is referred to as a laser assisted direct imprinting (LADI) process. In this process a region of a substrate is made flowable, e.g., liquefied, by heating the region with the laser. After the region has reached a desired viscosity, a mold, having a pattern thereon, is placed in contact with the region. The flowable region conforms to the profile of the pattern and is then cooled, solidifying the pattern into the substrate.  
         [0004]     An exemplary micro-fabrication technique is shown in U.S. Pat. No. 6,334,960 to Willson et al. Willson et al. discloses a method of forming a relief image in a structure. The method includes providing a substrate having a transfer layer. The transfer layer is covered with a polymerizable fluid composition. A mold makes mechanical contact with the polymerizable fluid. The mold includes a relief structure, and the polymerizable fluid composition fills the relief structure. The polymerizable fluid composition is then subjected to conditions to solidify and to polymerize the same, forming a solidified polymeric material on the transfer layer that contains a relief structure complimentary to that of the mold. The mold is then separated from the solid polymeric material such that a replica of the relief structure in the mold is formed in the solidified polymeric material. The transfer layer and the solidified polymeric material are subjected to an environment to selectively etch the transfer layer relative to the solidified polymeric material such that a relief image is formed in the transfer layer. The time required by this technique is dependent upon, inter alia, the time the polymerizable material takes to fill the relief structure.  
         [0005]     Thus, there is a need to provide an improved system for the filling of the relief structure with the polymerizable material.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is a system that selectively directs radiation of multiple wavelengths at a substrate to facilitate pattern formation. The system may include a wavelength discriminator to filter the radiation and an absorption layer to develop a localized heat source. The localized heat source may be employed to raise a temperature of an imprinting layer. This improves a flow rate and a fill factor of the material disposed within the imprinting layer, thus reducing the time required to fill the features defined on a mold. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a perspective view of a lithographic system in accordance with the present invention;  
         [0008]      FIG. 2  is a simplified elevation view of a lithographic system shown in  FIG. 1 ;  
         [0009]      FIG. 3  is a simplified representation of material from which a thin film layer, shown in  FIG. 2 , is comprised before being polymerized and cross-linked;  
         [0010]      FIG. 4  is a simplified representation of cross-linked polymer material into which the material shown in  FIG. 3  is transformed after being subjected to radiation;  
         [0011]      FIG. 5  is a simplified elevation view of a mold spaced-apart from the thin film layer, shown in  FIG. 1 , after patterning of the thin film layer;  
         [0012]      FIG. 6A  is a side view of an absorption layer disposed between a wafer and wafer chuck;  
         [0013]      FIG. 6B  is a side view of an absorption layer disposed between an imprinting layer and a wafer;  
         [0014]      FIG. 7  is a side view of a simplified lithographic system depicting dual radiation sources;  
         [0015]      FIG. 8  is a detailed view of a wafer having imprinting material disposed thereon shown in  FIG. 7 ;  
         [0016]      FIG. 9  is a side view of a simplified lithographic system depicting a single radiation source;  
         [0017]      FIG. 10 . is a detailed view of a wafer having imprinting material disposed thereon shown in  FIG. 9 ; and  
         [0018]      FIG. 11  is a flow diagram showing the method of increasing a flow rate of imprinting material in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]      FIG. 1  depicts a lithographic system  10  that includes a pair of spaced-apart bridge supports  12  having a bridge  14  and a stage support  16  extending therebetween. Bridge  14  and stage support  16  are spaced-apart. Coupled to bridge  14  is an imprint head  18 , which extends from bridge  14  toward stage support  16 . Disposed upon stage support  16  to face imprint head  18  is a motion stage  20 . Motion stage  20  is configured to move with respect to stage support  16  along X- and Y-axes. A radiation system  22  is coupled to lithographic system  10  to impinge radiation upon wafer  30 . As shown, radiation system  22  is coupled to bridge  14  and includes a power generator  23  connected to radiation system  22 .  
