Abstract:
Wetting and print transfer from a printing patterned transfer surface is enhanced by applying an ultraviolet radiation responsive material to the patterned transfer surface. Ultraviolet activation of the ultraviolet responsive coating is performed during a transfer of printing material to a substrate. The technique increases precision of the printing process and is useful for transfer of printing material to a substrate in order to establish printed circuit components such as circuit traces and printed circuit elements on the substrate. In a particular configuration the ultraviolet radiation responsive material can be made of azobenzene material or free radical initiators.

Description:
RELATED APPLICATION 
     The present Patent Application claims priority to Provisional Patent Application No. 61/071,923 filed May 27, 2008, which is filed by the inventors hereof and which incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present disclosure generally relates to contact printing, and more specifically to a surface modification of a stamp which assist printing material transfer from stamp to substrate. 
     2. Background 
     There are two major ways to lower the cost of electronics devices—increase the packaging efficiency of the integrated circuit (IC) components of the devices, and increase cost effectiveness by using volume production processes. For such a cost-efficient mass production and even more for thin, flexible polymer electronic systems, large area patterning processes may be a choice as an economic production method and will most probably play an important part in polymer electronics manufacturing. 
     Such electronic devices may include a substrate, printed circuit components such as circuit traces and printed circuit elements such as resistors, and discrete components such as electrodes and other active or passive circuit elements. In many cases, the devices are printed by use of a stamp capable of transferring printing material from the stamp to a substrate in order to establish printed circuit components such as circuit traces and printed circuit elements on the substrate. 
     High resolution patterning methods for defining the separation between interconnects in electronic devices are important because of the flexibility of the low cost flexible substrate. As simple and efficient as it is, contact printing does nevertheless present some problems, often related to the use of the soft polymer stamp. To ensure high efficiency of transfer of printing material from the stamp, controlling the surface wettability of the stamp is therefore of major interest.  FIGS. 1A-D  are diagrams showing the ideal a printing procedure of contact printing. During dipping, the printing material should be easily picked up by the stamp, which has a higher surface energy relative to the printing material. While printing on the substrate, the stamp surface should have a lower surface energy relative to the printing material in order to transfer the material to the substrate. To facilitate this material transfer mechanism, control of surface wettability on the stamp surface is a key process. 
     Stimuli-responsive surfaces make it possible to reversibly control the wettability of the surface and has been demonstrated by various methods, including light-irradiation. Among different controlling methods on surface wettability, light-irradiation is the most effective, especially in large volume production. 
     U.S. Pat. No. 7,361,724 discloses surface modification by providing a self-assembling monolayer. According to this disclosure, the modified surface cannot be activated by ultraviolet source which is essential to change its properties during the printing process. U.S. Pat. No. 6,951,666 discloses the precursor to modify the surface of a stamp; however, the precursor is limited to conductive features and its application is limited. U.S. Pat. No. 7,368,163 discloses a covalent bonded surface modified on a polymer for printing. According to this disclosure, the absorbed surface modifier is not photosensitive or able to change its state under ultraviolet radiation. U.S. Pat. No. 6,884,628 discloses multifunctional polymeric surface coatings, but its application is limited to sensor devices and the described surface modification cannot be activated by ultraviolet radiation. 
     SUMMARY 
     Printing of a pattern having electrical properties is accomplished using a stamp having a patterned transfer surface. The stamp and patterned transfer surface are capable of transfer of printing material from the stamp to a substrate in order to establish printed circuit components such as circuit traces and printed circuit elements on the substrate. An ultraviolet radiation responsive material is applied to the patterned transfer surface in a pattern corresponding to the circuit layout. Printing material is then applied to the patterned transfer surface over the ultraviolet radiation responsive material coating. The stamp is then used to apply the printing material on the patterned surface of a workpiece, and actinic light is used to activate the ultraviolet radiation responsive material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The embodiments disclosed herein are illustrated by examples shown in the following figures but not limited in these figures. These drawings are not necessarily drawn to scale. The disclosure will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIGS. 1A-1D  (prior art) are schematic cross section side views depicting contact printing. 
         FIGS. 2A-2D  are schematic cross section side views depicting printing with a treated stamp. 
         FIGS. 3A-3D  are schematic cross section side views depicting printing with a treated stamp and a treated substrate. 
         FIG. 4  is a schematic flowchart for a printing process with a treated stamp. 
