US20120125071A1

US20120125071A1 - Carbon molds for use in the fabrication of bulk metallic glass parts and molds

Info

Publication number: US20120125071A1
Application number: US13/260,537
Authority: US
Inventors: Jan Schroers; Golden Kumar; Marc Madou; Rodrigo Martinez-Duarte
Original assignee: Individual
Current assignee: Yale University; University of California
Priority date: 2009-03-27
Filing date: 2010-03-29
Publication date: 2012-05-24
Also published as: WO2010111701A1

Abstract

Novel molds and methods for Bulk Metallic Glass (BMG) molding using carbon templates obtained from pyrolyzed materials are provided. The method employs the Carbon MEMS (C-MEMS) technique to derive molds of different geometries and dimensions. The resultant carbon structures are stable at very high temperatures and have sufficient mechanical strength to be used as master molds for the thermoplastic forming of BMGs.

Description

FIELD OF THE INVENTION

The current invention is directed to master molds and methods of fabricating bulk metallic glass molds and parts using inexpensive carbon master molds.

BACKGROUND OF THE INVENTION

Bulk Metallic Glasses (BMGs) refer to a class of metal alloys, which exhibit high strength, large elastic strain limit, and high corrosion resistance owing to their amorphous nature. They are isotropic, homogeneous, and free from any crystalline defects down to atomic scales. These materials are therefore excellent candidates for small-scale applications such as micro- and nanostructures, MEMS (micro-electro-mechanical-systems), NEMS (nano-electro-mechanical-systems), precision surgery tools, watch movement components, and micro-nanomolds.
It has been shown that the BMGs can be thermoplastically formed like plastics. (See, e.g., J. Schroers, et al., Adv Materials & Proc, vol. 164, pp. 61-63, 2006, the disclosure of which is incorporated herein by reference.) Thermoplastic forming (TPF) takes place in the supercooled liquid region (SCLR) where the viscosity of BMG drops significantly allowing it to flow under small applied pressure. TPF of BMGs has been used for a wide range of applications including net-shape processing, extrusion, synthesis of amorphous metallic foams and blow molding. (See, e.g., Y. Saotome, et al, Intermetallics, vol. 10, pp. 11-12, 2002; J. Schroers, et al., APL, vol. 82, pp. 370-372, 2003; and J. Schroers, et al., Scripta Materialia, vol. 57, pp. 341-344, 2007, the disclosures of each of which are incorporated herein by reference.) TPF of BMGs has the potential to become an alternative to current metal forming techniques used in microforming such as electroplating. A first advantage of TPF of BMGs stems from its simplicity compared to electroplating. Secondly, electroplating of metals suffers from a rather limited menu for material selection, and often results in parts with non-uniform deposition at sharp edges and recessed areas, stresses that are very dependent on the electroplating current, and non-uniform mechanical properties imposed by the metal grain size. BMG molding can be carried out using complex alloys, which are intrinsically superior in strength, corrosion resistance and wear resistance compared to conventional electroplated metals. BMGs are also free of crystalline defects, which result in homogeneous and isotropic parts. Lastly, even when surface finish of molded BMG parts strongly depends on the surface finish of the mold used, their surface roughness can be reduced by re-heating above their glass transition temperature, an improvement not achievable with metal parts fabricated by electroplating. (See, e.g., G. Kumar, et al, APL vol. 92, pp. 031901-3, 2008, the disclosure of which is incorporated herein by reference.)
Despite the inherent advantages of TPF, significant hurdles remain in making it a commercially viable shaping technique. The main limiting factor in the TPF of BMGs is in the type of master molds required to guarantee stability under TPF temperatures (200-450° C.) and pressures (10-30 MPa). Unfortunately, under such conditions inexpensive polymer molds like the ones used for electroplating are not an option. Instead, the current standard is to use copper or silicon molds. While these molds are simple to manufacture when all that is desired is to obtain millimeter sized rods or basic geometric structure, forming micro and nano-sized structures using these copper molds rapidly increases the cost and complexity of fabricating the necessary master molds. Although LIGA (Lithography, Electroplating and Molding from the German Lithographie, Galvanoformung, Abformung) or silicon micromachining can be used to achieve desired mold structures, these techniques are extremely expensive. Additional drawbacks of the LIGA process are limited geometric complexity, inconsistent pattern transfer, and residual stresses in the metal product.
Accordingly, a need exists for improved fabrication techniques and molds that allow for the inexpensive and simple shaping of BMGs into parts and molds using TPF.

SUMMARY OF THE INVENTION

The current invention is directed to novel molds and methods for Bulk Metallic Glass (BMG) using carbon templates obtained from pyrolyzed polymeric materials.
In one embodiment, the method for molding the BMG includes patterning a master shape into a pyrolizable material, pyrolyzing the master shape into a carbon mold, thermoplastically forming the bulk-metallic glass material on the carbon mold to form a shaped bulk-metallic glass article. In such an embodiment, the polymeric material should have the ability to substantially maintain its shape during pyrolysis and the carbon mold should be capable of withstanding the temperatures and pressures necessary to shape the bulk-metallic glass under thermoplastic forming conditions. In one such an embodiment, the molding is performed on a milli, micro or nanoscale.
In another embodiment, the pyrolizable material is any material that can be pyrolized and can exhibit sufficient strength at molding temperatures such the BMG materials can be shaped. In one such embodiment, the pyrolizable material is a polymeric material selected from the group consisting of photoresists and organic polymers, and preferably from one of the following SU-8, poly(methyl methacrylate) (PMMA), phenolic resins, polyfurfuryl alcohols, cellulose, polyvinyl chloride and polyimides.
In still another embodiment, the material is polymeric and the patterning includes one of the following processes stamping, casting, machining, CNC machining, electrical discharge machining (EDM), electrochemical machining (ECM), wet bulk machining, milling, ion beam milling), lithography, photolithography, X-ray lithography, gray-scale lithography, electron beam lithography (EBL), nanoimprint lithography (NIL) and focused-ion beam (FIB).
In another such embodiment the pyrolizable material may include 2D and 3D patterned biomaterials.
In yet another embodiment, where the patterning is done by photolithography, the technique further includes inserting a filter between the photolithographic light source and the polymeric material to prevent T-topping in the polymeric master shape.
In still yet another embodiment, wherein the patterning is carried out by photolithography, the photolithographic pattern is preferably formed by a high-resolution chromium-on-quartz photomask patterned with an e-beam tool.
In still yet another embodiment the master shape comprises a free-standing structure. In one such embodiment, the master shape is formed on a substrate, and the substrate layer in contact with the master shape and the material of the master shape have good coefficient of thermal expansion matching. In another such embodiment, the substrate layer in contact with the master shape and the master shape are formed of the same material, and preferably the material is a polymeric material such as, for example, SU-8. In still another such embodiment, the substrate layer in contact with the master shape comprises a transparent polymeric film, and preferably the transparent polymeric film is selected from the group consisting of polyimide or polyester.
In still yet another embodiment, the master shape comprises undercuts and overlays. In one such embodiment, the master shape is patterned using a process selected from multi-layer photolithography and grayscale lithography.
In still yet another embodiment, the material is disposed on a substrate during patterning, and the substrate is made from a material selected from the group consisting of silicon, silicon dioxide, silicon nitride, glass, quartz, polyethylene terephthalate, polyimide and the polymeric material.
In still yet another embodiment, the pyrolizable material is patterned such that the walls of said master shape have a positive slope.
In still yet another embodiment, the separating process consists of wet immersion, plasma ion etching, reactive ion etching, isotropic etching, mechanical scraping, thermal heating, sonication, and a combination thereof.
In still yet another embodiment, the bulk-metallic glass is selected from the group consisting of Zr-based, Ti-based, Fe-base, Ni-based and Co-based alloys. In one such embodiment, the bulk-metallic glass has a supercooled liquid region (ΔTsc) of at least 30° C.
In still yet another embodiment, the thermoplastically forming includes one of net-shape processing, micro-replication, nano-replication, extrusion, and superplastic forming.
In still yet another embodiment, a further material the invention comprises shaping another material on the bulk-metallic glass article. In such an embodiment, the bulk-metallic glass article is a mold, and the further material is a polymer, metal or bulk-metallic glass having a molding temperature lower than that of the underlying bulk-metallic glass article.
In still yet another embodiment, the master structure may be only partially pyrolized and the unpyrolized material dissolved resulting in a porous carbon skeleton. In such an embodiment, the porous carbon skeleton can be infiltrated with the BMG material or can be filled by thermoplastic forming the BMG such that composite materials or BMG foams may be formed. In such an embodiment, the BMG need not be separated from the underlying carbon mold.
In still yet another embodiment, the carbon mold or BMG mold are used to form a new polymeric master shape and this new polymeric master shape is pyrolyzed such that the features of the polymeric master shape undergo isometric reduction in size. In one such embodiment, this process is repeated until the feature sizes of the polymeric master shape have the desired dimensions.
In still yet another embodiment, the critical dimensions of the features of the polymeric master shape are less than 100 nm.
The invention is also directed to a mold for thermoplastically forming a bulk-metallic glass. In one such embodiment, the mold comprises a carbonized master shape formed of a material capable of withstanding the temperatures and pressures necessary to shape the bulk-metallic glass under thermoplastic forming conditions.
In another embodiment, the carbonized master shape is formed from a glass-like-carbon, and the glass-like-carbon is formed by carbonizing a polymeric material selected from the group of photoresists and organic polymers.
In still another embodiment, the master shape comprises undercuts and overlays.
In yet another embodiment, the walls of the master shape have a positive slope.
In still yet another embodiment, the critical dimensions of the features of the carbonized polymeric master shape are less than 100 nm.

