Abstract:
An improved elastomeric mold for use in fabricating microstructures, the mold having first and second surfaces, the first surface including at least one recessed microchannel and the second surface including an access opening or filling member that extends through the mold to the first surface and communicates with the recessed microchannel. The mold is used by placing it onto a substrate with the recessed microchannel facing the substrate. The access opening of the mold is filled with a liquid material which is capable of solidifying. The access opening continuously introduces the liquid material into the space defined between the microchannel and the substrate. After the liquid material solidifies, the mold is removed from the substrate thereby leaving a microstructure formed from the solidified liquid material on the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 09/049,531 filed by the present applicants on Mar. 27, 1998, now U.S. Pat. No. 6,033,202. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to non-photolithographic fabrication of microstructures, and in particular, to an improved mold for fabricating microstructures on one or more substrates using micromolding in capillaries (MIMIC). The mold is especially useful for fabricating plastic microelectronic devices. 
     BACKGROUND OF THE INVENTION 
     The demand for inexpensive microelectronic devices has resulted in the development of organic materials potentially useful in electronic and optoelectronic systems. This has led to advances in microelectronic devices that make it possible to inexpensively produce microelectronic devices that occupy large areas and are easily fabricated on rigid or flexible plastic supports. These advances include the development of conducting, semiconducting, and dielectric organic materials. 
     Unfortunately, present methods for patterning these organic materials are less than adequate. One such method is screen printing. See, Z. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, A. J. Lovinger, “High-performance Plastic Transistors Fabricated by Printing Techniques,” CHEM. MATER. 9 (1997) at 1299-1301. But the use of screen printing in making microelectronic devices such as FETs is limited by relatively poor resolution (˜100 μm) of the screen printing method. 
     Another method which is capable of generating microstructures in a wide range of materials with feature sizes between one and several hundred microns is micromolding in capillaries (MIMIC). MIMIC involves forming capillary channels between a support and an elastomeric mold that contains recessed channels that emerge from the edges of the mold. A solution containing a solvent and a material (MIMIC solution) which forms the microstructure is applied to the channels at the edges of the mold. Once the solvent in the MIMIC solution evaporates, the mold is lifted from the substrate leaving a microstructure composed of the material. GaAs/AlGaAs heterostructure FETs with dimensions as small as ˜20 μm have been fabricated using MIMIC defined sacrificial polymer layers. The MIMIC defined polymer layers were used in “lift-off” procedures to form the gates and the electrodes of the FETs. 
     The conventional MIMIC technique, however, has several serious disadvantages. First, the conventional molds used in MIMIC may only be filled by repeatedly applying the MIMIC solution to the recessed channels at the edges of the mold as the solvent in the solution evaporates. Second, when a conventional MIMIC mold is removed, excess unusable material remains on the substrate where the edges of the mold were located. This material must then be removed by cutting it away from the substrate. Third, the MIMIC solution may have to travel a greater distance in an edge filled MIMIC mold which leads to very long filling times. Fourth, patterns made from more than one type of material are not possible with conventional MIMIC molds. Fifth, MIMIC molds can not be easily integrated with conventional printing methods such ink jet printing or screen printing. Sixth, the MIMIC solution can not be forced or drawn into a conventional MIMIC mold with a pressure or a vacuum. 
     Accordingly, there is a need for an improved mold for use in MIMIC that overcomes the deficiencies of conventional MIMIC molds. 
     SUMMARY 
     In accordance with the present invention, an improved mold for use in fabricating microstructures comprises a body of elastomeric material having first and second surfaces, the first surface including at least one recessed microchannel and the second surface including at least one mold filling member that extends through the mold to the first surface and communicates with the recessed microchannel. The mold is used by placing it onto a substrate with the recessed microchannel facing the substrate. The mold filling member of the mold is filled with a liquid material capable of solidifying. The filling member continuously introduces the liquid material into the space defined between the microchannel and the substrate. After the liquid material solidifies, the mold is removed from the substrate thereby leaving a microstructure formed from the solidified liquid material on the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments described in detail below, considered together with the accompanying drawings. In the drawings: 
     FIG. 1 is a perspective view of an elastomeric mold which may be used for micromolding in capillaries; 
     FIGS. 2A-2B illustrate the fabrication of the mold of FIG. 1; 
     FIG. 3 is a cross-sectional side view showing the mold of FIG. 1 positioned on a microstructure support substrate; 
     FIG. 4 is a perspective view of a mold according to a second embodiment of the invention; 
     FIGS. 5A-5F are transmission mode optical micrographs collected at several times after capillary channels and the micromold cavities of the mold fill with carbon particles from a MIMIC solution of carbon in ethanol; 
     FIG. 6 is a profilometer trace collected from three wires formed by one set of the capillary channels; 
     FIGS. 7A-7C are cross-sectional side views illustrating the fabrication of a plastic field-effect-transistor; 
     FIGS. 7D-7F are perspective views showing the use of the mold in MIMIC to fabricate source and drain electrodes for the plastic field-effect-transistor of FIG. 7C; 
     FIG. 8 shows a top plan view of the completed FET of FIG. 7F; 
     FIGS. 9A-9C are current-voltage curves from plastic FETs made using the mold of the present invention in MIMIC; and 
     FIG. 10 is a perspective view of a mold according to a third embodiment of the invention. 