         FIG. 5  is a schematic flowchart for a printing process with a treated stamp and treated substrate. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to configurations of a stamp structure with ultraviolet radiation responsive material coated thereon and methods for printing by adopting ultraviolet radiation responsive material on stamp and substrate surfaces. Details are set forth to provide a thorough understanding of the configurations described herein with the help of the drawings, but not limited thereto. The features, structures, materials, and characteristics of the technology can be combined in any suitable manner in one or more configurations. 
     Overview 
     Ultraviolet irradiation or other actinic light irradiation is applied to a surface which is coated with material sensitive to ultraviolet radiation. The coating should have an in-situ change of chemical structure or surface energy which assists the printing process. 
     The strategy is based on coating the stamp by ultraviolet radiation responsive material which modifies the stamp such that the printing material can be picked up in a more efficient fashion. During printing, ultraviolet light irradiates the ultraviolet radiation responsive material such that the material changes its chemical structure or chemical configuration which gives a different wetting behavior in contrast to its original state. This change of structure leads to less adhesion between stamp and printing material which leads to the efficient transfer of printing material from stamp to substrate. 
     The present disclosure provides a low cost and simple process printing solution by using an ultraviolet radiation responsive material on a stamp. The ultraviolet light is used to assist the volume of printing material transferred to and from the stamp. The height of the printed feature can be adjusted by volume picked up and transferred, through quick surface chemistry change on the stamp. This enhances efficient printing material transfer from the stamp to a substrate through ultraviolet irradiation. The residue of printing material can therefore be reduced, while avoiding a need for frequent cleaning or replacing the stamp with a new one. To modify the surface behavior of the stamp to suit different printing materials, the ultraviolet sensitive coating can be formulated according to the different material system. The ultraviolet sensitive coating is applied to the stamp and/or the workpiece, but is exposed to ultraviolet irradiation during the pressing process. 
     The substrate is the workpiece which is the object to be coated. The substrate may be, by way of non-limiting example, a conductive material for circuitry in electronic packages, semiconductor materials for circuits, a polymer material for building mechanical support or structure in devices, and other circuit devices which can be applied by printing. 
     In some configurations, the ultraviolet sensitive material is coated only on the surface of the stamp. The printing material can be transferred from stamp to substrate based on its mass and substrate adhesion force after ultraviolet radiation. 
     In some other configurations, both the stamp and the substrate can be coated by different ultraviolet sensitive material to further increase efficiency of material transfer from the stamp to the substrate. 
     References throughout this description to “stamp” mean a structure that has relief features or raised impression features which are used for pick up and transfer of printing material to the substrate. The stamp is advantageously transparent to ultraviolet radiation. References throughout this description to “printing material” mean a material transferred to or patterned on a substrate, e.g., a conductive material for circuitry in electronic packages, a polymer material for building mechanical support or structure in devices, etc. References throughout this description to “ultraviolet radiation responsive material” mean a material that is responsive to ultraviolet radiation, such that the material changes its state, chemical structure, chemical configuration or is ablated under ultraviolet radiation. References throughout this description to “substrate” mean a platform for printing material to be printed on. It can be made of any metal, polymer or composite. 
     U-V Responsive Material 
     The ultraviolet radiation responsive material is a material responsive to ultraviolet radiation. In one non-limiting example, the ultraviolet responsive material configuration responds to an optical radiation source with less than 400 nm wavelength, such that the material changes its state, chemical structure, chemical configuration or is ablated under ultraviolet radiation. Applying ultraviolet activation of the ultraviolet responsive material causes a reaction of the ultraviolet radiation responsive material. The ultraviolet responsive material may be, by way of non-limiting example, a material which reacts to ultraviolet energy by releasing free radicals. The free radicals infiltrate the printing material or coating material intended for printing, resulting in a reduction in the surface tension of the printing material. The material may be a small molecule organic material which reacts to ultraviolet energy by releasing free radicals, e.g. benzyol peroxide. Other examples of ultraviolet radiation responsive material are ultraviolet-sensitive azobenzene compounds and azobisisobutyronitrile. Suitable materials are responsive to ultraviolet and able to alter its surface energy or morphology to enhance material transfer during printing. The ultraviolet radiation responsive material may be selected so as to be able to form a uniform structure on the stamp. By using ultraviolet activation, it is possible for the stamp behavior to differ before and after application of ultraviolet energy. The actuation is based on the application of ultraviolet light. Prior to ultraviolet activation, the stamp is able to efficiently pick up material during dipping, whereas after ultraviolet activation, the ability of the stamp to release material is enhanced. Thus, during the pressing process, the actuation of the ultraviolet responsive material further facilitates the printing material release. 