         [0020]     Referring to both  FIGS. 1 and 2 , connected to imprint head  18  is a substrate  26  having a mold  28  thereon. Mold  28  includes a plurality of features defined by a plurality of spaced-apart recessions  28   a  and protrusions  28   b , having a step height, h, on the order of nanometers, e.g., 100 nanometers. The plurality of features defines an original pattern that is to be transferred into a wafer  30  positioned on motion stage  20 . To that end, imprint head  18  is adapted to move along the Z axis and vary a distance “d” between mold  28  and wafer  30 . In this manner, the features on mold  28  may be imprinted into a flowable region of wafer  30 , discussed more fully below. Radiation system  22  is located so that mold  28  is positioned between radiation system  22  and wafer  30 . As a result, mold  28  is fabricated from material that allows it to be substantially transparent to the radiation produced by radiation system  22 .  
         [0021]     Referring to both  FIGS. 2 and 3 , a flowable region is disposed on a portion of surface  32  that presents a substantially planar profile. In the present embodiment, however, the flowable region consists of a plurality of spaced-apart discrete droplets  33  of material  36   a  on wafer  30 , defining a flowable imprinting layer  34 . Imprinting layer  34  is formed from a material  36   a  that may be selectively polymerized and cross-linked to record the original pattern therein, defining a recorded pattern. Material  36   a  is shown in  FIG. 4  as being cross-linked at points  36   b , forming cross-linked polymer material  36   c.    
         [0022]     Referring to  FIGS. 2, 3  and  5 , the pattern recorded by imprinting layer  34  is produced, in part, by mechanical contact with mold  28 . To that end, imprint head  18  reduces the distance “d” to allow imprinting layer  34  to come into mechanical contact with mold  28 , spreading droplets  33  so as to form imprinting layer  34  with a contiguous formation of material  36   a  over surface  32 . In one embodiment, distance “d” is reduced to allow sub-portions  34   a  of imprinting layer  34  to ingress into and fill recessions  28   a.    
         [0023]     In the present embodiment, sub-portions  34   b  of imprinting layer  34  in superimposition with protrusions  28   b  remain after the desired, usually minimum distance “d”, has been reached, leaving sub-portions  34   a  with a thickness t 1 , and sub-portions  34   b  with a thickness t 2 . Thicknesses “t 1 ” and “t 2 ” may be any thickness desired, dependent upon the application.  
         [0024]     Referring to  FIGS. 2, 4 , and  5 , after a desired distance “d” has been reached, radiation system  22  produces actinic radiation that polymerizes and cross-links material  36   a , shown in  FIG. 3 , forming cross-linked polymer material  36   c . As a result, the composition of imprinting layer  34  transforms from material  36   a , shown in  FIG. 3 , to cross-linked polymer material  36   c , which is a solid, forming solidified imprinting layer  40 . Specifically, cross-linked polymer material  36   c  is solidified to provide side  34   c  of imprinting layer  40  with a shape conforming to a shape of a surface  28   c  of mold  28 , thereby recording the pattern of mold  28  therein. After formation of imprinting layer  40 , imprint head  18  is moved to increase distance “d” so that mold  28  and imprinting layer  40  are spaced-apart.  
         [0025]     Referring to  FIGS. 3 and 5 , as the features defined on mold  28  become substantially smaller, i.e., recessions  28   a  and protrusions  28   b , the time required to fill recessions  28   a  with material  36   a  increases, which is undesirable. Therefore, to reduce the time required to fill recessions  28   a , it is desirable to increase the flow rate of material  36   a . One manner in which to increase the flow rate of material  36   a  is to lower the viscosity of the same. To that end, the temperature of material  36   a  may be changed to be above the glass transition temperature associated therewith. Typically, material  36   a  is not increased to a temperature above 120° C.  
