Abstract:
The present invention provides a method for forming an optical coupling device on a substrate by disposing a material onto the substrate that is polymerizable in response to actinic radiation. A stack of the material is formed by contacting the material with a template having a stepped-recess formed therein. The material is then solidified into an optically transparent body with a surface having a plurality of steps by subjecting the stack to actinic radiation. To that end, the material may comprise an acrylate component selected from a set of acrylates consisting essentially of ethylene dio diacrylate, t-butyl acrylate, bisphenol A diacrylate, acrylate terminated polysiloxane, polydifluoromethylene diacrylate, perfluoropolyether diacrylates and chlorofluorodiacrylates. Alternatively, the material may include a silylated component selected from a group consisting essentially of (3-acryloxypropyltristrimethylsiloxy) silane.

Description:
BACKGROUND OF THE INVENTION 
     The field of invention relates generally to micro-fabrication of structures. More particularly, the present invention is directed to a functional patterning material suited for use in imprint lithographic processes to form optical components. 
     Optical communication systems include numerous optical devices, such as planar optical slab waveguides, channel optical waveguides, rib waveguides, optical couplers, optical splitters, optical switches, micro-optical elements and the like. Many of these optical devices are employed using standard photolithographic processes. As a result, many photopolymers have been developed. The photopolymers, such as acrylate materials, are light sensitive to facilitate recordation of a pattern therein. Furthermore, the photopolymers must demonstrate suitable operational and process characteristics. For example, it is desired that the photopolymers have good clarity and low birefringence over a range of temperatures. As a result, the thermal stability of the photopolymers is an important factor and should be such that the probability of color changes in the photopolymers is minimized during prolonged operation. Additionally, the photopolymers should withstand stresses so as not to crack during the baking process or during use. Finally, maximizing the miniaturization of the optical devices is desired. Recent advances in micro-fabrication techniques, have showed promising results in miniaturizing optical devices. 
     An exemplary micro-fabrication technique, commonly referred to as imprint lithography, is shown in U.S. Pat. No. 6,334,960 to Willson et al. Willson et al. disclose 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 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 and the minimum feature dimension provided by this technique is dependent upon, inter alia, the composition of the polymerizable material. However, Willson et al. does not disclose material suitable for use in forming optical devices employed in communication systems that may be formed using imprint lithography. 
     It is desired, therefore, to provide techniques to form optical devices using imprint lithographic processes. 
     SUMMARY OF THE INVENTION 
     The present invention includes a method for forming an optical coupling device on a substrate by disposing a material onto the substrate that is polymerizable in response to actinic radiation. A stack of the material is formed by contacting the material with a template having a stepped-recess formed therein. The material is then solidified into an optically transparent body with a surface having a plurality of steps by subjecting the stack to actinic radiation. To that end, the material may comprise a polymerizable acrylate component selected from a set of acrylates consisting essentially of ethylene di diacrylate, t-butyl acrylate, bisphenol A diacrylate, acrylate terminated polysiloxane, polydifluoromethylene diacrylate, perfluoropolyether diacrylates and chlorofluorodiacrylates. Alternatively, the material may include a silylated component selected from a group consisting essentially of (3-acryloxypropyltristrimethylsiloxy) silane. In yet another embodiment of the present invention, a stress relief layer may be disposed on the substrate before formation of the stack to reduce the probability of the stack cracking during operation. Thereafter, the material may be disposed on the stress relief layer. One embodiment of the stress relief layer may be formed from rubbers, such as polysiloxane rubber and fluorosilocane rubber. These and other embodiments are described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified elevation view of a lithographic system in accordance with the present invention; 