BRIEF DESCRIPTION OF THE FIGURES

Various examples of the present invention will be discussed with reference to the appended drawings and photograph reproductions. These figures depict only illustrative examples of the invention and are not to be considered limiting of its scope.

FIG. 1 provides a flowchart schematic of exemplary master mold fabrication and BMG molding techniques in accordance with the current invention.

FIG. 2 provides an image of a detail of a BMG part after plasma and sonication cleaning treatment.

FIG. 3 provides a schematic of a process for patterning a polymeric material in accordance with one embodiment of the current invention.

FIGS. 4A and 4B provide SEM reproductions comparing the wall roughness between the use of transparency (A) and e-beam patterned photomasks (B) in accordance with the current invention.

FIG. 5 provides a schematic of a process for patterning a polymeric material in accordance with another embodiment of the current invention incorporating an optical pathway between the light source and the polymeric film.

FIGS. 6A and 6B provide SEM reproductions of the T-topping effect (A) and its elimination by the use of a filler (B) in accordance with the current invention.

FIGS. 7A and 7B provide SEM reproductions of negative (A) and positive (B) slope walls on molds fabricated in accordance with the current invention.

FIG. 8 provides a series of schematics showing methods of obtaining different wall slopes using positive (A & B) and negative photoresists (C & D) in accordance with the current invention.

FIG. 9 provides schematics showing methods of fabricating holding substrates using a two-layer process in accordance with the current invention.

FIG. 10 provides schematics showing methods of fabricating free-standing structures using a grayscale lithography process in accordance with the current invention.

FIGS. 11A to 11J provide SEM reproductions of an exemplary fabrication sequence in accordance with the current invention, where A and B show SU-8 molds with holes of 19 (A) and 38 um (B) and gaps of 17 (A) and 7 um (B) fabricated with photolithography on a Si substrate, C and D show carbon molds with holes of 33 (C) and 46 (D) um and gaps of 7 (C) and 2.5 (D) um, obtained by pyrolysis of SU-8 conducted at 900° C. in N₂, E and F show BMG parts after release from the carbon mold, G and H show BMG parts after a 15 min oxygen plasma process with an etch rate of approximately 1 um/min, and I and J show final. BMG parts after a 5 min acetone bath with sonication.

FIGS. 12A to 12C provide SEM reproductions of BMG parts formed using carbon molds in accordance with an exemplary embodiment of the current invention, where A is the carbon mold obtained from SU-8, and B and C show BMG parts formed in the mold of A by TPF using a Pt-based (B) and Zr-based (C) BMG (the dashed squared focus on the reproduction of fine features of the carbon molds on the BMG part).

FIGS. 13A and 13B provide SEM reproductions of BMG parts formed using a carbon mold in accordance with an exemplary embodiment of the current invention, where (A) is the carbon mold obtained from SU-8, and (B) shows a BMG part formed in the mold of A by TPF using a Zr-based BMG (the dashed square focuses on the reproduction of fine features of the carbon molds on the BMG part).

FIGS. 14A to 14D provide SEM reproductions of original polymer molds and the final pyrolized carbon mold in accordance with the current invention using; (A) SU-8, (B) Kapton®, (C) Cirlex® and (D) Silicon as the substrate material.

FIGS. 15 a and 15 b provide images of SU-8 structures fabricated using gray-scale lithography in accordance with an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The current invention is directed to novel molds and methods for Bulk Metallic Glass (BMG) molding using carbon templates obtained from pyrolyzed materials that can be used in the shaping of BMGs by Thermoplastic Forming (TPF). The invention employs a Carbon MEMS (C-MEMS) technique to derive molds of different geometries and dimensions. It has been discovered that the resultant carbon structures are stable at very high temperatures and have sufficient mechanical strength to be used as master molds for the thermoplastic forming of BMGs. (For additional details see, Martinez Duarte, R., Thesis: Fabrication of Carbon Micro Molds, University of California, Irvine (2009), the disclosure of which is incorporated herein by reference.)

DEFINITIONS

Bulk Metallic Glasses (BMGs): For the purposes of this invention refer to a class of metal alloys which exhibit high strength, large elastic strain limit, and high corrosion resistance owing to their amorphous nature. They are isotropic, homogeneous, and free from any crystalline defects down to atomic scales. (Exemplary BMGs may be found in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, the disclosure of each of which are incorporated herein by reference.)
Thermoplastic Forming (TPF): For the purposes of this invention, is a shaping process that takes place in the supercooled liquid region (SCLR) of a BMG alloy where the viscosity of BMG drops significantly allowing it to flow under small applied pressure.
Supercooled Liquid Region (SCLR) or ΔTsc: For the purposes of this invention, is the difference of T_x(the onset of crystallization) and T_g(the onset of glass transition) as determined from standard DSC scans at 20° C./min.
Carbon-MicroElectroMechanical Systems (C-MEMS) and Carbon-NanoElectroMechnical Systems (C-NEMS): For the purposes of this invention, is a process for fabricating high strength carbon structures by pyrolyzing a precursor, such as, for example, a polymeric material. The precursor may be a naturally occurring pattern, or may be a patternable material, such as, for example, a polymer or photoresist. In such an embodiment, the polymeric material may be patterned using any known technique, for example using a combination of ultra violet (UV)/electron beam (EB) lithography.