    
    
     It should be understood that the drawings are for purposes of illustrating the concepts of the invention and are not to scale. 
     DETAILED DESCRIPTION 
     FIG. 1 shows an elastomeric mold  10  which may be used for micromolding in capillaries (MIMIC) to produce complex microstructures on one or more substrates. The mold  10  allows the microstructures to be fabricated from dilute solutions of organic material (MIMIC solutions). The mold  10  is especially useful for producing microstructures in MIMIC having dimensions substantially less than 100 μm. 
     The mold  10  includes a substrate contact surface  12  and a mold filling surface  14 . Recessed microchannel and micromold cavity structures  16 ,  18  are defined in the contact surface  12  of the mold  10 . Two access openings  20  extend through the mold  10  from the filling surface  14  to the contact surface  12 . Each access opening  20  communicates with a corresponding set of microchannels  16 . The recessed structures  16 ,  18  of the mold  10  may be shaped into simple and complex configurations according to the microstructure to be fabricated. 
     FIGS. 2A-2B illustrate the fabrication of the mold  10  of FIG.  1 . FIG. 2A shows a master  22  which may be used for reproducing one or more copies of the mold  10 . The master  22  comprises a rigid planar substrate  24  of Si/SiO 2 , glass or metal with a raised pattern  26 . The pattern  26  is substantially identical geometrically and dimensionally to the microstructures to be fabricated. 
     The raised pattern  26  may be fabricated on the substrate  24  from a photoresist material using an amplitude mask. Such photolithographic methods are capable of defining micrometer and submicrometer-scale photoresist patterns having complex structures. Accordingly, the pattern  26  may be easily generated with feature dimensions which are substantially less than 100 μm. For feature dimensions which are less than 0.5 μm, the raised pattern can be made using electron beam lithography to pattern a resist layer. 
     FIG. 2B shows the casting and curing of the mold  10  against the raised pattern  26  of the master  22  using known methods. See, D. Qin, Y. Xia, G. M. Whitesides, “Rapid Prototyping of Complex Structures with Feature Sizes Larger Than 20 μm,” ADVANCED MATERIALS 8, No. 11 (1996) at 917-919 which is incorporated herein by reference. The mold  10  may be fabricated from an elastomer such as polydimethylsiloxane (PDMS). The compliant nature of PDMS allows conformal contact between the mold and the substrate which supports the micro structure. The low reactivity of PDMS allows the mold to easily separate from the microstructures. 
     After curing, the mold  10  is removed from the master  22  and the access openings  20  are machined. It is also possible to mold the access openings  20  into the mold during casting. 
     FIG. 3 shows the mold  10  placed on a microstructure support substrate  28  with the recessed microchannel and micromold cavity structures  16 ,  18  facing the substrate  28 . The compliant nature of the mold  10  allows the contact surface  12  to conform to the surface  30  of the substrate  28  producing a plurality of sealed capillary channels  32  which lead into two sealed micromold cavities  34 . The access openings  20  are also sealed by the conformal contact between the surface  30  of the support substrate  28  and the contact surface  12  of the mold  10 . 
     The sealed access openings  20  in the mold  10  provide an improved means for introducing MIMIC solutions into the micromold cavities  34  via the capillary channels  32 . Dense, uniform solid microstructures may only be generated from the MIMIC solutions if the micromold cavities  34 , via the capillary channels  32 , are continuously supplied with the MIMIC solution as the solution cures by solvent evaporation or other means. This is accomplished with the mold  10  as each access opening  20  functions as a reservoir to maintain a continuous supply of MIMIC solution to a corresponding micromold cavity  34 . Moreover, the access openings  20  allow the mold  10  to be filled using conventional automated printing equipment. For example, the access openings  20  can be automatically filled using an ink jet printer or a screen printer (not shown). 