         FIG. 2  is a simplified representation of material from which an imprint layer, shown in  FIG. 1 , is comprised before being polymerized and cross-linked; 
         FIG. 3  is a simplified representation of cross-linked polymer material into which the material shown in  FIG. 2  is transformed after being subjected to radiation; 
         FIG. 4  is a simplified elevation view of an imprint device, shown in  FIG. 1 , in mechanical contact with an imprint layer disposed on a substrate, in accordance with one embodiment of the present invention; 
         FIG. 5  is a simplified elevation view of the imprint layer, shown in  FIG. 4 , after patterning; and 
         FIG. 6  is a simplified elevation view of material in an imprint device and substrate employed with the present invention in accordance with an alternate embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a lithographic system in accordance with an embodiment of the present invention includes a substrate  10 , having a substantially planar region shown as surface  12 . Disposed opposite substrate  10  is an imprint device  14  having a plurality of features thereon, forming a plurality of spaced-apart stepped-recesses  16  separated by a groove  18 , which should be deeper than stepped-recesses  16 , typically 10-20 μm. Although two stepped-recessed regions  16  are shown, any number may be present. The stepped recesses  16  extend parallel to groove  18 . A translation device  20  is connected between imprint device  14  and substrate  10  to vary a distance “d” between imprint device  14  and substrate  10 . A radiation source  22  is located so that imprint device  14  is positioned between radiation source  22  and substrate  10 . Radiation source  22  is configured to impinge radiation on substrate  10 . To realize this, imprint device  14  is fabricated from material that allows it to be substantially transparent to the radiation produced by radiation source  22 . 
     Referring to both  FIGS. 1 and 2 , an imprint layer  24  is disposed adjacent to surface  12 , between substrate  10  and imprint device  14 . Although imprint layer  24  may be deposited using any known technique, in the present embodiment, imprint layer  24  is deposited as a plurality of spaced-apart discrete beads  25  of material  25   a  on substrate  10 , discussed more fully below. Imprint layer  24  is formed from a material  25   a  that may be selectively polymerized and cross-linked to record a desired pattern. Material  25   a  is shown in  FIG. 3  as being cross-linked at points  25   b , forming cross-linked polymer material  25   c.    
     Referring to  FIGS. 1 ,  2  and  4 , the pattern recorded by imprint layer  24  is produced, in part, by mechanical contact with imprint device  14 . To that end, translation device  20  reduces the distance “d” to allow imprint layer  24  to come into mechanical contact with imprint device  14 , spreading beads  25  so as to form imprint layer  24  with a contiguous formation of material  25   a  over regions  12   a  of surface  12 , which are in superimposition with stepped-recesses  16 . A region  12   b  of surface  12  in superimposition with groove  18  is devoid of material  25   a . This occurs by providing beads  25  with a requisite volume so that stepped-recesses  16  become filled, but, due to capillary action, material from which beads  25  are formed does not enter groove  18 . As a result, the material of beads  25  are provided with the requisite viscosity that may vary, dependent, inter alia, upon the size of groove  18 , stepped-recess  16  and distance “d”. As a result there is a discontinuity, or hiatus  26 , in layer  24 . In one embodiment, distance “d” is reduced to allow sub-portions  24   a  of imprint layer  24  to ingress into and fill stepped-recesses  16 , while avoiding filling of groove  18 . 
     To facilitate filling of stepped-recesses  16  and avoiding the presence of material  25   a  in hiatus  26 , material  25   a  is provided with the requisite viscosity to completely fill stepped-recesses  16  in a timely manner, while covering regions  12   a  of surface  12  with a contiguous formation of material  25   a , on the order of a few milliseconds to a few seconds. In the present embodiment, sub-portions  24   a  of imprint layer  24  coextensive with regions  12   a  have a stepped profile and are separated from adjacent sub-portions  24   a  by hiatus  26 , after distance “d” has reached a desired distance, usually a minimum distance. After a desired distance “d” has been reached, radiation source  22  produces actinic radiation that polymerizes and cross-links material  25   a , forming cross-link polymer material  25   c , shown in FIG.  3 . 