DESCRIPTION

It is well-known that select BMGs can be thermoplastically formed like plastics. (See, e.g., J. Schroers, & N. Paton, Advanced Materials & Processes 164:61-63 (2006); J. Schroers & W. L. Johnson, Applied Physics Letters 84:3666-3668 (2004); J. Schroers, et al., Applied Physics Letters 87:61912 (2005); and B. Zhang, et al., Physical Review Letters 94 (2005), the disclosures of each of which are incorporated herein by reference.) Indeed, TPF of BMGs has been used for a wide range of applications including net-shape processing, extrusion, synthesis of amorphous metallic foams, superplastic forming of sheet material, synthesis of BMG composites and blow molding. Thermoplastic forming takes place in the supercooled liquid region (SCLR) where the viscosity of BMG drops significantly allowing it to flow under small applied pressure. (For a full discussion see, e.g., Waniuk, T., et al., Physical Review B 67(18):184203 (2003) and Schroers, J., JOM—Journal of Metals 57:35-39 (2005), the disclosures of each of which are incorporated herein by reference.) The BMG components produced by TPF are free of thermal stresses with excellent surface finish because of low processing temperatures and minimal shrinkage. It has also been shown that the surface of BMG parts can be smoothened by heat treatment in the SCLR. Thus, BMG parts and molds produced by TPF have superior mechanical, chemical, and surface properties compared to the parts and molds made by other processes. Therefore, TPF of BMGs offers an economical technology to fabricate BMG parts and mold-inserts with excellent surface and mechanical properties.
The main limiting factor in the use of TPF to form BMG parts and mold-inserts is that an expensive master mold must be fabricated. The master mold must be stable under TPF temperatures and pressures, which means that polymer molds are not practical. Typically, costly Si molds are created by lithography. Another alternative is to use LIGA to form polymer molds and then electroplate these molds with a metal capable of withstanding TPF conditions. However, conventional. LIGA requires the use of a synchrotron radiation source, which makes it impractical for commercial use. Moreover, while the advent of UV-sensitive polymer photoresists has extended the LIGA process to UV lithography, replacing expensive X-ray lithography, LIGA and its lower cost derivatives all still require the use of electroplating, which limits the material selection. Additional drawbacks of the LIGA process are limited geometric complexity, inconsistent pattern transfer, and residual stresses in the metal product.
It has recently been discovered that structures can be converted into amorphous carbon by heat treatment in an inert atmosphere, a process known as pyrolysis. (See, e.g., C. Wang, et al., J. Microelectromech. Syst., vol. 14, pp. 348-353, 2005, the disclosure of which is incorporated herein by reference.) It has now been discovered that the resultant carbon structures are stable at temperatures exceeding the processing temperatures for TPF of BMGs, and have sufficient mechanical strength to be used as master molds for TPF of BMGs. Furthermore, since polymeric materials may be used as precursors, a wide range of geometries and dimensions can be implemented through many types of fabrication processes, ranging from photolithography (UV, e-beam) to CNC machining. Moreover, carbon MEMS (C-MEMS) and NEMS (C-NEMS) structures with feature sizes ranging from millimeters down to few hundred nanometers can be synthesized by available fabrication techniques followed by pyrolysis. The current invention describes novel master molds and novel methods for Bulk Metallic Glass (BMG) molding using carbon templates obtained from pyrolyzed materials, and preferably from polymeric materials. The following description details exemplary embodiments of the molds and molding methods of the current invention.
FIG. 1. provides a schematic of an exemplary embodiment of the method for the mass production of high precision, high surface finish parts and molds utilizing TPF of BMG materials in accordance with the current invention. As shown, in a first step, a base material is formed into a desired master shape. In a second step, this master shape is converted into a carbon structure by pyrolysis. The carbon pattern is then transferred to a BMG by thermoplastic forming process over the carbon mold in a third step. The three dimensional. BMG microparts can then be separated from the mold by a suitable process if necessary, such as, for example, a scraping process in the supercooled liquid region of the BMG alloy. Alternatively, if a porous carbonized mold is formed, such a structure can be used to make carbon-BMG composites or BMG foams by infiltration or thermoplastic by means known in the art.
Moreover, while the process of the current invention offers an alternate viable technology for precision net-shaping, micromolding, and fabrication of high-aspect ratio BMG structures compared to the expensive LIGA process, which requires a synchrotron source and can only be used for metals that can be electroplated, as shown in step four of FIG. 1, in other optional embodiments of the invention, after release from the carbon mold, the BMG structures can also be used as precise molds or mold inserts to do further material processing. For example, in one embodiment, the BMG structures may themselves be used as a hot embossing template for mass production of polymer parts, or even as a mold for other BMGs with different softening behaviors, i.e., with TPF molding temperatures lower than those required to shape the underlying BMG structure. Alternatively, the BMG molds may be crystallized and used as a mold for the same BMG material. The good mechanical properties of BMGs make them excellent material for mold-inserts. As a result, with BMG mold inserts made in accordance with the current invention, innumerable plastic or BMG copies can be mass-produced with high precision and a relatively low cost using injection molding or hot-embossing technique.

Step 1: Shaping the Master

Turning to the specifics of the technique itself, step one of the process in accordance with the current invention requires that a material be formed into a desired master shape. With regard to this step, while the specific embodiments discussed in this disclosure use a polymeric material, and more particularly, an SU-8 polymer that is shaped by a photolithographic technique, it should be understood that any material or naturally occurring shape and shaping process suitable for use in conjunction with the pyrolysis necessary to transform the master pattern into a carbon mold may be used with the current invention. For example, any material that does not melt during the carbonization pyrolysis process, thus allowing the derivation of patterned or shaped carbon molds, may be used with the current invention, including polymeric materials, wood, clays, metals, etc. However, in a preferred embodiment, the current invention uses a polymeric material or photoresist such as, for example, SU-8 or poly(methyl methacrylate) (PMMA), or organic polymers, such as, for example, phenolic resins, polyfurfuryl alcohols, cellulose, polyvinyl chloride and polyimides. In addition, any technique may be used to imprint the desire initial shape onto these materials, including, but not limited to, stamping, casting, machining (such as, CNC machining, electrical discharge machining (EDM), electrochemical machining (ECM), and wet bulk machining), milling (such as focused ion beam milling), lithography based techniques (such as photolithography or X-ray lithography), and Next-Generation Lithography (NGL) techniques (such as electron beam lithography (EBL), nanoimprint lithography (NIL) and focused-ion beam (FIB)). (For an in-depth discussion of the relative merits of these techniques see, e.g., Fleischer J. and Kotschenreuther J. Int J Adv Manuf Technol 33:75-85 (2007); Giboz J., Copponnex T. and Mélé P., J Micromech Microeng 17:R96-R109 (2007); Heckele M. and Schomburg W. K. J Micromech Microeng 14:R1-R14 (2004); Piotter V., et al., Microsystems Technologies 10:547-551 (2004); Bilenberg B., et al., Microelectronic Engineering 83:1609-1612 (2006); Robinson A. P. G., et al., Microelectronic Engineering 83:1115-1118 (2006); Gierak J., et al., Microelectronic Engineering 84:779-783 (2007); Volkert C. A. and Minor A. M., MRS Bulletin 32:389-399 (2007); Madou M. J., Manufacturing Techniques for Microfabrication and Nanotechnology, CRC Press, Boca Raton, Fla. (2009); Gamero-Castaño M., J Fluid Mech 604:339-368 (2008); Gamero-Castaño M., Phys Fluids 20:032103 (2008); Chou S. Y., et al., Science 272:85-87 (1996); Chou S. Y., et al., Appl Phys Lett 67:3114-3116 (1995); and Chou S. Y., et al., J Vac Sci Technol B 15:2897-2904 (1997), the disclosures of each of which are incorporated herein by reference.)
The choice of technique and material is solely dictated by the cost, quality, complexity and final dimensions of the desired carbon mold, and thus the desired BMG part/mold. For example, the cost of obtaining carbon molds for BMGs can be controlled by using inexpensive polymers and patterning techniques such as casting, embossing or even CNC machining. Alternatively, molds with nanometer dimensions could be implemented using NGL techniques, such as, for example, EBL, FIB and NIL. In addition. (See, e.g., Kumar, G., et al., Nature 7231:868-872 (2009), the disclosure of which is incorporated herein by reference.) Different techniques could be explored depending on the desired dimension range.

