Patent Publication Number: US-2018030606-A1

Title: Microfabrication of Tunnels

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 13/420,696, filed on Mar. 15, 2012 and which will issue as U.S. Pat. No. 9,777,384 on Oct. 3, 2017, which claims the benefit of U.S. Provisional Patent Application No. 61/471,828, filed on Apr. 5, 2011, both of which are incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to forming hollow bores. More specifically, the invention relates to a method for forming any number of very small, precise high-aspect-ratio hollow bores, or “beam tunnels,” of arbitrary cross sectional shape and arbitrarily long length in metals for the purpose of transporting electron beams. 
     BACKGROUND 
     Vacuum electron devices that generate electromagnetic power in the gigahertz (GHz=10 9  Hz) to terahertz (THz=10 12  Hz) frequency range often rely on very small metal structures, known as “interaction circuits,” that support electromagnetic fields which interact with a beam of electrons in vacuum. Typically, the electrons are guided substantially by an external magnetic field through the interaction circuit(s) without physically touching the walls of the circuit by means of a tunnel, or “beam tunnel,” bored through the interaction circuit. The interaction circuit is often substantially metal, such as copper for low microwave loss and high thermal conductivity, or a metal-coated material such as silicon or diamond. At high electromagnetic frequencies, such as above 100 GHz, these beam tunnels become very small, approximately 0.002 inches to 0.050 inches in diameter for a round beam tunnel, and typically 0.1 inch to 5 inches in length. 
     Traditional methods for forming the beam tunnels through the interaction circuits include (a) drilling round holes, (b) ablating holes by sinker electrical discharge machining (SEDM), (c) forming the beam tunnel from two halves of the cross section by Wire EDM (WEDM) and then bonding the two halves together by brazing, mechanical fixturing, diffusion bonding or other means, (d) laser drilling the holes, (e) ablating holes by water jet, (f) casting the metal into a mold, (g) multi-layer lithography, electroforming and molding (LIGA) processes. However, these methods are all limited in their ability to reliably and precisely form very small holes of arbitrary shape for long lengths. Specific limitations of each of these methods are discussed below. 
     In the (a) drilling round holes methods, drilling a beam tunnel typically limits the cross sectional shape to round, and has a limited depth that the bore can be drilled to accurately. Furthermore, during drilling, there is a tendency to walk off-center, and the method relies on extremely fragile micro-drill bits. In the (b) SEDM method, as practiced in industry, there is typically a very limited beam tunnel length that can be fabricated (i.e., approximately 0.3 inches in length with a 0.005 inch diameter hole (aspect ratio 60:1) at the time of this application). Additionally, SEDM also tends to form a conical rather than cylindrical beam tunnel. The (c) WEDM method typically requires either a pilot hole to be drilled, which can be limited by the drilling techniques described in (a), or the method requires that two halves be bonded or brazed together. In brazing, stresses due to cyclical heating typically tend to separate the two halves where they were joined, which can reduce reliability. The (d) laser ablation method is typically limited in the depth of the cut. 
     The (e) water-jetting method typically produces a cone-shaped hole rather than a cylindrical one due to ballistic scattering. In the (f) casting method, molten metal around a mold requires the forms to be subjected to high temperatures, which can introduce issues of thermal expansion, can complicate the removal of the molds, and typically has a tendency to leave gaps and voids in the casting. In the case of some pure metals such as copper, cast melting tends to form large crystal grains, which are typically unable to fill the smallest features properly. Finally, (g) conventional multi-layer LIGA techniques require a minimum of three layers, each requiring a separate exposure step, electroforming step and polishing step, and these techniques are only capable of square or rectangular beam tunnel shapes or approximations to true cylinders. 
     Accordingly, there remains a need for an alternative method of forming beam tunnels in a metal structure. Preferably, this new method would allow for forming any number of very small, precise high-aspect-ratio hollow bores, or beam tunnels, of arbitrary cross sectional shape and arbitrarily long length in metals during a single photolithographic process along with the electromagnetic interaction circuit. Therefore, the method could circumvent many of the shortcomings of the prior methods along with providing a more efficient process. 