     Referring to  FIGS. 1 and 3 , an exemplary radiation source  22  may produce ultraviolet radiation. Other radiation sources may be employed, such as thermal, electromagnetic and the like. The selection of radiation employed to initiate the polymerization of the material in imprint layer  24  is known to one skilled in the art and typically depends on the specific application that is desired. After imprint layer  24  is transformed to consist of material  25   c , translation device  20  increases the distance “d” so that imprint device  14  and imprint layer  24  are spaced-apart. 
     Referring to  FIGS. 1 ,  2  and  3 , additional processing may be employed to complete the patterning of substrate  10 . For example, substrate  10  and imprint layer  24  may be etched to remove residual material (not shown) present on imprint layer  24  after patterning has been completed. Residual material (not shown) may consist of un-polymerized material  25   a , solid polymerized and cross-linked material  25   c , substrate  10  or a combination thereof. Well known etching processes may be employed to that end, e.g., argon ion milling, a plasma etch, reactive ion etching or a combination thereof. Further, removal of residual material (not shown) may be accomplished during any stage of the patterning. For example, removal of residual material (not shown) may be carried out before etching the polymerized and cross-linked material  25   c.    
     Referring to  FIGS. 2 ,  3 , and  5 , polymerization of material  25   a  solidifies the surface of sub-portions regions  24   a  with a shape conforming to a shape of stepped-recesses  16 . This provides sub-portions  24   a  with multiple steps, s 1 -s 14  having differing thicknesses, with thickness being measured in a direction parallel to distance “d”. Material  25   a  is selected so that steps s 1 -s 14  define an optical coupling device  28  that propagates optical energy impinging thereupon after polymerization into material  25   c . For example, optical energy may impinge upon surface  29  and propagate outwardly away from optical coupling device  28  through the surfaces associated with steps s 1 -s 14 . 
     As a result, optical coupling device  28 , which includes sub-portions  24   a  and regions  10   a  of substrate  10  that are coextensive with regions  12   a , shown in  FIG. 4 , should be formed with material that is transparent to desired optical frequencies. An exemplary embodiment forms optical coupling device  28  from materials that facilitates propagation of optical energy in a range of 850 nm to 1,550 nm. In addition, the materials should demonstrate operational characteristics so as to withstand various environmental stresses without varying the optical properties of the optical coupling device  28  by, for example, 5 to 10%. 
     Another operational characteristic that optical coupling device  28  should satisfy is maintaining structural integrity during operation. To that end, optical coupling device  28  should withstand thermal cycling without cracking. Thus, for a given material from which substrate  10  is formed, material  25   c  should maintain structural integrity when subjected changes in the ambient temperature, e.g., 0° C. to 70° C. in which optical coupling device  28  is employed. Material  25   c  should also maintain structural integrity when subjected to temperature changes due to the periodicity at which optical energy impinges upon optical coupling device  28 , as well as, differences in coefficient of thermal expansion (ΔCTE) of material  25   c  from which optical coupling device  28  is formed and the material from which substrate  10  is formed. 
     Referring to  FIGS. 1 ,  2 ,  3  and  4 , in addition to the operational characteristics that material  25   c  should satisfy, it is desirous to have material  25   a  satisfy numerous processing characteristics considering the unique deposition process employed. As mentioned above, material  25   a  is deposited on substrate  10  as a plurality of discrete and spaced-apart beads  25 . The combined volume of beads  25  is such that the material  25   a  is distributed appropriately over an area of regions  12   a , while avoiding the presence of material in region  12   b . As a result, imprint layer  24  is spread and patterned concurrently, with the pattern being subsequently set by exposure to actinic radiation, such as ultraviolet radiation. Thus, in addition to the operational characteristics mentioned above, it is desired that material  25   a  have certain processing characteristics to facilitate rapid and even spreading of material  25   a  in beads  25  over regions  12   a  while avoiding the presence of material  25   a  in region  12   b.    