     The printing material is suitable for transfer to or patterned on the substrate which, as indicated above, is the workpiece. The printing material is selected so that it is not directly affected by ultraviolet irradiation. Therefore, the ultraviolet should only affect the ultraviolet reacting material to change its chemical structure or surface energy of the stamp, with the printing material being unaffected by ultraviolet irradiation. The ultraviolet reacting material is tailored according to the printing material chemical composition. 
     U-V Material Applied to Stamp 
       FIGS. 2A-2D  are diagrams showing a configuration of printing with a stamp structure  211  which includes relief structure  213  or other patterned surface. The stamp  211  is coated with ultraviolet radiation responsive material  217  for printing onto a substrate  221  ( FIGS. 2C and 2D ). In this configuration, stamp structure  211  can be made of any material which is transparent to ultraviolet radiation, typically a radiation source of less than 400 nm wavelength. The relief structure  213  of the stamp structure  211  has a height and pitch ranging from submicron to millimeter size (around 1 nm to 1 mm). By way of non-limiting example, the stamp  211  may be made of glass; however other suitable substrates may be used. The glass may be selected as an ultraviolet transparent glass, in order to facilitate ultraviolet activation of the ultraviolet radiation responsive material  217 . The substrate  221  is a platform for printing material to be printed on. It can be made by way of non-limiting example, of any metal, polymer or composite, such as epoxy, epoxy composite, polyimide, other plastic resins, ceramics and glass. 
     The ultraviolet radiation responsive material  217  is the material responsive to ultraviolet radiation, typically a radiation source of less than 400 nm wavelength. The ultraviolet radiation responsive material  217  can be made of free radical initiators or azobenzene materials, which are able to change their state or structure under ultraviolet radiation. The ultraviolet radiation responsive material  217  may be applied in any convenient manner, for example by aerosol application or by dipping into a vessel or pot such as described below in connection with printing material  225 . 
     One feature of the ultraviolet radiation responsive material  217  is that it is responsive to the ultraviolet light so as to be able to change the surface energy of the stamp. In this respect, the surface energy of the ultraviolet radiation responsive material  217  is between the stamp and printing material before application of ultraviolet irradiation, and its surface energy will change to lower than the surface energy of printing material when ultraviolet irradiation is applied. 
     Free radicals are referred to as atomic or molecular species with unpaired electrons on an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so radicals are likely to take part in chemical reactions. Thus, free radical initiators are highly useful as the ultraviolet radiation responsive material  217 . The ultraviolet radiation responsive material  217  is able to form a uniform structure on stamp  211  through different coating and deposition methods. The ultraviolet radiation responsive material  217  should have the ability to change the surface energy of the stamp  211 . 
     Turning now to  FIG. 2A , the stamp  211  is first dipped into a vessel of printing material  225  inside a vessel  227  under pressure. The pressure is less than yield point of stamp  211 . The time of dipping is not limited, but may be less than one minute. The printing material  225  is then picked up when stamp  211  is lifted up ( FIG. 2B ) and the stamp  211  is stamped on to substrate  221  under pressure which is less than yield point of stamp  211  ( FIG. 2C ). The stamping time is not limited, but may be less than one minute. During stamping, an ultraviolet source having a wavelength less than 400 nm is applied on top of the stamp and activates the ultraviolet radiation responsive material  217 . The change of state or change of structure of ultraviolet radiation responsive material  217  results in a change of the wetting properties of the printing material  225 . The time for applying the ultraviolet radiation is not limited but may be long enough to change the state or structure of ultraviolet radiation responsive material  217 . The change the state or structure of ultraviolet radiation responsive material  217  should decrease the wetting of printing material  225  on stamp  211  which helps with the transfer of the printing material  225  to substrate  221 . After stamping, the stamp  211  is lifted up and the printing material  225  is transferred on to the substrate  221  with desired geometry ( FIG. 2D ). 
     The ultraviolet radiation responsive material  217  can be applied before each printing cycle, or can be reused. By way of non-limiting example, the ultraviolet radiation responsive material is reusable more than 10 printing cycles. 