     SUMMARY OF THE INVENTION 
     The invention satisfies the above-described and other needs by providing for a method of forming hollow bores by positioning one or more forms, such as fibers or sheets, above a substrate, and applying a photoresist over the substrate embedding the one or more forms. 
     According to another aspect of the invention, a system for forming hollow bores includes a substrate, one or more forms, such as fibers or sheet, located and secured above the substrate, and a photoresist applied over the substrate to embed the one or more forms. 
     According to another aspect of the invention, a method can be provided for forming beam tunnels by positioning one or more forms above a substrate, applying a photoresist over the substrate embedding the one or more forms; performing a single exposure LIGA process on the photoresist embedding the one or more forms over the substrate, and removing the one or more forms to leave one or more beam tunnels. 
     These and other aspects, objects, and features of the present invention will become apparent from the following detailed description of the exemplary embodiments, read in conjunction with, and reference to, the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top and side view of the methodology of the invention, in accordance with an exemplary embodiment of the invention. 
         FIG. 2  is a top and side view of the methodology of the invention, in accordance with an alternative exemplary embodiment of the invention. 
         FIG. 3  illustrates potential cross sectional shapes for the forms and beam tunnels that can be fabricated in accordance with an exemplary embodiment of the invention. 
         FIG. 4  illustrates a technique to position and secure fibers above a substrate in accordance with an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring now to the drawings, in which like numerals represent like elements, aspects of the exemplary embodiments will be described in connection with the drawing set. 
     In general, this invention is a method to form arbitrarily shaped cross sections of a hollow bore, or beam tunnel, of arbitrary length by means of forms, such as fibers or sheets, stretched over the surface that are substantially transparent to ultraviolet light such that a 3-D mold structure is formed by intersection of these forms with structures formed out of photoresist during an exposure to the ultraviolet light using photolithographic techniques. 
       FIG. 1  is a top and side view of the methodology of the invention in accordance with an exemplary embodiment of the invention. Specifically,  FIG. 1  represents individual steps, illustrated as panels, which will be discussed below. One of ordinary skill in the art will understand that some minor steps, such as cleaning steps, polishing steps, and/or baking on a hot plate have been excluded because they are well known to one of ordinary skill in the art. 
     In panel  1  of  FIG. 1 , a form  110 , such as a fiber (e.g., monofilament), or array of forms (i.e., more than one fiber, or monofilaments) of a desired size and shape of a final hollow bore, or beam tunnel, can be positioned at a desired height and a desired position above a substrate  105 , such as a polished copper wafer. The form(s)  110  can be positioned above the substrate  105  by locating and securing each of the one or more forms  110  with one or more form holders  115 . For example, the form holders  115  can include one or more height-setting rods, one or more wire guides, one or more posts, or other accurate means of fixturing. 
     Each form  110  can be made of a material that can pass the ultraviolet (UV) light used to activate the photoresist in the second step. Specifically, in an exemplary embodiment of the invention, the form  110  can be capable of passing ultraviolet light in the wavelength range of 350-380 nanometers. Furthermore, the form  110  can have other properties that allow it to survive high thermal excursions to approximately 100° C., a sulfuric acid-containing copper plating bath, and strong solvents that can be used to develop the photoresist. Additionally, the form  110  can possess an index of refraction similar to the photoresist to inhibit ultraviolet reflections, and can be removable by some means. A couple of examples of specific form materials that have the necessary characteristics to be suitable for this type of application include Ethylene Tetrafluoroethylene (ETFE), Polyvinylidene Fluoride (PVDF), Polyetherimide (PEI) and Polysulfone. Other form materials can also be utilized, for example, the form can be made from polymer materials, fluoropolymers, glass fibers, plastics, metals, photoresists, or other suitable materials. 
     In panel  2  of  FIG. 1 , a photoresist  120  can be applied over the surface of the substrate  105  thus embedding, or completely covering, the form(s)  110  in photoresist  120 . Subsequently, ultraviolet photolithography can be performed by means of a mask (not pictured) placed between a collimated ultraviolet source (not pictured) and the substrate  105  being processed. One of ordinary skill in the art will understand that ultraviolet photolithography is a common method used for manufacture of integrated circuits, micro-electro-mechanical systems (MEMS), and microfluidic systems. 