     The desirable processing characteristics include having a viscosity approximately that of water, (H 2 O), 1 to 2 centepoise (csp), or in some cases as high as 20-50 cps, dependent upon the lateral dimensions of the features, as well as the ability to wet surface of substrate  10  to avoid subsequent pit or hole formation after polymerization. To that end, in one example, the wettability of imprint layer  24 , as defined by the contact angle method, should be such that the angle, θ 1 , is defined as follows:
 
0≧θ 1 &lt;75°
 
With these two characteristics being satisfied, imprint layer  24  may be made sufficiently thin while avoiding formation of pits or holes in the thinner regions.
 
     Referring to  FIGS. 1 ,  2 ,  3  and  5 , another desirable characteristic that it is desired for material  25   a  to possess is thermal stability during further manufacturing processes and post process testing. To that end, it is desired that the structural integrity of optical coupling device  28  be maintained when subjected to wave soldering at 260° C. for ninety (90) seconds, e.g., three (3) intervals at thirty (30) seconds per interval. Additionally, it is desirous to have optical coupling device  28  maintain structural integrity when subjected to thermal cycling between −40° to 100° C. with a fifteen (15) minute dwell time at the endpoints of the temperature ranges and a five (5) minute transition time between temperatures. Finally, it is desirous to have optical coupling device  28  maintain structural integrity when subjected to an 85° C. ambient of 85% humidity for 1,000 hours. It is further desired that the wetting of imprint device  14  by imprint layer  24  be minimized. To that end, the wetting angle, θ 2 , should be greater than 75°. 
     Referring to  FIGS. 2 and 4 , the constituent components that form material  25   a  to provide the aforementioned operational and process characteristics may differ. This results from substrate  10  being formed from a number of different materials. For example, substrate  10  may be formed from silica, polymers, cadmium telluride, quartz and virtually any other electro-optic material. Additionally, substrate  10  may include one or more layers in regions  12   a , discussed more fully below. 
     Referring to  FIGS. 2 and 3 , in one embodiment of the present invention the constituent components of material  25   a  consist of acrylated polymerizable compositions and an initiator. The polymerizable compositions are selected to provide material  25   a  with a minimal viscosity, e.g., viscosity approximating the viscosity of water (1-2 cps) or up to 10-50 cps, and to provide the aforementioned operational and process characteristics. The initiator is provided to produce a free radical reaction in response to actinic radiation, causing the polymerizable compositions to polymerize and cross-link, forming cross-linked polymer material  25   c . In the present example, a photo-initiator responsive to ultraviolet radiation is employed. 
     Examples of polymerizable compositions include, but are not limited to, ethylene diol diacrylate, t-butyl acrylate, bisphenol A diacrylate, acrylate terminated polysiloxane, as well as compositions thereof. Other acrylates may also include fluorinated acrylates as described by Blomquist et al. in the article entitled FLUORINATED ACRYLATES IN MAKING LOW-LOSS, LOW-BIREFRINGENCE, AND SINGLE-MODE OPTICAL WAVEGUIDES WITH EXCEPTIONAL THERMO-OPTIC PROPERTIES, SPIE Vol. 3799, pp. 266-279 (1999). Examples of the fluorinated acrylates include polydifluoromethylene diacrylates, perfluoropolyether diacrylates and chlorofluorodiacrylates. The initiator may be any component that initiates a free radical reaction in response to radiation, produced by radiation source  22 , shown in  FIG. 1 , impinging thereupon and being absorbed thereby. Suitable initiators may include, but are not limited to, photo-initiators such as 1-hydroxycyclohexyl phenyl ketone or phenylbis(2,4,6-trimethyl benzoyl) phosphine oxide. The initiator may be present in material  25   a  in amounts of up to 5% by weight, but is typically present in an amount of 2-4% by weight. Were it desired to include silylated polymerizable compositions in material  25   a , suitable silylated polymerizable compositions may include, but are not limited to, 1,3-bis(3-methacryloxypropyl)tetramethyldisiloxane, (3-acryloxypropyl)tris(tri-methoxysiloxy)-silane. 