     The access openings  20  also eliminate the need to cut away excess material (the material forming the microstructure) which typically remains on the microstructure support substrate when conventional molds without access openings are used. This is because removal of the mold  10  eliminates all material that does not reside in the capillary channels  32 , the micromold cavities  34  and at the bottom of the access openings  20 . 
     The access openings  20  of the mold  10  also enable the MIMIC solution to be applied adjacent the locations where the solution is required and enable the mold  10  to be made with separate micromold cavities  34  which produce separate microstructures. Since each micromold cavity  18  is supplied by its own access opening  20  and set of microchannels  16 , each microstructure can be made from a different organic material if desired (using a different solution of organic material). 
     A single appropriately sized access opening may also be used for supplying one or more microcavities. FIG. 4 shows a mold  10 ′ with a single access opening  20 ′ that supplies two microchannel cavities  18 ′ via two corresponding sets of microchannels  16 ′. 
     Further reductions in mold filling times can be realized in the mold  10  by forcing or drawing the MIMIC solution into the mold. The MIMIC solution can be forced into the mold by pressurizing the access openings  20  of the mold  10  after filling them with MIMIC solution. The MIMIC solution can be drawn into the mold  10  by evacuating the micromold cavities. FIG. 10 shows a mold  10 ″ having a vacuum port  60  which leads into each micromold cavity  34 ″. A vacuum is applied to the vacuum ports  60  to evacuate the micromold cavities  34 ″. This in turn draws the MIMIC solution into the mold  10  from the access openings  20 ″. 
     FIGS. 5A-5F are transmission mode optical micrographs collected at several times after capillary channels and the micromold cavities of the mold  10  in contact with a transparent support, fill with carbon particles from a MIMIC solution of carbon in ethanol (about 2% solids). MIMIC solution flows through the two sets of three ˜100 μm long capillary channels  32  that lead from corresponding access holes (not shown) to corresponding 1 mm by 1 mm micromold cavities  34 . The flow stops at the edges of a ˜25 μm gap. Evaporation of the ethanol through the elastomeric mold  10  (PDMS is permeable to ethanol vapor) drives the flow of the solution from the access openings to the micromold cavities  34 . The time of fabrication can be reduced from about four (4) hours as indicated to about 30 minutes by increasing the solids content of the MIMIC solution. 
     FIG. 6 is a profilometer trace collected from three wires formed by one set of the capillary channels. The thickness of the carbon wires is slightly less (˜5 μm) than the depth of the recessed microchannel in the mold  10  (˜8 μm). 
     The mold  10  may be used in MIMIC to fabricate source and drain electrodes of a plastic field-effect-transistor (FET) from an organic material. The FET is first constructed using conventional screen printing processes. Fabrication begins in FIG. 7A, by providing a commercially available sheet  40  of poly(ethylene terephthalate) coated with a layer of indium-tin-oxide  42 . The PET sheet forms a mechanically flexible supporting substrate for the FET. The thickness of the PET sheet  40  may be approximately 50 μm. The ITO layer  42  forms a gate electrode for the FET and may be approximately 0.1 μm thick. The FET may also be supported on a mechanically rigid substrate such as silicon having a thickness of approximately 3 mm. 
     FIG. 7B shows a layer  44  of pre-imidized polyimide (PI) screen printed onto the ITO layer  42 . The PI layer  44  forms a thin dielectric approximately less than 1 μm in thickness. Pre-imidized PI is commercially available from Japan Synthetic Rubber Co. under the tradename OPTIMER AL 3046. The dielectric layer  44  may also be formed by a 0.1 μm thick layer of silicon dioxide (SiO 2 ). 
     FIG. 7C shows a layer  46  of regioregular poly(3-hexylthiophene) (PHT) screen printed or casted onto the PI dielectric layer  44  using well known methods. See, Z. Bao, Y. Feng, A. Dodabalapur, V. R. Raju, A. J. Lovinger, “High-Performance Plastic Transistors Fabricated by Printing Techniques,” CHEM. MATER. 9 (1997) at 1299-1301 which is incorporated herein by reference. The layer  46  of PHT has known semiconducting properties which makes it suitable for forming the channel region of the FET. See, Z. Bao, A. Dodabalapur, A. J. Lovinger, “Soluble and Processable Regioregular poly(3-hexylthiophene) for Thin Film Field-Effect Transistor Applications with High Mobility,” Appl. Phys. Lett. 69 (1996), at 4108-4110 which is incorporated herein by reference. The thickness of the PHT layer  46  may be approximately 30-100 nm. 