Step 2: Conversion of the Master to a Carbon Mold

Turning to the step of converting the master shape into a carbon mold, in accordance with the current invention, this conversion, or carbonization, is the process by which solid residues with a high content of carbon are removed from the underlying patterned material, usually by pyrolysis, in an inert atmosphere. (For a full discussion, see, e.g., Fitzer E., et al., Pure & Appl Chem 67:473-506 (1995), the disclosure of which is incorporated herein by reference.) As with all pyrolytic reactions, carbonization is a complex process with many reactions taking place concurrently, including dehydrogenation, condensation, hydrogen transfer and isomerization. (See, e.g., Lee L., J of Polymer Sci: Part A 3:859-882 (1965); Fitzer E. and Schafer W., Carbon 8:353-364 (1970); Gac N. A., et al., J Poly Sci A 8:593-608 (1970); Jenkins G. M., et al., Proc Roy Soc Lon A 327:501-517 (1972); Lyons A. M., et al., Thin Solid Films 103:333-341 (1983); Nakagawa H. and Tsuge S. J Anal Appl Pyrolysis 12:97-113 (1987); Chen K. S. and Yeh R. Z. J Hazard Mat 49:105-113 (1996); Ma C. M., et al., J Comp Mat 38:311-322 (2004), the disclosures of each of which are incorporated herein by reference.) The pyrolysis process of photoresists, and organic compounds in general, can be divided into three major steps: pre-carbonization, carbonization and annealing. The first step of pyrolysis is pre-carbonization. During pre-carbonization (T<573 K) molecules of solvent and unreacted monomer are eliminated from the polymer matrix. The carbonization step can be further divided into two stages. From 573 to 773 K, heteroatoms such as oxygen and halogens are eliminated causing a rapid loss of weight, but a minimal volume shrinkage, while a network of conjugated carbon systems is formed (i.e. carbon ribbons are formed). Hydrogen atoms start being eliminated towards the end of this stage. The second stage of carbonization, from 773 to 1473 K, completely eliminates hydrogen causing the carbon ribbons to move together. The crumbling of this carbon network causes a significant loss of volume but a minimal change in weight. At this point, permeability decreases and density, hardness, Young's modulus and electrical conductivity increase. The final step, annealing, is carried out at temperatures above 1473 K, to allow the gradual elimination of any structural defects and evolve further impurities. The final pyrolysis temperature determines the degree of carbonization and the residual content of foreign elements. For instance, at T˜1200 K the carbon content of the residue exceeds a mass fraction of 90% in weight, whereas at T˜1600 K more than 99% carbon is found. Accordingly, by modifying the heating rates and timing of the pyrolysis process, it is possible to tailor, to a degree, the amount of carbonization. (See, e.g., Nishikawa K., et al. Jpn J Appl Phys 37:6486-6491 (1998); Nakagawa H. and Tsuge S. J Anal Appl Pyrolysis 12:97-113 (1987); and Mehrotra B. N., et al., J Mat Sci 18:2671-2678 (1983), the disclosures of each of which are incorporated herein by reference.) In this way, fully or partially carbonized molds may be formed in accordance with the desired BMG shaping process. Although specific embodiments of a pyrolysis process are described in detail in the exemplary embodiments below, it should be understood that the only requirement for the pyrolysis step in accordance with the current invention is that the master substantially retain its shape, and that the material of the master be at least partially converted into a carbonized material capable of withstanding the processing conditions of the TPF shaping process.
A typical carbon material obtained through pyrolysis of polymeric materials is glass-like carbon. The properties of this material make it an ideal candidate for mold fabrication. It is impermeable to gases and extremely inert, with a remarkable resistance to chemical attack from strong acids such as nitric, sulfuric, hydrofluoric or chromic and other corrosive agents such as bromine. Moreover, its rates of oxidation in oxygen, carbon dioxide or water vapor are lower than those of any other carbon. Glass-like carbon has a hardness of 6 to 7 on the Mohs' scale, a value comparable to that of quartz. Its density ranges from 1.4 to 1.5 g/cm⁻³, and features a coefficient of thermal expansion of 2.2-3.2×10-6/K, which is comparable to that of Si and some BMGs. Its Young Modulus varies between 10 and 40 GPa. Because its thermal conductivity is only 7 W/mK it is considered thermally inert (compare to 401 W/mK of copper and 1 W/mK of Pyrex® glass). (See, e.g., Hull R. Properties of Crystalline Silicon. The Institution of Engineering and Technology, London (1999); Lide D. R., Handbook of Chemistry and Physics. CRC Press, Boca Raton, Fla. (1992); Yamada S. and Sato H., Nature 193:261-262 (1962); Lewis J. C., et al., Sol State Electron 6:251-254 (1963); Cowlard F. C. and Lewis J. C., J Mat Sci 2:507-512 (1967); Cahn W. and Harris B., Nature 221:132-141 (1969); and Halpin M. K. and Jenkins G. M. Proc Roy Soc Lond A 313:421-431 (1969), the disclosure of each of which are incorporated herein by reference.)
One additional consequential advantage of using the method of the current invention to form the BMG parts and molds relates to the natural shrinkage that occurs during pyrolysis. It is well-known that during pyrolysis all materials undergo some shrinkage. Different degrees of shrinkage and carbon yield (the ratio of the weight of carbon to the weight of the original material) are obtained during carbonization depending on the precursor used. In the case of photoresists, for example, volume shrinkage varies from 50 to 90%. Although this shrinkage can cause deformation of the desired shape, well-known methods exist to calculate and compensate for such shrinkage to form a correctly dimensioned final part. Ranganathan S., et al., J Electrochem Soc 147:277-282 (2000); Singh A., et al., J Electrochem Soc 149:E78-E83 (2002); Wang C., et al., J of MEMS 14:348-358 (2005); and Malladi K., et al., Carbon 44:2602-2607 (2006), the disclosures of each of which are incorporated herein by reference.) Moreover, by controlling the pyrolysis process it is possible to ensure that this shrinkage is isometric. Accordingly, it is possible to exploit this by-product of the pyrolysis process to obtain mold structures with the critical minimum dimensions far below the limits currently achievable using conventional patterning methods. For example, in one embodiment of the invention, one of the shaping techniques discussed above is used to pattern a polymer. This patterned polymer structure is carbonized thereafter resulting in a carbon structure with features having a smaller critical dimension than its polymer precursor and possessing mechanical properties that would allow for its use as a mold/stamp to produce other polymer master shapes. This phenomenon could then be exploited in a repeat fashion to obtain molds having features with ever-smaller critical dimensions.