         [0026]     Referring to  FIGS. 3 and 6 A, to increase a flow rate of material  36   a  in an imprint lithography process, infrared (IR) radiation is utilized. However, material  36   a , and hence droplets  33 , are substantially transparent to IR radiation; and thus, heating the same by exposure to IR radiation is problematic. Therefore, an absorption layer  42 , which is responsive to IR radiation is utilized. Absorption layer  42  comprises a material that is excited when exposed to IR radiation and produces a localized heat source. Typically, absorption layer  42  is formed from a material that maintains a constant phase state during the heating process which may include a solid phase state. Specifically, the IR radiation impinging upon absorption layer  42  causes an excitation of the molecules contained therein, generating heat. The heat generated in absorption layer  42  is transferred to material  36   a  in droplets  33  via heat conduction through wafer  30 . Thus, material  36   a  in droplets  33  may be heated at a sufficient rate to lower the viscosity of the same, thereby increasing the flow rate. As a result, absorption layer  42  and wafer  30  provide a bifurcated heat transfer mechanism that is able to absorb IR radiation and to produce a localized heat source sensed by droplets  33  to transmit heat through heat conduction. Absorption layer  42  may be permanently or removably attached. Exemplary materials that may be employed as absorption layer  42  include black nickel and anodized black aluminum. Also, black chromium may be employed as absorption layer. Black chromium is typically deposited as a mixture of oxides and is used coating of solar cells.  
         [0027]     Referring to  FIG. 6B , in another embodiment absorption layer  142  may be disposed between droplets  33  and wafer  30 . In this manner, absorption layer  142  creates a localized heat sources in surface  142   a . To that end, absorption layer  142  may be deposited using any known technique, including spin-on, chemical vapor deposition, physical vapor deposition and the like. Exemplary materials that may be formed from a carbon based PVD coating, organic thermo set coating with carbon black filler or molybdenum disulfide (MoS 2 ) based coating.  
         [0028]     Referring to  FIGS. 3, 5 , and  6 A, increasing the temperature of material  36   a  may be problematic due to, inter alia, evaporative loss. To reduce, if not avoid, evaporative loss of material  36   a  in droplets  33 , IR radiation may be impinged upon absorption layer  42  when mold  28  is in close proximity to droplets  33 . As a result of mold  28  and droplets  33  being in close proximity, the atmosphere between mold  28  and droplets  33  is reduced, thereby reducing a rate of evaporative loss of droplets  33 . Further, any evaporative losses of material  36   a  will most likely collect on mold  28 , thereby preventing loss of material  36   a . In a further embodiment, the atmosphere between droplets  33  and mold  28  may be reduced by partial or whole evacuation, further lessening evaporative loss of material  36   a  in droplets  33 .  
         [0029]     A second method of reducing the rate of evaporative loss of droplets  33  is to heat mold  28 , wherein the temperature of mold  28  is raised to a temperature greater than the temperature of wafer  30 . As a result, a thermal gradient is created in an atmosphere between template  28  and wafer  30 . This is believed to reduce the evaporative loss of material  36   a  in droplets  33 .  
         [0030]     Referring to  FIGS. 3 and 5 , after lowering the viscosity of material  36   a  and contacting the same with mold  28 , polymerization and cross-linking of material  36   a  may occur, as described above. Material  36   a , as mentioned above, comprises an initiator to ultraviolet (UV) radiation to polymerize material  36   a  thereto in response.  