     U-V Material on Stamp and Substrate 
       FIGS. 3A-3D  are diagrams showing a configuration of printing with inventive stamp structure  311  which includes relief structure  313  or other patterned surface. The stamp  311  is coated with ultraviolet radiation responsive material  317  and substrate  321  is coated with another type of ultraviolet radiation responsive material  323 . In this configuration, stamp structure  311  can be made of any material which is transparent to ultraviolet radiation, typically a radiation source of less than 400 nm wavelength. The relief structure  313  of the stamp structure  311  has the height and pitch ranging from submicron to millimeter size (around 1 nm to 1 mm). The ultraviolet radiation responsive material  317  is the material responsive to ultraviolet radiation, typically a radiation source of less than 400 nm wavelength. In a non-limiting example, the ultraviolet radiation responsive material  317  and substrate  321  is made of free radical initiators or azobenzene materials, which are able to change their state or structure under ultraviolet radiation. Other than the use of the other type of ultraviolet radiation responsive material  323 , the materials and structure are similar to that described in connection with  FIGS. 2A-2D . 
     The ultraviolet radiation responsive material  317  is able to form a uniform structure on stamp  311  through different coating and deposition methods. Examples of coating methods include spin-coating, dip-coating and other ultraviolet radiation responsive material deposition methods. The ultraviolet radiation responsive materials  317  and  323  should have the ability to change the surface energy of the stamp  311  and substrate  321 , respectively. 
     Turning now to  FIG. 3A , the stamp  311  is first dipped into a vessel of printing material  325  inside a vessel  327  under pressure which is less than yield point of stamp  311 . The time of dipping is not limited but may be less than one minute. The printing material  325  is then picked up when stamp  311  is lifted up ( FIG. 3B ) and the stamp  311  is stamped on to substrate  321  under pressure which is less than yield point of stamp  311  ( FIG. 3C ). The stamping time is not limited but may be less than one minute. During stamping, an ultraviolet source with its wavelength less than 400 nm is shined on top of the stamp  311  and activates the ultraviolet radiation responsive material  317  on the stamp  311  and the ultraviolet radiation responsive material  323  on substrate  321 . The change of state or change of structure of ultraviolet radiation responsive material  317  and  321  then changes its wetting properties with printing material  325 . The time for ultraviolet radiation is not limited but is long enough to change the state or structure of ultraviolet radiation responsive material  317 . The change in the state or structure of ultraviolet radiation responsive material  317  decreases the wetting of printing material  325  on stamp  311  and increase the wetting of printing material  325  on substrate  321 , which helps with the transfer of the printing material  325  to substrate  321 . After stamping, the stamp  311  is lifted up and the printing material  325  is transferred on to the substrate  321  with desired geometry ( FIG. 3D ). 
     Functional Operation 
       FIG. 4  is a schematic flowchart for a printing process with a treated stamp in accordance with the present subject matter. A stamp is prepared of desired dimensions (step  411 ). The stamp is first dipped into a solution of photosensitive material, such as an ultraviolet responsive material (step  413 ). The stamp, treated with the ultraviolet radiation responsive material is dipped (step  419 ) into a pot containing printing material. This causes the stamp to pick up printing material from the vessel. The stamp with the printing material is then pressed onto a substrate (step  421 ). While in contact with the substrate, ultraviolet light or other energy used to activate the ultraviolet responsive material which is applied to the top of stamp (step  423 ), resulting in activation of the ultraviolet radiation responsive material. The stamp is then separated from the substrate (step  425 ), resulting in the desired pattern being transferred to the substrate. 
       FIG. 5  is a schematic flowchart for a printing process with a treated stamp and treated substrate in accordance with the present subject matter. A stamp is prepared of desired dimensions (step  511 ). The stamp is first dipped into a solution of ultraviolet radiation responsive material, (step  515 ). At the same time, the surface of the substrate is treated with the same or a different type of ultraviolet responsive material. The stamp, treated with the ultraviolet responsive material, is then dipped (step  519 ) into a pot containing printing material. This causes the stamp to pick up printing material from the vessel. The stamp with the printing material is then pressed onto substrate (step  521 ). While in contact with the substrate, ultraviolet light or other energy used to activate the ultraviolet responsive material is applied to the top of stamp (step  523 ), resulting in activation of the ultraviolet responsive material. The stamp is then separated from the substrate (step  525 ), resulting in the desired pattern being transferred to the substrate. 
     CONCLUSION 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the subject matter, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. By way of example, the specific configuration of the stamp (e.g, stamp  211  depicted in  FIG. 2 ) can vary in accordance with a specific manufacturing technique. Variations can include the use of a raised structure instead of a relief structure in order to create a desired transfer surface on the stamp. Therefore, it will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the subject matter, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.