Step 3: Shaping the BMG

As discussed in reference to FIG. 1, above, once the carbon mold is formed, it is used to shape a BMG to fabricate a BMG part, foam, composite, or mold-insert. It should be understood in this step that any suitable forming process may be used with the carbon mold of the current invention. In one preferred embodiment, the forming process used is a TPF forming process. The ability to plastically form BMGs in their supercooled liquid region was recognized in the early days of metallic glass research and various terminologies are used, including superplastic forming, thermoplastic forming and hot-forming. (See, e.g., H. J. Leamy, et al., Metallurgical Transactions 3:699 (1972); C. A. Pampillo & H. S. Chen, Materials Science and Engineering 13:181-188 (1974); Patterson and Jones, Materials Research Bulletins, 13:583 (1978), the disclosures of which are incorporated herein by reference.) In turn, the ability to perform TPF on the carbon molds of the instant invention leads to a wide range of potential applications, including net-shape processing, micro- and nano-replication, extrusion, synthesis of amorphous metallic foams, superplastic forming of sheet material, and synthesis of BMG composites. (See, e.g., N. Nishiyama & A. Inoue, Materials Transactions Jim 40:64-71 (1999); Y. Saotome, et al., Scripta Materialia 44:1541-1545 (2001); Y. Saotome, et al., Journal of Materials Processing Technology 113:64-69 (2001); J. Schroers, et al., J. Mems 16:240 (2007); Y. Kawamura, et al., Applied Physics Letters 67:2008-2010 (1995); D. J. Sordelet, et al., Journal of Materials Research 17:186-198 (2002); I. Karaman, et al., Metallurgical and Materials Transactions A—Physical Metallurgy and Materials Science 35A:247-256 (2004); J. Schroers, et al., Journal of Applied Physics 96:7723-7730 (2004); T. Zhang, et al., Science Reports of the Research Institutes Tohoku University Series A—Physics Chemistry and Metallurgy 36:261-271 (1992); W. J. Kim, et al., Materials Science and Engineering A—Structural Materials Properties Microstructure and Processing 428:205-210 (2006); H. Soejima, et al., Journal of Metastable and Nanocrystalline Materials 24:531 (2005); J. Schroers, et al., Scripta Materialia 56:177-180 (2007); and A. A. Kundig, et al., Scripta Materialia 56:289-292 (2007), the disclosure of which are incorporated herein by reference.) The only limitation on the TPF forming process used and the application is that it must be compatible with the material properties of the underlying carbon mold. (Examples of BMG parts formed in accordance with the current invention are described in detail in the section of this disclosure entitled “Exemplary Embodiments”, below.)
An alternative to TPF forming would be infiltration of a porous carbon mold. Such a process would allow for the production of foams and composite materials. In such an embodiment of the invention, the carbon mold would be only partially pyrolized. The non-carbonized material could be dissolved or etched away, which would, in turn, lead to a porous carbon structure. Then a molten BMG could be infiltrated into the porous structure under pressure leading to BMG foams or carbon/BMG composites. In such an embodiment, the BMG material would not need to be separated from the underlying partially pyrolized carbon mold.
In turn, any suitable BMG material may be used with the carbon molds and molding methods of the instant invention so long as the material is capable of showing a glass transition in a Differential. Scanning calorimetry (DSC) scan at a forming temperature and under forming conditions compatible with the carbon molds described above. U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; 5,032,196; and 5,735,975, and WIPO Publication No. WO 2004/059019 (each of which are incorporated by reference herein) disclose families of BMGs having members with properties sufficient for use with the current invention, such as, for example, Zr-based, Ti-based, Mg-based, and Cu-based alloys. Another set of suitable bulk-solidifying amorphous alloys are compositions based on ferrous metals (Fe, Ni, Co). Examples of such compositions are disclosed in U.S. Pat. No. 6,325,868, (A. Inoue et. al., Appl. Phys. Lett., Volume 71, p 464 (1997)), (Shen et. al., Mater. Trans., JIM, Volume 42, p 2136 (2001)), and Japanese patent application 2000126277 (Publ. #0.2001303218 A), incorporated herein by reference. Finally, it is also possible to use bulk amorphous alloys comprising beneficial in-situ crystalline precipitates. One exemplary case is disclosed in (C. C. Hays et. al., Physical. Review Letters, Vol. 84, p 2901, 2000), which is incorporated herein by reference.
However, to fully practice the current invention the BMG materials used should be restricted to those highly processable BMGs with low viscosities and large supercooled liquid regions. There are a number of different methods of measuring the processability of a BMG. One method uses the size of the supercooled liquid region as a proxy for processablity. Under this measurement the feedstock of the BMG has a ΔTsc (supercooled liquid region) of more than about 30° C. as determined by DSC measurements at 20° C./min, and preferably a ΔTsc of more than about 60° C., and still most preferably a ΔTsc of about 90° C. or more. Another indirect measure of processability is the critical cooling rate of the material, namely, the rate at which the BMG material must be cooled to maintain its amorphous character. In this invention it is preferred that the BMG have a critical cooling rate of less than about 100° C./sec. Yet another method is the formability characterization method, where the final diameter of a BMG of 0.1 cm³, when formed between two parallel platens under a load of 1000 lb when heated through the supercooled liquid region, has a final diameter d>7, more preferable d>10 and most preferable d>12. (See, J. Schroers, Acta Materialia 56, p. 471 (2008), the disclosure of which is incorporated herein by reference.) Yet another way to select such materials is by reference to the viscosity of the material during processing. Preferably the BMG materials used in accordance with the current invention have viscosities below 10⁸Pascal·s, more preferably below 10⁷Pascal·s, and even more preferably below 10⁶Pascal·s. Finally, combining these parameters it is possible to provide a preferred formulation of BMG materials for use with the current invention. In accordance with the current invention, preferred materials are those BMGs having a viscosity when heated to within the supercooled liquid temperature region such that a flow stress of less than about 3 MPa may be used to achieve overall lateral strains of at least 100% prior to crystallization.
Step 4: Release of the BMG Part/Mold from the Carbon Mold
In non-composite embodiments of the invention, once the carbon mold has been used to shape a BMG part or mold-insert via TPF, the BMG part is released from the mold and the carbon mold either reused or discarded. Any suitable method for demolding of the BMG parts may be used with the instant invention, such as, for example, sacrificing the mold by wet immersion, plasma or reactive ion etching, isotropic etching, mechanical scraping, thermal heating and sonication, or a combination thereof. For example, in one preferred embodiment, the bulk removal of carbon residues from formed BMG parts was implemented with a combination of Inductive Conductive Plasma/Reactive Ion Etching, (ICP/RIE), followed by a sonicated bath in acetone. An SEM image of the detail of the surface of a BMG part cleaned in accordance with this method is provided in FIG. 2. As shown, the final. BMG part features only very small amounts of carbon residue on its walls. Although not to be bound by theory, it is believed such residues are mechanically interlocked and might be a direct consequence of the roughness of the walls of the part. Should surfaces having even lower concentrations of residue be desired, additional techniques can be used to facilitate the release of the BMG parts from the carbon mold, including, for example, the addition of a low-adhesiveness coatings such as polyester or PTFE to the carbon molds in order to facilitate the release of BMG parts after molding, the use of different polymer shaping processes or the use of positive slope mold features (an optional embodiment that will be discussed in greater detail below).