         [0031]     Referring to  FIGS. 1 and 7 , to that that end, one embodiment of radiation system  22  includes dual radiation sources, i.e., radiation source  50  and radiation source  52 . For example, radiation source  50  may be any known in the art capable of producing IR radiation. Radiation source  52  may be any known in the art capable of producing actinic radiation employed to polymerize and cross-link material in droplets  33 , such as UV radiation. Specifically, radiation produced by either of sources  50  and  52  propagates along optical path  54  toward wafer  30 . Typically, mold is disposed in optical path  54  and as a result, is transmissive to both UV and IR radiation. A circuit (not shown) is in electrical communication with radiation sources  50  and  52  to selectively allow radiation in the UV and IR spectra to impinge upon wafer  30 . In this fashion, the circuit (not shown) causes radiation source  50  to produce IR radiation when heating of material, shown in  FIG. 3 , is desired and the circuit (not shown) causes radiation source  52 , shown in  FIG. 7 , to produce UV radiation when polymerization and cross-linking of material, shown in  FIG. 3 , is desired. It is possible to employ the requisite composition of material  36   a  to allow cross-linking employing IR alone or in conjunction with UV radiation. As a result, material  36   a  would have to be heated with sufficient energy to facilitate IR cross-linking An exemplary material could include styrene divinylbenzene, both available from Aldrich Chemical Company, Inc. located at 1001 West Saint Paul Avenue, Milwaukee, Wis. and Irgacure 184 or 819 available from Ciba Specialty Chemicals, at 560 White Plains Road, Tarrytown, N.Y. 10591. The combination consists of, by weight, 75-85 parts styrene, with-80 parts being desired, 15-25 parts divinylbenzene, with 20 parts being desired, 1-7 parts Iragure, with 4 parts being desired, with the remaining portion of the composition comprising stabilizers to ensure suitable shelf-life.  
         [0032]     Referring to  FIG. 8 , in another embodiment, radiation system  22  consists of a single broad spectrum radiation source  60  that produces UV and IR radiation. An exemplary radiation source  60  is a mercury (Hg) lamp. To selectively impinge differing types of radiation upon wafer  30 , a filtering system  62  is utilized. Filtering system  62  comprises a highpass filter (not shown) and a lowpass filter (not shown), each in optical communication with radiation source  60 . Filtering system  62  may position highpass filter (not shown) such that optical path  54  comprises IR radiation or filtering system  62  may position lowpass filter (not shown) such that optical path  54  comprises UV radiation. Highpass and lowpass filters (not shown) may be any known in the art, such as interference filters comprising two semi-reflective coatings with a spacer disposed therebetween. The index of refraction and the thickness of the spacer determine the frequency band being selected and transmitted through the interference filter. Therefore, the appropriate index of refraction and thickness of the spacer is chosen for both the highpass filter (not shown) and the lowpass filter (not shown), such that the highpass filter (not shown) permits passage of IR radiation and the lowpass filter (not shown) permits passage of UV radiation. A processor (not shown) is in data communication with radiation source  60  and filtering system  62  to selectively allow the desired wavelength of radiation to propagate along optical path  54 . The circuit enables highpass filter (not shown) when IR radiation is desired and enables the lowpass filter (not shown) when UV radiation is desired.  
         [0033]     Referring to  FIGS. 3, 4 ,  6 A and  11 , in operation, imprinting material is deposited on wafer  30  at step  100 . Thereafter, at step  102 , mold  28  is placed proximate to droplets  33 . Following placing mold  28  proximate to droplets, IR radiation in impinged upon a target, which in the present case is the thermal absorption layer  42 . Typically, the temperature of material  36   a  in droplets is increased to provide a desired flow rate. This may be above a glass transition temperature associated with material  36   a . After material  36   a  has been heated to a desired temperature, contact is made between mold  28  and droplets  33  at step  104 . In this manner, material  36   a  is spread over wafer  30  and conforms to a profile of mold  28 . At step  106 , material  36   a  is transformed into material  36   c  by exposing the same to actinic radiation, e.g. UV radiation, to form imprinting layer  40 . If cooling of material  34   a  is desired, this may be accomplished through any method known in the art, such as natural convection/conduction through the wafer chuck or enforced convection/conduction with nitrogen (N 2 ) gas or a chilled substrate chuck. Further, cooling may occur before or after solidification of material  36   a . Thereafter mold  28  and imprinting layer  40  are spaced-apart at step  108 , and subsequent processing occurs at step  110 .  
         [0034]     While this invention has been described with references to various illustrative embodiments, the description is not intended to be construed in a limiting sense. For example, heating is described as occurring after the mold is placed proximate to droplets. However, heating may occur before the mold is placed proximate to the droplets. As a result various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.