     FIG. 7D shows the mold  10  of FIG. 1 placed onto the PHT layer  46  of the substrate  40  with the recessed microchannels  16  and micromold cavities  18  facing the PHT layer  46 . The compliant nature of the mold  10  allows it to conformably contact the PHT layer  46  of the substrate  40 . The mold  10  is located on the substrate so that micromold cavities  18  are disposed directly over the regions of the PHT layer  46  which form the source and drain regions of the FET. 
     FIG. 7E shows the access openings  20  of the mold  10  filled with a MIMIC solution  48  containing an organic material. The MIMIC solution  48  preferably comprises a carbon paint made up of ethanol solvent and about 2% solid carbon. However, other colloidal suspensions of conductive colloidal particles and conducting polymer solutions may be used for the electrodes. For example, the MIMIC solution  48  may also comprise m-cresol solvent and conducting polyaniline (PA). An advantage of using a colloidal suspension of carbon for the electrodes is that the carbon particles do not tend to seep between the electrode patterns. Seepage between the electrodes could cause problems with the operation of the FET. 
     The MIMIC solution  48  stored in the openings  20  wicks into the capillary channels  32  and the micromold cavities  34  defined between the PHT layer  46  and the recessed structures  16 ,  18  of the mold  10 . Evaporation of the solvent through the mold  10  from the filled capillary channels  32  and micromold cavities  34  forces the MIMIC solution  48  to flow from the access openings  20  into the capillary channels  32  and the micromold cavities  34 . The filled access openings  20  of the mold  10  maintain a continuous supply of MIMIC solution  48  in the capillary channels  32  and the microcavities  34 . 
     FIG. 7F shows the mold removed from the substrate  40 . The mold leaves a pattern  50  of solid carbon on the layer  46  of PHT. The pattern  50  includes source and drain electrodes  52 , contact pads  54 , and wires  56  connecting the contact pads  54  to the electrodes  52 . 
     FIG. 8 shows a top plan view of the completed FET of FIG.  7 F. The source and drain electrodes  52  consist of 1 mm by 1 mm squares separated by about 25 μm (defining a channel length of ˜25 μm). The contact pads  54  each consist of a 2 mm diameter solid carbon circle. The wires  56  connecting the electrodes  52  to the contact pads  54  are each about 100 μm in length. The electrodes and wire structures  52 ,  56  of the pattern  50  are approximately 5 μm thick (mold feature depth was ˜8 μm). 
     FIGS. 9A-9C are current-voltage curves from plastic FETs made using the mold of the present invention in MIMIC. The scaling of negative drain-source current with negative gate voltages indicates p-channel transistors. The field-effect mobilities of these transistors are in the order of 0.01 to 0.05 cm 2 /Vs. These characteristics are consistent with those of larger transistors fabricated using screen printing and conventional photolithographic methods and PHT semiconducting material. 
     FIG. 9A shows current-voltage curves of a FET with ˜5 μm thick conducting carbon electrodes, a ˜50 nm thick semiconducting PHT layer, a screen-printed ˜1 μm thick PI layer, and an ITO-coated ˜50 μm thick PET substrate. The FET had a channel width of about 2.5 mm, and a channel length of about 25 μm. 
     FIG. 9B shows current-voltage curves of a FET with ˜5 μm thick conducting carbon electrodes, ˜50 nm thick semiconducting PHT layer, a ˜0.3 μm thick SiO 2  layer, and an ITO-coated ˜3 mm thick Si substrate. The FET had a channel width of about 2.5 mm, and a channel length of about 25 μm. 
     FIG. 9C shows current-voltage curves of a FET with ˜5 μm thick doped PA electrodes, a ˜50 nm thick semiconducting PHT layer, a ˜0.3 μm thick SiO 2  layer, and an ITO-coated ˜3 mm thick Si substrate. The FET had a channel width of about 1.0 mm, and a channel length of about 50 μm. 
     The mold of the present invention is capable of producing features as small as 1 μm in MIMIC. The mold can be used with a broad range of materials and when used in MIMIC requires minimal or no access to clean room facilities. 
     It should be understood that the above described embodiments are illustrative of only a few of the many possible specific embodiments which can represent applications of the principles of the invention. For example, each access opening in the mold can be replaced by a filling member such as a syringe inserted through the mold which communicates with a corresponding micromold cavity. The syringe can be filled with MIMIC solution which is injected directly into the micromold cavity. This and other varied arrangements can be devised by those skilled in the art without departing from the spirit and scope of the invention.