Alternative Embodiments

Although the above description describes the formation of the original master pattern and its carbonization in broad terms, it should be understood that there are a number of techniques that can improve the quality of the master pattern mold or allow for greater complexity in the patterns formed.
Photolithographic Masks
For example, when using photolithography to pattern polymeric materials, the simplest setup, shown schematically in FIG. 3, consists of a UV lamp illuminating the resist-coated substrate through a mask without any lenses between the two. The purpose of the illumination systems is to deliver light with the proper intensity, directionality, spectral characteristics, and uniformity across the substrate, allowing a nearly perfect transfer or printing of the mask image onto the resist in the form of a latent image. The incident light intensity (in W/cm2) multiplied by the exposure time (in seconds) gives the incident energy (J/cm2) or dose across the surface of the resist film. As is well known, UV radiation induces a chemical reaction in the exposed areas of the polymeric material generating acids that initiate the cross-linking of the polymer. In such an embodiment, the first major step towards improving surface roughness of the mold walls is to use high quality masks. It is usually the case in research, especially when under economic restraints, to employ transparency masks for photolithography. These kinds of masks are plastic sheets that have been patterned (printed) with a high-resolution plotter and can be used directly to pattern photoresists. Even when they are affordable, the quality and resolution that can be achieved with transparency masks are highly dependent on the specifications of the plotter. Typical maximum resolution of commercially available plotter is currently 7 μm. Higher quality masks are fabricated by patterning a chromium film that has been evaporated on low UV-absorption quartz plates. For ultimate quality, patterning is done with an electron beam tool. As expected, the obtained results are significantly better than those achieved with transparency masks (see, e.g., FIG. 4, which compares patterns formed from transparency (A) and photomasks (B)). As shown, the use of chromium-on-quartz photomasks patterned with e-beam yields walls with minimal roughness.
T-Topping
Another phenomenon that can effect the quality of the molds is T-topping, which is an exaggerated negative slope at the top of the resist structure surface is (see, e.g., del. Campo A. and Greiner C., J Micromech Microeng 17:R81-R95 (2007), the disclosure of which is incorporated herein by reference), which can negatively impact photoresists such as SU-8, especially when deriving high aspect ratio structures. The reason behind T-topping is the fact that photoresists strongly absorb certain light wavelengths. For example, SU-8 strongly absorbs light that has a wavelength of less than 350 nm. If using a broadband light source for exposure, as it is usually the case, UV light shorter than 350 nm is strongly absorbed near the surface creating locally more acid that diffuses sideways along the top surface. Selective filtration of the light source can be used to eliminate these undesirable wavelengths. An easy and affordable way to accomplish this is to place a filler in between the light source and the mask, as shown schematically in FIG. 5. Accordingly, in one embodiment of the invention, suitable fillers may be used in the polymeric patterning, such as, for example, by using a 50-100 μm layer of SU-8 or commercial high pass fillers with cut-out wavelength of 360 nm are also available. As shown in FIG. 6, by using a 50 μm SU-8 layer on a quartz plate as a filler, T-topping (shown in FIG. 6A) was immediately eliminated (FIG. 6B). It will be understood that the selection of a filler will depend on the type of photoresist or polymeric material being used, and that when using such a filter it is necessary to take the attenuation factor of the filter into account and adjust the exposure dose accordingly.
Positive Slope Molds
Another improvement to the process is obtained by constraining the wall slope of the patterned molds to greater than 90°, preferably (90-95°) to facilitate demolding. The use of a mold featuring walls with positive slope (see FIG. 7A) facilitates the clean release of the part from the mold. (See, e.g., Yu L., et al., Polymer Engineering and Science 42:871-888 (2002), the disclosure of which is incorporated herein by reference.) This is in contrast to the use of walls with negative slopes, as is the case of the results just shown (FIG. 7B), that have been proven to mechanically lock the formed part in the mold. The use of these molds featuring positive slopes on its walls together with the inertness of glass-like carbon and its resistance to be wetted by most materials are expected to enable the clean release of the BMG part.
A number of methods are available to ensure positive slope walls on the final part. The selection of the technique depends on the type of photoresist being used. In the case of positive photoresists exposed from a top light source, diffraction effects generated at the mask-film interface cause a negative slope (<90°) on the mold wall that prevents a clean demolding step (see, FIG. 8A). However, if exposure is done from the back, by shining light through a transparent substrate, the wall slope reverses becoming positive (>90°) (FIG. 8B). Diffraction effects occur in both cases but they are exploited to the benefit of the mold when employing back-exposure. The application of the same diffraction principle discussed in the case of a positive photoresist would dictate walls with positive slopes when using top-exposure and negative slopes with back-exposure. (See, e.g., FIGS. 8C and 8D.) As shown in these schematics, the result is a negative wall slope that resembles the one obtained by taking diffraction effects into account with positive photoresists (FIG. 8A). These diffusion effects are expected to be responsible for the negative slopes commonly seen in SU-8 walls (see, e.g., FIG. 6A). Accordingly, by implementing back-exposure through a transparent substrate (FIG. 8D), positive wall slopes on the final. SU-8 mold can be obtained.
Free-Standing Molds
Another important refinement of the process includes the fabrication of carbon molds obtained from free-standing single polymeric material structures. Such development, besides reducing the process cost, eliminates stresses at the interface of different materials thus greatly improving carbon mold fidelity to the original designs.
In particular, traditional photolithography is usually conducted on rigid substrates such as silicon, quartz and glass. When using the system of the instant invention, it is important to choose a substrate that has a coefficient of thermal expansion (CTE) similar to that of the polymeric material, otherwise the patterns are formed with built in stresses that might cause film cracking. In some cases finding the right material match can be difficult. For example rigid substrates like silicon, quartz and glass have CTEs on the order of (<10 10⁻⁶/K), which is significantly different from that of an exemplary polymeric material such as SU-8 (50-52 10⁻⁶/K). Moreover, the importance of good CTE matching is more profound in the instant invention where it is necessary that the substrate material must either get carbonized together with the patterned polymer or get separated from the mold before carbonization, for instance by peeling the polymeric patterns from the substrate. Unfortunately, silicon, glass or quartz do not carbonize under a pyrolysis process and provide good adhesion to polymeric photoresists, such as SU-8. These facts, together with the fact that the substrate is rigid, make it quite challenging to remove the polymeric pattern in an easy, reliable way. Accordingly, even when a back-exposure procedure can be easily implemented with any transparent substrate, glass or quartz for instance, the use of transparent films, such as PET, provides a flexible substrate which enables the clean release of the polymeric mold from the film at the end of the polymer patterning process and before pyrolysis; a challenging fact when using glass or quartz. If the mold design features structures within structures, a mold for a nut or gear for example, and the goal is to obtain a free-standing polymeric part, a holding substrate must then be fabricated.
Several techniques may be implemented to achieve this. The simplest one is to fabricate the desired polymeric structures directly on polyimide or polyester films. The preferred alternative is to first pattern holding substrates, for example of 1 cm²squares, on a thick polymer material layer and then fabricate the desired polymeric structures from the same polymeric material, using top or back-exposure, on top of the squares (See, e.g., FIG. 9). After the process, the PET film that held both polymer layers, substrate and features, is peeled off yielding free-standing all-polymer molds featuring structures within bigger structures. Again, using an approach in which the same polymer material is used for both the substrate and structure material is preferred over the use of polyimide or polyester as holding substrates for smaller structures within larger ones. The main reason for this is that although both PI and PET carbonize, they do not share the same CTE with SU-8 and thus thermal expansion stresses are introduced in the carbonization process (see, e.g., Example 4). A part fabricated completely out of one polymer material, such as, for example, SU-8 does not suffer from these stresses and creates carbon molds with better fidelity to the intended design.
Another advantage of the two-layer polymeric process described above (FIG. 9) is that it also allows for the creation of complex molds featuring undercuts and overhangs and can be expanded to n-layers. (For examples, of the complex parts possible using such a technique, see Example 5.) This multi-layer process requires the individual processing of n-layers and the precise alignment of different masks, which might vary with layer, during exposure. Although it is possible to use a multi-layer photolithographic technique to create such complex parts, the approach can be tedious and lengthy especially if high aspect ratio structures are being fabricated. Alternatives to multi-layer photolithography, such as Grayscale Lithography, may also be used with the current invention.
In a first embodiment, Grayscale lithography consists of the sequential exposure of different masks on the same layer and at the same exposure step, i.e., top or back-exposure, or a combination of both, as shown schematically in FIG. 10. Another alternative Grayscale Lithography technique is the variation of light intensity across the SU-8 film to obtain topographies with two or more levels. An affordable and easy way to achieve this is by employing the SF-100 Maskless Lithography System from Intelligent Micro Patterning, LLC. The SF-100 systems are based on the Digital. Micromirror Device (DMD) chip from Texas Instruments Inc. (TI), and rely on the same spatial and temporal light modulation technology used in DLP (Digital Light Processing) projectors and HDTVs (high definition televisions).

Summary

In summary, independent of the specific type of master material used, or the precise nature of the master shaping process used to form the master pattern, or the pyrolysis technique employed to convert the master pattern into a carbon mold, or even the nature of the BMG material or TPF conditions used to form the BMG part/foam/composite/mold, the current invention allows for the manufacture of high precision, high surface finish, and high strength BMG parts, foams, composites, and molds using carbon-based structures, on any desired size scale (milli-, micro- and/or nano-), that themselves are derived directly from easily formed master structures. The use of carbon molds in accordance with the current invention allows for the fabrication of inexpensive parts with almost arbitrary lateral geometry and very high aspect ratios with heights on the millimeter range, which are expensive to obtain via conventional technologies such as silicon lithography or the LIGA process. The ability to thermoplastic form BMGs on carbon molds in accordance with the invention offers a versatile technology for the fabrication of inexpensive BMG parts and molds. In addition, the process of the current invention enables the use of different materials and systems than those accessible by conventional mold fabrication processes such as LIGA. Some specific advantages of the current process include:

- The carbon molds can withstand high temperatures in the range where conventional polymer-based molds would deform;
- The carbon molds offer a lower cost alternative to current silicon molds used for BMG;
- The carbon molds are easy to detach from a formed BMG part because of their low thermal expansion;
- The combination of a carbon mold and TPF offers a lower cost process than current mold fabrication options such as LIGA and electroplating;
- The non-destructive detachability of the carbon molds means that the molds can be reused multiple times; and
- The carbon molds can have high aspect ratios, and can be photolithographically defined down to nano-meter sized features.

It should be understood that the above alternative embodiments are not meant to be exclusive, and that other modifications to the basic apparatus and method that do not render the master mold fabrication process inoperative may be used in conjunction with this invention.

EXEMPLARY EMBODIMENTS

The present invention will now be illustrated by way of the following examples, which are exemplary in nature and are not to be considered to limit the scope of the invention.