     Other compositions that material  25   a  may consist of include epoxies, such as cyclo aliphatic epoxies. An exemplary cyclo aliphatic epoxy that may demonstrate the aforementioned operational and process characteristics is available from Union Carbide as part number ERL 4221. Additionally some vinyl ethers may demonstrate the aforementioned operational and process characteristics, e.g., polyvinyl ether and copolymers, such as isobutyl ethers used in conjunction with acrylics. 
     Specific examples of compositions for material  25   a  are as follows: 
     Composition 1 
     96% ethylene dio diacrylate+4% initiator 
     Composition 2 
     96% 1,3-bis(3-methacryloxypropyl)tetramethyldisiloxane+4% initiator 
     Composition 3 
     44% (3-acryloxypropyl)tris(tri-methoxysiloxy)-silane+15% ethylene dio diacrylate+37% t-butyl acrylate+4% initiator 
     Composition 4 
     48% acrylate Terminated poly siloxane+48% t-butyl acrylate+4% initiator 
     Composition 5 
     96% bisphenol A diacrylate+4% initiator 
     It should be understood that the relative mixture between the non-initiator components of the aforementioned compositions could vary by as much as 20%, dependent upon the stoichiometry. Also, the above-identified compositions may also include stabilizers that are well known in the chemical art to increase the operational life, as well as initiators. 
     Referring to  FIG. 6 , another embodiment in accordance with the present invention provides an optical coupling device  128  that includes a stress relief layer  130  disposed between substrate  110  and sub-portions  124   a . Stress relief layer  130  increases the selection of materials that may be employed to form optical coupling device  128 . Specifically, stress relief layer  130  typically has a classification temperature Tg that is lower than the operational temperature of optical coupling device  128 . This allows the use of materials having a Tg that is higher than the operational temperature of optical coupling device  128  without exacerbating the probability of cracking, because of the flexibility introduced by the presence of stress relief layer  130 . To provide stress relief layer  130  with optical transparence to the radiation that will be used during operation, stress relief layer  130  may be formed from polysiloxane rubbers, with an acrylic end group attached thereto to facilitate cross-linking when exposed to ultraviolet radiation. Alternatively, the polysiloxane rubber may be thermally cross-linked. Stress relief layer  130  may be disposed upon substrate  110  using spin-on techniques. It is desired that the material from which stress relief layer  130  is formed does not swell in acrylate polymerizable compositions. As a result, fluorosilocane rubbers may be beneficial to include in stress relief layer. 
     Referring to  FIGS. 2 and 6 , an additional benefit provide by stress relief layer  130  is that the same may function as a planarization layer. As a result, stress relief layer  130  may provide an additional function of ensuring surface  112  is planar. To that end, stress relief layer  130  may be fabricated in such a manner so as to possess a continuous, smooth, relatively defect-free surface that may exhibit excellent adhesion to sub-portions  124   a.    
     Referring again to  FIG. 1 , to ensure that imprint layer  24  does not adhere to imprint device  14 , imprint device  14  may be treated with a modifying agent. One such modifying agent is a release layer (not shown) formed from a fluorocarbon silylating agent. Release layer (not shown) and other surface modifying agents, may be applied using any known process. For example, processing techniques that may include chemical vapor deposition method, physical vapor deposition, atomic layer deposition or various other techniques, brazing and the like. In this configuration, imprint layer  24  is located between substrate  10  and release layer (not shown), during imprint lithography processes. 
     The embodiments of the present invention described above are exemplary. Many changes and modifications may be made to the disclosure recited above, while remaining within the scope of the invention. For example, although the present embodiment is discussed with respect to having fourteen steps, any number of steps may be formed. The scope of the invention should, therefore, be determined not with reference to the above description, but instead with reference to the appended claims along with their full scope of equivalents.