     In an exemplary embodiment, the mask can be a glass plate that transmits ultraviolet, being coated with a chrome film with the desired lithographic patterns etched into the chrome in order to shadow out areas of the photoresist from ultraviolet exposure. The ultraviolet source is preferably a collimated beam covering the whole area where the pattern is desired to be created. The ultraviolet exposure can also be achieved by other means, such as direct writing of the patterns into photoresist by a laser, or other focused ultraviolet light source. Sources other than ultraviolet, such as X-rays, can also be used. The photoresist can be an epoxy-based material that can undergo chemical reaction upon exposure to ultraviolet light, initiating a crosslinking reaction that can be strengthened by a baking step. 
     In an exemplary embodiment of the invention, SU-8 can be used as the photoresist  120 . In the case of SU-8, a negative photoresist, the mask can allow ultraviolet rays, such as those in the 350-380 nanometer range, to penetrate where one intends photoresist  120  to be hardened by crosslinking the molecular chains of SU-8. In an alternative exemplary embodiment, this described process can be equally achieved with positive-type photoresists  120  as well. Other types of photoresists  120  can also be utilized. For example, photoresists based on chemistries including but not limited to polymethyl methacrylate (PMMA), polymethyl glutarimide (PMGI), polyhydroxystyrene-based polymers, phenol formaldehyde resins such as diazonaphthoquinone (DNQ) or Novolac, or epoxy-based resins such as the previously discussed SU-8. 
     Following the ultraviolet exposure step, the substrate  105  can be baked to complete the crosslinking reaction in the SU-8 that was exposed to ultraviolet light. The un-crosslinked, or un-hardened, SU-8 photoresist  120  can be removed, or developed off by a solvent. In an exemplary embodiment of the invention, the developing solvent can be Propylene Glycol Methyl Ether Acetate (PGMEA). Other developing solvents or methods can also be utilized, leaving behind a crosslinked, hardened SU-8 structure on the copper substrate  105 . 
     In panel  3  of  FIG. 1 , after the solvent and un-crosslinked photoresist are removed, the remaining hardened SU-8 and the form(s)  110 , combined, can form a 3-D mold  125 . The crosslinked SU-8 portion can hold the shape for an electromagnetic interaction circuit, and the form(s)  110  can hold the shape of the electron beam tunnel(s). 
     In panel  4  of  FIG. 1 , the substrate  105  can be chemo-electrically plated with copper to fill the volume around the 3-D mold structure comprised of the combined SU-8 and form(s)  110 . Thus, the combination of the SU-8 mold and form(s)  110  can enable a true 3-D implementation of a lithography, electroforming, and molding process (as described in relation to panels  2 ,  3 , and  4  of  FIG. 1 ), which is collectively known to one of ordinary skill in the art by the German acronym LIGA, with as few as a single ultraviolet exposure step for efficiency. The single exposure LIGA process incorporates the one or more forms over the substrate having been embedded in the photoresist. 
     One of ordinary skill in the art will understand that the described techniques are not limited to only copper substrates  105  and copper electroforming. For example, one of ordinary skill in the art could seek to apply it to any number of substrate materials including, but not limited to, aluminum, silicon, germanium, titanium, glass, nickel, silver, gold, or any combination thereof. 
     In an exemplary embodiment, the electroformed metal can be overplated beyond the extent of the SU-8. Overplating and then grinding or polishing down the excess metal to the desired thickness can ensure a flat surface free of voids or other defects caused by possible non-uniform plating. 
     In panel  5  of  FIG. 1 , the crosslinked, or hardened, SU-8 photoresist that remains can be removed by a chemical, thermal, chemo-thermal, or plasma method. Other methods can also be utilized to remove the hardened photoresist. 
     In panel  6 , the form(s)  110  can be removed. For example, the form(s)  110  can be removed by the same chemical, thermal, chemo-thermal, or plasma method utilized above. Additionally, the form(s)  110  can be removed by heating to high temperatures, by dissolution with other chemicals, by mechanical pulling or destruction, by laser ablation, or exposure to X-rays. Other methods can also be utilized. In an exemplary embodiment of the invention, molten salts have been found effective at removing both SU-8 and fiber forms made from Ethylene tetrafluoroethylene (ETFE) or Polyvinylidene fluoride (PVDF). 