Methods and Materials

For the following examples, an organic negative photoresist, SU-8 2150 (Microchem), was used as carbon precursor. Traditional photolithography was employed to fabricate SU-8 structures. Materials employed as substrates for the photolithography process included: 1) 4″ Si wafer coated with a 5kA layer of SiO₂(Noel. Technologies), 2) 1.5 mm (0.0590″) thick Polyimide-based Cirlex®, 3) 127 um (0.005″) thick Polyimide (Kapton®) film, and 4) 70 um thick Polyester film (Mc. Master-Carr). A polymer film-holder apparatus was designed in-house and CNC-machined from a 1.78 mm (0.07″) thick aluminum sheet (Mc. Master-Carr).
While SU-8 patterns were fabricated and pyrolyzed on Si/SiO₂, polyimide and Cirlex® substrates, polyester film was only used as a low-adhesiveness release material to obtain free-standing SU-8 structures. (For a full description see, P. Abgrall, et al, J. Micromech. Microeng., vol. 16, pp. 113-121, 2006, the disclosure of which is incorporated herein by reference.) In this process, a thick layer of SU-8 was spun on the polyester film to act as holding substrate for the SU-8 patterns later fabricated on it. Polyester film was then released from SU-8 holding substrate and patterned structures before pyrolysis.
Pyrolysis was conducted on a Thermco Mini-Brute MB-71 diffusion furnace featuring a quartz tube. Nitrogen (Praxair) gas was flowed at 2000 sccm. All pyrolysis processes were conducted at 900° C. Heating ramp was conducted in two steps. First ramp was from 0 to 300° C. at 25° C./min while second ramp was from 300 to 900° C. at 12° C./min. Furnace was then held at 900° C. for 1 hour. Cooling ramp was set to 2° C./min.
BMG molding was conducted using custom heating plates (top and bottom) installed on a load cell of an Instron mechanical testing machine to allow a precise control of temperature and applied pressure during experiments. Carbon molds were heated to 430° C. by the bottom heating plate while a piece of Zr-BMG, an alloy of Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅also known as Vit1b, was placed on the heated mold. After allowing 30 s to equilibrate the temperature of mold and Vit1b, the applied load was increased to attain a preset pressure value of 10 MPa. The applied pressure was kept constant for varying time intervals depending on the mold type and features.
The bulk removal of carbon residues from formed BMG parts was implemented with a Minilock-Phantom III Inductive Conductive Plasma/Reactive Ion Etching, ICP/RIE (TRION Technology). Oxygen plasma was generated at 7 mT of oxygen pressure and 275 W of power. Resultant etch rate was 1.2 um/min. Subsequent carbon removal was implemented with a sonicated acetone bath. Polymer, Carbon and BMG parts were characterized with a Hitachi S-4700-2 FESEM Scanning Electron Microscope.

Example 1

Images from an exemplary fabrication sequence for two different BMG geometries are shown in FIG. 11. In this embodiment, SU-8 molds with holes of 19 (FIG. 11A) and 38 μm (FIG. 11B) and gaps of 17 (FIG. 11A) and 7 μm (FIG. 11B) were fabricated with photolithography on a Si substrate. Carbon molds with feature dimensions of holes of 33 (FIG. 2C) and 46 (FIG. 2D) μm and gaps of 7 (FIG. 11C) and 2.5 (FIG. 11D) μm were obtained by pyrolysis of the SU-8 molds and were then used to TPF the BMG. In this case, the alloy used was Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅(also known as Vit1b), which was molded at 430° C. and 15 MPa. The carbon molds were then released by mechanical means (FIGS. 11E and 11F). Oxygen plasma was then used to remove the bulk of the remaining carbon (FIGS. 11G and 11H) from the BMG while an acetone bath with sonication was used to remove any remaining carbon residues. Finished BMG parts are shown in FIGS. 11I and 11J. The final. BMG structure thickness is approximately 15 μm. As is shown, using the method and molds of the instant invention it is possible to transfer the underlying pattern with high-fidelity onto the final. BMG part.

Example 2

Images from a second set of BMG structures fabricated with the process described in Example 1, above, are shown in FIG. 12. In this example, a carbon mold in accordance with the current invention was used to fabricate parts by TPF for two different BMGs, a Zr-based and a Pt-based alloy. FIG. 12A shows an SEM image of the carbon mold with comb-type structures having spatial features ranging from 20 to 150 μm and having a depth of about 20 μm. A Pt-based alloy was thermoplastically formed on the carbon mold at 275° C. under 20 MPa. FIG. 12B shows and SEM image of the Pt-based BMG part after releasing from the carbon mold of the instant invention. The Zr-based BMG was formed at 435° C. and 20 MPa. FIG. 12C provides an SEM image of a Zr-based BMG part formed in the carbon molds of the instant invention. As marked by the dashed-boxes in FIGS. 12A to 12C, the TPF process replicates the wall roughness of the carbon molds on BMGs with excellent fidelity. This example also demonstrates that the molds of the instant invention may be used with different BMG materials.

Example 3

Images from a third set of BMG structures fabricate with the process described in Examples 1 and 2, above, are shown in FIG. 13. FIG. 13A shows an SEM image of the carbon mold with comb-type structures having spatial features ranging from 20 to 150 um and having a depth of about 20 μm. A Zr-based alloy was thermoplastically formed on the carbon mold at 275° C. under 20 MPa. FIG. 13B shows and SEM image of the Zr-based BMG part after releasing from the carbon mold of the instant invention. The Zr-based BMG was formed at 435° C. and 20 MPa. In each case, the BMG was released from the carbon mold easily because of low thermal expansion of carbon molds.
Wall roughness present in the final. BMG parts (dotted circle in FIG. 13B) is believed to be a direct cause of the mask used in the SU-8 photolithography process (a polymer sheet printed with a high definition photoplotter). Such conclusion was derived based on the images obtained in Example 4, below, where wall roughness can be seen as early in the process as in SU-8 molds. In the case of FIGS. 14B to 14D such roughness gets somewhat amplified after pyrolysis as shown in the respective carbon molds. However, as shown in FIG. 14A, the use of SU-8-only free-standing molds does not seem to amplify such roughness. These images do demonstrate that BMG molding is capable of replicating extremely tiny features present on the mold. Moreover, even when surface finish of molded BMG parts strongly depends on the surface finish of the mold used, BMG surface roughness can be improved by re-heating above their glass transition temperature, an improvement not achievable with metal parts fabricated by conventional methods such as electroplating. (See, e.g., G. Kumar, et al., APL vol. 92, pp. 031901-3, (2008), the disclosure of which is incorporated herein by reference.)

Example 4

As previously discussed, an important refinement to the process of the current invention includes the fabrication of carbon molds obtained from free-standing SU-8-only structures. Such development, besides reducing processing cost, eliminates stresses at the interface of different materials and greatly improves carbon mold fidelity to the original. SU-8 designs. In this example, the formation of free-standing structures using three different substrates was examined.
As discussed previously, the formation of free-standing structures first requires the proper selection of substrate. In particular, it is important to eliminate the stresses due to mismatches on the thermal properties of the precursor materials, such as the substrate and polymeric structures. FIG. 14 shows patterns exposed from similar blank disks on the mask and following the same process described above in Examples 1 to 3. The walls of the cylinder are intended to be vertical. As shown, the use of a SU-8 substrate gives the best results (FIG. 14A). Comparable results are obtained with Kapton® film substrates (FIG. 14B) as the CTE of polyimide approaches that of SU-8. However, the use of Cirlex®, a stack of polyimide films, gives the substrate a rigidity that negatively impacts wall verticality (FIG. 14C). The worst results are obtained with the use of silicon substrates (FIG. 14D). Therefore, the magnitude of the mismatch between the CTE of SU-8 and its substrate and the rigidness of the latter significantly impact SU-8 processing. The negative results obtained with a mismatch on the CTEs of the different materials in the mold are further amplified during pyrolysis as can be qualitatively concluded from FIG. 14 (right column). At the end, the use of SU-8 free-standing mold precursors greatly improves carbon mold fidelity to the original design albeit the isometric carbon shrinkage.
Another improvement studied is the incorporation of a releasable substrate. Based on the printed-circuit-board (PCB) industry experience, a PET film was selected to be used as peel-off substrate. As mentioned before, the use of PET film allowed for its easy and clean release from the SU-8 mold before pyrolysis. Polyester yielded the less adhesion of all of the materials used as substrates and greatly facilitated the peeling of SU-8 molds. Although Polyimide and Cirlex® were not meant to act as release layers, SU-8 molds could also be detached from them. However, such detachment proved to be more difficult than with polyester films and often led to only partial detachment and breakage of SU-8 molds. Detachment was not encountered at any degree on Si/SiO₂substrates. It is important to note however, that PET film should be released after the developing step and not prior. Attempts to remove it prior to development resulted in mechanical deformation of the mold as the adhesion of uncross-linked SU-8 is significantly higher than that of its cross-linked version. The use of a releasable PET film, besides reducing processing costs, eliminates stresses at the interface of different materials since the mold to be pyrolyzed is composed only of SU-8. As shown in this example, the use of a SU-8 substrate improves the fidelity of SU-8 patterns to the design intended originally.

Example 5

Finally, in a last example, millimeter-sized molds have been fabricated featuring undercuts of different heights (FIG. 15). The structures shown were fabricated on SU-8 using gray-scale lithography, and demonstrate that carbon plasma etching makes the use of sacrificial molds with undercuts viable. Moreover, such carbon etching by oxygen plasma allows for the implementation of sacrificial carbon molds featuring overhangs and undercuts of arbitrary size, shape and complexity. It is to be noted that in making these complex shapes, an etching system that attacks the carbon from all directions is preferred over a line-of-sight approach such as Reactive Ion Etching (RIE). Candidates for implementing an isotropic etch include oxygen plasma and xenon difluoride systems. In addition, molds with nanometer dimensions could be implemented using electron-beam, focused ion beam or nanoimprint lithography techniques. Obviously the choice of technique would depend on the desired dimension range.

CONCLUSION

This invention teaches methods and molds that allow for the use of low cost carbon templates to obtain high quality BMG parts. These BMG parts may be the final product or may be used as a further mold for other materials. The fabrication process demonstrated above represents an alternative to electroplating and LIGA. The BMG molding can be carried out using complex alloys, which are intrinsically superior in strength, corrosion resistance and wear resistance compared to conventional electroplated metals. BMGs are also free of crystalline defects and as a consequence are homogeneous and isotropic. The cost of carbon molds for BMGs can be even reduced further by exploring alternative polymers and patterning techniques such as casting, embossing or even CNC machining. The invention will be used for manufacturing high precision, high surface finish, and high strength BMG parts and molds at lower costs departing from carbon-based micro- and/or nano-structures. This use of carbon molds allows the fabrication of inexpensive parts with almost arbitrary lateral geometry and very high aspect ratios with heights on the millimeter range, which are expensive to obtain via silicon lithography or LIGA process. Possible applications include the use of lower cost BMG parts and molds in MEMS, NEMS, precision tools, precision molds, high precision microcomponents, non-silicon-based microfabrication technology, tool-making, mass production of polymer products, biomedical implants, watch movement components, surface patterning, nanoimprinting, and data storage.

DOCTRINE OF EQUIVALENTS

Those skilled in the art will appreciate that the foregoing examples and descriptions of various preferred embodiments of the present invention are merely illustrative of the invention as a whole, and that variations of the present invention may be made within the spirit and scope of the invention. For example, it will be clear to one skilled in the art that alternative molds and molding techniques or alternative configurations of the method and/or apparatus would not affect the improved molds and molding processes of the current invention nor render the method unsuitable for its intended purpose. Accordingly, the present invention is not limited to the specific embodiments described herein but, rather, is defined by the scope of the appended claims.

Claims

1. A method for shaping a bulk-metallic glass material using thermoplastic forming comprising:

patterning a master shape into a material, said material having the ability to substantially maintain its shape during pyrolysis;

pyrolyzing said master shape into a carbon mold, said carbon mold being capable of withstanding the temperatures and pressures necessary to shape the bulk-metallic glass under thermoplastic forming conditions; and

thermoplastically forming the bulk-metallic glass material on the carbon mold to form a shaped bulk-metallic glass article.

2. The method of claim 1, wherein the material is a polymeric material selected from the group consisting of photoresists and organic polymers.

3. The method of claim 2, wherein the polymeric material is selected from the group consisting of SU-8, poly(methyl methacrylate) (PMMA), phenolic resins, polyfurfuryl alcohols, cellulose, polyvinyl chloride and polyimides.

4. The method of claim 1, wherein the step of patterning includes a process selected from the group consisting of stamping, casting, machining, CNC machining, electrical discharge machining (EDM), electrochemical machining (ECM), wet bulk machining, milling, ion beam milling), lithography, photolithography, X-ray lithography, gray-scale lithography, electron beam lithography (EBL), nanoimprint lithography (NIL) and focused-ion beam (FIB).

5. The method of claim 1, wherein the step of patterning is carried out by photolithography, and further comprising inserting a filler between the photolithographic light source and the material to prevent T-topping in the master shape.

6. The method of claim 4, wherein the step of patterning is carried out by photolithography, and wherein the photolithographic pattern is formed by a high-resolution chromium-on-quartz photomask patterned with an e-beam tool.

7. The method of claim 1, wherein the master shape is formed of a pre-patterned biomaterial.

8. The method of claim 1, wherein the master shape comprises a free-standing structure.

9. The method of claim 8, wherein the master shape is formed on a substrate, and wherein the substrate layer in contact with the master shape and the material of the master shape have good coefficient of thermal expansion matching.

10. The method of claim 9, wherein the substrate layer in contact with the master shape and the master shape are formed of the same material.

11. The method of claim 10, wherein the material is SU-8.

12. The method of claim 9, wherein the substrate layer in contact with the master shape comprises a transparent polymeric film.

13. The method of claim 12, wherein the transparent polymeric film is selected from the group consisting of polyimide or polyester.

14. The method of claim 1, wherein the master shape comprises undercuts and overlays.

15. The method of claim 14, wherein the polymeric master shape is patterned using a process selected from the group consisting of multi-layer photolithography and grayscale lithography.

16. The method of claim 1, wherein the material is disposed on a substrate during patterning, and wherein the substrate is made from a material selected from the group consisting of silicon, silicon oxide, silicon nitride, glass, quartz, polyethylene terephthalate, polyimide and the polymeric material.

17. The method of claim 1, wherein the material is patterned such that the walls of said master shape have a positive slope.

18. The method of claim 1, further comprising separating the bulk-metallic glass article from said carbon mold.

19. The method of claim 18, wherein the step of separating uses a process selected from the group consisting of wet immersion, plasma ion etching, reactive ion etching, isotropic etching, mechanical scraping, thermal heating, sonication, and a combination thereof.

20. The method of claim 1, wherein the bulk-metallic glass is selected from the group consisting of Zr-based, Ti-based, Fe-base, Ni-based, Mg-based, Cu-based and Co-based alloys.

21. The method of claim 1, wherein the bulk-metallic glass has a supercooled liquid region (ΔTsc) of at least 30° C.

22. The method of claim 1, wherein the step of thermoplastically forming comprises a technique selected from the group consisting of net-shape processing, micro-replication, nano-replication, extrusion, and superplastic forming.

23. The method of claim 1, further comprising shaping a further material on said bulk-metallic glass article.

24. The method of claim 23, wherein the step of shaping includes using the bulk-metallic glass article as a mold.

25. The method of claim 23, wherein the further material is a polymer, metal or bulk-metallic glass having a molding temperature lower than that of the underlying bulk-metallic glass article.

26. The method of claim 24, wherein the bulk-metallic glass mold is crystallized and the further material is the same bulk-metallic glass material used to make the mold.

27. The method of claim 1, wherein the material is partially carbonized.

28. The method of claim 27, wherein the step of thermoplastic forming further includes infiltrating the bulk-metallic glass material into the partially carbonized mold.

29. The method of claim 28, wherein the bulk-metallic glass article formed is one of either a bulk-metallic glass foam or a carbon/bulk-metallic glass composite.

30. The method of claim 1, further comprising using the carbon mold to form a new master shape and pyrolyzing this new polymeric master shape such that the features of the master shape undergo isometric reduction in size prior to thermoplastically forming the bulk-metallic glass.

31. The method of claim 30, further comprising repeating the step of forming a new master shape until feature sizes of desired dimension are obtained.

32. The method of claim 1, wherein the critical dimensions of the features of the master shape are less than 100 nm.

33. A mold for thermoplastically forming a bulk-metallic glass comprising a carbonized master shape, wherein said carbonized master shape is formed of a material capable of withstanding the temperatures and pressures necessary to shape the bulk-metallic glass under thermoplastic forming conditions.

34. The mold of claim 33, wherein the carbonized master shape is formed from a glass-like-carbon, and wherein the glass-like-carbon is formed by carbonizing a material selected from the group consisting of photoresists and organic polymers.

35. The mold of claim 33, wherein the master shape comprises undercuts and overlays.

36. The mold of claim 33, wherein the walls of the master shape have a positive slope.

37. The mold of claim 33, wherein the critical dimensions of the features of the carbonized polymeric master shape are less than 100 nm.