     In an alternative exemplary embodiment, some fiber forms, such as ETFE, can be easily removed from the electroformed structure created in panel  4  prior to the removal of the hardened photoresist in panel  5 . For some materials, merely pulling the forms out with little force, such as by hand, is possible. 
     Finally, to complete the interaction circuit with an integrated beam tunnel, a top cover can be affixed by brazing, mechanical fixturing, diffusion bonding, or other means. One of ordinary skill in the art will understand that this technique can be applied to not only a single form, but also an array of forms constituting a multiple-beam distributed electron beam tunnel. 
       FIG. 2  is a top and side view of the methodology of the invention, in accordance with an alternative exemplary embodiment of the invention. Specifically,  FIG. 2  represents individual steps, illustrated as panels, which follows the same sequence of steps as described in  FIG. 1 . However, instead of fiber forms(s)  110 ,  FIG. 2  depicts a sheet-like form  210  with a rectangular cross section. The sheet-like form  210  can form a rectangular beam tunnel in the substrate  205  instead of a round fiber  110  that forms a round beam tunnel as represented in  FIG. 1 . One of ordinary skill in the art will understand that multiple rectangular beams formed by multiple sheet-like forms  210  are possible in this figure. 
       FIG. 3  illustrates potential cross sectional shapes for the forms and beam tunnels that can be fabricated from fibers and sheets in accordance with an exemplary embodiment of the invention. The potential cross sectional shapes can include a round beam, such as from a fiber, sheet (rectangular) beam, elliptical beam, dogbone beam tunnel to mitigate the effects of the diocotron instability on an electron beam, and multiple beams (i.e., distributed electron beam). Other shapes can also be fabricated. In addition to shapes, the size of the forms, and therefore, the size of the beam tunnels, may vary. One of ordinary skill in the art will recognize that determining the size, or thickness, of the fibers and sheets that are utilized can depend on ensuring that the material can pass the ultraviolet light used to activate the photoresist. 
       FIG. 4  illustrates a technique to position and secure forms  110  above a substrate in accordance with an exemplary embodiment of the invention. As noted previously, each of the one or more forms  110  can be positioned at a desired height and desired position above the substrate  105 . For example, the desired height can be set by placing the forms  110  over one or more height-setting rods  405 . Furthermore, the one or more height setting rods  405  can have notches, or slots, to secure and precisely position the forms  110  into place. 
     In an alternative embodiment, again one or more height-setting rods  405  can be utilized to set the desired height of the forms  110 . However, in this instance, the forms  110  can be secured and precisely positioned by placing them into one or more wire guides  410 . In an exemplary embodiment of the invention, the wire guides  410  can be formed from photoresist using a mask. Additionally, the forms  110  can be tensioned slightly by the wire guides  410  over which the forms  110  can be stretched in various ways to control tension. 
     In  FIG. 4 , the image  400  presents an example when the forms  110  have been positioned within +/−0.00016 ( 16/100,000) inches in height and +/−0.00008 ( 8/100,000) inches laterally with respect to each other. The precise positioning of the forms  110  subsequently allows precise multiple beam tunnels to be formed utilizing the steps described herein. 
     Other methods can be utilized to position and secure the forms  110  above the substrate  105 . In another example, the forms  110  can be positioned and secured by attaching them to one or more posts. 
     One of ordinary skill in the art will recognize that the system and methods have applicability to vacuum electron devices that require electron beams to be transported through hollow channels that pass through an electromagnetic slow-wave circuit. Specifically, these electron hollow channels, or beam tunnels, are shrinking toward sizes smaller than traditional techniques can manage as the operating frequencies push toward the THz. However, one of ordinary skill in the art will also recognize that other uses are relevant. For example, the system and method are relevant to microfluidic devices and other applications that require very small channels with aspect ratios of several hundred or more. The system and method are also relevant to creating waveguides, directional couplers, power splitters and combiners, filters, antennas, and other passive electromagnetic structures. 
     It should be understood that the foregoing relates only to illustrative embodiments of the present invention, and that numerous changes may be made therein without departing from the scope and spirit of the invention as defined by the following claims.